Photochemical properties of ultrathin TiO₂ films prepared by chemical vapor deposition

Thin films of TiO₂ were prepared on fused silica and titanium substrates by chemical vapor deposition (CVD) using Ti[OCH(CH₃)₂]₄ under conditions of anatase formation. The UV-visible transmission spectrum of the film deposited on the fused silica substrate showed an absorption edge 30 nm shorter than that of anatase powder. This blue shift may be attributed to size quantization, i.e. the film may consist of extremely small TiO₂ particles. The absorption edge shifted to slightly longer wavelength upon heating the film in air at high temperatures. The photocatalytic property of the film was examined using the photodeposition reaction of silver from aqueous AgNO₃. The silver photodeposition rate increased with the calcination temperature, maximizing at around 400°C and then decreasing at higher temperature. To compare the photocatalytic property with the photoelectrochemical property, photocurrents were measured in a KOH solution using the film deposited on the titanium substrate. They decreased after calcination at temperatures above 400°C. Compared to bulk TiO₂, the wavelength dependence of the current was shifted to considerably shorter wavelengths, in agreement with the absorption spectrum.