Band gap of C3N4 in the GW approximation

Ab initio calculations based on density functional theory are performed to study the stability of newly proposed C3N4 forms. Heptazine-based g-C3N4 was found to be energetically favored relative to other phases. The quasiparticle band energies of different C3N4 phases are calculated using the GW method. Among the seven phases of C3N4 studied, only the pseudocubic phase and g-h-triazine phase have direct band gaps, and all of the other phases have indirect band gaps. The band gap of α-C3N4, β-C3N4, cubic-C3N4, pseudocubic-C3N4, g-h-triazine, g-o-triazine and g-h-heptazine is 5.49 eV, 4.85 eV, 4.30 eV, 4.13 eV, 2.97 eV, 0.93 eV and 2.88 eV, respectively. From the viewpoint of band gap energies, both the g-h-heptazine and the g-h-triazine phases can be used as suitable photocatalysts for hydrogen production using water.

► Band gap characters of seven C3N4 polymorphs investigated by GW method.
► g-h-triazine phase of graphitic C3N4 has a direct band gap of 2.97 eV.
► g-h-heptazine phase of graphitic C3N4 has an indirect band gap of 2.88 eV.
► g-o-triazine phase of graphitic C3N4 has an indirect band gap of 0.93 eV.