Integrating carbon quantum dots with oxygen vacancy modified nickel-based metal organic frameworks for photocatalytic CO₂ reduction to CH₄ with approximately 100 % selectivity

Solar-light-driven reduction of CO₂ into renewable fuels has great potential in the production of sustainable energy, addressing the energy crisis and environmental problems simultaneously. However, it is a significant challenge to achieve high selectivity for the conversion of CO₂ into CH₄, which is a type of fuel with a high calorific value. Herein, carbon quantum dots (CQDs) were integrated with an oxygen vacancy modified nickel-based metal organic frameworks (NiMOFs) to form the CQDs-X/NiMOF_V series, which exhibited superior performance for CO₂ photoreduction into CH₄ compared with pure NiMOFs in the presence of hole scavengers under visible light irradiation. The highest yielding rate of CH₄ (1 mmol g^-1 h^-1) and selectivity (97.58 %) were obtained using a CQDs-25/NiMOF_V catalyst. Most importantly, in diluted CO₂ atmosphere, the yield of CH₄ was almost unchanged and the selectivity of CH₄ over CQDs-25/NiMOF_V was higher than that in pure CO₂. The superior performance of CQDs-25/NiMOF_V may be attributed to the following two factors: (i) both CQDs and oxygen vacancies facilitate the transmission of electrons to promote the eight-electron reaction producing CH₄, and (ii) oxygen vacancies can act as the electron trap to capture the photogenerated electrons to react with adsorbed CO₂ on Ni²⁺. This study offers a valuable strategy for designing efficient photocatalysts to convert CO₂ into CH₄ with superior selectivity.