Effect of annealing temperature on the photocatalytic activity of nanocrystalline tungsten oxide

Abstract

Simple acid precipitation method was adopted for the synthesis of nanocrystalline tungsten oxide with sodium tungstate dihydrate as precursor. To study the effect of temperature on the crystal structure and optical properties of tungsten oxide, as-prepared sample is annealed at 300,500 and 700 0C each for 2 hrs. Characterization techniques such as XRD, FTIR, UV–visible and PL are done to analyse the impact of annealing temperatures on the properties of annealed samples. The photocatalytic activity of the annealed samples (W300, W500 and W700) is evaluated by the degradation of MB dye under UV light irradiation. Low temperature annealed samples showed mixed phase of orthorhombic and monoclinic structure while phase pure monoclinic structure was obtained on increasing the annealing temperature to 700 0C. The high crystallinity of the samples is evident from the XRD pattern and the average crystallite size of the samples are obtained in the range 9–25 nm. The influence of crystal defects on the microstrain was carried out by using Williamson Hall analysis and it is found that microstrain decreased with increase of annealing temperature, indicating a lowering of defects arising due to cation or anion vacancies. The optical band gap energies of the samples are measured from diffused reflectance spectrum of the samples using Kubelka-Munk relation and its value was obtained high for the sample W500. Further, PL emission intensity for the excitation wavelength of 250 nm was also high for the sample W500. From the photocatalytic studies, it is evident that the degradation efficiency is high for the sample W500, suggestive of its use as an eminent tool for the treatment of water.