Encapsulating CuO quantum dots in MIL-125(Ti) coupled with g-C3N4 for efficient photocatalytic CO2 reduction

•CuO QDs are encapsulated in MIL-125(Ti) via a complexation-oxidation method.•Ternary g-C3N4/CuO@MIL-125(Ti) enables efficient photocatalytic CO2 reduction.•CuO QDs are crucial in altering the reaction pathway for the production C2+ .•The composite photocatalyst shows good stability and recyclability. Improving the stability of metallic oxide quantum dots (QDs) in a reaction system containing water is crucial for their practical applications in photocatalytic reduction of carbon dioxide. Herein, we use simple complexation-oxidation method to encapsulate CuO QDs in the pores of metal organic framework of MIL-125(Ti), and further combine it with g-C3N4 to form a composite photocatalyst, i.e., g-C3N4/CuO@MIL-125(Ti). Benefiting from the protection of the framework of MIL-125(Ti), the composite photocatalyst exhibits significantly improved stability in reaction systems containing water. In addition, due to the close contact of CuO QDs to the active catalytic site of Ti in MIL-125(Ti), the photogenerated electrons in the MIL-125(Ti) and g-C3N4 can be smoothly transferred to the confined CuO QDs, which remarkably enhances the photocatalytic activity of g-C3N4/CuO@MIL-125(Ti) for photocatalytic CO2 reduction in the presence of water. An optimization of the photocatalyst has led to the yields of CO, methanol, acetaldehyde and ethanol up to 180.1, 997.2, 531.5 and 1505.7 μmol/g, respectively. This work provides an effective strategy for improving the stability and charge separation property of metallic oxide-QDs modified photocatalyst toward efficient photocatalytic CO2 reduction.