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Thermally Evaporated Tungsten Oxide (WO3) Thin Films for Gas Sensing
Applications
Submitted in fulfilment of the requirements of Doctor of Philosophy
Mohammed Ahsan
School of Engineering Systems Faculty of Built Environment and Engineering
Queensland University of Technology Australia
2012
ii
Statement of Original Authorship
The present thesis reports the results of the work done during the years of my PhD
project. The work has been carried out mostly at Queensland University of
Technology although some experiments were performed also at University of
Queensland, Australian National University, Australian Nuclear Science and
Technology Organization and RMIT University.
I hereby declare that:
All the experiments have been performed during the PhD project.
All the results presented come directly from the experimental activity done.
All the interpretations and observations are based on the results of the
experiments and have been often referred, with the reported reference, to previous
literature results.
For these reasons, I can declare, at the best of my knowledge, that this work is
original and never submitted before in any other academic institution for higher
degree qualification purposes.
Mohammed Ahsan
________________________
28th April 2012
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Acknowledgements This work has been done with the invaluable help of many people. I wish to thank
them all.
First of all I wish to thank immensely to Dr. Tuquabo Tesfamichael for
providing me the opportunity to conduct my PhD and for his advice, support and
constructive feedback throughout my PhD. He has been a great support on all fronts
and made my PhD journey a memorable experience. Special thanks to my associate
supervisor Prof. John Bell for encouraging me to join QUT and his advice and
support throughout my PhD project. I wish to thank to my associate supervisor Prof.
Prasad Yarlagadda, for his positive feedback and advices.
I deeply acknowledge the funding provided by Prof. Nunzio Motta through
NIRAP Project “Solar powered Nano Sensors” for purchases and visits during the
project work.
I wish to thank Prof. Barry Wood from UQ, Prof. Wojtek Wlodarski from
RMIT, Dr. Mihail Ionescu from ANSTO and Nina De Caritat from ANU for their
immense help and support during experimental work at these facilities. I extend my
thanks to Peter Hynes, Cristina Theodoropoulos, Lambert Bekessy, Thor Bostrom
and Tony Raftery from AEMF, QUT for their continuous support and guidance
during my PhD. I would also like to extend my thanks to the technical staff of Built
Environment and Engineering Faculty for their support. I acknowledge all the lovely
people from Research Portfolio Office, for their support and guidance during my
PhD. I extend my sincere thanks to all my friends and colleagues in O401 for their
support and encouragement during my entire stay.
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Words are not sufficient to thank the most special person in my life, my wife,
Tehmeena whose sacrifice, love, encouragement and immense support made this
PhD an easy journey for me. I owe special thanks and immense love to my beautiful
children Ayman, Akmal and Hamnah who have been away from their dear father for
more than 2 years. This thesis could not have been accomplished with the immense
support of my father Amrullah Sharief, my mother Rabia Begum, my brother Akbar
and my sister Najma. I take this opportunity to thank all my relatives including my
father-in-law Prof. Sofi Ali, brother-in-laws Mansoor, Masood, Moudood and Late
Maqsood for their encouragement and support.
Mohammed Ahsan
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Abstract
In this thesis, the author proposed and developed gas sensors made of
nanostructured WO3 thin film by a thermal evaporation technique. This technique
gives control over film thickness, grain size and purity. The device fabrication,
nanostructured material synthesis, characterization and gas sensing performance have
been undertaken. Three different types of nanostructured thin films, namely, pure
WO3 thin films, iron-doped WO3 thin films by co-evaporation and Fe-implanted
WO3 thin films have been synthesized. All the thin films have a film thickness of 300
nm. The physical, chemical and electronic properties of these films have been
optimized by annealing heat treatment at 300ºC and 400ºC for 2 hours in air.
Various analytical techniques were employed to characterize these films. Atomic
Force Microscopy and Transmission Electron Microscopy revealed a very small
grain size of the order 5-10 nm in as-deposited WO3 films, and annealing at 300ºC or
400ºC did not result in any significant change in grain size. X-ray diffraction (XRD)
analysis revealed a highly amorphous structure of as-deposited films. Annealing at
300ºC for 2 hours in air did not improve crystallinity in these films. However,
annealing at 400ºC for 2 hours in air significantly improved the crystallinity in pure
and iron-doped WO3 thin films, whereas it only slightly improved the crystallinity of
iron-implanted WO3 thin film as a result of implantation. Rutherford backscattered
spectroscopy revealed an iron content of 0.5 at.% and 5.5 at.% in iron-doped and
iron-implanted WO3 thin films, respectively. The RBS results have been confirmed
using energy dispersive x-ray spectroscopy (EDX) during analysis of the films using
transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS)
revealed significant lowering of W 4f7/2 binding energy in all films annealed at 400ºC
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as compared with the as-deposited and 300ºC annealed films. Lowering of W 4f7/2 is
due to increase in number of oxygen vacancies in the films and is considered highly
beneficial for gas sensing. Raman analysis revealed that 400ºC annealed films except
the iron-implanted film are highly crystalline with significant number of O-W-O
bonds, which was consistent with the XRD results. Additionally, XRD, XPS and
Raman analyses showed no evidence of secondary peaks corresponding to
compounds of iron due to iron doping or implantation. This provided an
understanding that iron was incorporated in the host WO3 matrix rather than as a
separate dispersed compound or as catalyst on the surface.
WO3 thin film based gas sensors are known to operate efficiently in the
temperature range 200ºC-500 ºC. In the present study, by optimizing the physical,
chemical and electronic properties through heat treatment and doping, an optimum
response to H2, ethanol and CO has been achieved at a low operating temperature of
150ºC. Pure WO3 thin film annealed at 400ºC showed the highest sensitivity towards
H2 at 150ºC due to its very small grain size and porosity, coupled with high number
of oxygen vacancies, whereas Fe-doped WO3 film annealed at 400ºC showed the
highest sensitivity to ethanol at an operating temperature of 150ºC due to its
crystallinity, increased number of oxygen vacancies and higher degree of crystal
distortions attributed to Fe addition. Pure WO3 films are known to be insensitive to
CO, but iron-doped WO3 thin film annealed at 300ºC and 400ºC showed an optimum
response to CO at an operating temperature of 150ºC. This result is attributed to
lattice distortions produced in WO3 host matrix as a result of iron incorporation as
substitutional impurity. However, iron-implanted WO3 thin films did not show any
promising response towards the tested gases as the film structure has been damaged
vii
due to implantation, and annealing at 300ºC or 400ºC was not sufficient to induce
crystallinity in these films.
This study has demonstrated enhanced sensing properties of WO3 thin film
sensors towards CO at lower operating temperature, which was achieved by
optimizing the physical, chemical and electronic properties of the WO3 film through
Fe doping and annealing. This study can be further extended to systematically
investigate the effects of different Fe concentrations (0.5 at.% to 10 at.%) on the
sensing performance of WO3 thin film gas sensors towards CO.
viii
Table of Contents
Abstract ........................................................................................................................ v
Table of Contents ...................................................................................................... viii
List of Figures ............................................................................................................. xi
List of Tables .............................................................................................................. xv
List of Publication – Research Activities ................................................................. xvii
CHAPTER 1 : INTRODUCTION ........................................................................ 19
1.1 Motivation .................................................................................................... 19
1.2 Conductometric gas sensors ......................................................................... 20
1.3 Parameters expressing sensing properties .................................................... 21
1.4 Aims and objectives of the project ............................................................... 23
1.4 Thesis Organization ..................................................................................... 25
CHAPTER 2 : LITERATURE REVIEW ........................................................... 27
2.1 Introduction .................................................................................................. 27
2.2 Construction principles of conductometric metal oxide gas sensors ........... 27
2.2.1 Receptor function ................................................................................. 27
2.2.2 Transduction function .......................................................................... 28
2.3 Limitations of existing metal oxide gas sensors .......................................... 29
2.4 Basic characteristics of a metal oxide conductometric sensors ................... 31
2.5 Basic mechanisms of gas sensing in semiconductor metal oxide sensors ... 34
2.6 Role of additives in gas sensing ................................................................... 39
2.7 Importance of the dimension in gas sensing ................................................ 40
2.8 Structural properties of tungsten oxide ........................................................ 43
2.9 Metal oxide based gas sensors ..................................................................... 46
2.10 Tungsten oxide (WO3) based gas sensors ................................................... 47
2.11 Deposition techniques of nanostructured metal oxide films ....................... 53
2.11.1 Thin Film Processes ............................................................................. 54
ix
2.11.1.1 Solgel ................................................................................................ 54
2.11.1.2 Spray pyrolysis ................................................................................. 55
2.11.1.3 Chemical vapour deposition (CVD) ................................................. 55
2.11.1.4 Physical vapour deposition (PVD) ................................................... 55
2.11.2 Thermal evaporation ................................................................................... 56
2.12 Summary ..................................................................................................... 59
CHAPTER 3 : EXPERIMENTAL METHODS .................................................. 60
3.1 Introduction .................................................................................................. 60
3.2 Deposition of nanostructured WO3 thin films by thermal evaporation ....... 60
3.2.1 Material ................................................................................................ 60
3.2.2 Substrate ............................................................................................... 61
3.2.3 Film deposition .................................................................................... 62
3.2.4 Ion implantation ................................................................................... 62
3.2.5 Post deposition heat treatment of the films .......................................... 63
3.3 Characterization of nanostructured WO3 thin films ..................................... 63
3.3.1 Transmission Electron Microscopy (TEM) ......................................... 64
3.3.2 Atomic Force Microscopy (AFM) ....................................................... 64
3.3.3 Rutherford Backscattered Spectroscopy (RBS) ................................... 65
3.3.4 X-Ray Photoelectron Spectroscopy (XPS) .......................................... 65
3.3.5 X-Ray Diffraction (XRD) .................................................................... 66
3.3.6 Raman spectroscopy ............................................................................ 68
3.4 Gas sensing characterization ........................................................................ 68
3.5 Summary ...................................................................................................... 73
CHAPTER 4 : THIN FILM CHARACTERIZATION ....................................... 74
4.1 Introduction .................................................................................................. 74
4.2 Characterization of Structural Properties .................................................... 75
4.2.1 AFM and TEM analysis ....................................................................... 75
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4.2.2 GIXRD analysis ................................................................................... 80
4.2.3 Summary and outlook .......................................................................... 84
4.3 Characterization of chemical and electrical properties ............................... 86
4.3.1 Raman spectroscopy ............................................................................. 86
4.3.2 XPS analysis ......................................................................................... 89
4.3.3 Summary and outlook .......................................................................... 99
4.4 Characterization of compositional properties ........................................... 100
4.4.1 Rutherford backscattered spectroscopy .............................................. 100
4.4.2 Energy dispersive x-ray spectroscopy (EDX) .................................... 106
4.4.3 Summary and outlook ........................................................................ 107
CHAPTER 5 : GAS SENSOR CHARACTERIZATION ................................. 108
5.1 Introduction ................................................................................................ 108
5.2 Response to Hydrogen ............................................................................... 108
5.3 Response to Ethanol (C2H5OH) ................................................................. 120
5.4 Response to Carbon monoxide (CO) ......................................................... 129
5.5 Summary and outlook ................................................................................ 133
CHAPTER 6: CONCLUSIONS AND FUTURE WORK ....................................... 137
6.1 Conclusions ................................................................................................ 138
6.2 Recommendations for Future Work ........................................................... 140
BIBLIOGRAPHY .................................................................................................... 141
xi
List of Figures
Figure 2-1: A typical response curve of a conductometric gas sensor. ...................... 32
Figure 2-2: Schematic diagram showing the surface layer and corresponding electron
band structure, adapted from [48]. ............................................................................. 37
Figure 2-3: Schematic view of physical and electron band structure model for a
polycrystalline material [50]. ..................................................................................... 38
Figure 2-4: Schematic illustration of intergrain constriction situations: (a) Open neck
(b) Closed neck and (c) Conduction limited by point of contact. .............................. 39
Figure 2-5: Schematic of additive role in gas sensing [40]. ....................................... 40
Figure 2-6: The effect of particle size on gas sensitivity of SnO2 sensor exposed to
CO and H2, adapted from [56]. .................................................................................. 41
Figure 2-7: Influence of grain size on gas response of In2O3 film to NO2, adapted
from [57]. ................................................................................................................... 42
Figure 2-8: Tungsten oxide (WO3) crystal structure. ................................................. 44
Figure 2-9: A schematic of the classification of processes used for deposition of
nanostructured films. .................................................................................................. 54
Figure 2-10: A typical sketch of thermal evaporation chamber. ................................ 57
Figure 2-11: Thornton Zone diagram [186]. .............................................................. 59
Figure 3-1: Interdigitated Pt electrodes on Si substrate. ............................................ 62
Figure 3-2: Schematic of X-ray diffraction. ............................................................... 67
Figure 3-3: Schematic diagram of gas chamber setup. .............................................. 71
Figure 3-4: Schematic of sensor testing by flow through method. ............................ 72
Figure 4-1: AFM semicontact mode image (a) and TEM image (b) of as-deposited
nanostructured WO3 film. .......................................................................................... 76
Figure 4-2: AFM semicontact mode image (a) and TEM image (b) of nanostructured
WO3 film annealed at 300ºC for 2 hours in air. ......................................................... 76
Figure 4-3: TEM image of nanostructured WO3 film annealed at 400ºC for 2 hours in
air. .............................................................................................................................. 76
Figure 4-4: AFM semicontact mode image (a) and TEM image (b) of as-deposited
nanostructured Fe-doped WO3 film. .......................................................................... 77
Figure 4-5: AFM semicontact mode image (a) and TEM image (b) of nanostructured
Fe-doped WO3 film annealed at 300ºC for 2 hours in air. ......................................... 78
xii
Figure 4-6: TEM image of nanostructured Fe-doped WO3 film annealed at 400ºC for
2 hours in air. .............................................................................................................. 78
Figure 4-7: AFM semicontact mode image (a) and TEM image (b) of nanostructured
Fe-implanted WO3 film. ............................................................................................. 79
Figure 4-8: AFM semicontact mode image (a) and TEM image (b) of nanostructured
Fe-implanted WO3 film annealed at 300ºC for 2 hours in air. ................................... 80
Figure 4-9: TEM image of nanostructured Fe-implanted WO3 film annealed at 400ºC
for 2 hours in air. ........................................................................................................ 80
Figure 4-10: GIXRD pattern of nanostructured WO3 film, (a) as-deposited, (b)
annealed at 300ºC in air for 2 hours and (c) annealed at 400ºC in air for 2 hours. .... 81
Figure 4-11: GIXRD pattern of nanostructured Fe-doped WO3 film, (a) as-deposited,
(b) annealed at 300ºC for 2 hours in air and (c) annealed at 400ºC for 2 hours in air.
.................................................................................................................................... 83
Figure 4-12: Comparison of GIXRD patterns of nanostructured WO3 and Fe-doped
WO3 films annealed at 400ºC for 2 hours in air. ........................................................ 83
Figure 4-13: GIXRD pattern of nanostructured Fe-implanted WO3 film, (a) as-
deposited, (b) annealed at 300ºC for 2 hours in air and (c) annealed at 400ºC for 2
hours in air. ................................................................................................................. 85
Figure 4-14: Raman spectra of nanostructured WO3 films. ....................................... 87
Figure 4-15: Raman spectra of nanostructured Fe-doped WO3 films. ....................... 87
Figure 4-16: Raman spectra of nanostructured Fe-implanted WO3 films. ................. 89
Figure 4-17: XPS wide spectra of nanostructured WO3 films, (a) as-deposited, (b)
annealed at 300ºC for 2 hours in air and (c) annealed at 400ºC for 2 hours in air. .... 90
Figure 4-18: XPS wide spectra of nanostructured Fe-doped WO3 films, (a) as-
deposited, (b) annealed at 300ºC for 2 hours in air and (c) annealed at 400ºC for 2
hours in air. ................................................................................................................. 91
Figure 4-19: XPS wide spectra of nanostructured Fe-implanted WO3 films, (a) as-
deposited, (b) annealed at 300ºC for 2 hours in air and (c) annealed at 400ºC for 2
hours in air. ................................................................................................................. 92
Figure 4-20: W 4f core level high resolution spectra of nanostructured WO3 films. 93
Figure 4-21: W 4f core level high resolution spectra of nanostructured Fe-doped
WO3 films. .................................................................................................................. 94
xiii
Figure 4-22: W 4f core level high resolution spectra of nanostructured Fe-implanted
WO3 films. ................................................................................................................. 95
Figure 4-23: O 1s core level high resolution spectra of nanostructured WO3 films. . 96
Figure 4-24: O 1s core level high resolution spectra of nanostructured Fe-doped WO3
films. .......................................................................................................................... 97
Figure 4-25: O 1s core level high resolution spectra of nanostructured Fe-implanted
WO3 films. ................................................................................................................. 99
Figure 4-26: RBS spectrum of (a) as-deposited and (b) 300ºC annealed
nanostructured WO3 films. ....................................................................................... 101
Figure 4-27: RBS depth profile of (a) as-deposited and (b) 300ºC annealed
nanostructured WO3 films. ....................................................................................... 102
Figure 4-28: RBS spectrum of (a) as-deposited and (b) 300ºC annealed
nanostructured Fe-doped WO3 film. ........................................................................ 103
Figure 4-29: RBS spectrum (a) and depth profile (b) of nanostructured Fe-doped film
annealed at 300ºC for 2 hours in air. ........................................................................ 104
Figure 4-30: RBS spectrum (a) and corresponding depth profile (b) of nanostructured
Fe-implanted WO3 film. ........................................................................................... 105
Figure 4-31: EDX spectrum of for 2 hours in air. ................................................... 106
Figure 4-32: EDX spectrum of Fe-implanted WO3 film annealed at 400ºC for 2 hours
in air. ........................................................................................................................ 107
Figure 5-1: Dynamic response (a) and sensitivity (b) to H2 measured for a
nanostructured WO3 film annealed at 300ºC for 2 hours in air. .............................. 110
Figure 5-2: Dynamic response (a) and sensitivity (b) to H2 measured for a
nanostructured WO3 film annealed at 400ºC for 2 hours in air. .............................. 111
Figure 5-3: Dynamic response (a) and sensitivity (b) to H2 measured for a
nanostructured Fe-doped WO3 film annealed at 300ºC for 2 hours in air. .............. 114
Figure 5-4: Dynamic response (a) and sensitivity (b) to H2 measured for a
nanostructured Fe-doped WO3 film annealed at 400ºC for 2 hours in air. .............. 115
Figure 5-5: Dynamic response (a) and sensitivity (b) to H2 measured for a
nanostructured Fe-implanted film annealed at 300ºC for 2 hours in air. ................. 118
Figure 5-6: Dynamic response (a) and sensitivity (b) to H2 measured for a
nanostructured Fe-implanted WO3 film annealed at 400ºC for 2 hours in air. ........ 119
xiv
Figure 5-7: Dynamic response (a) and sensitivity (b) to ethanol measured for a
nanostructured WO3 film annealed 400ºC for 2 hours in air. .................................. 122
Figure 5-8: Dynamic response (a) and sensitivity (b) to ethanol measured for a
nanostructured Fe-doped WO3 film annealed at 300ºC for 2 hours in air. .............. 124
Figure 5-9: Dynamic response (a) and sensitivity (b) to ethanol measured for a
nanostructured Fe-doped WO3 film annealed at 400ºC for 2 hours in air. .............. 126
Figure 5-10: Comparison of ethanol sensitivities of nanostructured WO3 and Fe-
doped WO3 films annealed at 400ºC for 2 hours in air at an operating temperature of
150ºC. ....................................................................................................................... 127
Figure 5-11: Dynamic response (a) and sensitivity (b) to ethanol measured for a
nanostructured Fe-implanted WO3 film annealed at 400ºC for 2 hours in air. ........ 128
Figure 5-12: Dynamic responses of nanostructured Fe-doped WO3 films to CO, (a)
annealed at 300ºC for 2 hours in air and (b) annealed at 400ºC for 2 hours in air. . 131
Figure 5-13: Sensitivity of annealed (at 300ºC and 400ºC) nanostructured Fe-doped
WO3 films upon exposure to CO. ............................................................................. 132
xv
List of Tables
Table 2-1: Some selective publications of metal oxides used for gas sensing. ......... 46
Table 2-2: Summary of published results on gas sensing characteristics of WO3. .... 48
Table 3-1: Physical properties of WO3 and Fe powders. ........................................... 61
Table 3-2: Target gases and their concentrations. ...................................................... 69
Table 4-1: Comparison of the positions of O-W-O stretching vibration mode peaks
observed for nanostructured WO3 and Fe-doped WO3 films annealed at 400ºC for 2
hours in air. ................................................................................................................ 88
Table 4-2: Comparison of W 4f7/2 and O 1s peak positions of nanostructured WO3
and Fe-doped WO3 films annealed at 400ºC for 2 hours in air. ................................. 98
Table 5-1: Optimum operating temperature of WO3 thin film based gas sensors upon
exposure to H2. ......................................................................................................... 109
Table 5-2: Response and Recovery times of annealed (at 300ºC and 400ºC)
nanostructured WO3 films upon exposure to H2. ..................................................... 111
Table 5-3: Response and Recovery times of annealed (at 300ºC and 400ºC)
nanostructured Fe-doped WO3 films upon exposure to H2. ..................................... 115
Table 5-4: Response and Recovery times of annealed (at 300ºC and 400ºC)
nanostructured Fe-implanted WO3 films upon exposure to H2. ............................... 119
Table 5-5: Optimum operating temperature of nanostructured WO3 thin film based
gas sensors upon exposure to ethanol. ..................................................................... 121
Table 5-6: Response and Recovery times of 400ºC annealed nanostructured WO3
films upon exposure to ethanol. ............................................................................... 122
Table 5-7: Response and Recovery times of annealed (at 300ºC and 400ºC)
nanostructured Fe-doped WO3 films upon exposure to ethanol. ............................. 126
Table 5-8: Response and Recovery times of 300ºC annealed nanostructured Fe-
implanted WO3 film upon exposure to ethanol. ....................................................... 129
Table 5-9: Optimum operating temperature of nanostructured WO3 thin film based
gas sensors upon exposure to CO. ........................................................................... 130
Table 5-10: Response and Recovery times of annealed (at 300ºC and 400ºC)
nanostructured Fe-doped WO3 films upon exposure to CO. ................................... 132
Table 5-11: Optimum operating temperature of the films upon exposure to H2,
ethanol and CO in the tested temperature range of 100ºC-300ºC. ........................... 133
xvi
Table 5-12: Comparison of microstructural properties of 400ºC annealed films. ... 135
xvii
List of Publication – Research Activities
M. Ahsan, T. Tesfamichael, J. Bell, M. Ionescu and M.G. Blackford,
“Microstructural characterization of electron beam evaporated tungsten
oxide films for gas sensing applications”, 16th AINSE Conference on
Nuclear and Complementary Techniques of Analysis - 25 - 27 November
2009 at Lucas Heights, Sydney.
M. Ahsan, T. Tesfamichael, A. Ponzoni, and G. Faglia, “Sensing
Properties of E-Beam Evaporated Nanostructured Pure and Iron-Doped
Tungsten Oxide Thin Films”, Sensor Letters, 2011, Volume 9, pp.759-
762.
M. Ahsan, T. Tesfamichael, J. Bell, N. Motta, W. Wlodarski, “Iron doped
nanostructured WO3 thin film based conductometric hydrogen sensor”,
EMRS 2011 Fall Meeting, September 19-23, 2011, Warsaw University of
Technology, Poland.
M. Ahsan, T. Tesfamichael, M. Ionescu and J. Bell, “Annealing effect on
iron doped tungsten oxide thin films prepared by thermal evaporation”,
Materials Innovation in Surface Engineering 2011, 18-20 October 2011,
Melbourne, Australia.
M. Ahsan, T. Tesfamichael, M. Ionescu, J. Bell and N. Motta, “CO
sensitive nanostructured WO3 thin films doped with Fe”, NanoS-E3 -
Nanostructures for Sensors, Electronics, Energy and Environment, 12-16
September 2011, Kingscliff, Australia.
xviii
M. Ahsan, T. Tesfamichael, M. Ionescu, J. Bell and N. Motta, “Low
temperature CO sensitive nanostructured WO3 thin films doped with Fe”
Sensors and Actuators B, Volume 162, Issue 1, 2012, pp 14-21.
M. Ahsan, T. Tesfamichael, J. Bell and N. Motta, “Affect of annealing on
the sensing properties of nanostructured tungsten oxide (WO3) thin films
towards hydrogen and ethanol”, Sensors and Actuators B (Submitted).
T. Tesfamichael, A. Ponzoni, M. Ahsan, G. Faglia, “Gas Sensing
Characteristics of Fe-Doped Tungsten Oxide Thin Films”, Sensors and
Actuators B (Accepted).
19
CHAPTER 1 : INTRODUCTION
1.1 Motivation
Human living standards have grown remarkably due to the industrial revolution.
Industrialization also has a negative aspect on human health due to emission of gases
that pollute environment and pose risk to public health. Flammable gases also need
to be monitored to protect against unwanted incidence of explosion or fire.
Applications that ensure human health and safety, protect the environment, monitor
manufacturing processes and optimize the performance of control systems have
increased the demand for gas sensors in recent years [1-3]. The requirements of
these applications demand immediate, near continuous analysis to detect target gases.
Online analysis of gas mixtures is fundamental to quality control in industrial
production and assuring safety in the work place. In situ sensing of various gases is
important in many technological fields, such as the oil and petrochemical industry,
water treatment plants and biogas applications [4]. Modern alcohol breath analysers
based on ethanol sensors are replacing traditional detecting methods for the
identification of drunken drivers. Developments leading to electronic noses which
mimic the human olfactory system by using an array of electronic chemical sensors
and appropriate pattern-recognition electronics will further expand the requirements
for gas sensors [5, 6]. Poor indoor air quality due to emission of toxic gases, volatile
organic compounds and microbial contaminants can reduce the health and comfort of
occupants of a building. Hence monitoring of toxic and hazardous gases is a subject
of growing importance in both domestic and industrial built environments. To be
useful in providing timely feedback that enables effective control of these gasses,
20
sensors that are capable of continuously monitoring the levels and types of gases are
needed. The threshold limit values (TLV) for acceptable levels for some toxic gasses
can be as low as 20 ppb (1 ppm = 1 ml/m3 = 2.66 x 10-3 mg/m3) which requires high
performance sensing devices [7]. Together, this suite of applications indicates a
growing demand for the development of reliable and low power gas sensors with
excellent sensitivity and selectivity towards specific gases.
1.2 Conductometric gas sensors
Gas sensors operate on the principle of conversion of gas concentration into a
measurable signal. Gas sensor devices that have been developed so far include mass
sensitive sensors, optical sensors, electrolytic sensors and solid state sensors [8].
Among the solid-state gas sensors, semiconductor metal oxide gas sensors have
received the most attention as they show good potential for continuous monitoring of
gases. These sensors offer a wide variety of advantages over the traditional analytical
instruments which include lower cost, easier manufacturing, smaller size, short
response and faster recovery.
Conductometric metal oxide sensors are based on the principle of a change in
electrical resistance due to interaction between the surface and the target gas. The
change in sensor’s resistance which is linked to surface reactions depends on many
factors such as specific material reactivity, microstructure, free charge concentration,
sensitive layer morphology, etc. Most of the work on “conduction type” in these
sensors was performed on n-type semiconducting metal oxides such as SnO2, ZnO
and WO3. Unfortunately, the available conduction models [9] are not suitable to p-
type materials. Tin oxide (SnO2) is one of the most widely studied sensing material
since Taguchi built the first SnO2 sensor for Figaro Sensors in 1970 [10]. However,
recent research has shown other metal oxides such as TiO2, ZnO and WO3 also have
21
potential for high sensing properties. This is evident from the increasing number of
publications addressing the sensing properties of these metal oxides.
1.3 Parameters expressing sensing properties
It is important at this point to briefly explain sensing parameters in order to
develop a clear understanding of the effect of various factors on these parameters.
The response of a gas sensor is characterized by the following parameters [8]:
1. Sensitivity: It is defined as the ratio of ‘resistance change’ of a sensor upon
exposure to target gas to resistance in target gas for n-type materials and
reducing gas. For p-type materials and reducing gas, it is the ratio of
resistance change of sensor in target gas to that in air. Many authors use the
term ‘sensitivity’ to indicate response amplitude. However, in a true sense,
sensitivity of a gas sensor is defined as the derivative of the response to the
gas concentration [11]. Therefore, keeping in view the general trend of using
the definitions, the term ‘sensitivity’ would be used to indicate the response
amplitude of the sensors in this study.
2. Selectivity: It is expressed in terms of a dimension that compares the
concentration of the corresponding interfering gas that produces the same
sensor signal. It is expressed as ratio of sensitivity towards interfering gas to
sensitivity towards desired gas.
3. Response time: It is the time interval over which resistance of the sensor
material attains a fixed percentage (usually 90%) of final value when the
sensor is exposed to full scale concentration of the gas. A small value of
response time is highly desirable in applications such as detection of
flammable or combustible gases to prevent fire.
22
4. Recovery time: It is the time interval over which sensor resistance reduces to
10% of the saturation value when the target gas is switched off and the
sensor is placed in synthetic (or reference) air. A sensor should have a small
recovery time so that it can be ready for next detection.
5. Long term stability: It is the ability of a sensor to maintain its sensing
properties when operated continuously during its lifetime. Sensors are
designed to have long term stability that lasts up to several years without
showing a drift in sensor performance.
6. Short term stability: It is the ability of a sensor to reproduce its
characteristics during a certain period of time at working conditions, which
may include high temperature and presence of known target gas [12].
The operating temperature also plays an important role in sensing performance of
metal oxides. Most of the metal oxide sensors are operative only at high temperatures
in the range 200ºC-500ºC, as they are thermally activated in this temperature range.
At low temperatures, metal oxides do not show any response to target gases. The
overall gas sensing phenomenon is a combination of the following three functions
[13]:
Receptor function: It depends on the ability of the oxide surface to interact with
the target gas. It is largely affected by the surface properties of the oxide. Dopants
and defects such as interstitial cations or anion vacancies can play in important role
in enhancing the conductivity and hence sensitivity towards target gas [14, 15].
Transducer function: It depends on the ability to transport electrons through grain
boundaries. The electronic band structure and hence conductivity also depend on
particle size and porosity [16-20].
23
Utility function: It concerns the kinetic factor as a result of difference between
the diffusion rate and reaction rate. Film porosity plays an important role in order for
the target gas to penetrate the entire volume of the metal oxide layer and balance the
reaction rate with diffusion.
1.4 Aims and objectives of the project
The aim of the current research was to develop tungsten oxide (WO3)
conductometric gas sensors and optimize their sensing properties such as sensitivity,
selectivity, response and recovery times, and operating temperatures towards a few
selected gases (H2, ethanol and CO) by depositing suitable nanostructured thin films
using thermal evaporation technique, post-deposition heat treatment and
characterization of physical, chemical and electronic properties of the films. To
achieve this goal, gaps in the literature were identified. Based on the literature review
(Chapter 2), the following conclusions have been derived from available studies:
1. As with any semiconducting metal oxide, WO3 based gas sensors show good
performance only at elevated temperatures (>200ºC).
2. Sensors based on WO3 thin films have been used to detect a number of
oxidizing and reducing gases such as NH3, H2, NO2, H2S and ethanol.
However, there is very little evidence in the literature on their sensing
performance towards CO.
3. As in case of any semiconductor metal oxide, selectivity to a specific gas is a
major problem in sensors based on tungsten oxide (WO3).
4. Doping the WO3 thin films with noble metals such as Au, Ag and Pd has
shown improved sensing performance towards various gases which is mainly
attributed to the noble metal catalytic effect on the surface/gas interaction. In
the present study, iron has been used to dope the WO3 thin films. Since iron
24
has a similar atomic radius as W, therefore, it can be introduced as a
substitutional impurity in the WO3 matrix. The influence of Fe doping on
physical, chemical, electronic and gas sensing properties of WO3 thin films
has not been studied extensively.
In an attempt to address the above problems, the following objectives were set:
1. To synthesise nanostructured WO3 thin films by thermal evaporation method.
This method gives control over film thickness, grain size and porosity. These
process parameters are critical in gas sensing.
2. To modify the physical, chemical and electronic properties of the WO3 films
through doping with Iron (Fe) during thermal evaporation. As Fe has a
similar atomic radius as W, it can be introduced as a substitutional impurity in
the WO3 matrix and its influence on physical, chemical, electronic and gas
sensing properties can be investigated. Iron would also be incorporated by
implantation to investigate the effect of different doping techniques.
3. To optimise the film properties such as grain size, crystallinity, stoichiometry
and porosity by post deposition heat treatment. Post deposition heat treatment
also relieves the residual stresses and surface contamination.
4. To characterize the physical, chemical and electronic properties of these films
using various scientific instruments such as: Scanning Electron Microscopy,
Atomic Force Microscopy, Transmission Electron Microscopy, X-ray
Diffraction Analysis, X-ray Photoelectron Spectroscopy, Rutherford
Backscattered Spectroscopy and Raman Spectroscopy. This is valuable in
assessing the microstructural and electronic properties of the films.
25
5. To characterize the sensing properties (sensitivity, selectivity, response and
recovery times) of these films to H2, ethanol and CO in the temperature range
100ºC to 300ºC.
6. Investigate the effect of microstructure, doping and post deposition heat
treatment on the sensing performance and operating temperature of these
films.
1.4 Thesis Organization
This thesis consists of six chapters and is presented as follows:
Chapter 1 gives an overview of author’s motivation for performing this
research and specific objectives.
Chapter 2 presents the literature review on various aspects of gas sensing. It
includes the construction principle and basic characteristics of gas sensors,
gas sensing mechanisms, role of additives, importance of dimensions in gas
sensing and limitations of current metal oxide based gas sensors. The
structural properties of tungsten oxide and brief overview of various
deposition techniques with a particular focus on thermal evaporation are also
described in this chapter.
Chapter 3 outlines the processes and procedures involved in the synthesis and
characterization of tungsten oxide thin films investigated in this research.
This chapter is broadly divided into three sections which describe the thin
film deposition, thin film characterization and gas sensor characterization. In
the first section of this chapter, thermal evaporation of nanostructured
tungsten oxide thin films is described. It is followed by the description of
various analytical techniques such as Transmission Electron Microscopy,
26
Atomic Force Microscopy, Rutherford Backscattered Spectroscopy, X-ray
Diffraction, X-ray Photoelectron Spectroscopy and Raman spectroscopy
which were employed to characterize these films. The last section describes
the details of the conductometric gas sensing setup used in this study.
Chapter 4 focuses on the physical, chemical and electronic characterization of
nanostructured thin films. The characterization outcomes using various
analytical techniques are presented and the effect of heat treatment and
doping are discussed.
Chapter 5 presents the experimental results obtained from the gas sensor
characterization of nanostructured tungsten oxide thin film based gas sensors.
The results are linked to the various physical, chemical and electronic
properties of these films investigated in Chapter 4 and the influence of
various factors such as surface morphology, grain size, crystallinity and
stoichiometry on gas sensing performance are discussed.
Chapter 6 presents the conclusions of this thesis and suggestions for possible
future work.
27
CHAPTER 2 : LITERATURE REVIEW
2.1 Introduction
In Chapter 1, conductometric gas sensors were described and the parameters
expressing the sensor performance have been discussed. In this chapter, the
fundamental characteristics of conductometric metal oxide gas sensors are discussed.
Then, a critical review of the state of the technology of metal oxide gas sensors is
presented with a focus on tungsten oxide.
2.2 Construction principles of conductometric metal oxide gas
sensors
In principle, conductometric metal oxide gas sensors are constructed by two key
functions: Receptor function and Transduction function.
2.2.1 Receptor function
Receptor function transforms chemical information into a form of energy which
can be measured by the transducer [21]. It is determined through various interactions
between the surface and target gas such as adsorption, ion exchange or
electrochemical reaction. In air, oxygen is adsorbed on the oxide grains as negatively
charged ions, inducing a surface space charge layer depleted of electrons (space
charge layer) which leads to a band bending [22]. The band bending and thickness of
space charge layer created due to oxygen species are further decreased when the
metal oxide is exposed to a reducing gas as reducing gases inject electrons to the
conduction band. For oxidizing target gasses such as NO2 the conductivity decreases
as oxidising gases extract electrons. Variation of the height of the potential barrier is
28
believed to be the origin of the conductance response to gases [23]. The receptor
function of metal oxides is largely affected by their intrinsic electronic properties and
any deviation from their stoichiometric chemical properties. Defects such as oxygen
vacancies are inherent in metal oxides. This creates the space charge layer depleted
of electrons and negatively charged oxygen ions on the surface. The mobility of main
carriers is of primary importance for semiconductors, because it provides the
proportionality constant of the change of electrical conductivity when the number of
main carriers changes as a result of gas–solid interactions [24]. As the metal oxides
approach stoichiometry, the conductivity becomes extremely low (high resistance).
WO3 and TiO2 films have very small electron mobility (high resistivity) which
ranges between 0.03 and 0.2 cm2 V−1 s−1 [25, 26]. Dopants and defects such as
interstitial cations or anion vacancies can play an important role in enhancing the
conductivity [14, 15]. Dopants are important to the increased formation of oxygen
vacancies and modifying of the electronic structure and band gap energy of metal
oxides. It has been reported that doping of TiO2 with Fe increases the oxidation
activity of the oxide and this has been related to a higher density of oxygen vacancies
[27].
2.2.2 Transduction function
Transduction function is related to the ability to transport electrons through grain
boundaries. In polycrystalline metal oxide, gas sensing reaction takes place at the
surface of the individual particles and at grain boundaries and it becomes easy for
electrons to conduct through different grains. Experiments on various metal oxide
films exposed to specific gases suggest that the electronic band structure and hence
conductivity are also dependent on film microstructure (i.e. particle size and film
porosity) [16-19, 28]. This shows that the magnitude of the conductivity change
29
depends mainly on the ratio between particle size and Debye length (distance over
which charge separation occurs in a semiconductor). If the grain size is large (>>
Debye length) the depletion of the space charge region between the grain boundaries
controls the conductivity variation. If the radius of the grain is extremely small (less
than the Debye length, e.g. for WO3, is 10 nm), the entire particle is depleted and no
band bending occurs which results in high conductivity-change (or high sensitivity)
when exposed to target gas. In practice nanomaterials of particle size less than 10 nm
favours large gas-active surface area and provide high sensor sensitivity.
2.3 Limitations of existing metal oxide gas sensors
The functions discussed above in Section 2.2 strongly influence the sensitivity of
semiconducting metal oxides. Although most of the semiconductor metal oxide
sensors that have been investigated to date are promising, they do not show response
to gases at lower operating temperatures (100ºC-200ºC) and must be thermally
activated at higher temperatures (200ºC-500ºC). These high operating temperatures
cause grain growth and changes in material properties that lead to long term stability
problems of the sensors. The higher optimum operating temperatures [29, 30] also
demand higher power consumption, which makes them unsuitable for battery
operated sensor devices that would be advantageous in some in situ applications.
Lower operating temperature metal oxide gas sensors with an acceptable sensitivity
would overcome these stability and high power consumption problems. By analogy
with the current high temperature metal oxide gas sensors, a method for enhancing
sensitivity to gas at lower temperatures is modification of the electronic structure of
the metal oxides by using mixed metal oxides such as SnO2-ZnO, Fe2O3-ZnO and
ZnO-CuO [28, 31-33]. Composites of SnO2-ZnO and SnO2-In2O3 have shown
enhanced sensitivity when compared with single oxide sensors when exposed to
30
ethanol [34]. Sensitivity is improved by selective catalytic activity of one component
of the mixed oxide to a particular gas. However, this approach has not shown any
significant reduction in optimum working temperature of the sensors [33].
The catalytic activity of the gas sensor material can also be enhanced by doping
with noble metals such as Pt, Au, Pd and Ag [35-39]. The noble metals chemically
sensitize the metal oxide surface i.e., they activate target gases by enhancing their
spill-over (more surface coverage), so that they react with oxygen adsorbates more
easily. The oxygen supply can also be improved by metal additives, at the surface of
which oxygen molecules from the ambient can be dissociated and migrate to the
surface of metal oxide. In this way, the additive enhances the sensing properties of
the metal oxide.
Metal oxides can also be electronically sensitized to improve the gas response
and lower the operating temperature [40]. Addition of fine particles of some metals
results in a rise of the base resistance in air. The electron concentration in the oxide
surface layer is low, which corresponds to an increase in the space-charge depth as a
result of the transfer of electron from the metal oxide to the metal loaded onto its
surface. When the metal surface is covered with oxygen adsorbates at elevated
temperatures in air, its oxidation state changes (the metal is oxidized).The oxygen
adsorbates extract electrons from this metal, which in turn extracts electrons from the
metal oxide, leading to a further increase in the space charge depth. Consumption of
oxygen adsorbates on the metal, in addition to those on the metal oxide surface, by
reaction with gas, enhances the sensitivity.
Another limitation of current metal oxide sensors is that they are non-selective
i.e. they are sensitive simultaneously to a wide range of gases. Selectivity can be
improved by exploiting the influence of operating temperature on the sensitivity of
31
the sensor [41, 42]. A change in sensor resistance is expected with change in
temperature, as the reactions occurring at the surface of the sensor
(chemisorption/redox reaction) are functions of temperature [9]. The temperature
dependence of the sensitivity of different gases can therefore be exploited to obtain
the selectivity to a particular gas.The selectivity can also be improved by depositing
a diffusion filter layer, such as SiO2 on top of the metal oxide [43]. By doing so, only
small molecules such as H2 are able to reach the surface of sensing material.
However, this has the undesirable effect of reducing the total number of gas
molecules that reach the sensor, hence reducing sensitivity.
One of the recent developments in improving selectivity of metal oxide gas
sensors is the use of a neural network. The idea is inspired from the biological
olfactory system. In human and mammalian noses, there are thousands of receptors
with bad selectivity to different odours, but the brain is able to derive specific odour
identification by processing the signals from all the receptors. The technological
analogue is called ‘electronic nose’ and mimics the natural olfactory process [5, 6].
In this system, an array of sensors with different functionality is employed and their
data are processed by neural networks to determine the gas concentrations.
2.4 Basic characteristics of a metal oxide conductometric sensors
In general, the electrical resistance of a conductometric metal oxide gas sensor
changes upon exposure to the molecules of the target gas. The nature of sensor
material (n-type or p-type semiconductor metal oxide) and the target gas (oxidizing
or reducing) governs the increase or decrease in electrical resistance. For an n-type
semiconductor exposed to reducing gas, the resistance decreases, whereas, upon
exposure to oxidizing gas, the resistance increases. The variation of resistance of
32
sensor with time on exposure and withdrawal of target gas is depicted by a typical
response curve as shown in Fig. 2-1.
Figure 2-1: A typical response curve of a conductometric gas sensor.
The performance of a gas sensor is characterized by the following five parameters
[8]:
1. Sensitivity or Response Amplitude
2. Response time
3. Recovery time
4. Selectivity
5. Long term stability
A brief description of these parameters follows below:
1. Sensitivity(S):
It is defined as the ratio of resistance change of a sensor upon exposure to target
gas to the resistance in target gas for n-type materials [8]. For p-type materials, it is
the ratio of resistance change of sensor upon exposure to target gas to the resistance
in air.
Response time
Recovery time
Gas on
Gas off
Time
Res
ista
nce
33
)1.2()( materialstypenforR
RS
gas
)2.2()( materialstypepforR
RS
air
where ΔR is the change in sensor resistance upon exposure to target gas. Sensors of
high S value are desirable in order to sense low concentration of gases.
As pointed out in the Section 1.3, the term ‘sensitivity’ is often used to indicate
response amplitude. However, in a true sense, sensitivity of a gas sensor is defined as
the derivative of the response to the gas concentration [11]. For the purpose of
simplicity, the term ‘sensitivity’ would be used to indicate the response amplitude of
the sensors in this study.
2. Response time:
This is the time interval over which resistance of the sensor material attains a
fixed percentage (usually 90%) of final value when the sensor is exposed to full scale
concentration of the gas. It is usually expressed as T90, T80, etc. A T80 of 50s
means that the sensor exhibits 80% of saturation value of resistance in 50s.
3. Recovery time:
It is the time interval over which sensor resistance reduces to 10% of the
saturation value when the sensor is exposed to full scale concentration of the gas and
then placed in the clean air. A sensor should have a small recovery time so that it can
be used over and over again.
4. Selectivity:
Most of the chemiresistive sensors exhibit high value of sensitivity to many
gases under similar operating conditions. Thus, selectivity of a sensor towards target
gas is expressed in terms of dimension that compares the concentration of the
corresponding interfering gas that produces the same sensor signal. It is expressed as
34
(2.3) d gasthe desirey towards Sensitivit
ering gasfor interf the sensorof y SensitivitySelectivit
5. Long term stability:
The ability of a sensor to maintain its properties when operated continuously for
long durations is called its stability. Good sensors have long term stability that last up
to several years without showing a drift in sensor performance.
All the above sensing parameters depend on several factors including the following:
Sensing material i.e. intrinsic properties of the metal oxide.
Sensing mechanisms i.e. interaction between the gas and sensor surface
(details in Section 2.5).
Operating conditions i.e. temperature, type of target gas.
Film properties such as microstructural features, film type (thick or thin film),
stoichiometry, etc.
In order to control these parameters, scientific understanding of gas - sensor
interaction (sensing mechanism) needs to be addressed, which follows.
2.5 Basic mechanisms of gas sensing in semiconductor metal oxide
sensors
Semiconductor metal oxides are used for two different types of gas sensing
applications. Broadly speaking, these applications can be categorized as
Determination of partial pressure of oxygen.
Determination of concentration of a minor constituent (oxygen partial
pressure remains constant).
The materials that have been used commercially for determining partial pressure
of oxygen are TiO2, Cr2O3 and Ga2O3 [44]. If the sensor operates at a high
35
temperature (700ºC and above), the mechanism responsible for the detection is bulk
conduction. The oxygen partial pressure and electrical conductivity are related as
[44]
(2.4)1/m2
B
A OPTk
Eexpσσ
where, is the electrical conductivity,
* is a constant,
EA is activation energy for conduction,
KB is Boltzman’s constant,
P[O2] is oxygen partial pressure,
m is the oxygen vacancy constant dependent on dominant type of bulk defect
involved in the reaction between sensor and oxygen and
T is absolute temperature.
The second application of semiconductor metal oxide gas sensors involves
situations where the oxygen partial pressure is constant and concentration of minor
constituent gases such as H2, CO, CH4 and H2S are to be determined.
For an n-type metal oxide semiconductor, possible gas sensing mechanisms are
[45]:
Surface reactions with adsorbed gases.
Ion exchange.
Direct gas adsorption.
For semiconductor materials, the observed sensor effects are dominated by direct
gas adsorption and surface reactions with preadsorbed molecules. The sensor
characteristics will vary depending on whether the sensor material is n-type or p-type
and whether the interacting species are reducing or oxidizing gases.
36
For n-type sensor material and reducing gas, there will be preadsorbed oxygen
ions on the surface of the sensor. The gas will then react with oxygen ions to form
neutral molecules, leading to electron transfer to the sensor material and
consequently decrease the resistance. Following is the detailed description:
In air environment, oxygen molecules adsorb onto the surface of metal oxide
layer to form O2-, O- and O2- species by extracting electrons from the conduction
band depending on the temperature [46] and type of metal oxide (n-type or p-type).
These oxygen adsorbates play an important role in detecting gaseous species.
It has been experimentally confirmed by TPD, FTIR and ESR techniques that
dominant oxygen species are [47]:
Molecular (O2-) below 150ºC.
Atomic (O-) between 150ºC and 300ºC.
Atomic (O2-) above 300ºC.
The oxygen adsorption can be described by the following rate equations:
)6.2()()( 221 adsOgO k
)7.2()()( 222 adsOadsOe k
)8.2()(2)(2 32 adsOadsOe k
)9.2()()( 24 adsOadsOe k
The reaction of the target gas X to be detected can be represented by:
)10.2()()( 225 egXOadsOX k
)11.2()()( 6 egXOadsOX k
)12.2(2)()( 22 7 egXOadsOX k
In an n-type semiconductor metal oxide, electrons are transferred to the surface
and then ionize the oxygen adsorbates to form O2- and O- which results in a negative
37
charge being developed on the surface. The surface layer is therefore depleted of
electrons, and so called depletion layer is created [48].
Fig. 2-2 shows the schematic of the surface layer and the corresponding electron
band structure. The conduction band energy levels of the bulk and the surface are
represented by Ecb and Ecs, respectively. The valence band energy levels for the bulk
and the surface are represented by Evb and Evs, respectively, and Ef denotes the Fermi
energy. Et is trap energy level of electrons at surface states due to adsorbed oxygen,
eVs is the height in energy of the band bending at the surface, d is the distance from
the surface and dd is the depth of depletion. The difference between Ecb and Evb is the
band gap energy Eg.
Figure 2-2: Schematic diagram showing the surface layer and corresponding electron band structure,
adapted from [48].
If the surface belongs to an ideal bulk material (large d) without grain
boundaries, the influence of the depletion layer is of little or no importance for the
conduction along the surface. However, in case of polycrystalline film, each interface
between grains gives rise to a band bending as depicted in Fig. 2-3. At the
intergranular contact, the conduction is restricted by this Schottky potential barrier
Evs Evb
EF
Ecb
Ecs eVs
d
Surface
O2-
O-
Et
Depletion layer (for n-type
semiconductor)
dd
38
due to depletion layer and the electrons have to overcome the energy barrier, eVs.
The change of the barrier height makes the electrical resistance of the material
dependent on the gaseous atmosphere [49]. The resistance and hence, gas sensitivity
in this case are not dependant on particle size.
Figure 2-3: Schematic view of physical and electron band structure model for a polycrystalline
material [50].
Fig. 2-4 shows three situations with different influence on the depletion layer
[50]. Fig. 2-4a demonstrates the situation where the area of the depletion zone at the
contact is less than the contact area. The depletion layer extends into the grain to a
depth marked by the dashed line. The resistivity is about that of the undepleted
region in the center of the neck. Fig. 2-4b illustrates a closed neck, where, the
depletion layers from the surfaces overlap resulting in a higher resistance in the
center of the constriction. The above two situations imply that the porous structure
would respond to the gas in the same way as a thin film sensor. Fig. 2-4c illustrates a
Barrier
eVs
O-
O2-
O-
O- O2
-
O- O2-
O- O-
O2- O-
O- O2- O
- O- O2
- O-
O-
O-
O2-
O- O- O- O2
- O- O- O-
Electronic current
Conduction band electrons
Adsorbed oxygen
Depletion layer
Physical Model
Band Model
39
situation where conduction is limited by the point of contact. This type of situation is
applicable to a porous material.
Figure 2-4: Schematic illustration of intergrain constriction situations: (a) Open neck (b) Closed neck and (c) Conduction limited by point of contact.
Wang [51] investigated the transition from open neck, neck controlled to point of
contact limited conduction of metal oxide gas sensors. He showed that the sensitivity
to adsorbed gas increased rapidly as the grain size became smaller than 40 nm, at
which point the conduction mechanisms for neck controlled and point of contact
limited conduction exist together.
2.6 Role of additives in gas sensing
Appropriate amounts of metal additives such as Au, Pt, Pd, etc. when added to
metal oxide sensors have shown improvement for various kinds of gases.
Enhancement in sensor response and a decrease in operating temperature for
maximum sensor response have also been achieved, in addition to decrease in
response time and better selectivity. The main idea of using metal additives is to
enhance the reaction rate of the gases when they come in contact with the sensor
surface.
40
Shimizu et al [52] identified that metal additives can lead to two different
sensitization mechanisms: Electrical sensitization and Chemical sensitization. These
mechanisms are shown in Fig. 2-5.
Figure 2-5: Schematic of additive role in gas sensing [40].
In chemical sensitization, the metal cluster catalyzes the reaction and reaction
products are subsequently spill over the semiconducting metal oxide surface. These
reaction products then cause the gas sensing response [40].
Electronic sensitization occurs due to the alignment of Fermi energies of the
metal oxide and the additive. This is similar to the Schottky barrier influencing the
surface charge region in the semiconductor material. Oxidation or reduction of metal
additive controls the band bending of the metal oxide thereby controlling the sensing
mechanism [53].
2.7 Importance of the dimension in gas sensing
Nanomaterials are defined as those that have at least one of their dimensions
≤100nm. Thus we may visualize them as structures produced by reducing one, two,
or three dimensions of a bulk material, thereby resulting in 2D nanolayers, 1D
nanowires or 0D nanoclusters [54, 55]. At such small length scales, most of the
Electrical sensitization Chemical sensitization
O-
e-
O2
O- 2R+O2 2RO
O-
O- O- O-
R RO
Noble Metal Cluster
41
atoms are surface atoms, thus significantly increasing the effective number of sites
available for reactions. Increase in surface area to volume ratio with decrease in grain
size is very important in the context of gas sensing. Thus reducing the grain size
plays an important role in applications that involve surface reactions such as
catalysis, chemical gas sensing, etc.
Fig. 2-6 demonstrates the grain size dependence of sensitivity of SnO2 films
exposed to CO and H2 [56]. It can be observed that grain size reduction below 10 nm
results in a drastic increase in sensitivity.
Figure 2-6: The effect of particle size on gas sensitivity of SnO2 sensor exposed to CO and H2, adapted from [56].
Fig. 2-7 shows the response of electron beam evaporated In2O3 film to NO2 as a
function of grain size [57]. A remarkable increase in sensitivity is observed upon
grain size reduction from 20 nm to 5 nm. The depletion layer depth (Debye length)
plays an important role if the grain size is very small. For most nanostructures,
Debye length is of the order of the grain diameter (considering spherical particles,
nanowires or nanotubes) or their width in case of nanobelts and other flat
nanostructures. Under such conditions the surface chemical processes strongly
influence the electronic properties.
42
Figure 2-7: Influence of grain size on gas response of In2O3 film to NO2, adapted from [57].
A 2 to 3 order increase in sensitivity was observed in the case of In2O3 films
when the grain size was decreased from 60-80 nm to 10-50 nm [58].
For grains large enough to have a bulk region unaffected by the surface
phenomena, i.e. when the grain diameter d >> λD (Debye length), the surface charge
carrier density ns, is given by [9]
)13.2(exp
Tk
qVnn
B
sbs
where nb is the concentration of free charge carriers (electrons), qVs is activation
energy, kB is Boltzman's constant and T is temperature in Kelvin.
When the grain size d ≤ λD is comparable to depth of depletion layer (Debye length),
the activation energy ΔE is related to Debye length as [9]
)14.2(4
DB
DTkE
where D is the grain diameter.
43
If ΔE is comparable to the thermal energy then a homogeneous electron
concentration is attained in the grain and leads to the flat band case. For grain sizes
lower than 10 nm, complete depletion of charge carriers occurs inside the grain and a
flat band condition results in a wide range of temperatures. Also, as the gas sensing
mechanisms involve adsorption processes, the physical properties and the shape of
the material determine the response of the nanosensor. Higher area/volume ratio
favors gas adsorption (and change in conductivity), decrease the response time and
increase the sensitivity of the device. Additionally, the time taken for gas molecules
to diffuse into and out of the volume of nanostructures is minimized [59].
Because of the advantages nanostructured material based sensors have over the
same sensor built with bulk material, nanoparticles and thin films of metal oxides
(less than 1000 nm thick) have been used to detect a wide variety of gases. As a
result, nanotechnology offers sensor devices with improved sensing properties.
2.8 Structural properties of tungsten oxide
Amorphous tungsten oxide film has large open pores and constitutes clusters
which are built from 3-8 WO6 octahedra [60]. These octahedra are linked together at
corners or edges by W-O-W bonds or water bridges [61, 62]. Random packing of the
clusters results in open structure or voids which are usually filled with molecular
water taken from the air [61]. The ionic conduction of amorphous WO3 film is
carried out by proton transport through water bridges in pores. On the other hand, the
electronic conduction is done by clusters linked through W-O-W bonds.
Tungsten oxide exhibits a cubic perovskite-like structure based on the corner
sharing of WO6 octahedra, with the O atoms at the corner of each octahedron [63]. A
schematic view of WO3 crystal structure is shown in Fig. 2-8. The symmetry of
tungsten oxide is lowered by two distortions: tilting of WO6 octahedra and
44
displacement of tungsten from the center of its octahedron [64]. These distortions
result in a number of temperature dependant phases of WO3, which are listed below
[64-68]:
Monoclinic ε – WO3 phase below -50ºC.
Triclinic δ – WO3 phase (from -50 to 17ºC).
Monoclinic γ – WO3 phase (from 17 to 330ºC) stable at room temperature.
Orthorhombic β – WO3 phase (from 330-740ºC).
Tetragonal α – WO3 phase above 740ºC.
In addition to the above phases, a metastable hexagonal WO3 phase has also been
reported around 400ºC [69].
Figure 2-8: Tungsten oxide (WO3) crystal structure.
The electrical and optical properties of tungsten oxide are strongly dependent on
the crystalline structure. It has been demonstrated that resistivity decreases with
increasing temperature [70]. Migas et al [71] investigated the dispersion of bands
near the gap region of different phases of WO3. They found that the dispersion of
bands near the gap region is identical in all phases except the hexagonal WO3 phase,
x
y z
W
O
Absent
Corner sharing arrangement of WO6
45
which exhibited flat band structure in the last valence band and the first conduction
band.
One of the elementary defects in tungsten oxide structure, as in most metal
oxides, is the lattice oxygen vacancy, where an oxygen atom is absent from the
normal lattice site. The octahedral changes from corner sharing to edge sharing
lattice with the formation of crystallographic shear planes built up of edge and plane
sharing structures, which corresponds to partial replacement of W6+ to W5+ and W4+
ions. This leads to formation of non-stoichiometric tungsten oxides WO3-x and
strongly influences its electronic properties [16]. The influence of oxygen vacancies
on electronic properties of different phases of WO3 has been investigated in terms of
bonding-antibonding interactions [72]. Changes in band gap and positions of the
valence band maximum and the conduction band minimum have been revealed for
different phases of WO3 due to presence of oxygen vacancies [73]. From an
electronic point of view, an oxygen vacancy increases the electronic density on the
metallic (W) adjacent cations. This raises the Fermi level into the conduction band
where it partly crosses some bands and partly stays in the energy gap. It has been
observed that the energy gap shrinks by about 0.5 eV if an oxygen vacancy is formed
[71]. The absorption coefficient also shifted to the lower energy range if an oxygen
vacancy is present. This leads to formation of donor-like states slightly below the
edge of conduction band of the oxide and it acquires semiconducting properties [74].
In nanostructured WO3, the bandgap generally increase with reducing grain size [75].
This has been experimentally observed as a blue shift of the optical absorption
bandedge as the nanostructure dimensions are reduced.
The effect of doping on band gap shifts of WO3 has also been reported in the
literature [71, 76]. Hwang et al showed that Mg-doped WO3 has a band gap similar
46
to undoped WO3 [77]. Doping with Cu, Ag, Mo is also reported without much
success [71, 78, 79]. However, doping with 3.4% nitrogen significantly reduced the
band gap from 3.0 eV to 2.2 eV [80].
2.9 Metal oxide based gas sensors
The history of semiconducting metal oxide based gas sensors dates back to over
5 decades. Most of the metal oxides have been commercially in use for sensing
various gases. Table 2-1 is an attempt to encompass some selected metal oxides and
corresponding target gas based on the literature survey.
Table 2-1: Some selective publications of metal oxides used for gas sensing.
Reference Additives Target gas Metal Oxide
[81-90] Pt, Ag, Pd, Os, Fe, Au, In,
Ru, Bi2O3, CeO2, CuO
CO, CH4, SO2, N2O, CO2, NO2,
CH3OH, C2H5OH, H2, H2S, NH3,
LPG
SnO2
[91-96] La, Pt, Cr2O3, WO3 CH3OH, C2H5OH, O2, H2, NH3, NO2 TiO2
[97-103] Al, Sn, Cu, Pd, Fe2O3 NH3, H2, NO2, CH4, CO, H2S,
CH3OH, C2H5OH, LPG ZnO
[104-106] Ti NH3, CO, NO2 MoO3
[107-110] Au, Zn, Pt, Pd, RuO2 CH4, Benzene, toulene, CO, NO2 Fe2O3
[111-114] Al, SiO2/Si Humidity, CH4, NH3 Al2O3
[115, 116] TiO2 NO2, O2, NH3, Humidity Cr2O3
[117] ZnFe2O4 Ethanol CdO
[118-121] SnO2, Pd, Ta2O5, WO3, NiO O2, CO, CH4, NO, NH3 Ga2O3
[122-127] Mg, Zn, Mo, Re, Au, Pd NO2, NH3, H2S, O3 WO3
Tungsten oxide (WO3) is a well-known n-type semiconductor with a band gap of 2.6
-3.6 eV [128, 129] that has been used not only in catalytic/photocatalytic [130],
electrochromic applications [131] but also in solid state gas sensors. The number of
research publications where tungsten oxide is used for gas sensing applications has
increased dramatically during recent years leading this material to be the second
most studied metal oxide sensor for gas sensing applications after SnO2. As the main
47
focus of this PhD research is tungsten oxide, in the following section, the state of the
research of nanostructured tungsten oxide sensors will be explored.
2.10 Tungsten oxide (WO3) based gas sensors
The first report on WO3 for gas sensing was published by Shaver in 1967. He
showed that conductivity of Pt-activated WO3 increased by one order of magnitude
on exposure to H2. In the following years, several reports on WO3 based sensors have
been published. It has been found that WO3 can be used for detecting a variety of
gases such as H2, CH4, NH3, CO, NO, CHOH, O2, H2S, NO2, C2H5OH, O3, (CH3)3N,
SO2, Cl2. A summary of selected publications related to NH3, H2S and NO2 is
presented in Table 2-2. A detailed literature survey and state of the art of WO3 gas
sensors is given below.
The sensing properties of screen-printed WO3 films doped with Bi2O3 to NO
were investigated by Tomchenko et al [132]. The films showed good sensitivity to
NO over a wide range of temperature (200-500ºC). It was demonstrated that a bottom
thick-film layer of about 15µm determines the sensing characteristics of WO3 based
sensors. The authors also investigated fabrication of planar type WO3 based thick
film sensors for online monitoring of NO concentrations.
Bi2O3 doped WO3 sensors showed a good sensitivity to low NO concentrations
(2-300 ppm) at 300ºC. Penza et al [133] investigated NOx gas sensing properties of
WO3 thin films fabricated by reactive rf sputtering on glass substrates. Palladium,
platinum and gold were evaporated as activator layers onto the films. The sensor was
found to possess excellent sensitivity towards NO and NO2 gases in the temperature
range 100-300ºC. The optimum operating temperature was found to be 150ºC and
200ºC for Pt and Au doped WO3 films, respectively. They concluded that the
activator layers act as catalysts and speed up the response to NO and NO2 at low
48
temperatures and improve the selectivity with respect to other reducing gases (CO,
CH4, H2, SO2, H2S, NH3).
Table 2-2: Summary of published results on gas sensing characteristics of WO3.
Material Concentration
(ppm) Temperature
(ºC) Sensitivity* Year (Reference)
Method of preparation
NH3
WO3:Au 50 450 40 1992 ]134[ Powder-dip coating WO3 10 300 1.1 1995 [135] Sputtering WO3 1000 300 6 2000 [136] Drop coating
WO3:Mg 30 350 7.65 2000 [137] Powder-dip coating WO3 100 400 7 2003 [138] Sputtering
WO3 10 200 18.2 2005 [139] Modified thermal
evaporation WO3 100 220 7 2008 [140] Thermal oxidation
WO3:Pt 1000 260 1000 2006 [141] RF sputtering H2S
WO3 200 200 2 1990 [142] Powder dip coating WO3:Au 100 200 30 1993 [143] Sputtering
WO3:Au 100 250 50 2000 [144]
Sputtering
WO3: Au 3.5 250 46 2006 [141] RF sputtering WO3 10 200 10000 2001 [145] Deposition
NO2 WO3 80 300 97 1991 ]146[ Powder-dip coating WO3 100 350 20 1996 [147] Powder evaporation
WO3/TiO2 30 350 200 1999 [148] Powder-printing WO3:Mg 30 300 8 2000 [149] Powder evaporation
WO3 0.2 150 168 2011 [150] Hydrothermal
treatment WO3 100 250 33 2008 [140] Thermal oxidation
WO3: Au 10 150 430 2008 [151] Colloidal chemical
method WO3 0.045 200 0.8 2010 [152] Plasma spray
WO3:Ag 100 260 10 2006 [141] RF sputtering H2
WO3:Pt 200 200 8.5 2011 [153] RF sputtering WO3:Pd 200 200 10 2011 [154] Chemical Synthesis
WO3 10000 450 1.4 2011 [155] Thermal oxidation
WO3 1000 250 15 2010 [156] Electrochemical
anodizing CO
WO3:Pd 200 100 1.6 2011 [154] Chemical route
synthesis WO3 1000 150 0.2 2011 [157] Thermal evaporation
CoOHWO3:SWNT 1000 25 164
(S=ΔmV) 2009 [158] Hydrothermal
CoWO4 800 250 1.2 2010 [159] Pulsed laser deposition
*The sensitivity values presented in this table are absolute values as reported in the literature. Their magnitude cannot be compared as different authors have used different formula to calculate
sensitivity. The main intent of this table is to highlight the published results on WO3 based gas sensors.
Kawasaki et al [160] investigated NOx sensing properties of WO3 films
synthesized by Pulsed Laser Deposition (PLD) method. Their results demonstrated
that PLD technique is an efficient method to produce crystalline WO3 thin films
which are sensitive to NOx gases. It was also observed that substrate temperature (Ts)
49
is an important parameter in producing crystallinity in films and it increases with
increasing Ts. A sensor produced by using nanocrystalline WO3 and thin film
microfabrication technology showed a high degree of sensitivity to low NO2
concentration in the range from 50 to 550 ppb with relatively fast response and
recovery time [161]. The sensitivity was found to depend on surface structure, grain
size and geometrical heterogeneity of the films which were controlled by the
calcination temperature. The sensitivity was also found to depend on NO2 adsorbed
form on the surface which was affected by operating temperature. The optimal
sensing condition was found when the films were calcined at 550ºC for 1 hour and
operating the sensor at 300ºC. Ponzoni et al [139] obtained nanostructured WO3 gas
sensors by modified thermal evaporation technique which consisted of sublimation
from a metallic tungsten wire followed by oxidation in low vacuum conditions and
reactive atmosphere (PO2 = 0.22 mbar) with substrate heated at high temperature
(600ºC). The films were composed of agglomerates with nanometric size and present
high surface roughness and large effective area suitable for gas sensing applications.
Sensing measurements indicated high performance at a working temperature of
100ºC, high response towards sub-ppm concentrations of NO2 compared to lower
ones for NH3 and CO.
Iron-doped nanostructured WO3 thin films prepared by Electron Beam
Evaporation (EBE) technique were investigated towards acetaldehyde by
Tesfamichael et al [162]. Addition of 10 at.% Fe slightly decreased the band gap
energy and subsequent annealing at 300ºC for 1 hour in air further decreased the
band gap energy. The annealed Fe-doped WO3 sensor produced gas selectivity but a
reduced gas sensitivity towards acetaldehyde as compared to WO3 sensor. The NO2
sensing performance of pure and iron-doped WO3 thin films prepared by EBE
50
technique was investigated by Ahsan et al [163]. The pure WO3 films were found to
be highly sensitive to 5 ppm NO2 at lower temperature (150ºC). Doping with Fe was
found to decrease the film resistance significantly but also a reduced sensitivity. The
high sensitivity towards NO2 was attributed to the improved nanostructure obtained
through e-beam evaporation and subsequent annealing at 300ºC for 1 hour in air.
The H2S, N2O and CO sensing performance of Al-doped WO3 nanoparticle films
prepared by advanced gas deposition was investigated by Hoel et al [164]. A
maximum sensitivity towards H2S, N2O and CO was observed at temperatures
130ºC, 250ºC and 430ºC, respectively.
Iron addition lower than 10 at.% to WO3 films prepared by reactive RF
sputtering produced an enhancement in sensor response when exposed to NO2 [165].
Additionally, iron addition was found to be advantageous in sensing ozone, CO and
ethanol. NO2 and humidity sensing characteristics of WO3 thin films prepared by
vacuum thermal deposition and subsequent annealing in the temperature range of
300ºC-600ºC were investigated by Xie et al [166]. It was found that NO2 sensing was
strongly dependant on annealing and working temperature. Different WO3
mesoporous structures obtained by hard template route were used by Rossinyl et al
[167] to investigate their sensing response towards NO2. It was found that WO3 was
sensitive to NO2 even at low concentrations, although differences attributable to
different structures were observed. Introduction of copper as catalytic additive
improved both sensor response and response time [167].
Khatko et al [168] investigated the NO2, NH3 and ethanol sensing performance
of WO3 thin films deposited by reactive rf sputtering with interruptions during the
deposition process. Sensitivity was found to increase with increase in number of
interruptions and interruption time, which was attributed to observed grain size
51
reduction during interruption. In another study, the authors observed that the
response of these sensors to ozone is up to four times higher than that of the sensors
prepared using rf sputtering [169]. A high sensitivity to NO2 at a temperature of 50ºC
for a sensor made of WO3 particles of size ~36 nm was reported by Meng et al [170].
In this study, WO3 nanoparticles were prepared by evaporating tungsten filament
under a low pressure of oxygen gas, namely, by gas evaporation method. The
deposition was carried out under various oxygen pressures and samples were
annealed at different temperatures. The sensitivity was found to increase with
decreasing particle size, irrespective of oxygen partial pressure during deposition and
annealing temperature.
Solis et al [171] investigated the H2S response of nanocrystalline WO3 thick
films prepared by evaporation of tungsten metal by an electric arc discharge in
reactive atmosphere. The structure was found to consist of monoclinic and tetragonal
phases with a mean grain size of 40 nm. The influence of sintering temperature on
H2S sensitivity was studied. These films showed excellent sensing properties upon
exposure to low concentrations of H2S in air at room temperature. The conductance
of films sintered at 300ºC was found to increase by a factor of about 104 when
exposed to 10 ppm of H2S. A further rise in sintering temperature resulted in
decrease in sensor response. This effect was attributed to disappearance of the
tetragonal phase which may point at a specific crystal structure being responsible for
unique gas sensing properties of WO3. Nanoparticles and nanoplatelets of WO3 and
nanowires of WO2.72 were investigated for their H2S sensing characteristics by Rout
et al [172]. The WO2.72 nanowires emerged as good candidate for H2S sensors with
little effect of humidity (upto 60% relative humidity) as well as improved response
and recovery times.
52
The electrical response of WO3 based sensors for ozone detection was reported
by Boulmani et al [173]. Thin films (40 nm thick) of WO3 were deposited by rf
reactive magnetron sputtering on SiO2/Si substrate with Pt interdigitated micro
electrodes. The response towards ozone was found to strongly depend on film
morphology which depends on the oxygen concentration during the deposition
process. The sensor response was also affected by bias voltage, sputtering time and
oxygen concentration during deposition.
WO3 thin films with different effective surface area were deposited under
various discharge gas pressures at room temperature by using reactive magnetron
sputtering and their response towards H2 was investigated by Shen et al [174]. It was
observed that effective surface area and pore volume of WO3 thin films increased
with increasing discharge gas pressure. The peak sensitivity for H2 gas was observed
at 300ºC. The results indicate the importance of achieving high effective surface area
on improving the gas sensing performance. The hydrogen response of WO3
nanotextured thin films coated with a 2.5 nm Pt layer was investigated by Yaacob et
al [175]. The films exhibited gasochromic characteristics when tested in visible-NIR
(400-900 nm) range. The total absorbance in this range increased by 15% upon
exposure to 600 ppm H2 in synthetic air and 60% upon exposure to 10,000 ppm H2 in
synthetic air. The films were found to be highly sensitive with stable and repeatable
responses towards low concentrations of H2 at 100ºC. However, the recovery time
was found to be slow at room temperature.
The effect of cerium oxide additive on WO3 nanoparticles prepared by solgel
method towards Volatile Organic Compound (VOC) gases was investigated by Luo
et al [176]. The highest gas response of Ce-added WO3 samples was found to shift to
lower temperatures compared to pure WO3 samples. Grain boundaries were pinned
53
due to CeO2 which resulted in reduction in grain size and increase in surface area.
Complex impedence spectroscopy analysis indicated that grain boundary resistance
increased and grain boundary capacitance decreased with increasing concentration of
CeO2 which indicates that Ce ions mainly exist at WO3 grain boundaries and help to
improve the microstructure.
Tungsten oxide films have also shown a good sensing performance towards
ethanol [177-181]. The sensitivity towards ethanol has been attributed to the
desorption of oxygen at the surface of grains [181].
Carbon monoxide (CO) sensing characteristics of CoOOH-WO3 doped with Au
and SWCNT was investigated by Wu et al [158]. It was found that mixture with a
CoOOH-WO3 ratio of 2:1 had the highest sensor response at room temperature.
Doping with 1 wt% SWCNT and 0.1 wt% Au in CoOOH-WO3 was found to boost
the CO response by 3.6 times. Azad et al [182] investigated the sensing performance
of WO3 towards 100 ppm CO. The authors achieved sensitivity towards CO by
modulating ambient oxygen partial pressure to create oxygen deprivation on the
metal oxide surface. However, WO3 responded to CO only at 450ºC.
2.11 Deposition techniques of nanostructured metal oxide films
For the purpose of gas sensing, coatings or films of nanostructured metal oxides
are deposited on substrate. This section will briefly highlight the different processing
routes for deposition of metal oxide films. The methods can broadly be classified
into two main categories:
1. Thin film processes: Processes such as solgel, spray pyrolysis, physical
vapour deposition and chemical vapour deposition are classified under this
category.
54
2. Thick film processes: Processes such as screen printing and spin coating are
classified under this category.
Fig. 2-9 shows a schematic of the classification of different processing routes for
nanostructured materials. It is to be noted that there is a huge number of deposition
processes and only a few processes are discussed in the following sections.
Figure 2-9: A schematic of the classification of processes used for deposition of nanostructured films.
2.11.1 Thin Film Processes
2.11.1.1 Solgel
The process involves the hydrolysis of a metal organic compound such as a
metal alkoxide or inorganic salts such as chlorides to produce a colloidal solution
[183]. The hydrolysis can take place with the help of alcohol, acid or base. The sol is
then allowed to age and settle. This is called the gelation step. The sol can then be
coated on the substrate by either spin/dip coating to form a 'xerogel' film.
Alternatively, the solvent from the sol can be evaporated to precipitate particles of
uniform size and then these can be screen printed.
General Processing Routes for Nanostructured materials
Thin films Thick films
Wet process Vapour phase deposition
Solgel
Spray pyrolysis
Spin casting
Screen printing CVD PVD
RGTO
55
2.11.1.2 Spray pyrolysis
It involves atomization of a liquid precursor through a series of reactors, where
the aerosol droplets undergo evaporation, solution condensation within the droplet,
drying, thermolysis of the precipitate particle at higher temperature to form a
microporous particle which then gets sintered to give a dense film [184].
2.11.1.3 Chemical vapour deposition (CVD)
Chemical vapour deposition involves exposing a substrate of choice to a mixture
of volatile precursors that react or decompose on the substrate to give the desired
product. A wide variety of CVD techniques that are currently being in use are
Atmospheric pressure CVD (APCVD), Atomic layer CVD (ALCVD), Low pressure
CVD (LPCVD), Metal organic CVD (MOCVD), Microwave plasma assisted CVD
(MPCVD), Plasma enhanced CVD (PECVD) and Molecular Beam Epitaxy (MBE).
2.11.1.4 Physical vapour deposition (PVD)
Physical vapour deposition (PVD) uses physical means as opposed to chemical
vapour deposition techniques. The various techniques used in this method are
evaporation, sputtering and pulsed laser deposition. Thermal evaporation and
electron beam evaporation are classified under the category of evaporation.
The advantage of using CVD and PVD is that they offer an enormous amount of
control over film thickness, stoichiometry, and microstructure. Therefore, highly
controlled properties can be achieved using these methods. This project uses thermal
evaporation technique to deposit nanostructured WO3 thin films. A more detailed
description of thermal evaporation is given below.
56
2.11.2 Thermal evaporation
Thermal evaporation is a physical vapour deposition process in which the atoms
or molecules from a thermal vaporization source in a vacuum chamber reach the
substrate without collisions with residual gas molecules. This type of PVD process
requires a vacuum of better than 10-5 mbar. A high vacuum (10-7 mbar) or ultra high
vacuum (<10-9 mbar) is required if high purity films are desired. A schematic sketch
of the thermal evaporation chamber is shown in Fig. 2-10.
The system essentially consists of vaporization source, substrate holder,
substrate heater, a shutter and a thickness monitor enclosed in a chamber. Thermal
vaporization requires that the surface and generally a large volume of material must
be heated to a temperature where there is an appreciable vapour pressure. Common
heating techniques for evaporation include resistive heating, high energy electron
beams, low energy electron beams, inductive (rf) heating or arc deposition. The most
common way of heating materials that vaporize below about 1500ºC is by contact to
a hot surface that is heated by passing a current [183]. The evaporator must be able to
contain molten liquids without extensive reaction and also the molten liquid must be
prevented from falling from the heated surface. This is accomplished either by using
a container such as a crucible. The heated surface can be in the form of a wire,
usually stranded, boat, basket, etc., which can withstand high temperatures without
melting.
57
Figure 2-10: A typical sketch of thermal evaporation chamber.
Typical resistive heater materials are W, Mo, Ta, C and BN/TiB2 composite
ceramics. Typically, resistive heating is carried out at low voltage and very high
current (several hundreds of amperes) using AC transformer supplies. The vapour
travels from the source to the substrate in a straight line (line of sight). Physical
masks can be used to intercept the flux, producing defined patterns of deposition on
the surface. Thermally vaporized atoms may not always condense when they
impinge on the surface of substrate, instead they can be reflected or re-evaporate. Re-
evaporation is a function of the surface temperature and the flux of depositing atoms.
Hence, the substrate temperature should be carefully selected during deposition.
Deposition rates in vacuum can vary significantly. The rates can range from less than
one monolayer per second (MLS) (<3 Å/s) to more than 104 MLS (>3 µm/s). Various
factors such as power input to the source, system geometry, material, vacuum
Fixture temperature
monitor
Fixture heater
Substrate heater
(radiant)
Vacuum chamber
Thermocouple
Vacuum Guage High Vacuum
pumping Roughing pump
Shutter
Deposition rate
monitor
Substrate
View port
Vaporization source
Fixture rotation tooling
Gas Inlet
High Vacuum guage
58
condition, angle of deposition, substrate temperature and source to substrate distance
affect the deposition rate. In terms of deposition thickness, a uniform deposit over a
planar surface can be obtained by using multiple sources with overlapping patterns as
shown in Fig. 2-10. However this produces source control and flux distribution
problems [185]. By moving the substrate away, the uniformity over a given area can
be improved, but the deposition rate is decreased. The most common technique to
improve uniformity is to move the substrate in a random manner over the vapour
source using different fixture geometries. Thermally evaporated films have a residual
tensile stress. Compressive stresses exist only when deposition is done at very high
substrate temperatures. The thin film microstructure can be controlled by two factors:
the ratio of growth temperature to melting temperature (of the thin film material) and
the amount of added energy. This is depicted in Fig. 2-11 by the famous structure
zone model of Thornton [186], which is often used for thermal evaporation and
sputtering techniques. In case of thermal evaporation, the arrival energies are low
and the thin film structure would follow the high pressure side of the diagram. One
important feature that is often found in vacuum deposition chambers is the relatively
large distance between the heated source and the substrates. This is to minimize the
radiant heating from the source and allows elaborate fixture motion to randomize the
position of the substrate. In thermal evaporation, the temperature of the heating
filament is sufficient enough to sublimate metal or metallic compounds. Hence
semiconducting metallic oxides such as SnO2, TiO2, WO3, etc can be deposited using
thermal evaporation.
The physical and chemical properties of tungsten oxide thin films are largely
affected by a number of factors such as film thickness, substrate type, substrate
temperature, annealing temperature, annealing time and dopant addition [133, 187-
59
190]. Vacuum thermal evaporation offers the benefit of controlling the film
thickness, substrate temperature and co-evaporation of more than one species. Ultra
high purity films can also be achieved by improving the vacuum in the deposition
chamber.
Figure 2-11: Thornton Zone diagram [186].
2.12 Summary
In this chapter, a literature review on various aspects of gas sensors and state of
the art of WO3 based gas sensors is presented. The chapter starts with introduction to
the construction principle of gas sensors, basic characteristics, mechanisms of gas
sensing and limitations of current gas sensors. This is followed by literature review
on role of additives and importance of dimensions in gas sensing. The structural
properties of tungsten oxide and the state of the art in WO3 based gas sensing are
discussed in detail. This is followed by a brief overview of various deposition
techniques for thin film synthesis with emphasis on thermal evaporation.
60
CHAPTER 3 : EXPERIMENTAL METHODS
3.1 Introduction
This chapter is divided into three main sections:
Deposition and doping techniques (evaporation and implantation), which
discuss the details of WO3 thin film preparation (Section 3.2).
Characterization techniques to investigate the physical, chemical and
electronic properties of WO3 thin films (Section 3.3).
Characterization of gas sensing, which discusses the methodology adopted
for testing the sensor devices (Section 3.4).
It is beyond the scope of this thesis to give complete and detailed information on
each technique. Hence, the intent will be to give main information which is required
to understand the research carried out in this study.
3.2 Deposition of nanostructured WO3 thin films by thermal
evaporation
3.2.1 Material
In the present study, 99.9% purity WO3 and Fe powders of sizes 20 μm and 100
μm, respectively, were obtained from Sigma Aldrich Pty Ltd. Before deposition, the
powders were placed in dessicator in order to avoid any moisture and
decontamination. For the purpose of doping, iron was mixed with WO3 and the
mixture was evaporated. The physical properties of as-received WO3 and Fe powders
are listed in Table 3-1.
61
Table 3-1: Physical properties of WO3 and Fe powders.
WO3 Fe Density 7.16 g/mL at 25ºC 7.86 g/mL at 25ºC
Molecular weight 231.85 g/mol 55.85 g/mol
Melting point 1473ºC 1535ºC
3.2.2 Substrate
A gas sensor basically comprises of a sensitive layer deposited on a substrate
provided with two pre-printed electrodes for the measurement of the electrical
characteristics. The sensor device can be heated by its own heater, which is separated
from the sensing layer and the electrodes by an electrically insulating layer. Alumina,
silicon or glass is often used as substrate material. The electrodes are usually made of
Au or Pt, whereas Pt heating element is used for heating purpose. During the recent
years, interdigitated electrodes are being used more extensively than one finger
electrodes (see Fig. 3-1). In this case, instead of one electrode with two fingers, an
array of electrode fingers is used, thereby covering a larger surface area under the
film and improving the response time.
In this study, WO3 films were deposited by thermal evaporation at room
temperature on silicon substrate with interdigitated Pt electrodes (Electronics Design
Center, Case Western Reserve University, Cleveland, USA). A thin SiO2 film (10 nm
thick) was thermally grown on the substrate before printing interdigitated Pt
electrodes on it. The size of the substrate was 8 mm x 8 mm x 0.5 mm. The electrode
fingers have a line width and line thickness of 100 μm and a height of 300 nm,
respectively. A snapshot of the interdigitated Pt electrode on silicon substrate is
shown in Fig. 3-1.
62
Figure 3-1: Interdigitated Pt electrodes on Si substrate.
3.2.3 Film deposition
A bell jar type PVD unit (Varian Coater with AVT Control System, Australia)
was used to deposit pure and Fe-doped WO3 thin films. Prior to deposition, the
chamber was evacuated to 4 x 10-5 mbar. The substrates were mounted on a substrate
holder which was placed at a distance of 38 cm in line of sight from the evaporation
source. Before the actual deposition, a trial deposition was done to determine the
deposition parameters including the power required to evaporate tungsten oxide
powder. The powder was evaporated at about 650 W using constant voltage of 20 V
and variable current. 99.106 g WO3 powder mixed with 0.894 g of Fe was co-
evaporated during the Fe-doping. Deposition of all films was carried out at 4 x 10-5
mbar, 650 W, and the deposition rate of 35 nm per second. A thickness monitor
adopting crystal oscillator was used to adjust the thickness of the films to 300 nm. To
obtain uniformity of deposition, all the films were deposited under the same
conditions of vacuum, power and deposition rate.
3.2.4 Ion implantation
Implantation, which induce changes in the surface composition, morphology and
chemical bond structure, is an effective method to improve physical and chemical
63
properties of the material [191]. It has been reported that the optical band gap of the
films can be reduced by implantation [192].
In order to implant the WO3 thin films with small amount of iron, the ion
implantation facility at Australian Nuclear Science and Technology Organization
(ANSTO) was employed. Samples of WO3 thin film were mounted on a rotating
sample holder with a good electrical contact, which reduced charging effects during
ion implantation. Implantation was performed in a vacuum chamber, which was
evacuated to a base pressure of about 3 × 10-6 mbar. After achieving the required
vacuum, a high current was applied to the Fe cathode to generate Fe ion beam. The
beam was incident normally onto the sample surface at 40 KeV. Uniformity of the
implantation was assured by sweeping the sample across the ion beam.
3.2.5 Post deposition heat treatment of the films
Post deposition heat treatment of the films was carried out in order to improve
the microstructure and crystalline properties of the deposited films. Samples were
annealed at 300ºC and 400ºC in air for a period of 2 hours with a heating and
cooling rate of 2ºC per minute. It has been reported previously that annealing of WO3
film at temperatures below 300oC has not changed the particle size significantly
[128]. Annealing was performed in tube furnace at Microelectronics and Materials
Technology Center, RMIT University.
3.3 Characterization of nanostructured WO3 thin films
In order to characterize the WO3 thin films, various analytical techniques have
been used. Some of the most important techniques used for characterization are
discussed below:
64
3.3.1 Transmission Electron Microscopy (TEM)
Transmission electron microscopy is used to determine the nanostructure of the
films (grain size, structure and crystallinity). In transmission electron microscopy,
the sample is exposed to a beam of highly accelerated electrons. The electrons
interact with the material when they pass through the sample. As TEM works on
transmission principle, the sample thickness should be very small (less than 100 nm).
After interaction, the beam travels through the TEM column and produces an image
on a fluorescent screen and can be imaged using photographic film.
In the present work, a Jeol 1200 TEM was used at an accelerating voltage of 120
kV. Samples were investigated by scratching the film and placing it on TEM grid.
Three main features that have been investigated by this technique are size and shape
of WO3 nanoparticles and crystalline structure in the film.
3.3.2 Atomic Force Microscopy (AFM)
The surface morphology of the WO3 thin films was studied in ambient
conditions using atomic force mircroscopy. An NT-MDT P47 Solver Scanning Probe
Microscope was used for this purpose. This is a powerful technique with high spatial
resolution to study the surface morphology by scanning the surface with a special tip.
The AFM can be operated in different modes depending on sample type and surface
properties. In the present study, the WO3 film surface was scanned by a silicon tip
(radius of curvature 10 nm) in semi-contact mode over an area ranging from 500 nm2
to 2000 nm2. The mean grain size, grain distribution and surface roughness were
determined by using the Nova NT-MDT Image Analysis Software.
65
3.3.3 Rutherford Backscattered Spectroscopy (RBS)
RBS provides information on the profile of concentration as a function of depth
in a film. A beam of 2-3 MeV He+ ions is directed perpendicularly on the sample
surface. The He+ ions lose energy during collisions with electrons and occasionally
with nuclei. When the positively charged He+ ion comes close to the nucleus of an
atom, it is repelled by positively charged nucleus. These repulsive forces increase
with the mass of the target atom. By measuring the energy of the recoiled ions, the
information on the depth profile of the elements can be determined. In the present
study, the Ion Beam Analysis (IBA) facility at Australian Nuclear Science
Technology Organization (ANSTO) has been used to conduct RBS experiments. A
1.8 MeV He+ beam was directed on the film surface under a vacuum of 7 x 10-6
mbar. The concentration profile of constituent elements in WO3 film has been
determined using RBS. The experimental data was fitted using SIMNRA. The
sample was modelled by dividing it into a series of slabs, perpendicular to the normal
of the surface with each slab having a different thickness. The software calculates the
energy of the projectile on each facet of interaction. This is the basis of the calculated
spectrum. Once the spectrum is obtained, the depth profile is presented in terms of
slab concentration and thickness.
3.3.4 X-Ray Photoelectron Spectroscopy (XPS)
X-ray photoelectron spectroscopy (XPS) provides both elemental and chemical
state information of a material. When the sample is illuminated with monochromatic
x-rays, photoelectrons are emitted from the surface. The kinetic energy of these
emitted electrons is characteristic of the element from which the photoelectron
originates. The position and intensity of the peaks in the energy spectrum provide the
desired chemical state and quantitative information. The chemical state of an atom
66
alters the binding energy (BE) of a photoelectron which results in a change in the
measured kinetic energy (KE). The BE is related to the measured photoelectron KE
by the following equation.
)1.3(KEhBE
where h is the photon (x-ray) energy. These chemical shifts help to determine the
chemical or bonding information of elements.
In the present study, XPS analysis was performed using Kratos AXIS Ultra XPS
incorporating a 165 mm hemispherical electron energy analyser and using
monochromatic Al K X-rays (1486.6 eV) at 150 W (15 kV, 10 mA) incident at 45o to
the sample surface. Photoelectron data was collected at take off angle of 90o. Survey
(wide) scans were taken at analyser pass energy of 160 eV and multiplex (narrow)
high resolution scans at 20 eV. Survey scans were carried out over 1200-0 eV
binding energy range with 1.0 eV steps and a dwell time of 100 ms. Narrow high-
resolution scans were run with 0.05 eV steps and 250 ms dwell time. Base pressure
in the analysis chamber was maintained at 1.3 x 10-9 mbar and at 1.3 x 10-8 mbar
during sample analysis. Depth profiling of the film was also carried out by etching
the surface with an Ar source at a rate of 10 nm per second.
3.3.5 X-Ray Diffraction (XRD)
X-ray diffraction is widely used to characterize crystalline materials. The
technique gives information about crystallinity, orientation, composition, phases,
internal lattice strain and particle size. A schematic of X-ray diffraction is shown in
Fig. 3-2. The diffraction is represented by Bragg’s law as follows:
)2.3(2 rhkl nSind
Where dhkl is interplanar distance,
67
is the angle of reflection,
nr is the order of reflection and
is the wavelength of the incoming wave.
Figure 3-2: Schematic of X-ray diffraction.
For thin film analysis, grazing incident x-ray diffraction analysis (GIXRD) is
performed. In GIXRD measurement, the angle of incidence of the x-rays with respect
to the sample surface is kept fixed at a very low value. This value has to be higher
than the critical angle of total external reflection to ensure penetration of the x-rays
into the thin film. These measurements are performed in a parallel beam geometry,
which makes it non-sensitive to sample height displacement. Since the penetration
depth is significantly reduced due to low angle of incidence, x-ray beam is confined
within the thin film. The GIXRD technique is employed in this project as the film
thickness is very small (300 nm) and it is important to confine the x-ray beam within
the film.
In the present study, XRD analysis of WO3 thin films was performed on
PANanalytical XPert Pro Multi Purpose Diffractometer (MPD). A Cu K radiation of
wavelength 1.540 Å was used. The incident angle was kept at 2o and the 2 range
was kept between 10o to 85o with a step size of 0.05o.
dhklsindhkl
Incoming x-rays
Scatterd x-rays
68
3.3.6 Raman spectroscopy
Raman spectroscopy is a powerful tool to monitor the intensity and wavelength
of light that is scattered inelastically from molecules or crystals. It is used to
determine the chemical structure and physical state of the film. Raman effect occurs
when light strikes molecules and interacts with the electrons of the bonds of those
molecules. Subsequently, Raman scattering occurs due to these interactions and the
wavelength of the scattered light is shifted with respect to the wavelength of the
incident light. The change in wavelength is determined by analysing the spectrum of
the scattered light.
From the Raman spectrum, the wave numbers of Raman shifts are plotted
against their respective intensities generated from the interaction between phonons
and molecular vibrations. These phonon modes are related to the chemical bonding.
Therefore, information on molecules, bonds or defects can be obtained. Thus, this
technique can be applied to the identification of the type of bonds as well as
crystallinity in WO3 films.
In the present study, Raman measurements were performed using an
Oceanoptics QE 6500 spectrometer. A 532 nm line from an argon ion laser was used
as the excitation source. To avoid local heating of the samples, a power as low as
about 5 mW was incident onto the samples. A Raman shift between wavenumbers
200 cm-1 and 1200 cm-1 has been measured.
3.4 Gas sensing characterization
WO3 gas sensors prepared by the author were exposed to hydrogen (H2), ethanol
(C2H5OH) and carbon monoxide (CO). These gases at relatively low concentrations
(TLV: CO: 25 ppm (toxicity); ethanol: 200 ppm (toxicity), 6.7% (explosive limit);
69
H2: 4% (explosive limit)) [8] are considered harmful, not only to human life but to
the environment.
The primary physical hazards associated with hydrogen gas are its flammability
and potential for explosions. It forms flammable mixtures in air over a wide range of
concentrations and very low energy is needed to ignite hydrogen-air mixtures [193].
Hence, there is a need to detect hydrogen leaks to avoid any possible explosion.
Carbon monoxide is a major industrial gas that has significant fuel value. However,
CO is a toxic chemical, which is harmful to human health. Inhalation of CO can
cause interruptions in normal supply of oxygen in the bloodstream and influence
vital functions of the body [194]. Therefore, it requires early detection to prevent
health hazards. Ethanol is a major breakdown product of foodstuff where bacteria or
fungi develop. Sensitive ethanol sensors are necessary if the onset of spoilage is to be
detected as early as possible and remedial measures can be taken [195]. Stable
ethanol sensors can also be used as breathalysers to monitor human breath [196]. In
the present study, the WO3 sensor responses to various concentrations (10-10,000
ppm) of the above mentioned gases at various operating temperatures (100ºC to
300ºC) were measured. The relative humidity (RH) in the testing chamber during
the whole measurement was kept at zero (0% RH). All the gases were diluted in
synthetic air to achieve the desired concentrations. For all the experiments, the total
flow was adjusted to 200 sccm. Table 3-1 lists the gases and their concentrations
used in the present study.
Table 3-2: Target gases and their concentrations.
Target Gas Concentrations (ppm)
Hydrogen (H2) 600, 1250, 2500, 5000, 7500, 10000 Ethanol (C2H5OH) 12, 33, 67, 185
Carbon monoxide (CO) 50, 100, 250, 500, 1000
70
The sensor performance is typically expressed in terms of response parameters
such as response amplitude and response and recovery times. Many authors use the
term ‘sensitivity’ to indicate response amplitude. However, in a true sense,
sensitivity of a gas sensor is defined as the derivative of the response to the gas
concentration [11]. Therefore, keeping in view the general trend of using the
definitions, the term ‘sensitivity’ would be used to indicate the response amplitude of
the sensors in this study.
The sensitivity for reducing gases such as H2, ethanol and CO, denoted as
Sreducing is defined as the ratio of change in resistance to resistance in gas:
)3.3(gas
reducing R
RS
where Rair is the resistance in air under stationary conditions and Rgas represents the
resistance after the sensor is exposed to the target gas during a definite time.
Equation 3.3 can be applied ton-type material such as WO3 and reducing gases such
as H2, ethanol and CO. For the same type of material and oxidising gases, the
sensitivity is defined as:
)4.3(air
oxidizing R
RS
Fig. 3-3 shows the schematic diagram of the gas chamber setup. The gas cell was
built using 20 mm thick machined Teflon blocks and a fused quartz lid. The total cell
volume is approximately 30 mm2. The small volume of the gas chamber ensures
short gas equilibrium times within the chamber, and hence, the response times can be
considered to be the true response of the gas sensors. The sensor device was mounted
on a planar alumina micro-heater. The external power supply controlled the
71
operating temperature upto 400ºC. This gas chamber was then enclosed by an
environmental chamber to maintain 0% humidity and ambient temperature.
The response curve was recorded under a continuous flow of known amount of
target gas. This method is known as Flow through method [8]. Fig. 3-4 shows the
concept of the flow through method. The concentration of the gas is controlled by
mixing it with synthetic air using mass flow controllers. For recovery measurements,
the mass flow controller of the target gas is switched off. The sensor response can be
recorded repeatedly as a function of different concentrations.
Figure 3-3: Schematic diagram of gas chamber setup.
Fused quartz lid
Teflon
Printed circuit board
Gas Inlet
Gas Outlet
O-ring seal
Heater
Sensor
72
Figure 3-4: Schematic of sensor testing by flow through method.
In the present study, a sequence control computer was utilized to computerize
the pulse sequence of the target gas concentrations. Initially, synthetic air was passed
through the chamber at testing temperature until the stable baseline resistance was
observed. Then a sequence of target pulse for 10 minutes followed by synthetic air
pulse. This procedure was continued until a stable baseline was observed after
alternate pulses. This was followed by the experimental sequence of pulses and data
was recorded. Each sensor was tested at temperatures between 100ºC to 300ºC at
intervals of 50ºC under various concentrations of mentioned gases, and optimum
operating temperature of each sensor was determined. This was followed by two full
range tests for each sensor and corresponding gas at the optimum operating
temperature.
Chamber
Exhaust
Resistance measurement
and heater power
Sensor
Mass Flow Controller
Mass Flow Controller
Car
rier
gas
Tes
t gas
Control PC
73
3.5 Summary
This chapter describes the processes and procedures used in the deposition and
characterization of pure, Fe-doped and Fe-implanted WO3 thin films. Thermal
evaporation and ion implantation techniques were used to synthesize these films as
described in Section 3.2 of this chapter. This is followed by Section 3.3 which
describes the various analytical techniques employed to investigate the physical,
chemical and electronic properties of these films. The last section (Section 3.4)
describes the gas sensing setup and procedure employed in this study to investigate
the gas sensing performance of these films.
74
CHAPTER 4 : THIN FILM CHARACTERIZATION
4.1 Introduction
The physical, chemical and electronic properties of nanostructured metal oxide
thin films are strongly dependant on the deposition techniques, deposition
parameters, composition of the metal oxide and post-deposition processes (e.g. heat
treatment). These also directly affect the gas sensing properties and performance of
the metal oxides gas sensors. Thus, to understand the various properties of
nanostructured tungsten oxide thin films synthesized in this research work, it is
necessary to characterize their physical, chemical and electronic properties.
This chapter is aimed at presentation and discussion of results obtained by
various characterization techniques that have been employed to investigate the
various properties of thermally evaporated WO3 thin films. Following are film
properties characterized using different techniques:
1. Structural properties of films characterized by:
- Atomic Force Microscopy (AFM).
- Transmission Electron Microscopy (TEM).
- X-ray diffraction (XRD).
2. Chemical and Electronic properties of films characterized by:
- Raman spectroscopy.
- X-ray photoelectron spectroscopy.
3. Compositional properties of film characterized by:
- Rutherford backscattered spectroscopy (RBS).
- Electron Diffraction (EDX).
75
In this work, the author has extensively employed the above techniques to
characterize nanostructured tungsten oxide thin films. The characterization results
are subsequently linked with their gas sensing properties in Chapter 5. For the
purpose of identification, the various films are designated as follows:
WO3 film (pure WO3 film).
Fe-doped WO3 film.
Fe-implanted WO3 film.
4.2 Characterization of Structural Properties
4.2.1 AFM and TEM analysis
The AFM surface topography of as-deposited WO3 film (Fig. 4-1a) shows a
nanostructured surface with well defined grains of mean size and roughness of ~13 nm
and 0.5 nm, respectively, as obtained by NT-MDT Nova Image Analysis Software.
The TEM image of the film (Fig. 4-1b) reveals a dense structure and grain size of
about 10-15 nm. The broad and hazy ring around the center spot in selected-area
diffraction pattern (SADP) indicates highly amorphous nature of the WO3 film (inset in
Fig. 4-1b). Annealing of the as-deposited WO3 film at 300ºC for 2 hours in air resulted
in the reduction of the grain size from 13 nm to 10 nm as shown by the AFM (Fig. 4-
2a), but no change was observed in roughness. The TEM image (Fig. 4-2b) shows a
very compact and dense film. The halo diffraction pattern (inset in Fig. 4-2b) indicates
that the film is still amorphous. Annealing of WO3 films at 400ºC in air for 2 hours
resulted in crystalline grain growth with well defined grains of mean size of ~ 5 nm
(Fig. 4-3). The film appears to be loosely packed after annealing at 400ºC. The bright
spots in the diffraction pattern (inset in Fig. 4-3) shows that the film is highly
crystalline.
76
Figure 4-1: AFM semicontact mode image (a) and TEM image (b) of as-deposited nanostructured WO3 film.
Figure 4-2: AFM semicontact mode image (a) and TEM image (b) of nanostructured WO3 film
annealed at 300ºC for 2 hours in air.
Figure 4-3: TEM image of nanostructured WO3 film annealed at 400ºC for 2 hours in air.
77
The AFM surface topography of as-deposited Fe-doped WO3 film is shown in Fig.
4-4. The surface reveals well defined grain boundaries with an average grain size of 15
nm (Fig. 4-4a). However, the grains appear to be densely packed as compared to the
pure WO3 film (Fig. 4-1a). Addition of iron also resulted in an increase in roughness to
0.6 nm as compared to 0.5 nm for as-deposited WO3 film. The TEM image (Fig. 4-4b)
reveals a compact structure and a mean grain size of about 15 nm, which is consistent
with the AFM results. The halo pattern in the SADP indicates the amorphous nature of
Fe-doped WO3 film (inset in Fig. 4-4b).
Figure 4-4: AFM semicontact mode image (a) and TEM image (b) of as-deposited nanostructured Fe-
doped WO3 film.
The surface topography of Fe-doped WO3 film annealed at 300ºC for 2 hours in
air is shown in Fig. 4-5. Annealing at 300ºC decreased the grain size from 15 nm to
10 nm (Fig. 4-5a) and surface roughness from 0.6 to 0.5 nm. Doping with iron and
subsequent annealing at 300ºC results in a topography whcih is similar to the pure
WO3 film annealed at 300ºC (Fig. 4-2a). The TEM image of this film (Fig. 4-5b)
shows a densely packed structure. The film appears to be significantly amorphous, as
it is evidenct from dominant continuous rings (inset in Fig. 4-5b).
78
Figure 4-5: AFM semicontact mode image (a) and TEM image (b) of nanostructured Fe-doped WO3
film annealed at 300ºC for 2 hours in air.
Fig. 4-6 shows the TEM image of Fe-doped WO3 film annealed at 400ºC for 2
hours in air. A mean grain size of the order of 5-10 nm is observed. Annealing at
400ºC induced significant crystallinity in the film, indicated by bright spots in the
diffraction pattern (inset in Fig. 4-6). The film appears to have less dense structure
upon annealing at 400ºC, which was not observed in as-deposited or 300ºC annealed
Fe-doped WO3 films. The crystallinity of this film is similar to that of 400oC annealed
WO3 film but with a slightly larger grain size.
Figure 4-6: TEM image of nanostructured Fe-doped WO3 film annealed at 400ºC for 2 hours in air.
79
The surface topography of Fe-implanted WO3 film shows a nanostructured
surface with a grain size of less than 10 nm and a mean roughness of 0.2 nm (Fig. 4-
7a). The image was obtained in phase contrast mode as the semicontact topography
mode was unable to resolve the surface. The film shows a densely packed surface
and a very small grain size. The TEM image (Fig. 4-7b) also shows a very dense and
compact film characterized by a highly amorphous nature, as observed by the
diffraction pattern (inset in Fig. 4-7b).
Figure 4-7: AFM semicontact mode image (a) and TEM image (b) of nanostructured Fe-implanted
WO3 film.
Upon annealing the Fe-implanted WO3 film at 300ºC for 2 hours in air, a slight
improvement in surface topography, characterized by small clusters consisting of few
grains (Fig. 4-8) is observed. The grain size is less than 10 nm and roughness is
reduced to 0.1 nm. The TEM image (Fig. 4-8b) shows a dense and compact film with
a highly damaged surface. The film appears to be predominantly amorphous, as
observed from the diffraction pattern (inset in Fig. 4-8b). Fig. 4-9 shows the TEM
image of Fe-implanted WO3 film annealed at 400ºC for 2 hours in air. The film
appears to be very dense and highly amorphous as indicated by the inset in Fig. 4-9.
The grain size could not be quantified from TEM analysis in this case.
80
Figure 4-8: AFM semicontact mode image (a) and TEM image (b) of nanostructured Fe-implanted
WO3 film annealed at 300ºC for 2 hours in air.
Figure 4-9: TEM image of nanostructured Fe-implanted WO3 film annealed at 400ºC for 2 hours in air.
4.2.2 GIXRD analysis
The GIXRD patterns of WO3 films before and after annealing (at 300ºC and
400ºC) are shown in Fig. 4-10. The as-deposited and 300ºC annealed films do not
show any diffraction peak (Fig. 4-10a,b), which indicates that as-deposited WO3 film
is highly amorphous and annealing this film at 300ºC for 2 hours in air did not induce
any crystallinity in the film. This is consistent with the TEM observations. However,
81
significant crystallinity is observed in the film after annealing at 400ºC for 2 hours in
air (Fig. 4-10c).
0
50
100
150
200
10 20 30 40 50 60 70 80
Inte
ns
ity
(AU
)
2(degree)
(a) as-deposited WO3
0
50
100
150
200
10 20 30 40 50 60 70 80
Inte
nsi
ty (
AU
)
2 (degree)
(b) WO3 annnealed @ 300oC
0
200
400
600
800
1000
10 20 30 40 50 60 70 80
Inte
nsi
ty (
AU
)
2 (degree)
(c) WO3 annealed @ 400oC(2
00)
(111
)
(220
)
(22
1)
(112
)
(420
)
(312
)
Figure 4-10: GIXRD pattern of nanostructured WO3 film, (a) as-deposited, (b) annealed at 300ºC in air
for 2 hours and (c) annealed at 400ºC in air for 2 hours.
The peaks observed at 2 = 24.112º, 28.538º, 34.361º, 41.615º, 49.843º, 55.684º,
61.941º are closely associated with the monoclinic WO3 phase [197]. It should be
noted that the lattice parameters of orthorhombic WO3 phase are very similar to
monoclinic phase, and thus, these two phases cannot be distinguished within the
accuracy of GIXRD data. The two intense peaks usually observed at 2=24.278° and
34.117° belong to (2 0 0) and (2 2 0) monoclinic planes, corresponding to d=3.663 Å
82
and 2.626 Å, respectively [198]. The lattice parameters were found to be a = 7.375 Å,
b = 7.375 Å and c = 3.903 Å and its unit lattice volume is about 212.38 Å3.
The GIXRD patterns of Fe-doped WO3 films before and after annealing at 300ºC
and 400ºC for two hours in air are shown in Fig. 4-11. The as-deposited and 300ºC
annealed Fe-doped WO3 films do not show any diffraction peak (Fig. 4-11a,b),
which indicates that as-deposited Fe-doped WO3 film is highly amorphous and
annealing the Fe-doped WO3 film at 300ºC for 2 hours in air did not induce
crystallinity in the film. This result is consistent with the TEM observations.
However, after annealing at 400ºC, significant crystallinity is indicated by the
diffraction peaks in GIXRD pattern (Fig. 4-11c). The peak positions of 400ºC
annealed Fe-doped WO3 film closely match with that of 400ºC annealed WO3 film
(Fig. 4-12). The lattice parameters were found to be a = 7.372 Å, b = 7.372 Å and c =
3.897 Å and its unit lattice volume is about 211.87 Å3. The observed matching of
GIXRD pattern of WO3 and Fe-doped WO3 film can be explained from a
crystallographic viewpoint. The ionic radius of W6+ (0.62 Å) is similar to that of Fe3+
(0.64 Å). Moreover, the W6+ is octahedrally coordinated with O2-. In iron oxides, the
crystal field stabilization energy of Fe3+ is higher for octahedral orientation than for
tetrahedral orientation [199]. Therefore, Fe3+ can fulfil the same coordination as that
of W6+. Consequently, Fe-doped WO3 film shows the same crystal structure as that
of WO3 film. Similar crystal structures were also observed between pure ZnO and
Fe-doped ZnO by Han et al [200]. The observed shift in peak positions of Fe-doped
WO3 film compared to WO3 film, although very little (0.02 Å), can be attributed to
the ionic radius of W6+ and Fe3+.
83
0
50
100
150
200
10 20 30 40 50 60 70 80In
ten
sit
y (A
U)
2 (degree)
(a) as-deposited Fe-doped WO3
0
50
100
150
200
10 20 30 40 50 60 70 80
Inte
ns
ity
(AU
)
2 (degree)
(b) Fe-doped WO3 annealed @ 300 oC
0
200
400
600
800
1000
10 20 30 40 50 60 70 80
Inte
ns
ity
(AU
)
2 (degree)
(c) Fe-doped WO3 annealed @ 400oC(200)
(111
)
(220
)
(221
)
(112
)
(420
)
Figure 4-11: GIXRD pattern of nanostructured Fe-doped WO3 film, (a) as-deposited, (b) annealed at
300ºC for 2 hours in air and (c) annealed at 400ºC for 2 hours in air.
0
500
1000
1500
2000
10 20 30 40 50 60 70 80
Inte
ns
ity
(AU
)
2 (degree)
Fe-doped WO3 annealed at 400 oC
WO3 annealed at 400 oC
Figure 4-12: Comparison of GIXRD patterns of nanostructured WO3 and Fe-doped WO3 films
annealed at 400ºC for 2 hours in air.
84
The ionic radius of Fe3+ is slightly greater than that of W6+ and this can cause slight
distortion in the crystal lattice when WO3 is doped with Fe, and consequently a shift
the diffraction peaks. Such distortions can also produce a number of defects in the
film, making it a better candidate for gas sensing.
The GIXRD patterns of Fe-implanted WO3 films before and after annealing are
shown in Fig. 4-13. The implanted film appears to be highly amorphous (Fig. 4-13a)
as indicated by the diffraction pattern, where no characteristic peaks of WO3 are
observed. Upon annealing at 300ºC, there is no change in crystallinity of the film
(Fig. 4-13b). However, after annealing at 400ºC, the two characteristic peaks of
monoclinic planes of WO3 are observed (Fig. 4-13c). These two peaks observed at
2=24.025º and 33.875º belong to (2 0 0) and (2 2 0) monoclinic planes of WO3,
respectively [198]. Annealing at 400ºC appears to have induced a slight crystallinity
in the film, however, the crystallinity achieved in pure WO3 and Fe-doped WO3 films
after annealing at 400ºC could not be achieved in case of Fe-implanted WO3 film.
4.2.3 Summary and outlook
The as-deposited pure, Fe-doped and Fe-implanted WO3 thin films synthesized
by thermal evaporation are amorphous. The mean grain size in pure and Fe-doped
films is between 10-15 nm. The grain size of Fe-implanted film could not be
quantified due to its highly amorphous and damaged film structure. AFM, TEM and
GIXRD analyses have shown that annealing of these films at 300ºC for 2 hours in air
could not induce any crystallinity, indicating that the films are predominantly
amorphous. However, the grain size of the pure WO3 film reduced from 13 to 10 nm
and that of Fe-doped films from15 nm to 10 nm. Both the pure and Fe-doped WO3
films showed no change in porosity after annealing at 300oC. Annealing at 400ºC in
air for 2 hours significantly improved the crystallinity and porous structure in the
85
pure and Fe-doped WO3 thin films without any significant change in grain size.
However, Fe-implanted WO3 thin film annealed at this temperature (400ºC) remains
essentially amorphous. This is attributed to the highly damaged film structure caused
by implantation.
0
50
100
150
200
250
10 20 30 40 50 60 70 802 (degree)
Inte
nsi
ty (
AU
) (a) Fe-Implanted WO3
0
50
100
150
200
10 20 30 40 50 60 70 802 (degree)
Inte
nsi
ty (
AU
)
(b) Fe-Implanted WO3 annealed @ 300oC
050
100150200250300350400
10 20 30 40 50 60 70 80
Inte
nsi
ty (
AU
)
2 (degree)
(c) Fe-Implanted WO3 annealed @ 400oC
(200)
(220)
Figure 4-13: GIXRD pattern of nanostructured Fe-implanted WO3 film, (a) as-deposited, (b) annealed
at 300ºC for 2 hours in air and (c) annealed at 400ºC for 2 hours in air.
86
4.3 Characterization of chemical and electrical properties
4.3.1 Raman spectroscopy
The Raman spectra of as-deposited and annealed WO3 films are shown in Fig. 4-
14. Two characteristic bands are associated with WO3. The first band lies between
200-500 cm-1 and is associated with O-W-O bending vibration modes. The second
band lies in the range 600-1000 cm-1 and is associated with W-O stretching vibration
modes. The as-deposited WO3 film exhibited week and broad Raman band centred at
315 cm-1 and 799 cm-1. These features are characteristic of amorphous materials and
are usually assigned to O-W-O deformation modes and O-W-O stretching vibration
modes of monoclinic WO3 phase, respectively [201]. The amorphous nature of as-
deposited WO3 thin films is consistent to the results obtained by TEM and GIXRD
observations.
The film annealed at 300ºC also appears to be amorphous with a slight
broadening of peak at 315 cm-1. However, the crystallinity of this film increased after
annealing at 400ºC, as shown by the sharp peaks at 707 cm-1 and 799 cm-1 which are
characteristic of O-W-O stretching vibration modes [70], corresponding well with the
results obtained by GIXRD and TEM analysis.
The Raman spectra of as-deposited and annealed Fe-doped WO3 films are
shown in Fig. 4-15. The as-deposited Fe-doped WO3 thin film exhibited very weak
and broad Raman bands centered at 320 cm-1 and 804.4 cm-1, which are associated to
O-W-O deformation vibration modes and O-W-O stretching vibration modes,
respectively [201]. These weak and broad bands are indicative of the amorphous
nature of as-deposited Fe-doped WO3 thin film which is also confirmed by TEM and
GIXRD observations.
87
2000
2500
3000
3500
4000
4500
5000
5500
6000
200 400 600 800 1000 1200
WO3 annealed @ 300oC
WO3 annealed @ 400oC
as-deposited WO3
Inte
ns
ity
(AU
)
Raman shift (cm-1)
Si
Si
O-W-Odeformationmodes
O-W-Ostretchingmodes
799
707
315
Figure 4-14: Raman spectra of nanostructured WO3 films.
2400
2600
2800
3000
3200
3400
3600
200 400 600 800 1000 1200
as-deposited Fe-doped WO3
Fe-doped WO3 annealed @ 300oC
Fe-doped WO3 annealed @ 400oC
Inte
ns
ity
(AU
)
Raman shift (cm-1)
804.
4
712.
6320
394
O-W-Odeformationmodes
O-W-Ostretchingmodes
Si
Si
Figure 4-15: Raman spectra of nanostructured Fe-doped WO3 films.
Upon annealing at 300ºC for 2 hours in air, the intensity of these bands increased
slightly, indicating the onset of crystallinity growth of the film. Upon annealing at
400ºC for 2 hours in air, the peak intensity of O-W-O stretching modes at 712.6 cm-1
and 804.4 cm-1 increased significantly, indicating that the film is highly crystalline,
which corresponds well with the TEM and XRD observations. The O-W-O
stretching vibration mode peak positions of WO3 and Fe-doped WO3 films annealed
88
at 400ºC are compared in Table 4.1. A slight blue shift of about 5.5 cm-1 is observed
for both the peaks after doping with Fe. Such shifts are caused by shortening of O-
W-O bonds [202], which corresponds to slightly smaller cell parameters of Fe-doped
WO3 film as compared with WO3 film. The GIXRD analysis has shown that the
lattice parameters of Fe-doped WO3 film are slightly smaller than WO3 film.
However, the octahedral orientation of WO3 has been retained after doping with Fe,
indicating that the preferred oxidation state of Fe is Fe3+. This is evident from similar
XRD patterns of WO3 and Fe-doped WO3 films annealed at 400ºC and absence of
any Raman peaks associated with Fe in 400ºC annealed Fe-doped WO3 film.
Table 4-1: Comparison of the positions of O-W-O stretching vibration mode peaks observed for nanostructured WO3 and Fe-doped WO3 films annealed at 400ºC for 2 hours in air.
Raman Peak position (cm-1)
WO3 annealed at 400ºC 707 799 Fe-doped WO3 annealed at 400ºC 712.6 804.4
Blue Shift (cm-1) 5.5 5.5
Figure 4-16 shows the Raman spectra of Fe-implanted WO3 films. The as-
deposited film appears to be highly amorphous as no characteristic Raman peaks are
observed. Annealing the Fe-implanted film at 300ºC did not induce any crystallinity
in the film. However, after annealing at 400ºC, characteristic Raman peaks at 792
cm-1 and 706 cm-1 are observed. However, the intensity of these peaks is smaller than
those observed for WO3 and Fe-doped WO3 thin films annealed at 400ºC. This
indicates that the film is essentially amorphous even after annealing at 400ºC as
confirmed by GIXRD analysis.
89
2000
2500
3000
3500
4000
4500
5000
5500
6000
200 400 600 800 1000 1200
Fe-Implanted WO3
Fe-Implanted WO3 annealed @ 300oC
Fe-Implanted WO3 annealed @ 400oC
Inte
ns
ity
(AU
)
Raman shift (cm-1)
792
706
Si
Si
Figure 4-16: Raman spectra of nanostructured Fe-implanted WO3 films.
4.3.2 XPS analysis
Fig. 4-17 shows the XPS spectra obtained by wide survey scans on the surface of as-
deposited and annealed (300ºC and 400ºC) WO3 films between binding energies 0
and 1200 eV. Survey scan information is useful in identification of elements present
on the film surface. Peaks of O, N, C and W are observed in all the films. Presence of
carbon and nitrogen on the surface is attributed to atmospheric contamination. The C
peak measured at binding energy of 284.80 eV coincides with C 1s binding energy
reported in literature [203] and is, thus, used as a point for binding-energy reference.
Fig. 4-18 shows the XPS spectra of as-deposited and annealed (at 300ºC and 400ºC)
Fe-doped WO3 films between binding energies 0 and 1200 eV. Characteristic peaks
of O, N, C and W are observed as in the case of the pure WO3 film. However, no
characteristic peak of Fe was observed on the surface of all the films. The sensitivity
of XPS is only about 0.01 at.% over a depth of 10 nm. From RBS analysis, which is
discussed in the next section, the amount of Fe is only about 0.016 at.% over 10 nm
90
depth, which is much less than the critical amount that XPS can detect. This might
indicate the non-uniform deposition of Fe in the WO3. Hence, no Fe was observed on
the surface of the Fe-doped film as analysed using XPS.
0
5x104
1x105
1.5x105
2x105
2.5x105
3x105
020040060080010001200
as-deposited WO3
Co
un
ts/s
eco
nd
(A
U)
Binding energy (eV)
W 4dC 1s
N 1s
W 4f
O 1s (a)
0
5x104
1x105
1.5x105
2x105
2.5x105
3x105
020040060080010001200
Co
un
ts/s
eco
nd
(A
U)
Binding energy (eV)
W 4dC 1s
N 1s
W 4f
O 1sWO3 annealed @ 300oC (b)
0
5x104
1x105
2x105
2x105
020040060080010001200
Co
un
ts/s
eco
nd
(A
U)
Binding Energy (eV)
O 1s
N 1sC 1s
W 4d
W 4f
WO3 annealed @ 400oC (c)
Figure 4-17: XPS wide spectra of nanostructured WO3 films, (a) as-deposited, (b) annealed at 300ºC for
2 hours in air and (c) annealed at 400ºC for 2 hours in air.
91
0
5x104
1x105
2x105
2x105
3x105
3x105
020040060080010001200
Co
un
ts/s
eco
nd
(A
U)
Binding energy (eV)
W 4dC 1sN 1s
W 4f
O 1sas-deposited Fe-doped WO3 (a)
0
5x104
1x105
2x105
2x105
3x105
020040060080010001200
Co
un
ts/s
eco
nd
(A
U)
Binding energy (eV)
W 4dC 1s
N 1s
W 4fO 1s
Fe-doped WO3 annealed @ 300oC (b)
0
5x104
1x105
2x105
2x105
3x105
020040060080010001200
Co
un
ts/s
eco
nd
(A
U)
Binding Energy (eV)
W 4fO 1s
N 1s
W 4d
C1s
Fe-doped WO3 annealed @ 400oC (c)
Figure 4-18: XPS wide spectra of nanostructured Fe-doped WO3 films, (a) as-deposited, (b) annealed at
300ºC for 2 hours in air and (c) annealed at 400ºC for 2 hours in air.
The XPS spectra of Fe-implanted WO3 films are shown in Fig. 4-19. Characteristic
peaks of O, N, C, W and Fe are observed in all the films.
92
0
5x104
1x105
2x105
2x105
3x105
020040060080010001200
Co
un
ts/s
eco
nd
(A
U)
Binding energy (eV)
W 4dC 1s
N 1s
W 4f
O 1sFe-Implanted WO3
Fe 2p
(a)
0
1x105
2x105
3x105
4x105
020040060080010001200
Co
un
ts/s
eco
nd
(A
U)
Binding energy (eV)
W 4dC 1sN 1s
W 4f
O 1s
Fe-Implanted WO3 annealed @ 300oC
Fe 2p
(b)
0
1x105
2x105
3x105
4x105
020040060080010001200
Fe-Implanted WO3 annealed @ 400oC
Co
un
ts/s
eco
nd
(A
U)
Binding Energy (eV)
O 1s
C 1sN 1s
W 4fW 4d
Fe 2p
(c)
Figure 4-19: XPS wide spectra of nanostructured Fe-implanted WO3 films, (a) as-deposited, (b)
annealed at 300ºC for 2 hours in air and (c) annealed at 400ºC for 2 hours in air.
For the as-deposited WO3 film, the core level spectra of W 4f are observed at
binding energy Eb of 35.74 eV and 37.88 eV corresponding to W 4f7/2 and W 4f5/2,
respectively (Fig. 4-20). The Eb value for W 4f7/2 reported in literature is 35.8 eV
[204]. The measured value of 37.88 eV is in good agreement with those of WO3
powder, electron beam evaporated and electrodeposited WO3 films [205]. The W 4f
peak shapes get sharper with increasing annealing temperature, which indicates that
93
the surface becomes cleaner due to desorption of surface contaminants by annealing.
The broadening of peaks is associated with change in stoichiometry of the sample
surface, with the formation of different oxides such as WO2 or WO [206]. The W
4f7/2 peak of metallic tungsten is located at 31.50 eV [207]. The W 4f7/2 peaks located
at +4.5, +3 and +1.5 eV from the metallic tungsten W 4f7/2 peak are attributed to
W6+, W5+ and W4+ electronic states, respectively [208]. No significant change in
binding energy of W 4f7/2 is observed when the WO3 film is annealed at 300ºC.
However, annealing at 400ºC resulted in lowering of W 4f7/2 binding energy by 0.3
eV, indicating presence of mixed tungsten states [209]. The downshift of W 4f7/2
peak can be explained by the fact that, if an oxygen vacancy exists in the film, the
electronic density near its adjacent W atom increases. The 4f level binding energy is
expected to be at lower binding energy as the screening of its nucleus is higher
because of increased electronic density [210]. Oxygen vacancies play an important
role as adsorption sites for gaseous species and eventually, a minor shift of the
binding energy may imply greatly enhanced gas sensitivity [56].
0
1x104
2x104
3x104
4x104
5x104
6x104
323436384042
as-deposited WO3
WO3
annealed @ 300oC
WO3 annealed @ 400oC
Co
un
ts/s
eco
nd
(A
U)
Binding Energy (eV)
W 4f5/2
35.7437.88
W 4f7/2
Figure 4-20: W 4f core level high resolution spectra of nanostructured WO3 films.
94
0
1x104
2x104
3x104
4x104
5x104
6x104
7x104
32343638404244
as-deposited Fe-doped WO3
Fe-doped WO3 annealed @ 300oC
Fe-doped WO3 annealed @ 400oC
Co
un
ts/s
ec
on
d (
AU
)
Binding Energy (eV)
35.8
35.7
35.63
W 4f5/2
W 4f7/2
Figure 4-21: W 4f core level high resolution spectra of nanostructured Fe-doped WO3 films.
The W 4f core level high resolution spectra of as-deposited and annealed (at
300ºC and 400ºC) Fe-doped WO3 films is shown in Fig. 4-21. For the as-deposited
Fe-doped WO3 film, the W 4f7/2 and W 4f5/2 peaks are observed at 35.80 eV and
37.95 eV, respectively, closely matching with the values reported in literature [205] .
Similarly to the pure WO3 thin film, the W 4f peaks get sharper with increasing
annealing temperature, which indicates that the surface becomes cleaner due to
desorption of surface contaminants by annealing. The W 4f7/2 peak in Fe-doped WO3
film is located at +4.3 eV with respect to the 4f7/2 peak of metallic tungsten located at
31.5 eV. This indicates the formation of mixed W states upon doping with Fe [208].
Compared with the pure WO3 film (W 4f7/2 peak located at +4.2 eV from metallic
tungsten 4f7/2 peak), doping with Fe appears to have altered the stoichiometry
towards nominal WO3 (+4.3 eV). No significant change in binding energy of W 4f7/2
is observed when this film is annealed at 300ºC. However, annealing at 400ºC
resulted in lowering of W 4f7/2 binding energy by about 0.2 eV, indicating the
95
presence of mixed tungsten states and a corresponding increase in number of oxygen
vacancies on the surface [210].
0
1x104
2x104
3x104
4x104
5x104
6x104
7x104
323436384042
Fe-Implanted WO3
Fe-Implanted WO3 annealed @ 300oC
Fe-Implanted WO3 annealed @ 400oC
Co
un
ts/s
eco
nd
(A
U)
Binding Energy (eV)35
.83 35
.58
35.70
W 4f7/2W 4f5/2
Figure 4-22: W 4f core level high resolution spectra of nanostructured Fe-implanted WO3 films.
The W4f core level high resolution spectrum of Fe-implanted WO3 films is
shown in Fig. 4-22. The Eb value of W 4f7/2 (35.83 eV) obtained for this film closely
matches with the literature reported value [204]. The W 4f peak shapes get sharper
with increasing annealing temperature, indicating that the surface is cleaner due to
desorption of surface contaminants such as carbon by annealing, leading to change in
stoichiometry of the sample surface. A significant change is observed in binding
energy of W 4f7/2 after annealing at 300ºC (about 0.25 eV), indicating the presence of
mixed tungsten states (W6+, W5+, W4+, etc) [209]. However, after annealing at 400ºC,
the W 4f7/2 peak downshifted only by about 0.13 eV with respect to the implanted
film. Annealing at 400ºC appears to have oxidized the implanted film accompanied
with a composition approaching the stoichiometry.
96
0
1x104
2x104
3x104
4x104
5x104
6x104
7x104
8x104
524526528530532534
as-deposited WO3
WO3 annealed @ 300oC
WO3 annealed @ 400oC
Co
un
ts/s
ec
on
d (
AU
)
Binding Energy (eV)
O 1s
530.7
530.4
532.9
Figure 4-23: O 1s core level high resolution spectra of nanostructured WO3 films.
The O 1s core level high resolution spectrum of the as-deposited WO3 film (Fig.
4-23) shows Eb of 530.7 eV. The main maximum did not change upon annealing at
300ºC, which is similar to the trend observed for W 4f7/2 peak. However, annealing at
400ºC lowered the binding energy Eb by 0.3 eV, which is same as the downshift
observed for W 4f peak after annealing at 400ºC. This is most likely based on a shift
of the Fermi level, corresponding to band bending due to desorption of surface
contaminants during annealing at 400ºC [211]. A small shoulder centred at about
532.9 eV is observed in the as-deposited and 300ºC annealed film. This shoulder
transformed into a peak when the film is annealed at 400ºC. Such feature is
characteristic of substoichiometric monoclinic tungsten oxides [212]. The formation
and increasing intensity of this feature is in the sequence WO3WO2. XPS analysis
reveals that after annealing at 400ºC, the film surface is free from contamination and
has mixed W states. This indicates the presence of oxygen vacancies in the film,
which is highly beneficial for gas sensing.
97
0
2x104
4x104
6x104
8x104
1x105
524526528530532534
as-deposited Fe-doped WO3
Fe-doped WO3 annealed @ 300oC
Fe-doped WO3 annealed @ 400oC
Co
un
ts/s
eco
nd
(A
U)
Binding Energy (eV)
O 1s
530.
75
30.5
530.4
Figure 4-24: O 1s core level high resolution spectra of nanostructured Fe-doped WO3 films.
The O 1s core level high resolution spectra of as-deposited and annealed Fe-
doped WO3 films are shown in Fig. 4-24. The O 1s binding energy peak is observed
at Eb of 530.70 eV for as-deposited Fe-doped WO3 film, which is the same as that
observed for WO3 film. The peak intensity increased with annealing temperature,
which is attributed to the desorption of surface contaminants such as C during
annealing at 400ºC. Upon annealing at 300ºC, there is no significant shift in O 1s
peak position. However, when the Fe-doped WO3 film is annealed at 400ºC, the
binding energy of O 1s lowered by 0.3 eV as in the case of the pure WO3 film, in
which the lowered binding energy was ascribed to the shift in Fermi level [211]. The
W 4f7/2 and O 1s peak positions of pure WO3 and Fe-doped WO3 films annealed at
400ºC are compared in Table 4-2. The peak positions of both W 4f7/2 and O 1s did
not change significantly upon doping with Fe. This indicates that both the WO3 and
Fe-doped WO3 films have similar tungsten states after annealing at 400ºC.
98
XPS analysis reveals that the surface of Fe-doped WO3 film after annealing at
400ºC is free from surface contaminants and has mixed W states, similarly to the
WO3 film after annealing at 400ºC. This indicates the presence of oxygen vacancies
in both the films, which is highly beneficial for gas sensing.
Table 4-2: Comparison of W 4f7/2 and O 1s peak positions of nanostructured WO3 and Fe-doped WO3 films annealed at 400ºC for 2 hours in air.
XPS Peak position (eV)
W 4f7/2 O 1s
WO3 annealed at 400ºC 35.56 530.30
Fe-doped WO3 annealed at 400ºC 35.63 530.40
The O 1s core level high resolution spectrum of Fe-implanted WO3 films is
shown in Fig. 4-25. The Fe-implanted WO3 film shows an Eb value of 530.7 eV with
a significant shift of about 0.2 eV upon annealing at 300ºC. This shift, which is
similar to that observed in W 4f7/2, is due to the shift in Fermi level corresponding to
band bending caused by the desorption of surface contaminants such as C during
annealing [211]. However, after annealing at 400ºC, there is no change in O 1s peak
position, which indicates that annealing of Fe-implanted WO3 film at 400ºC shifted
the stoichiometry towards nominal WO3. The shift in peak position after annealing at
400ºC is similar to the change observed in W 4f7/2 peak (0.13eV) after 400ºC
annealing. Therefore, Fe-implanted WO3 film annealed at 400ºC has the same
tungsten states as in Fe-implanted WO3 film.
99
0
2x104
4x104
6x104
8x104
1x105
526528530532534
Fe-Implanted WO3
Fe-Implanted WO3 annealed @ 300oC
Fe-Implanted WO3 annealed @ 400oC
Co
un
ts/s
eco
nd
(A
U)
Binding Energy (eV)
530.7
530
.7 530.
5
O 1s
Figure 4-25: O 1s core level high resolution spectra of nanostructured Fe-implanted WO3 films.
4.3.3 Summary and outlook
Raman analysis has revealed that the as-deposited and 300ºC annealed pure, Fe-
doped and Fe-implanted WO3 films are amorphous. However, after annealing at
400ºC for 2 hours in air, the pure and Fe-doped WO3 thin films become crystalline,
whereas weak and broad peaks obtained in the Fe-implanted film annealed at 400ºC
are indicative of its amorphous nature. These results are in confirmation with TEM
and XRD characterization as discussed in Section 4.2. Iron addition by co-
evaporation and subsequent annealing at 400ºC slightly blue shifted the peaks by 5.5
cm-1. Both the Raman and GIXRD analyses reveal that iron is incorporated as a
substitutional impurity in the WO3 host matrix resulting in slightly smaller cell
parameters. This can lead to more distortions (defects) in the film structure and is
potentially beneficial for gas sensing.
XPS analysis reveals that annealing the films at 300ºC for 2 hours in air induced
a slight substoichiometry in all the films, and annealing at 400ºC for 2 hours in air
resulted in surface decontamination as observed by the high intensity of W and O
100
peaks. Moreover, the peak positions in both pure and Fe-doped WO3 thin films
shifted to lower binding energies indicating mixed tungsten states, correspondingly,
increase in number of oxygen vacancies. Both these films have similar tungsten
states after annealing at 400ºC. However, the Fe-implanted WO3 thin film annealed
at 400ºC has similar tungsten states as the as-deposited Fe-implanted WO3 thin film.
4.4 Characterization of compositional properties
4.4.1 Rutherford backscattered spectroscopy
The depth profile of the films was measured by using Rutherford Backscattered
Spectroscopy. The RBS spectra of as-deposited and 300ºC annealed WO3 films are
shown in Fig. 4-26. The as-deposited WO3 film spectrum (Fig. 4-26a) exhibits a
typical staircase structure with each step associated with an element in the film and
substrate. Well separated W peak with a high intensity is due to the higher mass
(atomic weight) of W compared with O and other trace elements such as N. The He+
particles are scattered with much higher recoil energy in the film than from the
substrate (Si) in this elastic scattering process. Upon annealing the WO3 film in air at
300ºC for 2 hours, the W peak becomes slightly sharper and more intense (Fig. 4-
26b) than the peak of the as-deposited film (Fig.4-26a).
Fig. 4-27 shows the depth profiles of the as-deposited and 300ºC annealed WO3
film. The depth profile of WO3 film (Fig. 4-27a) indicates the presence of O, N and
W. A high amount of nitrogen concentration is observed in the film. This can be due
to surface contamination of the film. Moreover, RBS is limited in resolution for light
elements with similar masses such as B, C, N and , hence other elements may
contribute to the high concentration of nitrogen. Presence of oxygen can be
attributed to adsorbed oxygen from the environment in addition to the lattice oxygen
101
existing in the film. The depth profile of 300ºC annealed WO3 film (Figure 4-27b)
shows a slight decrease in the amount of O and increase in the amount of W as
compared to the corresponding values for the as-deposited film (Figure 4-27a). This
reduced amount of oxygen may be attributed to desorption of surface contaminants
from the top few atomic layers.
0
500
1000
1500
2000
2500
3000
3500
4000
50 100 150 200 250 300 350 400
400 600 800 1000 1200 1400 1600 1800
ExperimentalSimulation
Co
un
ts (
AU
)
Channel
as-deposited WO3
Energy (KeV)
O+
N in
WO
3, SiO
2
O in
WO
3
Si i
n s
ub
stra
te
Si i
n S
iO2
Si s
ub
stra
teSiO
2
WO
3
He
W(a)
0
500
1000
1500
2000
2500
3000
3500
4000
50 100 150 200 250 300 350 400
400 600 800 1000 1200 1400 1600 1800
Experimental
Simulation
Co
un
ts (
AU
)
Channel
WO3 annealed @ 300oC
Energy (KeV)
O+
N i
n W
O3, S
iO2
O in
WO
3
Si i
n s
ub
stra
te
Si i
n S
iO2
W
Si s
ub
str
ate
SiO2
WO
3
He
(b)
Figure 4-26: RBS spectrum of (a) as-deposited and (b) 300ºC annealed nanostructured WO3 films.
102
0
0.2
0.4
0.6
0.8
0 50 100 150 200 250 300 350
Co
nc
entr
ati
on
(x
10
0 a
t%)
Depth (x1015 at/cm2)
O
W
N
Si
as-deposited WO3
(a)
0
0.2
0.4
0.6
0.8
0 50 100 150 200 250
Co
nc
entr
ati
on
(x1
00 a
t%)
Depth (x 1015 at/cm2)
OW
N
Si
WO3 annealed @ 300oC (b)
Figure 4-27: RBS depth profile of (a) as-deposited and (b) 300ºC annealed nanostructured WO3 films.
The RBS spectra of as-deposited and 300ºC annealed Fe-doped WO3 films are
shown in Fig. 4-28. The intensity of W peak in the as-deposited Fe-doped WO3 film
(Fig. 4.26a) is smaller than the other films (as-deposited WO3, Fig. 4.24a, 300oC
annealed WO3, Fig. 4.24b, and 300oC annealed Fe-doped WO3, Fig. 4.26b).
103
0
500
1000
1500
2000
2500
3000
3500
4000
50 100 150 200 250 300 350 400
400 600 800 1000 1200 1400 1600 1800
ExperimentalSimulation
Co
un
ts (
AU
)
as-deposited Fe-doped WO3
Energy (KeV)
Channel
O+
N in
WO
3, S
iO2
O in
WO
3
Si i
n s
ub
stra
te
Si i
n S
iO2
W
Si
su
bs
trat
e
SiO2
WO
3
He
(a)
0
500
1000
1500
2000
2500
3000
3500
4000
50 100 150 200 250 300 350 400
400 600 800 1000 1200 1400 1600 1800
ExperimentalSimulation
Co
un
ts (
AU
)
Fe-doped WO3 annealed at 300 oC
Energy (KeV)
Channel
O+
N in
WO
3, S
iO2
O i
n W
O3
Si i
n s
ub
stra
te
Si i
n S
iO2
W
Si s
ub
stra
te
SiO2
WO
3
He
(b)
Figure 4-28: RBS spectrum of (a) as-deposited and (b) 300ºC annealed nanostructured Fe-doped WO3
film.
The depth profiles of the as-deposited and 300ºC annealed Fe-doped WO3 films
are shown in Fig. 4-29. The depth profile of as-deposited Fe-doped WO3 film shows
O, W, N and Fe (Fig. 4-29a) and the amount of Fe in the film is only about 0.5 at.%.
This film also appears to be contaminated on the surface, hence a high amount of
nitrogen is observed owing to the poor resolution of RBS for lighter elements. AFM,
104
GIXRD and Raman analyses have shown that addition of Fe resulted in slight
increase in grain size and distortions in the crystal structure. Addition of Fe appears
to have slightly changed the stoichiometry of the film (change in amount of O and
W). The depth profile of 300ºC annealed Fe-doped WO3 film (Fig. 4-29b) shows that
the amounts of W and O are similar to those in the 300ºC annealed WO3 film. AFM
analysis also shows the same grain size of 10 nm in both the WO3 and Fe-doped WO3
films annealed at 300ºC. This finding indicates that the stoichiometry (Table 4-3) of
WO3 is improved by annealing and the film is similar to 300ºC annealed WO3 film.
0
0.2
0.4
0.6
0.8
0 50 100 150 200 250 300
Co
nc
entr
atio
n (
x100
at%
)
Depth (x1015 at/cm2)
O
W
Fe
N
Si
as-deposited Fe-doped WO3
(a)
0
0.2
0.4
0.6
0.8
0 40 80 120 160
Co
nc
entr
ati
on
(x
100
at%
)
Depth (x 1015 at/cm2)
O
W
FeSi
N
Fe-doped WO3 annealed at 300oC (b)
Figure 4-29: RBS spectrum (a) and depth profile (b) of nanostructured Fe-doped film annealed at
300ºC for 2 hours in air.
105
The RBS spectrum and corresponding depth profile of Fe-implanted WO3 film is
shown in Fig. 4-30. The spectrum (Fig. 4-30a) exhibits a typical staircase structure
with each step associated with an element in the sample. Well separated peaks of W
and Fe are observed in the spectrum. The depth profile (Fig. 4-30b) reveals the
presence of O, N, Fe and W. This film also shows high concentration of nitrogen as
surface contamination owing to the poor resolution of RBS for lighter elements.
From the RBS depth profile, the amount of Fe was estimated to be 5.5 at.%.
0
500
1000
1500
2000
2500
3000
3500
4000
0 50 100 150 200 250 300 350 400
0 400 800 1200 1600
ExperimentalSimulation
Co
un
ts (
AU
)
Fe-Implanted WO3
Energy (KeV)
Channel
O+
N in
WO
3, S
iO2
Si i
n S
iO2
Si i
n s
ub
stra
te
O i
n W
O3
Si
su
bs
tra
te
SiO2
WO
3
He
W
Fe
(a)
0
0.2
0.4
0.6
0.8
0 40 80 120 160
Co
nc
entr
ati
on
(x
100
at%
)
Depth (x1015 at/cm2)
O
W
Fe
N
Si
Fe-Implanted WO3
(b)
Figure 4-30: RBS spectrum (a) and corresponding depth profile (b) of nanostructured Fe-implanted
WO3 film.
106
4.4.2 Energy dispersive x-ray spectroscopy (EDX)
Energy dispersive x-ray spectroscopy (EDX) was carried out during TEM analysis to
determine the elemental composition of the films. However, it must be understood
that the depth of penetration in EDX is of the order of a few microns, and, hence, it is
difficult to determine the accurate composition of the thin films 300 nm thick
synthesized in this research. Nevertheless, the presence of various elements in the
films can be confirmed. Fig. 4-31 shows the EDX spectrum of Fe-doped WO3 film
annealed at 400ºC. Peaks corresponding to W, O, C, Fe and other contaminants are
revealed. From the EDX analysis, the amount of Fe was found to be about 0.7 at.%,
which is similar to the amount obtained by RBS (0.5 at.%). Fig. 4-32 is EDX
spectrum of Fe-implanted WO3 film annealed at 400ºC, showing, the presence of W,
O, C, Fe and other contaminants. The amount of Fe obtained by EDX was about 3
at.%, which is lower than the concentration obtained by RBS (5.5 at.%). It is to be
noted that the concentration obtained by RBS is more reliable than those obtained by
EDX.
Figure 4-31: EDX spectrum of for 2 hours in air.
Fe-doped WO3 film annealed @ 400ºC
107
Figure 4-32: EDX spectrum of Fe-implanted WO3 film annealed at 400ºC for 2 hours in air.
4.4.3 Summary and outlook
EDX analysis has revealed the presence of W, O, C and Fe in Fe-doped and
Fe-implanted WO3 films annealed at 400ºC. The XPS analysis could not reveal
the presence of Fe in Fe-doped film due to its low sensitivity (0.1 at.% per 10
nm). To accurately determine the elemental composition of the films, RBS
analysis was carried out. The results from both RBS analysis indicate that the
amount of Fe is about 0.5 at. % in Fe-doped WO3 thin film and 5.5 at.% in Fe-
implanted WO3 thin film. RBS analysis has revealed desorption of surface
contaminants after annealing at 300ºC. Iron incorporation in WO3 film slightly
changed the stoichiometry which is restored by annealing the film at 300ºC.
Fe-Implanted WO3 film annealed @ 400ºC
108
CHAPTER 5 : GAS SENSOR CHARACTERIZATION
5.1 Introduction
In this chapter, the gas sensing performance of the nanostructured tungsten oxide
thin films is presented. Nanostructured pure WO3, Fe-doped WO3 and Fe-implanted
WO3 thin films were deposited onto conductometric transducers by thermal
evaporation and subsequently annealed at 300ºC and 400ºC for 2 hours in air to
develop novel devices for gas sensing. These sensor devices have been tested
towards various concentrations of H2, ethanol and CO in a dry air in the temperature
range of 100–300ºC. The experimental setup and parameters for gas sensing
measurements have been detailed earlier in Section 3.4. Chapter 4 has provided
information about the physical, chemical and electronic properties of nanostructured
WO3 thin films which is a first step to evaluate their suitability for gas sensing.
In this chapter, the sensing performance of nanostructured thin film devices
based on WO3 towards various gases is presented. This will provide important static
and dynamic parameters such as sensitivity, optimum operating temperature,
response and recovery times and baseline resistance stability of these films. The gas
sensing performance will be linked to the physical, chemical and electronic
properties of the films to develop an understanding of the factors that contribute to a
better sensor performance.
5.2 Response to Hydrogen
The as-deposited and annealed pure, Fe-doped and Fe-implanted WO3 thin film
based gas sensors were exposed to H2 in the temperature range of 100–300ºC at
increments of 50ºC to determine the optimum operating temperature of these sensors.
109
Table 5-1 shows the optimum operating temperature obtained for various sensors
upon exposure to H2. The as-deposited films did not show any response towards H2
in the selected temperature range. It has been shown experimentally that amorphous
films have poor sensor response characteristics [213]. The amorphous nature of the
as-deposited films seems to lead these films being not suitable for gas sensing.
Table 5-1: Optimum operating temperature of WO3 thin film based gas sensors upon exposure to H2.
Optimum operating temperature (ºC) for H2
as-deposited WO3 - WO3 annealed at 300ºC 280 WO3 annealed at 400ºC 150 as-deposited Fe-doped WO3 - Fe-doped WO3 annealed at 300ºC 250 Fe-doped WO3 annealed at 400ºC 200 as-deposited Fe-implanted WO3 - Fe-implanted WO3 annealed at 300ºC 200 Fe-implanted WO3 annealed at 400ºC 100
An optimum response to H2 was observed for 300ºC annealed and 400ºC
annealed WO3 films at different temperatures. The dynamic responses and
sensitivities of 300ºC and 400ºC annealed WO3 films exposed to H2 at optimum
temperatures are shown in Figs. 5-1 and 5-2, respectively. Table 5-2 shows the
response dynamics (response and recovery times) of these films.
The 300ºC annealed film is found to be not stable as observed from its baseline
resistance as well as response (Fig. 5-1a). This is due to the amorphous nature of the
film. The 400ºC annealed film shows a stable response and a stable baseline
resistance (Fig. 5-2a), which is attributed to its crystalline properties (Fig. 5-2a). The
response dynamics (response and recovery times) of both the films exhibit some
fluctuation at high concentrations due to saturation caused by increased coverage of
the film surface at high H2 concentration which requires longer time to purge out H2
from the surface when synthetic air is injected between two cycles.
110
4
6
8
10
12
14
16
18
20
500 1500 2500
Re
sis
tan
ce
(K
Oh
ms
)
Time (seconds)
WO3 annealed @ 300oC exposed to H
2
Operating temperature: 280 oC
600
pp
m
1000
0 p
pm
7500
pp
m
5000
pp
m
2500
pp
m
120
0 p
pm
(a)
0
0.1
0.1
0.2
0.2
0.3
0.3
0 2000 4000 6000 8000 10000 12000
Se
nsi
tiv
ity
(
R/R
ga
s)
H2 concentration (ppm)
WO3 annealed @ 300oC exposed to H
2
Operating temperature: 280 oC
(b)
Figure 5-1: Dynamic response (a) and sensitivity (b) to H2 measured for a nanostructured WO3 film
annealed at 300ºC for 2 hours in air.
111
0
1
2
3
4
5
6
7
8
0 500 1000 1500 2000 2500 3000 3500
Re
sis
tan
ce (
M o
hm
s)
Time (seconds)
WO3 annealed @ 400oC exposed to H
2
Operating temperature: 150oC
600 12
00
120
0
250
0
5000
7500 10
000
(a)
0
2
4
6
8
10
12
14
0 2000 4000 6000 8000 10000 12000
Se
nsi
tiv
ity
(
R/R
air)
H2 Concentration (ppm)
WO3 annealed @ 400oC exposed to H
2
Operating temperature: 150oC
(b)
Figure 5-2: Dynamic response (a) and sensitivity (b) to H2 measured for a nanostructured WO3 film
annealed at 400ºC for 2 hours in air.
Table 5-2: Response and Recovery times of annealed (at 300ºC and 400ºC) nanostructured WO3 films upon exposure to H2.
H2 Concentration (ppm)
WO3 film annealed at 300ºC (Operating temperature: 280ºC)
WO3 film annealed at 400ºC (Operating temperature: 150ºC)
Response time (seconds)
Recovery time (seconds)
Response time (seconds)
Recovery time (seconds)
600 32 28 152 44 1200 28 24 152 72 2500 28 28 164 80 5000 24 32 176 56 7500 20 64 156 72
10000 40 44 140 80
112
The resistance does not saturate within a few tens of seconds after introduction of H2.
The WO3 film annealed at 300ºC showed an optimum response at an operating
temperature of 280ºC (Fig. 5-1a) with a sensitivity S=0.3 to 10000 ppm H2 (Fig. 5-
1b). However, when the film is annealed at 400ºC, an optimum response is obtained
at much lower operating temperature of 150ºC (Fig. 5-2a) with a high sensitivity
S=10 to 10,000 ppm H2 (Fig. 5-2b). Moreover, the sensitivity of the 400ºC annealed
film is much greater than the 300ºC annealed film for all concentrations of H2 (Fig.
5-2b). However, the response dynamics of the 400ºC annealed WO3 film is found to
be slow at lower operating temperature of 150ºC (Table 5-2). The response and
recovery time to 10000 ppm H2 are 140 s and 80 s at 150ºC for the 400°C annealed
film, and 40 s and 44 s at 280°C for the 300°C annealed film. It has been reported
that the response amplitude (sensor signal) of WO3 based gas sensors decreases
drastically with increasing temperature from 150°C to 300°C and that the sensor
indicates slow response and long recovery times at low operating temperatures of
150°C [214].
WO3 is an n-type semiconductor material and commonly operates as a gas
sensor in the temperature between 200°C -500°C [215]. When it is exposed to a
reducing gas such as H2, the oxygen adsorbates on the film surface interact with the
gas and release electrons back to the film, causing a drop in film resistance. This
behaviour is observed for the 300°C annealed WO3 film when exposed to H2 at
280ºC. However, the opposite behaviour (i.e. an increase in resistance) is observed
for the 400ºC annealed WO3 film upon exposure to H2 at 150ºC. Such behaviour
cannot be explained by merely considering the microstructural properties such as
grain size and porosity of the film.
113
In polycrystalline materials, surface barriers which electrons have to overcome
for taking part in the conduction are formed at the intergranular surfaces. The height
of surface barrier depends on the concentration of charge carriers (oxygen
adsorbates) at the surface, and, therefore, overall resistance changes can be correlated
with changes in surface band bending. The overall resistance, and, hence, surface
band bending increased exponentially when the polycrystalline WO3 surface was
exposed to increasing concentrations of oxygen [216]. The increase in resistance
observed for the 400ºC annealed WO3 film exposed to H2 at an operating
temperature of 150ºC might arise from various forms of oxygen adsorbates (O-, O2-
and O2-) on WO3 surface, which depend on temperature. At 150ºC, the most
dominant form of adsorbed oxygen is O2- [217]. Upon exposure to H2, the O2
-
species dissociates into O- with the formation of water, as per the following equation.
)1.5(222 OOHOH
Insitu Raman analysis of WO3 films annealed at 300ºC and 400ºC has shown
that the rate of water desorption above 100ºC is much faster than the rate of water
formation on the film surface when WO3 film is exposed to H2 [218]. At 150ºC, the
high concentration range of H2 (600 ppm – 10000 ppm) produces more O- species on
the surface, leading to increase in surface barrier height, consequently increasing the
resistance. Hence, this can be a reason why an increase in resistance was observed at
lower operating temperature.
The high sensitivity to H2 at 150ºC observed for the 400ºC annealed WO3 film is
attributed to its very small grain size (5 nm) and porous structure (see Fig. 4-3). The
film is also composed of mixed W states, as observed in XPS analysis, resulting in
increased number of oxygen vacancies on the surface, when compared with the as-
deposited or 300ºC annealed film.
114
The dynamic responses and sensitivities to H2 at the optimum temperature of
250ºC measured for 300ºC and 400ºC annealed Fe-doped WO3 films are shown in
Figs. 5-3 and 5-4, respectively. Table 5-3 shows the response dynamics (response
and recovery times) of these films. The response dynamics (response and recovery
times) of these films also exhibit variation similar to that observed for the pure WO3
films.
120
160
200
240
280
500 1000 1500 2000 2500 3000
Re
sis
tan
ce
(K
oh
m)
Time (seconds)
Fe-doped WO3 annealed @ 300oC exposed to H
2
Operating temperature: 250oC
600
pp
m
1200
pp
m
2500
pp
m
5000
pp
m
7500
pp
m
100
00 p
pm
(a)
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0 2000 4000 6000 8000 10000 12000
Se
nsi
tiv
ity
(
R/R
ga
s)
H2 concentration (ppm)
Fe-doped WO3 annealed @ 300oC exposed to H
2
Operating temperature: 250 oC (b)
Figure 5-3: Dynamic response (a) and sensitivity (b) to H2 measured for a nanostructured Fe-doped
WO3 film annealed at 300ºC for 2 hours in air.
115
0
0.5
1
1.5
2
0 1000 2000 3000 4000 5000 6000
Re
sis
tan
ce
(M
Oh
ms
)
Time (seconds)
Fe-doped WO3 annealed @ 400oC exposed to H
2
Operating temperature: 200 oC
600
pp
m
120
0 p
pm
2500
pp
m
5000
pp
m
7500
pp
m
1000
0 p
pm
(a)
2
2.5
3
3.5
4
4.5
5
5.5
6
0 2000 4000 6000 8000 10000 12000
Se
ns
itiv
ity
(R
/Rg
as)
H2 Concentration (ppm)
Fe-doped WO3 annealed @ 400oC exp to H
2
Operating temperature: 250 oC
(b)
Figure 5-4: Dynamic response (a) and sensitivity (b) to H2 measured for a nanostructured Fe-doped
WO3 film annealed at 400ºC for 2 hours in air.
Table 5-3: Response and Recovery times of annealed (at 300ºC and 400ºC) nanostructured Fe-doped WO3 films upon exposure to H2.
H2 Concentration (ppm)
Fe-doped WO3 film annealed at 300ºC
(Operating temperature: 250ºC)
Fe-doped WO3 film annealed at 400ºC
(Operating temperature: 200ºC) Response time
(seconds) Recovery time
(seconds) Response time
(seconds) Recovery time
(seconds) 600 56 64 84 228 1200 52 64 76 236 2500 66 60 68 244 5000 52 52 64 232 7500 84 40 60 148
10000 44 52 100 172
116
The response of the 300ºC annealed Fe-doped WO3 film is not stable (Fig. 5-3a)
due to its amorphous nature. The response is also characterized by spikes especially
at higher concentration, which is due to saturation of the film surface at higher H2
concentrations. This film shows an optimum response at an operating temperature of
250ºC (Fig. 5-3a) with a sensitivity S=0.26 to 10000 ppm H2 (Fig. 5-3b). The 400ºC
annealed Fe-doped WO3 film shows a stable response (Fig. 5-4a) which is attributed
to its crystalline properties, coupled with small grain size of 5-10 nm and porosity
compared to 300oC annealed Fe-doped WO3 film which is amorphous and dense film
with a grain size of 10 nm. The optimum response of this film is observed at 200ºC
(Fig. 5-4a) with a high sensitivity S=5.5 to 10000 ppm H2 (Fig. 5-4b). Moreover, the
sensitivity of this film is much higher than the 300ºC annealed Fe-doped WO3 film
while the response dynamic are slower for all concentrations of H2. It was mentioned
earlier that the response amplitude (sensor signal) of WO3 based gas sensors
decreases drastically as the operating temperature increases from 150ºC to 300ºC and
the sensor is characterized by slow response and long recovery times at low
operating temperatures [214]. At an operating temperature between 200ºC-300ºC, the
most dominant oxygen species on the film surface is O- [217]. The conduction
mechanism is governed by the following equation:
)2.5(22 eOHOH
Upon interaction with H2, the electron concentration in the film increases,
causing a drop in resistance. The higher sensitivity of 400ºC annealed Fe-doped WO3
film compared to that of the 300ºC annealed Fe-doped WO3 film is due to the
presence of high number of oxygen vacancies in the 400ºC annealed film, as
observed from XPS and Raman analysis. The baseline resistance of the as-deposited
and annealed (at 300ºC and 400ºC) Fe-doped WO3 films is also higher than the as-
117
deposited and annealed (at 300ºC and 400ºC) WO3 films in the tested temperature
range. The increase in resistance of Fe-doped film is due to the acceptor nature of Fe
in the host matrix [219]. Doping with lower valency cations, such as Fe3+ increases
the Debye length through a reduction in the carrier concentration, which increases
the film resistance [220] . XRD and Raman analysis have shown that Fe is
incorporated in the host WO3 matrix, rather than as a catalyst on the surface (no
evidence of Fe on film surface from XPS observations). The sensitivity of 400ºC
annealed WO3 is almost double than that of 400ºC annealed Fe-doped WO3 film to
10,000 ppm H2. This is due to smaller grain size (5 nm vs 10 nm) and larger porosity
of the 400ºC annealed WO3 film (Fig. 4-3) than the 400ºC annealed Fe-doped WO3
film (Fig. 4-6). Additionally, the 400ºC annealed WO3 contained higher number of
oxygen vacancies than the 400ºC annealed Fe-doped WO3 film, leading to higher
sensitivity.
The dynamic response and sensitivities of Fe-implanted WO3 films annealed at
300ºC and 400ºC are shown in Figs. 5-5 and 5-6, respectively. The 300ºC annealed
film shows an optimum response to H2 at an operating temperature of 200ºC (Fig. 5-
5a). However, the response is not stable and a drifting baseline resistance is
observed, which is due to its amorphous nature [213]. A maximum sensitivity
S=0.13 is obtained for 7500 ppm H2 (Fig. 5-5b), with response and recovery times of
32 s and 64 s, respectively (Table 5-4).
118
80
85
90
95
100
105
110
0 500 1000 1500 2000 2500 3000
Re
sis
tan
ce (
K O
hm
s)
Time (seconds)
Fe-Implanted WO3 annealed @ 300oC exposed to H
2
Operating temperature: 200oC
600
pp
m
1200
pp
m
250
0 p
pm
500
0 p
pm
100
00 p
pm
750
0 p
pm
(a)
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0 2000 4000 6000 8000 10000 12000
Se
nsi
tiv
ity
(
R/R
gas
)
H2 concentration (ppm)
Fe-Implanted WO3 annealed @ 300
oC exposed to H
2
Operating temperature: 200oC (b)
Figure 5-5: Dynamic response (a) and sensitivity (b) to H2 measured for a nanostructured Fe-implanted
film annealed at 300ºC for 2 hours in air.
119
60
65
70
75
80
0 500 1000 1500 2000 2500 3000
Res
ista
nce
(K
oh
ms)
Time (seconds)
Fe-Implanted WO3 annealed @ 400oC exposed to H
2
Operating temperature: 100oC
600
pp
m
1200
pp
m
2500
pp
m
5000
pp
m
7500
pp
m
1000
0 p
pm
(a)
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0 2000 4000 6000 8000 10000 12000
Se
ns
itiv
ity
(R
/Ra
ir)
H2 Concentration (ppm)
Fe-Implanted WO3 annealed @ 400oC exposed to H
2
Operating temperature: 100oC (b)
Figure 5-6: Dynamic response (a) and sensitivity (b) to H2 measured for a nanostructured Fe-implanted
WO3 film annealed at 400ºC for 2 hours in air.
Table 5-4: Response and Recovery times of annealed (at 300ºC and 400ºC) nanostructured Fe-implanted WO3 films upon exposure to H2.
H2 Concentration (ppm)
Fe-implanted WO3 film annealed at 300ºC
(Operating temperature: 200ºC)
Fe-implanted WO3 film annealed at 400ºC
(Operating temperature: 100ºC) Response time
(seconds) Recovery time
(seconds) Response time
(seconds) Recovery time
(seconds) 600 52 100 92 143
1200 44 56 120 196 2500 36 56 144 196 5000 36 64 136 212 7500 32 64 116 244
10000 36 72 97 136
120
The optimum response for the 400ºC annealed Fe-implanted film was observed
at a lower operating temperature (100ºC), characterized by a drifting baseline
resistance (Fig. 5-6a) and a lower sensitivity S=0.048 to 7500 ppm H2 (Fig. 5-6b),
with response and recovery times of 116 s and 244 s, respectively (Table 5-4). Both
the annealed films have a dense structure characterized by low porosity, and hence
lower sensing characteristics. However, the presence of higher number of oxygen
vacancies in the 300ºC annealed film, as observed from XPS analysis (Fig. 4-24)
improved the sensitivity of this film, although the response is characterized by noise,
which is attributed to its amorphous nature [213]. In case of 400ºC annealed Fe-
implanted WO3 film, a p-type sensor response is observed (Fig. 5-6a) which is
similar to that observed in case of 400ºC annealed WO3 film. At an operating
temperature of 100ºC, the dominant oxygen adsorbate is O2- [47]. Upon exposure to
H2, the O2- species dissociates into O- with the formation of water, as per equation
5.1. The high concentration range of H2 (600 ppm – 10000 ppm) produces more O-
species on the surface, leading to increase in surface barrier height, consequently
increasing the resistance. Hence, this can be a reason why an increase in resistance
was observed at lower operating temperature. At 100ºC, the rate of water desorption
from the surface is lower than rate of water formation [218]. Accumulation of water
on the surface of the film would increase the film resistance, hence, an increase in
baseline resistance is observed upon successive exposure cycles of H2.
5.3 Response to Ethanol (C2H5OH)
The as-deposited and annealed pure, Fe-doped and Fe-implanted WO3 thin film
based gas sensors were exposed to ethanol in the temperature range 100ºC-300ºC at
increments of 50ºC to determine the optimum operating temperature of these sensors.
Table 5-5 shows the optimum operating temperature obtained for various sensors
121
upon exposure to ethanol. The as-deposited films did not show any response to
ethanol in the selected temperature range. The highly amorphous nature of the as-
deposited films seems to make these films unsuitable for gas sensing [213]. The WO3
film annealed at 300ºC also did not show any response to ethanol. This may also be
attributed to the amorphous nature of the film. The dynamic response and sensitivity
of 400ºC annealed WO3 film exposed to ethanol at 150ºC are shown in Fig. 5-7. The
response and recovery times are shown in Table 5-6. Similarly to the response
dynamics upon exposure to H2, a variation in response and recovery times is also
observed upon exposure to ethanol at higher concentrations.
Table 5-5: Optimum operating temperature of nanostructured WO3 thin film based gas sensors upon exposure to ethanol.
Optimum operating temperature (ºC) for ethanol
as-deposited WO3 - WO3 annealed at 300ºC - WO3 annealed at 400ºC 150 as-deposited Fe-doped WO3 - Fe-doped WO3 annealed at 300ºC - Fe-doped WO3 annealed at 400ºC 150 as-deposited Fe-implanted WO3 - Fe-implanted WO3 annealed at 300ºC 250 Fe-implanted WO3 annealed at 400ºC -
The film exhibits a stable baseline resistance (Fig. 5-7a), due to its crystalline
properties. The response amplitude increased with increasing ethanol concentration,
reaching 0.2 for 185 ppm ethanol (Fig. 5-7b). The response and recovery times for
185 ppm ethanol are 180 s and 288 s, respectively (Table 5-6).
122
300
350
400
450
500
0 500 1000 1500 2000 2500 3000 3500
WO3 annealed @ 400oC exposed to ethanol
Operating temperature: 150 oC
Re
sis
tan
ce
(K
oh
ms
)
Time (seconds)
12 p
pm
30 p
pm
60
pp
m
185
pp
m
(a)
0
0.05
0.1
0.15
0.2
0.25
0 50 100 150 200
Se
nsi
tiv
ity
(
R/R
ga
s)
Ethanol Concentration (ppm)
WO3 annealed @ 400oC exposed to ethanol
Operating temperature: 150 oC
(b)
Figure 5-7: Dynamic response (a) and sensitivity (b) to ethanol measured for a nanostructured WO3
film annealed 400ºC for 2 hours in air.
Table 5-6: Response and Recovery times of 400ºC annealed nanostructured WO3 films upon exposure
to ethanol.
Ethanol Concentration (ppm)
WO3 film annealed at 400ºC (Operating temperature: 150ºC)
Response time (seconds) Recovery time (seconds)
12 120 296 30 144 260 60 164 292
185 180 288
123
As mentioned earlier, the dominant oxygen species on the film surface at 150ºC is
O2-, the conduction mechanism takes place by the following reaction.
)3.5(3252 eCOOHCHOOHHC
The above reaction increases the concentration of electrons, which decrease the
surface band bending, resulting in drop in resistance. Hence, a decrease in resistance
is observed upon exposure to ethanol. With increasing concentration of ethanol, a
further drop in resistance is expected, which is confirmed by the dynamic response
(Fig. 5-7a).
The sensitivity of 400ºC annealed WO3 film at 150ºC is higher to H2 than
ethanol. It is due to the porous structure of the film as well as small size of H2
molecule compared to ethanol. Hydrogen can easily diffuse through the surface and
interact with more surface area, hence, higher sensitivity is achieved.
Fig. 5-8 shows the dynamic response and sensitivity of 300ºC annealed Fe-
doped WO3 film exposed to ethanol at an optimum operating temperature of 250ºC.
It can be observed that the film shows an unstable response and a drifting baseline
resistance, due to its amorphous nature [213]. Moreover, the response behaviour is
typical of a p-type material. At 250ºC, the dominant oxygen species on the film
surface is O- [217]. Reaction of ethanol molecules with surface-adsorbed oxygen
produces water molecules with electrons according to the following equation [221]:
)4.5(2352 eOHCHOCHOOHHC ads
The above reaction is expected to result in production of free charges and
consequently decrease the film resistance. However, water molecules further interact
with the oxide surface and neutralize the effect of free charges according to the
following mechanism [221].
124
)5.5(0lattlattlatt2 V(W/Fe)OH2O2(W/Fe)OH
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 500 1000 1500 2000 2500 3000 3500
Res
ista
nce
(M
oh
ms
)
Time (seconds)
Fe-doped WO3 annealed @ 300oC exposed to ethanol
Opearating temperature: 250oC
12
pp
m
30
pp
m
60 p
pm
185
pp
m
(a)
0
0.2
0.4
0.6
0.8
1
0 50 100 150 200
Se
nsi
tiv
ity
(R
/Rai
r)
Ethanol Concentration (ppm)
Fe-doped WO3 annealed @ 300oC exposed to ethanol
Operating temperature: 250oC(b)
Figure 5-8: Dynamic response (a) and sensitivity (b) to ethanol measured for a nanostructured Fe-
doped WO3 film annealed at 300ºC for 2 hours in air.
The water molecule reacts with W/Fe and O at the lattice positions, which results in
the creation of a oxygen vacancy along an OH-(W/Fe) complex. These vacancies
neutralize the free charges produced through above reactions (equations 5.4 and 5.5)
and, consequently, increase the surface barrier height, provided that the rate of
vacancy formation is higher than the rate of adsorption on the surface. Below 200ºC,
125
the physisorbed water molecules can easily get desorbed from the surface, but the
temperature of desorption of chemisorbed water molecules is reported to be as high
as 500ºC to 600ºC [222]. This can cause a drift in baseline resistance as well as a
drop in sensitivity with increasing concentration of ethanol, which is observed in Fig.
5-8.
Fig. 5-9 shows the dynamic response and sensitivity of the 400ºC annealed Fe-
doped WO3 film upon exposure to ethanol at an optimum operating temperature of
150ºC. The film exhibits a stable baseline resistance (Fig. 5-9a), due to its crystalline
properties. The sensitivity increased with increasing ethanol concentration, reaching
S=5 for 185 ppm ethanol (Fig. 5-9b). The response and recovery times for 185 ppm
ethanol are 64 s and 108 s, respectively (Table 5-7).
The dominant oxygen species on the film surface at 150ºC is O2-, the conduction
mechanism takes place by the following reaction.
)6.5(3252 eCOOHCHOOHHC
The above reaction increases the concentration of electrons, which decrease the
surface band bending, resulting in drop in resistance. Hence, a decrease in resistance
is observed upon exposure to ethanol. With increasing concentration of ethanol, a
further drop in resistance is expected, which is confirmed by the dynamic response
(Fig. 5-9a).
126
0
0.4
0.8
1.2
1.6
2
2.4
0 500 1000 1500 2000 2500 3000
Res
ista
nc
e (
M o
hm
s)
Time (seconds)
Fe-doped WO3 annealed @ 400oC exposed to ethanol
Operating temperature: 150oC
12
pp
m
30
pp
m
60 p
pm
185
pp
m
(a)
0
1
2
3
4
5
6
7
0 50 100 150 200
Se
ns
itiv
ity
(R
/Rg
as)
Ethanol Concentration (ppm)
Fe-doped WO3 annealed @ 400oC exposed to ethanol
Operating temperature: 150oC(b)
Figure 5-9: Dynamic response (a) and sensitivity (b) to ethanol measured for a nanostructured Fe-
doped WO3 film annealed at 400ºC for 2 hours in air.
Table 5-7: Response and Recovery times of annealed (at 300ºC and 400ºC) nanostructured Fe-doped WO3 films upon exposure to ethanol.
Ethanol Concentration
(ppm)
Fe-doped WO3 film annealed at 300ºC
(Operating temperature: 250ºC)
Fe-doped WO3 film annealed at 400ºC
(Operating temperature: 150ºC) Response time
(seconds) Recovery time
(seconds) Response time
(seconds) Recovery time
(seconds) 12 68 108 48 52 30 40 100 60 56 60 32 96 76 84
185 32 116 64 108
127
The sensitivities of the 400ºC annealed WO3 and Fe-doped WO3 films to ethanol
at the operating temperature of 150ºC are compared in Fig. 5-10. The sensitivity of
the 400ºC annealed Fe-doped WO3 film is higher than the 400ºC annealed WO3 film
at the operating temperature of 150ºC. At this temperature, both the films show the
same magnitude of baseline resistance. However, the 400ºC annealed Fe-doped WO3
film contains more distortions, consequently contains higher number of oxygen
vacancies, thus enabling higher sensitivity at the same operating temperature of
150ºC.
0
1
2
3
4
5
6
0 50 100 150 200
WO3 annealed @ 400oC
Fe-doped WO3 annealed @ 400oC
Sen
siti
vit
y (
R/R
gas
)
Ethanol Concentration (ppm)
Target gas: ethanol
Operating temperature: 150 oC
Figure 5-10: Comparison of ethanol sensitivities of nanostructured WO3 and Fe-doped WO3 films
annealed at 400ºC for 2 hours in air at an operating temperature of 150ºC.
An optimum response to ethanol was obtained only by the Fe-implanted WO3
film annealed at 300ºC at an operating temperature of 250ºC. Fig. 5-11 shows the
dynamic response and sensitivity of the 300ºC annealed Fe-implanted WO3 film
upon exposure to ethanol. The response and recovery times are shown in Table 5-8.
The dynamic response is characterized by a slightly drifting baseline resistance and
noise. This is due to the amorphous nature of the film [213]. A maximum sensitivity
128
S=0.65 is obtained to 60 ppm ethanol at 250ºC. At this temperature, reaction of
ethanol molecules with surface adsorbed oxygen produces water molecules with
electrons according to equation (5.4) [221], resulting in production of free charges
and consequently decrease the film resistance. However, water molecules further
interact with the oxide surface and neutralize the effect of free charges according to
equation 5.5 [221].
16
24
32
40
48
56
64
72
80
0 500 1000 1500 2000 2500 3000
Fe-Implanted WO3 annealed @ 300oC exposed to
Re
sis
tan
ce (
K O
hm
s)
Time (seconds)
12
pp
m
30 p
pm
60 p
pm
185
pp
m
Operating temperature: 250 oC
ethanol
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8
0 50 100 150 200
Fe-Implanted WO3 annealed @ 300oC exposed to
Se
nsi
tiv
ity
(
R/R
air)
Ethanol Concentration (ppm)
Operating temperature: 250oC
ethanol
Figure 5-11: Dynamic response (a) and sensitivity (b) to ethanol measured for a nanostructured Fe-
implanted WO3 film annealed at 400ºC for 2 hours in air.
129
Table 5-8: Response and Recovery times of 300ºC annealed nanostructured Fe-implanted WO3 film upon exposure to ethanol.
Ethanol Concentration (ppm) Fe-implanted WO3 film annealed at 300ºC (Operating temperature: 250ºC)
Response time (seconds) Recovery time (seconds) 12 92 200
30 80 228 60 52 187 185 48 228
The water molecules react with W/Fe and O at the lattice positions which results in
the creation of a vacancy along an OH-(W/Fe) complex. These vacancies neutralize
the free charges produced through reactions (5.4) and (5.5) and consequently,
increase the surface barrier height, if the rate of vacancy formation is higher than rate
of adsorption on the surface. Between 100ºC-200ºC, the physisorbed water
molecules can easily get desorbed from the surface, but the temperature of desorption
of chemisorbed water molecules is reported to be as high as 500ºC to 600ºC [222].
This can cause a drift in baseline resistance with increasing concentration of ethanol,
which is observed in Fig. 5-11. The higher recovery times are associated with the
accumulation of water molecules on the film surface which prolong the recovery of
the film surface upon removal of ethanol from the chamber.
5.4 Response to Carbon monoxide (CO)
Various WO3 thin films were exposed to carbon monoxide in the temperature
range 100ºC-300ºC at increments of 50ºC to determine the optimum operating
temperature of these sensors. Table 5-9 shows the optimum operating temperature
obtained for various sensors upon exposure to CO.
130
Table 5-9: Optimum operating temperature of nanostructured WO3 thin film based gas sensors upon exposure to CO.
Optimum operating temperature (ºC) for CO as-deposited WO3 - WO3 annealed at 300ºC - WO3 annealed at 400ºC - as-deposited Fe-doped WO3 - Fe-doped WO3 annealed at 300ºC 150 Fe-doped WO3 annealed at 400ºC 150 as-deposited Fe-implanted WO3 - Fe-implanted WO3 annealed at 300ºC - Fe-implanted WO3 annealed at 400ºC -
It is shown in Table 5-9 that sensitivity to CO is achieved only by the Fe-doped
WO3 thin film sensor. It is widely accepted that pure WO3 thin film sensors are not
sensitive to CO. There is very little evidence of CO sensing performance of pure
WO3 thin films available in literature [158, 223]. CO-sensing characteristics of a
CoOOH-WO3 composite doped with Au and SWCNT was investigated by Wu et. al
[130]. In the present study, after doping with Fe, a response to CO has been
observed. Moreover, no response to CO was detected by the Fe-implanted WO3
films.
Fig. 5-12 shows the dynamic response of the annealed Fe-doped WO3 films
upon exposure to CO. The sensitivity of the films is shown in Fig. 5-13. The
dynamic response of the 300ºC annealed Fe-doped WO3 film shows a response
characterized with a noise and a drifting baseline (Fig. 5-12a) at an operating
temperature of 150ºC, which is due to the amorphous nature of the film [213]. The
400ºC annealed film shows a stable response at the same operating temperature. Its
sensitivity is also much higher (S=0.2) than 300ºC annealed Fe-doped WO3 film
(S=0.05) to 1000 ppm CO at an operating temperature of 150ºC (Fig. 5-13). The film
is also characterized by short response time of 64 s compared with 108 s for the
300ºC annealed film (Table 5-10). The crystalline nature, loosely packed film
structure and small grain size of the 400ºC annealed Fe-doped WO3 film contribute
131
to its improved sensing performance to CO. However, as evident from Table 5-10
that there are some variations in the response kinetics with increasing CO
concentration. This is attributed to delay in recovery of high amount of CO from the
film surface at operating temperature of 150ºC.
1.5
1.6
1.7
1.8
1.9
2
2.1
2.2
2.3
0 500 1000 1500 2000
Re
sis
tan
ce (
M o
hm
s)
Time (seconds)
100
pp
m
500
pp
m
250
pp
m
100
0 p
pm
Fe-doped WO3 annealed @ 400oC exposed to CO
Operating temperature: 150 oC (a)
4.8
4.9
5
5.1
5.2
5.3
5.4
5.5
5.6
0 500 1000 1500 2000
Re
sis
tan
ce
(M o
hm
s)
Time (seconds)
100
pp
m
250
pp
m
500
pp
m
100
0 p
pm
Fe-doped WO3 annealed @ 300oC exposed to CO
Operating temperature: 150oC (b)
Figure 5-12: Dynamic responses of nanostructured Fe-doped WO3 films to CO, (a) annealed at 300ºC
for 2 hours in air and (b) annealed at 400ºC for 2 hours in air.
132
0
0.05
0.1
0.15
0.2
0.25
0 200 400 600 800 1000 1200
Fe-doped WO3 annealed @ 300oC
Fe-doped WO3 annealed @ 400oC
Se
nsi
tiv
ity
(R
/Rg
as)
CO concentration (ppm)
Operating temperature: 150 oC
Figure 5-13: Sensitivity of annealed (at 300ºC and 400ºC) nanostructured Fe-doped WO3 films upon
exposure to CO.
As mentioned earlier, the most dominant species on the film surface at an
operating temperature of 150ºC is O2-. The conduction mechanism is governed by the
following equation [224].
)6.5(22 22 eCOOCO
Upon exposure to CO, carbon dioxide is formed with the production of free charge
carriers, which decreases the surface barrier height and causes a drop in resistance of
the film.
Table 5-10: Response and Recovery times of annealed (at 300ºC and 400ºC) nanostructured Fe-doped WO3 films upon exposure to CO.
CO Concentration (ppm)
Fe-doped WO3 film annealed at 300ºC
(Operating temperature: 150ºC)
Fe-doped WO3 film annealed at 400ºC
(Operating temperature: 150ºC) Response time
(seconds) Recovery time
(seconds) Response time
(seconds) Recovery time
(seconds) 100 64 148 140 88 250 64 140 44 36 500 68 104 48 48 1000 108 76 64 80
133
The response of rather non-response WO3 films towards CO is attributed to the
acceptor nature of Fe in the host matrix [219], which makes it easier for
measurement of the sensor signal specially at low temperatures. Hence, WO3 film,
which otherwise is not sensitive to CO, shows a response at low temperature upon
doping the film with Fe.
5.5 Summary and outlook
The optimum operating temperatures obtained for various films upon exposure
to the H2, ethanol and CO in the temperature range 100ºC-300ºC are shown in Table
5-11. Gas sensors based on as-deposited WO3 thin film did not show any response to
H2, ethanol and CO due to their amorphous nature.
Table 5-11: Optimum operating temperature of the films upon exposure to H2, ethanol and CO in the tested temperature range of 100ºC-300ºC.
Optimum temperature (ºC) H2 ethanol CO
as-deposited WO3 - - - as-deposited Fe-doped WO3 - - - as-deposited Fe-implanted WO3 - - - WO3 annealed at 300ºC 280 Fe-doped WO3 annealed at 300ºC 250 150 Fe-implanted WO3 annealed at 300ºC 200 250 WO3 annealed at 400ºC 100 150 Fe-doped WO3 annealed at 400ºC 200 150 150 Fe-implanted WO3 annealed at 400ºC - - -
The microstructural properties and gas sensing characteristics of various WO3
thin films are compared in Table 5-12. All the films have a very small grain size
ranging between 5-15 nm. No crystallinity is observed in any of the films after
annealing at 300ºC for 2 hours in air and their gas sensing response was poor.
However, the pure and Fe-doped WO3 films annealed at 400ºC exhibit high
crystallinity and stable sensor response. The Fe-implanted WO3 film annealed at
400ºC shows amorphous nature and a similar gas sensing characteristic to that of the
134
as-deposited WO3 film. This indicates that annealing the WO3 and Fe-doped WO3
films at 400ºC for 2 hours in air improved the crystalline properties without
significantly altering the grain size and achieved excellent sensor response as
compared with other films.
135
Table 5-12: Comparison of microstructural properties of 400ºC annealed films.
AFM XRD and TEM RBS XPS Baseline resistanc
e at 150ºC
(k)
Sensitivity (S) at optimum operating temperature (ºC)
Mean grain size (nm)
Crystalline Properties
Fe (at.%)
W 4f7/2
peak position
(eV)
S Target
Gas Concentratio
n (ppm) Temperatur
e (ºC)
as-deposited WO3 13 Amorphous - 35.74 15.6 - - - - as-deposited Fe-doped WO3
15 Amorphous 0.5 35.70 746 - - - -
as-deposited Fe-implanted WO3
8 Amorphous 5.5 35.83 163 -
WO3 annealed at 300ºC 10 Amorphous 35.71 128 0.3 H2 10,000 280
Fe-doped WO3 annealed at 300ºC
10 Amorphous 0.5 35.80 165 0.25 H2 10,000 250 0.25 ethanol 185 250 0.05 CO 1000 150
Fe-implanted WO3 annealed at 300ºC
8 Amorphous 5.5 35.58 43 0.12 H2 10,000 200
0.65 ethanol 185 250
WO3 annealed at 400ºC 5 Crystalline - 35.56 172 10 H2 10,000 150
0.22 ethanol 185 150
Fe-doped WO3 annealed at 400ºC
10 Crystalline 0.5 35.63 729 5.5 H2 10,000 200 5 ethanol 185 150
0.2 CO 1000 150 Fe-implanted WO3 Imp annealed at 400ºC
5 Amorphous 5.5 35.70 37 0.05 H2 10,000 100
136
However, the Fe-implanted film shows poor gas sensing response as it was highly
damaged and annealing at 300ºC or 400ºC is not sufficient to crystallize this film.
The W 4f 7/2 binding energies of 400ºC annealed films has the maximum downshift
as compared to as-deposited and 300ºC annealed WO3 films, indicating high number
of oxygen vacancies in this film. However, no significant change is observed in W
4f7/2 binding energy of Fe-implanted film, thus, its stoichiometry is essentially
similar to that of pure WO3.This indicates that 400ºC annealed WO3 and Fe-doped
WO3 films have higher number of oxygen vacancies as compared to as-deposited
WO3 film and showed better sensing performance towards the tested gases (H2,
ethanol and CO).
Highest sensitivity to H2 is demonstrated by the 400ºC annealed pure WO3 film
at an operating temperature of 150ºC, which is due to the very small grain size (5
nm) and high porosity of the film, combined with high number of oxygen vacancies.
The highest sensitivity to ethanol is shown by the 400ºC annealed Fe-doped WO3
film at operating temperature of 150oC. Response to CO is also observed by the
400ºC annealed Fe-doped WO3 thin film at an operating temperature of 150ºC.
137
CHAPTER 6: CONCLUSIONS AND FUTURE WORK
This thesis presents the evolution of the author’s PhD research. The research
program commenced with the aim of optimizing the physical, chemical and
electronic properties of WO3 thin film for improved gas sensing performance. The
specific aim of this project was to optimize the operating temperature of the sensor
device and improve its sensitivity towards H2, ethanol and CO. Although WO3 thin
film gas sensors have shown excellent performance in detecting various gases, the
operating temperature of these films is still very high (300ºC-500ºC) and thus low
operating temperature is desirable. In addition, very little evidence is available in
literature on the CO sensing performance of WO3 thin film gas sensors which is
worthy of investigation through modification of the pure film by doping.
Thermal evaporation technique was used to develop pure nanostructured WO3
thin films. Iron doping of the thin films was performed through two methods: by co-
evaporation during thermal evaporation and by ion implantation. The films were
annealed at 300ºC and 400ºC in air for 2 hours to improve their film properties. The
properties of the films were characterized using AFM, TEM, XRD, RBS, XPS and
Raman to understand and evaluate their suitability for gas sensing. A number of
factors such as the target gas, operating temperature, crystallinity, stoichiometry and
presence of oxygen vacancies on the film surface influenced the sensing performance
The developed sensors were tested towards various gas concentrations within the
TLV range of H2 (600-10,000 ppm), ethanol (12-185 ppm) and CO (50-1000 ppm) in
the temperature range of 100ºC-300ºC and relative humidity of 0%. The gas sensing
properties, namely, sensitivity, response and recovery times, and baseline resistance
were evaluated. The results were thoroughly discussed by understanding the gas
138
sensing mechanism of the films at various temperatures. The sensing mechanism is
largely dictated by the target gas and the dominant oxygen species at the specific
operating temperature, which can lead to an opposite sensor response, as observed in
the present study. The concluding major findings of this research and potential for
future work are summarised in the following sections.
6.1 Conclusions
The major findings of this research program are summarized below:
Highly amorphous nanostructured WO3, Fe-doped WO3 and Fe-implanted
WO3 thin films (300 nm thick) with a grain size less than 15 nm have been
synthesized using thermal evaporation technique. To the best of author’s
knowledge, this was the first attempt to dope WO3 film with Fe using thermal
evaporation technique.
The as-deposited WO3 films showed a highly unstable response towards H2
and ethanol, owing to their amorphous nature. These films did not show any
response towards CO.
Doping with small amount (0.5 at.%) of iron increased the film resistance and
considerably improved the sensing performance. The film characterization
revealed that Fe was incorporated as a substitutional impurity in the WO3
matrix, rather than as a catalyst on the film surface.
Upon implantation with 5.5 at.% of Fe, the film became highly amorphous,
however, no additional compounds were revealed from characterization,
indicating that implantation did not induce any chemical changes in the film,
however, the morphology and grain structure were highly distorted.
139
Annealing at 300ºC for 2 hours in air showed an onset of the crystalline
properties of pure and Fe-doped WO3 films and induced sub-stoichiometry in
these films. These films showed a response towards H2, ethanol and CO,
however, the response was characterized by noise and a drifting baseline.
Annealing at 400ºC for 2 hours significantly improved the crystalline
properties and altered the stoichiometry in the WO3 and Fe-doped WO3 films,
which increased the number of oxygen vacancies in the films. An increase in
number of oxygen vacancies is considered to be highly beneficial for gas
sensing.
The nanostructured WO3 film annealed at 400ºC showed maximum response
to H2 and ethanol at an optimum operating temperature of 150ºC and no
response to CO.
The Fe-doped WO3 film annealed at 400ºC showed maximum response to
H2 at an optimum operating temperature of 200ºC. A response to both ethanol
and CO was observed at an optimum operating temperature of 150ºC.
The Fe-implanted WO3 film annealed at 400ºC showed maximum response to
H2 at 100ºC, but the sensitivity is very low compared to other films and
sensor was characterized by a drifting baseline. This film did not show any
response to ethanol and CO in the temperature range 100ºC-300ºC.
Upon comparison the sensing performance of all the films towards various
gases, firstly, it is observed that the author has been able to achieve a lower
operating temperature of 150ºC towards various gases by depositing
nanostructured thin films and doping of the films with Fe and subsequent heat
treatment. Secondly, doping the WO3 film with Fe has demonstrated a
140
response towards CO, which otherwise is non-sensitive to CO as per
available literature reports and author’s knowledge.
6.2 Recommendations for Future Work
This thesis has presented advances in the field of nanostructured WO3 thin film
based gas sensors. Throughout the course of this research, several areas of interest,
which have tremendous research potential, have been identified. In this section, some
proposals will be made for possible future development of the current research.
These proposals for future work are listed below:
In the present study, the films were annealed at 300ºC and 400ºC for 2 hours
in air. There was no increase in grain size after annealing the film at 400ºC,
however, the films were completely crystalline. This project can be extended
to investigate the effect of annealing temperature between 300-600ºC on
change in grain size and gas sensing behaviour. This will allow to achieve an
optimum annealing temperature of the WO3 based films for improved gas
sensing properties.
The Fe-doped WO3 thin film gas sensor has demonstrated to be a potential
candidate for low temperature CO gas sensing. However, in the present study,
only a fixed Fe concentration (0.5 at.%) of co-evaporated tungsten oxide
films have been investigated. The study can be extended to vary the iron
concentrations (0.5 at.% - 10 at.%) and systematically investigate the
optimum effect of Fe doping on the sensing performance and operating
temperature of the tungsten oxide thin films towards CO.
141
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