Annealing and Plasma Effects on the Structural and Photocatalytic Properties of TiO2 Fibers Produced by Electrospinning

In this study, a combined method of heat treatment and plasma surface modification was used to improve the nanostructures and photocatalytic activity of electrospun TiO2 fibers. Based on the tuning effect of the annealing temperature from 500 to 800 °C, further improvements via the generation of H2 radiofrequency plasma reactions on the fiber’s surface were investigated. It was found that the anatase–rutile phase transition starts to occur at around 700 °C, which is higher than the common temperature for TiO2. The interfacial effect is generated by the symbiosis relationship between these two phases in the fibers, which can enhance photocatalytic activity since the anatase–rutile heterojunction in mixed-phase TiO2 is formed. The dramatic rise in oxygen vacancies on the fiber’s surface is created by the H2 plasma; this leads to the number of trapped electrons increasing and results in an accelerated separation between the photogenerated electrons and holes. Therefore, the photocatalytic mechanism, including the anatase–rutile heterojunction and the TiO2 fiber band structure containing oxygen vacancies, is predicted. The degradation rate was significantly enhanced (1.5 times) by increasing the annealing temperature up to 700 °C, which can be further improved upon after treatment with surface H2 plasma.