Photon recycling-assisted Z-scheme charge transfer in ZnCdO/CdS heterostructures for enhanced photocatalytic hydrogen evolution

A multi-phase ZnCdO/CdS (ZCS) heterostructure photocatalyst is synthesized via a thiourea-mediated sulfur anion exchange, resulting in a Z-scheme heterojunction for enhanced photocatalytic performance. This approach converts surface CdO into CdS and CdNCN, producing a heterojunction network composed of ZnO, CdO, CdS, and CdNCN. Optimization of solvent polarity and Zn:Cd precursor ratio is found to be critical, with the Z8C2 composition (50:50 methanol-water solvent) showing the best performance. The optimized sample exhibits a narrowed band gap (∼2.42 eV) for improving visible-light absorption and efficient electron-hole separation for leading to superior hydrogen evolution activity. In this configuration, photogenerated electrons in the CdS conduction band drive hydrogen reduction, while holes in the valence bands of ZnO and CdO facilitate water oxidation, maintaining strong redox potentials. This Z-scheme mechanism promotes efficient charge separation and suppresses recombinations in bulk states. In contrast, monolithic ZnO showed higher recombination, resulting in lower hydrogen production. A photon recycling effect was also identified, where ZnO emission is reabsorbed by CdS to generate additional charge carriers, further enhancing photocatalytic activity. Overall, this study demonstrates that sulfur anion exchange is an effective strategy for constructing Z-scheme heterojunctions and offers valuable insights into band structure engineering for efficient solar-driven hydrogen production.