Photocatalytic Ammonia Synthesis Boosted by Inorganic Metal–Solvent Complexes on Indium Phosphide Quantum Dots

Abstract

Surface modification of colloidal semiconductor nanocrystals (NC) with inorganic ligands is known to enhance charge transport in NC-based optoelectronic devices. We have adapted this strategy to the emerging field of quantum dot (QD) photocatalysis to boost ammonia synthesis under visible light irradiation. Herein, a positively charged inorganic Lewis acid metal-solvent complex is used as a surface ligand on indium phosphide (InP) QD to facilitate photoinduced charge transfer from QD photocatalyst to nitrate ions. Additionally, the cationic surface of metal-solvent complex-capped InP QDs assists the electrostatic channeling of nitrate ions toward the QD surface, thereby further enhancing the charge extraction. The nitrate-to-ammonia conversion approaches near completion within 30 min of visible light illumination, with ammonia as the sole product detected in both aqueous and gaseous phases. The use of inorganic ligands results in 16-fold and 4-fold enhancements in reaction rate and apparent quantum efficiency, respectively, compared to conventional organic alkyl ligands. The apparent quantum yield reached a maximum of ∼6%, which is one of the highest values reported to date for the visible light-driven reduction of nitrate to ammonia. Water is identified as the proton source. We further demonstrate the broader applicability of other Lewis acid metal halide complexes as surface ligands for InP QDs in photocatalytic ammonia synthesis. Overall, our study shows the significance of the “ligand of choice” approach and catalyst–reactant interactions in regulating the photocatalytic performance of QDs for multielectron reactions that demand efficient and directional electron flow.