Graphitic Carbon Nitride/Zinc Oxide-Based Z-Scheme and S-Scheme Heterojunction Photocatalysts for the Photodegradation of Organic Pollutants

Graphitic carbon nitride (g-C3N4), a metal-free polymer semiconductor, has been recognized as an attractive photocatalytic material for environmental remediation because of its low band gap, high thermal and photostability, chemical inertness, non-toxicity, low cost, biocompatibility, and optical and electrical efficiency. However, g-C3N4 has been reported to suffer from many difficulties in photocatalytic applications, such as a low specific surface area, inadequate visible-light utilization, and a high charge recombination rate. To overcome these difficulties, the formation of g-C3N4 heterojunctions by coupling with metal oxides has triggered tremendous interest in recent years. In this regard, zinc oxide (ZnO) is being largely explored as a self-driven semiconductor photocatalyst to form heterojunctions with g-C3N4, as ZnO possesses unique and fascinating properties, including high quantum efficiency, high electron mobility, cost-effectiveness, environmental friendliness, and a simple synthetic procedure. The synergistic effect of its properties, such as adsorption and photogenerated charge separation, was found to enhance the photocatalytic activity of heterojunctions. Hence, this review aims to compile the strategies for fabricating g-C3N4/ZnO-based Z-scheme and S-scheme heterojunction photocatalytic systems with enhanced performance and overall stability for the photodegradation of organic pollutants. Furthermore, with reference to the reported system, the photocatalytic mechanism of g-C3N4/ZnO-based heterojunction photocatalysts and their charge-transfer pathways on the interface surface are highlighted.

Automated summary

Graphitic carbon nitride (g-C3N4), a metal-free polymer semiconductor, has attracted significant attention as a promising photocatalytic material for environmental remediation. However, g-C3N4 faces challenges such as low specific surface area, limited visible-light utilization, and high charge recombination rate. To address these issues, researchers have explored the formation of heterojunctions between g-C3N4 and metal oxides. Zinc oxide (ZnO) has been widely studied as a suitable semiconductor photocatalyst to form heterojunctions with g-C3N4 due to its unique properties and simple synthetic procedure. The synergistic effect of g-C3N4/ZnO heterojunctions enhances the photocatalytic activity through improved adsorption and photogenerated charge separation. This review summarizes the strategies for fabricating g-C3N4/ZnO-based heterojunction photocatalytic systems and highlights the photocatalytic mechanism and charge-transfer pathways of these heterojunctions.