Investigating the Control by Quantum Confinement and Surface Ligand Coating of Photocatalytic Efficiency in Chalcopyrite Copper Indium Diselenide Nanocrystals

In the past few years, there has been immense interest in the preparation of sustainable photocatalysts composed of semiconductor nanocrystals (NCs) as one of their components. We report here, for the first time, the effects of structural parameters of copper indium diselenide (CuInSe₂) NCs on visible light-driven photocatalytic degradation of pollutants under homogeneous conditions. Ligand exchange reactions were performed replacing insulating, oleylamine capping with poly(ethylene glycol) thiols to prepare PEG-thiolate-capped, 1.8-5.3 nm diameter CuInSe₂ NCs to enhance their solubility in water. This unique solubility property caused inner-sphere electron transfer reactions (O₂ to O₂•-) to occur at the NC surface, allowing for sustainable photocatalytic reactions. Electrochemical characterization of our dissolved CuInSe₂ NCs showed that the thermodynamic driving force (-ΔG) for oxygen reduction, which increased with decreased NC size, was the dominant contributor to the overall process when compared to the contribution light absorption and the Coulombic interaction energies of electron-hole pair (J_e/h). A 2-fold increase in phenol degradation efficiency (from 30 to ∼60%) was achieved by controlled variation of the diameter of CuInSe₂ NCs from 5.3 to 1.8 nm. The surface ligand dependency of photocatalytic efficiency was also investigated, and a profound effect on phenol degradation was observed. Our PEG-thiolate-capped CuInSe₂ NCs showed photocatalytic activity toward other organic compounds, such as N,N-dimethyl-4-phenylenediamine, methylene blue, and thiourea, which showed decomposition under visible light.