Tio2 Photocatalysis: a Historical Overview and Future Prospects Tio2 Photocatalysis: a Historical Overview and Future Prospects

Printed with permission of The Institute of Pure and Applied Physics (IPAP) JAPANESE JOURNAL OF APPLIED PHYSICS Vol.44, No.12 (2005) pp.8269-8285[Part1] Invited Review Paper

TiO2 Photocatalysis: A Historical Overview and Future Prospects Kazuhito Hashimoto1,2, Hiroshi Irie2 and Akira Fujishima3

Photocatalysis has recently become a 1. Historical Background be- compared the photocatalytic activities of common word and various products using fore Honda–Fujishima Effect various TiO2 powders using twelve types of photocatalytic functions have been commer- commercial anatase and three types of rutile, cialized. Among many candidates for photo- TiO2 powders have been commonly used and concluded that the anatase activity of catalysts, TiO2 is almost the only material as white pigments from ancient times. They the autooxidation is much higher than that of suitable for industrial use at present and also are inexpensive, chemically stable and harm- rutile, suggesting a fairly high degree of prog- 4) probably in the future. This is because TiO2 has less, and have no absorption in the visible ress of the research. In those days, however, the most efficient photoactivity, the highest region. Therefore, they have a white color. the photocatalytic power of TiO2 might have stability and the lowest cost. More signifi- However, the chemical stability of TiO2 holds attracted only partially limited scientists’ atten- cantly, it has been used as a white pigment only in the dark. Instead, it is active under UV tion in the field of either catalysis or photo- from ancient times, and thus, its safety to light irradiation, inducing some chemical reac- chemistry, and the study of TiO2 photocatalysis humans and the environment is guaranteed tions. Such activity under sunlight was known had not developed widely in either academic by history. There are two types of photo- from the flaking of paints and the degrada- or industrial society. 1) chemical reaction proceeding on a TiO2 tion of fabrics incorporating TiO2. Scientific surface when irradiated with ultraviolet light. studies on such photoactivity of TiO2 have 2. Water Photolysis with TiO2 One includes the photo-induced redox reac- been reported since the early part of the 20th Electrode in 1970s tions of adsorbed substances, and the other is century. For example, there was a report on the photo-induced hydrophilic conversion of the photobleaching of dyes by TiO2 both in In the late 1960s, one of the present 2) TiO2 itself. The former type has been known vacuo and in oxygen in 1938. It was reported authors (AF) began to investigate the photo- since the early part of the 20th century, but that UV absorption produces active oxygen electrolysis of water, using a single crystal the latter was found only at the end of the species on the TiO2 surface, causing the n-type TiO2 (rutile) semiconductor electrode, century. The combination of these two func- photobleaching of dyes. It was also known because it has a sufficiently positive valence tions has opened up various novel applications that TiO2 itself does not change through the band edge to oxidize water to oxygen. It is of TiO2, particularly in the field of building photoreaction, although the ``photocatalyst’’ also an extremely stable material even in the materials. Here, we review the progress of the terminology was not used for TiO2 in the presence of aqueous electrolyte solutions, scientific research on TiO2 photocatalysis as report, but called a photosensitizer. much more so than other types of semicon- well as its industrial applications, and describe ductor that have been tried. The possibility of future prospects of this field mainly based on It is equivocal when and who started solar photoelectrolysis was demonstrated for the present authors’ work. utilizing first such a photochemical power the first time in 1969 with the system shown

of TiO2 to induce chemical reactions actively, in Fig. 1, which was exposed to near-UV KEYWORDS: titanium dioxide, Honda– but at least in Japan, there were a series of light, and was connected to a platinum black Fujishima effect, photoelectrol- reports by Mashio et al., from 1956, entitled counter electrode through an electrical load.5) 3) ysis, redox reaction, hydrophilic ``Autooxidation by TiO2 as a photocatalyst’’. Then, this electrochemical photolysis of water

conversion, environmental They dispersed TiO2 powders into various was reported in Nature by analogy with a application organic solvents such as alcohols and hydro- natural photosynthesis in 1972.6) In those carbons followed by the UV irradiation with days, crude oil prices ballooned suddenly, an Hg lamp. They observed the autooxida- and the future lack of crude oil was a serious tion of solvents and the simultaneous forma- concern. Thus, this became known as the time

tion of H2O2 under ambient conditions. It of ``oil crisis’’. Therefore, this report attracted is interesting to note that they had already the attention not only of electrochemists but

1 Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan 2 Department of Applied Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan 3 Kanagawa Academy of Science and Technology, 3-2-1 Sakado, Takatsu-ku, Kawasaki, Kanagawa 213-0012, Japan (Received August 11, 2005; accepted September 26, 2005; published December 8, 2005) ©2005 The Japan Society of Applied Physics

4 JSAP International No.14 (July 2006) JSAP International No.14 (July 2006) 5 also of many scientists in a broad area, and 1–103nm, depending on the carrier density It is desirable that the band gap of the numerous related studies were reported in the and dielectric constant of the semiconductor. semiconductor is near that for the optimum 1970s. If this electrode receives photons with ener- utilization of solar energy, which would be gies greater than that of the material band about 1.35eV. When semiconductor electrodes

When the surface of the rutile TiO2 elec- gap, EG, electron–hole pairs are generated are used as either photoanodes or photo- trode was irradiated with light consisting of and separated in the space-charge layer. In cathodes for water electrolysis, the bandgap wavelengths shorter than its band gap, about the case of an n-type semiconductor, the should be at least 1.23eV (i.e., the equilibrium 415nm (3.0eV), photocurrent flowed from the electric field existing across the space-charge cell potential for water electrolysis at 25°C and

platinum counter electrode to the TiO2 elec- layer drives photogenerated holes toward the 1atm), particularly considering the existence of trode through the external circuit. The direc- interfacial region (i.e., solid–liquid) and elec- polarization losses due to, for example, oxygen tion of the current revealed that the oxidation trons toward the interior of the electrode and evolution. Although various semiconductors

reaction (oxygen evolution) occurs at the TiO2 from there to the electrical connection to the with smaller band gaps were investigated, electrode and the reduction reaction (hydrogen external circuit. The reverse process occurs none succeeded for the water photoelectrol- evolution) at the Pt electrode. This observa- at a p-type semiconductor electrode. These ysis with visible light. This is because they were tion shows that water can be decomposed, fundamental processes of semiconductor in most cases corroded in an aqueous electro- using UV light, into oxygen and hydrogen, photoelectrochemistry have been discussed in lyte under irradiation, i.e., the photogenerated without the application of an external voltage, many recent reviews.7–11) holes oxidized the semiconductor itself. according to the following scheme.

– + If the conduction band energy ECB is One of the other approaches to utilize TiO2 +hν → e +h (1) higher (i.e., closer to the vacuum level, or longer wavelength light could involve the dye (at the TiO2 electrode) + + more negative on the electrochemical scale) sensitization of TiO2. This approach, although 2H2O+4h → O2 +4H (2) than the hydrogen evolution potential, photo- it is theoretically possible to use it for water (at the Pt electrode) + – generated electrons can flow to the counter photoelectrolysis, has practical problems, prin- 2H +2e → H2 (3) electrode and reduce protons, resulting in cipally that most photosensitizer dyes would The overall reaction is hydrogen gas evolution without an applied be far too unstable under these conditions.

potential, although, as shown in Fig. 2, ECB Therefore, the so-called regenerative photo- 2H2O+4hν → O2 +2H2 (4) should be at least as negative as –0.4V versus electrochemical cells, in which a single redox When a semiconductor electrode is in contact the standard hydrogen electrode (SHE) in couple exists, the reduced form is oxidized at

with an electrolyte solution, thermodynamic acid solution or –1.2V (SHE) in alkaline solu- the TiO2 photoanode and the oxidized form is

equilibration occurs at the interface. This may tion. Among the oxide semiconductors, TiO2 reduced at the counter electrode, were studied

result in the formation of a space-charge layer (acid), SrTiO3, CaTiO3, KTaO3, Ta2O5, and ZrO2 intensively. Chlorophyll and various organic within a thin surface region of the semicon- satisfy this requirement. On the other hand, dyes, such as xanthene dyes, were used as ductor, in which the electronic energy bands the employment of an external bias or of a sensitizers, but neither the solar conversion are generally bent upwards and downwards difference in pH between the anolyte and the efficiency nor the stability of the dyes was in the cases of the n- and p-type semicon- catholyte is required in the case of other mate- very high in those days. It is, however, to be ductors, respectively. The thickness of the rials in order to achieve hydrogen evolution. emphasized that this approach was intensively space-charge layer is usually of the order of researched later by Gratzel et al. using porous

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Fig.1. Schematic diagram of electrochemical photocell. (1) n-type Fig.2. Schematic representation of photoelectrochemical water electrolysis using illuminated oxide semiconductor TiO2 electrode; (2) platinum black counter electrode; (3) ionically electrode. An open circuit (or small current), pH 2, and illuminated conditions are shown for oxides with an ECB of conducting separator; (4) gas buret; (5) load resistance; and (6) volt- -0.65V (SHE) and an EVB of 2.35V (SHE). With the open circuit, a small excess potential (0.15V) is available for H2 evolu- meter. tion, assuming a reversible counter electrode.

4 JSAP International No.14 (July 2006) JSAP International No.14 (July 2006) 5 TiO2 electrodes and ruthenium complexes as efficiency is probably the higher reduction the reduction reaction was not necessarily sensitizers, and that the conversion efficiency potential of photogenerated electrons in the hydrogen production anymore. The platiniza- 12,13) reached 10–11% in the 1990s. former than in the latter, i.e., the bottom of tion of TiO2 was not necessary in this case,

the conduction band of anatase is located 0.1V and TiO2 powder itself was used under ambient 16) 3. Powdered TiO2 Photocatal- more negative than that of rutile. condition. Now both the reduction and oxida- ysis in 1980s tion sites are located on the TiO2 surface, and The first report on the efficient hydrogen the reduction of adsorbed oxygen molecules

The problem of light-assisted water split- production from water and organic compounds proceeds on the TiO2 surface. As already + ting has also been focused in the late 1970s was published in Nature at the time of the mentioned, the holes (h ) generated in TiO2

with the photocatalytic approach, i.e., essen- second oil crisis, and TiO2 photocatalysis drew were highly oxidizing, and most compounds tially with photoelectrochemistry but without the attention of many people as one of the were essentially oxidized completely, i.e., each an external circuit. Conceptually, this is similar promising methods for hydrogen production. constitution element of the compounds was to the process of corrosion, as being the However, even though the reaction efficiency oxidized to its final oxidation state. In addition, – combination of two short-circuited electro- is very high, TiO2 can absorb only the UV light various forms of active oxygen, such as O2 , • • • chemical reactions. In the case of corrosion, contained in a solar spectrum, which is only OH, HO2 and O , produced by the following

the two reactions might be anodic metal about 3%. Therefore, from the viewpoint of H2 processes may be responsible for the decom-

dissolution together with either cathodic H2 production technology, TiO2 photocatalysis is position reactions.

evolution or O2 reduction. Photocatalytic not very attractive. Various other semiconduc- – – e +O2 → O2 (ad) (5) water splitting has been studied intensively tors with a small band gap, such as CdS and – + • O2 (ad)+H → HO2 (ad) (6) with powdered anatase TiO2 suspensions, with CdSe, were investigated, but their efficiency + • + h +H2O → OH(ad)+H (7) Pt deposited on TiO2 as a cathodic catalyst. and stability were much lower than those of + – • h +O2 (ad) → 2 O(ad) (8) Although there were several experiments for TiO2, and the enthusiasm in the research of the

the simultaneous production of H2 and O2 in H2 production was over in the middle of the For the purpose of easy handling of

the powder systems, either such experiments 1980s. photocatalysts, the immobilization of TiO2 could not be reproduced or the reaction effi- powders on supports was carried out in the ciency was very low.14) Instead, the research shifted to the utiliza- late 1980s.19) Although many research studies tion of the strong photoproduced oxidation were on the purification of wastewater and

Kawai and Sakata examined, in 1980, power of TiO2 for the destruction of pollut- polluted air, TiO2 photocatalysis could not be the reasons why the water photolysis could ants. The first such reports were those of Frank developed to the stage of a real industrial not proceed in the powder system, although and Bard in 1977, in which they described technology in the 1980s. it could in the electrode one. They concluded the decomposition of cyanide in the presence 17) that the produced H2 and O2 gases might of aqueous TiO2 suspensions. In the 1980s, 4. TiO2 Film Photocatalysis recombine to regenerate water molecules detoxications of various harmful compounds under Weak UV Light in Early through the back reaction in the powder in both water and air were demonstrated 1990s

system, because the production sites of each using powdered TiO2 actively as potential gas are located close to each other. To solve purification methods of waste water and In 1990, we carefully determined the this problem, they added organic compounds polluted air.18) When the purpose shifted to reasons, in collaboration with TOTO Ltd., why

to the aqueous suspension of platinized TiO2. the oxidation reaction of harmful compounds, TiO2 photocatalysis could not be a practical

In this case, water is reduced, producing H2 at the Pt sites and the organic compounds are oxidized instead of water by photogenerated holes at the TiO2 sites. The H2 produc- �� tion proceeds surprisingly efficiently, with a �� quantum yield of more than 50% in the pres- � 15) � ence of ethanol. Most organic compounds, �� including biomass-derived compounds and �� even waste organisms, can enhance the � � hydrogen production efficiency. This is because �� the redox potential for photogenerated holes � �� is +2.53V versus SHE, as shown in the energy ��

band diagram for TiO2 (Fig. 3), and most � �� organic compounds can be oxidized finally � �� into CO2. In general, anatase (EG =3.2eV) �� gives better results than rutile (EG =3.0eV) for hydrogen production in powder photocatal-

ysis. One of the main reasons for the higher Fig.3. Schematic diagram of energy band for TiO2.

6 JSAP International No.14 (July 2006) JSAP International No.14 (July 2006) 7 Invited Review Paper

technology. We came to the conclusion that such as water or air. In this case, the absolute the daytime. This is low from the viewpoint of 15 it is fundamentally inadequate to utilize TiO2 amounts of the substances decrease, and thus, energy density, but corresponds to about 10 photocatalysis for either energy acquisition the rather weak UV light existing in an ordi- of photons/cm2 per second, which is a huge or the treatment of huge amounts of water nary environment could be a sufficient light amount compared to the number of mole-

and/or air, because light energy density is source for maintaining the TiO2 surface clean. cules adsorbed on the surface. Let us consider,

primarily low, and in addition, TiO2 can utilize We thus obtained the novel concept of light- for example, a highly packed organic mole- only the small amount of UV light contained in cleaning materials, which would be coated cule monolayer. Figure 4 shows the atomic 20,21) solar light. Based on such understanding, we with TiO2 film photocatalysts. force micrograph (AFM) images of mono-

conceived the idea of applying photocatalysis layer stearic acid prepared on a TiO2 rutile targeting only the substances adsorbed origi- 4.1 Photocatalytic cleaning effect (110) single crystal by the Langmuir–Blodgett 22,23) nally on surfaces. In other words, we could of TiO2-coated materials method. The thickness of the organic take the substances existing on two-dimen- substance was about 2nm, and the number of sional surfaces as the object of decomposition There exists typically several hundred stearic molecules was roughly estimated to be instead of those in three-dimensional spaces µW/cm2 of UV light even in outdoor shade in 1016–1017/cm2. When this was exposed to UV

Fig.4. AFM images of monolayer of stearic acid on rutile (100) surface (1mW/cm2).

2 light of about 2.5mW/cm , surface morpho- in the original purpose of the lighting, but is completely disappear on TiO2 after about one logical changes were clearly observed, i.e., sufficient to keep the surface clean when the week under a UV irradiation of 1mW/cm2. The

the inhomogeneous pitting of the film was cover glass is coated with the TiO2 photocata- cell deactivation can be achieved in a much very distinct. Further irradiation merged the lyst (Fig. 5).26) This light cleaning based on the shorter irradiation time, but still it takes nearly pits, producing a mosaic that eventually disap- photocatalytic decomposition effect is used 1h under outdoor UV light intensity.27) peared with the progress of the reaction. After now in other various commercial products 20min, the island structure of the LB film was such as window blinds. However, it is impor- In contrast, the typical indoor UV light no longer discernible, suggesting the complete tant to note that this function is effective only intensity is about several hundred nW/cm2,

decomposition of stearic acid to CO2. when the number of incident photons is much which is almost about three orders of magni- greater than that of filming mole- This observation suggests a potential utili- cules arriving on the surface per

zation of the photocatalysis of TiO2-coated unit time. In other words, the func-

materials, i.e., the surface coated with TiO2 tion is not effective when the flux could be kept clean. We first reported this of photons is insufficient compared type of photocatalytic cleaning material with to that of the organic substances, 24) a ceramic tile in 1992. Heller et al. also even though the TiO2 photocata- conceived a similar idea independently.25) One lyst is a very efficient one. of the first commercialized products using this effect was the self-cleaning cover glass 4.2 Photocatalytic anti- for tunnel light. In most tunnels in Japan, bacterial effect of copper- sodium lamps emitting yellow light are used and/or silver-deposited

for lighting, and the decrease in light inten- TiO2 film sity due to filming with exhaust compounds is one of the most serious problems. A high A photocatalytic decom- pressure sodium lamp also emits UV light of position reaction can be appli- about 3mW/cm2 at a position of its cover cable to microorganisms. In fact, Fig.5. Glass covers on highway tunnel lighting fixtures darkened by glass. This UV light had not been considered Escherichia coli (E. coli) cells can automobile exhaust without TiO2 and maintained clean with TiO2.

6 JSAP International No.14 (July 2006) JSAP International No.14 (July 2006) 7 ��

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Fig.6. Changes in survival rate of copper-resistant E. coli cells on Fig.7. Schematic illustrations of bactericidal process for copper-resistant E. coli cells on Cu/TiO2 thin film Cu/TiO2 thin film. Error bars: standard deviations of three replicate exper- under weak UV light irradiation. iments.

tude weaker than the outdoor one. Therefore, species photodeposited on the TiO2 film is a TiO2 surface before and after UV light irradia- the photocatalytic deactivation of microor- mixture of metallic copper (Cu0) and copper tion. With the discovery of this phenomenon, + 2+ ganism requires a much longer time under ions (Cu and Cu ). When this Cu/TiO2 film the application range of TiO2 coating has been indoor conditions than under outdoor ones; is irradiated, the copper ions can be reduced largely widen, as will be mentioned in detail thus, it cannot become a real practical tech- to metallic copper and metallic copper can be later. nology. However, the anti-bacterial function oxidized to copper ions by photogenerated

of a TiO2 photocatalyst is markedly enhanced electrons and holes, respectively. Although the The surface wettability is generally evalu- even with weak UV light, using a fluorescent oxidation state of copper might change under ated by the water contact angle (CA). The lamp and the aid of either silver or copper,28) UV light irradiation, copper ions (Cu2+) exist on CA (θ) is defined as the angle between the 28) which is harmless to the human body. Figure the Cu/TiO2 film. The enhancement of the solid surface and the tangent line of the liquid 6 shows the changes in the survival of copper- antibacterial effect under weak UV light was phase at the interface of the solid–liquid–gas

resistant E. coli cells on a Cu-photodeposited also observed on the Ag-deposited TiO2. This phases.

TiO2 (Cu /TiO2) film, indicating that E. coli antibacterial technique is safe and effective.

cells survived under dark conditions. However, It was observed that the number of bacteria A TiO2 thin film exhibits an initial CA of

when the Cu/TiO2 film is irradiated with very on the walls decreases to zero after installing several tens of degrees depending on the weak UV light, the survival rate begins to the photocatalytic tiles in an operating room. surface conditions mainly surface roughness. decrease. This demonstrates photocatalytic In addition, the bacterial count in air was When this surface is exposed to UV light, anti-bacterial activity even under weak UV also significantly decreased. The antibacterial water starts to exhibit a decreasing CA, that is,

light obtained from indoor lighting from fluo- ceramic tile coated with photocatalytic TiO2 it tends to spread out flat instead of beading rescent light bulbs. The survival decay curves containing Cu and/or Ag started full-scale up.29,30) Finally, the CA reaches almost 0°, as under irradiation in Fig. 6 are not simple manufacturing by TOTO Ltd. in 1995, and its shown in Fig. 8(a). At this stage, the surface exponential curves. Initial illumination had technology was exported to Western countries becomes completely non-water-repellant and a negligible effect on the survival rate, but widely. is termed ``highly hydrophilic’’. The surface after 2h, a marked decrease in survival was retains a CA of a few degrees for a day or two observed. We speculate that this cell deactiva- 5. Photo-induced Hydrophilicity under the ambient condition without being

tion proceeds as follows. The first step is that 5.1 Water wettability of TiO2 sur- exposed to UV light. Then, the CA slowly the reactive species generated by the TiO2 face increases, and the surface becomes the initial photocatalyst attacks the outer membrane less hydrophilic state again, as shown in Fig. [Fig. 7(b)]. The second step is the effective 5.1.1 What is a photo-induced high hydro- 8(b). At this point, the high hydrophilicity can uptake of the copper ions into the cytoplasmic philicity? be recovered simply by exposing the surface

membrane [Fig. 7(c)]. In this case, the TiO2 again to UV light. In short, this type of TiO2- photocatalytic reaction assists the intrusion of While investigating the novel concept of coated material is the only known practical

the copper ions into the cell, which is prob- light-cleaning materials coated with a TiO2 highly hydrophilic one that shows stable and ably why the E. coli cells are effectively killed film photocatalyst under weak UV light in the semi permanent properties.

on a Cu/TiO2 film even under very weak UV 1990s, we have found in 1995 by chance the light.28) It has been reported that the copper marked change in the water wettability of the How can we explain this phenomenon?

8 JSAP International No.14 (July 2006) JSAP International No.14 (July 2006) 9 Invited Review Paper

Just after applying oleic acid, both TiO2 and UV SrTiO3 surfaces were converted to the hydrophobic state with CAs of around 70° due to the hydrophobic property of the oleic acid adsorbed on the surfaces. When UV light dark was irradiated on the SrTiO3 surface, the CA decreased to 20°, which was almost the same �� as that observed before applying oleic acid. As �� a result of conventional photocatalytic oxida- �� tion processes, surface-adsorbed oleic acid �� was decomposed and almost removed, repro- �� ducing the initial surface. For the TiO2 surface, �� however, the CA decreased to 0°, which was � � �� lower than the initial value, showing that the � � � �� highly hydrophilic surface of TiO2 is not simply UV Irradiation Time/min Dark Storage Time/hour attributed to the effect of the photocatalytic oxidation power.

Fig.8. Changes in CA of TiO2 surface (a) under UV irradiation and (b) in the dark. (ii) Relationship between amount of stains and contact angle

We have vigorously attempted to find the TiO2 is explained using a simple photocatalytic answer by analyzing it from various points of decomposition mechanism.32–34) However, it Figure 10 shows the representative the 29–31,35–37,39,44,45,48–50,53,54) view. is important to note that a completely clean C 1s spectra of the TiO2 surface detected by surface cannot be obtained by the photo- X-ray photoemission spectroscopy (XPS) before 5.1.2 Characteristics of photo-induced high- catalytic reaction because the surface is easily and after UV light irradiations (60min and ly hydrophilic state contaminated by airborne stains under the 12h). Before UV light irradiation, the carbon ambient condition. We have obtained many peak derived from the stains was detected First, we considered that this highly hydro- experimental results showing that this highly and the CA on such surface was found to philic conversion originates from the clean hydrophilic state is not a simple clean surface be 56°. After 60min UV light irradiation, the surface produced by the decomposition of but is a structurally changed metastable one. CA was confirmed to be 0° when the distinct staining compounds adsorbed on the surface Some of these results will be described below. carbon peak was detected, although the through the conventional photocatalytic amount of stains decreased compared to that oxidation. This is because metal oxides have (i) Comparison of hydrophilic properties of before UV light irradiation. After further irra-

large surface energies in general, and thus, we TiO2 and SrTiO3 surfaces diation for 12h, the carbon peak still remained

considered that a clean TiO2 surface could be and the CA was 0°. These experiments clearly essentially hydrophilic. In fact, several reports Figure 9 shows the changes in CA indicate that the highly hydrophilic conversion have recently been published, in which the under UV light irradiation after applying oleic can be completed even if stains remain on the 35) photo-produced highly hydrophilic state of acid to the surfaces of TiO2 and SrTiO3. surface.

�� ater contact angle / deg

W � � �� Time / hour ��

Fig.9. Changes in water contact angle after applying oleic acid to surfaces of Fig.10. Change in XPS spectrum of TiO2 in C 1s peak region before and after 2 TiO2 and SrTiO3. UV illumination with an intensity of 2.0mW/cm .

8 JSAP International No.14 (July 2006) JSAP International No.14 (July 2006) 9 ��

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Fig.11. Change in XPS spectrum of TiO2 in C 1s peak region before and after Fig.12. Changes in water contact angle for TiO2 film subjected to alternate rinsing in warm 4M NaOH solution (50ºC). ultrasonic treatment in pure water () and UV light irradiation ().

Figure 11 shows the changes in the C Subjected to cycles of alternating ultrasonic surface increases gradually toward the initial 1s XPS spectra before and after the treat- treatment and UV light irradiation, the water value in the dark even without any external

ment of the TiO2 surface in a warm concen- contact angles switched between 0 and 10° stimuli. However, the rate of this back reac- trated NaOH solution. No distinct carbon reversibly. It is hardly believed that the ultra- tion strongly depends on the ambient temper- peak was detected within the experimental sonic treatment makes the surface fouler. ature. Figure 13 shows the result. Because error. Although the surface stains were nearly this measurement was carried out in a vessel completely removed by this treatment, we Similarly, the highly hydrophilic state is filled with synthetic air under the same condi- found that such a ``nearly clean’’ surface does turned to a less hydrophilic one by a ``wet tions other than temperature, even if the the CA of 20°, which is much higher than rubbing’’ method, which is a type of mechano- surfaces were adsorbed with stains, their influ- that of 0° induced by UV light.36) By consid- chemical surface treatment with clean lint-free ence could be ignored. It is obvious that the ering these findings, only the removal of stains paper. Kamei and Mitsuhashi reported that the apparent rate was enhanced by increasing the cannot lead to the highly hydrophilic state CA increases drastically from 3 to 80° by the temperature. To estimate the activation energy 38) under the ambient condition. wet rubbing method. These results indicate for this transformation, we used the rate (kb) that the photo-produced highly hydrophilic obtained by the reciprocal plot of CA against (iii) Stability of photo-produced highly hy- state is broken by external stimuli. the logarithm of the dark storage time.31,40)

drophilic TiO2 surface (iv) Temperature dependence of back reac- Figure 14 shows the natural logarithm of 39) When the highly hydrophilic TiO2 film tion in the dark kb obtained at CA of 20° versus the reciprocal prepared by UV light irradiation was soni- of the absolute temperature. According to Fig. cated in pure water, the CA was found to As was described previously, the CA of 14, the activation energy was calculated to be 37) –1 increase to around 10°, as shown in Fig. 12. the photo-produced highly hydrophilic TiO2 62.4kJmol .

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Fig.13. Ambient temperature dependence on less hydrophilic Fig.14. Activation energy of less hydrophilic conversion for TiO2 film kept in conversion for TiO2 film maintained in the dark. the dark.

10 JSAP International No.14 (July 2006) JSAP International No.14 (July 2006) 11 Invited Review Paper

Taking all the above experimental results has a constant value of 74mJ/m2. Therefore, phobic phases accounts for the highly hydro-

into consideration, we are certain that the by combining eqs. (9) and (10), the CA can be philic characteristic on the TiO2 surface.

photo-produced TiO2 surface is not completely simply expressed as

clean but contains some stains, although it is 1/2 (ii) Surface hardness change after UV light cosθ =cγ S –1 (c: constant). (11) highly hydrophilic with a CA of almost 0°. In irradiation other words, the removal of stains by oxida- This final equation shows that the CA

tive decomposition can make the surface decreases simply with increasing γ S. Therefore, Hardness changes before and after irradia- only moderately hydrophilic, with a CA of it can be considered that the highly hydrophilic tion with UV light on the surface of the rutile

10–20° under the ambient condition. In addi- state with a CA of 0° is achieved by the gener- TiO2(110) single crystal were also observed. tion, it seems that the highly hydrophilic state ation of some states with large surface ener- Figure 16 shows the repetition of UV light is not the most stable state but forms some gies when irradiated with UV light. As previ- on and off dependence of surface hardness metastable one. In fact, evidence has been ously mentioned, the highly hydrophilic state (within 50nm from the surface), inner hard- obtained, showing that the surface structure generated by UV light gradually returns to the ness (300nm from the surface), and CAs.44)

of TiO2 is changed by UV light irradiation, initial less hydrophilic state in the dark. More- When irradiated with UV light, CAs decreased which will be shown below. over, we showed that some external stimuli to 0°, accompanied by an increase in surface and a higher ambient temperature increase hardness. In contrast, the inner hardness 5.1.3 Wettability of solid surface the rate of the back reaction. All these suggest remained constant. These findings suggested that the photo-produced state with a large that the highly hydrophilic surface is subjected Before examining some evidence for surface energy is metastable. to compressive stress caused by the expansion surface structural changes accompanying the of the surface volume.

highly hydrophilic conversion, let us briefly 5.1.4 Structural changes on TiO2 surfaces ir- review a classical theoretical treatment for radiated with UV light 5.1.5 What is the metastable state? the wettability of the flat solid surface. It is commonly evaluated in terms of the CA, (i) Domain formation after UV light irradia- We have shown that the surface structure 41) which is given by Young’s equation: tion of TiO2 changes after UV irradiation, forming a metastable state. Then, what is this state? γ S =γ SL +γ L · cosθ, (9) To gain information about surface changes Thus far, we have not yet obtained a defi-

where γ S and γ L are the surface free energies at the microscopic level, friction force micros- nite answer. However, several experimental

per unit area of the solid and liquid, respec- copy (FEM) was utilized. A rutile TiO2(110) data indicate that the metastable state could

tively, and where γ SL is the interfacial free single crystal was used. Before UV light irra- be caused by an increase in the number of 31) energy per unit area of the solid–liquid inter- diation, no difference in contrast was observed, hydroxyl (OH) groups on the TiO2 surface.

face. In addition, γ SL can be approximated indicating microscopically homogeneous wetta- 42,43) using the Girifalco–Good equation, with γ S bility. After irradiation, the hydrophilic (bright) This increase could be detected by X-ray 36) and γ L, as and hydrophobic (dark) areas of 30–80nm were photoemission spectroscopy (XPS). The O 29) 1/2 clearly observed (Fig. 15). A gradual rever- 1s spectrum for the highly hydrophilic TiO2 γ SL =γ S +γ L −Φ(γ S /γ L ) . (10) sion to a smaller contrast was observed during surface exhibited a broad shoulder to the Here, Φ is a constant parameter ranging from the storage of the crystal in the dark. In this higher binding energy side of the main O 1s 0.6 to 1.1, depending on the solid. In addi- study, we concluded that the nanoscale sepa- peak. The shoulder was fitted with two bands,

tion, γ L is the water surface free energy, which ration between the hydrophilic and hydro- which are associated with the dissociatively

��

� � �� Fig.15. FFM images of rutile TiO2(110) single crystal surface before (left) and after (right) �� UV light irradiation. � ��

Fig.16. Relationship between changes in water contact angle and hardness at surface of TiO2 film (within 50nm) and inside (300nm).

10 JSAP International No.14 (July 2006) JSAP International No.14 (July 2006) 11 adsorbed water on the TiO2 surface as well as surface-enhanced IR absorption spectroscopy tion, and then a new OH group forms, leading the physically adsorbed molecular water on (SEIRAS).46) to the increase in the number of OH groups the dissociatively adsorbed OH groups. The at the surface. It is considered that the singly shoulder gradually decreased in size during Recently, Uosaki et al. reported the coordinated new OH groups produced by UV storage in the dark. Therefore, these photo- differences in interfacial water structures at light irradiation are thermodynamically less

generated surface OH groups were considered TiO2 surfaces between before and after UV stable compared to the initial doubly coor- to be thermodynamically metastable. light irradiation by sum frequency genera- dinated OH groups. Therefore, the surface 47) tion (SFG) spectroscopy. It was shown that energy of the TiO2 surface covered with the Fourier transformation infrared (FTIR) spec- UV light irradiation led to an increase in the thermodynamically less stable OH groups is

troscopy provides another method of investi- amount of the ordered adsorbed water on higher than that of the TiO2 surface covered

gating the relationship between hydrophilicity the TiO2 surface (a broad band with a peak with the initial OH groups. Because a water 45) –1 –1 and the amount of OH groups. TiO2 shows at 3400cm and a shoulder at 3200cm ), droplet is substantially larger than the hydro- IR bands positioned at 3695cm–1, assigned to confirming the increase in the hydrophilicity of philic (or hydrophobic) domains, as shown in the stretching of OH groups chemisorbed on the surface. Fig. 15, it instantaneously spreads completely the surface, 3300cm–1, assigned to hydroxyls on such a surface, thereby resembling a for both dissociated water and molecularly 5.1.6 Mechanism for highly hydrophilic con- two-dimensional capillary phenomenon. adsorbed water, and 1623cm–1, pertaining version to H–O–H bending for molecular water. This 5.2 Application observation denotes the coexistence of disso- On the basis of the previous studies

ciated water and molecular water on the TiO2 mentioned above and some electrochemical 5.2.1 Self-cleaning function surface. Storage in the dark for one week experiments,48) which show that the photo- resulted in the decrease of all the bands, generated holes, not the electrons, are The finding of the photo-induced hydro- correlated with both dissociated and molecular responsible for the highly hydrophilic conver- philicity has markedly widened the application 49) water desorptions. The significant increase in sion, we proposed a mechanism for the highly range of TiO2-coated materials. That is, the

CA after the dark storage is consistent with hydrophilic conversion under UV light irra- stains adsorbed on the TiO2 surface can easily this surface conversion. The surface conver- diation, as shown in Fig. 17.31) That is, the be washed by water, because water soaks

sion is ascribed to the removal of the chemi- photogenerated holes produced in the bulk between stain and the highly hydrophilic TiO2

sorbed hydroxyl groups. With UV light irradia- of TiO2 diffuse to the surface and are trapped surface. In other words, this has removed the –1 tion, however, the intensity of the 1623cm at lattice oxygen sites. Most trapped holes are limitation of the cleaning function of the TiO2 band decreased, whereas the 3695cm–1 band consumed to react with the adsorbed organics photocatalysis, that is, the function is limited increased, demonstrating the increase in the directly, or adsorbed water, producing OH radi- by the number of photons. Even though the amount of adsorbed dissociated water and cals. However, a small portion of the trapped number of photons is not sufficient to decom-

the decrease in that of adsorbed molecular hole may react with TiO2 itself, breaking the pose the adsorbed stains, the surface is main- water. Nakamura et al. also reported that the bond between the lattice titanium and oxygen tained clean when water is supplied there.

increase in the amount of chemisorbed H2O ions by the coordination of water molecules at

on the TiO2 surface after UV light irradia- the titanium site. The coordinated water mole- Thus, the TiO2-coated materials used tion was observed (a band at 3270cm–1), by cules release a proton for charge compensa- outdoors where they are exposed to rainfall

� � � � � � �� + � � � � �H �� Unstable OH H2O �� (metastable state) � � � � � hydrophilic ��

Fig.17. Schematic illustrations of reversible changes in amount of hydroxyl groups on TiO2 Fig.18. Conventional tent material (left) and TiO2-coated tent material (right). film under UV light irradiation and in the dark.

12 JSAP International No.14 (July 2006) JSAP International No.14 (July 2006) 13 Invited Review Paper

TiO2 that undergoes highly hydrophilic conversion has been required. We have intensively investigated this on the basis of the mechanism for the highly hydrophilic conversion, as described in §5.1.6.

As the reconstruction of surface OH groups is responsible for the hydrophilic conversion, different types of hydrophilic conversion behavior among the various single Fig.19. Anti-fogging effect of automobile side-view mirror: conventional mirror (left) and TiO2-coated mirror (right). crystal faces are expected to be observed by comparing their surface atom alignments. In fact, the (100) and (110) faces of a rutile single crystal exhibited a higher hydrophi- show a very effective self-cleaning function, catalysis is still carried out for developing licity than the (001) face, as the former faces i.e., stains are decomposed partially by the both its science and technology after the 21st have two-fold oxygens, which are higher in conventional photocatalytic reaction as well as century. In the following, some of the recent position and energetically more reactive than washed by rainwater. Such materials, which development will be introduced. their surrounding atoms and are so-called the we call ``photocatalytic building materials’’, ``bridging site oxygens’’, whereas the latter has e.g., exterior tiles, glass, aluminum walls, and 6.1 Design of nanostructure of three-fold oxygens, which are lower in posi- 50) PVC fabric (Fig. 18), have already begun to be TiO2 surface for highly sensitive hy- tion and energetically more inactive. There- commercialized. For example, the photocata- drophilicity fore, the (100) and (110) faces are favorable lytic exterior glass with an area of 20,000m2 for increasing the number of OH groups. This was installed in the terminal building of Chubu When utilizing the photooxidative decom- finding could serve as a guideline for selecting

International Airport opened in 2005. position properties, TiO2 has already been the appropriate faces of the TiO2 crystal for applied to indoor use with the aid of either surface exposure. 5.2.2 Anti-fogging function copper or silver for anti-bacterial purpose, as mentioned in §4.2. However, as for neither To obtain the appropriate faces, the Another function blessed with the photo- the decomposition of VOC nor the utilization photoetching technique was applied to the

induced hydrophilicity is the anti-fogging func- of hydrophilic properties, TiO2 has yet to be (001) face of a rutile single crystal. As shown

tion. The fogging of the surfaces of mirrors applied to indoor use. For example, the TiO2 in the inset of Fig. 20, a large number of rect- and glasses occurs when steam cools down surface does not undergo the highly hydro- angular porous holes shaped like wells, with on these surfaces to form many water drop- philic conversion when the UV light has an a size of 50–100nm, emerged with a fairly lets. On a highly hydrophilic surface, no water intensity of 1µW/cm2, available in an ordinary regular arrangement. According to Sugiura drops are formed. Instead, a uniform thin living environment. Thus, a highly sensitive et al.51) and Nakato et al.,52) rectangular holes film of water is formed on the surface. This uniform water film prevents the fogging. Once the surface turns into the highly hydrophilic state, it remains unchanged for several days or �� one week. Thus, we expect that various glass

products, mirrors and eyeglasses, for example, � �� can be imparted with antifogging functions using this technology, with simple processing �� and at a low cost. In fact, Japanese-made cars �� are being equipped with antifogging highly �� hydrophilic side-view mirrors, as shown in Fig. ��

19. � �� 6. Photocatalysis in 21st Cen- �� tury � � � � ��

As was described above, TiO2 photocatalysis has become a real practical technology �� after the middle of the 1990s, particularly in the field of building materials. It is, however, Fig.20. Changes in water contact angles on rutile (001) surface with and without photoetching treatments. The inset shows a SEM image of rectangular holes emerged by surprising that a research study on TiO2 photo- photoetching treatment on a (001) surface.

12 JSAP International No.14 (July 2006) JSAP International No.14 (July 2006) 13 59–61) proceed toward the 〈001〉 direction (c-axis regardless of the residual stresses. The surface films. Subsequently, nitrogen-doped TiO2 direction), and the (100) face or equivalents morphologies of the four films were controlled has attracted considerable attention.62–64) In are exposed selectively on the walls. This rect- to be the same, and therefore, the residual fact, similar photocatalysts sensitive to visible angular porous surface increased the sensitivity stress is an important factor influencing the light have already been reported in 1986.65,66) of the photoinduced hydrophilic reaction and highly sensitive photoinduced hydrophilicity of achieved a high hydrophilicity under UV light a film. Yellow and transparent nitrogen-doped 2 irradiation of 1µW/cm , whereas the surface TiO2 thin films can be fabricated by sputtering

without a photoetching treatment did not, The other highly sensitive system we a Ti or TiO2 target in a gas flow that contains 53) 61) as shown in Fig. 20. Because the surface produced is the heterogeneous TiO2/WO3 N2O or N2. The N 1s XPS peak at 396eV indi- roughness enhances the hydrophilicity, the one.55) As photogenerated holes are effectively cates that the dopant nitrogen is located at an 60,61) high sensitization for hydrophilic conversion accumulated on TiO2 without recombining oxygen site.

by the photoetching treatment was caused with electrons, and as WO3 could absorb by the exposure of faces with bridging site visible light contained in the fluorescent light Figure 22(a) shows the hydrophilic oxygens and the increase in surface rough- up to a wavelength of around 440nm due conversion behaviors of the nitrogen-doped

ness. This high sensitization was also observed to its bandgap of 2.8eV, the reaction respon- TiO2 thin film irradiated with visible light

on the rutile polycrystal by the photoetching sible for the increased hydrophilicity was (400–530nm). The nitrogen-doped TiO2 treatment, which suggested that this tech- greatly enhanced, and the high hydrophilicity thin film underwent hydrophilic conversion; nique is applicable to anatase thin films. was achieved on this system irradiated with however, irradiating only with visible light did extremely weak UV light with an intensity of not achieve the high hydrophilicity. Irradiating The introduction of residual tensile stress to 1µW/cm2 from a fluorescent lamp. As such, with UV light (300–400nm), the nitrogen-

anatase thin films is also a candidate method the high sensitization of TiO2 under weak UV doped TiO2 thin film underwent highly hydro- to increase the sensitivity. As the hydrophilic light is expected to increase the application philic conversion, as shown in Fig. 22(b). conversion is caused by the increase in the areas to indoor conditions. The photocatalytic activities (both oxidative number of surface OH groups, the Ti–O bond decomposition and hydrophilic conversion

length should be larger in the highly hydro- 6.2 Visible-light-sensitive TiO2 activities) generated by visible light-induced philic surface than in the less hydrophilic holes are inferior to those generated by UV surface. In fact, as previously mentioned, the The current area of interest in this field light-induced ones.67)

surface volume increased after UV light irra- has been the modification of TiO2 sensitive diation. Therefore, it can be expected that the to visible light. One approach was to substi- Before discussing the reason for this, let 56,57) TiO2 surface in the presence of tensile stress tute Cr, Fe or Ni for a Ti site. Another us discuss the origin of visible light sensitivity. is favorable for hydrophilic conversion. The approach was to form Ti3+ sites by introducing When the N atoms are substituted as N3– 58) 2– residual tensile stress in the anatase thin film an oxygen vacancy in TiO2. However, these ions at O sites, the impurity (accepter) level, fabricated by the sputtering technique greatly approaches were not widely accepted due composed of 2p orbitals of N (N 2p), is formed enhanced the hydrophilic behavior, compared to the lack of reproducibility and chemical above the valence band. Electron vacancies to a thin film without residual stress, as shown stability. exist at the acceptor level. Therefore, nitrogen- 54) in Fig. 21. In contrast, the compressive stress doped TiO2 should be a p-type semiconductor degraded the hydrophilic behavior. The photo- In 2001, several groups reported visible- as electrons are thermally excited from the

catalytic oxidation activities were identical light-sensitive TiO2-based powders and thin valence band. However, we have photoelec- trochemically and electrochemically deter-

mined that the nitrogen-doped TiO2 was an n-type semiconductor and the carrier concen-

�� tration increased as the dopant concentration increased. Actual TiO is known to be nonstoi- �� 2 �� chiometric and have oxygen defects. Oxygen �� defects generate electrons, which leads to the �� n-type semiconductor. The amount of elec- �� trons, originated from oxygen vacancies, is

CA / deg / CA �� considered to be larger than that of electron �� vacancies, originated from the nitrogen substi- � tution at O sites. Therefore, the nitrogen- � doped TiO2 should be an n-type semicon- � �� ductor. Thus, with increasing dopant nitrogen, �� the carrier concentrations should decrease. However, the experimental results are the opposite. When nitrogen is incorporated, Fig.21. Changes in water contact angles on TiO2 films when irradiated with UV light, without residual stress and with residual tensile/compressive stresses. because an increase in the amount of intersti-

14 JSAP International No.14 (July 2006) JSAP International No.14 (July 2006) 15 Invited Review Paper

will show the higher photocatalytic activities �� irradiated with visible light. This investigation is 72) �� now under way.

�� These anion-doped TiO are candidate �� 2 �� photocatalysts in terms of reproducibility and �� chemical stability. Therefore, further research

�� and practical applications are desirable. �� 6.3 Hydrophobic TiO2 surface �� As the discovery of the photo-induced �� hydrophilicity has markedly widened the application field of TiO2-coated materials, a highly

hydrophobic TiO2 surface could open a new Fig.22. Changes in water contact angles on TiO2 and nitrogen-doped TiO2 thin films irradiated with(a) vis and (b) UV lights. application field. In addition, if it is realized, we will be able to control the wettability on

a TiO2 surface using external stimuli, such as UV light, which we believe is both scientifically and practically very important.

tial titanium is clearly not possible, an oxygen Compared with the conventional TiO2,

vacancy is simultaneously produced. There- the advantage of nitrogen-doped TiO2 is the Let us try to design the hydrophobic TiO2 fore, the schematic illustration in Fig. 23 is the increased absorbed photon number, but the surface. As previously mentioned, the wetta- essential procedure when oxygen is replaced disadvantage is the decreased photocatalytic bility of a flat solid surface is determined by nitrogen. activity. Therefore, whether to use nitrogen- simply by the surface free energy of solid as

doped TiO2 or conventional TiO2 depends on described in eqs. (9) and (11). As for a non-flat The Ti 3d impurity (donor) level, derived the light source. surface, however, surface roughness affects from oxygen vacancies, is known to be nonvis- the wettability. Wenzel modified Young’s ible light sensitive. In fact, hydrogen-reduced In addition to nitrogen, the sulfur-doped equation by considering the surface roughness 73) TiO2 (TiO2-x) powders, which have a gray color, and carbon-doped TiO2 systems are reported to obtain do not show visible light sensitivity. The N 2p to be visible-light-sensitive photocatalysts.68–71) cosθ’=r cosθ, (12) level exists much deeper than the Ti 3d donor However, the visible-light-induced photocata-

level, from which we can consider that the N lytic activities of these anion-doped TiO2 are where θ’ is the apparent contact angle and 2p level has a much more oxidation power insufficient and should be further enhanced. r is the surface roughness ratio between the than the Ti 3d level. Therefore, the origin of This is because the introduced 3p

visible light sensitivity of nitrogen-doped TiO2 orbitals of S (S 3p) and 2p orbitals is considered to be the N 2p narrow band. of C (C 2p), similar to N 2p, also formed the localized narrow band �� Thus, the reason that irradiating the in the forbidden band, by not �� nitrogen-doped TiO2 with UV light resulted in mixing with the valence band � �� higher photocatalytic activities than irradiating composed of O 2p. Thus, it can �� with visible light is as follows. Irradiating with be speculated that mixing the �� UV light excites electrons in both the valence introduced anion 2p (or 3p) with �� band composed of O 2p and the narrow band O 2p, leading to the bandgap � composed of N 2p, but irradiating with visible narrowing, is the key method of � �� light only excites electrons in the narrow band enhancing the visible-light-induced �� of N 2p. In addition, the hole mobility in the photocatalytic activities. We have �� isolated narrow band should be low. There- found using the DOS calculations �� fore, the hydrophilic properties when irradi- that the simultaneous doping ��

ating with UV light were higher than when of nitrogen and sulfur into TiO2 �– �– �� irradiating with visible light. Although the could narrow the bandgap of TiO2 �� nitrogen concentration increases, it is not by mixing N 2p and S 3p with the �� – �� – �� suggested by the density of states (DOS) calcu- valence band composed of O 2p. � �� lations that the bandgap narrowing by mixing It can be expected that the broad �� N 2p with O 2p occurs. expanse of the electronic state, compared with the isolated one, Fig.23. Schematic illustration of nitrogen doping into oxygen site.

14 JSAP International No.14 (July 2006) JSAP International No.14 (July 2006) 15 nating between UV light and dark storage.75) At present, the hydrophobic conversion takes much time, and must be further enhanced.

In addition, the surface with a CA above 150° is called the super-hydrophobic surface, and such a surface is attracting considerable attention at present,76–84) because the limited contact area between the solid surface and the water could change chemical reactions and/or bond formation through water, intro- Fig.24. SEM images for polycrystalline TiO2 surfaces (a) before and (b) after etching treatments. ducing novel properties on the surface. There- fore, if one succeeds in the preparation of the

TiO2 surface with a CA of more than 150°, it

actual surface area and the apparent surface at room temperature in a clean vessel filled indicates that the surface property of TiO2 can area. This equation indicates that the surface with synthetic air. Maintaining the samples in be markedly controlled. roughness enhances the hydrophilicity of the dark for 15 days caused the CA to reach hydrophilic surfaces (θ<90°) and also enhances approximately 120° on the etched surface, 6.4 New environmental applica- the hydrophobicity of hydrophobic surfaces whereas the CA was around 80° on the tions (θ>90°) since the r value is always larger than 1. nonetched surface, indicating that we could

Because the CA of a flat TiO2 surface is always obtain the hydrophobic TiO2 surface, on which TiO2-coated materials practically used thus

less than 90°, the hydrophobic TiO2 surface Cassie’s mode dominates, by the photoetching far have been provided with self-cleaning, anti- cannot be obtained by considering only the process (Fig. 25). bacterial and/or anti-fogging functions based Wenzel equation. To realize the hydrophobic on the photo-induced decomposition reaction

TiO2 surface, we must introduce a different The UV irradiation of this hydrophobic and photo-induced hydrophilicity. It is impor-

concept to describe the CA (θ’) on a rough TiO2 markedly changed the surface wetta- tant to note that these functions are obtained surface. Cassie proposed an equation for a bility, i.e., the CA decreased to 0°. In addi- without using any chemicals, but with only 74) surface composed of solid and gas. When tion, storing the samples in the dark for a long sunlight and rainwater. Therefore, these TiO2- a unit area of the surface has an area friction, time caused the CA on the etched surface coated materials can be classified as being f, where a water droplet contacts the solid to further increase to approximately 140°, environmentally friendly. In this section, we

surface with a CA of θ, the θ’ on the surface whereas the CA on the nonetched surface will show further applications of TiO2 photo- can be expressed as follows, called Cassie’s remained constant at around 80°. We can catalysis, which can actively contribute to envi- equation, in which the CA for air is assumed thus control the surface wettability from highly ronmental preservation and/or improvement to be 180°: hydrophilic to hydrophobic repeatedly by alter- technologies.

cosθ’ = f cosθ +(1– f)cos 180°

= f cosθ +f –1. (13)

Considering eq. (13), the θ’ increases with decreasing f value. Thus, the hydrophobic

TiO2 requires surface roughness where air can intrude between the water droplet and the

� �� TiO2 surface. ��

To achieve such a rough surface, we �� applied a photoetching technique to a rutile �� pellet. Before the etching treatment, grains �� and grain boundaries were observed, which

�� is typical of a polycrystalline pellet, as shown � �� in Fig. 24(a). After etching, the surface struc- �� ture changed and was accompanied by an � � �� increase in surface roughness. Random direc- �� tions of digging ``ditches’’ as shown in Fig. 24(b) were plausible, considering the polycrystalline pellet. The pellets with the etched and Fig.25. Back reactions on etched and nonetched polycrystalline TiO2 surfaces as a non-etched surfaces were stored in the dark function of storage time in the dark.

16 JSAP International No.14 (July 2006) JSAP International No.14 (July 2006) 17 Invited Review Paper

6.4.1 Photocatalytic decomposition of pol- large surface area deposited with very photo- in Fig. 26. The wastewater from the hydro-

lutants active TiO2 nanoparticles; we were able to ponic tomato culture with a planting area of show that the wastewater solution is easily about 80m2 was introduced into a shallow Environment pollution, including water, purified using the photocatalytic mat under vessel with a bottom area of 4m2 and a depth air and soil is becoming an increasingly serious only solar light. The treatment method is of 10cm in which porous ceramic plates

problem today. There have been many reports very simple, i.e., the wastewater is poured coated with TiO2 photocatalyst nanopar-

aiming at applying TiO2 photocatalysis to pollu- onto mats that are spread over a wide area ticles were placed. The organic contaminants tion clean-up since the 1970s. However, the on the ground. The agricultural chemicals in the wastewater were easily decomposed purification of the three-dimensional spaces by were completely decomposed under sunlight under solar light, but the nutrient compounds photocatalysis is much more difficult than that in a few days. It is surprising that the initial containing nitrogen, phosphorous and potas- of the two-dimensional surface of building total organic carbon (TOC) values of several sium were not, because these components – materials due to the following two reasons. hundred to one thousand ppm decreased to existed in their most oxidized forms, NO3 , 3– + One is that photocatalytic reactions are nearly zero in one week. This could be one of PO4 and K , in the nutrient solution. There- surface reactions and thus the reactants must the simplest and easiest environmental tech- fore, this system is also expected to serve as a be captured by the photocatalyst surface. The nologies. nutrient-saving technology. other is that the total amount of reactant is, in general, higher in three-dimensional spaces (ii) Water treatment of hydroponic culture (iii) Treatment of VOC-polluted soil than on a two-dimensional surface, indicating system that much more light energy is necessary Volatile chlorinated organic compounds for the purification of the three-dimensional Most of the hydroponic culture installa- (VOCs), such as trichloroethylene and tetra- space. Therefore, practical technologies have tions are adopting run-to-waste systems (as chloroethylene, have been widely used as not been obtained in this field. However, we opposed to recirculating). The nutrient solu- solvents for the dry cleaning of clothes and have recently succeeded in the construction of tion contains highly concentrated nitrogen and washing of semiconductors. They had been practical purification systems for wastewater phosphorus compounds, and thus, the waste- indiscriminately disposed by the middle of the from agriculture and soil polluted by volatile water causes soil pollution. From the view- 1980s, causing soil and groundwater pollu- organic compounds (VOCs). These systems point of environmental problems, the recycling tion. These types of pollution are serious envi-

are based on TiO2 photocatalysts and use only of nutrient solutions is desirable. However, ronmental problems, because the carcinogenic solar light. organic contaminants accumulate in the solu- and toxic chemicals had been widely disposed. tion, inhibiting plant growth. The most common method of treating the Important keys for these successes are polluted soil is either the simple replacement

(1) the utilization of nanosized TiO2 photo- We succeeded in constructing a recy- with clean soil or the heating of the soil to catalyst powders dispersed on substrates cling system in a tomato hydroponic culture volatilize the VOCs into air. It is obvious that

with extremely large surface areas, and (2) system using the TiO2 photocatalyst and solar these methods do not truly purify the environ- spreading them on the ground widely to light. The system is schematically illustrated ment. collect sunlight. Because the concentrations of environmental pollutants are low in general, the UV light contained in sunlight is sufficiently

strong to decompose them by TiO2 photocatalysis, if we can collect the light from a broad area.

(i) Detoxification of wastewater used for rice hull disinfection �� Disinfection solutions for rice hulls (to stop �� the spread of plant diseases) contain highly concentrated agricultural chemicals. The corre- sponding wastewater is mostly disposed of by pouring onto the ground, which causes soil pollution. The amount of wastewater from rice hull disinfection amounts to about 500,000 metric tons/year in Japan, for example. Such �� a problem could also become much more �� serious in developing countries.

Fig.26. Novel photocatalytic water recycling system of tomato hydroponic culture. We developed a glass wool mat with a

16 JSAP International No.14 (July 2006) JSAP International No.14 (July 2006) 17 We designed photocatalytic sheets to surface area of water. Thus, the cultivation of was confirmed on a clear day in the middle purify the polluted soil on the ground using rooftop plants and/or the construction of arti- of summer that the temperature drops were sunlight. As shown in Fig. 27, the polluted soil ficial lakes or marshes are currently under way. 15ºC on window glass and 40–50ºC on black is dug up and covered with the sheet, which However, the areas of green tract land and roof-tile surfaces. Moist surfaces easily mildew,

is made of corrugated paper containing both water surfaces cannot be secured easily due to but since a TiO2 photocatalyst has oxidation

TiO2 powder adsorbed on activated carbon the high land value in urban areas. activity in addition to a high hydrophilicity, this powders. Then the covered soil is heated (for problem is not an issue. example, by mixing with calcium oxide), vola- We are proposing a method of securing tilizing the pollutant gases captured by adsorp- water surfaces by continuously sprinkling The cooling of the building surface also tion on activated carbon incorporated in the water onto the surfaces of buildings that will results in the reduction in the amount of elec-

sheet material. The sheet is allowed to remain be covered with a TiO2 photocatalyst (Fig. tricity consumed for air conditioning. We

undisturbed under sunlight, while TiO2 in the 28). The surface coated with TiO2 becomes measure this effect quantitatively with a real sheet decomposes the pollutants completely highly hydrophilic and plays an important house and building and observe that the by photocatalytic reaction. We have exam- role in minimizing the amount of sprinkling amount of electricity consumed is reduced by ined the efficacy of this method at an actual water required to form a water film. A very ten to several tens of percentage. Therefore,

polluted field and confirmed that the VOCs thin water layer of approximately 0.1mm TiO2-coated materials could also contribute to became completely harmless after several thickness can cover all of the highly hydro- the development of energy-saving technolo-

weeks. This purification method could be one philic TiO2 material; the water can be continu- gies. of the real environmental purification technol- ously supplied in small quantities for a vertical ogies. building. 7. Conclusions

6.4.2 Efficient water evaporation from hy- It is important to emphasize that the In this paper, an overview of the devel-

drophilic surfaces buildings and surroundings are not cooled by opment of TiO2 photocatalysis and its future water itself, but by the latent heat flux when prospects from both scientific and techno- Rapid increases in the absorption of solar water evaporates. In fact, it is often observed logical viewpoints are given. It is interesting energy and increases in energy consumption that the temperature of a wall surface is lower to note that this field has experienced major in large cities cause a temperature increase, than that of sprinkling water. The thinner developments every ten years for the last the so-called heat island phenomenon. The the water layer is, the more effectively the thirty years, namely, water photoelectrolysis former is due to the coverage of the ground building surface temperature and surrounding (Honda–Fujishima effect) in the early 1970s,

surface by artificial structures, such as build- air temperature decrease as the amount photocatalytic H2 production in the 1980s, and

ings and paved roads, and the latter is due of latent heat flux decreases to cool down the photocatalysis and hydrophilicity of TiO2 to the heat emissions from automobile traffic the thin water film. Although the margin of films in the 1990s. Based on the basic research and air conditioners. An effective method of the temperature drop depends on the color results, real industrial applications have been preventing the heat island phenomenon is and/or type of building material, as well as achieved since the end of the 1990s. In addi- increasing the area of green tract land or the the atmosphere temperature and humidity, it tion, this field is still being developed further in

the 21st century. We believe that TiO2 photocatalysis is one of the best examples how, on the time scale of tens of years, basic scientific knowledge can be developed into a techno- logical field and can produce a new industry.

Acknowledgment �� The authors gratefully thank many collab- �� orators in both academia and industry who �� have been involved in the present research. �� We also thank Dr. D. A. Tryk for carefully �� reading the manuscript.

��

Fig.27. Purification system for polluted soil utilizing solar energy and photocatalyst sheets.

18 JSAP International No.14 (July 2006) JSAP International No.14 (July 2006) 19 Invited Review Paper

Continuous sprinkling water onto TiO2-coated surfaces

Latent heat flux by water evaporation

Rainwater reservoir Fig.28. Energy-saving system using solar light and stored rainwater.

1 ) E. Keidel: Farben-Zeitung. 34 (1929) 1242. 14 ) This field remains active, with novel layered 28 ) K. Sunada, T. Watanabe and K. Hashimoto: En- 2 ) C. F. Doodeve and J. A. Kitchener: Trans. Fara- metal oxides being the favored materials. For viron. Sci. Technol. 37 (2003) 4785.

day Soc. 34 (1938) 902. example, a very efficient production of 2H and 29 ) R. Wang, K. Hashimoto, A. Fujishima, M. Chiku-

3 ) For example, S. Kato and F. Mashio: Abtr. Book O2 using deep UV light of around 290nm has ni, E. Kojima, A. Kitamura, M. Shimohigoshi and Annu. Meet. Chemical Society of Japan, 1956, been reported. T. Watanabe: Nature 388 (1997) 431. p.223. 15 ) T. Kawai and T. Sakata: Nature 286 (1980) 474. 30 ) R. Wang, K. Hashimoto, A. Fujishima, M. Chiku- 4 ) S. Kat and F. Masuo: Kogyo Kagaku Zasshi 67 16 ) L. Kavan, M. Gräzel, S. E. Gilbert, C. Klemenz ni, E. Kojima, A. Kitamura, M. Shimohigoshi and (1964) 1136 [in Japanese]. and H. J. Scheel: J. Am. Chem. Soc. 118 (1996) T. Watanabe: Adv. Mater. 10 (1998) 135. 5 ) A. Fujishima, K. Honda and S. Kikuchi: Kogyo 6716. 31 ) N. Sakai, A. Fujishima, T. Watanabe and K. Kagaku Zasshi 72 (1969) 108 [in Japanese]. 17 ) S. N. Frank and A. J. Bard: J. Am. Chem. Soc. 99 Hashimoto: J. Phys. Chem. B 107 (2003) 1028. 6 ) A. Fujishima and K. Honda: Nature 238 (1972) (1977) 303. 32 ) J. M. White, J. Szanyi and M. A. Henderson: J. 37. 18 ) Many reviews have been published. For ex- Phys. Chem. B 107 (2003) 9029. 7 ) W. Jaegermann: in The Semiconductor/ ample, M. A. Fox and M. T. Dulay: Chem. Rev. 33 ) C. Wang, H. Groenzin and M. J. Schltz: Lang- Electrolyte Interface: A Surface Science Ap- 93 (1993) 341; M. R. Hoffman, S. T. Martin, W. muir 19 (2003) 7330. proach, eds. R. E. White, B. E. Conway, J. O’M. Choi and D. W. Bahnemann: Chem. Rev. 95 34 ) T. Zubkov, D. Stahl, T. L. Thompson, D. Panayo- Bochris (Plenum Press, New York, 1996) Mod- (1995) 69. tov, O. Diwald and J. T. Yates, Jr.: J. Phys. Chem. ern Aspects of Electrochemistry, Vol.30, p.1. 19 ) R. W. Matthews: J. Phys. Chem. 91 (1987) B 109 (2005) 15454. 8 ) A. J. Nozik and R. Memming: J. Phys. Chem. 3328. 35 ) M. Miyauchi, A. Nakajima, A. Fujishima, K. 100 (1996) 13061. 20 ) T. Watanabe, K. Hashimoto and A. Fujishima: Hashimoto and T. Watanabe: Chem. Mater. 12 9 ) A. Fujishima and D. A. Tryk: in Photoelectro- in Photocatalytic Purification and Treatment of (2000) 3. chemical Conversion, ed. K. Honda (Springer, Water and Air, eds. D. F. Ollis and H. Al-Ekabi 36 ) R. D. Sun, A. Nakajima, A. Fujishim, T. Wata- Tokyo, 1999) Functionality of Molecular Sys- (Elsevier, Amsterdam, 1993). nabe and K. Hashimoto: J. Phys. Chem. B 105 tems, Vol.2, p.196. 21 ) A. Fujishima, K. Hashimoto and T. Watanabe: (2001) 1984.

10 ) N. Sato: Electrochemistry at Metal and Semi- TiO2 Photocatalysis: Fundamentals and Appli- 37 ) N. Sakai, R. Wang, A. Fujishima, T. Watanabe conductor Electrodes (Elsevier, Amsterdam, cations (BKC, Tokyo, 1999). and K. Hashimoto: Langmuir 14 (1998) 5918. 1998). 22 ) P. Sawunyama, L. Jiang, A. Fujishima and K. 38 ) M. Kamei and T. Mitsuhashi: Surf. Sci. 463 11 ) L. M. Peter: in Photoelectrochemical Kinetics at Hashimoto: J. Phys. Chem. B 101 (1997) 11000. (2000) L609. Semiconductor Electrodes, eds. R. G. Compton 23 ) P. Sawunyama, A. Fujishima and K. Hashimoto: 39 ) H. Irie, M. S. Utami, T. Shibata and K. Hashimo- and G. Hancock (Elsevier, Amsterdam, 1999) Langmuir 15 (1999) 3551. to: submitted to Chem. Commun.

Applications of Kinetic Modeling, Vol.37, p.223. 24 ) T. Watanabe, K. Hashimoto and A. Fujishima: 40 ) kb was obtained using θ and t described as the

12 ) M. K. Nazeeruddin, A. Kay, I. Rodicio, R. Proc. 1st Int. Conf. TiO2 Photocatalyst, ed. H. Al- CA and dark storage time (min), respectively. Humphry-Baker, E. Mueller, P. Liska, N. Vlacho- Ekabi, 1992. 1/θ =a–b×ln(t) poulos and M. Gräzel: J. Am. Chem. Soc. 115 25 ) A. Heller: Acc. Chem. Res. 28 (1995) 503. d(1/θ)/dt=–b×(1/t) (1993) 6382. 26 ) H. Honda, A. Ishizaki, R. Soma, K. Hashimoto

13 ) M. K. Nazeeruddin, P. Pechy and M. Gräzel: and A. Fujishima: J. Illum. Eng. Soc. (1998) 42. Combining these equations, kb can be Chem. Commun. 18 (1997) 1705. 27 ) K. Sunada, Y. Kikuchi, K. Hashimoto and A. Fu- described as a function of 1/θ:

jishima: Environ. Sci. Technol. 32 (1998) 726. –1 –1 kb(deg ·min )=–d(1/θ )/dt=b×exp((1/θ–a)/b).

18 JSAP International No.14 (July 2006) JSAP International No.14 (July 2006) 19 Invited Review Paper

41 ) Principles of Colloid and Surface Chemistry, ed. 56 ) E. Borgarello, J. Kiwi, M. Gratzel, E. Pelizzetti 70 ) T. Ohno, T. Mitsui and M. Matsumura: Chem. P. C. Hiemenz (Dekker, New York, 1986) p.307. and M. Visca: J. Am. Chem. Soc. 104 (1982) Lett. 32 (2003) 364. 42 ) L. A. Girifalco and R. J. Good: J. Phys. Chem. 61 2996. 71 ) H. Irie, Y. Watanabe and K. Hashimoto: Chem. (1957) 904. 57 ) H. Yamashita, Y. Ichihashi, M. Takeuchi, S. Kishi- Lett. 32 (2003) 772. 43 ) R. J. Good and E. Elbing: Ind. Eng. Chem. 62 guchi and M. Anpo: J. Synchrotron Radiat. 6 72 ) H. Irie and K. Hashimoto: Photocatalysis 15 (1970) 54. (1999) 451. (2004) 78 [in Japanese]. 44 ) T. Shibata, H. Irie and K. Hashimoto: submitted 58 ) I. Nakamura, N. Negishi, S. Kutsuna, T. Ihara, 73 ) R. N. Wenzel: J. Phys. Colloid Chem. 53 (1949) to J. Phys. Chem. B. S. Sugihara and K. Takeuchi: J. Mol. Catal. 161 1466. 45 ) K. Hashimoto and T. Watanabe: Hyomen (2000) 205. 74 ) A. B. D. Cassie: Discuss. Faraday Soc. 3 (1948) Kagaku 20 (1999) 27 [in Japanese]. 59 ) Y. Sakatani, K. Okusako, H. Koike and H. Ando: 11. 46 ) R. Nakamura, K. Ueda and S. Sato: Langmuir 17 Photocatalysis 4 (2001) 51 [in Japanese]. 75 ) H. Irie, T. S. Ping, T. Shibata and K. Hashimoto: (2001) 2298. 60 ) T. Ihara, M. Ando and S. Sugihara: Photocataly- Electrochem. Solid-State Lett. 8 (2005) D23. 47 ) K. Uosaki, T. Yano and S. Nihonyanagi: J. Phys. sis 5 (2001) 19 [in Japanese]. 76 ) A. Nakajima, K. Hashimoto and T. Watanabe: Chem. B 108 (2004) 19086. 61 ) R. Asahi, T. Ohwaki, K. Aoki and Y. Taga: Sci- Monatsh. Chem. 132 (2001) 31. 48 ) N. Sakai, A. Fujishima, T. Watanabe and K. ence 293 (2001) 269. 77 ) A. Nakajima, A. Fujishima, K. Hashimoto and T. Hashimoto: J. Phys. Chem. B 105 (2001) 3023. 62 ) H. Irie, Y. Watanabe and K. Hashimoto: J. Phys. Watanabe: Adv. Mater. 11 (1999) 1365. 49 ) A. Fujishima, K. Hashimoto and T. Watanabe: Chem. B 107 (2003) 5483. 78 ) A. Nakajima, K. Abe, K. Hashimoto and T.

TiO2 Photocatalysis, Fundamentals and Appli- 63 ) H. Irie, S. Washizuka, N. Yoshino and K. Hashi- Watanabe: Thin Solid Films 376 (2000) 140. cations (BKC, Tokyo, 1999). moto: Chem. Commun. 11 (2003) 1298. 79 ) A. Nakajima, K. Hashimoto, T. Watanabe, K. 50 ) R. Wang, N. Sakai, A. Fujishima, T. Watanabe 64 ) S. Sakthivel and H. Kisch: Chem. Phys. Chem. 4 Takai, G. Yamauchi and A. Fujishima: Langmuir and K. Hashimoto: J. Phys. Chem. B 103 (1999) (2003) 487. 16 (2000) 7044. 2188. 65 ) S. Sato: Chem. Phys. Lett. 123 (1986) 126. 80 ) N. Ogawa, M. Soga, Y. Takada and I. Nakaya- 51 ) T. Sugiura, T. Yoshida and H. Minoura: Electro- 66 ) H. Noda, K. Oikawa, T. Ogata, K. Matsuki and H. ma: Jpn. J. Appl. Phys. 32 (1993) L614. chemistry 67 (1999) 1234. Kamada: Nippon Kagaku Kaishi (1986) 1084 [in 81 ) K. Tadanaga, N. Katata and T. Minami: J. Am. 52 ) A. Tsujiko, T. Kisumi, Y. Magari, K. Murakoshi and Japanese]. Ceram. Soc. 80 (1997) 1040. Y. Nakato: J. Phys. Chem. B 104 (2000) 4873. 67 ) H. Irie, S. Washizuka, Y. Watanabe, T. Kako and 82 ) K. Tadanaga, N. Katata and T. Minami: J. Am. 53 ) T. Shibata, T. Watanabe, K. Hashimoto and A. K. Hashimoto: J. Electrochem. Soc. 152 (2005) Ceram. Soc. 80 (1997) 3213. Nakajima: Photocatalysis 4 (2001) 45 [in Japa- E351. 83 ) A. Hozumi and O. Takai: Thin Solid Films 303 nese]. 68 ) T. Umebayashi, T. Yamaki, H. Itoh and K. Asai: (1997) 222. 54 ) T. Shibata, H. Irie and K. Hashimoto: J. Phys. Appl. Phys. Lett. 81 (2002) 454. 84 ) A. Hozumi and O. Takai: Thin Solid Films 334 Chem. B 107 (2003) 10696. 69 ) T. Umebayashi, T. Yamaki, S. Tanaka and K. Asai: (1997) 54. 55 ) M. Miyauchi, A. Nakajima, K. Hashimoto and T. Chem. Lett. 32 (2003) 330. Watanabe: Adv. Mater. 12 (2000) 1923.

Kazuhito Hashimoto Hiroshi Irie studied Inor- Akira Fujishima received studied Chemistry at the ganic Materials Science his B.E. degree in Electro- University of Tokyo and re- and received his B.E. and chemistry from Yokoha- ceived his B.S., M.S., and M.E. degrees from Tokyo ma National University in Ph. D. degrees in 1978, Institute of Technology in 1966. He received his M.E. 1980, and 1985, respec- 1992 and 1994, respec- from the University of To- tively. From 1980 till 1989, tively. From 1994 to 1997, kyo in 1968 and his Ph. D. he served at Institute for he served at Sumitomo from the same University Molecular Science as a Metal Industries, Ltd., as a in 1971. He became a lec- technical associate and a research engineer. In 2000, turer in 1971 and an asso- research associate. He joined the University of Tokyo as he received his Ph. D. degree from the University of To- ciate professor in 1973 at Kanagawa University. Again, a lecturer in 1989. He became an associate professor at kyo in the Department of Interdisciplinary Studies. He he joined the University of Tokyo as a lecturer in 1975. the University of Tokyo from 1991 and was promoted to was a postdoctoral fellow with the University of Tokyo He became an associate professor at the University of a full professor in 1997. He served as a director of Pho- until the end of 2000 and a research staff member at Tokyo from 1978 and was promoted to full professor tochemical Conversion Project at Kanagawa Academy Kanagawa Academy of Science and Technology until in 1986. In 2003, he was awarded professor emeritus of Science and Technology from 1994 till 1999, and has 2001. He joined Research Center for Advanced Science from the University of Tokyo. Presently, he is the Chair- been a group leader of Research Laboratory for Opti- and Technology at the University of Tokyo as a research man of Kanagawa Academy of Science and Technology. cal Science at the same Academy from 1999 till now. At associate in 2001. Now, he belongs to Department of He served as the President of various scientific societies, present, he is the Director General of Research Center Applied Chemistry at the same University. He has been the Japanese Association of Photochemistry, the Electro- for Advanced Science and Technology (RCAST) at the awarded the Promotion of Research in the 19th Electro- chemical Society of Japan, and the Chemical Society of University of Tokyo. Also, he is a professor of RCAST and ceramics Research Conference of the Ceramic Society Japan. He has received several awards for his research Department of Applied Chemistry at the same Univer- of Japan. His current research interests are creations of excellence from various societies, such as the Electro- sity concurrently. He has received several awards for his high-performance photocatalytic TiO2, novel photocata- chemical Society of Japan, the Chemical Society of Ja- research excellence from various societies, such as the lytic materials, and TiO2 with new functions. pan, and the Federation of Asian Chemical Societies. In Japan Lightings Society, the Photochemical Society of Ja- 2004, he has been awarded the Japan Academy Prize pan, and the Electrochemical Society of Japan. Also, he and the Japan Prize for Science. His current research in- received the IBM Science Award from IBM Science Foun- terests include TiO2 photocatalysis and the electrochem- dation in 1997, the Nikkei Environmental Technology istry of boron-doped diamond electrodes. Award from Nihon Keizai Shinbun, Inc., in 2004, and the Prime Minister Award from Cabinet Office of Japan in

2004. His current research interests are TiO2 photocatalysis for environmental purifications, photo-functionalized molecular magnets and organic photovoltaic devices.

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