Abstract:

Solar energy is the largest renewable energy source in the world and the primary energy source of wind energy, tidal energy, biomass energy, and fossil fuel. Photocatalysis technology is a sunlight-driven chemical reaction process on the surface of photocatalysts that can generate H₂ from water, decompose organic contaminants, and reduce CO₂ into organic fuels. As a metal-free polymeric material, graphite-like carbon nitride (g-C₃N₄) has attracted significant attention because of its special band structure, easy fabrication, and low costs. However, some bottlenecks still limit its photocatalytic performance. To date, numerous strategies have been employed to optimize the photoelectric properties of g-C₃N₄, such as element doping, functional group modification, and construction of heterojunctions. Remarkably, these modification strategies are strongly associated with the surface behavior of g-C₃N₄, which plays a key role in efficient photocatalytic performance. In this review, we endeavor to provide a comprehensive summary of g-C₃N₄-based photocatalysts prepared through typical surface modification strategies (surface functionalization and construction of heterojunctions) and elaborate their special light-excitation and response mechanism, photo-generated carrier transfer route, and surface catalytic reaction in detail under visible-light irradiation. Moreover, the potential applications of the surface-modified g-C₃N₄-based photocatalysts for photocatalytic H₂ generation and reduction of CO₂ into fuels are summarized. Finally, based on the current research, the key challenges that should be further studied and overcome are highlighted. The following are the objectives that future studies need to focus on: (1) Although considerable effort has been made to develop a surface modification strategy for g-C₃N₄, its photocatalytic efficiency is still too low to meet industrial application standards. The currently obtained solar-to‑hydrogen (STH) conversion efficiency of g-C₃N₄ for H₂ generation is approximately 2%, which is considerably lower than the commercial standards of 10%. Thus, the regulation of the surface/textural properties and electronic band structure of g-C₃N₄ should be further elucidated to improve its photocatalytic performance. (2) Significant challenges remain in the design and construction of g-C₃N₄-based S-scheme heterojunction photocatalysts by facile, low-cost, and reliable methods. To overcome the limitations of conventional heterojunctions thoroughly, a promising S-scheme heterojunction photocatalytic system was recently reported. The study further clarifies the charge transfer route and mechanism during the catalytic process. Thus, the rational design and synthesis of g-C₃N₄-based S-scheme heterojunctions will attract extensive scientific interest in the next few years in this field. (3) First-principle calculation is an effective strategy to study the optical, electrical, magnetic, and other physicochemical properties of surface strategy modified g-C₃N₄, providing important information to reveal the charge transfer path and intrinsic catalytic mechanism. As a result, density functional theory (DFT) computation will be paid increasing attention and widely applied in surface-modified g-C₃N₄-based photocatalysts.

Key words: Photocatalysis, H₂ generation, CO₂ reduction, Surface modification, Heterojunction