Comparison of hydrophilic properties of amorphous TiO x films obtained by radio frequency sputtering and plasma-enhanced
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Dariusz Korzec and Jurgen Engemann Microstructure Center, University of Wuppertal, Obere Lichtenplatzer Strasse 336, 42287 Wuppertal, Germany (Received 26 September; accepted 30 November 2000)
The hydrophilic properties of amorphous TiOx films prepared by different methods, e.g., radio frequency (rf) sputtering and plasma-enhanced chemical vapor deposition (PECVD), were studied. It was found that the hydrophilicity strongly depends on the film structure. The best hydrophilicity was realized with the PECVD amorphous film having distorted Ti–O bonds due to a large amount of OH groups. These characteristics of the PECVD amorphous film suggest that such a low-density film including distorted Ti–O bonds could increase the photoenhancement efficiency by ultraviolet radiation. This reason is also supported from the results that a low-density rf sputtered film presented a higher hydrophilicity compared to a high-density radio frequency sputtered film. Furthermore, both electrical and chemical effects of OH groups will also contribute to the good hydrophilicity of the PECVD film.
I. INTRODUCTION
Recently, titanium dioxide (TiO2) has attracted much attention for its photocatalytic characteristics.1– 4 Roles of electron–hole pairs generated by the ultraviolet (UV) light irradiation have been studied in the basic and application field. It is also known that TiO2 films can be good hydrophilic materials by UV irradiation. The hydrophilic properties were reported as originated from the photoreduction while the photocatalytic effects originated from the photo-oxidation.5 However, the detailed mechanism for hydrophilic properties is not clear and some theories have been proposed. Wang et al. suggested that the hydrophilicity was caused by water adsorption at oxygen vacancies on the surface.6 The oxygen vacancies can be generated by the defection of bridging oxygen with UV irradiation. This theory was supported in comparison with the hydrophilic properties of rutile type TiO2 single crystals which have different surface properties with bridging oxygen.7 However, it was mentioned in the same paper that the anatase type TiO2 had a better hydrophilicity than the rutile type one though the anatase type TiO2 has less bridging oxygen sites on the surface. These aspects imply that other effects, such as photoen-
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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 16, No. 2, Feb 2001
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hancement efficiency, surface morphology, electrical properties, and so on, as well as the above-mentioned theory are related to the hydrophilicity. We previously reported that the electrical properties of the deposited films were strongly related to the hydrophilicity.8 In this report, our purpose is to get some information on the good hydrophilicity. The hydrophilic properties of amorphous TiOx films having different microstructures were investigated. The hydrophilic properties of amorphous TiO2 films have been seldom discussed. We present an
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