Z-Scheme Charge Flow Activates ZnCo₂S₄ as an Electron-Aggregation Site for Photocatalytic Hydrogen Evolution

Hydrogen energy, as a new, clean and renewable energy, has great potential to address the global energy crisis and environmental pollution. This study presents a ZnCo₂S₄/ZnIn₂S₄ (ZCS/ZIS) heterojunction photocatalyst, where the charge transfer mechanism ingeniously transforms the role of ZnCo₂S₄. While individually photocatalytically inactive, ZnCo₂S₄ becomes crucial in the ZCS/ZIS composite. Driven by a built-in electric field arising from their difference in Fermi level, a direct Z-scheme charge flow is established. This flow not only facilitates the separation of photogenerated electrons and holes at the interface but also, more importantly, activates ZnCo₂S₄ as a primary electron aggregation site. Consequently, strongly reductive electrons are efficiently enriched on ZnCo₂S₄ for the hydrogen evolution reaction. The optimized ZCS/ZIS composite achieves an exceptional H₂ production rate of 6.60 mmol g^-1 h^-1 under visible light, which is 9.3 times that of ZnIn₂S₄, with an apparent quantum yield of 7.96% at 400 nm. This work highlights the strategic design of charge dynamics to unlock the latent functionality of components within a Z-scheme system.