Unveiling the Origin of Co₃ O₄ Quantum Dots for Photocatalytic Overall Water Splitting

Spinel cobalt oxide displays excellent photocatalytic performance, especially in solar driven water oxidation. However, the process of water reduction to hydrogen is considered as the Achilles' heel of solar water splitting over Co₃ O₄ owing to its low conduction band. Enhancement of the water splitting efficiency using Co₃ O₄ requires deeper insights of the carrier dynamics during water splitting process. Herein, the carrier dynamic kinetics of colloidal Co₃ O₄ quantum dots-Pt hetero-junctions is studied, which mimics the hydrogen reduction process during water splitting. It is showed that the quantum confinement effect induced by the small QD size raised the conduction band edge position of Co₃ O₄ QDs, so that the ligand-to-metal charge transfer from 2p state of oxygen to 3d state of Co²⁺ occurs, which is necessary for overall water splitting and cannot be achieved in Co₃ O₄ bulk crystals. The findings in this work provide insights of the photocatalytic mechanism of Co₃ O₄ catalysts and benefit rational design of Co₃ O₄ -based photocatalytic systems.