Visible-light-driven Bi2O3/WO3 composites with enhanced photocatalytic activity

Article abstract of DOI:10.1039/c5ra13579f

Semiconductor heterojunctions (composites) have been shown to be effective photocatalytic materials to overcome the drawbacks of low photocatalytic efficiency that results from electron-hole recombination and narrow photo-response range. A novel visible-light-driven Bi₂O₃/WO₃ composite photocatalyst was prepared by hydrothermal synthesis. The composite was characterized by scanning transmission electron microscopy (STEM), scanning electron microscopy (SEM), powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET) surface area, Raman spectroscopy, photoluminescence spectroscopy (PL) and electrochemical impedance spectroscopy (EIS) to better understand the structures, compositions, morphologies and optical properties. Bi₂O₃/WO₃ heterojunction was found to exhibit significantly higher photocatalytic activity towards the decomposition of Rhodamine B (RhB) and 4-nitroaniline (4-NA) under visible light irradiation compared to that of Bi₂O₃ and WO₃. A tentative mechanism for the enhanced photocatalytic activity of the heterostructured composite is discussed based on observed activity, band position calculations, photoluminescence, and electrochemical impedance data. The present study provides a new strategy for the design of composite materials with enhanced visible light photocatalytic performance.

Full text of DOI:10.1039/c5ra13579f

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Adhikari, H. T. Dean, Z. D. Hood, R. Peng, K. L. More, I. N. Ivanov and Z. Wu, RSC Adv., 2015, DOI:
10.1039/C5RA13579F.
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towards visible light.22–27 Furthermore, the bismuth tungstate, mmol (0.23 g) of WO3 nanoparticles were then added while stirring.
Bi2WO6 which is formed by heating a mixture of Bi2O3 and WO3 in a The mixture was sonicated for 30 min, then transferred to a Teflon-
1/1 molar ratio at temperatures higher than 900 ○C was also lined stainless steel autoclave with an inner volume capacity of 100
reported to have good photocatalytic properties.28,29 Taking this mL. The autoclave was sealed and placed in an oven at 160 ○C for 5
into account, a composite of WO3 coated with Bi2O3 served as the h to deposit Bi2O3 over WO3. The solid obtained was separated by
basis for this study. Since WO3 and Bi2O3 are both visible light driven filtration and washed with distilled water and pure ethanol to
photocatalysts, the Bi2O3/WO3 composite was explored as
candidate for efficient photocatalytic activity under visible light.
a
remove any organics and dried under vacuum at 60 ○C. A bright
yellow Bi2O3/WO3 composite was obtained after calcination at 400
○C for 2 hours (0.68 g, ~ 97 % yield based on the weight of bismuth
nitrate and WO3 used). Pure Bi2O3 was also prepared similarly
without adding WO3. A portion of the composite was calcined at
900 ○C to obtain bismuth tungstate, Bi2WO6 as confirmed by PXRD.
In this work, a novel visible light active Bi2O3/WO3 composite was
synthesized using a hydrothermal process. The 1/1 molar ratio was
chosen with the rational that upon heating at higher temperature,
Bi2WO6 will be obtained and its photocatalytic properties can then
be studied. The photocatalysts were characterized by PXRD, To evaluate the role of heterojunction in the photocatalytic activity
TGA/DSC, STEM, SEM-EDS, TEM, DRS, Raman, PL, EIS, and BET. Their of Bi2O3/WO3 composite photocatalysts, a mechanically mixed Bi2O3
photocatalytic performance was evaluated using the photocatalytic and WO3 sample (blend) with 1 to 1 molar ratio was also prepared.
reduction of 4-nitroaniline (4-NA), and degradation of Rhodamine B
2.2 Characterization
(RhB) under visible light (λ ≥ 420 nm). The heterostructured
composite photocatalysts showed superior activity compared to
pristine tungsten oxide, bismuth oxide, and ternary bismuth
Bi2O3, WO3, Bi2O3/WO3 composite, and Bi2WO6, compounds were
characterized by powder X-ray diffraction (PXRD) using a Bruker D8
tungstate.
A possible mechanism for the enhancement of
diffractometer equipped with CuKα radiation (λ = 1.5418 Å) in the
range of 2 = 10–60°. The morphologies and structures of the
photocatalytic activity of Bi2O3/WO3 composite is proposed using
band position calculations, electrochemical impedance, and
photoluminescence data.
θ
obtained samples were determined with an SEM using a Hitachi S-
4800 FEG-SEM at 5 KV and a scanning transmission electron
microscope (STEM, JEOL 2200-FS) with aberration corrected at 200
KV. The chemical composition of the samples was determined by an
X-ray energy dispersion spectrometer (EDS, Bruker X-Flash, 30 mm2
SDD) attached to the STEM. X-ray photoelectron spectroscopy (XPS)
was performed using a Perkin Elmer PHI 570 ESCA/SAM System
with an Al Kα source operating at 15 kV and 300 W power output to
probe the elemental composition of the surface. Diffuse reflectance
UV-vis spectra were collected on a Cary 5000 instrument equipped
with a praying mantis kit in the range of 200 to 800 nm. Differential
scanning calorimetry (DSC) and thermogravimetric analysis (TGA)
were performed on a SDT-Q600 (TA instrument, USA) from 32-1000
○C under air flow and heating rate of 10 ○C/min. Nitrogen
adsorption–desorption isotherms at 77 K were investigated using
an autosorb-iQ instrument from Quantachrome. Raman scattering
2. Experimental Section
2.1 Synthesis
2.1.1 Reagents
All chemicals used were analytical grade reagents and were used as
received: Sodium tungstate (Na2WO4·2H2O, 99.7%, J.T. Baker
Chemicals, USA), Bismuth nitrate (Bi(NO3)3·5H2O, 99%, Johnson
Matthey Electronics, USA), Hexadecyl-trimethyl ammonium
bromide (CTAB, 98%, Alfa Aesar, UK), Ethylene glycol (C2H6O2, EG,
>95%, Fisher Scientific, USA), Glycerol (C3H8O3, 99.5%, Fisher
Scientific, USA), hydrochloric acid (HCl, Fisher Scientific, USA), and
pure ethanol (Pharmaco-AAPER, USA).
was collected on
a triple Raman spectrometer (Princeton
Instruments Acton Trivista-555) customized with an ellipsoidal
mirror and directed by a fiber optics bundle (Princeton Instruments)
using a 532 nm laser source. Photoluminescence (PL) spectra were
measured on the dual grating Fluorometer-3T (JY Horiba), with right
angle configuration of excitation and detection. The excitation was
made with 300 nm monochromatic light, with slits on the excitation
and detection monochromators set at 5 nm.
2.1.2 Synthesis of WO3 nanoparticles
WO3 nanoparticles were prepared according to
a previously
reported method with some modifications.30 In a typical procedure,
1.0 g (3.03 mmol) of Na2WO4·2H2O was dissolved in 10 mL of
distilled water. 0.25 g (0.68 mmol) of CTAB was added to 5 mL of
distilled water to make a clear solution. Subsequently, the two
solutions were combined to obtain a clear solution. A yellow
dispersion was formed after conc. HCl (3 mL) was added dropwise
under constant magnetic stirring for 30 min. The yellow slurry was
heated at 70 ○C for 3 hours to obtain a yellow solid which was
filtered, washed with deionized water and ethanol before drying
under vacuum at 60 ○C. The final dry mass was calcined at 400 ○C
for 4 hours to obtain pure WO3 nanoparticles as confirmed by
PXRD.
Electrochemical Impedance Spectroscopy (EIS) measurements were
conducted with a Gamry 3000 electrochemical workstation. The
Spectra were collected using the conventional three-electrode
system consisting of a modified indium-tin oxide (ITO) glass as the
working electrode, Ag/AgCl as the reference electrode, and
platinum wire as the counter electrodes. The EIS measurements
were performed between 100kHz – 0.1 Hz at 0.25 V in a 0.1 M KCl
4-
solution with 5 mM Fe(CN)63-/ Fe(CN)6 with an amplitude of 10
mV. The modified ITO working electrode was prepared by
dispersing 7 mg of each photocatalyst sample in 1.0 mL of 18.2
Milli-Q water, dripping 80 μL of the slurry on ITO glass (dimensions:
1 cm × 1 cm), and drying the modified ITO working electrode in a
vacuum oven at 60 °C for 5 hours prior to the measurement.
2.1.3 Preparation of Bi2O3/WO3 composite and ternary oxide
(Bi2WO6)
To obtain the composite, 2 mmol (0.97 g) of Bi(NO3)3·5H2O was
dissolved in a 60 ml mixture of water, glycerol and ethylene glycol
(1:1:1) under constant magnetic stirring to obtain a clear solution. 1
2 | J. Name., 2012, 00, 1-3
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2.3 Photocatalytic test and Electrochemical Impedance WO3 (JCPDS-072-0677) and monoclinic Bi2O3 (JCPDS-070-8243). No
spectroscopy (EIS)
additional peaks were observed, as confirmed by comparing Fig. 1a-
c. The Bi2O3/WO3 composite exhibits peaks from both Bi2O3 and
WO3 without any impurity peak. Fig. 1d corresponds to
orthorhombic phase of Bi2WO6 (JCPDS-079-2381). The refined
lattice parameters along with space groups of Bi2O3, WO3 and
Bi2WO6 are summarized in Table 1.
The photocatalytic activity of the catalysts was evaluated by
measuring the photocatalytic reduction of 4-nitroaniline (4-NA) and
photocatalytic degradation rate of Rhodamine B (RhB) and under
visible light irradiation. In
a typical experiment, 80 mg of
photocatalyst was dispersed in 50 mL of 4-NA aqueous solution (10
ppm) or RhB aqueous solution (10 ppm) in an 80 mL capacity quartz
glass photo-reactor cell. The suspension was stirred in the dark for
30 min before turning on the lamp to establish an adsorption–
desorption equilibrium. A 300 W Xenon lamp equipped with a UV
cut-off filter (λ ≥ 420 nm) was used as a source of visible light. The
degradation of 4-NA was performed under nitrogen (flow rate of 50
mL/min) in the presence of 10 mg of ammonium oxalate. 1 mL of
the suspension was withdrawn at predetermined time intervals (30
min. or 60 min.) and the catalyst was separated by centrifugation.
The change in 4-NA and RhB concentrations were monitored using
an Agilent-8453 UV-Vis spectrometer at λ = 380 nm (for 4-NA) and λ
= 553 nm (for RhB). In order to detect the active species generated
in the photocatalytic process, the scavengers, including disodium
ethylene diamine tetra acetate (Na2-EDTA), 1,4-benzoquinone (BQ)
and isopropyl alcohol (IPA) were added as quenchers of holes
(h+), superoxide radicals (•O2−), and hydroxyl radicals (•OH)
respectively. All the experiments with scavengers were conducted
in a similar manner of RhB degradation using Bi2O3/WO3 composite
photocatalyst (80 mg) using visible light as explained above. Fresh
RhB solutions were replaced in each experiment. The effects of
scavengers were concluded based on the percentage degradation
of RhB by Bi2O3/WO3 composite photocatalyst in 3 hours of visible
light (λ ≥ 420 nm) irradiation.
131
(d)
(c)
(b)
(a)
20
30
40
50
60
2
θ (degree)
Fig.1: PXRD patterns of (a) WO3, (b) Bi2O3, (c) Bi2O3/WO3 composite
and (d) Bi2WO6.
To further examine the Bi2O3/WO3 composite, the morphology and
microstructure were investigated by electron microscopy. SEM
images of the Bi2O3/WO3 composite are shown in Fig. 2a and b with
different resolutions. SEM images revealed that the composite
calcined at 400 ○C consists of elongated and spherical shapes
particles, approximately 40-60 nm. Also, cauliflower like
microstructures with 200-300 nm in size are evenly distributed in
the sample. To further confirm the heterojunction composition, the
samples were characterized by STEM analysis. The STEM study of
pure samples (Fig. 3a and b) shows that the as prepared Bi2O3 has
spherical morphology with particle size between 70 and 90 nm,
while the as synthesized WO3 is a mixture of spherical and needle
shaped particles with size between 40 and 60 nm.
3. Results and discussion
3.1 Characterization of photocatalysts
The identity and purity of prepared WO3, Bi2O3, Bi2O3/WO3
composite, and Bi2WO6 were determined using PXRD as shown on
Fig. 1. PXRD data indicate that the observed diffraction peaks of
Bi2O3/WO3 sample are in good agreement with those of monoclinic
Table 1: Crystal structure representation, refined lattice parameters, band gaps and colour of Bi2O3, WO3 and Bi2WO6.
Sample
Bi2O3
WO3
Bi2WO6
Crystal system
monoclinic
monoclinic
orthorhombic
Crystal
structure
representation
Space group
P
21/c
P
21/n
P ca21
Refined
parameters
lattice a = 5.8481(2) Å
b = 8.1651(2) Å
a = 7.3124(4) Å
a = 5.4443(8) Å
b = 16.4391(7) Å
c = 5.4603(2) Å
b = 7.5294(3) Å
c =7.6924(2) Å
β = 90.658(3) ○
c = 7.50921(3) Å
β = 113.57(2) ○
Band gaps
Color
2.80 eV
yellow
2.78 eV
yellow
2.93 eV
white
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STEM images of Bi2O3/WO3 composite as shown in Fig. 3c-f show
cauliflower-like structures (200-300 nm in size) as well as smaller
particles (40-60 nm) evenly distributed throughout the composite.
STEM-EDX mapping (Fig. 3f-h) on Bi2O3/WO3 composite
microcrystalline powders, show that tungsten and bismuth are
evenly distributed throughout the composite. In addition to even
distribution of the two oxides, elemental mapping clearly shows
that the cauliflower-like structures contain more bismuth oxide,
while the rod-like and spherical particles contain more tungsten
oxide than bismuth oxide. The observed lattice fringes with inter-
planar spacing of 0.325 and 0.361 nm calculated from high
resolution STEM images (Fig. 3e), correspond to the (120)
interplanar distance of the monoclinic phase of Bi2O3 and the (200)
interplanar spacing of the monoclinic phase of WO3 respectively.
This result is consistent with that obtained from the PXRD analysis
shown in Fig. 1.
Fig. 2: (a) and (b) SEM images of Bi2O3/WO3 composite.
(b)
(a)
Bi WO
2
6
Bi2O3/WO3
2.78 eV
Bi
O
/WO
3
2
3
Bi2O3
2.80 eV
Bi2WO6
2.93 eV
WO
3
Bi
O
2
3
WO3
2.77 eV
2.2
2.4
2.6
2.8
3.0
3.2
3.4
300
400
500
600
700
Energy (eV)
Wavelength (nm)
Fig. 4: (a) Diffuse reflectance spectra of composite, its components,
and ternary oxide, Bi2WO6; (b) Plot of (αh ) for the
2/n vs energy (
ν
)
hν
determination of the band gap energy of WO3, Bi2O3, and Bi2WO6.
The extrapolation of the (αhν)2/n vs. (hν) plot on the x intercepts
gives optical band gaps.
Diffuse reflectance UV-Vis spectra of Bi2O3, WO3, Bi2O3/WO3
composite, and Bi2WO6 (Fig. 4) display photo-absorption properties
at wavelength shorter than 443 nm, 447 nm, 446 nm and 423 nm,
respectively. This clearly demonstrates that the composite and
other studied catalysts had potential applications in the visible
light region (λ ≥ 420 nm)
. The band gap energy of a semiconductor
may be calculated from Equation 1.31
ahv = A(hv − Eg)n/2 (1)
Where α, h, ν, Eg and
constant, the light frequency, band gap energy, and
proportionality constant, respectively. The exponent
determined by the type of optical transition of a semiconductor (
1 for direct transition and
band-gap energies (Eg) of the samples could be estimated from
A are the absorption coefficient, the Planck’s
a
is
=
n
n
n
= 4 for indirect transition). Thus, the
2/n
plots of (ahv
taken as 1 for Bi2O3, and Bi2WO6 and 4 for WO3.32–36 In the case of
Bi2O3/WO3 composite, =4 was used, assuming that the composite
was an indirect band gap semiconductor. From the plot of (ahv
)
versus the photon energy (hv). The value of n is
n
2/n
)
versus (hv) (Fig. 4b), the band gap of Bi2O3, WO3, Bi2O3/WO3
composite, and Bi2WO6 were determined to be 2.80 eV, 2.77 eV,
2.78 eV, and 2.93 eV, respectively.
Fig. 3: STEM images of (a) Bi2O3, (b) WO3, (c, d, e and f) Bi2O3/WO3
composite, and (g, h) STEM mapping analysis for the composite
shown in (f), Bi (green) and W (red).
The DSC-TGA curves of Bi2O3/WO3 composite (Fig. 5) show a major
weight loss (approx. 10.5 %) before 400 ○C, which can be attributed
to dehydration and thermal decomposition of residual organic
4 | J. Name., 2012, 00, 1-3
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molecules present in the precursors used during the synthesis of Fig. 5: TGA and DSC data of the Bi2O3/WO3 composite.
the Bi2O3/WO3 composite. The DSC curve of the as prepared
XPS spectra of Bi2O3, WO3, and Bi2O3/WO3 composite (Fig. 6)
indicate the presence of W, O, and Bi in the Bi2O3/WO3 composite.
The binding energy of the WO3 sample was found to be about 38.7
eV, which corresponds to 4f7/2 of W(VI).37,38 The two peaks observed
at 160.09 and 165.80 eV correspond to the 4f7/2 and 4f5/2 of Bi(III),
respectively, and are consistent with previously reported binding
energies for Bi2O3.39,40 The binding energies of bismuth and
tungsten in the composite are slightly shifted when compared to
those found for pure samples, which may be attributed to the
intimate contact between the Bi2O3 and WO3 particles.41
Bi2O3/WO3 composite, shows three major endothermic peaks at
250 ○C, 320 ○C and 360 ○C which may be assigned to loss of organics
to form the Bi2O3/WO3 composite. Another endothermic peak at
830 ○C is attributed to the formation and crystallization of Bi2WO6.
Based on the DSC-TGA data, the as prepared composite was
calcined first at 400 ○C to form the composite, and at 900 ○C to
obtain Bi2WO6. The identity of the phases obtained was confirmed
by PXRD as shown on Fig. 1. The formation of pure ternary phase
(Bi2WO6) from the composite further supports the 1 to 1 molar ratio
of Bi2O3 and WO3 in composite (Equation 2).
BET surface area measurements were collected for all samples
using nitrogen adsorption–desorption isotherms as shown in Fig. 3S
(†ESI). The results indicate that the specific surface area of
composite (8.27 m2/g) is slightly smaller than that of WO3 (10.79
m2/g), which may be due to particles agglomeration upon
calcination. On the other hand, the surface area of composite is
higher than that of pure Bi2O3 (6.46 m2/g). The specific surface area
of bismuth tungstate (0.74 m2/g) is also small due to aggregation of
Bi2WO6 particles upon calcination at 900 ○C.
Bi2O3 + WO3 → Bi2WO6 (2)
102
100
98
96
94
92
90
88
86
84
82
80
40
30
20
10
0
endo
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
200
400
600
C)
800
Temperature (
°
(a)
Bi 4f5/2
160.90 eV
(b)
W 4f7
(c)
38.7 eV
Bi 4f7/2
165.95 eV
Bi2O3
WO3
160.20 eV
Bi2O3/WO3
WO3
165.12 eV
Bi2O3/WO3
38.4 eV
Bi2O3
0
200
400
600
800
1000
158
160
162
164
166
168
170
36
38
40
42
44
46
Binding Energy (eV)
Binding Energy (eV)
Binding energy (eV)
Fig. 6: (a) XPS survey spectra for Bi2O3/WO3 composite. (b) XPS spectra of Bi 4f region for WO3, Bi2O3, and Bi2O3/WO3 composite. (c) XPS
spectra of W 4f region for WO3, Bi2O3, and Bi2O3/WO3 composite.
Raman spectroscopy of WO3, Bi2O3, Bi2O3/WO3 composite and
Bi2WO6 (Fig. 7) was performed to determine the changes of
vibrational modes of Bi2O3 and WO3 in the composite. The Raman
data for pure WO3 sample has absorption bands at 276 and 332 cm-
1, which are attributed to W-O-W bending vibrational modes; two
other bands observed at 720 and 811 cm-1 are assigned to W-O-W
stretching vibrations.42 Similarly, the bands observed at 283, 319
and 452 cm-1 are due to the presence of Bi2O3.43,44 The Bi2O3/WO3
composite contains only bands of pure Bi2O3 and WO3, which
indicates that the composite consists of only these two oxides.
Furthermore, the absorption becomes broader and weaker when
WO3 is covered with Bi2O3 in the composite which suggests
increased crystalline defects within the framework. Such defects
may favour the capture of photo-generated electrons and inhibit
charge carriers recombination.17 For Bi2WO6 , the bands observed at
288, 311, 335, 723, 798, and 839 cm-1 correspond to the
orthorhombic phase of Bi2WO6.45,46
798
335
839
Bi WO
723
2
6
319
275
810
719
720
Bi O /WO
2
3
3
454
452
811
276
WO
332
318
283
3
Bi O
2
3
300
400
500
600
700
800
900 1000
Raman Shift (cm-1)
Fig. 7: Raman spectra of Bi2O3, WO3, Bi2O3/WO3 composite, and
Bi2WO6.
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3.2 Photocatalytic Testing
3.2.1 Activity
3.2.2 Kinetic and stability study
The photocatalytic activities of all synthesized compounds were The degradations of RhB and 4-NA agree with pseudo-first order
kinetics. Their photocatalytic degradation can be described by
Equation 3.
evaluated under visible light illumination (λ ≥ 420 nm) by
comparing the photocatalytic reduction 4-NA, and degradation
efficiencies of RhB (Fig. 8). Fig. 4S (supporting info) shows the
photocatalytic degradation of RhB by the Bi2O3/WO3 composite
ln (C○/C) = -kt
(3)
under visible light irradiation.
A decrease of RhB absorption
where C○ is the initial concentration of RhB (or 4-NA), C is the
concentration of RhB (or 4-NA) at time t, and k is the rate constant.
The corresponding rate constants (k) were calculated and are
reported in Table 2. Under the same experimental conditions, the
rate constant of RhB degradation with the Bi2O3/WO3 composite
was determined to be 3.12 times larger than that of pristine Bi2O3
and 2.78 times that of WO3. Similarly, the rate constant of 4-NA
degradation with Bi2O3/WO3 composite was found to be 4.32 times
as large as that of pure Bi2O3 and 3.32 times that of WO3. The rate
constants for the composites are 4.33 and 5.24 times greater than
that of Bi2WO6 for the degradation of RhB and 4-NA, respectively. It
is also clear that the rate constants for composites are higher than
those of physically mixed blend sample.
intensity at wavelength 553 nm is observed. Fig. 8a shows the
photocatalytic degradation efficiency of RhB with different catalysts
under visible light irradiation. In the absence of photocatalyst,
photolysis of RhB is negligible. Pure Bi2O3 and pure WO3 can
degrade RhB by 38.07 % and 41.37% in 180 min, respectively, while
the ternary oxide Bi2WO6 only degraded 27.15% of RhB when
irradiated by visible light for the same period of time. In contrast
76.67% of RhB decomposed after 180 min visible light irradiation in
the presence of Bi2O3/WO3 composite. The Bi2O3/WO3 composite
exhibited significantly higher photocatalytic activity compared to
pure Bi2O3, pure WO3, and Bi2WO6. For 4-nitroanilne (4-NA), 60% of
4-NA decomposed after five hours irradiation by visible light in the
presence of the Bi2O3/WO3 composite, compared to only 18%, 25%,
and 15% when equal amounts of Bi2O3, WO3, and Bi2WO6 were used
respectively (Fig. 8b). This result clearly indicates that the
Table 2: Pseudo first order rate constants for photocatalytic
degradation process of RhB or 4-NA under visible light irradiation (λ
≥ 420 nm).
Bi2O3/WO3
composite
photocatalyst
possesses
superior
photocatalytic capability in comparison to individual components as
well as the ternary oxide. It is notable that blend sample (physically
mixed) did not show notable enhanced photocatalytic activity as
compared to Bi2O3/WO3 composite. This implies that there might be
some interaction between Bi2O3 and WO3 in Bi2O3/WO3 composite
sample, which played important role in improving the
photocatalytic activity.
Sample
RhB degradation
Rate constants
(min-1)
4-NA degradation
Rate constants
(h-1)
Bi2O3
0.025
0.040
WO3
0.028
0.052
Bi2WO6
Bi2O3/WO3
blend
0.018
0.078
0.030
0.033
0.173
0.053
(a)
without
catalyst
1.0
0.8
0.6
0.4
0.2
0.0
Bi2WO6
(b)
100
Bi2O3/WO3 composite
(a)
Bi2O3
WO3
80
60
40
20
0
Blend
After
Bi2O3/ WO3
Before
0
30
60
90
120
150
180
210
Irradiation Time (min)
20
30
40
θ (degree)
50
60
2
1
2
3
4
5
Cycling runs
(b)
1.0
0.8
0.6
0.4
0.2
without
catalyst
Fig.
9: Photocatalyst stability. (a) Cycling runs of Bi2O3/WO3
Bi2WO6
Bi2O
composite under visible light irradiation for 3 hour; (b) XRD patterns
of Bi2O3/WO3 composite before and after photocatalytic
degradation of 4-nitroaniline (4-NA).
WO33
Blend
Bi2O3/ WO3
The stability of Bi2O3/WO3 composite was further investigated by
recycling the photocatalyst for RhB degradation under visible light
irradiation for 3 hours. During this stability test, fresh RhB solution
was used after each cycle of photocatalytic experiment. The results
are displayed in Fig. 9a. After five cycles, the photocatalytic ability
of the Bi2O3/WO3 composite was virtually unchanged (declined by
only 1.87%), indicating that the Bi2O3/WO3 composite photocatalyst
possesses excellent photocatalytic stability and recyclability.
0
1
2
3
4
5
6
Irradiation time, hr
Fig. 8: Photocatalytic degradation efficiency using different catalysts
activities of different catalysts under visible light irradiation; (a) RhB
and (b) 4-NA
6 | J. Name., 2012, 00, 1-3
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Furthermore, PXRD analyses of the Bi2O3/WO3 composite as the geometric mean of the absolute electronegativity of the
photocatalyst before and after the reaction (4-NA degradation) constituent atoms. The values for Bi2O3, WO3, and Bi2WO6 were
shown in Fig. 9b, clearly indicate that the Bi2O3/WO3 composite is calculated to be 6.23, 6.59 and 6.36, respectively, and the band gap
Х
photo- and chemically stable.
energies of Bi2O3, WO3 and Bi2WO6 were determined to be 2.80 eV,
2.78 eV, and 2.95 eV, respectively (Fig. 4 and Table 1). Using
equation (5) above, the conduction band bottom (ECB) of, Bi2O3,
WO3 and Bi2WO6 were calculated to be 0.33 eV, 0.70 eV, and 0.38
eV, respectively. Correspondingly, the valence band tops (EVB) were
calculated to be 3.13 eV, 3.48 eV, and 3.33 eV. Because of the small
band gaps in Bi2O3 and WO3, both are excited using visible light (λ ≥
420 nm) and corresponding photo-induced electrons and holes are
generated in each component of the composite. Photogenerated
electrons can be injected from the conduction band of Bi2O3 to the
conduction band of WO3 because of the intimate contact between
the two semiconductors. Simultaneously, holes generated in the
valence band of WO3 can be transferred to that of Bi2O3 because of
the relative band positions (Fig. 10). Thus, the photo-generated
charge carriers are efficiently separated and their recombination
rate is significantly reduced.8,9,35,56,57
3.2.3 Proposed mechanism of photocatalytic activity of RhB
degradation and 4-NA degradation
In the photocatalytic processes, a series of photo-induced reactive
species, including h+, •OH or •O2−, directly take part in the redox
process after the electron–hole pairs are generated. The electron
can be trapped by molecular oxygen to produce the superoxide
radical (•O2−), which acts as a strong oxidant to degrade dye
molecules. In the photocatalytic degradation of RhB using Bi2O3,
WO3, Bi2O3/WO3 composite, or Bi2WO6, the generation of hydroxyl
radicals (•OH) is almost impossible because the valence band
position of these catalysts lies much higher than the energy of
•OH/OH- (1.99 eV). Hence, the photocatalytic degradation of RhB is
primarily caused by superoxide radicals and photogenerated
holes.47
In order to survey the active species in the photodegradation
process of RhB over Bi2O3/WO3 composite, the trapping
experiments were conducted. Various scavengers, including
benzoquinone (BQ), disodium salt of ethylene diamine tetra acetic
acid (Na2-EDTA) and isopropanol (IPA) were added to as the
quenchers of superoxide radicals (•O2−), holes (h+) and hydroxyl
radicals (•OH) respectively.41,48 From Table 1S (supporting
information, ESI), it can be found that degradation efficiency of RhB
was slightly affected by the addition of IPA. On the other hand, the
degradation efficiency is significantly inhibited by the addition of BQ
and Na2-EDTA. These results indicated that superoxide radicals
(•O2−) and holes (h+) are the main active species in the degradation
of RhB.49
Similarly, the degradation of 4-NA also follows the same pathway
except that generation of the superoxide radical (•O2−) is unlikely
because the experiment was conducted under nitrogen flow.
Hence, the photogenerated holes (h+) located in the valence band
of the catalysts are solely responsible for 4-NA degradation. These
Fig. 10: Scheme for electron-hole separation and transport at the
visible light driven Bi2O3/WO3 composite photocatalyst with
calculated band positions.
-
holes (h+) can produce active carbon-dioxide anion radial (•CO2 ) in
the presence of oxalate (C2O42-) ion (Equation 4). These •CO2- radical
anions have a strong reductive ability and hence can reduce 4-
NA.50–52
Bi2WO6
Bi2O3
-
C2O42- + h+
→
CO2+ •CO2
(4)
3.2.4 Enhanced activity of composite
WO3
Bi2O3/WO3
To better understand the mechanism of the enhanced
photocatalytic activity of the Bi2O3/WO3 composite, the relative
band positions of the two semiconductors were determined since
the band-edge potential levels play a crucial role in determining the
pathway of the photogenerated charge carriers in a heterojunction.
The conduction band (CB) bottoms (ECB) were calculated empirically
according to Equation 5.53–55
350
400
450
500
550
Wavelength (nm)
Fig. 11: Photoluminescence (PL) spectra for Bi2O3, WO3, Bi2O3/WO3
composite and Bi2WO6.
ECB = X− 0.5 Eg + E○ (5)
Where Eg is the band gap energy of the semiconductor, E○ is a scale
factor relating the reference electrode’s redox level to the absolute
In order to investigate the lifetime of the photogenerated electron-
hole pairs, photoluminescence (PL) experiments were carried out
vacuum scale (E○ = −4.5 eV for normal hydrogen electrode), and
the electronegativity of the semiconductor, which can be expressed
Х is
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on all prepared phases. PL spectra are related to the transfer enhanced activity is attributed to the effective separation of
behaviour of the photogenerated electrons and holes. In PL electron−hole pairs in the composite giving the synergeꢀc effects
experiment, the electrons are excited to CB (or sub bands) from VB between Bi2O3 and WO3. Based upon the band positions,
at certain excitation wavelength. These electrons may go back to VB photoluminescence and electrochemical impedance data, a simple
giving rise to PL signal. High photoluminescence intensity is mechanism has been proposed that provides new insights for the
generally considered to reflect a high recombination rate of charge fabrication of composite materials that have enhanced
carriers.41,58–60 Fig. 11 shows the PL spectra for Bi2O3, WO3, photocatalytic performance under visible light thus maximizing the
Bi2O3/WO3 composite and Bi2WO6 with an excitation wavelength of use of solar energy.
300 nm. The main emission peak is centred at about 400-440 nm
for all samples. The PL emission intensity of the composite
Bi2O3/WO3 is significantly smaller compared to that of the
Acknowledgements
precursors Bi2O3, WO3, and ternary Bi2WO6. The PL emission
A portion of this research including the STEM, UV-Vis-DRS, Raman
intensity
was
found
to
be
in
the
order
and PL was conducted at the Center for Nanophase Materials
Sciences, which is a DOE Office of Science User Facility. The authors
would like to thank Dr. Corey Hewitt, Wake Forest University,
Department of Physics, for his support in collecting XPS data. The
authors gratefully acknowledge John Reynolds and Anna Österholm
for providing access to their potentiostat for EIS measurements.
Support from Phase II Triad Interuniversity Project (TIP) is
acknowledged.
Bi2O3/WO3<WO3<Bi2O3<Bi2WO6. In addition to the PL study, we
have also conducted electrochemical impedance spectra (EIS) to
understand the migration and transfer process of photogenerated
electron and holes. From Fig. 12, it could be seen that the diameter
of the Nyquist circle of composite was significantly lower than that
of the pristine Bi2O3 or WO3 and ternary oxide (Bi2WO6). The smaller
diameter of the arc implied lower resistance occurred on the
surface of electrode.31,41,48,61 This result further demonstrates that
introduction of Bi2O3 into WO3 can enhance the separation and
transfer efficiency of photogenerated electron and holes, which is
favourable for enhancing the photocatalytic activity. The results
from EIS are consistent with PL. These results suggest that the
composite has a much lower recombination rate of photogenerated
charge carriers, consistent with the observed higher photocatalytic
activity.
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The heterojunction formed between Bi2O3 and WO3 shows visible-light driven enhanced photocatalytic
performance in degradation of Rhodamine b (RhB) and 4 nitroaniline (4-NA).