Growth of hierarchical TiO2 nanostructures on anatase nanofibers and their application in photocatalytic activity

Three-dimensional hierarchical nanostructures of TiO2, consisting of high density nanorods on nanofibers, were synthesized by the combination of electrospinning, thermal annealing and hydrothermal methods. The morphologies, crystal structures, surface area, band gap and photocatalytic activity of the hierarchical nanostructures were characterized by Raman microscopy, X-ray diffraction, UV-vis spectroscopy, scanning electron microscopy, transmission electron microscopy and surface area analysis. The results revealed that the nanofibers, synthesized by the electrospinning method followed by the thermal annealing, were of anatase phase. Similarly, the TiO2 nanorods grown on the anatase nanofibers by hydrothermal reaction in Ti-HCl solution were of rutile phase, with tetragonal shape, and had top square facet morphologies. The diameter and the length of these rutile nanorods could be varied over the range, 10 nm to 400 nm, and 20 nm to 1 um, respectively, by changing the parameters of the hydrothermal reaction. The growth mechanisms of these rutile rods are discussed on the basis of (i) surface roughness of the anatase nanofibers, (ii) presence of the twin planes of anatase layer with structure similar to rutile phase and (iii) Cl/Ti ratio in the solution. The estimated band gap energies of the nanofibers and nanorods were close to 3.2 eV and 3.0 eV. The total surface area of the hierarchical nanostructures could reach up to similar to 20.41 m(2) g(-1); while, the pure the anatase nanofibers surface area is similar to 19.79 m(2) g(-1). For the first time, such TiO2 hierarchical nanostructures, rutile nanorods on anatase nanofibers, have been synthesized. The photocatalytic activity of the TiO2 hierarchical nanostructures was found to be superior to that of pure anatase nanofibers.