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°C, 500°C, 600°C, 700°C, and 800°C, with a ramp rate of 5°C min ^-1, and at a pressure of 1 mbar. Crystallographic structure, crystallite size, surface morphology and surface area of annealed TiO ₂ 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°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 (η) of 4.56% with a short circuit current (J_sc) of 8.61 mA cm^-2 was observed for highly ordered porous anatase TiO ₂ nanorods obtained upon annealing at 500°C under simulated AM1.5 G (100 mW cm^-2), confirming that surface area of TiO₂ resulted out of porous structure played dominant role.