Graphitic carbon nitride (g-C3N4) has emerged as a promising photocatalytic material for hydrogen evolution due to its unique layered structure, favorable electronic properties, and robust chemical stability. Composed of carbon and nitrogen arranged in a two-dimensional framework resembling graphite, g-C3N4 exhibits a bandgap of approximately 2.7 eV, enabling visible-light absorption. This property distinguishes it from many traditional photocatalysts, such as TiO2, which primarily absorb ultraviolet light. The material’s ability to harness solar energy efficiently while maintaining stability in aqueous and acidic environments makes it a compelling candidate for sustainable hydrogen production.

The layered structure of g-C3N4 consists of tri-s-triazine units connected by tertiary amines, forming extended sheets with strong covalent bonds within layers and weak van der Waals forces between them. This arrangement facilitates electron delocalization, enhancing charge carrier mobility. The conduction band position of g-C3N4 is sufficiently negative to reduce protons to hydrogen, while the valence band allows for water oxidation, making it suitable for overall water splitting. However, the efficiency of pristine g-C3N4 is often limited by rapid charge recombination and a relatively low surface area, which restricts active sites for photocatalytic reactions.

Synthesis methods for g-C3N4 significantly influence its photocatalytic performance. Thermal polymerization of nitrogen-rich precursors, such as melamine, urea, or thiourea, is the most common approach. Heating these precursors to temperatures between 500 and 600 degrees Celsius in an inert atmosphere results in the condensation of g-C3N4 layers. The choice of precursor and heating rate affects the material’s crystallinity, porosity, and defect density. For instance, urea-derived g-C3N4 tends to exhibit higher surface area and improved photocatalytic activity compared to melamine-derived samples due to the release of gaseous byproducts during polymerization, which creates porous structures.

Templating methods further enhance the surface area and porosity of g-C3N4. Hard templates, such as silica nanoparticles or mesoporous carbon, are incorporated during synthesis and later removed, leaving behind a porous architecture. Soft templates, including surfactants or ionic liquids, can also guide the growth of g-C3N4 nanostructures. These approaches yield materials with increased active sites and improved mass transfer properties, critical for efficient hydrogen evolution.

Modifications to g-C3N4 through elemental doping and copolymerization have been extensively explored to optimize its electronic structure and light absorption. Non-metal doping, such as sulfur, phosphorus, or boron, introduces new energy levels within the bandgap, narrowing it and extending light absorption into the visible spectrum. Metal doping, including iron or cobalt, can enhance charge separation by acting as electron traps. Copolymerization with organic monomers, such as barbituric acid or 2,4-diaminotriazine, alters the electronic configuration and improves photocatalytic activity by promoting charge delocalization.

Co-catalysts play a pivotal role in enhancing the hydrogen evolution performance of g-C3N4. Noble metals like platinum are highly effective due to their low overpotential for proton reduction, but their high cost limits scalability. Alternatives such as nickel sulfide (NiS), cobalt phosphide (CoP), or molybdenum disulfide (MoS2) have shown promising results. These co-catalysts provide active sites for hydrogen adsorption and reduce recombination by facilitating electron transfer. The integration of co-catalysts with g-C3N4 can be achieved through in-situ growth, photodeposition, or mechanical mixing, with each method influencing dispersion and interfacial contact.

The mechanism of photocatalytic hydrogen evolution over g-C3N4 involves several steps. Upon light absorption, electrons are excited from the valence band to the conduction band, leaving holes behind. The electrons migrate to the co-catalyst surface, where they reduce protons to hydrogen gas. Simultaneously, the holes participate in water oxidation or are scavenged by sacrificial agents, such as triethanolamine or methanol, which are often added to suppress recombination. The efficiency of this process depends on the separation and migration of charge carriers, which can be improved through nanostructuring and heterojunction design.

Despite its advantages, g-C3N4 faces limitations that hinder its practical application. The material’s low surface area restricts the number of active sites, while fast charge recombination reduces the quantum efficiency. Recent breakthroughs have addressed these challenges through nanostructuring techniques, such as exfoliation into ultrathin nanosheets or fabrication of quantum dots. These nanostructures exhibit higher surface areas and shorter charge migration paths, enhancing photocatalytic activity. Additionally, constructing heterojunctions with other semiconductors, such as TiO2, CdS, or WO3, improves charge separation by creating interfacial electric fields or Z-scheme mechanisms.

Recent advancements in g-C3N4-based photocatalysts have demonstrated remarkable improvements in hydrogen evolution rates. For example, sulfur-doped g-C3N4 coupled with NiS co-catalysts has achieved hydrogen production rates exceeding 1000 micromoles per hour under visible light. Similarly, porous g-C3N4 nanosheets prepared via templating methods have shown enhanced activity due to improved light absorption and charge carrier mobility. These developments highlight the potential of g-C3N4 as a cost-effective and sustainable photocatalyst for large-scale hydrogen production.

In summary, graphitic carbon nitride stands out as a versatile photocatalytic material for hydrogen evolution, offering visible-light absorption, chemical stability, and tunable electronic properties. Advances in synthesis methods, modifications, and co-catalyst integration have significantly improved its performance, while nanostructuring and heterojunction design address inherent limitations. Ongoing research continues to explore novel strategies to enhance efficiency and scalability, positioning g-C3N4 as a key player in the transition toward renewable hydrogen energy.