Physical properties and photocatalytic efficiencies of TiO 2 films prepared by PECVD and sol–gel methods

Abstract

TiO 2 thin-film photocatalysts coated onto glass were prepared either by plasma enhanced chemical vapour deposition (PECVD) deposition or by a dip-coating process using sol–gel solutions. The influence of the addition of a polymer and of a highly viscous solvent on physical properties (thickness, crystallite size and porosity) of TiO 2 thin films and on their photocatalytic efficiency was evaluated. The film thickness obtained after a single dipping was proportional to the viscosity of the dip-coating solution. The presence of highly viscous solvent such as α-terpineol did not modify the TiO 2 crystallite size, while smaller crystallites were formed in presence of a polymer. Both additives increased the porosity of the single dipping TiO 2 film. About twice higher TiO 2 crystallite sizes were formed by using PECVD method. All TiO 2 films prepared in this work by sol–gel methods were of the anatase phase and were orientated along the (1 0 1) plane. In contrast, anatase films prepared by PECVD and 520 nm thick were orientated along the (0 0 4) crystallographic plane was obtained in 520 nm film thickness. The photocatalytic activities of the TiO 2 thin films were evaluated by the photoxidation rate of 2-hydroxybutanedioı̈c acid (malic acid). They were strongly dependent on the film thickness, suggesting the participation of all the inner part of the film in the generation of e −/h + pair. No influence of the TiO 2 film porosity, in the range 10–70%, was observed because of a sufficient porosity to allow the total diffusion of organic compounds within the film. However, smaller TiO 2 particles seemed to slightly increase the photocatalytic activity, probably because of the increase in the surface area of the particulate film. PECVD TiO 2 films present smaller efficiencies than those prepared by the sol–gel method. No influence of textural properties on the photocatalytic mechanism was observed.