Síntese, estudo das propriedades físicas e decomposição térmica dos molibdatos K6Mo7O24.4H2O e KCu2OH(MoO4)2(H2O) com potenciais aplicações em filtros UV e supercapacitores

Abstract

Molybdates have attracted increasing interest due to their structural versatility and wide range of physicochemical properties, which confer them a prominent role in materials science and engineering. In this work, the molybdates K6Mo7O24·4H2O (KMO) and KCu2OH(MoO4)2(H2O) (KCMO) were investigated. Their structural, vibrational, morphological, physical, and thermal decomposition properties were analyzed and discussed. For the first compound, KMO crystals were synthesized through slow solvent evaporation. At 303 K, powder X-ray diffraction (PXRD) revealed that the material crystallizes in a monoclinic symmetry with space group P21/c. The optical transmittance spectroscopy results showed that the crystal exhibits low transmittance, especially in the ultraviolet region, suggesting its potential application in UV filters. Thermal analyses revealed two endothermic events and one exothermic event. These events were further investigated through in situ PXRD under atmospheric air and vacuum conditions, where phase transformation/transition associated with dehydration and recrystallization were observed. The phases were identified via Rietveld refinement. Additionally, a temperature-dependent Raman spectroscopy study was conducted and corroborated the thermal and structural findings. The results showed that KMO undergoes amorphization due to dehydration, followed by recrystallization into a composite of the anhydrous phases K2Mo3O10 and K2Mo2O7. Scanning electron microscopy (SEM) analysis revealed significant changes in morphology when the material was exposed to atmospheric air or vacuum. Vacuum conditions promoted dehydration and crystallization at lower temperatures, facilitating the formation of K2Mo2O7 and altering the growth patterns of rod-like structures. Regarding the second material, the new compound KCMO was synthesized via a sonochemical method. Its crystalline structure was identified by PXRD and Rietveld refinement as a layered transition metal molybdate polymorph (ϕy), crystallizing in a trigonal system with space group R-3. Morphology and elemental composition were examined using SEM and energy-dispersive X-ray spectroscopy (EDS). Vibrational properties were investigated using FT-IR and Raman spectroscopies. In addition, electrochemical properties were studied by cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy. The material exhibited a high specific capacitance of 967 F/g at a current density of 1 A/g, and cycling studies showed good stability after the first 1000 cycles. These results suggest that the new compound is a promising candidate for use in electrochemical energy storage devices. Furthermore, an extensive thermal decomposition study of KCMO was carried out. Thermal analyses indicated that dehydration occurs up to 655 K. In situ PXRD measurements under air revealed the formation of secondary phases Cu3Mo2O9 (triclinic, 𝑃1̅) and K4(MoO4)2 (monoclinic, C2/c) between 560 and 600 K. From 720 K, the phases K2Cu3(MoO4)4 (monoclinic, P21/a) and CuO (monoclinic, C2/c) appear. A heterostructure composed of K2Cu3(MoO4)4 and CuO was also obtained ex situ by heating KCMO to 780 K in atmospheric air. Under vacuum, dehydration and decomposition occurred at lower temperatures. Additionally, vacuum conditions favored the thermal reduction of Cu2+ to Cu1+ , resulting in the formation of Cu2O with a cubic structure.