Estudo dos métodos de síntese, sonoquímica e precursores poliméricos, na evolução estrutural e morfológica do ZnWO4

Abstract

Zinc tungstate (ZnWO4) that belongs to the wolframites class has drawn the attention of the scientific community due to its diverse technological applications, luminescent emissions, gas sensors, scintillators, photocatalysts, anticorrosives and electrochemical supercapacitors. In this work, ZnWO4 was synthesized by sonochemical and polymeric precursors methods, and a study on the phase formation pathway in this material with these two methods was carried out. The thermal behavior of the obtained ZnWO4 was analysed by Thermogravimetric Analysis and Differential Scanning Calorimetry techniques. Structural evolution was analyzed by X-Ray Diffraction (XRD) and Rietveld Refinement. The post morphology was analyzed by Field Emission Scanning Electron Microscopy (FE-SEM). In the sonochemichal method, the sonicated precursor presented amorphous phase. ZnWO4 structural evolution was studied through thermal treatments (400 to 500 °C), and an amorphous phase was observed, followed by an unidentified phase (intermediary phase) until the attainment of ZnWO4 single crystalline phase at 500 °C for 3 h, with all the well-defined diffraction peaks, indexed to the wolframite type monocyclic unit cell, spatial group P2/c (C4 2h). In polymeric precursors synthesis, a polymeric resin was obtained, which was pre-pyrolyzed at 350 °C for 3 h. The precursor material obtained exhibited amorphous material characteristic. For the study of ZnWO4 crystalline phase formation, the precursor was thermally treated at different temperatures, between 400 and 550 °C. The process of reaction and phase transformation in function of thermal treatments lead to two possible hypotheses, both based in the coexistence of more than one amorphous phase after pre-pyrolysis process, indicated by XRD and FE-SEM analysis. At the end of the reactional process, there was only the formation of crystalline compound ZnWO4, without the presence of spurious phases, indicating that the stoichiometric ratio [Zn] / [W] = 1 was conserved. The existence of amorphous metastable phases indicated a different formation pathway, not yet described in details in the literature for inorganic phases processed by the polymeric precursors method.