titania carbon nanotube composites for enhanced...
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TITANIA CARBON NANOTUBE COMPOSITES FOR ENHANCEDPHOTOCATALYSIS
By
GEORGIOS PYRGIOTAKIS
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOLOF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OFDOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
2006
Copyright 2006
by
Georgios Pyrgiotakis
I dedicate this work to my parents and sister,
and to the memory of my grandfather,
the first teacher I ever had.
ACKNOWLEDGMENTS
There are many persons that without their critical and influential support and
guidance this work would have never been accomplished.
I would like to first and foremost thank Dr. Wolfgang Sigmund whose work
ethic, compassion, support, understanding and guidance helped me through this
project. I would also like to thank my committee members, Drs. Milz, Norton,
Sinnott and Koopman for their constructive comments. Very special thanks go to
Dr. Koopman who very closely observed the whole project and whose suggestions
were always influential. Also I would like to thank Dr. Moudgil who always
challenged me to discover new pathways in science. I would also like to thank
Dr. Rinzler for all his help regarding the nanotubes. I would like to recognize
the help of the staff of MAIC (Materials Analytical Instrument Center) regarding
the characterization and the help of Maria Palazeulos regarding the Raman
Spectroscopy.
There are also a lot of students who without their help I would not have
finished this work. I thank Vijay Krishna and Jue Zao for the extensive discussions
about the problems we encountered and all the people in the Sigmund group,
especially Drs. S.-W. Lee, J.-M. Cho and S.-H. Lee. A very warm thank goes to
my dear friends Amit, Junhan, Isaac and Vasana, for their support and help during
my work. Also I would like to acknowledge all the past and current members in the
group for assisting me in many ways during my work.
Finally I would like to acknowledge my parents for their support through all
the rough moments of my life in the USA. Special thanks to my sister for cheering
me up all the time. And last but not least, I tank my friends all over the world
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(China, Germany, Cyprus, Greece, India, Japan, Korea, Taiwan, Turkey, UK and
USA) who constantly showed me love and support. Without them I would have
never accomplished this work. Finally I would like to thank all the people that
worked towards the discovery and perfection of coffee, my ultimate support through
my doctoral.
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TABLE OF CONTENTSpage
ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv
LIST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix
LIST OF FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi
ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xviii
1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Photocatalysis and Titania . . . . . . . . . . . . . . . . . . . . . . 21.2 Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . 21.3 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 PHOTOCATALYSIS ON TiO2 (TITANIA) SURFACE PRINCIPLESAND APPLICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1 Structure of Titania . . . . . . . . . . . . . . . . . . . . . . . . . . 62.1.1 Anatase . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.1.2 Rutile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2 Electronic Properties of Titania . . . . . . . . . . . . . . . . . . . 82.2.1 Anatase . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.2.2 Rutile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.3 Semiconductor Photocatalysis . . . . . . . . . . . . . . . . . . . . 102.3.1 Basic Principles . . . . . . . . . . . . . . . . . . . . . . . . 122.3.2 Enhancement of Photocatalysis . . . . . . . . . . . . . . . . 13
2.4 Applications of Photocatalysis . . . . . . . . . . . . . . . . . . . . 202.4.1 Environmental Applications . . . . . . . . . . . . . . . . . 202.4.2 Photovoltaic Cell . . . . . . . . . . . . . . . . . . . . . . . 21
3 CARBON NANOTUBES (CNTs): STRUCTURE AND ELECTRICALPROPERTIES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.1 Bonding, Structure and Physics of Single-Wall Carbon Nanotubes 243.1.1 Bonding in Carbon Materials . . . . . . . . . . . . . . . . . 243.1.2 Structure and Notation . . . . . . . . . . . . . . . . . . . . 253.1.3 Symmetries and Vibrational Frequencies . . . . . . . . . . 27
3.2 Electronic Properties of SWNT and MWNT . . . . . . . . . . . . 293.2.1 Electronic Properties of SWNT . . . . . . . . . . . . . . . . 293.2.2 Electronic properties of MWNT . . . . . . . . . . . . . . . 34
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3.3 Carbon Nanotubes Growth Mechanisms . . . . . . . . . . . . . . . 343.3.1 Arc Discharge . . . . . . . . . . . . . . . . . . . . . . . . . 353.3.2 CVD: Thermal CVD, PE-CVD . . . . . . . . . . . . . . . . 35
4 ANATASE COATED CARBON NANOTUBES (ANTs): SYNTHESISAND CHARACTERIZATION) . . . . . . . . . . . . . . . . . . . . . . 37
4.1 Design Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 374.2 Nanotube Selection and Preparation . . . . . . . . . . . . . . . . . 39
4.2.1 Materials Selection . . . . . . . . . . . . . . . . . . . . . . 404.2.2 Purification and Dispersion . . . . . . . . . . . . . . . . . . 404.2.3 Characterization of the Functionalized MWNTs . . . . . . 41
4.3 Sol-Gel Coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . 504.3.1 Precursor Selection . . . . . . . . . . . . . . . . . . . . . . 514.3.2 Coating Model . . . . . . . . . . . . . . . . . . . . . . . . . 544.3.3 Long MWNTs . . . . . . . . . . . . . . . . . . . . . . . . . 554.3.4 Short MWNTs . . . . . . . . . . . . . . . . . . . . . . . . . 58
4.4 Characterization of the Composites . . . . . . . . . . . . . . . . . 594.4.1 Short ANTs: TEM, XPS, BET . . . . . . . . . . . . . . . . 614.4.2 Long ANTs: TEM, XPS, BET . . . . . . . . . . . . . . . . 62
5 PHOTOCATALYTIC EVALUATION OF THE SYNTHESIZED PAR-TICLES WITH DYE DEGRADATION TESTS . . . . . . . . . . . . . 68
5.1 Experimental Setup, Materials and Procedures . . . . . . . . . . . 695.1.1 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . 695.1.2 Dye Selection . . . . . . . . . . . . . . . . . . . . . . . . . 705.1.3 Experimental Procedure . . . . . . . . . . . . . . . . . . . 72
5.2 Theory for the Photocatalytic Degradation of Dyes . . . . . . . . 745.3 Parameters that Influence the Photocatalytic Reaction . . . . . . 76
5.3.1 pH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 775.3.2 Initial Dye Concentration . . . . . . . . . . . . . . . . . . . 775.3.3 Intensity of the Radiation . . . . . . . . . . . . . . . . . . . 795.3.4 Solids Loading/Surface Area . . . . . . . . . . . . . . . . . 79
5.4 Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 815.4.1 Titania Nanoparticles and Carbon Nanotubes . . . . . . . . 835.4.2 Long ANTs: Photocatalysis under UV Light . . . . . . . . 875.4.3 Long ANTs: Photocatalysis under Visible Light . . . . . . 885.4.4 Long ANTs: Post UV Activity, Photocatalysis in Dark . . 925.4.5 Short Nanotubes: Photocatalysis under UV . . . . . . . . . 92
5.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
6 SPECTROSCOPIC TECHNIQUES TO EXPLAIN THE PHOTOCAT-ALYTIC EFFICIENCY OF THE ANTs. . . . . . . . . . . . . . . . . . 96
6.1 Raman Spectroscopy of the Carbon Nanotubes . . . . . . . . . . . 97
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6.1.1 General Theory of Raman Spectroscopy of Carbon Nan-otubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
6.1.2 Basic Raman Lines for Carbon Nanotubes . . . . . . . . . 1006.2 Raman Spectroscopy of the Anatase Structure of TiO2 . . . . . . 1046.3 Experimental Procedures . . . . . . . . . . . . . . . . . . . . . . . 106
6.3.1 Sample Preparation . . . . . . . . . . . . . . . . . . . . . . 1066.3.2 Mathematical Analysis and Manipulation . . . . . . . . . . 107
6.4 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . 1116.4.1 Long Nanotubes after the Acid Treatment . . . . . . . . . 1126.4.2 Short Nanotubes after the Acid Treatment . . . . . . . . . 1136.4.3 Long Nanotubes after the Coating . . . . . . . . . . . . . . 1176.4.4 Short Nanotubes after the Coating . . . . . . . . . . . . . . 1236.4.5 Summary of the Raman Spectra Analysis . . . . . . . . . . 130
6.5 X-Ray Photoelectron Spectroscopy (XPS) . . . . . . . . . . . . . 1316.5.1 Instrument, Sample Preparation and Mathematical Analysis 1326.5.2 XPS of the Reference Material . . . . . . . . . . . . . . . . 1336.5.3 XPS of the s-ANTs . . . . . . . . . . . . . . . . . . . . . . 1376.5.4 XPS of the ℓ-ANTs . . . . . . . . . . . . . . . . . . . . . . 143
6.6 Summary of the XPS Analysis . . . . . . . . . . . . . . . . . . . . 147
7 CONCLUSIONS AND FUTURE WORK . . . . . . . . . . . . . . . . . . 157
7.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1607.2 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
APPENDICES
A MATHEMATICA ALGORITHM USED FOR THE LOESS METHOD . 162
B RAMAN PEAKS OF CNTs . . . . . . . . . . . . . . . . . . . . . . . . . 165
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166
BIOGRAPHICAL SKETCH . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188
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LIST OF TABLESTable page
4–1 The calculated initial molecular ratio for the reactions for the ℓ-CNTs 55
4–2 The calculated initial molecular ratio for the reactions regarding theshort nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
5–1 The oxidation intermediates and their structure to be compared tothe initial dye structure in figure 5–2. . . . . . . . . . . . . . . . . . 74
5–2 Summary of the experiments performed . . . . . . . . . . . . . . . . . 84
5–3 Summary of the experimental results of this chapter. . . . . . . . . . . 94
6–1 The Raman frequencies fro anatase and rutile phase of titania. Thebrookite is not included here since is not a present form of TiO2
and it has in total 36 weak peaks. The notation in parenthesis isrepresenting the relative intensity of the peaks; w: weak; m: medium;s: strong; vs: very strong. Data are adapted from reference mate-rial and reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
6–2 The raw fitting parameters calculated with the Levenberg-Marquardtalgorithm for the acid treated ℓ-CNTs. The graphic representationof the results is in figure 6–6. The fit yielded χ2 =7.1333×104. Forconvenience at the data representation we use the symbol a
(2)L in-
stead of Γ that is used in equation 6−20. . . . . . . . . . . . . . . . 113
6–3 The raw fitting parameters calculated with the Levenberg-Marquardtalgorithm for the acid treated s-CNTs. The graphic representationof the results is in figure 6–7. The fit yielded χ2 =3.9138×107. . . . 115
6–4 The raw fitting parameters calculated with the Levenberg-Marquardtalgorithm for the titania coated ℓ-CNTs and the titania segment ofthe spectrum. The graphic representation of the results is in figure6–9. The fit yielded χ2 = 8.3378 × 104. . . . . . . . . . . . . . . . . 117
6–5 The raw fitting parameters calculated with the Levenberg-Marquardtalgorithm for the titania coated ℓ-CNTs. The graphic representa-tion of the results is in figure 6–6. The fit yielded χ2 = 8.3378×104.For convenience at the data representation we use the symbol a
(2)L
instead of Γ that is used in equation 6−20. . . . . . . . . . . . . . . 121
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6–6 The raw fitting parameters calculated with the Levenberg-Marquardtalgorithm for the acid treated ℓ-CNTs. The graphic representationof the results is in figure 6–12. The fit yielded χ2 =1.9924×108. . . . 125
6–7 The raw fitting parameters calculated with the Levenberg-Marquardtalgorithm for the coated s-CNTs. The graphic representation of theresults is in figure 6–12. The fit yielded χ2 =1.0956×105. . . . . . . 127
6–8 Summary of the Raman result. Here are listed the major peaks andshift both for titania and CNTs after the coating. . . . . . . . . . . 130
6–9 Summary of the XPS peaks . . . . . . . . . . . . . . . . . . . . . . . . 147
7–1 Electron affinity and work function for metals used to create rectify-ing contact with titania in order to increase the photocatalytic effi-ciency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
B–1 Properties of the various Raman features in graphite and SWNTs. . . 165
x
LIST OF FIGURESFigure page
2–1 The two basic titania structures. . . . . . . . . . . . . . . . . . . . . . 7
2–2 The electronic band structure of the two main phases of titania. . . . 8
2–3 Schematic diagram representing the main photocatalysts with theirbandgap energy. In order to photo-reduce a chemical species, theconductance band of the semiconductor must be more negative thanthe reduction potential of the chemical species; to photo-oxidize achemical species, the potential of the valence band has to be morepositive than the oxidation potential of the chemical species. Theenergies are shown for pH 0. . . . . . . . . . . . . . . . . . . . . . . 11
2–4 Schematic representation of the reactions taking place in titania. 1OLight strikes the semiconductor. 2O An electron-hole pair is formed.3O Electrons and holes are migrating to the surface. 4O The holesinitiate oxidation leading to CO2, Cl−H+, H2O. 5O The conductionband electrons initiate reduction reactions. 6O electron and holesrecombination to heat or light. . . . . . . . . . . . . . . . . . . . . 13
2–5 Titania band structure (a) before and (b) after doping. The transi-tion metals are interstitial or substitutional defects in the structureof titania and generate trapping levels in the bandgap. . . . . . . . 14
2–6 The principles of rectifying contact between titania (Eg=3.2 eV) anda metal with work function (φm), in this example 5 eV, greater thanthe affinity (χs) of titania. . . . . . . . . . . . . . . . . . . . . . . . 16
2–7 The principles of rectifying contact between anatase (α) titania (Eαg =3.2
eV) and and rutile (r) titania (Erg=3.0 eV). . . . . . . . . . . . . . 18
3–1 The 2D graphene sheets is shown with the a1 and a2 specifies thechirality of the nanotube. The chiral vector, Ch, is the OA, whilethe translation vector T is the OB. Also ψ is the rotation angleand τ the translation. Those two are constitute the symmetry op-eration R = (Ψ|τ). . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
xi
3–2 The graphene sheet is shown with the (n,m) pair which specifies thechiral nanotube. The pair of integer (n,m) in the figure specifiesthe chiral vector Ch for carbon nanotubes, including zigzag, arm-chair and chiral tubules. Below each pair of integer is listed thenumber of distinct caps that can be joined continuously to the cylin-drical carbon tubule denoted by (m,n) [ref]. It is also denoted theconduction state of every chirality. . . . . . . . . . . . . . . . . . . 28
3–3 The dispersion for graphite as calculated from equation 3−10. . . . . 30
3–4 The dispersion energies for two different chilarities. . . . . . . . . . . 32
4–1 SEM pictures of the two types of nanotubes. . . . . . . . . . . . . . . 42
4–2 TEM images of the s-CNTs. . . . . . . . . . . . . . . . . . . . . . . . 43
4–3 TEM images of the ℓ-CNTs. . . . . . . . . . . . . . . . . . . . . . . . 44
4–4 Immediate comparison of the two different kinds of nanotubes. . . . 45
4–5 The zeta potential for both the ℓ-CNTs (a) and s-CNTs (b). It showsthe shift of the IEP for the ℓ-CNTs (from 7 to 3.5) and the increaseat the surface charge for the s-CNTs (from -10 mV to -37 mv forph 4). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
4–6 The FTIR of the MWNTs after the acid treatment (only the s-CNTsresults are displayed). The bands that have been identified provethe reaction of the −COOH on the surface of the nanotubes. . . . . 47
4–7 The differential volume and number of the s-CNTs before and afterthe acid treatment. . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
4–8 The TGA/TDA data of the s-CNTs. The peak at the 600 indi-cates the burning temperature of the CNTs. It is observed about6% of the initial mass residue, which is the Fe catalyst. . . . . . . . 49
4–9 The different Sol-Gel precursors used in this research. . . . . . . . . . 53
4–10 Schematic diagram of the process for the coating of the ℓ-CNTs. . . . 56
4–11 Schematic diagram of the process for the coating of the s-CNTs. . . . 59
4–12 The TGA/TDA data of the s-ANTs. The peak at the 100 is fromthe water evaporation and therefore it is accommodated by a massreduction. At approximately 250 the phase transition is startingand carries on until the 500. . . . . . . . . . . . . . . . . . . . . . 60
4–13 TEM images of the coated s-CNTs. . . . . . . . . . . . . . . . . . . . 61
4–14 TEM images of the coated ℓ-CNTs. . . . . . . . . . . . . . . . . . . . 62
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4–15 The universal curve of the electrons, based on the calculations by M.P. Seah and W. A. Dench. The curve shows the mean free pathof the electrons as function of the kinetic energy (dashed lines).There are also experimental results that follow the same trend. Themean free path does not depend on the material. For Mg sourcethe X-Ray energy is 1253.6 eV, which give a mean free path of ap-proximately 10 A. . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
4–16 XRD patterns with and without the coating. . . . . . . . . . . . . . . 65
4–17 XPS survey for the s-ANTs. There is a significant amount of TiO2
(16.7% Ti). There is no direct stoichiometry with the oxygen (52%O) since the oxygen depends on the exposed crystallographic orien-tation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
4–18 XPS survey for the ℓ-ANTs. There is a significant amount of TiO2
(1.2% Ti). Again there is no stoichiometry with the oxygen (5.8%O). There is less TiO2 compared to the s-ANTs. . . . . . . . . . . . 67
5–1 Schematic diagram showing the basic elements of the photocatalyticdegradation chamber. . . . . . . . . . . . . . . . . . . . . . . . . . 69
5–2 Three-dimensional structure of the Brilliant Procion Red MX-5 molecule.As it can be seen it contains 3 benzene groups and a benzene groupwith three carbon atoms replaced by nitrogen atoms (s-triazine). . . 71
5–3 The absorption spectrum for a 5 ppm solution of the Procion RedMX-5B dye. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
5–4 The structure of several intermediate products of the photocatalyticreaction that show the destruction of the bonds and the size reduc-tion of the molecules. . . . . . . . . . . . . . . . . . . . . . . . . . . 73
5–5 Comparison between the numerical solution of the Langmuir-Hinshelwood(equation 5−1) and the approximation. The red lines represent theapproximation and the black is the numerical solution. The solidline represents the dye concentration while the dashed represents
reaction rate ddt
(CC0
)
. . . . . . . . . . . . . . . . . . . . . . . . . . 75
5–6 The main parameters that influence the oxidation rate. . . . . . . . . 78
5–7 The pH variation during the dye degradation. The initial value be-tween the ANTs and Degussa P25 since the specific surface area isdifferent. In the first case the pH is stabilized after 10 min while inthe second case that occurs after 20 min. In both cases the stablepH value is lower than the initial. . . . . . . . . . . . . . . . . . . . 80
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5–8 The dye spectrum during the different time intervals. The three dashedlines (513, 524 and 537 nm) are the three wavelengths that wereused for the C/C0 calculation. The data were obtained from a sam-ple of 3 mg Degussa P25 in a 50 ml of 5 ppm dye solution. . . . . . 81
5–9 Investigation of the dye degradation under the UV light for two dif-ferent dye concentrations. The UV is not having an apparent im-pact on the dye. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
5–10 The results for the experiments A-1 to A-4. . . . . . . . . . . . . . . . 85
5–11 Collective graph of the data presented above. . . . . . . . . . . . . . . 86
5–12 Investigation of the dye adsorption on the carbon nanotubes surface.The adsorption was not significant since it was only 5% reductionafter 90 min. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
5–13 Photocatalytic degradation of Degussa P25 and ℓ-ANTs under UVlight of 350 nm wavelength. . . . . . . . . . . . . . . . . . . . . . . 89
5–14 The photocatalytic results of the ℓ-ANTs and Degussa P25. The ℓ-ANTs clearly demonstrate photocatalytic activity with τ=152.31±6.13min. Degussa P25 is not demonstrating any obvious activity. . . . . 90
5–15 The dye degradation data in the dark for the ℓ-ANTs. Degussa is notincluded here since it never demonstrated behavior like such. Thedata were fitted with the equation 5−9. τDARK
ℓ−ANTs=1.29±0.24 days.The constant is 0.76±2.75×10−2. . . . . . . . . . . . . . . . . . . . 91
5–16 The dye degradation data in the UV light of 350 nm for the s-ANTs.τUVs−ANTs=177.41±10.00 mins. The photocatalysis is significantly
slower that all the previous cases. . . . . . . . . . . . . . . . . . . . 93
6–1 The different Raman scattering processes for CNTs. . . . . . . . . . . 98
6–2 Graphic representation of the major Raman modes. . . . . . . . . . 100
6–3 Typical Raman spectra from metallic and semiconducting SWNTs.The Radial Breathing Mode (RBM), the D Band and G Band arethe most important bands. The * is denoting bands that come formthe Si substrate. Due to the distinct structure of the semiconduct-ing nanotubes there are two additional bands M and iTOLA thatappear. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
6–4 The G Band split and how it is related to the conductivity of the tubes.101
6–5 Different options for the LOESS algorithm. . . . . . . . . . . . . . . . 109
xiv
6–6 The ℓ-CNTs after treated with nitric acid at 140 for 10 hours. TheD Band is showing at 1312 cm−1 and the G Band at about 1594cm−1. A very distinct split of the band can be seen with the G+ atthe 1584 cm−1 and G− at 1612 cm−1. . . . . . . . . . . . . . . . . . 114
6–7 The s-CNTs after treated with nitric acid at 100 for 6 hours. TheD Band is showing at 1305 cm−1 and the G Band at about 1586cm−1. Although the G Band looks like it consists on to overlap-ping peaks it still can be treated as one peak. . . . . . . . . . . . . 116
6–8 The Raman spectra of the coated long nanotubes. There are two sep-arate regions, (i) 0-1000 cm−1 that contain the titania peaks and(ii) 1000-1800 cm−1 that contain the carbon nanotubes peaks. Thepeak identification is done later in the chapter. . . . . . . . . . . . 118
6–9 The first region from figure 6–8. There are four major peaks but onlythree of them can be identified accurate. 149.56 cm−1, 628.65 cm−1
and 408.64cm−1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
6–10 The second region from figure 6–8. The D Band is at 1307 cm−1 andthe G Band is at the about 1590 cm−1. The band split still exists,with the G− at 1579 cm−1 and the G+ at 1606 cm−1. . . . . . . . . 122
6–11 The Raman spectra of the coated short nanotubes. There are twoseparate regions, (i) 0−1000 cm−1 that contain the titania peaksand (ii) 1000−1800 cm−1 that contain the carbon nanotubes peaks.This spectra has been obtained by the combination of two differentruns. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
6–12 The first portion of figure 6–11. There are 5 very distinctive peaks at150 cm−1, 202 cm−1, 393 cm−1, 510 cm−1 and 633 cm−1. . . . . . . 126
6–13 The second portion of figure 6–11. Although the carbon peaks arenot very clear we can still see them at the 1316 cm−1 the G Bandand at the 1582 cm−1 the G Band. The G Band seems to be split-ting in two peaks 1544 cm−1 and 1582 cm−1. The ratio betweenthe peaks is completely reversed but this is currently attributed tothe weak signal obtained by the s-CNTs in this case. . . . . . . . . 128
6–14 The C1s peak for the reference anatase nanoparticles. The major peakis at the 286.4 eV that is agreement with literature and several databases.134
6–15 The Si2p peak for the reference anatase nanoparticles. The majorpeaks are at the 98.5 eV for the Si2p1/2 and at 102.5 eV for theSi2p3/2 which are in agreement with literature and several databases. 135
xv
6–16 The O1s peak for the reference anatase nanoparticles. The major peaksare at the 529.6 eV, represents the lattice oxygen, and the 531.5eV for the surface oxygen. which are agreement in with literatureand several databases. . . . . . . . . . . . . . . . . . . . . . . . . . 136
6–17 The Ti2p peak for the reference anatase nanoparticles. The majorpeaks are at the 458.4 eV for the Ti2p1/2 and at 464.2 eV for theTi2p3/2 which are in agreement with literature and several databases. 138
6–18 The C1s peak for the s-ANTs. The major peak is appearing to the284.6 eV, which is again in great agreement with literature values.The peak at 285.9 eV is characteristic of the C−O bond while the289.5 eV peak is attributed to C−O−Ti. . . . . . . . . . . . . . . . 139
6–19 The O1s for the s-ANTs. The major peaks are again at 530.6 eV forthe O1s for the lattice oxygen and the 532.7 eV for the surface oxy-gen. The ratio between those two peaks reveals the surface are ofthe particle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
6–20 The Ti2p peak for the s-ANTs. The major peaks are at the 459.4 eVfor the Ti2p1/2 and at 465.1 eV for the Ti2p3/2. . . . . . . . . . . . 142
6–21 The C1s peak for the ℓ-ANTs. Again the major peak appears to beat 284.6 eV while there is a secondary peak at 285.2 eV. This peakis similar to the case of s-ANTs that appears to 285.9 eV. It is againattributed to the C−O bond or C=O bond. . . . . . . . . . . . . . 144
6–22 The O1s peak for the ℓ-ANTs. There are also two peaks observedat 532.7 eV and at 530.9 eV. Although both are from the oxygenthe 532.7 eV is attributed to surface oxygen while the other comesfrom lattice oxygen contribution. Relative to the case of s-ANTsthe surface oxygen and therefore the surface area is higher, some-thing that was confirmed with BET as well and is in agreementwith other researchers. . . . . . . . . . . . . . . . . . . . . . . . . . 145
6–23 The Ti2p peak for the ℓ-ANTs. The major peaks are at the 459.6 eVfor the Ti2p1/2 and at 465.2 eV for the Ti2p3/2 which are in signifi-cantly shifted compared to the reference material. . . . . . . . . . . 146
6–24 Collective representation if the XPS data regarding the coated longcarbon nanotubes. The upper row is the Ti2p and O1s peak of thereference material and the lower row is the data obtained by the s-ANTs. The shifts in both peaks are obvious and are summarizedin table 6–9. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
xvi
6–25 Collective representation if the XPS data regarding the coated shortcarbon nanotubes. The upper row is the Ti2p and O1s peak of thereference material and the lower row is the data obtained by the ℓ-ANTs. The shifts in both peaks are obvious and are summarizedin table. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150
6–26 Collective representation if the XPS data regarding the coated longand short carbon nanotubes. The upper row is the Ti2p and O1speak of the s-ANTs and the lower row is the data obtained by theℓ-ANTs. The peaks are similar regarding the position, but are sig-nificantly different in shape. . . . . . . . . . . . . . . . . . . . . . . 151
6–27 The C1s peak of the peak of the coated carbon nanotubes (both ℓ-ANTs and s-ANTs) and the reference material. The main differ-ence between the reference material and the samples are the peaksregarding the C−O and C=O bonds, that are appearing only forthe s-ANTs and ℓ-ANTs, and the peak at 289.7 eV (ℓ-ANTs) and289.5 eV (s-ANTs) that can be attributed to the C−O−Ti bond. . 152
6–28 The Si2p peak of the peak of the coated carbon nanotubes (both ℓ-ANTs and s-ANTs) and the reference material. Al the peaks areat the same energy, but the noise to signal ratio is a lot higher forthe both ℓ-ANTs and s-ANTs. The reason for that is the thicknessof the coating. The coated MWNTs were deposited in a thicker layer.153
6–29 Collective representation of the Raman spectra regarding the shortnanotubes before (top row) and after the coating (bottom row).The right column is for the G band and the left column is for theD band. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
6–30 Collective representation of the Raman spectra regarding the longnantubes before (top row) and after the coating (bottom row). Theright column is for the G band and the left column is for the D band.155
6–31 Collective representation if the XPS data regarding the coated longcarbon nanotubes. The upper row is the Ti2p and O1s peak of thereference material and the lower row is the data obtained by the s-ANTs. The shifts in both peaks are obvious and are summarizedin table. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
xvii
Abstract of Dissertation Presented to the Graduate Schoolof the University of Florida in Partial Fulfillment of theRequirements for the Degree of Doctor of Philosophy
TITANIA CARBON NANOTUBE COMPOSITES FOR ENHANCEDPHOTOCATALYSIS
By
Georgios Pyrgiotakis
May 2006
Chair: Wolfgang M. SigmundMajor Department: Materials Science and Engineering
Photocatalytic composites have been used for the past few decades in a wide
range of applications. The most common application is the purification of air
and water by removing toxic compounds. There is limited use however towards
biocidal applications. Despite their high efficiency, photocatalytic materials
are not comparable to the effectiveness of conventional biocidal compounds
such as chlorine and alcoholic disinfectants. On the other hand, nearly a decade
ago with the discovery of the carbon nanotubes a new vibrant scientific field
emerged. Nanotubes are unique structures of carbon that posse amazing electrical,
mechanical and thermal properties.
In this research carbon nanotubes are used as photocatalyitic enhancers. They
were coated with anatase titania to form a composite material. Two different types
of nanotubes (metallic versus non-metallic) were used and the photocatalytic ac-
tivity was measured. The metallic tubes demonstrated exceptional photocatalyitic
properties, while non-metallic tubes had low photocatalytic efficiency. The reason
for that difference was investigated and was the major focus of this research.
xviii
The research concluded that the reasons for the high efficiency of the carbon
nanotubes were (i) the metallic nature of the tubes and (ii) the possible bond
between the titania coating and the underlying graphite layers (C−O−Ti). Since
both composites had the same indications regarding the C−O−Ti bond, the
metallic nature of the carbon nanotubes is believed to be the most dominant factor
contributing to the enhancement of the photocatalysis. The composite material
may have other potential applications such as for sensing and photovoltaic uses.
xix
CHAPTER 1INTRODUCTION
The last few decades the demand for safer environmental conditions has
increased dramatically. One reason is the constantly increasing biological threats
that can be expressed in every aspect of the daily life, ranging from cases as simple
as food bacterial contamination (E. coli and salmonella) to extremely dangerous
such as epidemic outbreaks (Ebola and SARS ) and biological warfare (anthrax and
smallpox ). The need for effective and efficient disinfection is driving the industry in
the development of a wide range of products. These products can be divided in to
three major categories:
Chemical disinfectants: Chemical based disinfectants are the majority of they
have been used for the longest time. Most of them are chlorine, alcohol or
ammonium based products. They are in liquid form and therefore are limited
to surfaces. The majority are used to disinfect contaminated surfaces and not
to prevent contamination. Although their use is relatively simple and easy
they can still be dangerous if they are misused. Gasses can also be used for
the disinfection, but they are limited since they are extremely corrosive.
Radiation based disinfection: The radiation is a very effective technique since
it can immediately inactivate the majority of the contaminants without
damaging the surroundings. Still however the use is limited since it usually
requires expensive equipment and under certain conditions exposure to the
used radiation can be proved dangerous.
Passive disinfectants: Passive disinfectants are characterized those that do
not require a certain application (chemicals) or operation (radiation), but
constantly purify and clean surfaces, air and water. Activated carbon filters
1
2
are probably the best known and most widely used, since they are used
widely for water and air treatment. However they do not deactivate the
contaminants so constant replacement is required. If they are not replaced
regularly they can become a source of contamination rather than disinfection
medium.
The lack of efficient passive disinfectants has led the researchers to seek solu-
tions capable to provide both capturing and inactivating of biological contaminants.
One of the most promising and rapidly emerging fields is photocatalysis.
1.1 Photocatalysis and Titania
Photocatalysis is the type of reaction that takes place on the surface of a
certain type of material in the presence of a very specific range of radiation. There
are many materials that can display this type of reaction, but the most widely used
is titanium dioxide, TiO2, or titania. Titania in addition to the high efficiency is
cheap and environmentally safe.
There are significant limitations, however, to the application of titania since
the efficiency are not high enough or at least competitive with the results that
the chemical techniques can deliver. Chapter 2 will give a brief overview of the
principles of photocatalysis and specifically the catalysis on the surface of titania.
Emphasis will be given to the structure of titania and its electric properties, the
two primary reasons for the excellent photocatalytic properties. It will outline the
basic techniques that are currently used to improve the efficiency and finally will
discuss the major applications of titania.
1.2 Carbon Nanotubes
An also rapidly emerging field is the investigation of the properties of the
carbon nanotubes. They are a relative new material that has attracted great deal
of attention due to the unique shape and structure. Carbon nanotubes can be
visualized a graphite sheet that has been rolled seamlessly into a tube. It has been
3
more than a decade since the first report of nanotubes. Their unique properties,
that arise form their structure, have not yet completely understood. Probably the
most outstanding properties are the electronic. In addition, their needle-like shape
results in very high specific surface area. Both characteristics are very important to
the present research.
Although their properties are very unique and unmatchable, so far there is no
commercial application in small or large scale that takes full advantage of them.
Chapter 3 will explain in detail the structure and later the properties of the carbon
nanotubes. It will also give a short description of most popular methods used today
for nanotubes production.
1.3 Objectives
In this research those two unique materials will be combined in the form of a
nano-composite that will deliver a high efficient photocatalyst. There are several
researchers that have already achieved it, but the composites have never been
investigated in-depth. Therefore this research has the followings objectives: To synthesize TiO2-MWNTs composites To evaluate the photocatalytic efficiency To explain the behavior of the material
There are two distinct trends in combining those two materials: either in the
form of titania pellet with the nanotubes embedded, or in a more sophisticated
approach, the titania is applied as a coating on the nanotubes. In this research
the second approach was selected since it takes full advantage of the nanotube
properties, by creating a composite with a single nanotube as core.
To investigate the impact of the nanotube properties there are two different
composites synthesized. One has a pristine and highly crystalline core and the
other has of a less ordered tubular structure. The direct comparison of those
4
two composites will explain the effect of the electric properties, if any, at the
photocatalytic efficiency. All the synthesis is explained in detail in chapter 4.
The photocatalytic evaluation is done via dye reduction tests. Those types of
tests are very common and are preferred since they give fast, accurate and reliable
results. A drawback of those tests is the many parameters that can impact the
results and therefore they have to be monitored while the tests are executed, but
it is something that can easily be done. Those parameters and the experiments are
discussed in detail in chapter 5.
In order to explain the behavior of the material it is critical to select tech-
niques that directly or indirectly will determine the properties of the nanotubes.
One of the most recognized techniques for that is the Raman spectroscopy. In
addition to Raman, X-Ray Photoelectron Spectrometry (XPS) can be used to in-
vestigate the structure of the titania and point out structural differences that may
be related to the photocatalytic evaluation results. The complete analysis of those
two techniques, along with the necessary theory to understand Raman and XPS, is
discussed in chapter 6.
All the experimental results from chapters 4, 5 and 6 will be used to draw
conclusions on how the carbon nanotubes behave as a photocatalytic template and
what the impact of their electrical properties is on the final result.
CHAPTER 2PHOTOCATALYSIS ON TiO2 (TITANIA) SURFACE: PRINCIPLES AND
APPLICATIONS
Recently semiconductor photocatalysis has attracted a great deal of attention
since it has a wide range of applications [1, 2]. One of the most interesting mate-
rials is titania (TiO2) [3–5]. TiO2 is the material that is used here as coating on
the carbon nanotubes. It is widely available since it is used as pigment in many
applications and the production is fairly cheap [4]. Since 1972 when Fujishima et
al. reported the photocatalytic split of the water on TiO2 electrodes [6] a great deal
of research had been done to developeapplication and enhancing the properties of
titania. The applications range from photovoltaic cells to biological disinfection
[3–5].
One of the most popular applications is the microbial sterilization and self-
cleaning surfaces [7–15]. There are certain limitations however, coming primarily
from the electronic properties of the titania, that reduce the efficiency [4, 5]. The
biggest breakthrough came in 1985 by Matsunaga et al. [9] where by mixing the
titania with silver particles the observed significant enhancement of the catalysis.
Since that time the range of applications has increased dramatically.
This chapter covers the basic information necessary to explain the properties of
titania. The first section is about the crystal structure and the electronic properties
of titania. Later the chapter reviews the basic principles behind photocatalysis and
the recent advances towards the improvement of the efficiency. The last part of the
chapter gives a brief overview of the most important applications of titania.
5
6
2.1 Structure of Titania
Titanium dioxide (titania) exists in principle in eight phases rutile, anatase,
brookite, columbite, baddeleyite, flourite, pyrite, and cotunnite [16]. From those
eight phases thermodynamically more stable are rutile, anatase and brookite, with
rutile to be the most stable [17, 18]. Since photocatalytic activity is demonstrated
only from rutile and anatase, the analysis will focus on those two structures only.
The columbite, baddeleyite, flourite, pyrite, and cotunnite phases can be generated
only under very high temperatures and/or pressures, which is the reason those
phases do not occur naturally [19–21], but they still possess some very interesting
properties. Cotunnite for example is the hardest polycrystalline material known to
exist [16, 22].
2.1.1 Anatase
Figure 2–1(a) shows the crystal structure of anatase. It is tetragonal with
a = b = 3.782 A and c = 9.502 A and has a D194h-I41/amd symmetry. The building
block on anatase is the TiO6 which forms a slightly deformed octahedron (figure
2–1 (c)). The Ti atom that is in line with the two oxygen atoms (apical oxygen
atoms) has bond length of 1.996 A and the other four oxygen atoms (equatorial
oxygen atoms) have Ti−O bond lengths of 1.937 A. The widest angle of those two
bonds Ti−Oequatorial and Ti−Oapical is 102.308°. The angle between two consecutive
equatorial bonds is 92.604°or 87.394°. All the bond lengths and angles given above
represent the structure at room temperature.
Anatase is an unstable structure and it transforms to rutile at approximately
800 . While the temperature increases, the bond lengths are changing and grad-
ually the anatase turns into rutile [16, 17]. Rutile has a more compact structure
and therefore energy wise is more favorable. The transformation to rutile is an
irreversible process.
7
(a)
(b)1 0 2 . 3 0 8 9 2 . 6 0 41 . 9 3 7 A1 . 9 6 6 Ao o o
o(c)
9 8 . 9 3 9 01 . 9 4 6 A 1 . 9 8 3 Ao o oo(d)
Figure 2–1: The two basic titania structures (a) anatase and (b) rutile. Thedistorted octahedron that are shown are used to construct the (c)anatase and (d) the rutile.
2.1.2 Rutile
Rutile has also a tetragonal structure (2–1(b)), but it is a lot more compact
compared to anatase [16, 23–25]. The tetragonal structure has a = b = 4.584 A and
c = 2.953 A. It has D154h-P42/mmm symmetry [16, 25, 26]. Again the building block
of the crystal structure has an octahedral that is slightly distorted (figure 2–1(d)).
The apical oxygen atoms have Ti−O bond length of 1.983 A and the equatorial
Ti−O bond is 1.946 A. The equatorial and apical Ti−O bonds form a right angle
while the largest angle between the two equatorial bonds is 98.93°.
8
(a) (b)
Figure 2–2: The electronic band structure of the two main phases of titania (a)rutile and (b) anatase [25]. The calculation is based on first principlesself consistent OLCAO.
The bond length in rutile does not change significantly with the temperature.
It is therefore thermally a stable structure and all the different phases will turn into
rutile after annealing at high temperatures for an extended period.
2.2 Electronic Properties of Titania
The electronic structure of titania has been studied both experimentally
and theoretically. Experimentally it has been probed by X-Ray photoelectron
spectroscopy [27–30] (XPS), X-Ray induced Auger electron spectroscopy [28],
Auger electron spectroscopy [28], X-Ray emission [31, 32] (XES), absorption spec-
troscopy [33, 34] (XAS), electron energy loss spectroscopy [27, 35–37] (EELS),
ultraviolet photoelectron spectroscopy (UPS) [38] and resonant ultraviolet photo-
electron spectroscopy (RUPS) [38]. The theoretical analysis has been done mainly
with total-energy calculations within the LDA using pseudopotential plane wave
9
formalism [23, 24, 39], as well as the more recent Hartree-Fock pseudopotential cal-
culations [40]. Recently very accurate self-consistent ab initio calculations for TiO2
have been performed. Prior to those methods the attempts to theoretically predict
the electronic structure of titania were done based on the tight-binding [41–47]
(TB) calculations and the extended Huckel molecular orbital method [33, 36].
Certain defects in the crystal structure can impact the electric properties
of titania. Titania is an oxygen deficiency material and usually it is considered
n-type semiconductor. The Fermi-level therefore is not at a fixed value since the
production method will determine the oxygen deficiency and therefore the Fermi-
level shift. This is true for both anatase and rutile. In addition one of the most
common defects in titania is the Ti+4 substitution by Ti+3 (and often Ti+2 an
Ti+1) [48, 49], which also creates a charge imbalance that beyond for the electrical
properties, can affect spectroscopic techniques that rely on the electronic charge,
such as XPS. Those Ti cations can be generated by annealing, sputtering or
chemical reduction.
2.2.1 Anatase
Figure 2–2(b) shows the anatase band structure. The bandgap has been
experimentally measured and is 3.2 eV [50], while the theoretically determined
values can vary from 2.2 eV up to 3.89 eV [25]. Those differences are related
to the number of atoms accounted to the calculations and most important the
non-constant bond length in the crystal (section 2.1.1). For this research the
experimental value of 3.2 eV, which has been repeatedly confirmed [50], will be
accepted as the bandgap energy. The width of the valence band is 4.75 eV and
the distance between the uppermost conduction band state and the lowermost
valence band state is 17.7 eV [25]. Most of the theoretical calculations show that
the bandgap is almost indirect, which is not correct. It is often attributed to the
fact that anatase has a very unstable structure [25].
10
Anatase also has a very high carrier mobility, 80 cm2/V s [51], (89 times faster
than rutile) [52]. Since the bandgap is 3.2 eV the main absorption peak is at 395
nm. The Hall mobility is 20 cm2/V s at room temperature [53].
2.2.2 Rutile
Figure 2–2(a) shows the electronic structure of rutile. Rutile has a bandgap
that experimentally has been measured to be 3.0 [54] and with calculations it is
1.78 eV up to 3.73 eV [23, 55]. In this case the reason for the large variation is
primarily the number of atoms accounted in the calculation and secondarily bond
length variations. The upper valence band is composed of O2p orbital and has a
width of 5.4 eV. The lower O2s band is 1.94 eV wide [30]. The separation energy
between the upper conduction band and the minimum valence band has been
measured experimentally and is 16-18 eV [30]. The lowest conduction band consists
ofn two sets of Ti3d and is 5.9 eV wide [25].
2.3 Semiconductor Photocatalysis
The term photocatalysis is still under debate since strictly the term implies the
initiation of reactions in the presence of light only something that is not accurate
in the case of semiconductor photocatalysis, since in this case the presence of the
semiconductor is equally important [56]. But for the purpose of this research the
term photocatalysis will be used, and will denote the reaction that takes place on
the surface of a semiconductor in the presence of a certain range of radiation.
The first report on photocatalytic activity was by Becquerel in 1839 when
he observed voltage and electric current on a silver chloride electrode when it
is immersed in electrolyte solution in the presence of sunlight [57]. Technically
all semiconductors can display photocatalytic properties, but usually the oxides
and compound semiconductors are demonstrating significantly better results
[5, 58, 59]. The ability of a semiconductor to undergo photocatalytic oxidation is
governed by the band energy positions of the semiconductor and redox potentials
11
Figure 2–3: Schematic diagram representing the main photocatalysts with theirbandgap energy. In order to photo-reduce a chemical species, theconductance band of the semiconductor must be more negative thanthe reduction potential of the chemical species; to photo-oxidize achemical species, the potential of the valence band has to be morepositive than the oxidation potential of the chemical species. Theenergies are shown for pH 0.
of the acceptor species. The later is thermodynamically required to be bellow
(more positive than) the conduction band potential of the semiconductor [5, 59].
The potential level of the donor needs to be above (more negative than) the
valence band position of the semiconductor in order to donate an electron to
the vacant hole. Figure 2–3 are shows some of the most popular semiconductor
photocatalysts represented with their band energy positions. The internal energy
scale is given on the left for comparison to the Normal Hydrogen Electrode (NHE).
The positions are derived from the flat band potential in a contact to a solution
of aqueous electrolyte of pH 0 [59]. Among them TiO2 is the most popular. It is,
efficient, effective, requires shallow UV radiation, is very cheap to manufacture,
12
environmentally safe and easily incorporated with other materials. Since 1972 when
the ability to split the water under UV radiation was first discovered [6] there has
been great work in understanding the mechanism and the reactions that take place.
2.3.1 Basic Principles
Figure 2–4 schematically represents the steps of photocatalysis. Initially when
a photon of proper energy (hν ≥ Eg) strikes the surface of the semiconductor
it generates an electron hole pair (h+ − e−). Both electron and holes, either
recombined or migrate to the surface, where, they proceed with chemical reactions.
The holes are generating [OH•] and the electrons H2O2. A very important factor
for those processes is the required time. Here are summarized the main reactions
and the time required for each one [4]. The required time has been measured with
laser flash photolysis [60, 61]: Charge-carrier generation
TiO2 + hν → h+vb + e−cb, 10−15s (2−1) Charge-carrier trapping
h+vb+ > TiIVOH →
> TiIVOH•+
, 10 × 10−9s (2−2)
e−cb+ > TiIVOH →> TiIIIOH
, 100 × 10−12s (2−3)
h+vb+ > TiIV → > TiIII, 10 × 10−9s (2−4) Charge-carrier recombination
e−cb +> TiIVOH•+ → > TiIVOH, 100 × 10−9s (2−5)
h+vb +
> TiIIIOH
→ > TiIVOH, 10 × 10−9s (2−6) Oxidation or reduction
> TiIVOH•+
+ Red0 → > TiIVOH + Red•+, 100 × 10−9s (2−7)
e−tr + Ox → > TiIVOH + Ox•+, 10−3s (2−8)
According to the above proposed mechanism the overall quantum efficiency
depends on two major types of reactions, the carrier recombination and the
13
h+
e−
Ox
Ox•
red0
red+
CO2, Cl−
H+, H2O
1
2
3
3
4
5
6
Figure 2–4: Schematic representation of the reactions taking place in titania. 1OLight strikes the semiconductor. 2O An electron-hole pair is formed. 3OElectrons and holes are migrating to the surface. 4O The holes initiateoxidation leading to CO2, Cl−H+, H2O. 5O The conduction bandelectrons initiate reduction reactions. 6O electron and holesrecombination to heat or light.
[OH•]/H2O2 generation. The dominant reaction is the recombination of the e− and
h+ (1 ns) followed by the reduction reaction (10 ns) and oxidation (1 ms). Since
the recombination is also assisted by the localized crystal defects, the remaining
carriers are not enough for an efficient photocatalytic reaction.
2.3.2 Enhancement of Photocatalysis
It is necessary to enhance the photocatalytic efficiency of titania to obtain
a more effective material. Time-wise the oxidation coming from the holes is the
fastest degrading reaction [60]. It is reasonable therefore to favor this reaction
over the reduction reaction initiated by the electrons. Since the mechanism that
is responsible for the reduced efficiency is the recombination between the h+ and
e− all the previous research has focused on either scavenging the electrons away
from the system to prevent recombination, or just retarding the recombination so
the holes will generate [OH•] [4, 5, 58, 59]. Namely the best known ways are the
doping of titania, the coupling with a metal and the coupling of a semiconductor.
14
C.B.
Eg
φsχs
V.B.
Ef Eg
Vacuum
C.B.
Eg
φsχs
V.B.
Trap levels
Ef Eg
Vacuum
(a) (b)
Figure 2–5: Titania band structure (a) before and (b) after doping. The transitionmetals are interstitial or substitutional defects in the structure oftitania and generate trapping levels in the bandgap.
Since 1972 there has been extensive work towards all three types of photocatalytic
enhancement with the titania/semiconductor and titania/metal coupling more
dominant since they are easier to achieve.
Doping of titania. A great deal of work has been done the last few decades
to dope titania with transition metals, N [62] and C [63, 64]. In general transition
metals are incorporated in to the structure of titania and occupy substitutional
or interstitial positions. It is a very common defect in the case of semiconductors
since it generates trap levels in the bandgap. Figure 2–5(a) shows the electronic
structure of titania before the doping. After the doping (figure 2–5(b)) the bandgap
has been modified with the addition of the trapping levels. The trap levels are
usually located slightly below the lower edge of the conduction band and usually
are in a form of a narrow band.
There are several advantages to this modification. Before the modification
the required photon energy had to satisfy the condition hν ≥ Eg. After the
modification the required energy is going to be hν ≥ (Eg − Et) where Et is the
15
lower edge of the trapping level band. In addition the electrons that are excited at
those levels are trapped, and the holes have sufficient time for [OH•] generation.
Even in the case that hν ≥ Eg and the electron is excited to the conduction band,
then during the de-excitation process the electron is going to be transitioned from
the conduction band to the trap levels and then to the valence band which again
retards the recombination and therefore increases the overall efficiency.
The most common transition metals used are Fe+3, Cr+3 and Cu+2. Fe+3
doping of titania has been shown to increase the quantum efficiency for the
reduction of N2 [65–67] and methylviologen [65] and to inhibit the electron hole
recombination [60, 61, 68]. In the case of phenol degradation Scalfani et al. [66]
and Palmisano et al. [69] reported that Fe+3 had little effect on the efficiency.
Enhanced photoreactivity for water splitting and N2 reduction have been reported
with Cr+3 [69–72] doping while other reports mention the opposite result. Negative
effects have been also reported with the Mo and V doping, while Gratzel and Howe
reported inhibition of electron hole recombination. Finally Karakitsou and Verykios
noted a positive effect on the efficiency by doping of titania with cations of higher
valency than Ti+4 [73]. Butler and Davis [74] and Fujihira et al. [75] reported that
Cu+ can also inhibit recombination.
Coupling with a metal. In photocatalysis the addition of metals can affect
the overall efficiency of the semiconductor by changing the semiconductor surface
properties. The addition of metal which is not chemically bonded to the TiO2 can
selectively enhance the generation of holes by scavenging away the electrons. The
enhancement of the photocatalyis by metal was first observed using the Pt/TiO2
system [76, 77] by increasing the split of H2O to H2 and O2. In particular cases the
addition of metal can affect the reaction products.
Figure 2–6 demonstrates the effect on titania band structure when titania is
coupled with a metal. In general when a semiconductor that has work function φs
16
Ef
Vacuum
φmC.B.φs χs
V.B.
Eintf
Ef
Eg
V.B.
Vacuum
(a)
Ef
Vacuum
C.B.
V.B.
Vacuum
φs χs
Eg
φm
(b)
Figure 2–6: The principles of rectifying contact between titania (Eg=3.2 eV) and ametal with work function (φm), in this example 5 eV, greater than theaffinity (χs) of titania. (a) Before the contact, and (b) after thecontact, where a barrier is formed to prevent the electrons of crossingback to the semiconductor. The Eint
f is the Fermi level if titania is anintrinsic semiconductor and Ef is the Fermi level as an oxygen deficientmaterial.
is compared with a metal with work function of φm > φs the Fermi level of the
semiconductor, Esf , is higher than the Fermi level of the metal Em
f (figure 2–6(a)).
So when the two materials are brought in contact (figure 2–6(b)) there will be
17
electrons flowing from the semiconductor to the metal until the two Fermi energy
levels come to equilibrium. The electrons transition will generate an excess of
positive charge that creates an upward band bending. This bending creates a small
barrier (in the order of 0.1 eV) that excited electrons can cross and be transported
to the metal. From the moment the electrons migrate to the metal it is not possible
to cross back since the barrier for this action is larger and therefore the electrons
will remain in the metal. 1
The earliest work on titania metal was the Pt/TiO2 electrode for the split
of water [76, 77]. Currently the most effective metal/TiO2 interface is achieved
by colloidal suspension [78]. It was found that in the case of Pt/TiO2 system
the Pt particles are gathered in the form of clusters on the surface of TiO2 [79].
Other metals have also been investigated. Ag has been found to increase the
efficiency [80]. Other transition metals such as Cr+3 negatively modify the surface
by creating recombination sites. Although in principle all metals can be used, noble
metals are preferred since they have higher work function and better conductivity.
In all cases high solids loading will affect the kinetics of the system, the light
distribution and eventually decrease the overall efficiency [81].
Coupling with a semiconductor. Coupling a semiconductor with a pho-
tocatalyst is a very interesting way of assisting the photocatalysis. Figure 2–7
demonstrates the principles of the TiO2 coupling with another semiconductor. In
this example as titania is considered the anatase phase, while the the semiconduc-
tor is the rutile phase. When two semiconductors are brought together, as in the
previous case, the Fermi levels tend to balance so electrons are flowing from the
semiconductor with the highest Fermi level to the semiconductor with the lowest.
1 According to quantum mechanics there is a finite possibility that the electronscan cross back, but the number of the electrons that can do that is insignificant.
18
C.B. φαsχα
s
V.B.
Eαf E
αg
Vacuum
C.B.φrs χr
s
V.B.
ErfE
rg
Vacuum
(a)
C.B.
Eαg
φαsχα
s
V.B.
Ef Eαg
Vacuum
χrsφr
s
Erg
C.B.
(b)
Figure 2–7: The principles of rectifying contact between anatase (α) titania(Eα
g =3.2 eV) and and rutile (r) titania (Erg=3.0 eV). (a) Before the
contact and (b) after the contact, where a barrier is forming to preventthe electrons created in anatase crossing to the rutile. On the otherhand holes created into anatase can migrate to rutile. So the couple ofanatase-rutile is creating and effective electron-hole separation.
This charge transfer will create an excess of positive charge to the semiconductor
that had the highest Fermi level and an excess of negative charge to the semicon-
ductor that had the lowest energy (figure 2–7(b)). By light illumination, e− − h+
pairs are generated in both semiconductors. The barrier that forms separates the
19
electrons in the conduction band, but at the valence band the holes are free to
move and based on the energy diagram they move from the semiconductor with the
larger gap to the one with the smaller. In this case the composite material is acting
as a charge separator. The holes are gathered in the rutile where they create an
excess of holes, and despite the fact that the recombination is still the main process
the excess of holes will be enough to photo-oxidize the organic molecules.
In addition semiconductors can be used as a hole or electron injector. In order
to achieve optimum results a candidate semiconductor has to satisfy the following
criteria. Have a proper band-gap Have a proper position of the Fermi energy level Have proper relative position of the conduction and valence band to the
vacuum level.
The combination of the bandgap and Fermi level will determine if there are holes
or electrons that will be injected and towards which direction. Thus in order for
two coupled a semiconductor with titania in order to enhance the photocatalysis,
the semiconductor has to have very specific properties. This is the reason that this
technique, despite its simplicity, ease of manufacturing and very promising results,
is not very widely applied. Systems that have been developed are the TiO2/CdS
[82], TiO2/RuO2 [83] and Anatase-TiO2/Rutile-TiO2 [52, 84]. The last one is a
system commercially available from Degussa, known as Aeroxide P25, and is the
most powerful commercial, particulate, photocatalytic system [84]. The excellent
and uniform properties have established it as benchmark material to compare
photocatalytic efficiencies.
20
2.4 Applications of Photocatalysis
In this section are reviewed the main applications of the photocatalytic
systems that have been described above. The most popular uses are in environ-
mental application and photovoltaic cells. There are other applications such as
anti-fog coating and pigments in paints, but since they do not utilize the electrical
properties of titania, they are not going to be explained here.
2.4.1 Environmental Applications
During the last few decades the environmental applications of TiO2 have
attracted a great deal of attention since titania can be the base of low maintenance
systems. So far they mainly focus on water and air treatment and the objectives
are primarily the removal of organic contaminants [4, 85–87] and secondarily
biocidal applications [3, 8, 9, 11, 14, 88]. Although the systems can equally target
biological contaminants the effectiveness is less or equal to other competitive
technologies (chemical disinfection). So the biological applications, although they
are unique and interesting, are not widely utilized.
Several reactors configuration have been developed for the most effective
removal of the contaminants[89–91]. One of the most popular configurations,
mainly for experimental application, is the slurry reactor, where the water is
mixed and agitated with titania particles under the presence of UV radiation. The
main advantage of this configuration is the high surface area that allows faster
processing. The main disadvantage is the separation of the particles after the
reaction, which is a very tedious process. They can be separated by filtration,
centrifugation, coagulation and flocculation [86, 92, 93]. Recently magnetic core
has been used to assist the dispersion and recollection of the particles [94]. An
alternative to the slurry reaction is the flat bed reactor where the particles are
immobilized on a ceramic membrane [95]. The efficiency is lower compared to
the slurry reaction due to the lower surface area, but the system does not need
21
any kind of separation, which adds to the overall efficiency. Recently in order to
increase the surface area of the titania the particles have been coated on tubes [95],
glass beads [96], fiber or woven glass [97].
2.4.2 Photovoltaic Cell
Solar cells have been used the past few decades with great success in small
devices. In 1991 Gratzel and Oregan [98] reported a high efficiency solar cell based
on TiO2. The titania used in those cells is usually dye sensitized [99–101].
The basic titania cell consists of a sandwich of a TiO2, sensitizing dye,
electrolyte and the catalyst between two conductive transparent electrodes. The
substrate usually used for this application is a standard transparent electron
conductor (TEC) glass with high optical transmission and low resistance. Titania
is an excellent material to be used as base since it carries a good combination of
optical and electrical properties. The dye is required to absorb the sunlight and
inject electrons into titania with almost 100% efficiency. The oxidized dye molecule
is then reduced by the redox electrolyte. The electrolyte itself is then reduced at
the counter electrode. The cycle excitation-oxidation-reduction is then repeated.
Dye sensitized solar cells (DSSCs) continue to attract much attention as
viable systems for conversion of solar energy [102]. A titania cell that is sensitized
by a RuN3 dye achieves the highest efficiency. The best efficiency reproted is
10% [102]. Retartation of the recombination can further increase the efficiency
of the cell. The properties of these films depend on the phase, morphology and
preparation method that were used. There are a wide variety of techniques that
those films are synthesized. Traditional techniques include CVD, aerosol pyrolysis,
electrodeposition and sol-gel processing [100]. Most of them lead to amorphous,
partially crystallized or fully crystallized anatase. For the DSSC anatase TiO2
is still considered the best material, but recently brookite was reported to be
successfully used as the electrode material.
22
These processes are expected to be sensitive to the crystal structure, size and
morphology of the exposed lattice planes as it was shown, as well as to the bandgap
and to the flat band potentials. Solar cell photopotential is especially sensitive to
the nature of the semiconductor surface that determines largely the occurrence of
reverse reactions (i.e., recombination). The best actual solar cells work with the
I2/I− (or Br2/Br−) couple, because of a slow kinetics for I2 reduction on SnO and
especially on TiO2 surfaces.
CHAPTER 3CARBON NANOTUBES (CNTs): STRUCTURE AND ELECTRICAL
PROPERTIES OVERVIEW
Carbon nanotubes have been discovered by Iijima [103] in 1991 and since
their discovery they have attracted a great deal of attention due to the exceptional
electronic [104], thermal and mechanical properties [105]. Iijima reported the
creation of multiwall carbon nanotubes (MWNT) with outer diameter up to 55
A and inner diameter down to 23 A. Since that time extensive theoretical and
experimental research for the past decade has led to the creation of a rapidly
developing research field. In 1993 Bethune et al. [106] reported the discovery of
the singlewall nanotubes (SWNT). The very small diameter of the single nanotubes
and the very big length makes them behaving as quantum wires, giving them
very interesting properties. Due to the fact that the SWNT usually contain a
small number of carbon atoms (usually < 102), they have attracted almost all
the theoretical work. They possess some remarkable electronic, mechanical and
thermal properties that are related mainly to their diameter and chirality. Since the
nanotubes are the photocatalytic template, this chapter will give a general overview
of their unique electrical properties. Initially these properties will be described for
the SWNT that have been more intensively studied and understood. Later some of
the concepts will be expanded to include the MWNTs. Focus will also be given to
the physics of the nanotubes and especially the structure and how the structure is
related to the electric properties and the Raman active vibrational modes. The last
part of this chapter will discuses and compare the several production methods of
nanotubes and how those methods eventually will effect their properties.
23
24
3.1 Bonding, Structure and Physics of Single-Wall Carbon Nanotubes
To understand the structure of the nanotubes it is critical to review the
different bond structures of carbon. Explaining the physical properties of the
single and multi wall carbon nanotubes it is required to derive certain geometric
relation and explain the basic notation used for the NTs. It is important also to
describe several symmetries of the tubes, and how they correlate to the vibrational
frequencies. Those frequencies are crucial for explaining in chapter 6 in this
document bonding and electronic behavior.
3.1.1 Bonding in Carbon Materials
A carbon atom has six electrons from where the first two are occupying the
1s state and the other four are at the sp px and pz or sp2 and pz or sp3 hybridized
orbitals depending on the structure. The sp3 orbital is used for example at the
diamond structure, resulting three dimensional interlocking structure that is
responsible for the extreme hard nature of diamond [107]. In graphite, the three
outer shell electrons occupy the three sp2 orbitals, that is coplanar, and form
three in-plane bonds (σ bond) and one out-of-plane bond with the pz (π bond)
orbital that is perpendicular to the σ bond plane[108]. This results in honeycomb
structured carbon sheet (graphene sheet). The graphene sheets are held together by
van der Waals forces. The σ bond in the sp2 orbital is 0.14 nm long and has energy
of 420 kcal/mol, where in sp3 it is 0.15 nm and has energy of 360 kcal/mol. It is
obvious that the graphite sheet is stronger in the plane direction that diamond.
Since the carbon nanotubes are rolled graphene sheets the bonding is essen-
tially sp2. However, due to the curvature of the tube, the σ and π bonds are going
to be re-hybridized. The new structure push σ bonds out of the plane, all at the
same direction (towards the center of the tube). To compromise the charge shift
the π bond will be de-localized to the direction outside the tube. This configuration
will make the tubes mechanically stronger and electrically and thermally more
25
Ψ12
θhτ
Figure 3–1: The 2D graphene sheets is shown with the a1 and a2 specifies thechirality of the nanotube. The chiral vector, Ch, is the OA, while thetranslation vector T is the OB. Also ψ is the rotation angle and τ thetranslation. Those two are constitute the symmetry operationR = (Ψ|τ).
conducting than graphite. The flexibility of the σ bond allows the incorporation
of topological defects, such as pentagons or heptagons, that allow the formation of
caps, bend, toroidal or helical tubes [109].
The fullerenes C60 are made of 20 hexagons and 12 pentagons [110]. The
bonding is also sp2, although due to the high curvature it resembles sp3. This
unique structure gives to the fullerenes a very interesting set of properties.
3.1.2 Structure and Notation
A SWNT can be thought of as a graphene sheet rolled seamlessly in a cylinder
[111]. It usually has 10-40 carbon atoms in circumference and is capped. The
direction that the graphene sheet is rolled is called chirality and it is specified by
the chiral vector Ch (figure 3–1). The honeycomb structure is described by the
26
vectors a1 and a2 and all the vectors can be described as a linear combination of
those two vectors. Ch can be defined as (figure 3–1)
Ch = na1 +ma2 ≡ (n,m) (3−1)
which often is denoted with the (n,m) symbol. A very important variable is the
angle θ which is the angle of the chiral vector with the a1 direction [112]. The a1
direction is called zigzag. Consequently nanotubes rolled to that chiral direction
are called zigzag [113]. There are many possible directions that the graphene sheet
can be rolled with different properties (figure 3–2). The direction that has θ = 30
is called armchair [112]. All the other nanotubes for which 0 < θ < 30 are called
chiral. For angles θ > 30 and θ < 0 rotational symmetry rules apply. The tube
diameter dt can be written in terms of the integers (n,m) as:
dt =|Ch|π
=1
π
√3aCC
(m2 + nm+ n2
)1/2(3−2)
where aCC is the nearest neighbor C−C distance (1.42 A in graphite). From the
geometry in figure 3–1the cos θ and sin θ can be calculated,
sin θ =
√3m
2√m2 + nm+ n2
, cos θ =2n+m
2√m2 + nm+ n2
(3−3)
Consequently the chiral angle θ is
θ = tan−1
[ √3m
m+ 2n
]
(3−4)
The (dt, θ) pair can completely describe the nanotubes and occasionally it is used
as an alternative to the (n,m). The translation vector T is another important
vector, which on the nanotube denotes the longitudinal direction and is vertical to
the Ch (Ch · T = 0). It is defined as
T = t1a1 + t2a2 ≡ (t1, t2) (3−5)
27
where the coefficients t1 and t2 are related to the n and m by
t1 =(2m+ n)
dR
, t2 = −(2n+m)
dR
(3−6)
where dR is the greater common divisor of (2n+m, 2m+ n) and is given by
dR =
d, if n−m is not a multiple of 3d,
3d, if n−m is a multiple of 3d(3−7)
where d is the greatest common division of (n,m). The magnitude of T is |T | =
T =√
3Ch/dR. As unit cell of the nanotube is defined the area delineated by
the vectors T and Ch. So for instance in figure 3–1 the unit cell is defined by the
OBB’A parallelogram. The number of hexagons, N , contained within a unit cell of
a nanotube is determined by the integers (n,m) and is given by
N = 2(m2 + n2 + nm)
dR(3−8)
where dR is defined by equation 3−7. The carbon nanotubes are usually capped.
The cap can be thought of as a fullerene (C60) that has been bisected at the
equator. So for example if the C60 is bisected normal to a five fold symmetry axis
then that cap is suitable for armchair tube, while if it is bisected normal to the
3 fold symmetry axis then the resulting cap is suitable for a zigzag tube [112].
Since there are many diameters there are many different caps that can fit them
[112, 114]. Figure 3–2 shows several rolling directions and based on those direction
the number of distinct caps that can be joining them seamlessly.
3.1.3 Symmetries and Vibrational Frequencies
A very general way to simplify the analysis is to assume that the nanotubes
have very big length compared to the diameter and therefore ignore the caps. In
general we can distinguish two major types of symmetric groups, symmorphic or
non-symmorphic. The zigzag ((n, 0) tubes) and armchair ((n, n) tubes) belong to
28( 0 , 0 ) ( 1 , 1 )( 1 , 0 ) ( 2 , 0 ) ( 3 , 0 ) ( 4 , 0 ) ( 5 , 0 ) ( 6 , 0 ) ( 7 , 0 ) ( 8 , 0 ) ( 9 , 0 ) ( 1 0 , 0 ) ( 1 1 , 0 )( 2 , 2 ) ( 3 , 3 ) ( 4 , 4 ) ( 5 , 5 ) ( 6 , 6 )12( 2 , 1 ) ( 3 , 1 ) ( 4 , 1 ) ( 5 , 1 ) ( 6 , 1 ) ( 7 , 1 ) ( 8 , 1 ) ( 9 , 1 ) ( 1 0 , 1 )( 3 , 2 ) ( 4 , 2 ) ( 5 , 2 ) ( 6 , 2 ) ( 7 , 2 ) ( 8 , 2 ) ( 9 , 2 ) ( 1 0 , 2 )( 4 , 3 ) ( 5 , 3 ) ( 6 , 3 ) ( 7 , 3 ) ( 8 , 3 ) ( 9 , 3 ) ( 1 0 , 3 )( 1 0 , 1 )
( 5 , 4 ) ( 6 , 4 ) ( 7 , 4 ) ( 8 , 4 ) ( 9 , 4 )( 6 , 5 ) ( 7 , 5 ) ( 8 , 5 ) ( 9 , 5 )( 7 , 6 ) ( 8 , 6 )( 7 , 7 ) ( 8 , 7 )( 0 , 1 ) ( 0 , 2 ) ( 0 , 3 ) ( 0 , 4 ) ( 0 , 5 ) ( 0 , 6 ) ( 0 , 7 ) 1 1
1 331 31 5 3 2 8 73 7 4 31 75 7 1 81 0 1 91 7
8 04 82 0
Figure 3–2: The graphene sheet is shown with the (n,m) pair which specifies thechiral nanotube. The pair of integer (n,m) in the figure specifies thechiral vector Ch for carbon nanotubes, including zigzag, armchair andchiral tubules. Below each pair of integer is listed the number ofdistinct caps that can be joined continuously to the cylindrical carbontubule denoted by (m,n) [ref]. It is also denoted the conduction stateof every chirality.
the first group while the other chiral belong to the second. The basic difference
that in the case of symmorphic the translational (τ) and rotational (Ψ) operation
(both shown in figure 3–1) can each be executed independently, while for the
non-symmorphic this is not true.
The complete analysis is very complicated and is beyond the scope of this
research. Briefly here will be mentioned the very basic principles. Due to their
high complexity the chiral tubes are not going to be included in the analysis. From
equation 3−8 it can be calculated that for certain structures the N can be very
large. For example for the (30, 15) N = 210 [103, 115]. The symmetries for those
structures are very complicated [114]. For zigzag (n, 0) and armchair (n, n) are less
complicated. The (n, n) and (n, 0) the symmetry groups can be described by Dnh
or Dnd, that are even or odd, respectively.
29
The symmorphic symmetries usually have relative small area of 1D unit cell
(Ch · T ), therefore the number of phonon branches or number of electronic energy
bands are small. On the contrary for the chiral tubes that number is very big, since
the area of the 1D cell is large. For the zigzag tubes (n, 0) there are 4 × 3n = 12n
degrees of freedom with 60 phonon branches, having symmetry types (for n odd,
and thus Dnd symmetry) [114]:
Γvibn = 3A1g + 3A1u + 3A2g + 3A2u (3−9)
+ 6E1g + 6E1u + 6E2g + 6E2u
+ · · · + 6E[(n−1)/2]g + 6E[(n−1)/2]u
From those only 7 are non-vanishing modes that are infrared active and 15 that
are Raman active, but they are not all detectable. It was found that increasing
the diameter of the zigzag tubes the number of active modes does not increase.
This concept can be proved for armchair and chiral tubes, since it is a symmetry
imposed result. In chapter 6 are explained the major Raman lines that can be
detected.
3.2 Electronic Properties of SWNT and MWNT
3.2.1 Electronic Properties of SWNT
Their unique electronic properties are attributed to the different quantum
confinement of electrons. We can see three different directions that based on the
geometry it will result in, or not confinement. (i) In the radial direction, electrons
are confined by the mono-layer thickness of the graphene sheet. (ii) Around the
circumference of the nanotube, periodic boundary conditions come into play. As
seen in the previous section the radius, therefore the boundary conditions, depends
on the (n,m) configuration. For example, for a (5, 5) the radius, dt, is 6.78 A, for
a (10, 0) it is 7.83 A [115]. So the circumference boundary conditions vary even
for tubes that are at the same category (armchair or zig zag). (iii) Finally the
30
Π
Α
Π
2 Α
0-
Π
2 Α
-
Π
Α
kxΠ
ΑΠ
2 Α
0-
Π
2 Α
-
Π
Α
ky
-20
-10
0
10
20
EHeVL
2 Α
0-
Π
2 Α
-
Π
Α
ky
Figure 3–3: The dispersion for graphite as calculated from equation 3−10.
direction parallel to the axis (T direction), since it is considered infinite there is no
confinement.
Because of this 1D quantum confinement, the electrons can only propagate
along the nanotube axis designated by the vector T , and so their wavevectors k
point towards this direction. The resulting number of one-dimensional conduction
and valence bands effectively depends on the standing waves that are set up around
the circumference of the nanotube. These simple ideas can be used to calculate the
dispersion relations of the one-dimensional bands, which link wavevector to energy,
from the well known dispersion relation in a graphene sheet.
31
In the simplest model [113, 116, 117], the electronic properties of a nanotube
derived from the dispersion relation of a graphite sheets with wave vectors (kx, ky):
E(kx, ky) = ±γ0
1 + 4 cos
(√3kxa
2
)
cos
(kya
2
)
+ 4 cos2
(kya
2
)1/2
(3−10)
where γ0 is the neighbor-hopping parameter (usually γ0 = 2.5 − 3.2 eV, [113, 116–
118]) and a is the lattice constant a = 2.46 A. Figure 3–4 shows the plot of this
dispersion relation.
When the periodic boundary conditions are imposed along the tube circum-
ference (C direction) the k = (kx, ky) is quantized along that direction. It has to
satisfy the condition k · C = 2πq, where q is an integer. For the armchair (n, n) this
translates to
kmx =
m
Nx
2π√3a
(m = 1, . . . , Nx) with Nx = 5 (3−11)
replacing this value in equation 3−10, and simplifying ky with k we get
Earmm (k) = ±γ0
1 ± 4 cos(mπ
5
)
cos
(ka
2
)
+ 4 cos2
(ka
2
)1/2
(3−12)
where −π < ka < π and m = 1, . . . , 5 in which k is one-dimensional vector along
the axis of the tube (T direction). The plus and minus signs are denoting the
unfolded and folded energy bands, respectively.
Similarly for the case of the zigzag tubes we get the relation
kmy =
m
Ny
2π
a(m = 1, . . . , Ny) with Ny = 9 (3−13)
The energy dispersion relation in this case is calculated to be
Ezigm (k) = ±γ0
1 ± 4 cos
(√3ka
2
)
cos(mπ
9
)
+ 4 cos2
(ka
9
)1/2
(3−14)
where − π√3< ka < π√
3and m = 1, . . . , 9 in which k is one-dimensional vector
along the axis of the tube (T direction). In addition according to the circumference
32
ΓX
–3
–2
–1
0
1
2
3
k
E(k
)/γ 0
A1g+
E1g+
E2g+
A1g
E1g
E2g
A1u
E1u
E1u+
A1u+
E1u+
E2u+
X Γ–3
–2
–1
0
1
2
3
k
E(k
)/γ 0
A1g+
E1g+
E2g+
E3g+
E4g+
A1g
E1g
E4g
E2g
E3u, E3g
A1u+
E1u+
E2u+
E3u+
E4u+
A1u
E1u
E4u
E2u
(a) (b)
Figure 3–4: The dispersion energies for (a) armchair and (b) zigzag semiconductoras are calculated from equations 3−14 and 3−12. The differentbranches have been labeled according to [116].
direction boundary condition in order to have metallic tubes;
(n−m) = 3q (3−15)
That means that one third of the different nanotubes structures is metallic and two
thirds are semiconducting. Figure 3–2, shows the conductivity states for different
chiralities. For semiconducting tubes the band-gap (Eg) is [119–121]
Eg = 2dCCγ0
dt
(3−16)
So far for this approach the only weakness is that it did not account the re-
hybridization of the σ − π orbital due to the curvature. This effect can be included
in other approaches such as the first principle calculation ab-initio [122–125]. In
this new approach it is proved that for small diameter tubes (< 1.5 nm) a band
33
gap opens that is about 0.02 eV for non-armchair nanotubes, that still satisfies the
condition 3−15 [126]. However this phenomenon dissipates fast for larger diameters
tubes. Therefore the graphite model can be used as a good approach to describe
the SWNT with different chiralities. STM studies have confirmed the accuracy
of the model [123, 126] and also the existence of the small band-gap predicted by
ab-initio calculations [126].
It has been experimentally confirmed that a SWNT [127], a SWNT rope
[128] and a MWNT [129] behave like a quantum wire intrinsically. The conduc-
tance is given by
σ = σ0M =
(2e2
h
)
M (3−17)
where σ0 = (2e2/h) = (12.9 kΩ)−1 is quantized conductance. M is an apparent
number of conducting channels, that includes all the possible interactions, such
us electron-electron coupling, inter-tube coupling effects. For example for a
SWNT that value is 2. In a SWNT there are also impurities, structural defects,
coupling with the substrate that will further reduce the conductivity. Therefore the
experimental data have large variations from the predicted values, but they follow
the same trend.
The most important information that the graphite model can predict is the
density of states (DOS) [130–132]. According to that model the density of state
ρ(ǫ) is
ρ(ǫ) =4
l
2√3γ0a
+∞∑
m=−∞g(ǫ, ǫm) (3−18)
where,
g(ǫ) =
|ǫ|√ǫ2−ǫ2m
for |ǫ| > |ǫm|
0 for |ǫ| < |ǫm|(3−19)
and
|ǫ| =|3q − n +m| γ0a√
3dt
(3−20)
34
Calculations based on this model predict again that the armchair and zigzag
configurations have a continuous DOS while for the chiral a small band gap exists
[119, 133, 134]. Figure 3–2 shows the different directions that the graphene sheets
can be rolled and it is denoted if the tube is metallic or semiconducting.
3.2.2 Electronic properties of MWNT
It has already has mentioned in the previous chapter that the MWNT behave
as a wire with the conductance to follow the simple relation [129];
σ = σ0M =
(2e2
h
)
M (3−21)
For the case of the MWNT the value of M is significantly bigger than for the
SWNT to account for more conducting channels. In addition the multilayer
structure increases the probability to have armchair or zigzag tubes that will
increase the conductivity. While the diameter is increasing the electrons on the
tube are less confined and the electron distribution resembles more the structure
of graphite. this is due to the re-hybridization of the σ and π orbital, that is less
intense and the tubular structure approaches more the graphite structure. This is
obvious from equation 3−17 where while the tube diameter increases the energy
gap is diminishing even for the semiconducting tubes. So in general MWNT are
in their majority conducting and behave as nanowires. But there are still chances
that the tubes will be semiconducting, depending always on the arrangement of the
tubes certain defects and crystallinity.
3.3 Carbon Nanotubes Growth Mechanisms
There are two basic commercially available methods for producing carbon
nanotubes. The arc discharge and the Chemical Vapor Deposition (CVD). Both
have advantages and disadvantages that can be directly related to the properties
of the tubes. Generally speaking the two methods are competing at the quantity
35
versus quality, where CVD is designated for quantity and arc discharge is for
quality.
3.3.1 Arc Discharge
In general carbon nanotubes that are produced with carbon vapor that
is being created by the arc discharge, have fewer defects compared to other
techniques. The reason for that is the high growth process temperature that
ensures perfect annealing that eliminates most of the defects. The MWNT that
are produced via arc discharge are perfectly straight. The fewer defects have an
immediate dramatic impact on the tube properties such as, electric and mechanical.
One of the main disadvantages is the limited yield that this method has. Besides
the low yield it is a highly time consuming process. So in general if a a high yield
of nanotubes is required this method is not recommended, on the contrary if more
defined, and better properties is required then arc discharge is a very good solution
[135].
The most common set-up for arc-discharge two graphite electrodes of diameter
6-12 mm, that are kept in distance of 1-4 mm in a chamber that is filled with He-
lium. DC current operates the two electrodes. DC current and Helium are the two
factors that immediately influence the yield. While the positive electrode (anode)
is consumed a cylindrical slag is being deposited on the cathode. The alignment
of the electrodes does not effect the MWNTs but can effect the properties of the
single wall tubes [135].
3.3.2 CVD: Thermal CVD, PE-CVD
Since the application field of the nanotubes is growing the demand for higher
yield production methods is also growing. One of the most promising techniques
is the Chemical Vapor Deposition (CVD). It has a large knowledge base since it is
been used extensively in electronic applications for the last few decades.
36
The nanotubes that are CVD grown have a lot of structural defects due to the
low synthesis temperature during the growth process. An approach to improve this
is annealing the tubes, which will reduce the defects but in no case will have the
same results as the Arc-discharge [135].
The apparatus for CVD grown nanotubes is simple, which is also reducing a
lot of the cost of the production. In a quartz tube with very precise temperature
control, a substrate is placed in carbon containing gases, such as CO, CH4 or
higher order hydrocarbon, are flown in. To assist the reaction often a thermal
source is used, such us IR lamp (Thermal CVD) [135–137] or plasma (PE-CVD)
[138]. The growth rates can be controled precisely and can go from a few nm/min
up to 5 µm/min. In addition metal catalyst can further assist the yield. One of the
biggest advantages of CVD is the ability to grow on a patterned substrate, which is
desirable for microelectronic applications. The purification of the tubes in this case
is a necessity since they contain metal catalyst and different amorphous carbon
structures. There are many ways to purify the tubes; hydrothermal treatment [139],
H2O-plasma oxidation [140], acid oxidation [141], dispersion and separation by
micro-filtration [142] and high-performance liquid chromatography [143].
CHAPTER 4ANATASE COATED CARBON NANOTUBES (ANTs): SYNTHESIS AND
CHARACTERIZATION)
In the previous two chapters the main properties of titania and the carbon
nanotubes were reviewed. This chapter describes the process of combining those
two materials. There are many possible combinations, but in this research the
objective is to apply the titania in the form of a thin coating on the surface of
the MWNTs in order to maximize the contact between the two materials. There
are certain design parameters that have to be satisfied in order to obtain the
optimum results. The first section explains those parameters and following that
are explained the materials selection and preparation. Later a small introduction
to the Sol-Gel chemistry is given and based on that, the choice of chemicals and
precursors is explained. Finally fundamental characterization will follow to provide
arguments for the satisfaction or not of the design parameters and in what extend
it was achieved. The actual photocatalytic efficiency as well as the detailed study
of the interface between the MWNTs and the titania will be discussed in separate
chapters later since they are the main focus of this research.
4.1 Design Parameters
As stated in the introduction the purpose of this work is to combine those
materials and their properties to produce a highly efficient photocatalytic particle.
The main objective is to synthesize a thin coating of titania to cover the surface of
the MWNTs. The process has to satisfy certain criteria.
The coating has to be the anatase phase of titania: As seen in previous
chapter 2 anatase is the most photocatalytic active phase of titania. That
37
38
phase is also thermally very unstable and therefore obtaining anatase is a
non-trivial process with many parameters.
Thin coating will result better photocatalytic performance: The whole
photocatalytic process takes place in a thin layer of about 10 nm. If any
electron hole pair is generated in regions deeper than that, it is going to
recombine before it reaches the surface. In addition increasing the coating
thickness will result lighter color (since the coating will be less transparent)
and therefore the particle will absorb less light.
The coating has to be chemically bonded to the MWNTs: If the coating is
not chemically bonded on the surface of the MWNTs it is possible that it will
flake off. The coated nanotubes will have high tendency to coagulate since
the size is big enough to induce van der Waals forces. Therefore prolonged
sonication will be required to successfully disperse them, which might damage
loosely attached coating.
Individual MWNTs have to be coated: MWNTs have very high affinity into
coagulating. The hydrophobic nature of the tubes will also intensify the
phenomenon of coagulation especially when the solvent is water. In order to
maximize the surface area it is required to minimize the number of MWNTs
agglomerates and separate the bundles.
The number of free titania particles have to be kept minimum: Sol-Gel
is a process that balances between transport phenomena and reaction rate.
Ideally in order to achieve the coating the precursor molecules have to be
transported to the surface of the MWNTs and only after the anchoring they
should react. This balance can be controlled by reaction parameters such as
temperature and pH. However, regardless the values of those parameters there
is always a finite possibility of free anatase particles formation.
39
With those requirements in mind two distinct set of particles will be syn-
thesized. The first one will be consist of an arc discharge MWNT core and the
next one will consist of a CVD grown MWNT core. As described in the previous
chapter (section 3.3) the difference in the tube production can affect the electrical
properties of the carbon nanotubes. So the purpose of using those two different
nanotubes will be to examine the effect of the electrical properties of the tubes on
the photocatalytic activity. The CVD carbon nanotubes have been mechanically
and chemically shortened, which will result in a dramatic increase of the defects
on the surface of the tubes. The short nanotubes in addition will provide other
advantages. The high aspect ratio of the carbon nanotubes results in a particle
that interacts easily with molecules, but raises issues when is it used to deactivate
objects of comparable size such as spores and bacteria. Bacteria have very compli-
cated surfaces, that usually have fibrils of several µm length that can interfere and
prevent the coated tubes to reach the surface. In addition the spherical shape of
the spores does not allow the use of the whole available surface of the nanotubes.
So reducing the length of the MWNTs will result shorter in particles. Large scale
production of short nanotubes (daverage < 1 µm), cannot be achieved with neither
arc discharge method, nor with CVD. They have to be shortened with chemi-
cally assisted mechanical grinding. The short MWNTs will be occasionally called
s-CNTs and the long MWNTs will be called ℓ-CNTs.
4.2 Nanotube Selection and Preparation
The carbon nanotubes have to be properly modified to satisfy some of the
coating requirements. They have to be individually suspended, easily dispersed
in solvents and favor the anchoring of the precursor molecules. It is also critical
to characterize the tubes before the coating in terms of crystallinity and struc-
ture, something that can be used to explain differences in terms of the electrical
properties.
40
4.2.1 Materials Selection
Two different nanotubes were tested as photocatalytic template. The long
nanotubes were ordered from Alfa-Aecar (stock number: 42886) in soot form. The
CVD nanotubes were ordered from NanoMat (product number: 1236YJS) and
were delivered in powder form. According to the manufacturer the tubes were
shortened in a ball mill in a highly acid environment (nitric and sulfuric acid 1:3).
MWNTs from other manufacturer (Iljin Nanotech) were tried, but did not behave
desirably so they were not used. In addition highly conductive activated carbon
from Degussa was used, again with no desirable results.
4.2.2 Purification and Dispersion
The arc discharge nanotubes were obtained in the form of soot. In the soot
along with carbon nanotubes there were many other forms of carbon such as,
carbon fibers, fullerenes and amorphous carbon. Similarly is the situation for the
CVD grown nanotubes. In addition there is residue from the catalyst (in this case
Fe). In order to coat them they have to be purified and dispersed. Since most of
the impurities are carbon nature they can be easily oxidized by acid.
The main route was the same for both materials. The tubes were dispersed in
highly concentrated HNO3 (63% or 10N). The arc discharge nanotubes were in soot
form, so initially the soot was ground with molder and pestle to fine powder. After
that 50 mg of this powder was mixed in 200 ml of the nitric acid. The solution was
sonicated for 3 hours to further disperse the powder. The solution was refluxed
in an oil bath at 140 for 10 h. Then the heat was turned of and the solution
was left for additional 3 h until the temperature drops below 30. The solution
was then centrifuged and the excess nitric acid was removed. Triple washing with
di-ionized water followed.
The CVD nanotubes were already in powder form and therefore was no
need for grinding. In addition since they were already treated with acid for the
41
shortening there is no need for extensive purification, but still the acid treatment
is required for dispersion purposes. As previously 50 mg of tubes were dispersed
in 200 ml of HNO3 and sonicated for 3 h. After that the tubes were refluxed again
in oil bath of 100 for 6h and afterwards the solution was cooled down to 30.
Again the nitric acid was removed with centrifuge, and the tubes were washed with
ethanol three times.
In all cases the nanotubes were not removed from the solvent. During the
purification process there was a 40% weight reduction. So for the coating process
are left about 30 mg. This value was estimated, by drying and weighing the
remaining nanotubes.
4.2.3 Characterization of the Functionalized MWNTs
The characterization of the tubes was performed with SEM (FEG-SEM JEOL
JSM-6335F), TEM (JEOL TEM 2010F), FTIR (Nicolet MAGNA 760 Bench), Zeta
Potential measurements (Brookhaven ZetaPlus), particle sizing (Coulter Multisizer
III) and thermal gravitational analysis (Netzsch STA 449C Jupiter). The SEM
(figure 4–1) reveals roughly the general characteristic of the tubes. The s-CNTs,
figure 4–1 (b), appear more pure since they have undergone the acid treatment
twice, but they are not straight. On the contrary the ℓ-CNTs, figure 4–1 (a),
are straight. In both cases the tubes appear to be pure and there are no obvious
impurities. After the acid treatment the tubes appear purified with no obvious
impurities (at the order of 3 nm) (figure 4–3(d)) and HR-TEM shows the graphene
layers and the cap of the tubes. The TEM images showed an average diameter of
about 20 nm.
Figure 4–2 shows the TEM images of s-CNT before and after the acid treat-
ment. Before the treatment the tubes appear tangled (a) with many carbon
impurities on their surface (b). The acid removed most of the impurities and the
main features of the tubes such as the cavity are visible. The purity of the tubes
42
(a)
(b)
Figure 4–1: SEM pictures of the two types of nanotubes. (a) The long MWNTs(average lengh 1 µm). (b) The short MWNTs (average lengh 100 nm).
43
(a) (b)
(c) (d)
Figure 4–2: TEM images of the s-CNTs. (a) Agglomerate of s-CNTs. (b) Highmagnification of untreated s-CNTs where the impurities around thetube are visible. (c) Purified s-CNTs where there are almost noimpurities present. It can be seen that they are not straight and thatthey have been damanged. (d) Magnification of the treated s-CNTswhere the inner cavity is visible and the outer surface is almostcompletely free from impurities. From the images it can also beconcluded that the average diameter is 20 nm.
is demonstrated clearly in figure 4–2(c) where the tubes although are shown to be
aggregated they are free of impurities. It is also concluded that the tubes have an
44
(a) (b)
(c) (d)
Figure 4–3: TEM images of the ℓ-CNTs. (a) Agglomerate of ℓ-CNTs before theacid treatment. It is hold together by the carbon impurities. (b) Singleℓ-CNT covered by the carbon impurities. (c) After the acid treatmenta bundle on nanotubes. It is also visible some residue of the acidtreatment by products. (d) ℓ-CNTs after the treatment, where most ofthe surface carbon impurities have been removed. Again from thisimage we can see that the average ℓ-CNT diameter is about 15 nm.
average diameter of 15 nm, which is in agreement with the manufacturer specifi-
cations (10-20 nm with average 15 nm). Similar results can be derived from figure
4–3 for the ℓ-CNTs. Since they are arc discharge (60% by weight MWNTs) they
have more impurities than the short. In figure 4–3(a) the aggregates have big pieces
45
(a) (b)
(c) (d)
Figure 4–4: Immediate comparison of the two different kinds of nanotubes. Theimages (a) and (b) are for the ℓ-CNTs and the (c) and (d) for thes-CNTs. The end of the s-CNTs is usually open due to the catalyst(c), while the end of the ℓ-CNTs are capped (a). In addition thes-CNTs have damaged and not well defined walls (d), while theℓ-CNTs are very well defined and straight.
of the carbon impurities and in a characteristic picture of an individual tube (figure
4–3(b)) shows the surface to be covered in segments of the amorphous carbon
impurities.
46
1 2 3 4 5 6 7 8 9 10 11 12–100
–75
–50
–25
0
25
50
75
100
pH
Zet
a P
ote
nti
al (
mV
)
After the acid treatment
Before the acid treatment
(a)
1 2 3 4 5 6 7 8 9–70
–60
–50
–40
–30
–20
–10
0
10
pH
Zet
a P
ote
nti
al (
mV
)
After the acid treatment
Before the acid treatment
(b)
Figure 4–5: The zeta potential for both the ℓ-CNTs (a) and s-CNTs (b). It showsthe shift of the IEP for the ℓ-CNTs (from 7 to 3.5) and the increase atthe surface charge for the s-CNTs (from -10 mV to -37 mv for ph 4).
47
500 1000 1500 2000 2500 3000 3500 4000
Wavelength (cm1)
Ref
lact
ance
(a.
u.)
COO CH
COH
Figure 4–6: The FTIR of the MWNTs after the acid treatment (only the s-CNTsresults are displayed). The bands that have been identified prove thereaction of the −COOH on the surface of the nanotubes.
Finally the direct comparison of the nanotubes focuses the main difference on
the structure of the tube walls. In addition the s-CNTs appear to be occasionally
open ended, while the ℓ-CNTs are in the majority capped (4–4(c) and (a)). The
HR-TEM images (figure 4–4(b) and (d)) show clearly well defined graphene layers
for the ℓ-CNTs while the graphene layers for the s-CNTs appear damaged. In all
cases are visible small layers of carbonaceous impurities on the surface of the tubes
that are direct byproducts of the acid treatment [144, 145]. Although they can be
removed it is not necessary since it will be dissolve when the tubes are placed in a
solvent (water or ethanol).
The measurement of the isoelectric point (IEP) and surface charge it is
necessary to clarify if there is any surface modification of the CNTs. The results
48
0.01 0.1 1 10 100 10000
5
10
15
20
0
5
10
15
Diameter (µm)
Num
ber
(%
)
Dif
fere
nti
al V
olu
me
(%)
As obtained (Number)Acid treated (Number)As obtained (Differential Volume)Acid treated (Differential Volume)
Figure 4–7: The differential volume and number of the s-CNTs before and after theacid treatment.
for the ℓ-CNTs (figure 4–5(a)) clearly show a shift to lower values of the IEP and
higher surface charge. The results for the s-CNTs show that there was pre-existing
surface modification, as result of the mechanical-chemical shortening, and therefore
the second treatment just increased the amount of surface charge. In both cases the
change can be attributed to the generation of functional groups on the surfaces of
the MWNTs.
Since the acid used for the functionalization was HNO3 the surface groups
that have been generated on the surface have to be −COO−. DR-FTIR is utilized
to further investigate the surface groups on the surface of the carbon nanotubes.
Figure 4–6 shows the FTIR spectra of the s-CNTs. The bands that identified are
very typical of the −COOH group (1170 C−OH, 3450 O−H and 1720 −COOH)
49
100 200 300 400 500 600 700 800 900 10000
10
20
30
40
50
60
70
80
90
100
–1
–0.5
0
0.5
1
1.5
2
2.5
3
Temperature (oC)
TG
A (
%)
DT
A (µ
V)
6.02 %
Impuritiesburn out
Figure 4–8: The TGA/TDA data of the s-CNTs. The peak at the 600 indicatesthe burning temperature of the CNTs. It is observed about 6% of theinitial mass residue, which is the Fe catalyst.
[146, 147]. The other bands are characteristic of the carbon nanotubes (1460
C−H, 1640 C=C, 2850 C−H and 2970 C−H) [148, 149]. The band at 3450 O−H
is not proportional to the 1170 (C−OH) and 1720 (COOH) but this is due to the
atmospheric humidity. Similar results are obtained for the FTIR of the ℓ-CNTs and
they are in good agreement with the literature [146–149].
The final characterization was done by the Coulter Particle size analyzer. The
Coulter is utilizing a laser beam and with light scattering calculates the size of
the particles. The theory that is used at the Coulter instruments is similar to that
used at the Zeta Plus that was used for the measurement of the zeta potential. A
major assumption is that the particles are spherical or can be assumed as spherical.
This is completely wrong for the case of the carbon nanotubes, which are high
50
aspect ratio particles (1 : 150 for the ℓ-CNTs and 2 : 70 for the s-CNTs). In
addition since the limit of the instrument is 40 nm what is detected are mainly the
agglomerates and not the individual tubes. However, the instrument can be still
be used to showcase change in the dispersion of the CNTs. Due to the high aspect
ratio of the ℓ-CNTs the results cannot be considered accurate, and figure 4–7
shows only the s-CNTs case. The differential volume results are usually considered
more representative and according to the graph there is one order of magnitude
reduction in the diameter after the acid treatment. In both cases (zeta potential
measurements and particle measurements) the results are only used for qualitative
purposes.
In addition Thermo-Gravitational Analysis (TGA) showed that the ℓ-CNTs
are starting to burn at approximately 700 while the s-CNTs are burning at
approximately 600 and they have 6% weight residue that was identified as Fe2O3
which came for the catalyst used during the production (figure 4–8). That was in
agreement with manufacturer statements.
So from this section we can conclude that the two types of carbon nanotubes
used in this research are different regarding the overall structure. Although both
have a concentric tube structure and the characteristic cavity in the center, the
two types are different in quality; the ℓ-CNTs are very straight and have very
well defined structure, while the s-CNTs type has damaged walls as result of the
production method and the chemical mechanical shortening. In addition the acid
treatment was proved enough to remove carbon nature impurities and to cause
surface modification to stabilize the tubes, either by increasing the surface charge
(s-CNTs) and by shifting the IEP (ℓ-ANTs).
4.3 Sol-Gel Coating
The Sol-Gel [150] route is a very common and validated way to produce thin
coatings of amorphous and crystalline materials. For the titania there is a great
51
deal of attention to this method since the size of the produced particles can be
very accurately controlled and therefore nanosized particles can be easily produced
with very high yield and reproducibility [151, 152]. So for this research Sol-Gel
is the most appropriate method for the generating anatase titania coating on the
MWNTs. This section explains the materials selection and describes the process
that was followed to obtain the TiO2 coating.
4.3.1 Precursor Selection
There is are numerous different methods to produce anatase titania via the
Sol-Gel route. The precursors can be either organometallics or salts. The molecules
will undergo a variety of reactions that will result a three dimensional molecular
network. A common example is the hydrolysis and condensation reactions of metal
alkoxides to form larger metal oxide crystals. An alkoxide has an organic group
bonded to a negatively charged oxygen atom; when this oxygen is also bonded to a
metal it is called metal alkoxide. During the hydrolysis [153, 154] all or some of the
organic chains are replaced by the −OH groups.
M (OR)n + H2O → HO − M (OR)n−1 + ROH + . . .→ M (OH)n + nROH (4−1)
During condensation reaction [153, 154], the M(OH)n are reacting to produce the
metal oxide.
(HO)n−1 M−OH+HO−M (OH)n−1 → (HO)n−1 M−O−M (OH)n−1 +H2O (4−2)
Or alternatively the condensation can occur from the intermediates of the reaction
4−1 [150].
(RO)n−1 M−OH+HO−M (OR)n−1 → (RO)n−1 M−O−M (OR)n−1 +H2O (4−3)
(RO)n−1 M−OH+RO−M (OR)n−1 → (RO)n−1 M−O−M (OR)n−1 +ROH (4−4)
52
where M with valence n is the metal and the R are the organic chains. The
reaction is progressing with the hydrolysis and the condensation of all the −OR
groups of the (RO)n−1 M − O − M (OR)n−1 to result in the three dimensional
network. In the case of titania this reaction will produce the TiO6 octahedral,
which is the structural element of the anatase and rutile.
One of the factors that can determine the reaction rate is the length of the
organic chain. Usually increase in chain length will result in slower reaction rate.
The chain length is directly related to the mobility of the molecule. In addition the
three dimensional structure and complexity of the molecule will also effect the reac-
tion. More complex structures such as titanium bis-ammonium-lactato-di-hydroxide
(TALH) are less reactive. Significant differences in the reaction have been reported
even in the case of titanium isopropoxide (Ti
−O − CH <
CH3
CH3
4
) [155–158]
and titanium propoxide (Ti (−O − C3H7)4) [159, 160].
There is also the case of the salts that can be used such as titanium tetra-
chloride TiCl4 [161–164] and titanium sulphate Ti2(SO4)3 [165–167]. Titanium
tetrachloride can be directly hydrolyzed to yield the rutile phase of the TiO2
TiCl4 + H2O → Ti (OH)4 + 4HCl (Endothermic) (4−5)
Afterwards the reaction progresses similarly to the reaction 4−2. It can also be
used for the production of metal alkoxides that later can be hydrolyzed to produce
TiO2.
TiCl4 + 4ROH → Ti (OR)4 + 4HCl (4−6)
The titanium sulfate has more complicated structure and the reaction proceeds as
Ti2 (SO4)3 + 8H2O → 2Ti (OH)4 + 3H2SO4 + H2 (4−7)
53
(a) (b)
(c) (d)
(e) (f)
Figure 4–9: The different Sol-Gel precursors used in this research. (a) titaniumethoxide (Ti(OC2H5)4), (b) titanium isoproxide (Ti(OC3H7)4), (c)titanium butoxide (Ti(OC4H9)4), (d)titaniumbis-ammonium-lactato-dihydroxide ([CH3CH(O•)CO2NH4]2Ti(OH)2,(e) titanium sulphate (Ti2(SO4)3, (f) titanium tetrachloride.
In this research there were five different precursors used; titanium ethoxide
(Ti(−O−C2H5)4) titanium ispropoxide (Ti
−O − CH <
CH3
CH3
4
), titanium
butoxide (Ti(−O−C4H9)4), TALH, ((CH3CH(O•)CO2 =)2Ti(OH)2(NH4)2) and
titanium sulphate (Ti2(SO4)3) (figure 4–9).
54
Several precursors were tried for every case of MWNTs. Initial conditions and
precursor were selected based on the literature. For the titanium sulfate from Lee
et al. [168], for titanium isopropoxide, ethoxide and butoxide from Jitianu et al.
[169] and finally for TALH from Lee et al. [170].The results were judged based on
the repeatability, the coverage of the coating and the number of free particles. The
surface coverage and the free particle formation were checked with the TEM.
4.3.2 Coating Model
To estimate the amount of anatase required to coat the tubes a coating model
has to be developed. A uniform coating of approximately 5 nm will give the
optimum results. The nanotubes have a diameter of 20 nm and average length of 2
µm. The optimum coating will be around 5 nm thick. So
Vanatase = 5 nm 2 · π · ×10 nm × 2 µm (4−8)
= 2 · π · 5 × 10−9 · 10 × 10−9 · 2 × 10−6 m3 (4−9)
= 6.28 × 10−22 m3 (4−10)
Respectively the volume of a nanotube is
Vℓ−CNTs = π × (10 nm)2 × 2 µm ≈ 0.6 × 10−21 m3 (4−11)
The average density of the tubes (ρCNT) is 1.1 g/cm3. So 1 mg of MWNTs will
contain10−3 g/1.1 g
cm3
0.6×10−21 m3 ≈ 2 × 1012. So for every mg of ℓ-CNTs the required volume of
anatase is V totalanatase = 2 × 1012 · 6 × 10−22 m3 = 12 × 10−10 m3 = 1.2 × 10−3 cm3
of anatase. The density of the anatase (ρanatase) is 3.89 g/cm3, which translates to
approximately 4.67 × 10−4 g or 0.467 mg of anatase or 5.84×10−6 mol for every mg
of CNTs. In all cases minor adjustments were required to minimize the formation
of the free titania particles. In general the quantity that was used was less than
the estimated. The major difference between short and long tubes is the length
(which does not effect the coating model) and the diameter (Rℓ−CNT > Rs−CNT) so
55
Table 4–1: The calculated initial molecular ratio for the reactions for the ℓ-CNTs
Precursors ℓ-CNTs (mg) Solvent Precursor (µl) H2O (µl)Ti(OC2H5)4 30 mg Ethanol 300 ml 36.7 (N/A) 11.68Ti(OC3H7)4 30 mg Ethanol 300 ml 51.8 (44.0) 11.68Ti(OC4H9)4 30 mg Ethanol 300 ml 59.6 (N/A) 11.68Ti2(SO4)3
1 30 mg Water 300 ml 102.5 (106.0) N/A1Solution of 45% wt Ti2(SO4)3 in dilute sulfuric acid
the same model can be used for both types of MWNTs with some modification. If
mℓ−CNTanatase is the anatase required to coat 1 mg of ℓ-CNTs then the amount required
for 1 mg of s-CNTs is ms−CNTanatase =
Rℓ−CNT
Rs−CNTmℓ−CNT
anatase .
The equivalent volume of the MWNT can be considered as a sphere of
radius RGCNT = lCNT/2. The volume is calculated to be V G
CNT = 43π(RG
CNT
)3=
4.2 × 10−18 m3 = 4.2 × 10−12 cm3. So the total equivalent volume of 1 mg MWNTs
occupy is V = 4.2 × 10−12 cm3 · 2 × 1012 ≈8.2 ml. Therefore to ensure that the
30 mg of ℓ-CNTs (252 ml total volume) are not in contact during the coating the
tubes are suspended in 300 ml of solvent (water of 99.99% pure ethanol).
4.3.3 Long MWNTs
Based on the coating model the table 4–1 is constructed. Those values are the
starting values for the Sol-Gel chemistry. In parenthesis are listed the quantities
that are eventually proved to have the best results (based on surface coverage and
number of free particles). After the final washing the ℓ-CNTs suspension (30 mg
of ℓ-CNTs in 300 ml of water) were placed in a three way 300 ml flask. The flask
was placed in an oil bath at 40 and was refluxed under constant stirring speed.
After the temperature was stabilized the pH was fixed at ∼3 with 0.1N HNO3. The
precursor (Ti2S(O4)3) was injected and the reaction was carried for 1 hour. The
solution was divided into six 50 ml centrifuge tubes and was washed 3 times. The
composite was then let to dry at 40 for two days.
56
Grinding of thesoot
Sonication in200 ml HNO3
(10 N) for 3 h
Acid treatmentat 140 in 10 NHNO3 for 10 h
Triple wash withd.i. water
Refluxed at 40for 1h
Dispersing in 300 mlof d.i. waterAddition of
precursor solutionTi2(SO4)3
pH at 3
Washing withd.i. water
Drying at 40for two days
Figure 4–10: Schematic diagram of the process for the coating of the ℓ-CNTs.
The experiment was repeated with the Ti(OC3H9)4. For this case the nan-
otubes after the functionalization were washed with ethanol. The final solution
(30 mg of tubes and 250 ml of ethanol) placed again in a flask and refluxed at
40 under constant stirring until the temperature was stabilized at 40. The
appropriate amount of water was added and the pH was fixed at ∼3 with 0.1N
HNO3. The isopropoxide was placed in another beaker with 50 ml of ethanol and
was stirred for 10 mins. This was done to dissolve it so it will be less viscous and
57
Table 4–2: The calculated initial molecular ratio for the reactions regarding theshort nanotubes
Precursors ℓ-CNTs (mg) Solvent Precursor (µl) H2O (µl)Ti(OC2H5)4 30 mg Ethanol 300 ml 48.9 (40) 11.68Ti(OC3H7)4 30 mg Ethanol 300 ml 69.1 (58) 11.68Ti(OC4H9)4 30 mg Ethanol 300 ml 79.4 (62) 11.68Ti2(SO4)3
1 30 mg Water 300 ml 136.7 (140) N/A1Solution of 45% wt Ti2(SO4)3 in dilute sulfuric acid
less reactive. Then it was slowly injected into the flask to react for 30 min. The
process follows as before, triple washing and drying. The same experiment was
repeated again under nitrogen atmosphere. After the pH was fixed as previously
before nitrogen was let to flow in the container for 1 h (50 cc/min) and then the
isopropoxide solution was injected. Again the reaction was carried out for 30 min.
Then the same washing and drying steps followed. The nitrogen atmosphere did
not significantly affected the reaction results.
The TGA and XRD (figure 4–16) analysis showed that heat treatment at
500 with ramping rate 10K/min will completely transform the TiO2 to anatase.
The titanium ethoxide and titanium butoxide failed completely to achieve
coating in various conditions and therefore they were not used, although there is
a report of successfully using them to coat MWNTs [169]. The TALH was also
used, by following the method be Lee et al.[168] but the final result gave strongly
agglomerated particles.
From this part it is concluded that among all the precursors the most appro-
priate for the ℓ-CNTs is mainly the Ti2(SO4)3. The titanium isopropoxide although
it also yield good results, it was not consistent. From this point onwards as coated
ℓ-CNTs will be considered the tubes that have been coated with Ti2(SO4)3 as
precursor (ℓ-ANTs). Figure 4–10 summarizes the coating process.
58
4.3.4 Short MWNTs
For the s-CNTs a table similar to the ℓ-CNTs case is constructed (table 4–2).
The synthesis procedures for every precursor are identical to the previous so are
not going to be described again. The only difference is the pH that was fixed at
approximately 4. Again the optimum conditions for the crystallization were found
to be at 500 for 3 h with ramping temperature of 10 K/min (figures 4–16 and
4–12).
On the contrary to the previous section and the ℓ-CNTs the precursor that
shows the best results are the metal alkoxides. There is not a standalone reason for
that, but probably it is related to the different isoelectric points. The TALH was
not used for the s-CNTs.
Among the metal alkoxides the titanium isopropoxide displayed the most
stable performance (consistency, repeatability) and best result (number of free
particles). The titanium ethoxide was successful but it showed high sensitivity
to the pH, with sharp transitions from coated to uncoated nanotubes. On the
contrary the isopropoxide and butoxide were more stable in regards to the pH.
Butoxide, however, has high viscosity and slower reaction rate since it has a longer
organic chain. Therefore the isopropoxide was preferred for the short nanotubes.
Overall the coating of the short nanotubes seemed to be easier and more stable,
since the surface of an individual tube was significantly smaller than the surface
of the ℓ-CNTs. Additional advantage to this was the surface charge that for the
case of the s-CNTs it was higher (greater absolute value of the zeta potential)
for the selected pH value. Both kind of MWNTs showed better dispersion in the
ethanol compared to water and since the for the organic precursors the ethanol was
preferred as solvent, in the case of the s-CNTs are expected less coated bundles.
59
Sonication in200 ml HNO3
(10 N) for 3 h
Acid treatmentat 100 in 10 N
HNO3 for 6 h
Triple wash withethanol
Refluxed at 40for 1h
Dispersing in 250 ml ofabsolute ethanolAddition of
precursor solution:Ti(OC3H7)4 in 50 ml
pH at 4
Washing withwater
Drying at 50for two days
Figure 4–11: Schematic diagram of the process for the coating of the s-CNTs.
4.4 Characterization of the Composites
The very basic characterization of the composite material was done with XPS
(KRATOS XSAM 800), TEM (JEOL TEM 2010F), TGA/DTA and XRD (XRD
Philips APD 3720). The XRD and TGA/DTA will determine the crystal structure
and the required time for the heat treatment. XRD will also yield information
for the grain size via the Scherrer equation. This result is important not only for
the photocatalysis, but for the interpretation of certain spectra such as XPS and
Raman. Since the XRD has low detection limit, in order to determine the crystal
structure just particles were synthesized following the same process as the one
that the coating produced (figures 4–10 and 4–11). Figure 4–16 (I) shows the
60
0 50 100 150 200 250 300 350 400 450 5000.4
0.3
0.2
0.1
0
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Temperature (oC)
TG
A (
mg)
DT
A (µ
V)
Figure 4–12: The TGA/TDA data of the s-ANTs. The peak at the 100 is fromthe water evaporation and therefore it is accommodated by a massreduction. At approximately 250 the phase transition is startingand carries on until the 500.
results of the XRD of the coated tubes and figure 4–16 (II) shows the results of the
synthesized particles.
The TEM will confirm the coating uniformity and quality. The BET will de-
termine the specific surface area of the material (m2/g). This is critical since higher
surface area means more efficient photocatalysis. This value will be necessary for
the photocatalytic degradation tests that will be preformed on the same specific
surface area base.
In addition the XPS survey will show the composition of the material. The
detailed analysis of the peaks will be discussed in a separate chapter.
61
(a) (b)
Figure 4–13: TEM images of the coated s-CNTs. (a) The coating is approximately6 nm thick with very intense variation. (b) There are cases that thereare big particles nucleated on the surface of the nanotubes. That is inagreement with the BET results that showed specific surface area of183 m2/g.
4.4.1 Short ANTs: TEM, XPS, BET
The TEM of the short nanotubes revealed a coating with large variation in the
thickness ranging for 3 to 10 nm with average value of 6 nm (figure 4–13 (a)). It
also observed that there were spots that the coating was not complete and there
were uncoated regions on the surface of the nanotubes. Figure 4–17 shows the XPS
results that confirm the titania coating. In addition the XRD (figure 4–16(I), line
(b)) confirms the anatase phase, while there is no indication the of rutile phase.
The Scherrer’s formula will be used to estimate the grain size [171]
dgrain =Kλ
B cos θ(4−12)
where θ is the Bragg’ s angle, λ the wavelength (1.54 A), K is a constant (K =
2(
ln 2π
) 12 = 0.93) [171–173], B is the half value breadth of the most intense peak.
The grain size according to this calculation is 5 nm (figure 4–16, line (b)). This is
in agreement with the TEM results. The same calculations for the (figure 4–16(II),
62
(a) (b)
Figure 4–14: TEM images of the coated ℓ-CNTs. (a) The TEM images revealcoating of approximately 4 nm and it is uniform. (b) There are casesthat there are big particles nucleated on the surface of the nanotubes.That is in agreement with the BET results that showed specificsurface area of 172 m2/g.
line (b)) showed 53 nm average grain size. Although they were produced under the
same conditions they have different grain sizes that attributed to the presence of
the nanotubes.
Finally the BET revealed a surface are of 183 m2/g. The high surface area
is due to the needle like shape of the nanotubes and the rough surface that the
Sol-Gel chemistry generated.
4.4.2 Long ANTs: TEM, XPS, BET
The TEM showed a very uniform coating 4–14(a) of approximately 4 nm
thick. In contrast with the s-ANTs the coating is very uniform and has very small
variance (3 to 5 nm). Again there were cases of partially coated tubes, but less
compared to the s-ANTs. Again the XPS confirmed the elements of Ti, O and C.
Based on the elemental concentrations the amount of titania is about 12%. This
value however is not considered accurate since the XPS is very sensitive to the
thickness of the layers. The electrons depending on their energy can travel only
63
2 5 10 20 50 100 200 500 1000 20003
5
10
20
50
100
200
Kinetic energy (eV)
Mea
n fr
ee p
ath
λ ( A
)AgAlAuBeC
FeGeMoNiSe
Figure 4–15: The universal curve of the electrons, based on the calculations by M.P. Seah and W. A. Dench [174]. The curve shows the mean free pathof the electrons as function of the kinetic energy (dashed lines). Thereare also experimental results that follow the same trend. The meanfree path does not depend on the material. For Mg source the X-Rayenergy is 1253.6 eV, which give a mean free path of approximately 10A.
a certain distance in the material, regardless what the material is (figure 4–15).
The detected electrons are coming for only the few top nm [174]. The s-ANTs
have thicker coating and therefore the elemental analysis is not representative
composition. The XRD confirmed the anatase (figure 4–16(II), line (a)). According
to the Scherrer formula (equation 4−12) the grain size is 5 nm (figure 4–16(I), line
(a)). This is slightly contrasting the TEM result that was 4 nm. This is attributed
to the fact that the signal of the carbon nanotubes overpowered the signal of
titania and therefore the calculation is not considered exact but just a rough
estimate. The grain size that was calculated based on the XRD pattern from figure
4–16(II) (just the synthesized particles), line (a) is 23 nm.
64
Finally the BET gave a surface area of 172 m2/g. This is in agreement with
the expectations based on the TEM images and the respective result for the s-
ANTs. The lower value of the surface area is attributed to the smoother surface
that the Ti2(SO4)3 yielded. In case of the ℓ-ANTs there are less free particles as
result of the coating process (figure 4–10).
65
20 25 30 35 40 45 50 55 60 65 70 75
2θ (degrees)
Co
un
ts (
a.u
.)
(a)
(b)
(101) (100)
(101)
(103)
(004)
(112)(200)
(111)
(210)
(200)
(211)
(105)
(211)
(220)
(213)
(204)
(002)(220)
(221)(116) (220)
(112)
(301)
(320)
(107)
(311)
(I)
20 25 30 35 40 45 50 55 60 65 70 75
2θ (degrees)
Counts
(a.
u.)
(a)
(b)(101) (100)
(101)
(103)
(004)
(112)
(200)
(111)
(210)
(200)
(211)
(105)(211)
(220)
(213)
(204)
(002)
(220)
(221)(116) (220)
(112)(301)
(320)
(107)
(II)
Figure 4–16: XRD patterns with and without the coating. (I) XRD patterns of thenanotubes with the coating. (II) The XRD pattern of the particlesprepared by the same Sol-Gel method as the coating on thenanotubes. (a) Titanium sulfate (ℓ-ANTs) and (b) titaniumisopropoxide (s-ANTs). The solid lines denote the peaks for anatase(black line) and rutile (light gray) with the relative intensities.
66
01002003004005006007008009001000
Binding Energy (eV)
N(E
)
C 1s 31.2%
Ti 2p 16.7%
O 1s 52.0%
Ev/step:0.5 eV, Time/step: 30 ms, Sweeps: 10
Source: Mg, Pass Energy:89.45 eV, Work Function: 4.36 eV
Ti 3pNa KVV
Ti 2s
O KVV
Ti LVV
C KVV
Figure 4–17: XPS survey for the s-ANTs. There is a significant amount of TiO2 (16.7% Ti). There is no direct stoichiometrywith the oxygen (52% O) since the oxygen depends on the exposed crystallographic orientation.
67
01002003004005006007008009001000
Binding Energy (eV)
N(E
)
C 1s 91.0%
Ti 2p 1.2%
O 1s 5.8%
Ev/step:0.5 eV, Time/step: 30 ms, Sweeps: 10
Source: Mg, Pass Energy:89.45 eV, Work Function: 4.36 eV
Ti 3pNa KVV
Ti 2s
O KVV
Ti LVV
C KVV
Si 2p 2.0%
Figure 4–18: XPS survey for the ℓ-ANTs. There is a significant amount of TiO2 (1.2% Ti). Again there is no stoichiometrywith the oxygen (5.8% O). There is less TiO2 compared to the s-ANTs.
CHAPTER 5PHOTOCATALYTIC EVALUATION OF THE SYNTHESIZED PARTICLES
WITH DYE DEGRADATION TESTS
This chapter describes the series of experiments that were performed to
evaluate the photocatalytic efficiency of the synthesized particles. The method
used for this purpose is dye degradation, where a dye is being photocatalyitcally
degraded and its concentration is being monitored as function of time [175–179].
This technique was selected over the biocidal tests since it is fast, accurate and
depends primarily on the type and properties of particles and not on particle
interactions. Other methods that could have been used, such as spore or bacteria
inactivation, have many, not fully controlled, variables that can alter the results
[180, 181].
In the case of the biocidal test the length of the particles is comparable to the
diameter of the target bacteria or spores. This will affect the kinetics of the system
and the interaction between the particles and the bacteria by inducing steric forces
and occasionally electrostatic effects. In addition, the temperature and the pH
that can vary significantly during the experiments can dramatically affect the
behavior of the spores or bacteria. Especially for the spores, temperature increase
will trigger germination that will transform them into bacteria, making them more
vulnerable to the photocatalytic destruction. Biocidal tests are also time consuming
and require a highly specialized lab. So although the particle has been designed
primarily for biological applications, the biocidal tests are not an accurate way
to measure and compare the properties. Thus the dye degradation test was used
as a quick way to validate the photocatalytic properties of the particles, which
68
69
Figure 5–1: Schematic diagram showing the basic elements of the photocatalyticdegradation chamber.
are directly related to the structure and the electronic properties of the different
particles.
In the following sections, the experimental setup is described, followed by the
theory of the dye degradation and the parameters that can influence the results.
Subsequently the experimental results and finally some general conclusions are
derived.
5.1 Experimental Setup, Materials and Procedures
5.1.1 Experimental Setup
Figure 5–1 shows a sketch of the experimental setup (photocatalytic reaction
chamber). The whole structure consists of a light-insulating chamber where the
interior is black to absorb any scattered radiation. At the top of the chamber
is a 5W fan to maintain the temperature below 30. Inside the chamber are a
magnetic stirrer of variable speed and four UV lamps arranged over the stirrer
(figure 5–1). Depending on the test different lamps have been used:
70
UV 350 nm four fluorescence lamps of 350 nm peak wavelength and 8W power
each that in the current configuration gave 20 W/m2.
UV 305 nm four fluorescence lamps of 350 nm peak wavelength and 8W power
each that in the current configuration gave 20 W/m2.
Visible light two halogen lamps of light radiation and 100W power each that in
the current configuration gave 50 W/m2 that have built-in UV filter.
For all the different lamps the intensity was monitored as function of time.
It was found that the intensity increases with time for the first 30 min. After
this time has elapsed the intensity is stabilized at the power output given above.
Thus the lamps are always given a head start of minimum 30 min before the
experiment starts. Under those conditions a test with water demonstrated that
the temperature is maintained almost constant at approximately 25 with 1 to 2
degrees variation in one hour. Temperature is also a factor that can influence the
results, but not in a significant manner.
5.1.2 Dye Selection
In the literature there are many types of dyes used for this application. For
the present experiments the Brilliant Procion Red MX-5B (C19H13Cl2N6Na2O7S2)
was used [176, 182]. The color of the dye is magenta and absorbs strongly in the
510-540 nm (Figure 5–3). Figure 5–2 shows the molecular structure of the dye.
The presence of the three benzene and one s-triazine rings makes the dye more
resistant to degradation compared to other dyes with fewer rings, even for low
concentrations [183]. This is very critical since fast degradation means that the
system will not be fully stabilized (pH, temperature) before the degradation is over.
Very slow degradation however will give sufficient time for water evaporation that
will alter the dye concentration. An additional advantage is the existence of both
negatively (SO−24 ) and positively (Na+, NH+
4 ) charged chemical groups that will
induced adsorption on positively and negatively sites respectively.
71
Figure 5–2: Three-dimensional structure of the Brilliant Procion Red MX-5molecule. As it can be seen it contains 3 benzene groups and a benzenegroup with three carbon atoms replaced by nitrogen atoms (s-triazine).
Brilliant Procion Red MX-5B is one of the dyes that has been extensively
studied and the degradation byproducts are known [176, 185]. However in this
research there is no need to study the dye in this extend since all the necessary
information is available from the literature [176]. Table 5–1 shows the different
intermediates of the reaction in the order they appear in the solution during
degradation. The photocatalytic reaction proceeds in three steps. In the first step
the most active bonds are hydroxylated. Those bonds include the C−N bond linked
to the benzene ring or the naphthalene ring and the C−S bond of sulfonate group
linked to the naphthalene ring or the benzene ring, to form organic acids with or
without hydroxyl groups and the related ions (SO2−4 and NH+
4 ). In the second step,
the groups linked to the triazine ring are replaced by hydroxyl to yield cyanuric
acid, as in the case of the s-triazines herbicides, and the related ions (SO−3 , Cl−).
At the same time the aromatic acids produced from the first step subsequently
hydroxylated and led to the cleavage of aromatic rings to from aliphatic groups.
72
400 450 500 550 600
Wavelength (nm)
Ab
sorp
tio
n (
a.u
.)
Figure 5–3: The absorption spectrum for a 5 ppm solution of the Procion RedMX-5B dye.
The third step involves a further oxidation of the aliphatic acids to produce CO2
and water. Those steps are summarized in table 5–1 and figure 5–4 represents a
visualization of the degradation.
5.1.3 Experimental Procedure
Initially a mixture of dye solution and the particles that are being evaluated
are sonicated for 20 mins. Following the sonication the particles are placed in
the dark chamber (figure 5–1). While the solution is exposed to UV light, three
samples are obtained every certain time intervals, in 1.5 ml cuvettes. The cuvettes
were left for 2 days for the particles to settle. The dye concentration was measured
via UV-VIS spectroscopy and the reaction constant was estimated based on the
Langmuir-Hinshelwood theory. Since the particles tested here are nanosized, even
after 2 days there will still be suspended particles. Those particles can scatter or
73
p-Hydroxy-phenyl-3- 3-Hydroxy- 2-Hydroxy-benzoic acid-hydroxy-propanedioic acid -benzeneacetic acid
p-Hydroxy-cinnamic acid 1,2-Benzenedi- cyanuric acidcarboxylic acid
1-Propene-1,2,3- Propanedioic acid Propanoic acid-tricarboxylic acid
Malic acid Butenedioic acid Oxalic acid
Figure 5–4: The structure of several intermediate products of the photocatalyticreaction that show the destruction of the bonds and the size reductionof the molecules.
74
Table 5–1: The oxidation intermediates and their structure to be compared to theinitial dye structure in figure 5–2. Adapted from reference [184].
Step Photo-oxidation intermediates
Step-1
p-Hydroxy-phenyl-3-hydroxy-propanedioic acid3-Hydroxy-benzeneacetic acid2-Hydroxy-benzoic acidp-Hydroxy-cinnamic acid1,2-Benzenedicarboxylic acid
Step-2
Cyanuric acid1-Propene-1,2,3-tricarboxylic acidPropanedioic acidPropanoic acidMalic acidButenedioic acidOxalic acidAcetic acid
Step-3Aliphatic compounds to CO2 and H2Ominerals (S, Na)
absorb the light, which will alter the obtained spectrum. So for every experiment
a water solution with particle concentration equal to the ongoing experiment is
prepared. This solution is also left for 2 days and the obtained spectrum is used as
background.
5.2 Theory for the Photocatalytic Degradation of Dyes
Most experimental results agree that the rate of photocatalytic oxidation of
dyes can be approximated with the Langmuir-Hinshelwood (L-H) model [175–
178, 180–185]. The model assumes that the rate will depend on the adsorption
of the dye molecule on the TiO2 particle and the oxidation reaction. So if it is
assumed that k, is the reaction constant and K the adsorption constant then
according to the L-H kinetics model the oxidation rate is:
r = −dCdt
=kKC
1 +KC(5−1)
75
0 20 40 60 80 100 120 1400.0
0.2
0.4
0.6
0.8
1.0
–0.010
–0.008
–0.006
–0.004
–0.002
0.000
Time (min)
C/C
0
r=d(C
/C0 )/d
t
(a)
0 5 10 15 20 25 30 35 400.0
0.2
0.4
0.6
0.8
1.0
–0.1
–0.08
–0.06
–0.04
–0.02
0
Time (min)
C/C
0
r=d(C
/C0 )/d
t
(b)
Figure 5–5: Comparison between the numerical solution of theLangmuir-Hinshelwood (equation 5−1) and the approximation. Thered lines represent the approximation and the black is the numericalsolution. The solid line represents the dye concentration while the
dashed represents reaction rate ddt
(CC0
)
. Figure (a) is for large
concentrations (k=0.1, K=1, C0=10) and figure (b) is for smallconcentrations (k=0.1, K=1, C0=0.1).
76
In equation 5−1 k and r is mg/l min and K is in l/mg where C is the dye concen-
tration in mg/l. This model is non linear but it can be further simplified:
1
KC0ln
(C
C0
)
+
(C
C0− 1
)
= − kt
C0(5−2)
where C0 is the initial dye concentration. With the assumption that C0 → 0, then
1KC0
ln(
CC0
)
≫(
CC0
− 1)
and equation 5−2 simplifies to:
ln
(C
C0
)
= −Kkt (5−3)
which yields a simple exponential decay:
C (t) = C0e−kKt (5−4)
C (t) = C0e−kapp.t (5−5)
C (t) = C0e−t/τ (5−6)
Figure 5–5 shows a comparison of the approach for two different dye concentra-
tions. It is apparent that in the case of the low concentration (figure 5–5(a)) the
agreement between the exponential approach and the exact numerical solution is
very good, while for the case of the high concentration the difference is significant.
It has to be underlined that in the Langmuir-Hinshelwood model is assumed for
single reaction (AB A + B), which is not true for the case of the dye degra-
dation. As described before for this certain dye there are a lot more reactions
involved during the degradation. In this case it is just assumed that the k refers to
the slowest reaction.
5.3 Parameters that Influence the Photocatalytic Reaction
There are many parameters that can affect the reaction rate. The major pa-
rameters are the pH, the initial dye concentration, the solids loading and radiation
77
intensity. There also other parameters such stirring speed and temperature with
minor effect at the reaction rate.
5.3.1 pH
The pH is one of the most important parameters that influence photocatalytic
reactions. The pH can impact both the particles stabilization and the actual
reaction [180, 181]. Depending on the isoelectric point the particles will induce
coagulation that will significantly reduce the surface area of the particles. For
titania the isoelectric point ranges from 5 to 7. Therefore for pH values between
5.0 and 7.0 the photocatalytic reaction rate will be reduced. For pH values >7 and
< 5 the colloidal stability is optimum. In addition the surface charge impacts the
way the dye adsorbs on the titania particles. This is especially important for the
case of azo dyes, such as the one used here, since the have many polar groups. The
charged molecules (positively charged S and Na atoms) can be adsorbed well on the
surface with negative charge (in the case of titania means pH>7).
The pH can directly affect the reaction. A high pH will increase the amount of
OH−, and vise versa. In this reaction there are three steps with multiple reactions
within each step. Slight variations of the pH can have a significant impact on some
of the reactions that will immediately effect the overall reaction. It is obvious
that there is not a specific trend for the pH, since it depends on the dye and its
byproducts. So et al. however have investigated the pH effect of the Procion Red
MX-5B, and the results are in figure 5–6(a) [186]. There is approximately a 40%
variation at the reaction rate when the pH increases from 2 to 10.
5.3.2 Initial Dye Concentration
As it was already discussed smaller concentrations are more suitable for the
first order decay since it approaches more the simple exponential. However there
is a more physical dependence of the reaction rate to the dye concentration. While
the initial dye concentration increases it will increase the probability of a dye
78
2 4 6 8 10
Rea
ctio
n R
ate
(a.u
.)
pH0 10 20 30 40
Rea
ctio
n R
ate
(a.u
.)
C0 (ppm)
(a) (b)
0 10 20 30 40
Rea
ctio
n R
ate
(a.u
.)
Light Intensity (W/m2)
r=αΙ
r=βΙ1/2
0 0.1 0.2 0.3 0.4
Rea
ctio
n R
ate
(a.u
.)
φ (wt%)
1 µm
100 nm
10 µm
Radius increase
(c) (d)
Figure 5–6: The main parameters that influence the oxidation rate. (a) pHvariation, obtained from reference [180], for the Brilliant Procion RedMX-5B (b) as function of the initial dye concentration (c) as functionof the light intensity (d) as function of the surface area (datacalculated for Degussa P25).
molecule adsorbing on the surface and consequently leading to photocatalytic
degradation. Thus the reaction rate will increase. However, if the dye concentra-
tion increases further the solution will become darker resulting UV shielding and
therefore the rate will decrease. The increasing of the dye concentration, will also
increase the amount of adsorbed dye molecules on the surface of the particles,
which will reduce the available OH− sites and therefore reduce the [OH•] gener-
ation. So initially the reaction rate is increasing (figure 5–6(b)) almost linearly,
until it reaches a maximum and afterwards it decreasing almost exponentially. The
graph in figure 5–6(b) has been derived both with theoretical and experimental
79
data. The observed maximum, for the dye currently used is about 5 ppm. An addi-
tional advantage for using this concentration is that, as seen from the graph, small
variations (5±2 ppm) around this value do not have any impact on the reaction
rate
(
drdC0
∣∣∣C0=Cmax
= 0
)
.
5.3.3 Intensity of the Radiation
The light intensity is another parameter that can affect the reaction. It
is expected that low intensities (0 to 20 W/m2) will excite fewer electrons and
therefore the overall reaction rate will be low. While increasing the light intensity
the reaction rate will increase, till it reaches a maximum value and level out. The
way the light intensity influences the reaction rate cannot be derived directly from
first principles, but Ollis et al. [187] after reviewing several studies concluded that
three distinct regions can be delineated (figure 5–6(c)). (i) For low light intensities
the reaction rate increases proportionally to the light intensity (∝ I). (ii) At
intermediate light intensities and beyond a certain value (approximately 20 W/m2)
the rate intensity is proportional to the square root of the light intensity (∝√I)
and (iii) at higher intensities the light intensity does not have an impact on the
reaction rate.
5.3.4 Solids Loading/Surface Area
Many researchers have reported the effect of the solids loading on the pho-
tocatalytic efficiency [187–189]. It is, however, more valid hypothesis to assume
that the reaction constant depends on the available surface area and not the solids
loading. Generally increasing the number of particles (and consequently the avail-
able surface) the sites for adsorption and OH• generation will also increase and
therefore the overall reaction rate will increase. At higher solids loading, however,
there are other factors that come into play, such as more rapid coagulation of the
particles and UV light shielding, that will eventually impede the reaction rate, until
it reaches a plateau [188, 189].
80
0 10 20 30 40 504.0
4.5
5.0
5.5
6.0
6.5
Time (min)
pH
Degussa P25
ANTs
4.58
5.64
Figure 5–7: The pH variation during the dye degradation. The initial valuebetween the ANTs and Degussa P25 since the specific surface area isdifferent. In the first case the pH is stabilized after 10 min while in thesecond case that occurs after 20 min. In both cases the stable pH valueis lower than the initial.
The solids loading φ is correlated to the surface area per solution volume φS
with the equation:
φS
[m2
100ml
]
=3
ρRφ[ g
100ml
]
(5−7)
The relation between φS and φ is linear, but φS is also inversely proportional to
the particle radius R. So for the same solids loading the particle radius has a
tremendous impact on the reaction rate (figure 5–6(d)). So to avoid variations due
to surface area changes the experiments will be conducted on the same surface area
basis unless it is otherwise stated.
81
400 450 500 550 600 650
Wavelength (nm)
Ab
sorp
tio
n (
a.u
.)
524 (nm)
537 (nm)513 (nm)
60 min
50 min
40 min
30 min
20 min
10 min
0 min
Figure 5–8: The dye spectrum during the different time intervals. The three dashedlines (513, 524 and 537 nm) are the three wavelengths that were usedfor the C/C0 calculation. The data were obtained from a sample of 3mg Degussa P25 in a 50 ml of 5 ppm dye solution.
5.4 Experiments
For all the experiments the parameters discussed above (pH, initial dye
concentration, radiation intensity and solids loading) were either kept constant or
monitored to ensure the accuracy of the result. The dye concentration was always
kept at 5 ppm, the light intensity of the UV lamps was 20 W/m2 (50 W/m2 for the
visible radiation) and the pH was monitored during the experiments. Figure 5–7
shows the pH variation during the photocatalytic degradation. The stabilization
occurred, relatively fast, in 20 min for Degussa P25 and 10 min for the ANTs (both
short and long). The maximum difference between the reaction rates, due to the
different pH value (4.58 versus 5.64) will be only in the order of 10%. Other minor
parameters such as temperature and stirring speed were assumed insignificant
82
0 10 20 30 40 50 60 70 80 900
0.2
0.4
0.6
0.8
1
Time (min)
C/C
0
Dye itself (10 ppm)
Dye itself (5 ppm)
Figure 5–9: Investigation of the dye degradation under the UV light for twodifferent dye concentrations. The UV is not having an apparent impacton the dye.
and therefore they were just kept constant. The simplified approximation of the
Langmuir-Hinshelwood model (equation 5−6) was used to interpret the obtained
data. To avoid variations due to the initial dye concentration instead of the C(t)
the C(t)/C0 (∝ I(t)/I0) value was used to obtain the reaction rate. For every
experiment, three samples were collected and were measured with the Perkin-Elmer
Lambda 800 UV/VIS. Figure 5–8 represents a very typical series of the obtained
spectra. The dashed lines denote the three different wavelengths that were used.
So every data point (I(t)/I0) was the average value of 9 different intensities. The
different results were compared with the parameter τ which is the inverse reaction
constant 1/kapp.. Physically it is the required time for 67.21% destruction of the
dye. One of the concerns was the stability of the dye under UV. To investigate
whether the dye is UV stable, two solutions with different dye concentrations (5
83
ppm and 10 ppm) were exposed to UV and the dye concentration was measured
with the method previously described. The dye showed excellent stability in the
UV (350 nm) (figure 5–9) which is in agreement with Hu et al. [180, 184] and
Sivalingam et al. [178].
5.4.1 Titania Nanoparticles and Carbon Nanotubes
This set of experiments, will investigate whether the carbon nanotubes can be
used as photocatalysis enhancers. Anatase particles will be mixed with different
amounts of nanotubes and will degrade the dye under UV. These results will then
be compared with the respective results from the particles only. The particles
are anatase nanoparticles (obtained by Alfa-Aesar, product number: 44689) with
primary particle diameter 5 nm (α-TiO2). Since the particles are very small it is
expected that the band gap will be larger due to quantum effects. The change in
the band gap (∆Eg):
∆Eg =h2π2
2R2
(1
me
+1
mh
)
− 1.786e2
ǫR− 0.248E∗
RY (5−8)
where h is the Planck constant, R the particle radius, E∗RY the effective Rydberg
energy calculated to be 4.3 × 10−39 J, ǫ is the dielectric constant of anatase TiO2
which is 86, me and mh are the electron and hole masses, respectively [190]. Reddy
et al. [191] calculated the ∆Eg, for 5 and 10 nm particles and is 0.2 and 0.1 eV
respectively. So for 5 nm particles is required minimum of 346 nm1 . According to
So et al. however in order to effectively assist the photocatalysis are required UV
lamps with peak wavelength 305 nm [186].
There were in total four experiments performed. Table 5–2 lists all those ex-
periments with the amount of the particles and the result (τ). The first experiment
1 λ = hcEg
= 12.398 × 10−7 ⇒ λ[A] = 12,398Eg[eV ]
84
Table 5–2: Summary of the experiments performed
Experiment Anatase particles CNTs τ χ2
ID [min]A-1 3 mg 0 mg 52.40 ± 0.97 11.2615A-2 3 mg 1 mg 27.09 ± 0.26 64.5865A-3 3 mg 2 mg 53.46 ± 0.99 4.2614A-4 3 mg 3 mg 83.03 ± 0.56 3.9119
(A-1) is done to evaluate the photocatalytic activity of the anatase particles. 3 mg
of anatase particles were dispersed in 50 ml of 5 ppm dye solution and were placed
in the UV chamber. The same experiment was repeated again with the addition of
different amount of carbon nanotubes (1 mg, 2 mg and 3 mg).
Figure 5–10 shows the photocatalytic degradation results. The red lines denote
the fitting according to equation 5−6. The inserts are the logarithmic plot. For the
first experiment τ is 54.94 min (figure 5–10(b)). When 1 mg of nanotubes is added
in the solution the time τ drops to 27.54 min (figure 5–10(b)) which represents
a significant reduction to the parameter τ by 50%. This proves that the initial
hypothesis that the nanotubes can be used as photocatalytic carrier to enhance the
efficiency is true. However further increase of the nanotubes 2 mg and 3 mg is not
having the same effect (figures 5–10(c) and 5–10(d) respectively). This is attributed
to the fact that the presence of the high concentration of nanotubes is shielding the
UV light and makes the solution darker. Figures 5–11 (a) and (b) show the same
results collectively for immediate comparison.
One of the questions raised here is whether the dye adsorbs on the carbon
nanotubes instead of being destroyed by the titania particles. If that is true the
attribution of the dye concentration reduction to the enhancement of the photo-
catalysis is incorrect. It is necessary therefore to perform control measurements
for several carbon nanotubes solids loadingz. Figure 5–12 shows the results of the
controls, where the dye concentration does not change significantly during the
experiment. It is observed, however, a small, still questionable, reduction to the
85
0 10 20 30 40 50 60 70 80 900
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Time (min)
C/C
0
0 30 60 900.01
0.1
1
Time (min)
ln(C
/C0)
0 10 20 30 40 50 60 70 80 900
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Time (min)
C/C
0
0 30 60 900.01
0.1
1
Time (min)
ln(C
/C0)
(a) (b)
0 10 20 30 40 50 60 70 80 900
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Time (min)
C/C
0
0 30 60 900.01
0.1
1
Time (min)
ln(C
/C0)
0 10 20 30 40 50 60 70 80 900
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Time (min)
C/C
0
0 30 60 900.01
0.1
1
Time (min)
ln(C
/C0)
(c) (d)
Figure 5–10: The results for the experiments A-1 to A-4. (a) Just the anataseparticles (b) anatase particles and CNTs together(mα−TiO2 :mCNTs =3:1) (c) (mα−TiO2 :mCNTs =3:2) (d)(mα−TiO2 :mCNTs =1:1).
order of 2% in 90 min. Besides if that was the case, with the addition of the carbon
nanotubes in concentrations of 2 and 3 mg would further appear to increase the
photocatalytic efficiency. So from this experiments it is accurate to conclude that
the carbon nanotubes can indeed assist the photocatalytic efficiency.
86
0 10 20 30 40 50 60 70 80 900.01
0.1
1
Time (min)
C/C
0
0 mg1 mg2 mg3 mg
(a)
0 2 4 60
20
40
60
80
100
CNT solids loading (mg/100 ml)
1/τ
52.40 min
27.09 min
53.46 min
83.03 min
(b)
Figure 5–11: Collective graph of the data presented above. In figure (a) the dataare shown with the fitting and in figure (b) the bar chartdemonstrates the difference in the various τ .
87
0 10 20 30 40 50 60 70 80 90 1000
0.2
0.4
0.6
0.8
1
Time (min)
C/C
0
10 ppm CNT
5 ppm CNT
Figure 5–12: Investigation of the dye adsorption on the carbon nanotubes surface.The adsorption was not significant since it was only 5% reductionafter 90 min.
5.4.2 Long ANTs: Photocatalysis under UV Light
In this section the experiments are preformed to evaluate the photocatalytic
efficiency of the anatase coated long carbon nanotubes (ℓ-ANTs). The benchmark
material was the Degussa Aeroxide P25 from DuPont. There are two types of
experiments; same surface area basis and same mass basis. As it was discussed
previously the most accurate way to directly compare the particles is to keep
most of the parameters that influence the reaction rate, constant for both cases.
So in order to comply with this requirement we perform the experiments on the
same surface area basis. This will guarantee that the results depend only on the
photocatalytic properties of the material and not the possible higher specific
surface area. However the particles have been developed in a manner that they will
88
provide both high surface area and exceptional photocatalytic properties, thus the
same experiments were performed again on the same mass basis.
Figure 5–13(a) shows the results for photocatalytic degradation on the same
surface area basis. The light intensity used here was 20 W/m2 and the peak
wavelength of the lamp was 350 nm. The surface area of the ℓ-ANTs measured 172
2/g and for Degussa P25 it was found 52 m2/g. So the mass ratio used for those
experiments was approximately mℓ−ANTs : mP25 = 1 : 3. The results show that the
ℓ-ANTs performed well under UV compared to the Degussa P25. The τℓ−ANTs=19.1
min (±0.4 min) compared to Degussa P25 for which τP25=24.1 min (±0.4 min).
Since this experiment was done on the same surface area basis, this difference is
attributed to the photocatalytic properties of the particles. It is interesting to
compare this result to those for the carbon nanotubes/anatase particles mixture.
The surface area of the anatase particle is 70 m2/g, which means that the surface
ratio between ℓ-ANTs and anatase particles is Sanatase : Sℓ−ANTs = 2.14. Still,
however, we notice that the τℓ−ANTs is smaller. In the MWNTs/α−TiO2 mixture
the contact between the particles and the nanotubes is occurring due to Brownian
motion and it is instantaneous. In the case of the ℓ-ANTs the contact between the
coating and the carbon nanotubes is permanent.
Figure 5–13(b) shows the degradation data on the mass basis comparison
(1 mg of Degussa P25 and 1 mg of ℓ-ANTs). The photocatalytic efficiency, as
expected was significantly increased compared to the previous result. The reason
for this is the higher specific surface area of the ℓ-ANTs.
5.4.3 Long ANTs: Photocatalysis under Visible Light
This set of experiments investigates the activity of the particles under the
presence of visible light. The visible light source were two halogen lamps of 100 W
each and the total output power was 50 W/m2. According to the manufacturer of
the lamp, the lamp temperature is such, that the spectrum contains a small portion
89
0 10 20 30 40 50 600.0
0.4
0.8
Time (min)
C/C
0
ANTsDegussa P25
(a)
0 10 20 30 40 50 600
0.2
0.4
0.6
0.8
1
Time (min)
C/C
0
ANTs
Degussa P25
(b)
Figure 5–13: Photocatalytic degradation of Degussa P25 and ℓ-ANTs under UVlight of 350 nm wavelength. (a) ℓ-ANTs are shown to be moreeffective in destroying the dye with τ=19.1±0.4 min while Degussahas τ=24.1±0.4 min. (b) The same mass base results. τ=19.1±0.4min for the ℓ-ANTs while Degussa has τ=72.27±1.46 min.
90
0 40 80 1200.0
0.2
0.4
0.6
0.8
1.0
Time (min)
C/C
0
Degussa P25
ANTs
Figure 5–14: The photocatalytic results of the ℓ-ANTs and Degussa P25. Theℓ-ANTs clearly demonstrate photocatalytic activity withτ=152.31±6.13 min. Degussa P25 is not demonstrating any obviousactivity.
in the UV region. The lamp, however, includes a built in UV filter that blocks the
UV radiation. In addition a UV detector (detects radiation from 270 to 400 nm)
verified that there is no UV light present during the experiment. In this case an
amount of 3 mg ℓ-ANTs particles and 3 mg of Degussa P25 were used. Using less
quantity of the ℓ-ANTs will yield very poor results and the measured efficiency was
not reliable. According to the specific surface area of the particles the Degussa P25
had to be 9 mg, which however would have made the solution completely opaque,
resulting the high solids loading problems, such as coagulation and UV shielding
effects. Therefore the experiments were preformed on the same mass basis.
91
pro Fit T
RIA
L version
0 2 4 6 8 100.6
0.7
0.8
0.9
1.0
Time (days)
C/C
0
const.shou
lder
Figure 5–15: The dye degradation data in the dark for the ℓ-ANTs. Degussa is notincluded here since it never demonstrated behavior like such. Thedata were fitted with the equation 5−9. τDARK
ℓ−ANTs=1.29±0.24 days.The constant is 0.76±2.75×10−2.
The results are presented in the same manner in figure 5–14. For the ℓ-ANTs
τVISℓ−ANTs=151.2±4.7 min. Degussa P25 failed to demonstrate any photocatalytic
behavior (τ ≈ ∞). This is due to the white color of titania, which reflects almost
all the range of the visible light. On the contrary for the ℓ-ANTs, since the coating
is very thin (4-6 nm), the color of the composite is black and therefore absorbs all
the visible light. This result is very important, since a new property emerges for
the ℓ-ANTs. The range of the application can now be extended further since the
ℓ-ANTs can be easily used under the visible light.
92
5.4.4 Long ANTs: Post UV Activity, Photocatalysis in Dark
This experiment was designed and performed after it was observed that
cuvettes containing UV irradiated samples, left in the dark for long period of time
(days) appeared to contain no dye. So this experiment intents to measure the post
UV irradiation.
A solution of 1 mg of ℓ-ANTs was placed in the dark chamber and irradiated
for 13.5 min with UV (350 nm peak wavelength) and intensity of 20 W/m2. In this
time the dye concentration has decreased, according to the experiment described in
section 5.4.2, by 50%. The solution then was placed in a light insulated chamber
under a magnetic stirring, in a tightly sealed vial to prevent any water evaporation.
Three samples of 1.5 ml were collected every two days and were left for days so the
particles could settle. The samples were measured according to the protocol that
was described in section 5.1.3.
Figure 5–15 showcases the post-UV photocatalytic efficiency. The observed
data follows the first order reduction as before (equation 5−6), but it has to me
properly modified:
C (t) = C0e−t/τ + const. (5−9)
The constant is denoting that the photocatalytic degradation in the dark is
terminated after some period time has elapsed. There is also a shoulder at the
beginning, which denotes a delay of the mechanism responsible for the degradation.
For those experiments τ = 1.29 days. The delay is roughly about 2 days while the
degradation seems to stop at approximately 75%.
5.4.5 Short Nanotubes: Photocatalysis under UV
The same set of experiments as in section 5.4.2 were performed with the s-
ANTs. 1 mg of s-ANTs were dispersed in dye solution via sonication and then they
were placed in the reactor with lamps of 350 nm peak wavelength and total output
value 20 W/m2. Figure 5–16 shows the result of the photocatalytic degradation.
93
0 30 60 90 120 1500.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
Time (min)
C/C
0 0 30 60 90 120 1500.1
1
Time (min)
ln(C
/C0)
Figure 5–16: The dye degradation data in the UV light of 350 nm for the s-ANTs.τUVs−ANTs=177.41±10.00 mins. The photocatalysis is significantly
slower that all the previous cases.
The s-ANTs demonstrated photocatalytic results but very poor compared to the
ℓ-ANTs, Degussa P25 and even the α-TiO2. The inverse reaction constant was
found to be τ=177.41±10.00 min.
The s-ANTs were expected, to perform equally to the ℓ-ANTs since they
are both consisting on anatase coating on multi wall carbon nanotubes. However
the results are dramatically different. There is not an apparent reason for that.
The XRD (figure ) showed anatase crystal structure for both materials and the
XPS (figure ) survey showed that the particles consist only on titania and carbon
nanotubes. The different sol-gel precursors and the slightly different process can
have minor impact on the final result, since the anatase is in both cases the only
94
Table 5–3: Summary of the experimental results of this chapter.
Experiment LightMaterials
τχ2
ID Source [min]A-1
UV 305 nm
3 mg α-TiO2 52.40 ± 0.97 11.2615A-2 3 mg α-TiO2+1 mg CNT 27.09 ± 0.26 64.5865A-3 3 mg α-TiO2+2 mg CNT 53.46 ± 0.98 4.2140A-4 3 mg α-TiO2+3 mg CNT 83.03 ± 0.56 3.9119
AD-1
UV 350 nm
1 mg ℓ-ANTs 19.61 ± 0.20 8.9373AD-2 3 mg Degussa P25 24.06 ± 0.31 16.1456AD-3 1 mg ℓ-ANTs 19.61 ± 0.20 8.9373AD-4 1 mg Degussa P25 72.27 ± 1.42 0.0012V-1
Visible1 mg ℓ-ANTs 152.31 ± 6.13 0.6798
V-2 3 mg Degussa P25 N/A N/AD-1 Dark 1 mg ℓ-ANTs 1, 858± 346 0.0221SA-1 UV 350 nm 1 mg s-ANTs 177.41 ± 10.00
present phase. So the reason have to be sought on the difference between the two
kinds of tubes (s-CNTs and ℓ-CNTs).
5.5 Conclusion
In this chapter a series of experiments was preformed to quantify the photocat-
alytic activity of the synthesized particles. The photocatalytic evaluation was done
by the degradation studies of the azo dye, Brilliant Procion Red MX-5B. After
reviewing the parameters that will potentially influence the results, the conditions
were set to solely obtain results based on the photocatalytic properties of the
particles (the results are summarized in table 5–3). The following conclusions can
be derived. Carbon nanotubes can assist the photocatalysis by almost 50% when mixed
with anatase particles in 1 : 3 mass ratio according to experiments A-1, A-2,
A-3 and A-4. The ℓ-ANTs can function better under the UV (350 nm) compared to
Degussa P25 on the same surface are base (AD-1 and AD-2) and on the same
mass base (AD-3 and AD-4).
95 Also the ℓ-ANTs display photocatalytic activity under visible light (V-1)
although it is significantly lower than the UV-activity. Degussa P25 failed to
demonstrate such activity under those conditions. ℓ-ANTs displayed, what is named as post-UV activity, dye degradation in the
dark, after an initial dose of UV radiation. The experiment V-1 was repeated for the case for the s-ANTs (SA-1). The
results were completely different compared to ℓ-ANTs. Since the XRD
showed that both times we had anatase structure of TiO2 means that the
difference can be attributed to the CNTs.
CHAPTER 6SPECTROSCOPIC TECHNIQUES TO EXPLAIN THE PHOTOCATALYTIC
EFFICIENCY OF THE ANTs.
The use of the carbon nanotubes as carrier of photocatalyst had a dramatic
effect on the overall photocatalytic efficiency. The fact that the anatase coated
carbon nanotubes have performed better than the Degussa P25 on the same surface
area basis (section 5.4.2) showcased that there has to be something more than just
the high specific surface area. In order to ascertain the reason for this functionality
the investigation has to be focused onto the interface of the titania coating and
the carbon nanotubes. Many characterization techniques are available, but in
this case they are limited by the amount of carbon that the composite material
contains. The carbon will overpower the obtained spectra and consequently the
information cannot be considered accurate. This was already a problem during
the characterization of the composite particles with XRD. This research therefore
will mainly focus on the utilization of surface sensitive techniques. Since in that
case the majority of the information will come only from the top few nm of the
material the information will regard the TiO2 coating and the interface of the
CNTs and TiO2. The selected technique for this purpose is X-Ray Photoelectron
Spectrometry (XPS). In addition to the XPS, Raman Spectroscopy is used to
investigate the nature of the nanotubes and produce information regarding the
bonds. Literature, databases and reference material (anatase nanoparticles) are
used for the analysis of the data
The materials that will be investigated are the anatase coated short carbon
(s-ANTs) nanotubes and the anatase coated long nanotubes (ℓ-ANTs). The short
nanotubes have displayed very poor photocatalytic activity compared to the long
96
97
nanotubes (5.4.2 and 5.4.5). The shortening of the tubes was done with chemical-
mechanical processing which would have a significant impact on the structure
of the tubes although they maintained the tubular structure and the concentric
cylinder arrangement (figure 4–2). Structural information for the nanotubes and
the TiO2 can be obtained with Raman and more accurate bonding (CNT/Titania
coating) information can be obtained from XPS.
This chapter will initially give the general theory of the Raman spectroscopy
for both the carbon nanotubes and for the anatase phase of titania. A parameter
of major importance in Raman spectroscopy is the protocol that will be used to
analyze the obtained spectra (sample preparation, spectra smoothing and peak
recognition). Therefore a protocol is initially established and all the obtained
spectra are analyzed based on this. The last part of the chapter is dealing with
the X-Ray Photoelectron Spectrometry (XPS). The XPS was used primarily as
complimentary technique to Raman to reconfirm the results, and secondarily
to investigate the presence of stresses which arises from the bond between the
MWNTs and the TiO2 coating.
6.1 Raman Spectroscopy of the Carbon Nanotubes
The Raman spectroscopy is a very powerful and valuable tool for the investi-
gation of the carbon nanotubes properties [192–194]. Nanotubes can be thought
as very complex macromolecules with thousands of carbon atoms that will give
rise to many vibrational frequencies that are strongly depended on the structure
of the tubes. Although the carbon nanotubes are a relative new material, from the
extensive study of the Highly Ordered Pyrolytic Graphite (HOPG) there is suffi-
cient knowledge to study the properties of the nanotubes with Raman spectroscopy.
In addition computer simulations are a great complimentary tool to explain the
Raman spectra, since in the case of carbon nanotubes the analytical calculations
are very complicated and demanding [195].
98
1-phonon emission 2-phononsFirst Order Second Orderq k
qk q + k ; qk q + k qk q + kB q
q k EF E G F H H I JJ k L q k + qq
(a) (b) (c)
Figure 6–1: The different Raman scattering processes for CNTs. (a) First orderRaman scattering (b) and (c) are the second order Raman scattering.The k represents the momentum of the incident photon and q
represents the emitted phonon. The first row represents the incidentresonance, and the second the scattered resonance.
6.1.1 General Theory of Raman Spectroscopy of Carbon Nanotubes
The Raman spectra of graphite and SWNTs can provide information about
the exceptional 1D structure of carbon materials, such are phonon and electron
distributions. Since the conducting state (insulator, semiconductor conductor) is
directly related to the electronic structure Raman spectra can directly correlate
those properties to certain peaks and peak shapes [196–198]. Similarly, the me-
chanical and thermal properties are strongly correlated to the phonon interactions
and the phonon distribution, and therefore Raman spectra can provide very detail
information of the SWNTs regarding the thermal and mechanical properties.
Both Raman spectra and FTIR are inelastic scattering of the light. For a
Raman process and during a scattering event (i) an electron is excited from the
valence to the conduction band by absorbing a photon, (ii) the excited electron
is scattered by emitting (or absorbing) phonons, and (iii) the electron relaxes
99
to the valence band by emitting a photon. The observed scattered photon has
energy, which is smaller than the energy of the incident photon (when a phonon is
emitted during the de-excitation ). In Raman the intensity of the scattered photon
is measured as a function of the downshift of the energy (phonon emission). The
downshift is measured usually in cm−1. Those Raman peaks are called Stokes lines.
If the same process is repeated and this time the up-shift frequency (absorption of
phonons) is recorded then it is called anti-Stokes lines [199–201]. The anti-Stokes
and Stokes lines are symmetric to 0 cm−1 which represents the Rayleigh scattering.
In general, however, the adsorption of phonon is less likely to happen and the
intensity of the anti-Stoke lines is lower than the Stoke lines.
The number of emitted phonons (or absorbed) before the relaxation of the
lattice can be one, can be two, or more, which are called one phonon, two phonon
and multi-phonon Raman processes respectively. If there is only elastic scattering,
with no frequency shift involved it corresponds to Rayleigh scattering. Figure
6–1 shows the basic transitions that give rise to the Raman scattering for carbon
nanotubes; the first row represents the incident resonance (incident photon energy
is equal to the gap) and the second the scattered resonance (the emitted photon
energy is equal to the band gap). The × symbol in figure 6–1 symbolizes the
energy diagram with the conduction and valence band. Figure 6–1(a) demonstrates
the simplest first order Raman scattering. One photon (k) excites an electron to
a higher band, an inelastic scattering follows accompanied with the emission of a
phonon (q) and then the rest of the energy is emitted in form of a photon. The
energy of the emitted photon therefore is ERaman = h (k − q). In figures 6–1(b)
the same process is done, but in this case an elastic scattering is involved (dashed
lines). Those are processes that are known as second order Raman since there
are two scattering processes involved. Case (c) is another second order Raman
scattering where two phonons are emitted before the observed photon is emitted.
100
(a) (b)
Figure 6–2: Graphic representation of the major Raman modes. (a) Radialbreathing mode of a single wall nanotube. (b) G Band graphic forsinge wall nanotube. The D Band, since it involves two consequentvibrations, cannot be represented.
6.1.2 Basic Raman Lines for Carbon Nanotubes
Based on the previously explained theory the most important Raman active
bands will to be described. Most of them appear only for SWNTs, but there some
very significant peaks that are also present in the case of the MWNTs [202–204].
Figure 6–3 shows a typical spectrum for SWNTs with the most dominant
Raman features, the Radial Breathing Mode (RBM), the G band, both classified as
first order processes, and the D band, which is classified as second order.
The RBM is the coherent expansion and contraction of the nanotubes to the
radial direction [197, 206](figure 6–2). The RBM has been studied extensively since
it is related to the diameter of the nanotubes [197] and secondly on the density of
electronic states [206]. This is an easily observed mode in SWNTs and in certain
case for isolated double walled nanotubes. When the sample consists of multiwall
nanotubes then this frequency usually diminishes. These features are unique to the
carbon nanotubes and occur with frequencies ωRBM between 120 and 350 cm−1 for
101
Figure 6–3: Typical Raman spectra from metallic and semiconducting SWNTs.The Radial Breathing Mode (RBM), the D Band and G Band are themost important bands. The * is denoting bands that come form the Sisubstrate. Due to the distinct structure of the semiconductingnanotubes there are two additional bands M and iTOLA that appear.
(a) (b)
Figure 6–4: The G Band split and how it is related to the conductivity of thetubes. (a) The G Band split and how it is related to the conductivityof the tubes. (b) The difference between the ω+
G and ω−G. The ω+
G is notchanging but the ω−
G varies with the diameter and follows the equation6−2 [205].
102
tubes ranging from 0.7 nm< dt <2 nm [197, 207]. Empirical relations have relate
the diameter to the frequency of the RBM:
ωRBM =A
dt+B (6−1)
where A and B are constants that depending on the tubes [197, 199, 207]. Since it
is an out of plane bond stretching, for which all the carbon atoms move coherently
in the radial direction, involves only one scattering, and therefore is classified as
first order Raman scattering.
The next important Band is the G Band [208–210]. The G Band is coming
from the 2-D features of graphite and appear only also in nanotubes. It involves an
optical phonon exchange between two dissimilar carbon neighboring atoms A and
B in the unit cell (figure 6–4) [198, 211]. The corresponding mode in the case of
the tubular structure is the same. In contrast to the graphite structure, where the
G Band is a single frequency at around 1582 cm−1, at the nanotubes can consist
of several peaks that relate to the relative position of the two carbon atoms on the
tube. In general the frequencies that arise form vibration to a coaxial direction
(towards the T ) are lower compared to vibrations to circumferential direction
(towards the Ch). The G Band can be used for (i) diameter characterization, (ii) to
distinguish between metallic and semiconducting tubes, through strong difference
to their Raman lineshape and (iii) to probe charge transfer effects arising from
doping [211, 212]. In general the G band is splitting in two distinctive peaks G+
(ω+G around 1600 cm−1, depending on the tube structure) and G− (ω−
G around 1570
cm−1, again depending on the tube structure). The first is related mainly to carbon
atoms vibrations along the nanotube axis, and the frequency is sensitive to charge
transfer from dopant addition (up-shift at the G+ for acceptors and downshift
for donors) [213]. The later, G−, is related to vibrations along the circumferential
direction and their lineshape is mainly associated with the conducting nature of
103
the nanotubes (metallic semiconducting) [214]. If the G− lineshape is broader than
the G+ one and it is better approached by Breit-Wigner-Fano equation then it
means that the tubes are metallic [215] (6–4(a)). There are empirical relations that
correlate the difference between the ω+G and ω−
G with the diameter of the tubes.
ω+G − ω−
G =A±d2
(6−2)
where A± is 47.7 nm2/cm−1 [196] or 45.7 nm2/cm−1 [193] for semiconducting and
79.5 nm2/cm−1 [196] for metallic SWNTs (figure 6–4 (b)). If the split does not
appear indicates that the tubes are not metallic. Those features of the G Band
can be generalized to the case of the multiwall nanotubes and for very well defined
MWNTs it can be better than the SWNTs [216].
Another band with significant interest is the D Band [217]. The D band is
one of the second order Raman scattering and involves either one phonon and one
elastic scattering (figure 6–1 (b)) or two phonons (figure 6–1 (c)) [218, 219]. The
frequency where the Raman shift appears for the D Band depends on the laser
energy [207, 220]. A typical example of this feature is the D Band that shows at
1350 cm−1 and shifts by 53 cm−1, when the laser energy changes by 1 eV. The D
Band shows for amorphous carbon also, and it appears at the frequencies between
1285 cm−1 and 1300 cm−1 and the Full Width at Half Maximum (FWHM) is more
than 100 cm−1. For nanotubes this shows at frequencies between 1305 cm−1 and
1350 cm−1 and with FWHM about 30-60 cm−1. A very interesting feature arises
when the D band is compared to the G Band. The ratio between ID and IG is a
measure of the crystallinity of the nanotubes, meaning how pristine the nanotubes
are [205].
R =IDIG
(6−3)
104
Usually when R → 0 (R < 1) then the crystallinity is higher. Some researchers
define the same ratio as the
R =
∫ +∞−∞ fD(ω)dω∫ +∞−∞ fG(ω)dω
(6−4)
where fD(ω) and fG(ω) is the Lorentzian of the Raman D and G peak respectively.
There are other less significant peaks that can give more detailed structural
information, but since they are observed only for SWNTs they are not discussed
here. This theory, however, is enough to describe the behavior of the composite
materials (ℓ-ANTs and s-ANTs).
6.2 Raman Spectroscopy of the Anatase Structure of TiO2
The general theory of the Raman spectroscopy is similar for the titania
crystals, but in this case the vibrations are representing coherent lattice vibrations
instead of just bond vibrations. The TiO2 can exist in anatase, rutile and brookite,
with each structure having very distinct vibrational frequencies. As already
discussed in 2.1.1 section anatase is tetragonal (D194h) with two formula units per
unit cell and six Raman active modes (A1g + 2B1g + 3Eg) [221]. Rutile is also
tetragonal (D144h) and has two unit cell and four active modes (A1g +B1g +B2g +Eg)
[222]. Finally brookite is orthorhombic (D152h) and has eight formula units per unit
cell and shows 36 Raman active modes (9A1g +9B1g +9B2g +9B3g) [223]. Table 6–1
enlists the Raman frequencies and the relative intensity of the peaks for anatase
and rutile. The analytical calculations for those peaks are in great agreement with
experiments.
One of the most important characteristics is the peak at 144 cm −1. It was
recently discovered that it is very sensitive to the size of the grain and therefore
the size of the particles [224]. That sensitivity can be expressed in an asym-
metric broadening of the peak line shape and blue shift (towards higher wave
numbers)[225]. In an infinity crystal the phonons are free to travel in any direction
before they are absorbed back from the lattice. In the case of nano-sized however
105
Table 6–1: The Raman frequencies fro anatase and rutile phase of titania. Thebrookite is not included here since is not a present form of TiO2 and ithas in total 36 weak peaks. The notation in parenthesis is representingthe relative intensity of the peaks; w: weak; m: medium; s: strong; vs:very strong. Data are adapted from reference material and reference
Anatase D194h I41/amd Rutile D14
4h P42/mnmEg 144 cm−1 (vs) B1g 143 cm−1 (w)Eg 197 cm−1 (w) Eg 447 cm−1 (s)B1g 399 cm−1 (m) A1g 612 cm−1 (s)A1g 515 cm−1 (m) B2g 826 cm−1 (w)B1g 519 cm−1 (m) - -Eg 639 cm−1 (m) - -
crystals the phonons are confined in a space less than the required for uncon-
strained interactions [226]. The calculations for the line-shape change have to be
done in the reciprocal space. In this formulation the I(ω) is given by the equation
[225]:
I(ω) =
∫
B.Z.
|C(0, q)|2d3q
[ω − ω(q)]2 + a2L
(6−5)
where B.Z. denotes the limits for the 1st Brillouin zone, aL is the half width at
half maximum, ω(q) is the phonon dispersion curve and C(0, q) is the scattering
coefficient for first order scattering of spherical nanocrystals and it can be written
as:
|C(0, q)|2 = exp
(
− q2d2
16π2
)
(6−6)
ω(q) is the dispersion curve for titania. This result is too complicated to be di-
rectly calculated but it can be approached with the assumption that the dispersive
relation is a simple vibrational mode in a crystal, such us:
ω(q) = ω0 + ∆ × [1 − cos (|q × a|)] (6−7)
The 1st Brillouin zone can be approached by the Fermi sphere. So the limit for
the integral in equation 6−5 are 0 to kf
(
=√
2Ef me
h2
)
. With those assumptions
106
equation 6−5 can be modified to:
I(ω) =
∫r
2Ef me
h2
0
∣∣∣exp
(
− q2d2
16π2
)∣∣∣
2
d3q
(ω − ω0 + ∆ × [1 − cos (|q × a|)])2 + a2L
(6−8)
which further reduces since we are using the Fermi sphere for the 1st Brillouin zone
into:
I(ω) =
∫r
2Ef me
h2
0
∣∣∣exp
(
− q2d2
16π2
)∣∣∣
2
4πqdq
(ω − ω0 + ∆ × [1 − cos (|q × a|)])2 + a2L
(6−9)
The calculation of the function I(ω) is not trivial even in the case of the equation
6−9. Although there are a lot of assumptions and simplifications, depending
on the approaches (Brillouin zone, dispersion relations) there is an asymmetric
broadening and blue shift that strongly depends on the diameter of the particles.
The shift is 3.2 cm−1 for particles of average diameter 5 nm and has an additional
the broadening of 3 cm−1 (FWHM) towards lower energy values. This broadening
and shifting is relates only with the 144 cm−1 line and the size does not affect the
other bands. In addition since one of the assumptions was the spherical shape of
the particles, which is not accurate since there is no indication about spherical
titania particles on the nanotubes. It is however a good estimation of the order of
magnitude.
6.3 Experimental Procedures
This section explains the basic methods for preparing and obtaining the
Raman spectra. A protocol that summarizes all the mathematical models and
manipulation that will be used to analyze the data has to be established and
according to which all the data will be processed.
6.3.1 Sample Preparation
One of the biggest advantages of Raman Spectroscopy is the fact that it
requires very little sample preparation. In every case 5 mg of sample were mixed
with 1 ml of iso-propanol in a form of thin slurry. The slurry was placed on
107
a slide glass and left in room temperature to evaporate the iso-propanol. The
Raman spectra were obtained by the (Nicolet MAGNA 760 Bench with Spectra
Tech Continuum IR Microscope and FT-Raman) and the laser wavelength was
752 nm. Since the samples were black in color the full power of the laser was
used to maximize the obtained signal. Different spots of the same sample and
different samples of the same material yielded the same spectra, but with different
intensities and different noise levels.
6.3.2 Mathematical Analysis and Manipulation
Smoothing is a very sensitive manipulation of the data since over smoothing
may result disappearing of some peaks (6–5 (b)) and under-smoothing may show
pseudo peaks that may be misleading (6–5 (c)). Although there is commercial soft-
ware available to smooth and analyze the data, in this research manual smoothing
and fit was preferred so the data manipulation is fully controlled.
The algorithm for the smoothing was LOESS. The term is derived from the
term locally weighted scatter plot smooth. The method uses locally weighted linear
regression to smooth the data. The process is weighted because a regression weight
function is defined for the data points contained within the span. In addition to
the regression weight function, you can use a robust weight function, which makes
the process resistant to outliers. Finally, the method LOESS uses a quadratic
polynomial. If it uses a linear polynomial, it is called LOWESS. The algorithm
gives the option of using all the data or a certain section around the data point
(xi, yi), called span, α. For large span the data become smoother and the time
required for the calculations increases dramatically. For a series of data (xj , yj),
where j = 1, · · · , N and the point (xi, yi) the process has the in the steps:
1. The following distances are calculated:
di = |xk − xi|, i = 1, 2, · · · , N (6−10)
108
which then sorted into ascending order.2. The quantity q is calculated,
q = max(Truncate(αN), 1) (6−11)
3. This is used to calculate the distance scale
D =
dq α ≤ 1αdN α > 1
(6−12)
The steps 2 and 3 have only computational purposes and basically they willguaranty that the smallest distance D will not be smaller than d1
4. The weighted function for the data point is:
T (u) =
(1 − |u|3)3 |u| ≥ 10 |u| ≤ 1
(6−13)
and based on this equation the weights for the data points are then given by
wi = T(xi
D
)
(6−14)
5. For LOESS, the regression uses a second or third degree polynomial. The setof points used for the fit are in the form (xi, yi, wi) The difference betweenweighted least squares and the regular least squares is that the function thatis minimized is
F (a1, a2, · · · , aN) =
N∑
i=1
wi
(yi − f(xi; a1, a2, · · · , aM)
σi
)2
(6−15)
where f is the polynomial∑M
l=1 alxl, that is used for the fit and can be first,
second or third order.6. The process is repeated for the next point.
Figure 6–5 demonstrate the application of the LOWESS algorithm. Different
variations in the parameter α can have dramatic effect on the final result, (b)
under-smoothed, (c) over-smoothed and (d) nicely smooth. In the case of the
present data the parameter α was varying 0.03-0.02 depending on the noise to
signal ratio. The software that run the smooth algorithm is Mathematica 5.1 by
Worthram Research. The whole algorithm is at Appendix and was obtained by the
109
1000 1100 1200 1300 1400 1500 1600 1700 1800
Raman Shift (cm1)
Inte
nsi
ty (
a.u
.)
Raw data
1000 1100 1200 1300 1400 1500 1600 1700 1800
Raman Shift (cm1)
Inte
nsi
ty (
a.u
.)
Raw data
α=0.03, second order
(a) (b)
1000 1100 1200 1300 1400 1500 1600 1700 1800
Raman Shift (cm1)
Inte
nsi
ty (
a.u
.)
Raw data
α=0.3, second order
1000 1100 1200 1300 1400 1500 1600 1700 1800
Raman Shift (cm1)
Inte
nsi
ty (
a.u
.)Raw data
α=0.09, second order
(c) (d)
Figure 6–5: Different options for the LOESS algorithm. (a) Raw data as obtained.(b) Under-smoothed data, that give false peaks, (c) Over smootheddata that smooth out necessary peaks (d) nicely smoothed data withall the peaks showing nice. Always used quadratic equation for the fitand the variation was coming from the span α.
class notes of Dr. McQuarrie and is based on the algorithm by Cleaveland [227–
229]. It was slightly modified so it could handle larger number of data in shorter
time.
110
The obtained peaks in most of the cases were fitted with the Lorentz peak
profile. The equation describing that profile is
f(ω) =I0π
aL
(ω − ω0)2 + a2L
(6−16)
where I0 is the maximum intensity, aL is the half width at half height, and ω0 the
frequency where the peak appears. Although the aL can be directly measured and
obtained from the graph it is not recommended since the background has to be
first subtracted and then the exact height and width of the peak can be measured.
So in this case the aL is one of the fitting parameters. For n peaks equation 6−16
becomes:
f(ω) =1
π
n∑
i=1
I(i)0 a
(i)L
(ω − ω(i)0 )2 + (a
(i)L )2
(6−17)
For the background of Raman spectroscopy there are several approaches, but
in this case the best one found to be a simple polynomial equation that goes up to
the third order.k∑
i=0
ai · ωi (6−18)
where k = 0, 1, 2, 3. So the equation used to fit the obtained spectra is
f(ω) =1
π
n∑
i=1
I(i)0 a
(i)L
(ω − ω(i)0 )2 + (a
(i)L )2
︸ ︷︷ ︸
Lorentz Peaks
+
k∑
i=0
ai · ωi
︸ ︷︷ ︸
Background
(6−19)
where n is the number of peaks and k the order of the polynomial background
correction.
As stated in the previous section there are cases where the G− can be fitted
with the Breit-Wigner-Fano equation which is similar to Lorentz, but has an
asymmetric broadening. The equation of Breit-Wigner-Fano is
I(ω) = I0
(
1 + ω−ω0
qΓ
)2
1 +(
ω−ω0
Γ
)2 (6−20)
111
where I0 is the intensity, Γ is the half width at half maximum (HWHM), q is a
broadening parameter and ω0 is the frequency where the Raman peak appears. In
this case the equation used is:
f(ω) =1
π
n−1∑
i=1
I(i)0 a
(i)L
(ω − ω(i)0 )2 + (a
(i)L )2
︸ ︷︷ ︸
Lorentz Peaks
+
k∑
i=0
ai · ωi
︸ ︷︷ ︸
Background
+ I0
(
1 + ω−ω0
qΓ
)2
1 +(
ω−ω0
Γ
)2
︸ ︷︷ ︸
Breit-Wigner-Fano
(6−21)
In all cases for the background both second and third order polynomials were used.
Based on the parameter χ2 the order that was giving the best value was kept.
The algorithmic for the fitting was the Monte-Carlo, Levenberg-Marquardt and
Robust, which came as part of the software ProFit from QuantSoft. In most of
the cases all the algorithms gave the same results at the fitting parameters with
minor deviations. In some cases, certain algorithms (Monte-Carlo or Levenberg-
Marquardt) failed to converge and only the remaining algorithms were used.
Besides the obvious peaks the fit was attempted with more peaks, to ensure that
there are not any other hidden peaks. So when for example there are three obvious
peaks, the fit is attempted with three and in addition four or five but hidden or
overlapping peaks were never found.
The graphs are represented directly with the smoothed data. The black
solid line represents the smoothed data; the red line (dark grey) represents the fit
including the peaks and the background; and finally the dashed lines represent the
different peaks that have been identified.
6.4 Experimental Results
This section is divided in two subsections. The first one regards characteriza-
tion of the nanotubes before the coating and the other one after the coating. All
the data have been obtained and processed according to the protocol established in
section 6.3.
112
6.4.1 Long Nanotubes after the Acid Treatment
Figure 6–6 shows the Raman spectra for the long nanotubes after the acid
treatment. The acid treatment is expected to damage the surface of the nanotubes,
which will have an effect on the vibrational frequencies of the tubes. The low
frequencies such as the RBM did not appear so the 0−1000 cm−1 region is not
included. Since there is a split in the G Band the fit was attempted for both
line-shapes Lorentz (equations 6−19) and Breit-Wigner-Fano (equation 6−21).
Equation 6−21 gave better fit (χ2 parameter), thus the 6−21 fit was kept.
The first thing noticeable from figure 6–6 and table 6–2, is that the D Band
appears at the 1312 cm−1 and the aL is 22 cm−1 which is a very distinct char-
acteristic that the tubes consist on tubular arrangement of graphene sheets and
not amorphous carbon (e.g. carbon nanowires). The next very important feature
showing in the figure is the G Band. It shows roughly at 1594 cm−1 and has a very
distinct split. Breit-Wigner-Fano gave better fit results, which is a very profound
characteristic of the metallic nature of the carbon nanotubes. This is one of the
most important results since, it points out that the ℓ-CNTs are conductive in na-
ture. And based on the theory that was discussed in Chapter 2 addition of metals
to the photocatalyst can dramatically improve the overall performance.
The next step to the analysis is to calculate the ratios between the G Band
and D Band via the equation 6−3 and 6−4. This analysis will give the magni-
tude of the crystallinity of the tubes. According to 6−3 (as IG0 is considered the
IG−
0 +IG+
0
2) the ratio is 3.081. For the case of 6−4 the equation has to be modified as
R =
∫ +∞−∞ fD(ω)dω
∫ +∞−∞ fG+(ω)dω +
∫ +∞−∞ fG−(ω)dω
(6−22)
According to the equation 6−22 the ratio is 2.624 slightly lower than the previous.
In general the R < 1 is for very crystalline tubes and R > 1 is for tubes with
defects on the structure. This is reasonable since the acid treatment has shown the
113
Table 6–2: The raw fitting parameters calculated with the Levenberg-Marquardtalgorithm for the acid treated ℓ-CNTs. The graphic representation ofthe results is in figure 6–6. The fit yielded χ2 =7.1333×104. Forconvenience at the data representation we use the symbol a
(2)L instead of
Γ that is used in equation 6−20.
Fitted parameters Standard deviations
Backgrounda0 =-824.6088 ∆a0 =14.7875a1 =1.5224 ∆a1 =2.1812×10−2
a2 =-5.4635×10−4 ∆a2 =7.7575×10−6
D BandLorentz
I(1)0 =863.1688 ∆I
(1)0 =2.0660
ω(1)0 =1311.4516 ∆ω
(1)0 =5.2327×10−2
a(1)L =21.9585 ∆a
(1)L =9.0829×10−2
G Band (G−)BFW
I(2)0 =308.2726 ∆I
(2)0 =2.5939
ω(2)0 =1582.5923 ∆ω
(2)0 =0.1729
a(2)L =15.5437 ∆a
(2)L =0.2718
q =0.0342 ∆q =2.7345×10−6
G Band (G+)Lorentz
I(3)0 =252.0681 ∆I
(3)0 =3.4170
ω(3)0 =1611.2235 ∆ω
(3)0 =0.1401
a(3)L =9.6443 ∆a
(3)L =0.2420
destruction of the outer walls and the attachment of −COOH and −OH groups.
Another possible reason for the value of the ratio can be the presence of impurities
other than and acid treatment byproducts. But the XPS proved that there are no
other elements than carbon, and carbon impurities are in the form of thin layer
that cannot affect the Raman spectra in such a manner. It is accurate results to
conclude that the ℓ-CNTs are conducting, having distinct tubular structure with
very high density of surface defects as a result of the acid treatment.
6.4.2 Short Nanotubes after the Acid Treatment
Again in this case the Raman Spectroscopy did not shown the RBM frequency.
Besides the fact that RBM is not very easily identified in the case of MWNTs these
nanotubes have damaged structure not only from the acid treatment but also from
the shortening process, which utilizes acids (H2SO4 and HNO3) and mechanical
method. Thus the region of interest is from 1000 cm−1 to 1800 cm−1. From the
shape of the graph there is not a distinct split at the G Band and although both
114
1000 1100 1200 1300 1400 1500 1600 1700 1800
Raman Shift(cm1)
Inte
nsi
ty (
a.u
.)
Raw data
Peak at 1312 cm1
Peak at 1583 cm1
Peak at 1612 cm1
Fit
Figure 6–6: The ℓ-CNTs after treated with nitric acid at 140 for 10 hours. The DBand is showing at 1312 cm−1 and the G Band at about 1594 cm−1. Avery distinct split of the band can be seen with the G+ at the 1584cm−1 and G− at 1612 cm−1.
equations, 6−21 and 6−19 were used the 6−21 failed to give accurate and further
more the 6−19 could not be fitted when the number of peaks was set at 3.
From figure 6–7 and the table 6–3 again the first noticeable thing here is the
D Band that appears at 1305 cm−1 which is the lower limit for the D Band in the
case of MWNTs. The fit gave a aDL of about 31.3431 which is not broad enough
to conclude that this is amorphous carbon, but still it can be assumed that the
broadening is coming from the heavy damage that the tubes suffered due to the
115
Table 6–3: The raw fitting parameters calculated with the Levenberg-Marquardtalgorithm for the acid treated s-CNTs. The graphic representation ofthe results is in figure 6–7. The fit yielded χ2 =3.9138×107.
Fitted parameters Standard deviations
Background
a0 =-1.4326×104 ∆a0 =1403.8835a1 =35.2712 ∆a1 =3.2537a2 =-1.4479×10−2 ∆a2 =2.4599×10−6
a3 =-1.5079×10−6 ∆a3 =6.0625×10−7
D BandLorentz
I(1)0 =1.3329×104 ∆I
(1)0 =39.1153
ω(1)0 =1305.1773 ∆ω
(1)0 =9.1653×10−2
a(1)L =31.3431 ∆a
(1)L =0.1725
G BandLorentz
I(2)0 =5585.0378 ∆I
(2)0 =40.1330
ω(2)0 =1586.3196 ∆ω
(2)0 =0.2058
a(2)L =29.7420 ∆a
(2)L =0.4163
shortening process and the acid treatment. Even if the TGA and the XPS survey
showed the presence of iron (6.0% wt), the iron alone cannot affect directly the
Raman spectra.
The next characteristic is the G Band, which seems to be the overlapping of
too different peaks very close together and also very broad. All the fit algorithms
failed to recognize two peaks with variation in the smoothing parameters and
background. It is therefore accurate to conclude that there are is not a distinct
split of the band. From the table 6–3 we see that the peak shows at 1586 cm−1
which is expected for nanotubes. The interesting feature is that the broadening
of that peak is 30 cm−1 which is very large for D Band peak. Based on the fitting
parameters obtained from the table 6–3 the calculation of R, equations 6−3 and
6−4, will determine the quality of the tubes. The first approach (equation 6−3)
gives R = 2.38657 which is very small compared to the previous case. This
give initially the impression that the short nanotubes are less defective and their
structure is more defined than the long. But when the second approach (equation
6−4) is used then the R = 3.60633, which is more acceptable result regarding the
116
1000 1100 1200 1300 1400 1500 1600 1700 1800
Raman Shift(cm1)
Inte
nsi
ty (
a.u.)
Raw data
Peak at 1305 cm1
Peak at 1586 cm1
Fit
Figure 6–7: The s-CNTs after treated with nitric acid at 100 for 6 hours. The DBand is showing at 1305 cm−1 and the G Band at about 1586 cm−1.Although the G Band looks like it consists on to overlapping peaks itstill can be treated as one peak.
processing history. That shows that the s-CNTs have more defects compared to the
long nanotubes discussed in the previous section.
The most important result from these spectra however, remains the shape
of the G Band. The absence of the split (or at least a very distinct split) denotes
the very high possibility that those tubes lack of conducting properties. This is a
very important conclusion and can be correlated to the poor performance of the
ℓ-ANTs. The conclusion that the short nanotubes have poorer conductivity can
117
Table 6–4: The raw fitting parameters calculated with the Levenberg-Marquardtalgorithm for the titania coated ℓ-CNTs and the titania segment of thespectrum. The graphic representation of the results is in figure 6–9. Thefit yielded χ2 = 8.3378 × 104.
Fitted parameters Standard deviationsa0 =-8809.5084 ∆a0 =2.0077×104
Background a1 =9.1589 ∆a1 =16.7950a2 =2.9614×10−4 ∆a2 =4.6200×10−3
I(1)0 =427.8412 ∆I
(1)0 =5.2873
Eg ω(1)0 =150.1796 ∆ω
(1)0 =0.1802
a(1)L =16.4892 ∆a
(1)L =0.4071
I(2)0 =71.307 ∆I
(2)0 =0.881
B1g ω(2)0 =408.7834 ∆ω
(2)0 =0.3522
a(2)L =36.3452 ∆a
(2)L =1.2131
I(3)0 =85.568 ∆I
(3)0 =1.057
Eg ω(3)0 =629.1235 ∆ω
(3)0 =0.4801
a(3)L =22.3412 ∆a
(3)L =1.3041
be used to explain this behavior. Smallest conductivity means poorer ability to
transport the electrons away from the titania, which results less holes, consequently
less [OH•] and therefore lower photocatalytic activity.
6.4.3 Long Nanotubes after the Coating
Figure 6–8 shows the total spectrum of the ℓ-CNTs after the coating. In this
case the carbon nanotubes have a thin coating of titania and therefore all the
carbon nanotube peaks appear clearly. On the contrary the titania peaks are not
very clear and only the very strong and medium strength peaks appeared. The
spectra can be divided into two regions, 0−1000 cm−1 where are the titania peaks
appear and the 1000-1800 cm−1 where are the MWNTs peaks appear. A very
general characteristic is that the CNT peaks appear very clear and that they have
maintained their basic shape. Again all the analysis was done based on the same
protocol (section 6.3).
Figure 6–9 shows the spectra and table 6–4 summarizes the fit results.
The titania part is in the form of a very thin coating and therefore the only
118
0 200 400 600 800 1000 1200 1400 1600 1800
Raman Shift (cm1)
Inte
nsi
ty (
a.u.)
Figure 6–8: The Raman spectra of the coated long nanotubes. There are two separate regions, (i) 0-1000 cm−1 that containthe titania peaks and (ii) 1000-1800 cm−1 that contain the carbon nanotubes peaks. The peak identification isdone later in the chapter.
119
0 100 200 300 400 500 600 700 800 900 1000
Raman Shift (cm1)
Inte
nsi
ty (
a.u.)
Raw data
Fit
Peak at 150 cm1
Peak at 409 cm1
Peak at 629 cm1
Figure 6–9: The first region from figure 6–8. There are four major peaks but onlythree of them can be identified accurate. 149.56 cm−1, 628.65 cm−1 and408.64cm−1.
obvious peaks is Eg than the main peak at around 144 cm−1 which is nicely fitted
with a Lorentzian. The peak however shows at 150 cm−1 which indicates a 6
cm−1 blue shift compared to the literature value. There are two major reasons
for this irregularity. The surface termination of titania particles is imposing
constrains to the phonons, which results a more asymmetric peak and blue shift.
Calculations based on equation 6−9, show that this shift will occur for values
of 2-2.5 nm particles, which is in agreement with the literature [226] and more
detailed calculations. Grains of this size may exist but it is not the majority, since
120
the average grain size is 5 nm. Consequently we cannot conclude that the blue
shift comes only from the size of the particles. In addition for a shift of 5 cm−1
according to equation 6−9 we should observe a great asymmetric broadening of the
peaks. However this is not the case, since the peaks are nicely fitted with just a
single Lorentz line. That leads to one more reason for the shift. Shifts in Raman
spectroscopy can come from alteration of the symmetry, as a result of possible
bonding to a non-native element. In this case as it has been stated in chapter 4,
there are −COOH and −OH groups on the surface of the nanotubes. Those groups
are used as anchoring spots for the sol-gel chemistry of the titania crystals. So
it is possible to have a TiO2 bond in the form of C−O−Ti. Raman spectroscopy
provides evidence of that bond.
Additional proof comes from the other two peaks that have been identified
in the spectrum the one at 399 cm−1 and 639 cm−1. Those peaks are significantly
further than any rutile peaks (447 cm−1 and 612 cm−1 respectively) so there is
no doubt they belong to anatase. For both peaks we observe a shift that is not
towards the same direction. More specifically for the B1g peak is observed a blue
shift by 10 cm−1 (peak at 409 cm−1) and for the Eg peak it is observed a red shift
by 10 cm−1 (peak at 629 cm−1). Those peaks do not change due to the dimensions
of particle, the only reason they shifted can be the bonding to a non lattice
element. Therefore this argument can further justify the result for the CNT−TiO2
bond and that the blue shift of the first Eg frequency (144 cm−1) does not comes
exclusively from the size effect. The frequencies where the peaks appear is not
influenced by the Ti+3 or Ti+4 but only on the phonon distribution.
The other segment (1000−1800 cm−1)of the graph is about the nanotubes.
Analysis according to the protocol gave the results that are collectively represented
in table 6–5. Again we notice the position of the D band that is at 1307 cm−1 and
the aL is 26 cm−1 which is slightly larger than the value before the coating. The
121
Table 6–5: The raw fitting parameters calculated with the Levenberg-Marquardtalgorithm for the titania coated ℓ-CNTs. The graphic representation ofthe results is in figure 6–6. The fit yielded χ2 = 8.3378 × 104. Forconvenience at the data representation we use the symbol a
(2)L instead of
Γ that is used in equation 6−20.
Fitted parameters Standard deviations
Backgrounda0 =-1086.0823 ∆a0 =17.1629a1 =1.9385 ∆a1 =2.5408×10−2
a2 =-6.6503×10−4 ∆a2 =9.0231×10−6
D BandLorentz
I(1)0 =1091.2343 ∆I
(1)0 =2.0617
ω(1)0 =1307.0246 ∆ω
(1)0 =4.9037×10−2
a(1)L =26.2351 ∆a
(1)L =8.9694×10−2
G Band G−
BFW
I(2)0 =389.9422 ∆I
(2)0 =0.2694
ω(2)0 =1579.1871 ∆ω
(2)0 =0.3259
a(2)L =19.5770 ∆a
(2)L =0.3259
q =0.0546 ∆q =1.3445×10−6
G Band G+
Lorentz
I(3)0 =369.6873 ∆I
(3)0 =5.3204
ω(3)0 =1605.8443 ∆ω
(3)0 =0.1502
a(3)L =12.0105 ∆a
(3)L =0.2529
significant observation here is that the D band after the coating have a blue shift
by 4 cm−1. The reason for that is again the possible bond between the titania
coating and the nanotubes. The broadening of peak can also be attributed to the
same reason, since the width is directly correlated to the amount of coherence in
the vibrations. The coating will constrain those vibrations and consequently will
broaden the D- band.
The next band is the G band, which has maintained the split, a characteristic
of the conducting nature of the nanotubes. The G− is appearing at the 1589 cm−1
and the aL is 20 cm−1 while the G+ band is at the 1606 cm−1 and the aL is 12
cm−1. Compared to the values before the coating that indicates a red shift by 6
cm−1 for G− and a blue shift by 6 cm−1 for G+. It is interesting to compare the
relative intensity of the IG+ to IG− (R−/+) before and after the coating. Similarly
122
1000 1100 1200 1300 1400 1500 1600 1700 1800
Raman Shift (cm1)
Inte
nsi
ty (
a.u.)
Raw data
Fit
Peak at 1307 cm1
Peak at 1579 cm1
Peak at 1606 cm1
Figure 6–10: The second region from figure 6–8. The D Band is at 1307 cm−1 andthe G Band is at the about 1590 cm−1. The band split still exists,with the G− at 1579 cm−1 and the G+ at 1606 cm−1.
to the IG/ID ratio the
R−/+ =
∫ +∞−∞ fG−(ω)dω∫ +∞−∞ fG+(ω)dω
(6−23)
before and after the coating will give a measure on how the peaks have changed. So
before the coating this yields R−/+ = 1.97106 and after the coating R−/+ = 1.71925
which indicates that the relative intensity of the G+ to G− has increased. As
mentioned in section 6.1.2 the G+ is sensitive to charge transfer that comes form
sources such as dopant addition. In this case the change is also related to the
titania-CNT bond that can result charge transfer to the underlying nanotubes
123
from the TiO2. Another interesting point is the calculation of the ratio between
the G and D band. According to equation 6−4 the ratio is 3.9637 and according
6−3 is 2.8731. In both cases it is significantly higher than the values calculated
for the case of the bare nanotubes (2.64 and 3.081 respectively). This means that
the crystal structure of the nanotubes have been significantly distorted due to the
possible bonding with the titania coating.
However among all the different changes, the most outstanding is the fre-
quency shift. The shifts are significant indication of the existing of C−O−Ti bonds.
In similar cases, other researchers have reported similar peaks that have been
attributed to certain bonds. Yakovlev et al. [230] and Kamada et al. [231] have
worked with thin coatings of titania on silica and reported the existence of the
Si−O−Ti bond at 950 cm−1 but this bond was not accompanied by bond shift
at the titania or silicon peaks. This is most likely due to the fact that the film
was thick (700 nm) and therefore the bulk titania peaks (that appear in normal
frequencies) covered any shift due to the bonding. In this case we do not observe
any peak that can be directly attributed to a C−O−Ti bond. However the peak
shift alone is a very strong evidence for that bond.
6.4.4 Short Nanotubes after the Coating
Figure 6–11 shows the Raman spectra for the case of the short coated CNTs s-
ANTs. Again this spectrum can be divided into two regions; one from 0-1000 cm−1
regarding the titania peaks and a second from 1000-1800 cm−1 for the nanotubes.
One of the interesting results is that the titania peaks are a lot more obvious and
intense compared to the peaks before. The reason for that is the thicker coating the
s-ANTs (6 nm) have versus the ℓ-ANTs (4 nm). Raman is relative surface sensitive
(approximately 800 nm) technique and therefore the thickness of the coating will
impact the results. Therefore figure 6–11 required two different acquisitions with
different settings. Initially the titania overpowered the spectrum, so a second run
124
0 200 400 600 800 1000 1200 1400 1600 1800
Raman Shift (cm1)
Inte
nsi
ty (
a.u.)
Figure 6–11: The Raman spectra of the coated short nanotubes. There are two separate regions, (i) 0−1000 cm−1 thatcontain the titania peaks and (ii) 1000−1800 cm−1 that contain the carbon nanotubes peaks. This spectra hasbeen obtained by the combination of two different runs.
125
Table 6–6: The raw fitting parameters calculated with the Levenberg-Marquardtalgorithm for the acid treated ℓ-CNTs. The graphic representation ofthe results is in figure 6–12. The fit yielded χ2 =1.9924×108.
Fitted parameters Standard deviations
Background
a0 =-824.6088 ∆a0 =14.7875a1 =82.2700 ∆a1 =254.8581a2 =-0.1334 ∆a2 =2.9830×10−4
a3 =6.3907×10−5 ∆a3 =1.6551×10−6
Eg
I(1)0 =1.3060×105 ∆I
(1)0 =191.5728
ω(1)0 =149.9038 ∆ω
(1)0 =1.3982×10−2
a(1)L =9.6293 ∆a
(1)L =2.5634×10−2
Bg
I(2)0 =6099.0402 ∆I
(2)0 =211.8730
ω(2)0 =202.3874 ∆ω
(2)0 =0.2600
a(2)L =7.5055 ∆a
(2)L =0.4139
E1g
I(3)0 =5456.3761 ∆I
(3)0 =138.7558
ω(3)0 =392.5676 ∆ω
(3)0 =0.4355
a(3)L =17.4229 ∆a
(3)L =0.7914
B1g,A1g
I(4)0 =3461.7658 ∆I
(4)0 =155.9701
ω(3)0 =510.0127 ∆ω
(3)0 =0.5928
a(4)L =13.3065 ∆a
(4)L =0.5928
Eg
I(5)0 =1.0324 ×104 ∆I
(5)0 =125.1647
ω(5)0 =632.6918 ∆ω
(5)0 =0.2443
a(5)L =20.4046 ∆a
(5)L =0.4285
was need to focus on the CNTs part. The mathematical analysis again was done
according to the protocol described in section6.3.
Figure 6–12 shows the first part of the spectrum regarding the titania. In
this case since the titania coating was thicker, the peaks are significantly clearer
than before and all of the peaks listed in table 6–1 appear. In table 6–6 the
mathematical analysis of those peaks, shows again a large blue shift on the 144
cm−1 for 6 cm−1. Again this shift has two major contributions, size effect of
the coating and the bonding of titania on the nanotube. Regarding the first
contribution and according to chapter 4 the primary particle size is 4 to 8 nm
and according to equation 6−9 the shift has to be approximately 2 and 3 cm−1
respectively. So in this case since the shift is 6 cm−1 there has to be an additional
126
0 100 200 300 400 500 600 700 800 900 1000Raman Shift (cm1)
Inte
nsi
ty (
a.u.)
Raw data
Fit
Peak at 150 cm1
Peak at 202 cm1
Peak at 393 cm1
Peak at 510 cm1
Peak at 633 cm1
Figure 6–12: The first portion of figure 6–11. There are 5 very distinctive peaks at150 cm−1, 202 cm−1, 393 cm−1, 510 cm−1 and 633 cm−1.
reason for the peak shift. That reason again is bonding of the carbon nanotube
and titania. The other peaks appear also shifted. So the Eg is at 202 cm−1 shifted
by 5 cm−1, the B1g is at the 393 cm−1 shifted by 6 cm−1 (blue shift), the A1g is at
510 cm−1 shifted by 5 cm−1 (blue shift) and the Eg finally is at 633 shifted by 6
cm−1 (blue shift). An important characteristic, the shifts that are not equal and
they are not all at the same direction. Some of them are blue (B1g, A1g and Eg)
and some red (Eg). The shifts again, depends on the kind of vibration and on
how it is affected by the bond to the non lattice element, in this case carbon. A
very important peak is the small peak at the end of the spectrum (730 cm−1) that
127
Table 6–7: The raw fitting parameters calculated with the Levenberg-Marquardtalgorithm for the coated s-CNTs. The graphic representation of theresults is in figure 6–12. The fit yielded χ2 =1.0956×105.
Fitted parameters Standard deviations
Background
a0 =7561.0312 ∆a0 =147.6613a1 =-15.4746 ∆a1 =0.3227a2 =1.0866×10−2 ∆a2 =2.3081×10−4
a3 =-2.5298×10−6 ∆a3 =5.4214×10−8
D BandI
(1)0 =152.8715 ∆I
(1)0 =2.1243
ω(1)0 =1316.0874 ∆ω
(1)0 =0.5633×10−2
a(1)L =46.6845 ∆a
(1)L =1.3153
G− BandLorentz
I(2)0 =111.2193 ∆I
(2)0 =2.7395
ω(2)0 =1544.7456 ∆ω
(2)0 =0.6863
a(2)L =21.3581 ∆a
(2)L =1.2052
G+ BandLorentz
I(2)0 =543.3215 ∆I
(2)0 =3.8876
ω(2)0 =1582.2848 ∆ω
(2)0 =8.2552
a(2)L =10.9464 ∆a
(2)L =0.1459×10−2
was intentionally omitted from the fit, since it is not recognized as anatase, rutile,
brookite or any carbon vibrational mode. It is believed that it is the C−O−Ti
bond that is formed. Yakovlev et al. [230] mention the Ti−O−Si bond at 950
cm−1. Similarly it can be argued that this peak at 750 cm−1 is a Ti−O−X bond.
At this point only extensive mathematical calculations can prove the validity of
this concept and therefore is not going to be the main argument of the section.
On the contrary the bond shift is a very solid proof and therefore is going to be
the major argument. There is not an obvious asymmetric broadening of the peaks
as it was expected from nanosized particles which means that the peak shift is
primarily associated with the C−O−Ti. The reason for that is that the particles
are not expected to be spherical, and the dimension on the radial direction of the
tube is not necessary equal to the dimension at direction parallel to the tube. That
will effect the phonon confinement (by approaching a bulk crystal). Therefore it is
accurate at this point to conclude that peak shift is product of the C bonding and
not the phonon confinement.
128
1000 1100 1200 1300 1400 1500 1600 1700 1800
Raman Shift (cm1)
Inte
nsi
ty (
a.u
.)
Raw data
Fit
Peak at 1316 cm1
Peak at 1544 cm1
Peak at 1582 cm1
Figure 6–13: The second portion of figure 6–11. Although the carbon peaks are notvery clear we can still see them at the 1316 cm−1 the G Band and atthe 1582 cm−1 the G Band. The G Band seems to be splitting in twopeaks 1544 cm−1 and 1582 cm−1. The ratio between the peaks iscompletely reversed but this is currently attributed to the weak signalobtained by the s-CNTs in this case.
In figure 6–13 and in table 6–7 are the results regarding the CNTs part, in
this case the s-CNTs. The quality of the plot is not very good since the titania
layer was relative thick so the emitted radiation was not intense enough. Following
the same analysis as before the data were smoothed, but the smooth could not
eliminate a pseudo peak that appeared in the G Band (1544 cm−1). That peak
129
although it could be approached with a Lorentz peak, but not with a Breit-Wigner-
Fano peak, yields values for the width (aL) and frequency (ω−G) that for a G− Band
are not realistic. Still, however, since the G− is related mainly to charge transfer
of that split can be attributed to possible bond between the coating and the CNT.
Since the G− cannot be approached with the Breit-Wigner-Fano1 peak is secure
to conclude that the nanotubes are not changing state (semiconducting→metallic),
which is a physical acceptable result. In addition all the peaks have shifted
compared to the uncoated nanotubes. The D Band is showing a blue shift by 16
cm−1 and the G band (in the case after the coating will be considered the G+) is
showing red shift by 4 cm−1.
A very interesting result in this spectrum is the ratio between the D and G
Band (R). The ratio can be calculated by the equations 6−3 and 6−4. Equation
6−3 we obtain 0.2799 (assuming that IG ∼= IG+) and from equation 6−4 we get
0.857487. In both cases we obtained numbers smaller than 1, which means that
the nanotubes have very crystalline structure. This result cannot be representative
of those tubes specially accounting the processing history and the coating. The
coating as in the case of the long nanotubes have to increase the ration instead of
decreasing it. In this case though since the coating is very thick, many vibrational
modes have been prevented. Since D Band is the results of two consequent vibra-
tions (a phonon exchange between two dissimilar carbon atoms) it is expected to
have reduced intensity. So overall the Raman spectra for the case of the s-ANTs
demonstrated the same results as in the case of the ℓ-ANTs. All the nanotube
and titania peaks were shifted and in addition there were dramatic changes on the
1 In the case of Breit-Wigner-Fano fit, none of the algorithms could converge to arealistic result.
130
Table 6–8: Summary of the Raman result. Here are listed the major peaks andshift both for titania and CNTs after the coating.
ℓ-ANTs s-ANTsTitania ℓ-CNTs Titania s-ANTs
Band Shift Band Shift Band Shift Band ShiftEg +5 cm−1 D Band −5 cm−1 Eg +5 cm−1 D Band −5 cm−1
Eg − G− Band −4 cm−1 Eg +5 cm−1 G− Band AppearedB1g +10 cm−1 G+ Band −6 cm−1 B1g −6 cm−1 G+ Band −4 cm−1
A1g − A1g −B1g −10 cm−1 B1g −9 cm−1
Eg − Eg −6 cm−1
shape of the CNTs peak that conclude that there is a bond between the titania
coating and the CNTs.
6.4.5 Summary of the Raman Spectra Analysis
Table 6–8 shows collectively the results of the Raman spectroscopy. The most
interesting result comes when the spectra before and after coating are compared,
all the peaks (both titania and CNTs) were significantly shifted. The second
important result is that all the peaks have different shift not only in magnitude,
but in direction too. This basically eliminates the fact that the shift can occurred
due to a miss-calibration of the instrument. The fact that the two completely
different particles with different photocatalytic properties displayed similar results
in regards to the bonding information, leads to another reason for the difference in
the photocatalytic efficiency. That reason can be located to the split of the G Band
that occurs only in the case of the ℓ-CNTs (excellent photocatalytic properties) and
not for the case of s-CNTs (poor photocatalytic properties). The split was not only
very obvious with the two peaks to have almost similar intensity, but the G− was
fitted better with the Breit-Wigner-Fano lineshape compared to Lorentz. So from
the spectroscopic analysis we can conclude that: There is a bond between the titania coating and the carbon nanotubes
131 And that the ℓ-ANTs that performed better at the photocatalytic evaluation
consist on metallic carbon nanotubes, while the s-ANTs consist of non-
metallic properties carbon nanotubes.
At this point it is obvious that the C−O−Ti bond exists, but in order to reconfirm
that result in the following section X-Ray photoelectron spectroscopy is performed.
6.5 X-Ray Photoelectron Spectroscopy (XPS)
In this section we are using the photoelectron spectroscopy to confirm the
results of the Raman spectroscopy regarding the bonding information. XPS is also
a very surface sensitive technique so it will give information for the anatase crystal
and the interface. In XPS the emitted X-rays eject a core electron. This electron’s
energy is Ek = EX-Ray−Eb where Eb is the binding energy of the electron. Since the
EX-Ray is very well defined and the energy of the emitted photoelectron can be very
precisely measured the Eb is known with very high accuracy. The binding energy of
the electron on a very simplified model is:
Eb = −k2e4me
2h2
(Zeff
n
)2
(6−24)
Where Zeff is the effective nucleus charge, after the electrons cloud partially
shield the nucleus. In the case of the bond of one element to another the electron
distribution will immediately impact the effective charge and therefore the binding
energy will be changed. In XPS spectrum this change can be seen in two ways;
Peak shift: The major binding peak will shift, since the Zeff is changing. The
change in Zeff can come from possible bond to a different element or to bond
stress due to crystal confinement. The chemical shift depends on the amount
of stress or the number of bonds to the different element. Chemical shifts to
higher energies are attributed to bonds to elements that attract the electron
cloud and therefore increasing the Zeff which according to equation 6−24 will
increase the binding energy. Shifts to lower energies will similarly mean that
132
the element that the peak is coming from, is attracting the electronic cloud
and the Zeff is smaller therefore the energy shifts to lower bonding energies.
Extra peak: The origin of this peak is the same as the chemical shift but, in
this case not all the atoms are bonded to other elements so the initial peak
remains and just an extra peak appears, at slightly different energy.
The XPS spectrum of titania has been studied extensively already and there is a
large literature reference library about it. The major peaks are the Oxygen peak
O1s that has a major peak at 531.5 eV. There is a secondary peak at around 527-
529 eV. That peak is attributed to lattice oxygen, while the first one is attributed
to surface oxygen. The lattice oxygen is very sensitive to the size of the crystal
grain. So in order to investigate the XPS for nanosized particles it is recommended
to use a reference material with the same size to determine the exact position of
the lattice peak. The next peaks that are significant for the XPS is the titanium
peak Ti2p1/2 and Ti2p3/2. The Ti2p1/2 peak appears at 464.2 eV and is very
precise as it is in good agreement with literature database. The Ti2p3/2 is at
458-459 eV [232–236]. Sharp and intense peaks is a good indication that the TiO2
consist only on Ti+4. In addition in the case of Ti+3 and according to equation
6−24 the Zeff will be reduced which will shift the binding energy to lower energies.
So for this study we used three different samples, the 5 nm particles (α-TiO2), the
ℓ-ANTs and s-ANTs.
6.5.1 Instrument, Sample Preparation and Mathematical Analysis
The instrument used for this study is the Kratos Analytical Surface Analyzer
XSAM 800. For every spectrum two different samples were prepared and the peaks
were compared to ensure that the results are accurate. The sample preparation was
similar to the sample preparation followed for the Raman spectroscopy. Thin slurry
was prepared by mixing 5 mg of particles and 1 ml of iso-propanol. The slurry was
placed on a 1 cm × 1 cm silicon wafer (crystallographic plane (100)) and left dry.
133
The energy is calibrated usually with the Carbon 1s peak. In this case in
addition to that the Silicon 1s peak will also used for calibration. The reason is
that since the carbon nanotubes are bonded to the titania the binding energy of
carbon (C1s) might have been shifted.
The commercial software that came with the instrument was used to smooth
the data. The peaks were fitted with Gauss lineshape;
I(E) =I0
σ√
2πe−
(E−E0)2
2σ2 (6−25)
where I0 is the intensity (N(E)), E is the binding energy E0 the binding energy
where the peak appears and σ a parameter related with the width of the peak
(FWHM = σ√
2π). In certain cases in order to fit the tail of the peaks we are
using the Voigt peak, which is a mix of Lorentz and Gauss peak:
I(E) =
√π
2I0sl
sgV T
(E − E0)
sg
√2,sl
sg
√2
)
(6−26)
where
V T (y, x) =y
π
∫ +∞
−∞
e−t2
y2 + (x− t)2dt (6−27)
again E is the binding energy, E0 is the energy where the peak appears, sg and sl is
parameters of the fit. The default equation is the Gauss if not stated different. The
fit algorithms used are the Levenberg-Marquardt, Monte-Carlo and Robust. The
peak recognition was done based on the literature and on the commercial software.
6.5.2 XPS of the Reference Material
The first material analyzed with XPS is the anatase nano powder (α-TiO2).
Since the size is 5 nm it is expected that the lattice oxygen will have a slight shift
that comes from the size constrain while the surface oxygen will not be affected.
Figure 6–14 is shows the carbon peak. The carbon is not part of the material
composition, but comes from the atmosphere and it is expected to be present
in every XPS sample. The carbon peak at 284.6 eV is the typical C1s peak and
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)Raw Data
Fit
Peak at 284.6 eV
Peak at 288.4 eV
284.6 eV288.4 eV
Figure 6–14: The C1s peak for the reference anatase nanoparticles. The majorpeak is at the 286.4 eV that is agreement with literature and severaldatabases.
represents elemental carbon. There is a secondary smaller peak that is present
at 288.4 eV, which is also typical peak for carbon contaminated samples [237–
239]. The noise to signal ratio is relative high, which is expected for carbon of
this nature. It has to be noted that the secondary peak cannot be satisfactory
approached by any of the Gauss (equation 6−25) or the Voigt (equation 6−26)
peaks, since the noise to signal ratio is high.
The next important peak is the Si2p peak and is displayed in figure 6–15. It
comes from the substrate and it has also been used to calibrate the measurements.
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Binding Energy (eV)
N(E
)
Raw Data
Peak Fit
Peaks at 98.5 eV
Peak at 102.5 eV
102.5 eV
98.5 eV
Figure 6–15: The Si2p peak for the reference anatase nanoparticles. The major
peaks are at the 98.5 eV for the Si2p1/2 and at 102.5 eV for theSi2p3/2 which are in agreement with literature and several databases.
The two peaks are in a agreement with database values. There is some slight shift,
which is attributed to the formation of a thin oxide layer on top of the wafer. The
double calibration was done to re-ensure that the TiO2 peaks are correctly labeled
and located.
The next peak that is resolved from XPS is the O1s (figure 6–16). The noise
to signal ratio is very low and therefore fitting is very well. For O1s it is expected
only one peak since it is a single energy level (no spin-orbital coupling). However in
this case the peak appears split. The first peak observed at 529.6 eV, represents the
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N(E
)Raw data
Peak fitting
Peak at 529.6 eV
Peak at 531.6 eV
529.6 eV
531.5 eV
Figure 6–16: The O1s peak for the reference anatase nanoparticles. The majorpeaks are at the 529.6 eV, represents the lattice oxygen, and the 531.5eV for the surface oxygen. which are agreement in with literature andseveral databases.
lattice oxygen. It is shifted slightly compared to database values, but the reason
for that is the size, which is 5 nm. The other peak appears at 531.5 eV and is
attributed to the surface oxygen. In this case the energy is higher compared to the
bulk since there are open bonds. In addition since the material is nano-sized, it
has high surface area, there is a lot of surface oxygen and therefore the intensity
of the peak is higher as well. It has been argued that the ratio between the two
peaks can be correlated to the surface area of the material [234]. However this is
137
not absolutely correct since the ratio between bulk and surface oxygen depends on
the crystal orientation. Certain crystallographic orientations are richer on oxygen.
So the relative ratio of the two peaks will be a function of both the surface area
and the crystallographic orientation. Those peaks where fitted very well with the
Gaussian lineshape. From the fit results we can calculate the ratio of surface to
bulk oxygen (RSB)
RSB =ISurface
IBulk(6−28)
That ratio for the reference material is estimated to be 0.8307.
The last peak is the titanium peak (figure 6–17). There are two peaks for
titanium the 458.4 eV for the Ti2p1/2 and at 464.2 eV for the Ti2p3/2, both in good
agreement with the database. Again the fitting was excellent with the Gaussian
lineshape.
6.5.3 XPS of the s-ANTs
The next sample analyzed via XPS is the s-ANTs. The C1s will be used as
calibration since the Si2p is very weak and is insecure to be used for calibration (a
summary of all the peaks is given at the end of the chapter). In addition the 284.6
eV is a very characteristic peak of carbon based materials. Among all the carbon
peaks the 284.6 eV will always have the highest intensity since it is generated by
elemental carbon. Secondary peaks will represent other structures such as bonds
and functional groups.
Examining the C1s peak of the coated short carbon nanotubes (figure 6–18) it
is observed that there are two very distinct peaks at 284.6 eV and 285.9 eV. The
284.6 eV is the peak that is being attributed primarily to the elemental carbon sec-
ondarily to the graphite structure. In addition there is a very intense peak at the
285.9 eV. That peak can occur for two majors reasons. One is the C=O, −COOH
and −OH bonds [240–242] and the other is the presence of nitrides groups, such us
−NH2, that are attached on a benzene ring. The second option although it sounds
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Raw Data
Peaks Fit
Peak at 458.4 eV
Peak at 464.2 eV
458.4 eV464.2 eV
N(E
)
Figure 6–17: The Ti2p peak for the reference anatase nanoparticles. The major
peaks are at the 458.4 eV for the Ti2p1/2 and at 464.2 eV for theTi2p3/2 which are in agreement with literature and several databases.
reasonable (HNO3 was used for the purification), is not acceptable since the survey
of the sample did not reveal any nitrogen. So consequently the peak has to be
attributed to C=O, −COOH and −OH bonds. Those groups are expected to be
present after the acid treatment of the nanotubes as part of the −COOH groups
that have been formed on the surface and are responsible for the stabilization of
the nanotubes in a suspension. They were also confirmed by FTIR (figure 4–6).
Another very interesting peak that appears in the spectra is the one at 289.7
eV. This peak is far form the elemental carbon region and it has to be due to the
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Binding Energy (eV)
N(E
)Raw data
Fit
Peak at 284.6 eV
Peak at 285.9 eV
284.6 eV
285.9 eV
Figure 6–18: The C1s peak for the s-ANTs. The major peak is appearing to the284.6 eV, which is again in great agreement with literature values.The peak at 285.9 eV is characteristic of the C−O bond while the289.5 eV peak is attributed to C−O−Ti.
bond of carbon to another element. In the literature there are many references
for this peak most of are about fluorite bonded directly to carbon [243] and some
metals like Na and Li that are also directly bonded to carbon [244]. There are some
references that report this peak as an oxygen bond. However none of the previous
reasons can give a satisfactory explanation. Since form the Raman analysis there
was strong evidence of the bond of the MWNTs to the TiO2 it is believed that this
peak has the same origin. Since at the same region usually reported the C−Metal
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Binding Energy (eV)
N(E
)Raw data
Fit
Peaks at 530.7 eV
Peaks at 532.6 eV
530.6 eV532.7 eV
Figure 6–19: The O1s for the s-ANTs. The major peaks are again at 530.6 eV forthe O1s for the lattice oxygen and the 532.7 eV for the surfaceoxygen. The ratio between those two peaks reveals the surface are ofthe particle.
bonds a suitable option for the bond is the Ti−C bond, which however appears at
281.3 eV. As already discussed, in figure 6–12 there was a peak at 730 cm−1 and
it was attributed to C−O−Ti bond. So at this point, there are evidences, strong
enough, to attribute the peak at 289.7 eV to a possible C−O−Ti bond. Again
very detailed analytical work could prove the concept, which however is beyond the
purpose of this research.
141
The next important peak is O1s (figure 6–19). The shape of the peak is
significantly different compared to the line shape of the reference material (figure
6–16). It looks like a single peak. There are two different possibilities to fit that
peak. One is to assume that there is only one peak at 530.6 eV and attribute it
to the lattice oxygen. In this case the fitting will be done with the Voigt equation
to include the asymmetric broadening. This approach failed to give reliable
results. The other approach is to start with the assumption that the broadening
comes from a second overlapping peak with lower intensity, the surface oxygen.
That is more reasonable approach and yields nice fit. The background was not
fitted properly but that is because we assumed polynomial background where
in XPS it can be more complicated (Shirley). The peak appears at 532.7 eV. If
we estimate the ISurface/Ibulk ratio is found to be 0.2526 where in the case of the
reference material it was 0.8307. This means that the nanoparticles have more
surface oxygen, something that contradicts with the BET specific surface area
measurements, which gave higher surface area for the s-ANTs. This contradiction
however can be explained on the crystallographic orientation, which in the case of
the s-ANTs can have orientation to expose the surfaces less oxygen rich.
The most important result however, is the shift (compared to the reference
material values) that has occurred for both peaks. The first peak at 530.6 eV,
regarding the lattice oxygen, is shifted by +1.0 eV (original value 529.6 eV ) and
the second peak that is at 532.7 eV, surface oxygen, has been shifted by +1.1 eV
(original value at 531.5 eV). This shift again can be attributed to bonding to a non
native element which in this case is C. Another source of this shift could be the
dimensions that are 6 nm, which is large enough to eliminate nanosized effects. But
in this case the shift would occur towards lower energies. So it is safe to conclude
that the shift is due to the bond between TiO2 and MWNTs. Those oxygen peaks
can have a significant contribution from the oxygen that comes from thin layer of
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N(E
)
Raw data
Fit
Peak at 459.4 eV
Peak at 465.1 eV
459.4 eV465.1 eV
Figure 6–20: The Ti2p peak for the s-ANTs. The major peaks are at the 459.4 eV
for the Ti2p1/2 and at 465.1 eV for the Ti2p3/2.
SiO2. However the peaks of the silicon are very weak and the contribution of the
SiO2 oxygen, if any, can be neglected.
The next peak is the one from titanium (figure 6–20). There are two peaks
that appear in titanium and are at energies 459.4 eV for the Ti2p1/2 and the other
465.1 eV Ti2p3/2. Comparing those peaks with the peaks at the reference material
both appear shifted. The Ti2p1/2 is shifted by 1.0 eV and the Ti2p3/2 is shifted by
1.1 eV. The shift is again significant and since for titanium peak the literature does
not report any size effects then the only reason for the peak shift is the bond to the
143
underlying graphite. So from the analysis of the XPS spectra for the short carbon
nanotubes there are strong evidences that there is bond between the titania and
the MWNTs. And since the very characteristic peak of C−Ti is not present, the
bond should be C−O−Ti.
6.5.4 XPS of the ℓ-ANTs
In this section we examine the XPS spectra from the ℓ-ANTs. Again the
C1s peak was used to calibrate the spectra since the Si peak is very weak. The
MWNTs used in this sample are different as well as the titania precursor. But since
structurally the final result is not very different it can be expected that the two
spectra will be similar.
Starting again from the C1s (figure 6–21) peak we see the main graphite peak
at 284.6 eV. Since the major analysis of this peak is the same as in the case of
the s-ANTs, only the major differences will be analyzed. In this case the peak
at 285.2 eV is slightly shifted compared to the previous case (285.9 eV). This
has to do however with the amount of −COOH and it therefore is related to
the treatment of the tubes. The s-CNTs have been treated with sulfuric acid in
addition to the nitric acid. The ℓ-CNTs were treated only with the nitric acid.
Therefore it was expected for that peak to be less intense compared to the s-
ANTs. Since the scale is in arbitrary units the peaks cannot be directly compared
but the relative height to the main carbon peak can. In the case of the s-ANTs
that ratio is I1Cs/IC=0,−COOH =1.1026 and for the ℓ-ANTs that ratio becomes
I1Cs/IC=0,−COOH =0.3273. That ratio can be related directly to the number of the
−COOH groups, and it is a very strong evidence that the longer tubes have less
carboxylic groups on the surface. The next important peak is the one that shows
at 289.7 eV. This is almost at the same position as the peak that in the previous
section was attributed to the C−O−Ti peak. The peak here is broader than before
and a lot less intense. However, since the reduction of −COOH was followed by the
144
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N(E
)Raw data
Fit
Peak 284.6 eV
Peak 285.2 eV
Peak 289.7 eV
286.4 eV
285.2 eV
289.7 eV
Figure 6–21: The C1s peak for the ℓ-ANTs. Again the major peak appears to be at284.6 eV while there is a secondary peak at 285.2 eV. This peak issimilar to the case of s-ANTs that appears to 285.9 eV. It is againattributed to the C−O bond or C=O bond.
289.7 eV peak reduction it can be assumed that those two peaks are closely related.
So it is again safe to conclude that the 289.7 eV is indeed a peak that comes from
the C−O−Ti bond.
The following two peaks are for the titanium and oxygen. Since, as mentioned
before, the titania in this sample is less than the s-ANTs the intensity of the peaks
are lower than before and that can be seen from the noise to signal ratio, which
is higher (figure 6–22). But still some important features are recognizable. The
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Binding Energy (eV)
N(E
)Raw data
Fit
Peak at 530.9 eV
Peak at 532.7 eV
530.9 eV
532.7 eV
Figure 6–22: The O1s peak for the ℓ-ANTs. There are also two peaks observed at532.7 eV and at 530.9 eV. Although both are from the oxygen the532.7 eV is attributed to surface oxygen while the other comes fromlattice oxygen contribution. Relative to the case of s-ANTs thesurface oxygen and therefore the surface area is higher, somethingthat was confirmed with BET as well and is in agreement with otherresearchers.
peaks in this case are also significantly shifted compared to the reference material.
The peak that comes from the surface oxygen is at 532.7 eV located at the same
energy as the surface oxygen peak for the s-ANTs. The second peak, regarding
the lattice oxygen, is at the 530.9 eV and is very close where the respective peak
for the s-ANTs is (530.6 eV). Again the contribution of the SiO2 in this spectrum
146
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N(E
)Raw data
Peak Fit
Peak 459.6 eV
Peak 465.3 eV
459.6 eV465.3 eV
Figure 6–23: The Ti2p peak for the ℓ-ANTs. The major peaks are at the 459.6 eV
for the Ti2p1/2 and at 465.2 eV for the Ti2p3/2 which are insignificantly shifted compared to the reference material.
is negligible so the intensity of the peaks is attributed almost exclusively from the
TiO2 peaks. The other very important result is the relative intensity of the two
peaks. The ratio ISurface/Ibulk ratio is found to be 1.5636 where in the case of the
reference material it was 0.8307 and in the case of the s-ANTs that was 0.2526.
That is in agreement with researchers that report that among several precursors
the Ti2(SO4)3 yields higher surface area. The relative high noise to signal ratio did
not allow for good fit of the background but the peaks were very nicely fitted with
147
Table 6–9: Summary of the XPS peaks
PeakReference s-ANTs ℓ-ANTsPeak [eV] Peak [eV] Shift [eV] Peak [eV] Shift [eV]
C1sGraphite 284.6 284 .6 0.0 284.6 0.0C1sC−O,C=O N/A 285.9 - -C1sC−O−Ti N/A 289.5 - -O1sSurface 531.5 532.7 1.1 532.7 1.1O1sBulk 529.6 530.6 1 530.9 1.3
Ti2p1/2 458.4 459.4 1. 459.6 1.2
Ti2p3/2 464.2 465.1 0.9 465.3 1.1
the Gauss. Still the main result of those peaks remains the shift of the peaks to
higher energies.
The final peak is again the Ti2p (figure 6–23). The major peaks are at the
459.6 eV for the Ti2p1/2 and at 465.2 eV for the Ti2p3/2 that are very close to the
respective values of the s-ANTs (459.4 eV and 465.1 eV respectively). Again it is
obvious that the noise is slightly increased compared to the reference material and
the s-ANTs due to the relative less amount of titania in the sample. But overall
the peak shifts, 1.2 eV for the Ti2p1/2 and 1 eV for the Ti2p3/2, is denoting again
that there is a bond between TiO2 and MWNT.
6.6 Summary of the XPS Analysis
The last section of this chapter was devoted in the XPS analysis of both
the ℓ-ANTs and s-ANTs. The XPS confirmed the results of the Raman. All the
peaks showed displacement compared to the reference material (table 6–9). Since
the grain size was not significantly different those shifts can be attributed to the
bond of titania on the carbon nanotube. In addition to the shift, a new peak
that is not explained reasonable from the databases, appeared at approximately
289.7 eV. It is not accurate to attribute this peak to the C−O−Ti bond. The
combination, however of Raman and XPS can lead to such a conclusion, which can
be backed up from theoretical calculations. We can safely conclude therefore that
the TiO2 coating is bonded to the MWNTs. Furthermore the bond is in the form
148
of C−O−Ti bond. This is directly related to the production process. The −COOH
and −OH groups have been successfully used as anchoring points during the sol-gel
process.
149
Tip2O1sReference Material
528530532534536538540542
Binding Energy (eV)
N(E
)
Raw data
Peak fitting
Peak at 529.6 eV
Peak at 531.6 eV
529.6 eV
531.5 eV
455458461464467470
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Raw Data
Peaks Fit
Peak at 458.4 eV
Peak at 464.2 eV
458.4 eV464.2 eV
N(E
)
Long ANTs
528530532534536538540542
Binding Energy (eV)
N(E
)
Raw data
Fit
Peak at 530.9 eV
Peak at 532.7 eV
530.9 eV
532.7 eV
455458461464467470
Binding Energy (eV)
N(E
)
Raw data
Peak Fit
Peak 459.6 eV
Peak 465.3 eV
459.6 eV465.3 eV
Figure 6–24: Collective representation if the XPS data regarding the coated longcarbon nanotubes. The upper row is the Ti2p and O1s peak of thereference material and the lower row is the data obtained by thes-ANTs. The shifts in both peaks are obvious and are summarized intable 6–9.
150
Tip2O1sReference Material
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N(E
)
Raw data
Peak fitting
Peak at 529.6 eV
Peak at 531.6 eV
529.6 eV
531.5 eV
455458461464467470
Binding Energy (eV)
Raw Data
Peaks Fit
Peak at 458.4 eV
Peak at 464.2 eV
458.4 eV464.2 eV
N(E
)
Short ANTs
527529531533535537539541
Binding Energy (eV)
N(E
)
Raw data
Fit
Peaks at 530.7 eV
Peaks at 532.6 eV
530.6 eV532.7 eV
455458461464467470
Binding Energy (eV)
N(E
)
Raw data
Fit
Peak at 459.4 eV
Peak at 465.1 eV
459.4 eV465.1 eV
Figure 6–25: Collective representation if the XPS data regarding the coated shortcarbon nanotubes. The upper row is the Ti2p and O1s peak of thereference material and the lower row is the data obtained by theℓ-ANTs. The shifts in both peaks are obvious and are summarized intable.
151
Tip2O1sShort ANTs
527529531533535537539541
Binding Energy (eV)
N(E
)
Raw data
Fit
Peaks at 530.7 eV
Peaks at 532.6 eV
530.6 eV532.7 eV
455458461464467470
Binding Energy (eV)N
(E)
Raw data
Fit
Peak at 459.4 eV
Peak at 465.1 eV
459.4 eV465.1 eV
Long ANTs
528530532534536538540542
Binding Energy (eV)
N(E
)
Raw data
Fit
Peak at 530.9 eV
Peak at 532.7 eV
530.9 eV
532.7 eV
455458461464467470
Binding Energy (eV)
N(E
)
Raw data
Peak Fit
Peak 459.6 eV
Peak 465.3 eV
459.6 eV465.3 eV
Figure 6–26: Collective representation if the XPS data regarding the coated longand short carbon nanotubes. The upper row is the Ti2p and O1s peakof the s-ANTs and the lower row is the data obtained by the ℓ-ANTs.The peaks are similar regarding the position, but are significantlydifferent in shape.
152
C1s
Reference Material
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N(E
)
Raw Data
Fit
Peak at 284.6 eV
Peak at 288.4 eV
284.6 eV288.4 eV
280285290295300
Binding Energy (eV)
N(E
)
Raw data
Fit
Peak 284.6 eV
Peak 285.2 eV
Peak 289.7 eV
286.4 eV
285.2 eV
289.7 eV
Short ANTs
280282284286288290292294296298300Binding Energy (eV)
N(E
)
Raw Data
Fit
Peak at 284.6 eV
Peak at 288.4 eV
284.6 eV288.4 eV
Long ANTs
280282284286288290292294296298300
Binding Energy (eV)
N(E
)
Raw data
Fit
Peak at 284.6 eV
Peak at 285.9 eV
284.6 eV
285.9 eV
Figure 6–27: The C1s peak of the peak of the coated carbon nanotubes (bothℓ-ANTs and s-ANTs) and the reference material. The main differencebetween the reference material and the samples are the peaksregarding the C−O and C=O bonds, that are appearing only for thes-ANTs and ℓ-ANTs, and the peak at 289.7 eV (ℓ-ANTs) and 289.5eV (s-ANTs) that can be attributed to the C−O−Ti bond.
153
Si2p
Reference Material
9698100102104106108
Binding Energy (eV)
N(E
)
Raw Data
Peak Fit
Peaks at 98.5 eV
Peak at 102.5 eV
102.5 eV
98.5 eV
9698100102104106108
Binding Energy (eV)
N(E
)
Raw data98.6 eV
102.4 eV
Short ANTs
9698100102104106108
Binding Energy (eV)
N(E
)
Raw Data
Peak Fit
Peaks at 98.5 eV
Peak at 102.5 eV
102.5 eV
98.5 eV
Long ANTs
9698100102104106108
Binding Energy (eV)
N(E
)
Raw data98.6 eV
102.5 eV
Figure 6–28: The Si2p peak of the peak of the coated carbon nanotubes (bothℓ-ANTs and s-ANTs) and the reference material. Al the peaks are atthe same energy, but the noise to signal ratio is a lot higher for theboth ℓ-ANTs and s-ANTs. The reason for that is the thickness of thecoating. The coated MWNTs were deposited in a thicker layer.
154
G BandD Band
Acid Treated s-CNTs
1200 1220 1240 1260 1280 1300 1320 1340 1360 1380 1400
Raman Shift(cm1)
Inte
nsi
ty (
a.u.)
1500 1520 1540 1560 1580 1600 1620 1640 1660 1680 1700
Raman Shift(cm1)
Inte
nsi
ty (
a.u.)
CNTs Segment of s-ANTs
1200 1220 1240 1260 1280 1300 1320 1340 1360 1380 1400
Raman Shift (cm1)
Inte
nsi
ty (
a.u.)
1500 1520 1540 1560 1580 1600 1620 1640 1660 1680 1700
Raman Shift (cm1)
Inte
nsi
ty (
a.u.)
Figure 6–29: Collective representation of the Raman spectra regarding the shortnanotubes before (top row) and after the coating (bottom row). Theright column is for the G band and the left column is for the D band.
155
G BandD Band
Acid Treated ℓ-CNTs
1200 1220 1240 1260 1280 1300 1320 1340 1360 1380 1400
Raman Shift(cm1)
Inte
nsi
ty (
a.u.)
1500 1520 1540 1560 1580 1600 1620 1640 1660 1680 1700
Raman Shift(cm1)
Inte
nsi
ty (
a.u.)
CNTs Segment of ℓ-ANTs
1200 1220 1240 1260 1280 1300 1320 1340 1360 1380 1400
Raman Shift (cm1)
Inte
nsi
ty (
a.u
.)
1500 1520 1540 1560 1580 1600 1620 1640 1660 1680 1700
Raman Shift (cm1)
Inte
nsi
ty (
a.u
.)
Figure 6–30: Collective representation of the Raman spectra regarding the longnantubes before (top row) and after the coating (bottom row). Theright column is for the G band and the left column is for the D band.
156
ℓ-ANTss-ANTs
Reference Material
0 100 200 300 400 500 600 700 800 900 1000
Raman Shift (cm1)
Inte
nsi
ty (
a.u.)
Eg 148 cm1
Eg 197 cm1
B1g 399 cm1
A1g 515 cm1
Eg 639 cm1
0 100 200 300 400 500 600 700 800 900 1000
Raman Shift (cm1)In
tensi
ty (
a.u.)
Eg 148 cm1
Eg 197 cm1
B1g 399 cm1
A1g 515 cm1
Eg 639 cm1
CNTs Segment of ℓ-ANTs
0 100 200 300 400 500 600 700 800 900 1000Raman Shift (cm1)
Inte
nsi
ty (
a.u
.)
Raw data
Fit
Peak at 150 cm1
Peak at 202 cm1
Peak at 393 cm1
Peak at 510 cm1
Peak at 633 cm1
0 100 200 300 400 500 600 700 800 900 1000
Raman Shift (cm1)
Inte
nsi
ty (
a.u
.)
Raw data
Fit
Peak at 150 cm1
Peak at 409 cm1
Peak at 629 cm1
Figure 6–31: Collective representation if the XPS data regarding the coated longcarbon nanotubes. The upper row is the Ti2p and O1s peak of thereference material and the lower row is the data obtained by thes-ANTs. The shifts in both peaks are obvious and are summarized intable.
CHAPTER 7CONCLUSIONS AND FUTURE WORK
The main objective of this research, as outlined in the introduction, is to
combine the two different materials, MWNTs and TiO2, in one composite that
will deliver high photocatalytic efficiency. This new composite will take advantage
of the excellent electronic properties and high specific surface area. In general
photocatalysis can be improved by the increasing the surface area, or by improving
the [OH•]. The later is directly correlated to the rate at which the e− and h+ are
generated and recombined. This rate can be mathematically expressed as
φquant. ∝kCT
kCT + kR
(7−1)
where kCT is the charge transfer rate, and the kR is the recombination rate. So
by minimizing the recombination rate (kR → 0) the quantum efficiency will
increase (limkR→0kCT
kCT+kR= 1). As seen in Chapter 5, if shielding and coagulation
phenomena are neglected, the efficiency dependence to surface area is just a linear
relationship.
φsurf. ∝ S (7−2)
The overall efficiency will be
φtot. ∝kCT
kCT + kR
× S (7−3)
In order to maximize the overall efficiency it is necessary to minimize the recombi-
nation rate and increase the surface area. The ways to minimize the recombination
rate have been already explained and they are; the incorporation of transition
metals (Cu+3, Cr+3 and Fe+3), N or C in the crystal structure of titania and the
157
158
Table 7–1: Electron affinity and work function for metals used to create rectifyingcontact with titania in order to increase the photocatalytic efficiency.
Element work Function (φ) [eV] Electron Affinity (χ) [eV]Pt 5.55 2.128Au 5.38 2.309Ag 4.63 1.302Al 4.17 0.441
C (amorphous) 5.00 1.263C60 7.74 2.780 (2.650 ± 0.020)†
SWNT (9,0) 5.10 -SWNT (5,5) ⋆ 4.780 2.840-2.660
MWNT 4.80-5.05 -† Experimental value
Conducting zig-zag⋆
Conducting armchair
coupling with a metal. According to the theory of photocatalysis, work function is
a critical parameter to the creation of the rectifying contact. Table 7–1 compares
the work function of the nanotubes to the work function of other traditional metals
among which are Pt and Au, both used to improve photocatalysis. Carbon nan-
otubes are standing the comparison very well, since they are slightly bellow Au.
Therefore the utilization of carbon nanotubes as the core of the photocatalytic
composite is expected to enhance the photocatalysis since it has the ability to
increase the efficiency by both methods mentioned earlier, high specific area and
metallic properties.
The photocatalytic degradation experiments that were carried out (chapter 5)
demonstrated the validity of this assumption. The addition of 1 mg of nanotubes
in the solution of 3 mg of anatase nanoparticles, enhanced the efficiency by nearly
doubling the reaction rate. Furthermore the ℓ-ANTs showed exceptional photocat-
alytic properties compared to the MWNTs/TiO2 nanoparticles mixture and even
compared to Degussa P25. However the s-ANTs displayed poor photocatalytic
activity.
159
During the synthesis of the titania coated carbon nanotubes −COOH groups
were generated on the surface of the tubes by the acid treatment. Those groups act
later as anchoring points for the sol-gel precursors. This is the fundamental reason
why a bond between the carbon nanotubes and the titania coating is formed. The
bond was confirmed by Raman spectroscopy, which indicated a significant shift of
the titania and nanotubes peaks, and by XPS, which also displayed peak shifts in
addition to a new peak at 289.6 ± 0.1 eV which is attributed the C−O−Ti. Since
both techniques showed the existence of the bond between TiO2 and MWNTs it is
accurate to conclude that this bond exist in the form of C−O−Ti.
The characterization of the nanotubes in chapter 4, before the coating,
revealed that both types of nanotubes (long and short) have a concentric structure,
but the s-CNTs had significantly more damaged structure, which will affect
primarily their electric properties. In chapter 6 the Raman spectra verified this
hypothesis. The G Band did not demonstrate a distinct split, which is a very direct
indication for the absence of metallic properties. The ℓ-CNTs on the contrary, not
only showed that they have well defined structure, but in addition the G Band
split was very distinct. The G− band was better approximated with the Breit-
Wigner-Fano peak model which further justifies the validity this argument about
the metallic nature of the nanotubes.
The most important experimental results of this work can be summarized at
the following points: The MWNTs can enhanced the photocatalysis behavior The TiO2 coating was bonded on the MWNTs The ℓ-CNTs were metallic while the s-CNTs did not have any indication of
similar properties.
160
7.1 Conclusions
According to the previous discussion the following conclusions can be summa-
rized. The high work function of the nanotubes and the conducting properties is the
main reason that the nantubes can assist the photocatalysis when they are in
colloidal suspension. Applying the TiO2 as coating on the carbon nanotubes, yield very high
photocatalytic efficiency. This is due to the bond (C−O−Ti) that is created
between the MWNTs and the TiO2. The bond makes the underlined carbon
atoms dopants to the structure of titania. The metallic nature of the carbon nanotubes is more critical than the bond.
Both samples prepared and tested here (ℓ-ANTs and s-ANTs) displayed
the same evidences for the C−O−Ti bond. However the s-ANTs did not
had conducting properties, and therefore they had very poor photocatalytic
activity. Overall carbon in the form of carbon nanotubes can be a very promising way
to enhance the photocatalyis. For this to happen, the carbon nanotubes must
be very well defined with distinct structure and good electrical properties.
7.2 Future Work
The concepts explained and investigated in this research are based almost
exclusively on experimental results It is therefore necessary to investigate the main
principles on theoretical base. One of them is how the XPS peaks will shift and
where the C−O−Ti peak will appear. To derive this information it is required to
know the electronic structure of the composite material, something that currently
can be derived with computer simulations. In addition the direction and the
amount of the shifts in the titania and MWNTs peaks and the appearance of the
C−O−Ti in the Raman spectra needs to be theoretically explained.
161
From an experimental perspective the results from this dissertation can be
applied in many ways. Since titania has such a wide range of uses, this research can
be the foundation for many applications. The most immediate work that can be
done, is to test these composite particles on a wide range of bacteria, spores and
other biological contaminants, and examine the interactions. Another application is
to combine the large knowledge base regarding the solar cell application of titania
to produce cell with very high, energy conversion. In a more engineering approach,
ways to mass produce the product and commercialize the product can be sought.
This has to be done, however, in respect to the recently raised potential issues
about the toxicity of the nanotubes.
APPENDIX AMATHEMATICA ALGORITHM USED FOR THE LOESS METHOD
Needs["Statistics‘ContinuousDistributions‘"];
DataRange[x_] := Min[x], Max[x]
LoessFit[(x_)?NumberQ, data_, \[Alpha]_:0.75, \[Lambda]_:1] :=
WLSFit[data, LoessWts[x, data, \[Alpha]], \[Lambda], x]
LoessFit[(x_)?VectorQ, data_, \[Alpha]_:0.75, \[Lambda]_:1] :=
Table[LoessFit[x[[i]], data, \[Alpha], \[Lambda]], i, Length[x]]
SLPlot[fits_, res_, p_:0.5, \[Alpha]_:1] :=
Module[a, f, r, data2, s, lines,
data2 = Sort[Transpose[fits, res], First[#1] < First[#2] & ];
f, r = Transpose[data2]; a = If[p == 0, Log[Abs[r]], Abs[r]^p];
data2 = Transpose[f, a]; s = RobustLoessFit[data2, \[Alpha]];
lines = Line[Transpose[f, s]];
ListPlot[data2, PlotRange -> All, Axes -> False, Frame -> True,
FrameLabel -> "fit", "Abs[res]^p",
PlotStyle -> PointSize[0.02], RGBColor[0, 0, 1],
Epilog -> RGBColor[0, 1, 0], Thickness[0.02], lines]; BWPlot[a - s]]
RobustLoessFit::MaximumReached=
"The maximum number of iterations of the IRWLS algorithm, as
specified by the option MaxIterations, has been reached and without
convergence of the algorithm.
You could try increasing MaxIterations.";
RobustLoessFit[data_, \[Alpha]_:0.75, \[Lambda]_:1, (opts___)?OptionQ] :=
Module[x, y, \[Delta], res, rsum = 0, data2, iter = 0, rprev = 1,
r = Table[1, Length[data]],
maxiter = MaxIterations /. opts /. Options[RobustLoessFit];
data2 = Sort[data, First[#1] < First[#2] & ]; x, y = Transpose[data2];
\[Delta] = Table[LoessWts[x[[i]], data, \[Alpha]], i, 1, Length[x]];
While[++iter < maxiter && Abs[rsum - rprev] > 0.001,
res = Table[y[[i]] - WLSFit[data, \[Delta][[i]]*r, \[Lambda], x[[i]]],
i, 1, Length[x]]; r = BiSquare[res/(6*Median[Abs[res]])];
162
163
rprev = rsum; rsum = Abs[Plus @@ r; ]];
If[iter==maxiter,Message[RobustLoessFit::MaximumReached]];
y - res]
Options[RobustLoessFit] = MaxIterations -> 25
LoessSummary[data_, \[Alpha]_:0.75, \[Lambda]_:1] :=
With[res = LoessResiduals[data, \[Alpha], \[Lambda]],
res -> res, \[Sigma] -> Sqrt[Plus @@ (res^2)/Length[res]],
\[Mu] -> 1.199999999999999*(\[Lambda] + 1)/\[Alpha]]
LoessResiduals[data_, \[Alpha]_:0.75, \[Lambda]_:1] :=
Last[Transpose[data]] - LoessFit[First[Transpose[data]], data, \[Alpha],
\[Lambda]]
RobustLoessPlot[data_, \[Alpha]_:0.6, \[Lambda]_:1, opts___] :=
Module[x, y, fits, data2, lines,
data2 = Sort[data, First[#1] < First[#2] & ]; x, y = Transpose[data2];
fits = RobustLoessFit[data2, \[Alpha], \[Lambda]];
lines = Line[Transpose[x, fits]];
ListPlot[data2, PlotRange -> All, Axes -> False, Frame -> True,
PlotStyle -> PointSize[0.02], RGBColor[0, 0, 1],
Epilog -> RGBColor[0, 1, 0], Thickness[0.02], lines,opts]]
LoessPlot[data_, \[Alpha]_:0.75, \[Lambda]_:1, numvalues_:30, opts___] :=
With[x = EquispaceVector[First[Transpose[data]], numvalues],
ListPlot[data, opts, PlotStyle -> PointSize[0.05],
PlotRange -> ScaleRectangle[data], Frame -> True, Axes -> False,
Epilog -> Thickness[0.02], RGBColor[0, 1, 1],
Line[Transpose[x, LoessFit[x, data, \[Alpha], \[Lambda]]]],opts]]
WLSFit[data_, wts_, ldegree_:1, x_] :=
Fit[Transpose[(wts*#1 & ) /@
Join[Table[1, Length[data]], Transpose[data]]],
Join[u, Table[v^i, i, ldegree]], u, v] /. u -> 1, v -> x
LoessWts[x_, data_, \[Alpha]_] :=
Tricube[(x - First[Transpose[data]])/LoessDistance[x, data, \[Alpha]]]
Tricube = Compile[x, _Real, 1,If[Abs[#]<1, (1-#^2)^2, 0]& /@x];
Bisquare = Compile[x, _Real, 1,
If[Abs[#]<1, (1-Abs[#]^3)^3, 0]& /@x];
LoessDistance[x_, data_, \[Alpha]_] :=
164
Module[A = Max[1, \[Alpha]], X = First[Transpose[data]], q,
q = Min[Length[X], Ceiling[\[Alpha]*Length[X]]]; A*Sort[Abs[X - x]][[q]]]
EquispaceVector[(x_)?VectorQ, numvalues_:30] :=
Range[Min[x], Max[x], N[(Max[x] - Min[x])/numvalues]]
ScaleRectangle[data_] :=
With[x = Transpose[data], (AddEps[#1] & ) /@
DataRange[First[x]], DataRange[Last[x]]]
AddEps[xlo_, xhi_] :=
With[\[Epsilon] = (xhi - xlo)*0.05, xlo - \[Epsilon], xhi + \[Epsilon]]
BWPlot[data_] :=
Module[datadim, k, datapts, whiskers, box, outsidepts, medpt, dmax, dmin,
coldata, Q1, Q3, uplim, dnlim, outside, jitter, drange, epsilon,
boxwidth = 0.4, datadim = Dimensions[data];
k = If[Length[datadim] == 2, datadim[[2]], 1];
datapts = outsidepts = whiskers = box = medpt = dmax = dmin = ;
Do[coldata = If[k == 1, data, Column[data, i]];
datapts =
Join[datapts, Transpose[coldata, Table[i, Length[coldata]]]];
medpt = Join[medpt, PointSize[0.04],
Point[Quantile[coldata, 0.5], i]];
Q1 = Quantile[coldata, 0.25]; Q3 = Quantile[coldata, 0.75];
box = Join[box, RGBColor[0.690207, 0.7685929999999999, 0.870602],
Polygon[Q1, i - boxwidth, Q1, i + boxwidth,
Q3, i + boxwidth, Q3, i - boxwidth]];
step = 1.5*(Q3 - Q1); uplim = Q3 + step; dnlim = Q1 - step;
upadj = Max[Select[coldata, #1 <= uplim & ]];
dnadj = Min[Select[coldata, #1 >= dnlim & ]];
whiskers =
Join[whiskers, Thickness[0.005], Line[dnadj, i, Q1, i],
Line[Q3, i, upadj, i]];
outside =
Join[Select[coldata, #1 > uplim & ], Select[coldata, #1 < dnlim & ]]\
; jitter = Table[i + (Random[] - 1/2)/6, Length[outside]];
outsidepts =
Join[outsidepts, (Circle[#1, Offset[5, 5]] & ) /@
Transpose[outside, jitter]]; dmax = Max[dmax, Max[coldata]];
(dmin = Min[dmin, Min[coldata]]; ), i, k];
epsilon = (dmax - dmin)*0.03; drange = dmin - epsilon, dmax + epsilon;
ListPlot[datapts, Ticks -> Automatic, None,
PlotStyle -> AbsolutePointSize[0], PlotRange -> drange, 0, k + 1,
Axes -> True, False, Epilog -> outsidepts, box, medpt, whiskers]]
APPENDIX BRAMAN PEAKS OF CNTs
Table B–1: Properties of the various Raman features in graphite and SWNTs.
Name ω (cm−1) Resonance dω/dEL
iTA 288 DR1 129LA 453 DR1 216RBM 248/dt SR 0IFM− 750 DR2 −220oTO 860 DR1 0iFM+ 960 DR2 180D 1350 DR1 53LO 1450 DR1 0BWF 1550 SR 0G 1582 SR −0M− 1732 DR2 −26M+ 1755 DR2 0iTOLA 1950 DR2 230G’ 2700 DR2 1062LO 2900 DR2 02G 3180 DR2 0Adapted from [245].
165
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BIOGRAPHICAL SKETCH
Georgios Pyrgiotakis was born in Heraklion, Greece, in 1977. In 2000 he
graduated with a B.S. degree in physics from University of Crete. In 2003 he
earned his M.S. from University of Florida in materials science and engineering. He
enjoys cooking and mixing music.
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