Silver enhanced TiO2 thin films: photocatalytic characterization using aqueous solutions of tris(hydroxymethyl)aminomethane

Abstract

The photocatalytic activity in aqueous solutions of TiO₂ and Ag enhanced TiO₂ sol–gel produced films was characterized using tris(hydroxymethyl)aminomethane (Tris) under black light (365 nm) and the observed differences in efficiency were further investigated by O₂ adsorption studies using the same probe. Hydrogen abstracting species, such as hydroxyl radicals formed upon photocatalysis, are able to abstract hydrogen from Tris. This reaction leads to the formation of formaldehyde which was detected and quantified through a modified version of the Hantzsch reaction. It was found that the Ag enhanced TiO₂ film increased the apparent quantum yield from 7% to 12%, partly as a result of a Schottky barrier formation at the metal–semiconductor interface and partly as the sensitizing effect of Ag nanoparticles extends the visible light absorption, which through electron transfer processes enable an efficient charge separation in the TiO₂ by attracting acceptor species more efficiently than pure TiO₂. The O₂ adsorption studies in this paper showed that the Ag enhanced TiO₂ film has a stronger adsorption affinity than pure TiO₂ towards O₂, which make the reduction of O₂ more efficient with a subsequent enhanced electron–hole lifetime. It was also found that the Ag enhanced TiO₂ film had a poorer adsorption affinity for Tris than the pure TiO₂ film, which is a consequence of fewer available surface adsorption sites due to the Ag coverage at 64% which agrees well with the obtained adsorption equilibrium constants (K_LH(TiO2) = 615 M⁻¹ and K_{LH(Ag–TiO2)} = 320 M⁻¹).