What does graphitic carbon nitride really look like?

Abstract

Graphitic carbon nitrides (g-CNs) have become popular light absorbers in photocatalytic water splitting cells. Early theoretical work on these structures focused on fully polymerized g-C₃N₄. Experimentally, it is known that the typically employed melamine polycondensation does not go toward completion, yielding structures with ∼15 at% hydrogen. Here, we study the conformational stability of “melon”, with the [C₆N₉H₃]_n structural formula using DFT. Referencing to a 2D melon sheet, B3LYP-dDsC and PBE-MBD computations revealed the same qualitative trend in stability of the 3D structures, with several of them within 5 kJ mol⁻¹ per tecton. Fina's orthorhombic melon is the most stable of the studied conformers, with Lotsch' monoclinic melon taking an intermediate value. Invoking a simple Wannier–Mott-type approach, Fina's and Lotsch' structures exhibited the lowest optical gaps (2.8 eV), within the error margin of the experimental value (2.7 eV). All conformers yielded gaps below that of the monolayer's (3.2 eV), suggesting Jelley-type (“J”) aggregation effects.