First-Principles Prediction Of New Graphitic C₃N₄ Allotropes And Electronic Properties

Two-dimensional graphitic carbon nitride(g-C3N4)has attracted many research interest in recent years.They are excellent materials for applications in clean energy and environmental purification.With appealing electronic band structures,g-C3N4 can produce hydrogen from water with visible-light irradiation without sacrificial donor.Previous experimental works have reported the synthesizing of both triazine-based g-C3N4 and heptazine-based g-C3N4,and confirmed their excellent physical properties,such as metal-free photoactivity and intrinsic piezeoelectricity.Due to the easy regulation of chemical components and band structures,the structure of two-dimensional graphite phase carbonitrides is diverse and rich in physical properties.With constraint of stoichiometric C:N ratio of 3:4,we discover three new hexagonal carbon nitride structures((?)13-C3N4,(?)16-C3N4 and(?)21-C3N4)and studied the electronic structure and photocatalytic properties of these three new structures and explored the modulation of the electronic properties of the three structures by biaxial strain.Three new graphitic C3N4 allotropes are identified in stochastic group and graph constrained searches and systematically investigated through our recently developed RG2 code.These three new C3N4 allotropes are structurally quite similar to the widely investigated graphitic C3N4 structures based on triazine((?)4-C3N4)and heptazine((?)9-C3N4).The calculated total energies of(?)13-C3N4,(?)16-C3N4 and(?)21-C3N4 are 55meV/atom,12meV/atom and 47meV/atom higher than that of the ground state(?)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(?)13-C3N4,(?)16-C3N4 and(?)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(?)21-C3N4 with remarkable stability and proper electronic properties,comparable to the widely used(?)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.We perform first-principles calculation to study the effects the electronic properties of three new graphite-phase C3N4 allotropes by biaxial strain.Under the application of-8%to +8%strain,we found that when adding tensile strain,three new structures can not withstand excessive tensile strain.(?)13-C3N4 is greater than?xy=+2%,the structure will break.As the(?)16-C3N4 and(?)21-C3N4 is greater than?xy=+4%.The band gap of the structures increases follow the strain increases when compressive strain is added.When compressive strain is added,the band gaps of the three structures monotonically decrease as the strain increases.When(?)16-C3N4 is added the strain at ?xy=-2%,the bandgap type is transformed from an indirect bandgap to a direct bandgap.When(?)21-C3N4 is added the strain at ?xy=-4%,the bandgap type changes from indirect bandgap to direct bandgap.As the strain increases to -6% and-8%,the bandgap is still a direct bandgap.