Tunable hierarchical TiO2 nanostructures by controlled annealing of electrospun fibers: formation mechanism, morphology, crystallographic phase and photoelectrochemical performance analysis

Highly crystalline hierarchical TiO2 nanostructures of morphology ranging from one-dimensional regular fibers, hollow tubes, porous rods and spindles were achieved from electrospun TiO2/composite fibers by annealing at temperatures ranging from 400 degrees C, 500 degrees C, 600 degrees C, 700 degrees C, and 800 degrees C, with a ramp rate of 5 degrees C min(-1), and at a pressure of 1 mbar. Crystallographic structure, crystallite size, surface morphology and surface area of annealed TiO2 nanostructures were analysed by X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM) and Brunauer-Emmett-Teller (BET) method. The analysis of post-annealing process on electrospun TiO2 nanofibers showed an orderly change in the crystallographic phase transformation with corresponding change in their surface morphologies. XRD and HRTEM analysis confirmed the phase transformation of highly crystalline anatase phase to rutile with crystallite size varied from 11 nm to 36 nm upon tuning the annealing temperature. Interestingly, TiO2 nanostructures annealed at 700 degrees C showed the formation of biphasic TiO2 hollow tubes with stoichiometry phase compositions of 45.74% anatase and 54.25% rutile. A possible formation mechanism was proposed based on series of temperature-dependent experiments. To evaluate the potential use of these TiO2 nanostructures, dye sensitized solar cell (DSSC) was fabricated using the post-annealed TiO2 nanostructures as photoanode. A higher conversion efficiency (eta) of 4.56% with a short circuit current (J(sc)) of 8.61 mA cm(-2) was observed for highly ordered porous anatase TiO2 nanorods obtained upon annealing at 500 degrees C under simulated AM1.5 G (100 mW cm(-2)), confirming that surface area of TiO2 resulted out of porous structure played dominant role.