Structure and optical properties of polymeric carbon nitrides from atomistic simulations

Computational investigation of the structural and photophysical properties of polymeric carbon nitride (PCN) materials can help to establish understanding-driven material optimization strategies. However, the plethora of structural motifs found in synthesized PCNs complicates atomistic simulations. Performing hybrid DFT studies, we systematically investigate formation energy trends and optical properties of PCNs as a function of dimensionality. Thermochemical calculations predict that a mixture of structural motifs including the melon string structure, poly(heptazine imide), and g-C3N4 motifs is stable under typical synthetic conditions. Lateral condensation reduces the bandgap while out-of-plane corrugation increases both stability and the bandgap. The key result of this work is that already small domains of strongly condensed PCN embedded in a less condensed framework can give rise to desirable optical properties. This result reconciles conflicting literature reports indicating that the calculated bandgap of the thermodynamically favorable melon structure is too large compared to experiments, while the g-C3N4 structure, for which bandgap calculations are in better agreement with experiments, does not agree with the measured chemical composition of synthesized PCNs. Finally, we postulate a new computational model for PCNs which combines the most important structural motifs and for which we calculate a bandgap of ca. 2.9 eV.