First-principles prediction of three new graphitic C3N4 allotropes with potentials for application in sun-light-driven water splitting

Three new graphitic C3N4 allotropes are identified in stochastic group and graph constrained searches and systematically investigated by first-principles calculations. They are named as root 13- C3N4, root 16- C3N4 and root 21-C3N4 according to their structure features that can be constructed through introducing atomic defects in root 13-, root 16- and root 21- super cells of a hypothetical hexagonal carbon nitride structure, respectively. These three new C3N4 allotropes are structurally quite similar to the widely investigated graphitic C3N4 structures based on triazine (root 4-C3N4) and heptazine (root 9-C3N4). The calculated total energies of root 13-C3N4, root 16-C3N4 and root 21-C3N4 are 55 meV/atom, 12 meV/atom and 47 meV/atom higher than that of the ground state root 9-C3N4, which indicate that they are energetically stable. They are also confirmed to be thermo-stable phases at 300 K temperature condition through ab initio molecular dynamics (MD) simulations. The HSE06 based calculations show that root 13-C3N4, root 16-C3N4 and root 21- C3N4 are semiconductors with indirect band gaps of 1.921 eV, 2.056 eV and 2.808 eV, respectively. Their band gaps and band edge positions are proper for sun-light driven water splitting. The root 21 C3N4 with remarkable stability and proper electronic properties, comparable to the widely used root 9-C3N4, for sun-light-driven water splitting at both acidic and neutral environments is highly expected to be synthesized in future experiment for application in sun-light-driven water splitting.