What does graphitic carbon nitride really look like?

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 similar to 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.

Automated summary

This study investigates the conformational stability and optical properties of melon structures in graphitic carbon nitrides using DFT calculations. The results show that Fina's orthorhombic melon is the most stable conformer, with the lowest optical gap, while Lotsch's monoclinic melon falls in between. Both conformers exhibit optical gaps lower than the experimental value, suggesting potential aggregation effects.