Abstract:

In recent years, photocatalytic degradation of organic pollutants has attracted considerable attention because of its potential application for solving environmental problems. Among various semiconductor photocatalysts, TiO₂ is considered a promising candidate due to its excellent structural stability. Many researchers have focused on improving the visible-light catalytic efficiency of TiO₂, because the large band gap of TiO₂ limits its utilization of visible light energy. Recently, it has been proved that intrinsic defects like oxygen vacancies in TiO₂ can trigger the visible light activity. TiO₂ hollow microspheres with large surface areas have shown high photocatalytic efficiencies in the degradation of organic pollutants. To date, the photocatalytic performance of TiO_2-x hollow microspheres has not been investigated. The kinetics of photocatalytic degradation of organic dyes is usually depicted by the pseudo-first-order kinetic equation. However, a few studies have demonstrated the impact of light absorption by the dye itself on photocatalytic performance in terms of the rate equation. In this study, defective TiO_2-x hollow microspheres were prepared by the hydrogen reduction process to effectively promote photocatalytic activity under visible light irradiation. The structure and properties were characterized by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) analysis, X-ray photoelectron spectroscopy (XPS), electron spin-resonance (ESR), Raman spectrometry, ultraviolet-visible diffuse-reflectance spectroscopy (UV-Vis DRS), and electrochemical tests. The photocatalytic performance was evaluated based on the photocatalytic degradation of methylene blue (MB) solution under visible light irradiation. The mechanism underlying the enhancement of photocatalytic activity was also discussed. The results show that the visible-light photocatalytic activity of TiO_2-x, and TiO_2-x hollow microsphere benefit from the presence of oxygen vacancies on the surface. The photocatalytic activity of TiO_2-x hollow microspheres is better than that of TiO_2-x, attributed to the formation of hollow structures with higher specific surface areas. The mechanism of MB degradation occurring on the TiO_2-x hollow microsphere surface was also investigated. The results show that the MB molecules are photodegraded by the photogenerated hole (h⁺), reactive superoxide radical (•O₂^-), and hydroxyl radicals (•OH), and that the •OH radicals, produced only by photogenerated holes, play an essential role in the degradation of MB. Based on the discussion of the effect of initial concentration of MB on the degradation process, a new kinetic model was proposed for the photocatalytic degradation of dye, considering the effect of visible light absorbed by MB molecules, because the data estimated by pseudo-first-order kinetic equation do not fit well with the experimental data. The Runge-Kutta method was used to obtain the numerical solution of the kinetic model. The results show that the kinetic model proposed for photocatalytic dye degradation gives a more realistic description of the photocatalytic degradation of MB because the calculated results fit better with the experimental data. The rate constant (k_app) of the pseudo-first-order kinetic equation decreases with increasing initial concentration of MB, indicating that k_app is affected by the light absorption properties of MB, because an increase in the initial concentration of MB will lead to increased absorption of visible light by MB molecules rather than by TiO_2-x hollow microsphere. Unlike the rate constant k_app, the rate constant k_a in the proposed model describes the process of photocatalytic dye degradation more effectively because it does not depend on the initial dye concentration.

Key words: TiO₂ hollow microsphere, Oxygen vacancy, Photocatalysis, Degradation