**2. The Principles of H2 Generation via Water Splitting**

Photocatalytic reactions can be divided into three parts. The first step is to obtain photons with energies that exceed the photocatalyst's band gap of the electron–hole pairs, the second step is the separation of the carrier in the photocatalyst by transfer, and the third step is the reaction between the carrier and H2O. In addition, the electron–hole pairs are concurrently combined with each other. The photocatalyst is involved in the production of hydrogen, but the lowest position of the CB should

be lower than the reduction position of H2O/H2 and the position of the VB should be higher than the potential of H2O/O2 [34–40].

Figure 2 shows the band gap and band edge positions of various semiconductor photocatalysts [41]. A variety of these, such as ZnO, TiO2, and WO3 have been studied for solar hydrogen production and degradation of organic pollutants [42–45]. However, although there are exceptions for some semiconductor photocatalysts, most of the semiconductor photocatalysts have low efficiency under visible-light irradiation. Therefore, it is a major challenge to develop photocatalysts that efficiently exploit solar energy.

**Figure 2.** A schematic illustration of the band-gap energy of several typical semiconductor photocatalysts. Reproduced with permission from [41].

Recently, g-C3N4, which has a unique electron band structure for photo-oxidation and reduction, has been confirmed by several researchers as an efficient photocatalyst for visible-light activation for photochemical reactions [46]. This achieves the photoexcited state when creating electron–hole pairs where photogenerated electrons are involved in the reduction process while the holes are consumed in the oxidation process [47]. The excited electrons and holes act as reactive species that are highly oxidizing and reducing. The excited electrons and holes travel to the active sites on the surface, thereby splitting the water into oxygen and hydrogen (Figure 3) [48]. However, despite its excellent electron and optical properties, g-C3N4 has low efficiency for visible-light utilization and a high recombination speed of photoelectric carrier, resulting in the poor formation of radical species causing redox reaction during the photocatalytic reaction [49]. It has a low specific surface area, provides fewer reactive sites, and reduces light harvesting. In addition, the low bandgap (2.7 eV) of g-C3N4 is still quite large for efficient visible-light harvesting and has limited use, leaving much of the visible-light spectrum unexploited.

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**Figure 3.** Schematic illustration of the charge-transfer mechanism of neat g-C3N4 as a photocatalyst. Reproduced with permission from [48]; copyright (2016), Royal Society of Chemistry Advances.
