Systematic Characterization of the Crystalline Phase Transformation, from Amorphous to Rutile through Anatase, of TiO2 Nanofibers Synthesised by Electrospun Technique

TiO2 nanofibers were synthesised by means of the electrospun technique, which were annealed at high temperatures to achieve the crystalline phase transformation from amorphous to rutile through anatase and the phase mixture. The chemical stoichiometry of electrospun TiO2 nanofibers was estimated by EDS, finding that at low annealing temperatures excess of oxygen was detected and at high temperatures excess of titanium that originates oxygen vacancies. The TEM images showed clearly the formation of TiO2 nanofibers (NF's) that exhibit a homogeneous and continuous aspect without the presence of crystalline defects, whose surface morphology depends strongly on the annealing temperature. The crystalline phase transformation was studied by Raman spectroscopy, which revealed that annealed TiO2 NF's showed a crystalline phase transformation from amorphous, pure anatase, anatase-rutile mixed, to pure rutile as the annealing temperature increased, which was corroborated by X-ray diffraction and high-resolution TEM. The average grain size, inside the NF's, increased with the crystalline phase transformation from 10 to 24 nm for anatase-TiO2 and from 30 to 47 nm for rutile-TiO2, which were estimated by using the Scherrer-Debye equation. By absorbance measurements at room temperature the band gap energy (E-g) was obtained, which is ranged in 3.75-2.42 eV, caused by the amorphous -> anatase -> anatase-rutile mixed -> rutile crystalline phase transformation.