Rational Synthesis of S-Scheme CdS/SnS2 Photocatalysts with Isolated Redox Cocatalysts for Enhanced H2 Production

A novel quaternary composite (10-NT6/CC8) comprising a heterostructure 1D CdS and 2D SnS2 embedded with spatially separated redox cocatalysts (cobalt phosphate (CoPi) and nickel phosphide (Ni2P)) was prepared by successive hydrothermal steps. The structural and elemental characteristics have been investigated by transmission electron microscopy, X-ray/ultraviolet photoelectron spectroscopy, UV–vis diffuse reflectance spectra, and N2 adsorption–desorption isotherms. The charge transfer at the interfaces resulted in reduced recombination and decay lifetimes, leading to a high charge carrier density, low charge transfer resistance, and better photocurrent density as evidenced by the photocurrent and photoluminescence studies. The in situ ESR measurements confirmed the production of highly capable redox charge carriers in the composite photocatalysts and confirmed the hole-mediated oxidation of lactic acid to produce hydrogen. Gibbs free energy from density functional theory calculations suggested that CoPi and Ni2P acted as favorable adsorption sites, which ultimately reduced the activation energy for the OER and HER reactions, respectively. 10-NT6/CC8 showed commendable hydrogen (H2) production activity (360.75 μL h–1) that was 30 times to that of the bare CdS (12.25 μL h–1). The 10-NT6/CC8 photocatalyst even produced 44.81 μL h–1 H2 from pure water (in the absence of a lactic acid scavenger) under solar light irradiation. The continuous time-on-stream hydrogen production experiments suggested the photostability of 10-NT6/CC8 even after 7 days of continuous photoactivity. The data obtained by Mott–Schottky and UPS analysis displayed an S-scheme charge transfer mechanism between the CdS and SnS2 heterojunction. The present work contributes to the design and development of various novel combinations of composite heterostructures and redox catalysts for improved photocatalytic hydrogen production.