Preparation of RuO₂/TiO₂ Mesoporous Heterostructures and Rationalization of Their Enhanced Photocatalytic Properties by Band Alignment Investigations

Abstract

Nanoporous RuO₂/TiO₂ heterostructures, in which ruthenium oxide acts as a quasi-metallic contact material enhancing charge separation under illumination, were prepared by impregnation of anatase TiO₂ nanoparticles in a ruthenium­(III) acetylacetonate solution followed by thermal annealing at 400 °C. Regardless of the RuO₂ amount (0.5–5 wt %), the as-prepared nanocatalyst was made of a mesoporous network of aggregated 18 nm anatase TiO₂ nanocrystallites modified with RuO₂ according to N₂ sorption, TEM, and XRD analyses. Furthermore, a careful attention has been paid to determine the energy band alignment diagram by XPS and UPS in order to rationalize charge separation at the interface of RuO₂/TiO₂ heterojunction. At first, a model experiment involving stepwise deposition of RuO₂ on the TiO₂ film and an <i>in situ</i> XPS measurement showed a shift of Ti 2p_3/2 core level spectra toward lower binding energy of 1.22 eV which was ascribed to upward band bending at the interface of RuO₂/TiO₂ heterojunction. The band bending for the heterostructure RuO₂/TiO₂ nanocomposites was then found to be 0.2 ± 0.05 eV. Photocatalytic decomposition of methylene blue (MB) in solution under UV light irradiation revealed that the 1 wt % RuO₂/TiO₂ nanocatalyst led to twice higher activities than pure anatase TiO₂ and reference commercial TiO₂ P25 nanoparticles. This higher photocatalytic activity for the decomposition of organic dyes was related to the higher charge separation resulting from built-in potential developed at the interface of RuO₂/TiO₂ heterojunction. Finally, these mesoporous RuO₂–TiO₂ heterojunction nanocatalysts were stable and could be recycled several times without any appreciable change in degradation rate constant that opens new avenues toward potential industrial applications