Hydrogen is gathering strong momentum as a pivotal energy transition pillar driven by the global shift toward decarbonization. Nevertheless, 85% of H₂ is produced from fossil fuel combustion, which generates roughly 500 metric tons of carbon dioxide every year and proffers a challenge and obstacle toward the sustainable living of future generations [132]. Solar-driven photocatalytic H₂ generation as a promising technology has received extensive attention in addressing the global energy crisis [133,134]. Photocatalytic water splitting for the energy transformation from solar to eco-friendly fuels has been studied for decades with various semiconductor photocatalysts. As a type of semiconductor photocatalyst, g-C₃N₄ is simple and inexpensive to fabricate, and has an adequate bandgap (≈2.7 eV) for activation upon sunlight irradiation. Wang’s group first utilized g-C₃N₄ in photocatalytic H₂ evolution [15,135]. Nonetheless, pristine g-C₃N₄ is far from satisfactory energy conversion because of its low light energy utilization, low density active sites, and ineffective isolation of the photogenerated excitons. Thus, researchers have proposed numerous strategies to boost the photocatalytic activity of g-C₃N₄-based materials for H₂ production. For example, the g-C₃N₄/carbon-dot-based nanocomposites, which possess enormous visible light absorption and applicable energy structures, have been prepared and serve as efficacious photocatalysts in photocatalytic water splitting for H₂ generation under light illumination [128,134,136,137]. Gao et al. reported hexagonal tubular g-C₃N₄/CD-based nanocomposites which exhibited nine times higher than bulk g-C₃N₄ in H₂ production rate [134] and related results indicated that CDs performed as both photosensitizer and electron acceptor. CDs could absorb long wavelength light to extend the visible-light response region and suppress the recombination of electron-hole pairs. Hussien et al. [138] combined four different strategies (non-metal doping, porosity generation, functionalization with amino groups, and thermal oxidation etching) in a one-pot thermal reaction and successfully prepared amino-functionalized ultrathin nanoporous B-doped g-C₃N₄ by using NH₄Cl as a gas bubble template, together with a thermal exfoliation process to produce ultrathin sheets. According to the process, the surface area, adsorption capacity, and charge migration of the as-prepared photocatalyst have been improved, and a 3800 µmol g⁻¹ h⁻¹ H₂ generation rate and 10.6% prominent quantum yield were recorded. Li et al. [139] decorated carbon self-doping g-C₃N₄ nanosheets with gold-platinum (AuPt) nanocrystals through a photo-deposition route and compared the photocatalytic H₂ evolution performance of Pt/CCN, Au/CCN, Au/Pt/CCN, and Pt/Au/CCN, in which AuPt/CCN stood out and gave the highest H₂ generation rate (1135 μmol h⁻¹). The excellent performance can be ascribed to the non-plasmon-related synergistic effect of Au and Pt atoms in AuPt nanocrystals. Sun et al. [140] assessed the arrangements of metal- and non-metal-modified g-C₃N₄ composites in hydrogen evolution and found that the contribution of dye conjugation in non-metallic g-C₃N₄ composites favored their performance. However, the co-catalyst doping strategy was recommended for metallic g-C₃N₄ composites. In addition, the hybrid of MOF materials and g-C₃N₄ is also a good approach to develop novel photocatalysts. For example, Devarayapalli et al. [141] reported a g-C₃N₄/ZIF-67 nanocomposite and obtained a 2084 μmol g⁻¹ H₂ production, which is 3.84-fold greater than that of bare g-C₃N₄.

Rising atmospheric levels of CO₂ and the consumption of fossil fuels raise a concern about the continued reliance on the utilization of fossil fuels for both energy and chemical production [173]. Photocatalytic reduction of CO₂ is a promising strategy to meet increasing energy needs and reduce the greenhouse effect [174]. Through photocatalytic reduction, CO₂ can be converted to light oxygenates and hydrocarbons. Photocatalytic CO₂ reduction is a multielectron transfer process. Fu et al. [175] have listed the possible reaction and corresponding redox potentials and stated that CO₂ was complicated to reduce at room temperature due to its stable chemical structure. For the complex reaction, five factors, comprising the matching of band energy, separation of charge carrier, kinetic of e- and hole transfer to CO₂ and reductant, the basicity of photocatalyst, and the strength and coverage of CO₂ adsorption, are considered to be crucial [176]. As a hot member of photocatalysts, g-C₃N₄ has been applied to CO₂ photo-reduction in recent years because the CB of g-C₃N₄ is sufficient to reduce CO₂ to various hydrocarbons, such as CH₃OH, CH₄, HCHO, and HCOOH, etc. [177].

However, metal-free g-C₃N₄ is limited for CO₂ reduction activity due to its poor ability to activate the C-O bond of CO₂. To improve the photocatalytic movement of CO₂ conversion, different metal units have been composited with g-C₃N₄ for broadening the absorption response range, and accelerating the charge separation and transfer, such as Pt/g-C₃N₄ [178], Co²⁺/g-C₃N₄ [179,180], Au/g-C₃N₄ [181], etc. Metal nanoparticles acting as cocatalysts could effectively improve the photocatalytic activity and selectivity of CO₂ reduction. In addition, other methods, including doping, loading cocatalysts and nanocarbons, constructing Z-scheme, and heterojunction, have also been employed [16,182,183,184,185,186,187,188,189]. For example, Fu et al. [190] prepared hierarchical porous O-doped g-C₃N₄ nanotubes (OCN-Tube) through continuing thermal oxidation exfoliation and curling condensation of bulk g-C₃N₄. Due to the higher specific surface area, better light harvesting, higher CO₂ uptake capacity, and superior separation efficiency of photogenerated charge carriers, the OCN-Tube exhibits excellent photocatalytic CO₂ reduction performance into CH₃OH. The CH₃OH evolution rate was as high as 0.88 µmol g⁻¹ h⁻¹, five times higher than the bulk (0.17 µmol g⁻¹ h⁻¹). Huo et al. [191] fabricated amine-modified step-scheme (S-scheme) porous g-C₃N₄/CdSe-diethylenetriamine (A-PCN/CdSe-DETA) by a one-step microwave hydrothermal method. The modification by amine and formation of S-scheme heterojunction contributed to the remarkable photocatalytic performance of A-PCN/CdSe-DETA composite in CO₂ reduction and a CO production rate of 25.87 μmol/(h g) was achieved under visible-light irradiation. Wang et al. [174] reviewed different modification methods of g-C₃N₄-based photocatalysts for CO₂ reduction. They discussed each method (including morphology adjustment, co-catalysts, heterostructures, and doping) and compared the theoretical calculations and experimental results. By morphology adjustment, g-C₃N₄ with various shapes can be fabricated, such as rods, tubes, nanosheets, hollow spheres, and honeycomb-like structures. Due to the advantage of cocatalysts (e.g., Au, Ag, Pt, Pd, MXene, AuCu alloy, Pd-Ag), g-C₃N₄ with co-catalysts can be widely applied to activate CO₂ on the surface. Heterojunction with different types is also an effective method to improve the properties of g-C₃N₄-based materials. In addition, elemental doping is considered a common method to enhance photocatalytic quantum efficiency by changing the energy band, surface electronic property, and electrical conductivity.

Along with rapid population growth and significant industrialization development, large numbers of toxic, hazardous, and endless contaminants invade the environment, threatening to human life, especially a variety of pollutants present in water that are difficult to eliminate or degrade naturally. Photocatalytic degradation of contaminants is a green and efficient technology for coping with sewage [128,223]. Different kinds of g-C₃N₄-based materials have been exploited to increase the photodecomposition efficiency of pollutants, such as the constructed heterojunction, loading O₂-reduction co-catalysts, g-C₃N₄/CDs-based nanocomposites, and so on [182,224,225,226]. Generally, under the irradiation of visible light, the photogenerated electrons (e⁻) on the g-C₃N₄ catalyst will be excited from VB to CB, leaving holes (h⁺) in the VB. The holes can oxidize pollutants directly or react with H₂O/OH⁻ to form hydroxyl radicals [227]. When the REDOX potential of g-C₃N₄ composites is more negative than O₂/O₂⁻, the photogenerated electrons in the material can react with O₂ to produce O₂⁻ with strong oxidation capacity [228]. In addition, the resulting O₂⁻ could be protonated to produce OH [229]. Finally, the RhB dye is degraded to CO₂ and H₂O under the action of these free radicals. Chen et al. [230] fabricated a BiFeO₃/g-C₃N₄ heterostructure through mixing-calcining and compared its performance with BiFeO₃. Around 30% higher photocatalytic efficiency toward RhB dye was observed for the BiFeO₃/10% g-C₃N₄ heterostructure, which was assigned to the contribution of a higher concentration of O₂⁻. Zhang et al. [231] studied the selective reduction of molecular oxygen on g-C₃N₄ and probed its effect on the photocatalytic phenol degradation process. Compared with bulk g-C₃N₄, the exfoliated nanosheet yielded a three times improvement in photocatalytic phenol degradation. It has been demonstrated that bulk g-C₃N₄ prefers to reduce O₂ to O₂⁻via one-electron reduction. At the same time, the photoexcited g-C₃N₄ nanosheet facilitates the two-electron reduction of O₂ to yield H₂O₂ because of the formation of 1,4-endoperoxide species. The two-electron reduction of O₂ on the nanosheet surface boosts hole generation and thus accelerates phenol oxidation degradation [231,232]. Thus, to improve the photocatalytic performance of g-C₃N₄, more effort should be devoted to strengthening the solid O₂-reduction reactions. For example, Liu et al. [83] reported a heterojunction material of K-doped g-C₃N4 nanosheet -CdS and degraded tetracycline with 94% degradation under visible light in 30 min. In addition, due to the electronegativities, ionic radius differences, and impurity states, element doping is also an effective method to manipulate the electronic structure and physicochemical performance of g-C₃N₄-based materials. Gao et al. [68] synthesized Fe-doped g-C₃N₄ nanosheets and obtained 1.4- and 1.7-fold higher degradation rates of MB than that of pure g-C₃N₄ nanosheets and bulk g-C₃N₄, which indicated that the exploitation of efficient g-C₃N₄-based photocatalysts with high stabilization and degradation under visible light irradiation would significantly contribute to sewage disposal. Zhang et al. [117] synthesized a novel hybrid of Zr-based metal-organic framework with g-C₃N₄ (UiO-66/g-C₃N₄) nanosheets and applied a photodegradation of methylene blue, by which a 100% photodegradation was achieved within 4 h under visible light. This research has provided a new insight into the design of g-C₃N₄-based photocatalysts to deal with organic dyes in the environment.