Covalently linking CuInS₂ quantum dots with a Re catalyst by click reaction for photocatalytic CO₂ reduction

Covalently linking photosensitizers and catalysts in an inorganic-organic hybrid photocatalytic system is beneficial for efficient electron transfer between these components. However, general and straightforward methods to covalently attach molecular catalysts on the surface of inorganic semiconductors are rare. In this work, a classic rhenium bipyridine complex (Re catalyst) has been successfully covalently linked to the low toxicity CuInS₂ quantum dots (QDs) by click reaction for photocatalytic CO₂ reduction. Covalent bonding between the CuInS₂ QDs and the Re catalyst in the QD-Re hybrid system is confirmed by UV-visible absorption spectroscopy, Fourier-transform infrared spectroscopy and energy-dispersive X-ray measurements. Time-correlated single photon counting and ultrafast time-resolved infrared spectroscopy provide evidence for rapid photo-induced electron transfer from the QDs to the Re catalyst. Upon photo-excitation of the QDs, the singly reduced Re catalyst is formed within 300 fs. Notably, the amount of reduced Re in the linked hybrid system is more than that in a sample where the QDs and the Re catalyst are simply mixed, suggesting that the covalent linkage between the CuInS₂ QDs and the Re catalyst indeed facilitates electron transfer from the QDs to the Re catalyst. Such an ultrafast electron transfer in the covalently linked CuInS₂ QD-Re hybrid system leads to enhanced photocatalytic activity for CO₂ reduction, as compared to the conventional mixture of the QDs and the Re catalyst.