The stability of nonporous and macroporous titania thin films in aqueous electrolyte solutions

Abstract

A simple electrochemical approach was used to evaluate the stability and porosity of titania and silica thin films spin coated on electrode surfaces. This approach involved monitoring the magnitude of the Faradaic current of diffusing redox probes at the modified electrode surfaces over the course of a week to 4.5 months. Relatively nonporous films were examined as well as films templated with polystyrene latex spheres. The results show that templated titania films were significantly more porous compared to non-templated films. After the defect sites in the templated films were blocked, their long-term stability in aqueous electrolyte was evaluated. For titania, blocking was done by spin coating a dilute titania sol on the top of the film whereas for silica, the film was soaked in octyltrimethoxysilane. Both types of titania films (templated and non-templated) were found to be significantly more stable than the corresponding silica films, showing no signs of deterioration in simple electrolyte solutions during the entire evaluation period. In contrast, silica films showed significant deterioration in as little as 3 days. The enhanced stability of the titania films relative to silica films in near neutral electrolyte solutions was attributed to the differences in the point of zero charge of the oxide films.