Defect Engineering in Two-Dimensional Graphitic Carbon Nitride and Application to Photocatalytic Air Purification

Since the pioneering work on polychlorinated biphenyl photodegradation by Carey in 1976, photocatalytic technology has emerged as a promising and sustainable strategy to overcome the significant challenges posed by energy crisis and environmental pollution. In photocatalysis, sunlight, which is an inexhaustible source of energy, is utilized to generate strongly active species on the surface of the photocatalyst for triggering photo-redox reactions toward the successful removal of environmental pollutants, or for water splitting. The photocatalytic performance is related to the photoabsorption, photoinduced carrier separation, and redox ability of the semiconductor employed as the photocatalyst. Apart from traditional and noble metal oxide semiconductors such as P25, bismuth-based compounds, and Pt-based compounds, 2D g-C3N4 is now identified to have enormous potential in photocatalysis owing to the special pi-pi conjugated bond in its structure. However, some inherent drawbacks of the conventional g-C3N4, including the insufficient visible-light absorption ability, fast recombination of photogenerated electron-hole pairs, and low quantum efficiency, decrease its photocatalytic activity and limit its application. To date, various strategies such as heterojunction fabrication, special morphology design, and element doping have been adopted to tune the physicochemical properties of g-C3N4. Recent studies have highlighted the potential of defect engineering for boosting the light harvesting, charge separation, and adsorption efficiency of g-C3N4 by tailoring the local surface microstructure, electronic structure, and carrier concentration. In this review, we summarize cutting-edge achievements related to g-C3N4 modified with classified non-external-caused defects (carbon vacancies, nitrogen vacancies, etc.) and external-caused defects (doping and functionalization) for optimizing the photocatalytic performance in water splitting, removal of contaminants in the gas phase and wastewater, nitrogen fixation, etc. The distinctive roles of various defects in the g-C3N4 skeleton in the photocatalytic process are also summarized. Moreover, the practical application of 2D g-C3N4 in air pollution control is highlighted. Finally, the ongoing challenges and perspectives of defective g-C(3)N(4 )are presented. The overarching aim of this article is to provide a useful scaffold for future research and application studies on defect-modulated g-C3N4