Naruhiro Koi

Molecular orbital calculations of hydrogen storage in carbon and boron nitride clusters

Abstract Hydrogen gas storage ability in carbon and boron nitride (BN) clusters was investigated by molecular orbital calculations. From single point energy calculations, H2 molecules would enter from hexagonal rings of C60 and B36N36 clusters and octagonal rings of B24N24 cluster because of lower energy barrier. Chemisorption calculation of hydrogen for BN clusters showed that hydrogen bonding with nitrogen atoms was more stable than that with boron atoms. Stability of H2 molecules in BN clusters seems to be higher than that of carbon clusters.

Boron Nitride Nanocage Clusters, Nanotubes, Nanohorns, Nanoparticles, and Nanocapsules

Various types of boron nitride (BN) nanostructured materials such as nanocage clusters, nanotubes, nanohorns, nanoparticles, and nanocapsules were synthesized by arc melting, thermal annealing, and chemical vapor deposition methods, which were characterized by high-resolution electron microscopy and molecular orbital calculations, and their properties were discussed. The BN clusters consisted of 4-, 6-, 8- and 10-membered BN rings satisfying the isolated tetragonal rule, which was optimized by molecular orbital calculations. Total energy calculation showed that some elements stabilize and expand the B36N36 structure. Bandgap energies of the B36N36 clusters were found to be reduced by introducing a metal atom inside the cluster, which indicates controllability of the energy gap. Chiralities of BN nanotubes with zigzag- and armchair-type structures were directly determined from high-resolution images, and structure models are proposed. Total energies of BN nanotubes with a zigzag-type structure were lower than those of armchair-type structure, and these results agreed well with the experimental data. Cup-stacked BN nanotubes and Fe-filled BN nanotubes were also produced, and the atomic structures, structural stability, and electronic property were investigated and discussed. BN nanohorns were synthesized, and multiwalled BN nanohorns would be stabilized by stacking of BN nanohorns. Formation and structures of multiply twinned nanoparticles with fivefold symmetry in chemical vapor-deposited BN were also investigated. A new process for Fe or Co nanoparticles coated with BN layers in large quantity was developed, and they exhibited a soft magnetic characteristic and good oxidation resistances. These unique structures would be suitable materials for nanoelectronics devices, magnetic recording media, and biological sensors with excellent protection against oxidation and wear. Possibility of hydrogen gas storage in BN clusters was also investigated by molecular orbital calculations, which indicated possibility of hydrogen storage of ~5 wt%. The new BN nanostructured materials would be expected as future nanocale devices.