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Effect of stoichiometry and microstructure on hydrolysis in MoO3 films
Chemical Physics Letters 418 (2006) 170–173 www.elsevier.com/locate/cplett
Effect of stoichiometry and microstructure on hydrolysis in MoO3 films Tarsame S. Sian, G.B. Reddy
*
Thin Film Laboratory, Department of Physics, Indian Institute of Technology, Hauz Khas, New Delhi 110 016, India Received 29 July 2005; in final form 20 October 2005 Available online 17 November 2005
Abstract The effect of microstructure and stoichiometry of MoO3 films on hydrolysis has been thoroughly investigated and reported in this Letter. MoO3 films having different cluster size, stoichiometry and crystal structure were prepared and deliberately exposed to identical high humidity conditions for different durations. Infrared spectroscopy was used to monitor hydroxylation process in these films. Polycrystalline films with large grain size were found to be inert to humidity conditions establishing the role of structure in hydroxylation process. All amorphous films adsorb water, part of which is dissociated to form Mo–OH, Mo–O–H bonds giving rise to two closely placed absorption peaks in 2500–4000 cm1 spectral range. The concentration of these bonds and free water are strongly influenced by film stoichiometry and microstructure. 2005 Elsevier B.V. All rights reserved.
1. Introduction Oxides are the best-known functional materials that have long been used in various devices viz. sensors [1], electrochromic devices [2], transparent conductors [3], components of various optical devices [4], catalyst in reduction/ oxidation processes [5]. Some devices viz. window coatings, humidity sensors are outdoor devices that are not only exposed constantly to continuously changing ambient conditions but also operated in cyclic manner in these ambients. The major drawback of some well-known electrochromic oxides is that they show relatively higher affinity for water vapor, which leads to degradation in their performance as reported by Yoshiike and Kondo [6] and Arnoldussen [7]. Very limited studies have been conducted to investigate and understand how adsorbed water affects device performance and itÕs lifetime. Based on XPS studies on H2O adsorbed titanium oxide surfaces, Kuntz and Heinrich [8] concluded that dissociation of adsorbed H2O molecules occurs only at defect sites. Noguera [9] further suggested that H+ and OH interact with the oxygen and cation of *
Corresponding author. Fax: +91 11 26581114. E-mail address: [email protected] (G.B. Reddy).
0009-2614/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.10.112
the oxide, respectively, to form hydrolyzed bonds. The complete process; dissociation of water into H+ and OH and formation of the hydrolyzed bonds with host is generally referred as hydrolysis. Therefore kinetics of both, dissociation and interaction with cations/anions processes must be interlinked. The progress of the second process is expected to depend on the oxidation state of cation, as lower valency state (LVS) will curtail the formation of hydrolyzed bonds because of the presence of electrons in their valence shells [9]. It has already been reported by the authors that relative concentration of LVS depends on stoichiometry and crystallinity of the films [10]. Although Arnoldussen [7] reported deleterious effect of water on electrochromic properties of MoO3, the effect of stoichiometry on hydrolysis was not investigated by him. The objectives of present work are: (i) to find out the role of cluster/crystallite size and stoichiometry on water dissociation rate (ii) to understand the interaction of dissociated (H+ and OH) species with MoO3 as a function of film stoichiometry. Such a study relating the hydrolysis to the stoichiometry has not been reported for any oxide so far. Fulfillment of these objectives will enable to realize more efficient EC devices with longer life times. MoO3 films with different microstructures and stoichiometry were intentionally exposed to ambient with 70%
T.S. Sian, G.B. Reddy / Chemical Physics Letters 418 (2006) 170–173
171
relative humidity (RH) at room temperature. The resulting chemical changes were monitored using FTIR at regular intervals. 2. Experimental For Fourier transform infrared (FTIR) spectroscopic studies, 1-lm (±0.01 lm) thick MoO3 films were deposited on indium doped tin oxide (ITO) coated glass substrates. Four different samples were prepared by: (i) evaporating MoO3 powder (Merck, 99.99%) at a base vacuum of 2 · 106 mbar (referred as F1 films) (ii) annealing F1 films in air at 150 C (referred as F2 films) (iii) annealing F1 annealed in air at 350 C (referred as F3 films) (iv) evaporating MoO3 powder thermally in the presence of oxygen plasma at 40 mtorr using activated reactive evaporation (ARE) technique (referred as F4 films) [11]. Annealing was carried out using programmable furnace with temperature accuracy of ±1 C. Annealing duration, specimen heating and cooling rates were maintained constant at 60 min, 10 C/min and 2 C/min, respectively. Infrared spectra of the films was measured in reflectance mode in the spectral region from 4000 to 2500 cm1 using Perkin– Elmer (Model spectrum BX2) spectrophotometer. Minimum in reflectance (R) in a recorded spectrum is referred as Ôabsorption peakÕ in this Letter. The instrumental resolution was fixed at 2 cm1 and the number of spectra for one measurement was 256 cycles. Transmission electron microscopy (TEM) studies were carried out using Philips Model CM 12 electron microscope. Photoelectron spectroscopic studies were carried out on Perkin–Elmer Model PHI 1257 spectrometer, with a dual anode Mg/Al Ka 25 kV X-ray source and high-resolution hemispherical section analyzer. The radiation used for the present study was Mg Ka at 150 W. The resolution of the instrument was better than ±0.05 eV. Probing X-ray photons were incident at an angle of 45 to the substrate normal and the axis of the hemispherical analyzer was kept parallel to the substrate normal. 3. Results and discussion XPS elemental scan recorded on the surface of freshly prepared MoO3 film given in Fig. 1 shows the presence of photoelectron peaks corresponding to Mo 3d, O 1s and C 1s. Also the Auger transitions corresponding to these elements are seen in the figure. The micrograph and selected area diffraction (SAD) pattern, recorded with transmission electron microscopic (TEM) on F1, F2, F3 and F4 films are shown in Fig. 2a–d, respectively. SAD pattern of F1, F2 and F4 films reveal their amorphous nature and that of F3 films exhibit polycrystalline form. As seen in micrographs, F1 films are more porous as compared to other films. The average size of the clusters is approximately 6 nm in F1 and F2 films. F3 films contain large crystallites with average size of 100 nm and the distribution of cluster size in F4 films vary from 4 to 14 nm. In an earlier report
Fig. 1. XPS elemental surface scan recorded on freshly MoO3 films.
[10], the authors have confirmed the variation in stoichiometry of these films through optical measurements. It was observed that F2 films had the highest non-stoichiometry followed by that in F1. Both F4 and F3 films were found to be nearly stoichiometric [11]. Therefore, the study of hydrolysis process in these films should clearly bring out the role of porosity, cluster/crystallite size and stoichiometry. Infrared reflectance (R) recorded on F1, F2, F4 and F3 films in 2500–4000 cm1 spectral region before and after exposing the films to high humid ambient for different durations, are shown in Fig. 3 a–d, respectively. Curve 1, 2, 3 and 4 in all cases correspond to R of fresh films and those recorded after 7, 21 and 40 days of continuous exposure, respectively. As seen from Fig. 3d , F3 films do not show any absorption peaks in this spectral range even after continuous exposure for 40 days. In all other films three absorption peaks, labeled as peak I, peak II and peak III (Fig. 3), are observed in spectral windows: 3400–3500, 3200–3250 and 3020–3040 cm1, respectively. It is known that peak I is due to the stretching mode of H2O [12]. The presence of two absorption peaks II and III, located closely with varying intensities, suggest the formation of two different hydrolyzed bonds in exposed films. These bonds are formed due to attachment of the water-dissociated species: H+ and OH with anion and cation in the host, respectively, [9]. From the figure, it is clear that peak II grows faster than peak III in all-amorphous films. Protons being lighter are able to penetrate inside films at a much faster rate as compared to OH species. Therefore, peaks II and III are assigned to Mo–O–H and Mo–OH bonds, respectively. The hydroxylation of Mo–O bonds can be represented by the following reactions: H2 OðadÞ ! Hþ þ OH ðO–MoÞ þ OH ! O–Mo–OH
ð1Þ ð2Þ
ðO–MoÞ þ Hþ ! Mo–O–H
ð3Þ
172
T.S. Sian, G.B. Reddy / Chemical Physics Letters 418 (2006) 170–173
Fig. 2. TEM micrographs of F1 (a), F2 (b), F4 (c) and F3 (d) along with their respective selected area diffraction pattern.
Fig. 3. Measured IR reflectance (R) spectra of F1 (a), F2 (b) F4 (c) and F3 (d) films. Curves 1, 2, 3 and 4 in (a), (b) and (c) refer to the R spectrum of fresh films and that recorded after 7, 21 and 40 days of continuous exposure to humid ambient. In F3 films, curves 1 and 2 refer to the R spectrum of fresh and 40 days exposed films. All spectra shown in the figure are baseline fitted.
where H2O(ad) is the water adsorbed in the film. Infrared measurements do not show any evidence of existence of either free water or Mo–OH and Mo–O–H bonds in F3 films (Fig. 3d). R spectra of the films in Fig. 3 were deconvoluted, to obtain intensities of individual peaks. The heights of these peaks are assumed to be proportional to the amount of
adsorbed free water, Mo–O–H and Mo–OH bonds, respectively. The intensity of peaks I and III are monitored as a function of exposure duration in all films and the same are shown in Figs. 4 and 5, respectively. As seen in Fig. 4, among fresh films the intensity of peak I is highest in F1 films followed by that observed in F4 and F2 films, in that order. In F2 films, water related peak was found to grow
T.S. Sian, G.B. Reddy / Chemical Physics Letters 418 (2006) 170–173
Fig. 4. Plot of intensity of absorption peak associated with water (peak I) vs. exposure duration of F1, F2 and F4 films.
173
and F2 films. This indicates that water adsorbed by these films is highest. The present study clearly established the following: (i) complete inertness towards humidity exhibited by F3 films was due to the combined effect of both crystal structure and large grain size, (ii) high porosity of F1 films allowed adsorption of more water immediately after venting the films to atmosphere, as observed from IR studies (Fig. 3a, curve 1) and (iii) the formation of low concentration of Mo–OH bonds observed in highly non-stoichiometric F2 films inspite of having high amount of adsorbed water confirms the role of stoichiometry in controlling the dissociation of water. This can be explained as follows: In order to form Mo–OH bonds, the unpaired electron in the OH group must be transferred to Mo 4d orbitals, in accordance with acid base concept [9]. It has already been reported that in non-stoichiometric films these orbitals are partially occupied and electrons in them dissuade transfer of electron from OH group, causing slow growth rate of hydrolyzed bonds. Therefore, the growth rate of Mo–OH bonds will be inversely proportional to the degree of nonstoichiometry in the films. As a consequence of this, less water is dissociated leading to higher concentration of free water as observed in Fig. 4. 4. Conclusions From the present work it is established that; (i) polycrystalline films exhibit better chemical inertness to humidity as compared to amorphous films and (ii) higher the non-stoichiometry lower is the rate of hydrolysis. References
Fig. 5. Plot of Intensity of absorption peak associated with Mo–OH bonds (peak III) vs. exposure duration of F1, F2 and F4 films.
much slowly attaining highest intensity confirming the presence of more free water in these films. The intensity of absorption peak associated with Mo–OH bonds was found to be lowest at any instant (Fig. 5), which implies that consumption (dissociation of water) of free water is minimum in these films. In F4 films intensity of the absorption peak corresponding to Mo–OH bonds is highest compared to that of other films and also the intensity of free water related peak is in between those observed in F1
[1] M.K. Jain, M.C. Bhatnagar, G.L. Sharma, Appl. Phys. Lett. 73 (1998) 3854. [2] P.M.S. Monk, R.J. Mortimer, D.R. RosseinskyElectrochromism, Fundamentals and Applications, VCH, Weinheim, 1995. [3] K.L. Chopra, S.S. Major, D.K. Pandya, Thin Solid Films 102 (1983) 1. [4] R.M. Roth, T. Izuhara, R.L. Espionala, D. Djukic, R.M. Osgood Jr., S. Bakhru, H. Bakhru, Opt. Lett. 30 (2005) 994. [5] G. Mestl, P. Ruiz, B. Belmon, H. Knozinger, J. Phys. Chem. B 98 (1994) 11269. [6] N. Yoshiike, S. Kondo, J. Electrochem. Soc. 130 (11) (1983) 2283. [7] T.C. Arnoldussen, J. Electrochem. Soc. 123 (4) (1976) 527. [8] R.L. Kuntz, V.E. Heinrich, Phys. Rev. B 26 (12) (1982) 6682. [9] C. Noguera, Physics and Chemistry at the Oxide Surfaces, Cambridge University Press, New York, 1996 (p.183)(Chapter 6). [10] T.S. Sian, G.B. Reddy, Sol. Energ. Mater. Sol. Cells 82 (2004) 375. [11] T.S. Sian, G.B. Reddy, J. Appl. Phys 98 (2005) 26104. [12] T.S Sian, G.B. Reddy, Solid State Ionics 167 (2004) 399.
Effect of stoichiometry and microstructure on hydrolysis in MoO3 films Tarsame S. Sian, G.B. Reddy
*
Thin Film Laboratory, Department of Physics, Indian Institute of Technology, Hauz Khas, New Delhi 110 016, India Received 29 July 2005; in final form 20 October 2005 Available online 17 November 2005
Abstract The effect of microstructure and stoichiometry of MoO3 films on hydrolysis has been thoroughly investigated and reported in this Letter. MoO3 films having different cluster size, stoichiometry and crystal structure were prepared and deliberately exposed to identical high humidity conditions for different durations. Infrared spectroscopy was used to monitor hydroxylation process in these films. Polycrystalline films with large grain size were found to be inert to humidity conditions establishing the role of structure in hydroxylation process. All amorphous films adsorb water, part of which is dissociated to form Mo–OH, Mo–O–H bonds giving rise to two closely placed absorption peaks in 2500–4000 cm1 spectral range. The concentration of these bonds and free water are strongly influenced by film stoichiometry and microstructure. 2005 Elsevier B.V. All rights reserved.
1. Introduction Oxides are the best-known functional materials that have long been used in various devices viz. sensors [1], electrochromic devices [2], transparent conductors [3], components of various optical devices [4], catalyst in reduction/ oxidation processes [5]. Some devices viz. window coatings, humidity sensors are outdoor devices that are not only exposed constantly to continuously changing ambient conditions but also operated in cyclic manner in these ambients. The major drawback of some well-known electrochromic oxides is that they show relatively higher affinity for water vapor, which leads to degradation in their performance as reported by Yoshiike and Kondo [6] and Arnoldussen [7]. Very limited studies have been conducted to investigate and understand how adsorbed water affects device performance and itÕs lifetime. Based on XPS studies on H2O adsorbed titanium oxide surfaces, Kuntz and Heinrich [8] concluded that dissociation of adsorbed H2O molecules occurs only at defect sites. Noguera [9] further suggested that H+ and OH interact with the oxygen and cation of *
Corresponding author. Fax: +91 11 26581114. E-mail address: [email protected] (G.B. Reddy).
0009-2614/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.10.112
the oxide, respectively, to form hydrolyzed bonds. The complete process; dissociation of water into H+ and OH and formation of the hydrolyzed bonds with host is generally referred as hydrolysis. Therefore kinetics of both, dissociation and interaction with cations/anions processes must be interlinked. The progress of the second process is expected to depend on the oxidation state of cation, as lower valency state (LVS) will curtail the formation of hydrolyzed bonds because of the presence of electrons in their valence shells [9]. It has already been reported by the authors that relative concentration of LVS depends on stoichiometry and crystallinity of the films [10]. Although Arnoldussen [7] reported deleterious effect of water on electrochromic properties of MoO3, the effect of stoichiometry on hydrolysis was not investigated by him. The objectives of present work are: (i) to find out the role of cluster/crystallite size and stoichiometry on water dissociation rate (ii) to understand the interaction of dissociated (H+ and OH) species with MoO3 as a function of film stoichiometry. Such a study relating the hydrolysis to the stoichiometry has not been reported for any oxide so far. Fulfillment of these objectives will enable to realize more efficient EC devices with longer life times. MoO3 films with different microstructures and stoichiometry were intentionally exposed to ambient with 70%
T.S. Sian, G.B. Reddy / Chemical Physics Letters 418 (2006) 170–173
171
relative humidity (RH) at room temperature. The resulting chemical changes were monitored using FTIR at regular intervals. 2. Experimental For Fourier transform infrared (FTIR) spectroscopic studies, 1-lm (±0.01 lm) thick MoO3 films were deposited on indium doped tin oxide (ITO) coated glass substrates. Four different samples were prepared by: (i) evaporating MoO3 powder (Merck, 99.99%) at a base vacuum of 2 · 106 mbar (referred as F1 films) (ii) annealing F1 films in air at 150 C (referred as F2 films) (iii) annealing F1 annealed in air at 350 C (referred as F3 films) (iv) evaporating MoO3 powder thermally in the presence of oxygen plasma at 40 mtorr using activated reactive evaporation (ARE) technique (referred as F4 films) [11]. Annealing was carried out using programmable furnace with temperature accuracy of ±1 C. Annealing duration, specimen heating and cooling rates were maintained constant at 60 min, 10 C/min and 2 C/min, respectively. Infrared spectra of the films was measured in reflectance mode in the spectral region from 4000 to 2500 cm1 using Perkin– Elmer (Model spectrum BX2) spectrophotometer. Minimum in reflectance (R) in a recorded spectrum is referred as Ôabsorption peakÕ in this Letter. The instrumental resolution was fixed at 2 cm1 and the number of spectra for one measurement was 256 cycles. Transmission electron microscopy (TEM) studies were carried out using Philips Model CM 12 electron microscope. Photoelectron spectroscopic studies were carried out on Perkin–Elmer Model PHI 1257 spectrometer, with a dual anode Mg/Al Ka 25 kV X-ray source and high-resolution hemispherical section analyzer. The radiation used for the present study was Mg Ka at 150 W. The resolution of the instrument was better than ±0.05 eV. Probing X-ray photons were incident at an angle of 45 to the substrate normal and the axis of the hemispherical analyzer was kept parallel to the substrate normal. 3. Results and discussion XPS elemental scan recorded on the surface of freshly prepared MoO3 film given in Fig. 1 shows the presence of photoelectron peaks corresponding to Mo 3d, O 1s and C 1s. Also the Auger transitions corresponding to these elements are seen in the figure. The micrograph and selected area diffraction (SAD) pattern, recorded with transmission electron microscopic (TEM) on F1, F2, F3 and F4 films are shown in Fig. 2a–d, respectively. SAD pattern of F1, F2 and F4 films reveal their amorphous nature and that of F3 films exhibit polycrystalline form. As seen in micrographs, F1 films are more porous as compared to other films. The average size of the clusters is approximately 6 nm in F1 and F2 films. F3 films contain large crystallites with average size of 100 nm and the distribution of cluster size in F4 films vary from 4 to 14 nm. In an earlier report
Fig. 1. XPS elemental surface scan recorded on freshly MoO3 films.
[10], the authors have confirmed the variation in stoichiometry of these films through optical measurements. It was observed that F2 films had the highest non-stoichiometry followed by that in F1. Both F4 and F3 films were found to be nearly stoichiometric [11]. Therefore, the study of hydrolysis process in these films should clearly bring out the role of porosity, cluster/crystallite size and stoichiometry. Infrared reflectance (R) recorded on F1, F2, F4 and F3 films in 2500–4000 cm1 spectral region before and after exposing the films to high humid ambient for different durations, are shown in Fig. 3 a–d, respectively. Curve 1, 2, 3 and 4 in all cases correspond to R of fresh films and those recorded after 7, 21 and 40 days of continuous exposure, respectively. As seen from Fig. 3d , F3 films do not show any absorption peaks in this spectral range even after continuous exposure for 40 days. In all other films three absorption peaks, labeled as peak I, peak II and peak III (Fig. 3), are observed in spectral windows: 3400–3500, 3200–3250 and 3020–3040 cm1, respectively. It is known that peak I is due to the stretching mode of H2O [12]. The presence of two absorption peaks II and III, located closely with varying intensities, suggest the formation of two different hydrolyzed bonds in exposed films. These bonds are formed due to attachment of the water-dissociated species: H+ and OH with anion and cation in the host, respectively, [9]. From the figure, it is clear that peak II grows faster than peak III in all-amorphous films. Protons being lighter are able to penetrate inside films at a much faster rate as compared to OH species. Therefore, peaks II and III are assigned to Mo–O–H and Mo–OH bonds, respectively. The hydroxylation of Mo–O bonds can be represented by the following reactions: H2 OðadÞ ! Hþ þ OH ðO–MoÞ þ OH ! O–Mo–OH
ð1Þ ð2Þ
ðO–MoÞ þ Hþ ! Mo–O–H
ð3Þ
172
T.S. Sian, G.B. Reddy / Chemical Physics Letters 418 (2006) 170–173
Fig. 2. TEM micrographs of F1 (a), F2 (b), F4 (c) and F3 (d) along with their respective selected area diffraction pattern.
Fig. 3. Measured IR reflectance (R) spectra of F1 (a), F2 (b) F4 (c) and F3 (d) films. Curves 1, 2, 3 and 4 in (a), (b) and (c) refer to the R spectrum of fresh films and that recorded after 7, 21 and 40 days of continuous exposure to humid ambient. In F3 films, curves 1 and 2 refer to the R spectrum of fresh and 40 days exposed films. All spectra shown in the figure are baseline fitted.
where H2O(ad) is the water adsorbed in the film. Infrared measurements do not show any evidence of existence of either free water or Mo–OH and Mo–O–H bonds in F3 films (Fig. 3d). R spectra of the films in Fig. 3 were deconvoluted, to obtain intensities of individual peaks. The heights of these peaks are assumed to be proportional to the amount of
adsorbed free water, Mo–O–H and Mo–OH bonds, respectively. The intensity of peaks I and III are monitored as a function of exposure duration in all films and the same are shown in Figs. 4 and 5, respectively. As seen in Fig. 4, among fresh films the intensity of peak I is highest in F1 films followed by that observed in F4 and F2 films, in that order. In F2 films, water related peak was found to grow
T.S. Sian, G.B. Reddy / Chemical Physics Letters 418 (2006) 170–173
Fig. 4. Plot of intensity of absorption peak associated with water (peak I) vs. exposure duration of F1, F2 and F4 films.
173
and F2 films. This indicates that water adsorbed by these films is highest. The present study clearly established the following: (i) complete inertness towards humidity exhibited by F3 films was due to the combined effect of both crystal structure and large grain size, (ii) high porosity of F1 films allowed adsorption of more water immediately after venting the films to atmosphere, as observed from IR studies (Fig. 3a, curve 1) and (iii) the formation of low concentration of Mo–OH bonds observed in highly non-stoichiometric F2 films inspite of having high amount of adsorbed water confirms the role of stoichiometry in controlling the dissociation of water. This can be explained as follows: In order to form Mo–OH bonds, the unpaired electron in the OH group must be transferred to Mo 4d orbitals, in accordance with acid base concept [9]. It has already been reported that in non-stoichiometric films these orbitals are partially occupied and electrons in them dissuade transfer of electron from OH group, causing slow growth rate of hydrolyzed bonds. Therefore, the growth rate of Mo–OH bonds will be inversely proportional to the degree of nonstoichiometry in the films. As a consequence of this, less water is dissociated leading to higher concentration of free water as observed in Fig. 4. 4. Conclusions From the present work it is established that; (i) polycrystalline films exhibit better chemical inertness to humidity as compared to amorphous films and (ii) higher the non-stoichiometry lower is the rate of hydrolysis. References
Fig. 5. Plot of Intensity of absorption peak associated with Mo–OH bonds (peak III) vs. exposure duration of F1, F2 and F4 films.
much slowly attaining highest intensity confirming the presence of more free water in these films. The intensity of absorption peak associated with Mo–OH bonds was found to be lowest at any instant (Fig. 5), which implies that consumption (dissociation of water) of free water is minimum in these films. In F4 films intensity of the absorption peak corresponding to Mo–OH bonds is highest compared to that of other films and also the intensity of free water related peak is in between those observed in F1
[1] M.K. Jain, M.C. Bhatnagar, G.L. Sharma, Appl. Phys. Lett. 73 (1998) 3854. [2] P.M.S. Monk, R.J. Mortimer, D.R. RosseinskyElectrochromism, Fundamentals and Applications, VCH, Weinheim, 1995. [3] K.L. Chopra, S.S. Major, D.K. Pandya, Thin Solid Films 102 (1983) 1. [4] R.M. Roth, T. Izuhara, R.L. Espionala, D. Djukic, R.M. Osgood Jr., S. Bakhru, H. Bakhru, Opt. Lett. 30 (2005) 994. [5] G. Mestl, P. Ruiz, B. Belmon, H. Knozinger, J. Phys. Chem. B 98 (1994) 11269. [6] N. Yoshiike, S. Kondo, J. Electrochem. Soc. 130 (11) (1983) 2283. [7] T.C. Arnoldussen, J. Electrochem. Soc. 123 (4) (1976) 527. [8] R.L. Kuntz, V.E. Heinrich, Phys. Rev. B 26 (12) (1982) 6682. [9] C. Noguera, Physics and Chemistry at the Oxide Surfaces, Cambridge University Press, New York, 1996 (p.183)(Chapter 6). [10] T.S. Sian, G.B. Reddy, Sol. Energ. Mater. Sol. Cells 82 (2004) 375. [11] T.S. Sian, G.B. Reddy, J. Appl. Phys 98 (2005) 26104. [12] T.S Sian, G.B. Reddy, Solid State Ionics 167 (2004) 399.