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Influence of TiCl4 Treatment on Surface Defect Photoluminescence in Pure and Mixed-Phase Nanocrystalline TiO2 Fritz J. Knorr, Dongshe Zhang, and Jeanne L. McHale* Department of Chemistry, Washington State UniVersity, Pullman, Washington 99164-4630 ReceiVed January 31, 2007. In Final Form: May 4, 2007 Room-temperature UV-excited photoluminescence spectra are reported for nanocrystalline films of anatase, rutile, and mixed-phase TiO2 (Degussa P25) before and after treatment with TiCl4 solution. The surface defect luminescence of anatase in the visible region is suppressed by TiCl4 treatment, indicating a decrease in surface traps. A similar anatase surface-defect emission is observed in the mixed-phase nanoparticles but is completely quenched following TiCl4 treatment and replaced by emission characteristic of rutile. Our results suggest that TiCl4 treatment of mixed-phase TiO2 may result in a surface layer of rutile and that radiative recombination of electron-hole pairs formed in the bulk anatase region of nanocrystallites occurs after electrons migrate to newly formed rutile surfaces.

Introduction In recent years, there has been tremendous emphasis on the use of metal oxide nanoparticles, particularly TiO2, for a variety of energy-related applications. Since the initial report by Fujishima and Honda1 in 1972 on the photolysis of water by TiO2, rapid advances in nanotechnology and interest in solar energy for the production of hydrogen have spawned intense study of the photovoltaic and photocatalytic properties of metal oxide nanoparticles.2,3 Many of the potential uses of nanoparticulate TiO2, including dye-sensitized solar energy conversion4-8 and photocatalysis,9-11 depend on interfacial electron transfer at the semiconductor/solvent or semiconductor/gas interface. Because the efficiencies of these processes depend on the large surface area-to-volume ratios afforded by nanoparticulate TiO2, surface defect states12-14 take on extreme importance in determining electrical, optical, and electrochemical properties. Intra-bandgap states associated with localized defects can act as traps for free carriers affecting both transport and recombination.13-18 The defects leading to electron trapping in nanocrystalline TiO2 are located predominately on the particle surface;13,14 hence, many empirical surface modification strategies have been * Corresponding author. E-mail: [email protected]. Phone: 509 335 4063. Fax: 509 335 8867. (1) Fujishima, A.; Honda, K. Nature 1972, 238, 37-38. (2) Bard, A. J.; Fox, M. A. Acc. Chem. Res. 1995, 28, 141-145. (3) Woodhouse, M.; Herman, G. S.; Parkinson, B. A. Chem. Mater. 2005, 17, 4318-4324. (4) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737-740. (5) Gra¨tzel, M. Nature 2001, 414, 338-344. (6) Hagfeldt, A.; Gra¨tzel, M. Acc. Chem. Res. 2000, 33, 269-277. (7) Hagfeldt, A.; Gra¨tzel, M. Chem. ReV. 1995, 95, 49-68. (8) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Ma¨uller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 63826390. (9) Fox, M. A.; Dulay, M. T. Chem. ReV. 1993, 93, 341-357. (10) Anderson, C.; Bard, A. J. Phys. Chem. 1995, 99, 9882-9885. (11) Linsebigler, A. L.; Lu, G.; Yates, J. T., Jr. Chem. ReV. 1995, 95, 735758. (12) Diebold, U. Surf. Sci. Rep. 2003, 48, 53-229. (13) Kopidakis, N.; Neale, N. R.; Zhu, K.; van de Lagemaat, J.; Frank, A. J. Appl. Phys. Lett. 2005, 87, 202106/1-212106/3. (14) Zhu, K.; Kopidakis, N.; Neale, N. R.; van de Lagemaat, J.; Frank, A. J. J. Phys. Chem. B 2006, 110, 25174-25180. (15) Tachibana, Y.; Haque, S. A.; Mercer, I. P.; Durrant, J. R.; Klug, D. R. J. Phys. Chem. B 2000, 104, 1198-1205. (16) Green, A. N. M.; Palomares, E.; Haque, S. A.; Kroon, J. M.; Durrant, J. R. J. Phys. Chem. B 2005, 109, 12525-12533. (17) Haque, S. A.; Tachibana, Y.; Willis, R. L.; Moser, J. E.; Gra¨tzel, M.; Klug, D. J. Phys. Chem. B 2000, 104, 538-547. (18) Nelson, J.; Haque, S. A.; Klug, D. R.; J. R. Durrant, J. R. Phys. ReV. B 2001, 63, 205321/1-2053231/9.

employed19-23 to optimize carrier transport, including surface treatment of TiO2 films by aqueous TiCl4.24-30 Treatment by TiCl4 might improve electron transport in dyesensitized solar cells (DSSCs) by improving interparticle connectivity, akin to the dramatic improvements in current collection observed for sintered films compared to that for films prepared at room temperature.31,32 Recently, Sommeling et al. concluded that the TiCl4 treatment results in improvements in the performance of DSSCs by lowering the conduction band edge and enhancing electron injection rather than by improving electron transport through the nanocrystalline film.29 Others have reported that TiCl4 treatment suppresses the recombination current by healing defects.28 In our recent study of DSSCs using sintered and unsintered TiO2 films,31 we observed the UV-excited visible photoluminescence of TiO2 films to be strongly diminished after sintering, consistent with the removal of defects associated with oxygen vacancies. In this work, we present photoluminescence spectra of TiO2 films before and after TiCl4 treatment to explore the influence of this surface treatment on defects associated with radiative recombination. The performance of nanoparticulate TiO2 in photocatalysis33 and photoelectrochemical energy conversion34 is dependent on (19) Schlichtho¨rl, G.; Huang, S. Y.; Sprague, J.; Frank, A. J. J. Phys. Chem. B 1997, 101, 8141-8155. (20) Huang, S. Y.; Schlichtho¨rl, G.; Nozik, A. J.; Gra¨tzel, M.; Frank, A. J. J. Phys. Chem. B 1997, 101, 2576-2582. (21) Zhang, Z.; Zakeeruddin, S. M.; O’Regan, B. C.; Humphrey-Baker, R.; Gra¨tzel, M. J. Phys. Chem. B 2005, 109, 21818-21824. (22) Palomares, E.; Clifford, J. N.; Haque, S. A.; Lutz, T.; Durrant, J. R. J. Am. Chem. Soc. 2003, 125, 475-448. (23) Ramakrishnan, G.; Singh, A. K.; Palit, D. K.; Ghosh, H. N. J. Phys. Chem. B 2004, 108, 1701-1707. (24) Barbe, C. J.; Arendse, F.; Camte, P.; Jirousek, M.; Lenzmann, F.; Shklover, V.; Gra¨tzel, M. J. Am. Ceram. Soc. 1997, 80, 3157-3171. (25) Park, N-G.; Schlichtho¨rl, G.; van de Lagemaat, J.; Cheong, H. M.; Mascarenhas, A.; Frank, A. J. J. Phys. Chem. B 1999, 103, 3308-3314. (26) Kambe, S.; Nakade, S.; Wada, Y.; Kitamura, T.; Yanagida, S. J. Mater. Chem. 2002, 12, 723-728. (27) Ito, S.; Liska, P.; Comte, P.; Charvet, R.; Pe´chy, P.; Bach, U.; SchmidtMende, L.; Zakeeruddin, A. M.; Kay, A.; Nazeeruddin, M. K.; Gra¨tzel, M. Chem. Commun. 2005, 4351-4353. (28) Lin, Y.; Xiao, X. R.; Li, W. Y.; Wang, W. B.; Li, X. P.; Cheng, J. Y. J. Photochem. Photobiol., A 2003, 159, 41-45. (29) Sommeling, P. M.; O’Regan, B. C.; Haswell, R. R.; Smit, H. J. P.; Bakker, N. J.; Smits, J. J. T.; Kroon, J. M.; van Roosmalen, J. A. M. J. Phys. Chem. B 2006, 110, 19191-19197. (30) Zhang, D.; Ito, S.; Wada, Y.; Kitamura, T.; Yanagida, S. Chem. Lett. 2001, 1042-1043. (31) Zhang, D.; Downing, J. A.; Knorr, F. J.; McHale, J. L. J. Phys. Chem. B 2006, 110, 21890-21898. (32) Gutie´rrez-Tauste, D.; Zumeta, I.; Vigil, E.; Herna´ndez-Fenollosa, M. A.; Dome`nech, X.; Ayllo´n, J. A. J. Photochem. Photobiol., A 2005, 175, 165-171.

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the crystal structure of the nanoparticles, which may exist in the rutile or anatase phase or in widely employed mixed-phase particles such as Degussa P25, which is 75-80% anatase and 20-25% rutile. The band gap of rutile is lower than that of anatase, but anatase is preferred in the DSSC owing to faster electron transport and a larger surface area-to-volume ratio for anatase nanoparticles.35 However, P25 TiO2 is widely used in DSSCs because it is inexpensive and commercially available and results in comparable performance to that of anatase-based solar cells. Evidence exists that P25 and similar mixed-phase TiO2 consists of nanoparticles containing both phases rather than a mixture of separate pure anatase and pure rutile nanoparticles.35 EPR evidence has been presented that points to the presence of unique defect sites at the interface between the rutile and anatase phases in P25 nanoparticles.36 To address differences in defect structure and the effect of TiCl4 treatment, we report photoluminescence (PL) spectra of pure and mixed-phase TiO2 before and after exposure to TiCl4. Surprisingly, TiCl4 treatment of P25 nanoparticles will be shown to lead to complete conversion of the PL from anatase-like to rutile-like even though the newly formed rutile content is barely detectable by Raman spectroscopy. The UV-excited PL of bulk crystalline TiO2 is frequently observed at cryogenic temperatures because it is strongly quenched by thermally activated nonradiative transitions at room temperature.37,38 The weak PL of nanocrystalline TiO2, however, is observable at room temperature but is low in intensity and subject to complications from scattering artifacts.39-43 We have found that the broad visible PL of mixed-phase P25 particles at room temperature, which peaks in the green at about 540 nm, is completely quenched by oxygen but can be measured reproducibly in an argon atmosphere with care to avoid stray light. In agreement with refs 40-43, we have assigned this emission to radiative recombination of self-trapped excitons associated with oxygen vacancies. In this picture, the breadth and large Stokes shift of the visible PL, which will be demonstrated here to be similar in P25 and anatase, results from the coupling of a defect-associated phonon mode to the radiative transition. In contrast, UV band gap excitation of rutile results in strong PL in the near-infrared (∼840 nm), which has been assigned to the radiative recombination of conduction band electrons with surface hydroxyl radicals or to oxygen vacancies.44-46 A recent report47 assigns the visible PL of anatase to surface oxygen vacancies, (33) Furube, A.; Asahi, T.; Masuhara, H.; Yamashita, H.; Anpo, M. J. Phys. Chem. B 1999, 103, 3120-3127. (34) Park, N.-G.; van de Lagemaat, J.; Frank, A. J. J. Phys. Chem. 2000, 104, 8989-8994. (35) Zhang, J.; Li, M.; Feng, Z.; Chen, J.; Li, C. J. Phys. Chem. B 2006, 110, 927-935. (36) Hurum, D. C.; Gray, K. C.; Rajh, T.; Thurnauer, M. C. J. Phys. Chem. B 2005, 109, 977-980. (37) Ghosh, A. K.; Wakim, F. G.; Addiss, R. R. Phys. ReV. 1969, 184, 979988. (38) Anpo, M.; Tomonari, M.; Fox, M. A. J. Phys. Chem. 1989, 93, 73007302. (39) Tang, H.; Berger, P. E.; Schmid, Levy, F.; Burri, G. Solid State Commun. 1993, 87, 847-850. (40) Abazovic´, N. D.; C ˇ omor, M. I.; Dramic´anin, M. D.; Jocanovic´, D. J.; Ahrenkiel, S. P.; Nedeljkovic´, J. M. J. Phys. Chem. B 2006, 110, 25366-25370. (41) Jung, K. Y.; Park, S. B.; Anpo, M. J. Photochem Photobiol., A 2005, 170, 247-252. (42) Pan, D.; Zhao, N.; Wang, Q.; Jiang, S.; Ji, S.; An, L. AdV. Mater. 2005, 17, 1991-1995. (43) Nakato, Y.; Ogawa, H.; Morita, K.; Tsubomura, H. J. Phys. Chem. 1986, 90, 6210-6216. (44) Nakamura, R.; Nakato, Y. J. Am. Chem. Soc. 2004, 126, 1290-1298. (45) Montoncello, F.; Carotta, M. C.; Cavicchi, B.; Ferroni, M.; Giberti, A.; Guidi, V.; Malagu, C.; Martinelli, G.; Meinardi, F. J. Appl. Phys. 2003, 94, 1501-1505. (46) Hurum, D. C.; Agrios, A. G.; Crist, S. E.; Gray, K. A.; Rajh, T.; Thurnauer, M. C. J. Electron. Spectrosc. Relat. Phenom. 2006, 150, 155-163. (47) Shi, J.; Chen, J.; Feng, Z.; Chen, T.; Lian, Y.; Wang, X.; Li, C. J. Phys. Chem. C 2007, 111, 693-699.

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whereas the near-IR PL of rutile was concluded to arise from intrinsic defects. Nakamura et al.,44 however, demonstrated the sensitivity of single-crystal rutile PL to surface roughness and the atomic structure of crystal planes where nucleophilic attack by water quenches the radiative recombination of conduction band electrons with surface hole traps lying about 1.5 eV below the conduction band edge. There appears to be strong evidence that both the visible anatase PL and the near-IR rutile PL are surface phenomena. Experimental Section P25 films were cast on quartz microscope slides (Chemglass) from an ethanol dispersion and sintered in air at 450 °C as previously described,31 except that a 1:8 mole ratio of TiO2 to ethanol was used. Anatase films were prepared from a commercial aqueous dispersion (Solaronix) containing nominal 9 nm nanoparticles deposited on quartz and sintered in air at 450 °C. Rutile films were prepared from these anatase films by sintering them in air at 950 °C for 4 h, and the conversion of anatase to rutile was verified using Raman spectroscopy.35 TiCl4 treatment consisted of soaking the prepared films in 0.2 M aqueous TiCl4 for 12 h, followed by sintering at 450 °C for 30 min. PL and Raman spectra were measured using instrumentation previously described.31 For PL measurements, the sample was excited at 350 nm with 4 mW incident power from a Kr ion laser, the scattered light was reduced with a long-pass filter, and the emission was measured with a single monochromator and CCD detection using backscattering geometry. The reported spectra are not corrected for instrumental response. The samples were mounted in a quartz cuvette and purged with argon. It should be noted that the PL of TiO2 at room temperature is very weak (on the order of Raman scattering), so care must be taken to avoid instrumental artifacts arising from the strongly scattering samples. Raman spectra were obtained using 413 nm excitation and a double monochromator equipped with photomultiplier tube detection. For measurements of dark current, TiO2 films were deposited on conductive glass and assembled in a dye-free cell using a platinumcoated conductive glass counter electrode and an electrolyte consisting of 0.5 M LiI and 0.05 M I2 in acetonitrile. Dark currents were measured as described in more detail in ref 31.

Results and Discussion Figure 1 shows the room-temperature PL of the sintered nanocrystalline films: (a) a P25 mixed-phase film, (b) anatase, and (c) rutile, excited at 350 nm. The three types of films all exhibit an emission band at approximately 420 nm, which has been attributed to shallow trap emission.35-37 This emission is observed in all three samples and is not sensitive to the sample environment, suggesting that it is a bulk phenomenon. The very broad visible emission, centered at 540 nm in the P25 sample and at approximately 560 nm for the pure anatase sample, is assigned to the radiative recombination of an exciton trapped at an undercoordinated surface defect site.39,40,47 This emission is highly sensitive to its environment and is completely and reversibly quenched when exposed to room oxygen.40 A similar oxygen-quenched emission has been reported for rutile TiO2 at cryogenic temperatures;41 however, it was not observed at room temperature, nor is it seen in this room-temperature study. Instead, the rutile sample shows a distinctive strong emission in the nearIR, centered at around 840 nm. Note that the rutile emission in Figure 1 is decreased by a factor of 100 to place it on the same scale as for the anatase and P25 emission. Although P25 is approximately 25% rutile by bulk measurement such as X-ray diffraction, there is no indication of the intense rutile emission at 840 nm. Assuming that rutile emission in the near-IR is a (48) Kavan, L.; Gra¨tzel, M.; Gilbert, S. E.; Klemenz, C.; Scheel, H. J. J. Am. Chem. Soc. 1996, 118, 6716-6723.

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Figure 1. Emission spectra in argon of nanocrystalline films excited at 350 nm: (a) P25, (b) anatase, and (c) rutile.

surface phenomenon as has been suggested,44 our results indicate that there is little rutile on the surface of P25 nanoparticles as received. Alternatively, conduction band electrons created in the rutile phase could migrate to the anatase phase and recombine there. The second explanation seems less plausible because the conduction band edge of rutile is 0.2 eV lower than that of anatase.48 The nanocrystalline P25 and anatase film samples were then subjected to aqueous TiCl4 treatment for 12 h, and the resulting UV-excited PL spectra are shown in Figure 2. For the anatase film, the broad emission at 560 nm is nearly completely suppressed. It has been proposed28 that the TiCl4 treatment heals surface defects through the precipitation of more ordered anatase TiO2 on the surface of the nanocrystallites. If the broad visible emission is associated with electrons trapped at surface defect sites, then the suppression of the emission upon TiCl4 treatment is consistent with a decrease in the number of surface defects. In a previous study, we have observed a decrease in this emission upon sintering of the nanocrystalline P25 film, which reduces the number of defect sites.31 The PL emission of the P25 film shows much different behavior than that of the anatase film after treatment with TiCl4. The broad anatase-like emission of P25 at 540 nm, which is associated with electrons trapped at surface defects, is completely suppressed, more so than that of the pure anatase film. More striking, however, is the appearance of a strong rutile emission at 840 nm. As shown in Figure 3, the Raman spectrum of the TiCl4treated P25 film compared with that of the untreated film shows only a small increase in the overall rutile content, despite the dramatic appearance of the rutile surface emission. A reasonable explanation of this result is that treatment of P25 with TiCl4 results in a thin layer of rutile on the surface. Alternatively, the treatment could result in more facile electron transport from the anatase to the preexisting rutile phase, perhaps sequestered in separate nanoparticles from those of anatase, where electrons then recombine with hole traps to generate the near-IR emission. The second explanation seems less likely on the basis of the complete absence of rutile PL in untreated P25 nanoparticles as well as the complete absence of anatase PL in the treated ones. Therefore, the influence of TiCl4 treatment on the performance of P25-based DSSCs could conceivably result from the deposition of a very thin layer of rutile. The same TiCl4 treatment of the pure anatase sample does not result in the formation of a rutile overlayer, as indicated by the absence of near-IR emission. In

Figure 2. Photoluminescence spectra of TiCl4-treated TiO2 films excited at 350 nm: (a) anatase and (b) P25.

Figure 3. Raman spectra of nanocrystalline mixed-phase (P25) films, untreated and treated with TiCl4. The excitation wavelength was 413.1 nm. The curves are displaced vertically for illustration. The arrows marked R mark the positions of rutile Raman bands.

both samples, the broad visible emission due to self-trapped excitons associated with anatase surface defects is suppressed (for anatase) or quenched completely (for P25). Surface defect states exert deleterious effects in dye-sensitized solar cells by enhancing the recombination of electrons with oxidized species in the contacting electrolyte solution, resulting in lower photocurrents and photovoltages. The observation of dark current under reverse bias reveals the recombination of electrons with the oxidized form of the redox mediator (I3-) at

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Figure 4. Dark current versus applied potential in dye-free “solar cells” prepared from anatase and P25, with and without TiCl4 treatment.

the conductive glass or TiO2 surface. Figure 4 reveals the consequences of TiCl4 treatment on the dark current under negative bias from “dye-free” solar cells employing anatase or P25. In both cases, treatment results in a reduction of dark current, which is generally less for the mixed-phase than for the anatasebased cell. A similar decrease in dark current on TiCl4 treatment of a P25 film was reported in ref 30. In contrast to the conclusions of ref 29, TiCl4 treatment does not significantly change the onset potential for the dark current as would be expected if there were a shift in the conduction band. The current at a given voltage is lower after treatment, an effect that is likely correlated to a decrease in surface defects where recombination occurs.13,14 An additional benefit of TiCl4 treatment might be the coverage of solvent-exposed conductive glass substrate by TiO2, but this was not studied in the present work. On the basis of an absorption coefficient on the order of 106 m-1 at 350 nm,7 the initial excitation of electron-hole pairs when TiO2 is illuminated in the UV takes place throughout the bulk of the nanoparticles, which are respectively about 9 and 25 nm in diameter for the anatase and P25 samples used here. Similarly, Raman spectra with excitation at 413 nm sample the entire nanoparticle volume rather than just the surface. The radiative recombination associated with anatase luminescence, however, takes place at surface trap sites associated with oxygen vacancies. Our results provide evidence that TiCl4 treatment of mixed-phase TiO2 nanoparticles results in a very thin shell of rutile on particles that are mostly anatase within the bulk. The UV-excited luminescence of untreated P25 is dominated by anatase and shows no rutile-like emission despite being about 25% rutile. We conclude that the rutile content of untreated P25 may be largely contained in the interior of the nanoparticle. This is in agreement with Zhang et al.,35 who studied the thermally assisted phase transition of TiO2 from anatase to rutile and

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concluded that the rutile phase forms on the interior of the TiO2 nanoparticles. Reference 35 also determined that the phase transition from anatase to rutile in the ∼550-900 °C temperature range is suppressed when surface defects on the anatase nanoparticles are passivated. We find that TiCl4 treatment of both anatase and P25 nanoparticles appears to heal surface defects of the anatase phase that are associated with visible photoluminescence. However, rutile luminescence appears only when mixed-phase TiO2 is treated, perhaps as a result of trace amounts of surface seed rutile, as depicted in the table of contents graphic. Our interpretation of the nature of untreated P25 particles differs slightly from the conclusions of ref 46, where the unique surface defects assigned to undercoordinated Ti3+ were associated with the interface between separate nanocrystallites of rutile and anatase rather than an interface between the two phases within the same nanoparticle. Owing to the much stronger intensity of the rutile near-IR emission compared to the anatase emission in the visible, the presence of separate nanoparticles of the two phases should have resulted in significant rutile emission in untreated P25, in contrast to what we observe. We note that our previously reported PL studies of sintered and unsintered P25 films showed similar visible luminescence in the unsintered films, much stronger than that of the sintered films, and no evidence of rutile emission. Given the poor electrical connectivity of unsintered nanoparticles, it is hard to rationalize the absence of rutile emission in the unsintered P25 film unless the rutile phase is already in intimate contact with the anatase phase before sintering or TiCl4 treatment. Our results can be compared to those of Park et al.,25 who report that more prolonged (2 days) exposure of anatase nanocrystalline films to aqueous TiCl4 results in a rutile overlayer detectable by Raman spectroscopy. The treatment in ref 25 consisted of exposure to 0.2 M TiCl4 for 2 days, whereas we used 0.1 M TiCl4 and 12 h of exposure time. We also used ethanol-stirred dispersions for casting P25 films, in contrast to frequently used aqueous dispersions. As previously reported,31 P25 films cast from ethanol as opposed to those cast from water have fewer surface defects.31 As reported in ref 34, surface defects promote the anatase-to-rutile phase transition. Thus, the failure of TiCl4 treatment to result in a rutile overlayer on pure anatase, in the present work, could result from the lower defect concentration and the shorter exposure times of our films compared to those used in ref 25. In ref 25, improvements in photon-to-current conversion efficiencies of DSSCs using TiCl4-treated TiO2 films were attributed to enhanced film thickness and light scattering. In the present work, both Raman and luminescence spectra argue against the formation of a rutile surface layer when pure anatase TiO2 is treated with TiCl4, perhaps because we use a shorter exposure time or because of the different surface properties of the untreated films. Our results also have important implications for the performance of mixed-phase TiO2 in photocatalysis and photoelectrochemical applications. The luminescence measurements reported here provide experimental evidence that TiCl4 treatment suppresses surface defect states that are likely associated with oxygen vacancies. In addition, our studies suggest that while electron-hole pairs are created in the treated P25 nanoparticle interior, which is predominantly anatase, radiative recombination occurs within a thin rutile overlayer. The flatband potential of anatase is reported to be 0.2 V more negative than that of rutile48 such that the difference in the band gaps in the two materials essentially arises from a lower (more positive) conduction band edge in rutile than in anatase. Conduction band electrons created within the nanoparticle interior are thus driven to the rutile surface

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by the decrease in the conduction band edge. Thus, in addition to removing surface defects that promote recombination, TiCl4 treatment of P25 may also influence carrier transport in dyesensitized solar cells by driving injected electrons toward the rutile phase that may reside on the nanoparticle surface. Further studies of the influence of TiCl4 and other surface treatments on carrier dynamics in anatase and P25 will be of great interest to understanding how to improve the performance of TiO2-based photoelectrochemical and photocatalytic systems.

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nanoparticles of rutile and anatase. We propose that the rutile content is sequestered within the anatase phase within the same nanoparticle. TiCl4 treatment of P25 nanoparticles might then result in a thin shell of rutile, explaining the complete conversion of photoluminescence from anatase-like to rutile-like. UV-excited photoluminescence of nanoparticulate TiO2 is a surface-sensitive technique and will be of great interest in further studies of photocatalytic and photoelectrochemical properties for which the state of the surface plays a crucial role.

Conclusions Photoluminescence experiments on TiCl4-treated anatase and mixed-phase nanoparticles of TiO2 reveal that treatment reduces the surface defects of anatase. The absence of rutile emission in untreated P25-TiO2 argues against the existense of separate

Acknowledgment. Support from the Washington State University Department of Chemistry and College of Science is gratefully acknowledged. LA700274K