**3. Hydrogen Generation of g-C3N4-Based Photocatalysts**

In recent years, a review of the technological improvements of the photocatalytic efficiency of g-C3N4-based materials has been published, mostly focusing on contaminant removal, the reaction mechanisms, principles of photocatalysis, and the effects of operational parameters [50,51]. Therefore, the aim of this review is to summarize recent trends in the improvement of hydrogen production by photocatalysts using various methods for the purpose of improving g-C3N4-based photocatalytic hydrogen production: (1) modification of g-C3N4; (2) heterojunctions from g-C3N4/semiconductors; and (3) metal- and non-metal-doped g-C3N4.
