Sonophotocatalytic degradation of Rhodamine B dye on MgWO4 crystals modified with AgNPs

Abstract

In recent years, environmental problems caused by organic pollutants, especially industrial organic dyes, have caused severe environmental issues because they are potentially toxic and non-biodegradable carcinogens. Therefore, in this work, the magnesium tungstate (MgWO4) crystals were synthesized by the polymeric precursor method, followed by heat treatment at 900 °C for 2 h. Then, the obtained crystals were modified with silver nanoparticles (AgNPs) by the photoreduction method using silver nitrate solutions at ratios (m/m%) of 0.5%, 2%, and 4%. X-ray diffraction patterns and Rietveld refinement confirmed the presence of MgWO4 crystals with a wolframite-type monoclinic structure, emphasizing that the modification did not promote structural changes, only superficial ones. Raman and Fourier-transform infrared spectroscopies showed twenty-four active vibrational modes overall, seventeen active Raman modes, and seven IR-active modes. X-ray photoelectron spectroscopy (XPS) identified the peaks related to Mg, W, and O elements in all samples, and ascribed to Ag in the modified samples. In addition, field emission scanning electron microscopy (FE-SEM) images showed that both materials present a set of small aggregated particles with irregular shapes. Moreover, the UV–Vis absorption spectra showed a shift of the maximum band to a higher wavelength value from the modification with 2% and 4% of AgNPs with direct forbidden band energy between 3.87 and 4.00 eV. The crystals modified with 2% AgNPs present improved the catalytic properties that pristine MgWO4 crystals since they showed the best sonophotocatalytic (SPC) activity with the 92% degradation rate of Rhodamine B (RhB) dye in 240 min. Finally, we demonstrate for the first time that SPC activity in these crystals enables 74% RhB degradation even after four reuse cycles, while the radical scavengers assays indicated that holes (h) and hydroxyl (OH) radicals are the main ones responsible for the chemical reactions to RhB dye degradation.