Ultrasonics Sonochemistry 38 (2017) 256–270
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Ultrasonics Sonochemistry

journal homepage: www.elsevier .com/ locate/ul tson
Structural evolution, growth mechanism and photoluminescence
properties of CuWO4 nanocrystals
http://dx.doi.org/10.1016/j.ultsonch.2017.03.007
1350-4177/� 2017 Elsevier B.V. All rights reserved.

⇑ Corresponding author.
E-mail address: laeciosc@bol.com.br (L.S. Cavalcante).
E.L.S. Souza a, J.C. Sczancoski b, I.C. Nogueira c, M.A.P. Almeida d, M.O. Orlandi e, M.S. Li f, R.A.S. Luz a,
M.G.R. Filho a, E. Longo e, L.S. Cavalcante a,⇑
a PPGQ-CCN-GERATEC, Universidade Estadual do Piauí, Rua: João Cabral, N. 2231, P.O. Box 381, 64002-150 Teresina, PI, Brazil
bDQ-UFSCar, Universidade Federal de São Carlos, P.O. Box 676, São Carlos, SP 13565-905, Brazil
c ICE-Universidade Federal do Amazonas, Av. Rodrigo Otávio Japiim, P.O. Box 670, 69077-000 Manaus, AM, Brazil
dCCT-Universidade Federal do Maranhão, P.O. Box 322, 65080-805 São Luís, MA, Brazil
eDepartamento de Físico-Química, Universidade Estadual Paulista, 14800-060 Araraquara, SP, Brazil
f IFSC-Universidade de São Paulo, P.O. Box 369, 13560-970 São Carlos, SP, Brazil

a r t i c l e i n f o a b s t r a c t
Article history:
Received 31 January 2017
Received in revised form 6 March 2017
Accepted 6 March 2017
Available online 8 March 2017

Keywords:
CuWO4 crystals
Sonochemistry
Clusters
Raman spectroscopy
TEM images
Photoluminescence properties
Copper tungstate (CuWO4) crystals were synthesized by the sonochemistry (SC) method, and then, heat
treated in a conventional furnace at different temperatures for 1 h. The structural evolution, growth
mechanism and photoluminescence (PL) properties of these crystals were thoroughly investigated. X-
ray diffraction patterns, micro-Raman spectra and Fourier transformed infrared spectra indicated that
crystals heat treated and 100 �C and 200 �C have water molecules in their lattice (copper tungstate dihy-
drate (CuWO4�2H2O) with monoclinic structure), when the crystals are calcinated at 300 �C have the pres-
ence of two phase (CuWO4�2H2O and CuWO4), while the others heat treated at 400 �C and 500 �C have a
single CuWO4 triclinic structure. Field emission scanning electron microscopy revealed a change in the
morphological features of these crystals with the increase of the heat treatment temperature.
Transmission electron microscopy (TEM), high resolution-TEM images and selected area electron diffrac-
tion were employed to examine the shape, size and structure of these crystals. Ultraviolet–Visible spectra
evidenced a decrease of band gap values with the increase of the temperature, which were correlated
with the reduction of intermediary energy levels within the band gap. The intense photoluminescence
(PL) emission was detected for the sample heat treat at 300 �C for 1 h, which have a mixture of
CuWO4�2H2O and CuWO4 phases. Therefore, there is a synergic effect between the intermediary energy
levels arising from these two phases during the electronic transitions responsible for PL emissions.

� 2017 Elsevier B.V. All rights reserved.
1. Introduction published papers have reported on the structural features,
Hydrous copper tungstate (CuWO4�xH2O) crystals can be natu-
rally found as a mineral, presenting colors from bright yellowish
to green [1]. This mineral, also known as ‘‘Cuprotungstite”, has been
discovered and analyzed by Whitney T. Schaller in 1892. However,
the same designation also is adopted for copper tungstate
(CuWO4), i.e., an oxide material belongs to wolframite sub-group
[2]. Whitney T. Schaller verified that CuWO4�xH2O crystals are
composed of two water molecules; therefore, it was posteriorly
described as copper tungstate dihydrate (CuWO4�2H2O) [1,2]. In
general, CuWO4�2H2O crystals are easily produced by the reaction
between copper nitrate and sodium tungstate precursors in aque-
ous solutions, using specific pH and temperature conditions. Some
morphological aspects and electronic properties of this tungstate
[3–12]. Particularly, some of these studies [3–7]. consider the
CuWO4�2H2O as a raw or precursor precipitate formed during the
initial synthesis stages due to its favorable thermodynamic condi-
tion [8–12].

In past years, CuWO4 crystals were obtained by several tradi-
tional methods, mainly including oxide mixture or solid state reac-
tion [13–17], flux growth technique [18,19], melting at a high
temperature [20], and Czochralski process [21]. Generally, these
techniques require high temperatures, long processing times and
sophisticated equipment with high maintenance costs [22]. On
the other hand, to overcome these drawbacks, simple methods
were developed for the preparation of CuWO4 micro- and
nanocrystals, such as precipitation reaction [23–26], polyol-
mediated at low-temperature [27], hydrothermal conventional
[28,29], and microwave-assisted synthesis [30].

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E.L.S. Souza et al. / Ultrasonics Sonochemistry 38 (2017) 256–270 257
The wolframite-type monoclinic structure is commonly found
in tungstates composed of transition metals belonging to the
fourth period of the periodic table (MnWO4 [31], FeWO4 [32],
CoWO4 [33], NiWO4 [34], and ZnWO4 [35]). The only exception is
CuWO4, which crystallizes in a triclinic structure at room temper-
ature [36–38]. In addition, CuWO4 crystals exhibit a phase transi-
tion from triclinic structure at low-pressure to monoclinic
structure at high-pressure (9.9 GPa) [39]. Therefore, when
subjected to extreme pressure environments, the researchers
[40–43] have reported that these crystals have a monoclinic struc-
ture characterized by space group (P2/c), point group symmetry
(C4

2h) and two molecular formula units per unit cell (Z = 2)
[43,44]. On the other hand, under low-pressure conditions at room
temperature, these crystals exhibit a triclinic structure with space
group (P�1), point group symmetry (Ci) and two molecular formula
units per unit cell (Z = 2) [45–47]. Moreover, CuWO4 crystals with
triclinic structure are influenced by Jahn-Teller effect due to the
presence of Cu2+ ions, which promote distortions on octahedral
[CuO6] clusters. Consequently, this phenomenon gives rise to a d-
orbital splitting, in which the degeneracy ofr-antibonding orbitals
is broken [48,49]. According to the literature [50,51], the Pauli
exclusion principle can provide that the Cu2+ ions have an electron
with unpaired spin occupying the dx

2
�y
2 orbital, indicating that this

electronic level could produce a mid-gap band state. The additional
stabilization is greater in a Jahn–Teller-elongated Cu2+ ions (where
3dz2 contains two electrons) [52].

Currently, the scientific studies on the electronic properties of
pure and doped CuWO4 have been mainly focused on the photocat-
alytic (PC) degradation of organic dyes (Rhodamine B, eosin yellow
dye and methylene blue) under ultraviolet and visible light [53–
55], magnetic [56–59], photoelectrochemical water splitting [60–
64], visible and solar-assisted water splitting [65,66], photoanode
for solar water oxidation [67,68], electrical transport [69], and pho-
toluminescence (PL) [24,53,70]. An important point to be consid-
ered is that the theoretical studies [16,71–75], performed by
means of ab initio calculations based on the density-functional the-
ory (DFT) for the electronic structure of CuWO4 crystals, have
shown that the conduction band (CB) of this oxide is composed
of 3d orbitals (Cu atoms) and 5d orbitals (W atoms), while the
valence band (VB) is formed of 2p orbitals (O atoms).

Therefore, the aim of the present study was to investigate the
structural, morphological and optical properties of CuWO4 crystals.
These crystals were initially synthesized by the sonochemistry (SC)
method (30 min), and then, heat treated in a conventional furnace
at different temperatures for 1 h. A crystal growth mechanism was
proposed in order to explain the evolution of particle shape/size
with the increase of the heat treatment temperature. Finally, the
optical properties of these crystals were investigated by
Ultraviolet-Visible spectroscopy and PL measurements at room
temperature.

2. Experimental details

2.1. Synthesis of CuWO4 crystals

The synthesis of CuWO4 crystals is described as follows:
1 � 10�3 mols of sodium tungstate dihydrate (Na2WO4�2H2O;
99.5% purity, Sigma-Aldrich) and 1 � 10�3 mols of copper nitrate
trihydrate [Cu(NO3)2�3H2O; 99% purity, Sigma-Aldrich] were sepa-
rately placed in two plastic tubes (Falcon – capacity of 50 mL) and
dissolved with deionized water. The two solutions (pH = 6) were
transferred into a beaker (250 mL) and ultrasonicated for 30 min
by means of an ultrasonic cleaner (model CPX-1800H, Branson –
USA) at frequency of 42 kHz. These suspensions containing bright
green precipitates were seven times washed (water and acetone)
and centrifuged (8500 rpm for 10 min), and then, dried in a single
hot plate (60 �C for 30 min). Finally, the obtained CuWO4�2H2O
precipitates were heat treated at different temperatures (100 �C,
200 �C, 300 �C, 400 �C, and 500 �C for 1 h), maintaining a heating
rate of 5 �C.min�1. The increase in the heat treatment temperature
was employed to monitor the crystallization process of single
CuWO4 phase.

CuWO4�2H2O crystals were synthesized via chemical reaction
between hexaaquacopper(II) complex ion ([Cu(H2O)6]2+) and tung-
state ions (WO2�

4 ) in aqueous solution. These ions were originated
by means of the complete dissolution of their respective chemical
precursors [Na2WO4�2H2O and Cu(NO3)2�3H2O], as described in
Eqs. (1)--(3):

Na2WO4 � 2H2OðsÞ !H2O 2NaþðaqÞ þWO2�
4ðaqÞ þ 2H2O ð1Þ

CuðNO3Þ2:3H2OðsÞ �!H2O ½CuðH2OÞ6�2þ þ 2NO�3ðaqÞ þ 3H2O ð2Þ
The heat treatment performed at low temperatures (from 100 �C to
200 �C) was responsible for the partial removal of water molecules
belonging to CuWO4�2H2O structure, according to Eq. (4):

CuWO4 � 2H2OðsÞ . . . x:H2OðadsÞ �!�100�Cand200�C=1h
CuWO4:xH2OðsÞ þ 2H2OðgÞ

ð4Þ
The water removal (dehydration process) continues at low tem-

peratures (�300 �C); however, there is a mixture of CuWO4�xH2O
and CuWO4 phases, as proposed in Eq. (5):

2CuWO4 � xH2OðsÞ �!�300�C=1h
CuWO4 � xH2OðsÞ þ CuWO4ðsÞ ð5Þ

The formation of single CuWO4 crystals occurs at temperatures
above 400 �C, according to Eq. (6):

CuWO4 � xH2OðsÞ þ CuWO4ðsÞ �!�400�Cand500�C=1h
2CuWO4ðsÞ þ xH2OðgÞ

ð6Þ
2.2. Characterizations of CuWO4 crystals

CuWO4 nanocrystals were structurally characterized by X-ray
diffraction (XRD) with a DMax/2500PC diffractometer (Rigaku,
Japan), using Cu-Ka radiation (k = 0.15406 nm). Data were col-
lected over 2h ranging from 10� to 70�, employing a step scan of
2�.min�1. Rietveld analysis was performed over 2h ranging from
5� to 120�, at a scan step and step size of 1�.min�1 and 0.02�,
respectively. Thermogravimetric analysis (TGA) and differential
thermal analysis (DTA) were carried out in a STA 409 thermal ana-
lyzer (Netzsch, Germany). These thermal measurements were per-
formed from room temperature to 550 �C under synthetic air flow
(15 cm3.min�1), maintaining a heating rate of 10 �C.min�1. Micro
Raman (M-Raman) spectra were recorded using a LabRAM HR
800 spectrometer (Horiba Jobin Yvon, France). These spectra were



258 E.L.S. Souza et al. / Ultrasonics Sonochemistry 38 (2017) 256–270
obtained from 50 cm�1 to 1000 cm�1 with an Ar+ laser of 514.5 nm
(model CCD DU420AOE325), maintaining a maximum output
power of 6 mW. A 50 lm lens was used to prevent sample over-
heating. Fourier Transform infrared (FT-IR) spectra were performed
from 200 cm�1 to 1000 cm�1 with a Bomem–Michelson spec-
trophotometer operated in transmittance mode (model MB-102).
Ultraviolet–Visible (UV–Vis) spectra were taken using a Cary 5G
spectrophotometer (Varian, USA) operated in diffuse reflectance
mode. The morphological features were examined by using a Supra
35-VP field-emission scanning electron microscope (FE-SEM) (Carl
Zeiss, Germany) operated at 10 kV, and with a CM200 transmission
electron microscope (TEM) (Philips/FEI, Netherlands) operated at
200 kV. The shape, average size and crystal growth directions of
CuWO4 crystals were determined using the selected-area electron
diffraction (SAED) and high resolution (HR)-TEM, respectively. The
samples for TEM and HR-TEM were prepared depositing
(dropwise) dilute suspensions of CuWO4 crystals in acetone on
300-mesh Cu grids. The photoluminescence (PL) spectra were
conducted at room temperature by using a Monospec 27
monochromator (Thermal Jarrel Ash, USA) coupled to a R955
photomultiplier (Hamamatsu Photonics, Japan). A krypton-ion
laser (Coherent Innova 90 K; k = 350 nm) was used as an excitation
source, maintaining a maximum output power at 500 mW. The
laser beam passed through an optical chopper, so that the maxi-
mum power incident on the sample was maintained at 14 mW.
Fig. 1. XRD patterns of (a) CuWO4�2H2O and (b) CuWO4 crystals, respectively. The
vertical lines in red color indicate the position and relative intensity of XRD patterns
for CuWO4�2H2O phase reported in Joint Committee on Powder Diffraction
Standards (JCPDS) card No. 33-0503. The symbol (�) is assigned to CuWO4 phase.
The vertical lines in black color show the position and relative intensity of XRD
patterns for CuWO4 phase described in Inorganic Crystal Structure Database (ICSD)
card No. 16009.
3. Results and discussions

3.1. Long-range structural analyses

Fig. 1(a, b) shows XRD patterns of CuWO4�2H2O and CuWO4

crystals heat treated at different temperatures, respectively.
According to the literature [76], the degree of structural order/

disorder or periodicity of a crystalline lattice in oxide materials can
be analyzed by means of X-ray diffraction. In Fig. 1(a), the precip-
itated crystals heat treated at 100 �C and 200 �C for 1 h revealed
the presence of wide XRD peaks assigned to CuWO4�2H2O mono-
clinic structure. This widening can be due to the presence of water
molecules bonded in this crystalline structure (TGA and DTA pro-
files in Support Information Fig. SI-1(a, b)) as well as because of
effects of order–disorder [77], i.e., these crystals have not a com-
plete long-range structural ordering. Increasing the heat treatment
temperature up to 300 �C (Fig. 1(a)), XRD patterns revealed a mix-
ture of CuWO4�2H2O and CuWO4 phases. Therefore, in this temper-
ature occurs a significant elimination of water molecules in
CuWO4�2H2O. Chen and Xu [28] described the CuWO4�2H2O crys-
tals as a crystalline phase with monoclinic structure referring to
JCPDS Card No. 33-0503. When the heat treatment was performed
at 400 �C and 500 �C for 1 h, all XRD patterns showed diffraction
peaks ascribed to CuWO4 triclinic structure, in agreement with
the ICSD card No. 16009 [78]. In order to confirm this triclinic
structure, the structural refinement by means of Rietveld method
[79] was performed for CuWO4 crystals heat treated at 500 �C for
1 h.

The Rietveld method is based on the construction of diffraction
patterns calculated according to a structural model [80]. The calcu-
lated patterns are adjusted to the observed pattern, providing the
structural parameters of the desired material and its diffraction
profile. In the present study, the Rietveld method was applied to
estimate the atomic positions, lattice parameters, and unit cell vol-
ume of CuWO4 crystals. The Rietveld refinement was performed
using the general structure analysis software (GSAS) program
[81], in which the refined parameters were scale factor, back-
ground, shift lattice constants, profile half-width parameters (u,
v, w), isotropic thermal parameters, lattice parameters, strain ani-
sotropy factor, preferential orientation, factor occupancy, and
atomic functional positions. The background was corrected using
a Chebyschev polynomial of the first order. The peak profile func-
tion was modeled using a convolution of the Thompson–Cox–Hast
ings pseudo-Voigt (pV-TCH) function [82] with the asymmetry
function described by Finger et al. [83] In order to explain the ani-
sotropy in the half width of the reflections, the model by Stephens
[84] was used.

Fig. 2 shows the Rietveld refinement plot for CuWO4 crystals
synthesized by the SC method and heat treated at 500 �C for 1 h.

All structural refinement results obtained by the Rietveld
method [80] are consistent with ICSD No. 16009 reported by Kihl-
borg and Gebert [78]. According to the literature [85], single
CuWO4 crystals have a triclinic structure, presenting a space group
(P�1), point group symmetry (Ci) and two molecular formula units
per unit cell (Z = 2). The structural refinement confirmed the
triclinic structure for CuWO4 crystals (Fig. 2). In general, slight



Fig. 2. Rietveld refinement plot of CuWO4 crystals heat treated at 500 �C for 1 h.

Fig. 3. Schematic representation of CuWO4 triclinic structure.

E.L.S. Souza et al. / Ultrasonics Sonochemistry 38 (2017) 256–270 259
differences in the intensity scale were identified between
experimental and calculated XRD patterns, as described by the line
(YObs–YCalc). However, the quality of the structural refinement can
be accurately determined by the R-values (Rwp, RBragg, Rp, v2, and S).
More details on the Rietveld refinement results are displayed in
Table 1.

In this table, the fit parameters (RBragg, Rwp, Rp, v2, and S) sug-
gest that refinement results are very reliable. In general, small vari-
ations in atomic positions of O atoms were identified, while Cu and
W atoms are fixed in their respective positions within the struc-
ture. In this case, the O atoms are able to induce distortions on both
O–Cu–O or O–W–O bonds, resulting in distorted octahedral [CuO6]
and [WO6] clusters.
Fig. 4. Micro-Raman spectra of CuWO4 crystals heat treated at (a) 100 �C, (b)
200 �C, (c) 300 �C, (d) 400 �C and (e) 500 �C for 1 h. Inset shows a typical symmetric
stretching vibration of O–W–O bonds in octahedral [WO6] clusters. The vertical
dotted lines with symbol ( ) indicate the relative positions and intensities of
Raman-active modes of CuWO4 crystals, while the symbol (*) show the presence of
water molecules within these crystals, respectively.
3.2. Structural representation, and coordination of clusters in CuWO4

Fig. 3 shows a schematic representation of CuWO4 triclinic
structure.

This triclinic structure was modeled through the visualization
system for electronic and structural analysis (VESTA) software
(version 3.4.0 for version of Windows 7-64-bit) [86,87], using the
lattice parameters and atomic positions obtained from the Rietveld
refinement data listed in Table 1. In Fig. 3, Cu and W atoms are
both coordinated to six O atoms, forming distorted octahedral
[CuO6] and [WO6] clusters, which are octahedron-type polyhe-
drons with 6-vertices, 8-faces and 12-edges [88]. In principle, these
CuWO4 crystals heat treated at different temperatures are able to
present variations in both (O–Cu–O)/(O–W–O) bond angles and
lengths. This behavior implies in distortions on octahedral [CuO6]
and [WO6] clusters with distinct degrees of order–disorder in the
lattice.
Table 1
Rietveld refinement results for CuWO4 crystals obtained by the sonochemistry method an

Atoms Wyckoff Site

Cu 2i 1
W 2i 1
O1 2i 1
O2 2i 1
O3 2i 1
O4 2i 1

a = 4.7062(7) Å, b = 5.8432(8) Å, c = 4.8829(4) Å, a = 91.6680(3)�, b = 92.4985(3)�, c = 82.
and S = 1.08166.
3.3. Short-range structural analysis

Fig. 4(a–e) shows the M-Raman spectra of CuWO4 crystals heat
treated at different temperatures.

The short-range structural ordering in the lattice of any mate-
rial can be analyzed via Raman spectroscopy [89]. According to
group theory calculations, tungstates with triclinic structure, space
group (P�1) and point group symmetry (Ci) have 36 different
d heat treated at 500 �C for 1 h.

x y z

0.4959(5) 0.6589(9) 0.2450(6)
0.0202(8) 0.1729(2) 0.2538(3)
0.2472(9) 0.3569(4) 0.4257(0)
0.2126(9) 0.8846(4) 0.4321(0)
0.7334(9) 0.3837(4) 0.0993(0)
0.7334(9) 0.9131(4) 0.0545(0)

7871(3)�; V = 133.054(7)Å3, RBragg (%) = 2.50, Rwp (%) = 8.84, Rp (%) = 6.89, v2 = 1.17



Fig. 5. FT-IR spectra of CuWO4 crystals heat treated at (a) 100 �C, (b) 200 �C, (c)
300 �C, (d) 400 �C and (e) 500 �C for 1 h. Inset shows a typical anti-symmetric
stretching vibration of O–W–O bonds in octahedral [WO6] clusters. The vertical
dotted lines with symbol (�) indicate the relative positions and intensities of IR-
active modes.

260 E.L.S. Souza et al. / Ultrasonics Sonochemistry 38 (2017) 256–270
{(Raman) and [infrared]} vibrational modes, as described in Eq. (7)
[40,41,71].

CfðRamanÞþ½Infrared�g ¼ fð18AgÞ þ ½18Au�g ð7Þ

where Ag are Raman-active vibrational modes, and Au are infrared-
active vibrational modes. Therefore, 18 Raman-active vibrational
modes are expected for CuWO4 crystal, shown in Eq. (8):

CðRamanÞ ¼ 18Ag ð8Þ
According to the literature [71,90], vibrational modes observed

in Raman spectra of tungstates are classified into two groups,
external and internal modes. The vibrational external modes are
related to lattice phonons, corresponding to the motion of dis-
torted octahedral [CuO6] clusters (Oh symmetry) and rigid units.
Vibrational internal modes are ascribed to vibrations of distorted
octahedral [WO6] clusters in the lattice, assuming the center of
mass in a stationary state. Distorted octahedral [WO6] clusters
have point group symmetry (Ci), in which the vibrations are com-
posed of internal modes (Ag); the other vibrations are external
modes (Ag) [91]. As can be observed in Fig. 4(a–e), there are
Raman-active modes for the two crystals (CuWO4.2H2O and
CuWO4). Firstly, the samples heat treated at 100 �C and 200 �C
have vibrational modes arising from water molecules (⁄H2O) in
CuWO4.2H2O crystals (Fig. 4(a, b)). The disappearing or lower
intensity of these Raman-active modes in the spectrum illustrated
in Fig. 4(c) indicates that the temperature of 300 �C is critical for a
phase transition from CuWO4�2H2O (monoclinic structure) to
CuWO4 crystals (triclinic structure). In Fig. 4(d, e), when the sam-
ples were heat treated at 400 �C and 500 �C for 1 h, the Raman-
active vibrational modes are only ascribed to CuWO4 triclinic
structure. Another important point to be considered is that the
heat treatment at 500 �C resulted in more intense and well-
defined Raman-active bands for CuWO4 crystals. In this case, the
evolution of temperature also leads to an increase of short-range
structural ordering for this oxide. Table 2 shows a comparative
between the respective positions of Raman-active vibrational
modes for CuWO4 reported experimentally and theoretically in
papers published in the literature [41] with those obtained in our
present study.
Table 2
Comparative data of the respective positions of Raman-active modes of CuWO4 reported i

M – THEO

T (�C) – –

t (h) – –

R A1g 95.3 81
a A1g 127.6 115.6
m A1g 149.1 137.7
a A1g 179.2 164.4
n A1g 191 178

A1g 223.8 209.2
A A1g 282.6 263.5
c A1g 292.6 276.2
t A1g 316.2 294.3
i A1g 358.2 341
v A1g 397.5 374.9
e A1g 403.4 391.9

A1g 479.9 454.1
M A1g 549.8 525.2
o A1g 676.7 633.6
d A1g 733.1 695.8
e A1g 778.9 763.2
s A1g 905.9 854.4

Ref. [40] [41]

M = Method; t = time; Raman modes = (cm�1); THEO = Theoretical, TSSG = top-seede
= References.
In this table, there is a good agreement between our experimen-
tal Raman-active vibrational modes with the reported in other
papers previosly published [40–42,92]. The slight displacements
in the positions of these vibratiol modes can be related to varia-
tions in the degree of structural order-disorder on both octahedral
[CuO6] and [WO6] clusters, as a consequence of the peculiarities of
each synthesis condition (temperature, time) adopted for the for-
mation of CuWO4 phase.

3.4. FT-IR spectroscopy analyses

Fig. 5(a–e) illustrate FT-IR spectra of CuWO4 crystals heat trea-
ted at different temperatures.
n the literature with those obtained in the present study.

TSSG SC-C SC-C SC-C

1000 500 400 500

48 5 1 1

– – 87.4 86.8
– – 118.2 118.3
– – 140.1 140.2
180 – 170.2 170.2
192 190 181.1 181.2
224 223 213.1 213.4
283 279 272.2 272.4
293 314 304.8 305.5
315 395 331.2 332
358 447 346.8 347.2
398 547 380.3 381.2
405 447 392.8 391.2
479 547 472.1. 472.3
550 673 538.1 538.8
676 731 665.7 667.6
733 778 728.8 729.1
779 805 769.1 771.5
906 905 900.9 901.2
[42] [92] z z

d solution growth; SC-C = Sonochemical-Calcination; [z] = This work and Ref.



E.L.S. Souza et al. / Ultrasonics Sonochemistry 38 (2017) 256–270 261
According to the literature [93], IR spectra also is able to provide
information on the degree of structural order-disorder in atomic
bonds of ABO4 materials. Eq. (7) shows that the CuWO4 crystals
with triclinic structure have 18 Raman-active vibrational modes
and 18 IR-active vibrational modes, as indicated by Eq. (9) [94]:

C½Infrared� ¼ 18Au ð9Þ
As illustrated in Fig. 5(a–e), only eleven IR-active vibrational

bands [Au modes] were detected in our IR spectra. This behavior
is explained by the low symmetry of CuWO4 lattice and phonon
pattern associated with each mode that is in general complex
and involves the whole unit cell [94]. However, the atom dynamics
associated to highest energy modes can be understood on the basis
of the main atomic shifts [94,95]. The band located at around
910 cm�1 is related to symmetric stretching vibrations
( O W?O?) in distorted octahedral [WO6] clusters. It was
noted a smaller band at 812 cm�1, which cannot be related to opti-
cal modes at zone center [94]. Another band of low intensity at
720 cm�1 is related to anti-symmetric stretching vibrations in dis-
torted octahedral [WO6] clusters (Inset Fig. 5). Also, it was noted a
shoulder at 631 cm�1 [Au mode], which cannot be attributed to
optical modes [94]. The symmetric stretching vibrations
( O W?O W?O?) between [WO6]–[WO6] clusters were ver-
ified at 558 cm�1. On the other hand, the symmetric stretching
vibrations ( O Cu?O?) of distorted octahedral [CuO6] clusters
were detected at 478 cm�1. The small band at 417 cm�1 is arising
from symmetric bending vibrations (-O-W%O%) in [WO6] clus-
ters.[94] Finally, the last four modes (from 275 cm�1 to 377 cm�1)
are assigned to anti-symmetric (?O?Cu?O-Cu-O-) and sym-
metric ( O Cu?O Cu?O?) stretching vibrations between
[CuO6]–[CuO6] clusters, and symmetric bending vibrations
(-O-Cu%O%) in [CuO6] clusters, respectively. A comparison
between the respective positions of IR-active vibrational modes
of CuWO4 obtained in our study with others published in the liter-
ature [41] are listed in Table 3. As expected in this table, our exper-
imental IR-active vibrational modes are in good agreement with
the results verified in other papers [41,94–97]. In addition, we have
noted that some of these infrared vibrational modes of CuWO4

nanocrystals are similar to isostructural CuMoO4 crystals [98].
Table 3
Comparative data of the respective positions of IR-active modes of CuWO4 reported in the

M THEO TSSG CP-C

T (�C) – 1000 450

t (h) – 48 6

I A1u – – –
n A1u – – –
f A1u – – –
r A1u 101.4 – –
a A1u 157.3 – –
r A1u 214.2 – –
e A1u 239 253 –
d A1u 266.1 275 –

A1u 281 290 –
Ac A1u 320.5 355 –
ti A1u 332.3 – –
ve A1u 383.8 395 –

A1u 438.9 466 500
M A1u 474.8 540 –
o A1u 516.5 600 –
d A1u 639.6 722 –
e A1u 727.7 760 748
s A1u 852.7 911 876

Ref. [40] [94] [95]

M = method; T = temperature; t = time; IR-active modes = (cm�1); TSSG = Top-seeded so
C = Sonochemical-Calcination; THEO = Theoretical, and [z] = this work.
3.5. FE-SEM images analyses

Fig. 6(a–e) shows FE-SEM images of CuWO4 crystals heat trea-
ted at different temperatures.

FE-SEM micrographs can be used as a powerful tool to accom-
pany the particle shape evolution and growth process of CuWO4

nanocrystals. Fig. 6(a) shows that the CuWO4�2H2O microcrystals
obtained at 100 �C have a similar aspect of irregular flowers, which
are formed by aggregated assemblies of several crystals (as
‘‘petals”). These ‘‘petals” show many imperfections and surface
defects, as a direct result of both uncontrollable formation and
interaction of nanocrystals caused by the chemical synthesis
employed, solvent nature or intrinsic morphological feature of
CuWO4�2H2O microcrystals [64]. These final structures similar to
flowers have an average size of 4.7 lm, while their petals have
an average size of 860 nm (Support Information Fig. SI-2(a, b)).
When the heat treatment was performed at 200 �C, both shape
and size of these crystals were modified. This morphological
change can be due to the initial stage of elimination of water mole-
cules in CuWO4�2H2O, resulting in irregular stone-like microcrys-
tals with average crystal size of 3.3 lm (Fig. 6(b)). These
microcrystals are composed of several aggregated nanocrystals
with average size of approximately 7.7 nm (Support Information
Fig. SI-2(c, d)). In Fig. 6(c), the increase of heat treatment temper-
ature up to 300 �C was able to maintain the morphological feature
of stone-like microcrystals in the samples. In this case, it was esti-
mated for these microcrystals an average size of approximately
6.6 lm, in which their aggregated nanocrystals presented an aver-
age crystal size of 11 nm (Support Information Fig. SI-2(e, f)). In
Fig. 6(d), the presence of a large number of small crystals was ver-
ified on the surface of stone-like microcrystals heat treated at
400 �C for 1 h. As detected by XRD patterns (Fig. 1(b)), in this tem-
perature occurs only the presence of CuWO4 triclinic structure.
Therefore, the counting of these stone-like CuWO4 microcrystals
revealed an average size at around 9.3 lm, which are formation
by irregular nanocrystals with average size of 18.3 nm (Support
Information Fig. SI-2(g, h)). Finally, for the temperature of 500 �C,
it was possible to prove that these large stone-like microcrystals
are composed of several flake-like CuWO4 nanocrystals. For these
literature with those obtained in the present study.

CP-C SSR SC-C SC-C

800 800 400 500

8 36 1 1

– – – –
– – – –
– – – –
– – – –
– – – –
– – – –
– – – –
– 270 274 275
– 290 296 297
– 340 344 345
– 375 376 377
– 415 416 417
– – 477 478
545 550 557 558
– 605 630 631
710 740 719 720
799 800 811 812
900 910 909 910
[96] [97] z z

lution growth; CP-C = Co-precipitation-Calcination; SSR = Solid state reaction; SC-



Fig. 6. FE-SEM images of CuWO4 crystals heat treated at different temperatures: (a) 100 �C, (b) 200 �C, (c) 300 �C, (d) 400 �C and (e) 500 �C for 1 h, respectively.

262 E.L.S. Souza et al. / Ultrasonics Sonochemistry 38 (2017) 256–270
nanocrystals was found an average size of approximately 41 nm
(Support Information Fig. SI-2(i)). All results on the average size
distribution of CuWO4 crystals are summarized in Table 4 and
(Support Information Fig. SI-2(a–i)).

All experimental results obtained in our study are good agree-
ment with the literature [99–103]. In addition, all statistical data
obtained through the counting of particle sizes using FESEM and
TEM images were well-described by the log-normal distribution
[104].
3.6. TEM and HR-TEM images analyses

Fig. 7(a–j) show TEM and HR-TEM images of CuWO4 crystals
heat treated at different temperatures.

Based on the TEM characterization for the material obtained at
100 �C, it was possible to confirm that the ‘‘petals” are originated
by the joint between several nanocrystals, which are able to aggre-
gate to form the flower-like microcrystals. Fig. 7(a) shows the pres-
ence of some nanoparticles attached in a common crystallographic



Table 4
Comparative results between the morphological features (crystal size and shape) and optical band gap energy (Egap) of CuWO4 crystals heat treated at different temperatures for
1 h obtained in the present study with those reported in the literature.

M Crystal shape Average crystal size (nm)*/(lm) j T(�C) t(h) Egap (eV) Ref.

HC Hollow microspheres 300* 180 18 2.3 [99]
SC-C Nanoparticles 30–50* 500 2 3.2 [100]
HC/CC Nanoflakes 30*/1j 450 1 2.22 [101]
MI-C Nanoparticles 31.8* 500 2 – [102]
P-C Nanoparticles 50* 500 1 2.0 [103]
SC-C Petal-like/Flower-like 860*/4.68j 100 1 2.45 z
SC-C Flakes-like/Rough stones-like 7.664*/3.32j 200 1 2.28 z
SC-C Flakes-like/Rough stones-like 11*/6.653j 300 1 2.34 z
SC-C Flakes-like/Rough stones-like 18.3*/9.313j 400 1 2.24 z
SC-C Flakes-like 41* 500 1 2.19 z

M =method; T = temperature; t = time; HC = Hydrothermal conventional; SC-C = Sonochemistry-Calcination; CC = Chemical conversion; MI-C = Microwave irradiation-Cal-
cination; P-C = Polyol-Calcination; [z] = This work.

E.L.S. Souza et al. / Ultrasonics Sonochemistry 38 (2017) 256–270 263
orientation. HR-TEM image presented in Fig. 7(b) shows the pres-
ence of nanocrystals with diameters of approximately 4 nm. The
interplanar distance for these nanocrystals was estimated in
approximately 0.23 nm, which correspond to (200) crystallo-
graphic plane of CuWO4 triclinic phase, which is present in small
points locally in the hydrated global lattice. The selected area elec-
tron diffraction (SAED) image (inset in Fig. 7(b)) was indexed also
as CuWO4 phase. In this case, it is important to emphasize that
both TEM and SAED focused on some nanocrystals locally indicate
the CuWO4 phase, while X-ray patterns, due to be a long-range
technique, indicated the CuWO4�2H2O phase. The low resolution
TEM image for the sample obtained at 200 �C showed several
aggregated nanoparticles (Fig. 7(c)). SAED (not shown here) indi-
cated the presence of low intensity rings related to nanocrystalline
particles, which is supported by HR-TEM image in Fig. 7(d). The
materials heat treated at 300 �C are illustrated in Fig. 7(e, f). Again,
the micrographs revealed that the stone-like CuWO4 microcrystals
are clearly formed by randomly distributed aggregated nanoparti-
cles, as confirmed by low intensity rings in SAED (Inset in Fig. 7(f)).
HR-TEM image also revealed the same interplanar distance
(0.23 nm), as observed in Fig. 7(b), i.e., proving the CuWO4 triclinic
phase. Exactly, this same behavior was also identified for the mate-
rials obtained at 400 �C (Fig. 7(g, h)). For the material heat treated
at 500 �C, the existence of larger particles was identified, in which
some of them are well-faceted (high degree of crystallinity). TEM
images also showed these particles are agglomerated instead of
aggregated, presenting an interplanar distance of 0.31 nm related
to (�1�11) plane of CuWO4 triclinic phase. The other TEM and HR-
TEM images can support the explanations above as found in Sup-
port Information Figs. SI-3(a–j). All experimental results obtained
in this study are in good agreement with the literature and pre-
sented in Table 4 [99–103]. Therefore, TEM images provides some
advantages over SEM images, specially confirming that larger par-
ticles for samples obtained from 200 �C to 400 �C are composed of
smaller nanoparticles. On the other hand, the particles observed at
500 �C are single-crystalline. SAED agrees with XRD results, in
which the synthesis method allowed the formation of crystalline
nanoparticles of CuWO4.

3.7. Crystal growth mechanism

Fig. 8(a–k) shows a schematic representation of all stages
involved in the synthesis and growth of CuWO4 nanocrystals syn-
thesized by the SC method and heat treated at different tempera-
tures for 1 h.

Initially, there is the coulomb interaction between [Cu(H2O)6]2+

and WO2�
4 complex ions in aqueous solution, promoting the forma-

tion of first CuWO4�2H2O nuclei (Fig. 8(a, b)). These nuclei control
the kinetics of nucleation and growth of CuWO4 nanocrystals. In
the next growth stage (Fig. 8(c, d)), the crystals in the aqueous
medium are able to rotate and align to find a common crystallo-
graphic plane via self-assembly process. As this process is uncon-
trollable, there is the random and spontaneous aggregation of
nanocrystals, resulting in petal-like CuWO4�2H2O microcrystals
(Fig. 8(e)). After heat treatment performed at 100 �C for 1 h,
flower-like CuWO4�2H2Omicrocrystals are formed, which are com-
posed of several petal-like crystals (Fig. 8(f)). The initial stage of
elimination of water molecules in CuWO4�2H2O due to the heat
treatment temperature at 200 �C resulted in irregular stone-like
microcrystals (Fig. 8(g)). The progressive removal of these water
molecules in the lattice with the increase of heat treatment pro-
moted a phase transition from CuWO4�xH2O (monoclinic structure)
to CuWO4 crystals (triclinic structure). However, for temperatures
of 300 �C and 400 �C, the morphological aspect of irregular stone-
like microcrystals is maintained (Fig. 8(i, j)). Finally, when the
materials were heat treated at 500 �C, CuWO4 crystals grew by
means of nanocrystals. These results can be proved through FESEM,
TEM and HR-TEM images (Figs. 6(e) and 7(i, j)).

3.8. Uv–vis spectra and optical band gap energy

The optical band gap energy (Egap) was calculated using the
method proposed by Kubelka and Munk [105]. This methodology
is based on the transformation of diffuse reflectance measure-
ments to estimate Egap values of semiconductors with good accu-
racy [106]. The Kubelka–Munk Eq. (10) for any wavelength is
defined as:

FðR1Þ 	 ð1� R1Þ
2R1

¼ k
s

ð10Þ

where F(R1) is the Kubelka–Munk function or absolute reflectance
of the sample. In our case, magnesium oxide (MgO) was adopted as
standard in reflectance measurements. R1 = Rsample/RMgO, where R1
is the reflectance, k is the molar absorption coefficient, and s is the
scattering coefficient. The optical band gap and absorption coeffi-
cient of semiconductor oxides [107] can be calculated using the fol-
lowing Eq. (11):

ahm ¼ C1ðhv � EgapÞn ð11Þ
where a is the linear absorption coefficient of the material, hm is the
photon energy, C1 is a proportionality constant, and n is a constant
associated to the type of electronic transition (n = 0.5, 2, 1.5, and 3
for direct allowed, indirect allowed, direct forbidden, and indirect
forbidden transitions, respectively). According to Lalic et al. [72]
and Lacomba-Perale et al. [108] CuWO4 crystals have an optical
absorption spectrum governed by indirect electronic transitions



Fig. 7. TEM/HR-TEM images and SAED of CuWO4 crystals heat treated at different temperatures: (a, b) 100 �C, (c, d) 200 �C, (e, f) 300 �C, (g, h) 400 �C and (i, j) 500 �C for 1 h,
respectively.

264 E.L.S. Souza et al. / Ultrasonics Sonochemistry 38 (2017) 256–270



Fig. 7 (continued)

E.L.S. Souza et al. / Ultrasonics Sonochemistry 38 (2017) 256–270 265
between the valence band (VB) and conduction band (CB). After the
electronic absorption process, the electrons located in the
maximum-energy states in VB return indirectly to minimum-
energy states in CB (different points in the Brillouin zone) [109].
Based on this information, Egap values of CuWO4 crystals were cal-
culated using n = 2 in Eq. (11). Finally, using the remission function
described in Eq. (10) with the term k = 2a and C2 as a proportional-
ity constant, the modified Kubelka–Munk equation can be obtained,
as indicated by Eq. (12):

½FðR1Þhm�
1
2 ¼ C2ðhm� EgapÞ ð12Þ

By finding the F(R1) value from Eq. (12) and plotting [F(R1)
hm]1/2 against hm, the Egap value of CuWO4 crystals was determined.

Fig. 9(a–f) shows the UV–vis diffuse reflectance spectra of
CuWO4 crystals heat treated at (a) 100 �C, (b) 200 �C, (c) 300 �C,
(d) 400 �C, and (e) 500 �C for 1 h, (f) optical band gap values as a
function of temperature.

In this figure, a slight decrease in Egap values with the increase
in the heat treatment temperature was detected. This behavior,
for samples heat treated from 100 �C to 300 �C, is related to remo-
tion of water molecules (dehydration process) in the lattice, result-
ing in a phase transition from CuWO4�xH2O to CuWO4, modifying
the number and organization of intermediary energy levels
between the VB and CB (Fig. 9(a–c)). Moreover, the exponential
optical absorption profile as well as Egap are controlled by the
degree of structural order-disorder in the lattice [110]. For samples
heat treated at 400 �C and 500 �C, there is only single CuWO4 tri-
clinic phase; therefore, the decrease in Egap values (2.24 and
2.19 eV) can be explained by the presence of low symmetry and
distortions on both octahedral [CuO6] and [WO6] clusters in the
lattice. However, the contributions of electronic levels in these
crystals can be achieved only by means of theoretical calculations,
which will be perforated in a future study.
3.9. Photoluminescence properties

Fig. 10(a–e) illustrates PL spectra at room temperature of
CuWO4 crystals heat treated at different temperatures.

In the last years, the experimental results previosly reported in
the literature [26,53,70] have explained the key factors or variables
responsible for PL properties of CuWO4 crystals. Pourmortazavi
et al. [26,70] and, ReddyPrasad and Naidoo [53] described the PL
emission of these crystals with the electronic transitions within
complex structure of WO2�

4 anion molecular as well as by the trap-



Fig. 8. Schematic representation of the growth mechanism of CuWO4�2H2O and CuWO4 crystals obtained by the SC method: (a) reaction between complex ions; (b)
appearance of the first CuWO4�2H2O nuclei; (c) rotation and alignment of nanocrystals, sharing common crystallographic planes; (d) self-assemble process; (e) aggregation of
petal-like crystals; (f) formation of flower-like CuWO4�2H2O microcrystals; (g) crystal growth via heat treatment, (h) irregular stone-like CuWO4�xH2O microcrystals, (i)
stone-like CuWO4�xH2O microcrystals and flake-like CuWO4 nanocrystals; (j) formation of aggregated CuWO4 nanocrystals; (k) crystal growth of flake-like CuWO4

nanocrystals.

266 E.L.S. Souza et al. / Ultrasonics Sonochemistry 38 (2017) 256–270
ping and recombination of photoinduced electrons and holes in the
semiconductor. CuWO4 crystals published in these papers were
excited with wavelengths of 290 nm � 4.276 eV and
350 nm � 3.543 eV, exhibiting maximum PL emissions at 483 nm
and 455 nm, respectively. Particularly, all these explanations were
directly correlated to WO2�

4 groups (ions), but CuWO4 is a crys-
talline solid composed of interconnected (. . .[WO6]–[CuO6]–
[WO6]. . .) clusters (Fig. 3). Therefore, in our study, we assume the
distorted octahedral [WO6] clusters have an important role in the
electronic transitions involving the energy levels located between
the VB and CB. The existence of interconnections between octahe-
dral [WO6] and [CuO6] clusters in the triclinic lattice indicates that
any distortion caused on [WO6] clusters also promotes a slight
deformation on O–Cu–O bonds ([CuO6] clusters). As O–Cu–O bonds
have a covalent bond character than ionic, promoting a high degree
of distortion on them. This Jahn-Teller effect, along the z-axis, are
able to induce a symmetry break in the lattice, leading to the
appearance of intermediate levels within the band gap. In Fig. 10
(a), CuWO4�2H2O microcrystals have a medium PL emission at
456 nm (blue) with a narrower profile in relation to other samples.
Moreover, the increase of temperature at 200 �C promoted a
decrease and a displacement of the PL emission for 529 nm (green
region) (Fig. 10(b)) [38]. Therefore, these modifications in PL profile
for samples heat treated at 100 �C and 200 �C are related to pro-
gressive removal of water molecules in CuWO4�2H2O crystals.
The highest and widest PL spectrum was detected for the sample
heat treated at 300 �C (Fig. 10(c)). This optical feature can be due
to the effective contribution in electronic transitions between the
intermediary energy levels arising from both CuWO4�xH2O and
CuWO4 crystals. In this case, this mixture of phases leads to differ-
ent levels of short-range structural ordering in CuWO4 lattice,
resulting in a favorable condition for the formation of intermediate
levels responsible for recombination processes (electron/hole),
improving the PL behavior at room temperature. On the other
hand, a significant reduction or quenching in the PL intensity
was observed for single CuWO4 crystals, i.e., for samples heat trea-
ted at 400 �C and 500 �C, respectively (Fig. 10(d, e)). Also, it was
noted a change in the color of the powders from dark green to
black (digital photos in Fig. 10(a–e)). According to black body the-
ory [111], a hypothetical material (black) absorbs all the electro-
magnetic radiation. Therefore, the laser employed in the
excitation process was more absorbed and not reflected in PL mea-
surements of single CuWO4 crystals. In addition, this PL behavior
can be associated to other factors, such as: inhomogeneous crystal
size distribution, crystallographic orientation, and morphological
changes (see Supporting Information Figs. SI-2(a–i) and Fig. 3(a–j)).

4. Conclusions

In summary, CuWO4�2H2O and CuWO4 crystals were obtained
by the sonochemistry method, followed by heat treatment per-
formed at different temperatures (from 100 �C to 500 �C) for 1 h.
XRD patterns, Rietveld refinement, micro-Raman and FT-IR spec-
troscopies proved that CuWO4 crystals crystallize in a triclinic
structure with space group (P�1). XRD patterns revealed that the
temperature of 300 �C is responsible for a phase transition from
CuWO4�2H2O (monoclinic) to single CuWO4 (triclinic). M-Raman
spectra presented eighteen Raman-active vibrational modes for
CuWO4 nanocrystals, which are caused by external modes of dis-
torted octahedral [CuO6] clusters and internal modes ascribed to
distorted octahedral [WO6] clusters. These M-Raman spectra also
showed the existence of short-range structural ordering in these
crystals. FT-IR spectra detected eleven IR-active vibrational modes
for CuWO4 crystals, which are related to anti-symmetric, symmet-
ric stretch, interaction forces in a chain and symmetric bending,
which are present due to (O–Cu–O and O–W–O) bonds. FE-SEM
images showed a dependence between the morphological aspects
with the heat treatment temperature. In these micrographs, it



Fig. 9. UV–vis spectra of CuWO4 crystals synthesized by the SC-C method and heat treated at (a) 100 �C, (b) 200 �C, (c) 300 �C, (d) 400 �C and (e) 500 �C for 1 h, and (f) Egap as a
function of heat treatment temperature.

E.L.S. Souza et al. / Ultrasonics Sonochemistry 38 (2017) 256–270 267
was verified that CuWO4�2H2O crystals have irregular flower- and
stone-like shaped at 100 �C and 200 �C, respectively. On the other
hand, CuWO4 crystals exhibited only the stone-like shaped. All
these morphological shapes are composed of uncountable aggre-
gated nanocrystals, which grew by means of self-assembly process,
as observed by TEM and HR-TEM images. The decrease in Egap
values with the increase of the temperature was caused by the
reduction of intermediary energy levels between the VB and CB.
CuWO4 crystals heat treated at 300 �C for 1 h exhibited the highest
PL emission at room temperature. This behavior was favored due to
the effective participation of intermediary energy levels arising
from both CuWO4�2H2O and CuWO4 crystals.



Fig. 10. PL emission spectra at room temperature of CuWO4 crystals heat treated at
(a) 100 �C, (b) 200 �C, (c) 300 �C, (d) 400 �C and (e) 500 �C for 1 h, respectively. Insets
show the digital photos of the powders, which exhibit different colors, depending
on the heat treatment temperature. (For interpretation of the references to colour in
this figure legend, the reader is referred to the web version of this article.)

268 E.L.S. Souza et al. / Ultrasonics Sonochemistry 38 (2017) 256–270
Acknowledgements

The authors acknowledge the financial support of the Brazilian
research financing institutions: CNPq (479644/2012-8 and
304531/2013-8), FAPESP (2012/14004-5 and 2013/07296-2), and
CAPES-PNPD (1268069).
Appendix A. Supplementary data

Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.ultsonch.2017.03.
007.

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	Structural evolution, growth mechanism and photoluminescence properties of CuWO4 nanocrystals
	1 Introduction
	2 Experimental details
	2.1 Synthesis of CuWO4 crystals
	2.2 Characterizations of CuWO4 crystals

	3 Results and discussions
	3.1 Long-range structural analyses
	3.2 Structural representation, and coordination of clusters in CuWO4
	3.3 Short-range structural analysis
	3.4 FT-IR spectroscopy analyses
	3.5 FE-SEM images analyses
	3.6 TEM and HR-TEM images analyses
	3.7 Crystal growth mechanism
	3.8 Uv–vis spectra and optical band gap energy
	3.9 Photoluminescence properties

	4 Conclusions
	Acknowledgements
	Appendix A Supplementary data
	References