Effects of annealing temperature on the crystal structure, optical and photocatalytic properties of Bi 2 O 3 needles

This study investigated the synthesis of α-Bi2O3 needles by a co-precipitation method. The isolated needles were annealed at temperatures ranging from 200 to 700 °C and the possible changes in spectroscopic, structural and optical properties were investigated. Thermogravimetric analysis, X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, diffuse reflectance, X-ray photoelectron spectroscopy (XPS), electron spin resonance spectroscopy (ESR) and photoluminescence (PL) were used to characterise the synthesized needles. An increase in annealing temperature resulted in a decrease in the band gap values from 2.90 to 2.81 eV. ESR and XPS results confirmed the presence of Bi3+ and oxygen vacancies in the synthesized products, which were responsible for the PL and the observed photocatalytic degradation of the organic dye. The photocatalytic degradation of Rhodamine B dye using the synthesized Bi needles followed pseudo first order kinetics. Results indicated that the ‘as-synthesized’ Bi2O3 had the highest degradation efficiency (98%) compared to the annealed products and was attributed to the optimum concentration of oxygen vacancies and thus higher surface area. Photocatalysis was found to be very suitable for the removal of the poisonous substances such as dyes from wastewater. © 2020 Elsevier B.V.