Abstract
Tungsten trioxide (WO3) has attracted great attention due to its promising and wonderful properties. In this study, the influences of calcined temperature on the structural, morphological and optical properties of WO3 were investigated. Sodium tungstate dehydrates (Na2WO4.2H2O, 99.9%) powder was used as starting precursor. A sol–gel spin coating technique was used to synthesize pure WO3 thin films. The as- deposited films have been annealed at 300 oC, 400 oC, 500 oC, 550 oC and 600 oC for 2 hours. The structural properties of thin film samples have been studied by the X-ray diffractometer (XRD) with Cu Kα radiation of wavelength 1.542 A. X- ray diffraction data has been used to find out changes in the crystallite size and to determine the phases present in the films. Field emission- scanning electron microscopy (FE-SEM) has been performed to investigate the surface morphology of the prepared samples. The optical band gap energy of the thin films has been investigated and analyzed using UV-Vis spectrophotometer equipped with an integrating sphere and a Spectral reflectance standard. The absorbance of the films was measured in a wavelength range of 190–2500 nm. From the results it was found that the characterized peaks were associated to the crystalline planes of the monoclinic phase of WO3. The crystallite sizes increased with increasing calcination temperatures. It was observed that the WO3 had a sphere-like structure composed of numerous nanoparticles. The absorbance spectra show that the there is a red shift in the absorption edge as the calcined temperature increases The optical energy gap was decreased as the calcined temperature increased. The estimated energy gap values for the thin film sample under consideration was about 2.95, 2.58, and 2.023 for sampled calcined at 400 0C 500 0C, and 550 0C respectively.
Highlights
The oxides of transition metals such as tungsten trioxide (WO3) are very important for the development of many functional materials and smart devices because of its chemical stability, low cost, non-toxicity, semiconducting, electrochemical and optoelectronic properties (Chen et al, 2016; Pal et al, 2018)
In the X-ray diffractometer (XRD) pattern for WO3 thin films calcined at 300°C shows one diffraction peak observed at 2θ = 41.74°, that associated to the (121) crystalline planes of the monoclinic phase of WO3
The energy gap values for the thin film sample calcined at 400 oC is about 2.95 generally consistent with that reported for the WO3 (Eg=2.8 eV), and it was 2.58, and 2.023 for samples calcined at 500 oC, 550 oC, respectively
Summary
The oxides of transition metals such as tungsten trioxide (WO3) are very important for the development of many functional materials and smart devices because of its chemical stability, low cost, non-toxicity, semiconducting, electrochemical and optoelectronic properties (Chen et al, 2016; Pal et al, 2018). Tungsten trioxide (WO3) is feasible electrochromic materials which can be applied for many potential utilizations due to their high optical reversible color change induced by electrochemical process. Many methods have been reported for the synthesis of WO3 nanostructures, such as sputtering (Garg et al 2005), chemical vapor deposition (Joraid, 2009), pulsed spray pyrolysis (Patil et al, 2005), sol-gel route (Işık et al, 2009) and Spin Coating Technique (Çırak et al, 2006). Riech et al (2013) have proposed the synthesis of WO3 thin films by using a sol gel spin coating deposition method. WO3 films were prepared by using spin coating and screen printing method in the presence of organic additive such as polyvinylpyrrolidone (Go et al, 2016). Saleem et al (2015) have successfully prepared WO3 thin films using Thermal evaporation method
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