A Comprehensive Review on the Boosted Effects of Anion Vacancies in Photocatalytic Solar Water Splitting: Focus on g-C3N4 with Carbon and Nitrogen Vacancies

As one of the best sustainable approaches for visible-light production of hydrogen (H 2 ) to meet energy demands, semiconductor-based photocatalysis has received broad interest in recent decades. The fundamental restrictions of graphitic carbon nitride (g-C 3 N 4 ) as a promising metal-free photocatalytic semiconductor in water splitting, like insufficient visible-light harvesting and high electron–hole (e/h) pairs recombination, have limited its applications for this goal. In this regard, the optical, charge separation, and surface features of g-C 3 N 4 can be tuned via engineering C/N vacancies, which is reviewed here for water splitting by g-C 3 N 4 . Reports confirm that the enhanced surface features of g-C 3 N 4 resulting from the engineering of C/N vacancies are helpful for water adsorption on its surface, improving the water-splitting kinetics. First in this Review, recent improvements to the structural and optical characteristics of g-C 3 N 4 by introducing C/N vacancies, especially nitrogen vacancies, will be discussed to illustrate its better photocatalytic performance. Then, various strategies for creating and controlling C/N vacancies will be reviewed. The critical roles of C/N vacancies for optimizing photocatalytic performance will also be described, and finally advances in defective photocatalysis water oxidation will be addressed.