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

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 degrees 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 N-2 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 in situ 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.