Reduced {001}-TiO2-x photocatalysts: noble-metal-free CO2 photoreduction for selective CH4 evolution.

The preparation of reduced TiO2 photocatalysts with high Ti3+ concentration is a great challenge due to their instability in air. Here we report a new approach for the synthesis of reduced TiO2 with {001} facets exposed via a hydrothermal process. By the introduction of fluoride atoms, {001} and {101} facets are formed, which act as hole and electron collectors, respectively, for charge separation. By adjusting the volume of HF added, a rutile-anatase transition is observed for the first time. EPR spectra confirm the generation of Ti3+ species in the bulk of TiO2, and Ti3+ signals are studied in the anatase and rutile phases separately. The quantified EPR shows that reduced TiO2 samples present 14 000-fold more spins compared to the pristine TiO2, and the intensity can reach as high as 24.6 × 1019 spins per g. The obtained samples also have a unique disordered layer with a thickness of 1-2 nm on their surfaces, which contributes to the stabilization of the formed Ti3+ species by preventing their oxidation in air. In addition, the synthesized reduced TiO2 samples also exhibit wide-spectrum solar light absorption, especially in the near-infrared region. Owing to their enhanced solar light absorption, improved electron-hole separation and special facet exposure, these samples exhibit enhanced photocatalytic CO2 reduction performance and high CH4 selectivity under solar light irradiation, in the absence of a noble metal Pt as a co-catalyst.