However, the photocatalytic activities of the pristine g-C3N4 are still mediocre and plagued by problems related to limitations in light absorption, charge separation, and carrier-induced surface reactions, although efforts have been made in recent decades to increase efficiency. Hydrogen has become an excellent renewable fuel due to its higher gravimetric heating value (141.9 MJ.kg-1) compared to most conventional fossil fuels, for example methane (55.5 MJ.kg-1). Hopefully this effort will prompt further research. of novel g-C3N4-based photocatalysts, which can efficiently utilize the abundant solar energy and overcome the disadvantage of charge recombination in mediating the surface reactions; Such exceptional materials will open newer avenues for the large-scale era of solar-powered fuel production.
These processes consist of four main steps: (i) a photocatalyst collects incident light containing energy higher than that of its band value to excite electrons in the valence band (VB) to the conduction band (CB); (ii) photogenerated electron-hole pairs migrate to the surface; (iii) electron-hole recombination leading to loss of charge carriers. This step has been considered as the bottleneck problem that significantly limits the photocatalytic efficiency; and (iv) surface chemical reactions occurring between available charge carriers and reactants.50, 51 Eqn. 1) describes the hydrogen reaction production while hydroxyl radicals (OH·), which is one of the most powerful oxidizing species, are produced by oxidation reactions between photogenerated holes with H2O/OH- or H2O2, as described in Eqns. Considering the band structure of g-C3N4, the photogenerated electrons in the conduction band are useful for the formation of H2, O2· and H2O2, while the valence band is more positive than that of O2 formation.
The photocatalytic performance of g-C3N4 materials is essentially governed by three aspects, namely (i) light-harvesting ability: The absorption edge of g-C3N4 is ~460 nm corresponding to the band gap of ~2.7 eV, which accounts for a small part of visible region. Therefore, extending the light absorption of g-C3N4-based photocatalysts, which fulfills the visible region of the solar spectrum, has become a critical issue to increase the number of generated charge carriers.56 (ii) Charge separation and transport: Photogenerated electron-hole pair has a tend to recombine after the charge photogeneration.
Such improvement arose from longer visible light absorption and charge lifetime, anchored in n-π* excitation, as shown in Figure 6C, and an improvement in the separation of the photogenerated electron-hole pairs (Figure 6D -E).91 In particular, the formation of g-C3N4 with an n-π* transition feature is strongly related to the preparative method used. Structural vacancies (i.e., carbon or nitrogen vacancies) significantly change the optical and charge separation behavior of the g-C3N4 materials. Furthermore, the conduction band potential of the sample CNPS-NH2 was found to be more negative than the bulk g-C3N4, indicating a strong thermodynamic driving force for hydrogen production (Figure 8B).
These advantages result in a significant improvement in charge transport and extend the lifetime of the photogenerated electron (Figure 8C). Furthermore, a strong shift of the CN-K band structure results in significant changes in the reduction and oxidation capabilities. The conjugated copolymer thus increases light absorption and improves charge separation ability; the hydrogen production via these materials appears to be significantly higher than that of the bulk g-C3N4.
It is clear that the incorporation of the above molecules leads to expansion of the conjugation system, which causes delocalization and polarization of electrons, which strongly affects the light absorption and charge separation properties. The presence of the higher number of π-electrons in the g-C3N4 system prolongs the light collection and inhibits the charge recombination.126, 127 Recently, Li et al. These synergistic features significantly stimulated visible light-driven photocatalytic hydrogen production, which ~20 times higher than that of the conventional g-C3N4.128 Therefore, the tactic of increasing the π-electrons in the g-C3N4 system reveals a great opportunity to simultaneously address light absorption and charge separation issues.
Such structure modifications cause the remarkable change in the light absorption behavior, thereby extending the lifetime of the photogenerated electrons and improving the charge separation leading to extraordinary photocatalytic performance for hydrogen production, as shown in Figure 15B; The generation of the GD-C3N4 is exceptionally higher than that of the conventional g-C3N4.
As a proof-of-concept, the proper band structure of the integrated semiconductor is the crucial consideration in composite preparation. In recent years, sulfide-based semiconductor/gC3N4, as one of the influential heterojunctions, has shown a configuration that can provide both: the strong absorption of sunlight due to the small band gap of metal sulfide catalysts and the efficient charge separation. Indeed, the evolved hydrogen was found to be exceptionally higher than that of the bulk g-C3N4, and is one of the most efficient g-C3N4-based type II photocatalysts; This extraordinary outcome is due to the robust interfacial charge separation and strong light absorption capacity (Figure 16B).
The photogenerated electrons in the CB of semiconductor I (SI), the lower semiconductor, migrate to the mediator to recombine with the photogenerated holes in semiconductor II (SII). The charge density plot at the W18O49/g-C3N4 interface, shown in Figure 18F, confirms the electron transport path and the presence of the C-O bond at the interface.172 Thus, such excellent properties enhance the lifetime of charge carriers, contributing significantly to stimulating photo activity. In another study, She et al. found that α-Fe2O3/g-C3N4, in which α-Fe2O3 nanosheets form an intimate contact with 2D g-C3N4, enables efficient interfacial charge transfer of the photogenerated electrons.
It was found that the prepared composite exhibited a significant improvement in charge separation, resulting in a remarkable improvement in the photocatalytic activity. It is noteworthy that the hydrogen produced is significantly higher than for the sample Pt/g-C3N4 and Ti3C2/g-C3N4. Ni2P nanoparticles were a valuable co-catalyst when coupled to g-C3N4, which hinders the charge recombination and acts as hydride and proton acceptor centers inducing acceleration of surface chemical reactions.
Such remarkable achievements were consolidated in the presence of RhPx cocatalysts, which form Rh(δ+)-C(δ-) bonds that promote charge separation and maintain stability, while RhPx . nanoparticles show the ability to accelerate hydrogen production reactions.198 Similarly, g-C3N4 embedded in FeP and Co2P, Cu3P, NiCoP nanoparticles showed a remarkable improvement in photocatalytic efficiency, which means superior properties of metal sulfide cocatalysts.199- 202. Monatomic Pt with a size less as 0.2 nm deposited on g-C3N4 could be observed by HAADF-STEM image as shown in Figure 22A; The Pt atoms were localized on top of the five-membered rings as shown in (Figure 22B). It is worth noting that the evolved hydrogen on the PtSA/g-C3N4 sample, which contains an extremely low percentage of Pt, is significantly higher than that of the Pt-g-C3N4 nanoparticles with a higher amount of loaded Pt (Figure 22D).
In particular, the negatively charged surface of g-C3N4-K and the use of the anionic precursor of Pt are the primary reasons to sanction the cation exchange and limit the aggregation of PtSA due to the adjacent layer-initiated space effect. Interestingly, the PtSA/g-C3N4 samples exhibited the excellent photocatalytic activity compared to the PtNPs/g-C3N4 (Figure 24C); the amount of hydrogen produced was 22.65 mmol.g-1.h-1 over the sample PtSA/g-C3N4 with 8.7% Pt, which is 14.29 times higher than that of PtNPs/g-C3N4 with 3, 1% Pt. The DFT calculations indicate that the electronic structure and charge density of Pt atoms located in the interior is significantly changed compared to Pt located on the surface model. Such favorable properties promote proton adsorption and reduction through reduction of the reaction energy barrier, as shown in Figure 24D-E).209.
The photocatalytic hydrogen production of different PtSA/g-C3N4. samples compared g-C3N4 and PtNPs/g-C3N4; energy profiles of Pt placed D) on the surface layer and E) inner layer of carbon nitride. The development of the leading preparative method, which can stabilize the shape of single atoms, could be considered the key to solving this obstacle.
Conclusions and future perspectives 1.Conclusions
Future perspectives
Also, embracing such structures to form a heterojunction can be considered as a means to further improve light absorption and charge separation issues. In this context, the development of 3D architecture based on g-C3N4 can be a feasible approach because the unique structure further improves light harvesting, reactant diffusion and active sites. Among structural engineering strategies, intermolecular tailoring by creating a donor-acceptor configuration and g-C3N4-linked graphitic rings has seen growth as a vital tactic to address light absorption and charge separation problems.
In particular, the new structural features are highly associated with the deployed preparation methods, and searching such routes in an accelerated manner may be the most crucial aspect that will help introduce new milestones in the photocatalytic results. Moreover, the combination of construction technique and preparation of nanocomposite is also a smart way to prepare promising materials with added emphasis on the emergence of a burning topic related to the use of single atom cocatalysts. Undoubtedly, single atom-based cocatalysts will be the holy grail in the development of robust g-C3N4-based photocatalysts.
Most current efforts are focused on Pt and the other metals await exploration. Stabilization of the single atom metal species on the g-C3N4 surface will be the most difficult challenge that can potentially be addressed via the innovative preparation techniques. The use of theoretical research provides an excellent tool to identify active sites and the underlying reaction mechanisms.
The combination of theoretical and experimental works will provide a road map towards the next generation of g-C3N4 photocatalysts, and this fertile area awaits the exploration of the pioneers. The authors would like to thank Henan Province Engineering Research Center for Biomass Value-Add Products and Universiti Malaysia Terengganu under the Golden Goose Research Grant Scheme (GGRG) (Vot 55191) for supporting Dr.
