Method for Visible Light-Induced Photocatalytic Degradation of Methylparaben in Water Using Nanostructured Ag/AgBr@m-WO3

Article abstract of DOI:10.1111/php.13118

An efficient method of photocatalytic degradation of methylparaben in water using Ag nanoparticles (NPs) loaded AgBr-mesoporous-WO₃ composite photocatalyst (Ag/AgBr@m-WO₃), under visible light is presented. In this process, quantific

Full text of DOI:10.1111/php.13118

Article type : Method Article
METHOD ARTICLE
Method for Visible Light Induced Photocatalytic Degradation of Methylparaben in
Water Using Nanostructured Ag/AgBr@m-WO
3
a
b*
b
c
M. A. Suliman , M. A. Gondal , M. A. Dastageer , Gaik-Khuan Chuah ,
a
C. Basheer
a
Department of Chemistry, King Fahd University of Petroleum and Minerals, P.O Box 5047,
Dhahran 31261, Saudi Arabia
b
Laser Research Group, Department of Physics, King Fahd University of Petroleum and
Minerals, P.O Box 5047, Dhahran 31261, Saudi Arabia
c
Department of Chemistry, National University of Singapore, Science Drive 2, Kent Ridge,
Singapore 117543
Corresponding author’s email: magondal@kfupm.edu.sa (Mohammed Gondal)
Abstract
An efficient method of photocatalytic degradation of methylparaben in water using Ag
nanoparticles (NPs) loaded AgBr-mesoporous-WO composite photocatalyst (Ag/AgBr@m-
3
3
WO ), under visible light is presented. In this process, quantification of methylparaben in
water was carried out by high-performance liquid chromatography (HPLC) and the HPLC
results showed a significant reduction of methylparaben in water due to the enhanced of
3
photo-catalytic degradation efficiency of Ag/AgBr@m-WO . For the material synthesis,
highly ordered mesoporous-WO (m-WO ) was initially synthesized by sol-gel method and
3
3
AgBr nanoparticles (NPs) were subsequently introduced in the pores of m-WO , and finally,
3
the Ag nanoparticles were introduced by light irradiation. The enhanced photocatalytic
degradation of methylparaben in water is attributed to the formation of surface plasmonic
This article has been accepted for publication and undergone full peer review but has not
been through the copyediting, typesetting, pagination and proofreading process, which may
lead to differences between this version and the Version of Record. Please cite this article as
doi: 10.1111/php.13118
This article is protected by copyright. All rights reserved.

resonance (SPR) due to the introduction of Ag NPs on the surface of the catalyst. Also, the
formation of heterojunction between AgBr and mesoporous-WO in Ag/AgBr@m-WO
3
3
significantly inhibited the recombination of light-induced electron-hole pairs in the
semiconductor composite. The morphological and optical characterizations of the synthesized
3 2
photocatalysts (Ag/AgBr@m-WO ) were carried out using SEM, TEM, XDR, N adsorption-
desorption, UV-VIS diffuse reflectance spectroscopy (DRS). Also, the photocatalytic studies
•-
using radical scavengers were carried out and the results indicated that O
2
is the main
reactive species.
Keywords:
Heterogeneous Photo-catalysis, Surface Plasmon Resonance, Photo-degradation,
3
Nanocomposites, Ag/AgBr@-m-WO , Methylparaben
INTRODUCTION
Application of heterogeneous photocatalysis in water treatment has been increasingly
popular, due to its inherent simplicity, low operational cost, mild experimental conditions
(
temperature and pressure) and complete mineralization of the pollutants without any residual
secondary pollution. Ever since Fujishima and Honda (1) reported water splitting using TiO2
electrode, many pure, doped and composite variants of TiO have been synthesized and used
for various photocatalytic and solar energy applications due to many positive attributes of
TiO , like low cost, physical and chemical stability and high efficiency. However, one of the
major constraints of TiO as a photocatalyst is its wide band gap energy, that limits its
activity in the UV spectral region, and hence it cannot be a good photocatalyst that can be
used to harness the abundant solar radiation. Also, TiO in particular shows low adsorption
on the hydrophobic contaminants when used as photocatalyst for the purification of water. In
addition to numerous variants of doped and composite TiO , many other semiconductor
have been used for various
2
2
2
2
2
2 3 2 2 3 2 3
photocatalysts like CdS, SnO , WO , SiO , ZnO, Nb O , Fe O
photocatalytic applications.
This article is protected by copyright. All rights reserved.

The nanostructured WO
photocatalyst due to its narrow band gap energy (between 2.4 and 2.8 eV), in addition to its
stable physicochemical properties, and resistance to photo corrosions (2) (3). However, the
inherent band edges of WO are not compatible with the oxidation and reduction potential,
3
has been proven to be an excellent visible-light-responsive
,
3
that favors the chemical reactions that lead to the elimination of organic substances from
water. In order to overcome this problem, and at the same time harnessing other positive
attributes of WO , many metals loaded WO like Pd/WO (4) Pt/WO (5),(6) and its
3
3
3
,
3
,
composite forms like CuO/WO
3
(7)
,
and CaFe
2
4 3
O /WO (8) were used as a photocatalyst for
the degradation of organic contaminants from water.
It is well-known that due to surface plasmonic resonance (SPR) effect, the noble metal (Au
and Ag) decorated semiconductor materials exhibit a significant enhancement of light
absorption in the wide spectral range and this breakthrough opened up a new field of
plasmonic photocatalysis (9–11). In SPR, when the incident light radiation of appropriate
wavelength falls on the metal surface, the coherent oscillations of electrons in the noble metal
set in, and the interaction between these electronic oscillations with the incident light
promotes the light absorption due to resonance effect. Another advantage of plasmonic
photocatalysis is the enhancement of the desired charge separation, promoted by the
formation of Schottky junction between the metal and semiconductor and the internal electric
field in the junction (12),(13). Another positive feature that can be used for photocatalysis is
the mesoporosity of the material and many metal oxides semiconductors with ordered
mesoporous structures have shown increased photocatalytic activity in many applications
(
14) (15). This is due to the increased specific surface area in the mesoporous materials and
,
its positive effect on the creation of more active sites and the enhancement of the adsorption
of reactant molecules on the semiconductor surface (16). Also, ordered mesoporous channels
promote quick intraparticle molecular charge transfer, and thereby prolong the lifetime of the
This article is protected by copyright. All rights reserved.

separated electron-hole pairs by impeding their recombination, which also further helps the
improvement of photocatalytic activity (17–19). Ordered mesoporous WO has been
3
extensively used for gas sensing application, and its increased affinity with gas molecules,
and also due to increased surface area, but only a few studies have been performed for photo-
catalytic applications (20–23).
The enhancement of the photocatalytic activity due to the increased visible light absorption
brought about by the surface plasmonic effect led to the applications of many silver/silver
halide (Ag/AgX, X= Cl, Br, I) nanomaterials for the photocatalytic degradation of organic
pollutants. In this line, silver bromide (AgBr) as such, is proven to be an excellent
photoactive material, as it readily generates electron-hole pair when the visible radiation falls
on it. The photo-generated electrons combine with interstitial silver ions to form silver atom
and this makes AgBr quite unstable in its pure crystal structure (24). The following chemical
equations explain the above process:
ꢀ
ꢁ
AgBr + hv → AgBr (h + e )
(1)
(2)
ꢁ
ꢃ
ꢀ
e + Ag → Ag (Ag : interstitial silver ions)
ꢂ
ꢄ
AgBr → Ag + ꢅ Brꢅ
(3)
However, many reports have confirmed that the stability of AgBr can be improved and the
consequent enhancement of photocatalytic activity can be achieved by dispersing it with
certain support materials like in the form of composite materials like Ag/AgCl/WO
Ag@AgBr/Bi WO (26), Ag@AgBr/g-C (27), Ag–AgBr@hierarchical hollow silica
spheres (Ag–AgBr@HHSS) (28), AgBr/WO (29) Ag/AgBr/WO .3H O (30), mesoporous
Ag/WO (31), and Ag-WO /SBA-15 (32),which were reported to have an excellent visible
3
(25),
2
6
3 4
N
3
,
3
2
3
3
light photocatalytic activity for various applications. It was reported in some studies that the
This article is protected by copyright. All rights reserved.

3
deposition of a trace amount of silver on the surface of AgBr@m-WO has affected the
photocatalytic activity of AgBr@-m-WO (4, 33).
3
In the present work, the mesoporous WO
was used as a support material for AgBr to form AgBr@m-WO
atomic Ag was introduced in the AgBr@m-WO composite by photo-reduction to produce
Ag/AgBr@m-WO as a final product (Fig. 1).
3
was initially synthesized by sol-gel method and it
3
composite and subsequently,
3
3
3
The Ag/AgBr@m-WO (with three different Ag content) were used as photocatalysts under
visible light radiation for the photocatalytic removal of methylparaben in water and this
photocatalytic removal was found to be quite substantial. Parabens have been used as
preservatives in personal care products, pharmaceutical, cosmetic, and food stuff for a long
time, due to their ability to prevent bacterial and fungal growth. Although parabens have been
extensively used in most of the personal care products, the recent studies have shown that
they have some adverse health effect by disrupting the endocrine system by mimicking
certain hormones in the body. Particularly, parabens mimic the activity of estrogen in the
body cell, which is largely associated with breast cancer. Also, there are a few reports about
the ecotoxicity of trace level of paraben in water (in ng/L to µg/L level), particularly about its
adverse effects on various microorganism due to continuous discharge into the aquatic
,
environment (4) (34). Parabens are considered as “pseudo-persistent” pollutants as a result of
their continuous ejection into the water bodies and this became a cause for eco-toxicological
concerns (35–37).
The motivations to use Ag/AgBr@m-WO as photocatalyst for this photocatalytic
3
degradation process are (i) the enhancement of visible absorption due to surface plasmonic
effect brought about by the introduction of atomic Ag (ii) the reduction of charge
This article is protected by copyright. All rights reserved.

recombination due to the formation of metal-semiconductor Schottky junction and also the
presence of mesoporosity in WO and (iii) the enhancement of active sites due to the
3
increased surface area due mesoporosity. Morphological and optical characterizations of the
3
synthesized Ag/AgBr@m-WO were also carried out to explain the observed enhancement in
the photocatalytic degradation of methylparaben in water.
EXPERIMENTAL SECTION
Synthesis of mesoporous WO
3
catalyst. A measure of 1.0 g of Pluronic P-127 (Sigma-Aldrich)
was dissolved in 18 ml of ethanol by stirring for 30 minutes at room temperature and to this
solution, 0.01 mol of WCl
6
(Sigma-Aldrich) was added. After vigorous stirring for 2 hours,
o
the resulting sol solution was left to gel in an open Petri dish at 40 C overnight in which,
WCl
6
hydrolyzes and polymerizes into a tungsten oxide network. In order to remove the
o
surfactant species, the sample was calcined at 450 C for 6 hours in air at a rate of 2.0
o
C/minute.
3 3
Synthesis of Ag/AgBr@m-WO . A measure of 0.25 g of m-WO (prepared above) and 0.12 g
of Cetyltrimethylammonium bromide (CTAB) (Sigma-Aldrich) were added into 100 ml of
distilled water and after stirring the mixture for 30 minutes at room temperature, 1.0 ml of
3
AgNO (0.1008 M) was added and stirred for a further 20 minutes. Subsequently, the above
product was irradiated using visible light from a solar simulator (150 W xenon lamp) for
different duration in order to produce catalysts with different contents of Ag through the
+
0
photo-reduction of Ag ions in AgBr into Ag nanoparticles. The catalyst was washed with
o
o
distilled water, dried at 80 C for 24 hours in the dark and finally, calcined in air at 300 C for
0
3
3
hours to obtain the Ag/AgBr@m-WO catalyst with different contents of metallic Ag
nanoparticles in it.
This article is protected by copyright. All rights reserved.

Material characterization. The phase and crystallinity of the synthesized materials were
investigated by X-ray diffraction (XRD, Siemens D5005) equipped with Cu anode and
variable slits. The morphology and the microstructure of the synthesized materials were
studied by scanning electron microscopy (SEM), Energy dispersive spectroscopy (EDX) and
high-resolution transmission electron microscopy (HR-TEM) (JEM2100F, JEOL, operated at
200 KV). The diffuse reflectance spectra (DRS) were taken using a Shimadzu (UV-2450)
UV-Visible spectrophotometer, where the BaSO fine powder was used as a standard for the
4
baseline and the spectra were recorded in the 200 to 800 nm wavelength range. A
2
Micromeritics Tristar 3000 system was used to measure the N adsorption/desorption
isotherms of the samples. Prior to the measurement, the samples were degassed for 4 h at 300
°C for the composites. The surface area was calculated using the Brunauer-Emmett-Teller
o
(
BET) equation and the pore volume was calculated from P/P ~ 0.95.
Photocatalytic degradation of methylparaben under visible light. For the study of
photocatalytic degradation of methylparaben, 25 ml of 10 ppm methylparaben solution and
2
5 mg catalyst were taken in a beaker and the suspension was magnetically stirred for 15 min
in the dark to establish the adsorption-desorption equilibrium of methylparaben at the surface
of the catalyst. A sample was removed after the equilibration period and constituted time
zero. The suspension was irradiated at room temperature with a broadband of light source
(450 W Xe arc lamp) equipped with the UV cutoff filter to allow the wavelength of light
above to 400 nm, and aliquots were removed at regular time intervals, centrifuged at 13,500
rpm for 5 minutes and the suspension was filtered with a 0.22 µm filter to remove any
catalyst particles in the test solution. The concentration of methylparaben in the samples was
quantified using high-performance liquid chromatography (HPLC, (Prominence Modular
HPLC, Shimadzu Corporation, Japan) equipped with a photodiode array detector. A C18
column (250 x 4.6 mm, particle size 5 µm) was employed. The elution phase is the mixture of
This article is protected by copyright. All rights reserved.

methanol/water (70:30, v/v) phase at a flow rate of 1.0 mL/min and the injection volume was
0 µL. The retention time of methylparaben under these conditions was 4.0 min. For
scavenger studies, 1.0 mM of EDTA, isopropyl alcohol (IPA) and 1,4-benzoquinone (BQ)
2
+
•
were prepared for mechanistic studies for the holes (hvb ), hydroxide ( OH ) and superoxide
−
2
•
radicals (O
) respectively.
RESULTS AND DISCUSSION
Morphological characterization
XRD patterns of mesoporous WO and the Ag/AgBr-loaded WO samples are shown in Fig.
3
3
2
. The XRD pattern for m-WO
3
3
fits with that of monoclinic WO (PDF 01-072-0677).
Loading with Ag/AgBr did not change the crystal structure of the support although the
intensity of the WO peaks decreased with higher loadings of Ag/AgBr (Figs. 2b-d). At the
same time, three new peaks at 31.0 , 44.2
3
o
o
,
o
and 72.2 , appeared which can be assigned to face-
centered cubic AgBr (PDF: 06-0438). However, the XRD peaks for Ag nanoparticles are not
o
shown due to low amounts of the Ag nanoparticles and/or their small size as reported in
2 3 ,
earlier works of Ag/AgBr/TiO , AgBr/BiOBr and Ag/AgBr/WO nanocomposites (38) (39).
3
High-magnification scanning electron microscopy (SEM) images were taken of m-WO (Fig.
3
a) and Ag/AgBr@m-WO taken after 30 minutes of irradiation (Fig. 3b). The Ag/AgBr@m-
3
WO
3
composite consists of spherical particles, whose grain size cannot be approximately
estimated from the SEM. SEM images with higher resolution were not obtained as AgBr may
get decomposed by the high-energy electron beam. SEM-EDX elemental mappings (Figs. 3c-
f) confirm the presence of W, O, Br and Ag, respectively. On comparing Fig. 3(e) with 3(f),
This article is protected by copyright. All rights reserved.

we can observe that the distribution of Ag is higher than that of Br. This is because Ag is
present as Ag metal as well as AgBr on the surface of mesoporous WO . Fig. 3(f) also shows
3
that the distribution of Ag particles is homogeneous.
The mesoporous structure of the catalyst was further studied by HR-TEM (Fig. 4). The TEM
image of WO
3
shows rectangular plate-like structure (Fig. 4a) while the 30 min.
Ag/AgBr@m-WO shows a combination of oval and spherical shapes (Fig. 4b). The particle
3
sizes were in the range of 10 - 50 nm. The inter-planar distances determined from the high-
resolution image were found to be 0.362, 0.283 and 0.251 nm, which are consistent with the
3 ,
spacing of m-WO (200), AgBr (200),(25) (40) and Ag (111) planes, respectively. This
confirms the homogenous distribution of Ag/AgBr nanoparticles on the surface of mesoporous
WO . The specific surface and the pore volumes of m-WO and Ag/AgBr@m-WO are
3
3
3
summarized in Table 1.
Fig. 5 shows the typical deconvoluted XPS spectra of Ag and Br in 30 min Ag/AgBr@ m-
WO , which confirms the coexistence of Ag and Br in the surface of mesoporous WO
3
3
surface. The peak of Ag3d5/2 and Ag3d3/2 appeared at 367.3 and 373.8 eV, respectively. The
Ag 3d 3/2 peak resolved into 373.3 and 373.8 eV, and two peaks of 367.8 and 368.4
contributed to the peak Ag 3d5/2, respectively. According to Zhang et al (41), the peak at
0
3
73.8 and 367.8 eV were attributed to Ag and the separation of 6.0 eV which also indicates
3
the existence of metallic silver in the 30 min Ag/AgBr@m-WO (42). The peak of Ag 3d
+
which appeared at 373.3 and 367.7 eV belong to Ag , suggesting the existence of AgBr on
the photocatalyst (43)(44).
This article is protected by copyright. All rights reserved.

Optical characterization
In order to eliminate the effect of light scattering by a particle in the absorption spectra, the
diffuse reflectance spectra were transformed into Kubelka-Munk function (45–47). From Fig.
6
3 3
, it is quite clear that as compared to pure m-WO , Ag/AgBr@-m-WO shows enhanced
absorbance, particularly in the visible region of the spectrum. Moreover, Ag/AgBr@-m-WO
3
with 30 minutes of irradiation has much stronger absorption in the visible region, compared
to one with 15 minutes of irradiation. This indicates that Ag nanoparticles were effectively
deposited into the photocatalyst composite and the SPR of Ag nanoparticles in Ag/AgBr@m-
WO
shows the Tauc plots (49) for m-WO
the materials were determined and listed in Table 1. The band gap energy of m-WO
3
is responsible for the enhancement of the visible light absorption (48). The inset in Fig.
and Ag/AgBr@m-WO . The band gap energies of
is 2.6
6
3
3
3
eV, and giving allowance for the statistical error, the change of bandgap energy due to the
introduction of Ag/AgBr is not appreciable.
Photocatalytic degradation of methylparaben
3
The Ag/AgBr@m-WO composites with different contents of atomic Ag (due to different
photo- reduction time during synthesis) and pure m-WO were used as photocatalysts for the
3
degradation of methylparaben in water under visible radiation of wavelength λ ≥ 400 nm. As
can be seen from Fig. 7, the photocatalytic degradation of methylparaben over m-WO
3
is not
appreciable compared to the Ag/AgBr@m-WO composites. Ag/AgBr@m-WO with the
3
3
photoreduction time of 30 minutes (during synthesis) was the best performer with 85%
degradation of methylparaben after 180 minutes under visible light radiation.
This article is protected by copyright. All rights reserved.

This result indicates that the presence of Ag/AgBr plays a significant role in the
photocatalytic degradation of methylparaben. The degradation rate of 30 min Ag/AgBr@m-
3 3 o
WO was three times faster than the m-WO . From the linear plots of -ln(C/C ) vs irradiation
time, it can be deduced that the photodegradation of methylparaben follows pseudo-first
order kinetics (Fig. 8). The rate constants for the photodegradation can be obtained from the
slopes of linear plots and are listed in Table 1. The photocatalytic degradation of
methylparaben in water can be ranked in the following order: 30 min Ag/AgBr@m-WO > 45
3
3 3 3
min Ag/AgBr@m-WO > 15 min Ag/AgBr@m-WO > m-WO .
In Ag/AgBr@m-WO
atomic silver is smaller compared to Ag/AgBr@m-WO
Consequently, the SPR effect is reduced which leads up to less visible light absorption (Fig.
), and weaker photocatalytic degradation of methylparaben in water. On the other hand,
when the Ag content in Ag/AgBr@m-WO is beyond a certain limit, as for the sample with
5 minutes of photoreduction, a portion of the Ag nanoparticles become active sites that
3
with 15 minutes of photo-reduction (during synthesis) the content of
3
with 30 minutes of photo-reduction.
6
3
4
promote the undesirable electron-hole recombination and consequently the photocatalytic
activity is diminished. Also, a higher content of AgBr in m-WO will lead to the formation of
3
nanoclusters of Ag/AgBr species, which reduces the active sites on the surface of m-WO
3
,
through agglomeration of the particles (50) (51). The weight % of AgBr on 15 min, 30 min
3
and 45 min of Ag/AgBr@m-WO catalyst was 0.65 %, 0.81% and 1.0 % AgBr respectively.
In addition to the photocatalytic activity, the stability and the reusability of Ag/AgBr@m-
WO were investigated (Fig. 9). The photoactivity of the catalyst remained quite stable even
3
after five cycles of use.
This article is protected by copyright. All rights reserved.

3 3
The degradation rate of 30 min Ag/AgBr@m-WO was three times faster than the m-WO .
Th degradation rate follows pseudo-first order reaction which indicates that the ratio Ag and
AgBr were significant parameters on the rate of photodegradation. In order to quantify the
photocatalytic degradation of methylparaben in water with three different variants of
Ag/AgBr@m-WO
3
, we transformed the photocatalytic degradation curves in Fig. 7 in a linear
form (-ln (C/C ) vs. irradiation time) and are shown in Fig. 8. The photocatalytic degradation
0
of methylparaben follows the pseudo-first-order kinetic model and the slopes of the above
linear graphs represent the photocatalytic degradation rate constants, which are listed in Table
1
for all the catalysts used in this work. The photocatalytic degradation rate constant for
-1
3
methylparaben with 30 min Ag/AgBr@m-WO is 0.00991 min , which is much higher than
3
the same with m-WO . In addition to the photocatalytic activity, we examined the stability
and the reusability of Ag/AgBr@m-WO and the results are summarized in Fig. 9, which
3
shows that all the catalysts are quite stable even after five cycles of use with the
photocatalytic reaction time of 10 minutes for each cycle. The photoactivity of the catalyst
remained quite stable even after five cycles of use.
Mechanism of methylparaben degradation
Free radical and holes scavenging experiments. In order to understand the interaction
3
between Ag/AgBr and mesoporous WO and the active species generated during the
3
photocatalyzed degradation of methylparaben using Ag/AgBr (using m-WO ) and
+
Ag/AgBr@m-WO
3
, we carried out the holes (hvb ) scavenging and free radical experiments
and the results are in Fig. 10.
This article is protected by copyright. All rights reserved.

In this part of the study, EDTA, isopropyl alcohol (IPA) and 1,4-benzoquinone (BQ) were
+
•
used as holes (hvb ) scavenger, hydroxyl radical scavenger ( OH), and superoxide radical
•
-
•
scavenger (O
bring about any change in the photocatalytic degradation of methylparaben, while BQ (O
scavenger) and EDTA (holes scavenger) decreased the catalytic activity (4) (5). Generally,
the more the role of the reactive species, the more reduced would be the photoactivity. Hence,
2 ,
) (2) (3). The addition of isopropyl propyl alcohol ( OH scavenger) did not
•-
2
,
•
•-
on this basis, our results indicate that the OH radical has no active role and O was the main
2
active species in the degradation of methylparaben. As the photodegradation was also
+
suppressed by the addition of EDTA, it shows that holes (hvb ) also play a role. Knowing that
+
the holes (hvb ) mostly accumulate on the valence band of AgBr and it can oxidize bromide
ion to bromine atoms, which function as a reactive species to degrade methylparaben
(
52) (53). This explains the minor effect of the holes in the degradation of methylparaben.
,
Charge transfer mechanism. Fig. 11 describes the proposed charge transfer mechanism for
the photodegradation of methylparaben over the Ag/AgBr@m-WO composite catalyst. In
Ag/AgBr@m-WO , the charge separation is promoted by the formation of Schottky junction
3
3
between the metal and semiconductor and by the formation of a heterojunction between AgBr
and mesoporous-WO in Ag/AgBr@m-WO . These two routes of charge separation
3
3
effectively reduce the photo-generated electron-hole recombination and make them available
for efficient photocatalytic degradation of methylparaben. When Ag/AgBr@m-WO is
3
irradiated with visible light, photoexcited electron-hole pairs are generated in AgBr, m-WO3,
and plasmon excited Ag nanoparticles. The photo-generated electrons from Ag nanoparticles
•-
migrate to the conduction band of WO and convert O adsorbed on WO to O . At the same
3
2
3
2
+
time, the photo-generated holes (hvb ) are trapped by AgBr particles due to surface plasmon
state of silver nanoparticles (54) and this mechanism results in the efficient charge separation.
Another charge separation is mediated by the migration of photogenerated electron into the
This article is protected by copyright. All rights reserved.

+
metallic silver on the surface of WO
3
(55) and at the same time, holes (hvb ) from AgBr
-
0
oxidize Br to Br . In addition to these charge transfer mechanisms, methylparaben can
+
+
directly be oxidized by the plasmon-induced h (or Ag ) on the silver nanoparticles, and this
also accelerates the process of transforming photo-oxidized Ag nanoparticles back to their
3
initial state. Mesoporous WO provides more adsorption active sites for pollutant due to the
higher specific surface area. The photocatalytic studies using radical scavengers were carried
•-
out and the results indicated that O is the main reactive species. However, the role of AgBr
2
is to mediate the spatial charge separation of photo-excited electron-hole pairs and thereby
reducing the charge recombination and make more photo-generated charges available for the
•-
2
degradation process through the formation of O .
Major by-products of methylparaben and reaction pathways. To characterize photocatalytic
-1
by-products of this reaction, experiments were carried out at ambient conditions (1.0 mg L
-
3
of Ag/AgBr@m-WO as photocatalyst) using the stock solution of methylparaben (10 mg L
1
)
in the visible spectral region. Samples were withdrawn for analysis after 180 and 240
minutes of irradiation and were analyzed by LC-MS to identify the intermediates (Fig. 12).
In the current study, an effort was made to explain the mechanism and reaction pathways
through the by-products identification, formed during the photocatalytic treatment procedure
of methylparaben. Fig. 12 shows the suggested chemical formula and the value of detected
ion exact mass for identified products of the degradation, during the methylparaben
photocatalytic degradation process. Overall, five of the by-products was tentatively identified
from the screening of the treated samples, followed by the reaction pathway of the
methylparaben degradation in analyzed samples. As T. Velegraki et al (56) reported the
This article is protected by copyright. All rights reserved.

methylparaben degradation proceed by three different ways: (i) hydroxide radical attack on
the methylparaben, (ii) the reaction of methylparaben with photo-generated hole generated in
3
WO and (iii) direct oxidation of methylparaben by dissolved oxygen in water. During the
treatment, the methylparaben toxicity or at least part of it remains in the solution because
most of the by-products of methylparaben contain the core structure of the compound.
When the methyl group of methylparaben and hydrogen in the aromatic ring are attacked by
hydroxide radical, 4-hydroxybenzoic acid (1) and (2) dihydroxybenzoic acid are produced.
Also, the last product is further oxide to phenol (3). The formation of 1, 2-
benzenedicarboxylic acid bis-(2 methylpropyl) ester (4) and benzendicarboxylic acid (5)
could be attributed to coupling at high concentration (57). Furthermore, the formation of (4)
and (5) could be attributed to the addition of 3-methylbutanoic acid group at the aromatic ring
at C-2 and the di-hydroxylation and consequently addition of isobutene and methyl moieties
at methyl acetate group, respectively.
CONCLUSIONS
In this study, highly ordered mesoporous WO
subsequently, deposition-precipitation method was employed to synthesize AgBr@m-WO3.
Different levels of atomic silver were generated on the surface of AgBr@m-WO by
photoreduction method to get Ag@AgBr-m-WO . The synthesized material was used as a
photocatalyst for the degradation of methylparaben in water. It was observed that the
presence of Ag/AgBr on the surface of m-WO brought about a significant enhancement in
3
was synthesized by sol-gel method and
3
3
3
the photocatalytic performance of Ag/AgBr@m-WO nanocomposite. The enhancement of
3
photocatalytic degradation can be attributed to the SPR due to atomic Ag and consequent
enhancement of visible absorption, the reduced of charge recombination due to the formation
This article is protected by copyright. All rights reserved.

of metal-semiconductor Schottky junction and the enhancement of surface area due to
mesoporosity. Morphological and optical characterizations of the synthesized Ag/AgBr@m-
3
WO were also carried out to explain the efficiency of this photocatalytic material in the
process of removal of methylparaben in water.
Acknowledgements
We thankfully acknowledge the support of this work by KFUPM through the project #
NUS15109 and NUS 15110 under the Center of Excellence for Scientific Collaboration with
NUS. The support of Chemistry and Physics Departments of KFUPM is acknowledged.
References
1
. Fujishima, a and Honda, K. (1972) Electrochemical photolysis of water at a
semiconductor electrode. Nature 238, 37–38.
. Pan, C. and Zhu, Y. (2010) New Type of BiPO4 Oxy-acid salt photocatalyst with high
photocatalytic activity on degradation of dye. Environ. Sci. Technol. 44, 5570–5574.
. Chen, C., Wang, Q., Lei, P., Song, W., Ma, W. and Zhao, J. (2006) Photodegradation of
2
3
dye pollutants catalyzed by porous K3PW 12O40 under visible irradiation. Environ. Sci.
Technol. 40, 3965–3970.
4
. Li, G., Wong, K. H., Zhang, X., Hu, C., Yu, J. C., Chan, R. C. Y. and Wong, P. K. (2009)
Degradation of Acid Orange 7 using magnetic AgBr under visible light: The roles of
oxidizing species. Chemosphere 76, 1185–1191.
5
. Cao, J., Xu, B., Luo, B., Lin, H. and Chen, S. (2011) Preparation, characterization and
visible-light photocatalytic activity of AgI/AgCl/TiO2. Appl. Surf. Sci. 257, 7083–7089.
. Wang, P., Huang, B., Zhang, X., Qin, X., Jin, H., Dai, Y., Wang, Z., Wei, J., Zhan, J.,
6
Wang, S., Wang, J. and Whangbo, M. H. (2009) Highly efficient visible-light plasmonic
photocatalyst Ag@AgBr. Chem. - A Eur. J. 15, 1821–1824.
7
. Liu, H., Cao, W., Su, Y., Wang, Y. and Wang, X. (2012) Synthesis, characterization and
This article is protected by copyright. All rights reserved.

photocatalytic performance of novel visible-light-induced Ag/BiOI. Appl. Catal. B Environ.
111–112, 271–279.
8. Zang, Y. and Farnood, R. (2008) Photocatalytic activity of AgBr/TiO2 in water under
simulated sunlight irradiation. Appl. Catal. B Environ. 79, 334–340. 9. Wang, P., Huang, B.,
Zhang, Q., Zhang, X., Qin, X., Dai, Y., Zhan, J., Yu, J., Liu, H. and Lou, Z. (2010) Highly
efficient visible light plasmonic photocatalyst Ag@Ag(Br,I). Chem. - A Eur. J. 16, 10042–
1
1
0047.
0. Xiao, M., Jiang, R., Wang, F., Fang, C., Wang, J. and Yu, J. C. (2013) Plasmon-enhanced
chemical reactions. J. Mater. Chem. A 1, 5790.
1. Wang, H., Liu, Y., Li, M., Huang, H., Xu, H. M., Hong, R. J. and Shen, H. (2010)
1
Multifunctional TiO2 nanowires-modified nanoparticles bilayer film for 3D dye-sensitized
solar cells. Optoelectron. Adv. Mater. Rapid Commun. 4, 1166–1169.
1
2. Zhang, N., Liu, S. and Xu, Y.-J. (2012) Recent progress on metal core@semiconductor
shell nanocomposites as a promising type of photocatalyst. Nanoscale 4, 2227.
3. Linic, S., Christopher, P. and Ingram, D. B. (2011) Plasmonic-metal nanostructures for
efficient conversion of solar to chemical energy. Nat. Mater. 10, 911–921.
4. Tsung, C. K., Fan, J., Zheng, N., Shi, Q., Forman, A. J., Wang, J. and Stucky, G. D.
2008) A general route to diverse mesoporous metal oxide submicrospheres with highly
crystalline frameworks. Angew. Chemie Int. Ed. 47, 8682–8686.
https://doi.org/10.1002/anie.200802487.
5. Tang, J., Wu, Y., Mcfarland, W., Stucky, G. D., Lafayette, W. and May, A. (2004)
1
1
(
-
1
Synthesis and photocatalytic properties of highly crystalline and ordered mesoporous TiO
thin films. 1670–1671.
2
16. Li, L., Krissanasaeranee, M., Pattinson, S. W., Stefik, M., Wiesner, U., Steiner, U. and
3
Eder, D. (2010) Enhanced photocatalytic properties in well-ordered mesoporous WO . Chem.
Commun. (Camb). 46, 7620–2.
1
7. Chen, X. and Mao, S. S. (2007) Titanium dioxide nanomaterials: Synthesis, properties,
modifications and applications. Chem. Rev. 107, 2891–2959.
8. Carp, O., Huisman, C. L. and Reller, A. (2004) Photoinduced reactivity of titanium
dioxide. Prog. Solid State Chem. 32, 33–177.
9. Chen, X., Yu, T., Fan, X., Zhang, H., Li, Z., Ye, J. and Zou, Z. (2007) Enhanced activity
of mesoporous Nb2O5 for photocatalytic hydrogen production. Appl. Surf. Sci. 253, 8500–
506.
1
1
8
This article is protected by copyright. All rights reserved.

2
0. Qin, Y., Wang, F., Shen, W. and Hu, M. (2012) Mesoporous three-dimensional network
of crystalline WO nanowires for gas sensing application. J. Alloys Compd. 540, 21–26.
1. Yan, X., Zong, X., Qing, G., Lu, M. and Ã, L. W. (2013) Progress in Natural Scienceꢀ:
3
2
Materials International Ordered mesoporous tungsten oxide and titanium oxide composites
and their photocatalytic degradation behavior. Prog. Nat. Sci. Mater. Int. 22, 654–660.
2
2. Usami, Y., Hongo, T. and Yamazaki, A. (2012) Microporous and Mesoporous Materials
Phosphate constituent effects on the structure and photocatalytic properties of mesoporous
tungsten oxides. Microporous Mesoporous Mater. 158, 13–18.
23. Yan, X., Zong, X., Lu, G. Q. (Max) and Wang, L. (2012) Ordered mesoporous tungsten
oxide and titanium oxide composites and their photocatalytic degradation behavior. Prog.
Nat. Sci. Mater. Int. 22, 654–660.
2
2
4. Proudfoot, N. C. Handbook of photographic science and engineering.
5. Adhikari, R., Gyawali, G., Sekino, T. and Wohn Lee, S. (2013) Microwave assisted
hydrothermal synthesis of Ag/AgCl/WO3 photocatalyst and its photocatalytic activity under
simulated solar light. J. Solid State Chem. 197, 560–565.
2
6. Lin, S., Liu, L., Hu, J., Liang, Y. and Cui, W. (2015) Nano Ag@AgBr surface-sensitized
Bi WO photocatalyst: Oil-in-water synthesis and enhanced photocatalytic degradation. Appl.
Surf. Sci. 324, 20–29.
7. Yang, Y., Guo, W., Guo, Y., Zhao, Y., Yuan, X. and Guo, Y. (2014) Fabrication of Z-
2
6
2
scheme plasmonic photocatalyst Ag @ AgBr / g-C 3 N 4 with enhanced visible-light
photocatalytic activity. J. Hazard. Mater. 271, 150–159.
2
8. Zhang, J., Li, B. and Yang, W. (2014) One-pot synthesis of novel Ag–AgBr@HHSS with
superior visible photocatalytic activity. Microporous Mesoporous Mater. 194, 66–71.
9. Cao, J., Luo, B., Lin, H. and Chen, S. (2011) Photocatalytic activity of novel AgBr/WO
2
3
composite photocatalyst under visible light irradiation for methyl orange degradation. J.
Hazard. Mater. 190, 700–706.
3
0. Wang, P., Huang, B., Qin, X., Zhang, X., Dai, Y. and Whangbo, M. H. (2009)
Ag/AgBr/WO -H O: Visible-light photocatalyst for bacteria destruction. Inorg. Chem. 48,
3
2
1
0697–10702.
1. Sun, S., Wang, W., Zeng, S., Shang, M. and Zhang, L. (2010) Preparation of ordered
and its highly efficient degradation of acetaldehyde under visible-light
irradiation. J. Hazard. Mater. 178, 427–433.
2. Gondal, M. A., Suliman, M. A., Dastageer, M. A., Chuah, G.-K., Basheer, C., Yang, D.
3
mesoporous Ag/WO
3
3
and Suwaiyan, A. (2016) Visible Light Photocatalytic Degradation of Herbicide (Atrazine)
This article is protected by copyright. All rights reserved.

using Surface Plasmon Resonance Induced in Mesoporous Ag-WO3/SBA-15 Composite. J.
Mol. Catal. A Chem. 425, 208–216.
3
3. Zang, Y., Farnood, R. and Currie, J. (2009) Photocatalytic activities of AgBr/Y-zeolite in
water under visible light irradiation. Chem. Eng. Sci. 64, 2881–2886.
4. Gmurek, M., Bizukojć, M., Mosinger, J. and Ledakowicz, S. (2015) Application of
3
photoactive electrospun nanofiber materials with immobilized meso-tetraphenylporphyrin for
parabens photodegradation. Catal. Today 240, 160–167.
3
5. Terasaki, M., Makino, M. and Tatarazako, N. (2009) Acute toxicity of parabens and their
chlorinated by-products with Daphnia magna and Vibrio fischeri bioassays. J. Appl. Toxicol.
29, 242–247.
3
6. Dobbins, L. L., Usenko, S., Brain, R. A. and Brooks, B. W. (2009) Probabilistic
ecological hazard assessment of parabens using Daphnia magna and Pimephales promelas.
Environ. Toxicol. Chem. 28, 2744–53.
3
7. Fatta-Kassinos, D., Bester, K. and Kümmerer, K. (2013) Mass Flows, Environmental
Processes, Mitigation and Treatment Strategies ‘Xenobiotics in the Urban Water Cycle’.
2
8. Zhang, Y., Tang, Z. R., Fu, X. and Xu, Y. J. (2011) Nanocomposite of Ag-AgBr-TiO as
3
a photoactive and durable catalyst for degradation of volatile organic compounds in the gas
phase. Appl. Catal. B Environ. 106, 445–452.
39. Ye, L., Liu, J., Gong, C., Tian, L., Peng, T. and Zan, L. (2012) Two Different Roles of
Metallic Ag on Ag /AgX / BiOX ( X = Cl , Br ) Visible Light Photocatalystsꢀ: Surface
Plasmon Resonance and Z- Scheme Bridge. ACS Cata. 2, 1677−1683.
4
0. Wang, P. (2016) Ag–AgBr/TiO
photocatalytic activity and aggregation evaluation. J. Environ. Sci.
1. Wang, D., Duan, Y., Luo, Q., Li, X. and Bao, L. (2011) Visible light photocatalytic
activities of plasmonic Ag / AgBr particles synthesized by a double jet method. DES 270,
2
/RGO nanocomposite: Synthesis, characterization,
4
1
4
74–180.
2. Xu, Y., Xu, H., Li, H., Xia, J., Liu, C. and Liu, L. (2011) Enhanced photocatalytic activity
of new photocatalyst Ag / AgCl / ZnO. J. Alloys Compd. 509, 3286–3292.
3. Yang, Y., Guo, W., Guo, Y., Zhao, Y., Yuan, X. and Guo, Y. (2014) Fabrication of Z-
scheme plasmonic photocatalyst Ag@AgBr/g-C with enhanced visible-light
photocatalytic activity. J. Hazard. Mater. 271, 150–159.
4. Rong, X., Qiu, F., Zhang, C., Fu, L., Wang, Y. and Yang, D. (2015) Preparation of Ag –
AgBr/TiO –graphene and its visible light photocatalytic activity enhancement for the
4
3
N
4
4
2
degradation of polyacrylamide. 639, 153–161.
This article is protected by copyright. All rights reserved.

4
5. Xu, H., Li, H., Wu, C., Chu, J., Yan, Y. and Shu, H. (2008) Preparation, characterization
4
and photocatalytic activity of transition metal-loaded BiVO . Mater. Sci. Eng. B Solid-State
Mater. Adv. Technol. 147, 52–56.
46. Li, H., Zhang, X., Huo, Y. and Zhu, J. (2007) Supercritical Preparation of a Highly
2
Active S-Doped TiO Photocatalyst for Methylene Blue Mineralization. Environ. Sci.
Technol. 41, 4410–4414.
7. Wang, S. (2008) Application of Solid Ash Based Catalysts in Heterogeneous Catalysis.
Environ. Sci. Technol. 42, 7055–7063.
8. Wang, P., Huang, B., Dai, Y. and Whangbo, M.-H. (2012) Plasmonic photocatalysts:
harvesting visible light with noble metal nanoparticles. Phys. Chem. Chem. Phys. 14, 9813–
4
4
9
4
825.
9. Tauc, J. (1968) Optical properties and electronic structure of amorphous Ge and Si.
Mater. Res. Bull. 3, 37–46.
0. Zhu, J., Liu, S., Yang, Q., Xu, P., Ge, J. and Guo, X. (2016) Colloids and Surfaces Aꢀ:
5
2 6
Physicochemical and Engineering Aspects Fabrication of flower-like Ag@AgCl/Bi WO
photocatalyst and its mechanism of photocatalytic degradation. 489, 275–281.
5
1. Zhuang, J., Dai, W., Tian, Q., Li, Z., Xie, L., Wang, J., Liu, P., Shi, X. and Wang, D.
(
2010) Photocatalytic degradation of RhB over TiO bilayer films: Effect of defects and their
2
location. Langmuir 26, 9686–9694.
5
2. Jiang, J., Li, H. and Zhang, L. (2012) New insight into daylight photocatalysis of
AgBr@Ag: Synergistic effect between semiconductor photocatalysis and plasmonic
photocatalysis. Chem. - A Eur. J. 18, 6360–6369.
53. Zhou, Z., Long, M., Cai, W. and Cai, J. (2012) Synthesis and photocatalytic performance
of the efficient visible light photocatalyst Ag–AgCl/BiVO . J. Mol. Catal. A Chem. 353–354,
4
2
2–28.
4. An, C., Peng, S. and Sun, Y. (2010) Facile synthesis of sunlight-driven AgCl: Ag
5
plasmonic nanophotocatalyst. Adv. Mater. 22, 2570–2574.
5. Hou, Y., Li, X., Zhao, Q., Quan, X. and Chen, G. (2011) TiO
component nanojunction for efficient photoconversion. J. Mater. Chem. 21, 18067.
6. Velegraki, T., Hapeshi, E. and Fatta-Kassinos, D. (2015) Solar-induced heterogeneous
photocatalytic degradation of methyl-paraben. Appl. Catal. B Environ. 178, 2–11.
7. Lin, Y., Ferronato, C., Deng, N., Wu, F. and Chovelon, J.-M. (2009) Photocatalytic
5
2
nanotube/Ag–AgBr three-
5
5
2
degradation of methylparaben by TiO : Multivariable experimental design and mechanism.
Appl. Catal. B Environ. 88, 32–41.
This article is protected by copyright. All rights reserved.

Figure Captions:
Figure 1. Schematic for the synthesis of Ag/AgBr@m-WO photocatalyst.
3
Figure 2. XRD patterns of (a) monoclinic m-WO
Ag/AgBr@ m-WO (d) 45 min Ag/AgBr@ m-WO
3
(b) 15 min Ag/AgBr@m-WO
3
.
3
(c) 30 min
3
3
Figure 3. (a) High magnification SEM image of m-WO particle, (b) SEM of 30 min
Ag/AgBr@m-WO (c) SEM-EDX elemental mapping image of W La1 on this area, (d) SEM-
3
EDX elemental mapping image of O Ka1 on this area, (e) SEM-EDX elemental mapping
image of Br Ka1 on this area and (f) SEM-EDX elemental mapping image of Ag La1 on this
area.
Figure 4. TEM images of high magnification photocatalyst (a) m@WO (b) 30 min
3
Ag/AgBr@m-WO (c-d) d-spacing of m-WO and 30 min Ag/AgBr@WO respectively.
3
3
3
Figure 5. XPS spectra of 30 min Ag/AgBr@ m-WO
3
(a) Ag 3d and (b) Br 3d binding energy
regions
Figure 6. UV-VIS diffuse reflectance spectra represented as Kubelka-Munk Function for (a)
m-WO3 (b) 15 min Ag/AgBr@WO3 (c) 30 min Ag/AgBr@WO3 and (d) 45 min
Ag/AgBr@WO Inset shows the Tauc plots for m-WO and 30 min Ag/AgBr@WO in the
3
3
3
order (a) and (c).
Figure 7. Photocatalytic degradation of methylparaben in aqueous suspension under visible
light irradiation with different photocatalysts under visible light.
Figure 8. Plots of −ln(C/C
) vs. irradiation time for the photo-catalytic degradation of
0
methylparaben with different photocatalysts under visible light.
3
Figure 9. The reusability of the photo-catalyst for 30 min Ag/AgBr@m-WO .
Figure 10. Effect of different scavenger on the photocatalytic degradation in the presence of
0 min Ag/AgBr@m-WO3.
3
Figure 11. The photo-catalytic reaction mechanism for the degradation of methylparaben.
Figure 12. Schematic for the mechanism of photocatalytic degradation of methylparaben
3
with Ag/AgBr@m-WO under visible light.
This article is protected by copyright. All rights reserved.

This article is protected by copyright. All rights reserved.

This article is protected by copyright. All rights reserved.

This article is protected by copyright. All rights reserved.