Determination of Crystal Structure of Graphitic Carbon Nitride: Ab Initio Evolutionary Search and Experimental Validation

Although graphitic carbon nitride (g-C3N4) is a promising photofunctional material, its structure is poorly understood. Here, we present a systematic study of stable crystal structures of g-C3N4 by ab initio evolutionary searching. It was discovered that off-plane distortion of heptazine units is a characteristic of the most stable structure, which explains a known discrepancy between the lattice parameters determined by X-ray diffraction (XRD) patterns and the planar structures modeled in previous studies. A phase transition from a metastable phase to the global minimum phase provides a reasonable explanation for the observed red shift in photoabsorption edges upon high-temperature annealing. The recently suggested salt-melt synthesis for g-C3N4 is subject to the contamination of hydrogen, chlorine, and lithium according to our detailed analysis of the crystal structures of C6N9H3-Li3Cl and C6N9H3-LiCl in comparison with the measured XRD patterns of these samples. Finally, a viable synthesis pathway for purifying high-crystallinity g-C3N4 is proposed.