Insights into Photocatalytic Mechanism of H2 Production Integrated with Organic Transformation over WO3/Zn0.5Cd0.5S S-Scheme Heterojunction

Abstract

Developing novel nanostructures to enhance the efficiency of solar-tochemical conversion through integrated photocatalytic hydrogen (H₂) evolution and organic transformation holds great promise in addressing pressing energy and environmental crises. Ternary metal sulfides have garnered considerable attention in photocatalytic H₂ production due to their tunable bandgap and excellent visible light response. Among them, Zn_0.5Cd_0.5S stands out as a reduction photocatalyst with a narrow bandgap, a high conduction band level, and excellent resistance to photocorrosion. However, unitary Zn_0.5Cd_0.5S suffers from a high recombination rate of photogenerated electron/hole pairs, resulting in only a small fraction of charge carriers being involved in the photoreactions, leading to a low quantum efficiency that falls short of practical demand. WO₃, a typical oxidation photocatalyst with a lower valence band position and strong oxidization ability, is an ideal candidate for constructing an S-scheme heterojunction with Zn_0.5Cd_0.5S. Herein, a core-shell structured WO₃/Zn_0.5Cd_0.5S heterojunction with Zn_0.5Cd_0.5S nanosheets vertically growing out of WO₃ nanofibers is fabricated through electrospinning and hydrothermal methods. The distinct disparity in work functions leads to the transfer of electrons from Zn_0.5Cd_0.5S to WO₃ upon contact, creating an interfacial electric field (IEF) and simultaneously bending the energy bands at the interface. As a consequence of IEF, bent energy bands, and coulomb attraction, the photogenerated electrons in the conduction band of WO₃ migrate to the valence band of Zn_0.5Cd_0.5S and recombine with its photoinduced holes, signifying the formation of an S-scheme heterojunction between WO₃ and Zn_0.5Cd_0.5S and enabling efficient separation of powerful charge carriers, as evidenced by in situ irradiated X-ray photoelectron spectroscopy, electron paramagnetic resonance, and time-resolved fluorescence spectroscopy analyses. Benefiting from the unique S-scheme photocatalytic mechanism, along with the effective chemisorption and activation of reactants on the catalyst, the optimized WO₃/Zn_0.5Cd_0.5S heterostructures exhibit exceptional photocatalytic performance in H₂ production (715 μmol·g⁻¹·h⁻¹) and the transformation from lactic acid to pyruvic acid without the need for any noble metal cocatalyst, achieving the full utilization of photoinduced electrons and holes. In situ diffuse reflectance infrared Fourier transform spectroscopy, as well as density functional theory simulations, reveal the photoreaction mechanism of H₂ production and organic transformation. This work offers valuable insights into the design and investigation of the mechanism behind novel S-scheme heterojunction photocatalysts, enabling high-performance H₂ production and simultaneous organic transformation.

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