Graphitic carbon nitride (g-C3N4) synthesis and heterostructures, principles, mechanisms, and recent advances: A critical review

Graphitic carbon nitride (g-C3N4) is a semiconductor polymeric photocatalyst with an attractive electronic band structure, a moderate band gap energy, facile synthesis, and functionalization which can be applied as a photocatalyst in the visible light of the spectrum. The main problem of the g-C3N4 photocatalyst is the recombination of the photogenerated electron-hole pairs. The photogenerated electrons in the conduction band (CB) tend to return to the valence band (VB) with subsequent recombination which is unfavored for photocatalysis. It is difficult for a single-component photocatalyst to harvest a large portion of the sunlight spectrum, and simultaneously, possess a suitable spatial charge separation and efficient redox ability. Constructing a heterojunction aims to satisfy the above three factors in a photocatalyst heterojunction system. Constructing g–C3N4–based heterostructures can promote electron-hole pair separation through the charge transfer across the interface of the g-C3N4/semiconductor. In this review, the photocatalysis mechanism is discussed and several types of g-C3N4/semiconductor heterostructures including type II, Z-scheme, and S-scheme heterostructures are explained. Recent advances in different types of g–C3N4–based heterostructures have been addressed. The synthesis methods for mesoporous, 0D, 1D, and 3D g–C3N4 and different modifications on this photocatalyst are reviewed.