In this work, we report the room-temperature synthesis of pure calcium tungstate (CaWO 4 ) and copper-doped calcium tungstate solid solution (Ca 0.99 Cu 0.01 WO 4 ) by using a sonochemistry method. These materials were structurally characterized by X-ray diffraction (XRD) and Raman spectroscopy. The obtained XRD patterns were submitted to a Rietveld refinement showing, in both materials, a tetragonal phase with space group and point group of I4 1 /a and C 4h 6 , respectively. Microscopy images of both materials, obtained by field emission scanning electron microscopy, showed spherical agglomerated structures composed by spherical nanoparticles, while calcium and tungstate elements were identified by energy-dispersive X-ray spectroscopy for pure calcium tungstate and copper, calcium, and tungstate for Ca 0.99 Cu 0.01 WO 4 solid solution. The decrease of optical band gap (E gap ) from 4.0 eV (CaWO 4 ) to 3.45 eV (Ca 0.99 Cu 0.01 WO 4 ) confirmed the substitution of calcium atoms for copper atoms in the clusters [CaO 8 ]. Maximum photoluminescence (PL) emission was shifted from 522 nm in the pure CaWO 4 to 475 nm in the Ca 0.99 Cu 0.01 WO 4 solid solution. Consequently, there was an increase of PL emissions intensity in the blue and green regions of the visible spectrum, due to electronic transitions between the orbitals O 2p, Cu 3d, and W 5d.

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This paper reports easy and fast synthesis of PbMoO4 nanocrystals by microwave-assisted hydrothermal (MH) method at different synthesis times (1, 10, 30 and 60 min) at 100 °C. X-ray diffraction, Rietveld refinement and Raman spectroscopy confirm all characteristics of diffraction peaks and active vibrational modes of the pure scheelite structure (tetragonal, I41/a) for all synthesized PbMoO4 nanocrystals. The optical bandgap calculated directly from the samples is close to 3.5 eV. The images collected by scanning electron microscopy show particles with mean length from 159.90(8) nm to 303.02(3) nm with greater exposure of planes (111), (100), (011) and (110). The photocatalytic activity of PbMoO4 nanocrystals against RhB and RBBR dyes resulted in successful degradation in short time intervals using ultraviolet light, where the best performance was achieved for the PbMoO4-10 sample, which was 29.2 and 51.8 times more effective than photolysis. The contribution of oxidant species was monitored by radical scavengers, which confirms that holes (h+) are the main oxidative species in photodegradation of RhB and RBBR dyes, while reuse of the catalyst against RhB and RBBR dyes confirms high stability of the catalyst, although recycled four times.