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Redox properties of nanoporous TiO2 (anatase) surface modified with phosphotungstic acid
Thin Solid Films 323 Ž1998. 141–145
Redox properties of nanoporous TiO 2 ž anatase/ surface modified with phosphotungstic acid Henrik Lindstrom Rensmo a , Sten-Eric Lindquist a , Anders Hagfeldt ¨ a, Hakan ˚ b Anders Henningsson b, Sven Sodergren , Hans Siegbahn b ¨ a
a,)
,
Department of Physical Chemistry, Uppsala UniÕersity, P.O. Box 532, S-751 21 Uppsala, Sweden b Department of Physics, Uppsala UniÕersity, P.O. Box 530, S-751 21 Uppsala, Sweden Received 2 July 1997; accepted 11 December 1997
Abstract Nanostructured anatase TiO 2 electrodes surface modified with inorganic metal oxide clusters, i.e., phosphotungstic acid ŽPWA., was studied by XPS and spectroelectrochemistry. The surface modifiers were electroactive and could be addressed reversibly by changing the applied bias. The formation of new states in the electronic structure of TiO 2 and the PWA was monitored by XPS using a novel preparation technique. The coloration efficiency of the deposit was about 20 cm2rC. Prolonged electrochemical cycling degraded the electrical contact between the PWA and the TiO 2 surface. q 1998 Elsevier Science S.A. All rights reserved. Keywords: XPS; Anatase; Phosphotungstic acid; Lithium intercalation; Nanoporous; Polyoxometalates
1. Introduction Nanoporous TiO 2 electrodes represent an interesting outgrowth from the fields of electrochemistry on colloidaland macroscopic-semiconductors. These novel class of systems exhibit hybrid properties in the sense that they combine a large electrode surface-to-volume ratio with the possibility of injecting or extracting charges via an external back contact. The electrodes typically consist of a m m-thin, highly permeable network Žporosity around 50%. of interconnected nanometer-sized semiconductor particles. The supporting electrolyte solution enters the pore channels and establishes contact with each particle in the film. Especially, the nanoporous electrode geometry renders a two-phase redox reaction to occur at the inner surface throughout the entire volume of the electrode. Previously, Gratzel et al. developed and exploited the advantages of ¨ these systems for dye-sensitized solar cells w1x and rechargeable batteries w2,3x. However, work in other directions is in progress; successful attempts to anchor organic molecules onto the TiO 2 surface have been made, e.g.,
)
Corresponding author.
0040-6090r98r$19.00 q 1998 Elsevier Science S.A. All rights reserved. PII S 0 0 4 0 - 6 0 9 0 Ž 9 8 . 0 0 3 5 3 - 8
adsorption of vitamin-B12 w4x for electrochemical synthesis or adsorption of viologens w5x, for smart windowrdisplay applications. In this paper metal oxide clusters of phosphotungstic acid were used for surface modification. This compound belongs to a family of substances called polyoxometalates which exhibit properties unique in their versatility mainly due to the electron- and proton transferrstorage abilities and high thermal stability w6x. Many of the solution properties of polyoxometalates are also retained in the solid state, i.e., the crystalline salts, since the large anions tend to have very weak attractions for the counter ions and solvate molecules. The properties of polyoxometalates are desirable in fields such as catalysis w7,8x, analytical chemistry w9x, electronics w10x, and medicine w6x. Concerning electrochromic devices, solid state cells containing compressed phosphotungstic acid w10–12x and cells based on PWA incorporated into a conducting polymer w13x or in water solution w14x have been reported previously. The idea behind this initial study was to combine the unique properties of nanoporous electrodes and metal oxide clusters. The aim was mainly to see if PWA communicates electrically with the inner surface of the nanoporous TiO 2 electrode.
142
H. Lindstrom ¨ et al.r Thin Solid Films 323 (1998) 141–145
2. Experimental 2.1. Electrode preparation The TiO 2-film was deposited on F-doped SnO 2 glass Žsheet resistance 8 VrI, transmittance about 80% in the visible.. The preparation of the TiO 2 colloidal solution has been described elsewhere w15x. The TiO 2 sol–gel was spread out onto the substrate using scotch-tape as a frame and spacer. The excess solution was raked off with a glass rod. The tape was subsequently removed and the plate was fired at 4508C in a heat gun for 30 min. The resulting film was transparent, mechanically stable, homogeneous, approximately 5 m m thick and had a porosity Žpercentage of voids. of 40.4%. The structure was pure anatase as determined by X-ray diffraction. The particle size was about 8 nm from TEM pictures and the X-ray diffractogram using Scherrer’s formula. 2.2. Surface modification After firing, the film was post-treated with solutions of phosphotungstate ŽPWA., as described below. The PWA was dissolved in HCl ŽpH s 1. to avoid decomposition of the heteropolyanions w16x. The concentration of precursor was varied between 0.05 M and 0.20 M. Visual inspection of the samples showed that the samples turned opaque above 0.3 M, indicating that the pore channels were filled up with PWA at higher concentrations. The amount of metal-oxide cluster deposited in the film was determined by weighing the electrode before and after the surface treatment. Fig. 1 shows how the amount of deposit Žexpressed as mass percent of the total amount of TiO 2 . varies with the concentration of the precursor solution. The sintered nanoporous film was completely covered with the precursor solution. The excess solution was then slung away during the spin coating. Due to the low viscosity of the solutions the spin coating resulted in an optically homogeneous deposition. The modified electrode was heated at 2008C in air for 10 min to establish electrical contact between the clusters and the TiO 2 surface and to remove the crystal water. This procedure is likely to
produce the anhydrous phosphotungstate w17x. The electrodes were removed from the oven while still hot, i.e., 2008C and was transferred directly to the electrochemical cell in a small desiccator containing a drying agent. 2.3. Spectroelectrochemistry The spectroelectrochemistry was performed using a three-electrode system connected to a multichannel ECO Chemie AutolabrGPES electrochemical interface. A 5 cm quartz cuvette with a specifically designed sealing Teflon cover was used as the electrolyte vessel. The atmosphere in the vessel was continuously purged in a stream of dry nitrogen Ž- 5 ppm water. during all measurements. The electrolyte solution was prepared in a glove box using anhydrous acetonitrile Ž- 50 ppm water. and anhydrous LiClO4 supplied by Aldrich, and was stored in a sure seal bottle ŽAldrich. under N2 atmosphere. Acetonitrile was used as the solvent because of its high electrochemical stability. All chemicals were of reagent grade and used as received. Electrolyte Ž15 ml. was injected into the cuvette using a syringe. The counter electrode consisted of glassy carbon and was enclosed with a glass frit to avoid the influence of reaction products. The reference was a AgrAgCl in saturated LiCl ŽMerck. in anhydrous acetonitrile. The LiCl was dried at 2008C before use. All potentials are referred to the AgrAgCl electrode. The optical transmission of the electrodes was recorded in situ on a Carry 2000. The absorbance on each freshly prepared electrode was set to zero before any electrochemical measurement was performed. 2.4. XPS Measurements The influence of the incorporated charge on the electronic states of TiO 2 and PWA was examined by XPS. By using a novel preparation technique it was possible to perform the electrochemistry within the XPS spectrometer, allowing for analysis within minutes after the reaction. The electrode was immersed in the electrolyte and a constant potential was applied for 5 min. The excess electrolyte was removed by rinsing in acetonitrile. All electrochemical preparation steps were performed in the atmosphere of the electrolyte. For a detailed description of this method and instrument see Ref. w18x.
3. Results and discussion 3.1. Cyclic Õoltammetry
Fig. 1. The variation of the amount of deposited PWA with the concentration of the precursor solution, see Section 2.
Voltammograms were collected for pure and modified TiO 2 , Fig. 2. The applied potential was swept linearly between q0.7 V and y1.0 V at 20 mVrs. It can be seen that the currents for the TiO 2 Ža. are small compared to the
H. Lindstrom ¨ et al.r Thin Solid Films 323 (1998) 141–145
143
Fig. 2. Voltammograms for pure and modified TiO 2 . The concentration of the precursor solution was 0.2 M. Scan rate 20 mVrs.
PWA treated electrode Žb.. The insertionrextraction charge reversibility was higher than 97% showing that the residual currents are small. The amount of injected electronic charge is shown as a function of the cycle number in Fig. 3. It can be seen that the amount of injected charge decreases with the cycle number. Presumably, the electrical contact between the PWA and the TiO 2 surface is degraded after prolonged cycling or it could be that the PWA disconnects from the TiO 2 surface by slow dissolution into the electrolyte. The absorbance at 780 nm was measured during the electrochemical cycling. At the end of the cathodic sweep the absorbance reached a maximum. The obtained absorption maxima during cycling are displayed as a function of cycle number, Fig. 4. As expected, the optical density increases with the amount of deposited PWA. The inset shows that the absorbance at 780 nm relates linearly to the amount of injected electrons. The data were obtained by charging the electrode at y1 V for different time periods. The injected charge was then extracted during an anodic sweep Ž20 mVrs. while simultaneously monitoring the corresponding absorbance change. The collected integrated anodic was correlated with the difference in optical density, inset in Fig. 4. Coloration efficiencies were calculated for different loads of modifier, Fig. 5. The coloration efficiency is defined as the ratio between the optical density or absorbance and the amount of injected charge Žin Coulombs.
Fig. 3. The amount of injected charge as a function of the cycle number.
Fig. 4. The maximum obtained absorbance Žat 780 nm. during the cathodic sweeps as a function of the cycle number. The inset shows the relation between the absorbance and the injected charge.
per square centimeter geometric electrode area. The maximum efficiency was obtained for a surface treatment with a 0.2 M solution. Both phosphotungstate and anatase TiO 2 have been long known to exhibit deep coloration upon reduction. One question to be addressed for the present system is whether the observed color originates exclusively from reduced Ti 3q states or if reduced tungsten ions are also involved in the coloration process. The object of the measurements below was to determine if the electronic charge is transferred to Ti- or W-states and to study the potential dependence of these reaction pathways. 3.2. XPS measurements The Ti 2 p peak in the core level spectrum of TiO 2 was recorded under various conditions ŽFig. 6.. Spectrum Ža. and Žb. were obtained for pure and PWA modified TiO 2 Ž0.2 M solution. before any contact with the electrolyte. A comparison between Ža. and Žb. shows that the Ti 2 p peaks are not affected significantly by the presence of PWA. Curves Žc–f. were collected after immersing the PWA modified electrode into an electrolyte and applying different negative biases. Essentially, the Ti 2 p peaks are
Fig. 5. Calculated coloration efficiencies for different loads of PWA.
144
H. Lindstrom ¨ et al.r Thin Solid Films 323 (1998) 141–145
Fig. 6. Ti 2 p core levels for different electrochemical treatment, see XPS measurements in Section 2 and Section 3.
unperturbed down to y0.9 V. However, decreasing the potential down to y1.3 V allows for intercalation of lithium ions in the TiO 2 . This process gives rise to an additional peak in the Ti 2 p spectrum, shifted to lower binding energy. The appearance of this new Ti 2 p peak originates from the reduction of Ti 4q to Ti 3q w19x. By reversing the applied potential to q0.7 V, a similar spectrum to that in Žb. is obtained. Thus, the initial electronic structure is recovered showing that the reduced titanium ions are re-oxidized reversibly to their original states. Spectra of the W 4 f peaks were recorded on pure PWA Ža. and TiO 2 modified with PWA Žb., Fig. 7. The alteration of the spectrum in Žb. compared to Ža., is merely due to the overlap in binding energies of the Ti 3 p and the W 4 f states. The superimposed Ti 3 p peaks in Žb. contribute to about 25% of the total intensity. Applying potentials of y0.7 V and y0.9 V to the PWA modified electrode is accompanied by gradual changes of the W 4 f peaks as shown in Žc–d.. Note that the Ti 2 p states are unaffected at these conditions, cf. Fig. 6. Analogously to the discussion above, the negative shifts in binding energies of the W 4 f states are ascribed to the reduction of tungsten ions w20x. With this interpretation, the small difference in Žd. and Že. may be explained by saturation of the PWA in terms of the electrochemical reduction of tungsten ions. Since mainly the W peaks are affected at y0.7 V and y0.9 V Žcf. Fig. 6., the electronic charge is preferentially transferred to PWA at these potentials. Consequently, the observed color changes down to y0.9 V can be attributed to the reactions of PWA. Reversal of the potential to q0.7
Fig. 7. W 4 f core levels for different electrochemical treatments, see XPS measurements in Section 2 and Section 3.
V Žf., gives a similar spectrum to that in Žb. and thus, also the tungsten ions are re-oxidized reversibly.
4. Conclusions The electrochemical redox properties of nanoporous anatase TiO 2 electrode surfaces modified with phosphotungstate ŽPWA. were examined. The potential dependence of different reaction pathways, i.e., the reduction of the TiO 2 or the reversible charge transfer of electrons between the TiO 2 and the deposited PWA, was well clarified by means of a new XPS method. The obtained coloration efficiency was about 20 cm2rC. The contact between the PWA and the TiO 2 surface was gradually deteriorated after extended electrochemical cycling.
Acknowledgements This work was supported by the Swedish Research Council for Engineering Sciences ŽTFR., the Commission of the European Community Joule III program and the Swedish Natural Science Research Council ŽNFR..
References w1x M.K. Nazecruddin, A. Kay, I. Rodicio, B.R. Humphry, E. Mueller, P. Liska, N. Vlachopoulos, M. Gratzel, J. Am. Chem. Soc. 115 ¨ Ž1993. 6382–6390.
H. Lindstrom ¨ et al.r Thin Solid Films 323 (1998) 141–145 w2x S.Y. Huang, L. Kavan, I. Exnar, M. Gratzel, J. Electrochem. Soc. ¨ 142 Ž1995. 142–144. w3x S.-Y. Huang, L. Kavan, A. Kay, M. Gratzel, Accepted for publica¨ tion in J. Act. Passive Electron. Compon. w4x A. Mayor, A. Hagfeldt, L. Waldner, M. Gratzel, Chimia 50 Ž1996. . ¨ w5x A. Hagfeldt, L. Walder, M. Gratzel, in: Proc. Soc. Photo-Opt. ¨ Instrum., San Diego, CA, USA, 1995, p. 2531. w6x M.T. Pope, A. Muller, Angew. Chem., Int. Ed. Engl. 30 Ž1991. ¨ 34–48. w7x K. Young Lee, T. Arai, S.-I. Nakata, S. Asaoka, T. Okuhura, M. Misono, J. Am. Chem. Soc. 114 Ž1992. 2836–2842. w8x N. Mizuno, T. Watanabe, M. Misono, J. Phys. Chem. 89 Ž1985. 80–85. w9x A.G. Fogg, N.K. Bsebsu, Talanta 28 Ž1981. 473–476. w10x B. Tell, J. Electrochem. Soc. 127 Ž1980. 2451–2454.
145
w11x B. Tell, J. Appl. Phys. 50 Ž1979. 5944–5946. w12x B. Tell, S. Wagner, Appl. Phys. Lett. 33 Ž1978. 837–838. w13x T. Shimidzu, A. Ohtani, M. Aiba, K. Honda, J. Chem. Soc., Faraday Trans. 1 84 Ž1988. 3941–3949. w14x P. Maheswari, M.A. Habib, Solar Energy Mater. 18 Ž1988. 75–82. w15x B. O’Regan, M. Gratzel, Nature ŽLondon. 353 Ž1991. 737–740. ¨ w16x M.T. Pope, M. Varga, Inorg. Chem. 5 Ž1966. 1249–1254. w17x W.H. Knoth, R.D. Farlee, Inorg. Chem. 23 Ž1984. 4765–4766. w18x S. Sodergren, H. Rensmo, H. Siegbahn, Surface and Interface ¨ Analysis, in press. w19x S. Sodergren, H. Rensmo, H. Lindstrom, A. Hagfeldt, S.-E. ¨ ¨ Lindquist, H. Siegbahn, J. Phys. Chem. B 101 Ž1997. 3087–3090. w20x S. Sodergren, A. Stashans, S. Lunell, H. Rensmo, L. Vayssieres, H. ¨ Lindstrom, ¨ A. Hagfeldt, H. Siegbahn, manuscript in preparation.
Redox properties of nanoporous TiO 2 ž anatase/ surface modified with phosphotungstic acid Henrik Lindstrom Rensmo a , Sten-Eric Lindquist a , Anders Hagfeldt ¨ a, Hakan ˚ b Anders Henningsson b, Sven Sodergren , Hans Siegbahn b ¨ a
a,)
,
Department of Physical Chemistry, Uppsala UniÕersity, P.O. Box 532, S-751 21 Uppsala, Sweden b Department of Physics, Uppsala UniÕersity, P.O. Box 530, S-751 21 Uppsala, Sweden Received 2 July 1997; accepted 11 December 1997
Abstract Nanostructured anatase TiO 2 electrodes surface modified with inorganic metal oxide clusters, i.e., phosphotungstic acid ŽPWA., was studied by XPS and spectroelectrochemistry. The surface modifiers were electroactive and could be addressed reversibly by changing the applied bias. The formation of new states in the electronic structure of TiO 2 and the PWA was monitored by XPS using a novel preparation technique. The coloration efficiency of the deposit was about 20 cm2rC. Prolonged electrochemical cycling degraded the electrical contact between the PWA and the TiO 2 surface. q 1998 Elsevier Science S.A. All rights reserved. Keywords: XPS; Anatase; Phosphotungstic acid; Lithium intercalation; Nanoporous; Polyoxometalates
1. Introduction Nanoporous TiO 2 electrodes represent an interesting outgrowth from the fields of electrochemistry on colloidaland macroscopic-semiconductors. These novel class of systems exhibit hybrid properties in the sense that they combine a large electrode surface-to-volume ratio with the possibility of injecting or extracting charges via an external back contact. The electrodes typically consist of a m m-thin, highly permeable network Žporosity around 50%. of interconnected nanometer-sized semiconductor particles. The supporting electrolyte solution enters the pore channels and establishes contact with each particle in the film. Especially, the nanoporous electrode geometry renders a two-phase redox reaction to occur at the inner surface throughout the entire volume of the electrode. Previously, Gratzel et al. developed and exploited the advantages of ¨ these systems for dye-sensitized solar cells w1x and rechargeable batteries w2,3x. However, work in other directions is in progress; successful attempts to anchor organic molecules onto the TiO 2 surface have been made, e.g.,
)
Corresponding author.
0040-6090r98r$19.00 q 1998 Elsevier Science S.A. All rights reserved. PII S 0 0 4 0 - 6 0 9 0 Ž 9 8 . 0 0 3 5 3 - 8
adsorption of vitamin-B12 w4x for electrochemical synthesis or adsorption of viologens w5x, for smart windowrdisplay applications. In this paper metal oxide clusters of phosphotungstic acid were used for surface modification. This compound belongs to a family of substances called polyoxometalates which exhibit properties unique in their versatility mainly due to the electron- and proton transferrstorage abilities and high thermal stability w6x. Many of the solution properties of polyoxometalates are also retained in the solid state, i.e., the crystalline salts, since the large anions tend to have very weak attractions for the counter ions and solvate molecules. The properties of polyoxometalates are desirable in fields such as catalysis w7,8x, analytical chemistry w9x, electronics w10x, and medicine w6x. Concerning electrochromic devices, solid state cells containing compressed phosphotungstic acid w10–12x and cells based on PWA incorporated into a conducting polymer w13x or in water solution w14x have been reported previously. The idea behind this initial study was to combine the unique properties of nanoporous electrodes and metal oxide clusters. The aim was mainly to see if PWA communicates electrically with the inner surface of the nanoporous TiO 2 electrode.
142
H. Lindstrom ¨ et al.r Thin Solid Films 323 (1998) 141–145
2. Experimental 2.1. Electrode preparation The TiO 2-film was deposited on F-doped SnO 2 glass Žsheet resistance 8 VrI, transmittance about 80% in the visible.. The preparation of the TiO 2 colloidal solution has been described elsewhere w15x. The TiO 2 sol–gel was spread out onto the substrate using scotch-tape as a frame and spacer. The excess solution was raked off with a glass rod. The tape was subsequently removed and the plate was fired at 4508C in a heat gun for 30 min. The resulting film was transparent, mechanically stable, homogeneous, approximately 5 m m thick and had a porosity Žpercentage of voids. of 40.4%. The structure was pure anatase as determined by X-ray diffraction. The particle size was about 8 nm from TEM pictures and the X-ray diffractogram using Scherrer’s formula. 2.2. Surface modification After firing, the film was post-treated with solutions of phosphotungstate ŽPWA., as described below. The PWA was dissolved in HCl ŽpH s 1. to avoid decomposition of the heteropolyanions w16x. The concentration of precursor was varied between 0.05 M and 0.20 M. Visual inspection of the samples showed that the samples turned opaque above 0.3 M, indicating that the pore channels were filled up with PWA at higher concentrations. The amount of metal-oxide cluster deposited in the film was determined by weighing the electrode before and after the surface treatment. Fig. 1 shows how the amount of deposit Žexpressed as mass percent of the total amount of TiO 2 . varies with the concentration of the precursor solution. The sintered nanoporous film was completely covered with the precursor solution. The excess solution was then slung away during the spin coating. Due to the low viscosity of the solutions the spin coating resulted in an optically homogeneous deposition. The modified electrode was heated at 2008C in air for 10 min to establish electrical contact between the clusters and the TiO 2 surface and to remove the crystal water. This procedure is likely to
produce the anhydrous phosphotungstate w17x. The electrodes were removed from the oven while still hot, i.e., 2008C and was transferred directly to the electrochemical cell in a small desiccator containing a drying agent. 2.3. Spectroelectrochemistry The spectroelectrochemistry was performed using a three-electrode system connected to a multichannel ECO Chemie AutolabrGPES electrochemical interface. A 5 cm quartz cuvette with a specifically designed sealing Teflon cover was used as the electrolyte vessel. The atmosphere in the vessel was continuously purged in a stream of dry nitrogen Ž- 5 ppm water. during all measurements. The electrolyte solution was prepared in a glove box using anhydrous acetonitrile Ž- 50 ppm water. and anhydrous LiClO4 supplied by Aldrich, and was stored in a sure seal bottle ŽAldrich. under N2 atmosphere. Acetonitrile was used as the solvent because of its high electrochemical stability. All chemicals were of reagent grade and used as received. Electrolyte Ž15 ml. was injected into the cuvette using a syringe. The counter electrode consisted of glassy carbon and was enclosed with a glass frit to avoid the influence of reaction products. The reference was a AgrAgCl in saturated LiCl ŽMerck. in anhydrous acetonitrile. The LiCl was dried at 2008C before use. All potentials are referred to the AgrAgCl electrode. The optical transmission of the electrodes was recorded in situ on a Carry 2000. The absorbance on each freshly prepared electrode was set to zero before any electrochemical measurement was performed. 2.4. XPS Measurements The influence of the incorporated charge on the electronic states of TiO 2 and PWA was examined by XPS. By using a novel preparation technique it was possible to perform the electrochemistry within the XPS spectrometer, allowing for analysis within minutes after the reaction. The electrode was immersed in the electrolyte and a constant potential was applied for 5 min. The excess electrolyte was removed by rinsing in acetonitrile. All electrochemical preparation steps were performed in the atmosphere of the electrolyte. For a detailed description of this method and instrument see Ref. w18x.
3. Results and discussion 3.1. Cyclic Õoltammetry
Fig. 1. The variation of the amount of deposited PWA with the concentration of the precursor solution, see Section 2.
Voltammograms were collected for pure and modified TiO 2 , Fig. 2. The applied potential was swept linearly between q0.7 V and y1.0 V at 20 mVrs. It can be seen that the currents for the TiO 2 Ža. are small compared to the
H. Lindstrom ¨ et al.r Thin Solid Films 323 (1998) 141–145
143
Fig. 2. Voltammograms for pure and modified TiO 2 . The concentration of the precursor solution was 0.2 M. Scan rate 20 mVrs.
PWA treated electrode Žb.. The insertionrextraction charge reversibility was higher than 97% showing that the residual currents are small. The amount of injected electronic charge is shown as a function of the cycle number in Fig. 3. It can be seen that the amount of injected charge decreases with the cycle number. Presumably, the electrical contact between the PWA and the TiO 2 surface is degraded after prolonged cycling or it could be that the PWA disconnects from the TiO 2 surface by slow dissolution into the electrolyte. The absorbance at 780 nm was measured during the electrochemical cycling. At the end of the cathodic sweep the absorbance reached a maximum. The obtained absorption maxima during cycling are displayed as a function of cycle number, Fig. 4. As expected, the optical density increases with the amount of deposited PWA. The inset shows that the absorbance at 780 nm relates linearly to the amount of injected electrons. The data were obtained by charging the electrode at y1 V for different time periods. The injected charge was then extracted during an anodic sweep Ž20 mVrs. while simultaneously monitoring the corresponding absorbance change. The collected integrated anodic was correlated with the difference in optical density, inset in Fig. 4. Coloration efficiencies were calculated for different loads of modifier, Fig. 5. The coloration efficiency is defined as the ratio between the optical density or absorbance and the amount of injected charge Žin Coulombs.
Fig. 3. The amount of injected charge as a function of the cycle number.
Fig. 4. The maximum obtained absorbance Žat 780 nm. during the cathodic sweeps as a function of the cycle number. The inset shows the relation between the absorbance and the injected charge.
per square centimeter geometric electrode area. The maximum efficiency was obtained for a surface treatment with a 0.2 M solution. Both phosphotungstate and anatase TiO 2 have been long known to exhibit deep coloration upon reduction. One question to be addressed for the present system is whether the observed color originates exclusively from reduced Ti 3q states or if reduced tungsten ions are also involved in the coloration process. The object of the measurements below was to determine if the electronic charge is transferred to Ti- or W-states and to study the potential dependence of these reaction pathways. 3.2. XPS measurements The Ti 2 p peak in the core level spectrum of TiO 2 was recorded under various conditions ŽFig. 6.. Spectrum Ža. and Žb. were obtained for pure and PWA modified TiO 2 Ž0.2 M solution. before any contact with the electrolyte. A comparison between Ža. and Žb. shows that the Ti 2 p peaks are not affected significantly by the presence of PWA. Curves Žc–f. were collected after immersing the PWA modified electrode into an electrolyte and applying different negative biases. Essentially, the Ti 2 p peaks are
Fig. 5. Calculated coloration efficiencies for different loads of PWA.
144
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Fig. 6. Ti 2 p core levels for different electrochemical treatment, see XPS measurements in Section 2 and Section 3.
unperturbed down to y0.9 V. However, decreasing the potential down to y1.3 V allows for intercalation of lithium ions in the TiO 2 . This process gives rise to an additional peak in the Ti 2 p spectrum, shifted to lower binding energy. The appearance of this new Ti 2 p peak originates from the reduction of Ti 4q to Ti 3q w19x. By reversing the applied potential to q0.7 V, a similar spectrum to that in Žb. is obtained. Thus, the initial electronic structure is recovered showing that the reduced titanium ions are re-oxidized reversibly to their original states. Spectra of the W 4 f peaks were recorded on pure PWA Ža. and TiO 2 modified with PWA Žb., Fig. 7. The alteration of the spectrum in Žb. compared to Ža., is merely due to the overlap in binding energies of the Ti 3 p and the W 4 f states. The superimposed Ti 3 p peaks in Žb. contribute to about 25% of the total intensity. Applying potentials of y0.7 V and y0.9 V to the PWA modified electrode is accompanied by gradual changes of the W 4 f peaks as shown in Žc–d.. Note that the Ti 2 p states are unaffected at these conditions, cf. Fig. 6. Analogously to the discussion above, the negative shifts in binding energies of the W 4 f states are ascribed to the reduction of tungsten ions w20x. With this interpretation, the small difference in Žd. and Že. may be explained by saturation of the PWA in terms of the electrochemical reduction of tungsten ions. Since mainly the W peaks are affected at y0.7 V and y0.9 V Žcf. Fig. 6., the electronic charge is preferentially transferred to PWA at these potentials. Consequently, the observed color changes down to y0.9 V can be attributed to the reactions of PWA. Reversal of the potential to q0.7
Fig. 7. W 4 f core levels for different electrochemical treatments, see XPS measurements in Section 2 and Section 3.
V Žf., gives a similar spectrum to that in Žb. and thus, also the tungsten ions are re-oxidized reversibly.
4. Conclusions The electrochemical redox properties of nanoporous anatase TiO 2 electrode surfaces modified with phosphotungstate ŽPWA. were examined. The potential dependence of different reaction pathways, i.e., the reduction of the TiO 2 or the reversible charge transfer of electrons between the TiO 2 and the deposited PWA, was well clarified by means of a new XPS method. The obtained coloration efficiency was about 20 cm2rC. The contact between the PWA and the TiO 2 surface was gradually deteriorated after extended electrochemical cycling.
Acknowledgements This work was supported by the Swedish Research Council for Engineering Sciences ŽTFR., the Commission of the European Community Joule III program and the Swedish Natural Science Research Council ŽNFR..
References w1x M.K. Nazecruddin, A. Kay, I. Rodicio, B.R. Humphry, E. Mueller, P. Liska, N. Vlachopoulos, M. Gratzel, J. Am. Chem. Soc. 115 ¨ Ž1993. 6382–6390.
H. Lindstrom ¨ et al.r Thin Solid Films 323 (1998) 141–145 w2x S.Y. Huang, L. Kavan, I. Exnar, M. Gratzel, J. Electrochem. Soc. ¨ 142 Ž1995. 142–144. w3x S.-Y. Huang, L. Kavan, A. Kay, M. Gratzel, Accepted for publica¨ tion in J. Act. Passive Electron. Compon. w4x A. Mayor, A. Hagfeldt, L. Waldner, M. Gratzel, Chimia 50 Ž1996. . ¨ w5x A. Hagfeldt, L. Walder, M. Gratzel, in: Proc. Soc. Photo-Opt. ¨ Instrum., San Diego, CA, USA, 1995, p. 2531. w6x M.T. Pope, A. Muller, Angew. Chem., Int. Ed. Engl. 30 Ž1991. ¨ 34–48. w7x K. Young Lee, T. Arai, S.-I. Nakata, S. Asaoka, T. Okuhura, M. Misono, J. Am. Chem. Soc. 114 Ž1992. 2836–2842. w8x N. Mizuno, T. Watanabe, M. Misono, J. Phys. Chem. 89 Ž1985. 80–85. w9x A.G. Fogg, N.K. Bsebsu, Talanta 28 Ž1981. 473–476. w10x B. Tell, J. Electrochem. Soc. 127 Ž1980. 2451–2454.
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