Ligand enhanced mode selective photocatalytic CO2 reduction by quantum dots

Photocatalytic CO2 reduction has sparked significant interest in the generation of chemical fuels through solar energy harvesting. However, the activity, stability and selectivity of products are heavily dependent upon the efficiencies of light harvesting ability, charge migration and surface reactions. Herein, colloidal CdS quantum dots (QDs) have been interfacially engineered with four different capping ligands viz. mercaptoacetic acid (MAA), 3-mercaptopropanoic acid (MPA), l-cysteine (LC) and oleic acid (OA). Compared to other ligands, MAA functionalization introduces deep trap states by developing interactions with the surface Cd atoms. These states enhance the lifetime of photogenerated electrons for substantial CO2 reduction with an activity of approximately 2120 mol g(-1) h(-1). Computational analysis suggests the rate of electron transfer to CO2 from CdS QDs is much faster as compared to the nuclear degrees of freedom, and the bending angle of CO2 enhances electron transfer. The maximum electron transfer efficiency was observed at the bent angle of 130 degrees. The bent geometry is supported by the surface proton shutter provided by the acid group of the thiol/acid bifunctional ligand. This work tries to comprehend the crucial effect of ligand-induced deep trap states and change in the bending angle of CO2, which makes the C-Cd distance shorter and significantly promotes the photocatalytic reduction of CO2 to formaldehyde.