Raman spectra of polycrystalline bismuth titanate nanotubes

The Raman spectra of bismuth titanate Bi$_4$Ti$_3$O$_{12}$ nanotubes synthesized by the template method on an Al$_2$O$_3$ membrane have been studied. The samples prepared represent an array of oriented $\sim$50-$\mu$m-long nanotubes with coalescing smooth walls. The nanotube wall thickness is varied from 20 to 50 nm, and the nanotube inside diameter is about 200 nm. The Raman spectra of the nanotubes demonstrate a broadening of the lines corresponding to the stretching and bending vibrations of TiO$_6$ octahedra. In the low-frequency spectral range corresponding to vibrations of Bi ions with respect to the oxygen octahedra, a transformation due to strong mechanical stresses generated during the formation of the nanotubes has been observed.