NiWO 4 was prepared using the polymeric precursor method and studied in terms of physical and chemical properties to verify its stability for industrial applications as pigments. The characterization was accomplished using thermal analyses, X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL) and UV-Vis spectroscopies, colorimetric coordinates, and Raman spectra. Increasing the temperature, successive exothermic reactions were observed and they are related with thermal decomposition of the organic compound. The stability was reached at ~700 ℃. The material is verified to become completely free of second phase at ~800 ℃. The end NiWO 4 powders showed an intense charge transfer (CT)-related tail centered in the ultraviolet region, resulting in a yellow product. In addition, the powders exhibited broad excitation band and broad deep blue-green emission band, which were enhanced with increasing powders' crystallinity.
Calcium tungstate (CaWO 4) crystals were prepared by microwave-assisted hydrothermal (MAH) and polymeric precursor methods (PPM). These crystals were structurally characterized by X-ray diffraction (XRD), N 2 adsorption, X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) measurements. The morphology and size of these crystals were observed by field emission scanning electron microscopy (FE-SEM). Their optical properties were investigated by ultraviolet visible (UV-Vis) absorption and photoluminescence (PL) measurements. Moreover, these materials were employed as catalysts towards gas phase toluene oxidation reaction. XRD indicates the purity of materials for both preparation methods and MAH process produced crystalline powders synthesized at lower temperatures and shorter processing time compared to the ones prepared by PPM. FE-SEM images showed particles with rounded morphology and particles in clusters dumbbellslike shaped. PL spectra exhibit a broad band covering the visible electromagnetic spectrum in the range of 360 to 750 nm. XANES and EXAFS results show that preparation method does not introduce high disorders into the structure, however the H 2-TPR results indicated that the catalyst reducibility is affected by the preparation method of the samples.
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