Carbon Nitrides

Carbon nitride is a type of triazine or heptazine-based polymer containing carbon and nitrogen, usually called graphite carbon nitride (g-C₃N₄), which is a layered crystal structure based on heptazine (Figure 1a). However, in fact, the material related to photocatalysis, even if it is called g-C₃N₄, contains a large amount of hydrogen and appears to be composed of melon (Figure 1b).

Photocatalytically active carbon nitride materials usually have poor crystallinity or X-ray amorphous. According to experimental characterization and modeling, CN_xH_y can be used to represent it more accurately. CN_xH_y is an effective substitute for graphite and amorphous carbon in different fields such as catalyst support.

Figure 1. The structure of different (hypothetical) carbon nitride polymorphs. (a) hypothetical graphite C₃N₄ structure. (b) Melon, possible structure of carbon nitride material prepared by thermal decomposition experiment of nitrogen-containing precursor. [1]

The physical basis of carbon nitride photocatalytic activity

In the polymerization of CN_xH_y, the heptazine unit is only cross-conjugated via secondary or tertiary amino bridges. Unlike classical wide band gap semiconductors, molecular singlet excitons limited to heptazine units are sufficient to explain the PL behavior of CN_xH_y, which is consistent with the typical low dielectric constant of organic materials and the resulting local Frenkel-like excitons. [2]

Applications

g-C₃N₄ has broad application potential in the following fields:

Research Cases

• Photocatalytic hydrogen evolution

Shaodong Sun et al. demonstrated for the first time a general carbon beverage-assisted hydrothermal reforming commercial melamine (monoclinic) strategy for the synthesis of mpg-C₃N₄ nanosheets. The carbon beverage involved contains sugar components, which are used to construct new orthorhombic MA and new melon building blocks, and finally form mpg-C₃N₄ nanosheets in the air by calcining the reformed MA precursor. Although the amount of carbon beverage used to synthesize the orthorhombic MA precursor is very small, the photocatalytic activity of the mpg- C₃N₄ product is significantly improved.

Figure 2. Mesoporous graphite carbon nitride (g- C₃N₄) nanosheets for enhanced photocatalytic hydrogen evolution. [3]

• Heterogeneous photocatalysis

Carbonitrides are used as photocatalysts for the photochemical decomposition of water and some mild and selective photooxidation reactions catalyzed by g-C₃N₄. Among them, the use of visible light photocatalysts to directly split water is a panacea for the production of hydrogen fuel, thereby converting sunlight into storable and transportable energy in chemical bonds.

Figure 3. Strategies to achieve visible light-induced photocatalysis: (a) hydrogen evolution from water, (b) photochemical activation of O₂, and (c) photocatalytic oxidation or degradation of organic substrates.[4]

References

1. Yiou Wang. (2019). "Current understanding and challenges of solar-driven hydrogen generation using polymeric photocatalysts," Nature Energy 5: 633.
2. C. Merschjann. (2013). "Photophysics of polymeric carbon nitride: An optical quasimonomer," Physical Review B 87: 205204.
3. Shaodong Sun. (2019). "Mesoporous graphitic carbon nitride (g-C₃N₄) nanosheets synthesized from carbonated beverage-reformed commercial melamine for enhanced photocatalytic hydrogen evolution," Materials Chemistry Frontiers 3: 597-605.
4. Xinchen Wang. (2012). "Polymeric Graphitic Carbon Nitride for Heterogeneous Photocatalysis," ACS Catalysis 2(8): 1596-1606.