Soluções sólidas Ca1-xCuxWO4 (0 ≤ x ≤ 0,05) obtidas por síntese sonoquímica: um estudo acerca das propriedades estruturais e fotoluminescentes

Abstract

Technological advances, mainly in the field of energy storage, electronics and biomedical, have driven several studies aimed the synthesis, characterization, and application of semiconductors. In this context, was investigate a simple, fast, and low-cost route to obtain solid solutions composed by copper ions (Cu2+) doped calcium tungstates (Ca1-xCuxWO4). As also, their properties of interest for electronic and biomedical applications. Therefore, pure calcium tungstates were initially synthesized, using the sonochemistry method in aqueous medium at room temperature for 1, 10, 20 and 30 min. Then, the time of 30 min was adopted for the synthesis of solid solutions (Ca1-xCuxWO4), 0.005 ≤ x ≤ 0.05 mol Cu2+. These materials were structurally characterized by X-ray diffraction (XRD) and Rietveld refinement, Raman vibrational and near-infrared spectroscopy (NIR-FTIR), UV-Vis by diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM) and photoluminescence spectroscopy (PL). The tetragonal structure (scheelite) characteristic of calcium tungstate was confirmed for all pure materials and solid solutions by XRD and Rietveld refinement. The addition of Cu2+ ions has been changed the intensity and width of the main diffraction peaks. However, XRD peaks associated with secondary phase or copper tungstate were not identified. The vibrational active modes by Raman and near infrared spectroscopy (NIR-FTIR), corroborated to the results presented in the structural analysis (XRD and Rietveld method), considering that all modes identified were characteristic of calcium tungstate with space group and point group of I41/a and C_4h^6, respectively. The FE-SEM images for calcium tungstate and solid solutions revealed the obtaining of materials on a nanometric scale, with occurrence of microspheres composed of several nanoparticles. The optical band gap (Egap) for pure calcium tungstate for UV-Vis by diffuse reflectance spectroscopy was 4.0 eV, however, the increase of Cu2+ ions in the tetragonal structure led to the decrease of optical band gap, where the Egap = 3.46 eV was reached for the solid solution with x = 0.05 mol Cu2+. Through the PL spectroscopy, the characteristic emission of CaWO4 was verified in the wavelengths associated to blue, green, and red region. However, the formation of solid solutions implied in the shift of the PL emission, enhancing the emission in the blue and green region, with the addition of Cu2+ ions. This fact is due to the d-d electronic transitions between O2p → Cu3d → W5d orbitals, therefore, suppressing the electronic transitions with photons in the red region. From the chromaticity diagram, is confirmed that there was a significant change in the correlated color temperature (CCT), from 5765 K (x = 0) to 11332 K, when x = 0.05 mol of Cu2 +. Therefore, based on these results, it can be confirmed that these synthesized materials, being unpublished, can be considered promising for technological applications, including biomedical, as well as, development of light emitting diodes in blue or green region, that is, cool colors.