Self-doped TiO2 nanowires in TiO2-B single phase, TiO2-B/anatase and TiO2-anatase/rutile heterojunctions demonstrating individual superiority in photocatalytic activity under visible and UV light

Abstract

TiO2 nanowires possess structurally well-defined physical and chemical properties related to their dimensional confinements. However, the obscure phase transformation mechanism and the wide optical band gap of TiO2 nanowires limit their wide applications as a photocatalytic material, particularly under the visible solar spectrum. The present investigation deals with the systematic synthesis of TiO2 nanowires with their different phases e.g., TiO2-B, anatase and rutile, by the hydrothermal technique and subsequent thermal annealing in vacuum at different temperatures, and probing on their characteristic photocatalytic performances. Pure single phase TiO2-B nanowires of high crystallinity have been obtained at TA = 600 °C in which the oxygen vacancies generated via reduction of Ti4+ centers to Ti3+ facilitate enhancing absorption of visible light by virtue of their reduced optical band gaps. In addition, large surface area out of their porous structures provides enhanced adsorptive and reactive sites and accelerates the photocatalytic activity in the visible region, extending a photodegradation efficiency of RhB to ∼88% with a rate constant ∼0.013 min−1. Purely rutile phase of TiO2 formed at TA = 900 °C, exhibiting an intermediate band gap, demonstrates moderate degradation efficiency, ∼55% under visible light and ∼45% under UV exposure on RhB. However, the TiO2-B/anatase heterostructured nanowires grown by partial phase transformation at TA = 700 °C, exhibit a significantly enhanced UV photocatalytic ability by virtue of their wide optical band gaps. Migration of photogenerated holes from the anatase to the TiO2-B facilitated by the differences in two distinct phases reduces the charge recombination and enhances the photocatalytic activity with a high degradation efficiency ∼96% and a rate constant ∼0.058 min−1 under the UV light exposure. The self-transformed TiO2 heterostructured nanowires grown on thermal annealing are significantly promising for advanced photocatalytic applications, by virtue of controllable optical band gap, high porosity and self-doping characteristics.