The role of the annealing process in different gas environments on the degradation of the methylene blue organic pollutant by brookite-TiO₂ photocatalyst

Abstract

Among sustainable technologies, TiO2 photocatalysts are promising materials as an alternative to conventional methods that are being used for eliminating problems such as water, air, and environmental pollution originating from industrial waste. In this work, TiO2 film was produced by ultrasonic spray pyrolysis technique and annealed in oxygen, argon, and nitrogen atmospheres. The effect of the annealing process in different environments was determined by investigating their structural, optical, surface, and photocatalytic properties. It was determined from X-ray diffraction patterns that highly (004)-oriented anatase-TiO2 films were successfully obtained with annealing of polycrystalline brookite-TiO2 film. Thickness and refractive index values were determined by spectroscopic ellipsometry. According to absorbance and photoluminescence spectra, indirect optical band gaps were shifted to the visible region, and the oxygen vacancy and Ti3+ cations were formed as surface defects, respectively. Atomic force microscopy and energy-dispersive X-ray spectroscopy were used to determine surface properties and elemental compositions, respectively. Besides, to investigate the use potentials of TiO2 films in photocatalytic applications, photocatalytic tests were made using methylene blue as organic pollutant. Consequently, the structural, optical, and surface properties of the TiO2 films are strongly dependent on the annealing medium, and TiO2 films annealed in oxygen and nitrogen environments with higher photocatalytic activities are promising materials for photocatalytic applications. We also conclude that determination of the physical and photocatalytic properties of brookite-TiO2 film is one of the important outputs due to the difficulties encountered in obtaining of the unstable brookite phase.