Effect of Sheet Distance on the Optical Properties of Vanadate Nanotubes

We measured the optical properties of mixed-valent vanadium oxide nanotubes to investigate the charge degrees of freedom in these novel materials. The electronic structure strongly resembles that of other bulk, low-dimensional, and molecular vanadates. The 1.2-eV band is assigned as a superposition of V4+ d → d and V4+ → V5+ charge-transfer excitations, and the features above 3 eV are attributed to O 2p → V 3d charge-transfer excitations. The 5-eV excitation shows a modest sheet distance dependence, red-shifting with increasing sheet distance. We find that the optical gap is ∼0.56 eV at room temperature and ∼0.65 eV at 4.2 K. It does not depend systematically on tube size. At the same time, selected V−O−V stretching modes sharpen and red-shift with increasing sheet distance. We attribute these trends to the microscopic manifestations of strain, which changes with curvature. A low-frequency mode is observed at 113 cm-1, which is assigned as the radial breathing mode of the VO x nanotubes.