Effect of Annealing on the Surface Morphology, Optical and and Structural Properties of Nanodimensional Tungsten Oxide Prepared by Coprecipitation Technique

Tungsten oxide (WO3) nanoparticles with monoclinic structure have been synthesized by using an inexpensive coprecipitation process. The obtained nanoparticles were annealed at 400 degrees C, 500 degrees C, 600 degrees C, 700 degrees C, 800 degrees C, and 900 degrees C for 1h under the same physical conditions. The morphology, structure, and optical properties of the synthesized nanoparticles were studied by x-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), ultraviolet-visible (UV-Vis) spectrophotometry, and Raman spectroscopy. The XRD results confirmed that the synthesized nanomaterial was crystalline in nature with monoclinic phase. The crystallite size varied from 14nm to 87nm when changing the annealing temperature. Williamson-Hall analysis was used to investigate the change in lattice strain and crystallite size. The optical performance was investigated by using UV-visible spectroscopy. The bandgap of the prepared nanomaterials varied from 2.51eV to 3.77eV with the annealing temperature, due to the variation of the effect of oxygen vacancies on the electronic band structure. SEM revealed formation of uniform and irregular-sized nanoparticles. HRTEM analysis revealed that the nanoparticles grew along the [002] plane with d-spacing of 0.39nm for the material annealed at 500 degrees C and along the [200] plane with spacing of 0.36nm when annealed at 800 degrees C. The crystalline nature of the synthesized nanomaterial was confirmed by uniform and clear fringes obtained in TEM micrographs. The correlation between the peak position and width of the key band at 806cm(-1) in Raman spectroscopy band is discussed. These enhancements in the properties of WO3 nanomaterial make it an efficient material for many potential applications, e.g., in photocatalysis, electro- and photochromic devices, etc.