Abstract:

Sustainable photocatalytic H₂ evolution has attracted extensive attention in recent years because it can address both energy shortage and environmental pollution issues. In particular, metal sulfide solid-solution photocatalysts have been widely applied in photocatalytic hydrogen generation owing to their excellent light harvesting properties, narrow enough band gap, and suitable redox potentials of conduction and valance bands. However, it is still challenging to develop low-cost and high-efficiency sulfide solid-solution photocatalysts for practical photocatalytic hydrogen evolution. Recently, 1D Mn_xCd_1-xS nanostructures have shown superior light absorption, charge separation, and H₂-evolution activity owing to their shortened diffusion pathway of carriers and high length-to-diameter ratios. Thus, 1D Mn_xCd_1-xS nanostructures have been applied in photocatalytic H₂ evolution. However, a single Mn_xCd_1-xS photocatalyst still has some disadvantages for photocatalytic H₂ evolution, such as the rapid recombination of photogenerated electron-hole pairs and low quantum efficiency. Herein, to further boost the separation of photogenerated charge carriers and H₂-evolution kinetics, an in situ solvothermal method was used to synthesize the 1D/2D Schottky-based heterojunctions between the Mn_0.2Cd_0.8S nanorods (MCS NRs) and Ti₃C₂ MXene nanosheets (NSs). Furthermore, various characterization methods have been used to investigate the crucial roles and underlying mechanisms of metallic Ti₃C₂ MXene NSs in boosting the photocatalytic H₂ evolution over the Mn_0.2Cd_0.8S nanorods. X-ray Diffraction (XRD), Transmission Electron Microscope (TEM), High Resolution Transmission Electron Microscopy (HRTEM), element mapping images, and X-ray Photoelectron Spectroscopy (XPS) results clearly demonstrate that hybrid low-cost Schottky-based heterojunctions have been successfully constructed for practical applications in photocatalytic H₂ evolution. Additionally, the photocatalytic hydrogen evolution reaction (HER) was also carried out in a mixed solution of Na₂SO₃ and Na₂S using as the sacrificial agents. The highest hydrogen evolution rate of the optimized 1D/2D Schottky-based heterojunction is 15.73 mmol·g^-1·h^-1, which is 6.72 times higher than that of pure MCS NRs (2.34 mmol·g^-1·h^-1). An apparent quantum efficiency of 19.6% was achieved at 420 nm. The stability measurements of the binary photocatalysts confirmed their excellent photocatalytic stability for practical applications. More interestingly, the UV-Vis diffuse reflection spectra, photoluminescence (PL) spectrum, transient photocurrent responses, and Electrochemical Impedance Spectroscopy (EIS) Nyquist plots clearly confirmed the promoted charge separation between the MCS NRs and Ti₃C₂ MXene NSs. The linear sweep voltammetry also showed that the loading of MXene cocatalysts could greatly decrease the overpotential of pure MCS NRs, suggesting that the 2D Ti₃C₂ NSs could act as an electronic conductive bridge to improve the H₂-evolution kinetics. In summary, these results show that the 2D/1D hybrid Schottky-based heterojunctions between metallic Ti₃C₂ MXene NSs and MCS NRs can not only improve the separation of photogenerated electrons and holes but also decrease the H₂-evolution overpotential, thus resulting in significantly enhanced photocatalytic H₂ generation. We believe that this study will inspire new ideas for constructing low-cost Schottky-based heterojunctions for practical applications in photocatalytic H₂ evolution.

Key words: Photocatalytic hydrogen evolution, 1D Mn_0.2Cd_0.8S nanorods, Ti₃C₂ MXene NSs, 2D/1D Schottky-based heterojunctions, Solar fuel