14.1.1 Photocatalysis

The term photocatalysis implies that light is acting as a catalyst in a reaction, which is not the case.^12,13 However, the term photocatalysis will always be used to describe the process that semiconductor materials such as TiO₂ undergo when irradiated by light of a certain wavelength. It is a term that implies photon-assisted generation of catalytically active species.

In photocatalysis light of energy greater than the band gap of the semiconductor excites an electron from the valence band to the conduction band (Fig. 14.1) by the following reaction:.

Scheme 14.1

The excited electron leaves a positive hole in the valence band and these charge carriers can migrate to the catalyst surface and initiate redox reactions on absorbents such as water and oxygen. Positive holes generated by light become trapped by surface adsorbed H₂O. The H₂O becomes oxidized by h⁺VB, producing H⁺ and OH• radicals (Eq. [14.1]), which are extremely powerful oxidants (Table 14.1). The hydroxyl radicals subsequently oxidize organic species from the surrounding environment to CO₂ and H₂O (Eq. [14.3])¹⁴ and in most cases these are the most important radicals formed in TiO₂ photocatalysis.

Electrons in the conduction band can be rapidly trapped by molecular oxygen adsorbed on the particle. Trapped molecular oxygen will be reduced by excited electrons to form superoxide (O^2–•) radicals (Eq. [14.2]) that may further react with H⁺ (Eq. [14.4]), to generate peroxide radicals (•OOH) and hydrogen peroxide, H₂O₂ (Eq. [14.5]).^15,16

Recombination of the excited electron and the photo-generated hole is a major limitation in semiconductor photocatalysis as it reduces the overall quantum efficiency of the photocatalyst because of the high recombination rate of photoinduced electron–hole pairs at the surface of the photocatalyst.¹⁷ The photocatalytic efficiency can be significantly enhanced if recombination is reduced. Doping with ions,^18–20 heterojunction coupling²¹ and nanosized crystals²² have all been reported to promote separation of the electron–hole pair, reducing recombination and therefore improving the photocatalytic activity of the semiconductor material.

When recombination occurs, the excited electron reverts to the valence band without reacting with adsorbed species (Scheme 14.2).²³ Radiation may be emitted when an excited electron recombines with the valence band. As such, photoluminescence may be successfully employed to monitor recombination and, in general, low intensity photoluminescence signals indicate lower recombination rates.¹⁷

Scheme 14.2

[14.1]

[14.2]

[14.3]

[14.4]

[14.5]

[14.6]

[14.7]

Equations [14.1]–[14.7] schematize the whole process.^14,24,25 Hydroxyl radicals produced by the photocatalytic process will oxidize the majority of volatile organic compounds (VOC) until complete mineralization. Recombination competes strongly with the photocatalytic process. It may occur on the surface in the bulk and is in general catalysed by impurities, defects, or all factors which introduce bulk or surface imperfections into the crystal.²⁶ The fact that the process can only be initiated by UV light is also a limiting factor in the process. It is desirable to produce a photocatalyst that can be activated by visible light to make full use of the solar spectrum.

14.1.2 Practical use of photocatalysts for tiles and glasses

A major drawback in commercializing conventional TiO₂ photocatalysts for community care applications is the large band gap. Titanium dioxide can only be activated upon irradiation with a photon of light < 390 nm, limiting its use on commodity materials such as tiles and glasses.^27–29 Ultraviolet light makes up only 3–5% of the solar spectrum, whereas the spectrum consists of ~ 40% visible light. Therefore, in order to utilize TiO₂ to its full potential it is necessary to decrease the band gap size facilitating visible light absorption.

Non-metal doping has shown great promise in achieving visible light activated photocatalysis, with nitrogen being the most effective dopant. Asahi et al. were the first to show visible light absorption through N doping. They reported nitrogen-doped TiO₂ promoted photocatalytic activity up to λ = 520 nm.³⁰ The nitrogen substitutional doping of TiO₂ was initially claimed as a method for narrowing the band gap by exclusively changing the valence band structure; fine electronic details of this are, however, under discussion. Asahi et al. claimed that the presence of nitrogen narrows the band gap of TiO₂, thus making it capable of performing visible light-driven photocatalysis.³⁰ However, Ihara et al. suggested that it is the oxygen vacancies that contributed to the visible light activity, and the doped nitrogen only enhanced the stabilization of these oxygen vacancies.³¹ They also confirmed this role of oxygen vacancies in plasma-treated TiO₂ photocatalysts.³¹

In addition, that the structural oxygen vacancy caused visible light photocatalytic activity was also reported by Martyanov et al.³² Currently there appears to be some agreement on the mechanism of nitrogen-doped visible light absorption explained by Irie et al.³³ and Nakamura et al.³⁴ They explained that TiO₂ oxygen lattice sites substituted by nitrogen atoms form an occupied midgap (N-2p) level above the (O-2p) valence band. Irradiation with UV light excites electrons in both the valence band and the narrow (N-2p) band, but irradiating with visible light only excites electrons in the narrow (N-2p) band.^33,34

It has also been shown that F doping improves both UV and visible light photocatalytic activity. However, their mechanisms are still under discussion. Previous studies have shown that N-F-co-doped TiO₂ powders demonstrated excellent photocatalytic activity no matter what kind of light source was used. This seems to be a consequence of the perfect combination of some beneficial effects induced by both N and F dopants.³⁵

Carbon, phosphorous and sulphur have also shown positive results for visible light responsive TiO₂.³⁶’³⁷ The non-metal dopants effectively narrow the band gap of TiO₂ (< 3.2 eV).³⁸ Change of the lattice parameters and the presence of trap states within the conduction and valence bands from electronic perturbations give rise to band gap narrowing.²⁷ Not only does this allow for visible light absorption, but the presence of trap sites within the TiO₂ bands increases the lifetime of photoinduced charge carriers.

Doping of TiO₂ with transition metals such as Cr, Co, V and Fe has extended the spectral response of TiO₂ well into the visible region, also improving photocatalytic activity.^27,39 However, transition metals may also act as recombination sites for the photoinduced charge carriers, thus lowering the quantum efficiency. Transition metals have also been found to cause thermal instability to the TiO₂ nanomaterials.¹⁵ Even though a decrease in band gap energy has been achieved by many groups through metal doping, photocatalytic activity has not been remarkably enhanced because the metals introduced were not incorporated into the TiO₂ framework. In addition, metals remaining on the TiO₂ surface cover photo reaction sites.

14.1.3 Applications of titanium dioxide for tiles and glasses

Despite the great promise shown by the self-cleaning abilities of TiO₂ surfaces, there are certain limitations. Because TiO₂ is a wide band gap (3.2 eV) semiconductor material, the self-cleaning process can only be initiated by light of wavelength ~ 390 nm or less. This causes substantial reduction in the efficiency of the product as light of such energy, ultraviolet light (UV), makes up only 3–5% of the solar spectrum. Therefore, in order to improve the efficiency of these materials, it is necessary to either reduce the band gap or to introduce mid-band gap energy levels that act as a stepping stone between the energy levels, facilitating visible light absorption.

Titanium dioxide can be incorporated into construction materials such as glasses and tiles to produce anti-bacterial surfaces.^12,13 It can be used to coat hospital surfaces and provide anti-bacterial protection against harmful bacteria such as E. coli and MRSA.⁴⁰ By applying TiO₂ to roadside partitions and lights, the surfaces can be kept clean while having the added advantage of reducing harmful exhaust gases such as NO_x and VOCs.

14.1.4 Commercial photocatalytic tiles and glass

Pilkington Glass have utilized titanium dioxide technology to develop a range of self-cleaning windows known as Pilkington Activ.⁴¹ Self-cleaning glass clearly displays the benefits of titanium dioxide’s self-cleaning and superhydrophilic properties. In 2001 Pilkington started to develop photocatalytic glass, which has been available in the market since then. The Pilkington Activ was successfully trialled in many countries such as the UK, Ireland and North America. The product is currently available worldwide and is applied in many commercial and private buildings. This is one of the most successful photocatalytic products available in the market. The coating is composed of a 15 nm layer of titanium dioxide deposited by chemical vapour methods.

Saint Gobain Glass’s Bioclean is another significant photocatalytic glass product available in the market. These glasses were produced by coating a transparent layer of photocatalytic or hydrophilic material on the glass. An Italian manufacturer, Gambarelli, has developed photocatalytic tiles using titanium dioxide ceramic particles.

TOTO’s (Japan) HYDROTECT tiles, with a photocatalytic antibacterial function, has been available since 1993. HYDROTECT finds applications in TOTO’s own tile building materials, paints, and coatings. They also licensed the technology to over 100 companies in Japan and overseas. About 270 patents have been registered in the photocatalytic technology domain by TOTO Ltd.⁴² Their representative products are white ceramic tiles for exterior walls and home environments. They are fabricated by spraying a liquid suspension containing TiO₂ powder or gel on the surface and then heated to 600–800 °C. Through the heat treatment, the TiO₂ is sintered and strongly attached to the tile surface, forming a micrometer thick layer.⁴³

Turkish-based ceramic tile manufacturer VitrA has launched a photoactive tile (http://www.vitratilescpd.com/faq/whats-photoactivity-2) to inhibit bacterial growth and eliminate microorganisms through oxidation. Air pollutants such as oxides of nitrogen and sulphur can also be removed using this technology.

14.2.1 Temperature stability of the photocatalyst

One of the key issues with using titanium dioxide as a photocatalytic material in tiles and glass is the transition of the photocatalytically active phase, anatase, to the non–photocatalytically active phase, rutile, which typically occurs at 600–700 °C. This is 300 °C below the typical processing temperatures for tiles.

The retention of a high stable anatase phase with visible light active photocatalytic properties up to a temperature as high as ≥ 1000 °C has not yet been published or patented. Many proposed innovative and commercial applications for photocatalytically active stable titania-coated materials such as bathroom tiles, sanitary wares, and self-cleaning glass for the control of organic contaminants require high processing temperatures and hence high-temperature stability.2.^4,17 High consumer demand is projected for these materials.

The second area to investigate is the stability of the anatase at high temperature. This can be investigated in many ways. Previous X-ray photoelectron spectroscopy (XPS) studies by the authors’ group have indicated that only 0.3 at% F of doping has achieved when 16 mole time (1:16) F precursors trifluoroacetic acid (TFA). This 0.3% F doped sample showed stability up to 900 °C and a small increase in dopant content is expected to give even higher temperature stability. Adding dopants above 0.5% by addition of high-temperature stable dopant precursors should be developed. Therefore both the levels of dopants (e.g., N-F, S-F, S-N, C-F, C-N, C-F, etc.) and various precursors (e.g., TFA, TiCl₄) should be looked at and incorporated into the titania matrix.

The solid state chemistry at high temperature can also be tuned by changing precursors or employing various annealing schedules such as step heating without grain growth. Step annealing was previously employed successfully to sinter Y₂O₃ materials without significant grain growth. A similar heating strategy can be designed to produce anatase materials with less or little grain growth at higher temperatures (≥ 1000 °C) thus preventing rutile formation. New sintering techniques such as ramp-sustain-decay can be applied. The development of photocatalysts (with lower band gap) which can be activated under visible light (> 400 nm) is desired in order to make use of the main part of the solar spectrum, and to extend their applications to room interiors where there is relatively poor lighting illumination.

The anatase-to-rutile phase transformation in TiO₂ is an area of both scientific and technological interest.^44,45 The anatase-to-rutile transformation (ART) is kinetically defined and the reaction rate is determined by parameters such as particle shape/size,⁴⁶ purity,⁴⁷ source effects,⁴⁸ atmosphere⁴⁹ and reaction conditions.⁵⁰ It is agreed that the mechanism for phase transformation of titania is one of nucleation and growth.^51,52 Anatase nanocrystals coarsen, grow and then transform to rutile only when a critical size is reached.⁵³ Therefore, phase transformation is dominated by effects such as defect concentration,⁵⁴ grain boundary concentration⁵⁵ and particle packing.⁴⁹ Rutile is the thermodynamically stable phase, while anatase and brookite are both metastable, transferring to rutile under heat treatment at temperatures typically ranging between 600 and 700 °C.⁵⁶ Anatase is widely regarded as the most photocatalytically active of the three crystalline structures.^57,58

The generally accepted theory of phase transformation is that two Ti–O bonds break in the anatase structure, allowing rearrangement of the Ti–O octahedra, which leads to a smaller volume, forming a dense rutile phase. The removal of oxygen ions, which generate lattice vacancies, accelerates the transformation. The transition follows first order kinetics, with an activation energy of ~ 418 kJ mol^− 1. The breaking of these bonds can be affected by a number of factors, including the addition of dopants, synthesis method and thermal treatment.⁵²

Table 14.2 presents the results of a patent search using the relevant terms, for photocatalytic tiles and glass. There are many patents which relate to high temperature stable anatase titania such as WO2009061707 (A1), but they do not indicate visible light active photocatalytic properties or their intended use in building materials, for example. Most disclosures relate to a process for making titanium dioxide in an anatase crystalline form which is stable at temperatures above 1000 °C. Another invention (CN 101205083 (A)) relates to a technology for preparing a nanometre-scale material, in particular to a preparation method in which, after a high temperature (above 700 °C) process, nanometre crystal titanium oxide with small crystal size and high surface area, the main anatase phase can still be retained, something that has proved hard to obtain. The invention can realize large-scale preparation at high temperatures (700–1,500 °C) and produces the nanometre crystal titanium oxide which is characterized by the main anatase phase, the small crystal size, the high surface area, high crystallinity, low surface state distribution, etc. The nanometre titanium oxide grains prepared by the invention are expected to be applied in photocatalysis fields such as a sensitized solar energy battery, and hydrogen prepared by water splitting.

There are not many reported patents on materials that are stable, antibacterial and can be activated by visible/room light. The CREST-DIT, Ireland patented materials are the first to claim anti-microbial visible light photocatalytic action and it is believed that they could represent the best available technology to deal with the persistent problems of E. coli and MRSA.

There are some interesting reports that tackle the problem of the high temperature stable anatase titania, and spraying the solution directly onto the ceramic surfaces. One such report (JP 2001031483 (A)) set out to solve the following problem: to form a strong photocatalytic layer taking advantage of the remaining heat immediately after burning by directly spraying a photocatalyst liquid on a ceramic substrate during cooling immediately after burning. The reported solution is that the ceramic substrate is a ceramic building material necessitating high-temperature burning such as tile, brick, ceramic plate or roofing tile. Preferably, a mixture of anatase TiO₂ sol and SiO₂ sol is used as the photocatalyst liquid for a ceramic substrate having coarse porous surface, and a photocatalyst liquid containing peroxotitanic acid is used for a ceramic substrate having a smooth surface. The surface temperature suitable for the formation of the photocatalytic layer is preferably 350–500 °C. The photocatalytic layer formed on a coarse porous surface of a substrate is resistant to the peeling from the substrate because the TiO₂ particles are fixed to the pore of the porous surface of the substrate to semi-permanently exhibit an excellent effect. The photocatalyst liquid containing peroxotitanic acid preferably forms fine particles of anatase TiO₂ by heat treatment to form a photocatalytic film having high activity. The product has air-cleaning, stain-proofing and antibacterial characteristics.

Another report (KR 20020043133 (A)) provides a novel method of making photocatalytic titanium oxide sol by improving the existing hydrolysis process of the sol–gel method. It is a method for making acidic titanium oxide sol and basic titanium oxide sol, inducing the crystal growth of anatase type at high temperature and pressure. The above titanium oxide sol for the photocatalyst has excellent properties of dispersion and coating and photocatalytic ability to decompose various organic materials.

It can be seen that there are numerous reports/patents that deal with high temperature stable anatase titania, but no such reports include activation due to visible light and photocatalytic properties at high temperatures and that are also used in the application area of anti-bacterial ceramic tiles.

14.3.1 Self-cleaning properties

It is known that titanium dioxide surfaces display excellent anti-fogging and self-cleaning abilities because of the superhydrophilic attributes of TiO₂ surfaces.⁵⁴ Wang et al. have reported that the transition between the hydrophobic and hydrophilic states could possibly be connected to photoactive electronic transition across the energy gap, i.e., the conversion of Ti^4 + sites into Ti^3 + on the surface under UV illumination.^59,60 Therefore in terms of UV activation, there are common features between the photocatalytic mechanism and hydrophilicity.⁶¹

Recently, however, there has been some consensus that the basic mechanism of these two phenomena may not be the same. According to Watanabe et al.,⁶² the existence of sodium ions in TiO₂ showed very different effects on these photoinduced reactions, suggesting two different photoinduced defect reaction mechanisms on the surface. The essential photocatalytic mechanism could be explained in terms of bulk properties, such as the charge transfer efficiency of a wide gap semiconductor. Therefore it seems photocatalysis of TiO₂ is more dependent on bulk properties, while the hydrophilicity of TiO₂ is an inherently interfacial property, limited to the interface between TiO₂ surface (solid) and water (liquid).

The hydrophilic mechanism is believed to be as follows; electrons reduce the Ti (IV) cations to the Ti (III) state, and the holes oxidize the O^2 − anions. In the process, oxygen atoms are ejected and oxygen vacancies are created (Fig. 14.2). Water molecules can then occupy these oxygen vacancies, producing adsorbed OH groups, which tend to make the surface hydrophilic.⁶³

14.3.2 Anti-bacterial action

There are two principal ways to realize self-cleaning material surfaces: the development of superhydrophobic or superhydrophilic materials. By transferring the microstructure of selected plant surfaces to practical materials like tiles and façade paints, superhydrophobic surfaces were obtained (Lotus effect). Superhydrophilic materials were developed by coating glass, ceramic tiles or plastics with the semiconducting photocatalyst titanium dioxide (TiO₂). If TiO₂ is illuminated by light, grease, dirt and organic contaminants are decomposed and can easily be swept away by water (rain).

TiO₂-coated ceramic tiles are considered to be very effective against organic and inorganic materials, as well as against bacteria. There is general interest in the application of these tiles in hospitals and care facilities to reduce the spread of infections and the threat to patients whose immune system has been weakened, in public and commercial facilities and schools to improve the hygienic conditions and in residential kitchens, baths and floors to promote family hygiene and to reduce housework. Furthermore, these tiles show superhydrophilic behaviour. Grease, dirt and other staining materials can easily be swept away with a stream of water. Superhydrophilicity, combined with the strong photocatalytic oxidizing properties, makes this tile self-cleaning in exterior applications.

With the increasing concern for human health and quality of life, the use of TiO₂ for disinfection becomes more and more important. In the ceramic and building industries, there is a special interest in the photoinduced bactericidal effect of TiO₂.⁴⁹ This is particularly true when the ceramic is going to be placed in microbiologically sensitive environments, such as medical facilities, and food industries where biological contamination must be prevented.⁵⁰

Applying antibacterial TiO₂ building materials to indoor furnishings has been shown to be an effective way to decrease bacterial counts to negligible levels. It was reported that in an operating room in a hospital the number of bacteria on the wall surface was reduced to zero and the bacteria in the air was also decreased significantly after installing photocatalytic tiles. The longer term effect was much better than the spraying of disinfectants.⁵¹ Several companies, such as TOTO, Karpery and Biocera, have commercialized the concept of a deposited thin film semiconductor photocatalyst on ceramics as an antimicrobial agent. Their semiconductor photocatalyst thin film ceramic products exhibit both UV light-induced antimicrobial agent and deodorizing properties.⁵⁴ The light-induced bactericidal activity of TiO₂ can also be used to control biological growth on concrete surfaces. A schematic mechanism of photocatalytic anti-bacterial action is given in Fig. 14.3. Unsightly stains due to the growth of biofilm may cause the loss of aesthetic beauty, particularly in places where design features or maintenance faults result in frequent wetting of the building surface.⁵³ This could also trigger chemical changes of concrete surfaces and decrease their durability.⁵⁵ Photosynthetic algae can only grow where sunlight is available, so that photocatalytic technology is an ideal control method.

Besides self-cleaning cementitious materials, TiO₂-based self-cleaning exterior building products including tiles and glass have been widely commercialized and applied. The self-cleaning and stain-free performance are confirmed by samples suspended outdoors for six months.⁵⁹ For interior tiles used in washrooms or bathrooms, soilage and dirt are always a problem. The fatty acids from soap can form chemical bonds with calcium and magnesium in hard water and adhere to the tile surface, which are difficult to clean after the accumulation of dirt. Tiles with a TiO₂ film surface can break the binding between the organic compounds and the ceramic tiles, which make the washing process easier. Compared with the other two major applications, less research work has been conducted in the area of antimicrobial building materials. So far, standardized protocols for evaluating the light-induced anti-bacterial activity have not been established. The stated efficiency of different self-disinfecting products cannot be verified and compared. Moreover, effective and reliable coating techniques are needed to anchor the nano-photocatalysts to interior building surfaces in the event that the dispersion of fallen nanoparticles causes potential health threats.

1. Pelaez, M., Nolan, N.T., Pillai, S.C., Seery, M.K., Falaras, P., Kontos, A.G., Dunlop, P.S.M., Hamilton, J.W.J., Byrne, J., O’Shea, K., Entezari, M.H., Dionysiou, D.D. Applied Catalysis B. 2012; 125:331.

2. Kamat, P.V. J. Phys. Chem. C. 2007; 111:2834.

3. Parkin, I.P., Palgrave, R.G. J. Mater. Chem.. 2005; 15:1689.

4. Pillai, S.C., Periyat, P., George, R., McCormack, D.E., Seery, M.K., Hayden, H., Colreavy, J., Corr, D., Hinder, S.J. J. Phys. Chem. C. 2007; 111:1605.

5. Chen, X., Mao, S.S. Chem. Rev.. 2007; 107:2891.

6. Yamagishi, M., Kuriki, S., Song, P.K., Shigesato, Y. Thin Solid Films. 2003; 442:227.

7. Bach, U., Corr, D., Lupo, D., Pichot, F., Ryan, M. Adv. Mater.. 2002; 14:845.

8. Wang, X., Yu, J.C., Ho, C., Hou, Y., Fu, X. Langmuir. 2005; 21:2552.

9. Graetzel, M. Nature. 2001; 414:338.

10. Hoffmann, M.R., Martin, S.T., Choi, W., Bahnemann, D.W. Chem. Rev.. 1995; 95:69.

11. Yang, S.W., Gao, L.J. Am. Ceram. Soc.. 2005; 88:968.

12. Bokhimi, X., Morales, A., Lopez, T., Gomez, R., Navarrete, J. J. J. Sol–gel Sci. Tech.. 2004; 29:31.

13. Bokhimi, X., Morales, A., Novaro, O. Chem. Mater.. 1997; 9:2616.

14. Cozzoli, P.D., Comparelli, R., Fanizza, E., Curri, M.L., Agostiano, A. Mat. Sci. Eng. C. 2003; 23:707.

15. Hoffman, A., Carraway, E.R., Hoffman, M. Environ. Sci. Technol.. 1994; 28:776.

16. Mahdavi, F., Burton, T.C., Li, Y. J. Org. Chem.. 1993; 58:744.

17. Liqiang, J., Yichun, Q., Baiqi, W., Shudan, L., Baojiang, J., Libin, Y., Wei, F., Hing-gang, F., Jiazhong, S. Solar Energy Materials & Solar Cells. 2006; 90:1773.

18. Choi, W., Termin, A., Hoffman, M.R. J. Phys. Chem. B. 1994; 98:13669.

19. Soria, J., Conesa, J.C., Augugliaro, V., Palmisano, L., Schiavello, M., Sclafani, A. J. Phys. Chem.. 1991; 95:274.

20. Yu, J.C., Yu, J.G., Ho, K.W., Jiang, Z.T., Zhang, L.Z. Chem. Mater.. 2002; 14:3808.

21. Vinodgopal, K., Kamat, P.V. Environ. Sci. Technol.. 1995; 29:841.

22. Maira, A.J., Yeung, K.L., Lee, C., Yue, P.L., Chan, C.K. J. Catal.. 2000; 192:185.

23. Li, Y., Hwang, D.S., Lee, N.H., Kim, S.J. Chem. Phys. Lett.. 2005; 404:25.

24. Testino, A., Bellobono, I.R., Buscaglia, V., Canevali, C., D’Arienzo, M., Polizzi, S., Scotti, R., Morazzoni, F. J. Am. Chem. Soc.. 2007; 129:3564.

25. Sclafani, A. J. Phys. Chem.. 1996; 100:13655.

26. Emilio, C.A., Litter, M.I., Kunst, M., Bouchard, M., Colbeau-Justin, C. Langmuir. 2006; 22:3606.

27. Hamal, D.B., Klabunde, K.J.J. Colloid Interface Sci.. 2007; 311:514.

28. Kuo, Y.-L., Chen, H.-W., Ku, Y. Thin Solid Films. 2007; 515:3461.

29. Luo, H., Takata, T., Lee, Y., Zhao, J., Domen, K., Yan, Y. Chem. Mater. 2004; 16:846.

30. Asahi, R., Morikawa, T., Oikawa, K., Aoki, K., Taga, Y. Science. 2001; 293:269.

31. Ihara, T., Miyoshi, M., Iriyama, Y., Matsumoto, O., Sugihara, S. Appl. Catal. B. 2003; 42:403.

32. Martyanov, I.N., Uma, S., Rodrigues, S., Klabunde, K.J. Chem. Commun.. 2004; 7:2476.

33. Irie, H., Watanabe, Y., Hashimoto, K. J. Phys. Chem. B. 2003; 107:5483.

34. Nakamura, R., Tanaka, T., Nakoto, Y. J. Phys. Chem. B. 2004; 108:10617.

35. Li, D., Ohashi, N., Hishita, S., Kolodiazhnyi, T., Haneda, H. J. Sol. State Chem.. 2005; 178:3293.

36. Irie, H., Watanabe, Y., Hashimoto, K. Chem. Lett.. 2003; 32:772.

37. Sakthivel, S., Kisch, H. Angew. Chem. Int. Ed.. 2003; 42:4908.

38. Morikawa, T., Asahi, R., Ohwaki, T., Aoki, K., Taga, Y. Jpn. J. Appl. Phys.. 2001; 40:L561.

39. Borgarello, E., Kiwi, J., Gratzel, M., Pelizzetti, E., Visca, M. J. Am. Chem. Soc.. 1982; 104:2996.

40. Machida, M., Norimoto, W.K., Kimura, T. J. Am. Ceram. Soc.. 2005; 88:95.

41. Riegel, G., Bolton, J.R. J. Phys. Chem.. 1995; 99:4215.

42. , Patent licensing of super hydrophilic photocatalyst technology. Hydrotech. 2008 TOTO Ltd., Available at: http://www.toto.co.jp/docs/hyd_patent_en/case_001.htm

43. Shimohigoshi, M., Saeki, Y. Research and application of photocatalyst tiles. In: Baglioni P., Cassar L., eds. RILEM Int. Environment and Construction Materials, Italy: Symp. on Photocatalysis; 2007:291.

44. Takahaschi, Y., Matsuoka, Y. J. Mater. Sci. 1988; 23:2259.

45. Takahaschi, Y., Kiwa, K., Kobayashi, K., Matsuki, M. J. Am. Ceram. Soc.. 1991; 74:67.

46. Sanchez, C., Livage, J., Henry, M., Babonneau, F. J. Non-Cryst. Solids. 1988; 100:65.

47. Guilment, J., Pencelot, O., Rigola, J., Truchet, S. Vib. Spectrosc.. 1996; 11:37.

48. Nolan, N.T., Seery, M.K., Pillai, S.C. J. Phys. Chem. C. 2009; 113:16151.

49. Chen, J., Poon, C.-S. Building and Environment. 2009; 44:1899.

50. Amézaga-Marid, P., Nevárez-Moorillón, G.V., Orrantia-Borunda, E., Miki-Yoshida, M. FEMA Microbiology Letters. 2002; 211:183.

51. Fujishima, A., Hashimoto, K., Watanabe, T. TiO2 Photocatalysis: Fundamentals and Applications. Japan: BKC, Inc.; 1999.

52. Mills, A., Lee, S.K. Photochem Photobiol A. 2002; 152:233.

53. Dubosc, A., Escadeillas, G., Blanc, P.J. Cem Concr Res. 2001; 31:1613.

54. Reidy, D.J., Holmes, J.D., Nagle, C., Morris, M.A. J. Mater. Chem.. 2005; 15:3494.

55. Kurth, J.C., Giannantonio, D.J., Allain, F., Sobecky, P.A., Kurtis, K.E. Mitigating biofilm growth through the modification of concrete design and practice. In: Baglioni P., Cassar L., eds. RILEM Int. Symp. on Photocatalysis. Italy: Environment and Construction Materials; 2007:195.

56. Padmanabhan, S.C., Pillai, S.C., Colreavy, J., Balakrishnan, S., McCormack, D.E., Perova, T.S., Gun’ko, Y., Hinder, S.J., Kelly, J.M. Chem. Mater.. 2007; 19:4474.

57. Wang, H., Miao, J.J., Zhu, J.M., Ma, H.M., Zhu, J.J., Chen, H.Y. Langmuir. 2004; 20:11738.

58. Baiju, K.V., Sibu, C.P., Rajesh, K., Pillai, P.K., Mukundan, P., Warrier, K.G.K., Wunderlich, W. Mater. Chem. Phys.. 2005; 90:123.

59. Wang, R., Hashimoto, K., Fujishima, A., Chikuni, M., Kitamura, A., Shimohigoshi, M., Watanabe, T. Adv. Mater.. 1998; 2:135.

60. Wang, R., Hashimoto, K., Fujishima, A., Cjikuni, M., Kojima, E., Kitamura, A., Shimohigoshi, M., Watanabe, T. Nature. 1997; 388:431.

61. Lee, Y.C., Hong, Y.P., Lee, H.Y., Kim, H., Jung, Y.J., Ko, K.H., Jung, H.S., Hong, K.S. J. Colloid and Interface Science. 2003; 267:127.

62. Watanabe, T., Fukayama, S., Miyauchi, M., Fujishima, A., Hashimoto, K. J. Sol–gel Sci. Technol. 2000; 19:71.

63. Misook, K. Mat. Lett.. 2005; 59:3122.