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Electrochimica Acta 55 (2010) 5786–5792

Contents lists available at ScienceDirect

Electrochimica Acta

journa l homepage: www.e lsev ier .com/ locate /e lec tac ta

nvestigation on hydrophilicity of micro-arc oxidized TiO2

ano/micro-porous layers

.R. Bayati a,∗, Roya Molaeia, Amir Kajbafvalab,∗∗, Saeid Zanganehc, H.R. Zargard, K. Janghorbane

School of Metallurgy and Materials Engineering, Iran University of Science and Technology, P.O. Box 16845-161, Tehran, IranDepartment of Materials Science and Engineering, North Carolina State University, 911 Partners Way, Raleigh, NC 27695-7907, USASchool of Engineering, University of Connecticut, 261 Glenbrook Rd., Storrs, CT, USADepartment of Materials Engineering, University of British Columbia, Vancouver, CanadaDepartment of Materials Science and Engineering, Shiraz University, Shiraz, Iran

r t i c l e i n f o

rticle history:eceived 18 February 2010eceived in revised form 3 May 2010ccepted 4 May 2010vailable online 11 May 2010

a b s t r a c t

Titania porous layers with a rough surface were synthesized via micro-arc oxidation (MAO) and the effectof the applied voltage and electrolyte concentration on surface structure, and chemical composition ofthe layers was studied. Morphological and topographical investigations, performed by SEM and AFM,revealed that pore size and surface roughness of the layers increased with the applied voltage and theelectrolyte concentration. Based on the XRD and XPS results, the grown layers consisted of anatase and

eywords:itanium oxideicro-arc oxidation

orous materials

rutile phases with varying fractions depending on growth conditions. It was found that anatase/rutilerelative content reached its maximum value at medium applied voltages or electrolyte concentrations.Finally, hydrophilicity of the grown layers was determined using a water contact angle apparatus, anda correlation between measured contact angles and MAO-parameters was suggested. It was observed

d undhest h

ydrophilic that the layers synthesize10 g l−1 exhibited the hig

. Introduction

Titanium dioxide (TiO2) has been widely studied due to its greatariety of applications by many researchers. Photocatalysis [1,2],olar cells [3,4], gas sensors [5], lithium batteries [6], antibacte-ial activity [7,8], and self-cleaning and antifogging surfaces [9–11]re some of its applications. Compared with its powder form, TiO2lms can be easily separated, recovered, and efficiently recycled12]. In 1997, Wang et al. [13] reported that UV-irradiation of titaniaurfaces produces a super-hydrophilic surface. So far, hydrophilicitanium dioxide layers have attracted many theoretical and prac-ical applications such as self-cleaning and antifogging mirrors.owever, the hydrophilicity of the TiO2 layers needs to be furthernhanced for practical applications [14]. One way to improve itsydrophilicity is by fabricating porous layers. Since photo-chemicaleactions mainly occur on the surface, the surface properties of

iO2 such as surface area, defects, surface acidity, surface func-ional groups, particle size, and crystalline phase will greatly affecthoto-chemical activity and related mechanisms [12,15]. Yu andark studied the effect of specific surface area on photocat-

∗ Corresponding author. Tel.: +98 21 77920727.∗∗ Corresponding author. Tel.: +1 919 515 7217.

E-mail addresses: bayati@iust.ac.ir (M.R. Bayati),kajbaf@ncsu.edu (A. Kajbafvala).

013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2010.05.021

er the applied voltage of 400 V in the electrolytes with a concentration ofydrophilicity.

© 2010 Elsevier Ltd. All rights reserved.

alytic performance and hydrophilicity of titania and demonstratedthe more the specific surface area the better the photo-inducedhydrophilicity [16,17]. Titanium dioxide with varying morpholo-gies has been synthesized by different methods including sol–gel[18,19], chemical vapor deposition [20,21], physical vapor depo-sition [22,23], hydrothermal process [24,25], electrochemicalmethods [26,27], liquid phase deposition [28], and spray pyrolysis[29,30]. TiO2 can also be obtained via micro-arc oxidation (MAO)process [31,32].

MAO is an electrochemical technique for formation of anodicfilms by spark/arc micro-discharges which move rapidly on thevicinity of the anode surface [33–36]. It is characterized by highproductivity, economic efficiency, ecological friendliness, highhardness, good wear resistance, and excellent bonding strengthwith the substrate [37–39]. This process is carried out at voltageshigher than the breakdown voltage of the gas layer enshrouding theanode. Since the substrate is connected to positive pole of the recti-fier as anode, the gas layer consists of oxygen. When the dielectricgas layer completely covers the anode surface, electrical resistanceof the electrochemical circuit surges and the process continues pro-viding that the applied voltage defeats the breakdown voltage of

the gas layer. Applying such voltages leads to formation of electri-cal discharges via which electrical current could pass the gas layer.MAO process is characterized by these electrical sparks [34,40]which are responsible for formation of the structural pores [41,42].Due to strong electrical field (106–108 V m−1) between anode and

M.R. Bayati et al. / Electrochimica Acta 55 (2010) 5786–5792 5787

Fig. 1. SEM top-view of the TiO2 layers grown in the electrolytes with a concentration of 5 g l−1 under applied voltages of (a) 350 V, (b) 400 V, (c) 450 V, (d) 500 V, and (e)550 V.

Fig. 2. SEM top-view of the TiO2 layers grown in the electrolytes with a concentration of 10 g l−1 under applied voltages of: (a) 300 V, (b) 350 V, (c) 400 V, (d) 450 V, (e) 500 V,and (f) 550 V.

5788 M.R. Bayati et al. / Electrochimica Acta 55 (2010) 5786–5792

F on of 2(

cwaoobpe

cvpbo

2

epl(tecawcraudpie

if

ig. 3. SEM top-view of the TiO2 layers grown in the electrolytes with a concentratif) 500 V, and (g) 550 V.

athode, electrolyte anions are drawn into the structural poreshere they can attend electrochemical reactions. Structural pores

re formed by electron avalanches taking places on the vicinityf the anode. Characteristics of electrolyte have a great influencen the film formation kinetics. Phosphates, sulfates, silicates andorates are four conventional kinds of electrolytes employed inrevious researches, and the formed TiO2 films usually contain thelement of the electrolytes (P, S, Si, B, etc.) [34,36,37,40].

In this research, results of growth, characterization, and espe-ially hydrophilicity performance of the TiO2 layers fabricatedia MAO process were discussed. A correlation between MAO-arameters and hydrophilicity of the layers was proposed. To theest of our knowledge, this is the first study on the hydrophilicityf the MAO-grown titania layers.

. Experimental

A DC-rectifier with a maximum output of 600 V/30 A wasmployed as current source. 3 cm × 3 cm × 0.5 mm commerciallyure grade 2 titanium pieces, surrounded by an ASTM 316 stain-

ess steel cylindrical container (cathode), was also used as substrateanode). Three sodium phosphate (Na3PO4·12H2O, Merck) solu-ions with different concentrations were used as electrolyte. Thelectrolyte temperature was fixed at 70 ± 3 ◦C employing a waterirculating system. Prior to MAO treatment, substrates underwentcleaning process including mechanical polishing followed byashing in distilled water. Afterward, the titanium plates were

hemically etched in diluted HF solution (HF:H2O = 1:20 vol.%) atoom temperature for 30 s, and then washed in distilled watergain. In the last stage of cleaning procedure, the substrates wereltrasonically cleaned in acetone for 15 min and finally washed byistilled water. After surface cleaning, MAO treatment was accom-lished under different voltages in a range of 250–550 V with +50 V

ntervals. Meanwhile, the deposition time for each run was consid-red as 3 min.

Surface morphology and topography of the layers were exam-ned by scanning electron microscope (TESCAN, Vega II) and atomicorce microscope (Veeco auto probe) in contact mode with a 10 nm

0 g l−1 under applied voltages of: (a) 250 V, (b) 300 V, (c) 350 V, (d) 400 V, (e) 450 V,

radius silicon tip. X-ray diffraction (Philips, PW3710) and X-rayphotoelectron spectroscopy (VG Microtech, Twin anode, XR3E2 X-ray source, using AlK� = 1486.6 eV) techniques were used to studyphase structure and chemical composition of the synthesized lay-ers. The photo-induced hydrophilicity of the layers was evaluatedby screening photos and measuring the contact angle of DI-waterdroplets. The grown layers were first UV-irradiated by a 25 W lamp(� = 365 nm) for 1 h; then, the hydrophilicity photos were obtainedby a water contact angle apparatus. The hydrophilicity studies wereperformed in the atmosphere.

3. Results and discussion

SEM morphologies of the layers synthesized at different appliedvoltages for 3 min in electrolytes with concentrations of 5, 10, and20 g l−1 are shown in Figs. 1–3. No pore was observed in the struc-ture of the layers grown at applied voltages less than 350 V in theelectrolytes with a concentration of 5 g l−1, but the samples whichwere fabricated under higher applied voltages were porous, and thepore size increased with the applied voltage. It should be mentionedthat no electrical sparking occurred at the applied voltages less than350 V, and increasing the applied voltage resulted in generation ofstronger and long-living sparks. It is to emphasize that applyinghigher voltages caused electrical sparks with higher energy dueto higher electrical current passing through the electrochemicalcell. Stronger electric avalanches resulted in the formation of widerpores. A similar behavior was also observed for the layers synthe-sized under various voltages in electrolytes with concentrations of10 and 20 g l−1 for 3 min. It was noted that the voltage at whichstructural pores began to appear decreased with the electrolyteconcentration. As the electrolyte electrical resistance, and, hence,total resistance of the electrochemical cell decreased with increas-ing the electrolyte concentration, the voltage which was applied on

the anode surface increased and reached the breakdown voltage ofthe surface gas layer more suddenly. As a consequence, the elec-trical sparks appeared at lower applied voltages. Furthermore, itwas observed that the pore size increased with the electrolyte con-centration. Any decrease in circuit resistance resulted in increased

M.R. Bayati et al. / Electrochimica Acta 55 (2010) 5786–5792 5789

F oncen(

caebht

ig. 4. AFM surface topography of the TiO2 layers grown in the electrolytes with a ce) 500 V, and (f) 550 V.

urrent which passed the cell, and the power of the electrical

valanches. As a consequence, larger pores were formed in thickerlectrolytes. According to the SEM images presented in Fig. 1, it cane deduced that the layers grown under medium applied voltagesad the highest pore density, smallest pore size, and, consequently,he highest surface area.

Fig. 5. XRD patterns of TiO2 layers grown under different conditions: (a) 5

tration of 10 g l−1 under applied voltages of (a) 300 V, (b) 350 V, (c) 400 V, (d) 450 V,

Fig. 4 shows the AFM surface topography of the layer syn-

thesized with different applying voltages in electrolytes with aconcentration of 10 g l−1 in a scale of 10 �m × 10 �m. Other AFMimages are not presented here. The results depict a rough surfacewhich is usual for MAO-grown layers. Using statistical analysis, itwas found that the average surface roughness of the layers (ASR)

50 V, (b) 500 V, (c) 450 V, (d) 400 V, (e) 350 V, (f) 300 V, and (g) 250 V.

5790 M.R. Bayati et al. / Electrochimica Acta 55 (2010) 5786–5792

Fc

iaclge

saasAmXrmavpihs

luA24cvOdatgscoIatraTt2ri

hydrophilicity of the layers, reached its maximum value at medium

ig. 6. Anatase mass fraction as a function of applied voltage for different electrolyteoncentrations.

ncreased with the applied voltage because applying higher volt-ges increased the electrical current passing the electrochemicalell. Higher electrical current generated more heat in the oxideayer resulting in sequential local melting and solidifying of therowing layers in the surrounding electrolyte which made the lay-rs porous.

XRD patterns, depicted in Fig. 5, demonstrate that the synthe-ized layers consisted of anatase and the rutile phases. Since thenatase form of the TiO2 is known as the only photoactive phasemong its other forms, the mass fraction of anatase in the synthe-ized layers determines the sample hydrophilicity performance.natase mass fraction of the layers was calculated using the for-ula WA = (1 + 1.265IR/IA)−1 [43] where IR and IA are the normalizedRD peak intensities of rutile and anatase phases, respectively. Theesults are shown in Fig. 6. The results revealed that the anataseass fraction reached a maximum value at a certain applied volt-

ge (Vm) and then decreased at very high voltages. Applying higheroltages warms the anode up more due to higher electrical currentassing the electrochemical circuit and more electrical sparks tak-

ng place on the vicinity of the anode [34]. Because of this extraeat, the anatase which is a meta-stable phase transforms to rutiletable phase at higher temperatures.

To further confirm the stoichiometry of the synthesized TiO2ayers grown in the electrolytes with a concentration of 10 g l−1

nder different applied voltages, XPS technique was employed.ll of the binding energies were referenced to the C(1 s) peak at85.0 eV. Fig. 7 depicts the Ti(2p3/2) core level binding energy at58.7 eV. Hence, existence of titanium in the form of Ti+4 state isonfirmed. The O(1 s) peak, shown in Fig. 8, can be resolved intoarious components using original software with Gaussian rule. All(1 s) peaks are wide and asymmetric demonstrating that there areifferent kinds of O-binding states in the layers. The peak A, locatedt 530.1 eV, is assigned to the crystal lattice oxygen (Ti–O), whilehe peak B, located at 531.6 eV, represents the oxygen in hydroxylroups (O–H) formed by adsorbed O2/OH on the surface. Oxide freeurfaces contacting with the atmosphere are always hydrated, i.e.ontain water molecules and hydroxyl groups. There are two typesf OH-groups on the surface: single M–OH and double OH–M–OH.t was observed that the proportion of this peak increased withpplied voltage. The peaks C at the binding energy of 532.7 eV revealhe existence of O− species on the surface. Finally, the peak D rep-esents oxygen in water molecules. Since the layers are porous andre grown in aqueous solutions, water may trap inside the pores.he proportions of the surface area under the peaks OA to that of

he peaks Ti(2p3/2) were calculated and approximately obtained as.09, 1.98, and 1.83 for the applied voltage of 200, 350, and 550 V,espectively. The reason for decreasing the O/Ti value with increas-ng the voltage is that higher applied voltages warms the anode up.

Fig. 7. XPS Ti(2p3/2) core level binding energy of the layers grown in the electrolyteswith a concentration of 10 g l−1 under different voltages.

It has been suggested [44] that titania begins to lose its oxygen athigher temperatures. Moreover, applying such voltages increasesthe growth rate resulting in formation of surface defects namelyoxygen vacancies. It was observed that the amount of –OH-groups,present on the surface of the films, increased with the appliedvoltage. This phenomenon asserts the formation of more surfaceoxygen vacancies under higher applied voltage, because hydroxylgroups occupy the sites corresponding to such vacancies.

Results of the hydrophilicity tests are presented in Fig. 9 wherethe measured contact angles are plotted as a function of the appliedvoltage for different electrolytes concentrations. It was deducedthat water contact angle decreased with the applied voltages andreached its minimum value at medium applied voltages, and,then, increased at higher voltages. Two reasons were proposed forsuch a behavior. First, anatase/rutile relative content, affecting the

voltages, as elucidated in Fig. 6. The mechanism of photo-inducedhydrophilicity of TiO2 layers has been previously studied [45–47].It was suggested that preferential adsorption of water moleculeson the photo-generated surface defective sites led to the formation

M.R. Bayati et al. / Electrochimica Acta 55 (2010) 5786–5792 5791

he ele

oawwm[

T

O

T

aigwtBsbrbsmlahtim

Ft

Fig. 8. XPS O(1 s) core level binding energy of the layers grown in t

f highly hydrophilic TiO2 films [16,48]. Photo-generated electronsnd holes could either recombine or move to the surface to reactith species adsorbed on the surface. Some of the electrons reactith lattice metal ions Ti4+ to form Ti3+ defective sites [49]. The for-ation processes of defective sites on TiO2 surface was expressed

16] as follows:

iO2 + h� → e− + h+ (1)

2− + 2 h+ → 1/2O2 + oxygen vacancy (2)

i4+ + e− → Ti3+ (3)

In air, the surface trapped electrons tend to react immedi-tely with O2 adsorbed on the surface to form O2

− or O22−

ons. Meanwhile, water molecules may coordinate into the oxy-en vacancy sites, which lead to dissociative adsorption of theater molecules on the surface [50,51]. This process gives rise to

he increase of hydroxyl content on the illuminated TiO2 surface.ased on this mechanism, electrons and holes are two prerequi-ites of hydrophilicity reactions. Since, they are mostly generatedy anatase phase, the layers which have the highest anatase/rutileatios exhibit the maximum hydrophilicity. Secondly, as explainedefore, the layers grown under medium applied voltages had higherurface areas where the hydrophilicity reactions take place. Theore the surface area the higher the hydrophilicity. Although the

ayers grown under the applied voltage of 250 V had the highest

natase relative content (see Fig. 6), they did not reveal a highydrophilicity due to their low effective surface area. Results showhat the MAO-synthesized titania layers exhibit a high hydrophilic-ty due to their rough surface. Wenzel has proposed a theoretical

odel to describe the contact angle (�) of a rough surface [52].

ig. 9. Water contact angle as a function of the applied voltage for different elec-rolyte concentrations.

[[

[[

[

ctrolytes with a concentration of 10 g l−1 under different voltages.

He modified Young’s equation as follows:

cos(�) = r(�SV − �SL)�LV

= r cos(˛) (4)

where r is defined as the surface ratio, �SV, �SL, and �LV are theinterfacial free energy per unit area of solid–vapor, solid–liquid,and liquid–vapor interfaces, respectively, and ˛ is the contact angleof a smooth area. This equation indicates that the surface rough-ness enhances the hydrophilicity of a hydrophilic surface (� < 90◦)and the hydrophobicity of a hydrophobic one (� > 90◦) because r isalways greater than 1.

4. Conclusions

Micro-arc oxidation was used to grow titania layers with aporous and rough surface which are suitable for hydrophilicityapplications. The layers consisted of anatase and rutile phaseswhose fraction was observed to vary with the applied voltage andelectrolyte concentration. It was found that the samples fabricatedunder medium applied voltages had highest anatase/rutile rela-tive content. In addition, pore size as well as surface roughness ofthe layers increased with voltage and electrolyte concentration. Itwas also found that the layers prepared under the applied voltageof 400 V in the electrolytes with a concentration of 10 g l−1 fromsodium three phosphate salt exhibited maximum hydrophilicity.

Acknowledgement

The authors would like to thank the personnel in the advancedceramic synthesis laboratory of Iran University of Science and Tech-nology for their technical assistance.

References

[1] V. Stengl, S. Bakardjieva, N. Murafa, Mater. Chem. Phys 114 (2009) 217.[2] F. Meng, Z. Sun, Mater. Chem. Phys 118 (2009) 349.[3] W.H. Baek, I. Seo, T.S. Yoon, H.H. Lee, C.M. Yun, Y.S. Kim, Sol. Energy Mater. Sol.

Cells 93 (2009) 1587.[4] M.S. Roy, P. Balraju, Manish Kumar, G.D. Sharma, Sol. Energy Mater. Sol. Cells

92 (2008) 909.[5] N.E. Stankova, I.G. Dimitrov, T.R. Stoyanchov, P.A. Atanasov, Appl. Surf. Sci. 254

(2007) 1268.[6] L. Kavan, K. Kratochilova, M. Gratzel, J. Electroanal. Chem 394 (1995) 93.[7] W. Su, S.S. Wei, S.Q. Hu, J.X. Tang, J. Hazard. Mater 172 (2009) 716.[8] R.A. Damodar, S.J. You, H.H. Chou, J. Hazard. Mater 172 (2009) 1321.[9] J. Yu, M. Zhou, H. Yu, Q. Zhang, Y. Yu, Mater. Chem. Phys 95 (2006) 193.10] K. Guan, Y. Yin, Mater. Chem. Phys 92 (2005) 10.

11] D. Vernardou, G. Kalogerakis, E. Stratakis, G. Kenanakis, E. Koudoumas, N. Kat-

sarakis, Solid State Sci. 11 (2009) 1499.12] Y. Chen, Feng Chen, J. Zhang, Appl. Surf. Sci. 255 (2009) 6290.13] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M.

Shimohigoshi, T. Watanabe, Nature 388 (1997) 431.14] F. Meng, Z. Sun, Appl. Surf. Sci. 255 (2009) 6715.

5 imica

[

[

[

[[[[[[

[[[

[[[

[

[[[[[[

[

[[

[[

[

[

[

[

[

[

[

[Catal. A: Gen 314 (2006) 40.

792 M.R. Bayati et al. / Electroch

15] M. Houmard, D. Riassetto, F. Roussel, A. Bourgeois, G. Berthomé, J.C. Joud, M.Langlet, Surf. Sci 602 (2008) 3364.

16] J.C. Yu, J. Yu, W. Ho, J. Zhao, J. Photochem. Photobiol. A: Chemistry 148 (2002)331.

17] M. Wark, J. Tschirch, O. Bartels, D. Bahnemann, J. Rathousky, Micropor. Mesopor.Mater 84 (2005) 247.

18] J. Sun, Y. Wang, R. Sun, S. Dong, Mater. Chem. Phys 115 (2009) 303.19] M. Hamadanian, A. Reisi-Vanani, A. Majedi, Mater. Chem. Phys 116 (2009) 376.20] M. Zhou, X. Ma, Electrochem. Commun 11 (2009) 921.21] Y. Su, S. Han, X. Zhang, X. Chen, L. Lei, Mater. Chem. Phys 110 (2008) 239.22] B. Liu, L. Wen, X. Zhao, Mater. Chem. Phys 106 (2007) 350.23] P. Kasemanankul, N. Witit-Anan, S. Chaiyakun, P. Limsuwan, V. Boonamnuayvi-

taya, Mater. Chem. Phys 117 (2009) 288.24] H. Tian, J. Ma, K. Li, J. Li, Mater. Chem. Phys 112 (2008) 47.25] H. Yun, C. Lin, J. Li, J. Wang, H. Chen, Appl. Surf. Sci. 255 (2008) 2113.26] H.J. Oh, J.H. Lee, Y.J. Kim, S.J. Suh, J.H. Lee, C.S. Chi, Mater. Chem. Phys 109 (2008)

10.27] H. Oh, J. Lee, Y. Kim, S. Suh, J. Lee, C. Chi, Mater. Chem. Phys 109 (2008) 10.28] M. Pourmand, N. Taghavinia, Mater. Chem. Phys 107 (2008) 449.29] L. Castaneda, J.C. Alonso, A. Ortiz, E. Andrade, J.M. Saniger, J.G. Banuelos, Mater.

Chem. Phys 77 (2002) 938.30] I. Oja Acik, A. Junolainen, V. Mikli, M. Danilson, M. Krunks, Appl. Surf. Sci. 256

(2009) 1391.31] F. Jin, H. Tong, L. Shen, K. Wang, P.K. Chu, Mater. Chem. Phys 100 (2006) 31.

32] D. Wei, Y. Zhou, D. Jia, Y. Wang, Mater. Chem. Phys 104 (2007) 177.33] L. Wan, J.F. Li, J.Y. Feng, W. Sun, Z.Q. Mao, Mater. Sci. Eng. B 139 (2007) 216.34] A.L. Yerokhin, A. Leyland, A. Matthews, Surf. Coat. Technol 200 (2002) 172.35] X. Sun, Z. Jiang, S. Xin, Z. Yao, Thin Solid Films 471 (2005) 194.36] P. Gupta, G. Tenhundfeld, E.O. Daigle, D. Ryabkov, Surf. Coat. Technol 201 (2007)

8746.

[

[

[

Acta 55 (2010) 5786–5792

37] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, Surf. Coat. Technol 130 (2000)195.

38] W. Xue, C. Wang, R. Chen, Z. Deng, Mater. Lett 52 (2002) 435.39] W. Xue, Z. Deng, R. Chen, T. Zhang, Thin Solid Films 372 (2000)

114.40] A.L. Yerokhin, V.V. Lyubimov, R.V. Ashitkof, Ceram. Int 24 (1998) 1.41] M.R. Bayati, F. Golestani-Fard, abd A. Z. Moshfegh, Mater. Chemi. Phys 120

(2010) 582.42] M.R. Bayati, F. Golestani-Fard, A.Z. Moshfegh, Appl. Surf. Sci. 256 (2010)

4253–4259.43] J. He, Q.Z. Cai, Y.G. Ji, H.H. Luo, D.J. Li, B. Yu, J. Alloys Compd 482 (2009)

476.44] Wi.S. Epling, C.H.F. Peden, M.A. Henderson, U. Dieboldb, Surf. Sci 412/413 (1998)

333.45] N. Sakai, A. Fujishima, T. Watanabe, K. Hashimoto, J. Phys. Chem. B 105 (2001)

3023.46] R. Wang, N. Sakai, A. Fujishima, T. Watanabe, K. Hashimoto, J. Phys. Chem. B

103 (1999) 2188.47] N. Sakai, R. Wang, A. Fujishima, T. Watanabe, K. Hashimoto, Langmuir 14 (1998)

5918.48] M.R. Hoffmann, S.T. Martin, W.Y. Choi, D.W. Bahnemann, Chem. Rev 95 (1995)

69.49] S.D. Sharma, D. Singh, K.K. Saini, C. Kant, V. Sharma, S.C. Jain, C.P. Sharma, Appl.

50] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M.Shimohigoshi, T. Watanabe, Adv. Mater 10 (1998) 135.

51] R.D. Sun, A. Nakajima, N. Sakai, A. Fujishima, T. Watanabe, K. Hashimoto, J. Phys.Chem. B 105 (2001) 1984.

52] R.N. Wenzel, J. Phys. Colloid Chem 53 (1949) 1466.