PREPARATION OF PHOTOCATALYTIC THIN FILMS OF TRICALCIUM PHOSPHATE BY DOPING TITANIUM(IV) ION

Abstract

Thin films of tricalcium phosphate (TCP) with doping Ti(IV) ion were prepared. These prepared films were characterized by XRD, UV-vis transmission, FT-IT (RAS), and wettability. These characterization indicated that phase separation such as TiO2 did not occur and that substitution with Ti(IV) occurred at Ca site. Photocatalytic activity was evaluated by methylene blue decomposition, and the results suggested that the activity was enhanced by doping Ti(IV). (Received Jan. 16, 2014; Accepted Mar. 5, 2014) INTRODUCTION Previously we have reported preparation of photocatalytic thin film 1 based on a novel apatite Ti-doped calcium hydroxyapatite (TiHAP), 2 which have calcium hydroxyapatite (HAP) structure partially substituted with Ti 4+ ions at calcium sites. This photocatalyst is found to be different from composite systems of TiO2 and an adsorbent (apatite, activated carbon, zeolite, and so on). Yoneyama et al. have revealed that stronger adsorbents lowered the photocatalytic activity of the composite because it prevents migration of adsorbed pollutants on the composite surface. 3 The problem of this composite system is attributable to a distance between photocatalytic site and adsorptive site. However, this apatite-based photocatalyst is not thought to have such problem, and the strong adsorptive ability of apatite may be effectively available. Several groups reported catalytic activities of calcium phosphates such as thermal oxidation reaction of alcohols, trichloroethylene, and so on. 4,5 Nishikawa suggested a possibility of OH radical reactions, 6 that is, OH radical forms thermally at the surface of CP and attacks adsorbed substrates. In the case of TiHAP, it may form photochemically. Doping of metal ion was reported by Suzuki et al 7 and Wakamura et al 8,9 for HAP with many divalent and trivalent metal ions such as Pb 2+ , Sn 2+ , Cu 2+ , Mn 2+ , Ni 2+ , Co 2+ , Al 3+ , La 3+ , Fe 3+ , and Cr 3+ , and Hashimoto et al 10,11 for -TCP for Li + , Na + , K + , Mg 2+ , Sr 2+ , and Al 3+ , for example. In many cases, limits of the concentration for solid solutions are not so high, and especially for TiHAP, it was suggested that the substitution with Ti 4+ was occurred at columnar site probably with a certain amount of Ca-deficiency. 12 For –TCP, the substitution site may depend on radius of doped metal ion. 10 Anyway, doping of metal ion causes changes in lattice constants and perturbation of electronic states, so that it is plausibly expected that doping of metal ions promote photochemical generation of OH radical. As a photocatalytic coating, self-cleaning property is also important. TiO2 photocatalytic coating has been developed as a self-cleaning material, in terms of its photocatalytic activity and photo-induced superhydrophilicity. 13,14 Small amount of water can spread over its surface due to photo-induced superhydrophilicity, which can easily sweep surface stains away. The photocatalytic activity contributes to the self-cleaning property by decomposition of the residual stains. In this way, the superhydrophilicity of the surface plays an important role of self-cleaning property of the practical TiO2 coating. However, a fatal problem of the hydrophilic self-cleaning material is an inherent higher affinity to stains due to the high surface energy. Another approach for self-cleaning material is preparing a composite of a photocatalyst and hydrophobic material. 15,16 Low affinity of the hydrophobic surface to stains would prevent adhesion of those, and a large amount of stains would be easily swept away from the surface. From this point of view, the combination of hydrophobicity and photocatalyst may be more preferable as a self-cleaning material, although any composites with TiO2 have not shown enough properties for practical usage because of strong phtocatalytic activity (photocatalytic decomposition of hydrophobic