Imidazolium-modification enables enhanced photocatalytic CO2 reduction on ZnSe quantum dots

Colloidal photocatalysts are a promising, low-cost material to utilize solar light for the conversion of CO2 to carbon-based fuels, but controlling the product selectivity for CO2 reduction remains challenging, in particular in aqueous solution. Here, we present an organic surface modification strategy to tune the product selectivity of colloidal ZnSe quantum dots (QDs) towards photocatalytic CO2 reduction even in the absence of transition metal complex co-catalysts. Besides H2, imidazolium-modified ZnSe QDs evolve up to 2.4 mmol CO gZnSe−1 (TONQD > 370) after 10 h of visible light irradiation (AM 1.5G, lambda > 400 nm) in aqueous ascorbate solution with a CO-selectivity of up to 20%. This represents a four-fold increase in CO-formation yield and 13-fold increase in CO-selectivity compared to non-functionalized ZnSe-BF4 QDs. The ligand-QD interactions are characterized quantitatively using 1H-NMR spectroscopy and isothermal titration calorimetry revealing that a subset of 10 to 17 ligands interact strongly with the QDs. Transient absorption spectroscopy reveals an influence of the ligand on the intrinsic charge carrier dynamics through passivating Zn surface sites. Density functional theory calculations indicate that the imidazolium capping ligand plays a key role in stabilizing the surface-bound *CO2- intermediate, increasing the yield and selectivity toward CO production. Overall, this work unveils a powerful tool of organic ligand capping to modify the chemical environment on colloids, thus enabling control over the product selectivity within photocatalyzed CO2 reduction.