Hydrothermal-assisted sol–gel synthesis of Cd-doped TiO2 nanophotocatalyst for removal of acid orange from wastewater

Abstract

This research is based on synthesis of titania (TiO2) nanophotocatalyst followed by cadmium (Cd) doping to activate the photocatalyst in visible part of the light spectrum. Therefore, the Cd–TiO2 nanophotocatalyst was synthesized in various Cd/Ti molar ratios of 0, 0.05, 0.1, and 0.15 using hydrothermal-assisted sol–gel method. The characterization analyses of X-ray diffraction, field emission scanning electron microscopy, brunauer-emmett-teller, diffuse reflectance spectroscopy, Fourier transform infrared spectroscopy, thermogravimetric-derivative thermogravimetric, transmission electron microscopy, and energy dispersive X-ray were carried out to evaluate the physical, chemical, and optical properties of the catalysts. The X-ray diffraction analysis resulted a decrease in the crystallinity due to cadmium doping. Field emission scanning electron microscopy images have confirmed the nano-sized structure of the catalysts. The analysis also has demonstrated particle size distribution enhancement with a meaningful low average particle size for Cd/TiO2 (0.15) sample. This sample has represented the highest brunauer-emmett-teller surface area. According to energy dispersive X-ray dot mapping analysis, no aggregation was detected on the catalyst surface. Diffuse reflectance spectroscopy analysis has indicated a moderate decrease in band gap, after Cd loading. The photocatalytic degradation of acid orange 7 dye from the synthetic wastewater was carried out to evaluate the photocatalytic activity. The most effective parameters on the process, such as pH, catalyst loading, and dye concentration, were investigated. According to the characterization analyses and degradation tests, Cd/TiO2 (0.15) was selected as a sample having best structural properties and photocatalytic activity. Finally, the experiments resulted the optimum conditions of pH = 2, catalyst loading of 1 g/L, and dye concentration of 10 mg/L, indicating 95 % of dye degradation after 120 min of experiment. The electron–hole pair generation via transition of electron between the valence and conduction bands (band gap) is the fundamental of any photocatalytic process. After Cd doping, the conduction band position shifts toward the valence band. Then, a valence electron in narrow band gap photocatalyst needs lower energy of light to transfer to conduction band, it means that using light in higher wavelengths (visible light spectra) will be available. In photocatalytic wastewater treatment process, degradation occurs in two ways: the first mechanism is direct oxidation-reduction through electron–hole generation, and the second one would be the oxidation through produced reactive species such as hydroxyl radicals, this route of reaction organizes the major part of wastewater treatment.