*3.3. Metal- and Non-Metal-Doped g-C3N4*

Among the strategies for making g-C3N4 as a photocatalyst capable of effective hydrogen production, sufficient doping with metallic and nonmetallic elements is known to enhance the photocatalytic activity of g-C3N4. Metal doping is an effective strategy to adjust the electronic structure of g-C3N4 and promotes surface kinetics to accelerate photogenerated electron transfer and provide active sites for better photocatalytic hydrogen production. In addition, the light transmittance can be maximized since the spatial distribution and the particle size of the metal can be finely controlled to provide a sufficient active size.

In 2016, Li et al. [84] reported water splitting by Cu- and Fe-doped g-C3N4 visible-light-activated photocatalysts. Figure 9a shows the mechanism of water splitting by light-driven catalysis with Fe- and Cu-doped g-C3N4. Under visible-light irradiation, water is converted to H2 and H2O2, and then H2O2 is further converted to O2 and H2O via the photocatalytic imbalance path. After absorbing visible light, g-C3N4 forms excited electrons and holes by electron catalysis, and the electrons move from the energy potential difference between g-C3N4 and Fe or Cu to the metal Fe or Cu sites. The potential of these electrons is around −0.25 eV and has enough force to induce H2O2 disproportionation to form ·OH and OH−. In the hole catalytic process (HCP), OH<sup>−</sup> and H2O2 could form the ·O2 − and H2O species reaction with the holes. Finally, O2 <sup>−</sup> and OH can recombine to form O2. Electron catalysis is an energy-consuming process whereas HCP and recombination processes can be viewed as energy-releasing processes. Figure 9b shows the oxygen and hydrogen evolution rates of Fe/C3N4 (0.37 wt%) and Cu/C3N4 (0.42 wt%) under visible-light irradiation (λ ≥ 420 nm) for 12 h. In this case, the production of hydrogen and oxygen by the Cu/C3N4 and Fe/C3N4 photocatalysts were 1.4 and 0.5 μmol, and 2.1 and 0.8 μmol, respectively. In addition, the potential of the Fe/g-C3N4 photocatalyst is obviously lower than those of the g-C3N4 and Cu/g-C3N4 photocatalysts, which leads to the O2 and H2 evolution activity over the Fe/g-C3N4 photocatalyst being clearly higher than that over the g-C3N4 and Cu/g-C3N4 photocatalysts. The findings of this study give new insight into the designing of efficient catalysts for overall water splitting.

**Figure 9.** (**a**) A schematic diagram of the water splitting mechanism by Fe/C3N4 and Cu/C3N4 photocatalysts under visible-light irradiation. (**b**) Production of H2 and O2 by water splitting by the Fe/C3N4 and Cu/C3N4 photocatalysts under visible-light irradiation for 12 h. Reproduced with permission from [84]; copyright (2015), American Chemical Society.

Non-metal doping is a useful strategy to adjust the electronic structure of g-C3N4 and to increase the photocatalytic effect by promoting the reaction surface. When the non-metal elements B, N, O, P, and S are used to dope g-C3N4, the photocatalyst is efficiently optimized by lowering the charge recombination rate due to optical absorption and accelerated charge mobility, and thus the amount of H2 produced can be increased [85,86]. Consequently, the potential of the Fe/g-C3N4 photocatalyst is obviously lower than those of the g-C3N4 and Cu/g-C3N4 photocatalysts. This indicates that the Fe/g-C3N4 photocatalyst has higher activity on photocatalytic hydrogen evolution than the g-C3N4 and Cu/g-C3N4 photocatalysts. The findings of this study give new insights into designing efficient photocatalytic hydrogen generation and catalysts through overall water splitting.

In 2018, Feng et al. [87] reported P nanostructures with P-doped g-C3N4 as light photocatalysts for H2 evolution. P nanostructures and P-doped g-C3N4 (P@P-g-C3N4) were synthesized via a solid reaction, and P@P-g-C3N4 showed increased optical absorption, high-efficiency transmission, and efficient separation of photogenerated electron–hole pairs. When C atoms are replaced with P atoms (the gray and red balls in Figure 10a, respectively) in the base frame of g-C3N4, the extra electrons are decentralized into a π-conjugated triazine ring and generate a positive-charge P<sup>+</sup> center, thereby facilitating rapid separation of the photogenerated excited electrons. Furthermore, efficient band gap transfers between the P and P-doped g-C3N4 leads to a significant improvement in photoactivity (Figure 10b). P-doped g-C3N4 photoexcited electrons can be delivered to phosphorus via intimate contact because the CB edge of g-C3N4 (−1.2 V vs. normal hydrogen electrode (NHE)) is more negative than P (−0.25 V vs. NHE) which provides an interface under the buildup of the internal electric field. Thus, the extra electrons superimposed on the P surface can easily be captured by the oxygen molecules in the solution and react with ·O2<sup>−</sup> and ·OH. Figure 10c,d shows the hydrogen evolution yield and the improvement in hydrogen production ability of the photocatalysts prepared at different weight ratios of P/g-C3N4. P@P-g-C3N4-15 showed the highest hydrogen production rate (941.80 μmol h−<sup>1</sup> g−1), which is around four times that of conventional g-C3N4.

**Figure 10.** (**a**,**b**) A schematic of the mechanism of H2 evolution by the P@P-g-C3N4 catalyst; (**c**) comparison of the evolution rates of H2; and (**d**) H2 evolution rate of the P@P-g-C3N4 composites. Reproduced with permission from [87]; copyright (2018), American Chemical Society.

#### **4. Summary and Perspectives**

Photocatalytic action is a key factor for the future of environmental pollution and hydrogen generation due to water splitting. Over the past several years, photocatalytic reactions have emerged as a promising method to generate hydrogen, and interest in the photocatalyst g-C3N4 has received attention in a variety of scientific disciplines. However, a major problem that limits the rate of production of H2 by g-C3N4-based photocatalysis is the fast recombination of photoexcited electron–hole pairs. This problem can be solved in a variety of ways, including modification, heterojunctions, and metal and non-metal doping. Table 1 summarizes the literature on the photocatalytic H2 generation of g-C3N4-based materials. We reviewed the rational design of photocatalysts for efficient H2 generation though a variety of methods. Furthermore, the improvement of g-C3N4-based photocatalysts will likely result from advances in science. Herein, we have covered the recent progress of g-C3N4-based materials involved in hydrogen production in improving their overall photocatalytic activity and have characterized their performance and importance. We hope that this report will support further research efforts related to photocatalytic development.



*Catalysts* **2019**, *9*,

805

**Funding:** This research was supported by the Technology Innovation Program (or Industrial Strategic Technology Development Program) (10080293), Development of carbon-based non phenolic electrode materials with 3000 m2g−<sup>1</sup> grade surface area for energy storage device funded by the Ministry of Trade, Industry and Energy (MOTIE, Korea) and the Commercialization Promotion Agency for R&D Outcomes (COMPA) funded by the Ministry of Science and ICT (MSIT) 2018\_RND\_002\_0064, Development of 800 mA·h·g−<sup>1</sup> pitch carbon coating.

**Conflicts of Interest:** The authors declare no conflict of interest.
