Modulation of the Reduction Potential of TiO2- 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 TiO2- 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 TiO2- 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.

One-pot in-situ hydrothermal synthesis of ternary In2S3/Nb2O5/Nb2C Schottky/S-scheme integrated heterojunction for efficient photocatalytic hydrogen production.

MXene-derived photocatalysts continue to fascinate the research community in developing photo-driven green and sustainable fuel production. However, the efficiency of MXene-derived photocatalyst is still low due to the wide bandgap and high recombination rate of photo-excited charge carriers. Here, we have synthesized the Nb2C MXene-derived ternary photocatalyst via one-pot in-situ hydrothermal method for photocatalytic hydrogen (H2) evolution. The partial oxidation of Nb2C MXene into Nb2O5 nanorods and coupling with In2S3 nanoparticles via in-situ chemical anchoring were the key factors toward high efficiency and long-term stability during photocatalytic H2 evolution. The optimized ternary photocatalyst composite manifested the highest H2 evolution efficiency at 68.8 µmol g-1 h-1, which was 11 and 7.5 times higher than the Nb2O5/Nb2C (NNC) and pure In2S3 photocatalyst, respectively. Moreover, the photocatalytic stability of the optimized ternary photocatalyst composite was analyzed for five consecutive cycles, and above 87% activity retention was observed even after the fifth cycle without any obvious decline. The separation efficiency of photoexcited charge carriers could be attributed to the synergic effect of the In2S3/Nb2O5 heterojunction and the redox reactions at different sites of the composite. More importantly, the participation of Schottky junction and S-scheme heterojunction charge transfer for the obtained novel ternary photocatalyst was evaluated via ultraviolet photoelectron spectroscopy (UPS) and electron paramagnetic resonance (EPR). This research will provide additional insight into the extended potential of MXene-derived ternary photocatalysts towards efficient H2 production to meet future global energy demands.