Modification Strategies of g-C3N4 for Potential Applications in Photocatalysis 299

band gaps and improved surface area for efficient charge transfer. The adsorption efficiency of Na­

doped g-C3N4 produced from cyanamide and NaCl as precursors were reported by (Fronczak et al.

2017). Textural characteristics of Na-doped g-C3N4 were studied using N2-adsorption/desorption

isotherms. The findings showed that Na concentration was increased. The photocatalytic activity

and visible light absorption of g-C3N4 were significantly enhanced by deprotonation with Na⁺.

Similarly (Hu et al. 2014), created potassium (K) doped g-C3N4 with a band gap optimized

for removing RhB dye under visible light irradiation using dicyandiamide (DCDA) monomer and

potassium hydrate as precursors. Due to its enhanced electronic structure and redox potentials for

adequate consumption of photogenerated electrons, it was inferred from several investigations that

Na-doped g-C3N4 demonstrated higher photocatalytic capacity. Strong interactions between metal

dopants and lone pair electrons on g-C3N4 nitrogen pots make it easy for metal cations to get into

the framework. Alkali metal addition showed stable chemical activity despite being very reactive,

which justifies the conclusion that it will increase the photodegradation of organic contaminants.

17.4.2.2 Transition Metal Ion Doping

In addition to alkali-metal doping, other metals, including Fe, W, Zr, Pd, Cu and so on, have also

been widely used to change the optical and electrical properties of g-C3N4 (Sudrajat 2018, Gonçalves

et al. 2018). For significant photocatalytic activity, metal doping can effectively minimize the

band gap, boost light absorption, speed up charge movement and extend the lifetime of charge

carriers (Shandilya et al. 2019, Zhang et al. 2015). The strong interactions between the negatively

charged nitrogen atoms and the cations are attributed to lone pairs. Using conventional precursors

and a basic pyrolysis procedure, molybdenum-doped g-C3N4 catalysts were synthesized by (Zhang

et al. 2015). They discovered that adding Mo species can significantly lower the rate at which

photogenerated charges recombine, expand the response to visible light, increase surface area,

produce mesoporous structure and provide narrow band gap energy. As a result, the CO2 reduction

activity of g-C3N4 catalysts doped with Mo was considerably higher for electrons in the nitrogen

pots of g-C3N4, causing the compound to capture the ions readily. The g-C3N4 was functionalized

with better carrier mobility, improved electron-hole separation and a reduced band gap using

noble metals like Pt and Pd (Hu et al. 2014). Noble metals like Pt and Pd doping can enhance the

g-photocatalytic C3N4’s activity, but its expensive price makes it impractical for everyday use.

17.4.2.3 Rare Earth Metal Doping

By thermally condensing urea and cerium sulfate precursors, the rare earth metal Cerium (Ce) was

integrated into pure g-C3N4, which led to 90% Rhodamine B (RhB) degradation (Jin et al. 2015).

The interstitial occupancy of Ce⁺³ coupled to a lone pair of N atoms was revealed by XPS spectrum

analysis. Ce⁺³’s ionic radius, which is substantially bigger than that of N and C, was also discovered

to be 0.103 nm. As a result, the Ce dopant is encouraged to have a higher electron density and

interstitial attachment. Xu et al. (2013) doped Europium (Eu) in g-C3N4 and thoroughly examined

the photocatalytic mechanism and influence of impurity concentration. The optimized composite

showed a high rate of degradation of MB dye (81.57%) compared to bulk g-C3N4’s efficiency of

just 53%.

To create a heterogeneous photocatalyst, Ruthenium (Ru) was used in complex formation

with g-C3N4. The Ru-doped g-C3N4 increased apparent quantum yield to 5.7% and the efficiency

of CO2 reduction to formic acid (Kuriki et al. 2015). Recently, synthetic Samarium (Sm⁺³) doped

g-C3N4 has been used to remove Methylene Blue (MB), Rhodamine B (RhB) and eosin yellow

dyes with remarkable success. It was discovered that Sm⁺³’s empty 4f orbital effectively captured

photogenerated e^–, which decreased the recombination rate and enhanced photocatalytic activity

(Thomas et al. 2018).

By integrating NH4F into the g-C3N4 framework, fluorine-doped g-C3N4 was synthesized (Das

et al. 2018). Due to nitrogen’s and fluorine’s electronegativity, the doped fluorine will attach to

carbon rather than nitrogen, partially converting C-sp2 to C-sp3. Due to this, fluorine doping caused