CdS/ZnSe Quantum Dot Assembled Clusters vs. Dot-on-Rod: Charge Separation and Utilization for Efficient Photocatalytic NO₃ ^--to-NH₃ Conversion.

Abstract

Photocatalytic NO₃ ^- reduction with semiconductor nanocrystals has promising prospects for ammonia (NH₃) synthesis, which typically relies on broad light absorption, efficient charge separation, and high surface reactivity. Represented herein is, however, contrary to the widely accepted facts that long-lived charge separation favors higher photocatalytic efficiency, i.e. ZnSe@CdS dot-on-rods with better charge separation unexpectedly yield NH₃ with much lower efficiency (4.10 mmol h^-1 g_cat. ^-1) than CdS/ZnSe assembled clusters (53.85 mmol h^-1 g_cat. ^-1). Mechanistic studies reveal that the intimate binding of ZnSe on CdS in dot-on-rods accelerates charge separation by 3 orders of magnitude, while the electron transfer from CdS to NO₃ ^- and the hole transfer from ZnSe to 1-phenylethanol proceed at 10⁸ s^-1. As a result, the comparable charge transfer rates in the assembled cluster of ZnSe and CdS quantum dots enable effective utilization of separated electrons and holes timely for photocatalytic NO₃ ^--to-NH₃ reaction, while the imbalance of fast charge separation and slow utilization of electrons and holes in dot-on-rods leads to inferior NH₃ yield. The kinetic balance for photocatalytic NO₃ ^--to-NH₃ reaction offers valuable guidance for orchestrating multi-step photochemical events to realize elegant transformations.