Prediction of ultra-flexible 2D carbon nitride and tunable molecular sieve

Abstract

Based on the evolutionary algorithm and first-principles calculation, we have carried out a comprehensive structural search for two-dimensional C3N4 system and found three structures, i.e., P 6/ m -C3N4, P 2/ m -C3N4 and P 6 ¯ 2 m -C3N4, which are more stable than graphitic-C3N4 under ambient pressure. More importantly, through deformation testing, it is found that all three structures exhibit ultra-flexible characteristics. Their flexibility is much better than graphitic-C3N4, and the minima in the energy landscapes are remarkably broad. When the lattice constants change ±5%, the energy (stress) changes of P 6/ m -C3N4, P 2/ m -C3N4, and P 6 ¯ 2 m -C3N4 are just ±23.52, ±42.21, and ±3.41 meV/atom (±0.33, ±0.53, and ±0.07 GPa ), respectively, far less than the average value of graphitic-C3N4, 125.24 meV/atom ( 3.27 GPa ). In other words, the stress could be 46 times smaller than that of graphitic-C3N4. This remarkable behavior is attributed predominantly to the pronounced angular flexibility of the N-C-N bond linking the C3N3 rings. In addition, the connected C3N3 rings formed hexagonal ( 128 A2 ), quadrilateral ( 112 A2 ) and trigonal ( 108 A2 ) tunable pores, respectively, which make the new structures highly promising tunable microporous molecular sieve material. Finally, we analyze the variation of band structure during the deformation of new structures. The band gap of P 6 /m- C3N4 and P 2/ m -C3N4 decrease linearly, about 0.02 eV per step, when strain change from −5% to +3%. But for P 6 ¯ 2 m -C3N4, the band gap increases first and then decreases when strain change from −5% to +5%. Our research provides new insights into a better understanding of the two-dimensional C3N4-based porous materials and will stimulate a new round of theoretical and experimental research on graphite-like carbon nitride.