Controlled synthesis of electrospun TiO₂ nanofibers and their photocatalytic application in the decolouration of Remazol Black B azo dye

The obtaining of monocrystalline TiO₂ nanofibers with high specific surface area synthesized by the electrospinning technique with controlled crystalline structure and morphology is reported. The nanofibers were annealed at 500, 700 and 900 °C in a controlled atmosphere in the presence of air for two hours to achieve crystalline phase transformation. By means of TEM, it was possible to clearly corroborate the presence of solid nanofibers with a well-defined shape, a smooth surface and without the presence of interconnections and defects called beads. 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. By Raman scattering was found that the TiO₂ nanofibers showed a crystalline phase transformation from pure anatase, mixed anatase-rutile and pure rutile as the annealing temperature is increased, caused by the generation of oxygen vacancies, which was corroborated by X-ray diffraction. The band gap energy (E_g), obtained from optical absorption spectra, decreases as the temperature is increased, which is ranged in 2.42 ≤ E_g ≤ 3.27 eV, caused by the anatase→rutile crystalline phase transformation. Photoluminescence shows that the radiative bands -present a gradual red shift as the temperature increases due to the E_g reduction. In addition, the photocatalytic properties of the annealed TiO₂ nanofibers were evaluated in the decolouration of the Remazol Black B azo dye. Changes in absorption spectra were noticeable as the measurement time increases. Absorbance spectra showed a decrease in the intensity of the main absorption band at 589 nm, which gradually decreases until it completely disappears, indicating that the decomposition of the organic compound is complete, while physically there is an absence of color from the solution. The anatase crystalline phase was the one with the highest specific surface area and the highest photocatalytic activity.