Reduced {001}-TiO_2-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.