Tuning the local chemical environment of ZnSe quantum dots with dithiols towards photocatalytic CO2 reduction

Sunlight-driven CO2 reduction to renewable fuels is a promising strategy towards a closed carbon cycle in a circular economy. For that purpose, colloidal quantum dots (QDs) have emerged as a versatile light absorber platform that offers many possibilities for surface modification strategies. Considerable attention has been focused on tailoring the local chemical environment of the catalytic site for CO2 reduction with chemical functionalities ranging from amino acids to amines, imidazolium, pyridines, and others. Here we show that dithiols, a class of organic compounds previously unexplored in the context of CO2 reduction, can enhance photocatalytic CO2 reduction on ZnSe QDs. A short dithiol (1,2-ethanedithiol) activates the QD surface for CO2 reduction accompanied by a suppression of the competing H-2 evolution reaction. In contrast, in the presence of an immobilized Ni(cyclam) co-catalyst, a longer dithiol (1,6-hexanedithiol) accelerates CO2 reduction. H-1-NMR spectroscopy studies of the dithiol-QD surface interactions reveal a strong affinity of the dithiols for the QD surface accompanied by a solvation sphere governed by hydrophobic interactions. Control experiments with a series of dithiol analogues (monothiol, mercaptoalcohol) render the hydrophobic chemical environment unlikely as the sole contribution of the enhancement of CO2 reduction. Density functional theory (DFT) calculations provide a framework to rationalize the observed dithiol length dependent activity through the analysis of the non-covalent interactions between the dangling thiol moiety and the CO2 reduction intermediates at the catalytic site. This work therefore introduces dithiol capping ligands as a straightforward means to enhance CO2 reduction catalysis on both bare and co-catalyst modified QDs by engineering the particle's chemical environment.