Análise estrutural, morfologia, propriedades ópticas, fotocatalíticas e sonocatalíticas dos cristais de α-Al2Mo3O12 sintetizados pelo método sonoquímico

Abstract

In this work, we report on the dialuminium alpha-trismolybdate (α-Al2Mo3O12) crystals synthesized by the sonochemical method with post-heat treatment from 100 to 600 ºC for 2 h. The structure, morphology, optical, photocatalytic and sonocatalytic properties of α-Al2Mo3O12 crystals are investigated in detail. X-ray diffraction, Rietveld refinement, micro-Raman and Fourier transform infrared spectroscopy confirm a monoclinic structure with eighteen active Raman vibrational modes (Ag and Bg) and five active infrared vibrational modes (Au and Bu) for the α-Al2Mo3O12 crystals calcined at 600 °C for 2 h. Rietveld refinement data were used to model the clusters and the electron density map. The electron density map showed the presence of distorted octahedral [AlO6] and distorted tetrahedral [MoO4] clusters in the monoclinic lattice, and inhomogeneous distributions of electronic charges on the crystal surface. Field emission scanning electron microscopy images revealed that these α-Al2Mo3O12 crystals are formed by the aggregation of several nanocrystals leading to the growth of microcrystals with irregular size, non-uniform shapes and an agglomerated nature. The ultraviolet-visible diffuse reflectance spectroscopy shows optical band energy (Egap) values between 3.64 and 3.18 eV with non-linear behavior for the Egap values. Photoluminescence (PL) measurements at room temperature indicate that α-Al2Mo3O12 crystals heat treated at 600 ºC present a more intense PL emission, which is related to the charge transfer process between the distorted-ordered octahedral [AlO6]d[AlO6]o and clusters distorted-ordered tetrahedral [MoO4]d[MoO4]o clusters. Moreover, we investigated the photocatalytic (PC) and sonocatalytic (SC) properties of these crystals thermally at 600 ºC. In comparative terms, the best electronic property was related to the SC process, with a superior activity of ≈65% of Rhodamine B (RhB) dye degradation, and only ≈24% for the PC process. Finally, it was demonstrated for the first time in the literature that SC activity can be increased by approximately 96% within 240 min for RhB dye degradation with α-Al2Mo3O12 crystals as a catalyst and ammonium oxalate as a hole scavenger.