Using Dynamics to Determine the Ground-State Properties of g-C₃N₄

Graphitic carbon nitride (g-C3N4) is a promising photocatalyst with unique structural, electronic, and thermodynamic properties that currently requires a favorable semiconducting cocatalyst for efficient water-splitting performance due to its poor charge mobility and fast photogenerated charge recombination. g-C3N4 may exist in three phases, but there are many outstanding questions regarding its ground-state properties. Here, we perform a thorough investigation of the ground-state structural and electronic properties of all three phases of g-C3N4, including bulk and single-layer systems, using a range of density functional theory approaches. By investigating second-order phase transitions, we determine the dynamically stable ground-state configurations of each phase, finding that the system will “buckle” substantially even in the bulk phases, and that this buckling or corrugation occurs within moieties, not just between them. We find that the fundamental band gap varies by over 2 eV, depending on the structure. Moreover, the bulk and surface energetics indicate that real samples may contain multiple phases and structures and that the measured band gap is most likely an average value. We also compute band alignments with regard to the redox potential lines that vary strongly with structure and indicate that preferential buckling may be beneficial for photocatalytic water splitting. Finally, analysis of the charge distributions of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) suggests that buckled structures may offer more favorable water oxidation and reduction sites, although cocatalysts will be required for efficient activity. Our study provides key information on this important material that may help in the design of efficient photocatalysts for H2 production.