Controlled growth of single-crystalline Bi.333(Bi6S9)Br nanorods under hydrothermal conditions for enhanced photocatalytic reduction of Cr (VI)

Article abstract of DOI:10.1016/j.jallcom.2020.155879

The rational design of composition and structure of the photocatalytic materials is especially critical for the practical application of photocatalytic technology. Herein, uniform single-crystal Bi_.333(Bi₆S₉)Br nanorods were synthesized for the first time using a one-pot hydrothermal method. By prolonging the hydrothermal reaction time, the evolution process of Bi_.333(Bi₆S₉)Br from nanoparticles to short nanorods, and finally to long nanorods was observed. Bi_.333(Bi₆S₉)Br nanorods can effectively inhibit the recombination of photogenerated electron-hole pairs and effectively adsorb of Cr (VI) due to the presence of surface defects and unique structure, thereby promoting the photocatalytic activity. Bi_.333(Bi₆S₉)Br nanorods exhibited excellent photocatalytic performance as well as promising recyclability in rapidly reducing aqueous Cr (VI) to Cr (III) under visible light. The degradation rate of Cr (VI) reached 90.96percent within 60 min, and the structure of Bi_.333(Bi₆S₉)Br did not change after six photocatalytic cycles. The photocatalytic reaction mechanism of nanorods was proposed based on the band structure of Bi_.333(Bi₆S₉)Br and electron spin resonance radical scavenging assay.

Full text of DOI:10.1016/j.jallcom.2020.155879

Journal of Alloys and Compounds 842 (2020) 155879
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Controlled growth of single-crystalline Bi.333(Bi S )Br nanorods under
6 9
hydrothermal conditions for enhanced photocatalytic reduction of Cr
(
VI)
*
*
*
Lili Ai , Dianzeng Jia , Nannan Guo , Mengjiao Xu , Su Zhang , Luxiang Wang , Lixia Jia
Key Laboratory of Energy Materials Chemistry, Ministry of Education Key Laboratory of Advanced Functional Materials, Autonomous Region Institute of
Applied Chemistry, Xinjiang University, Urumqi, Xinjiang, 830046, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 February 2020
Received in revised form
The rational design of composition and structure of the photocatalytic materials is especially critical for
the practical application of photocatalytic technology. Herein, uniform single-crystal Bi.333(Bi
nanorods were synthesized for the first time using a one-pot hydrothermal method. By prolonging the
hydrothermal reaction time, the evolution process of Bi.333(Bi )Br from nanoparticles to short nano-
rods, and finally to long nanorods was observed. Bi.333(Bi )Br nanorods can effectively inhibit the
recombination of photogenerated electron-hole pairs and effectively adsorb of Cr (VI) due to the pres-
6 9
ence of surface defects and unique structure, thereby promoting the photocatalytic activity. Bi.333(Bi S )
6 9
S )Br
23 May 2020
6 9
S
S
6 9
Accepted 31 May 2020
Available online 4 June 2020
Keywords:
Br nanorods exhibited excellent photocatalytic performance as well as promising recyclability in rapidly
reducing aqueous Cr (VI) to Cr (III) under visible light. The degradation rate of Cr (VI) reached 90.96%
Bi.333(Bi
6 9
S )Br
Nanorods
Photocatalysis
Cr (VI)
within 60 min, and the structure of Bi.333(Bi
photocatalytic reaction mechanism of nanorods was proposed based on the band structure of
Bi.333(Bi )Br and electron spin resonance radical scavenging assay.
6 9
S )Br did not change after six photocatalytic cycles. The
6 9
S
©
2020 Elsevier B.V. All rights reserved.
1
. Introduction
most commonly used photocatalysts, which are typically effective
under ultraviolet light due to their wide band gaps. Various doping
and modifications have been performed to improve their quantum
efficiency and spectral response range, which, however, remain as
major challenges for their large scale applications [12,13]. Hence,
more and more studies have been focusing on the exploration of
alternative materials.
Photocatalysis using semiconductive oxides, sulfides etc. as
active materials is one of the most promising approaches in tack-
ling energy and environment problems [1,2]. While photocatalytic
water splitting and CO reduction remain highly challenging for
2
practical applications, the degradation of harmful pollutants using
photocatalysis has demonstrated high applicability for various
organic compounds and inorganic cations [3e5]. For example, in
wastewater treatment the abatement of heavy metal pollutants like
hexavalent chromium Cr (VI) can be achieved by photocatalytic
process [6e8]. It is noteworthy that Cr (VI) is extremely toxic to
microorganisms as well as to human beings. Exposure to Cr (VI)
may cause various health problems such as chromosome aberra-
tions, digestive tract diseases, lung cancer, etc [9e11]. Up till now,
titanium oxide- and zinc oxide-based semiconductor oxides are the
Recently, Bi-based semiconductors have received extensive
attention due to their stable chemical structure and excellent
photocatalytic performance [14,15]. Bismuth oxyhalides (BiOX,
X ¼ F, Cl, Br, I) as the most notable semiconductors among all bis-
2
þ
layers sandwiched be-
muth compounds, consisting of [Bi
2
O
2
]
tween two slabs of halogen ions [16e19]. As compared to oxide,
bismuth oxyhalides exhibit a higher photocatalytic activity due to
the presence of halogens. Another class of interesting Bi com-
pounds in photocatalysis is bismuth chalcogenides, which have a
good electrical conductivity, an outstanding thermoelectric per-
formance and a narrow band gap [20e22]. For example, Luo and
2 3
coworkers prepared hollow Bi S nanospheres via Kirkendall effect,
which was used for photocatalytic reduction of Cr (VI) in waste-
water in electroplating industry and showed an outstanding per-
*
Corresponding author.
*
* Corresponding author.
E-mail addresses: jdz0991@gmail.com (D. Jia), wangluxiangxju@163.com
(
L. Wang).
2 3
formance [23]. Jin and coworkers reported Bi S nanoribbons with
https://doi.org/10.1016/j.jallcom.2020.155879
925-8388/© 2020 Elsevier B.V. All rights reserved.
0

2
L. Ai et al. / Journal of Alloys and Compounds 842 (2020) 155879
different aspect ratios synthesized by solvothermal technique, and
subsequent photocatalytic studies revealed that Bi nanocatalysts
with the largest aspect ratio exhibited the highest activity in the
reduction of CO into methanol [24]. Wang et al. reported a general
ionic-assisted microwave-ultrasonic combined synthetic strategy
to fabricate Bi
(E ¼ S, Se, and Te) hierarchitectures, and the
prepared hierarchical Bi nanospheres can be used in the actual
2 3
S
2
2 3
E
2 3
S
Cr(VI)-containing wastewater treatment [25]. These promising re-
sults motivated us to work a step further in this direction, namely,
to make use of the advantages of both oxyhalides and chalcogen-
ides. It is believed that the halogens added to Bi chalcogenides will
play an essential role in improving the photocatalytic activity of Bi
chalcogenide like the BiOX system [26,27]. Nevertheless, bismuth
sulfidehalides have been rarely reported in literatures so far.
6 9
In this work, the single-crystal Bi.333(Bi S )Br nanorods were
synthesized by a one-pot hydrothermal method for the first time.
The amount of raw materials used in this synthesis was calculated
based on the stoichiometric ratio of Bi and S elements in
6 9
Bi.333(Bi S )Br. The effect of reaction time on the formation of
nanorods was explored in detail, and the formation mechanism of
nanorods was subsequently put forward. An outstanding photo-
catalytic performance was recorded in the reduction of Cr (VI) to Cr
Fig. 1. XRD patterns of the as-prepared samples obtained by hydrothermal reaction for
different reaction durations.
(
III) under visible light irradiation. Moreover, the photocatalytic
mechanism of the nanorods is proposed. The successful synthesis
and application of the highly active single crystalline nanorods in
this work may stimulate more studies on Bi-based photocatalysts.
respectively. X-ray photoelectron spectroscopy (XPS, Thermo
ESCALAB 250Xi, USA) was performed to investigate the surface
chemical composition and binding state of the samples. The elec-
tron paramagnetic resonance (EPR) spectra were recorded on a
Bruker EMX nano EPR spectrometer at 298 K, and electron spin
resonance (ESR) spectra were obtained using an EPR spectrometer
2
. Experimental
6 9
2.1. Synthesis of Bi.333(Bi S )Br nanorods
(
JEOL JES-FA200, Japan) at the same temperature.
All chemicals are of analytical grade and used as received
without further purification. K Cr (99.5%) was used to simulate
wastewater in photocatalytic tests. The Bi.333(Bi )Br nanorods
were synthesized by a one-pot hydrothermal process. In a typical
preparation process, 0.486 g of Bi(NO $5H O (1 mmol) and
.350 g of polyvinylpyrrolidone [(C NO) , PVP] were dissolved
into 25 mL of mannitol (C , 0.1 mol/L) solution under vigorous
stirring. 5 mmol of NaBr was slowly added into the above mixture
yielding a uniform reseda suspension. Afterwards, 1.4 mmol of
3 2
thioacetamide (CH CSNH , TAA) was added to the above reseda
2
2 7
O
2
.3. Photocatalytic activity evaluation
6 9
S
A photocatalytic reaction system (Xujiang Electromechanical
3
)
3
2
Plant, Nanjing, China) was used to evaluate the photocatalytic
reduction of Cr (VI) under visible light illumination from a 300 W
Xenon lamp with a 420 nm cutoff filter (about 11 mW/cm ). In a
typical process,10 mg of photocatalyst was added into a quartz tube
reactor containing 50 mL of Cr (VI) solution (10 mg L based on Cr
in a dilute K Cr solution). In order to ensure the establishment
0
6
H
9
n
6 14 6
H O
2
ꢁ
1
2 7
O
suspension and vigorous stirring for 1 h for sulfuration. Subse-
2
of an adsorption-desorption equilibrium, the suspension was son-
icated for 10 min and magnetically stirred in the dark for 30 min
before irradiation. During the reaction process, 2 mL aliquot of the
suspension was taken out at given time intervals. The supernatant
liquid was obtained after centrifugation and used for further
analysis. The concentration of Cr (VI) ions in the supernatant was
determined by the diphenylcarbazide (DPC) method (see Sup-
porting Information) and the corresponding UV-vis absorption
spectra were recorded on a UV-vis spectrophotometer. After each
reaction cycle, the photocatalyst was separated by centrifugation
and employed for Cr abatement in a fresh Cr (VI) solution
quently, the mixture was sealed in a 50 mL Teflon-lined stainless-
steel vessel and treated at 160 C for 10 min, 1 h, 4 h and 8 h,
respectively. The products were washed with distilled water,
centrifuged and dried in a freeze drier. For comparison, pure oxy-
ꢀ
2 3
halide (BiOBr) and pure chalcogenide (Bi S ) were synthesized
ꢀ
under the same hydrothermal conditions (160 C for 4 h) except for
the absence of TAA and NaBr, respectively.
2.2. Characterization
X-ray diffraction (XRD) was performed using a SmartLab-SE
ꢁ1
(10 mg L ). Parallel experiments were performed to obtain enough
Diffractometer (Rigaku, Japan) with a Cu K
a
radiation source
¼ 1.5406 Å). A field emission scanning electron microscopy
FESEM, Hitachi S-4800, Japan) was used to examine the
catalysts for the reuse.
(l
(
morphology of the samples. The high resolution transmission
electron microscopy (HRTEM) and corresponding Fast Fourier
Transform (FFT), and high angle annular dark field-scanning
transmission electron microscope (HAADF-STEM) were carried
out with a JEM 2100F microscope equipped with an energy
dispersive spectrometer (EDS). The solid-state diffuse reflectance
spectra (DRS) and photoluminescence emission spectra (PL) were
measured on a UV-vis spectrophotometer (Hitachi U-3010, Japan)
and a fluorescence spectrophotometer (Hitachi F-4500, Japan),
3. Results and discussion
6 9
3.1. Characterization of Bi.333(Bi S )Br samples
The XRD patterns of the samples obtained by hydrothermal
ꢀ
reaction at 160 C for different reaction durations are shown in
Fig. 1. All the reflections of the samples can be readily assigned to
the hexagonal Bi.333(Bi
6
S
9
)Br (JCPDS card No. 70-0202, space group
ꢀ
ꢀ
P63/(173), a ¼ b ¼ 15.545 Å, c ¼ 4.019 Å,
a
¼
b
¼ 90 ,
g
¼ 120 ). It

L. Ai et al. / Journal of Alloys and Compounds 842 (2020) 155879
3
1 1 2 2 6 9
Fig. 2. FESEM images (a -d ) and TEM images (a -d ) of Bi.333(Bi S )Br nanorods synthesized by hydrothermal method for different reaction durations: (a) 10 min, (b) 1 h, (c) 4 h
and (d) 8 h.
can be seen from the XRD patterns that with increasing reaction
time the reflections became stronger, and the full width at half
maxima (FWHM) of diffraction peak of (130) facet became nar-
rower, indicating continuous growth of crystals and increasing
raw materials during hydrothermal synthesis.
The development of the nanorods was evidenced by FESEM and
TEM studies as shown in Fig. 2. Nanoparticles of Bi.333(Bi )Br were
formed initially and the nanoparticles tend to form short nanorods
(Figs. 2a and 10 min), which were grown up into one dimensional
short nanorods as the reaction time was extended to 1 h (Fig. 2b).
As the reaction time increased further to 4 h, the length of the
nanorods increased significantly (Fig. 2c). Finally, well-defined
nanorods with high aspect ratios were obtained after reaction for
6 9
S
crystallinity. The crystal structure of Bi.333(Bi
Fig. S1. It consists of BiS and Bi(S Br ) polyhedra, as well as BiS
Bi(S Br ) polyhedra connected consecutively by coedges. The XRD
results demonstrated that highly crystalline Bi.333(Bi )Br can be
prepared by stoichiometrically applying Bi and S elements in the
S
6 9
)Br is depicted in
8
5
2
8
and
5
2
6 9
S

4
L. Ai et al. / Journal of Alloys and Compounds 842 (2020) 155879
6 9
Fig. 3. Schematic illustration of the growth processes of the Bi.333(Bi S )Br nanorods.
6 9
Fig. 4. Microscopic characterization of Bi.333(Bi S )Br nanorods: (a-b) TEM images, (c) HRTEM image, (d) FFT image, (e) HAADF-STEM image and EDS elemental mapping images.
8
h (Fig. 2d). Based on the above analysis, it can be concluded that
6 9
Bi.333(Bi S )Br nanorods with high aspect ratios and high crystal-
the nanorods were formed through initial nucleation followed by
anisotropic growth. The growth of the nanorods is schematically
linity are finally formed after hydrothermal reaction for more than
4 h.
illustrated in Fig. 3. Firstly, the Bi(NO
3
)
3
$5H
2
O was dissolved into
For photocatalysts, a larger particle size may increase the
transport path and the recombination probability of photo-
generated charge carriers, which are detrimental for photocatalytic
reactions [28]. Thus, grain size is an important factor in photo-
ꢁ
mannitol solution and reacted with the large quantity of the Br
2ꢁ
and S ion to form the Bi.333(Bi
6 9
S )Br nucleates in the solution.
Subsequently, the Bi.333(Bi )Br nucleates could be further grown
6 9
S
into nanoparticles under hydrothermal conditions, followed by the
formation of short nanorods with increasing reaction time. The
catalysis. In this work, the microstructure of Bi.333(Bi
tained by hydrothermal reaction for 4 h was further analyzed in
6 9
S )Br ob-

L. Ai et al. / Journal of Alloys and Compounds 842 (2020) 155879
5
6 9
Fig. 5. XPS spectra of the as-prepared Bi.333(Bi S )Br: (a) survey spectrum, (b) Bi 4f, (c) Br 3d, (d) S 2s.
detail. As shown in Fig. 4a and b, the TEM images demonstrate that
the obtained Bi.333(Bi )Br nanorods are homogeneous in shape
and size with diameter of about 10 nm and a typical length of about
50 nm. The clear lattice fringes with a lattice spacing of 3.73 Å in
HRTEM image corresponds to the (130) facet of Bi.333(Bi )Br
crystal (Fig. 4c), and the diffraction spot corresponding to the d-
value of 3.73 Å can be observed in the FFT pattern (Fig. 4d). The
corresponding EDS elemental mapping images reveal that the Bi, S
and Br elements are evenly distributed throughout the whole
nanorods (Fig. 4e).
162.1 eV are well matching to the S 2p3/2 and S 2p1/2 peaks [33,34].
The Br 3d spectrum is deconvoluted into two peaks at 67.6 and
6 9
S
68.5 eV (Fig. 5c), which can be ascribed to Br 3d3/2 and Br 3d1/2
,
1
respectively [35,36]. The S 2s spectrum in Fig. 5d consists of a single
2
ꢁ
S
6 9
peak at 225.3 eV, revealing the presence of the S species [11,37].
The UV-vis DRS was used to investigate the optical properties of
6 9 6 9
the Bi.333(Bi S )Br nanorods. As shown in Fig. S2a, the Bi.333(Bi S )
Br shows strong photo absorption in both the ultraviolet and visible
light regions. It is known that the band gap energy of the direct
semiconductor is linearly proportional to the square of the ab-
sorption coefficient, and the indirect semiconductor band gap en-
ergy is linearly proportional to the square root of the absorption
coefficient [2,38]. Analysis of the converted Kuberka-Munk func-
tion and light energy shows that the square of the absorption co-
efficient versus the energy function is more linear than the square
root of the absorption coefficient versus the energy function (inset
6 9
The surface composition and chemical states of Bi.333(Bi S )Br
were investigated by XPS. The survey XPS spectrum of the sample is
shown in Fig. 5a, and there are no obvious impurities in the sample
except for O and C, which are common for materials after exposure
to ambient air. It was reported that surface defects could be eval-
uated by the Bi/S/Br ratio derived from XPS [29]. The Bi/S/Br atomic
ratios of the as-obtained Bi.333(Bi
6
S
9
)Br differ greatly from the
of Fig. S2a). We believe that the Bi.333(Bi
band gap semiconductor. The band gap (E
Bi.333(Bi )Br nanorods is derived to be about 1.65 eV (inset of
Fig. S2a). To further explore the band structure of Bi.333(Bi )Br,
XPS valence band (VB) spectrum was collected and linearly
extrapolated to determine the VB position (EVB) of the sample. As
6
S
9
)Br catalyst is a direct
theoretical values as disclosed by XPS analysis (insert in Fig. 5a).
The lower concentration of S element indicates the presence of
sulfur vacancies in the surface region. Fig. 5b shows the high-
resolution Bi 4f spectrum. The doublet at 158.3 and 163.6 eV of
g
) of the as-prepared
6 9
S
6 9
S
6 9
Bi.333(Bi S )Br can be assigned to Bi 4f7/2 and Bi 4f5/2, respectively
[
30e32]. The shoulder at higher binding energies may be tenta-
6 9
displayed in Fig. S2b, the EVB of Bi.333 (Bi S ) Br is about 0.91 eV vs.
tively assigned to BieS defect because the BieS defect is more
electropositive than the lattice Bi and therefore appears at higher
binding energies [33]. The binding energies located at 161.0 and
vacuum level. The EVB vs. NHE could be calculated to be 0.86 eV
according to the contact potential difference between the sample
and the XPS analyzer. According to the equation ECB ¼ EVB - E
g
, the

6
L. Ai et al. / Journal of Alloys and Compounds 842 (2020) 155879
ꢀ
Fig. 6. (a) UV-vis absorption spectra of Cr (VI) solution as a function of reaction time using Bi.333(Bi
6
S
9
)Br (obtained at 160 C for 4 h) as catalyst under visible light (>420 nm)
irradiation; (b) Reduction curves of Cr (VI) derived from (a), BiOBr and Bi S were also tested for comparison; (c) Degradation rate of Cr (VI) after reaction for 60 min, (d) Pseudo-
2 3
first-order kinetics fitting curves.
corresponding conduction-band (CB) position (ECB) of Bi.333(Bi
Br is ꢁ0.79 eV.
6
S
9
)
Bi.333(Bi
dark, which may be due to the presence of surface defects on the
Bi.333(Bi )Br nanorods that enable to effectively adsorb and acti-
vate Cr (VI), thereby promoting the photocatalytic reaction. 90.96%
of Cr (VI) was reduced to Cr (III) after 60 min using Bi.333(Bi )Br as
photocatalyst, while 13.14% and 33.92% were reduced at the same
time using BiOBr and Bi as photocatalyst, respectively (Fig. 6c).
In addition, the reduction curves of Cr (VI) catalyzed by Bi.333(Bi
Br, BiOBr and Bi after eliminating the adsorption are shown in
Fig. S6. The Bi.333(Bi )Br exhibits the best photocatalytic perfor-
mance with 76.20% Cr (VI) reduction in 60 min after eliminating the
adsorption. The plot of -ln (C/C ) as a function of reaction time
suggests that the Cr (VI) reduction reaction fits well with pseudo-
first-order kinetics, i.e. -ln(C/C
) ¼ kt, where C is the concentra-
tion of the Cr (VI) at time t, C is the initial concentration of the Cr
VI) solution, and the slope k is the apparent reaction rate constant
11]. As shown in Fig. 6d, the k constant of Bi.333(Bi )Br is deter-
6 9
S )Br nanorods have the highest adsorption capacity in the
6 9
S
6 9
3.2. Photocatalytic performance of Bi.333(Bi S )Br nanorods
6 9
S
To evaluate the photocatalytic performance of Bi.333(Bi
6 9
S )Br,
2 3
S
the reduction of Cr (VI) was performed under visible light
6 9
S )
(
>420 nm). Among all the samples, the one obtained by hydro-
2 3
S
ꢀ
thermal reaction at 160 C for 4 h showed the highest conversion
efficiency (Fig. S3). The sample obtained at 160 C for 10 min is
6 9
S
ꢀ
unstable under visible light irradiation because the crystal struc-
ture was destroyed after reaction (Fig. S4). It was demonstrated that
different hydrothermal reaction time led to different grain size, and
the effect of grain size on catalytic performance can be clearly seen
from the catalytic tests. Obviously, the sample of 4 h outperformed
other samples thanks to its grain size and unique structural and
electronic properties related to its dimensions.
0
0
0
(
[
6 9
S
ꢁ2
ꢁ1
mined to be 2.35 ꢂ 10 min , which is 12.6 times that of BiOBr
UV-vis absorption spectra of Cr (VI) solution were recorded at
ꢁ
3
ꢁ1
ꢁ3
(
k ¼ 1.86 ꢂ 10 min ) and 5.6 times that of Bi
2
S
3
(k ¼ 4.19 ꢂ 10
min ), respectively. Compared with literature results (Table S1),
the Bi.333(Bi )Br sample shows great advantages in the photo-
different time intervals during photocatalytic reduction using
ꢁ
1
ꢀ
Bi.333(Bi
6
S
9
)Br (obtained at 160 C for 4 h) as catalyst (Fig. 6a). For
6 9
S
comparison, the photocatalytic performances of the BiOBr nano-
sheets and Bi nanorods (see XRD and FESEM results in Fig. S5)
catalytic reduction of Cr (VI).
2 3
S
The recyclability and stability of photocatalyst are very impor-
tant for practical applications. Recycle experiments were therefore
carried out to assess the stability of the sample. As displayed in
synthesized under the same conditions were also investigated. The
concentration of Cr (VI) derived from the absorption spectra de-
creases with increasing reaction time (Fig. 6b). Moreover, adsorp-
tion of aqueous phase pollutants is also important for efficient
Fig. 7a, the degradation rates of Cr (VI) over Bi.333(Bi
6 9
S )Br show a
slight decrease after 6 cycles due to incomplete collection and
2 3
photocatalytic process [39e43]. Compared to BiOBr and Bi S ,

L. Ai et al. / Journal of Alloys and Compounds 842 (2020) 155879
7
ꢀ
Fig. 7. (a) Degradation rate of Cr (VI) in the presence of Bi.333(Bi
6
S
9
)Br (obtained at 160 C for 4 h) under visible light irradiation for 6 cycles; (b) XRD patterns of Bi.333(Bi
6
S
9
)Br before
6 9
and after 6 cycles of photocatalytic reaction; XPS analysis of Bi.333(Bi S )Br after photocatalytic reaction: (c) survey spectrum and (d) Cr 2p spectrum.
washing of the photocatalyst after each cycle. XRD studies revealed
no structural change of the sample after photocatalytic tests as
indicated by nearly identical diffraction patterns (Fig. 7b). The
sample after recycling tests was further characterized by XPS. As
shown in Fig. 7c, Bi, Br, S, and Cr were detected in the survey
spectrum. In Fig. 7d, the Cr 2p3/2 and Cr 2p1/2 peaks were observed
at 575.7 and 585.3 eV, respectively, which can be assigned to Cr (III)
species [44,45]. The presence of Cr (III) species on the catalyst is
obviously due to the successful abatement of Cr (VI) under visible-
light illumination.
6 9
of much higher concentration of defects in Bi.333(Bi S )Br. Lots of
reports show that the defects can adsorb and activate the pollutants
[48e50], which implies more adsorption sites and enhanced
6 9
reactivity of the Bi.333(Bi S )Br provided by abundant rationally
created surface defects, namely sulfur defects.
ꢁ
Moreover, the reaction intermediates $OH and $O
2
were
measured by ESR, where 5, 5-dimethyl-1-pyrroline-N-oxide
(DMPO) was used as the radical scavenger. As described in Fig. 8c
ꢁ
and d, both DMPO-$O
2
(1:1:1:1) and DMPO-$OH (1:2:2:1) signals
could not be observed in the dark, while they could be clearly
To understand the photocatalytic performance, PL and EPR
detected under visible light irradiation. Moreover, the illumination
ꢁ
spectra of Bi.333(Bi
of BiOBr and Bi
recombination rate of the photogenerated electrons and holes
under irradiation. Fig. 8a shows that the PL intensity of Bi.333(Bi
Br is much lower than those of BiOBr and Bi , indicating that
Bi.333(Bi )Br is able to separate photogenerated electrons and
6
S
9
)Br were measured and compared with those
time significantly affects the production of $OH and $O
2
. A much
2
S
3
. PL spectroscopy can be used to evaluate the
higher amount of radicals were detected at 10 min than at 5 min.
Considering the band structure of Bi.333(Bi )Br, the redox abilities
of electrons on conduction band (CB) and holes on valence band
6 9
S
6 9
S )
ꢁ
ꢁ
2
2
S
3
(VB) are stronger than the potential of O
2
/$O
2
(ꢁ0.33 eV) and HO
/
6
S
9
$OH (0.12 eV) as well as H
2
O
2
/O
2
(0.70 eV) [51], respectively. In this
radicals can be generated, which is consistent
ꢁ
holes more effectively. EPR was employed to examine the presence
of crystal defects and their relative concentrations in the samples
case, $OH and $O
2
with the results of ESR spin-trap measurements.
(
2
Fig. 8b). The g-factor of Bi.333(Bi
.0048, which may be related to the sulfur defects. This is consis-
tent with the XPS results discussed in Fig. 5. Compared to
Bi.333(Bi )Br, the shift of the EPR signal may be attributed to
different types of defects [46,47]. In contrast, no signal was detected
for Bi , indicating that there are little defects. The Bi.333(Bi )Br
showed a much stronger signal than BiOBr, pointing to the presence
6
S
9
)Br was determined to be
Based on these results, a possible photocatalytic reaction
mechanism is proposed to elucidate the high photocatalytic activity
of Bi.333(Bi S )Br nanorods (Fig. 9). First, when the Bi.333(Bi S )Br
6 9 6 9
6
S
9
was irradiated under visible-light, the photogenerated electrons
ꢁ
(e ) were excited from the VB of Bi.333(Bi
6
S
9
)Br to its CB and the
þ
2
S
3
6
S
9
holes (h ) remained in the VB (Equation (1)). Then, the adsorbed Cr
(VI) cations will be reduced rapidly to non-toxic Cr (III) by the

8
L. Ai et al. / Journal of Alloys and Compounds 842 (2020) 155879
ꢁ
Fig. 8. (a) PL and (b) EPR spectra of the Bi.333(Bi
6
S
9
)Br, BiOBr and Bi
2
S
3
; ESR spectra of (c) DMPO-$O
2
and (d) DMPO-$OH using Bi.333(Bi
6
S
9
)Br as photocatalyst under visible light
irradiation for different time (0 min means before irradiation in the dark).
6 9
Fig. 9. Schematic diagram of photocatalytic mechanism of Bi.333(Bi S )Br nanorods.

L. Ai et al. / Journal of Alloys and Compounds 842 (2020) 155879
9
highly reducing electrons on the CB (Equation (2)), and simulta-
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with other people or organizations that can inappropriately influ-
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any nature or kind in any product, service and/or company that
could be construed as influencing the position presented in the
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[
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