Band alignment investigations of heterostructure NiO/TiO₂ nanomaterials used as efficient heterojunction earth-abundant metal oxide photocatalysts for hydrogen production

Abstract

Earth-abundant NiO/anatase TiO₂ heteronanostructures were prepared by a straightforward one-pot sol-gel synthetic route followed by a suitable thermal post-treatment. The resulting 0.1-4 wt% NiO-decorated anatase TiO₂ nanoparticles were characterized by X-ray diffraction, electron microscopy, Raman and UV-visible spectroscopy and N₂ sorption analysis, and showed both nanocrystallinity and mesoporosity. The careful determination of the energy band alignment diagram by a suitable combination of XPS/UPS and absorption spectroscopy data revealed significant band bending at the interface of the p-n NiO/anatase TiO₂ heterojunction nanoparticles. Furthermore, these heterojunction photocatalysts exhibited an improved photocatalytic activity in H₂ production by methanol photoreforming compared to pure anatase TiO₂ and commercial P25. Thus, an average H₂ production rate of 2693 μmol h^-1 g^-1 was obtained for the heterojunction of a 1 wt% NiO/anatase photocatalyst, which is one of the most efficient NiO/anatase TiO₂ systems ever reported. An enhanced dissociation efficiency of the photogenerated electron-hole pairs resulting from an internal electric field developed at the interface of the NiO/anatase TiO₂ p-n heterojunctions is suggested to be the reason of this enhanced photocatalytic activity.