**1. Introduction**

As interest in the fossil fuel depletion and environmental pollution has increased, the development of clean energy has also recently attracted increased attention. It is important to find new alternative energy sources because of the increased use of energy, depletion of fossil fuels, and the need for sustainable energy development [1]. Among the many alternative energy sources, hydrogen-based energy systems are considered candidates for future energy because they are nonpolluting, inexhaustible, efficient, and can provide high-quality energy services in a wide range of applications [2,3]. However, most hydrogen production processes are based on natural gas [4], coal [5], crude oil [6], or the electrolysis of water [7], and unfortunately, the application of most of these processes is limited because heat and electrical energy are required. Thus, photocatalytic hydrogen production using solar energy, a clean energy resource for the foreseeable future, is considered to be an attractive way of solving the global energy issue and environmental pollution [8,9].

The overall water splitting by a photocatalyst under sunlight irradiation enables the production of environmentally friendly molecular hydrogen and does not use fossil fuel [10]. A photocatalytic system should consider the following prerequisites. First, to absorb as many photons as possible, the photocatalyst must have a narrow band-gap; to generate hydrogen from water splitting, the bottom of the conduction band (CB) must be more negative than the reduction potential of H+/H2 and the top of the valence band (VB) must be more positive than the oxidation potential of H2O/O2 [11]. Second, efficient charge separation and fast charge transport that simultaneously avoid bulk and surface charge recombination are essential to transfer the photogenerated charge to the surface reaction site [12]. Third, because the charge carriers at the interface lack the capacity to boost the transportation process, the charge carriers mostly move via a random path and require a surface chemical reaction that is active between the charge carrier and the water or other molecules [13]. A variety of semiconductor materials such as TiO2, ZnO, CdS, and WO3 have been extensively studied for hydrogen generation via photocatalytic water splitting [14–17]. Among them, WO3 absorbs visible light but has a problem in that the CB is not useful for hydrogen production because it is lower than the H reduction potential [18,19]. In addition, hydrogen evolution through photocatalytic water splitting has been extensively studied for metal oxides, quantum dots, and metal–organic frameworks, etc. However, some methods are difficult to use due to their low efficiency under visible light and the fast recombination rate of the electron–hole pairs [20–25]. Therefore, it is a major challenge to develop photocatalysts that exhibit stable water-splitting performance under visible-light irradiation for the efficient use of solar energy.

Recently, graphitic carbon nitride (g-C3N4) has attracted attention as a hydrogen-generating photocatalyst via water splitting. g-C3N4 is synthesized by the thermal condensation of nitrogen-rich precursors with a tri-s-triazine ring structure such as cyanamide, dicyandiamide, urea, or thiourea, resulting in a graphene-like structure after exfoliation (Figure 1) [26]. In addition, it has a band gap of ~2.7 eV corresponding to 460 nm in the visible range and high thermal and chemical stability [27].

**Figure 1.** Schematic illustration of the synthesis process from the possible precursors of g-C3N4. Reproduced with permission from [26]; copyright (2016), the American Chemical Society.

However, there are some drawbacks to using g-C3N4 as a water-splitting photocatalyst. The relatively large band-gap and low charge-carrier mobility limit the electron and hole separation and transport and thus limit the effective use of visible light [28]. Thus, increasing hydrogen production during photocatalytic water splitting under visible-light irradiation is necessary through a variety of methods such as creating heterojunctions with semiconductors and doping with other elements [29–33]. As a result, the focus of this review is on summarizing the current and prospective advances in photocatalysis research based on g-C3N4 that make it effective even under visible-light irradiation.
