Raman spectroscopy characterisation of oriented bundles of single-crystal rutile-phase TiO_2 nanorods prepared by hydrothermal bath deposition on transparent conducting substrates

摘要

Rutile-phase TiO2 rods prepared by hydrothermal bath deposition on F:SnO2 are in fact bundles of single-crystal TiO2 prismatic nanorods of similar to 4nm diameter each. These bundles of rutile-phase TiO2 nanorods have tetragonal morphology and are strain-free (or the strain in all the samples is the same). Bundles of TiO2 that are oriented tetragonally with respect to the substrate's surface, can be prepared for optimized growth times. It is observed that bundles of TiO2 nanorods act as single entities from the point of view of the Raman spectra (and thus lattice vibrations). Single prismatic nanorods of similar to 4nm width, as observed via TEM, appear to act as larger crystals to the lattice phonons once stacked together to form large oriented bundles of TiO2. Hence, the phonon confinement effect cannot be observed because translational symmetry is preserved at the grain boundaries of the nanorods that make each TiO2 bundle. Moreover, an increase in the density of bundles growing preferentially in the [002] direction, perpendicular to the substrate surface, results in an unusual increase of room and low (77K) temperature Raman band E-g/A(1g) intensity ratios.