New structure canditates for the experimentally synthesized heptazine-based and triazine-based two dimensional graphitic carbon nitride

Abstract

The widely used structure model for both heptazine-based and triazine-based two-dimensional graphic carbon nitride (g-C 3 N 4 ) is the flat P-6m2 structure. However, the experimentally synthesized g-C 3 N 4 possess thickness ranging in 0.2-0.5 nm, indicating that the theoretically used flat P-6m2 configurations are not correct ground states. In this work, we propose three new corrugated structures P321, P3m1 and Pca21 with energies of about 65 meV/atom, 70 meV/atom and 74 meV/atom lower than those of the corresponding flat P-6m2, respectively. These corrugated structures have very similar periodic patterns with the flat P-6m2 ones and they are difficult to be distinguished from their top-views. The optimized thicknesses of the three corrugated structures ranging in 1.347-3.142 Å are in good agreement with the experimental results. The first-principles results show that these corrugated structure candidates are also insulators with band gaps slightly larger than those calculated based on the flat P-6m2 model. Furthermore, they also possess suitable band edge positions for sun-light-driven water-splitting at both pH = 0 and pH = 7 environment as the widely used P-6m2. Our results show that these three new structures are more promising candidates for the experimentally synthesized g-C 3 N 4 .