Enhanced visible light photocatalytic activity of AgBr on {001} facets exposed to BiOCl

Article abstract of DOI:10.1016/j.jallcom.2017.04.126

The facet dominated AgBr/BiOCl composites with exposed {001} (BiOCl-001) and {101} (BiOCl-101) facets were prepared by facile solvothermal and chemical precipitation process. Microstructures, morphologies, photoelectron and band gap energy of the as-prepared composites were characterized by various experimental methods. The AgBr/BiOCl heterojunctions showed a much higher photocatalytic performance than pure AgBr and BiOCl in photodegradation of Rhodamine B (RhB) under visible light irradiation. In addition, the AgBr-{001}BiOCl with the mole ratio of 1:2 showed the highest photocatalytic performance with the RhB completely degraded in 15?min. The better photocatalytic performance of AgBr-{001}BiOCl may attribute to the fact that {001} facet of BiOCl was beneficial for the separation of photo-generated carriers and more oxygen vacancy can be formed on the surface {001} facets of BiOCl. Moreover, the photocatalytic mechanism of AgBr-{001}BiOCl composites were also discussed.

Full text of DOI:10.1016/j.jallcom.2017.04.126

Journal of Alloys and Compounds 712 (2017) 535e542
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Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Enhanced visible light photocatalytic activity of AgBr on {001} facets
exposed to BiOCl
,
*
Yi Ling Qi a, Yi Fan Zheng b, Hao Yong Yin c, Xu Chun Song a
a Department of Chemistry, Fujian Normal University, Fuzhou 350007, PR China
b Research Center of Analysis and Measurement, Zhejiang University of Technology, Hangzhou 310014, PR China
c Institute of Environmental Science and Engineering, Hangzhou Dianzi University, Hangzhou, 310018, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The facet dominated AgBr/BiOCl composites with exposed {001} (BiOCl-001) and {101} (BiOCl-101) facets
were prepared by facile solvothermal and chemical precipitation process. Microstructures, morphologies,
photoelectron and band gap energy of the as-prepared composites were characterized by various
experimental methods. The AgBr/BiOCl heterojunctions showed a much higher photocatalytic perfor-
mance than pure AgBr and BiOCl in photodegradation of Rhodamine B (RhB) under visible light irradi-
ation. In addition, the AgBr-{001}BiOCl with the mole ratio of 1:2 showed the highest photocatalytic
performance with the RhB completely degraded in 15 min. The better photocatalytic performance of
AgBr-{001}BiOCl may attribute to the fact that {001} facet of BiOCl was beneficial for the separation of
photo-generated carriers and more oxygen vacancy can be formed on the surface {001} facets of BiOCl.
Moreover, the photocatalytic mechanism of AgBr-{001}BiOCl composites were also discussed.
© 2017 Elsevier B.V. All rights reserved.
Received 19 January 2017
Received in revised form
4 April 2017
Accepted 13 April 2017
Available online 15 April 2017
Keywords:
BiOCl
AgBr
Heterojunction
Photocatalytic
Facet
1. Introduction
light region [2], and the rapid recombination of photo-generated
electrons and holes makes it far from satisfaction. Therefore, it is
Semiconductor photocatalysis is a promising technology using
solar light energy through storing photon energy in the chemical
bonds to mitigate the environment deterioration and alleviate the
energy crisis, therefore it has attracted much more attention for its
wide application. Over the past several years, BiOCl has drawn
considerable interests for its unusual electrical and optical prop-
erties [1e4]. Besides, excellent photocatalytic properties have been
obtained on BiOCl and their compounds under ultraviolet or visible
light illumination. For instance, Bismuth oxychloride (BiOCl) ex-
hibits higher activity for organic dyes degradation than TiO2 under
UV light illumination due to its unique layer structure [5e7].
Recently, some researchers reported that BiOCl nanosheets with
highly exposed {001} facets displayed more excellent photo-
catalytic activity under ultraviolet irradiation, which is mainly due
to the fact that the {001} facets might facilitate the generation of
oxygen vacancies [8]. Furthermore, it has been exhibited that the
density of oxygen (O) atoms on {001} facets of bismuth oxychloride
was much bigger than on other facets [9]. However, the wide
bandgap (3.5 eV) of BiOCl limits its further application in visible
highly demanded to tailor BiOCl semiconductor materials with
suitable method to improve its photocatalytic activity under visible
light irradiation. In previous reports, Hu et al. have selectively
synthesized BiOCl nanoplates with exposed {001} and {102} facets
by adding sodium citrate via a facile hydrothermal route, the BiOCl
with higher percentage of {001} facets showed the higher photo-
catalytic property for degrading methyl orange (MO) [10]. Kim et al.
also successfully synthesized ternary nanohybrid composites [11]
and graphene-based nanocomposites [12,13] which have demon-
strated the wide applications in the environmental issues. Gener-
ally speaking, the high density of oxygen atoms on the exposed
{001} facet of BiOCl makes the oxygen vacancies more evident [14].
These oxygen vacancies are considered to be relevant to the
improved photocatalytic activity. So a higher oxygen vacancy
density exhibits higher visible-light photoactivity [9,15].
More recently, AgX (X ¼ Cl, Br and I) photocatalysts [16e18],
well known as the photosensitive materials, have been extensively
investigated due to their potential practical applications in the
photocatalysis. A variety of catalysts were reported to combine with
AgX to improve their photocatalytic performance. For instance,
AgX/BiOX [19], AgI/BiOI [20e22], Ag@AgI/ZnS [23], AgCl/ZnO [24],
AgX/Ag2CO3 [25] and AgBr/Ag2O [26] all displayed significantly
* Corresponding author.
E-mail address: songxuchunfj@163.com (X.C. Song).
http://dx.doi.org/10.1016/j.jallcom.2017.04.126
0925-8388/© 2017 Elsevier B.V. All rights reserved.

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Y.L. Qi et al. / Journal of Alloys and Compounds 712 (2017) 535e542
increased photocatalytic activity in the degradation of organic
pollutant after being combined with the AgX. These results may be
due to the heterojunction formed between the different semi-
conductor photocatalysts. The heterojunction can facilitate the
separation of photogenerated charges and reduce the distance of
separation, finally improve the photocatalytic performance.
In present work, we prepared AgBr/BiOCl composites with
exposed {101} facets on BiOCl microspheres and AgBr/BiOCl with
high exposed {001} facets on BiOCl nanosheets by using a simple
solvothermal method in ethylene glycol (EG) and water system. As
expected, AgBr indeed promoted visible-light response and
improved the photocatalytic activities of both composites under
the visible light irradiation. Furthermore, the AgBr-{001}BiOCl
showed higher photocatalytic activity than that of AgBr-{101}BiOCl.
Photocurrent experiments were carried out to investigate the
charge transfer in the heterojunction and to get a better under-
standing over the photocatalytic reactions. A possible photo-
catalytic mechanism of the enhanced AgBr-{001}BiOCl
heterojunctions was also proposed according to the experimental
results.
sample was washed, collected and dried at 60 ꢀC.
2.2. Characterization
Thermo ARL SCINTAG X'TRA X-ray diffractometer was used to
analyze the X-ray diffraction patterns spectrum of as-prepared
samples at room temperature by using Cu K a radiation. The Hita-
chi S-4700 SEM was used to characterize scanning electron mi-
croscope images of the morphologies and structures over samples.
Lambda 850 UVevis spectrophotometer was used to test the
UVevis diffuse reflectance spectra with BaSO4 as a reference. XPS
characterization was collected by performing on a X-ray photo-
electron spectroscopy analyzer (KRATOS AXIS ULTRA DLD). The
photocurrent experiment was analyzed on a CHI-660C electro-
chemical work station (Made in China), using a common three-
electrode chemical cell, and 0.1 mol/L Na2SO4 aqueous liquor
works as electrolyte solution. The 0.1 mg of as-prepared sample
was coupled on ITO glass that acted as the working electrode. The
reference electrode was a saturated AgeAgCl electrode and the
counter was a platinum wire.
2. Experimental section
2.3. Photocatalytic experiments
2.1. Sample preparation
In a typical photocatalytic experiment, the RhB and phenol were
used to evaluate the photocatalytic performance of the as-prepared
samples under visible/solar light irradiation. Firstly, 0.1 g of the
photocatalyst was dispersed in 200 mL 2 ꢁ 10ꢂ5 mol/L RhB (or
50 mL 50 mg/L phenol) solution, then the suspension was stirred
intensely in darkness for 30 min prior to irradiation toward
establishing the adsorptionedesorption equilibrium. And during
the procedure of photodegradation, 3 mL suspension was collected
per 3 min, the concentration of RhB was detected using a UV759S
UVeVis spectrophotometer after centrifuged, and a 300 W Xe lamp
with a 420 nm cut off filter worked as the visible-light source.
The degree of RhB mineralization was also measured through
the total organic carbon (TOC) in the irradiated solutions at a
certain time intervals using a TOC-VCPH analyzer (Shimadzu Co.,
Japan). A typical photocatalytic experiment system is consisted of
200 mL 2 ꢁ 10ꢂ5 mol/L RhB solution and 0.1 g of the photocatalyst.
2.1.1. Synthesis of {101} BiOCl and {001} BiOCl
All chemicals were of analytical grade and applied without other
purification. BiOCl microspheres with exposed {101} facets were
synthesized via a facile solvothermal process. In a typical proced-
ure, 5 mmol Bi(NO3)3$5H2O and 5 mmol KCl were dissolved in
20 mL ethylene glycol (EG) under magnetic stirring, respectively.
After 30 min, the KCl solution was slowly added to the
Bi(NO3)3$5H2O solution drop by drop. Then the mixture was
transferred to a 50 mL Teflon-lined stainless steel autoclave after
stirring for another 30 min at room temperature. Subsequently, the
mixture was carried out at 160 ꢀC for 12 h. The product was
collected by centrifugal separation, washed with distilled water and
absolute ethanol for several times, then dried at 60 ꢀC overnight.
This sample was labelled as {101}BiOCl. And the {001}BiOCl was
prepared through a simple hydrothemal method. Similarly, 5 mmol
Bi(NO3)3$5H2O and 5 mmol KCl were dispersed into 20 mL deion-
ized water with continually stirring, respectively. Then the latter
was dropped to the previous solution after stirring for 30 min. At
the same time, 2 mol/L NaOH was added to this suspension to
adjust the pH value to 7.0. And other conditions were the same with
that of {101} BiOCl. The sample was labelled as {001}BiOCl.
3. Results and discussion
As shown in Fig. 1, the crystallographic structure of the as-
prepared samples synthesized in water was characterized by
XRD. It can be seen that the diffraction peaks of curve a and b in the
XRD patterns are readily indexed to the AgBr (JCPDS card No. 06-
0438) and BiOCl (JCPDS card No. 06-0249), respectively. For the
BiOCl, the characteristic peaks of (001), (101) and (102) are sharper
2.1.2. Synthesis of AgBr-{101} BiOCl and AgBr-{001} BiOCl
Taking AgBr-{101}BiOCl (1:1) as an example, 1 mmol AgNO3 and
1 mmol {101}BiOCl were dispersed equably in 10 mL deionized
water in a beaker to form suspension A, and 1 mmol KBr was dis-
solved in 10 mL distilled water to form solution B at the same time.
Then solution B was added into suspension A drop by drop with
strongly stirring. The mixture was continuously stirred for 0.5 h.
And the AgBr-{101}BiOCl and AgBr-{001}BiOCl composites with
different mole ratios of AgBr were successfully achieved in the
same way. Finally, the samples were washed with deionized water
for three times, collected and dried at 60 ꢀC. The mole ratios of the
obtained samples were considered to be AgBr-{101}BiOCl (1:1),
AgBr-{101}BiOCl (1:2), AgBr-{101}BiOCl (1:4), AgBr-{001}BiOCl
(1:1), AgBr-{001}BiOCl (1:2) and AgBr-{001}BiOCl (1:4).
and stronger. For the AgBr, the peaks at 2
q
¼ 26.78ꢀ, 30.98ꢀ, 44.40ꢀ
and 55.08ꢀ were assigned to the (111), (200), (220) and (222)
planes, respectively. The XRD patterns of AgBr/BiOCl hetero-
structured photocatalyst samples with different mole ratios 1:4, 1:2
and 1:1 are shown in Fig. 1 c to e. It is clearly seen that the samples c
to e have the pure phases of both AgBr and BiOCl. Meanwhile, it is
noteworthy that the (200) peak belonging to the AgBr is obviously
strengthened as the increased content of AgBr. These observations
indicates that the AgBr has been successful grafted onto BiOCl. No
other characteristic peaks of impurities can be detected, demon-
strating high purities of all samples. And the sharp diffraction peaks
suggest that the as-prepared samples are well-crystallized. The
intensity ratio of the (001) to (101) peaks for the b to e is 0.91, 0.97,
0.96 and 0.94, respectively. It can be inferred that the BiOCl which
prepared in water had high percentage of {001} facets.
In comparison, the AgBr photocatalyst was also synthesized.
Firstly, 5 mmol AgNO3 and 5 mmol KBr got dissolved in 10 mL
deionized water, respectively. Then the solutions were mixed under
magnetic stirring with continuously stirring for 30 min. Finally, the
Fig. 2 shows the XRD patterns of the samples which prepared in
ethylene glycol (EG). The samples ranging from a to e are AgBr,

Y.L. Qi et al. / Journal of Alloys and Compounds 712 (2017) 535e542
537
For comparison, the morphology of BiOCl prepared in water was
also observed which can be seen from Fig. 3d that the sample was
assembled of interlaced nanosheets. And the SEM images of AgBr-
{001} BiOCl (1:2) (Fig. 3e) also depict that AgBr are adhered well to
the {001}BiOCl. Besides, Fig. 3c and e also exhibit that the
morphology of the composites does not change after embedding
AgBr particles. Furthermore, XPS was performed to explore the
chemical compositions and valence states of as-prepared AgBr-
{001}BiOCl (1:2). From the full scan spectrum exhibited in Fig. 3f,
the peaks of Bi (4p), O (1s), Bi (4d), Ag (3d), Cl (2p), O (2s) and C (1s)
elements can be identified in the heterojunction as a proof of the
chemical purity of AgBr-{001}BiOCl (1:2). The C signal primarily
derives from the hydrocarbon of XPS instrument itself. As shown
the inset in Fig. 3f, the binding energy values of Ag 3d3/2 and Ag 3d5/
2 are observed at 374.2 eV and 368.5 eV, respectively, which are in
good agreement with the values reported for Agþ. And no obvious
peaks belonging to the Ag0 was observed, indicating that the AgBr-
{001}BiOCl heterostructures are rather stable.
It is known that the optical absorption property of catalysts is
one of the vital factors that determine their photocatalytic prop-
erties. Hence, the UVevis diffuse reflectance spectra of the catalysts
with different amounts of AgBr particles are shown in Fig. 4. As
showed in Fig. 4a, the pure AgBr shows strong absorption edge at
about 478 nm in whole range of visible light region which is in good
agreement with the previously reported results [27]. The {101}
BiOCl microsphere without AgBr has an absorption wavelength at
about 390 nm. As for AgBr-{101}BiOCl (1:1), AgBr-{101}BiOCl (1:2)
and AgBr-{101}BiOCl (1:4), the optical absorption edges shift
slightly to longer wavelengths, showing an apparent red-shift
compared with {101}BiOCl, and this red-shift exhibits inconspic-
uous change with different content of AgBr. By contrast, {001}BiOCl
(Fig. 4b) shows the maximum absorbance at about 360 nm, which
seems that the {001}BiOCl possessed the favorable photo-
absorption property in the UV light region. The maximum absor-
bance edges of the AgBr-{001}BiOCl composites with different mole
ratio of 1:1, 1:2 and 1:4 are 380, 377 and 375 nm, respectively.
Compared to the {001}BiOCl, the existence of the AgBr can increase
its photoabsorption with absorption edges shifting to larger
wavelengths. The results showed that all samples exhibit strong UV
photoabsorption. However, {101}BiOCl and {001}BiOCl displayed
quite different light absorption in the visible light region
(360e800 nm). Moreover, the {101}BiOCl and the AgBr-{101}BiOCl
heterostructures displayed a broader absorption wavelengths from
the UV light region to the visible light region than that of {001}
BiOCl and the AgBr-{001}BiOCl.
Fig. 1. XRD diffraction patterns of the AgBr (a), BiOCl (b) and AgBr/BiOCl samples with
different mole ratio of 1:4 (c), 1:2 (d) and 1:1 (e) synthesized in water.
BiOCl and AgBr/BiOCl heterojunctions with different mole ratios of
1:4, 1:2 and 1:1, respectively. Compared to the diffraction peaks in
Fig. 1, it can be found that the diffraction peaks intensity of (001)
and (101) in Fig. 2 are lower. Meanwhile, the diffraction intensity
ratio of (001)/(101) planes for the BiOCl and AgBr/BiOCl hetero-
junctions prepared in EG are 0.43, 0.48, 0.45 and 0.56. This means
that the BiOCl prepared in EG may grow along the (101) plane,
suggesting that the as-prepared samples have a high exposure
percentage of {101} facets, for which the sample was labelled as
{101}BiOCl.
The morphology and structure of the resultant samples were
observed by SEM. It is clearly seen that well-defined AgBr particles
(see Fig. 3a) are in a small size. Fig. 3b shows that the {101}BiOCl
consists of microsphere with diameters about 7 mm. The AgBr-{101}
BiOCl (1:2) heterojunction was also characterized by SEM (Fig. 3c)
which indicates that AgBr is well embedded in {101}BiOCl surface.
It is proverbial to figure out the band gap energies of AgBr, {001}
BiOCl and {101} BiOCl on the basis of the DRS result by using the
following equation [28].
Aðhn
ꢂ EgÞn=2
¼
a
hn
(1)
where a, h, A, v and Eg are adsorption coefficient, Plank constant,
proportionality constant, light frequency and band gap, respec-
tively. In the equation, n is decided by the type of optical transition
in a semiconductor, where n ¼ 4 for indirect transition and n ¼ 1 for
direct transition. Fig. 4c displays the plots of (ahy) y for
1/2 against h
the samples. The band gap energies of AgBr, {001}BiOCl and {101}
BiOCl are calculated to be 2.56, 3.44 and 3.51 eV, respectively.
Generally speaking, the optical band gap in crystals is anisotropic
and decided by the direction of the crystal [29]. The small differ-
ence of Eg values between the {001}BiOCl and {101}BiOCl samples
may be attributed to the difference of the morphology and the
percentage of exposed {101} facets or {001} facets. In addition, the
electric potentials of the VB and CB of the samples can be calculated
by Mulliken electronegativity theory [30,31]:
Fig. 2. XRD diffraction patterns of the AgBr (a), BiOCl (b) and AgBr/BiOCl samples with
different mole ratio of 1:4 (c), 1:2 (d) and 1:1 (e) synthesized in EG.

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Y.L. Qi et al. / Journal of Alloys and Compounds 712 (2017) 535e542
Fig. 3. SEM images of AgBr (a), {101}BiOCl (b), AgBr-{101}BiOCl (1:2) (c), {001}BiOCl (d) and AgBr-{001}BiOCl (1:2) (e), together with XPS survey spectra of AgBr-{001}BiOCl (1:2)
and Ag 3d (f).
adsorption-desorption equilibrium. Fig. 5a displays the variation of
ECB ¼ X ꢂ EC ꢂ 0:5Eg
EVB ¼ ECB þ Eg
(2)
(3)
RhB concentration (C/C0) versus the irradiation time of AgBr-{101}
BiOCl (1:1), AgBr-{101}BiOCl (1:2), AgBr-{101}BiOCl (1:4) and AgBr,
respectively. AgBr-{101}BiOCl (1:2) shows a relatively better pho-
tocatalytic activity, with a degradation ratio of 87.9% after 15 min.
Obviously, different facets of AgBr/BiOCl composites have different
photocatalytic activity for the photodegradation of RhB under the
visible light irradiation. For comparison, Fig. 5b shows the photo-
degration abilities of {001}BiOCl and their heterojunctions. The
concentration of RhB decrease rapidly in the presence of the AgBr-
{001}BiOCl (1:2) samples, which shows the highest photocatalytic
activity. For pure BiOCl sample and AgBr, the total degradation of
RhB can only reach 84.4% and 57.9% after 15 min of visible light
irradiation. Besides, the physical mixture of {001}BiOCl and AgBr
photocatalysts show lower photocatalytic activity than the AgBr-
{001}BiOCl (1:2) heterojunction. As for the heterojunctions fabri-
cated by BiOCl and AgBr, they all showed better photocatalytic
performance than that of pure AgBr or BiOCl.
where Eg and X are the band gap energy and the electronegativity
of semiconductor, respectively. ECB is the conduction band and EVB
is the valence band edge potentials, respectively. Ec is the energy of
free electrons about 4.5 eV on the hydrogen scale. The values of X
for AgBr and BiOCl are 5.81 and 6.36 respectively. Therefore, the ECB
and EVB of AgBr are calculated to be 0.03 eV and 2.59 eV. Through
potential calculation, it is found that the conduction band of {101}
BiOCl is located at 0.11 eV, which is more negative than that of {001}
BiOCl (0.14 eV). Meanwhile, the valence band of {101}BiOCl is
3.62 eV, which is more positive than the VB potential of {001}BiOCl
(3.58 eV).
The photocatalytic activity of all samples was evaluated by
photodegrading RhB under visible light. All the samples are firstly
The kinetics of RhB photodegradation was calculated
disposed for 0.5
h without light irradiation to achieve an

Y.L. Qi et al. / Journal of Alloys and Compounds 712 (2017) 535e542
539
Fig. 5. Photodegradation efficiencies of RhB over the samples prepared in EG (a) and
water (b).
lnðC=C0Þ ¼ kt
(4)
where k is the apparent first-order rate constant, t, C and C0 are the
degradation time, concentration and initial concentration of RhB
during the photodegradation reaction, respectively. Table 1 exhibits
the photodegradation rate constants over different catalysts. The
rate constants for the degradation of RhB over the {101}BiOCl, AgBr-
{101}BiOCl (1:1), AgBr-{101}BiOCl (1:2) and AgBr-{101}BiOCl (1:4)
are 0.0663, 0.0818, 0.1368 and 0.1341 minꢂ1, respectively. While the
photocatalytic degradation constants of {001}BiOCl, AgBr-{001}
BiOCl (1:1), AgBr-{001}BiOCl (1:2) and AgBr-{001}BiOCl (1:4) are
Table 1
The photocatalytic degradation rate constant of RhB in the presence of samples.
Fig. 4. Diffuse reflectance spectra of the samples prepared in EG (a) and water (b); (c)
Plots of (ahv)1/2 versus hv for the AgBr, {001}BiOCl and {101}BiOCl.
Photocatalyst
k/(minꢂ1
)
Photocatalyst
k/(minꢂ1
)
AgBr
0.0573
0.0818
0.1368
0.1341
0.0663
AgBr
0.0573
0.1236
0.2533
0.1308
0.1223
AgBr-{101}BiOCl (1:1)
AgBr-{101}BiOCl (1:2)
AgBr-{101}BiOCl (1:4)
{101}BiOCl
AgBr-{001}BiOCl (1:1)
AgBr-{001}BiOCl (1:2)
AgBr-{001}BiOCl (1:4)
{001}BiOCl
quantitatively by fitting the experimental data with the pseudo-
first-order model [32,33], as expressed by

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Y.L. Qi et al. / Journal of Alloys and Compounds 712 (2017) 535e542
calculated to be 0.1223, 0.1236, 0.2533 and 0.1308 minꢂ1, respec-
tively. And the rate constant for AgBr is 0.0573 minꢂ1. It is obvious
that photocatalytic activity of all the AgBr-{001}BiOCl composites is
higher than that of the AgBr-{101}BiOCl. Besides, the photocatalytic
activity of the AgBr-{001}BiOCl (1:2) composites is approximately 2
times of the AgBr-{101}BiOCl (1:2). It can be seen from the results
that AgBr-{001}BiOCl (1:2) possesses the best photocatalytic ac-
tivity under visible light irradiation, which may be due to syner-
getic effect of the higher percentage of exposed {001} facetson the
catalyst's surface and the photosensitization of AgBr.
To investigate the mineralization of organic compounds on the
AgBr-{001}BiOCl (1:2) composites, the Total Organic Carbon (TOC)
analysis was carried out to determine the mineralization of RhB
during the photocatalytic reaction (Fig. 6). As can be seen that the
TOC removal increases with the irradiation time and arrives at
21.48% through consecutive irradiation of visible light within
15 min (Fig. 6). The degree of mineralization reached 98% for RhB
after 210 min of visible light irradiation. The results demonstrate
that not only the chromophoric group but also the aromatic groups
in the RhB molecules are attacked in the photocatalytic reaction. In
addition, these results also indicate that the completely destruction
of RhB is possible. In this experiment, the AgBr-{001}BiOCl (1:2)
composite with the highest photocatalytic activity was also used to
evaluate the photocatalytic degradation of phenol in order to rule
out the photosensitization effect under light irradiation. As shown
in Fig. 7, the phenol degradation efficiency is nearly 100% when the
AgBr-{001}BiOCl (1:2) composite was irradiated for 150 min which
indicates that the AgBr-{001}BiOCl (1:2) composites also exhibit
superior activities for phenol degradation.
Fig. 7. The UVeVis absorption spectra of phenol under solar light irradiation using
AgBr-{001}BiOCl (1:2) as photocatalyst.
In addition, the reusability and stability of a photocatalyst is a
significant parameter for practical applications. So in order to
evaluate the reusability and stability of the AgBr-{001}BiOCl (1:2)
samples during photocatalytic degradation process, a recycling test
of the degradation of RhB was carried out, and the experimental
results are shown in Fig. 8. After each run of repetitive photo-
catalysis reactions, all of the photocatalysts were collected and
centrifuged for the next recycling test. A slight decrease in photo-
degradation efficiency can be observed after two cycles for the
degradation of RhB, which is attributed to the loss of catalysts
during the decontamination. Recycling test shows that the AgBr-
{001}BiOCl (1:2) photocatalysts possess relatively good stability.
For the purpose of investigating the structural stability of the AgBr-
Fig. 8. The cycling stability of AgBr-{001}BiOCl (1:2) photocatalyst for degradation of
RhB under visible light irradiation.
{001}BiOCl (1:2) photocatalyst after three recycling measurements,
the catalyst was centrifuged and washed with distilled water for
several times, collected and dried at 60 ꢀC overnight. Fig. 9 shows
XRD patterns of AgBr-{001}BiOCl (1:2) after three recycling mea-
surements. It can be seen that there exhibit similar diffraction
peaks in Fig. 9 compared with Fig. 1d, suggesting similar crystal
structures. Not only the diffraction peaks of AgBr but also the
diffraction peaks of BiOCl can be observed in the XRD patterns. In
particular, the (200) and (220) diffraction peaks in the XRD patterns
belonging to the AgBr can be observed. No any impurity can be
detected, which indicates that the structure of AgBr-{001}BiOCl
(1:2) photocatalyst is relatively stable.
As is know that the photocurrent experiment is related to the
transfer of the photo-generated electrons and holes, therefore, it
can represent the separation and recombination of the photo-
generated carriers. The transient photocurrent responses of the
samples were also recorded under simulated visible light irradia-
tion under the light-on and light-off cycles. In Fig. 10a, the photo-
electrode of the AgBr-{101}BiOCl (1:2) shows the highest
photocurrent density compared with the AgBr and {101}BiOCl
Fig. 6. The TOC evolution as a function of irradiation time in AgBr-{001}BiOCl (1:2)
photocatalysis process.

Y.L. Qi et al. / Journal of Alloys and Compounds 712 (2017) 535e542
541
photoelectrodes, indicating the efficient separation of photo-
induced charges and the faster transfer of charges in the AgBr-
{101}BiOCl (1:2) composite. It is commonly believed that the higher
photocurrent density leads to a higher charge separation efficiency,
resulting in higher catalytic performance. Similarly, the AgBr-{001}
BiOCl (1:2) heterojunction also displays the highest photocurrent
intensity than AgBr and {001}BiOCl as shown in Fig. 10b. It is clearly
that the AgBr-{001}BiOCl (1:2) shows the better photocatalytic
activity than that of AgBr-{101}BiOCl (1:2) according to the exper-
iment result (Fig. 5a and b), which may indicate that the BiOCl
sheets with high percentage of {001} facets exposed are beneficial
to the photocatalytic activity. As is known that Bi and O atoms are
normally arranged orderly in the (001) facet of the BiOCl, this
means that more oxygen atoms are exposed. Therefore, the oxygen
vacancies can be formed easily on the surface of BiOCl, coming from
the low chemical bonding energy of Bi-O [34]. Moreover, under the
irradiation, disorder of the BiOCl surface might be caused since the
energy is very powerful [35]. As a result, the length of Bi-O bonds is
increased and the Bi-O bond can be considered broken to a certain
degree. This particularly facilitates the release of O atoms which
come from the surface of BiOCl, and then the oxygen vacancies can
be generated [10]. Oxygen vacancy is always considered as one kind
of point defect which plays an important role in the photocatalytic
activity for the degradation of RhB. It has been reported that the
oxygen vacancy in BiOCl can extend the photoabsorption of BiOCl to
visible-light region and facilitate the separation efficiency of photo-
generated electrons and holes [2], subsequently enhance the pho-
tocatalytic performance. Therefore, the tremendous enhancement
of photocatalytic activity on AgBr-{001}BiOCl are mainly attributed
to the synergy between the high {001} exposed and defects (oxy-
gen vacancy).
Fig. 9. XRD diffraction patterns of AgBr-{001}BiOCl (1:2) samples after three recycling
measurements.
To further identify the photocatalytic mechanism, different
scavengers were introduced to evaluate the contributions of active
species on the photodegradation, the results are shown in Fig. 11.
The system was added 2 mg benzoquinone (BQ), 2.13 mg ammo-
nium oxalate and 15 mL isopropanol (IPA) which act as superoxide
radicals (ꢃOꢂ2 ) scavenger, holes (hþ) scavenger and hydroxyl radicals
(ꢃOH) scavenger, respectively. As shown in Fig. 11, benzoquinone
suppresses the degradation of RhB, indicating that superoxide
radicals play an important role in the photodegradation reaction. It
ꢃ
is well known that the isopropanol is a quencher of OH, the pho-
todegradation rate does not decreased obviously when isopropanol
Fig. 10. Photocurrent response for pure AgBr, AgBr-{101}BiOCl (1:2) and {101}BiOCl
Fig. 11. Effects of radical scavengers in the degradation process of RhB over the AgBr-
(a); AgBr, AgBr-{001}BiOCl (1:2) and {001}BiOCl (b) under visible light irradiation.
{001}BiOCl (1:2) heterojunction.

542
Y.L. Qi et al. / Journal of Alloys and Compounds 712 (2017) 535e542
light region. The oxygen vacancies plays a positive role in the dye
degradation processes. The synergetic effect of the highly exposed
crystal facets and photosensitization of AgBr can improve the
photocatalytic performance of AgBr-{001}BiOCl composites and
facilitate its practical applications in the environmental protection
issues in future.
Compliance with ethical standard
Conflict of interest the authors declare that they have no con-
flict of interest.
Acknowledgment
This work is financially supported by the National Nature Sci-
ence Foundation of China (No. 21273034).
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Fig. 12. Schematic illustration of the AgBr-{001}BiOCl (1:2) photocatalytic reaction
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In summary, a series of AgBr/BiOCl composites with different
facets were successfully synthesized through a solvothermal and
chemical precipitation method by hybridizing BiOCl with AgBr. The
as-prepared AgBr-{001}BiOCl composites with high exposed {001}
facets exhibit higher photocatalytic activity than that of AgBr-{101}
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{001} facets of BiOCl, which can significantly enhance the light
absorption and boost the interfacial charge transfer in the visible-