Electrochemical desorption of a self-assembled monolayer of alkanethiol in ionic liquids

Available online at www.sciencedirect.com Journal of

Electroanalytical Chemistry Journal of Electroanalytical Chemistry 615 (2008) 110–116 www.elsevier.com/locate/jelechem

Electrochemical desorption of a self-assembled monolayer of alkanethiol in ionic liquids Daisuke Oyamatsu a, Takeshi Fujita a, Satoshi Arimoto a,b, Hirokazu Munakata a, Hajime Matsumoto c, Susumu Kuwabata a,b,* a

Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Suita, Osaka 5650871, Japan b CREST, JST, Kawaguchi, Saitama 3320012, Japan c Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31, Midorigaoka, Ikeda, Osaka 5638577, Japan Received 17 July 2007; received in revised form 28 November 2007; accepted 4 December 2007 Available online 8 December 2007

Abstract Electrochemical desorption of a self-assembled monolayer (SAM) of n-alkanethiols was investigated in four kinds of ionic liquids. It was found for the first time by linear potential sweep voltammetry that reductive desorption of the SAM took place even in ionic liquids. The potential peak was negatively shifted with an increase in the alkyl chain length of the alkanethiol SAM in a manner similar to that for the case of reductive desorption of the SAM in an aqueous solution. However, the cathodic wave broadened with an increase in chain length, although its width became narrow in the case of measurements in an aqueous solution. The contrary behaviors of reductive desorption in an ionic liquid and in an aqueous solution were discussed by comparing the experimentally obtained voltammograms with numerically simulated ones. Based on the results obtained, we propose a plausible reaction scheme in which cations work mainly as charge compensators for generated alkanethiolates and anions work as a solvent. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Self-assembled monolayer; Reductive desorption; Alkanethiol; Ionic liquid; Numerical simulation

1. Introduction Ionic liquid, which is a room temperature molten salt, is receiving considerable attention in various chemical fields because of its remarkable features such as non-volatility, non-combustibility, high ionic conductivity and capability to dissolve many kinds of substances [1–8]. In the electrochemical field, measurements of electrochemically active species in ionic liquid, including simple redox reaction of ferrocene derivatives, have revealed that ionic liquid is utilizable well as an electrolyte [9–12]. Attempts have, therefore, been made to apply this non-volatile electrolyte to * Corresponding author. Address: Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Suita, Osaka 5650871, Japan. Tel./fax: +81 6 6879 7372. E-mail address: [email protected] (S. Kuwabata).

0022-0728/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2007.12.003

electrochemical devices such as lithium ion batteries [13– 17], fuel cells [18,19], and solar batteries [20–22]. However, its property as an electrolyte has not yet been completely elucidated because an electrolyte without any solvent has not been considered in conventional theories of electrochemistry. Therefore, further studies are required to completely understand the roles of anion and cation of the ionic liquid as an electrolyte. A self-assembled monolayer (SAM) of n-alkanethiol formed on a metal substrate (Au, Pt, or Cu) has been intensively studied since publication of the first report by Allara and Nuzzo in 1983 [23]. The SAM possesses sufficient stability due to its highly organized structure developed by van der Waals forces between alkyl chains. However, as reported first by Porter et al. [24], the closely packed monolayer can easily be desorbed by electrochemical reduction formulated by

D. Oyamatsu et al. / Journal of Electroanalytical Chemistry 615 (2008) 110–116

R-S-M þ e ! R-S þ M

ð1Þ

where R and M represent an alkyl chain and a metal electrode, respectively. This reaction gives a definite cathodic wave in linear sweep voltammetry of the SAM-coated electrode in an aqueous electrolyte solution, and its shape and peak potential are sensitive to several parameters, including length of the alkyl chain, kind of substituted functional group, and kind of metal substrate [24–54]. Also, our previous studies have revealed that conditions of the electrolyte solution such as pH and kind of dissolved ionic species have significant effects [43,44]. Regarding cathodic waves appeared in linear sweep voltammetry, Aoki and Kakiuchi have developed a method to simulate them with consideration of the above-mentioned van der Waals forces between neighboring alkanethiol molecules. The computational drawing expresses well the characteristic cathodic wave of the voltammogram, which is negatively shifted and narrowed with an increase in alkyl chain length of the SAM [55]. Furthermore, Kakiuchi has reported the modified way with consideration of fine structure of the SAM that can be improved by its annealing [56]. Although extensive studies have been made concerning ionic liquid as an electrolyte and electrochemical desorption of n-alkanethiol SAM, combination of both materials has not been studied, so far. Then, we have attempted it, focusing on roles of cations and anions in the desorption reaction. As shown in this paper, we found that the SAM was certainly desorbed by electrochemical reduction even in ionic liquid but that the desorption features were different from those observed in an aqueous electrolyte solution. In order to know factors causing such the differences, the experimentally obtained voltammograms with numerically simulated voltammogram curves. As mentioned above, Aoki and Kakiuchi groups have established two ways to simulate voltammograms with consideration of van der Waals interaction between alkyl chains of the SAM [55,56]. In this study, we adopted the first way developed by using the honeycomb model, in which a parameter (uR) representing interaction between alkyl chain of the SAM as well as that (uV) representing interaction between adsorbed thiol molecules and electrolyte solution are considered.

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arated into two layers and the resulting ionic liquid isolated as a lower layer was washed with dichloromethane. The hydrophilic ionic liquids, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF4) and 1-buthyl-3-methylimidazolium tetrafluoroborate (BMI-BF4), were purchased from Solvent Innovation Co., Ltd. All ionic liquids were dried under vacuum at 105 °C for 3 h at least prior to use. Four kinds of alkanethiols, n-propanethiol, n-hexanethiol, noctanethiol and n-decanethiol, purchased from Wako Pure Chemical Ind., were of reagent grade and used without further purification. Their alkyl chain lengths will be indicated in this paper by numbers of carbon atoms (n of CnH2n+1SH). 2.2. SAM preparation and electrochemical measurements An Au/mica electrode substrate having a quasi (1 1 1) surface was prepared by vacuum evaporation of Au on a freshly cleaved natural mica sheet (Nilaco Co.) heated at 290 °C. The electrode coated with the SAM of n-alkanethiol was prepared by immersing the Au/mica substrate in 1 mmol dm3 alkanethiol/ethanol solution for 3 h at room temperature. The electrochemical cell used was a Pyrex glass tube (/ 1.5 cm  9 cm), both ends of which were open. The SAMcoated electrode was placed at the bottom hole of the cell with a silicone rubber O-ring (apparent electrode area of 0.36 cm2), and the top hole was tightly fitted with a silicon rubber stopper having a Pt foil counter and Ag/Ag+ (0.1 M) reference electrodes [59–62]. The effective surface area of the electrode, which was determined from the electric charges of anodic oxidation of chemically adsorbed iodine [63], was 0.40 cm2. Linear sweep voltammetry was conducted using a computer-controlled potentiostat (Hokuto Denko HSV-100) in a glovebox circulated with purified argon gas (inner temperature of 30 °C). 2.3. Numerical simulations of linear sweep voltammograms Computational simulation of linear sweep voltammograms was carried out using a program that was composed by Visual Basic 6.0 based on a formula considering the stabilizing energies due to thiol–thiol and thiol–solvent interactions [24,45–55].

2. Experimental 3. Results and discussion 2.1. Chemicals The hydrophobic ionic liquids, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMI-TFSI) and 1-buthyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMI-TFSI), were synthesized by mixing aqueous solutions containing corresponding cation (EMIBr or BMI-Br, Solvent Innovation Co., Ltd) and anion (Li-TFSI, Kishida Kagaku Co., Ltd.) species so as to give equivalent molar amounts of each species [7,20,57,58]. After vigorous mixing, the solution was spontaneously sep-

3.1. Reductive desorption of n-alkanethiol SAM in KOH aqueous solution Fig. 1 shows linear sweep voltammograms taken in 0.5 M KOH solution for Au(1 1 1) electrodes coated with n-alkanethiol SAM having different alkyl chain lengths. Each voltammogram shows a cathodic wave indicating reductive desorption of the alkanethiols, and its peak potential is negatively shifted and its width becomes narrow with an increase in chain length. When the alkyl chain

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Fig. 1. Linear sweep voltammograms for reductive desorption of a selfassembled monolayers of n-alkanethiols having different chain lengths. All voltammograms were obtained in 0.5 M KOH aqueous solution with a scan rate of 0.1 V s1.

length of alkanethiol SAM increases, van der Waals interaction between alkyl chains becomes large, resulting in an increase in stability of the SAM. By considering such an interaction using an appropriate coordination model of adsorbed alkanethiol molecules, Aoki and Kakiuchi have developed a method to numerically simulate the voltammogram of the reductive desorption of the alkanethiol SAM [46,55], which will be shown later. 3.2. Reductive desorption of n-alkanethiol SAM in ionic liquid

Fig. 2. Linear sweep voltammograms for reductive desorption of SAMs of n-alkanethiols (n-C3H7SH, n-C6H11SH, n-C8H17SH, and n-C10H21SH) obtained in ionic liquids (a) EMI-BF4, (b) BMI-BF4, (c) EMI-TFSI and (d) BMI-TFSI. The scan rate was 0.1 V s1 for all voltammograms.

Reductive desorption of the alkanethiol SAM formed on Au(1 1 1) was attempted in four kinds of ionic liquids. In order to compare the desorption potentials in different ionic liquids, the redox potential of ferrocene/ferricinium (Fc/Fc+) couple was measured by cyclic voltammetry in each ionic liquid and all electrode potentials were given respect to this redox potential. Fig. 2 shows linear sweep voltammograms of the SAM-coated Au(1 1 1) electrodes taken in (a) EMI-BF4, (b) BMI-BF4, (c) EMI-TFSI and (d) BMI-TFSI. All voltammograms show definite cathodic waves, indicating clearly that electrochemical desorption of the alkanethiol SAM takes place even in ionic liquid. In analogy with the case of the reductive desorption in an aqueous solution, negative potential shifts were observed with an increase in the alkyl chain length. However, changes in shape of the cathodic wave were different from those in an aqueous solution; the width tended to broaden as the alkyl chain became long. The area of each cathodic wave was estimated by comparing the voltammogram for the SAM-coated Au(1 1 1) electrode and that for a naked Au(1 1 1) electrode taken in the same ionic liquid. The obtained values ranging from 0.69  109 to 9 2 0.79  10 mol cm were closepto the coverage (0.77  ffiffiffi pffiffiffi 109 mol cm2) expected for a ð 3  3ÞR30 overlayer structure of alkanethiols absorbed on the Au(1 1 1) surface [24,42–46,56,64]. In voltammograms of n-decanethiol SAM-coated Au(1 1 1) taken in EMI-BF4 and BMI-BF4, shoulder peaks appeared with the main peaks. It has been

reported by Poter et al. that such the peak splitting was observed even in aqueous solution especially for long alkylthiol SAM [64]. They speculated that appearance of the peak splitting indicated presence of different structure in the SAM which have difference in accessibility of protons and/or electrolyte cationic species to the gold/sulfur interface. Similar argument might be made for voltammograms taken in ionic liquid but further investigation must be required for elucidation of the voltammetry behavior. Plots of peak potentials and peak widths of the cathodic waves as a function of chain length of the alkanethiol SAM are shown in Fig. 3. The peak width at half height was estimated using the voltammogram obtained for the naked Au(1 1 1) electrode as a base line. As mentioned above, however, cathodic waves obtained for decanethiol SAMs in EMI-BF4 and BMI-BF4 exhibited shoulder peaks, making it difficult to determine precisely the main peak widths. We tried to remove the shoulder peaks with geometric means, but the values obtained by such the intentional treatment should include some errors. Then, the widths were estimated from five time experiments and they are given in Fig. 3 with error bars. As mentioned above, negative shifts of the peak potential were observed for all cases, but the ratio of potential shift per increase in number of methylene unit was slightly different between the electrolytes used. The largest ratio was seen for aqueous KOH solution and it decreased in the order of BMI-BF4, EMIBF4, EMI-TFSI and BMI-TFSI, as shown in Fig. 3a.

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Scheme 1. The honeycomb model used for numerical calculation of linear sweep voltammograms. In this case, the center thiol molecule is stabilized by three neighboring thiol molecules (3uR) and three vacancy sites occupied by solvent molecules (3uV), respectively.

Fig. 3. (a) Changes in peak potentials and (b) peak width at half height with an increase in chain length. The reductive desorption of n-alkanethiol SAMs was conducted in 0.5 M KOH aqueous solution (M) and ionic liquids (EMI-BF4 (d), BMI-BF4 (j), EMI-TFSI (s) and BMI-TFSI (h)).

The relationship between peak width and alkyl chain length as shown in Fig. 3b indicates clearly differences in the peak width changes among voltammograms taken in aqueous solution and ionic liquids. The width change tendencies can be classified into three types; decrease (in aqueous KOH solution; type I), increase (in EMI-TFSI and BMI-TFSI; type II), and no change (EMI-BF4 and BMI-BF4; type III) with an increase in chain length of alkanethiol. The type I is widely observed for electrochemical desorption of the alkanethiol SAM in aqueous solutions. In general, it was explained that stabilization of the SAM by the van der Waals interactions between alkyl chains concerns both negative peak potential shifts and the peak narrowing. However, it would be better to consider separately the effect of chain length on the peak potential shifts and that on the wave width for voltammograms obtained in ionic liquids. The results shown in Fig. 3b suggest strongly that the anion species of the ionic liquids has a significant effect on the width of the cathodic wave. 3.3. Numerical simulation of reductive desorption of n-alkanethiol SAM In order to understand the above-mentioned effects of chain length on reductive desorption of the alkanethiol SAM in ionic liquid, we attempted the numerical simulation using the method developed by Aoki and Kakiuchi, which expressed well the reductive desorption behavior of

the alkanethiol SAM based on a model of hexagonally packed thiol molecules as shown in Scheme 1. Before desorption, one thiol molecule is surrounded by six thiol molecules. When the thiol molecule is stabilized by a neighboring thiol molecule with the stabilization energy of uR, the thiol molecule in a perfect SAM is stabilized by 6uR. It is assumed here that the center thiol molecule remains, while the surrounding thiol molecules are desorbed, allowing solvent molecules to occupy the generated vacancy sites. If the stabilization energy of the center thiol molecule by the solvent is denoted by uV, desorption of one thiol molecule changes the total stabilization energy for the center thiol molecule to 5uR + uV. The energy is then continuously changed to 4uR + 2uV, 3uR + 3uV, 2uR + 4uV, uR + 5uV, and 6uV with progress of the desorption. Based on such a model, the electrode potential (E) and current (I) can be formulated as functions of the coverage of thiol SAM (x) * ( RT aFv wx þ 1   ln E ¼ E0  ln 6uV aF ð w þ 1Þx kC exp kB T RT )+ 5  X 1 1 þ ð2Þ n  n nð w þ 1Þ nðwx þ 1Þ n¼1 ( 5  X aFv wx þ 1 1 ln þ I ¼ F C n RT ðw þ 1Þx n¼1 nðw þ 1Þ  1 6  ð3Þ xðwx þ 1Þ n nðwx þ 1Þ   uR  uV w ¼ exp 1 kBT where E0 is the standard electrode potential of the desorption reaction, R is the gas constant, T is the absolute temperature, F is Faraday’s constant, a is the transfer coefficient (0.84 in this case [55]), k is the rate constant of the desorption reaction, C is the surface density of the closely packed thiol SAM (7.7  1010 mol dm2), kB is the Bolzmann constant and v is rate of potential scan. Values of E and I are obtained by substituting values from 1 to 0 into x in Eqs. (2) and (3). Then, plots of I values as a function of E give a simulation curve of the linear sweep voltammogram.

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In the case of desorption in aqueous solution, it is assumable that solvent is the same even if supporting electrolyte and solution pH are different. Based on this assumption, in the first paper regarding this model [55], u = uR  uV is treated as the parameter representing the interaction energy between alkanethiol molecules relative to the energy between alkanethiol molecules and vacant, which takes always negative value. However, when ionic liquid containing no solvent is used as an electrolyte, it is required to consider that the uV is changed by changing kind of ionic liquid used as an electrolyte. Simulated voltammogram curves exhibit how uR and uV influence the peak potential and the shape of the cathodic wave of the voltammogram. Some typical examples are shown in Fig. 4. The negative shift of the uR value with an increase in chain length means more stabilization of the SAM due to van der Waals interaction between alkyl chains. Increase in chain length also increase numbers of solvent molecules that can touch to alkyl chains, resulting in proportional changes in uV value to the chain length. As shown in Fig. 4, negative shift of both uR and uV with alkyl chain length causes negative shift of the peak poten-

Fig. 4. Changes in potential and width of simulated desorption peak with an increase in stabilizing energies due to thiol–thiol (uR) and thiol–solvent (uV) interactions in the cases of (a) DuR > DuV, (b) DuR = DuV and (c) DuR < DuV.

tial, while relationship between uR and uV values determines tendency of the width changes. As mentioned in the previous section, the electrochemical desorption of alkanethiol SAM in the ionic liquids was classified to three types. The simulated voltammograms shown in Fig. 4 seem to express well the three types; Fig 4a, b, and c depict the voltammograms belonging to the types I, III, and II, respectively. 3.4. Plausible reaction model for SAM desorption in ionic liquid The comparison of experimentally obtained voltammograms shown in Fig. 2 and numerically simulated voltammograms shown in Fig. 4 gives several hints for considering reductive desorption behavior of the alkanethiol SAM. The alkanethiol SAM is definitely desorbed by electrochemical reduction even in ionic liquid but it is required to consider that the reaction takes place under the condition without any solvent. Negative charges of the desorbed alkanethiolate molecules are, of course, compensated by cationic species of ionic liquid. However, both anionic and cationic species should concern dissolution of the alkanethiolate molecules. In addition, the both species also should interact with the alkanethiol SAM on the Au electrode, as shown in Scheme 2. In other words, although the parameter uV concerns interaction between the alkyl

Scheme 2. Schematic illustration of proposed mechanism of reductive desorption of a SAM in ionic liquid.

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chains of the SAM and solvent for the desorption in aqueous solution, it must concern interaction between alkyl chains of the SAM and anionic and cationic species in the case of the desorption in ionic liquid. The most notable influence of ionic liquid on the SAM desorption is that anionic species determines the desorption behavior; the desorption in ionic liquids containing BF 4 belongs to the type II and the reaction in ionic liquids containing TFSI belongs to the type III. As well known, EMI-BF4 and BMI-BF4 commingle with water but EMITFSI and BMI-TFSI are completely separated from water,  indicating that anionic species, i.e. BF 4 and TFSI determine hydrophobicity of the four ionic liquids used in this study. This fact allows us to speculate that TFSI possessing more hydrophobic than BF 4 interacts more strongly with alkyl chain of the SAM and dissolved alkanethiolate. Such the situation might enlarge slope of the changes in uV value with an increase in chain length of the SAM. It was a little difficult to estimate precisely the slope of uV changes and that of uR changes from the voltammograms obtained in this study. However, it is likely that higher interaction between TFSI and the alkyl chain of the SAM is main reason why the cathodic peak broadened with an increase in alkyl chain length. Although the ionic liquids of BF 4 -salts are mixed with water, hydrophobicities of this organic species must be higher than that of water. Therefore, the slope of uV changes for these ionic liquids might be larger than that for water, resulting in almost no changes in cathodic wave width. 4. Conclusion Electrochemical reductive desorption of n-alkanethiol SAMs having various chain lengths was investigated by linear sweep voltammetry in four ionic liquids. In aqueous KOH solution, the peak potentials shifted in a negative direction and peak width narrowed with an increase in alkyl chain length of the SAM. This means that the stabilizing energy due to thiol–thiol interaction became dominant compared to that due to thiol–solvent (water) interaction. On the other hand, although negative shifts of the peak potential with an increase in chain length were also observed in voltammograms taken in all ionic liquids, the peak width was not notably changed in EMI-BF4 and BMI-BF4, and the peak width broadening was observed in EMI-TFSI and BMI-TFSI. The numerical simulations of the voltammograms were made with consideration interactions between neighboring alkyl chains in the SAM (uR) and those between alkyl chains and electrolyte (uV). Comparison of them with the experimentally obtained data suggested that hydrophobicity of the ionic liquid was main factor influencing change in uV value. In particular, anionic  species that are BF 4 and TFSI influence strongly hydrophobicity of the ionic liquids used in this study and they determine predominantly the uV value. In the case of the desorption of alkanethiol SAM in aqueous solution, solvent (water) is the sole factor that

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influences change in uV value. From this viewpoint, the desorption reaction in the ionic liquids used in this study, it is likely that anionic species of the ionic liquids work as solvent and cationic species work as charge compensators of alkanethiolate generated by the electrochemical desorption. As already mentioned in the introduction section, theories of electrochemical reactions without any solvent have not yet been established. Probably choice of appropriate electrochemical reactions like desorption reaction adopted in this study would tell roles of anionic and cationic species of ionic liquid on the reactions. Acknowledgements This work was supported by Grant-in-Aid for Scientific Research on Priority Area (452) ‘‘Science of Ionic Liquids” and Grant-in-Aid for Scientific Research (18201022) from Japanese Ministry of Education, Culture, Sports, Science and Technology. One of the authors (S.A.) expresses his special thanks for The Global COE (center of excellence) Program ‘‘Global Education and Research Center for Bio-Environmental Chemistry” of Osaka University. References [1] W. Chen, L. Xu, C. Chatterton, J. Xiao, Chem. Commun. (1999) 1247. [2] M.J. Earle, P.B. McCormac, K.R. Seddon, Chem. Commun. (1998) 2245. [3] C.M. Gordon, A. McCluskey, Chem. Commun. (1999) 1431. [4] K. Hong, H. Zhang, J.W. Mays, A.E. Visser, C.S. Brazel, J.D. Holbrey, W.M. Reichert, R.D. Rogers, Chem. Commun. (2002) 1368. [5] T. Kitazume, G. Tanaka, J. Fluorine Chem. 106 (2000) 211. [6] M.D. Sliger, S.J. P’Pool, R.K. Traylor, J. McNeill, S.H. Young, N.W. Hoffman, M.A. Klingshirn, R.D. Rogers, K.H. Shaughnessy, J. Organomet. Chem. 690 (2005) 3540. [7] T. Welton, Chem. Rev. 99 (1999) 2071. [8] F. Zulfiqar, T. Kitazume, Green Chem. 2 (2000) 137. [9] J. Fuller, R.T. Carlin, R.A. Osteryoung, J. Electrochem. Soc. 144 (1997) 3881. [10] V.M. Hultgren, A.W. Mariotti, A.M. Bond, A.G. Wedd, Anal. Chem. 74 (2002) 3151. [11] Y. Shen, T. Tajima, M. Atobe, T. Fuchigami, Electrochemistry 72 (2004) 849. [12] B.K. Sweeny, D.G. Peters, Electrochem. Commun. 3 (2001) 712. [13] R.T. Carlin, H.C.D. Long, J. Fuller, P.C. Trulove, J. Electrochem. Soc. 141 (1994) L73. [14] V.R. Koch, C. Nanjundiah, G.B. Appetecchi, B. Scrosati, J. Electrochem. Soc. 142 (1995) L116. [15] H. Matsumoto, H. Sakaebe, K. Tatsumi, J. Power Source 146 (2005) 45. [16] H. Sakaebe, H. Matsumoto, Electrohem. Commun 5 (2003) 594. [17] S. Seki, Y. Kobayashi, H. Miyashiro, Y. Ohno, A. Usami, Y. Mita, M. Watanabe, N. Terada, Chem. Commun. (2006) 544. [18] A. Noda, M.H. Susan, K. Kudo, S. Mitsushima, K. Hayamizu, M. Watanabe, J. Phys. Chem. B 107 (2003) 4024. [19] R.F. Souza, J.C. Padilha, R.S. Goncalves, J. Dupont, Electrochem. Commun. 5 (2003) 728. [20] P. Bonhoˆte, A.-P. Dias, N. Papageorgiou, K. Kalyanasundaram, M. Gra¨zel, Inorg. Chem. 35 (1996) 1168. [21] H. Matsumoto, T. Matsuda, T. Tsuda, R. Hagiwqara, Y. Ito, Y. Miyazaki, Chem. Lett. (2001) 26.

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