g-C3N4 and Others: Predicting New Nanoporous Carbon Nitride Planar Structures with Distinct Electronic Properties

We report a theoretical investigation aiming at the prediction of graphene-like carbon nitride structures. Using a combination of model and ab initio calculations, we search among hundreds of thousands of CxNy structures in a wide (x, y) compositional range and find several structures with formation energies similar to those of the existing tri-s-triazine and s-triazine g-C3N4 nanoporous structures. Our results indicate that the increase of nitrogen concentration causes the coalescence of vacancies and therefore increases the size of the nanopores. We also find clear correlations between formation energies, the relative concentrations of graphitic and pyridinic nitrogen sites, and the Fermi level of stable structures. These correlations indicate that charge doping plays an important role in the stability of carbon nitride planar structures. We also find that the majority of the stable structures are metallic, but several are semiconducting structures that may absorb in the visible light region.