Photo-Functionalized Materials Using Nanoparticles: Photocatalysis†

K. Mori Surface Finishing R&D Center Central Research Laboratories Nihon Parkerizing Co., Ltd.*

Abstract

Among photo-functionalized materials, photocatalysts in particular have been researched and developed by many researchers in various fields. After the discovery of the Honda-Fujishima effect, their effectiveness became apparent, not only in water decomposition but also in sanitation and purification of the environment, for example, through antibacterial, self-cleaning, and deodorizing effects as well as NOx removal. Recently, novel nano-size photocatalysts, with performance superior to that of conventional types, have been developed, and examples include nitrogen-doped photocatalysts responsive to visible light and brookite-type photocatalysts with higher photocatalytic activity. Moreover, when applying a photocatalyst, it must be fixed to a substrate and blocking of the latter avoided. When photocatalysts with high photocatalytic performance are fixed to plastics, papers and textiles, the substrate can be decomposed and may be prone to peel off when exposed to irradiation with light, owing to the oxidation action of the photocatalysts. To prevent damage to the substrate resulting from photocatalytic oxidation, we have developed a photocatalyst whose particles are coated with inorganic compounds that can be blended with organic substances.

Key words: Photocatalyst, Environment, Antibacterial, Self-cleaning, Deodorizing

and environmental purification applications such as 1. Introduction antibacterial, self-cleaning and deodorizing operations, Among photo-functionalized nanoparticle materials, as well as the removal of NOx. Thus, researchers in photocatalysts in particular are expected to provide a various technical fields have been studying the poten- useful environment conserving technology. This tech- tial applications of photocatalysts. TiO2 is mainly used nical field has recently been developing rapidly as as a photocatalyst because of its strong photo-oxida- active efforts have been made for research and devel- tion power, chemical stability and safety for the human opment and commercial applications; in particular in body, and its most common form is an anatase type Japan. Following research into the utilization of photo- powder with particles measuring several to tens of catalysis, beginning with the discovery of the TiO2- nm in crystal size. However, as development in photo- based Honda-Fujishima effect, it has been reported catalyst applications progresses, demand is mounting that the photocatalysis of photo-functionalized nano- for those that are capable of effective photocatalysis particle photocatalysts is effective not only for the in weak light or in the absence of ultraviolet radiation. photodecomposition of water but also for sanitation To address this problem, laboratories and manufac- turers of ceramics materials in Japan have been

actively researching higher functions with TiO2-based photocatalytic materials. * 2784, Ohgami, Hiratsuka, Kanagawa 254-0012, Japan TEL: 81-463-55-4431, FAX: 81-463-54-7328 Consequently, novel nanoparticle photocatalysts E-mail: [email protected] with functions more advanced than those of conven- † This report was originally printed in J. Soc. Powder Tech- tional photocatalysts have been commercialized, and nology, Japan, 41, 750-756 (2004) in Japanese, before examples include visible light-responsive photocata- being translated into English by KONA Editorial Commit- tee with the permission of the editorial committee of the lysts, prepared by doping TiO2 particles with a dissim- Soc. Powder Technology, Japan. ilar element such as nitrogen, and brookite type TiO2

KONA No.23 (2005) 205 photocatalysts, featuring high photocatalytic activity. 2. Operating Principle and Types of Incidentally, to allow photocatalyst particles to be Photocatalysts applied to final products, the particles must be fixed to a substrate and at the same time deterioration of When a given photosemiconductor is irradiated the latter must be avoided. However, if a highly active with light of energy greater than the band gap energy photocatalyst material is carried by an organic sub- of the photosemiconductor, charge separation occurs. strate, such as resin, paper or cloth, the oxidizing Then, utilizing the electrons and holes generated by power of the photocatalyst will lead to decomposition the charge separation, the photocatalyst triggers an of the substrate, causing the photocatalyst particles to oxidation-reduction reaction. In particular, TiO2, with come off. To prevent this problem, a unique powder a relatively large band gap energy of 3.0 to 3.2 eV as photocatalyst to be mixed with organic material has shown in Fig. 11), can achieve a powerful oxidation- been developed, wherein the particles of this photo- reduction reaction with the ultraviolet rays present in catalyst are of a core-shell structure; prepared by allow- our living environment. ing inert particles of silica, hydroxyapatile or similar It is known that active oxygen and radical species to be deposited on the surface of photocatalyst parti- existing in the presence of oxygen and water take part cles. Photocatalysts with a surface layer of inert parti- in the oxidation-reduction reaction, and that various cles are generally high-performance photocatalysts. It functions of the photocatalysts are realized by this 2) is also possible to enhance the adsorption of bacteria reaction. Various types of TiO2 are listed in Table 1 and NOx by choosing appropriate inert particles to be as representative photocatalysts. Ordinary TiO2 for deposited on the surface of TiO2 particles. pigment is of the rutile type and its crystal size mea-

V 2.0

GaP ZrO2 1.0 Si KTa0.77Nb0.23O3 CdS SrTiO3 KTaO3 CdSe TiO2 ZnO 2.2eV 5.0eV 1.1eV Nb2O5 Fe2O3 0 H2/H2O WO3 SnO2 Conduction band

1.0 2.5eV 3.4eV 3.2eV 1.7eV 3.2eV 3.0eV 3.4eV 3.2eV 2.2eV O2/H2O

2.0 2.5eV 3.5eV

3.0 Potential (vs. Standard Hydrogen Electrode)

4.0 pH0 Valence band

Fig. 1 Energy structures of various photosemiconductors1)

Table 1 Types and physical properties of titanium oxide

Properties Rutile Anatase Brookite

Crystalline form Tetragonal system Tetragonal system Orthogonal system

Density (g/cm3) 4.27 3.90 4.13

Refractive index 2.72 2.52 2.63

Mohs’ hardness 7.07.5 5.56.0 5.56.0

Permittivity 114 48 78

Melting point (°C) 1825 Transformation to rutile Transformation to rutile

206 KONA No.23 (2005) sures several hundreds of nm in size. TiO2 for photo- 0.1 to several µm, they still boast excellent functions catalysts, meanwhile, is of the anatase type with very by controlling their crystal structure. small crystal size, measuring several to 20 nm. For this reason, TiO for photocatalysts is more trans- 2 3. Functions and Features of Photocatalysts parent than conventional TiO2 for white pigment, and a colorless transparent photocatalytic layer may be 3.1 Self-cleaning, hydrophilic, and antibacterial obtained if the layer thickness measures 1 µm or less. characteristics Various manufacturing processes for titanium oxides Titanium oxide particles decompose organic mat- for photocatalysts have recently been investigated, ters when irradiated with ultraviolet rays. This fact and the rutile, brookite and amorphous types (for pre- has long been known as a choking phenomenon that cursors) of TiO2 for photocatalysts have been devel- occurs with a paint containing a TiO2 pigment. During oped, other than the anatase type. a self-cleaning process with a photocatalyst, contami- Studies on photocatalysts began with research on nants on a substrate are photodecomposed, wherein the decomposition of water into hydrogen and oxygen radicals and active oxygen generated by ultraviolet by irradiation with light, and are still underway. No irradiation on a photocatalyst decompose the organic photocatalyst for the photodecomposition of water contaminants into carbon dioxide, thereby allowing has yet been commercialized because of a lack in effi- the surface of the substrate to remain clean. ciency, high production cost, etc. Nevertheless, the It has been reported that a positive hydrophilic progress attained in photocatalysts to date is based on effect appears when a TiO2 photocatalyst is irradiated the results of these studies. Various photocatalysts for with ultraviolet rays. This characteristic is widely water decomposition have been disclosed and devel- applied to glasses, mirrors and building materials. oped, many of which utilize TiO2, and the examples of If a TiO2 photocatalytic coating material is used in which comprise TiO2 particles carrying a metal such an outdoor location exposed to rainwater, organic con- as Pt or Rh3) 4) as shown in Fig. 2 or an oxide such as taminants on the surface of coating are decomposed

RuO2. by light irradiation and the residual inorganic parti-

Recently, there are many studies concerning TiO2 cles are readily washed away by the rainwater, hence photocatalysts that feature higher efficiency and that the coated surface exhibits the expected self-cleaning are capable of responding to visible light. Certain exam- effect. Fig. 3 schematically illustrates a self-cleaning ples of such photocatalysts already disclosed include model with an outdoor application, and Fig. 4 shows a TiO2 photocatalyst, whose response to visible light an example of a self-cleaning effect with an exterior 5) is improved with TaON , a TiO2 photocatalyst that wall consisting of tiles coated with a TiO2 photocat- 6) utilizes In1XNiXTaO4 , and a TiO2 photocatalyst, alytic material. whose quantum yield is improved with La-doped Radicals and active oxygen generated by the activ- 7) NaTaO3 . Though these examples have photocata- ity of a TiO2 photocatalyst are effective in decompos- lysts with relatively large particle sizes, ranging from ing and preventing the propagation of bacteria and fungi. Because of their small selectivity against bacteria species and their ability to decompose the toxins

produced by bacteria, TiO2 photocatalysts have been increasingly used for interior finishing materials in hospitals and medical equipment. Pt R UV rays On the other hand, the intensity of ultraviolet rays available indoors is one digit lower compared with Red e that available outdoors, as shown in Fig. 5, and it is Conduction band difficult to offer sufficient photocatalytic effect with ultraviolet rays in a room alone. In addition, there are TiO 2 many places where the intensity of ultraviolet rays is lower than expected such as in cars that are Valence band equipped with UV-cut glasses. To operate effectively R’ h in these locations, novel photocatalysts responsive to Ox visible light, and capable of performing within the visible light spectrum in the region of 500-600 nm wave-

Fig. 2 Structure of Pt-carrying TiO2 particles length as well as the UV band have been developed

KONA No.23 (2005) 207 Sunlight Rainwater Organic fouling

Inorganic particles

Fig. 3 Model of fouling removal by self-cleaning effect (outdoor)

Uncoated TiO2 photocatalyst coating

Fig. 4 Self-cleaning effect of wall tiles coated with TiO2 photocatalyst (1 year after installation: top joint is filled with silicone sealant)

and commercialized. It should be understood from from the presence of impurities by doping a TiO2 pho- the spectra of sunlight 8) in Fig. 6 that the energy tocatalyst with another element such as nitrogen or efficiency of photocatalysts under sunlight will be sulfur9) 10). The photocatalyst thus obtained is yel- improved with expansion of the effective wavelength lowier compared with conventional anatase-type pho- to 500-600 nm. tocatalysts, and capable of absorbing a portion of the Visible light-responsive photocatalysts can be pro- visible light. Usually, the crystal size of the visible light- duced, for example, through change in the band struc- responsive photocatalysts is designed to be somewhat ture with the addition of new energy levels resulting larger than that of conventional anatase-type photocat-

208 KONA No.23 (2005) Inside a car situated outdoors (directly below windshield)

Indoor (below fluorescent lamp)

Outdoor (fine weather cloudy weather)

0 0.5 1 1.5 2 2.5 3 3.5

UV ray intensity (µW/cm2)

Fig. 5 Amounts of UV light in living environments

Light energy 70% Heat energy 30%

Outer space

Ground Sunlight energy

Visible light UV ray IR ray

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

Wavelength (µm)

Fig. 6 Sunlight spectra

alysts. One reason for this arrangement is to avoid a 11 nm) layer has been virtually completely decom- shift of the optical absorption edge to a shorter wave- posed following irradiation with a fluorescent lamp. length side, resulting from the quantum size effect Furthermore, a fingerprint was printed on a glass occurring when the crystal size is less than 10 nm. In substrate coated with a visible light-responsive photo- order to investigate the photocatalytic activity of a vis- catalyst and decomposed by irradiation with a fluores- ible light-responsive photocatalyst, a dye (Methylene cent lamp. The result is visually illustrated in Fig. 8. Blue) was allowed to be adsorbed onto a layer of Components in fingerprints comprise organic contam- visible light-responsive photocatalyst, and was then inant matters, such as fatty acids, as well as inorganic decomposed with a white fluorescent lamp. The result contaminant particles, such as those of salts. The is shown in Fig. 7. From this illustration, we can pre- organic contaminants, which form the major con- sume that decolorization is minor on the irradiated stituents of fingerprints, were found to have been area (upper half of the photo in the left) of the anatase decomposed, and it was visually apparent that the fin-

TiO2 (diameter: 7 nm) layer and that the dye in the gerprints had disappeared. In addition, the antibac- irradiated area (upper half of the photo in the right) of terial performance of the visible light-responsive the visible light-responsive photocatalyst (diameter: photocatalyst (a property important in indoor appli-

KONA No.23 (2005) 209 Anatase type (conventional) Visible light-responsive type

Fig. 7 Dye decomposing ability of visible light-responsive photocatalyst coating layer (after 12 hours of irradiation onto the upper half with a white fluorescent lamp)

Before irradiation After irradiation

Fig. 8 Fingerprint decomposing ability of visible light-responsive photocatalyst Evaluated with an optical microscope [magnifying power: 50] Substrate: glass (12 hours of irradiation with a white fluorescent lamp, 3,000 lx)

cations) was tested and the results are summarized commercialized applications of antibacterial visible in Fig. 9. As can be understood from this figure, light-responsive photocatalyst include window blinds antibacterial performance is obtained with a white flu- and wallpapers. To be able to form a photocatalyst orescent lamp rated at about 1000 lx. Examples of layer on the surface of these resin-coated products, a

210 KONA No.23 (2005) honeycomb or porous substrate; hence allowing UV 106 rays to penetrate their deeper portions. Currently, 105 these filters are required to have higher performance

104 under weak light, to be more responsive to visible light and to have a boosted decomposition ability 103 against various indoor pollutants including VOC.

102 Fig. 10 summarizes the acetaldehyde decomposition Non-irradiated rate of a visible light-responsive photocatalyst (mean 10 Irradiated diameter: 11 nm) exposed to sunlight transmitted 1 through a UV-cut glass (car windshield), by compar- Number of live bacteria (bacteria/piece) Immediately 24 hours later ing the acetaldehyde decomposition rate with that of a after inoculation conventional anatase type photocatalyst (mean diameter: 7 nm). Fig. 9 Antibacterial performance of visible light-responsive coat- Furthermore, interior finishing materials and tex- ing material (MRSA) Light irradiation: white fluorescent lamp, 1,000 lx tile products capable of air purification can be man- Method: film contact method ufactured by blending photocatalyst particles into Executed by: Japan Food Research Laboratories organic materials such as wallpapers, shouji (Japanese paper screen doors) and floor panels during their production processes or by finishing the interior of build- ings with a resin coating containing photocatalysts. With this type of application, however, the substrate transparent primer coating agent, composed princi- may be gradually degraded owing to irradiation with pally of inorganic components, is applied to form an light. Moreover, the strength of the substrate can intermediate layer to improve durability, and then a decrease or the adhesion of the photocatalyst-contain- coating solution, containing photocatalyst particles, is ing material to the substrate may decrease, rendering applied to form the top layer. the material prone to drop off. To address this problem, there have been new developments in photocata-

3.2 Fog-proofing lysts: muskmelon-like TiO2 particles produced by

Since the expression of super-hydrophilicity with coating TiO2 photocatalyst particles with inert porous

TiO2 photocatalyst was presented in 1997 by silica and thus preventing them from coming into 11) 14) Hashimoto et al. , research into this feature has contact with the substrate and surface-coated TiO2 been active. Consequently, it has been increasingly applied to fog-proofing and self-cleaning applications for mirrors, including road mirrors (curve mirrors) and door mirrors on cars, as well as window glass panels. Since the hydrophilicity of TiO2 photocata- 2.5 lysts is more positively maintained by the addition of

SiO2 or a more porous structure of TiO2 particles, 2 improvement in the composition and layer forming method for TiO2 photocatalysts is now underway. 1.5

Additionally, in order for TiO2 photocatalysts to be effective under weak indoor lighting, tungstic oxide 1 12) may be joined with TiO2 , or TiO2 surface may be Visible light- provided with a nanoporous structure13) by a pho- 0.5 responsive type Conventional type toetching technique. log (acetaldehyde concentration) 0 01020304050 3.3 Air purification Time (min) Air purification is one example of the most advanced applications of photocatalysts. For example, photocatalysts are used in deodorizing filters in air-purifiers Fig. 10 Acetaldehyde decomposition rate in a car Fine weather: under sunlight, sample size: 100 cm2 incorporating UV lamps to eliminate aldehyde or VOC (A 3-L glass cell was placed directly below the wind- in indoor air. Such filters are used in the form of a shield.)

KONA No.23 (2005) 211 particles, prepared by depositing hydroxyapatite on was irradiated with UV rays with black light and the 15) the surface of TiO2 particles . These products are resin decomposition inhibition effect provided by the already commercially used for plastics and textile coated TiO2 photocatalyst particles was compared products and electron-microscopic photos of the latter with that of conventional anatase-type TiO2 photocata- are shown in Figs. 11 (a) and (b). Note that with lyst particles. The result is summarized in Fig. 12. these TiO2 photocatalyst particles types, individual particles are not fully covered and their coating layers are gas-permeable. These coated TiO photocatalyst particles are Uncoated TiO2 2 Muskmelon-like formed through the aggregation of primary TiO2 par- substance-coated TiO2 ticles whose particles size measures from several nm Hydroxyapatite-coated TiO2 to 20 nm and a coating of aggregated particles. Con- 100 sequently, despite their relatively large particle size, they can be treated like pigment particles. 20% of each 99.5 of these particle types was mixed in an acrylic resin, 99 and each mixture was applied to a glass substrate to a thickness of 100 µm. Subsequently, each specimen 98.5

Residual resin (mass %) 97.5

97 Before irradiation After UV irradiation

Fig. 12 Resin substance decomposition inhibition effect with

coated TiO2 photocatalyst particles

3.4 Elimination of NOx Since the report concerning the successful elimination of low-concentration NOx in outdoor environments by Dr. Ibusuki, Dr. Takeuchi et al., expectations for photocatalysts as environmental cleaning materials a) Muskmelon-like substance-coated TiO2 have been mounting, and air purification-capable paving materials and soundproof wall materials for roads have been developed, while research efforts for the application of NOx-eliminating equipment in road tunnels are underway.

A TiO2 photocatalyst oxidizes NO into NO2 and

eventually into NO3, hence removing NO from the air. Since the nitric acid ions generated are adsorbed and accumulated in the surface of the photocatalyst, the latter must be rinsed at regular intervals with water to prevent deterioration in performance due to an increase in adsorbed nitric acid ions. When used

in outdoors, a TiO2 photocatalyst will be cleaned by sunlight and rainwater, and its NOx elimination effect will be guaranteed without any maintenance work b) Hydroxyapatite-coated TiO2 required. One potential problem when NO is eliminated with

Fig. 11 Surface-coated photocatalyst particles a TiO2 photocatalyst is that if the NO2 adsorption

212 KONA No.23 (2005) power of the photocatalyst type is low, the reaction odes, and utilization in dye-sensitized solar cells. only progresses to NO2 (an intermediate product) Photocathode corrosion prevention is a unique stage and the resultant NO2 concentration on the technique where if a photocatalyst layer consisting of

TiO2 photocatalyst will be higher than before adsorp- a n-type semiconductor forms on the surface of metal, tion. The known methods and means to avoid this the potential on the metal surface drops as the sur- problem include a TiO2 photocatalyst particle type face is irradiated with light, promoting a non-sacrifi- that carries another metal or metal oxide or a method cial cathode corrosion prevention effect21). Currently, involving mixing the TiO2 photocatalyst particles with research is in progress for the commercial utilization adsorptive particles such as active carbon16) or blend- of this effect. ing them into mortar. The dye-sensitized solar cells (Graetzel cells) are wet-type solar cells that use electrodes comprising

3.5 Water purification and soil decontamination photocatalyst particles, such as TiO2 particles, that Various researches have been conducted for the have adsorbed dye. This solar cell type has been purification of water with titanium oxide photocata- developed as a type of next-generation solar cell; lysts, wherein the examples of water being purified since it will realize higher efficiency and lower cost. include effluent water from factories and sewage and environmental water such as ground and river water. 4. Conclusion Much research is associated with low concentration chlorinated organics in effluent water from factories As discussed above, photocatalysts boast many and in groundwater. Such research has helped to unique functions among photo-functionalized materi- prove that titanium oxide photocatalysts can decom- als and satisfy the environmental conservation and pose endocrine disrupting chemicals such as bis- energy saving requirements. Therefore, they will be phenol A17), meaning commercial water purification utilized in a diversity of industrial fields familiar to us. applications involving this type of photocatalyst are The current market size for photocatalysts in Japan expected, although progress in water purification is estimated at tens of billions yen per year, and will applications is lagging behind that of air purification dramatically expand as photocatalyst technology is equivalents. This is because the reaction efficiency of increasingly recognized as a typical key technology; a titanium oxide photocatalyst decreases in water and indispensable for environmental conservation and the performance of a titanium oxide photocatalyst energy saving following progress in research and easily deteriorates when the surface of photocatalyst development work associated with this technology. particles becomes fouled up. To overcome these We hope this paper will assist in the further develop- problems, certain methods have been commercial- ment and creation of a new market for photo-function- ized, wherein air is bubbled into groundwater or soil alized materials. water and volatile organic chemicals, which are taken into the air phase, are eliminated using a photocata- References lyst filter similar to an air purifying photocatalyst fil- 18) ter . The examples of photocatalysts developed for 1) A. Fujishima, K. Hashimoto, T. Watanabe: “TiO2 Photo- water treatment include a water purifying substance catalysis Fundamentals and Applications,” p. 128, BKC, that is prepared by allowing an inorganic adsorbent Inc. (1999) consisting of silica gel particles carrying photo- 2) M. Kiyono: “Sankatitan,” Gihodo-shuppan, Japan, p. 52 (1991) catalyst particles to enhance pollutant adsorbing 3) S. Sato, J. M. White, Chem. Phys. Lett., 72, p. 83 (1980) 19) performance . This substance, however, has the dis- 4) K. Yamaguchi et al., JCF Faraday Trans. 1, 81, p. 1237 advantage of the tendency of the photocatalyst parti- (1985) cles to peel off or be damaged by water flow. To solve 5) G. Hitoki, A. Ishikawa, T. Takata, J. N. Kondo, M. Har, 20) this problem, a high-strength titania fiber material and K. Domen: “Ta3N5 as a Novel Visible Light-Driven boasting sufficient strength and durability, and capa- Photocatalyst (λ600 nm),” Chem. Lett., p. 736 (2002) ble of withstanding a high-speed water flow, has been 6) Z. Zou, J. Ye, H. Arakawa: “Photocatalytic behavior of a new series of In M TaO (MNi, Cu, Fe) photocata- developed. 0.8 0.2 4 lysts in aqueous solutions,” Catal. Lett., 75, 209 (2001) 7) H. Kato and A. Kudo: “Highly Efficient Water Splitting 3.6 Other applications into H2 and O2 over Lanthanum-Doped NaTaO3 Photo- Other potential applications for the photocatalyst catalysts with High Crystallinity and Surface Nanos- powder include corrosion prevention by metal cath- tructure,” J. Am. Chem. Soc., 125, 3082 (2003)

KONA No.23 (2005) 213 8) A. Fujishima: Kagaku-sosetsu “Muki-Hikarikagaku,” low-concentration NOx from ambient air by the photo-

No. 39, Gakkai Shuppan Center, Japan, p. 98 (1983) catalytic oxidation of NOx using TiO2 active carbon mix- 9) R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Tage: ture,” Proc. Fukuoka Int. Symp. ’90, pp. 253-254 (1990) “Visible-Light Photocatalysis in Nitrogen-Doped Tita- 17) I. Ando, Y. Ohko, T. Nakashima, Y. Kubota, T. Yamamura, nium Oxides,” SCIENCE, Vol. 293, 269 (2001) T. Tatsuma, A. Fujishima: “Degradation of bisphenol-A

10) Y. Sakatani, K. Okusako, H. Koike, H. Ando: “Develop- in water by TiO2 photocatalysis,” Proceedings of the 6th

ment of a Visible Light Responsive TiO2 Photocatalyst,” Symposium on Recent Development in Photocatalysis Kaiho Hikari Shokubai, Vol. 4, p. 51 (2001) (Photo-Functionalized Materials Society), p. 134 (1999)

11) T. Watanabe: “Super-hydrophilic TiO2 Photo-Catalyst 18) H. Tomioka, H. Yamazaki, K. Okamoto, K. Ito, M. and Its Application,” Bull. Chem. Soc. Jpn., 31, 837-840 Murabayashi: “Purification of underground water conta- (1996) minated with organic chloride by the gas-phase photo- 12) M. Miyauchi, A. Nakajima, K. Hashimoto, T. Watanabe: catalytic reaction,” Proceedings of the 6th Symposium “A Highly Hydrophilic Thin Film under 1 µW/cm2 UV on Recent Development in Photocatalysis (Photo-Func- Illumination,” Adv. Mater, 12, 1923 (2000) tionalized Materials Society), p. 122 (1999) 13) T. Shibata, A. Nakajima, T. Watanabe, K. Hashimoto: 19) Y. Zhang et. al.: “Fixed-Bed Photocatalysts for Solar

“Sensitization for photo-induced hydrophilicity of TiO2,” Decontamination of Water,” Environ. Sci. Technol., 28, Kaiho Hikari Shokubai, Vol. 4, p. 45 (2001) 435 (1994) 14) N. Yamashita, K. Hayashi, S. Imaizumi, H. Nimura, T. 20) H. Yamaoka: “Development of Strong Photocatalytic Umemura, H. Sakurai, M. Takahashi: “Development of Fiber and Environmental Purification,” Proceedings of muskmelon-like photocatalyst,” JETI, Vol. 49, No. 2, p. the 9th Symposium on Recent Development in Photo- 97 (2001) catalysis (Photo-Functionalized Materials Society), p.

15) T. Nonami et al: “Apatite formation on TiO2 photocata- 206 (2002) lyst in a pseudo body solution,” Material Research Bul- 21) T. Imokawa, R. Fujisawa, T. Shinohara, S. Tsujikawa, A. letin, 33, 125-131 (1998) Suda: Proceedings of the 39th Japan Conference on 16) T. Ibusuki, S. Kutsuna, and K. Takeuchi: “Removal of Corrosion and Protection, p. 277 (1992)

Author’s short biography

Kazuhiko Mori K. Mori graduated in industrial chemistry from the Nagoya Institute of Technol- ogy, Japan, in 1980 and has been developing material surface treatments for over 20 years. His expertise spans a range of surface treatment and coating technologies, including photocatalytic coating, ceramic composite plating, and sol-gel coating. He is currently a research manager in surface treatment and functionalized coating technologies in laboratories at Nihon Parkerizing Co., Ltd.

214 KONA No.23 (2005)