Recent developments in titanium oxide-based photocatalysts

Applied Catalysis A: General 325 (2007) 1–14 www.elsevier.com/locate/apcata

Review

Recent developments in titanium oxide-based photocatalysts Masaaki Kitano a, Masaya Matsuoka b, Michio Ueshima a, Masakazu Anpo b,* b

a Industry-University Cooperation Organization, Osaka Prefecture University, 1-2 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8570, Japan Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan

Received 9 March 2007; accepted 11 March 2007 Available online 15 March 2007

Abstract Recent development in titanium oxide-based photocatalysts was reviewed concerning the development in the highly dispersed titanium oxide photocatalysts prepared on or within zeolites and the visible light-responsive TiO2 photocatalysts. The unique and high reactivities of titanium oxide species anchored or incorporated in the zeolite for various photocatalytic reactions such as reduction of CO2 with H2O and direct decomposition of NOx into N2 and O2 were discussed focusing on the relationship between the reactivity and local structures of the catalysts. Moreover, the preparation of the visible light-responsive TiO2 photocatalysts by applying ion-engineering techniques such as an ion-implantation and an RF magnetron sputtering deposition method was discussed focusing on its unique reactivity for the decomposition of water into H2 and O2 with a separate evolution under sunlight irradiation. # 2007 Elsevier B.V. All rights reserved. Keywords: Highly dispersed titanium oxide photocatalysts; CO2 reduction; NOx decomposition; Visible light-responsive TiO2; Metal ion implantation; RF magnetron sputtering; Nitrogen substitution; Water splitting

Contents 1. 2.

3.

4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Titanium oxide photocatalysts anchored in the cavities of various zeolites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Photocatalytic reduction of CO2 with H2O on highly dispersed titanium oxides anchored on various zeolites . . . . . . . . 2.2. Photocatalytic decomposition of NO on highly dispersed titanium oxides anchored on various zeolites . . . . . . . . . . . . Design of visible light-responsive TiO2 photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Application of metal ion-implantation techniques to TiO2 photocatalysts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Preparation of visible light-responsive TiO2 thin film photocatalysts by an RF magnetron sputtering deposition method . 3.3. Preparation of nitrogen-substituted TiO2 thin film photocatalysts by an RF magnetron sputtering deposition method . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction In recent years, there has been great concern over the many serious environmental problems and a lack of natural energy resources we face on a global scale. The increase in world population and industrial development have all led to

* Corresponding author. E-mail address: [email protected] (M. Anpo). 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2007.03.013

. . . . . . . . . . .

. . . . . . . . . . .

. . . . . . .

1 3 3 6 7 7 9 11 12 12 13

accelerated energy consumption and the unabated release of toxic agents into the air and water, leading to such averse effects as pollution-related diseases and global warming. It is, therefore, of the utmost urgency to develop ecologically clean and safe chemical technologies, materials and processes to sustain our present level of population and economic expansion. In line with these objectives, the application of pollution-free photocatalysis in many chemical processes has recently attracted a great deal of attention. Thus, the high activity and stability as well as availability of TiO2 photocatalysts are especially interesting to

2

M. Kitano et al. / Applied Catalysis A: General 325 (2007) 1–14

researchers. In fact, TiO2 photocatalysts have many advantages for such practical applications as the purification of toxic compounds in polluted water and air [1–3], the photocatalytic decomposition of water [3–5], the development of anti-fogging films [6,7] and photovoltaic cells [8–10]. In the pioneering work of Honda and Fujishima [4], UV light irradiation of a TiO2 photoelectrode in aqueous solution led to the production of H2 and O2 on a Pt electrode and TiO2 photoelectrode, respectively, onto which a small electric voltage was applied. Bard’s concept [11–13], which emerged in 1979, could then be applied to the design of photocatalytic systems using TiO2 particles or powders. ATiO2 particle loaded with Pt can be thought of as a ‘‘short-circuited photoelectrochemical cell’’ providing both the oxidizing and reducing sites for the reaction. Schematic representations of photoelectrochemical cells (PEC) and Pt-loaded TiO2 particle systems (Pt/TiO2) are depicted in Fig. 1. Comparing the two, the Pt/TiO2 particulate system is simpler and less expensive to construct and use than PEC. Also, such Pt/TiO2 particulate systems can be applied for a wide range of photocatalytic reactions. In fact, Sato and White have reported that TiO2 semiconducting nanopowdered photocatalysts loaded with small amounts of Pt worked to decompose H2O into H2 and O2 as a mixture gas under UV irradiation of wavelengths shorter than 400 nm [14– 16]. Wagner et al. also reported that UV irradiation of platinized SrTiO3 single crystals coated with films of NaOH led to the

Fig. 1. Schematic representations of: (A) a photoelectrochemical cell (PEC) and (B) the Pt-loaded TiO2 (Pt/TiO2) particle system.

production of H2 [17–19]. These findings suggest that Pt-loaded semiconductors envisioned as ‘‘short-circuited photochemical cells’’ could decompose water into H2 and O2 by irradiation of UV light. Studies have also been carried out on TiO2 nanoparticles as well as on various binary oxides including extremely small TiO2 moieties such as TiO2/SiO2, TiO2/Al2O3 and TiO2/B2O3 [7,20–30]. In these systems, the photocatalytic activity increased as the diameter of the TiO2 particles decreased, ˚ . The absorption and photoluminesespecially below 100 A cence spectra of the catalysts exhibit blue shifts as the diameter of TiO2 becomes smaller, especially below a particle size of ˚ , suggesting that a size quantization effect occurs [22,25– 100 A 27]. Moreover, TiO2 photocatalysts with well-defined and highly dispersed Ti-oxide species anchored onto supports, such as SiO2, porous glasses and zeolites, exhibited high photocatalytic activity and selectivity as compared with bulk TiO2 semiconducting powders. Especially, Ti-containing micro- and mesoporous zeolites have been found to demonstrate efficient and selective photocatalytic reactivity for several significant reactions such as the reduction of CO2 with H2O, NO decomposition and the selective photoepoxidation of alkene with O2 [31–35]. It is also known that TiO2 thin films can induce a highly hydrophilic surface with a water contact angle of 08 under UV light irradiation, referred to as super-hydrophilicity [36–38]. Such super-hydrophilic properties enable TiO2 thin films to be utilized for applications such as anti-fogging or self-cleaning glass, mirrors and the walls of architectural structures. In line with such work, various techniques have been utilized for TiO2 film fabrication such as sol–gel [39], spray pyrolysis [40], chemical vapor deposition [41] and sputtering [42]. Ti-based oxide materials were found to be efficient photocatalysts for various photocatalytic reactions. However, due to a large band gap of 3.20 eV, only a small UV fraction of around 2–3% solar light that reaches the earth can be utilized. Sensitization of the TiO2 to react to a much larger visible region is, therefore, of great importance. As an approach in the development of such visible light-responsive catalysts, various photosensitizing dyes adsorbed on semiconducting catalysts have been investigated [43–45]. Gra¨tzel and co-workers have reported that dye-sensitized porous TiO2 solar cells show excellent solar power conversion efficiency of about 11% [46]. In these systems, the dyes absorb visible light to form electronically excited states from which electrons are injected into the conduction band of the semiconducting catalyst, producing photosensitized catalysts which are able to work under visible light [8–10]. However, these photosensitizing dyes are not thermally or photochemically stable. Although numerous investigations have been carried out to develop visible light-responsive TiO2 photocatalysts by adding small amounts of various components such as cations or metal oxides, no significant results could be obtained and these initial trials were found to have limitations [47–50]. In contrast, the physical doping of transition metal ions such as V, Cr, Fe, Co and Ni into TiO2 by an advanced ion-implantation technique has been shown to enable TiO2 photocatalysts to work under

M. Kitano et al. / Applied Catalysis A: General 325 (2007) 1–14

visible light [3,51–61]. Another promising approach involves the anion doping of the TiO2 films and nanostructures. Efforts have also been made to narrow the band gap by doping with carbon [62–64], nitrogen [65–72] and sulfur [73–75]. Such procedures shift the valence band to a less positive potential, thereby, decreasing the band gap. Domen and co-workers have also reported that Ti-based oxysulfide (Sm2Ti2S2O5) exhibited photocatalytic activity under visible light irradiation (l < 650 nm) for H2 or O2 evolution from aqueous solutions containing a sacrificial electron donor or accepter [76]. Furthermore, visible light-responsive TiO2 thin film photocatalysts have also been successfully prepared by a radio frequency magnetron sputtering deposition method [60,77– 85]. These TiO2 thin films can be considered a significant photofunctional material for applications in H2 production systems from water and in the purification of toxic compounds in polluted water and air using the most abundant and safe resource, solar energy. Charts 1 and 2 show the advances made in the research of TiO2 photocatalysts and, in the present review, we have focused on the development and improvement of effective TiO2 photocatalysts mainly carried out by the present authors.

3

2. Titanium oxide photocatalysts anchored in the cavities of various zeolites 2.1. Photocatalytic reduction of CO2 with H2O on highly dispersed titanium oxides anchored on various zeolites The design of highly efficient and selective photocatalytic systems that work without any loss of the utilized solar energy through chemical storage is of vital interest. In particular, the photocatalytic reduction of CO2 with H2O, a significant reaction as a means of artificial photosynthesis, has been attempted in light of the importance of carbon resource storage [31–34,86–89]. In this section, a special attention has been focused on the preparation of Ti-b zeolites and their application in the photocatalytic reduction of CO2. Two types of Ti-b zeolites were synthesized under hydrothermal conditions using OH ions and F ions as anions of the structure-directing agents (SDA). The Ti-b zeolites are denoted according to their kind of SDA, i.e., Tib(OH) and Ti-b(F). Detailed experimental procedures have been introduced in previous literature [88].

Chart 1. Advances in the development of efficient TiO2 photocatalysts.

4

M. Kitano et al. / Applied Catalysis A: General 325 (2007) 1–14

Chart 2. Advances in the development of efficient TiO2 photocatalysts.

Ti-b(OH) and Ti-b(F) catalysts exhibit an absorption band in the wavelength region of 200–260 nm, attributed to the ligand-to-metal charge transfer band (LMCT) of the highly dispersed tetrahedrally coordinated titanium oxide species. The FT-EXAFS spectra revealed that both Ti-b(OH) and Ti-b(F) ˚ assigned to the catalysts exhibit a peak at around 1.6 A neighboring oxygen atoms (Ti–O bond), indicating the presence of an isolated Ti oxide species in these catalysts. From the results obtained by curve-fitting analysis of the EXAFS spectra, it was found that the titanium oxide species incorporated within the zeolite framework exists in tetrahedral ˚ for Ti-b(OH) coordination with a Ti–O bond distance of 1.84 A ˚ for Ti-b(F), while differences in the coordination and 1.83 A geometry of these species for both Ti-b(OH) and Ti-b(F) could scarcely be observed. The Ti-b(OH) and Ti-b(F) catalysts also exhibited a photoluminescence spectra at around 480 nm by excitation at around 260 nm at 77 K. The observed photoluminescence can be attributed to the radiative decay process from the chargetransfer excited state to the ground state of the highly dispersed

Ti oxide species in tetrahedral coordination, as shown below: ðTi4þ O2 Þ @ ðTi3þ O Þ



The photoluminescence yield of the Ti-b(OH) catalyst is much higher than that of Ti-b(F) and the yields may be related to the concentration of the charge-transfer excited complexes (Ti3+– O)*. Differences in the environment around the Ti sites for these zeolites and the presence of F atoms on Ti-b(F) may be related to the low intensity of the photoluminescence. Also, as shown in Figs. 2 and 3, the addition of CO2 and H2O molecules on Ti-b(OH) and Ti-b(F) led to a quenching of the photoluminescence at 298 K. At the same time, it was also observed that the lifetime of the charge-transfer excited state was shortened by the addition of CO2 and H2O molecules. Such a quenching of the photoluminescence suggests that the added CO2 and H2O interact with the titanium oxide species incorporated within the Ti-b zeolite in its excited state. As also shown in Fig. 2, the quenching of the photoluminescence of Tib(OH) and Ti-b(F) with CO2 molecules shows almost the same

M. Kitano et al. / Applied Catalysis A: General 325 (2007) 1–14

5

Fig. 3. Effect of the addition of H2O molecules on the photoluminescence spectra of: (A) Ti-b(OH) and (B) Ti-b(F) catalysts at 298 K. Excitation: 260 nm. Amount of added H2O: (A) 0, 0.11, 0.27 and 0.53 mmol/g; (B) 0, 0.27, 0.52 and 1.04 mmol/g (top to bottom, respectively). Fig. 2. Effect of the addition of CO2 molecules on the photoluminescence spectra of: (a) Ti-b(OH) and (b) Ti-b(F) catalysts at 298 K. Excitation: 260 nm. Amount of added CO2: (a) 0, 0.05, 0.21 and 1.1 mmol/g; (b) 0, 5.3, 10.5 and 15.8 mmol/g (top to bottom, respectively).

efficiency. On the other hand, the quenching of the photoluminescence of the Ti-b(OH) catalyst with H2O molecules is much more efficient than that with Ti-b(F) (Fig. 3). These results clearly show that the efficient quenching by the addition of H2O for Ti-b(OH) can be attributed to its hydrophilic properties which are due to the easy accessibility and interaction of the H2O molecules with the excited state of the tetrahedrally coordinated titanium oxide species. Fig. 4 shows the yields of the main products in the photocatalytic reduction of CO2 with H2O at 323 K. UV irradiation of Ti-b zeolite photocatalysts in the presence of CO2 and H2O led to the formation of CH4 and CH3OH as well as trace amounts of CO, C2H4 and O2. As shown in Fig. 4, Tib(OH) exhibits higher reactivity compared to Ti-b(F). On the other hand, the selectivity for the formation of CH3OH from Tib(F) (41%) is higher than from Ti-b(OH) (11%). Moreover, the selectivity for the formation of CH3OH from Ti-b(F) is higher than that of the other Ti-containing zeolites and molecular sieve catalysts [TS-1 (23%), Ti-MCM-41 (31%), Ti-MCM-48 (29%)]. The higher reactivity of Ti-b(OH) over Ti-b(F) is attributed to the higher concentration of the charge-transfer

Fig. 4. Yields of CH4 and CH3OH in the photocatalytic reduction of CO2 with H2O at 323 K on the Ti-b(OH), TS-1, Ti-b(F) and TiO2 (P-25) catalysts. Intensity of light: 265 mW cm2. Reaction time: 6 h.

6

M. Kitano et al. / Applied Catalysis A: General 325 (2007) 1–14

excited complexes for Ti-b(OH), as observed by the photoluminescence. From these results, it was found that the highly efficient and selective photocatalytic reduction of CO2 with H2O into CH3OH can be achieved using Ti-containing zeolite catalysts involving highly dispersed tetrahedrally coordinated Ti oxides in their frameworks as the active species. 2.2. Photocatalytic decomposition of NO on highly dispersed titanium oxides anchored on various zeolites The removal of NOx (NO, N2O, NO2) from the exhaust of internal combustion engines or industrial boilers is one of the most urgent issues we face since NOx is an especially harmful atmospheric pollutant and the main cause of acid rain and photochemical smog. And in fact, the removal of NOx, e.g., the direct decomposition of NOx into N2 and O2, has been a great challenge for many researchers. As a promising way to address such concerns, highly dispersed titanium oxides prepared in zeolite cavities, such as Y-zeolites, MCM-41, HMS and TS-1, etc., are being investigated as effective photocatalysts for the decomposition of NOx [35,89–95]. Such systems exhibit the advantage of having a high dispersion of titanium oxides due to their high internal surface area and nano-scale pore reaction fields. Here, we have investigated the relationship between the local structure of the Ti4+ centers in MCM-41 type materials and their photophysical and photocatalytic characteristics by applying various spectroscopic investigative techniques. MCM-41 and Ti-MCM-41(X) (Ti content as wt.%: X = 0.15, 0.60, 0.85, 2.00, 5.00) were synthesized by using tetraethyl orthosilicate (TEOS) and tetraisopropyl orthotitanate (TPOT) as the starting materials and cetyltrimethylammonium bromide as the structure directing agent (SDA). Details of the experimental procedures have been published elsewhere [94]. UV irradiation of the Ti-MCM-41 catalysts in the presence of NO was observed to lead to the formation of N2, O2 and N2O at 295 K, their yields increasing in proportion to the irradiation time. No products could be detected either under dark conditions or UV light irradiation of the siliceous zeolite. These results clearly indicate that the reaction proceeds photocatalytically on Ti-MCM-41. Fig. 5 shows the diffuse UV–vis spectra of the Ti-MCM-41 catalysts. The absorption band was seen to increase with an increase in the Ti content, resulting in a progressive shift of the absorption maximum from 205 to 208 nm [Ti-MCM-41(0.15) and Ti-MCM-41(0.60)] (Fig. 5a and b) to 215 nm [Ti-MCM41(2.0)] (Fig. 5d), while a shoulder at ca. 240 nm became evident in the spectrum of Ti-MCM-41(0.85) (Fig. 5c), showing a larger contribution in the spectrum of the Ti-MCM-41(2.0) sample by reflecting a larger extent of the Ti-oxide species observed at ca. 250 nm. The diffuse reflectance UV–vis spectra revealed that Ti-MCM-41 has absorption bands associated with the ligand-to-metal charge transfer process of the tetrahedrally coordinated Ti-oxide species involving an electron transfer from O2 to Ti4+ to form a charge-transfer excited state (Ti3+– O)*, in UV light regions (l < 250 nm). The absorption bands

Fig. 5. Diffuse reflectance UV–vis spectra of Ti-MCM-41 with different Ti contents: (a) 0.15, (b) 0.60, (c) 0.85 and (d) 2.0 wt.%.

at 200–210 and 220–230 nm are attributed to the isolated tetrahedrally coordinated Ti-oxide species with different sites [TiO(Si)4 and Ti(OH)–(OSi)3], while the band at ca. 250 nm is assigned to the tetrahedral titanium dimers or small oligomers. Fig. 6 shows the XANES spectra of the Ti-MCM-41 catalysts which, at the Ti K-edge, show several well-defined pre-edge peaks related to the local structure surrounding the Ti atom. The number, position and relative intensity of these preedge peaks provide vital information on the coordination environment of Ti. In tetrahedral symmetry, one intense single pre-edge peak corresponding to a dipolar-allowed transition from the 1s to t2 molecular levels built from the 3d and 4p metal orbital could be observed. Ti-MCM-41(0.60) and Ti-MCM41(2.00) exhibited an intense single pre-edge peak, indicating that the Ti oxide species exists in tetrahedral coordination. These two samples show a pre-edge peak of the same intensity and position, indicating that they have the same titanium coordination. On the other hand, Ti-MCM-41(5.0) shows three characteristic weak pre-edge peaks attributed to the crystalline anatase TiO2 in octahedral coordination. Fig. 6 also shows the FT-EXAFS spectra of the Ti-MCM-41 catalysts. All of the ˚ (without phase catalysts exhibit a strong peak at around 1.6 A shift correction) that can be assigned to the neighboring oxygen atoms (Ti–O). The Ti-MCM-41(0.60) and Ti-MCM-41(2.0) catalysts exhibited only a Ti–O peak while Ti-MCM-41(5.0) ˚ due to the exhibited an additional strong peak at around 2.7 A aggregation of the Ti oxide species. From the curve-fitting analysis of the EXAFS spectra, it was found that Ti-MCM-41 having Ti content of less than 2.0 wt.% consists of fourcoordinate titanium ions with an atomic distance of about ˚. 1.80 A Photoluminescence investigations of Ti-containing zeolite catalysts were also carried out and they were shown to exhibit a photoluminescence spectrum at around 400–600 nm by excitation at around 230–260 nm due to the isolated

M. Kitano et al. / Applied Catalysis A: General 325 (2007) 1–14

7

Fig. 6. XANES (left) and FT-EXAFS (right) spectra of Ti-MCM-41 with various Ti contents: (A, a) 0.60; (B, b) 2.0; (C, c) 5.0 wt.%.

tetrahedrally coordinated Ti-oxides species highly dispersed in the silica matrices. The photoluminescence spectra can be attributed to the reverse radiative decay process from the charge transfer excited triplet to ground state of the isolated Ti-oxides in tetrahedral coordination. The addition of NO molecules onto the Ti-containing zeolite catalyst led to an efficient quenching of the photoluminescence as well as a shortening of its lifetime, the extent depending on the amount of NO added, indicating that the added NO molecules interact with the Ti-oxide species in its charge transfer excited triplet state. After degassing of NO, the photoluminescence recovered to the original intensity level, suggesting that the NO dynamically or collisionally interact with the excited Ti-oxides species. These results show the reaction mechanism for the photocatalytic decomposition of NO on the isolated tetrahedral titanium oxide species to be as follows: the NO species are able to adsorb onto these oxide species as weak ligands to form the reaction precursors. Under UV-irradiation, the charge-transfer excited complexes of the oxides (Ti3+–O)*, are formed. Within their lifetimes, the electron transfer from the trapped electron center, Ti3+, into the p-anti-bonding orbital of NO takes place and, simultaneously, the electron transfer from the p-bonding orbital of another NO into the trapped hole center, O, occurs. These electron transfers lead to the direct

decomposition of two sets of NO on (Ti3+–O)* into N2 and O2 under UV-irradiation even at 275 K. On the other hand, with the aggregated or bulk TiO2 catalysts, the photoformed holes and electrons rapidly separate from each other with large space distances between the holes and electrons, thus, preventing the simultaneous activation of two NO on the same active sites and resulting in the formation of N2O and NO2 in place of N2 and O2. The decomposed N and O species react with the NO on different sites to form N2O and NO2, respectively. These results clearly demonstrate that zeolites used as supports enable the anchoring of titanium oxide species within their cavities in a highly dispersed state. Such tetrahedrally coordinated titanium oxide photocatalysts are, thus, promising candidates for unique and practical applications in the reduction of toxic NOx elements. 3. Design of visible light-responsive TiO2 photocatalysts 3.1. Application of metal ion-implantation techniques to TiO2 photocatalysts Intensive studies of visible light-responsive photocatalysts have been carried out in order to harness the abundant and safe potential of solar energy. In line with such work, there have

8

M. Kitano et al. / Applied Catalysis A: General 325 (2007) 1–14

been various attempts to sensitize TiO2 for much larger visible light regions. One of these investigations has been the chemical doping of TiO2 with transition metals ions or oxides [47–50]. Although metal ion chemically doped TiO2 can induce visible light response, most of these catalysts do not show long-term stability nor have sufficiently high reactivity for a wide range of applications. When metal ions or oxides are incorporated into the TiO2 by a chemical doping method such as impregnation, a small absorption band appears at 400–550 nm as a shoulder peak due to the formation of the impurity energy levels in the band gap of TiO2. Such impurity energy levels may act as the recombination centers for the excited electrons and holes, thus, decreasing the photocatalytic activity [47]. In order to modify the electronic structure of TiO2 to absorb and utilize visible light, an advanced ion-implantation method with various transition metal ions at high energy acceleration (50–200 keV) was carried out on TiO2 semiconductor powders [3,51–61]. When TiO2 is bombarded with such high energy transition metal ions accelerated by high voltage, the ions can be implanted into the lattice without destroying the surface structure of the TiO2. As shown in Fig. 7, the absorption band of TiO2 physically implanted with such metal ions as Cr, V, Fe and Ni was found to shift smoothly to visible light regions (up to 600 nm) depending on the kind of metal implanted, indicating that the band gaps of the physically ion-implanted TiO2 are much smaller than that of the original TiO2. However, the spectra of chemically ion-doped TiO2 are quite different from the spectra of physically ion-implanted TiO2, exhibiting a shoulder peak in visible light regions, although it is not a smooth shift. The band structures of the original TiO2, chemically ion-doped TiO2 and physically ion-implanted TiO2 are shown in Fig. 8. Such an effective narrowing of the

Fig. 7. Diffuse reflectance UV–vis spectra of the TiO2 photocatalysts implanted with: (a) V, (b) Cr, (c) Fe, (d) Ni ions with the same amount of 1.3 mmol/ g catalyst and (e) TiO2 doped with Cr ions with 4.9 mmol/g catalyst prepared by impregnation.

Fig. 8. Band structures of TiO2, chemically ion-doped TiO2 and physically ionimplanted TiO2.

band gap is attributed to the substitution of the Ti ions in the TiO2 lattice with the metal ions. It was also found that the application of the metal ion-implantation method allows the modification of the electronic states of both the TiO2 thin film catalyst [95] and the Ti/zeolite catalyst involving highly dispersed Ti-oxides within its framework to absorb visible light [60,61]. The chemically ion-doped TiO2 photocatalysts showing a shoulder in the visible light region exhibited a drastic decrease in photocatalytic reactivity under UV irradiation (i.e., band gap irradiation) as compared with that of the undoped original TiO2 since the doped ions induce the impurity energy levels within the band gap and the energy level plays an important role in the recombination of the photoformed electrons and holes, leading to a drastic decrease in the photocatalytic reactivity. However, under UV light irradiation, the physically ion-implanted TiO2 catalysts exhibited the same photocatalytic reactivity for the decomposition of NO and the complete oxidation reaction of 2propanol into CO2 and H2O as the unimplanted original TiO2 photocatalysts, clearly indicating that the implanted metal ions did not act as the electron-hole recombination centers since they were present in the TiO2 catalysts in a highly dispersion state. On the other hand, visible light irradiation (l  450 nm) of the physically ion-implanted TiO2 in the presence of NO at 275 K led to the decomposition of NO into N2, O2 and N2O with a good linearity against the light irradiation time. Under the same conditions, no reaction proceeded on the unimplanted original TiO2 photocatalyst. Spectroscopic investigations, such as SIMS, XAFS and ESR, showed that the substitution of octahedrally coordinated Ti ions in the bulk TiO2 lattice with the implanted metal ions was a major factor in the modification of TiO2 to absorb and operate under visible light irradiation. Moreover, ab initio molecular orbital calculations on the basis of a density functional theory method revealed that the mixing of the Ti(d) orbital of the Ti-oxide and the metal(d) orbital of the implanted metal ions under a low electric charge was essential in decreasing the energy gap between the Ti(d) and O(p) orbitals of the Ti-oxide [96]. These results clearly indicate that the substitution of Ti ions with the isolated metal ions implanted into the lattice position of TiO2 was the origin of the effective narrowing of the band gap.

M. Kitano et al. / Applied Catalysis A: General 325 (2007) 1–14

3.2. Preparation of visible light-responsive TiO2 thin film photocatalysts by an RF magnetron sputtering deposition method In recent years, the development of visible light-responsive TiO2 photocatalysts have been investigated by the substitutional doping of metals or nonmetals with the aim of extending the absorption edge into visible light regions and to improve photocatalytic reactivity. However, most of these modified TiO2 photocatalysts are in powder form and, thus, have limitations for practical or industrial applications. The development of stable and easily applicable TiO2 thin films which can operate not only under UV but also visible light is, therefore, of great importance. With this objective in mind, a radio frequency magnetron sputtering deposition (RF-MS) method was applied to prepare visible light-responsive TiO2 thin film photocatalysts using a TiO2 plate as the source material and pure Ar gas as the sputtering gas [60,77–85]. Fig. 9 shows the UV–vis transmission spectra of the TiO2 thin films prepared on quartz substrates under various substrate temperatures and Ar gas pressures. The TiO2 thin films prepared at 473 K were colorless and transparent to visible light, thus enabling the absorption of only UV light of wavelengths shorter than 380 nm (hereafter, designated UV–TiO2). On the other hand, the TiO2 thin films prepared at higher substrate temperature (>673 K) were yellow-colored and exhibited considerable absorption in wavelength regions longer than 400 nm, enabling the absorption of visible light. In fact, the absorption band at visible light regions shifted toward longer wavelength regions at around 600 nm with an increase in the substrate temperatures. Among the three types of TiO2 thin films developed, the film prepared at a substrate temperature of 873 K exhibited an absorption edge at the longest wavelength regions (hereafter, Vis–TiO2). Control of the substrate temperature during the simple onestep TiO2 deposition process was, thus, found to be one of the major factors in controlling the efficiency of visible light absorption. Furthermore, the absorption band of Vis–TiO2

Fig. 9. UV–vis transmission spectra of TiO2 thin films prepared on quartz substrates under various Ar gas pressures (PAr) and substrate temperatures (TS). TS (K): (a) 473, (b) 673 and (c–e) 873; PAr (Pa): (c) 2.0, (d) 3.0 and (e) 1.0.

9

shifted toward longer visible wavelength regions as the Ar gas pressure decreased (Fig. 9c–e) but the substrate temperature was kept at 873 K [84]. It is known that sputtered atoms from the target material experience many collisions with the sputtering gas molecules before they reach the substrate. The amount of sputtered atoms that reach the substrate and their kinetic energy may, thus, decrease with an increase in the sputtering gas pressure. Under conditions of low sputtering gas pressure, the TiO2 thin film is easily reduced by bombardment with the large number of high energy sputtered atoms in the deposition process. In fact, SIMS investigations revealed that Vis–TiO2 exhibited a unique declined O/Ti composition from the surface to the deep inside bulk [77–81]. The kinetic energy of the sputtered atoms also increases with a decrease in the target-to-substrate distance (DT-S: mm). In fact, visible light absorption for Vis–TiO2 increased with a decrease in the DT-S and these films are referred to as Vis–TiO2–DT-S [85]. Fig. 10 shows the SIMS depth profiles of UV–TiO2 and Vis–TiO2–DT-S. SIMS investigations revealed that the concentration of O2 ions for Vis–TiO2–80 and Vis– TiO2–70 gradually decreases from the top surface (O/Ti ratio of 2.00  0.01) to the inside bulk, although no significant changes were observed for UV–TiO2 which is composed of stoichiometric TiO2 (2.00  0.01). These results clearly indicate that the higher the kinetic energy of the sputtered atoms, the lower the O/ Ti ratio of the TiO2 thin films, accompanied by a large shift in their absorption band toward visible light regions. Such a unique anisotropic structure was seen to play an important role in the modification of the electronic properties, thus, enabling the absorption of visible light [77–85]. Both the UV–TiO2 and Vis–TiO2 thin film photocatalysts loaded with Pt were found to be effective in decomposing water

Fig. 10. Depth distribution profiles of 18O and 48Ti for: (a) UV–TiO2, (b) Vis– TiO2–80 and (c) Vis–TiO2–70 thin films, as determined by SIMS measurements.

10

M. Kitano et al. / Applied Catalysis A: General 325 (2007) 1–14

Table 1 Quantum yields for H2 and O2 evolution from aqueous solutions of methanol and silver nitrate on TiO2 thin films under UV and visible light irradiation Photocatalyst

Quantum yield (%) l = 360 nm H2

UV–TiO2 Vis–TiO2 c

a

26.2 34.2

l = 420 nm O2

b

12.6 60.0

H2 a

O2 b

0.00 1.25

0.00 2.43

a

From 50 vol% aqueous methanol solutions. From 0.05 M aqueous silver nitrate solution. c Vis–TiO2 was prepared under the optimum conditions (substrate temperature, 873 K; Ar gas pressure, 2.0 Pa; target-to-substrate distance, 80 mm). b

into H2 and O2 under UV light irradiation. On the other hand, the photocatalytic evolution of H2 and O2 from water, including sacrificial reagents such as CH3OH or AgNO3, proceeded only with Pt-loaded Vis–TiO2 under visible light irradiation. The electronic band gap of Vis–TiO2 can be considered significantly smaller than that of UV–TiO2 (i.e., 3.2 eV). However, the conduction band edge as well as the valence band edge is so located as to facilitate the decomposition of H2O into H2 and O2. The apparent quantum efficiencies for the photocatalytic evolution of H2 or O2 from water involving the sacrificial reagents are summarized in Table 1. We have also investigated the photocatalytic activity of Vis– TiO2–DT-S for O2 evolution under visible light (l = 420nm). The apparent quantum yield of Vis–TiO2–80 was 2.43% and higher than the 0.48% determined for Vis–TiO2–70. SIMS analysis (Fig. 10) showed the O/Ti values of the inside bulk at 1.93 and 1.70 for Vis–TiO2–80 and Vis–TiO2–70, respectively. These results clearly show that a slight decrease in the O/Ti ratio of the TiO2 thin films plays an important role in the preparation of visible light-responsive TiO2 thin films. The photoelectrochemical properties of the Vis–TiO2 films prepared on a Ti foil substrate (Vis–TiO2/Ti) under visible light were also investigated in 0.1 M HClO4 aqueous solution. An anodic photocurrent was observed even under visible light irradiation up to 520 nm, determining the band gap energy to be about 2.5 eV. Current–voltage characteristics support the findings that the bottom of the conduction band and top of the valence band have enough potentials for the decomposition of water under visible light. Moreover, the apparent quantum efficiencies, or incident photon to current conversion efficiency (IPCE), of the Vis–TiO2/Ti electrode are defined as the number of electrons collected per incident photon and determined by measuring the photocurrent (Iph) of the electrodes at each excitation wavelength using the following equation [82]:  IPCE ð%Þ ¼

1240  I ph ðA=cm2 Þ l ðnmÞ  I inc ðW=cm2 Þ

In order to investigate the long-term stability of Vis–TiO2, the relationship between the anodic photocurrent and irradiation time was measured under visible light irradiation. The anodic photocurrent was observed to be constant during approximately 20 h consecutive measurement under visible light longer than 450 nm, indicating that Vis–TiO2 exhibits long-term stability [84]. Moreover, repetitive use of the Vis– TiO2 thin films was possible for the photocatalytic reaction while the photocatalytic activity remained stable even after a year. In fact, no noticeable differences were seen in the UV–vis spectra and XRD patterns of Vis–TiO2 before and after photoelectrochemical measurements. The separate evolution of H2 and O2 from water was investigated with a TiO2 thin film device prepared by the RFMS method [81,83–85]. Fig. 11 shows the schematic model of the photocatalytic decomposition of water on this TiO2 thin film device. This device consists of the Ti foil deposited on one side with the TiO2 thin film acting as the oxidation site while, on the other side, the Pt acts as the reduction site. The TiO2 side of the thin film device was immersed in 1.0 M NaOH aqueous solution and the Pt side was immersed in 0.5 M H2SO4 aqueous solution in order to add a small chemical bias between the two aqueous solutions. As shown in Fig. 12, H2 and O2 could be separately produced on such a Vis–TiO2 thin film device under visible light (l  420 and 450 nm), while no reaction proceeded on a similar UV–TiO2 thin film device. However, in the initial stage, the ratio of H2/O2 shows a deviation from the stoichiometric value of 2 (up to 4 h). A small amount of organic compounds involved in the aqueous solution are considered to be decomposed during the initial stage. After 4 h, the stoichiometric decomposition of water could be observed. In

  100

(1)

where Iinc is the incident light intensity (W/cm2) and l is the excitation wavelength (nm). The IPCE of Vis–TiO2 at 1.0 V (versus SCE) was found to be ca. 40% at 360 nm and 5.6% at 420 nm.

Fig. 11. TiO2 thin film photocatalytic system, prepared on a Ti metal substrate with an oxidation site (TiO2) on the irradiated side and a reduction site (Pt particles) on the back side, for the decomposition of H2O into O2 and H2.

M. Kitano et al. / Applied Catalysis A: General 325 (2007) 1–14

11

Fig. 12. Time profile of the photocatalytic decomposition of H2O with the separate evolution of H2 and O2 under visible light on a Vis–TiO2 thin film device in an H-type glass container of two aqueous phases having different pH values (TiO2 side, 1.0 M NaOH aq; Pt side, 0.5 M H2SO4 aq).

fact, the rates of H2 evolution were 0.18 mmol/h (l > 420 nm) and 0.088 mmol/h (l > 450 nm) and O2 evolution rates were 0.090 mmol/h (l > 420 nm) and 0.043 mmol/h (l > 450 nm). Field experiments were conducted outdoors under natural sunlight irradiation on a clear sunny day in March. Light irradiation was carried out with a sunlight-gathering system that could remove the UV light in sunlight. Sunlight irradiation of this device successfully led to the stoichiometrical evolution of H2 and O2 from the Pt and Vis–TiO2 side, respectively [81]. It was, thus, confirmed that Vis–TiO2 thin film photocatalysts are, indeed, an effective photofunctional material for applications in clean and safe H2 production systems from water using the most abundant energy source, solar light. 3.3. Preparation of nitrogen-substituted TiO2 thin film photocatalysts by an RF magnetron sputtering deposition method In recent years, much work has been carried out in the development of visible light-responsive TiO2 by the doping of various kinds of anions, such as N, S or C, as a substitute for oxygen in the TiO2 lattice [62–75]. For these anion-doped TiO2 photocatalysts, the mixing of the p states of the doped anion (N, S, C) with the O 2p states was reported to shift the valence band edge upwards to narrow the band gap energy of TiO2. However, most of these nitrogen-doped TiO2 exhibited visible light absorption as a shoulder in the wavelength range of 400– 600 nm [65–71], indicating that the isolated N 2p orbitals are formed above the O 2p orbitals due to the limited concentration of nitrogen that could be doped into the TiO2 lattice at a very low range of <2%. However, we have successfully prepared high nitrogen-substituted TiO2 (N-TiO2) thin films by an RFMS method in an N2/Ar gas mixture [97,98]. Fig. 13 shows the UV–vis absorption spectra of the N-TiO2 and TiO2 thin films prepared on quartz substrates with a film thickness of 1.2 mm. The TiO2(O2/Ar) thin films prepared under an O2/Ar gas mixture were colorless and transparent to visible light, thus

Fig. 13. UV–vis absorption spectra of TiO2(O2/Ar) and N-TiO2 thin films substituted with various concentrations of nitrogen by an RF-MS method in an N2/Ar gas mixture. The concentration of the substituted nitrogen (%): (a) 0.5, (b) 2.0, (c) 6.0, (d) 11.5 and (e) 16.5.

enabling the absorption of only UV light of wavelengths shorter than 390 nm. On the other hand, N-TiO2 thin films with low concentrations of substituted nitrogen (<2%) exhibited visible light absorption as a shoulder in the wavelength range of 400– 500 nm, indicating that the isolated N 2p orbitals are formed above the O 2p orbitals. Furthermore, the absorption band of NTiO2 with high concentrations of substituted nitrogen (2%) shifted smoothly towards visible light regions, as shown in Fig. 13b–e. The steep absorption edges in the visible light region are clear evidence of the narrowing of the band gap energy. The band gap energy of the N-TiO2 thin films was roughly estimated to be 2.58–2.25 eV from the onset of the absorption edges, which is much smaller than that for

Fig. 14. Relative photocurrent, as a function of the cutoff wavelength of the incident light, for the N-TiO2/ITO electrode measured in 0.25 M K2SO4 aqueous solution at +1.0 V vs. SCE. Broken line shows the UV–vis absorption spectrum of N-TiO2/ITO.

12

M. Kitano et al. / Applied Catalysis A: General 325 (2007) 1–14

Fig. 15. Control of the electronic structure of TiO2 photocatalysts by the substitution of metal ions or anions in the design of visible light-responsive TiO2 photocatalysts.

TiO2(O2/Ar) (3.2 eV). The narrowing of the band gap is considered to be due to the mixing of the N 2p states with the O 2p states on the top valence band [65]. These N-TiO2 thin films were found to exhibit effective photocatalytic activity for the liquid-phase degradation of 2-propanol diluted in water under visible or sunlight irradiation [97]. The photoelectrochemical properties of the N-TiO2/ITO electrode with substituted nitrogen of 6.0% were examined using an aqueous solution of 0.25 M K2SO4 (pH 6.7). Fig. 14 shows the photocurrent observed for the N-TiO2/ITO electrode as a function of the incident light wavelength which was controlled by cut-off filters. These measurements were carried out with a bias of +1.0 V versus SCE. Dark currents were negligible under this condition. The photoelectrochemical onset of N-TiO2 was located at approximately 550 nm and showed a good parallel relationship between the photoresponse and the absorption spectrum. These results clearly show that the observed photocurrent originated from a band gap transition. The quantum yield, or absorbed photon to current conversion efficiency (APCE), defined as the number of electrons collected per absorbed photons was determined by measuring the photocurrent of the N-TiO2/ITO electrode at each excitation wavelength using the following equation:  APCE ð%Þ ¼

number of reacted electrons number of absorbed photons

  100

(2)

The APCE at 1.0 V (versus SCE) reached 25.2 and 22.4% under UV (l = 360 nm) and visible light irradiation (l = 420 nm), respectively. It was, thus, demonstrated that the RF-MS method could enable the preparation of high nitrogen-substituted TiO2 thin films which are considered to be promising photocatalysts for the purification of toxic compounds in polluted water and air as well as for the production of H2 in the photoelectrochemical splitting of water under sunlight irradiation. Significantly, it could be seen that visible light-responsive TiO2 photocatalysts could be successfully prepared by both a metal-ion-implantation and an RF-MS method. The local

structures of the TiO2, metal ion-substituted TiO2 and anionsubstituted TiO2 are schematically depicted in Fig. 15. The substitution of O2 ions with anions or defects as well as the substitution of Ti ions with the metal ions into the lattice position of TiO2 were found to have significant impact on the modification of the electronic structure of TiO2, thus, enabling the absorption of visible light. 4. Conclusions Highly dispersed titanium oxide species within zeolite cavities were found to demonstrate efficient and selective photocatalytic reactivity for several significant reactions such as the reduction of CO2 with H2O and NO decomposition. Furthermore, visible light-responsive TiO2 (Vis–TiO2) photocatalysts were successfully developed by a metal ion-implantation method as well as a radio frequency magnetron sputtering (RF-MS) deposition method. Vis– TiO2 was found to act as an efficient photocatalyst for the decomposition of NO, the mineralization of organic compounds and the decomposition of water into H2 and O2 under visible or sunlight irradiation. Moreover, Vis–TiO2 thin film photocatalysts prepared by the RF-MS method were seen to enable the separate evolution of H2 and O2 from H2O under visible or sunlight irradiation. These photocatalyst could be applied for the production of H2 in an ‘‘onsite production system’’, leading to the application of H2 as a fuel cell. Also, such visible light-responsive TiO2 thin films could also be applied in new building materials that can reduce or eliminate toxic agents in the environment as an important eco-material. Acknowledgements This work was supported by a ‘Grant-in-Aid for the Creation of Innovations through Business-Academic-Public Sector Cooperation’ (No. 13308) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. M.A. would like to express his thanks for their kind support.

M. Kitano et al. / Applied Catalysis A: General 325 (2007) 1–14

References [1] M. Anpo, in: P. Tundo, P. Anastas (Eds.), Green Chemistry—Challenging Perspectives, Oxford University Press, Oxford, 2000, pp. 1–19. [2] M. Anpo, M. Takeuchi, H. Yamashita, S. Kishiguchi, A. Davidson, M. Che, in: V. Ramamurthy, K.S. Schanze (Eds.), Semiconductor Photochemistry and Photophysics, Marcel Dekker Inc., New York, 2003, pp. 283–299. [3] M. Anpo, S. Dohshi, M. Kitano, Y. Hu, M. Takeuchi, M. Matsuoka, Annu. Rev. Mater. Res. 35 (2005) 1. [4] A. Fujishima, K. Honda, Nature 238 (1972) 37. [5] S.C. Moon, Y. Matsumura, M. Kitano, M. Matsuoka, M. Anpo, Res. Chem. Intermed. 29 (2003) 233. [6] R. Wrang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Nature 431 (1997) 388. [7] M. Takeuchi, S. Dohshi, T. Eura, M. Anpo, J. Phys. Chem. B 107 (2003) 14278. [8] B. O’Regan, M. Gra¨tzel, Nature 335 (1991) 737. [9] U. Bach, D. Lupo, P. Comte, J.E. Moser, F. Weissortel, J. Salbeck, H. Spreitzer, M. Gra¨tzel, Nature 395 (1998) 583. [10] M. Gra¨tzel, Nature 414 (2001) 338. [11] A.J. Bard, J. Photochem. 10 (1979) 59. [12] A.J. Bard, Science 207 (1980) 139. [13] A.J. Bard, J. Phys. Chem. 86 (1982) 172. [14] S. Sato, J.M. White, Chem. Phys. Lett. 72 (1980) 83. [15] S. Sato, J.M. White, J. Catal. 69 (1981) 128. [16] K. Yamaguchi, S. Sato, J. Chem. Soc., Faraday Trans. 181 (1985) 1237. [17] F.T. Wagner, S. Ferrer, G.A. Somorjai, Surf. Sci. 101 (1980) 462. [18] F.T. Wagner, G.A. Somorjai, Nature 285 (1980) 559. [19] F.T. Wagner, G.A. Somorjai, J. Am. Chem. Soc. 102 (1980) 5494. [20] M. Anpo, N. Aikawa, S. Kodama, Y. Kubokawa, J. Phys. Chem. 84 (1984) 569. [21] C. Yun, M. Anpo, S. Kodama, Y. Kubokawa, J. Chem. Soc. Chem. Commun. (1980) 609. [22] M. Anpo, T. Shima, Y. Kubokawa, Chem. Lett. (1985) 1799. [23] S. Kodama, H. Nakaya, M. Anpo, Y. Kubokawa, Bull. Chem. Soc. Jpn. 58 (1985) 3645. [24] M. Anpo, M. Yabuta, S. Kodama, Y. Kubokawa, Bull. Chem. Soc. Jpn. 59 (1986) 259. [25] M. Anpo, T. Shima, S. Kodama, Y. Kubokawa, J. Phys. Chem. 91 (1987) 4305. [26] M. Anpo, H. Nakaya, S. Kodama, Y. Kubokawa, K. Domen, T. Onishi, J. Phys. Chem. 90 (1988) 1633. [27] M. Anpo, T. Kawamura, S. Kodama, K. Maruya, T. Onishi, J. Phys. Chem. 92 (1988) 438. [28] H. Yamashita, S. Kawasaki, Y. Ichihashi, M. Harada, M. Takeuchi, M. Anpo, G. Stewart, M.A. Fox, C. Louis, M. Che, J. Phys. Chem. B 102 (1998) 5870. [29] S.C. Moon, H. Mametsuka, E. Suzuki, M. Anpo, Chem. Lett. (1998) 117. [30] S. Dohshi, M. Takeuchi, M. Anpo, Catal. Today 85 (2003) 199. [31] M. Anpo, H. Yamashita, K. Ikeue, Y. Fujii, S.G. Zhang, Y. Ichihashi, D.R. Park, Y. Suzuki, K. Koyano, T. Tatsumi, Catal. Today 44 (1998) 327. [32] H. Yamashita, Y. Fujii, Y. Ichihashi, S.G. Zhang, K. Ikeue, D.R. Park, K. Koyano, T. Tatsumi, M. Anpo, Catal. Today 45 (1998) 221. [33] S.G. Zhang, Y. Fujii, H. Yamashita, K. Koyano, T. Tatsumi, M. Anpo, Chem. Lett. (1997) 659. [34] M. Anpo, H. Yamashita, Y. Ichihashi, Y. Fujii, M. Honda, J. Phys. Chem. B 101 (1997) 2632. [35] M. Matsuoka, M. Anpo, J. Photochem. Photobiol. C: Photochem. Rev. 3 (2003) 225. [36] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Nature 431 (1997) 388. [37] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Adv. Mater. 10 (1998) 135. [38] M. Takeuchi, K. Sakamoto, G. Martra, S. Coluccia, M. Anpo, J. Phys. Chem. B 109 (2005) 15422. [39] N. Negishi, T. Iyoda, K. Hashimoto, A. Fujishima, Chem. Lett. (1995) 841.

13

[40] C. Natarajan, N. Fukunaga, G. Nogami, Thin Solid Films 322 (1998) 6. [41] T.W. Kim, M. Jung, H.J. Kim, T.H. Park, Y.S. Yoon, W.N. Kang, S.S. Yom, H.K. Na, Appl. Phys. Lett. 64 (1994) 1407. [42] S. Takeda, S. Suzuki, H. Odaka, H. Hosono, Thin Solid Films 392 (2001) 338. [43] T. Kajiwara, K. Hashimoto, T. Kawai, T. Sakata, J. Phys. Chem. 86 (1982) 4516. [44] K. Hashimoto, T. Kawai, T. Sakata, Chem. Lett. (1983) 709. [45] T. Shimizu, T. Iyoda, Y. Koide, J. Am. Chem. Soc. 107 (1985) 35. [46] M.K. Nazeeruddin, F.D. Angelis, S. Fantacci, A. Selloni, G. Viscardi, P. Liska, S. Ito, B. Takeru, M. Gra¨tzel, J. Am. Chem. Soc. 127 (2005) 16835. [47] W.Y. Choi, A. Termin, M.R. Hoffman, J. Phys. Chem. 84 (1994) 13669. [48] J. Soria, J.C. Conesa, V. Augugliaro, L. Palmisano, M. Schiavello, A. Sclafani, J. Phys. Chem. 95 (1991) 274. [49] M.I. Litter, Appl. Catal. B: Env. 23 (1999) 89. [50] A. Fuerte, M.D. Herna´ndez-Alonso, A.J. Maira, A. Martı´nez-Arias, M. Ferna´ndez-Garcı´a, J.C. Conesa, J. Soria, Chem. Commun. (2001) 2718. [51] M. Anpo, Catal. Survey Jpn. 1 (1997) 169. [52] M. Anpo, Y. Ichihashi, M. Takeuchi, H. Yamashita, in: H. Hattori, K. Otsuka (Eds.), Science and Technology in Catalysis, vol. 121, Elsevier, Amsterdam, 1998, p. 305. [53] M. Anpo, Y. Ichihashi, M. Takeuchi, H. Yamashita, Res. Chem. Intermed. 24 (1998) 143. [54] M. Anpo, M. Takeuchi, S. Kishiguchi, H. Yamashita, Surf. Sci. Jpn. 20 (1999) 60. [55] H. Yamashita, Y. Ichihashi, M. Takeuchi, S. Kishiguchi, M. Anpo, J. Synchrotron. Rad. 6 (1999) 451. [56] M. Anpo, in: A. Corma, et al. (Eds.), 12th International Congress on Catalysis, vol. 130, Elsevier, Amsterdam, 2000, p. 157. [57] M. Anpo, Pure Appl. Chem. 72 (2000) 1265. [58] M. Anpo, Pure Appl. Chem. 72 (2000) 1787. [59] H. Yamashita, M. Harada, J. Misaka, M. Takeuchi, K. Ikeue, M. Anpo, J. Photochem. Photobiol. A 148 (2002) 257. [60] M. Anpo, M. Takeuchi, J. Catal. 216 (2003) 505. [61] M. Anpo, Bull. Chem. Soc. Jpn. 77 (2004) 1427. [62] S.U.M. Khan, M. Al-Shahry, W.B. Ingler Jr., Science 297 (2002) 2243. [63] H. Irie, Y. Watanabe, K. Hashimoto, Chem. Lett. 32 (2003) 722. [64] S. Sakthivel, H. Kisch, Angrew. Chem. Int. Ed. 42 (2003) 4908. [65] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269. [66] T. Morikawa, R. Asahi, T. Ohwaki, K. Aoki, Y. Taga, Jpn. J. Appl. Phys. 40 (2001) L561. [67] H. Irie, S. Washizuka, N. Yoshino, K. Hashimoto, Chem. Commun. (2003) 1298. [68] T. Ihara, M. Miyoshi, Y. Iriyama, O. Matsumoto, S. Sugihara, Appl. Catal. B-Env. 42 (2003) 403. [69] H. Irie, Y. Watanabe, K. Hashimoto, J. Phys. Chem. B 107 (2003) 5483. [70] S. Sakthivel, H. Kisch, ChemPhysChem 4 (2003) 487. [71] O. Diwald, T.L. Thompson, E.G. Goralski, S.D. Walck, J.T. Yates, J. Phys. Chem. B 108 (2004) 52. [72] C.D. Valentin, G. Pacchioni, A. Selloni, S. Livraghi, E. Giamello, J. Phys. Chem. 109 (2005) 11414. [73] T. Umebayashi, T. Ymaki, H. Itoh, K. Asai, Appl. Phys. Lett. 81 (2002) 454. [74] T. Umebayashi, T. Ymaki, S. Tanaka, K. Asai, Chem. Lett. 32 (2003) 330. [75] T. Umebayashi, T. Ymaki, S. Yamamoto, A. Miyashita, S. Tanaka, K. Asai, J. Appl. Phys. 93 (2003) 5156. [76] A. Ishikawa, T. Takata, J.N. Kondo, M. Hara, H. Kobayashi, K. Domen, J. Am. Chem. Soc. 124 (2002) 13547. [77] M. Takeuchi, M. Anpo, T. Hirao, N. Itoh, N. Iwamoto, Surf. Sci. Jpn. 22 (2001) 561. [78] M. Kitano, M. Takeuchi, M. Matsuoka, J.M. Thomas, M. Anpo, Chem. Lett. 34 (2005) 616. [79] M. Matsuoka, M. Kitano, M. Takeuchi, M. Anpo, J.M. Thomas, Mater. Sci. Forum 486–487 (2005) 81. [80] M. Matsuoka, M. Kitano, M. Takeuchi, M. Anpo, J.M. Thomas, Top. Catal. 35 (2005) 305. [81] M. Kitano, H. Kikuchi, T. Hosoda, M. Takeuchi, M. Matsuoka, T. Eura, M. Anpo, Key Eng. Mater. 317–318 (2006) 823.

14

M. Kitano et al. / Applied Catalysis A: General 325 (2007) 1–14

[82] H. Kikuchi, M. Kitano, M. Takeuchi, M. Matsuoka, M. Anpo, P.V. Kamat, J. Phys. Chem. B 110 (2006) 5537. [83] M. Kitano, K. Tsujimaru, M. Anpo, Appl. Catal. A: Gen. 314 (2006) 179. [84] M. Kitano, M. Takeuchi, M. Matsuoka, J.M. Thomas, M. Anpo, Catal. Today 120 (2007) 133. [85] M. Matsuoka, M. Kitano, M. Takeuchi, K. Tsujimaru, M. Anpo, J.M. Thomas, Catal. Today 122 (2007) 51. [86] K. Sayama, H. Arakawa, J. Phys. Chem. 97 (1993) 531. [87] K. Ikeue, H. Yamashita, M. Anpo, Chem. Lett. (1999) 1135. [88] K. Ikeue, H. Yamashita, M. Anpo, T. Takewaki, J. Phys. Chem. B 105 (2001) 8350. [89] K. Ikeue, S. Nozaki, M. Ogawa, M. Anpo, Catal. Lett. 80 (2002) 111. [90] H. Yamashita, Y. Ichihashi, M. Anpo, M. Hashimoto, C. Louis, M. Che, J. Phys. Chem. 100 (1996) 16041.

[91] H. Yamashita, Y. Ichihashi, S.G. Zhang, Y. Matsumura, Y. Souma, T. Tatsumi, M. Anpo, Appl. Surf. Sci. 121 (1997) 305. [92] N.U. Zhanpeisov, M. Matsuoka, H. Yamashita, M. Anpo, J. Phys. Chem. 102 (1998) 6915. [93] J.L. Zhang, Y. Hu, M. Matsuoka, H. Yamashita, M. Minagawa, H. Hidaka, M. Anpo, J. Phys. Chem. B 105 (2001) 8395. [94] Y. Hu, G. Martra, J.L. Zhang, S. Higashimoto, S. Coluccia, M. Anpo, J. Phys. Chem. B 110 (2006) 1680. [95] M. Takeuchi, H. Yamashita, M. Matsuoka, M. Anpo, T. Hirao, N. Itoh, N. Iwamoto, Catal. Lett. 67 (2000) 135. [96] H. Yamashita, M. Anpo, Catal. Survey Asia 8 (2002) 35. [97] M. Kitano, K. Funatsu, M. Matsuoka, M. Ueshima, M. Anpo, J. Phys. Chem. B 110 (2007) 25266. [98] M. Kitano, T. Kudo, M. Matsuoka, M. Ueshima, M. Anpo, Mater. Sci. Forum 544–545 (2007) 107.