Density functional theory studies on C with substitutional TiN impurities.

In this paper, we have performed systematic theoretical surveys of C and its CTiN nanocages with n = 1-8 at DFT. Full optimization indicates none of the structures collapse to open deformed as segregated heterofullerene. Also, in order to avoid the resulted strain of fused five-pentagon configuration, some of them deform their cage at the Ti-N bonds and appear cubic-like. Binding energy (E) increases, and the absolute heat of atomization │ΔH│ of the designed CTiN structures decreases, respectively, as the number of substituting Ti-N units increases. The calculated E of 57.05 eV/atom and │ΔH│ of 2437.40 kcal/mol display CTiN as the most thermodynamic stable heterofullerene where including eight separated Ti-N units through two double C═C bonds. In contrast, the calculated band gap of 2.06 eV shows CTiN as the best-insulated heterofullerene. Here isolable or extractable open-shell CTiN heterofullerene must be kinetic stable species, and closed-shell CTiN should be thermodynamic stable species. Compared to the suggested Ti-decorated B fullerene as a high capacity hydrogen storage material with large E (5.67 eV/atom), our studied CTiN heterofullerenes show the higher E with a range of 13.78 to 57.05 eV/atom, the higher stability, and the higher capacity hydrogen storage. Each Ti-N unit can bind up to two hydrogen molecules with an average adsorption energy of 0.073 eV/H. While the CTiN fullerene substituted with 8 Ti-N units can store 16 H molecules, the hydrogen gravimetric density (the hydrogen storage capacity) reaches up to 5.61 wt% with an average adsorption energy of 0.587 eV/H. Based on these results, we infer that CTiN fullerene is a potential material for hydrogen storage with high capacity and might motivate active experimental efforts in designing hydrogen storage media.