Defective Nb₂C MXene Cocatalyst on TiO₂ Microsphere for Enhanced Photocatalytic CO₂ Conversion to Methane

Sustainable and scalable solar-energy-driven CO₂ conversion into fuels requires earth-abundant and stable photocatalysts. In this work, a defective Nb₂C MXene as a cocatalyst and TiO₂ microspheres as photo-absorbers, constructed via a coulombic force-driven self-assembly, is synthesized. Such photocatalyst, at an optimized loading of defective Nb₂C MXene (5% def-Nb₂C/TiO₂), exhibits a CH₄ production rate of 7.23 µmol g^-1 h^-1, which is 3.8 times higher than that of TiO₂. The Schottky junction at the interface improves charge transfer from TiO₂ to defective Nb₂C MXene and the electron-rich feature (nearly free electron states) enables multielectron reaction of CO₂, which apparently leads to high activity and selectivity to CH₄ (sel. 99.5%) production. Moreover, DFT calculation demonstrates that the Fermi level (E_F) of defective Nb₂C MXene (-0.3 V vs NHE) is more positive than that of Nb₂C MXene (-1.0 V vs NHE), implying a strong capacity to accept photogenerated electrons and enhance carrier lifetime. This work gives a direction to modify the earth-abundant MXene family as cocatalysts to build high-performance photocatalysts for energy production.