Optical investigation of Na2V3O7 nanotubes

We report the electronic and vibrational properties of Na2V3O7 nanotubes and compare the response with other layered and nonlayered vanadates. The electronic structure of Na2V3O7 displays a strong similarity to that of nontubular vanadates. We assign the 1.2-eV band as a V d --> d excitation and the 3.3- and 3.9-eV bands as O 2p --> V 3d charge-transfer structures. Although band structure calculations predict additional fine structure in the density of states because of the tubular morphology, such features are not observed in the absorption spectrum. The vibrational spectrum displays triplet mode splitting as a result of reduced site symmetry, consistent with the three sightly different vanadium atoms that form the basic structural unit. A low-frequency rattling mode is observed in Na2V3O7 at 88 cm(-1). This unique characteristic of the Na+-ion intercalated tubes might be connected with the ionic conductivity.