Effect of Photoirradiation on the Properties of Layered Titanate Thin Films from Transparent Aqueous Titanate Sols

UV irradiation on films prepared by a spin-coating method using aqueous titanate sols was investigated. The transparent sols used in this study were prepared by a reaction of titanium tetraisopropoxide (TIP) with tetramethylammonium hydroxide (TMAOH) or alkylamines, and consisted of a layered compound. The refractive indices and hardening of the films increased with the irradiation time, indicating that photoirradiation accelerated densification of the films. It was found that these phenomena were caused by the photocatalytic decomposition of alkylammonium ions located between the titanate sheets to contract the interlayer distance. Interestingly, when the film prepared from the TIP-TMAOH sol containing 30 mol% of sucrose to Ti was irradiated through a patterning mask and subsequently heated in air at 500 °C, the irradiated part of the film selectively turned dark brown due to residual carbon. It was thought that sucrose molecules were confined within the host titanate layers densificated by UV irradiation and that the oxidation of the sucrose by oxygen in air was difficult under heating because the densificated titanates might suppress the diffusion of oxygen. On the other hand, such coloring of the films by sucrose was not found for films prepared from an aqueous suspension of crystalline TiO2 (anatase), an aqueous solution of TIP-lactic acid (LA)-NH3, and alcoholic sols such as the TIP-diethanolamine (DEA) system.