Structural Refinement, Morphological Features, and Optical, Photo- and Sonophotocatalytic Properties of (Ca1-xSrx)WO4 Synthesized by the Sonochemical Method

Abstract

Aims: We investigate the structural and morphological features as well as the optical, photo- and sonophotocatalytic properties of strontium-modified CaWO4, labeled as (Ca1-xSrx)WO4 with (x = 0.25; 0.50; 0.75 and 1 mol%), synthesized by the sonochemical method (SC). The as-synthesized samples were structurally characterized by X-ray diffraction (XRD), Rietveld refinement, micro-Raman, and Fourier-transform infrared (FT–IR) spectroscopies. Field-emission scanning electron microscopy (FE–SEM) and energy-dispersive X-ray spectroscopy (EDX) were used to analyze the morphological aspects (particle shape and size, average particle size distribution) and elemental chemical composition, respectively. Background: Currently, problems related to environmental issues have been one of the main concerns in the world, especially the contamination of freshwater supplies. Faced with this challenge, there is an enormous interest of environmental scientists in the progress of wastewater treatment technologies with superior performance to remove harmful substances in contaminated water resources. Particularly, synthetic organic dyes and pigments, widely employed as coloring agents in leather or textile industries, have a severe impact on the environment because of their toxic and carcinogenic nature. When are discharged into water sources, such chemical pollutants have an irreversible and devastating impact on flora and fauna of local ecosystems. As an alternative route to minimize these negative impacts, the heterogeneous photocatalysis has emerged as a low-cost, safe and versatile wastewater treatment approach. Objective: To analyze the effect of different strontium (Sr) contents (x = 0, 0.25, 0.50, 0.75, and 1 mol%) on the structural, morphological , optical, photo- and sonophotocatalytic (SPC) properties of (Ca1-xSrx)WO4 samples. Methods: (Ca1-xSrx)WO4 (x = 0, 0.25, 0.50, 0.75, and 1 mol%) samples were synthesized by the sonochemical method at 40 ºC for 3 h. Results: XRD patterns, Rietveld refinement data, and micro-Raman and FT–IR spectra proved that all samples are structurally organized in a scheelite-type tetragonal structure without deleterious phases. FE–SEM images showed the growth of flower-like and spindle-like crystals. UV–Vis spectra indicated that the increase of Egap (from 4.68 to 4.94 eV) in (Ca1-xSrx)WO4 samples was proportional to the amount of Sr (x concentration) inserted in the lattice. Conclusion: Among all samples, the CaWO4 exhibited the highest SPC performance for the degradation of RhB solution under UV irradiation. When irradiated for 200 min, a degradation rate at around 96% was achieved for this sample. The ultrasound-assisted photocatalysis proved to be a promising approach for the degradation of organic dyes.

Synthesis and characterization of solid solution of Sr(Mo1-xWx)O4 crystals obtained by the sonochemical method and their sonophotocatalytic properties

“…The bands around 133 cm − 1 (E g ) and 75 cm − 1 (B g ) were assigned to symmetric stretching, and symmetric bending vibrations of O-Sr-O bonds, respectively. The band around 100 cm − 1 (E g ) was attributed to free motion (x-, y-, z-axis) of deltahdral [SrO 8 ] clusters [1,3,40,47,[59][60][61]. Therefore, for all solid solution of Sr(Mo 1 − x W x )O 4 (x = 0, 0.25, 0.50.…”

“…5 here> Three infrared-active vibrational modes identi ed were considered internal modes because they originate from movements related to vibrational within the tetrahedral [MoO 4 ] and/or [WO 4 ] clusters. The wide and intense band located between 950 and 750 cm − 1 (1Eu and 1Au) and a narrow located between 400 e 415 cm − 1 [1(Au + Eu)] were attributed, respectively, to the anti-symmetric stretching and symmetric bending vibration of the O-Mo-O and/or O-W-O bonds [12,40,62,63]. The proportional replacement of Mo 6+ by W 6+ cations promoted a shift in the band close to 400 cm − 1 for higher wavenumbers.…”

“…The Figs. 7(a-e) show the UV-Vis absorption spectra of the Sr(Mo 1 − x W x )O 4 (x = 0; 0.25; 0.50; 0.75; and 1) crystals constructed applying the methodology proposed by Kubelka and Munk [70], as described by De Sousa et al [40]. In sequence, the E gap values were estimated by extrapolation the linear portion of the spectra to zero absorption coe cient.…”

“…Previously, the authors focused their attention on the investigation of the structural, morphological, optical, and sonophotocatalytic (SPC) properties of calcium and strontium tungstate (Ca 1 − x Sr x )WO 4 with (x = 0; 0.25; 0.50; 0.75; and 1) synthesized by the sonochemical method, evaluating the in uence of the gradual replacement of Ca 2+ by Sr 2+ cations on such properties [40]. As a way of continuing this study, this work attention was directed to the solid solution of strontium molybdate-tungstate [Sr(Mo 1 − x W x )O 4 ] crystals with (x = 0; 0.25; 0.50; 0.75; and 1), also synthesized by the sonochemical method.…”