Synthesis and photocatalytic properties of sunlight-responsive BiOBr–CoWO4 heterostructured nanocomposites

Article abstract of DOI:10.1002/aoc.5436

Efficient sunlight-responsive BiOBr–CoWO4 heterostructured nanocomposite photocatalysts were prepared via a chemical precipitation route at 100°C in 4 hours. The prepared BiOBr–CoWO4 heterostructures were characterized for phase iden

Full text of DOI:10.1002/aoc.5436

Received: 13 December 2018
DOI: 10.1002/aoc.5436
Revised: 9 October 2019
Accepted: 4 December 2019
F U L L P A P E R
Synthesis and photocatalytic properties of sunlight-
responsive BiOBr–CoWO heterostructured nanocomposites
4
Arpita Paul Chowdhury | Baban H. Shambharkar
Chemistry Department, National Institute
Abstract
of Technology Silchar, Silchar, 788 010,
Assam, India
Efficient sunlight-responsive BiOBr–CoWO heterostructured nanocomposite
4
ꢀ
photocatalysts were prepared via a chemical precipitation route at 100 C
Correspondence
in 4 hours. The prepared BiOBr–CoWO heterostructures were characterized
B. H. Shambharkar, Chemistry
Department, National Institute of
Technology Silchar, Silchar 788 010,
Assam, India.
4
for phase identification, chemical composition, surface morphology, optical
properties and surface area using various techniques. The X-ray diffraction pat-
tern of the BiOBr–CoWO nanocomposite was composed of diffraction peaks
Email: baban.shambharkar@gmail.com
4
equivalent to both the tetragonal phase of BiOBr and the monoclinic phase of
CoWO nanoparticles. X-ray photoelectron spectral study of the BiOBr–CoWO
4
4
nanocomposite revealed orbitals of both BiOBr and CoWO compounds. Trans-
4
mission electron microscopy images revealed that spherical particles of CoWO4
(20–25 nm) were dispersed on the surface of BiOBr. UV–visible–near-infrared
spectral study of the BiOBr–CoWO nanocomposite showed good visible-light
4
absorption. Among the manufactured materials, BiOBr–CoWO -2 nanocom-
4
posite showed better charge carrier separation efficiency, as demonstrated
by photoluminescence and time-resolved fluorescence. To study the
practical utility of the prepared materials, their photocatalytic capability was
examined for the degradation of rhodamine B (RhB) aqueous solution under
sunlight irradiation. The photodegradation results showed that BiOBr–
CoWO -2 nanocomposite degraded 98.69% RhB solution and the degradation
4
−
1
constant was 0.067 min , which was 5.6 and 22.5 times larger than that of
pure BiOBr and CoWO nanoparticles, respectively, after 60 minutes of sun-
4
light irradiation. The superior photoactivity was facilitated by electron–hole
pair separation and transfer driven by the heterostructure interface between
BiOBr particles and CoWO nanoparticles. The removal of RhB was initiated
4
+
•−
•
by photogenerated h , O2 and OH reactive species based on the scavenger
effect.
K E Y W O R D S
BiOBr–CoWO
4
, heterostructure, material characterization, nanocomposite, photodegradation
1
| INTRODUCTION
for the degradation of hazardous organic contaminants
from wastewater. Photocatalysts can capture solar energy
Sunlight-induced semiconductor photocatalysis is pre-
ferred since biodegradation of pollutants is very sluggish
and convert it into chemical energy and thus solve issues
related to the energy crisis and environmental pollution.
Appl Organometal Chem. 2020;e5436.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
1 of 11
https://doi.org/10.1002/aoc.5436

2
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PAUL CHOWDHURY AND SHAMBHARKAR
The photoinduced electron–hole pairs of a photocatalyst
can drive the redox reaction of pollutants without the
need for addition of external oxidizing reagents. Since the
sunlight. For the BiOBr–CoWO nanocomposite, BiOBr
4
is the main photocatalyst, whereas CoWO nanoparticles
4
function as a sensitizer for absorbing visible light. It is
introduction of TiO as a photocatalyst, the search for
expected that the prepared heterostructured BiOBr–
2
new heterogeneous photocatalysts with suitable bandgaps
has begun to produce the required efficiency for practical
applications. From the viewpoint of enhancing separation
efficiency of photogenerated charge carriers, various
CoWO photocatalysts will have an improved response to
4
visible light, larger surface area and enhanced charge sep-
aration efficiency and photoactivity compared to their
parent compounds.
[1]
3+
photocatalysts such as Cu S/Zn Cd S, BiOBr:Yb ,-
2
0.5
0.5
[3]
3+
[2]
Er /g-C N ,
CoFe O /Cd Zn S,
Bi/BiOBr:Yb,-
3
4
2
4
0.9
0.1
[
4]
[5]
[6]
Er/C, Cu S/Zn Cd S, BiOBr–Ag SnS , etc., have
2 | EXPERIMENTAL
2.1 | Materials
2
0.5
0.5
8
6
been explored for harvesting visible light for H produc-
2
tion and elimination of organic contaminants from
wastewater.
Bismuth oxybromide (BiOBr) is one of the bismuth
oxyhalide-based p-type indirect semiconductors and is
For the preparation of CoWO nanoparticles and BiOBr–
CoWO4 nanocomposites, sodium tungstate dihydrate
4
[
7]
widely studied as a photocatalyst for CO reduction
(Na WO ꢁ2H O), bismuth nitrate (Bi(NO ) ꢁ5H O) and
2
2
4
2
3 3
2
[
8]
and dye degradation. BiOBr material exhibits a wide
ethylene glycol were purchased from Himedia Ltd. Cobal-
[9]
bandgap of 2.64–2.91 eV.
Although BiOBr exhibits
tous chloride hexahydrate (CoCl ꢁ6H O) and KBr were
2
2
marked UV and visible-light photodegradation
purchased from Merck. RhB was procured from Sigma
Aldrich and used as received.
[10–12]
activities,
its utilization of visible light is limited
and undergoes rapid recombination of electrons and
holes. Therefore, BiOBr shows low photocatalytic per-
formance with visible light, thus limiting its use for
practical applications in environmental remediation.
Researchers have reported that the BiOBr performance
2.2 | Synthesis of CoWO nanoparticles
4
and BiOBr–CoWO nanocomposites
4
[13]
can be improved by metal ion doping,
noble metal
and developing semiconductor compos-
These approaches have helped in tailoring
CoWO4 nanoparticles were synthesized using a co-
[14]
[26]
deposition
ites.
precipitation route.
In this method, 0.1 M CoCl ꢁ6H O
2
2
[
15,16]
solution (100 ml) was added to a 250 ml beaker. After-
band-edge position and enabling charge separation and
ultimately increasing the photoactivity of BiOBr.
ward, 0.1 M Na WO ꢁ2H O (100 ml) was added slowly.
2
4
2
The mixture was magnetically stirred at room tempera-
ture for 4 hours. A reddish violet precipitate of CoWO4
nanoparticles was obtained and collected by centrifuga-
Cobalt tungstate (CoWO ) is a p-type semiconductor
4
and displays paramagnetic behavior at room temperature.
[17]
The bandgap energy of CoWO is about 2.24 eV, which
tion. The obtained precipitate of CoWO nanoparticles
4
4
is suitable for visible-light absorption. Therefore, CoWO
was washed with distilled water, dried in a hot-air oven
4
[20]
[18,19]
ꢀ
has been used as a photocatalyst,
electrocatalyst
and finally calcined at 500 C for 2 hours. Black CoWO
4
[
21]
and photoanode in a photovoltaic electrochemical cell.
nanoparticles were obtained.
The construction of BiOBr–CoWO heterostructures by
The BiOBr–CoWO heterostructured nanocomposite
4
4
[
27]
merging the narrow bandgap of CoWO nanoparticles
was prepared using a precipitation route.
The as-
4
with the wide bandgap of BiOBr is a promising approach
for utilizing visible light and encouraging photoinduced
electron–hole pair separation, which can result in
enhanced photoactivity. In order to enhance the activity
of BiOBr, a variety of metal tungstate nanoparticles have
been used to couple with BiOBr to form heterostructures,
prepared CoWO4 nanoparticles (100 mg) were added
into a beaker containing 0.02 M bismuth nitrate solution
(100 ml), ethylene glycol (20 ml) and 1 g of potassium
bromide. The reaction mixture was heated at 100 C and
magnetically stirred for 4 hours. The molar ratio of bis-
muth nitrate to CoWO4 was 0.02:0.0032. Finally, a
ꢀ
[22]
[23]
namely BiOBr nanodots–Bi WO ,
BiOBr–FeWO4,
BiOBr–CoWO composite was obtained and denoted as
2
6
4
[24]
[25]
CdWO –BiOBr,
BiOBr–ZnWO4,
etc.
BiOBr–CoWO -2. Similarly, BiOBr–CoWO -1 (0.02:0.
4
4
4
Inspired by these studies, to investigate the practical
utility of BiOBr, we prepared BiOBr-based materials in
0016), BiOBr–CoWO -3 (0.02:0.0048) and BiOBr–
4
CoWO -4 (0.02:0.0065) composites were prepared using
4
the
form
of
BiOBr–CoWO4
heterostructured
precipitation
50, 150 and 200 mg of CoWO nanoparticles, respec-
tively. For comparison, pure BiOBr was also prepared
using a similar procedure but without addition of
4
nanocomposite photocatalysts using
method and their photocatalytic activity was studied for
the removal of rhodamine B (RhB) from solution under
a
CoWO nanoparticles.
4

BIOBR-COWO4
3 of 11
2.3 | Characterization of samples
An X-ray diffraction (XRD) study was conducted to deter-
mine the crystal structure and purity of BiOBr, CoWO4
nanoparticles and BiOBr–CoWO nanocomposites. The
4
size and surface morphology of the compounds were
observed using transmission electron microscopy (TEM).
The photoabsorption ability of the compounds was evalu-
ated using a Varian Cary 5000 UV–visible–near-infrared
spectrophotometer. The nitrogen adsorption–desorption
isotherms of samples were measured with a Micro-
meritics ASAP 2020. The degassing temperature was
ꢀ
1
50 C. Photoluminescence (PL) responses of the prepared
photocatalysts were determined using a Horiba Jobin
Fluorolog and Edinburgh F900. The time-resolved fluo-
rescence of the photocatalysts was studied at an excita-
tion wavelength of 360 nm and an emission wavelength
of 434 nm. X-ray photoelectron spectroscopy (XPS) was
conducted to identify the surface chemical composition
and the chemical state of the elements present in the
−
1
composites. Finally, photocatalytic RhB (20 mg l ) deg-
radation was carried out over BiOBr, CoWO4
nanoparticles and BiOBr–CoWO heterostructures under
4
sunlight. The photocatalyst (100 mg) was mixed with
8
0 ml of RhB solution in a beaker and stirred to establish
adsorption–desorption equilibrium between BiOBr–
CoWO nanocomposite and RhB in the absence of light.
4
Around 5 ml of suspension was removed after sunlight
irradiation and the absorption of RhB was quantified by
assessing the change in absorbance with a UV–visible
spectrophotometer (Cary 100 Bio).
3
| RESULTS AND DISCUSSION
Figure 1 shows the XRD patterns of the synthesized
CoWO4 nanoparticles, BiOBr and BiOBr–CoWO4
nanocomposites. The pattern of the CoWO nanoparticles
4
ꢀ
ꢀ
ꢀ
ꢀ
shows diffraction peaks at 15.60 , 19.17 , 23.82 , 24.71 ,
ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ
FIGURE 1 XRD patterns of (a) BiOBr, (b) BiOBr–CoWO
4
-1,
3
5
0.77 , 36.48 , 38.63 , 41.30 , 44.36 , 45.96 , 48.87 ,
(
c) BiOBr–CoWO
f) CoWO nanoparticles
4 4 4
-2, (d) BiOBr–CoWO -3, (e) BiOBr–CoWO -4 and
ꢀ
ꢀ
ꢀ
ꢀ
0.62 , 52.10 , 61.84 and 65.19 corresponding to reflec-
tions from (0 1 0), (0 0 1), (−1 1 0), (0 1 1), (−1 1 1), (12
(
4
0
3
), (0 0 2), (−2 0 1), (−2 1 1), (−1 1 2), (−2 2 0), (0 2 2), (0
1), (−3 1 1) and (−2 3 1) planes, respectively, in accor-
are shown in Figure 2. The XPS survey spectrum indi-
cates the presence of Bi, Br, O, Co and W elements in
dance with JCPDS file 00-015-0867. The XRD patterns of
the BiOBr–CoWO composites show the diffraction peaks
BiOBr–CoWO -2. The spectrum of Bi 4f orbital exhibited
4
4
of both BiOBr and CoWO phases. The peaks at
two intense peaks that appeared at binding energies of
157.39 and 162.79 eV which can be ascribed to the spin–
orbital splitting of Bi 4f7/2 and Bi 4f5/2, respectively
(Figure 2b). For Bi 4f7/2 and Bi 4f5/2 orbitals, the binding
energies were found to be slightly shifted to lower values
for the composite when compared with reported values
4
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
2θ = 19.17 , 23.82 , 30.77 , 36.48 and 41.32 correspond
to monoclinic CoWO nanoparticles in tetragonal BiOBr
4
(JCPDS file 01-078-0348) indicating the formation of
BiOBr–CoWO nanocomposites.
4
The XPS survey spectrum of BiOBr–CoWO -2 and
4
core-level spectra of Bi 4f, Br 3d, O 1s, Co 2p and W 4f
of 159.3 and 164.6 eV for Bi 4f7/2 and Bi 4f5/2,

4
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PAUL CHOWDHURY AND SHAMBHARKAR
FIGURE 2 XPS spectra of BiOBr–CoWO
4
-2 nanocomposite: (a) survey scan, (b) Bi 4f, (c) Br 3d, (d) Co 2p, (e) W 4f and (f) O 1s
[28]
respectively, for pure BiOBr,
which can be attributed
binding energies of 32.87 and 35.06 eV due to W 4f7/2 and
to chemical interaction between BiOBr and CoWO4
nanoparticles. The high-resolution spectrum of Br 3d
W 4f5/2 of CoWO , respectively, showing that tungsten
element is in the W oxidation state (Figure 2e). How-
4
6+
(Figure 2c) shows peaks at 66.46 and 67.40 eV attributed
ever, for BiOBr–CoWO -2, the spectrum of W 4f orbital
4
to Br 3d5/2 and Br 3d3/2, respectively, corresponding to
shows two peaks at 33.60 and 35.67 eV attributed to W
─
[29]
Br in tetragonal BiOBr.
orbital (Figure 2d) observed at 779 and 782 eV were
ascribed to Co 2p and Co 2p1/2 of CoWO , respec-
The XPS peaks for Co 2p3
4f7/2 and W 4f5/2 of CoWO , respectively, and an addi-
4
tional peak located at 27.35 eV was observed due to W
4f7/2 orbital which indicates the lower oxidation state of
3
/2
4
[30]
4+
[31,32]
tively.
The high-resolution spectrum of W 4f orbital in
tungsten element W in the composite.
The forma-
CoWO4 showed two intense peaks corresponding to
tion of lower oxidation state W element in the composite
4 4
FIGURE 3 TEM images of (a) BiOBr, (b, c) CoWO nanoparticles and (d–f) BiOBr–CoWO -2 nanocomposite

BIOBR-COWO4
5 of 11
may be due to adsorption of bromide ion on the surface
corresponding to (1 1 2) and (0 0 1) crystal planes of
BiOBr and CoWO4 nanoparticles, respectively. The
occurrrence of these two types of lattice fringe together in
the composite shows the formation of interfacial connec-
defect sites of CoWO nanoparticles in BiOBr–CoWO -2
4
4
that changed the coordination environment of W. Fig-
ure 2f shows the XPS peak at a binding energy of
5
28.22 eV ascribed to O 1s present in BiOBr and CoWO4.
tion between BiOBr and CoWO nanoparticles in the
4
From the above results obtained from the XPS study, it
was confirmed that composite consisted of BiOBr and
composite which will be useful for the movement of pho-
toinduced charge carriers between the two components.
The energy-dispersive X-ray spectrum of BiOBr–
CoWO nanoparticles.
4
Figure 3 shows TEM images of BiOBr, CoWO and
CoWO (Figure 4b) shows that the composite comprises
4
4
BiOBr–CoWO -2 nanocomposite. Figure 3b,c depicts
TEM images of CoWO4 nanoparticles indicating a
elements Bi, O, Br, Co and W. This indicated that CoWO4
was loaded on the BiOBr surface forming the composite.
4
sphere-like structure with
a
diameter of around
The optical absorption edge of CoWO nanoparticles
4
2
0–25 nm. In the case of BiOBr–CoWO nanocomposite
was found at wavelengths of 464 and 828 nm, while
BiOBr had an absorption edge located at 437 nm. On
4
(Figure 3d–f), the TEM images show that CoWO4
nanoparticles are firmly attached to the surface of BiOBr
particles (300 nm) and well distributed in the composite
indicating a good interaction occurs between BiOBr and
adding CoWO to the composite, the absorption edge was
4
slightly shifted to 454 nm for BiOBr–CoWO -2 and
4
462 nm for BiOBr–CoWO -3 (Figure 5a). The Tauc equa-
4
CoWO nanoparticles during the precipitation reaction.
BiOBr surface morphology was not affected by the addi-
tion was used to determine the bandgap energy (E ) of
4
g
[
33]
1/n
the prepared samples : (αhν) = A(hν − E ), where A,
g
tion of CoWO nanoparticles. Figure 3f shows the lattice
α, h and ν signify a constant, absorption coefficient,
4
fringes with interplanar spacing of 0.228 and 0.46 nm
Planck constant and light frequency, respectively. For
FIGURE 4 Energy-dispersive X-ray
patterns of (a) CoWO
4
nanoparticles and
(
b) BiOBr–CoWO -2 nanocomposite
4

6
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PAUL CHOWDHURY AND SHAMBHARKAR
FIGURE 5 (a) UV–visible–near-infrared absorption spectra of BiOBr, CoWO
4
nanoparticles, BiOBr–CoWO
4 4
-2 and BiOBr–CoWO -3.
1
/2
(
b) Plots of (αhν) versus hν of BiOBr, CoWO
4
nanoparticles, BiOBr–CoWO
4
-2 and BiOBr–CoWO -3
4
BiOBr, the value of n is set as 2 as it is an indirect semi-
higher in the CoWO nanoparticles. On incorporating
4
1/2
conductor. Plots of (αhν) versus incident photon energy
hν) are shown in Figure 5b. On the introduction of
CoWO , E was found to be slightly lowered in the com-
CoWO nanoparticles in the composite, the PL emission
4
(
intensity of BiOBr–CoWO -2 was lowered. This suggests
4
that coupling between BiOBr and CoWO diminishes the
4
g
4
−
+
posites which arose from the combined effect of CoWO4
nanoparticles and BiOBr interface interactions (Table 1).
The decreased bandgap energy increases the visible light
recombination efficiency of photogenerated e –h pairs.
An additional amount of CoWO in BiOBr–CoWO -3 ele-
4
4
−
+
vated the PL intensity since the segregated e –h pairs
tend to recombine at this concentration. Thus, BiOBr–
absorption capability of BiOBr–CoWO nanocomposites.
4
To study the charge recombination of the BiOBr–
CoWO -2 showed a higher separation efficiency of
4
−
+
CoWO nanocomposites, PL spectra of the materials were
photogenerated e –h pairs and it can be expected that
this composite should have the best photocatalytic
performance.
4
obtained (Figure 6). Normally, PL having lower intensity
indicates a greater separation efficiency of photoinduced
[
34]
charges and, consequently, a higher photoactivity.
In
The charge separation efficiency in the BiOBr–
this study, CoWO nanoparticles had the maximum PL
CoWO composites was further investigated using time-
4
4
intensity with a strong emission peak at 497 nm. This
indicated that recombination efficiency for e –h pairs is
resolved fluorescence measurements (Figure 7). These
measurements showed that fluorescence emission decays
−
+
4
TABLE 1 Band gap energy, absolute electronegativity, calculated VB and CB potentials of BiOBr and CoWO nanoparticles; surface
area and kinetics data of BiOBr, CoWO nanoparticles and BiOBr–CoWO heterostructures
4 4
Absolute
electronegativity, χ
VB position CB position Indirect band gap
Surface area
k RhB
2
−1
(min−1)
Sample
(eV)
6.17
6.32
(eV)
3.12
3.16
—
(eV)
0.21
0.49
—
energy, E
g
(eV)
(m g
)
BiOBr
2.91
0.05
21.83
—
0.012
0.003
—
CoWO
BiOBr–
.003
BiOBr–
.067
BiOBr–
.065
BiOBr–
.044
P25
4
2.67
CoWO -1
4
4
4
4
—
0
CoWO -2
—
—
—
—
—
—
—
—
—
—
—
—
2.90
2.89
—
12.70
12.84
—
0
CoWO -3
0
CoWO -4
0
—
—
0.017

BIOBR-COWO4
7 of 11
CoWO -3 were of type IV (Figure 8). The Brunauer–
4
Emmett–Teller surface area was improved upon adding
CoWO4 to the BiOBr–CoWO4 composites when com-
pared with BiOBr, which may be favorable for the
adsorption of dye (Table 1). The pore size was about 17,
2
1.6 and 21.8 nm for CoWO , BiOBr–CoWO -2 and
4
4
BiOBr–CoWO -3, respectively. This suggested that meso-
4
pores are present in the BiOBr–CoWO composite mate-
4
rials. The cumulative pore volume for BiOBr–CoWO -2,
4
BiOBr–CoWO -3 and CoWO nanoparticles was found to
4
4
3
−1
be 0.067, 0.069 and 0.102 cm g , respectively.
An N,N,N,N-tetraethylated RhB solution showed
absorption maxima at 553 nm (Figure 9). This character-
istic peak position decreased in wavelength with photo-
reaction time in the presence of BiOBr–CoWO photo
4
FIGURE 6 PL spectra of (a) BiOBr, (b) BiOBr–CoWO
4
-2,
catalyst and reached 497 nm after 30 minutes of reac-
tion and no further change in the peak position was
observed. However, the peak intensity decreased to zero
after 60 minutes of photoreaction. The concomitant
wavelength was shifted gradually to shorter wavelength
which was due to the de-ethylation of RhB on the sur-
(
4 4
c) BiOBr–CoWO -3 and (d) CoWO nanoparticles
with time. The average fluorescence lifetime of charge
carriers for BiOBr, BiOBr–CoWO -2, BiOBr–CoWO -3
4
4
and BiOBr–CoWO -4 was 12.9, 16.7, 15.4 and 14.9 ns,
4
respectively. Increasing the CoWO4 content in BiOBr
face of BiOBr–CoWO -2, and the color of the dye chan-
4
reduces the fluorescence lifetime of the composites. Simi-
ged from red to light green–yellow. This reaction is
initiated by the photosensitizing effect in the presence
of sunlight. RhB degradation is predominantly a bulk
phenomenon whereas de-ethylation is a surface phe-
[35]
lar effects were reported by Pokhrel et al.
The decay
curves of the BiOBr–CoWO nanocomposites revealed
4
that energy transfer and decay processes were dependent
[
36,37]
on the amount of CoWO nanoparticles in the composite.
nomenon.
The de-ethylated intermediates of RhB
4
The BiOBr–CoWO -2 composite had longer decay time
for photogenerated charge carriers among the prepared
may account for the absorption peak between 496 and
553 nm. The decrease in the absorption spectra after the
N-de-ethylation step was primarily due to the degrada-
tion reaction. This suggested that the visible-light photo-
sensitization effect co-occurs with the degradation
process. The photosensitized degradation of RhB over
4
materials which indicates that the recombination of
−
+
photogenerated e –h pairs is hindered in the composite
photocatalyst.
The nitrogen adsorption–desorption isotherms of
CoWO4 nanoparticles, BiOBr–CoWO -2 and BiOBr–
BiOBr–CoWO -2 reached 98.69% after 60 minutes of
4
4
FIGURE 8 Nitrogen adsorption–desorption isotherms of
FIGURE 7 Time-resolved fluorescence spectra of BiOBr,
(a) CoWO
CoWO -3
4 4
nanoparticles, (b) BiOBr–CoWO -2 and (c) BiOBr–
BiOBr–CoWO
4
-2, BiOBr–CoWO
4
-3 and BiOBr–CoWO
4
-4
4

8
of 11
PAUL CHOWDHURY AND SHAMBHARKAR
FIGURE 9 UV–visible absorption spectra of RhB in presence of (a) BiOBr, (b) CoWO
CoWO -2, (e) BiOBr–CoWO -3, (f) BiOBr–CoWO -4 and (g) P25
4 4
nanoparticles, (c) BiOBr–CoWO -1, (d) BiOBr–
4
4
4
photoreaction. For comparison, the photosensitized deg-
radation of RhB by BiOBr and P25 was very slow and
just 34% and 27.74% of the dye was degraded, respec-
tively. The photodegradation efficiency of BiOBr–
100 mg of CoWO in BiOBr–CoWO -2 led to the highest
degradation efficiency towards RhB, due to efficient iso-
lation and transfer of electron–hole pairs at the interface
4
4
of BiOBr and CoWO nanoparticles. Further loading of
4
CoWO heterostructures was higher than that of com-
CoWO nanoparticles in BiOBr–CoWO -3 and BiOBr–
4
4
4
mercial P25. Further, it was found that the presence of
CoWO -4 led to a decrease in their photoactivity
4
low concentration of CoWO nanoparticles (50 mg) in
(Figure 10a). This might be due to the increased recom-
bination process of electrons and holes. A blank experi-
ment was performed by keeping the RhB solution
4
the BiOBr–CoWO -1 composite was not effective for the
4
photosensitized degradation of RhB, while adding
FIGURE 10 (a) Degradation
efficiency of samples. (b) Plot of ln(C /
0
C) versus time. (c) Results of scavenging
experiments. (d) Catalyst recyclability

BIOBR-COWO4
9 of 11
4
FIGURE 11 Band diagram and electron–hole transfer process in BiOBr–CoWO heterostructured composite in the presence of RhB
without photocatalyst in sunlight, which does not stimu-
late self-photodegradation. The photodegradation perfor-
mance of the materials was found in the order:
harness the visible light and effectively transfer photoin-
duced electrons from the CB of CoWO to the CB of
4
BiOBr. Meanwhile, the photogenerated holes in the
valence band (VB) of CoWO4 moved to the VB of
BiOBr. This process causes an electric field at the inter-
CoWO4 < BiOBr<P25 < BiOBr–CoWO -1 < BiOBr–
4
CoWO -4 < BiOBr–CoWO -3 < BiOBr–CoWO -2. The
4
4
4
photodegradation process of RhB was fitted with
face of CoWO nanoparticles and BiOBr for charge sepa-
ration. Studies of RhB degradation reported that it
4
[20]
pseudo-first-order kinetics
and the rate constant (k)
for each sample was calculated (Figure 10b). BiOBr–
degrades via an N-de-ethylation sensitization mecha-
CoWO -2 catalyst exhibited the maximum degradation
nism in the presence of visible light, and an appropriate
4
−1
[41]
rate (k = 0.067 min ) which was about 5.6 times higher
semiconductor
through sensitization of the dye,
−1
than that of BiOBr (k = 0.012 min ) and about 22.5
charge transport to the semiconductor and development
of active radicals on the semiconductor. Absorption of
light by RhB generates highest occupied molecular
times higher than that of CoWO4 nanoparticles
−
1
(
k = 0.003 min ) (Table 1).
The band-edge potentials for BiOBr and CoWO
orbital (HOMO) and lowest unoccupied molecular
4
[38,39]
[42]
nanoparticles
were calculated according to the
orbital (LUMO).
The RhB and excited RhB* redox
results of diffuse reflectance spectroscopy (Table 1). The
potentials versus NHE reference electrode were reported
[
43]
conduction band (CB) potential of CoWO nanoparticles
to be 0.95 and −1.42 eV, respectively.
The LUMO
4
(
(
0.49 eV) was found to be higher than that of BiOBr
0.21 eV). When CoWO4 nanoparticles were coupled
potential of RhB is more negative than that of BiOBr
and CoWO nanoparticles. Therefore, from the LUMO
4
with BiOBr, the exchange of electrons and holes could
occur at the two-phase interface of the composite. The
energy band alignment of CoWO4 and BiOBr brings
of RhB*, electrons are transferred to the CB of BiOBr
and CoWO4 nanoparticles. In the CB of BiOBr, the
excited-state electrons get congested and move to the
[40]
•−
about the creation of a diffusion potential.
When the
surface and produce superoxide anion radicals (O2 )
BiOBr–CoWO nanocomposite is illuminated by visible
which could react with RhB to afford intermediates and
eventually decomposing the dye via photosensitization.
4
light, only CoWO nanoparticles can absorb the radia-
4
tion because the experimental Eg value of CoWO4
nanoparticles is narrower than that of BiOBr particles.
Hence, this reflects that the BiOBr acted as the major
Moreover, the holes in the VB of CoWO can react with
4
•
H O to generate reactive hydroxyl radicals ( OH) to oxi-
2
+
•−
dize RhB. Hence, highly reactive h and O2 radicals
decompose the RhB solution into CO and H O. There-
photocatalyst, while the CoWO nanoparticles acted as a
4
2
2
sensitizer for absorbing visible-light energy in the
fore, we propose a band structure diagram for BiOBr–
BiOBr–CoWO4 system. CoWO4 nanoparticles can
CoWO4 heterostructure to describe the photocatalytic

10 of 11
PAUL CHOWDHURY AND SHAMBHARKAR
mechanism and transfer of electrons and holes in the
presence of RhB solution (Figure 11).
XRD, TEM, PL, time-resolved fluorescence, XPS and
photocatalytic measurements. TEM observations showed
+
•
•−
To investigate the role of h , OH and O
during
that CoWO nanoparticles were deposited on the BiOBr
2
4
the photocatalytic process, scavengers isopropanol (Pr)
surface and an interface between the two semiconduc-
tors was formed. The photocatalytic experiments showed
•
+
for OH radicals, ammonium oxalate (AO) for h and
•−
1
,4-benzoquinone (BQ) for O2
radicals were added
that BiOBr–CoWO -2 heterostructured photocatalyst
4
into the RhB solution containing BiOBr–CoWO -2
nanocomposite, as shown in Figure 10c. In the pres-
ence of AO, BQ and Pr, only 20.9%, 17.39% and
demonstrated higher activity than commercial P25 in
decomposing RhB after 60 minutes of solar irradiation.
The greater activity could be due to improved solar light
absorption, high surface area and the establishment of a
4
6
4.13% RhB decomposition occurred, respectively.
Therefore AO, BQ and Pr inhibited the degradation
heterostructure between BiOBr and CoWO nanoparti-
4
+
•
•−
reaction, indicating that h , OH and O
were active
cles that hindered the recombination of photoinduced
2
−
+
species involved in the photocatalytic reaction. The
photodecomposition of RhB was repeated to study the
e –h pairs. BiOBr–CoWO -2 possesses the longest
4
time-resolved fluorescence, which indicated that the
construction of a heterojunction is favorable for the pho-
toinduced charges to diffuse quickly to the surface and
participate in the photoreaction. The prepared photocat-
alyst could be reused for up to three runs to carry out
the degradation experiments. This study presents a new
approach in the design and preparation of BiOBr–
CoWO4 heterostructures capable of harvesting visible
light and also provides a promising system for the puri-
fication of water in the presence of sunlight.
reusability of the BiOBr–CoWO photocatalyst. It could
4
be reused for up to three cycles of the experiment
which indicates its potential for practical applications
(Figure 10d).
The probable photocatalytic reactions taken place on
the surface of BiOBr-CoWO nanocomposite can be des-
4
ignated as below:
À
Á
−
þ
VB
BiOBr−CoWO þhν ! BiOBr ðe CBÞþCoWO4
h
4
ACKNOWLEDGEMENT
One of the authors (APC) is grateful to the Director, NIT
Silchar for financial assistance.
RhBþhν ! RhB*
RhB ðadsÞ*þBiOBr−CoWO
4
−
!
RhB ðadsÞ*þBiOBr ðe CBÞ
ORCID
Baban H. Shambharkar https://orcid.org/0000-0003-
2779-4990
−
ꢁ−
2
O þe ! O
2
þ
ꢁ
þ
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