No abstract

Titanium dioxide has been extensively studied in recent decades for its important photocatalytic application in environmental purification. The search for a method to narrow the optical band-gap of TiO 2 plays a key role for enhancing its photocatalytic application. The optical band gap of epitaxial rutile and anatase TiO 2 thin films deposited by helicon magnetron sputtering on sapphire and on SrTiO 3 substrates was correlated to the lattice constants estimated from HRTEM images and SAED. The optical band-gap of 3.03 eV for bulk-rutile increased for the thin films to 3.37 on sapphire. The band gap of 3.20 eV for bulk-anatase increases to 3.51 on SrTiO 3 . In order to interpret the optical band gap expansion for both phases, ab-initio calculations were performed using the Vienna ab-initio software. The calculations for rutile as well anatase show an almost linear increase of the band gap width with decreasing volume or increasing lattice constant a. The calculated band gap fits well with the experimental values. The conclusion from these calculations is, in order to achieve a smaller band-gap for both, rutile or anatase, the lattice constants c has to be compressed, and a has to be expanded.

The atomic and electronic structure of the ␤-silicon nitride (␤-Si 3 N 4 ) crystal have been determined using the ab initio pseudopotential method based on the density functional theory. We have obtained the stable lattice parameters and the stable positions of 14 atoms in the unit cell for the structure P6 3 /m for the first time. The electronic structure and the charge distribution indicate that the Si-N bond has both ionic and covalent characters. The band structure is in good agreement with the other first-principles results and consistent with the experiments.

The relative stability between the P6 3 /m and P6 3 structures of ␤-silicon nitride (␤-Si 3 N 4 ) crystal has been examined using the ab initio pseudopotential method. We have performed the relaxation of atomic positions in the unit cell according to the Hellmann-Feynman forces for several sets of lattice constants around the experimental ones. For each set of lattice constants, the P6 3 /m structure is naturally recovered through the relaxation from the P6 3 initial configurations. There exist no energy minima of the P6 3 structure for the examined sets of lattice parameters. Thus it is concluded that the ground-state structure of ␤-Si 3 N 4 has P6 3 /m symmetry contrary to a recent orthogonalized-linear combination of atomic orbitals calculation. However, the energy increases by the displacements of the z coordinates of N atoms constituting a coplanar configuration on the ͑0001͒ plane are very small. This should be concerned with the conflicting experimental observations.

In order to clarify the adsorption mechanism of the O 2 molecule on Co-polypyrrole composite metallo-organic catalyst, we have investigated the interaction between the molecule and Co-(n)pyrrole model clusters (n ¼ 4; 6) using the density functional theory. The stable adsorption site of the O 2 molecule on Co-(4)pyrrole is found to be at the O-O center of mass located on top of the Co atom in side-on configuration, while for the case of Co-(6)pyrrole cluster, the O 2 molecule is slightly deviated from the side-on configuration. The O-O bonds of the O 2 /Co-(4)pyrrole and the O 2 /Co-(6)pyrrole systems have elongated by 10.84 and 9.86%, respectively. The elongation mechanism of O 2 on Co-(n)pyrrole is induced by the interaction between the cobalt d-orbitals and the O 2 anti-bonding à orbital, which results in a charge transfer from the cobalt atom toward the O 2 molecule. This effect seems important in the adsorption of the O 2 molecule on Co-(n)pyrrole. It is likely that the extra charge in the O 2 molecule would fill its anti-bonding orbital and consequently weaken the O-O bond. In Co-(4)pyrrole, the elongation of the O 2 bond is larger than that of Co-(6)pyrrole since a complete side-on configuration has more symmetric overlapping between the cobalt d-orbitals and the O 2 anti-bonding orbital.