Electronic structure, optical and sonophotocatalytic properties of spindle-like CaWO4 microcrystals synthesized by the sonochemical method

Article abstract of DOI:10.1016/j.jallcom.2020.157377

In this letter, the electronic structure, optical, and sonophotocatalytic properties of calcium tungstate (CaWO₄) microcrystals synthesized by the sonochemical method are reported. Structural and morphological characterization techniques revealed that CaWO₄ has a tetragonal structure and composed of several spindle-like microcrystals. The ultraviolet–visible spectroscopy showed an optical band gap energy E_gap(exp) of 4.69 eV. The theoretical calculations were performed to describe the electronic band structure, the density of states, and Infrared/Raman vibrational modes. The theoretical models were based on optimized and defect-based structures. The theoretical optical band gap energy (E_gap) confirmed the existence of direct electronic transitions (Γ?Γ points in Brillouin zone). The optimized structure exhibited an E_gap(theo) value of 5.70 eV due to the participation of energy levels arising from O and Ca atoms in the valence band as well as W and O atoms in the conduction band. A decrease from 5.70 to 4.29 eV was observed for the defect-based structure. The sonophotocatalytic properties of CaWO₄ microcrystals were investigated for the first time with respect to degradation of Rhodamine B dye and they revealed a degradation capacity of approximately 96% after 200 min. Finally, our electron density maps indicate that the presence of structural defects induces a polarization phenomenon and inhomogeneous distribution of electron charge between the [CaO₈]–[WO₄] clusters.

Full text of DOI:10.1016/j.jallcom.2020.157377

Journal of Alloys and Compounds 855 (2021) 157377
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Electronic structure, optical and sonophotocatalytic properties of
spindle-like CaWO4 microcrystals synthesized by the sonochemical
method
P.B. de Sousa a, A.F. Gouveia b, J.C. Sczancoski c, I.C. Nogueira d, E. Longo c,
, e, *
M.A. San-Miguel b, L.S. Cavalcante a
a
^
Universidade Federal do Piauí, Campus Ministro Petronio Portella, Ininga, 64049-550, Teresina-PI, Brazil
Institute of Chemistry, State University of Campinas, 13083-970, Campinas, Sao Paulo, Brazil
b
c
~
~
~
DEMA-DQ-Universidade Federal de Sao Carlos, P.O. Box 676, Sao Carlos, SP, 13565-905, Brazil
d
e
ꢀ
Universidade Federal do Amazonas, Av. Rodrigo Otavio Japiim, P.O. Box 670, 69077-000, Manaus-AM, Brazil
Universidade Estadual do Piauí, Rua Joao Cabral, N. 2231, P.O. Box 381, 64002-150, Teresina-PI, Brazil
~
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 July 2020
Received in revised form
7 September 2020
Accepted 27 September 2020
Available online 28 September 2020
In this letter, the electronic structure, optical, and sonophotocatalytic properties of calcium tungstate
(CaWO4) microcrystals synthesized by the sonochemical method are reported. Structural and morpho-
logical characterization techniques revealed that CaWO4 has a tetragonal structure and composed of
several spindle-like microcrystals. The ultravioletevisible spectroscopy showed an optical band gap
energy Egap(exp) of 4.69 eV. The theoretical calculations were performed to describe the electronic band
structure, the density of states, and Infrared/Raman vibrational modes. The theoretical models were
based on optimized and defect-based structures. The theoretical optical band gap energy (Egap
)
Keywords:
CaWO4 crystals
confirmed the existence of direct electronic transitions ( points in Brillouin zone). The optimized
G4G
structure exhibited an Egap(theo) value of 5.70 eV due to the participation of energy levels arising from O
and Ca atoms in the valence band as well as W and O atoms in the conduction band. A decrease from 5.70
to 4.29 eV was observed for the defect-based structure. The sonophotocatalytic properties of CaWO4
microcrystals were investigated for the first time with respect to degradation of Rhodamine B dye and
they revealed a degradation capacity of approximately 96% after 200 min. Finally, our electron density
maps indicate that the presence of structural defects induces a polarization phenomenon and inho-
mogeneous distribution of electron charge between the [CaO8]e[WO4] clusters.
Optical band gap
Band structure
Sonophotocatalysis properties
© 2020 Elsevier B.V. All rights reserved.
1. Introduction
technological and socio-environmental application possibilities,
due to their interesting luminescent and structural peculiarities [3],
The tungstates are inorganic oxides formed by divalent metal
cation with general formula AWO4 (where A2þ are Ca, Sr, Ba, Pb and
Cd). Among these, the calcium tungstate (CaWO4) is the most
important compound at group of the scheelites, a specific tetrag-
onal structure with space group (I41/a), possessing four molecular
formula per unit cell (Z ¼ 4) in which the W atoms are coordinated
to four equivalent O atoms, forming tetrahedral [WO4] clusters
isolated from each other, while the Ca atoms are coordinated to
eight O atoms, forming the deltraedral [CaO8] clusters [1,2]. Such
oxide materials have been extensively studied and found numerous
such as diode lasers [4], scintillators [5], fluorescent lamps [6], drug
carrier [7], temperature sensing [8] and catalysts [9].
In recent years, CaWO4 crystals with different sizes and shapes
have been prepared by a wide variaty of synthesis methods,
including radio-frequency sputtering [10], sol-gel [11], hydrother-
mal [7], solid state reaction [12], co-precipitation [13], polymeric
precursor [14], solvothermal [15], microwave assisted synthesis
[16], electrospinning [6], and pulse laser ablation [17]. However,
among the previously mentioned methods, the sonochemical (SC)
method has been shown to be more efficient due to its simplicity
and low cost, resulting in products with high purity and crystal-
linity, dispensing long periods of synthesis, high temperatures and
pressures or special equipment, conditions not suitable for large
scale productions or industrial applications [2,18]. The SC method is
based on the acoustic cavitation phenomenon, which consists in
* Corresponding author. Universidade Federal do Piauí, Campus Ministro Pet-
^
ronio Portella, Ininga, 64049-550, Teresina-PI, Brazil.
E-mail address: laeciosc@gmail.com (L.S. Cavalcante).
https://doi.org/10.1016/j.jallcom.2020.157377
0925-8388/© 2020 Elsevier B.V. All rights reserved.

P.B. de Sousa, A.F. Gouveia, J.C. Sczancoski et al.
Journal of Alloys and Compounds 855 (2021) 157377
space group (I41/a) and point-group symmetry (C46h) being in
agreement with the Inorganic Crystal Structure Database (ICSD) file
no. 60547 [27]. No deleterious or intermediate phase was detected.
Moreover, detected sharp and well-defined diffraction peaks indi-
cate a high degree of structural order and periodicity of the CaWO4
crystalline lattice at long-range [28]. Fig SI-1 illustrate the Rietveld
refinement plots for the CaWO4 microcrystals with observed pat-
terns versus the calculated patterns. Moreover, the profiles of XRD
patterns experimentally observed and theoretically calculated
display small differences between the observed y-scale values
(YObs) and calculated y-values (YCalc) on the intensity scale near
zero, as illustrated by the (YObseYCalc) line. Tables SDe1 presents the
the formation, growth and implosive collapse of vapor bubbles in a
liquid, in a cycle that lasts throughout the period of ultrasonic
irradiation [19]. Immediately after the collapse of the cavitation
bubbles, the so-called “hot spots” (microscopically located liquid
regions that have high temperatures and pressures: 5000 K and
1000 atm, approximately) are generated, capable of significantly
increasing the chemical reactions rates [20,21], a fact by which the
ultrasound has also been used as an aid in organic pollutants
degradation processes [22].
Therefore, this letter reports the synthesis of spindle-like
CaWO4 microcrystals by the SC method and the evaluation of its
sonophotocatalytic (SPC) activity in the degradation of Rhodamine
B (RhB) dye under UV-C illumination. These CaWO4 microcrystals
were characterized by X-ray diffraction (XRD), Rietveld refinement,
fit parameters (Rwp, Rp, RBragg,
c2 and S), which suggest that the
refinement results are very reliable (Supplementary data Figs. SD-1).
In Fig. 1(b), we detect eleven Raman-active vibrational modes, the
Ag mode at 911 cmꢁ1 is assigned to the ()O)W/O/) sym-
metric stretching vibrations within the tetrahedral [WO4] clusters,
while the Bg mode located at 836 cmꢁ1 and the Eg mode located at
797 cmꢁ1 are ascribed to the ()O)W)O))/(/O/W/O/)
anti-symmetric stretching vibrations. The Bg mode located at
399 cmꢁ1 and the Ag and Bg modes located both at 332 cmꢁ1 are
associated, respectively, to the anti-symmetric and symmetric
bending vibration in the tetrahedral [WO4] clusters. The Raman
shift due to the free rotation of tetrahedral [WO4] clusters is noted
at 209 cmꢁ1 and 194 cmꢁ1 referring to Eg and Bg modes, respec-
tively. The Eg mode at 115 cmꢁ1 (Eg) is assigned to free motion at (x,
y, z-axis) of the deltahedral [CaO8] clusters [28]. Finally, the Raman-
active Bg mode at 86 cmꢁ1 is related to symmetric bending vibra-
tions of (OeCaeO) bonds in deltahedral [CaO8] clusters [9,30,31].
Fig. 1(c) presents the FT-IR spectrum, which depicts only three of
eight possible IR-active modes were verified. These three IR-active
vibrational modes were identified for our CaWO4 crystals exhibited
an overlapping of two intense absorption bands [n3(1Eu and 1Au)]
in the range from 794 cmꢁ1 to 854 cmꢁ1, which were ascribed to
the ()O)W)O))/(/O/W/O/) anti-symmetric stretching
vibrations within the tetrahedral [WO4] clusters (Fig. 3(d)). The
n4[1(Au)] modes located at approximately 438 cmꢁ1 are ascribed to
symmetric bending vibrations within the tetrahedral [WO4] clus-
ters [29].
micro-Raman,
Fourier
transform-infrared
(FT-IR)
and
ultravioletevisible (UVeVis) spectroscopies. The morphology as-
pects and elemental composition analysis were investigated by
field emission-scanning electron microscopy (FE-SEM) and energy-
dispersive X-ray spectrometry (EDXS), respectively. In addition, we
report on first-principles quantum mechanical calculations based
on density functional theory (DFT) the electronic band structure
(EBS), density of states (DOS), infrared/Raman vibrational modes
and electronic density map to better explain the experimental re-
sults. For the first time, the enhanced SPC properties of spindle-like
CaWO4 microcrystals for the degradation of RhB under UV-C illu-
mination were described in detail.
2. Experimental details
2.1. Synthesis of CaWO4 crystals
CaWO4 microcrystals were synthesized by the SC method
described as follows: 1.0 ꢀ 10ꢁ3 mol of sodium tungstate (VI)
dihydrate (Na2WO4$2H2O; 99.0% purity, Sigma-Aldrich) and
1.0 ꢀ 10ꢁ3 mol of calcium nitrate tetrahydrate [Ca(NO3)2$4H2O; 99%
purity, Sigma-Aldrich] were separately placed in two Falcon tubes
(capacity of 50 mL) and dissolved in 50 mL of deionized water (DI-
H2O) at room temperature. The solution containing WO24ꢁ complex
anion was transferred to a graduated borosilicate reagent bottle of
100 mL, partially immersed in an ultrasonic bath (Bransonic
CPX1800H Digital) operating at 40 kHz and 70 W. Then the solution
containing the Ca2þ ions was slowly dripped (one drop per second,
approximately), with the aid of a graduated burette, into of the
Borosilicate bottle containing the WO24ꢁ ions. At the end of the drip,
the system was maintained at 40 ꢂC for 3 h under sonication. The
theoretical calculation of the total-energy to study the electronic
structure and optical properties of CaWO4 crystals was performed
with the CRYSTAL17 software package [23,24] and the methodol-
ogy is based on the DFT associated with the hybrid functional
B3PYP [25,26] (See Supplementary data for more details about
Synthesis, characterizations, sonophotocatalytic measurements,
theoretical calculations and average size distribution).
3.2. Optical analysis: Experimental and theoretical
The experimental optical band gap energy [Egap(exp)] of CaWO4
crystals was estimated by the modified Kubelka-Munk [32] equa-
tion (1):
À
Á
n
½FðR∞Þh
n
ꢃ ¼ C1
hn
ꢁ Egap
(1)
where F(R∞) is the Kubelka-Munk function or absolute reflectance
of the sample, R∞ is the reflectance when the sample is infinitely
thick, hn is the photon energy, C1 is a proportionality constant, Egap
is the optical band gap and n is a constant associated with different
types of electronic transitions (n ¼ 0.5 and n ¼ 2 for direct and
indirect allowed transitions; n ¼ 1.5 and n ¼ 3 for direct and indi-
rect forbidden transitions, respectively). Since barium sulfate
(BaSO4) was the standard sample in reflectance measurements: R∞
¼ Rsample/Rstandard ¼ Rsample/RBaSO4 [33,34]. For our CaWO4 micro-
crystals, the theoretical calculations shown in this work indicate an
optical diffuse reflectance spectra governed by direct electronic
transitions, in agreement with other theoretical studies [34,35] i.e.
after the photon absorption process, the electrons located in the
valence band (VB), when excited to the conduction band (CB), do so
under the same point in the Brillouin zone [36]. Thus, using n ¼ 0.5
3. Results and discussions
3.1. Structural analysis: Experimental and theoretical
XRD patterns, micro-Raman and FT-IR spectra of spindle-like
CaWO4 microcrystals synthesized by the SC method are shown in
Fig. 1(aec). Fig. 1(d and e) display the comparison between the
relative positions of theoretical and experimental Raman and IR-
active modes, respectively.
The diffraction patterns shown in Fig. 1(a) indicate that the
CaWO4 sample have a scheelite-type tetragonal structure with
in equation (1) and plotting the graph of [F(R∞)hn n, it is
]2 against h
2

P.B. de Sousa, A.F. Gouveia, J.C. Sczancoski et al.
Journal of Alloys and Compounds 855 (2021) 157377
Fig. 1. (a) XRD patterns, (b) micro-Raman spectrum, and (c) FT-IR spectrum for CaWO4 crystals synthesized by the SC method. The vertical lines (|) in red color in Fig. 1(a) indicate
the position and relative intensity of XRD patterns for the CaWO4 phase reported in ICSD file No. 60547. (e,d) Relative positions of experimental and theoretical Raman and IR-active
modes. The error bars in the experimental Raman and IR-modes indicate the difference between the theoretical Raman and IR modes obtained by DFT calculations, respectively.
possible to estimate the Egap values of the CaWO4 microcrystals by
extrapolation of linear portion of the UVeVis curve of the resulting
graph to the y-axis intercept [33].
Fig. 2(a) shows the UVeVis diffuse reflectance spectrum of
CaWO4 microcrystals and its optical band gap estimated (Egap(exp)).
Fig. 2(b and c) display the optimized and defected EBS of CaWO4
microcrystals and Fig. 2(d) shows DOS for optimized and defected
theoretical models projected over all atoms involved in the elec-
tronic structure of the CaWO4 microcrystals.
transitions with a quasi-vertical absorption profile typical of
semiconductor crystalline materials [37] with value of
a
Egap(exp) ¼ 4.69 eV. The literature [38,39] reports that CaWO4
crystals exhibit different optical band gap values ranging from 4.0
to 5.89 eV all ascribed to a direct optical band gap, suggesting that
our Egap(exp) value of the CaWO4 crystals synthesized by SC method
and reported in this study is within acceptable values. Considering
the theoretical calculations, the optimized electronic band struc-
ture of CaWO4 microcrystals is characterized by direct electronic
transitions between the valence band (VB) and conduction band
Fig. 2(a) indicates that the UVeVis spectrum of our CaWO4
microcrystals is characterized by well defined direct electronic
(CB) (G
4G
points in the Brillouin zone) and Egap(theo) ¼ 5.70 eV
3

P.B. de Sousa, A.F. Gouveia, J.C. Sczancoski et al.
Journal of Alloys and Compounds 855 (2021) 157377
Fig. 2. (a) UVeVis spectrum of CaWO4 microcrystals synthesized by the SC method, (b) optimized EBS, (c) defected EBS, and (c) Total DOS for the optimized structure and defected
structure projected on the Ca, W, and O atoms of CaWO4 microcrystals, respectively.
(Fig. 2(b)). This value is higher than the experimental value because
in the theoretical calculations the system is in the ideal arrange-
ment without local defects, like distortion in the angles and bonds
distances. The Egap values are controlled by the degree of structural
organization of the crystalline lattice: the greater the structural
organization, i.e. crystallinity, the greater the value of Egap [38]. The
reduction of the band gap value due to the structural defects that
generated energy levels between the VB and CB. In order to
investigated this effect on the electronic properties, the defect
model was proposed with a distortion in the O atom that is shared
between the two [CaO8] and [WO4] clusters. As can be seen in
Fig. 2(c), the local defect on the O atom induced the creation of new
intermediate levels between the VB and the CB, which caused the
reduction of the band gap value from Egap(theo) ¼ 5.70 eV to
Egap(theo) ¼ 4.29 eV.
regarding O 2p states in the VB, while unoccupied W 5d states at
the bottom of the CB was found to be characteristic of other
tungstates, in particular, CuWO4, FeWO4, CoWO4, and ZnWO4
crystals [40e43].
The O 2p orbitals in the VB are mainly antibonding and the W 5d
orbitals in the CB are bonding and antibonding. Focusing on the
electronic density near the band edges, the top of VB is predomi-
nantly composed of the O 2px and O 2py orbitals, whereas the
bottom of the CV is formed by 4s from Ca atoms and 5d2z orbitals
from W atoms. From this result, it is possible to affirm that the
atomic orbitals of all atoms in the CaWO4 structure were not
degenerated, with distinct energies. Projected DOS of the CaWO4
defect structure (bottom part of Fig. 2(d)) allows observation of the
new intermediate energy levels created by the disorder provoked
by the defect on the O atom mainly on the VB, where the contri-
bution of the O atoms is major. The CB also was affected due to the
disorder in the [WO4] clusters with new energy levels in this region.
Thus, the reduction of Egap is directly proportional to the concen-
tration of defects in the material.
The study of the energy levels in the VB and CB can be better
evaluated by the analysis of the DOS, as shown in Fig. 2(d). From the
analysis of the DOS, it is possible to known which orbitals are
involved in each band, VB and CB, in the electronic transitions and
also which atoms are in the band gap region. Fig. 2(d) shows that
projected total DOS for both models, optimized and defected
structures, illustrate that O 2p states are principal contributors to
the VB and with minor input from the electronic states associated
with Ca atoms, while the CB is mainly determined by W 5d states
with minor contributions of the unoccupied electronic states
originated from O and Ca atoms. Meanwhile, the same feature
3.3. FE-SEM images and EDXS spectrum for surface and chemical
analysis
Fig. 3(aec) show the low, medium, and high magnification FE-
SEM images and Fig. 3(d) EDXS spectrum for spindle-like CaWO4
microcrystals, respectively.
4

P.B. de Sousa, A.F. Gouveia, J.C. Sczancoski et al.
Journal of Alloys and Compounds 855 (2021) 157377
Fig. 3. (a) FE-SEM image at (a) low magnification (10.000ꢀ), (b) medium magnification (40.000ꢀ), (c) medium magnification (80.000ꢀ), and (d) EDXS spectrum of several spindle-
like CaWO4 microcrystals, respectively.
The FE-SEM images illustrated in Fig. 3(a) indicate the presence
of several CaWO4 microcrystals with an agglomerate nature,
polydisperse shape, in which the large majority is composed of
spindle-like CaWO4 microcrystals. These images also suggest that
these microcrystals are formed by a large quantity of small flake-
like nanocrystals, resulting from the growing quickly in an
aqueous solution after the sonication and fast precipitation reaction
as shown in Fig. 3(b and c). Moreover, some of these spindle-like
CaWO4 microcrystals grow by means of the self-aggregation pro-
cess to the formation of the star-like CaWO4 microcrystals and
subsequent growth of these crystals leads to the growing of flower-
like CaWO4 microcrystals [44]. Therefore, these FE-SEM image
suggests that the flower-like CaWO4 microcrystals are formed by an
aggregate of spindles-like rods by means of self-assembled in a
radial form similar to that proposed in the literature [42]. The large
majority of our CaWO4 microcrystals present the shape of the
irregular spindle with an average size distribution in the range from
3.4. Sonophotocatalytic process and electronic density map:
Experimental and theoretical
Fig. 4 displays the temporal evolution of the UVeVis absorption
spectra of RhB dye solution after 200 min of illumination under UV-
C light (
l
¼ 254 nm z 4.88 eV) for (a) photolysis (P) process; only
with ultrasonic radiation for (b) sonolysis (S) process; under ul-
trasonic radiation and UV-C light for (c) sonophotolysis (SP) pro-
cess; under UV-C light and CaWO4 microcrystals as catalysts for (d)
photocatalysis (PC) process; under ultrasonic radiation, UV-C light,
and CaWO4 microcrystals as catalysts for (e) sonophotocatalysis
(SPC) process. Fig. 4(f) illustrates the evolutions for all the degra-
dation rate (Cn/C0 vs. time) and the insets show kinetic (k) constants
values for all processes employed to degradation of the RhB dye
solution using the CaWO4 microcrystals as catalysts, respectively.
Fig. 4(g and h) show the electronic density maps on the (110) plane
for optimized structure and defected structure with the presence of
bonds elongated in CaeO and WeO, respectively.
1.5 to 3.1 mm and an average size of 2.18 mm. While several CaWO4
nanocrystals aggregated by selfassembly reveal an average size
distribution being in the range from 14 to 30 nm with an average
size of 21.7 nm (Supplementary data Figs. SD-2(a,b)). Finally,
Fig. 3(d) shows an EDXS spectrum analysis for these spindle-like
CaWO4 microcrystals. This EDXS spectrum revealed that the pow-
ders are chemically composed of calcium (Ca), tungsten (W), and
oxygen (O) atoms. Inset in Fig. 3(d) show the approximate values of
elementary percentage composition (weight %). Therefore, this
result confirms that the SC method is able to allow the formation of
pure CaWO4 microcrystals.
As shown in Fig. 4(a), the degradation rate for the RhB dye so-
lution was almost insignificant after 200 min by the P process
(about 4% degradation), indicating a large resistance of RhB dye
when it is employed only UV-C light [45]. However, when the S
process is employed, we have noted a considerable improvement of
z37% degradation of RhB dye solution, as shown in Fig. 4(b). Ac-
cording to the literature [46], the water sonolysis produces H* atom
and hydroxyl (HOꢄ) radicals. In other paper reported [47] has been
observed through electron paramagnetic resonance spectroscopy
that the production rate (molar) of HOꢄ radicals increase with the
5

P.B. de Sousa, A.F. Gouveia, J.C. Sczancoski et al.
Journal of Alloys and Compounds 855 (2021) 157377
Fig. 4. (aee) UVeVis absorption spectra of RhB dye solution after 200 min of UV-C illumination/ultrasonic radiation, (f) degradation rate (Cn/C0 vs. time) and kinetics k-constants
values for all processes involved with and without the CaWO4 catalyst. Electronic density map for (g) optimized structure and (h) defected structure, respectively.
6

P.B. de Sousa, A.F. Gouveia, J.C. Sczancoski et al.
Journal of Alloys and Compounds 855 (2021) 157377
rise of ultrasound energy. However, the production rate (molar) of
HO* radicals increases until a certain limit sonication time to each
chemical reaction case. Fig. 4(c) shows an increase in the degra-
dation rate of the RhB dye solution after 200 min by the SP process
(at around 48% degradation) when ultrasound and UV-C light are
used simultaneously. This behavior is due to the synergistic effect
between the sonolysis and photolysis improves the sonochemistry
processes and promotes the bond cleavage of water (H2O) mole-
cules and adsorbed oxygen (O2) gases at H2O molecules causing the
broken and decomposition into an H* atom and HOꢄ radicals and
superoxide anion (O02) radicals resulting in the formation of per-
hydroxyl (O2H*) radical [48]. Fig. 4(d) illustrates the PC process
(about 8% degradation) when UV-C light and CaWO4 microcrystals
are used. According to previous investigations reported in the
literature [49e52], the pure CaWO4 crystals have low photo-
catalytic efficiency or are not able to the photodegradation of RhB
dyes or other organic molecules under UV and/or Vis light. As
displayed in Fig. 4(e), a high degradation rate at around 96% is
achieved when the SPC process is employed. We attribute this
considerable improvement in the catalytic properties to the bene-
ficial and synergic effect of ultrasonic radiation and UV-light allow
to activation of active sites of our CaWO4 catalysts and the rise of
the amount of H* atom, HOꢄ radicals at VB and O02 radicals at CB are
more available, which allows a good catalytic performance to the
CaWO4 microcrystals, not yet reported in the literature. Therefore,
we present in Tables SDe2 a comparative of photocatalysis and
sonophotocatalysis properties obtained in this work, with those
reported in the literature for CaWO4 crystals synthesized by other
methods (Supplementary data Tables SDe2).
presents
a
following ascending order: k(P)
¼
2.87
ꢀ
10ꢁ4
minꢁ1 < k(PC) ¼ 3.65 ꢀ 10ꢁ4 minꢁ1 < k(S) ¼ 2.18 ꢀ 10ꢁ3
minꢁ1 < k(SP) ¼ 3.01 ꢀ 10ꢁ3 minꢁ1 < k(SPC) ¼ 1.43 ꢀ 10ꢁ2 minꢁ1).
Therefore, we attributed that the synergistic effect of ultrasound
radiation and UV-C light promotes the activation of catalytic sites
favoring the formation of more H* atom, HOꢄ radicals at VB, and O20
radicals at BC, which are necessary for the degradation of the RhB
dye in aqueous solution. Finally, Fig. 4(g and h) displays the elec-
tronic density map on the Ca, W, and O atoms in the (110) plane for
the optimized and defected structure. On the optimized structure
(Fig. 4(g)), a homogeneous electron and charge distribution be-
tween the atoms of the [CaO8] and [WO4] clusters are observed and
the isolines demonstrated that the bonds between the atoms are of
the covalent type. However, with the bond elongated in the CaeO
and WeO caused by the defect in the O atom, inhomogeneous
electron and charges distribution in this region are observed
instead (Fig. 4(h). The covalent character of the CaeO bond is lost
and becomes an ionic bond, while the WeO continues with a co-
valent character but weaker due to the bond elongated.
4. Conclusions
In summary, spindle-like CaWO4 microcrystals were success-
fully synthesized by the SC method at 40 ꢂC for 3 h. XRD patterns,
micro-Raman, FT-IR and EDXS spectra confirmed that our CaWO4
microcrystals are monophasic with scheelite-type tetragonal
structure. The Raman-active and IR-active modes proved the exis-
tence of local order at a short-range for our CaWO4 microcrystals.
Moreover, the relative positions of experimental Raman and IR-
active modes were corroborated with those theoretically calcu-
lated by the DFT method. FE-SEM images revealed that the CaWO4
microcrystals with spindles-like morphologies have an average
The ultrasound facilitates the transport of RhB dye molecules to
the surface of the CaWO4 catalyst microcrystals, where it can
absorb, and react with the ꢄOH radicals generated by the reaction of
holes (hꢄ) with adsorbed H2O molecules. Moreover, the micro-
convection may also assist desorption of the product from the
catalyst surface, making it accessible to the next RhB dye molecules,
prevent the agglomeration of catalyst CaWO4 microcrystals and still
increase the effective surface area of these particles, with the
consequent increase in the extent of adsorption of RhB dye mole-
cules [53,54].
crystals size of 2.18 mm and these CaWO4 nanocrystals aggregated
by self-assembly presents an average crystals size 21.7 nm,
respectively. The experimental optical band gap Egap(exp) was found
to be 4.69 eV with the possible existence of intermediary energy
states within the band gap. The theoretical calculation indicated
that the EBS of CaWO4 microcrystals is characterized by direct
electronic transitions, with an Egap(theo) equal a 5.70 eV to optimized
structure, while the defected structure presents an Egap(theo) equal a
4.29 eV where is much closer to Egap(exp) value, demonstrating that
the band gap value is related to the order/disorder of the material
and a decrease of this value is due to the presence of intermediated
energy levels between the gap region (VB and CB). According to the
DOS analyses, the energy states in the VB is constituted mainly from
O 2p orbitals, while in the CB has a major contribution related to (W
5d) orbitals. When is provoked a local defect in the O atom, it’s
noted that affects both VB and CB which is referent to the structural
defect model due to the presence of new intermediate energy levels
between these bands. From the electronic density map, it is
possible to affirm that the local defects, such as a bond elongated,
causes an inhomogeneous distribution of the electronic density and
charge between the [CaO8] and [WO4] clusters, that is related to the
capacity of the material to generated electron and holes pars.
Finally, the SPC process using CaWO4 catalyst microcrystals was
more promising than the traditional PC process, due to the syner-
gistic effect between ultrasound radiation and UV-C light to
unblocking the active catalytic sites, assist desorption of the
product from the catalyst surface and promoting the formation of
more H* atom, HOꢄ radicals at VB and O02 radicals, which are
necessary conditions for the high degradation of almost 96% RhB
dye until 200 min.
According to the previous works [55,56],
a high photo-
degradation rate for RhB dye or cationic dyes can be achieved using
visible/irradiation light, activated carbon bimetallic nano-
composite, presence of hydrogen peroxide (H2O2) and scavengers,
such as dimethyl sulphoxide (DMSO), benzoquinone (BQ), tri-
ethanolamine (TEOA) and potassium dichromate (PD).
To quantitatively understand the reaction kinetics for the
discoloration of the RhB dye by the CaWO4 catalyst microcrystals,
as illustrated in Fig. 4(f), we applied the pseudo-first order model
expressed in equation (2) to obtain the rate constants (k):
ꢀ
ꢁ
Cn
C0
ꢁln
¼ kt
(2)
where C0 and Cn(%) is the initial and different concentration of the
dye solution of UV-C illumination, t is the exposure time, and k is
the pseudo-first-order rate constant. This equation is generally
used for a P, S, SP, PC, and SPC processes if the initial concentration
of the pollutant is low (1 ꢀ 10ꢁ5 mol Lꢁ1). According to equation (2),
a plot of ½ꢁInðCn =C0Þꢃ as a function of t gives a straight line where
the slope is k. The results obtained by the equation (2) for the k
values for each process using our CaWO4 catalyst microcrystals are
illustrated in Fig. 4(f). As it can be noted in the insets Fig. 4(f) the
SPC process is more effective for the degradation of RhB dye.
Moreover, the rate constants values for the degradation of RhB dye
7

P.B. de Sousa, A.F. Gouveia, J.C. Sczancoski et al.
Journal of Alloys and Compounds 855 (2021) 157377
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Supporting Information
Supporting Information is available online.
Declaration of competing interest
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
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8 (2018) e1360, e1361-36.
Acknowledgements
The authors acknowledge the financial support of the Brazilian
research financing institutions: CNPq (479644/2012-8 and 304531/
2013-8), FAPESP (2019/01732-1, 2016/23891-6, and 2013/07296-2),
and CAPES-PNPD (1268069).
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9