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Send Orders for Reprints to reprints@benthamscience.net 114
 
Journal of Photocatalysis, 2021, 2, 114-129 
 
RESEARCH ARTICLE 
Structure, Morphology Features and Photocatalytic Properties of α-
Ag2WO4 Nanocrystals-modified Palygorskite Clay 
 2665-9778/21 $65.00+.00 © 2021 Bentham Science Publishers 
Amanda Carolina Soares Jucá1, Francisco Henrique Pereira Lopes1, Herbert Vieira Silva-Júnior2, 
Lara Kelly Ribeiro Silva3, Elson Longo3, Júlio César Sczancoski3 and Laecio S. Cavalcante1,* 
1PPGQ-CCN-DQ-GERATEC, Universidade Estadual do Piauí, Rua: João Cabral, N. 2231, P.O. Box 381, CEP: 
64002-150, Teresina, PI, Brazil; 2PPGEM-IFPI, Instituto Federal do Piauí, CEP: 64.000-040, Teresina, PI, Brazil; 
3DQ-UFSCar, Universidade Federal de São Carlos, P.O. Box 676, CEP: 13565- 905, São Carlos, SP, Brazil 
 Abstract: Aims: In the present study, we investigate the photocatalytic properties of α-Ag2WO4 nano-
crystals-modified Palygorskite (PAL) clay synthesized by the impregnation method. The PAL clay 
was chemically purified and heat-treated (500 ºC for 2 h), which served as an excellent supporting 
matrix for loading α-Ag2WO4 (α-AWO) nanocrystals. 
Background: Water contamination is one of the most serious problems affecting human health, eco-
system survival, and the economic growth of societies. Industrial effluents, such as textile dyes, when 
not treated and improperly discharged into water resources are considered the main cause of water 
pollution. Thus the scientific community has been developing effective remediation technologies 
based on advanced oxidative processes to reduce the harmful effects of these organic pollutants. 
Objective: This study aimed to improve the photocatalytic activity of PAL clay with α-AWO nano-
crystals to degradation of Rhodamine B (RhB) dye. 
Methods: We purified and heat-treated the PAL clay, synthesized nanocrystals of α-AWO nanocrys-
tals and modified PAL clay with 30% α-AWO nanocrystals by the impregnation method. The modi-
fied PAL clay was able to improve RhB dye degradation. The materials were characterized by XRD, 
RAMAN, FE-SEM, FT-IR, XRF, etc. The samples were used as photocatalysts under UV-C lamps for 
the degradation of RhB dye in order to analyze their catalytic performances. 
Results: The PAL clay modified with 30% α-AWO nanocrystals showed a catalytic efficiency of 
79%, and degradation kinetics about 16 times higher when compared to PAL-500 only purified and 
heat-treated at 500 ºC. In this way, this PAL-modified is an alternative as a low-cost photocatalyst for 
the degradation of RhB dye. 
Conclusion: Ultraviolet-visible spectra revealed that our materials have optical band gap energies 
controlled by indirect and direct electronic transitions and suitable to be activated under ultraviolet il-
lumination. The adequate amount (30 wt.%) of α-Ag2WO4 nanocrystals added to PAL brought signif-
icant improvement in the photocatalytic activity for the degradation of rhodamine B. Finally, a photo-
catalytic mechanism was proposed in detail. 
A R T I C L E H I S T O R Y 
Received: October 20, 2020 
Revised: December 24, 2020 
Accepted: January 05, 2021 
 
DOI: 
10.2174/2665976X02666210210163001 
Keywords: Palygorskite, α-Ag2WO4 nanocrystals, optical band gap, photocatalysis, ultraviolet-visible, rhodamine B. 
1. INTRODUCTION 
Currently, the energy crisis and environmental degrada-
tion have become two major challenges for governmental 
authorities, scientific communities, and ecological entities. 
From the point of view of environmental pollution, water 
contamination is one of the most serious problems affecting 
 
*Address correspondence to this author at the PPGQ-CCN-DQ-
GERATEC, Universidade Estadual do Piauí, Rua: João Cabral, N. 2231, 
P.O. Box 381, CEP: 64002-150, Teresina, PI, Brazil; Tel: +55 86 3213-
7942; Cell: +55 86 99940−9556; E-mail: laeciosc@gmail.com 
human health, ecosystem survival and the economic growth 
of societies [1-3]. Generally, the untreated industrial efflu-
ents inappropriately dumped into water resources are con-
sidered the main aggressors. Among the different types of 
hazardous effluents found in polluted waters, the synthetic 
organic dyes employed in specific industries (textiles, tan-
neries, and pharmaceuticals) have a high contamination po-
tential because of their undesirable features such as high 
toxicity, carcinogenicity, d mutagenic action, and being 
non-biodegradable [4-6]. 
Structure, Morphology Features and Photocatalytic Properties Journal of Photocatalysis, 2021, Vol. 2, No. 2 115 
Over the years, research, motivated to change this criti-
cal scenario, developed effective remediation technologies 
based on biological, physical and chemical phenomena to 
reduce the deleterious effects of these organic pollutants [7, 
11]. Among these different approaches, heterogeneous pho-
tocatalysis is very well-accepted in the scientific community 
because of its simplicity, versatility and environmental-
friendly nature [12, 13]. In general, photocatalysis is de-
scribed as a photon-activated reaction triggered in the pres-
ence of a light-sensitive semiconductor (photocatalyst) [14]. 
The milestone in the field of photocatalysis was the research 
published in 1972 by Fujishima and Honda [15], which em-
ployed n-type TiO2 electrodes for the production of hydro-
gen via photocatalytic water splitting. This pioneer study 
inspired the discovery and development of new metal oxide-
based photocatalysts [16-19]. 
From a practical point of view, the success of any mate-
rial chosen as photocatalyst is directly related to some spe-
cific features as crystal structure, suitable bandgap for light 
absorption, high yielding of charge separation, surface-
related adsorption properties, surface area, porosity, chemi-
cal stability and reuse cycles [20, 21]. Among the wide vari-
ety of photocatalysts found in the literature, silver-based 
metal oxides have demonstrated an excellent photocatalytic 
performance for the degradation of different organic dyes 
[22-24]. Particularly, the silver tungstate nanocrystals (α-
Ag2WO4) (referred to as α-AWO nanocrystals) are a prom-
ising candidate for this purpose. This α phase is the most 
thermodynamically stable polymorphic form of this tung-
state, crystallizing in an orthorhombic structure with space 
group Pn2n and presenting an optical band gap of approxi-
mately 3.1 eV [25-27]. Moreover, this material has a strong 
tendency towards anisotropic growth of one-dimensional 
structures [28, 29]. 
Within the knowledge available about heterogeneous 
photocatalysis, several studies point out that clay minerals 
act as solid supports to immobilize the particles and improve 
the photocatalytic performance of metal oxide-based photo-
catalysts [30-32]. A well-studied support is the palygorskite 
(PAL) clay, a hydrated magnesium aluminum silicate min-
eral whose chemical composition normally varies within 
certain limits according to the geographical region of extrac-
tion. This clay is scientifically attractive because of its high 
adsorption capacity for organic compounds, large specific 
area, and excellent chemical stability [33-35]. On the other 
hand, the rhodamine B (RhB) dye is a compound organic 
that has been widely used as printing and dyeing in textile, 
paper, paints, leathers, etc. However, its high water solubili-
ty makes it potentially dangerous due to its disposal in an 
aquatic environment, which can lead to cause serious envi-
ronmental and biological problems as carcinogens and toxic. 
In this way, the removal of organic dye from water is a great 
challenge and a pressing task. The convention methods for 
the removal of RhB dye contain biochemical and physical-
chemical methods, such as liquid membrane, ozonation, and 
adsorption, which are expensive and not very effective. An 
alternative and widely used path is heterogeneous photoca-
talysis due to the presentpromising application for organic 
dye decomposition with superior activity. 
Therefore, in the present study, the photocatalytic poten-
tial of α-Ag2WO4 nanocrystals-modified PAL clay was ex-
plored for the degradation of rhodamine B (RhB) dye under 
ultraviolet (UV) light. Moreover, both structural and mor-
phological properties were explored in detail. Taking into 
account our experimental data, a photocatalytic reaction 
mechanism was proposed for this system. 
2. EXPERIMENTAL DETAILS 
2.1. Preparation of PAL Clay 
The natural PAL clay was extracted from the “Grotão do 
Angico” mine by “Mineradora Indústria Coimbra de Miné-
rio Ltda” located at “Guadalupe-PI” (northeast Brazil) [36, 
37]. Initially, PAL was deagglomerated with mortar/pistil, 
and then passed through a 325-mesh (45 μm) sieve. In order 
to remove the organic matter, the clay was treated with 25 
mL of hydrogen peroxide (Dinâmica®; 35%) diluted in 250 
mL of deionized water at room temperature for 24 h [38]. 
Successive washes with deionized water by means of cen-
trifugation were performed to eliminate some impurities. 
Thereafter, the final product was dried in a lab oven at 70 ºC 
for 24 h. After this chemical purification stage, PAL was 
again deagglomerated and heat-treated at 500 ºC for 2 h in a 
muffle furnace using a heating rate of 10 ºC.min-1. This 
sample was referred to as PAL-500 clay. 
2.2. Synthesis of αα-Ag2WO4 Nanocrystals 
The α-AWO nanocrystals were synthesized by the con-
trolled precipitation (CP) method. Silver nitrate (AgNO3) 
(99.8%, Sigma-Aldrich), citric acid (C6H8O7) (99.5%, Sig-
ma-Aldrich), sodium tungstate dihydrate (Na2WO4.2H2O) 
(99%, Sigma-Aldrich) and ammonium hydroxide (NH4OH) 
(99%, Synth) were chosen as starting chemical precursors 
and used without further purification. For the preparation of 
silver citrate solution, AgNO3 (2×10
-3 mol) was dissolved in 
100 mL of deionized water contained in a beaker, which 
was posteriorly added dropwise to C6H8O7 (2.2×10
-4 mol). 
The pH was adjusted close to 8 with NH4OH. In another 
beaker, Na2WO4.2H2O (1.0×10
-3 mol) was dissolved in 200 
mL of deionized water. Both the systems were maintained at 
room temperature under vigorous stirring for 10 min. Final-
ly, silver citrate complexes anion [ 2Ag(C6H5O7)
-] was 
dripped into the solution containing Na(aq)
+ �and WO4 (aq)
2-
�ions, 
which was maintained at 60 ºC for 3 h until the formation of 
precipitates. All chemical reactions responsible for the for-
mation of α-AWO nanocrystals are described in the follow-
ing equations: 
���������
���
��� ��
� � ��������
� � (1) 
 
���������
����������������� ��
�
��� ������ ��
� +�� ��
� (2) 
 
������� �������
���
��� ��
� ��������
�� � ���� (3) 
 
��� ������ ��
� � ��� ��
� � ���� ��
� ���� ��
�� � ����
���
�� � ������ ���� ��
� � ���� ��
� � ���� (4) 
116 Journal of Photocatalysis, 2021, Vol. 2, No. 2 Jucá et al. 
Finally, the solid precipitates were separated from the 
liquid phase via centrifugation, washed several times with 
deionized water and acetone, and dried in a lab oven at 65 
°C for 10 h. 
2.3. Preparation of α-Ag2WO4 Nanocrystals-modified 
PAL-500 Clay 
The PAL-500 modified with AWO nanocrystals were 
prepared synthesized using the impregnation method [39]. 
Firstly, 2 g of purified PAL-500 clay was modified with 30 
wt.% of α-AWO nanocrystals. This PAL-500 containing α-
AWO nanocrystals was placed in a beaker containing 15 mL 
of deionized water and 20 mL of acetone (99%, Synth®). 
This mixture was subjected to an ultrasonic bath (model 
1510, Branson, USA) on a frequency of 42 kHz for 1 h. Fi-
nally, the final product was dried in a lab oven at 70 ºC for 
12 h. This sample was referred to as PAL-500 clay/30% α-
AWO nanocrystals. 
2.4. Characterizations 
The structural behavior of all samples was analyzed by 
means of X-ray diffraction (XRD) by using a 
DMax/2500PC diffractometer (Rigaku®, Japan) with CuKα 
radiation (λ = 0.154184 nm). Data were collected over 2θ 
ranging from 10º to 70º, employing an angular step of 0.02º 
and a scanning rate of 0.02º.min-1, respectively. Fourier-
transform infrared (FT-IR) spectra were acquired on a 
VERTEX 70V spectrophotometer (Bruker®, Germany) and 
operated in transmittance mode. Raman spectra were rec-
orded on a Raman scattering microscope (Bruker Optik®, 
Germany) equipped with a laser of 532 nm and operated at 2 
mW. The morphological aspects were analyzed by using a 
field-emission scanning electron microscopy (FE-SEM) 
with a Supra 35-VP microscope (Carl Zeiss®, Germany) 
operated at 5 kV. The elemental chemical composition was 
checked on an Epsilon 3X X-ray fluorescence spectrometer 
(PANalytical®, Netherlands) operated at 50 kV and 3 mA. 
Ultraviolet-visible (UV-Vis) spectra were recorded on a 
UV-2600 spectrophotometer (Shimadzu®, Japan) operated 
in diffuse-reflection mode. 
2.5. Photocatalytic Activity Measurements 
The photocatalytic activity of our samples was investi-
gated by using UV irradiation and RhB solution as a pollu-
tant. In these tests, 50 mg of each catalyst was dispersed in 
50 mL of RhB solution (1×10-5 mol/L and pH = 4) with an 
ultrasonic bath (1510 model, Branson®, USA) for 5 min. 
Before illumination, the suspensions were maintained in the 
dark for 30 min to establish the adsorption-desorption equi-
librium between dye and photocatalyst. In this condition, the 
first aliquot (3 mL) was collected and referred to as time 
zero. Afterwards, the suspensions were placed inside the 
photocatalytic reactor equipped with four ultraviolet lamps 
(UV-C Moran Light®, 15 W and λ = 254 nm) and controlled 
temperature at 20 ºC. At predetermined intervals, aliquots of 
3mL were collected and centrifuged (8,000 rpm for 10 min) 
to separate the precipitates from the RhB solution. The ab-
sorption band maximum of the aliquots wascmonitored by 
using a UV-2600 double-beam spectrophotometer (Shimad-
zu®, Japan). 
3. RESULTS AND DISCUSSION 
3.1. Structural Behavior at Long-range 
XRD patterns were employed to identify all phases pre-
sent in our samples, as shown in Figs. 1(a–c). 
As can be observed in Fig. 1(a), even with thermal 
treatment, the purified PAL-500 clay presented a mixture of 
five different crystallographic phases. Besides the majority 
phase ([(Mg0.669Al0.331)4(Si4O10)2(OH)2(H2O)8] - monoclinic 
structure; ICSD No. 75974) [40], XRD patterns revealed the 
coexistence of keatite phase (SiO2 - tetragonal structure; 
ICSD No. 34889) [41], cordierite phase ([(Mg1.91Fe0.09) 
Al4Si5O18] - orthorhombic structure; ICSD No. 100489) 
[42], enstatite (Fe3Mg7Si5O18 - orthorhombic structure; 
ICSD No. 16971) [43] and pyrolusite (β-MnO2-tetragonal 
structure; ICSD No. 393) [44]. These additional phases are 
arising from the extraction region of PAL-500 clay (“Grotão 
do Angico” mine), which naturally contains a rich concen-
tration of these oxides [45]. As expected, XRD peaks corre-
sponding to α-AWO nanocrystals were perfectly indexed to 
orthorhombic structure with space group Pn2n (Inorganic 
Crystal Structure Database ICSD No. 14000) [46] (Fig. 1b). 
In addition, typical reflections of crystal planes of both α-
AWO nanocrystals and PAL-500 clay (with the respective 
additional phases) were detected in PAL-500 clay/30% α-
AWO nanocrystals (Fig. 1c). This experimental information 
provides good evidence that the α-AWO nanocrystals can 
be randomly distributed on the surface of PAL-500 clay. 
The structural representation of each of these oxides is 
shown in Topic 3.5. When XRD patterns are merged, it is 
possible to notice small displacements when compared to 
the standard of PAL-500 and α-AWO nanocrystals. This 
behavior is probably due to the incorporation of nanocrys-
tals in the interlayer spacing of PAL clay (as shown in Sup-
plementary data in Fig. 1-S1). 
3.2. Unit Cell Representation and the Symmetry, Geom-
etry, and Coordination of the Clusters inPAL-500 Clay 
and αα-Ag2WO4 Nanocrystals 
Figs. 2(a–f) show the schematic representations for: (a) 
monoclinic [(Mg0.669Al0.331)4(Si4O10)2(OH)2(H2O)8] unit cell, 
(b) tetragonal SiO2 unit cell, (c) orthorhombic [(Mg1.91Fe0.09) 
Al4Si5O18] unit cell, (d) orthorhombic (Fe3Mg7Si5O18) unit 
cell, (e) tetragonal β-MnO2 unit cell, and (f) orthorhombic 
α-Ag2WO4 unit cell, respectively. 
These unit cells shown in Figs. 2(a–f) were modeled 
through the visualization system for electronic and structural 
analysis (VESTA) program (version 3.5.2 for 64-bit version 
of Windows 7 [47, 48]. As can be seen in Fig. 2(a), the main 
and majority crystalline structure for the heat-treated PAL-
500 clay is composed of monoclinic [(Mg0.669Al0.331)4 
(Si4O10)2(OH)2(H2O)8] unit cells. In this unit cells, we have 
the presence of some water molecules (H2O) in two forms: 
the first is ascribed to H2O molecules coordinated to the 
cations formed by the distorted octahedral [MgO6]/[AlO6] 
clusters with the symmetry group (Oh) and the second is 
related to H2O molecules of the zeolitic-type present in the 
channels in which it interacts both with the H2O molecule 
coordinated with the distorted tetrahedral [SiO4] clusters 
with the symmetry group (Td). In addition, we have the 
presence of some hydroxyls (OH) groups linked to the 
Structure, Morphology Features and Photocatalytic Properties Journal of Photocatalysis, 2021, Vol. 2, No. 2 117 
 
Fig. (1). XRD patterns of (a) PAL-500 clay, (b) α-Ag2WO4 nanocrystals, (c) PAL-500 clay /30% Ag2WO4. The vertical lines indicate the 
positions and intensities found in Inorganic Crystal Structures Database (ICSD) cards, such as: | ICSD Nº. 75974 for 
[(Mg0.669Al0.331)4(Si4O10)2(OH)2(H2O)8]; | ICSD Nº. 34889 for SiO2, | ICSD Nº. 100489 for [(Mg1.91Fe0.09)Al4Si5O18], | ICSD Nº. 16971 for 
(Fe3Mg7Si5O18), | ICSD Nº. 393 for (β-MnO2) and | ICSD Nº. 4165 for (α-Ag2WO4). (A higher resolution / colour version of this figure is 
available in the electronic copy of the article). 
118 Journal of Photocatalysis, 2021, Vol. 2, No. 2 Jucá et al. 
 
 
 
 
 
 
 
Fig. (2). contd…. 
 
Structure, Morphology Features and Photocatalytic Properties Journal of Photocatalysis, 2021, Vol. 2, No. 2 119 
 
 
 
 
 
Fig. (2). Schematic representations for (a) monoclinic [(Mg0.669Al0.331)4(Si4O10)2(OH)2(H2O)8] unit cell, (b) tetragonal SiO2 unit cell, (c) 
orthorhombic [(Mg1.91Fe0.09)Al4Si5O18] unit cell, (d) orthorhombic (Fe3Mg7Si5O18) unit cell, (e) tetragonal β-MnO2 unit cell, and (f) ortho-
rhombic α-Ag2WO4 unit cell, respectively. (A higher resolution / colour version of this figure is available in the electronic copy of the article). 
PAL-500 clay structure at the center of the distorted octahe-
dral [MgO6]/[AlO6] clusters in good agreement with the 
literature [40, 49, 50]. It is possible to observe in greater 
detail that the atoms of Mg and Al are located in the same 
atomic occupation site, with approximately 33% referred to 
Al atoms and 67% related to Mg atoms (Fig. 2a). According 
to several works reported in the literature [51-55], in the 
structure of palygorskite clay, there is a general consensus 
that OH groups and H2O molecules are part of the coordina-
tion. Although, the positions of the H atoms cannot be accu-
rately determined from the powder X-ray data (Rietveld 
refinement method), because the dispersion efficiency of the 
H atoms is low for X-rays, even for higher quality materials 
and using powerful diffractometers. Atomic positions for H 
atoms are accurate using neutron diffraction experiments, 
even if the sample is not very crystalline, as is generally the 
120 Journal of Photocatalysis, 2021, Vol. 2, No. 2 Jucá et al. 
case with clays and some minerals [53]. Fig. 2(b) displays 
the tetragonal unit cell for SiO2 related to Keatite as a sili-
cate mineral [56], which is composed of several polyhe-
drons interconnected related to distorted tetrahedral [SiO4] 
clusters with the symmetry group (Td). Moreover, when 
observing this tetragonal network in more detail along the x-
axis, it is possible to observe three-dimensional tunnels or 
channels that are formed through the connection between 
five distorted tetrahedral [SiO4] clusters [57]. The presence 
of a third mineral in the PAL-500 clay is ascribed to Cordi-
erite with an orthorhombic structure and [(Mg1.91Fe0.09) 
Al4Si5O18] unit cell as shown in Fig. 2(c). This phase of 
magnesium iron aluminosilicate is formed by two types of 
coordination polyhedra, where the lattice formers are consti-
tuted by distorted tetrahedral [SiO4]/[AlO4] clusters [58-60], 
while the lattices modifiers Fe and Mg atoms are located in 
the same atomic occupation site and are composed by the 
distorted octahedral [FeO6]/[MgO6] clusters with the sym-
metry group (Oh) [61, 62]. Fig. 2(d) displays the orthorhom-
bic (Fe3Mg7Si5O18) unit cell for the fourth mineral present in 
the PAL-500 clay is assigned to Enstatite, which this mag-
nesium iron catena-silicate phase is composed of two types 
of coordination polyhedra, where the lattice formers are 
only formed by distorted tetrahedral [SiO4] clusters [63], 
while the lattices modifiers Fe and Mg atoms are located in 
the same atomic occupation site forming distorted octahe-
dral [FeO6]/[MgO6] clusters [64]. Fig. 2(e) displays the unit 
cell for the fifth mineral presenting in a minor percentage in 
the PAL-500 clay and is related to Pyrolusite with a rutile-
type tetragonal structure, which is formed only by one type 
of coordination polyhedra attributed to distorted octahedral 
[MnO6] clusters for beta (β) phase from MnO2 crystal [65]. 
Fig. 2(f) illustrates that the orthorhombic α unit cell, 
where all the W (W1, W2, and W3) atoms are coordinated 
to only six O atoms, thus forming distorted octahedral 
[WO6] clusters in the lattice with the symmetry group (Oh). 
The differences in the O−W−O bond lengths and angles can 
lead to different degrees of distortion or intrinsic or-
der−disorder in this type of the lattice. Moreover, the Ag 
atoms in these unit cells can exhibit four types of cluster 
coordination. The Ag1 and Ag2 atoms are coordinated to 
seven O atoms, forming distorted deltahedral [AgO7] clus-
ters, which are distorted pentagonal bipyramidal polyhedra 
with the symmetry group (D5h). The Ag3 atoms are bonded 
to six O atoms, which form distorted octahedral [AgO6] 
clusters with the symmetry group (Oh). The Ag4 and Ag5 
atoms are coordinated to four O atoms, forming distorted 
tetrahedral [AgO4] clusters with the symmetry group (Td). 
Also, it was observed that the Ag4 atoms form distorted 
tetrahedral [AgO4] clusters more often than the Ag5 atoms 
(Fig. 2f), whereas the Ag6 atoms bond to two O atoms to 
form angular [AgO2] clusters with the symmetry group (C2v) 
and an O−Ag−O bond angle of 170.5o [66]. 
3.3. Structural Behavior at Short-range Analyses 
Fig. 3 illustrates the Raman spectra, presenting the re-
spective active vibrational modes, of PAL-500 clay, α-
AWO nanocrystals and PAL-500/30% α-AWO nanocrys-
tals. 
According to literature [67], the Palygoskite has 54 Ra-
man-active modes (Ag and Bg). The Ag modes are related to 
crystal parallel polarization, while the Bg modes are ob-
served in the cross-polarization. As can be noted in Fig. 
3(a), the Raman spectrum of PAL-500 revealed only two 
Raman modes. The first Bg mode at 142 cm
-1 is dominated 
by sheet shear around Mg sites, while the other Bg mode at 
around 541 cm-1 is ascribed to (Mg–O) stretching vibrations 
in [MgO6] octahedra. Other vibrational bands in this clay 
were not identified in our Raman spectrum, which can be 
hidden by the background fluorescence arising from the 
laser employed in these measurements [68-70]. 
 
 
Fig. (3). Raman spectra of (a) PAL-500 clay, (b) α-Ag2WO4 nano-
crystals and (c) PAL-500 clay /30% Ag2WO4 nanocrystals. The 
dotted vertical lines denote the respective positions of Raman-
active modes. (A higher resolution / colour version of this figureis 
available in the electronic copy of the article). 
According to a study [71], the α-phase for AWO crystals 
with orthorhombic structure exhibit space group (Pn2n) and 
symmetry point group (���
��). These crystals present 21 Ra-
man-active modes (6 A1g, 5 A2g, 5 B1g, and 5 B2g). The pres-
ence of seven of these vibrational modes (1 A1g, 1 A2g, 2 
B1g, and 3 B2g) was recorded in our spectrum (Fig. 3b). The-
se active Raman modes can exhibit a variable intensity due 
to the rotation of x-, y- and z-axes in α-AWO crystals occur-
ring in the different scattering of the tensors and compo-
nents. The most intense A1g mode at 898 cm
-1 is ascribed to 
(←O←W→O→) symmetric stretching vibrations in distort-
ed octahedral [WO6] clusters [72]. The second A2g mode at 
around 800 cm-1 is related to (�O�W�O�W�O�; 
O�W�O�) antisymmetric stretching vibrations in distorted 
octahedral [WO6]–[WO6] clusters [73]. The B1g mode locat-
ed at 664 cm-1 is associated with (W–O–O–W) stretching 
vibrations bonds in distorted octahedral [WO6]–[WO6] clus-
ters. Another B1g mode found at 515 cm
-1 is arising from 
out-of-plane wagging vibrations due to (W–O–O–W) 
asymmetrical stretching vibrations. The B2g mode at 334 
cm-1 is caused by (�O�W�O�) bending vibrations in dis-
torted octahedral [WO6] clusters [74]. Other B1g and B1g 
modes at around 260 and 91 cm-1 are described as external 
Structure, Morphology Features and Photocatalytic Properties Journal of Photocatalysis, 2021, Vol. 2, No. 2 121 
modes of the lattice because the movements of [(AgOy) y = 
2, 4, 6, and 7)] clusters [75]. 
In relation to PAL-500 clay/30% α-AWO nanocrystals, 
the Raman spectrum showed the predominance of signals 
ascribed to vibrational bands of α-AWO nanocrystals. Such 
feature can be arising from the complexity structural exhib-
ited by the PAL-500 clay as well as a direct response to the 
distribution of α-AWO nanocrystals on the surface of this 
clay [68]. 
To corroborate with the Raman data, FT-IR measure-
ments were also executed. FT-IR spectra of PAL-500 clay, 
α-AWO nanocrystals and PAL-500 clay/30% α-AWO 
nanocrystals are shown in Fig. 4. 
 
 
Fig. (4). FT-IR spectra of (a) PAL-500, (b) α-Ag2WO4 nanocrys-
tals and (c) PAL-500 clay /30% α-Ag2WO4 nanocrystals. The dot-
ted vertical lines symbolize the respective positions of IR-active 
modes. (A higher resolution / colour version of this figure is available 
in the electronic copy of the article). 
For PAL-500 clay (Fig. 4a), the absorption bands locat-
ed at 3.709 cm-1 and 3.080 cm-1 are arising from (Mg-OH) 
group and (O–H) stretching vibrations of adsorbed H2O, 
respectively. The band centered at around 1629 cm-1 is due 
to coordinated water and absorbed water molecules. The 
intense vibrational band at around 1.032 cm-1 is assigned to
(Si–O–Si) stretching vibrations in [SiO4] tetrahedra. The 
low intensity band at 876 cm-1 is assigned to (M–OH) 
stretching vibrations (M = Al, Fe and/or Mg) [76-80]. 
The main absorption bands of α-AWO nanocrystals 
were detected in the range from 866 and 806 cm-1 (Fig. 4b). 
The two A1u modes related to (←O←W←O←; 
→O→W→O→) anti-symmetric stretching vibrations in 
distorted octahedral [WO6] clusters were confirmed at 866 
and 806 cm-1 [68, 77-79]. The absorption bands detected at 
750 cm-1 and 677 cm-1 are dominated by (O–W–O) and (O–
W–O–W–O) bonds, respectively. The band identified at 633 
cm-1 corresponds to bridging O atoms in (�O�W��
O�W�O�) asymmetric stretching vibrations [81-83]. 
As was a probable result, the respective absorption 
bands of PAL-500 clay and α-AWO nanocrystals were also 
identified in PAL-500 clay/30% α-AWO nanocrystals, pre-
senting slight shifts in their respective positions (Fig. 4c). 
This information reinforces the concept a possible interac-
tion between the α-AWO nanocrystals with silanol groups 
at the external surface of PAL-500 clay occurs [84-86]. 
3.4. XRF Elemental Analysis 
A semi-quantitative chemical analysis of the samples 
was performed by using XRF spectrometry (Figs. 5a-c) 
[87]. 
According to XRF spectrometry data, the PAL-500 clay 
is chemically composed of the following elements (decreas-
ing order): silicon (Si = 65.28%), magnesium (Mg = 
16.53%), iron (Fe = 10.90%), aluminum (Al = 7.07%), and 
manganese (Mn = 0.22%) (Fig. 5a). These results are in 
good agreement with other published papers [88, 89]. The 
chemical composition of α-AWO nanocrystals exhibited the 
presence of silver (Ag = 51.92%) and tungsten (W = 
48.08%) (Fig. 5b). For PAL-500 clay/30% α-AWO nano-
crystals, there is a predominance of elements belonging to 
PAL-500 clay (Si = 54.76%, Mg = 13.10%, Fe = 7.34%, Al 
= 5.25%, and Mn = 0.14%) than the α-AWO nanocrystals 
(Ag = 10.43% and W = 8.98%). This evidence implies that 
the desired concentration between both materials was suc-
cessfully obtained by impregnation method. 
3.5. Morphological Features Analyses 
The morphological aspects of all samples were analyzed 
via FE-SEM images (Fig. 6a–c). 
As well-reported in the literature [90, 91], the natural 
PAL clay is a system characterized by several one-
dimensional and elongated microfibers. In our case, these 
microfibers remained even after heat treatment performed at 
500 ºC for 2 h (Fig. 6a). FE-SEM images of α-AWO nano-
crystals revealed a largely agglomerated arrangement (dis-
ordered assembly), the vast majority presenting irregularly-
shaped quasi-spherical and rod-like nanocrystals, with aver-
age sizes of approximately 80 nm (Fig. 6b) [25, 83]. Alt-
hough α-AWO nanocrystals have been synthesized in a re-
action medium containing C6H8O7, this chelating agent was 
able to prevent the particle-particle interactions in order to 
acquire a stabilized system [92]. FE-SEM images of PAL-
500 /30% α-AWO nanocrystals proved that the irregularly-
shaped quasi-spherical and rod-like α-AWO nanocrystals 
were dispersed homogeneously on the surface of PAL-500 
clay microfibers (Fig. 6c). 
It is important to highlight that the FE-SEM images were 
also acquired for other PAL-500/x Ag2WO4 (x = 10, 20 and 
40 wt.%) as shown in Fig. 2(a–c)-SI (Supplementary Data-
SD). An interesting consideration is that the amount of α-
AWO nanocrystals has a significant impact on the disper-
sion level in PAL-500 clay microfibers. Particularly, when 
the α-AWO nanocrystals were added up to 40 wt.%, a non-
homogeneous system was again perceived, resulting in the 
appearance of isolated groups composed of several α-AWO 
nanocrystals in the agglomerated state. 
122 Journal of Photocatalysis, 2021, Vol. 2, No. 2 Jucá et al. 
 
Fig. (5). XRF spectrometry data of (a) PAL-500 clay, (b) α-
Ag2WO4 nanocrystals and (c) PAL-500 clay /30% α-Ag2WO4 
nanocrystals. (A higher resolution / colour version of this figure is 
available in the electronic copy of the article). 
3.6. UV-Vis Spectra Analyses 
The optical band gap energy (Egap) values were calculat-
ed by the Kubelka-Munk theory [93], which is based on the 
transformation of diffuse reflectance measurements to esti-
mate Egap values with good accuracy. Particularly, it is used 
in limited cases of infinitely thick samples. The Kubelka–
Munk method for any wavelength is described by: 
 � �� �
����
�
���
≅ �
�
������ (5) 
where F(R∞) is the Kubelka−Munk function or absolute 
reflectance of the sample, R∞ is the reflectance, k is the mo-
lar absorption coefficient and s is the scattering coefficient. 
In our case, BaSO4 was adopted as the standard sample in 
reflectance measurements (R∞ = RSample/RBaSO4). In a para-
bolic band structure, the optical band gap and absorption 
coefficient of semiconductor oxides [94] can be calculated 
by: 
��� � ����� � �����
��� (6) 
where α is the linear absorption coefficient of the material, 
hν is the photon energy, C1 is a proportionality constant, Egap 
is the optical band gap and n is a constant associated with 
different kindsof electronic transitions (n = 0.5 for direct 
allowed, n = 2 for indirect allowed, n = 1.5 for direct forbid-
den, and n = 3 for indirect forbidden transitions). From theo-
retical calculations, α-AWO nanocrystals have an optical 
absorption spectrum governed by direct electronic transi-
tions between the valence band and the conduction band 
[95]. Thus, Egap values of our samples were estimated by 
using n = 2 in Eq. (7). 
The absolute reflectance function described in equation 
(6) with k = 2α in modified Kubelka-Munk equation is given 
by [96]: 
�� �� ���
� � �� �� � ���� �� (7) 
Hence, F(R∞) value can be obtained from Eq. (8) and 
plotting a graph of [F(R∞)hν]2 against hν. The Egap values for 
PAL-500 clay, α-AWO nanocrystals and PAL-500 
clay/30% α-AWO nanocrystals were calculated by extrapo-
lating the linear portion of UV-Vis diffuse reflectance spec-
tra curves (Fig. 7a–c). 
According to some studies [88, 97], the natural Palygor-
skite has a typical Egap in the range from 3.38 to 3.8 eV. 
Based on our UV-Vis results, the Egap of PAL-500 clay was 
estimated at around 2.26 eV (Fig. 7a). This value suggests 
the contribution of energy levels in the bandgap arising from 
other oxides, (Mg,Al)2Si4O10(OH).4H2O, SiO2, 
(Mg1.7Fe0.3Al4)Si5O18, β-MnO2 and (Fe3Mg7)SiO3, as shown 
previously in XRD patterns (Fig. 1a). An Egap of 3.17 eV 
was verified for AWO nanocrystals, which is in perfect 
agreement with other published studies [25, 85, 98] (Fig. 
7b). When compared to PAL-500 clay, the PAL-500 
clay/30% α-AWO nanocrystals presented a slight increase 
in Egap (2.46 eV), indicating synergic participation between 
the energy levels of both materials. 
3.7. Photocatalytic Activity Analyses 
In order to explore the photocatalytic effectiveness of the 
samples, especially of PAL-500 clay/30% α-AWO nano-
crystals, the tests were performed under UV light by adopt-
ing the RhB as target pollutant. Initially, insignificant pho-
tolysis of 8% (Error Bars ± 0.0002) was noted for RhB after 
exposure to UV illumination for 40 min. On the other hand, 
when the system (photocatalyst and RhB solution) was 
maintained in the dark for 30 min to reach the adsorp-
tion/desorption equilibrium, a pronounced degradation rate 
was detected (∼ 40%). Moreover, the reaction kinetic data 
indicated that the degradation process proceeds following a 
pseudo-first order rate law (Fig. 8a). As was expected, PAL-
500 clay showed a poor photocatalytic response (Fig. 8b) 
[88]; however, a curious behavior was the minor photocata-
lytic activity of 79% (Error Bars ± 0.001667) for PAL-500 
clay/30% α-AWO nanocrystals in relation to pure α-AWO 
nanocrystals of 100% (Error Bars ± 0.045) (Fig. 8c). This 
result denotes that the increase in concentration of α-AWO 
nanocrystals photocatalyst plays a key role in providing 
more active sites in PAL-500 clay (actuating as natural sup-
Structure, Morphology Features and Photocatalytic Properties Journal of Photocatalysis, 2021, Vol. 2, No. 2 123 
port) [99]. This hypothesis is consistent with other data dis-
played in Fig. 3(a–c)-SI (Supplementary data), taking into 
account that the maxim limit of good dispersion of AWO 
nanocrystals in PAL-500 clay reached up to 30 wt.% (Fig. 
5(c) in XRF analysis). 
 
 
 
Fig. (6). FE-SEM images of (a) PAL-500 clay, (b) Ag2WO4 nano-
crystals and (c) PAL-500/30% α-Ag2WO4 nanocrystals. (A higher 
resolution / colour version of this figure is available in the electronic 
copy of the article). 
 
Fig. (7). UV-Vis spectra of (a) PAL-500 clay, (b) α-Ag2WO4 
nanocrystals and (c) PAL-500 clay/30% α-Ag2WO4 nanocrystals. 
Based on the Langmuir–Hinshelwood kinetic model, the 
following equation was employed to describe the reaction 
order [100]: 
124 Journal of Photocatalysis, 2021, Vol. 2, No. 2 Jucá et al. 
���
��
��
� ����� (8) 
where k is the reaction rate constant, t is the time, Cn and C0 
are the concentrations at time t and 0 of the organic dye, 
respectively. 
 
 
 
Fig. (8). (a) Photocatalytic activity, (b) photocatalytic efficiency 
and, (c) pseudo-first-order kinetics for degradation of RhB dye 
under UV-C illumination with and without the photocatalysts. (A 
higher resolution / colour version of this figure is available in the elec-
tronic copy of the article). 
The k values (min-1) of all samples were graphically de-
termined from the slope of – ln(Ct/C0) as a function of the 
illumination time. Thus, the performance efficiency was 
ranked in decreasing order as: k(AWO nanocrystals) = 
3.83×10-2 min-1 > k(PAL-500/30% AWO nanocrystals) = 
1.20×10-2 min-1 > k(PAL-500 clay) = 7.53×10-4 min-1 > 
k(Photolysis) = 6.86×10-4 min-1. This result proves that the 
PAL-500 clay/30% α-AWO nanocrystals yielded an im-
proved photocatalytic performance for the degradation of 
RhB solution compared to other samples (Figs. 8c and Figs. 
8c and SI3). 
3.8. Proposed Photocatalysis Mechanism 
Based on our experimental data a photocatalytic mecha-
nism was proposed for PAL-500 clay/30% α-AWO nano-
crystals (Fig. 9). 
The PAL-500 clay has inactive catalytic sites, serving as 
a natural support for impregnation of α-AWO nanocrystals. 
The presence of distortions on [(AgOy) y = 2, 4, 6, and 7)] 
and [WO6] clusters provokes the polarization of α-AWO 
nanocrystals lattice, leading to the appearance of intermedi-
ary energy levels in the bandgap. When exposure to UV 
illumination, electronic transitions involving the participa-
tion of energy levels arising from connections between or-
dered (ord) and disordered (dis) clusters are stimulated 
(formation of photogenerated e´/h• pairs). The effective 
charge separation process and low recombination rate be-
tween photogenerated e´/h• pairs are responsible for the pho-
tocatalytic efficiency. These e´/h• pairs play an important 
role in the production of hydroxyl radicals (O��), superox-
ide radicals (��
� ), and hydroperoxyl radical (���
�) radicals, 
the most oxidizing species in chemical reactions for the deg-
radation or mineralization of organic dyes [26, 101, 102]. 
Thus, the photodegradation of RhB molecules by using the 
PAL-500 clay/30% α-AWO nanocrystals as photocatalyst 
under UV light can be well-described by the following 
equations: 
����������
 ����������������� � �� � �• ����������������� �� �������������
• � 
 (9) 
����������
 
����α � ���������������� � ��� � �� � �• ����������������� �� �������������
• (10) 
����������
��� dye → ���* dye (11) 
��� � �������������
• �� �������������
• ����������� (12) 
�������������
� � ���������� �������������
� � �������
� � ������
� (13) 
�������������
� � �������
� � �������������
� � �������
� (14) 
�������������
� � ������� �� �������������
� � �������
� (15) 
�������
� � ������
� �� ��������
� (16) 
���������
� �� ������� � ��������� (17) 
��������� � ��������
� � �������
� � �������
� � �������� (18) 
����������
���* dye + �������� � �������� � ��������� � CO2(g) + H2O(l) (19) 
In our photocatalytic reaction where the surface of the 
catalyst can polarize the lattice and lead to possible electron-
ic transitions at the molecular level between disordered 
Structure, Morphology Features and Photocatalytic Properties Journal of Photocatalysis, 2021, Vol. 2, No. 2 125 
[clusters����
� and ordered [clusters����
′ . When UV−light is 
absorbed by the catalyst, the following electronics process 
of charge transfer between between species involved can 
occur, as expressed in equations (10,11). After, this process 
the disordered [clusters����
� in the VB react with the ad-
sorbed H2O and/or RhB dyes, while the ordered 
ters����
′ located in theCB interact with the adsorbed oxygen 
(O2) species. Moreover, the semiconductor surface is able to 
react with the O2(ads) molecules by means of the electron 
transference process. Before this process, the H� species in 
equations (12–16) are able to interact with the superoxide 
radical anion (��
� ) resulting in the formation of perhydroxyl 
radical (���
�) presented in equations (17–19). Finally, after 
several photooxidation cycles (100 min), the discoloration 
of RhB* dyes by O��, ��
� and ���
� radicals occurs, as in-
dicated by equations (20). Moreover, according to the litera-
ture [103, 104], a high photodegradation rate of cationic dye 
can be achieved using visible/irradiation light, activated 
semiconductor, presence of H2O2 and scavengers, such as: 
dimethyl sulphoxide (DMSO), pbenzoquinone (BQ), tri- 
ethanolamine (TEOA) and potassium dichromate (PD). 
However, these experimental tests will be reported in a fu-
ture paper. 
CONCLUSION 
In summary, PAL-500 clay/30% α-AWO nanocrystals 
were successfully prepared by the impregnation method. 
The purified PAL-500 clay revealed other crystalline com-
pounds in its composition as a consequence of the geograph-
ic region of the clay-content soil. On the other hand, the α-
AWO nanocrystals crystallized in a single orthorhombic 
phase. The coexistence of both PAL-500 clay and α-AWO 
nanocrystals in PAL-500 clay/30% α-AWO nanocrystals 
was perfectly identified by means of XRD patterns. Such a 
combination of these materials was confirmed by the re-
spective vibrational modes distinguished in Raman and FT-
IR spectra. All our samples exhibited Egap values in the 
range from 2.26 to 3.17 eV, which can be easily activated 
by UV light. The impregnation with 30wt.% of irregularly-
shaped quasi-spherical and rod-like α-AWO nanocrystals on 
the surface of PAL-500 clay improved the photocatalytic 
activity for the degradation of RhB in relation to PAL-500 
clay. Therefore, there is a synergic interaction between the 
active sites of α-AWO nanocrystals with the high adsorp-
tion capacity of PAL-500 clay for organic compounds (i.e., 
RhB molecules). 
CONSENT FOR PUBLICATION 
Not applicable. 
AVAILABILITY OF DATA AND MATERIALS 
Not applicable. 
FUNDING 
This work was supported by Conselho Nacional de 
Desenvolvimento Científico e Tecnológico (CNPq) 
(408036/2018-4). 
CONFLICT OF INTEREST 
The authors declare no conflict of interest, financial or 
otherwise. 
ACKNOWLEDGEMENTS 
The authors are grateful to CAPES, CETEM, GER-
ATEC-UESPI, FAPESP (2013/07296-2; 2012/14004-5; 
408036/2018-4) for the financial support. 
SUPPLEMENTARY MATERIAL 
 Supplementary material is available on the publisher’s 
web site along with the published article. 
Fig. (9). A proposed photocatalytic mechanism for degradation of RhB dye by using PAL-500 clay/30% α-Ag2WO4 nanocrystals under UV-
C illumination. (A higher resolution / colour version of this figure is available in the electronic copy of the article). 
126 Journal of Photocatalysis, 2021, Vol. 2, No. 2 Jucá et al. 
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	Structure, Morphology Features and Photocatalytic Properties of α-Ag2WO4 Nanocrystals-modified Palygorskite Clay
	Abstract: Aims
	Background:
	Objective:
	Methods:
	Results:
	Conclusion:
	Keywords:
	1. INTRODUCTION
	2. EXPERIMENTAL DETAILS
	3. RESULTS AND DISCUSSION
	Fig. (1).
	Fig. (2).
	Fig. (3).
	Fig. (4).
	Fig. (5).
	Fig. (6).
	Fig. (7).
	Fig. (8).
	Fig. (9).
	CONCLUSION
	CONSENT FOR PUBLICATION
	AVAILABILITY OF DATA AND MATERIALS
	FUNDING
	CONFLICT OF INTEREST
	ACKNOWLEDGEMENTS
	SUPPLEMENTARY MATERIAL
	REFERENCES