Modification Strategies of g-C3N4 for Potential Applications in Photocatalysis 301
For the fabrication of S doped g-C3N4 photocatalyst (Wang et al. 2015), used melamine and
thiourea precursors calcined at 520°C. The findings of photocatalytic reduction of CO2 showed that
the CH3OH yield with pure g-C3N4 was 0.81 mol g–1, whereas it was 1.12 mol g–1 for g-C3N4 doped
with S. Using melamine and thiourea as common precursors (Liang et al. 2016), created a series of
S doped g-C3N4 grafted with zinc phthalocyanines (ZnTNPc). S-doped g-C3N4 and ZnTNPc showed
a synergistic relationship for the photocatalytic elimination of Methylene blue dye, which was
4.5-fold more than that of zinc phthalocyanines (ZnTNPc). According to Mott-Schottky Relationship,
adding S atoms narrows the band gap and lowers the Conduction Band (CB) from 1.04V to 0.83V,
which facilitates the photocatalytic activity of MB.
By condensing oxalic acid and urea at a high temperature of 550°C (Qiu et al. 2017), created
porous O-doped g-C3N4. The band gap was reduced from 2.91 eV to 2.07 eV due to the inclusion of
the O atom into the g-C3N4 lattice. To enable the oxidation of benzene to phenol and other non-toxic
chemicals under the influence of visible light, a fluorinated g-C3N4 heterogeneous photocatalyst was
produced. The preparation was initiated by adding NH4F by thermal precursor in a consistent, easy
one-pot facile thermal polymerization technique to design and create B-doped g-C3N4 nanosheets.
17.5 Application of Metal Oxide-Based g-C3N4 Nanocomposites
17.5.1 Photocatalysts
17.5.1.1 H2 Generation via Water Splitting
More and more individuals are calling for the use of reliable, cost-effective and renewable
energy sources instead of finite fossil fuel supplies. This substitution is a successful treatment for
greenhouse gas emissions and global warming. The energy content of hydrogen is higher than that
of hydrocarbon fuels, ranging from 120 to 142 MJ kg–1. Thus, it is predicted that by 2080, hydrogen
will produce 90% of all energy. As a result, several researchers (Paul et al. 2020, Shi et al. 2021)
considered H2 creation a unique and ecologically favorable study issue. The photocatalytic water
splitting approach employing metal oxide-g-C3N4 heterojunctions and abundant light sources is
one of the most current methods for producing hydrogen (Ji et al. 2018). The photocatalysts’ band
positions should be changed to make the CB position more favorable than the H2O oxidation potential
for O2 production and more harmful than the H2O reduction potential for H2 production. Several
heterojunctions are more anodic than H2O reduction potential to demonstrate good activity under
visible light, when considering the band edges position of some metal oxide g-C3N4 composites
identified earlier. TiO2-g-C3N4 heterojunctions are a prime example of how well they work as a
photocatalytic for H2 evolution. According to (Marchal et al. 2018) analysis of Au/(TiO2-g-C3N4),
optimum component ratios and contact quality improved visible light absorption with the appropriate
band locations for photogenerated charge carriers. Table 17.1 lists other studies on applying metal
oxide composites based on g-C3N4 for water splitting.
17.5.1.2 Reduction of CO2
One of the leading environmental issues brought on by fossil fuels is CO2 emission, that raises
the surface temperature of the globe. For two reasons, photocatalytic CO2 reduction offers a green
solution to this issue. The production of energy fuels like CH4, CH3OH and other fuels helps to meet
future energy demands and reduce CO2 emissions. Different metal oxide g-C3N4-based systems
convert CO2 via photocatalysis significantly as they have advantageous band edge locations. As
g-C3N4-based composites for CO2 conversion, ZnO and TiO2 are frequently employed (Mulik et al.
2021). TiO2 and g-C3N4 were combined to create a photocatalyst by (Wang et al. 2020) utilizing ball
milling and calcination. High CH4 and CO evolution yields of 72.2 and 56.2 mol g–1 are achieved due
to the heterostructure between TiO2 and C3N4's low charge recombination rate and high separation.
As shown in Table 17.2, other scientists have also studied the CO2 reduction of photocatalysts based
on g-C3N4-metal oxide.