*2.1.2 Template-free method*

A simple thermal treatment of dicyandiamide is used to make porous g-C3N4. The synthesized porous g-C3N4 has a large pore volume and a high BET surface area (0.50–0.52 m3 g−1). A simple template-free technique to make g-C3N4 nanofibers (GCNNFs). Melamine was first reacted with ethanol, then annealed at 450°C for 2 hours to produce GCNNFs, which had a 1D structure and a large specific surface area. Thermal calcinations were also used to make graphene-modified porous g-C3N4 (porous g-C3N4/graphene). The polymerization process was carried out at various temperatures in this approach, with high calcination temperatures yielding porous g-C3N4.

### **Figure 2.**

*Scheme of two representative synthesis routes for ordered mesoporous materials: (a) soft templating method and (b) hard templating (nanocasting) method. Reproduced from [8] with permission from the Royal Society of Chemistry.*

#### *2.1.3 Fractional thermal polymerization method*

Using melamine, guanidine carbonate, and dicyandiamide as starting ingredients, a fractional thermal polymerization process was used to create g-C3N4 particles with a large surface area. Melamine, guanidine carbonate, and dicyandiamide were polymerized to create g-C3N4 at 515, 550, and 515 degrees Celsius, respectively. At the temperatures given, no residual component of precursors could be established. Around 200–240°C, all of the products for these three precursor materials had the structure of C3H6N6, which converted to tri-s-trizines at 350–500°C. The dense packing between the conjugated aromatic system of g-C3N4 became stable using this fractional thermal polymerization approach. After 120 minutes of irradiation, the MO of g-C3N4-M (600°C) could reach 54.67 %, whereas that of g-C3N4-G (550°C) and g-C3N4-D (590°C) could reach 23.46 % and 22.16 %, respectively.

#### *2.1.4 Simple pyrolysis method*

Simple pyrolysis of affordable, environmentally friendly, active oxygen-evolving urea in a covered crucible yielded porous g-C3N4 with a band gap of 2.87 eV. The photocatalytic hydrogen evolution activity of g-C3N4 produced from urea as a precursor was higher than that of thiourea or melamine in the presence of methanol as a sacrificial reagent and Pt as a co-catalyst. This is due to the structure's porous nature and large surface area. The g-C3N4 from urea has a somewhat lower degree of polymerization, resulting in more structural flaws acting as active photocatalytic sites for the Pt nanoparticle co-catalyst photodeposition as well as hydrogen production, according to XRD, TGA, XPS, and NMR data.

### *2.1.5 Ionothermal method*

The synthesis of highly crystalline graphitic carbon nitride by dicyandiamide self-condensation in a salt melt of lithium chloride and potassium chloride has been demonstrated, and the resulting g-C3N4 has been compared to LiebigLs melon made using the typical bulk condensation technique. The product's FTIR and elemental analysis point to a structure with few flaws and unreacted end groups, indicating a highly condensed framework. Powder XRD analysis and high-resolution TEM reveal pronounced in-plane ordering with a repeat distance of d = 7.30 K, which corresponds to the separation of co-planar, covalently linked heptazine units, and a planar graphitic interlayer distance of d = 3.36 K, which corresponds to the separation of co-planar, covalently linked heptazine units.
