Enhanced photocatalytic activity in hybrid composite combined BiOBr nanosheets and Bi2S3 nanoparticles

Article abstract of DOI:10.1016/j.jpcs.2018.05.023

To improve photocatalytic activity of 2D BiOBr nanosheets, Bi₂S₃ nanoparticles were introduced to fabricate the hybrid composite using co-precipitation method. The Bi₂S₃ loading 15 wt% amount of as-prepared comp

Full text of DOI:10.1016/j.jpcs.2018.05.023

Journal of Physics and Chemistry of Solids 121 (2018) 163–171
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids
journal homepage: www.elsevier.com/locate/jpcs
Enhanced photocatalytic activity in hybrid composite combined BiOBr
nanosheets and Bi S nanoparticles
2
3
a
a
b,∗
a
a,∗∗
Shuai-Ru Zhu , Qi Qi , Wen-Na Zhao , Yuan Fang , Lei Han
a
State Key Laboratory Base of Novel Functional Materials and Preparation Science, School of Materials Science & Chemical Engineering, Ningbo University, Zhejiang
15211, China
Key Laboratory for Molecular Design and Nutrition Engineering of Ningbo, Ningbo Institute of Technology, Zhejiang University, Zhejiang 315100, China
3
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
Photocatalyst
BiOBr
Bi S
2 3
Rhodamine B
2 3
To improve photocatalytic activity of 2D BiOBr nanosheets, Bi S nanoparticles were introduced to fabricate the
hybrid composite using co-precipitation method. The Bi2S3 loading 15 wt% amount of as-prepared composite
shows the optimal visible light-driven photocatalytic activity for the degradation of Rhodamine B (RhB), tet-
racycline hydrochloride (TC) and ciprofloxacin (CIP). Radicals trapping experiments are further conducted to
detect the reactive active species generated during the irradiation of BiOBr/Bi
2
S
3
-15 photocatalyst. It demon-
Tetracycline hydrochloride
strated that h+ and ∙O
−
play the key role in photocatalytic degradation of pollutants. Photocurrent response,
2
2 3
EIS and PL spectra reveal that the enhanced photocatalytic performance of BiOBr/Bi S hybrid material shows
higher separation efficiency of the photo-generated electron–holes pairs.
1. Introduction
conduction band of Bi
BiOBr, and the valence band of Bi
2
S
3
is more cathodic than the conduction band of
is less anodic than the valence
2 3
S
Two-dimensional (2D) layered materials have attracted much at-
band of BiOBr. This suitable band structure is beneficial for charge
separation and transfer efficiency to enhance photocatalytic perfor-
mance. To the best of our knowledge, the growth of Bi S nanoparticles
2 3
on 2D BiOBr nanosheets to create nanocomposite photocatalysts was
rarely investigated [34–38].
With the rapid development of industrialization, a large number of
waste water contains different types of pollutants. Thereinto, dyes and
antibiotic pollutants can damage circulation of animals and plants in
the biosphere. Especially, drug-resistant pathogens can be induced
through residues of antibiotic in ecosystem, which may cause possible
risks to human health. Therefore, it is very significant to remove these
tention due to their promising applications in catalysis, electronics and
energy storage devices [1–4]. BiOBr is one of popular 2D layered
semiconductors materials for photocatalytic energy conversion due to
its good chemical stability and excellent photocatalytic activity under
visible light irradiation [5–9]. Recently, some strategies have been
realized to improve photocatalytic activity of BiOBr, especially by the
fabrication of BiOBr-based heterostructured composites [10–15]. These
photocatalysts integrating the synergistic effects of each components,
which could result in the increased visible light harvesting ability and
the enhanced photocatalytic activity [16]. In general, the large surface
area of 2D BiOBr nanosheets allow the robust anchoring of nano-
particles onto its surface, which facilitates perfect interface separation
centers and combines the excellent catalytic performances of both na-
nosheets and nanoparticles simultaneously [17–24]. As a proof of
2 3
pollutants [39–41]. In this work, 2D BiOBr nanosheets integrating Bi S
nanoparticles hybrid materials were designed and synthesized via a
simple co-precipitation method. This is significant to explore the de-
pendence of catalytic activity with nano-size of Bi
gradation of organic pollutants under visible light irradiation. The
BiOBr nanosheets incorporated with Bi nanoparticles showed ex-
2 3
S towards de-
2 3
concept, our strategy focuses on combining Bi S nanoparticles with 2D
BiOBr nanosheets via self-assembly growth to create hybrid nano-
composites.
2 3
S
cellent catalytic activity for Rhodamine B (RhB), tetracycline hydro-
Bi
S
2 3
nanoparticles have attracted significant interest due to the
chloride (TC) and ciprofloxacin (CIP) photodegradation than individual
unique band gap and photocarrier trapping effect [25,26], and exhibit
potential applications in various fields, such as photocatalytic activity,
hydrogen generation, solar cells, etc. [27–33]. Meanwhile, the
BiOBr nanosheets, which highlight the indispensable role of Bi
noparticles in improving the photocatalytic performance.
2 3
S na-
∗
Corresponding author.
∗
∗
Corresponding author.
E-mail addresses: wnzhao@nit.zju.edu.cn (W.-N. Zhao), hanlei@nbu.edu.cn (L. Han).
https://doi.org/10.1016/j.jpcs.2018.05.023
Received 11 February 2018; Received in revised form 9 May 2018; Accepted 17 May 2018
Available online 19 May 2018
0022-3697/ © 2018 Elsevier Ltd. All rights reserved.

S.-R. Zhu et al.
Journal of Physics and Chemistry of Solids 121 (2018) 163–171
2. Experimental
2.5. Photocatalytic experiments
2
.1. Materials
The photocatalytic activity of the photocatalyst was measured by
photodegradation of Rhodamine B (RhB), tetracycline hydrochloride
(TC) and ciprofloxacin (CIP). 40 mg of sample was dispersed into
aqueous solution in a photochemical reactor containing 200 mL RhB
(20 mg/L), TC (10 mg/L) or CIP (10 mg/L). A 500 W Xe lamp using an
BILON photochemical reactor (Shanghai BILON Instrument
Corporation) was provided as a visible light source. The pH value was
not adjusted when the reaction was operated. Experiments were carried
out at 25 °C with a circulating water system to prevent thermal catalytic
effects. Prior to the irradiation, the solution was ultrasonicated for
Bismuth nitrate pentahydrate (Bi(NO
3
)
3
·5H
2
O), acetic acid (HAc),
·H O, 25–28%),
KBr, elemental sulfur (S), ammonium hydroxide (NH
dimethyl sulfoxide (DMSO) were purchased from Sinopharm and
Aldrichand used without further purification.
3
2
2
2 3
.2. Synthesis of Bi S nanoparticles
Elemental sulfur (0.0116 g) was dissolved in 120 mL of DMSO at
40 °C, then, 0.1188 g of Bi(NO ·5H O was incorporated into this
OH aqueous solu-
5
min and stirred continuously magnetically for 30 min in the dark to
1
3
)
3
2
get a saturated pollutants absorption onto the photocatalyst surface. At
certain time intervals, 6 mL of suspensions were collected and cen-
trifuged to remove the products. The pollutants concentration of the
absorbance at the wavelength of 553 nm (RhB), 357 nm (TC) and
solution. After stirred for 30 min, 3.0 mL of the NH
4
tion was added to the reaction mixture which turned from transparent
to a light brown color. The formed colloid was held at 140 °C for 15 min
and then cooled to room temperature naturally. All the synthesis pro-
cess occurred under constant magnetic stirring.
272 nm (CIP) were analyzed by a TU-1901 spectrometer (Beijing
Purkinje General Instrument). The blank experiment was also treated
under the same processes, but no photocatalyst was mixed. The pho-
todegradation efficiency was evaluated by dividing C/C
0
, where C is the
2
.3. Preparation of BiOBr/Bi
2
S
3
nanocomposite
2 3
·5H O and a specified amount of Bi S
remained dye concentration and C is the starting dye concentration.
0
Typically, 0.243 g Bi(NO
3
)
3
2
2.6. Active species trapping experiments
nanoparticles was dissolved in 0.75 mL acetic acid (HAc), and the re-
sulting solution was added to 7.5 mL de-ionized water containing
0
room temperature, the suspension was transferred into a Teflon-lined
stainless steel autoclave (20 mL capacity) and heated at 120 °C for 6 h.
The resulting precipitate was filtrated, washed thoroughly with distilled
water to remove any possible ionic species in the product, and then
To explore the photocatalytic mechanism of the photocatalytic re-
action, radicals trapping experiments are further conducted to detect
the reactive active species generated during the irradiation of BiOBr/
.060 g KBr with ultrasonication for 10 min. After stirring for 20 min at
Bi
2 3
S -15 photocatalyst. 1 mM of hydroxyl radicals (∙OH) scavenger
+
(
isopropyl alcohol, IPA) [14], a holes (h ) scavenger (triethanolamine,
−
TEOA) [42] and a superoxide radicals (∙O
2
) scavenger (benzoquinone,
dried at 70 °C overnight. BiOBr/Bi
varying the Bi nanoparticles mass percent to 5.0% (BiOBr/Bi
0.0% (BiOBr/Bi -10), 15.0% (BiOBr/Bi -15) and 20.0% (BiOBr/
Bi -20). For comparison, pure BiOBr nanosheets were synthesized
under the same conditions in the absence of Bi nanoparticles. To
further understand BiOBr/Bi system, we use Bi nanoparticles as
O under the above same
2
S
3
-x system were prepared by
BQ) [43] were added to the RhB solution. Otherwise, the method was
similar to the described above-mentioned experiment of photo-
degradation of dye.
S
2 3
2 3
S -5),
1
2
S
3
2 3
S
2 3
S
2 3
S
2.7. Photoelectrochemical measurements
2
S
3
2 3
S
3
+
Bi sources of BiOBr instead of Bi(NO
conditions, and noted as BiOBr-B.
3 3 2
) ·5H
The electrochemical impedance spectroscopy (EIS) were analyzed
on an electrochemical analyzer (CHI660E, Chenhua, Shanghai, China)
using a standard three-electrode system with the samples as the
working electrodes, a saturated calomel electrode (SCE) as the re-
ference electrode, and a Pt wire as the counter electrode, respectively.
ITO and Ni film was used as the current collector of working electrode
for measuring photocurrent response and EIS, respectively. For the
fabrication of the working electrodes, 80 wt% of active materials, 10 wt
% of acetylene black (conductive agent) and 10 wt% of vinylidene
fluoride (binder)were dispersed in 1-methyl-2-pyrrolidinone to form
homogeneous slurry. Then the slurry was dotted on the Ni film and
dried for 24 h at room temperature. EIS was performed from 0.01 Hz to
100 kHz at an open circuit potential of 0.3 V and alternating current
(AC) voltage amplitude of 5 mV. The electrodes were immersed in
2
.4. Characterization
The powder X-ray diffraction (XRD) patterns of the products were
investigated by a Bruker D8 advance X-ray diffractometer equipped
with Cu Kα radiation (λ = 1.5418 Å). The FT-IR spectrum experiments
were performed on a Nicolet 6700 F T-IR spectrometer in the range of
−1
4
00–4000 cm . The morphology of the as-prepared samples was car-
ried out by Hitachi S-4800 field-emission scanning electron micro-
graphs (FESEM) at an acceleration voltage of 10.0 kV.
Thermogravimetric analysis (TGA) was measured with SII TG/DTA
7
300 instrument at a heating rate of 10 °C/min. UV–Vis diffuse re-
flectance spectra were recorded by a Perkin Elmer Lambda 950 UV/Vis/
NIR spectrometer using BaSO as reference in the wavelength of
00–800 nm. The chemical component and dispersive state was oper-
0.2 M of Na
lamp [44].
2 4
SO aqueous solution and illuminated by using a 500 W Xe
4
2
ated at Energy Dispersive Spectrometer using EDAX analyzer. X-ray
photoelectron spectroscopy (XPS) measurements were recorded on a
Thermo Fisher Scientific corporation Escalab 250Xi instrument. All
binding energies were calibrated using the C 1s peak at 284.8 eV.
Photoluminescence (PL) spectra of the obtained samples were detected
at room temperature with a Hitachi F-4600 fluorescence spectrometer.
3. Results and discussion
The X-ray diffraction data of BiOBr samples prepared via hydro-
thermal method are shown in Fig. 1a. The peaks with 2θ values at 10.7°,
21.6°, 25.0°, 31.6°, 32.2°, 39.2°, 46.3°, 56.2°, 57.2° were respectively
indexed to (001), (002), (011), (012), (110), (112), (020), (114) and
(112) crystal planes of tetragonal BiOBr (JCPDS 73–2061) [45,46]. The
broad and superimposed signals in the diffractogram corresponding to
N
2
adsorption–desorption
isotherms
were
evaluated
with
Brunauer–Emmett–Teller (BET) and pore size distributions measure-
ments using an ASAP 2020 apparatus. Transmission electron micro-
scopy (TEM) images were determined at a FEI corporation TF20.
Raman spectra was recorded on a Renishaw inVia-reflex micro-Raman
spectrometer with laser excitation of 785 nm.
the Bi
order, which is consistent with a previous report [26]. For BiOBr/Bi
15 photocatalyst, no typical diffraction peaks of Bi nanoparticles
were observed, indicating that the very low-range crystalline order of
2 3
S nanostructures account for the very low-range crystalline
2 3
S -
2 3
S
164

S.-R. Zhu et al.
Journal of Physics and Chemistry of Solids 121 (2018) 163–171
Fig. 1. (A) XRD patterns and (b) FT-IR spectra of samples: (A) BiOBr-B, (B) BiOBr/Bi
2
S
3
-5, (C) BiOBr/Bi
2
S
3
-10, (D) BiOBr/Bi
2
S
3
-15, (E) BiOBr/Bi
2
S
3
-20, (F) Bi
2
S
3
, (G)
BiOBr.
(
Fig. 2). As for the Bi
2
S
3
/BiOBr composite, the peak intensity of BiOBr
nanoparticles. Fur-
2 3
decreased significantly with introduction of Bi S
−1
thermore, compared to signal at 118 and 165 cm of pure BiOBr, the
corresponding Raman peaks in BiOBr/Bi -15 composite slightly right
shift, which is caused by the nano-size effect of Bi nanoparticles.
The surface morphology and microstructure of the as-synthesized
pure BiOBr and BiOBr/Bi -x materials were examined by field emis-
2 3
S
2 3
S
2 3
S
sion scanning electron microscopy (SEM) and transmission electron
microscopy (TEM). It can be seen that pristine BiOBr is flake-like with
an average diameter of approximately 1 μm (Fig. S1a). By the co-pre-
cipitation method, the BiOBr nanoflakes are doped with a certain
amount of Bi
with BiOBr, BiOBr/Bi
diameter of about 600 nm (Fig. S1b). When the mass percent of Bi
nanoparticles to BiOBr was 10%, the average size of the nanoflakes
decreased slightly to 400 nm (Fig. S1c). As the content of Bi Nano-
particles was increased to 15%, the morphology of BiOBr/Bi re-
vealed obvious changes to three dimensional overlapped nanosheets
structure (Figs. S1e–f). Clearly observation discloses that BiOBr/Bi -x
has a smaller size diameter than pure BiOBr, demonstrating that the
Bi nanoparticles content in BiOBr/Bi -x has a significant effect on
the size of the BiOBr materials. At the same time, the formation of this
D structure may be attributed to a certain number of Bi nano-
particles effectively promote the anisotropic growth of BiOBr. When the
mass percent was further increased to 20%, 3D overlapped nanosheets
structure could also be observed (Fig. S1d). In order to explain this
anisotropic growth phenomenon, we synthesize BiOBr-B sample in Fig.
S2. It can be clearly observed that many smaller 3D overlapped na-
2
S
3
nanoparticles to form BiOBr/Bi
2 3
S -x system. Compared
2 3
S
-5 exhibits a similar nanoflake with average
2 3
S
Fig. 2. Raman spectra of samples.
2 3
S
Bi
the FT-IR spectra of Bi
investigate the material compositions and structures. The peaks at
2
S
3
nanoparticles exist in the BiOBr/Bi
S
2 3
-15 composite. Fig. 1b gives
2 3
S
2
S
3
nanoparticles, BiOBr and BiOBr/Bi
2 3
S
-15 to
2 3
S
−1
around 3430 and 1628 cm can be assigned as the O–H stretching and
bending vibrations respectively due to a small amount of water ad-
sorbed on the surface of the catalysts. The absorption band at 513 cm
is attributed to the Bi–O stretching mode [46]. Moreover, the peaks at
S
2 3
2 3
S
−1
3
2 3
S
−
1
1
382 and 1115 cm
bration band of the Bi–S. About the formation of BiOBr/Bi
the vibration peak intensity of the Bi–S bond increased slightly with the
increase of Bi nanoparticles content from 5 to 20%, indicating that
the Bi nanoparticles and BiOBr have been coupled together suc-
cessfully [37]. Also, it can be found that the BiOBr-B prepared by using
Bi
nanoparticles as Bi3+ sources displayed the vibration band of the
Bi–S, implaying that still a few Bi nanoparticles can be maintained
in the spectrum of pure Bi
2
S
3
belong to the vi-
2 3
S
-x system,
2 3
S
2 3
S
nosheets was generated, implying that Bi
perse in solution as growth center of Bi
2
S
3
nanoparticles can be dis-
sources to promote the ani-
sotropic growth of BiOBr. Furthermore, the typical TEM images
Fig. 3a) of BiOBr/Bi -15 further observed that Bi nanoparticles
became smaller size by erosion effect of BiOBr (indicated by arrows in
Fig. 3b). It also can be implied that Bi nanoparticles as growth center
are dispersed into BiOBr nanosheets and is consistent with the SEM
analysis. Fig. S3 shows the HRTEM image of the BiOBr/Bi -15.
2
S
3
3+
2 3
S
after hydrothermal treatment. In order to further investigate micro-
scopic structure of samples, Raman spectra analysis are applied. The
(
2
S
3
2 3
S
2 3
Raman peaks of Bi S nanoparticles were located at the broad peak
2 3
S
−
1
regions (170–280 cm ). This result is close to the previous literature
26], also suggesting that the Bi nanoparticles was synthesized
[
2 3
S
2 3
S
2 3
Fig. 3. (A-b) TEM images of BiOBr/Bi S -15.
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S.-R. Zhu et al.
Journal of Physics and Chemistry of Solids 121 (2018) 163–171
2 3
Fig. 4. EDS analyses and mapping of BiOBr/Bi S -15 composite.
Meanwhile, The red circles show the variational and distorted lattice
fringes, which further confirm the existence of the Bi and BiOBr in
composites. Energy dispersed spectroscopy (EDS) of the BiOBr/Bi -15
indicates they contain Bi, O, S and Br elements (Fig. 4). The peak of C is
ascribed to the conducting resin used in the EDS characterization.
Moreover, the elemental mappings of Bi (Fig. 4c), Br (Fig. 4d), O
trace amount of Bi
2 3
S nanoparticles in Fig. 5e. The S 2s peak for BiOBr/
2
S
3
Bi -15 have a higher in binding energy, which can be attributed to the
2 3
S
S
2 3
strong chemical bonding by erosion effect of BiOBr nanosheets [34,35].
Furthermore, weight percentage of S element was 0.93% by XPS spectra
analysis. This result is lower than EDS analysis, indicating that surface
content of Bi
particles as growth center for BiOBr nanosheets was consistent with FT-
IR, EDS, SEM and TEM analysis. Fig. 5f shows the ad-
2 3 2 3
S nanoparticles less than its internal. Thus, Bi S nano-
(
Fig. 4e), and S (Fig. 4f) further confirm the elements distribution of the
sample, implying that the Bi nanoparticles are well-distributed on
S
2 3
N
2
BiOBr nanosheets. Based on the EDS analyses, the actual weight per-
centage of S element was estimated to be about 1.5%, which was
slightly lower than the theoretical percentage.
sorption–desorption isotherms of the photocatalyst samples. The spe-
cific surface area and pore average diameter (Figs. S4–S5) of BiOBr/
2
−1
Bi
16.22 nm and 1.86 m g , 33.59 nm, respectively, which indicated
that Bi nanoparticles effectively change the microstructure of the
S
2 3
-15 and pristine BiOBr are determined to be 10.68 m g
,
2
−1
X-ray photoelectron spectroscopy (XPS) was used to study the sur-
face composition and chemical state of the as-prepared BiOBr/Bi
hybrid nanosheets. The typical XPS survey spectrum in Fig. 5a presents
that BiOBr/Bi -15 was composed of Bi, Br, O and S elements. In
S
2 3
-15
2 3
S
BiOBr nanosheets. Meanwhile, the enhanced surface area and appro-
priate pore size can facilitate to the adsorption and transfer of dye
molecules and abundant active sites to improve the photocatalytic
performance to some extent.
2 3
S
Fig. 5b, the O 1s binding energy of 530.4 eV was attributed to the
crystal lattice O atoms (Bi–O) in BiOBr, while the O 1s binding energy
of 531.8 eV might represent surface eOH [14]. The Br 3 d peaks of the
sample fitted into 68.6 eV and 69.5 eV in Fig. 5c are ascribed to Br 3d5/
UV–vis diffuse reflectance spectra (DRS) was measured to analyze
the optical absorption of pure BiOBr, Bi
Bi -15 photocatalysts, as shown in Fig. 6. The BiOBr and BiOBr/Bi
15 samples have very close absorption edges in the 430 nm, which is in
agreement with the previous literature [12]. Bi nanoparticles have
2 3
S nanoparticles and BiOBr/
2
and Br 3d3/2, respectively. The Bi 4f peaks of the sample appeared at
64.8 and 159.5 eV in Fig. 5d can be attributed to the spin orbital
S
2 3
2 3
S -
1
splitting photoelectrons of the Bi 4f5/2 and Bi 4f7/2, respectively.
However, the Bi 4f5/2, Bi 4f7/2 and Br 3d5/2, Br 3d3/2 peaks for BiOBr/
2 3
S
more strong absorption over the whole visible light region. In addition,
the band gap of semiconductors also can be estimated by Kubelk-
Bi
2 3
S -15 have a slight positive shift to high in binding energy, which
n/2
can be attributed to the strong chemical bonding by coupling with Bi
S
2 3
a–Munk (KM) expression [44]: α(hv) = A (hv–Eg) , where α, h, v, A
nanoparticles into BiOBr nanosheets. Meanwhile, the weak peak of S 2s
XPS spectrum could be assigned to 231.4 eV binding energy from a
and Eg are absorption coefficient, Planck's constant, photon frequency,
constant and band gap, respectively. For BiOBr, the value of n is 4 for
166

S.-R. Zhu et al.
Journal of Physics and Chemistry of Solids 121 (2018) 163–171
Fig. 5. XPS spectra of BiOBr/Bi
2
S
3
-15sample:(a) survey scan, (b) O1s, (c) Br3d, (d) Bi4f, (e) S2s, (f) Nitrogen adsorption-desorption isotherms of samples.
the indirect transition. For Bi
transition [34,35]. The band gaps of BiOBr and BiOBr/Bi
calculated to be 2.85 and 2.79 eV, respectively. Due to the nano-size
effect, the Eg values are about 1.48 eV for Bi nanoparticles (Fig. S6).
To demonstrate the photocatalytic activities of the BiOBr/Bi
2
S
3
the value of n is 1 for the direct
clearly blue shift in RhB absorption band at 553 nm was observed,
which can be regarded as the step-by-step de-ethylation process (Fig.
S7) [47]. It shows excellent photocatalytic efficiency, as about 77.8% of
RhB (20 mg/L) was degraded under visible light irradiation for
100 min. By comparison, degradation efficiency of BiOBr-B is similar
2 3
S -15 are
2 3
S
2 3
S
composites, we choose the RhB as a model organic pollutant to de-
compose under visible-light irradiation, as shown in Fig. 7. Before ir-
radiation, saturation adsorption/desorption equilibrium between the
photocatalyst and the dye molecules in solution was achieved. During
the photodegradation, unpromoted photolysis of RhB is negligible. An
with BiOBr, indirectly indicating that presence of Bi
played an significant role to improve the catalytic performance of
BiOBr. In addition, the Bi nanoparticles has an effect on the photo-
catalytic activity of as-prepared BiOBr/Bi composites, following an
2 3
order of BiOBr/Bi -15 > BiOBr/Bi -10 > BiOBr/Bi S -
2 3
S nanoparticles
2 3
S
2 3
S
2
S
3
2
S
3
Fig. 6. (A) UV–Vis diffuse reflectance spectra. (b) The band gap energies of the samples.
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S.-R. Zhu et al.
Journal of Physics and Chemistry of Solids 121 (2018) 163–171
Fig. 7. (A) Photocatalytic degradation efficiencies of RhB with different sample sunder visible-light irradiation. (b) Comparison of the rate constant k in the presence
of different samples. Degradation of (c) TC and (d) CIP in the presence of pure BiOBr, BiOBr/Bi -15 under visible light irradiation.
2 3
S
2 3 2 3
20 > BiOBr/Bi S -5 > BiOBr. The BiOBr/Bi S -15 photocatalyst
shows the highest photocatalytic activity. This indicates that the pho-
tocatalytic activities are mainly influenced by the appropriate content
of the Bi
parent reaction constants (k) were quantitatively investigated by a
pseudo-first-order model (−ln (C/C ) = kt) about RhB degradation,
and the data are summarized in Fig. 7b. The reaction constant of BiOBr/
Bi -15 is about 3.04 times higher than that of the pure BiOBr.
Antibiotic agent have been widely used in human and veterinary
2 3
S nanoparticles existed in the BiOBr. The corresponding ap-
0
2 3
S
applications against infectious diseases, and their residues left in the
environment can induce the proliferation of drug-resistant pathogens
and may cause serious threats to the human health and ecosystem [48].
Thereinto, tetracyclines (TCs) have the strongest toxicity on 20 kinds of
commonly used antibiotics [49]. Therefore, tetracycline hydrochloride
(
TC) and ciprofloxacin (CIP) were selected as a typical representative
for evaluating the photocatalytic performances of BiOBr/Bi -15 ma-
terial. As shown in Fig. 7c and d, the blank test showed that the self-
photolysis of CIP and TC can be neglected. The BiOBr/Bi -15 com-
posite exhibited higher activity for TC and CIP degradation as com-
pared to pure BiOBr with visible light exposure. The enhanced photo-
2 3
S
2 3
S
Fig. 8. Trapping experiment of active species during the photodegradation of
RhB over BiOBr/Bi
light for 100 min.
2 3
S -15 and BiOBr photocatalyst under irradiation by visible-
2 3
activity of BiOBr/Bi S -15 material may become valuable photocatalyst
in potential elimination for pollutants in the future.
To further study the main active species over BiOBr/Bi
S
2 3
-15 pho-
2 3
the arc of BiOBr/Bi S -15 is reflects the lower charge-transfer resistance
tocalyst, IPA, TEOA and BQ were used as the trapping scavengers for
than that of the BiOBr, indicating that there exists a fast interfacial
charge transfer and results in an effective separation of the photo-
generated electrons and holes by the introduction of Bi S nanoparticles
+
−
∙
2
OH, h and ∙O , respectively [14,42]. As shown in Fig. 8, the rate of
RhB degradation abruptly decreased by adding BQ or TEOA. While,
only slight decrease was observed by adding IPA. These results deduced
2
3
[17,50]. The PL spectrum was consistent with the characterization re-
sults of photocurrents and EIS measurements. As shown in Fig. 9b, the
BiOBr/Bi S -15 sample presents lower PL intensity than BiOBr under
+
−
2
that both the h and ∙O
radicals acted as the main active species
during photocatalysis. Meanwhile, it is also observed that the same
phenomenon was occurred at pure BiOBr.
2
3
the excitation wavelength of 300 nm. These results further reveal the
modification of Bi2S3 is an effective way to improve the separation ef-
ficiency of electron–hole pairs. To measure the stability and lifetime of
the BiOBr/Bi S -15, the photocatalyst was reused for photocatalytic
Due to nano-size effect of Bi
excellence of BiOBr/Bi -x composite over BiOBr in the charge carrier
transfer. The photocurrent response of the BiOBr/Bi -15 is about 1.79
times as high as that of the BiOBr nanosheets. The increased current
indicates that BiOBr/Bi -15 restrains the recombination rate of pho-
2 3
S , we further explore the extremely
2 3
S
2 3
S
2
3
reaction under the same conditions, and the test is shown in Fig. 9d.
2
S
3
The photocatalytic efficiency of the BiOBr/Bi S -15 slightly decreases
2
3
togenerated electron–hole pairs and prolongs the life time of photo-
generated carriers (Fig. 9c). Electrochemical impedance spectroscopy
about 8.4% after four cycles. This situation may be due to the mass loss
during the photocatalytic processes.
(
EIS) pure BiOBr and BiOBr/Bi
2
S
3
-15 hybrid material was carried out to
On the basis of the above experimental results, a suitable me-
chanism for photodegradation of RhB over BiOBr/Bi2S3 system is
the interfacial charge transfer resistance (Fig. 9a). The smaller radius of
168

S.-R. Zhu et al.
Journal of Physics and Chemistry of Solids 121 (2018) 163–171
Fig. 9. (A) Electrochemical impedance spectroscopy of samples. (b) Photoluminescence (PL) spectra of samples recorded at room temperature with the excitation
wavelength of 300 nm. (c) Photocurrent responses for samples (d) Reusability of BiOBr/Bi S -15 for the photocatalytic degradation of RhB.
2 3
discussed. Under visible-light irradiation, holes were generated in va-
lence band (VB) and electrons were transferred into conduct band (CB)
in BiOBr and Bi
VB edge potential can be acquired by the formula EVB = X–E +0.5 Eg,
CB = Eg–EVB [37], where X is the electronegativity of the semi-
2 3
S (Eqs. (1) and (2)). The theoretical calculated CB and
e
E
conductor that is the geometric average of the absolute electro-
negativity of the constituent atoms, ECB and EVB is the conduct band and
valence band edge potential, Eg is the band gap energy of BiOBr
e
(
2.85 eV) and Bi
S
2 3
(1.48 eV), E is the energy of free electrons on the
hydrogen scale (ca. 4.5 eV). Thus the VB and CB of BiOBr is 3.15 and
0
0
.3 eV, while the VB and CB of Bi
.04 eV, respectively. Due to the band edge potentials normally move
and BiOBr at pH = 7
2 3
S is estimated to be 1.52 and
2 3
with pH, the new potentials of VB and CB for Bi S
were calculated using an equation: Efp = Efp −0.05915 pH, where Efp
is the flat band potential of photocatalyst when the pH is 0. In which the
0
0
2 3
Scheme 1. Plausible mechanism for photocatalytic degradation of BiOBr/Bi S -
1
5 under visible irradiation.
pH was 7.0, the corresponding EVB and ECB of Bi
−
−
2
S
3
were 1.11 and
0.37 eV while those of BiOBr were estimated to be 2.74 and
0.11 eV, respectively. Therefore, the holes on the VB top of BiOBr
. The photo-
generated electrons can be transferred efficiently from the CB of Bi
−
(e + h+
Bi
S
2 3
+ hv → Bi
S
2 3
)
(2)
(3)
(4)
(5)
(6)
2 3
transferred in the opposite direction to the VB top of Bi S
RhB + hv → RhB* → RhB+ + e−
2 3
S
−
−
and then introduced into the more positive CB of BiOBr. Clearly, the
suitable energy band match is beneficial for effectively restrains the
recombination of photoinduced electron–hole pairs, resulting in an
enhanced photocatalytic activity. In this process, holes could initiate
e
2 2
+ O → ∙O
h+ + RhB/RhB+ → intermediates → CO
2 2
+ H O
−
2
+ RhB/RhB+ → intermediates → CO
2 2
+ H O
•O
the direct oxidation of RhB to form the ultimate degradation products
−
2
(
Eq. (5)). E
0
(O
2
/∙O
) is about −0.048 eV vs. NHE. The CB potential of
BiOBr and Bi
2 3 2
S are to reduce the O molecule to the oxygen radical by
4
. Conclusion
the photo-excited electrons. Moreover, the photosensitization of RhB is
not negligence because RhB (0.95 eV vs. NHE) could also be excited to
RhB* (−1.42 eV vs. NHE) injects electrons into CB of Bi
2 3
In summary, Bi S nanoparticles were introduced to fabricate the
2
S
3
(Eq. (3))
hybrid composite to improve photocatalytic activity of 2D BiOBr na-
nosheets. The 15 wt% BiOBr/Bi hybrid nanosheets show the optimal
visible light-driven photocatalytic activity for the degradation of RhB
and TC. The results from the photocurrent response, EIS and PL spectra
reveal that the enhanced photocatalytic performance of BiOBr/Bi
hybrid materials show higher separation efficiency of the photo-gen-
erated electron–holes pairs by introduction of Bi nanoparticles. The
radicals trapping experiments disclose that h+ and ∙O
role in photocatalytic degradation of pollutants. We expect that the
[
51,52]. As follow, these photoexcited electrons will react with O
2
−
2 3
S
adsorbed on the interface of BiOBr/Bi
moving into the CB of BiOBr nanosheets (Eq. (4)). Then, ∙O
further oxidize the RhB molecules (Eq. (6)). Therefore, the plausible
reaction mechanism could be summarized as follows, and illuminated
in Scheme 1.
2
S
3
to form ∙O
2
radicals while
−
2
could
2 3
S
2 3
S
BiOBr + hv → BiOBr (e + h+
−
)
(1)
−
2
play the key
169

S.-R. Zhu et al.
Journal of Physics and Chemistry of Solids 121 (2018) 163–171
synthesis strategy developed in this study could be extended to promote
other nanoparticles with 2D ultrathin nanosheets for better photo-
catalytic degradation or solar energy utilization.
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