Abstract:

Photocatalytic hydrogen evolution is a scalable pathway to generate hydrogen fuels while mitigating environmental crisis. Strategies based on modification of the host photocatalyst surface are key to improve the adsorption/activation ability of the reaction molecules and the efficiency of charge transport, so that high-efficiency photocatalytic systems can be realized. Cadmium sulfide (CdS), a visible light-responsive semiconductor material, is widely used in photocatalysis because of its simple synthesis, low cost, abundant raw materials, and appropriate bandgap structure. Many researchers have focused on improving the photocatalytic efficiency of CdS because the rapid charge recombination in this material limits its applications. Among the various strategies proposed in this regard, surface modification is an effective and simple method used to improve the photocatalytic performance of materials. In this work, polyvinyl pyrrolidone (PVP)-capped CdS (denoted as P-CdS) nanopopcorns with hexagonal wurtzite (WZ)-cubic zinc blende (ZB) homojunctions were fabricated via one-step gamma-ray radiation-induced reduction under ambient conditions. Subsequent alkaline treatment under ambient conditions led to a dramatic improvement in the activity of the alkalized PVP-capped CdS (MP-CdS) photocatalyst. The structure and properties of the photocatalyst were determined by X-ray diffraction (XRD) analysis, field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) analysis, Brunauer-Emmett-Teller (BET) specific surface area measurements, and photoelectric tests. The photocatalytic performance was evaluated based on the photocatalytic H₂ evolution under visible-light irradiation. The mechanism underlying the enhancement of the photocatalytic activity is also discussed. The results showed that after the alkaline treatment, the crystal structure of CdS with WZ-ZB homojunctions was preserved, but PVP on the surface of CdS hydrolyzed to form PVP hydrolysis product (MPVP) with carboxyl and amino groups. Owing to the increased alkaline solubility, a portion of MPVP dissolved into the solution and was removed from the surface of MP-CdS, exposing a greater number of active sites of the WZ-ZB phase junctions with a larger specific surface area. On the other hand, the carboxyl groups in MPVP coordinated with CdS could affect the bandgap and valence band position of CdS to facilitate the photocatalysis. Because of the synergistic effects of the exposure of WZ-ZB phase junctions and band structure engineering, the alkalized samples at a 1 mol·L^-1 concentration of NaOH showed a H₂ evolution rate of 477 μmol·g^-1·h^-1 under visible-light illumination, which was twice that obtained for the pristine P-CdS photocatalysts. This simple and low-cost post-synthesis strategy can be extended to the preparation of diverse functional photocatalysts. The present work is expected to contribute to the practical application of sulfide photocatalysts.

Key words: CdS, Homojunction, Alkaline treatment, Photocatalytic hydrogen evolution