Thickness effect in the photocatalytic activity of TiO 2 thin films derived from sol-gel process
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.Z Chem. Eng., 1816), 91)3-907 (2001)
Thickness Effect in the Photocatalytic Activity of TiOz Thin Films Derived from Sol-Gel Process Wen Xu Xianyu, Myun Kyu Park and Wan In Lee ~ Department of Chemistry and Center for Chemical Dynamics, lnha University, lnchon 402-75 [, Korea (Received 5 September 200t 9 ac~'epted t3 October 2001)
Abstraet-TiO~ films in ;arious thicknesses were prepared by sol-gel method, and their photocatalytic activities in the decomtvasition of gaseous 2-propanot were evaluated. It was tbund that the photocatalytic activities of transparent riO, films increase with the increase of film thickness: The pht~tocatalytic activity of TiO~ fihns in 670 nln-thickness was 3,7 times that of" films in 7t1 nm-thickness. We proposed that the higher photocatalytic activities lbr the thicker Ti()~ films originate li'om the greater amount of phomgenerated electron and hoIe pairs, which are transferred fi'om the inside to the surface ofTi(), films. Wc also provided some experimental evidences supporting this mechanism, Key words: Photocatalyst. TiOe, Thickness, Thickness Ell'oct, Photocatalytic Fihn
INTRODUCTION
EXPERIMENTAL
It has been known that TiQ, dr one of the highest photocatalytic activities among semiconductors for the efimination of organic pollutants in aqueous soltNon or in gas phase [Matthew, f985; Nozik, 1978, Hoffmann et al., 1995; Chat et al., 2000: Lee et al., 2000; Fujishima et aL, 1997: Anpo et at., f997]. Especially, TiO= photocatalysts in thin film form have promising application for the decomposition of gas phase pollutants or as a self-cIeaning smart material [Lee Et aI., 2001; Jtmg et al., 19971. Moreover, Ti(~ surface presents super-hydrophilic character with an irradiation of light [Fujishima et al., 1997, 1998]. Hence, the Ti({ thin films coated on window glass, light bulbs, ceramic tiles, or walls can purify the polluted air in the room, and can self-clean or defog their surfaces, For these industrial applications ofphotocataFytic TiQ fihns, many practical problems, such as fabrication of transparent TiQ fiIms, adhesion to gIass substrate, blocking of impurity migration from the substrate, determination of optimum film thickness, etc,, have to be solved. Among them, in this work, we have focused on tJ~e correlation of photocatalytic activity with film thickness. In the case of thin TiQ, films, the photocatalytic reactions take place only on the surfaces. 1bus, it is expected that the photocatalytic activity is irrelevant to the thickness of T i ~ film. since the surface area of film is not appreciably affected by the change of thickness. HowEver, it has been found that photocatalytic activity is strongly dependent on the thickness of tilm. In the literature, a few researchers reported on the thickness dependence on the pholocatalyric activity of TiO_~ fihns [Yu et al., 2000; Cut et aI., 1993], but they did not mention the reason why thicker films demonstrate higher photocatalytic activities in the photocatalytic oxidation J,eactions. In this work we made a scrupulous observatio
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