Facile microwave-assisted synthesis of monodispersed ball-like Ag@AgBr photocatalyst with high activity and durability

Article abstract of DOI:10.1016/j.apcata.2013.01.035

We reported a rapid one-step microwave-assisted approach to synthesize a plasmonic photocatalyst of ball-like AgBr nanoparticles (ca. 290 nm in average diameter) with a small amount of metal Ag anchored on the surface. The obtained Ag@AgBr nanocomposites were characterized by means of X-ray diffraction, scanning electron microscopy, Transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy and UV-visible diffuse reflectance spectroscopy. The shape, size, and compositions of the Ag@AgBr photocatalysts could be controlled by tuning the microwave irradiation time and the concentrations of polyvinylpyrrolidone (PVP) in the reaction solution. The as-prepared Ag@AgBr plasmonic photocatalysts show excellent visible-light photocatalytic performance and good reusability for decomposing organic pollutant of Rhodamine B (RhB) due to the surface plasmon resonance (SPR) effect of Ag nanoparticles. Meanwhile, the possible degradation pathways of RhB and a mechanism of the plasmonic photocatalytic process were also proposed.

Full text of DOI:10.1016/j.apcata.2013.01.035

Applied Catalysis A: General 455 (2013) 183–192
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Facile microwave-assisted synthesis of monodispersed ball-like
Ag@AgBr photocatalyst with high activity and durability
Xiang Xua, Xiaoping Shena,∗, Hu Zhoub, Dezhou Qiua, Guoxing Zhua, Kangmin Chenc
a School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, PR China
b School of Materials Science and Engineering, Jiangsu University of Science and Technology, Zhenjiang, 212003, PR China
c School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
We reported a rapid one-step microwave-assisted approach to synthesize a plasmonic photocatalyst of
ball-like AgBr nanoparticles (ca. 290 nm in average diameter) with a small amount of metal Ag anchored
on the surface. The obtained Ag@AgBr nanocomposites were characterized by means of X-ray diffraction,
scanning electron microscopy, Transmission electron microscopy, energy dispersive X-ray spectroscopy,
X-ray photoelectron spectroscopy and UV-visible diffuse reflectance spectroscopy. The shape, size, and
compositions of the Ag@AgBr photocatalysts could be controlled by tuning the microwave irradiation
time and the concentrations of polyvinylpyrrolidone (PVP) in the reaction solution. The as-prepared
Ag@AgBr plasmonic photocatalysts show excellent visible-light photocatalytic performance and good
reusability for decomposing organic pollutant of Rhodamine B (RhB) due to the surface plasmon res-
onance (SPR) effect of Ag nanoparticles. Meanwhile, the possible degradation pathways of RhB and a
mechanism of the plasmonic photocatalytic process were also proposed.
Received 1 December 2012
Received in revised form 27 January 2013
Accepted 28 January 2013
Available online xxx
Keywords:
Nanoparticles
Photocatalysis
Silver
Silver bromide
Microwave
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
Noble metals (such as Ag and Au) nanoparticles have been
extensively studied over the past decades due to their unique
During the past decades, semiconductor photocatalysts based
on harnessing and converting solar energy into chemical energy
have attracted increasing attention because of their applications
in solving the energy crisis and environmental pollution problems
[1–6]. Typical example, TiO2, is the most popular photocatalyst
due to its peculiarities of chemical inertness, longterm stability
against photocorrosion, abundance in nature, low cost, and non-
toxicity [7–11]. However, the band gap of TiO2 is larger than 3.0 eV,
which means it can only absorb about 3–5% of sunlight in the
ultraviolet region and the resultant low photocatalytic efficiency
[12]. From the viewpoint of the efficient utilization of solar energy
and higher photocatalytic efficiency, the development of visible-
light-driven photocatalysts has recently become a very important
topic of research. Consequently, anion-doping with C [13], N [14], S
[15], and B [16] or cation-doping with transition metals [17] has
been widely used to functionalize TiO2 as a visible-light-driven
photocatalysts. However, these doped TiO2 materials show low
absorption of visible-light; moreover, their photocatalytic activi-
ties are still very low due to massive charge carrier recombination.
Therefore, it remains a great challenge to develop other effective
visible-light-driven photocatalysts.
surface plasmon resonance (SPR) properties that can dramati-
cally amplify the absorption of visible-light [18,19]. When noble
metal nanoparticles are anchored on the surface of large-bandgap
photocatalysts, they can act as a sensitizer to extend the light
absorption region. For instance, Wang et al. have successfully
synthesized Ag-loaded Bi2WO6 nanoarchitectures with enhanced
photocatalytic activities [20]. Chen et al. have also reported plas-
monic photocatalysts Au/ZrO2 and Au/SiO2 with high oxidation
degradation efficiency for organic contaminants [21]. Recently,
Huang and co-workers have developed a further type of plasmonic
photocatalyst silver/silver halides (Ag@AgX; X = Cl, Br, I) by an ion-
exchange reaction between aqueous solutions of Ag2MoO4 and
HX followed by UV irradiation [22–26]. Sun and co-workers have
reported a sunlight-driven AgCl/Ag photocatalyst by a direct reac-
tion between AgNO3 and NaCl under conventional oil-bath heating
[27]. Since AgX can be in situ partially converted to plasmonic
Ag nanoparticles, the interface between AgX and in situ-formed
Ag is clean and well-defined, and the absorption coefficient of
the resulting nanocomposites in the visible region can be signif-
icantly enhanced, which make Ag@AgX exhibit high photocatalytic
activity and stability. Thus, the development of visible-light driven
Ag@AgX photocatalysts has recently become a very important topic
of research [28–33].
However, the reported Ag@AgX photocatalysts are often with
large size and wide size distribution [22–26]. To the best of our
∗
Corresponding author.Tel.: +86 511 88791800; fax: +86 511 88791800.
E-mail address: xiaopingshen@163.com (X. Shen).
0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.01.035

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X. Xu et al. / Applied Catalysis A: General 455 (2013) 183–192
◦
radiation (ꢀ = 1.5406 A) at a scanning rate of 4 min−1. The com-
position, morphology, and size of the products were examined
by scanning electron microscopy (SEM, JSM-6480), transmission
electron microscopy (TEM, JEOL JEM-2100), energy dispersive X-
ray spectroscopy (EDS, attached to SEM), and X-ray photoelectron
spectroscopy (XPS, PHI 5000). The photocatalytic activity measure-
ment was conducted on a GHX-2 photochemical reactor (Science
and Education equipment Co., Ltd., YangZhou, China) (Fig. S1) with
a 500 W of tungsten lamp (Small500WR7S, Philips) as the light
source.
˚
knowledge, there is no report on the monodispersed ball-like
AgBr nanocrystals coupled with silver as plasmonic photocata-
lysts (abbreviated as Ag@AgBr). Herein, for the first time, we
report a simple and direct microwave-assisted method for the
synthesis of monodispersed Ag@AgBr nanocomposites as effi-
cient photocatalysts, which can drive degradation of Rhodamine
B (RhB) under visible-light irradiation. By rationally tuning the
microwave irradiation time and the content of polyvinylpyrroli-
done (PVP) in the precursor solution, we could control the shape,
size, and constituents of the Ag@AgBr products. Their strong
response towards visible-light is ascribed to the strong SPR effect
of metallic Ag components in the nanocomposites. Meanwhile,
the as-prepared plasmonic photocatalysts exhibit high activity and
durability towards decomposition of RhB. The possible degradation
pathways of RhB and a mechanism of the plasmonic photocatalytic
process under the visible-light irradiation are proposed.
2.4. Photocatalytic activity measurement
The photocatalytic activities of the samples were evaluated by
the degradation of RhB under visible-light irradiation. A 500 W
tungsten lamp with a cutoff filter as the visible-light source
(ꢀ > 420 nm) was positioned at ca. 10 cm away from the reaction
cell to trigger the photocatalytic reaction. There is a water layer
between the reaction system and the lamp to remove the thermal
effect of light. The light intensity on the surface of the reaction mix-
ture is ca. 120 mW/cm2, estimated with a radiometer (FZ-A, China).
The experiments were performed at room temperature as follows:
0.1 g of the photocatalysts was added to 100 mL of RhB solution
(10 mg L−1, at natural pH of 5.0) in a Pyrex reactor. Before illumi-
nation, the suspensions were stirred at the speed of 1000 rpm in
the dark for 30 min to ensure the establishment of an adsorption-
desorption equilibrium between the photocatalyst and RhB. The
dispersion was then exposed to visible-light irradiation under stir-
ring. At given time intervals, 4 mL of the suspension was pipetted
into a centrifuge tube and centrifuged at 8000 rpm for 2 min to
remove the remnant photocatalyst. The concentrations of RhB in
the supernatant were monitored by checking the absorbance at
553 nm using a UV-2450 (Shimadzu) spectrophotometer. In the
recycle experiments, after Ag@AgBr composites were separated
from solution, they were washed with ethanol and deionized water
2. Experimental
2.1. Materials
Silver nitrate (AgNO3), sodium bromide (NaBr), ethylene gly-
col (EG), Rhodamine B (RhB), polyvinylpyrrolidone K30 (PVP) and
ethanol were purchased from Shanghai Chemical Reagent Co. Ltd.
(China). All chemicals in the study were analytical reagent grade
and were used without further purification. Deionized water was
used in all the experiments. The nitrogen-doped TiO2 (N-TiO2) was
prepared according to the method reported in the literature [34].
2.2. Preparation of Ag@AgBr photocatalysts
Ag@AgBr photocatalysts were synthesized by a facile and rapid
microwave-assisted nonaqueous route. In a typical procedure,
15 mL of EG was cooled to 0 ◦C in an ice bath, then 2 mmol of
AgNO3 was dissolved in it to form a clear solution. In a sepa-
rate beaker, 2 mmol of NaBr and 3 mmol of PVP were added to
15 mL of EG, followed by sonication at room temperature to form a
homogeneous solution, and then was slowly dropped to the AgNO3
solution under vigorous stirring for 30 min. The resulting mixture
was transferred to a 100 mL flask, and the flask was placed into a
XH-MC-1 microwave reactor system (XiangHu Science and Tech-
nology Equipment Co., Ltd., Beijing, China). The mixture was heated
to 160 ◦C by microwave irradiation and kept at the temperature for
10 min. Then the reaction system was quickly cooled to room tem-
perature by an ice bath. The obtained products were collected by
centrifugation, washed with absolute ethanol several times, and
then dried in vacuum at 45 ◦C for 12 h. The sample was denoted as
T-10. In the control experiment, the time series of samples denoted
as T-20 and T-30 represent the samples prepared for 20 min and
30 min, respectively, with identical other conditions.
For comparison, AgBr nanoparticles were also prepared by a pre-
cipitation method without microwave irradiation. 30 mL of EG was
cooled to 0 ◦C in an ice bath, then 4 mmol of AgNO3 was dissolved
in it with magnetic stirring to form a clear solution. In addition,
4 mmol of NaBr and 6 mmol of PVP were dissolved in 30 mL of EG
at room temperature to form a homogeneous solution, which then
was slowly dropped to the AgNO3 solution under vigorous stirring
for 30 min, resulting in the formation of a primrose yellow disper-
sion containing AgBr nanoparticles. The obtained products were
collected by centrifugation, washed with absolute ethanol several
times, and then dried in vacuum at 45 ◦C for 12 h.
before being re-dispersed in the dye solution (100 mL, 10 mg L−1
for another cycle.
)
2.5. Analytical methods
The final intermediates were conducted by a Agilent gas
chromatography interfaced a HED-EM mass spectrometer GC/MS
(Agilent 7890A/5975C, USA). The pre-treatment process was as fol-
lows: the reacted solution was centrifuged to remove the remnant
photocatalyst and the pH was adjusted to 2.0 with 10% HCl. 30 mL
of the solution was then extracted with 30 mL dichloromethane
for three times and the extracted solution was dehydrated using
anhydrous sodium sulphate. Afterward, the dehydrated solution
was concentrated to 1 mL. Then, trimethylsilylation was carried
out at 50 ◦C for 30 min using 0.5 mL bis(trimethylsilyl) trifluo-
roacetamide (BSTFA). 1.0 ␮L of the final sample was injected into
GC equipped with DB-5MS column (fused-silica capillary column,
30 mm × 0.25 mm i.d., 0.25 ␮m film thickness with a 5% equivalent
polysilphenylene siloxane) with splitless mode. The tempera-
ture program of the column was set as follows: at 60 ◦C, hold
time = 1 min, from 60 to 260 ◦C, rate = 10 ◦C/min.
3. Results and discussion
In our study, a direct precipitation reaction between Ag+ cations
and Br− anions is used to grow monodispersed ball-like AgBr
nanoparticles in ethylene glycol with the assistance of polymeric
surfactant PVP. It has been widely reported that metal ions could
be reduced by ethylene glycol [35]. In the present study, ethylene
glycol is proposed to play a key role in the formation of Ag@AgBr.
2.3. Characterization
The phase of as-synthesized products were characterized
using X-ray diffraction (XRD, Shimadzu XRD-6000) with Cu K␣

X. Xu et al. / Applied Catalysis A: General 455 (2013) 183–192
185
It acts as a solvent, a microwave-absorber, and a reducing agent
in the partial reduction of AgBr to metallic Ag under a moderate
temperature.
The phase structure and crystallinity of the as-synthesized
Ag@AgBr photocatalysts obtained at different microwave irradi-
ation times were characterized by XRD. As shown in Fig. 1, AgBr
phase was detected for all of the Ag@AgBr samples. the XRD peaks
at 2ꢁ about 26.8, 31.1, 44.6, 55.4, 64.7 and 73.5◦ can be indexed to
the (1 1 1), (2 0 0), (2 2 0), (2 2 2), (4 0 0) and (4 2 0) crystal planes of
AgBr (JCPDS card no. 06-0438), respectively. The result is in good
agreement with the XRD patterns of AgBr crystals reported previ-
ously [28,32]. No reflections assignable to Ag metal were present
in the XRD patterns of samples T-10 and T-20, possibly because the
low Ag content was below the detection limit of XRD. However, in
the XRD pattern of sample T-30, one additional reflection located at
2ꢁ = 38.1◦ can be assigned to the (1 1 1) plane of Ag crystal (JCPDS
card no. 65-2871), though the diffraction peak was rather weak.
This result implies that the content of metal Ag is improved with
the increase of microwave irradiation time.
SEM has been used to observe the morphologies of the as-
synthesized samples. Typical SEM images of the as-synthesized
Fig. 1. XRD patterns of the as-prepared Ag@AgBr samples obtained at different
microwave irradiation times. (a) 10 min; (b) 20 min; (c) 30 min.
Fig. 2. SEM images and EDS patterns of the as-preapred Ag@AgBr samples obtained with different microwave irradiation times. (a) and (d) 10 min; (b) and (e) 20 min; (c)
and (f) 30 min. The insets show the magnified SEM images of the samples.

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X. Xu et al. / Applied Catalysis A: General 455 (2013) 183–192
Ag@AgBr photocatalysts obtained at different microwave irra-
diation time are shown in Fig. 2. Fig. 2a shows that the
sample synthesized in 10 min consists of a large amount of ball-
like nanocrystals with an average diameter of 290 nm. As the
microwave irradiation time was increased to 20 min, the diameter
of AgBr particles was increased to 350 nm, and Ag nanoparti-
cles were distributed uniformly on the surface of AgBr (Fig. 2b).
However, when the microwave irradiation time was prolonged to
30 min, the size of the obtained Ag@AgBr particles was increased
to around 440 nm, and the morphology became irregular (Fig. 2c).
The average diameter of AgBr particles in our experiment is much
smaller than the AgBr polyhedra in Zhang’s study [30], which may
be due to the high viscosity of ethylene glycol in low temperature.
The high viscosity of ethylene glycol cause lower diffusion rates of
both Ag+ ions (from AgNO3) and Br− ions (from NaBr), and thus
the formation of AgBr has a burst nucleation and a lower precipi-
tation rate, which make the AgBr sample have uniform and small
size [36]. We also examined the Ag@AgBr samples by TEM. Unfor-
tunately, we failed to obtain exact TEM images of the samples (Fig.
S2) because the AgBr nanoparticles were quickly destroyed by the
high-energy electron beam during the TEM measurement. Similar
phenomena have also been reported by other researchers before
[31,37]. EDS has been used to confirm the chemical composition
of the as-synthesized samples. The EDS spectra of the Ag@AgBr
photocatalysts obtained at different microwave irradiation time are
shown in Fig. 2. Carbon element in the EDS spectra comes from
the conductive adhesive used as a support for the samples in the
SEM measurement. The EDS analysis shows that the sample was
composed of Ag and Br elements.
The chemical compositions and surface chemical states of the
Ag@AgBr samples were further examined by XPS. The survey spec-
tra of the three Ag@AgBr samples obtained at different microwave
irradiation time are similar and confirm Ag and Br as the main
components (Fig. 3a). As shown in Fig. 3b, the peaks at 367.5
and 373.3 eV are assigned to those of Ag 3d5/2 and Ag 3d3/2
binding energies, respectively [38]. These two bands could be fur-
ther deconvoluted into two peaks, respectively, at 366.8, 368.5 eV
and 372.8, 374.5 eV, where the bands at 366.8 and 372.8 eV are
attributed to the Ag+ of AgBr, and those at 368.5 and 374.5 eV are
ascribed to the metallic Ag0 [39–42]. From the XPS peak areas,
the surface Ag0 contents were calculated to be 1.6, 3.9 and 5.7 at%
for T-10, T-20 and T-30, respectively, confirming that increasing
microwave irradiation time can promote the conversion of AgBr to
Ag (Table S1), which is consistent with the XRD results (Fig. 1).
It is found that PVP in the precursor solution also has a signif-
icant influence on the size of AgBr nanocrystals. By adjusting the
amount of PVP, AgBr with different sizes can be obtained accord-
ingly, and the corresponding SEM images are shown in Fig. 4a–d.
For clarity, the samples denoted as PVP-0, PVP-1, PVP-3 and PVP-5
represent the products synthesized with 0 mmol, 1 mmol, 3 mmol
and 5 mmol of PVP contents in the precursor solution at 160 ◦C for
10 min, respectively. The sample synthesized without PVP (PVP-0)
takes on a sphere-like morphology with a wide size distribution and
an average diameter of about 910 nm (Fig. 4a and e). The addition
of PVP can reduce the particle size and improve the size unifor-
mity, as shown in Fig. 4b–d. The sizes of the AgBr nanocrystals
are 340, 290, and 260 nm for samples PVP-1, PVP-3, and PVP-5,
respectively (Fig. 4f–h). In the reaction system, PVP may serve
as a polymeric capping agent, the long polymeric chains of the
molecules increase the overall viscosity of the reaction solution
to further slow down the precipitation reaction and facilitate the
formation of monodisperse-sized AgBr nanoparticles [36].
Fig. 3. Typical XPS spectra of the as-prepared Ag@AgBr samples: (a) survey and
(b)Ag 3d.
nanostructured metals have a positive effect on the optical absorp-
tion of plasmonic-metal-semiconductor composite photocatalysts
[33]. The formation of Ag metal on the surface of AgBr can extend
and enhance absorption capacity. The existence of Ag metal in the
products can be further confirmed by UV/vis diffuse reflectance
spectra (UV/vis DRS). As shown in Fig. 5b, the Ag@AgBr nanocrystals
exhibit enhanced absorption in the visible-light region in compar-
ison with AgBr nanocrystals. The strong absorption in the region
of 450–650 nm should result from the strong SPR of Ag nanopar-
ticles, which is consistent with previous report [33]. Since AgBr is
very sensitive to UV light and can be reduced to metal Ag under UV
light, it is difficult to obtain the exact UV/vis DRS of pristine AgBr
sample.
Although various structures of Ag@AgBr photocatalysts have
been prepared by a variety of methods [26,39,41,42], the monodis-
persed and uniform Ag@AgBr ball-like photocatalysts have not
been reported before. Meanwhile, previous study were all focus
on the photo-degradation of methyl orange (MO) as a probe reac-
tion, no investigation on the photodegradation of RhB. As a kind
of the most commonly used xanthenes dyes in textile industry,
RhB has been found to be potentially toxic and carcinogenic and
is being widely studied as a representative water pollutant [43].
Fig. 5a shows the photographs of the reaction solution before
and after microwave irradiation. The plum color of the colloid after
microwave irradiation indicates that some Ag species are formed
and embedded in the AgBr matrix. It was reported that plasmonic

X. Xu et al. / Applied Catalysis A: General 455 (2013) 183–192
187
Fig. 4. SEM images and particle size distribution of the as-prepared Ag@AgBr samples obtained under microwave irradiation of 10 min with different PVP contents in the
precursor solution. (a) and (e) 0 mmol; (b) and (f) 1 mmol; (c) and (g) 3 mmol; (d) and (h) 5 mmol.
In the present work, the visible-light photocatalytic activity of the
Ag@AgBr plasmonic photocatalysts was investigated in terms of
photodegradation of RhB in aqueous solution. Fig. 6a displays the
temporal evolution of the spectral changes taking place during
the photodegradation of RhB mediated by the typical Ag@AgBr
nanocomposites under visible-light irradiation. The sharp decrease
in the maximum absorption band implies that RhB molecules suf-
fered a rather cleavage of the whole conjugated chromophore.

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X. Xu et al. / Applied Catalysis A: General 455 (2013) 183–192
Fig. 5. (a) Photographs of the reaction solution before and after microwave irradiation; (b) UV/vis DRS of Ag@AgBr and AgBr samples.
Furthermore, the absorption maximum of the degraded solution
exhibited hypsochromic shifts (Fig. 6b), which is presumed to result
from the formation of a series of N-de-ethylated intermediates in a
stepwise manner. Similar phenomena were also observed in litera-
ture and our previous research in Bi2WO6–Fe3O4/RhB system [44].
Nevertheless, hypsochromic shifts of the absorption maximum in
our experiment were slight before the complete decomposition.
The possible reason is that during visible-light irradiation the N-
de-ethylation reaction is just a minor process and cycloreversion of
RhB is the major process [45]. The color of the dispersion nearly dis-
appeared when the time of visible-light irradiation reached 50 min,
indicating the excellent photocatalytic activity and the complete
photocatalytic decolorization of RhB aqueous solution during the
reaction (insert of Fig. 6a).
The final organic products after photocatalytic reaction were
identified by GC/MS. Six benzenoid removal intermediates were
identified, including cyclohexanol, cyclohex-1-enol, benzeneacetic
acid, 2-phenyl-butyric acid, terephthalic acid and dibutyl phtha-
late. Besides the intermediates discussed above, some other smaller
organic acids such as 2-hydroxy-propionic acid, propane-1,2-diol,
hydroxy-acetic acid, butane-1,3-diol, and 4-methyl-pentan-1-ol
were also identified by GC/MS (Table 1). On the basis of the above
experimental results and the previous studies [44,46], we tenta-
tively propose the possible pathways of degradation of RhB under
visible-light illumination as depicted in Fig. 7. Under visible-light
irradiation, two pathways occurred during the photoreaction: the
N-de-ethylation process and the cycloreversion process [47]. On the
one hand, the dye is degraded by a series of successive de-ethylation
reaction, from N,N,Nꢀ,Nꢀ-tetraethylated rhodamine to rhodamine
[45,46]. On the other hand, the conjugated xanthene structure
of both RhB and the N-de-ethylated intermediates were further
degraded into smaller organic molecules during the decolorization
process.
The concentration of RhB (C) is assumed proportional to the
maximum absorbance (A) at 553 nm. However, it should be point
out that since the absorbance may also be contributed by other
absorbing components of the intermediates from photodegrada-
tion, and thus the concentration determined by UV–vis method
should actually be the total concentration of all absorbing com-
ponents. In this study, we approximatively represent the change
of RhB concentration (C/C0) by the variation of absorbance (A/A0),
where A is the absorbance at the wavelength of 553 nm during
photocatalytic degradation, and A0 was the absorbance of original
RhB solution at 553 nm. To evaluate the photocatalytic activity
of Ag@AgBr, we examined the decomposition of RhB in solution
over the Ag@AgBr sample under visible-light irradiation as a
function of time (Fig. 8). For comparison, we also carried out the
decomposition of RhB over AgBr particles and the N-TiO2 reference
photocatalyst under the same reaction condition. It can be seen that
the degradation of RhB was extremely slow without photocatalyst
under visible-light illumination, and no concentration changes
were detected if the experiment was conducted without light
irradiation, confirming that the degradation of RhB was caused
by photocatalytic reaction. For the Ag@AgBr photocatalysts, the
adsorption for RhB is much weaker than that of the AgBr. A possible
reason is that the pure AgBr particles have more Br atoms on their
surfaces than the Ag@AgBr particles, and thus can provide more
RhB adsorption sites because of strong electrostatic interaction
Fig. 6. (a) Temporal UV–vis absorption spectra for RhB solution (10 mg/L, 100 mL, at natural pH of 5.0) in the presence of Ag@AgBr nanocomposite under visible-light
irradiation. (b) Wavelength shifts of the absorption peaks as a function of the photocatalytic time.

X. Xu et al. / Applied Catalysis A: General 455 (2013) 183–192
189
Table 1
Identification of the degradation intermediates of RhB by GC/MS.
m/z
Retention time
Identified intermediates
Benzeneacetic acid
Structural formula
O
136
3.436
HO
HO
90
4.065
4.123
Butane-1,3-diol
OH
HO
O
164
2-Phenyl-butyric acid
100
4.386
5.153
Cyclohexanol
HO
HO
O
90
2-Hydroxy-propionic acid
OH
98
76
5.233
5.296
Cyclohex-1-enol
HO
HO
OH
OH
Hydroxy-acetic acid
O
HO
76
5.496
8.197
Propane-1,2-diol
OH
102
4-Methyl-pentan-1-ol
O
OH
O
166
20.156
Terephthalic acid
HO
O
278
22.959
Dibutyl phthalate
O
O
O

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X. Xu et al. / Applied Catalysis A: General 455 (2013) 183–192
Fig. 7. Proposed degradation pathways for photocatalytic degradation of RhB with Ag@AgBr plasmonic photocatalysts.
between the electron lone pairs of Br atoms and cationic charged
RhB molecules. However, it can be found that the Ag@AgBr
nanocomposite exhibit higher photocatalytic activities than AgBr
and N-TiO2 during the photocatalytic degradation process.
In order to reveal the relationship between the composition and
photodegradation efficiency, the photocatalytic performances of
the different Ag@AgBr nanocomposites were investigated at the
identical conditions. Fig. 9 shows the degradation of RhB using time
series Ag@AgBr samples under visible-light irradiation. Among the
time series samples, sample T-30 showed lowest photocatalytic
activity than samples T-10 and T-20. The possible reason may be
that the increasing microwave irradiation time can enhance the
Ag content of the Ag@AgBr nanocomposites, and thus enlarge the
interface between Ag and AgBr, which leads to the increasing hole
capture by the negative surface charge on the Ag nanoparticles,
and reduces the efficiency of charge separation [31]. Moreover,
the excessive amounts of Ag can produce photonic shielding effect
in the photocatalytic reaction, resulting in photocatalytic perfor-
mance degradation. The relatively bigger size of T-30 would also
decrease its photodegradation efficiency.
Fig. 8. The photocatalytic degradation efficiencies of RhB as a function of irradiation
time over Ag@AgBr nanocomposites and N-TiO2. The photocatalytic experiments
were performed under visible-light illumination, and the initial RhB concentration
was 10 mg/L at natural pH of 5.0 in all cases.
Besides the excellent visible-light-driven photocatalytic activ-
ity, the renewable photocatalytic activity after photocatalytic

X. Xu et al. / Applied Catalysis A: General 455 (2013) 183–192
191
Fig. 10. Schematic illustration of the Ag@AgBr photocatalytic reaction process
under visible-light irradiation.
Fig. 9. Visible-light-driven photodegradation curves of RhB over Ag@AgBr photo-
catalyst obtained with various synthetic time. The photocatalytic experiments were
performed under visible-light illumination, and the initial RhB concentration was
10 mg/L at natural pH of 5.0 in all cases.
generate electrons in the semiconductor conduction band and holes
in the valence band, respectively [30]. Then the electrons can be
transferred to the surface of the Ag nanoparticles that together
with the injected SPR electrons will initiate the catalytic reaction.
In addition, because of the in situ production of Ag nanoparticles on
the surface of AgBr, the well-defined interface facilitates the inter-
facial electron transfer and promotes charge carrier separation,
which may make another contribution to the high photocatalytic
activity [51]. So, the SPR of Ag nanoparticles, the quality of AgBr,
and the well-defined interface between Ag and AgBr are assumed
to promote the photocatalytic efficiency.
The Ag@AgBr plasmonic photocatalyst is also stable in degrad-
ing organic pollutants under visible-light. When subjected to visible
light irradiation, Ag@AgBr nanocomposites are excited due to
the localized SPR of Ag nanocrystals and generate electron-hole
pairs. On the one hand, owing to the excellent conductivity of Ag
nanoparticles and the negative charge of the surface of AgBr, the
photogenerated electrons could be far from the surface of AgBr and
transferred quickly to the present molecular oxygen to form the
reactive oxygen species [22]. Thus, an Ag+ ion inside AgBr could
avoid being reduced, which may be one factor for the high stability
of the Ag@AgBr composites. On the other hand, though the pho-
togenerated holes could oxidize the Br− ions inside AgBr into Br0
atoms, they as reactive radical species could oxidize the RhB dye
and hence be reduced to Br− ions again [32]. This good cycle of
bromine may be another factor in the stability of the photocatalyst.
In a word, the SPR effect of Ag nanoparticles plays an important
role in the high photocatalytic activity and stability of the Ag@AgBr
photocatalyst.
reactions is also important to the application of a photocatalyst
[48]. To evaluate the stability and reusability of Ag@AgBr plasmonic
photocatalyst, the circulating runs in the photocatalytic degrada-
tion of RhB under visible-light irradiation were also investigated
with sample T-10. All the processes and parameters were kept
unchanged during the cycling tests. The photocatalytic activity of
the Ag@AgBr composites did not show obvious decrease after four
catalytic recycles for the photodegradation of RhB (Fig. S3). At the
4th cycle, about 93% of the original RhB was degraded, while it
was 98% for the 1st run. These results indicate that photocatalytic
activity of Ag@AgBr was highly stable.
The XRD pattern (Fig. S4a) of the Ag@AgBr nanocomposites after
recycle experiments exhibits almost identical character to that of
the as-prepared sample. The typical SEM images of the Ag@AgBr
composites after photocatalytic reaction clearly showed that they
keep almost the same morphology as the freshly prepared sample
(Fig. S4b). These facts imply that the Ag@AgBr composites are stable
during the photocatalytic degradation of the model dye molecules,
which is of advantage to their practical applications. Therefore, it is
believed that the Ag@AgBr nanocomposite synthesized under our
experimental conditions is a highly efficient and stable photocata-
lyst under visible-light irradiation.
The high photocatalytic activity of the Ag@AgBr plasmonic
photocatalysts are mainly attributed to the existence of Ag loaded
on the AgBr surface (Fig. 10). Under visible-light illumination, the
Ag@AgBr nanocomposite has a strong absorption in the visible-
light region due to the SPR effect of Ag nanoparticles, and then a
number of electron–hole pairs are generated [22,49]. Because of the
synergy of the excellent conductivity of Ag nanoparticles and the
polarization field provided by the AgBr, the photogenerated elec-
trons are transferred to the surface of the Ag nanoparticles far from
the Ag@AgBr interface and trapped by oxygen molecules (O2) in the
solution to form superoxide ions (O2−) and other reactive oxygen
species [50], which could also oxidize the RhB dye molecules. At
the same time, the photogenerated hole can not only oxidize the
RhB dye directly, but can also be transferred to the AgBr surface
to oxidize bromine ions (Br−) to bromine atoms (Br0), which are
also reactive radical species that could oxidize the RhB molecules
[32]. On the other hand, besides this plasmonic photocatalytic
process, the n-type AgBr semiconductor photocatalysis process
simultaneously occurred because AgBr with a small bandgap could
also be directly photoexcited under visible-light illumination to
4. Conclusions
In summary, we have successfully synthesized Ag@AgBr plas-
monic photocatalysts with ball-like morphology through a simple
one-step microwave-assisted approach. The shape, size, and com-
position of the Ag@AgBr nanocomposites were well controlled by
tuning the microwave irradiation time and the concentrations of
PVP. The as-prepared Ag@AgBr plasmonic photocatalysts show
excellent photocatalytic performance under visible light irradia-
tion, which is attributed to the SPR effect of the metallic silver
component. Meanwhile, the possible degradation pathways of RhB
and a mechanism of the plasmonic photocatalytic process was also
proposed. It is believed that the obtained Ag@AgBr plasmonic pho-
tocatalyst with high activity and strong durability has potential

192
X. Xu et al. / Applied Catalysis A: General 455 (2013) 183–192
applications in the degradation of organic contaminations and envi-
ronmental cleaning.
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