Novel hollow mesoporous 1D TiO₂ nanofibers as photovoltaic and photocatalytic materials

Hollow mesoporous one dimensional (1D) TiO₂ nanofibers are successfully prepared by co-axial electrospinning of a titanium tetraisopropoxide (TTIP) solution with two immiscible polymers; polyethylene oxide (PEO) and polyvinylpyrrolidone (PVP) using a core-shell spinneret, followed by annealing at 450 °C. The annealed mesoporous TiO₂ nanofibers are found to having a hollow structure with an average diameter of 130 nm. Measurements using the Brunauer-Emmett-Teller (BET) method reveal that hollow mesoporous TiO₂ nanofibers possess a high surface area of 118 m² g^-1 with two types of mesopores; 3.2 nm and 5.4 nm that resulted from gaseous removal of PEO and PVP respectively during annealing. With hollow mesoporous TiO₂ nanofibers as the photoelectrode in dye sensitized solar cells (DSSC), the solar-to-current conversion efficiency (η) and short circuit current (J_sc) are measured as 5.6% and 10.38 mA cm^-2 respectively, which are higher than those of DSSC made using regular TiO₂ nanofibers under identical conditions (η = 4.2%, J_sc = 8.99 mA cm^-2). The improvement in the conversion efficiency is mainly attributed to the higher surface area and mesoporous TiO₂ nanostructure. It facilitates the adsorption of more dye molecules and also promotes the incident photon to electron conversion. Hollow mesoporous TiO₂ nanofibers with close packing of grains and crystals intergrown with each other demonstrate faster electron diffusion, and longer electron recombination time than regular TiO₂ nanofibers as well as P25 nanoparticles. The surface effect of hollow mesoporous TiO ₂ nanofibers as a photocatalyst for the degradation of rhodamine dye was also investigated. The kinetic study shows that the hollow mesoporous surface of the TiO₂ nanofibers influenced its interactions with the dye, and resulted in an increased catalytic activity over P25 TiO₂ nanocatalysts.