Graphitic carbon nitride (g-C3N4) has emerged as a promising photocatalyst due to its visible-light absorption, chemical stability, and tunable electronic structure. Its photocatalytic activity stems from a combination of optical and electronic properties, including a moderate bandgap (approximately 2.7 eV), suitable conduction and valence band positions, and efficient charge separation under illumination. The material’s polymeric nature, composed of tri-s-triazine units connected by tertiary amines, provides abundant active sites for redox reactions.
In water splitting, g-C3N4 facilitates hydrogen evolution by absorbing photons to generate electron-hole pairs. The excited electrons reduce protons to H2, while holes oxidize water or sacrificial agents. However, rapid charge recombination and slow surface kinetics limit efficiency. Strategies to enhance performance include nanostructuring to increase surface area, such as creating porous or ultrathin nanosheets, which shorten charge migration distances. Cocatalysts like Pt, Ni, or MoS2 are often loaded to lower the overpotential for hydrogen evolution. Nitrogen vacancies or doping with elements like sulfur or phosphorus can also modify the band structure, improving light absorption and charge separation.
For CO2 reduction, g-C3N4 converts CO2 into hydrocarbons like CO, CH4, or CH3OH under visible light. The process relies on the conduction band electrons reducing CO2, while valence band holes are scavenged by sacrificial reagents. The challenge lies in the high activation energy for CO2 reduction and competing hydrogen evolution. Morphology control, such as introducing mesopores or defects, enhances CO2 adsorption and active site exposure. Cocatalysts like Cu or Co oxides promote selective reduction pathways. Hybridizing g-C3N4 with conductive materials like graphene accelerates electron transfer, while heterojunctions with other semiconductors (e.g., TiO2 or CdS) widen the light absorption range and suppress recombination.
Pollutant degradation leverages g-C3N4’s strong oxidative holes to mineralize organic contaminants. The holes generate hydroxyl radicals or directly oxidize pollutants, while electrons reduce oxygen to superoxide radicals. Efficiency is improved by optimizing crystallinity to reduce bulk recombination and engineering surface functional groups to enhance pollutant adsorption. Metal-free modifications, such as oxygen doping or creating carbon-rich domains, enhance visible-light activity and stability.
Key strategies to boost photocatalytic efficiency across these applications include:
1. **Morphology Control**
- Porous structures increase surface area and active sites.
- Ultrathin nanosheets reduce charge diffusion distances.
- Hierarchical architectures improve light scattering and absorption.
2. **Cocatalyst Loading**
- Noble metals (Pt, Au) enhance hydrogen evolution.
- Transition metal compounds (NiOx, CoP) lower overpotentials.
- Dual cocatalysts separate reduction and oxidation sites.
3. **Defect Engineering**
- Nitrogen vacancies improve charge separation.
- Doping (B, P, S) adjusts band positions and conductivity.
4. **Heterojunction Design**
- Type-II heterojunctions (e.g., g-C3N4/WO3) enhance charge separation.
- Z-scheme systems maintain high redox potentials.
5. **Surface Modification**
- Functional groups (-NH2, -COOH) enhance reactant adsorption.
- Amorphous-crystalline interfaces promote charge transfer.
Despite progress, challenges remain in scaling up synthesis, ensuring long-term stability, and achieving high quantum yields. Future research may focus on precise atomic-level modifications and advanced characterization to unravel reaction mechanisms at the nanoscale. The versatility of g-C3N4 positions it as a key material in sustainable photocatalysis, provided efficiency and scalability hurdles are addressed.
The development of g-C3N4-based systems exemplifies how structural and electronic tuning can unlock multifunctional photocatalytic performance. By integrating material design with mechanistic insights, researchers can further optimize its applications in energy conversion and chemical synthesis.