3.1. XRD patterns of the solid solution of Sr(Mo1 − xWx)O4 crystals
Figures 1(a–e) show the XRD patterns of the solid solution of Sr(Mo1 − xWx)O4 crystals (x = 0; 0.25; 0.50; 0.75; and 1) samples in the 2θ range from 10 to 70°.
<Insert Figs. 1(a–e) here>
For the pure SrMoO4 crystals, all the diffraction peaks were in good agreement with those registered and reported in the Inorganic Crystal Structure Database (ICSD) card No. 250491 [47]. Already for our SrWO4 crystals, the diffraction peaks were matched with those registered according to the ICSD card No.291536 [48]. These pure crystals presented the scheelite-type tetragonal structure, space group I41/a, symmetry point group \({C}_{4h}^{6}\), four molecular formula units per unit cell (Z = 4), and No. 88 at International Tables of Crystallography. The solid solutions of Sr(Mo0.25W0.75)O4, Sr(Mo0.50W0.50)O4, and Sr(Mo0.75W0.25)O4 crystals present their crystallographic information files (CIF) generated after Rietveld refinement using both ICSD cards of the samples pure proportionally to the desired stoichiometric information for each one. It is possible to observe that with the increase in the amount of Mo6+ cations, the diffraction peaks located around 18º and 54º almost disappeared, confirming the replacement. However, the other peaks exhibit practically identical locations and profiles due to the similarity between the ionic radii of Mo6+ and W6+ cations (0.41 Å and 0.42 Å, respectively, when the coordinate number for both is 4) [49]. The absence of additional peaks suggests the chemical homogeneity of solid solutions. The sharp diffraction peaks indicate a good crystallinity of the obtained products [50].
3.2 Rietveld refinements of solid solution Sr(Mo1 − xWx)O4 crystals
Figures 2(a–e) display the Rietveld refinement plots of the solid solution of Sr(Mo1 − xWx)O4 crystals with (x = 0; 0.25; 0.50; 0.75; and 1) obtained by the sonochemical method.
<Insert Figs. 2(a–e) here>
The structural study and investigation for all solid solution of Sr(Mo1 − xWx)O4 crystals synthesized by the sonochemical method was performed by means of XRD with Rietveld refinement. The measured diffraction patterns were adjusted to the ICSD No. 250491 [41] and ICSD No. 291536 [42]. As displayed in Figs. 2(a–e), the Rietveld refinement plots of these crystals showing good agreement between the experimentally observed XRD patterns and theoretically fitted results and presented in Table 1, which indicate the success of this method.
<Insert Table 1 here>
The good quality of the structural refinements is also indicated by low deviations of the statistical parameters (Rwp, Rp, RBragg, and χ2). These parameters, listed in Table 1, indicate that measured diffraction patterns are well adjusted and corroborated to the ICSD No. 250491 [47], and ICSD No. 291536 [48]. Also, Table 1 contains the lattice parameters, unit cell volumes, and site occupancy factor by W and Mo atoms for each solution solid in the crystal lattice calculated by the Rietveld refinement. The Rietveld refinement showed that solution solids Sr(Mo1 − xWx)O4 crystals with (x = 0; 0.25; 0.50; 0.75; and 1) present a scheelite-type tetragonal structure with the Hermann–Mauguin symmetry space group I41/a and four molecular formula units per unit cell (Z = 4), without the presence of secondary phases. Thus, it was proved that there was the substitution of Mo6+ for W6+ cations in the SrMoO4 crystal lattice, forming solid solutions of Sr(Mo1 − xWx)O4 crystals. It was also observed that the substitution of Mo6+ by W6+ ions on the crystal lattice, causes changes in the lattice parameters and consequently in the unit cell volume.
In the Table 2 is presented the data which contains the atomic coordinates of atoms of the solid solution of Sr(Mo1-xWx)O4 crystals calculated by the Rietveld refinement. It was observed variations in the atomic positions related to O atoms, while Sr, Mo and W atoms have fixed atomic positions. This suggests the existence of structural distortions on the tetrahedral [MoO4]/[WO4] clusters and deltahedral [SrO8] clusters.
<Insert Table 2 here>
3.3. Unit cell representation of the solid solution of Sr(Mo1 − xWx)O4 crystals
Figures 3(a–e) show the schematic representations of the unit cells for solid solutions of Sr(Mo1 − xWx)O4 crystals with (x = 0; 0.25; 0.50; 0.75; and 1), in which exhibit the scheelite-type tetragonal structure, as previously identified by XRD patterns and confirmed by the Rietveld refinement. All structures and clusters were modeled by means of VESTA program (version 3.5.7) using the lattice parameters and atomic positions listed in Table 1 and Table 2.
<Insert Figs. 3(a–e) here>
As can be observed in Figs. 3(a–e), in all unit cells the Mo and W atoms are lattice formers at tetragonal crystal lattice, and are bonded to four O atoms, forming distorted tetrahedral [MoO4] and/or [WO4] clusters (4 vertices, 4 faces, and 6 edges) with Td symmetry groups e different internal (O–Mo–O) and/or (O–W–O) bond angles [51]. However, the solid solutions of Sr(Mo0.75Sr0.25)O4, Sr(Mo0.50W0.50)O4 and Sr(Mo0.25W0.75)O4 crystals exhibit tetrahedral [MoO4]/[WO4] clusters more distorted that in pure crystals. Moreover, in all unit cells the Sr atoms are lattice modifiers and are coordinated to eight O atoms, resulting in distorted deltahedral [SrO8] clusters (8 vertices, 12 faces, and 18 edges) and D2d symmetry group [52]. As previously discussed, these distortions at the long-range are caused by variations in the (O–Mo–O) and (O–W–O) and (O–Sr–O) bonding angles due to disturbances in the atomic positions at O atoms after the substitution of Mo6+ by W6+ ions (of greater ionic radius and electronic density), providing different levels of structural order-disorder in crystal lattices [52, 53].
3.4. Micro-Raman and FT-IR spectra of solid solution of Sr(Mo1 − xWx)O4 crystals
The Raman spectroscopy technique is of fundamental importance for providing information about the structural order-disorder variations at short range in the crystal lattice of the materials, complementing the data obtained by XRD and Rietveld refinement [54].
According to group theory, molybdate and tungstate crystals with the scheelite-type tetragonal structure present 26 different vibration modes, as described in the equation below:
$$\text{Г}\text{=3}{\text{A}}_{\text{g}}\text{+5}{\text{A}}_{\text{u}}\text{+5}{\text{B}}_{\text{g}}\text{+3}{\text{B}}_{\text{u}}\text{+5}{\text{E}}_{\text{g}}\text{+5}{\text{E}}_{\text{u}} \text{(2)}$$
The 13 modes Ag, Bg and Eg are only active in the Raman spectra, while the modes Au and Eu are displayed only in the Infrared spectra. The three Bu modes are prohibited, and one Au and one Eu modes are acoustic (of zero frequency). A and B modes are nondegenerate while E modes are doubly degenerate. The “g” and “u” subscripts indicate the parity under inversion in centrosymmetric crystals [55, 56].
It was noted by means of Raman spectra, the presence of a weak coupling between the ionic groups, which is related to distorted tetrahedral [MoO4] and/or [WO4] clusters and the metallic cations, which is related to deltahedral distorted [SrO8] clusters in the scheelite-type structures.
The vibrational modes active in the characteristic Raman spectra of these crystals can be classified as one of seven internal modes, which correspond to the oscillations inside the tetrahedral [MoO4] and/or [WO4] clusters with an immovable mass center, or as one of the six external modes, which correspond to the motion of the metallic cations in relation to rigid molecular units [57, 58].
Figure 4 shows the Raman spectra of the solid solution of Sr(Mo1 − xWx)O4 (x = 0; 0.25; 0.50; 0.75; and 1) samples with the corresponding band assignments.
<Insert Fig. 4 here>
Twelve active Raman modes were detected for the Sr(Mo1 − xWx)O4 (x = 0, 0.25, 0.50. 0.75 and 1) samples obtained by the sonochemical method in this work. The intense bands located at 921 cm− 1 (Ag) in the Raman spectrum of SrWO4 and at 887 cm− 1 (Ag) in the Raman spectrum of SrMoO4 were attributed to the symmetric stretching vibrations of the O–W–O and/or O–Mo–O bonds within the tetrahedral [WO4] and/or [MoO4] clusters, as displayed in Fig. 4 and Inset Fig. 4. It is interesting to note that both are present in the Raman spectra of the Sr(Mo0.25W0.75)O4, Sr(Mo0.50W0.50)O4 and Sr(Mo0.75W0.25)O4 samples, whose intensities are directly proportional to the Mo/W ratio. The Raman modes at 837 (Bg) and 798 cm− 1 (Eg) were attributed to the anti-symmetric stretch vibrations of the O–W–O and/or O–Mo–O bonds. The bands located at 382 (Eg) and 372 cm− 1 (Bg) were associated with anti-symmetric bending vibrations of the O–W–O and/or O–Mo–O bonds, while the band at 337 cm− 1 (Ag + Bg) was associated with symmetric bending vibrations. The modes at 237 cm− 1 (Eg) and 190 cm− 1 (Bg) were assigned to the free rotation motions of the distorted tetrahedral [WO4] and/or [MoO4] clusters. The external modes generated with the interactions between the Sr2+ cations and the distorted tetrahedral [WO4] and/or [MoO4] clusters are shown between 150 and 65 cm− 1, as shown in Fig. 4. The bands around 133 cm− 1 (Eg) and 75 cm− 1 (Bg) were assigned to symmetric stretching, and symmetric bending vibrations of O–Sr–O bonds, respectively. The band around 100 cm− 1 (Eg) was attributed to free motion (x-, y-, z-axis) of deltahdral [SrO8] clusters [1, 3, 40, 47, 59–61]. Therefore, for all solid solution of Sr(Mo1 − xWx)O4 (x = 0, 0.25, 0.50. 0.75 and 1), the Raman spectra showed characteristic vibrational active modes related to scheelite-type tetragonal structure in good agreement with the literature [53] and our XRD patterns and Rietveld refinement.
The FT-IR spectra for the Sr(Mo1 − xWx)O4 crystals with (x = 0; 0.25; 0.50; 0.75; and 1), at the wavenumber range of 400–1000 cm− 1, are shown in Fig. 5(a) and inset Fig. 5(b).
<Insert Fig. 5(a) and inset Fig. 5 here>
Three infrared-active vibrational modes identified were considered internal modes because they originate from movements related to vibrational within the tetrahedral [MoO4] and/or [WO4] 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 Mo6+ by W6+ cations promoted a shift in the band close to 400 cm− 1 for higher wavenumbers. This is due to the difference in electronegativity between the atoms of Mo (2.16) and W (2.36) [64]. Since the W atom is more electronegative, the O–W–O bond tends to be stronger than the O–Mo–O bond, so they vibrate at higher frequencies (in regions with higher wavenumbers) [65, 66].
3.5. FE-SEM images analysis of the solid solution of Sr(Mo1 − xWx)O4 crystals
The FE-SEM images of the solid solution of Sr(Mo1 − xWx)O4 crystals with (x = 0; 0.25; 0.50; 0.75; and 1) are shown in Fig. 6(a-j).
<Insert Figs. 6(a–j) here>
As shown in Figs. 6(a,b), the SrMoO4 samples (x = 0) formed mostly octahedral-type morphologies (3 µm long and 2 µm wide, approximately) with an apparently smooth surface. Similar results were achieved by Jiang et al. [67] also using the sonochemical method and by Rendón-Angeles et al. [66] through the hydrothermal method. As the x content increased, the octahedron assumed a spindle-like morphology (3 µm long and 1 µm diameter, approximately) with a rougher surface, Fig. 6(c–h), until some evolved into dumbbell-like morphologies (3.5 µm long and 1 µm diameter, approximately), predominant in SrWO4 (x = 1) samples, Fig. 6(i,j). Mukherjee, et al. [68], by the sonochemical method, and Karthik et al. [12], by the coprecipitation method, obtained similar morphologies for SrWO4 crystals. A few star-like and flower-like morphologies also emerged (with a mean size of approximately 3 µm) possibly due to agglomeration of spindle-like morphologies [69]. However, according to Dos Santos et al. [55], star-like morphologies can also arise from a single spindle after dividing its ends, generating new tips and so on, until the morphologies evolve into flower-like crystals.
3.6. UV-Vis spectra of the solid solution of Sr(Mo1 − xWx)O4 crystals
The Figs. 7(a-e) show the UV–Vis absorption spectra of the Sr(Mo1 − xWx)O4 (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 Egap values were estimated by extrapolation the linear portion of the spectra to zero absorption coefficient. All spectra showed characteristic profiles of semiconductor crystalline materials with direct transitions between the valence band (VB) and conduction band (CB) [71].
<Insert Figs. 7(a–e) here>
The estimated Egap values of the Sr(Mo1 − xWx)O4 (x = 0; 0.25; 0.50; 0.75; and 1) crystals were 4.27; 4.30; 4.39; 4.53; and 5.01 eV, respectively. The gradual replacement of the Mo6+ by the W6+ cations caused a directly proportional increase in the Egap values. An expected behavior considering the Egap values reported in the literature for SrMoO4 and SrWO4 crystals. Muralidharan and Sivaji [72] synthesized SrMoO4 and SrWO4 by precipitation method with Egap values of 4.25 and 4.85 eV, respectively. Wang et al. [73] synthesized SrMoO4 by hydrothermal method with Egap of 4.32 eV. Khobragade et al. [74] synthesized SrWO4 by solid state reaction method with Egap value of 5.80 eV.
Its known that Egap values are associated with the existence of intermediate energy levels within the gap of semiconductor materials that are directly affected by the degree of structural organization of the crystal lattice (oxygen vacancies, binding distortion and angle deformation) which in turn are affected by synthesis method, particle size, morphology, treatment temperature and pH of the precursor solution [72]. Therefore, the increase in the Egap value proportional to the replacement of the Mo6+ by the W6+ cations suggest an increase in the degree of structural organization, although the difference between the ionic radii is small [75].
Dos Santos et al. [55] and Zhang et al. [50] synthesized SrMoO4 microcrystal by the sonochemical method with Egap of 4.05 eV and 3.72 eV, respectively, but, to date, no Egap values associated with SrWO4 crystals synthesized by the sonochemical method on the micro scale have been reported. Theoretical studies estimate Egap values for SrMoO4 crystals between 3.69 and 5.35 eV [5, 76–78] and for SrWO4 between 4.41 and 5.37 eV [79–82]. Therefore, the data discussed so far suggest that the Egap values estimated in this work are within the range of expected values.
3.7. Sonophotocatalytic activities of the solid solution of Sr(Mo1 − xWx)O4 crystals
The RhB dye was subjected to different degradation processes and the results obtained are shown in Figs. 8(a–d). The degradation rates (Ct/C0(%) vs. time) only under UV-C (photolysis – P) or ultrasonic (sonolysis – S) radiation, as well as under the combined radiations (sonophotolysis – SP), is shown in Fig. 8(a). Figures 8(b,c) show the degradation rates after photocatalysis (PC) and sonophotocatalysis (SPC) process, respectively, using the Sr(Mo1 − xWx)O4 crystals as catalysts. The percentage of degradation of the processes assisted by sonication without and with catalysts were summarized in Fig. 8(d).
<Insert Figs. 8(a–d) here>
As can be seen from Fig. 8(a), UV-C radiation alone was not enough to degrade the RhB dye completely (under the conditions employed in this work), since the P process showed an efficiency of only 8%. Even after addition of the Sr(Mo1 − xWx)O4 crystals as photocatalysts (PC, Fig. 8(b)) suggesting that the recombination rate between the e‒↔h+ pairs photogenerated for these crystals is high [83].
On the other hand, the processes assisted by ultrasonic radiation proved to be promising, starting with S and SP with degradation rates of 31% and 42%, respectively. This is due to the appearance of highly reactive species (such as H•, HO• e HO2•) formed from the cleavage of H2O and dissolved O2 molecules during acoustic cavitation events. However, it is known that the thermolytic degradation of the pollutant is also an option, given the high temperatures generated after the collapse of the cavitation bubbles [84].
The SP confirmed the benefits of the simultaneous use of UV-C and ultrasonic radiation and, after addition of Sr(Mo1 − xWx)O4 crystals as catalysts, a synergistic effect was observed during SPC process of the RhB dye illustrated in Figs. 8(c). This effect is due to the ability of the combined systems (ultrasound, light and catalyst) to compensate for the deficiencies presented by each degradation process [85].
One of the limitations of the photocatalysis is the high recombination rate between e‒↔h+ pairs photogenerated on the catalyst surface, but reactive species formed during sonolysis can delay this recombination, contributing to the generation of more reactive species. Furthermore, the shock waves created after implosion of cavitation bubbles i) lead to fragmentation and deagglomeration of catalyst particles, increasing their surface area; ii) contribute to the continued cleaning of their active sites, preventing the accumulation of pollutants and their intermediates produced during degradation; and iii) accelerate the mass transfer between the organic pollutants and the catalyst surface. Meanwhile, the catalysts can act as additional cores to form more cavitation bubbles potentiating all the effects previously mentioned [31, 86].
The better SPC performance was observed for the Sr(Mo0.25W0.75)O4 crystals (98%) followed by SrMoO4 (90%), SrMo(0.75W0.25)O4 (71%), SrWO4 (67%), and Sr(Mo0.50W0.50)O4 crystals (59%). According to Fig. 8(d), starting from pure SrMoO4 crystals, such a sequence seems to be inversely proportional to the increase of the x value and raise of Egap value, with the exception of the Sr(Mo0.25W0.75)O4 crystal. In view of this and based in our structural data, clusters modeling, FE-SEM imagens and UV-Vis spectra we can attribute that the solid solution Sr(Mo0.25W0.75)O4 crystals exhibits favorable condition to the best performance catalytic, as specific defects on the crystal surfaces and optical band gap values near the energy (4.88 eV) of UV-C illumination. So, its structural organization seems to be ideally favorable (among the conditions investigated in this work) to achieve the highest SPC performance. Therefore, it was the catalyst chosen to carry out the concentration tests, whose results are shown in Fig. 9, in order to find the best proportion between the amount of catalyst (25, 50, 75 and 100 mg) and volume of solution (100 mL) in the SPC of RhB dye.
<Insert Figs. 9 here>
The SPC degradation rate increased proportionally to the increase in the amount of catalyst up to the ratio of 50 mg/100 mL, and thereafter it decreased in the same proportion. This may be related to the fact that when the catalyst dosage increases, the number of available active sites also increases, as well as additional nuclei for the formation of cavitation bubbles, increasing the production of oxidizing radicals and, therefore, the rate of degradation. However, this synchronism seems to be efficient to a certain extent: as the catalyst dosage increases even more, the particle deagglomeration process seems to lose its efficiency, decreasing the catalyst surface area and, consequently, the number of active sites. Furthermore, catalyst concentration beyond an ideal value can cause light scattering due to increased solution turbidity, making it difficult for some catalyst particles to absorb light, thus reducing degradation efficiency [85, 87, 88].
After discovering the ideal ratio (under the conditions of this work) and to confirm the synergistic effect during SPC process, Sr(Mo0.25W0.75)O4 crystals were also used in the sonocatalysis (SC: only ultrasound and catalyst) of the RhB dye and the result shown in Fig. 10. The SC process presented a considerable degradation rate of 76% (2.45 times greater than S process). However, as expected, the degradation efficiency in the SPC process was superior to all other processes.
<Insert Figs. 10 here>
In general, photogenerated e‒↔h+ pairs and superoxide (O2•‒) and hydroxyl (HO•) radicals are the predominant reactive species in the degradation process. However, to determine the main species during SC and SPC processes with Sr(Mo0.25W0.75)O4 crystals (in the proportion of 50 mg/100 mL), experiments were carried out with AO, AgNO3, BQ, and ISO scavengers of h+, e‒, O2•‒, and HO• radicals, respectively. These results were shown in Fig. 11.
<Insert Figs. 11 here>
The inhibition of RhB dye degradation in both SC and SPC process was not as pronounced after addition of AO and AgNO3 showing that the photogenerated e‒↔h+ pairs are not the main reactive species in both processes, although they contribute to a certain degree, even in the absence of UV-C light, which can be explained by the phenomenon of sonoluminescence. When the light energy emitted during sonoluminescence equals or exceeds the Egap of the catalyst, e‒ can be excited from VB to CB and generate e‒↔h+ pairs [32, 89, 90].
In SC process, the lowest rate of inhibition was observed after addition of BQ, indicating that the O2•‒ radical is the least participative reactive species, possibly because it is present in a smaller amount. The opposite was observed during SPC, which was shown to be the main species, followed by the HO• radicals. Most of the O2•‒ radicals are formed by e‒ in CB, generated in greater numbers in SPC due to the presence of UV-C light [91]. As expected, in SC the HO• radical was shown to be the main degradation agent.
Figures 12(a–d) show the UV‒Vis spectra of the RhB dye after the S, SP SC, and SPC processes using the solid solution of Sr(Mo0.25W0.75)O4 crystals as a catalyst.
<Insert Figs. 12(a–d) here>
Displacements in the RhB dye maximum absorption bands at 554 nm (characteristic of the chromophore group) were not observed in any of the processes, suggesting that there was no formation of secondary products [92]. Its considerable reduction, as well as the more discrete band located at 259 nm (attributed to aromatic rings), during SPC process is one of the indications of complete mineralization of RhB dye [93].