Photocatalytic Degradation of Toluene by a TiO₂ p-n Homojunction Nanostructure

The development of catalysts for the treatment of volatile organic compounds (VOCs) by photocatalytic oxidation is very important for treating industrial flue gases. Although defect engineering has been developed into an effective method to improve the photocatalytic activity of TiO2 semiconductor materials, few holistic studies have been performed on the structure and properties of TiO2. Herein, a one-step hydrothermal method was used to synthesize three types of nanoscale TiO2 (p-TiO2 with only Ti vacancies, n-TiO2 with only O vacancies, and pn-TiO2 with dual Ti/O vacancies) for the photocatalytic removal of gaseous toluene to determine the relationship between the redox capacity of the electron–hole pairs and the type of conductive semiconductors. It was found that pn-TiO2 exhibited a higher gaseous toluene conversion (99.6%) within 2 h, with 2.8-fold faster photocatalytic kinetics than p-TiO2 and 1.26-fold faster than n-TiO2. Based on the results of an experimental study and theoretical calculations, it was verified that the dual vacancies in pn-TiO2 enabled the valence and conduction bands to be fully exploited to generate ·OH and ·O2–, respectively, for the synergistic degradation of gaseous toluene. Because of the characteristics of the p-n homojunction, charge transfer and separation were efficiently increased in pn-TiO2, resulting in the effective treatment of gaseous toluene. In addition, after five cycles of photocatalytic degradation of toluene, the degradation rate by pn-TiO2 remained over 80%, indicating potential applications for pn-TiO2. In this study, a rational pathway is demonstrated for photocatalysts containing dual vacancies to degrade toluene based on the synergistic effect of free radicals, providing an inspiration for designing materials with vacancies or homojunctions and providing means of eliminating VOCs in practical applications.