Hetero-nanostructured Films of Titanium and Manganese Oxide

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Hetero-nanostructured Films of Titanium and Manganese Oxide Nanosheets: Photoinduced Charge Transfer and Electrochemical Properties Nobuyuki Sakai, Katsutoshi Fukuda, Yoshitomo Omomo, Yasuo Ebina, Kazunori Takada, and Takayoshi Sasaki* International Center for Materials Nanoarchitectonics and Nanoscale Materials Center, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ReceiVed: December 21, 2007; In Final Form: January 30, 2008

Multilayer heterostructured films of two different nanosheets, semiconducting Ti0.91O2 nanosheets and redoxable MnO2 nanosheets, were fabricated by the layer-by-layer sequential adsorption method using a polyelectrolyte as a linker. Successful deposition of these two nanosheets in various sequences was confirmed by monitoring UV-visible absorption spectra and X-ray diffraction data on the films. We examined how nanostructures of the heteroassembled films affect the behaviors of photoinduced electron transfer between the two types of nanosheets and electrochemical properties of composite film electrodes. The excitation of Ti0.91O2 nanosheets under light irradiation at 280 nm was found to cause a gradual decrease in intensity of the optical absorption peak at 372 nm, which can be ascribed to the reduction of Mn4+ to Mn3+ of MnO2 nanosheets as confirmed by XANES spectra. This behavior can be understood by a scheme that the excited electrons produced in Ti0.91O2 nanosheets are injected into MnO2 nanosheets. The injection rate depended on the nanostructures of the multilayer films. Electrochemical studies of composite multilayer films showed an additional oxidation peak in comparison with those for films solely composed of MnO2 nanosheets, which may be accounted for by the extraction of Li+ ions inserted into the interlayer gallery with a hetero-environment between Ti0.91O2 and MnO2 nanosheets.

Introduction Layered heterostructures including semiconductor superlattices1 have been investigated extensively to enhance the properties of individual components and/or to create new functionalities as a consequence of the cooperative action of different properties of constituent components. Such materials are important for various applied fields such as photonic crystal heterostructure devices,2 high electron mobility transistors (HEMT),3 thermoelectric materials,4 and hard coatings5 as well as photocatalysis,6 photovoltaics,7 and photoluminescence.8 The materials incorporated range from organic polymers to inorganic metal oxides. Oxide nanosheets are important and promising materials as a component for constructing such heterostructures. Various nanosheets based on transition metal oxides have been synthesized recently by delaminating precursor crystals of a layered oxide into its elementary layers.9-12 The oxide nanosheets have an extremely high two-dimensional anisotropy of the crystallites: the thickness is ∼1 nm while the lateral size ranges from submicrometers to several tens of micrometers. This structural and morphological aspect makes the nanosheets a suitable building block for designing nanostructured films. Sequential layer-by-layer assembly is one of the most powerful methods of fabricating nanostructured multilayer films with precisely controlled composition, thickness, and architecture on a nanometer scale.13,14 Through this process, nanosheets can be combined with a wide range of polyions such as organic * To whom correspondence should be addressed. Phone: +81-29-8604313. Fax: +81-29-854-9061. E-mail: [email protected].

polyelectrolytes, metal complexes, clusters, and even oppositely charged nanosheets, which is a major advantage of this approach.15 Various interesting and useful properties have been developed by organizing or assembling transition metal oxide nanosheets into composite materials. Titanium oxide nanosheets flocculated with lanthanide cations emitted intense photoluminescence at room temperature through effective energy transfer from the semiconducting host.8 Reassembled titanium or manganese oxide nanosheets, either with or without carbon, were reported to have as large a capacity as Li-ion batteries.16 Ruthenic acid nanosheets showed high performance as electrochemical supercapacitors.17 Transparent multilayer films of Co- or Fesubstituted titanium oxide nanosheets exhibited gigantic magnetooptical effects in response to ultraviolet light.18 Highly stable photoinduced charge separation was attained in a composite film of restacked titanium oxide nanosheets and mesoporous silica or clay minerals, in which electron donors and acceptors are spatially separated at a distance of micrometers.19 Cooperative interaction between nanosheets with different properties in a multilayer system with a tailored nanoarchitecture is of particular interest. Recently, we have reported that Ti0.91O2 nanosheets possess semiconducting properties similar to those of bulk TiO2 such as rutile and anatase except for some modifications due to size quantization.20 Ti0.91O2 nanosheets generate anodic photocurrent by band gap excitation under light irradiation with wavelengths shorter than 320 nm, corresponding to a wider band gap energy of 3.8 eV.20b In contrast, MnO2 nanosheets have a broad absorption peak centered at 372 nm, which results from d-d transitions in the MnO2 nanosheets. The electrochemical reduction and reoxidation of MnO2 nanosheets give rise to fading and enhancement of the optical

10.1021/jp7119894 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/12/2008

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absorption of MnO2 nanosheets.21 This electrochromic efficiency has been estimated to be 64.2 cm2 C-1 at 385 nm,22 which is a relatively high value among manganese oxides. In the present study, we deposited two types of nanosheets, semiconducting Ti0.91O2 nanosheets and redoxable MnO2 nanosheets, to produce superlattice-like assemblies, and examined how the layering sequence of the nanosheets affects photoinduced electron transfer from Ti0.91O2 nanosheets into MnO2 nanosheets and electrochemical reduction/oxidation behaviors. Experimental Section Reagents. Chemicals such as Cs2CO3, K2CO3 (Wako Pure Chemical Co., Japan), TiO2, and Mn2O3 (High Purity Chemical Co., Japan) of 99.9% purity were used as received in the synthesis of starting layered oxides of Cs0.7Ti1.82500.175O4 (0: vacancy at Ti site) and K0.45MnO2. Polyethylenimine (PEI) (50% solution in water) and poly(diallyldimethylammonium) (PDDA) chloride (20% solution in water) were purchased from Aldrich and used without further purification. All other chemicals and solvents were of analytical grade. Ultrapure water, filtered by a Milli-Q reagent water system at a resistivity of >18 MΩ cm, was used throughout the experiments. Nanosheets. Oxide nanosheets of Ti0.91O2 and MnO2 were prepared by a well-established procedure reported previously.10,11c Starting layered materials of Cs0.7Ti1.82500.175O4 and K0.45MnO2 were synthesized via conventional solid-state calcination.23,24 Subsequent acid leaching using 1 mol dm-3 HCl solution converted them into protonated forms of H0.7Ti1.82500.175O4‚ H2O and H0.13MnO2‚0.7H2O, respectively. The resulting protonic product (0.4 g) was shaken vigorously in 100 cm3 of an aqueous tetrabutylammonium hydroxide (TBA+OH-) solution at ambient temperature. The suspension containing MnO2 nanosheets was centrifuged at 10 000 rpm for 30 min to remove any unexfoliated material. Exfoliated single-layer nanosheets of Ti0.91O2 and MnO2 with a molecular thickness of approximately 1.2 and 0.8 nm, respectively, were dispersed in the resulting stable colloidal suspensions. Fabrication of Multilayer Films. Quartz glass substrates were cleaned by dipping in a bath of methanol/HCl (1:1 in volume) and then concentrated H2SO4 for 30 min each. Indiumtin oxide (ITO)-coated quartz glass slides were cleaned by ultrasonic treatment in acetone, ethanol, and pure water for 15 min each. The multilayer films were fabricated via the sequential adsorption procedure reported previously.15a-c Because the oxide nanosheets are negatively charged, polycations such as PEI and PDDA were used as linkers. The substrate was dipped first in a PEI solution (2.5 g cm-3, pH 9) for 20 min to make the surface positively charged. MnO2 nanosheets and Ti0.91O2 nanosheets were adsorbed in various sequences from their colloidal suspension (0.08 g dm-3, pH 9) with intervention of dipping in a PDDA solution (20 g dm-3, pH 9). The substrate was immersed in each solution for 20 min and then rinsed thoroughly with ultrapure water. Film Characterizations. Ultraviolet-visible (UV-visible) absorption spectra of multilayer films fabricated on a quartz glass substrate were recorded using a Hitachi U-4100 spectrophotometer. X-ray diffraction (XRD) data were collected using a Rigaku Rint 2000 powder diffractometer with monochromatized Cu KR radiation (λ ) 0.15404 nm). X-ray absorption near edge structure (XANES) spectra were obtained with synchrotronradiated X-ray using an extended X-ray absorption fine-structure facility installed at beam line 12C at the Photon Factory, KEK, Japan. The spectra for the films were obtained at room temperature in a total-reflection fluorescent mode using a 19element Ge solid-state detector.

Figure 1. (A) UV-visible absorption spectra in the multilayer buildup process. Red lines: spectra for the films after deposition of MnO2 nanosheet; blue lines: spectra for the films after deposition of Ti0.91O2 nanosheet. Experimental conditions employed for this film growth: deposition time of 20 min each and nanosheet concentrations of 0.08 g dm-3. (B) Peak-top absorbance at 372 and 255 nm as a function of the number of nanosheet layers. Red and blue symbols represent the data after deposition of MnO2 and Ti0.91O2 nanosheets, respectively.

Light Irradiation and Electrochemical Measurements. Monochromatic light (fwhm ) 15 nm) obtained from a 500 W Xe lamp (XEF-501S, San-ei Electric) through a monochromator (H-10UV, Jobin Yvon) was used for optical excitation of the multilayer nanosheet films. The irradiated light intensity was calibrated by a spectroradiometer (USR-30, Ushio). Electrochemical measurements were carried out in a conventional threeelectrode, single-compartment glass cell, fitted with a synthesized quartz window, using a potentiostat (SI1287, Solartron). The nanosheet film electrodes served as the working electrode. The counter and reference electrodes were platinum black wire and Ag/Ag+/acetonitrile ()+0.49 V vs NHE), respectively. Propylene carbonate containing 0.1 mol dm-3 LiClO4 was employed as the supporting electrolyte. Results and Discussion Film Fabrication. The layer-by-layer assembly process of oxide nanosheets such as Ti0.91O2 and MnO2 could be monitored from the UV-visible absorption spectra. Figure 1A shows the data when Ti0.91O2 and MnO2 nanosheets were deposited in an alternating fashion using PDDA as an electrostatic glue. The spectral profile observed can be understood in terms of the superimposition of the profile of each nanosheet. The absorption peaks at 255 and 372 nm are attributable to Ti0.91O2 and MnO2 nanosheets, respectively, as is clear from the data for the multilayer films composed of each nanosheet (Supporting Information, Figure S1).15a-c The enhancement of absorbance at 372 nm was observed only after the deposition of MnO2

Hetero-nanostructured Films

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Figure 2. XRD patterns for the multilayer assembly of (a) (Ti0.91O2/ PDDA/MnO2/PDDA)9/Ti0.91O2/PDDA/MnO2/PEI, (b) (Ti0.91O2/PDDA)9/ Ti0.91O2/PEI, and (c) (MnO2/PDDA)9/MnO2/PEI on quartz glass substrate.

nanosheets (Figure 1B). Ti0.91O2 nanosheets do not show absorption at this wavelength. The increment of absorbance at 372 nm was 0.086 ( 0.009 per deposition, which is almost the same as that ()0.082 ( 0.014) for multilayer films composed only of MnO2 nanosheets. Alternatively, the absorbance at 255 nm was enhanced by both the deposition of MnO2 and Ti0.91O2 nanosheets. The increment of absorbance at 255 nm was 0.055 ( 0.009 and 0.103 ( 0.013 after the deposition of MnO2 and Ti0.91O2 nanosheets, respectively, which are again similar to those for multilayer films composed only of MnO2 or Ti0.91O2 nanosheets. Hence, the deposition of each nanosheet enhanced its associated absorbance, and its progressive enhancement as a function of the number of layers is evidence of the successful heterostructure buildup of the two types of nanosheets, (Ti0.91O2/ PDDA/MnO2/PDDA)9/Ti0.91O2/PDDA/MnO2/PEI/quartz. The XRD data for the obtained films exhibited a Bragg peak centered at 2θ ) 8.2°, as shown in Figure 2a. The d spacing of 1.1 nm for this peak is clearly different from the multilayer repeat distance for the films of each component of Ti0.91O2 (Figure 2b, d ) 1.3 nm) and MnO2 (Figure 2c, d ) 0.94 nm) nanosheets. Because the obtained value of d ) 1.1 nm is close to half of the repeating unit of Ti0.91O2/PDDA/MnO2/PDDA (d ) 2.24 nm ) 1.3 + 0.94 nm), the reflection observed for the heterostructured films may be interpreted as the second-order line for the heterostructure. The apparent absence of the first-order peak may be explained by fairly similar scattering amplitudes of Ti0.91O2 and MnO2 nanosheets in the film. The intensity (I) of the basal diffraction series of the multilayer films are calculated as

I ) FF* F)

∑j nj fj exp{2π izj(2 sin θ/λ)}

(1) (2)

where F is the structure factor, nj, fj, and zj are population, atomic scattering factor, and position along the stacking direction for the jth atom, and θ and λ are the diffraction angle and wavelength of the X-ray ()0.15405 nm). The structure models illustrated in Figure 3 are based on the host layer architectures reported for the starting layered materials of obtained Cs0.7Ti1.82500.175O4 and K0.31MnO2.23,25 Intervening layers of PDDA are ignored because our previous study indicated that it makes a negligible contribution.15b The intensity ratio of first-, second-, and third-order lines was calculated as 100, 22, 1 and 100, 32, 5 for the multilayer films of (Ti0.91O2/PDDA)n (Figure 3b) and (MnO2/PDDA)n (Figure 3c), respectively, whereas values of 0.01, 100, 2 were obtained

Figure 3. Structure models of (a) heteroassembled system of Ti0.91O2/ MnO2 and multilayer structure of (b) Ti0.91O2 and (c) MnO2 nanosheets. In the heteroassembled structure, populations for Ti and O atoms in Ti0.91O2 nanosheets are normalized to MnO2 nanosheets. The twodimensional unit cell area is 0.112 nm2 ()0.3760 × 0.2967) for the former and 0.698 nm2 ()0.2857 × 0.2857 × sin 120°) for the latter.15f,26

Figure 4. Changes of absorbance at 372 nm of the multilayer assembly of Ti0.91O2 and MnO2 nanosheets as a function of irradiation time. Irradiation wavelength: (a) 280 nm (circles) and (b) 370 nm (triangles). Plots (c) represent the absorbance change of the multilayer assembly of MnO2 nanosheets without Ti0.91O2 nanosheets under light irradiation at 280 nm (squares).

for the multilayer film of (Ti0.91O2/PDDA/MnO2/PDDA)n (Figure 3a), which is in agreement with the observed profile. Photoinduced Electron Injection. We examined the effect of light irradiation on the hetero-assembled films composed of alternating Ti0.91O2 and MnO2 nanosheets. Exposure to light with λ ) 280 nm caused a gradual decay of the absorption peak at 372 nm, which is ascribed to MnO2 nanosheets (Figure 4a). In contrast, light irradiation at λ ) 370 nm caused no apparent change in absorption (Figure 4b). In addition, the multilayer films fabricated only with MnO2 nanosheets did not show any change in spectral profile or intensity when irradiated by light at λ ) 280 nm (Figure 4c). These results strongly suggest that

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Figure 5. Mn K-edge XANES spectra of the multilayer assembly of Ti0.91O2 and MnO2 nanosheets for (a) as-grown film and (b) after UV irradiation. The spectra for (c) β-MnO2 and (d) Mn2O3 are also presented for reference.

the observed decay of optical absorbance at 370 nm in the hetero-assembled film is associated with the excitation of Ti0.91O2 nanosheets. The basal peak observed in XRD measurements remained unchanged after the light irradiation (λ ) 280 nm), thus precluding the possibility of decomposition or peeling-off of MnO2 nanosheets. Another plausible explanation is the reduction of Mn4+ to Mn3+ in the MnO2 nanosheets. As reported previously,21 MnO2 nanosheets are an anodic electrochromic material, showing the bleached state by the electrochemical reduction of Mn4+ to Mn3+. Actually, a XANES study confirmed that such reduction induced by light irradiation (λ ) 280 nm) occurred in MnO2 nanosheets (Figure 5). The jumpup energy for Mn K-edge in the as-grown film was very similar to that of β-MnO2 as the standard material, indicating that the oxidation state before UV irradiation was very close to +4.

Sakai et al. Upon irradiation, the edge was shifted to the lower energy and became similar to that of Mn2O3. Therefore, the observed phenomena can be understood by the injection of excited electrons generated in Ti0.91O2 nanosheets into MnO2 nanosheets, reducing Mn4+ to Mn3+. The photogenerated holes in the valence band of Ti0.91O2 nanosheets may be consumed through oxidation of the adsorbed water and/or the polycation layers between the nanosheets.27 The amount of decomposed PDDA should be so small that it would not decrease the interlayer distance. It is of interest to estimate the efficiency of the photoinduced electron transfer from Ti0.91O2 nanosheets to MnO2 nanosheets. The rate of decrease of absorbance was 6.2 × 10-4 min-1 estimated from the slope during the irradiation time from 20 to 120 min. On the basis of the decoloration efficiency of 64.2 cm2 C-1,21,22 0.96 mC cm-2 of electrons were injected into MnO2 nanosheets during 100 min of light irradiation. Because the light intensity was 42.2 µW cm-2 at λ ) 280 nm, it was estimated that 1.7% of incident photons was used for the photoinduced electron transfer. Films with Different Sequences of Nanosheets. The nanostructures in the heteroassembled films, for example, the number of nanosheet layers and their sequences, can be tailored through layer-by-layer deposition. Figure 6A-D shows the UV-visible absorption spectra of 12 layers of nanosheets composed of MnO2 and Ti0.91O2 nanosheets with six layers each in a different order as examples of various nanostructures. The deposition of Ti0.91O2 nanosheets of (A) 1, (B) 2, (C) 3, and (D) 6 layers was carried out after the deposition of MnO2 nanosheets of the same number of layers, respectively. This procedure was repeated until the total number of deposited layers reached 12. In all cases, the deposition of MnO2 nanosheets enhanced the absorbance at 372 nm as well as at 255 nm, whereas the deposition of Ti0.91O2

Figure 6. UV-visible absorption spectra in the multilayer buildup process. Ti0.91O2 and MnO2 nanosheets with a total of 12 layers were assembled layer-by-layer every (A) 1, (B) 2, (C) 3, and (D) 6 layers of each nanosheet with a PDDA layer sandwiched as a countercation. Red lines: spectra for the films after deposition of MnO2 nanosheet; blue lines: spectra for the films after deposition of Ti0.91O2 nanosheet. The insets indicate the designed stacked structure of the nanosheets. The intervening PDDA is not drawn for clarity.

Hetero-nanostructured Films

Figure 7. Decay rate of absorbance at 372 nm under light irradiation at λ ) 280 nm. The sample notation corresponds to that in Figure 6.

nanosheets enhanced the absorbance only at 255 nm, and their absorbance gains were comparable to each other. The spectral changes observed clearly indicate that the films grew as designed. The overall absorption profile of each spectrum was similar for all of the films fabricated and independent of the deposition order of each nanosheet, suggesting that the optical absorption of the multilayer films does not depend on how the nanosheets are layered but on what nanosheets the multilayer films are composed of. We examined the photoinduced electron injection rate from Ti0.91O2 nanosheet to MnO2 nanosheet by using these four samples with different nanostructures. The decay of absorbance at 372 nm was observed for each sample when exposed to light at 280 nm. The slope of the decay is plotted in Figure 7. A smaller rate of decrease in sample A compared with that in Figure 4 is due to the smaller number of total layers of nanosheets. Samples A and B showed almost the same rate, whereas the rates of samples C and D were about half. In samples A and B, the MnO2 nanosheets are always present as neighbors of Ti0.91O2 nanosheets, and therefore the distance between Ti0.91O2 and MnO2 nanosheets is the smallest, giving the highest rate for photoinduced electron injection from Ti0.91O2 to MnO2 nanosheets. In samples C and D, however, some layers of MnO2 nanosheets are farther from the layer of Ti0.91O2 nanosheets because of additional intervening layers of MnO2 nanosheets. This structural aspect may be responsible for the smaller rate of electron injection. The result for D, that is, not showing a lower rate than that for C, is unexpected and the exact reason is unknown at present. To sum up, the behavior of photoinduced electron injection from Ti0.91O2 nanosheets to MnO2 nanosheets depends on the nanostructure of multilayer films or how the nanosheets are assembled. In other words, we can control the diffusion of carriers by designing the heteroassembly of the nanosheets. Electrochemical Behaviors of Heteroassembled Films. The heteroassembled film electrodes were subjected to electrochemical reduction and oxidation processes by sweeping their potential in a nonaqueous electrolyte. Two different composite films fabricated on ITO substrates were examined: each composite film consisted of three layers of Ti0.91O2 and MnO2 nanosheets, although the layering order was different: (A) (Ti0.91O2/PDDA/ MnO2/PDDA)2/Ti0.91O2/PDDA/MnO2/PEI/ITO and (B) (Ti0.91O2/ PDDA)3/(MnO2/PDDA)2/MnO2/PEI/ITO. Figure 8a and b depicts cyclic voltammograms (CV) of these films. The latter data is similar to a CV curve for the multilayer film composed only of MnO2 nanosheet except for a small oxidation peak at around +0.15 V. The well-defined reduction and oxidation peaks at -0.52 and -0.15 V, respectively, can be ascribed to the reduction/oxidation of the Mn4+/Mn3+ of the MnO2 nanosheets,

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Figure 8. Cyclic voltammograms of the heteroassembled films of (a) (Ti0.91O2/PDDA/MnO2/PDDA)2/Ti0.91O2/PDDA/MnO2/PEI and (b) (Ti0.91O2/PDDA)3/(MnO2/PDDA)2/MnO2/PEI coated on conductive ITO substrate in propylene carbonate containing 0.1 mol dm-3 LiClO4. Sweep rate: 20 mV s-1.

Figure 9. Optical absorption changes at 372 nm for Ti0.91O2/MnO2 nanosheet composite films: as-grown film (closed square), after UV irradiation (open circles), and after anodic oxidation at +1.0 V (open triangles).

accompanied by insertion/extraction of Li+ ions into/from the interlayer gallery of MnO2 nanosheets.21 Alternatively, the CV curve (a) has an additional oxidation peak at around +0.21 V, which is close to the small oxidation peak of curve b. This peak may be ascribed to the extraction of Li+ ions accommodated in the interlayer gallery encircled by Ti0.91O2 and MnO2 nanosheets because film A, which has five interlayer galleries sandwiched by Ti0.91O2 and MnO2 nanosheets, has a larger area of oxidation peak at +0.21 V than that of film B, which has only one such space. The oxidation peak at -0.15 V of the CV curve (a) may arise from the Li+ ions accommodated in overlapped parts of MnO2 nanosheets. The area of reduction/oxidation peak for sample A was smaller than that for sample B although the number of MnO2 nanosheets was identical for these two films. This may also be a consequence of the interlayer environment having Ti0.91O2 and MnO2 nanosheets. Li ions accommodated in such space may be less stable than those in the gallery of MnO2 nanosheets. These results indicate that the layering with different nanosheets modifies the electrochemical potential of MnO2 nanosheets as a consequence of hetero-interlayer environments. The reduced state of MnO2 nanosheets generated by the photoinduced electron injection from Ti0.91O2 nanosheets could be oxidized by the electrochemical method. As shown in Figure 9, the absorption peak at 372 nm weakened by light irradiation at λ ) 280 nm was enhanced by the anodic polarization at +1.0 V. The enhancement is derived from the electrochemical oxidation of Mn3+ to Mn4+ in MnO2 nanosheets. The reduction and enhancement of the absorption peak could be repeated by

5202 J. Phys. Chem. C, Vol. 112, No. 13, 2008 the alternating UV irradiation and electrochemical oxidation, and the process was stable and reversible. Conclusions We fabricated composite films with tailored nanostructures by layer-by-layer assembly of two different nanosheets, semiconducting Ti0.91O2 nanosheets and redoxable MnO2 nanosheets. We found that the designed nanostructures of the multilayer films affected the behaviors of photoinduced electron transfer between two different kinds of nanosheets as well as the electrochemical properties of the composite multilayer film electrodes. The present work may shed light on the chemistry of self-organized systems composed of different components assembled in a nanometer scale. Acknowledgment. This work was supported by CREST of Japan Science and Technology Agency (JST). The XANES experiments were performed under the approval of the Photon Factory Program Advisory Committee (2001G324). Supporting Information Available: Figures showing the layer-by-layer assembly of multilayer films composed of each nanosheet: Ti0.91O2 and MnO2 (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Dumelow, T.; Parker, T. J.; Smith, S. R. P.; Tilley, D. R. Surf. Sci. Rep. 1993, 17, 151. (b) Albuquerque, E. L.; Cottam, M. G. Phys. Rep. 1993, 233, 67. (c) Steslicka, M.; Kucharczyk, R.; Akjouj, A.; DjafariRouhani, B.; Dobrzynski, L.; Davison, S. G. Surf. Sci. Rep. 2002, 47, 93. (2) Istrate, E.; Sargent, E. H. ReV. Mod. Phys. 2006, 78, 455. (3) Mimura, T. Jpn. J. Appl. Phys. 2005, 44, 8263. (4) Dresselhaus, M. S.; Chen, G.; Tang, M. Y.; Yang, R.; Lee, H.; Wang, D.; Ren, Z.; Fleurial, J.-P.; Gogna, P. AdV. Mater. 2007, 19, 1043. (5) Ziebert, C.; Ulrich, S. J. Vac. Sci. Technol., A 2006, 24, 554. (6) (a) Weller, H. AdV. Mater. 1993, 5, 88. (b) Kamat, P. V. Chem. ReV. 1993, 93, 267. (c) Kamat, P. V. J. Phys. Chem. C 2007, 111, 2834. (d) Miyauchi, M.; Nakajima, A.; Hashimoto, K.; Watanabe, T. Chem. Mater. 2002, 14, 4714. (e) Tatsuma, T.; Saitoh, S.; Ohko, Y.; Fujishima, A. Chem. Mater. 2001, 13, 2838. (7) (a) Spanhel, L.; Weller, H.; Henglein, A. J. Am. Chem. Soc. 1987, 109, 6632. (b) Gopidas, K. R.; Bohorquez, M.; Kamat, P. V. J. Phys. Chem. 1990, 94, 6435. (c) Sant, P. A.; Kamat, P. V. Phys. Chem. Chem. Phys. 2002, 4, 198. (d) Gra¨tzel, M. Nature 2001, 414, 338. (8) (a) Xin, H.; Ma, R.; Wang, L. Z.; Ebina, Y.; Takada, K.; Sasaki, T. Appl. Phys. Lett. 2004, 85, 4187. (b) Matsumoto, Y.; Unal, U.; Kimura, Y.; Ohashi, S.; Izawa, K. J. Phys. Chem. B 2005, 109, 12748. (c) Ida, S.; Araki, K.; Unal, U.; Izawa, K.; Altuntasoglu, O.; Ogata, C.; Matsumoto, Y. Chem. Commun. 2006, 3619. (d) Ida, S.; Unal, U.; Izawa, K.; Altuntasoglu, O.; Ogata, C.; Inoue, T.; Shimogawa, K.; Matsumoto, Y. J. Phys. Chem. B 2006, 110, 23881. (9) Sasaki, T. J. Ceram. Soc. Jpn. 2007, 115, 9. (10) (a) Sasaki, T.; Watanabe, M.; Hashizume, H.; Yamada, H.; Nakazawa, H. J. Am. Chem. Soc. 1996, 118, 8329. (b) Sasaki, T.; Watanabe, M. J. Am. Chem. Soc. 1998, 120, 4682. (c) Tanaka, T.; Ebina, Y.; Takada, K.; Kurashima, K.; Sasaki, T. Chem. Mater. 2003, 15, 3564. (11) (a) Liu, Z.-H.; Ooi, K.; Kanoh, H.; Tang, W.-P.; Tomida, T. Langmuir 2000, 16, 4154. (b) Yang, X.; Makita, Y.; Liu, Z.-h.; Sakane, K.; Ooi, K. Chem. Mater. 2004, 16, 5581. (c) Omomo, Y.; Sasaki, T.; Wang, L. Z.; Watanabe, M. J. Am. Chem. Soc. 2003, 125, 3568. (d) Oaki, Y.; Imai, H. Angew. Chem., Int. Ed. 2007, 46, 4951. (e) Liu, Z.; Ma, R.; Ebina, Y.; Takada, K.; Sasaki, T. Chem. Mater. 2007, 19, 6504.

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