Electron Transport and Recombination in Dye-Sensitized TiO2

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J. Phys. Chem. C 2008, 112, 20505–20509

20505

Electron Transport and Recombination in Dye-Sensitized TiO2 Solar Cells Fabricated without Sintering Process Shogo Mori,*,† Kenji Sunahara,† Yosuke Fukai,† Taisuke Kanzaki,‡ Yuji Wada,‡ and Shozo Yanagida*,§ DiVision of Chemistry and Material, Faculty of Textile Science and Technology, Shinshu UniVersity, Ueda, 386-8567, Japan, Material and Life Science, Graduate School of Engineering and Center for AdVanced Science and InnoVation, Osaka UniVersity, Suita, Osaka 565-0871 Japan ReceiVed: July 24, 2008; ReVised Manuscript ReceiVed: September 11, 2008

Nanoporous TiO2 electrodes were prepared from a colloidal suspension of TiO2 nanoparticles with heating processes at 80 °C for 12 h (regarded as nonsintering process) or 450 °C for 30 min (as sintering process) in air, and dye-sensitized solar cells (DSCs) were prepared from them. Electron diffusion coefficients (D) and lifetime (τ) in the DSCs were measured under various light intensities. The DSCs with nonsintered TiO2 showed lower values of D and τ than those in the sintered TiO2 at the matched electron density. A plot of open-circuit voltage (Voc) vs electron density of the DSCs showed that the nonsintered TiO2 gave higher Voc at the matched electron density under high light intensity and lower Voc under low light intensity. These results suggest that nonsintered TiO2 has less density of shallow trap sites and sintering process will form the traps at the grain boundary during the growth of the necks between TiO2 particles. Introduction Nanoporous metal oxide semiconductor electrodes have been the subject of intensive studies due to their possibilities to enhance the functionalities of various applications, in addition to their unique material properties originated from the nanostructures.1–4 Dye-sensitized solar cell (DSC) is one of the applications.1 DSCs typically consist of a dye-adsorbed nanoporous TiO2 electrode immersed in an electrolyte containing I-/I3- redox couple and a platinized counter electrode. The nanoporous TiO2 electrodes can be prepared by applying a TiO2 nanoparticle dispersed solution onto a transparent conductive oxide (TCO), followed by sintering at between 450 and 550 °C in air. Under light irradiation, excited electrons in the sensitizing dye molecules are injected into the conduction band of TiO2 and diffuse to the TCO. At the same time, the resulting oxidized dye cations are reduced by I- and the concomitant I3- species are regenerated at the Pt electrode. In order to reach the TCO for the injected electrons, electron diffusion length, which is (Dτ)1/2, where D is the electron diffusion coefficient and τ is the electron lifetime, should be longer than the thickness of the TiO2.5 Thus, the values of D and τ and the parameters influencing these have been studied with substantial efforts.3–11 Electron transport has been considered with diffusion since there is negligible band bending at the TiO2 surface due to the nanosize of the TiO2 particles, and the cationic species, which are the counter charges of I-/I3- redox couple in the electrolyte solution, surround the nanoporous TiO2 electrode and screen the injected electrons in the TiO2 effectively.6,7 The electron diffusion coefficient has been measured by various groups, showing light intensity dependence and very low values of the D in the TiO2 electrode.8,9 The properties can be interpreted with intraband charge traps where the electron transport occurs * Corresponding authors. E-mail: [email protected], [email protected] † Shinshu University. ‡ Graduate School of Engineering, Osaka University. § Center for Advanced Science and Innovation, Osaka University.

with the events of trapping and detrapping.10–13 Measured electron lifetimes have also shown light intensity dependence.14 The observations can be explained also by the charge traps, where the traps play a role to reduce the collision frequency between the injected electrons and I3- or dye cations at the surface of the TiO2 particles.13,15 Here, the point is that the charge recombination would occur mostly through the interfacial charge transfer from the conduction band and the trapped electrons in the TiO2 are isolated from encountering the charge acceptors under typical light intensities. In practice, it is important to understand how TiO2 electrode preparation methods influence the value of D and τ. For example, larger TiO2 particles results in larger value of D and smaller value of τ.16 Influence of TiO2 sintering process on the electron transport properties has been also an important issue. This has been motivated to apply the DSC on a flexible plastic substrate, which limits the sintering temperature.17–20 Between the annealing temperatures of 150 to 550 °C, the value of D was increased with the increase of the temperature.21 Plausible explanation of the results is that the electron transfer is facilitated with the growth of the neck between the particles. In view of the electron lifetime, a shorter value of electron lifetime was observed in low temperature annealed electrodes.21,22 Interpretation of the short electron lifetime is not straightforward, and it has not been concluded yet. If the lower values of the D in low temperature annealed TiO2 were mainly due to higher charge trap density, in addition to the poor necking, the electron lifetime would be longer with lower annealing temperature. This is because that since there are essentially no holes in the TiO2 particles and when the traps are located inside the particles, the traps are no longer a recombination center.13,15,23 In the case of low-temperature annealed TiO2, the experimental data showed opposite. The open circuit voltage (Voc) of the DSC prepared by low temperature processes showed relatively high values, despite the low values of short circuit current (Jsc), which is also inconsistent with the assumption of higher trap density. On the other hand, several papers have shown that the low

10.1021/jp8065629 CCC: $40.75  2008 American Chemical Society Published on Web 11/24/2008

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Mori et al.

TABLE 1: I-V Characteristics of DSCs Using TiO2 Films Prepared with and without Sintering Processesa annealing tetemperature 80 °C 450 °C

electrolyteb E50 E25 E15 E50 E25 E15

thickness, µm

Jsc, mA cm-2

Voc, V

FF

Eff, %

3.7 3.7 3.8 3.9 4.0 4.0

5.5 6.0 6.1 7.6 8.3 8.2

0.69 0.74 0.74 0.76 0.77 0.78

0.71 0.71 0.71 0.70 0.68 0.70

2.7 3.2 3.2 4.1 4.3 4.4

a Measured under light intensity of 100 mW/cm2 (AM 1.5). b Electrolytes were 0.1 M LiI, 0.6 M DMPImI, and 0.5 M tBP, in AN, with 0.05 M I2 (E50), 0.025 M I2 (E25), and 0.015 M I2 (E15).

temperature or nonannealed TiO2 electrodes have higher trap density than annealed TiO2.24,25 In order to elucidate the origin of the short electron lifetime, measurements of trap density are inevitable. In this paper, we prepared nonsintered nanoporous TiO2 electrodes and discuss the measured values of D and τ based on the experimentally estimated density of charge traps.

the values were divided by the volume of the TiO2 electrodes. Capacitance, which relates with density of states, of DSCs was also measured by a pulsed laser, based on the method in a paper by O’Regan et al.31 Note that the porosity was not taken into account for the calculation. These measurements were repeated under various light intensities controlled by a set of neutral density filters.

Experiments Nanoporous TiO2 electrodes were prepared from a colloidal suspension of TiO2 nanoparticles (P25, average diameter 21 nm, Nippon Aerosil) in distilled water without surfactant. The preparation method of suspension was described elsewhere,26 except that we used a planetary ball mill (Gokin Planetaring Inc.) instead of a paint shaker. The ball mill was operated with 6 g of P25, 10 g of water, and 70 g of ZrO2 balls in a ZrO2 cup for 100 rpm for 2 min, and then 380 rpm for 60 min. The suspension was applied onto a transparent conducting glass (SnO2:F,) by a doctor blade techniques, and the resulting films were heated at 80 °C for 12 h in air (regarded as non-sintering process). For comparison, the identical films were annealed at 450 °C for 30 min in air (as sintering process). Note that for the case of P25, morphology is probably not affected by the annealing because the heating process up to 450 °C did not change the BET surface area.21 Then, the films were dipped into a solution containing 0.3 mM of a Ru dye (N719, Peccell Technologies, Inc.) for one night. Solar cells were prepared by placing a Pt sputtered conducting glass on the dye adsorbed films, and the two glasses were attached by a thermal adhesive film. Electrolytes were introduced from a drilled hole on the Pt glass and the hole was sealed by the thermal adhesive film with a piece of slide glass. We prepared electrolytes: 0.1 M of LiI, 0.6 M of 1,2-dimethyl-3-propylimidazorium iodide (DMPImI), and 0.5 M of tert-butyl pyridine tBP in acetonitril with three different concentrations of I2. We denote the electrolyte containing 50 mM of I2 as E50, 25 mM E25, and 15 mM E15. Energy conversion efficiency was measured under simulated one sun conditions (AM 1.5, 100 mW/cm2, Yamashita denso). Electron diffusion coefficients and lifetimes were measured by stepped light induced transient measurements of photocurrent and voltage (SLIM-PCV).27 By the method, small fraction of the intensity of irradiated laser (635 nm) was stepped down and induced transient was fitted with a single exponential function, exp(-t/τc), where t is time. Then, from the current transient at short circuit, the value of D was calculated from L2/(2.77τc), where L is the thickness of the TiO2. Electron lifetime is directly obtained from τc for voltage decay at open circuit. Electron density (n) in the DSCs at open circuit conditions was measured by charge extraction methods,28 which was introduced to DSCs by Peter et al.29,30 In short, accumulated electrons in a DSC at open circuit conditions were released by switching to short circuit conditions with turning off the laser intensity simultaneously. The resulting current transients were integrated, and

Results and Discussion Table 1 summarizes I-V characteristics of the DSCs prepared with and without sintering processes. Jsc of the nonsintered DSCs was lower than those of annealed DSCs.32 Among the sintered DSCs, Jsc was independent from [I3-].33 On the other hand, among the DSCs without sintering, Jsc was increased when [I3-] was reduced from 50 mM to 25 mM and comparable between 25 and 15 mM. This suggests that charge recombination is negligible when [I3-] is at least less than 25 mM, at short circuit conditions. The lower Jsc from nonsintered DSCs with low [I3-] may be partially due to lower amount of adsorbed dyes because of incomplete dehydration of the TiO2 surface.22,34 Figure 1 shows the electron diffusion coefficients at short circuit conditions and electron lifetime at open circuit conditions under various irradiated light intensities. The values of D and τ in the nonsintered DSCs were lower than those in the 450 °C sintered DSCs. These results are consistent with the previous results obtained from the comparison study between the annealing temperature of 150 and 450 °C.26 For both preparation temperatures, the lifetime was increased when [I3-] was decreased. In the case of sintered TiO2, the decrease of [I3-] results in little influence on Jsc. This is because the electron diffusion length was already longer than the TiO2 thickness.35 In the case of nonsintered TiO2, the calculated diffusion length with E50 and E15 under the conditions giving 1.6-1.7 mA cm-2 was 3.4 and 5.5 µm, respectively. These values are consistent with the plot of energy conversion efficiency as a function of TiO2 thickness in the previous paper, that is, the efficiency decreases when the thickness becomes thicker than the L.26 Figure 2 shows the electron density at short and open circuit conditions as a function of Jsc. At short circuit, the n values in nonsintered DSCs were much higher than the n in the sintered DSCs. Diffusion current is described by D times the gradient of n with respect to distance. Since the D is lower in the nonsintered DSCs, in order to have high current, the electron density should be high. This is consistent with the experimental results here. Then the high n in the nonsintered DSCs at short circuit conditions increase the recombination rate in comparison to the sintered DSC. When [I3-] was reduced, the L was increased to be longer than the thickness of the employed TiO2 films, that is, 4 µm, suggesting high charge collection efficiency for the nonsintered DSCs. Figure 3 shows the plot of Voc vs the electron density in the DSCs. At the matched electron density, nonsintered DSCs

Electron Transport and Recombination in TiO2 Solar Cells

J. Phys. Chem. C, Vol. 112, No. 51, 2008 20507

Figure 3. Open circuit voltage of DSCs as a function of electron density. Symbols were the same as in Figure .

Figure 1. Electron diffusion coefficients at short circuit conditions and lifetime at open circuit conditions in the DSCs using TiO2 films prepared with (open symbol) and without (closed symbol) sintering processes. Electrolytes were 0.1 M LiI, 0.6 M DMPImI, and 0.5 M tBP, in AN, with 0.05 M I2 (square), 0.025 M I2 (circle), and 0.015 M I2 (triangle).

Figure 2. Electron density at open and short circuit. Symbols were the same as in Figure 1. Colored and gray symbols show the values at open and short circuit conditions, respectively. Electron density at open circuit was plotted with Jsc obtained under the same laser intensity to measure the electron density at open circuit.

showed higher and lower Voc at higher and lower electron density, respectively. The Voc of the DSCs scales with the potential difference between the Fermi level of the TiO2 and the redox potential of I-/I3- redox couple. The Fermi level is determined by the electron density in the TiO2 and the potential

Figure 4. Capacitance of DSCs using TiO2 without (closed symbol) and with (open) sintering process with electrolyte containing 15 mM of I2.

of the conduction band edge (Ecb) of the TiO2. The electron density is related with electron lifetime through roughly G times τ, where G is the electron injection (generation) rate. The Ecb is related with the species of cations dissolved in electrolyte solution.36,37 When cations, such as Li+, are adsorbed on the TiO2 surface, it shifts the conduction band edge positively, and thus it decreases Voc. In addition, the Fermi level of the TiO2 is related with the density and energetic distribution of intraband charge traps, e.g., under matched electron density and energetic distribution of traps, the Fermi level becomes lower with higher trap density. In the case of Figure 3, the difference of the Voc between the DSCs with and without sintering cannot be explained only with the shift of the conduction band, because Voc dependence on the n is not the same. This suggests that difference exists in the trap distribution and density. To support this, the cell capacitance was measured (Figure 4). The capacitance has been regarded as the density of states of dyed TiO2 electrode.31 Figure 4 suggests that nonsintered DSCs have lower and higher trap densities located at higher and lower energy levels, respectively, in comparison to the sintered DSCs. This is consistent with the results in Figure 3. There is still a possibility that the difference observed in Figures 3 and 4 might be the combined results of the shift of the Ecb and change of trap distribution. Here, the shift might be caused during the sintering process at 450 °C, for example, some contaminants would be on the surface and burn out, causing the different surface potentials. In order to check the possibility, we first annealed dry TiO2 particles at 450 °C for 30 min and

20508 J. Phys. Chem. C, Vol. 112, No. 51, 2008 prepared a colloidal suspension from the particles. Then we did the same comparison performed in Figures 1 and 2 and found the same trend (Supporting Information). Thus, we excluded the possibility of the Ecb shift. Another possible reason to increase the Voc for nonsintered TiO2 might be due the adsorption of water at the TiO2 surface. However, to increase 50 mV, more than 0.5 M of water in the electrolyte is needed.37 Therefore, we excluded the possibility also. The low values of D in nanoporous TiO2 in comparison to that in bulk TiO2 have been attributed to the intraband charge traps. In addition, the large number of grain boundaries in the electrodes would influence the values of D. Previously, the easiness of the neck growth at the grain boundaries was suggested to be a factor controlling the value of the D.21 By calculation, Cass et al. showed that the condition of the grain boundaries has an evident effect on D.38 In comparison to the D in the sintered DSCs, the lower values of D in the nonsintered DSCs cannot be explained only by the multiple trapping model, and the influence of the particle-particle boundary is probably needs to be taken into account. The shorter electron lifetime observed with nonsintered TiO2 film can be explained only by the less density of traps, that is, the electrons can move more freely in each particle through the conduction band and thus the electrons will more frequently meet the acceptors like I3at the surface. The origin of the intraband charge traps has not been elucidated yet. Location of the traps has been speculated at the surface of the nanoparticles. Recently, it was reported that there was a good correlation between the surface area of the nanoparticles and the values of D, supporting the hypothesis of the surface located traps.39 In the case of the nonsintered TiO2 electrodes, traps are probably located at the surface and in the bulk, especially for P25,40 but few were at the grain boundaries. This is because the particles’ surface structure facing other particles would not be changed by heating at 80 °C. On the other hand, by heat treatment at 450 °C, the necks would grow at the boundaries among particles and it could be accompanied with the formation of charge traps at the boundaries, e.g., by lattice mismatch, causing the difference of the density of charge traps. Note that the origin of the traps would differ depending on the synthesis method of TiO2 nanoparticles, e.g., P25 is produced by gas-phase reaction. Several reports have shown that nonannealed or lowtemperature processed TiO2 has higher trap density.24,25 However, the traps mentioned by the papers are “deep” trap sites, which are located at well below the Ecb. Our result in Figure 3 is consistent with the reports, that is, higher deep trap density in nonsintered films. In addition, Figure 4 shows that density of “shallow” trap sites which are located close to the Ecb is lower than that in high temperature annealed film. Since the density of deep trap site is much lower than that of shallow traps, the higher density of the deep trap sites in the low temperature annealed films would not be significant on the charge transport and recombination in the DSCs under normal operation conditions. Conclusions Dye-sensitized solar cells prepared from nonsintered nanoporous TiO2 electrodes showed lower Jsc, comparable Voc, lower electron diffusion coefficients, and shorter lifetime in comparison to those prepared from sintered electrodes. The plot of Voc vs electron density in the DSCs suggested that the nonsintered TiO2 has lower density of shallow traps. Based on that, the observed D can be explained by higher electrical resistance at grain boundaries. The shorter values of the τ can

Mori et al. be simply explained by the lower density of the traps, which increases the electron concentration on the conduction band. In order to increase the efficiency of the DSCs, these results suggest that the decrease of the charge trap density, which gains the Voc, is one strategy. On the other hand, the decrease of the traps enhances the interfacial charge transfer to the dye cation and/or I3-, decreasing electron lifetime. Thus, simultaneous control of the traps and the interfacial charge transfer rate is important. Supporting Information Available: Additional data of I-V characteristics and IPCE with E50, E25, and E15. Electron diffusion coefficients and lifetime and extracted charge in the DSCs prepared from a colloidal suspension, which contains P25 particles annealed at 450 °C for 30 min in air. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (2) Garcia-Belmonte, G.; Kytin, V.; Dittrich, T.; Bisquert, J. J. Appl. Phys. 2003, 94, 5261. (3) Cinnsealach, R.; Boschloo, G.; Rao, S. N.; Fitzmaurice, D. Sol. Energy Mater. Sol. Cells 1999, 57, 107. (4) Tachikawa, T.; Tojo, S.; Fujitsuka, M.; Majima, T. J. Phys. Chem. B 2004, 108, 11054. (5) Fisher, A. C.; Peter, L. M.; Ponomarev, E. A.; Walker, A. B.; Wijayantha, K. G. U. J. Phys. Chem. B 2000, 104, 949. (6) So¨dergren, S.; Hagfeldt, A.; Olsson, J.; Lindquist, S. E. J. Phys. Chem. 1994, 98, 5552. (7) van de Lagemaat, J.; Park, N. G.; Frank, A. J. J. Phys. Chem. B 2000, 104, 2044. (8) Cao, F.; Oskam, G.; Meyer, G. J.; Searson, P. C. J. Phys. Chem. 1996, 100, 17021. (9) Kopidakis, N.; Schiff, E. A.; Park, N. G.; Van de Lagemaat, J.; Frank, A. J. J. Phys. Chem. B 2000, 104, 3930. (10) Schwarzburg, K.; Willig, F. Appl. Phys. Lett. 1991, 58, 2520. (11) Nelson, J. Phys. ReV. B 1999, 59, 15374. (12) van de Lagemaat, J.; Frank, A. J. J. Phys. Chem. B 2000, 104, 4292. (13) Nelson, J.; Haque, S. A.; Klug, D. R.; Durrant, J. R. Phys. ReV. B 2001, 63, 2053211. (14) Schlichtho¨rl, G.; Huang, S. Y.; Sprague, J.; Frank, A. J. J. Phys. Chem. B 1997, 101, 8141. (15) Bisquert, J.; Zaban, A.; Greenshtein, M.; Mora-Sero, I. J. Am. Chem. Soc. 2004, 126, 13550. (16) Nakade, S.; Saito, Y.; Kubo, W.; Kitamura, T.; Wada, Y.; Yanagida, S. J. Phys. Chem. B 2003, 107, 8607. (17) Pichot, F.; Pitts, J. R.; Gregg, B. A. Langmuir 2000, 16, 5626. (18) Lindstro¨m, H.; Holmberg, A.; Magnusson, E.; Lindquist, S. E.; Malmqvist, L.; Hagfeldt, A. Nano Lett. 2001, 1, 97. (19) Miyasaka, T.; Kijitori, Y. J. Electrochem. Soc. 2004, 151, A1767. (20) Zhang, D.; Yoshida, T.; Oekermann, T.; Furuta, K.; Minoura, H. AdV. Funct. Mater. 2006, 16, 1228. (21) Nakade, S.; Matsuda, M.; Kambe, S.; Saito, Y.; Kitamura, T.; Sakata, T.; Wada, Y.; Mori, H.; Yanagida, S. J. Phys. Chem. B 2002, 106, 10004. (22) Park, N. G.; Schlichtho¨rl, G.; Van De Lagemaat, J.; Cheong, H. M.; Mascarenhas, A.; Frank, A. J. J. Phys. Chem. B 1999, 103, 3308. (23) When the traps are located on the surface, the traps could act as a recombination center. Light intensity dependent electron lifetime in DSCs can be also explained by dealing the traps as a recombination center (see ref 14). The model predicts that when the trap density increases, both electron diffusion coefficients and electron lifetime in the DSCs increase. However there are several experimental results which are inconsistent with the prediction (e.g., ref 16). (24) Takeshita, K.; Sasaki, Y.; Kobashi, M.; Tanaka, Y.; Maeda, S.; Yamakata, A.; Ishibashi, T. A.; Onishi, H. J. Phys. Chem. B 2004, 108, 2963. (25) Zhang, D.; Downing, J. A.; Knorr, F. J.; McHale, J. L. J. Phys. Chem. B 2006, 110, 21890. (26) Kanzaki, T.; Nakade, S.; Wada, Y.; Yanagida, S. Photochem. Photobiol. Sci. 2006, 5, 389. (27) Nakade, S.; Kanzaki, T.; Wada, Y.; Yanagida, S. Langmuir 2005, 21, 10803. (28) Nakade, S.; Kanzaki, T.; Kubo, W.; Kitamura, T.; Wada, Y.; Yanagida, S. J. Phys. Chem. B 2005, 109, 3480. (29) Duffy, N. W.; Peter, L. M.; Rajapakse, R. M. G.; Wijayantha, K. G. U. Electrochem. Commun. 2000, 2, 658.

Electron Transport and Recombination in TiO2 Solar Cells (30) Peter, L. M.; Duffy, N. W.; Wang, R. L.; Wijayantha, K. G. U. J. Electroanal. Chem. 2002, 524-525, 127. (31) O’Regan, B. C.; Bakker, K.; Kroeze, J.; Smit, H.; Sommeling, P.; Durrant, J. R. J. Phys. Chem. B 2006, 110, 17155. (32) The FF of the nonsintered DSCs showed slightly higher values than that of sintered DSCs, although the electron diffusion coefficients in the nonsintered DSCs gave lower values. The value of FF is related with internal electrical resistance. Thus, one might expect lower FF from nonsintered DSCs. On the other hand, the current of the DSCs is determined mostly by diffusion current, that is, low diffusion coefficients can be compensated by the increased charge density. This probably explains that the FF is less sensitive to the electrical resistance of the TiO2 films. (33) The value of Jsc for the cell prepared with 450 °C heating process with E50 seems to be reduced from the cells with E25 and E15. This was due to the sample variations. In the Supporting Information, we showed values for two samples prepared at the same time for each condition (Table S1). Another concern would be that the value of Jsc could be reduced due to the absorption of I3-. In order to check the effect, we prepared DSCs again with 450 °C heating process with E50, E25, and E15, and measured I-V curves and incident-photon to current-conversion efficiency (IPCE).

J. Phys. Chem. C, Vol. 112, No. 51, 2008 20509 We observed the decrease of IPCE at less than 500 nm. However, the influence on the values of Jsc was a few percent (Supporting Information, Figure S1). (34) The influence of annealing temperature on the amount of dye adsorption was measured, and the difference was 11% between the films having 450 and 80 °C heating processes. (35) From the values of D and τ in Figure 1, estimated diffusion length for sintered DSCs with E50 was more than 20 µm. (36) Watson, D. F.; Meyer, G. J. Coord. Chem. ReV. 2004, 248, 1391. (37) Liu, Y.; Hagfeldt, A.; Xiao, X. R.; Lindquist, S. E. Sol. Energy Mater. Sol. Cells 1998, 55, 267. (38) Cass, M. J.; Walker, A. B.; Martinez, D.; Peter, L. M. J. Phys. Chem. B 2005, 109, 5100. (39) Kopidakis, N.; Neale, N. R.; Zhu, K.; van de Lagemaat, J.; Frank, A. J. Appl. Phys. Lett. 2005, 87, 1. (40) Hurum, D. C.; Gray, K. A.; Rajh, T.; Thurnauer, M. C. J. Phys. Chem. B 2005, 109, 977.

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