Bismuth MOF-derived BiOBr/Bi24O31Br10heterojunctions with enhanced visible-light photocatalytic performance

Article abstract of DOI:10.1039/d0cy00019a

In this work, a hierarchical BiOBr/Bi24O31Br10 heterojunction is prepared for the first time from a bismuth MOF (Bi-BTC) by a facile bromination-annealing route. Since cetyltrimethylammonium bromide (CTAB) is used as the Br element source, the heterojunction constructed of BiOBr and Bi24O31Br10 nanosheets maintains the rod-like morphology of the original MOF. In photocatalytic degradation tests, the BiOBr/Bi24O31Br10 heterojunction exhibits much higher photocatalytic activity than pure Bi2O3, and the heterojunctions annealed at 450 °C show the best photocatalytic activity among samples prepared at various temperatures. The heterojunction also shows good pH adaptability, good reusability, and structural stability for RhB degradation, and the TOC results and full UV-vis spectra confirm its good mineralization performance under visible-light irradiation. Based on the photo-electrochemical characterization and free radical tests, a photocatalytic enhancement mechanism over the heterojunction and different photocatalytic reactions in the presence of photo-induced h+, O2- and 1O2 are reasonably proposed.

Full text of DOI:10.1039/d0cy00019a

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Bismuth MOF-derived BiOBr/Bi O Br
2
4 31 10
heterojunctions with enhanced visible-light
Cite this: DOI: 10.1039/d0cy00019a
photocatalytic performance†
Ganghong Huang, Zishun Li, Kaun Liu, *ab Xuekun Tang,
ab
ab
ab
ab
Jing Huang and Guofan Zhang*
In this work, a hierarchical BiOBr/Bi24
MOF (Bi-BTC) by a facile bromination–annealing route. Since cetyltrimethylammonium bromide (CTAB) is
used as the Br element source, the heterojunction constructed of BiOBr and Bi24 31Br10 nanosheets
maintains the rod-like morphology of the original MOF. In photocatalytic degradation tests, the BiOBr/
31Br10 heterojunction exhibits much higher photocatalytic activity than pure Bi and the
O31Br10 heterojunction is prepared for the first time from a bismuth
O
Bi24O
2 3
O ,
heterojunctions annealed at 450 °C show the best photocatalytic activity among samples prepared at
various temperatures. The heterojunction also shows good pH adaptability, good reusability, and structural
stability for RhB degradation, and the TOC results and full UV-vis spectra confirm its good mineralization
performance under visible-light irradiation. Based on the photo-electrochemical characterization and free
Received 6th January 2020,
Accepted 22nd June 2020
DOI: 10.1039/d0cy00019a
radical tests,
a photocatalytic enhancement mechanism over the heterojunction and different
+
2−
1
rsc.li/catalysis
photocatalytic reactions in the presence of photo-induced h , ˙O and O are reasonably proposed.
2
The common strategies of heterojunction preparation
Introduction
2
0
21
include solvothermal synthesis,
chemical etching,
co-
2
2
23
Photocatalysis has been extensively studied for energy
conversion and environmental management over the past
precipitation or precipitation–deposition, ion exchange,
etc. Among these, the chemical etching method presents its
superiority. On the one hand, the chemical etching method
can utilize elements from the substrate directly without
subsequent addition, which ensures the close combination of
new phases and substrates and thus increases the contact
area between two phases. On the other hand, compared with
other disordered synthesis methods, the functional products
of chemical etching are usually distributed in the outer layer
of materials, which is more conducive for visible light
absorption during the catalysis. Previous research studies
reported the preparation of heterojunctions containing BiOBr
via chemical etching methods, and corrosive HBr was usually
utilized as an etching agent due to the strong covalent bonds
1
–4
decades.
Bismuth oxybromide (BiOBr), which shows an
appropriate band gap for visible-light excitation, high
chemical stability, and a special layered structure, has been
regarded as a promising photocatalyst with efficient visible-
light responsiveness and high chemical durability. However,
it was reported that the single-phase BiOBr still showed
limited photocatalytic performance due to its insufficient
electron yield and high recombination of the photogenerated
5
–8
9
,10
electron–hole
pairs.
Compounding
with
other
semiconductors with matched energy band structures to
construct new heterojunctions is a feasible strategy that can
significantly increase the visible light absorption capacity and
reduce the charge recombination and consequently enhance
and stable chemical properties of common substrates, e.g.,
1
1–13
24,25
the photocatalytic activity under visible light irradiation.
For example, a variety of heterojunction systems, including
Bi O .
Therefore, to facilely prepare BiOBr heterojunctions
2
3
via chemical etching methods, appropriate substrates which
are easily chemically modified need to be explored.
1
4
15
16
17,18
TiO
2
-based,
3
g-C N
4
-based,
CdS-based,
2
Ag O-based,
1
9
BiOBr-based heterostructures, etc. were reported.
Metal–organic frameworks (MOFs) have attracted great
2
6
research interest in various applications, such as catalysis,
2
7,28
29,30
energy storage,
and drug delivery,
in recent years
a
School of Minerals Processing and Bioengineering, Central South University,
due to their flexible coordination ways and high specific
surface areas. Since MOFs are coordination products of
metal ions and organic ligands, their weak coordination
bonds and poor chemical stability in water enables them to
be chemically etched by common salts instead of corrosive
Changsha 410083, China. E-mail: kliu@csu.edu.cn, zhangguofan2002@163.com
b
Hunan Key Laboratory of Mineral Materials and Application, Central South
University, Changsha 410083, China
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0cy00019a
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acids. In addition, the activity and morphology of
photocatalytic materials are closely related. Therefore, MOFs
have been widely used as the precursor to prepare
photocatalytic materials with specific morphology as well as
unique structure, and the researchers also pointed out that
the improvement of photocatalytic activity is mainly due to
2.2 Synthesis of BiOBr/Bi O Br
24 31 10
BiOBr/Bi24O31Br10 was prepared via a partial bromination
treatment using CTAB as the Br source. 2 g CTAB and 50 mL
deionized water were first put in a 250 mL three-neck flask
with vigorous stirring, and then 0.5 g of prepared Bi-BTC was
dispersed into the CTAB solution with another 10 min
stirring. The three-neck flask was later heated to 90 °C and
maintained for 6 h. After solid–liquid separation and drying
in vacuum at 60 °C, the obtained light-yellow powder was
annealed in a muffle furnace at 450, 500, and 600 °C,
respectively, for 2 h with a heating rate of 1 °C min . For
comparative purposes, the pure Bi O was also prepared by
annealing in a muffle furnace at 450 °C without the
bromination treatment by CTAB.
3
1–37
the MOF precursors.
Inspired by the ideas mentioned
above, a modified chemical etching route for the facile
preparation of BiOBr heterojunctions was proposed by using
the appropriate bismuth MOFs as the substrates.
Meanwhile, thanks to the advances in the study of bismuth-
based MOFs in recent years, bismuth has been proved to be
−1
2
3
3
8–40
a
good candidate for MOF synthesis,
and a new
bismuth MOF ([BiIJBTC)IJDMF)]·DMF·(CH OH) , Bi-BTC)
3
2
which showed outstanding properties in several fields, such
4
1
42
as ion adsorption and electrocatalysis, was chosen in
this work.
2.3 Photocatalytic measurement
Herein,
a
novel BiOBr/Bi24O31Br10 photocatalyst is
Typically, 50 mg of photocatalysts were first dispersed into
prepared for the first time from a MOF precursor (Bi-BTC) via
a facile chemical etching process, and Bi-BTC acts both as
the bismuth source and as the frame of the heterojunction.
Cetyltrimethylammonium bromide (CTAB), which is a typical
surfactant and structure-directing agent in the preparation of
−
1
1
00 mL RhB solution (10 mg L ) under ultrasound assistance
and magnetic stirring. The above mixture was transferred into
a 100 mL quartz tube in a photoreaction instrument for
another 30 min of stirring until adsorption equilibrium was
reached. Then, photocatalytic degradation was started by
turning on a 500 W Xe lamp with a cut-off filter (λ >420 nm).
The water samples were taken using a syringe and filter
4
3
mesoporous materials, is used as the Br source and the
etching agent in this study. The organic ion introduced by
CTAB can be easily removed in the heat treatment, and its
mild chemical nature allows the etching reaction to proceed
in a controllable rate without changing the overall MOF
morphology. To the best of our knowledge, this preparation
route is original, and no bismuth-based heterojunctions with
visible-light photocatalytic activity derived from Bi-MOFs were
membrane (0.45 μm) and analyzed by
a
UV-visible
spectrophotometer (UV-2600) based on the absorption at 554
nm. Experiments at different initial pH were carried out using
the same process, and the pH of the RhB solution was
adjusted with HCl and NaOH before the addition of catalysts.
reported before. The prepared heterojunction shows
a
hierarchical structure with a rod-like overall morphology,
which is constructed of BiOBr and Bi O Br nanosheets. In
2.4 Characterization
2
4 31 10
addition, BiOBr/Bi24
O
31Br10 is found to exhibit enhanced
Powder X-ray diffraction (XRD) analysis was carried out using
a Bruker AXS D8 ADVANCE diffractometer (CuKα source).
The morphologies of samples were observed using a TESCAN
MIRA3 LMU scanning electron microscope (SEM) and a
JEOL1200EX transmission electron microscope (TEM).
Thermal analysis of the samples was performed on a Tarsus
TG209F3 thermal analyzer from 30 °C to 800 °C with a
photocatalytic activity for RhB degradation under visible light
irradiation after the introduction of the bromine element.
Furthermore, the photocatalytic mechanism of the composite
has been proposed and discussed in detail via structural and
electrochemical characterization.
−
1
heating rate of 10 °C min under air atmosphere. X-ray
Experimental
photoelectron spectroscopy (XPS) analysis of samples was
performed via a VG ESCALAB250Xi electron spectrometer. N
2
2
.1 Synthesis of Bi-BTC
adsorption analysis of samples was conducted on an ASAP
4
4
The synthesis of Bi-BTC is modified from previous studies.
In a typical experiment, 1.2 g BiIJNO ·5H O was first
2020 surface area and a porosimeter system with N at 77 K.
2
)
3 3
2
UV-vis diffuse reflection spectroscopy (DRS) tests were carried
dissolved in 45 mL of dimethylformamide (DMF), termed
out on
a
Thermo Scientific evolution 220 UV-vis
solution A. Then, 1.6 g H BTC was dissolved in 15 mL of
spectrophotometer. Photoluminescence (PL) spectra were
detected using a Hitachi F-4500 fluorescence spectrometer.
Electrochemical tests were carried out on a Gamry Interface
1010E electrochemical workstation. The Fourier transform
infrared (FTIR) spectra of samples were measured using a
Nicolet Nexus 670 spectrometer. Electron spin resonance
(ESR) analysis was carried out on a Bruker A300 electronic
paramagnetic resonance spectrometer.
3
pure methanol, termed solution B. After stirring for 30 min,
solution B was poured into solution A with further stirring
for 1 h. Subsequently, the resulting transparent mixture was
transferred into a 100 mL Teflon-lined autoclave and then
heated to 130 °C and kept for 48 h. Finally, a white powder
was obtained after multiple washes with deionized water
and ethanol.
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Results and discussion
3
.1 Structural and morphological characterization
The crystal structure of the samples and phase
transformation during the preparation was discussed via the
XRD measurement. Fig. 1a shows the XRD patterns of as-
prepared Bi-BTC and pure Bi
O
2 3
without bromination
treatment. The XRD pattern of the prepared Bi-BTC is found
4
4
to be almost identical to that reported in the literature,
indicating the successful preparation of Bi-BTC via the
modified synthesis method. Subsequently, the oxidic roasting
products of Bi-BTC are perfectly indexed with the standard
2 3
pattern of alpha-Bi O without interfering peaks, and no
typical peaks of the organic ligands in Bi-BTC are observed.
Fig. 1b shows the XRD patterns of products after
bromination and the subsequent roasting process at various
temperatures. The phase of the 400 °C sample is composed
of BiOBr and residual organic matter, as shown in Fig. S1.†
The samples obtained at 450 °C and 500 °C show similar
XRD patterns and all their peaks can be well indexed with
the standard patterns of BiOBr and Bi O Br , which
Fig. 2 SEM images of Bi-BTC (a), Bi O3 (b), BiOBr/Bi24O31Br10 (450 °C)
2
(d and e) and corresponding EDS analysis (c) and elemental mapping
images (f); TEM and HRTEM images of BiOBr/Bi24 31Br10 (450 °C) (g
O
and h); and TG–DTA curves of CTAB-treated Bi-BTC (i).
2
4 31 10
confirms the successful preparation of the BiOBr/Bi24O31Br10
composite. It is worth noting that the pattern of the sample
prepared at 600 °C matches well with the pattern of the pure
Bi24O31Br10 phase without a trace of BiOBr. This might be
ascribed to the higher oxidic temperature that can make Br
escape from the composite structure, and thus BiOBr
temperature, the rod-like morphology was partially destroyed.
Some irregular plate-like morphology was formed in the
sample (500 °C) (Fig. S2†). In magnified images (Fig. 2e), it
can be found that the rods show a hierarchical structure with
uniform micro-sheets of BiOBr embedded on the surface of
the composite. EDS mapping analysis of BiOBr/Bi O Br
4 31 10
transforms into the Bi-rich bismuth oxyhalide (Bi24O31Br10).
2
Based on the above XRD analysis results, it can be seen that
the phase transition process of Bi-BTC bromination and
calcination at different temperatures was from BiOBr/residual
organic matter to BiOBr/Bi O Br and then to Bi O Br .
(
450 °C) reveals the even distribution of Bi, O, and Br
elements over the whole rod-like structure (Fig. 2f), and the
atomic ratios also match well with the phase composition in
XRD analysis (Fig. 2c). Moreover, Fig. 2g and h show the TEM
and HRTEM images of a typical surface area in BiOBr/
Bi O Br (450 °C). The lattice fringe spacing of 0.353 nm is
24
31 10
24 31 10
It shows that the successful bromination of the MOF
structure and the oxidic temperature at around 450 to 500 °C
is conducive to the photocatalytic activities of BiOBr/
Bi O Br heterojunctions.
2
4 31 10
45
in line with the (101) plane of BiOBr, and the (21 ¯3 ) crystal
24 31 10
plane of Bi24 31Br10 with a lattice fringes spacing of 0.299 nm
O
Fig. 2a shows the SEM images of the prepared Bi-BTC. Bi-
BTC shows a unique rod-like structure with a length of more
can also be observed. The TG–DTA curves of CTAB-treated Bi-
BTC were recorded under air atmosphere to measure the
weight loss and reaction during the oxidizing roasting process
than 10 μm. Bi O derived from Bi-BTC well maintains the
2
3
rod-like morphology and shows a porous structure (Fig. 2b).
After the bromination–roasting process, the obtained BiOBr/
(
Fig. 2i). There is a sharp weight loss of 51.5% from 250 °C to
around 450 °C in the TG curve associated with two
endothermic peaks at 374.3 °C and 426.1 °C in the DTA curve.
The weight loss of 51.5% is due to the oxidation of organic
ligands in Bi-BTC, and endothermic peaks at 374.3 and 426.1
Bi O Br (450 °C) still maintains the rod-like overall
4 31 10
2
morphology (Fig. 2d). However, with the increase of annealing
°
C are attributed to the formation of BiOBr and Bi24O31Br10,
respectively. No obvious weight loss is observed after the
temperature rises to 450 °C, indicating that the organic
compounds have been removed before the temperature rises
to 450 °C. TG–DTA analysis confirms that the lowest roasting
temperature for the formation of BiOBr and Bi24O31Br10
heterojunction is around 450 °C, where no organic residue is
left and the synthesis process has just finished.
2
Fig. 3 shows the N adsorption–desorption isotherm plots
and the corresponding pore diameter distribution results
calculated by BJH methods. All four plots show type IV
isotherm with H3 hysteresis loops, according to the IUPAC
2 3
Fig. 1 XRD patterns of Bi-BTC and Bi O (a) and samples obtained at
different annealing temperatures (b).
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surface composition and chemical state. Fig. 4a shows the
survey spectra of the three samples. The peaks of Bi 4f and O
1s can be observed at all three curves at the binding energy
of around 162 eV and 530 eV, respectively. The Br 3d peaks
can be detected on the spectra of BiOBr/Bi O Br obtained
2
4 31 10
at a different temperature, while it cannot be observed on
Bi O , indicating the successful introduction of the Br
2
3
element during the bromination process. Fig. 4b shows the
high-resolution Bi 4f spectra of three samples. All curves
show two individual symmetrical peaks belonging to Bi 4f5/2
and Bi 4f7/2, respectively, which reveals that the main
chemical state of the bismuth element in three samples are
4
9
tri-valent. Compared with pure Bi O , the Bi 4f peaks of
2
3
BiOBr/Bi O Br 450 °C shift 0.75 eV to higher binding
24 31 10
Fig. 3
N
2
adsorption–desorption isotherms and the corresponding
, and BiOBr/
energy, and when the phase transforms to pure Bi24
600 °C), the Bi 4f peaks shift back to lower binding energy
by 0.60 eV, while their spin separation energies are still the
same as that of pure Bi and there is no asymmetrical peak
O31Br10
pore-size distribution (inset) curves of Bi-BTC, pure Bi
Bi24 31Br10 (450 °C and 500 °C).
2 3
O
(
O
2 3
O
observed in the spectrum. The peak shift after bromination
might be ascribed to the introduction of the Br element and
the interaction between two components in BiOBr/
4
6
classifications. Bi-BTC exhibits the largest specific surface
2
−1
area of over 130 m g among the three samples, which is
ascribed to the unique frame structure of MOFs. This value
decreases significantly to 3.07 m g after the oxidic roasting
Bi ), which might be caused by the removal of Bi-BTC
organic ligands. This is a common phenomenon in the
transformation of MOF materials to inorganic materials.
However, after the bromination–roasting process, the specific
5
0
Bi O Br . The lower Br content in Bi O Br can lead to
24
31 10
x
y
z
2
−1
the lower binding energy of bismuth, so the Bi 4f peaks of
pure Bi24 31Br10 (600 °C) shift back to the location close to
that of pure Bi O . Fig. 4c shows the Br 3d high-resolution
(
O
2 3
O
2
3
spectra of three samples. No obvious signal of the Br element
was detected in pure Bi , and two peaks at 69.82 eV (or
69.59 eV) and 68.74 eV (or 68.51 eV) can be assigned to the
2 3
O
surface area of BiOBr/Bi O Br (450 °C) and BiOBr/
4 31 10
2
2
−1
2
−1
51
Bi24O
31Br10 (500 °C) increases to 9.68 m g and 3.65 m g ,
Br 3d3/2 and 3d5/2, respectively.
After the roasting
respectively. This phenomenon indicates that the
construction of hierarchical structures alleviates the sudden
temperature rises to 600 °C, the locations of the Br 3d5/2 and
3d3/2 slightly shift to lower values by 0.35 eV, which is also
attributed to the escape of Br atoms from the lattice. Fig. 4d
compares the O 1s XPS envelops of the three samples. After
the XPS-peak-differentiation-imitating analysis, all O 1s peaks
can be fitted into two peaks representing the lattice oxygen
4
7
decrease in specific surface area. In addition, all four
samples possess average pore diameters of from 10 to 30 nm,
4
8
indicating the mesoporous structures in the samples.
The XPS spectra of pure Bi and BiOBr/Bi24
obtained at 450 and 600 °C were measured to analyze the
O
2 3
O31Br10
52–54
(bottom) and surface oxygen (top) of samples.
Furthermore, the O 1s peaks also show a similar shift trend
to that of Bi 4f with the change of phases, which further
confirms the interaction of the Br element.
3.2 Photocatalytic degradation properties
Fig. 5a shows the RhB removal performances of different
samples by adsorption and the subsequent visible-light
photocatalysis process. The system has reached adsorption
equilibrium before light irradiation (Fig. S3†). It can be
observed that BiOBr/Bi O Br (450 °C) exhibits the best
2
4 31 10
adsorption and photocatalytic performance among other
samples. After irradiation for 60 min, the RhB removal ratios of
BiOBr/Bi O Br
(450 °C), BiOBr/Bi O Br
(500 °C),
2
4
31 10
24 31 10
Bi24O31Br10 (600 °C) and pure Bi O reach 92.4%, 65.9%, 24.7%
2 3
and 19.9%, respectively. A pseudo-first-order kinetic model is
employed to evaluate the degradation rate of the photocatalytic
process (Fig. 5b and d). The fitted k value of BiOBr/Bi24
O31Br10
(
450 °C) is about 13.1 times higher than that of Bi O ,
2 3
Fig. 4 XPS spectra of Bi O , BiOBr/Bi24O31Br10 (450 °C) and Bi24O31Br10
2 3
(600 °C): survey (a), Bi 4f (b), Br 3d (c) and O 1s (d) of samples.
indicating that the degradation activity of the Bi-BTC-derived
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irradiation, the full UV-vis spectra during the photocatalytic
degradation process are illustrated in Fig. 6a. The UV-vis
spectra of the initial RhB solution shows a typical absorption
peak at 554 nm with a maximum absorbance value of 2.1 (a.
u.). During the photocatalytic process, the typical absorption
peak shows an obvious blue shift and the maximum
absorbance value also significantly decreases. At 180 min, the
absorbance peak disappears completely. Meanwhile, the color
of the RhB solution goes through the process of pink, orange,
light yellow and totally transparent. In addition, the total
organic carbon (TOC) results show that more than 60% TOC
in the RhB solution is removed within 180 min of irradiation.
All these results confirm the high mineralization activity of
BiOBr/Bi O Br (450 °C) under visible-light irradiation.
24 31 10
The stability and recyclability of materials are prerequisite
55
for their practical application in wastewater treatment, and
is also the main drawback of original MOF structures. After
the roasting and bromination process, the stability of the
MOF-derived materials in this study was measured, and
Fig. 5 Photocatalytic removal performances of RhB over different
samples (a and b) and RhB removal performances of BiOBr/Bi24
450 °C) at different pH values (c and d).
O31Br10
(
Fig. 6c presents the reuse test results. The BiOBr/Bi24O31Br10
materials increases significantly because of the introduction of
the Br element. Meanwhile, the photocatalytic activity
decreases gradually with the rising of roasting temperature.
The rod-like morphology was partly damaged (500 °C, Fig. S2†),
and the specific surface area decreased (500 °C, Table 1). As the
maintains about 80% photocatalytic activity in 4 reuse cycles.
Meanwhile, no obvious peak changes can be observed in the
XRD pattern of the used photocatalyst compared with the
fresh one. In addition, although a few structures of BiOBr/
Bi O Br (450 °C) after 4 reuse cycles were destroyed, the
24 31 10
temperature keeps rising, single-phase Bi24
obtained at about 600 °C (Fig. 1). These results reveal that the
O
31Br10 was
morphology of the samples did not apparently change in
general (Fig. S5†), indicating that BiOBr/Bi24 31Br10 (450 °C)
O
obtained heterojunction structure and the rod-like morphology
possesses good chemical stability and reusability. The
outstanding stability and reusability might be ascribed to the
stable chemical nature of bismuth-based composites. In
addition, the roasting process also leads to higher
crystallinity as well as the better stability of the MOF-derived
structure. All these results confirm the good reusability as
enhanced the photocatalytic activity of BiOBr/Bi24
O31Br10 (450
°C). To further investigate the photocatalytic activity of the
samples, excluding the effect of dye sensitization, the colorless
tetracycline hydrochloride (TC) was used to evaluate the
photocatalytic performance (Fig. S4†). The results show that
BiOBr/Bi O Br (450 °C) also has a good photocatalytic
well as stability of BiOBr/Bi O Br in practical application.
2
4
31 10
24 31 10
activity for TC.
The initial pH value of wastewater is a critical factor
affecting the adsorption performance and surface chemical
features of the photocatalysts. The effect of the initial pH
3
.3 Optical and electrochemical analysis
Fig. 7a shows the UV-vis DRS spectra of pure Bi O and
2
3
value on the photocatalytic activity of BiOBr/Bi24
C) was discussed, and the experimental results are shown in
Fig. 5c and d. Although the photocatalytic activity of BiOBr/
31Br10 (450 °C) decreases slightly with the rise of the
initial pH value, this heterojunction still shows considerable
photocatalytic activity at wide pH range (3 to 11),
O
31Br10 (450
samples obtained at 450, 500, and 600 °C. All samples,
°
including pure Bi O , show strong absorption in the visible-
2
3
light region, indicating their visible-light-responsive activity.
The estimated absorption edges for pure Bi , Bi24
(600 °C), BiOBr/Bi O Br (500 °C) and BiOBr/Bi O Br
Bi24
O
O
2 3
O31Br10
2
4 31 10
24 31 10
a
(450 °C) are 454, 458, 468 and 480 nm, respectively. It can be
seen that BiOBr/Bi24 31Br10 (450 °C) shows the widest
absorption edges among the four samples, while the pure
demonstrating its outstanding adaptability to pH conditions
and good practical potential.
O
1/2
To further understand the mineralization process of RhB
Bi O shows the narrowest. According to the plot of (αhν)
2 3
in the BiOBr/Bi24O31Br10 system under visible light
vs. photon energy (hν) using the transformed Kubelka–Munk
Table 1 Detailed BET parameters of the four samples
2
−1
3
−1
Samples
Specific surface area (m g
)
Pore volume (cm g
)
Average pore diameter (nm)
Bi-BTC
BiOBr/Bi24
BiOBr/Bi24
2 3
Bi O
137.8
9.68
3.65
3.07
0.146
0.067
0.016
0.013
11.4
27.8
17.3
17.7
O
O
31Br10 (450 °C)
31Br10 (500 °C)
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Fig. 6 Full UV-vis spectra with optical photo (a) and TOC removal
results (b) and the photocatalysis recycling test results (c) of BiOBr/
Fig. 8 The photoluminescence spectra of various samples with an
excitation wavelength of 300 nm.
Bi24
O31Br10 (450 °C); XRD patterns of BiOBr/Bi24O31Br10 (450 °C)
before and after recycling tests (d).
photocurrent of BiOBr/Bi24O31Br10 (450 °C) suggests that the
introduction of Br and the heterojunction structure can lead
to higher electron–hole separation efficiency and a lower rate
of charge carrier recombination under visible light
irradiation. In addition, the photocurrent values of the
samples rapidly boost with the light turned on and decay to
zero once the light is turned off, which reveals that the
carriers' generation at the surface of BiOBr/Bi O Br is
24 31 10
5
7
photo-dependent and sustainable. To confirm the stability
of the photocurrent of BiOBr/Bi24 31Br10 (450 °C) in a long-
term irradiation process, the chronoamperometry curves of
BiOBr/Bi24 31Br10 (450 °C) and pure Bi were tested with
uninterrupted light irradiation (Fig. 9b). During the 600 s of
irradiation, both Bi O and BiOBr/Bi O Br (450 °C)
1/2
Fig. 7 UV-vis DRS spectra (a) and corresponding plots of (α(hν))
versus energy (b) of pure Bi and samples prepared at 450, 500, and
00 °C.
O
2 3
O
6
O
2 3
O
2
3
24 31 10
function (Fig. 7b), the estimated band gap energies (E
calculated to be 2.63 eV (450 °C), 2.65 eV (500 °C), 2.69 eV
600 °C) and 2.73 eV (pure Bi O ). Photoluminescence (PL)
g
) are
maintain a constant photocurrent without obvious decay,
(
2
3
measurement was conducted to detect the separation and
recombination efficiency of photo-induced charge carriers in
various samples, and the results are shown in Fig. 8. BiOBr/
Bi24O
31Br10 (450 °C) possesses the lowest peak at about 470
nm, while pure Bi shows the highest recombination
efficiency, indicating the best carrier separation efficiency of
2 3
O
5
6
BiOBr/Bi24O31Br10 (450 °C) among these samples. All these
optical analyses reveal that the higher content of Br in the
BiOBr/Bi O Br composite can significantly optimize the
2
4 31 10
photoabsorption ability as well as charge separation
efficiency of the heterojunction, which consequently results
in the enhancement of photocatalytic activity for the
degradation of contaminants.
The transient photocurrent responses were tested via a
typical on–off cycle of visible light irradiation (λ >420 nm)
Fig. 9 The transient photocurrent responses via on–off cycles (a) and
chronoamperometry curves (b) of the different photocatalysts under
visible light illumination and the electrochemical impedance spectra
(
Fig. 9a). The photocurrent curve of BiOBr/Bi24
O31Br10 (450
−
1
°C, 0.11 mA cm ) is about 10 times stronger than that of
(EIS) plots (c) in Na
2 3 2
SO (0.1 M) and Na S (0.1 M) mixed aqueous
pure Bi O and significantly higher than those of the samples
2
3
solution, the Tafel polarization curves of the various photocatalysts (d)
prepared at other temperatures. The much higher
in Na
2
SO
4
(0.1 M) aqueous solution.
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5
8
confirming the sustainability of the photocurrent. BiOBr/
Bi O Br (450 °C) shows a higher chronoelectric current
24 31 10
2 3
compared with Bi O , which is consistent with the results in
the transient photocurrent test. The charge transfer capability
of the photocatalysts in this research was characterized via
electrochemical impedance spectroscopy (EIS). Fig. 9c shows
the EIS curves of the prepared BiOBr/Bi O Br (450 °C) and
24 31 10
other contrastive samples obtained with or without visible
light illumination, in which a larger Nyquist curve radius
means
a
higher transfer resistance of photo-generated
5
9
charge. BiOBr/Bi O Br (450 °C) shows the smallest plot
24 31 10
radius compared with the other samples, revealing the
quickest charge transfer capacity compared with the
contrasts. It can be found that the radius distribution trend
of the different samples highly corresponds to their
photocatalytic activity and optical features measured above.
Meanwhile, the plots tested under light irradiation show a
smaller radius than those measured under dark, which
indicates the better transport performance of charge carriers
under visible-light irradiation. The Tafel analysis of pure
Fig. 10 Photo-degradation of RhB in the presence of various radical
1
−
scavengers (a). ESR spectra of TEMPO– O
c) over BiOBr/Bi24 31Br10 (450 °C) in the dark and under visible-light
2
(b) and DMPO–˙O adducts
2
(
O
irradiation. ESR signal of the BiOBr/Bi24O31Br10 (450 °C) for surface
oxygen vacancy (d).
2 3
Bi O and the BiOBr/Bi24O31Br10 (450 °C) was conducted to
determine the corrosion density value during the
measurement (Fig. 9d). The corrosion density value was
calculated via the intersection between the linear portion of
the anode and the cathode curve. A larger corrosion density
photocatalytic degradation process in the system. All these
results indicate that the functional radical species in the
BiOBr/Bi O Br system under visible-light irradiation are
24 31 10
−
2
−
1
of BiOBr/Bi24
O
O
31Br10 (450 °C) (35.78 μA cm ) compared with
the photo-induced holes, ˙O
2
and
2
O , and the photo-
−
2
the pure Bi
2
3
(1.994 μA cm ) indicates higher catalytic
induced holes are the main radical species in the system.
To further verify the results in scavenging experiments,
ESR analysis was conducted in the dark and under light
irradiation to detect the presence of various radical species
activity and quicker carriers transfer capacities after the
bromination process. All these photo-electrochemistry
measurement results reveal the outstanding photocurrent
response, better charge transfer capability and lower charge
and clarify the relationship between their generation and
1
carrier recombination rate of BiOBr/Bi24
heterojunction under visible-light irradiation.
O
31Br10 (450 °C)
visible-light irradiation. The TEMPO– O
2
peaks are clearly
6
0
observed under 15 min of visible light irradiation (Fig. 10b),
while they cannot be detected under dark conditions,
1
revealing the presence of O
2
in the BiOBr/Bi24O31Br10 system
3
.4 Photocatalytic mechanism
and light irradiation is indispensable for its generation. In
−
The functional radical species in the photo-degradation were
determined by several radical scavenging experiments, and KI,
isopropyl alcohol (IPA), superoxide dismutase (SOD) and
histidine were utilized as scavengers for the photo-induced hole
Fig. 10c, typical peaks for DMPO–˙O adducts are also
2
−
observed, indicating that ˙O
BiOBr/Bi24
produced without light irradiation. In addition, no signal of
DMPO–˙OH can be observed in the BiOBr/Bi24 31Br10 system
under visible light irradiation (Fig. S6†). BiOBr/Bi24 31Br10
2
radicals are generated in the
−
1
O
31Br10 system, and neither ˙O
2
2
nor O can be
+
−
(
(
h ), hydroxyl radical (˙OH), superoxide (˙O ) and singlet oxygen
O
2
1
2
O ) scavengers, respectively. Fig. 10a shows the RhB removal
O
−
1
efficiencies after adding various radical scavengers. After adding
IPA (˙OH scavenger) into the BiOBr/Bi O Br (450 °C) system,
the photo-degradation efficiency of RhB remained unchanged
compared with the control without scavengers, indicating that
the degradation of organic compounds in this system is
independent of the generation of ˙OH.
shows great productivity of ˙O2 and O2 under visible-light
irradiation according to the above analysis, which can be
ascribed to the good absorption capacity for free oxygen of
the catalysts. The surface oxygen vacancy of BiOBr/Bi O Br
4 31 10
2
4 31 10
2
was also measured, and the ESR curve is shown in Fig. 10d.
BiOBr/Bi24 31Br10 shows two distinct symmetrical peaks of
O
However, the photocatalytic degradation efficiency of RhB
surface oxygen vacancy in the ESR patterns, indicating that
the oxygen vacancy is generated on the surface during the
preparation of BiOBr/Bi24O31Br10. This result confirms that
significantly decreases to 70% of the blank control in the
+
presence of KI (h
scavenger), indicating that the
photogenerated hole is the main functional radical species in
the system. Sharp decreases in the RhB degradation
performance are observed in the system in the presence of
SOD (about 45%) and histidine (about 50%), revealing that
the surface oxygen vacancy also improves the surface
chemical activity and subsequent photocatalytic performance
of BiOBr/Bi24O31Br10 when the catalyst is stimulated by visible
light. The abundant oxygen vacancies of the composite might
be ascribed to the introduction of Br into the bismuth
−
1
both ˙O2 and O2 are generated and contribute to the
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composites, in which the BiOBr and Bi O Br phases show
24 31 10
unique layer structures that allow more lattice oxygen to be
exposed to the atmosphere. Moreover, the roasting process in
the muffle furnace further accelerates the release of oxygen
atoms in the lattice.
To determine the energy band structure of the BiOBr/
Bi O Br heterojunction, valence band (VB) analysis was
24 31 10
carried out during the XPS measurement, and the results are
shown in Fig. 11a. The VB edges can be defined by the
intersection between the fitting line of the VB spectrum and
the background line, and the VB edges are estimated to be
Fig. 12 The energy band structure of the whole BiOBr/Bi24O31Br10
heterojunction (left) and the separation process of electron–hole pairs
under visible light (right).
about 2.49 eV (BiOBr/Bi24O31Br10 (450 °C)) and 2.01 eV
(
Bi O Br (600 °C)). Since the bandgap energies were
2
4 31 10
calculated to be 2.63 and 2.69 eV via UV-vis DRS analysis, the
conduction band (CB) edges would be located at −0.14 eV
According to the above analysis, the separation processes
of charge carriers and the photocatalytic mechanism in the
(
BiOBr/Bi O Br (450 °C)) and −0.68 eV (Bi O Br (600
whole BiOBr/Bi O Br
heterojunction are proposed, as
2
4
31 10
24 31 10
24 31 10
°
C)). To further confirm the energy band structure of BiOBr/
31Br10 (450 °C), Mott–Schottky analysis was also carried
out, and the plot is shown in Fig. 11b. The Mott–Schottky
plot shows a positive slope that reveals the BiOBr/Bi24
shown in Fig. 12. Under the irradiation of visible light,
electrons in the VB of BiOBr and Bi24 31Br10 can be
transferred to the CB of corresponding phases with different
reduction potentials (eqn (2)). For Bi24 31Br10, it shows a CB
Bi24O
O
O
31Br10
O
6
1
(
450 °C) belongs to n-type semiconductors, which matches
potential (−0.68 eV) which is more negative than the standard
−
well with previous reports about BiOBr and Bi-rich bismuth
redox potential of O /˙O (−0.36 eV); hence the O caught by
2
2
2
6
2
oxyhalides. The flat-band potential of semiconductors can
be obtained via the intersection of the fitted line with the
X-axis, and the flat-band potential of BiOBr/Bi O Br (450
the abundant oxygen vacancies on the surface of catalysts
can react with electrons to form ˙O
usually generates O in the catalytic oxidation system (eqn
−
−
2
(eqn (3)), and the ˙O
2
1
24
31 10
2
6
4
°
C) is −0.62 eV (vs. Ag/AgCl). The corresponding normal
(4)).
Meanwhile, the photo-induced holes with strong
5
1
hydrogen electrode (NHE) can be calculated via eqn (1):
oxidation ability are left on both VBs of BiOBr and
Bi O Br as the main functional radical in the system.
24 31 10
0
E
NHE = EAg/AgCl + 0.059pH + EAg/AgCl
(1)
Subsequently, RhB molecules in the system are degraded into
−
various products in the presence of photo-induced holes, ˙O
2
0
1
where EAg/AgCl = +0.199 V; the initial pH value is 6 in this
test. The flat-band potential of n-type semiconductors is
about 0.1 V more negative than the minimum of the CB, so
the CB edge potential of BiOBr/Bi O Br (450 °C) can be
and O2 (eqn (5)). In addition, both the CB and VB of
31Br10 are more negative than those of BiOBr, which
Bi24O
allows the photogenerated holes in BiOBr to transfer to the
VB of Bi O Br , and the photoinduced electrons can also
2
4
31 10
24 31 10
calculated to be −0.167 eV (vs. NHE). The corresponding VB
edge potential can be calculated to be 2.463 eV by adding E
pass from the CB of Bi24O31Br10 to the CB of BiOBr. The
g
simultaneous transfer of carriers on different semiconductors
significantly improves the charge separation efficiency, while
the recombination of electron–hole pairs is greatly
suppressed. For these reasons, the BiOBr/Bi24O31Br10
heterojunction exhibits a much higher photocatalytic activity
than the single-phase catalyst under visible light irradiation.
(
2
2.63 eV) and ECB (−0.167 eV), and this value is very close to
.49 eV obtained via the valence band spectrum in XPS
of pure BiOBr is about 2.60 eV,
analysis. In addition, the E
referring to the results of previous reports, so the E values
g
6
3
g
of BiOBr/Bi24O31Br10 (450 °C) and BiOBr/Bi24O31Br10 (500 °C),
which are equal to 2.63 and 2.65 eV, respectively, are just
located in the middle of two separate phases.
−
+
hν + catalysts → eCB + h
(2)
(3)
(4)
CB−
2
+ O → ˙O
2−
e
2−
+
2
2
1
2
2
˙O + 2H → H
O
+ O
+
−
1
−
h + ˙O + O + RhB → intermediates → CO + H O + Cl (5)
2
2
2
2
Conclusions
In summary, we successfully prepare a BiOBr/Bi24O31Br10
heterojunction from a novel bismuth MOF (Bi-BTC) for the
first time, and cetyltrimethylammonium bromide (CTAB) is
used as the Br element source in the bromination–annealing
Fig. 11 The valence band spectra of BiOBr/Bi24
Bi24 31Br10 (600 °C) (a) and Mott–Schottky plots of BiOBr/Bi24
450 °C) (b).
O
31Br10 (450 °C) and
O
O31Br10
(
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treatment. Thanks to the unique structure of Bi-BTC, the as-
9 H. Huang, X. Han, X. Li, S. Wang, P. K. Chu and Y. Zhang,
ACS Appl. Mater. Interfaces, 2015, 7, 482–492.
prepared BiOBr/Bi O Br is constructed of BiOBr and
4 31 10
2
Bi24
O
31Br10 nanosheets and shows a hierarchical rod-like
morphology. In photocatalytic activity tests, BiOBr/Bi O Br
4 31 10
10 F.-t. Li, Q. Wang, J. Ran, Y.-j. Hao, X.-j. Wang, D. Zhao and
S. Z. Qiao, Nanoscale, 2015, 7, 1116–1126.
2
annealed at 450 °C shows much higher RhB degradation
efficiency than other samples without bromination treatment
11 M. Ao, K. Liu, X. Tang, Z. Li, Q. Peng and J. Huang, Beilstein
J. Nanotechnol., 2019, 10, 1412–1422.
(
Bi O ) or obtained at different temperatures, and it
12 J. Zhang, L. Zhang, X. Shen, P. Xu and J. Liu, CrystEngComm,
2016, 18, 3856–3865.
13 Y. Y. Yang, X. G. Zhang, C. G. Niu, H. P. Feng, P. Z. Qin, H.
Guo, C. Liang, L. Zhang, H. Y. Liu and L. Li, Appl. Catal., B,
2020, 264, 1–15.
2
3
maintains the outstanding photocatalytic performance in a
wide pH range. The TOC and full UV-vis results confirm the
good mineralization performance of BiOBr/Bi O Br under
24 31 10
visible-light irradiation. In addition, BiOBr/Bi O Br
4 31 10
2
exhibits good reusability and structural stability in four
recycling tests. Optical analyses and electrochemical
measurements reveal that the annealing temperatures can
significantly affect the photo absorption ability, charge
separation efficiency and transportation of carriers in the
BiOBr/Bi O Br heterojunction, and the sample annealed
14 Y. Zhang, Z. Shi, L. Luo, Z. Liu, D. K. Macharia, G.
Duoerkun, C. Shen, J. Liu and L. Zhang, J. Colloid Interface
Sci., 2020, 561, 307–317.
15 C. Liang, H. P. Feng, H. Y. Niu, C. G. Niu, J. S. Li, D. W.
Huang, L. Zhang, H. Guo, N. Tang and H. Y. Liu, Chem. Eng.
J., 2020, 384, 123236.
2
4 31 10
at 450 °C shows superiority in all these aspects, which is in
good agreement with the photocatalytic performances in RhB
degradation. According to the energy band structure of
heterojunctions and analysis of free radicals, the
heterojunction structure of BiOBr/Bi O Br enables it to
16 Y. Zhang, L. Luo, Z. Shi, X. Shen, C. Peng, J. Liu, Z.
Chen, Q. Chen and L. Zhang, ChemCatChem, 2019, 11,
2855–2863.
17 H. Guo, C. G. Niu, D. W. Huang, N. Tang, C. Liang, L.
Zhang, X. J. Wen, Y. Yang, W. J. Wang and G. M. Zeng,
Chem. Eng. J., 2019, 360, 349–363.
2
4 31 10
exhibit higher photoresponse and longer electron–hole
lifetimes compared with single-phase photocatalysts. The
18 H. Guo, H. Y. Niu, C. Liang, C. G. Niu, D. W. Huang, L.
Zhang, N. Tang, Y. Yang, C. Y. Feng and G. M. Zeng,
J. Catal., 2019, 370, 289–303.
+
−
1
photo-induced h , ˙O
and O2 are proved to be the
2
functional free radicals in RhB degradation.
1
2
2
2
2
2
2
9 Z. Shi, Y. Zhang, X. Shen, G. Duoerkun, B. Zhu, L. Zhang, M.
Li and Z. Chen, Chem. Eng. J., 2020, 386, 124010.
0 R. Zha, R. Nadimicherla and X. Guo, J. Mater. Chem. A,
Conflicts of interest
There are no conflicts to declare.
2
015, 3, 6565–6574.
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Eng. J., 2016, 288, 264–275.
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4 X. Tang, Z. Wang, N. Wu, S. Liu and N. Liu, Catal. Commun.,
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (No. 51774330) and the
Fundamental Research Funds for the Central Universities of
Central South University (No. 2018zzts804).
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