Microwave-Assisted Solution Combustion Synthesis of WO3 Nanoparticles: Optical and Colorimetric Characteristics

Tungsten oxide (WO3) and tungsten oxide hydrate (WO3.H2O) nanoparticles were synthesized via microwave-assisted solution combustion in comparison with the acidic precipitation method. Oxalic acid was used as a surfactant and forming agent in the acidic precipitation method. In addition to oxalic acid, glycine and citric acid were also used as fuels in the microwave-assisted combustion method. The synthesis process was investigated by thermogravimetric (TG) and Differential Thermal Analysis (DTA) analysis. The obtained nanoparticles were analyzed using the scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. The sample synthesized via the acidic precipitation method showed an orthorhombic crystal structure. One of the samples synthesized via the microwave-assisted solution combustion method was monoclinic and the two others were amorphous. The acidic precipitation method resulted in uniform plate-like structures while the combusted samples indicated irregular spherical morphology. Fourier-transform infrared (FTIR) analysis revealed stretching-vibrating bands relating to W-O bonds in the synthesized tungsten oxide nanoparticles. The bandgap energy of the nanoparticles calculated using UV-Vis spectra and Tauc plot extrapolation increased with decreasing the particle size. The data of reflectance and colorimetry had good agreement with the maximum peak position in the absorption spectra. The results indicated that the acidic precipitation method controls the particle's morphology as well as the size distribution. Although the combustion of fuels releases a lot of heat, the synthesis by solution combustion can control the size and shape of the nanoparticles, which can be an appropriate method for mass production of nanoparticles.