Theoretical studies of the CNx nanotube with four-nitrogen divacancy (4ND) defects

The formation energies, electronic properties, and applications in hydrogen storage of four-nitrogen divacancy (4ND) defects in a series of CNTs are investigated by density functional theory (DFT) calculations. We find that the formation energies of 4ND defects in the (n, 0) CNT are always smaller than those of the (n, n) CNT, suggesting that the formers might be easily formed. For (n, n) CNTs, the formation energies of 4ND defect are nearly independent of the tube diameters for s, while the values increase monotonically with the increased tube diameters in (n, 0) CNTs. Furthermore, the incorporation of 4ND structures to the CNT introduces the acceptor states to the band structures, resulting in the decrease of the band gaps of pure CNTs in various ways. By studying the hydrogen adsorption on the Ca-dispersed CNx nanotube with 4ND structures, we suggest that up to five H2 molecules can be bound to per Ca atom with the adsorption energy of −0.263 eV. The present results indicate that the Ca-dispersed CNx nanotube with 4ND defects might be a promising candidate as hydrogen storage media.