Theoretical Modeling of Direct Z-Scheme B,F-Doped <i>g</i>-C₃N₄/CoN₄ Composites for Promoting Photocatalytic Water Splitting Reaction

The Z-scheme mechanism bestows a perfect band alignment for photocatalysis due to the high separation efficiency of the photoexcited electron–hole pair and concomitantly preserves the redox ability toward reduction of electrons and oxidation of holes. However, the challenge of modeling and building of Z-scheme photocatalysts hinges on the correct band alignment and the direction of the inherent electric field. Keeping the efficacy of the advanced metaGGA functional to illustrate the band gap of layered materials in mind, we model a Z-scheme photocatalytic mechanism using density functional theory calculations using R2SCAN method. The type I g-C3N4/CoN4 heterojunction with limited photocatalytic activity is switched into a direct Z-scheme configuration through boron and F doping in g-C3N4 and enhances the utilization ratio of visible light for the g-C3N4/CoN4 photocatalyst. The built-in electric field in the B,F-g-C3N4/CoN4 nanocomposite facilitates interface charge transfer and forbids the rapid recombination of photoinduced carriers, causing the B,F-g-C3N4 monolayer with a negative charge and the CoN4 (111) surface with a positive charge. The higher band edge potentials of CoN4(111) surface compared to that of the B,F-g-C3N4 monolayer ensure a stronger redox ability and the built-in electric field along with satisfy the essentiality of B,F-g-C3N4/CoN4 nanocomposite to be a typical Z-scheme heterostructure. In the presence of visible light irradiation, electrons are excited to the conduction band minimum (CBM) of the CoN4 (111) surface enduring the hydrogen evolution reaction (HER), and the holes remain in the valence band maximum (VBM) of the B,F-g-C3N4 monolayer ministering the oxygen evolution reaction (OER). Investigation on the absorption spectra further manifests a large enhancement of the visible light absorption efficiency after boron and fluorine doping. A systematic theoretical analysis of electronic and optical properties, charge transfer, differential charge density distribution, work function, and photocatalytic mechanisms manifests that B,F-g-C3N4/CoN4 is a potential Z-scheme photocatalyst for water splitting reaction under visible light.