Modulation of the Reduction Potential of TiO_{2- x} by Fluorination for Efficient and Selective CH₄ Generation from CO₂ Photoreduction

Photocatalytic reduction of CO₂ holds great promises for addressing both the environmental and energy issues that are facing the modern society. The major challenge of CO₂ photoreduction into fuels such as methane or methanol is the low yield and poor selectivity. Here, we report an effective strategy to enhance the reduction potential of photoexcited electrons by fluorination of mesoporous single crystals of reduced TiO_2-x. Density functional theory calculations and photoelectricity tests indicate that the Ti³⁺ impurity level is upswept by fluorination, owing to the built-in electric field constructed by the substitutional F that replaces surface oxygen vacancies, which leads to the enhanced reduction potential of photoexcited electrons. As a result, the fluorination of the reduced TiO_2-x dramatically increases the CH₄ production yield by 13 times from 0.125 to 1.63 μmol/g·h under solar light illumination with the CH₄ selectivity being improved from 25.7% to 85.8%. Our finding provides a metal-free strategy for the selective CH₄ generation from CO₂ photoreduction.