Extending the Optical Absorption Limit of Graphitic Carbon Nitride Photocatalysts: A Review

Due to its chemical/thermal stability, metal-free nature, abundance, low toxicity, cost-effectiveness, high surface area, tunable properties, and ease of synthesis, graphitic carbon nitride (g-C₃N₄) is a promising material for applications in catalysis, sensing, energy storage and optoelectronics. By virtue of its optical band gap, g-C₃N₄ has been extensively used to drive a range of photocatalytic reactions, such as H₂ generation, CO₂ reduction, N₂ fixation, and organic transformation, to name a few. Despite these prospects, significant improvement in extending its optical absorption is essential because g-C₃N₄ absorbs light only up to 460 nm (≈10% of the incoming sunlight). Thus, reducing its band gap to harness a wider part of the sunlight is a key approach to advancing the solar energy conversion efficiency of g-C₃N₄. In this direction, the effect of (i) improving the degree of polymerization, (ii) molecular and elemental doping, (iii) distorting planar structure, (iv) inducing nitrogen vacancies, and (v) tuning the C/N ratio of g-C₃N₄ on red-shifting the optical absorption is analyzed in detail. A comprehensive correlation between the synthesis approach/conditions-structure-optical property is established. Rational guidelines on extending the spectral response of g-C₃N₄ toward the visible/near-infrared region and using low-energy photons to drive photocatalytic reactions efficiently are detailed. The insights presented will help to utilize the full potential of g-C₃N₄ to enhance the solar fuel generation efficiency and in understanding the mechanism of light absorption.