Density functional theory-based modeling of the half-metallic g-C3N4/CoN4 heterojunction for photocatalytic water splitting reaction

Abstract

Using density functional theory (DFT), we have investigated the structural, optical, electronic and magnetic properties of a graphitic carbon nitride (g-C₃N₄) and CoN₄ composite to explore the effect of the heterojunction on the photocatalytic performance of g-C₃N₄. The structure of g-C₃N₄ is modified while complexing with CoN₄ and the corresponding stabilization is confirmed through adhesion energy calculation. The phonon spectra analysis furthermore guaranteed the lattice-dynamic stability of the CoN₄ bulk and the CoN₄ slab. Pristine g-C₃N₄ is a wide band gap semiconductor, which becomes half metallic upon CoN₄ inclusion. The metallicity in the g-C₃N₄/CoN₄ composite originates from the spin down channel, keeping the spin up channel in a semiconducting state. The charge density analysis and work function calculation suggest a substantial amount of charge transfer from g-C₃N₄ to the CoN₄ unit in the g-C₃N₄/CoN₄ heterojunction. The model heterojunction of the g-C₃N₄/CoN₄ composite can enhance the utilization ratio of visible light for the g-C₃N₄ photocatalyst. In g-C₃N₄/CoN₄, the valence band maximum (VBM) has a more positive potential compared to O₂/H₂O (+1.23 V) on the normal hydrogen electrode (NHE) scale. However, the conduction band minimum (CBM) displays a more negative potential compared to H⁺/H₂ (0 V) on the NHE scale. The details of the band structure, density of states and band edge position determining calculations confirm that the g-C₃N₄/CoN₄ composite forms a type 1 heterojunction, making it a suitable photocatalyst for water splitting reaction. The practical application of the g-C₃N₄/CoN₄ heterostructure as a photocatalyst was substantiated in the presence of polar solvent (water) by calculating the band gap, charge transfer interaction and charge density difference. There is a significant decrease of charge transfer and thereby charge density difference in the g-C₃N₄/CoN₄ heterojunction in the presence of water; however, it still holds potential for use as a photocatalyst for water splitting reaction. The state-of-the-art theoretical modeling of the g-C₃N₄/CoN₄ heterojunction is the first theoretical study incorporating the CoN₄ crystal.