S-Doping of the N-Sites of g-C3N4 to Enhance Photocatalytic H2 Evolution Activity

Abstract

The use of solar energy as an inexhaustible resource to conduct photocatalytic water splitting in hydrogen (H₂) production can alleviate the worldwide energy crisis and achieve carbon neutrality. However, research in photocatalytic H₂ evolution reaction (HER) is extremely challenging in terms of exploring the current development of an active and durable graphitic carbon nitride (g-C₃N₄)-based photocatalyst. Several parameters of pristine g-C₃N₄ require structural, physical, and chemical improvements, such as optimization of the surface area, electron transfer, and photo-generated carrier recombination, to render the g-C₃N₄ suitable for photocatalysis. In this study, the development of an efficient and robust S-doped g-C₃N₄ (S-g-CN) catalyst was pursued that involves doping nitrogen (N) active sites of g-C₃N₄ with sulfur (S) dopants via one-step calcination of the sulphate and melamine precursors. A combination of structural and spectroscopic fingerprints was established to distinctly determine the realization of S-doping onto the g-C₃N₄ structure. We obtained the optimum Gibbs free energy of adsorbed hydrogen (ΔG_H*) for S-g-CN at the S active sites, which is nearly zero (~0.26 eV), suggesting that the filled S dopants play an essential role in optimizing the adsorption and desorption processes of H-active intermediates. The results of atomic force and transmission electron microscopies (AFM and TEM) demonstrated that the produced S-g-CN catalyst has an ultrathin nanosheet structure with a lamellar thickness of approximately 2.5 nm. A subsequent N₂ sorption isotherms test revealed a substantial increase in the specific surface area after the integration of S dopants into the g-C₃N₄ nanoskeleton. Moreover, the incorporation of S atoms into the g-C₃N₄ significantly increased the carrier concentrations, improving the transfer, separation, as well as the oxidation and reduction abilities of the photo-generated electron-hole pairs. Leveraging the favorable material characteristics of the S-doped two-dimensional nanostructures, the resulting S-g-CN achieved a high H₂ evolution rate of 4923 μmol·g⁻¹·h⁻¹, which is 28 times higher than that of the pristine g-C₃N₄. Additionally, the developed S-g-CN possessed a high apparent quantum efficiency (3.64%) at visible-light irradiation. When compared to the recently reported S-doped g-C₃N₄-based photocatalysts, our optimal S-g-CN catalyst (S-CN_1.0) showed one of the best HER catalytic activities. Our rational design is based on an effective strategy that not only explored a promising HER photocatalyst but also aimed to pave the way for the development of other high-performance g-C₃N₄ based catalysts.

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