Synthesis of one-dimensional Bi2O3-Bi5O7I heterojunctions with high interface quality

Article abstract of DOI:10.1039/c8ce00819a

One-dimensional Bi₂O₃-Bi₅O₇I heterostructures with enhanced visible light photocatalytic performance were synthesized by high temperature calcination of BiOI-Bi(OHC₂O₄)·2H₂O precursors. The Bi₅O₇I nanosheets uniformly grew on the Bi₂O₃ porous rods. The photocatalytic performance of the obtained products was evaluated by degradation of methyl orange (MO) and phenol under visible light irradiation. The results show that the Bi₂O₃-Bi₅O₇I heterostructure displays higher photocatalytic activity than pure phase Bi₂O₃ and Bi₅O₇I, and MO and phenol with high concentration can be completely degraded in 60 min under visible light irradiation using the Bi₂O₃-Bi₅O₇I (DS-2) heterostructure as a photocatalyst. This enhanced photocatalytic performance is ascribed to the synergistic effect of the suitable band alignment of Bi₂O₃ and Bi₅O₇I, high interface quality and one-dimensionally ordered nanostructure. Radical scavenger experiments indicate that holes (h⁺) and superoxide radicals (O₂^-) were the main active species for MO and phenol degradation during the photocatalytic process. This work will offer a simple route to design and synthesize junction structures with high interface quality for photocatalytic applications.

Full text of DOI:10.1039/c8ce00819a

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Synthesis of one-dimensional Bi2O3–Bi5O7I
heterojunctions with high interface quality†
Cite this: DOI: 10.1039/c8ce00819a
*
Yin Peng,
Yan Ge Mao and Ting Liu
One-dimensional Bi2O3–Bi5O7I heterostructures with enhanced visible light photocatalytic performance
were synthesized by high temperature calcination of BiOI–BiIJOHC2O4)·2H2O precursors. The Bi5O7I nano-
sheets uniformly grew on the Bi2O3 porous rods. The photocatalytic performance of the obtained products
was evaluated by degradation of methyl orange (MO) and phenol under visible light irradiation. The results
show that the Bi2O3–Bi5O7I heterostructure displays higher photocatalytic activity than pure phase Bi2O3
and Bi5O7I, and MO and phenol with high concentration can be completely degraded in 60 min under visi-
ble light irradiation using the Bi2O3–Bi5O7I (DS-2) heterostructure as a photocatalyst. This enhanced photo-
catalytic performance is ascribed to the synergistic effect of the suitable band alignment of Bi2O3 and
Bi5O7I, high interface quality and one-dimensionally ordered nanostructure. Radical scavenger experiments
indicate that holes (h+) and superoxide radicals (˙O2−) were the main active species for MO and phenol deg-
radation during the photocatalytic process. This work will offer a simple route to design and synthesize
junction structures with high interface quality for photocatalytic applications.
Received 18th May 2018,
Accepted 6th July 2018
DOI: 10.1039/c8ce00819a
rsc.li/crystengcomm
picoseconds, so the fast transfer rate requires the interface to
provide the smallest energy barrier (large coupling area).5,6
Introduction
Environmental pollution and energy shortage are the two key
problems to be solved due to rising global population and
fast-growing industry. Thus, it is urgent to develop renewable
and environmentally friendly technologies for environmental
remediation and green energy production. Compared with
other proposed technologies, photocatalytic technology is con-
sidered as the most promising technology for environmental
remediation due to its highly efficient, eco-friendly and low-
cost approach.1
A variety of strategies have been developed in the past de-
cade in order to improve efficiencies of photocatalysts, such
as doping, co-deposition of noble metals, formation of semi-
conductor heterojunctions and crystal facet engineering.
Among the proposed strategies, the formation of semicon-
ductor heterojunctions has been proved to be one of the
most promising ways because of its feasibility and effective-
ness for the spatial separation of electron–hole pairs.2–4 How-
ever, practical applications of semiconductor heterojunctions
are still limited by their low photocatalytic activity and insta-
bility due to low interfacial quality. The interfacial charge
transfer between semiconductor interfaces is performed in
Therefore, the interfacial quality plays a key role in improving
photocatalytic activity.
Over the past decade, fabrication of a high interface qual-
ity in heterostructured catalysts has attracted increasing
attention.7–15 For example, Rohrer et al. studied and com-
pared the photocatalytic activity of AFeO3/TiO2 and ATiO3/
TiO2 heterojunctions. It was found that ATiO3/TiO2 had a
higher reactivity than AFeO3/TiO2 under the same condi-
tions.15 The reason was that chemically strong Ti–O–Ti bonds
could be formed between ATiO3 and TiO2 at the interface in-
stead of AFeO3 and TiO2. Peng et al. synthesized Bi2O3–
Bi2O2.33 heterostructures through high temperature calcina-
tions.10 The obtained Bi2O3–Bi2O2.33 heterostructures
exhibited outstanding photocatalytic performance to degrade
phenol and MO dyes with high concentration in a short time
under solar/visible light irradiation. The enhanced photocata-
lytic activity was ascribed to the small interfacial lattice
mismatch between Bi2O3 and Bi2O2.33 which enhanced the
interface quality. So, according to the reported results, it is
believed that the similar composition (or structure) on both
sides of the interface is beneficial to form heterostructures
with high interfacial quality. Namely: low interfacial defects,
chemically-strong bonds and large contact areas in the
interface.
The Key Laboratory of Functional Molecular Solids, Ministry of Education,
College of Chemistry and Materials Science, Anhui Normal University, Wuhu
241000, China. E-mail: kimipeng@mail.ahnu.edu.cn; Fax: +86 553 3937135
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8ce00819a
To date, most heterojunctions are obtained by a solution-
phase method in which components of the heterojunction
are prepared separately and then integrated with the other.
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This kind of operation was feasible and straight-forward, but
may lead to the limited contact interface area between indi-
vidual components and thus decreased the interface quality
of semiconductor heterostructures.16 Recently, our group has
of electrons and holes. This work provided a simple route to
design and synthesize Bi-based heterostructures with high
interface quality for photocatalytic applications.
proved that the high temperature calcining precursor method Experimental section
could effectively improve the interface quality.10,11,17–19 Dur-
ing the calcination process, precursors decompose to respec-
Sample preparation
All reagents were of analytical grade and used without any
further purification in our experiment.
tive calcined-products. Simultaneously, these respective
calcined-products and precursors involve a chemical reaction
with each other, and then form new heterostructures via
epitaxial growth at the interface. So, the high temperature
calcining precursor method utilizes structural rearrangement
at the atomic level, which is helpful to obtain chemically-
strong bonds across interfaces with large contact areas. More-
over, high temperature is beneficial to decrease the interface
defect density of semiconductor heterostructures, and then
improve the interface quality.
Bi5O7I is a newly found visible-light-driven photocatalyst,
and its dispersed hybridized valence bands with a moderate
band gap (∼2.87 eV) endow Bi5O7I with efficient photocata-
lytic activity in the decomposition of pollutants under visible
light.20–29 However, Some drawbacks in Bi5O7I still exist, such
as deficient light absorption, low transfer efficiency of photo-
generated charge carriers, and so forth. Therefore, construc-
tion of heterojunction structures by combining Bi5O7I with
other appropriate semiconductors may be a constructive and
instructive method to further improve the photocatalytic ac-
BiIJOHC2O4)·2H2O nanorods were synthesized according to
our earlier report.17 Afterwards, BiIJOHC2O4)·2H2O–BOI com-
posites were prepared by a hydrothermal method. Firstly,
0.35 g BiIJOHC2O4)·2H2O nanorods were dispersed in 25 ml
ethylene glycol (EG) solution to be a homogeneous solution
under magnetic stirring. Secondly, BiIJNO3)3·5H2O was added
to the above solution, followed by stirring for 30 min.
Thirdly, 25 mL of KI mannitol solution (0.1 M) was slowly
added into the above mixture, followed by stirring for 60
min. Finally, the mixture was sealed into a Teflon-lined stain-
less steel autoclave and treated at 100 °C for 10 h. After being
cooled down to room temperature naturally, the products
were collected and washed several times with deionized water
and absolute ethanol, and dried at 60 °C for 4 h. The as-
obtained samples were labeled S-1, S-2 S-3 and S-4 when the
molar ratio of BiIJOHC2O4)·2H2O : BiOI is 100 : 1.5, 100 : 2.5,
100 : 5 and 100 : 15, respectively.
Bi2O3–Bi5O7I heterojunctions were obtained by calcining
S-1, S-2, S-3 and S-4 precursors at 350 °C for 3 h in air, and
the corresponding calcined-products were labeled DS-1, DS-2,
DS-3 and DS-4, respectively.
Bi2O3 rods were obtained by calcining BiIJOHC2O4)·2H2O
precursors at 350 °C for 4 h in air.
tivity of Bi5O7I. Recently, some Bi5O7I-based heterostructures,
30
such as Bi5O7I/BiOBrxI1−x
,
Ag2O/Bi5O7I p–n junction,31 core–
shell-like BiOCl0.5I0.5/Bi5O7I,32 Sr2TiO4–Bi5O7I,33 AgI/Bi5O7-
I,34,35 Ag/AgBr/Bi5O7I,36 Bi4O5I2/Bi5O7I,37,38 BiOI/Bi5O7I,39
g-C3N4/Bi5O7I,40 and Bi5O7I/Bi2O3,41,42 have been synthesized.
However, most of the obtained Bi5O7I-based heterostructures
still have low photocatalytic activity due to their low interface
quality.
Test of photocatalytic activity
Photocatalytic performance of the obtained Bi2O3–Bi5O7I
heterostructures was tested by degrading MO dye and toxic
phenol under visible light irradiation. A 300 W Xe lamp (CEL-
HXF300F, Beijing China Education Au-light Co., Ltd) with a
400 nm cut off filter was used as a visible light source. In a
typical photocatalytic experiment, 50 mg of photocatalyst and
50 mL MO (30 mg L−1) or phenol (20 mg L−1) solution were
mixed together under magnetic stirring. Prior to illumina-
tion, the suspension was stirred for 30 min in the dark to
reach an adsorption–desorption equilibrium between MO (or
phenol) and the photocatalyst. At each irradiation time inter-
vals, the above suspension (3 mL) was collected and
centrifuged, then the supernatant was monitored by UV-vis
absorption spectroscopy. All the photocatalytic reactions
were carried out at room temperature by cooling water
circulation. Additionally, in order to test the durability, the
photocatalyst was reused five times to degrade MO dye mole-
cules. The experiments of trapping active species were similar
to the photocatalytic tests. Ammonium oxalate (AO),
p-benzoquinone (BQ) and t-butanol acted as the scavengers of
In this work, we synthesized one-dimensional Bi2O3–
Bi5O7I heterostructures with high interface quality by high
temperature calcination of BiOI–BiIJOHC2O4)·2H2O precursors.
Methylene orange (MO) and phenol with high concentration
were used as model compounds to investigate the photocata-
lytic activities of the Bi2O3–Bi5O7I1 heterostructure under visi-
ble light irradiation. The results demonstrated that the
Bi2O3–Bi5O7I heterostructures exhibited more excellent photo-
catalytic activity than the recently reported Bi5O7I-based
heterostructures for degrading high concentration MO dyes
and phenol.28,30,33,38–40,43 The enhanced photocatalytic per-
formance was attributed to the synergistic effect of the fol-
lowing three factors: (1) the formation of p–n junctions
between Bi2O3 and Bi5O7I effectively drives the photo-
generated electrons and holes to reach the most stable energy
configuration and then leads to a spatial charge separation;
(2) the high interface quality provides a small penetration
barrier for the photogenerated carrier transfer across the
interface of the Bi2O3–Bi5O7I heterojunction; and (3) a one-
dimensional ordered nanostructure is desirable for facilitat-
ing highly efficient and directional transport and separation
hole (h+), the superoxide radicals (˙O2 ) and hydroxyl radicals
−
(˙OH), respectively.
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Photocatalyst characterization
Field emission scanning electron microscopy (FE-SEM) im-
ages were recorded on a Hitachi S-4800 microscope. Trans-
mission electron microscopy (TEM) images, high-resolution
transmission electron microscopy (HRTEM) images were
performed using a JEOL-2010 microscope with an accelerat-
ing voltage of 200 kV. X-Ray powder diffraction (XRD) was
carried out using a Rigaku (Japan) D/max-γA X-ray diffractom-
eter with Cu-Kα radiation (λ = 0.154178 nm). UV-vis diffuse-
reflectance spectrum was recorded with a UV-2450 spectro-
photometer in the wavelength range of 200–800 nm at room
temperature. BaSO4 was used as the reflectance standard ma-
terial. X-Ray photoelectron spectroscopy (XPS) was performed
using a Perkin-Elmer RBD upgraded PHI-5000C ESCA system.
FT-IR spectroscopy was performed with a FT-IR spectrometer
using KBr disks.
Fig. 1 The XRD patterns of Bi2O3, Bi5O7I, Bi2O3–Bi5O7I heterostructures
and standard card of Bi2O3 and Bi5O7I.
Fig. 2 displays the SEM images of the Bi2O3–Bi5O7I hetero-
junctions and their precursors. BiIJOHC2O4)·2H2O is a rod-like
structure with a smooth surface (Fig. S2a†). After calcinations
at 320 °C for 4 h, BiIJOHC2O4)·2H2O transformed into porous
Bi2O3 rods due to gas removal (Fig. S2b† and 1b). The BiOI–
BiIJOHC2O4)·2H2O precursors are also 1D rod-like structures,
and the BiOI nanosheets grow dispersedly on the BiIJOHC2O4)
·2H2O nanorod (Fig. 2a). Meanwhile, with the increase of the
loaded-BiOI content, more and more BiOI nanosheets grow
on the BiIJOHC2O4)·2H2O rods (Fig. 2c, e and g). After
calcining BiOI–BiIJOHC2O4)·2H2O precursors at 350 °C for 3 h,
the shape of the obtained Bi2O3–Bi5O7I heterostructures were
similar to the Bi2O3 porous rods (Fig. 2b, d, f and h), and
Bi5O7I not BiOI nanosheets grow on the Bi2O3 porous rods
according to the result of the XRD patterns.
Electrochemical impedance spectroscopy (EIS) measurements
Electrochemical impedance spectroscopy (EIS) measurements
were conducted at a frequency between 0.1 Hz and 100 kHz
using a CHI660D instrument. A three-electrode system was
used, in which a Bi2O3–Bi5O7I electrode, a saturated calomel
electrode and a platinum sheet were employed as the work-
ing electrode, the reference electrode and the auxiliary
electrode, respectively. The working electrode was prepared
by pasting a slurry mixture of the as-prepared Bi2O3–Bi5O7I
sample (80 wt%), acetylene black (15 wt%) and poly-
tetrafluoroethylene binder (5 wt%) using ethanol as a solvent
onto a piece of nickel foam and dried under vacuum at 50 °C
for 6 h. 0.5 M Na2SO4 was used as the detecting electrolyte.
As we know, BiOI can't decompose at low-temperature
(less than 400 °C).45 So, the above results clearly reveal that
chemical reaction occur between BiIJOHC2O4)·2H2O and BiOI
during calcinations. The Bi2O3 could be obtained directly by
calcining BiIJOHC2O4)·2H2O nanorods (reaction eqn (1)). At
the same time, Bi2O3 would react with the loaded-BiOI to ob-
tain the Bi5O7I phase at 350 °C.
Results and discussion
BiOI–BiIJOHC2O4)·2H2O heterostructures were synthesized by
a hydrothermal method at 100 °C for 10 h using BiIJOHC2O4)
·2H2O rods as precursors. Fig. S1† shows the XRD patterns of
the BiOI–BiIJOHC2O4)·2H2O heterostructures, pure BiOI and
BiIJOHC2O4)·2H2O. From Fig. S1a,† it can be found that all the
diffraction peaks can be indexed to the bismuth oxalate
BiIJOHC2O4)·2H2O precursor.44 When the molar ratio of
BiIJOHC2O4)·2H2O : BiOI is 100 : 15 (S4 sample), a new diffrac-
tion peak (Fig. S1d†) appears which can be indexed to tetrag-
onal BiOI (Fig. S1g,† JCPDS No. 10-0445).
m
m
3
mBi OHC O 2H O  O2  Bi2O3  2m CO2  m H2O

4 
2
2
2
2
2
(1)
BiOI–BiIJOHC2O4)·2H2O precursors (S-1, S-2, S-3 and S-4)
were calcined at 350 °C for 3 h in air and the respective
calcined-products were obtained which were labeled DS-1,
DS-2 and DS-3, respectively. Fig. 1 shows their XRD patterns.
It can be found that two sets of XRD peaks exist in the DS-4
samples, which are indexed to tetragonal Bi2O3 (JCPDS No.
27-0050) and orthorhombic Bi5O7I (JCPDS No. 40-0548). How-
ever, no BiOI XRD peaks are discovered, implying the change
of BiOI into Bi5O7I during the calcination process. No Bi5O7I
peaks are found in the DS-1, DS-2 and DS-3 samples, which
are due to the low loaded-BiOI content according to Fig. S1.†
m
n  m
2
3n  m
2
(2)
Bi2O3 
BiOI  BinO
Im
2
where n is the initial molar amount of Bi3+, m is the molar
amount of BiIJOHC2O4)·2H2O added. Consequently, the O-rich
BiOI compounds can be formed when n > m.46 According to
the reported result, Bi5O7I can't be obtained until the reac-
tion temperature reaches 500 °C if the BiOI precursor is cal-
cined.45 Thus, we supply a simple and low-temperature
method to obtain Bi5O7I.
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Fig. 2 FE-SEM images of the precursors and their respective calcined products, S-1 (a), DS-1 (b), S-2 (c), DS-2 (d), S-3 (e), DS-3 (f), S-4 (g) and DS-
4 (h).
Surface compositions and chemical states in the Bi2O3,
DS-2 and Bi5O7I samples were measured by XPS. A type full
XPS spectrum of the above samples is shown in Fig. 3a. It
can be seen that Bi and O elements exist in all the samples,
but I element only exists in DS-2 and Bi5O7I. The high resolu-
tion XPS spectra of Bi, I and O are shown in Fig. 3b–d. The
two peaks of DS-2 located at 164.2 and 158.9 eV are ascribed
to Bi 4f5/2 and Bi 4f7/2 (Fig. 3b), respectively, which prove
that the Bi ions in the DS-2 sample are in form of Bi3+. More
importantly, after Bi2O3 and Bi5O7I are in contact with each
other, the peaks of Bi 4f in the DS-2 sample slightly shift to
higher (or lower) binding energies compared to the Bi2O3 (or
Bi5O7I) sample, indicating the strong chemical interaction
between Bi2O3 and Bi5O7I in the interface. A similar phenom-
enon is discovered in the O1s spectrum (Fig. 3d). The high
resolution XPS spectrum of I 3d is displayed in Fig. 3c, and
the peaks at 630.7 and 619.3 eV are assigned to I 3d3/2 and I
3d5/2, respectively. In the DS-2 sample, the peaks of I 3d3/2
and I 3d5/2 also appear, but the intensity of the peaks are
weaker and the position of the peaks shifts to the lower bind-
ing energy than that in Bi5O7I (Fig. 3c). This result not only
proves that small Bi5O7I exists in the DS-2 sample, but also
shows that the strong chemical interaction is formed between
Bi2O3 and Bi5O7I in the interface. Furthermore, according to
the XPS result of DS-2, the molar ratio of Bi and I in the DS-2
sample is 19.02 : 0.69 (27.57 : 1), more than the molar ratio of
Bi and I in the Bi5O7I sample, implying only few Bi5O7I exis-
tences in the DS-2 sample, in good accordance with the XRD
and SEM results.
In order to prove the formation of the Bi5O7I–Bi2O3 hetero-
structure, the IR spectra of pure Bi2O3, Bi5O7I and DS-4 sam-
ples are measured (Fig. 4). We can see that the peaks at
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Fig. 3 Survey XPS spectra of Bi2O3, DS-2, and Bi5O7I samples (a) and their high-resolution XPS spectra of (b) Bi 4f, (c) I 3d and (d) O 1s.
419.0, 508.1, 572.1 and 615.2 cm−1 can be observed in the IR
spectrum of Bi5O7I,40 which can be ascribed to the vibration
of Bi–O bonds. In the IR spectrum of pure Bi2O3, 419.0, 520
and 631 cm−1 are assigned to the vibration of Bi–O bonds.47
After Bi2O3 and Bi5O7I are combined with each other, the
main characteristic peaks ascribed to Bi2O3 (419.0 cm−1) and
Bi5O7I (615.2 cm−1) appear in the Bi5O7I–Bi2O3 hetero-
structure. However, the characteristic peaks centered at 572.1
cm−1 (Bi5O7I) shift to the higher wavenumber (574.9 cm−1),
while the characteristic peak at 520 cm−1 (Bi2O3) shifts to the
lower wavenumbers. This result implies the formation of the
strong interfaces via strong chemical interaction in the
Bi5O7I–Bi2O3 heterostructures.11 This conclusion is consistent
with the result of XPS analysis.
TEM technology was used to obtain structural information
of the obtained DS-2 sample in detail. The TEM and HRTEM
images of DS-2 are shown in Fig. 5. It is clearly noted that
the Bi5O7I nanosheets grow onto the surface of the Bi2O3 po-
rous rod, which is consistent with the SEM and XRD results.
Fig. 5b shows the HRTEM image of DS-2. The lattice spacing
of 0.347 nm corresponds to (210) planes of Bi2O3, and 0.318
nm corresponds to the (312) plane of Bi5O7I, respectively. The
Fig. 4 IR spectra of pure Bi2O3, Bi5O7I and DS-4 heterostructures.
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Fig. 5 TEM (a) and HRTEM (b) images of the DS-2 heterostructure.
interfacial crystal lattice matches well (see the red ring in
Fig. 5b) between Bi2O3 and Bi5O7I, indicating the
crystallographic-oriented growth of Bi5O7I from the Bi2O3 sur-
face, which favors the formation of chemically strong bonds
across the large contact interfaces.
The photocatalytic activity of the obtained hetero-
structures was evaluated by degrading MO and phenol.
Fig. 6a and c display the degradation curve of MO and phenol
in the presence of photocatalysts. It can be found that DS-2
displays excellent photocatalytic performance among all the
samples. When using DS-2, Bi2O3 and Bi5O7I as photo-
catalysts, the degradation efficiencies of MO (30 mg L−1) are
98%, 80% and 48% in 60 min under visible light irradiation
(Fig. 6a and b), respectively. In addition, 99%, 62% and 33%
of phenol (20 mg L−1) can be degraded in 60 min (Fig. 6c).
Moreover, the absorption peaks of phenol at 208 nm almost
disappear in 60 min (Fig. 6d) using the DS-2 sample as the
photocatalyst. This indicates that all the phenol molecules
are completely degraded to small molecular compounds,
such as CO2 and H2O. Compared with the reported Bi-based
Fig. 6 Photocatalytic degradation curves of MO (30 mg L−1) (a) and phenol (20 mg L−1) (c) using different photocatalysts; UV-vis absorption spectra
of MO (b) and phenol (d) using DS-2 as the photocatalyst under visible light irradiation.
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photocatalysts for degradation of phenol with the similar
photocatalytic conditions, the DS-2 sample shows excellent
photocatalytic activity. This enhanced photocatalytic activity
is ascribed to the formation of high interface quality in the
DS-2 sample, which can be seen from the SEM and HRTEM
images (Fig. 2 and 5).
red arrows). This may be due to the Bi2O3 reaction with CO2
which comes from the decomposition of MO in the process
of photocatalysis.
UV-vis diffuse-reflectance spectroscopy (DRS) was used to
measure the optical properties of Bi2O3, Bi5O7I and Bi2O3–
Bi5O7I samples. As shown in Fig. 8a, Bi2O3–Bi5O7I hetero-
junctions show good visible light absorption. The optical
band gaps of Bi2O3 and Bi5O7I (Fig. 8b) can be determined by
the following equation:
In addition, the photocatalytic activity of Bi2O3–Bi5O7I
photocatalysts was also investigated by degrading RhB and
MB solutions under visible light irradiation (Fig. S3†). It is
found that RhB (10 mg L−1) and MB (10 mg L−1) can be de-
graded completely within 100 and 60 min, respectively, using
the DS-2 sample as the photocatalyst. This result shows that
the obtained DS-2 selectively degrades MO and phenol.
To investigate the stability of photocatalytic performance
in the visible light region, the DS-2 sample was used to de-
grade MO in five repeated cycles, and the results are shown
in Fig. 7. The DS-2 photocatalyst retains its excellent photo-
catalytic activity after 5 repeated cycles (Fig. 7), implying that
the DS-2 photocatalyst possesses favorable stability. Small
amounts of Bi2O2CO3 are found to be present in the XRD pat-
tern of DS-2 after recycling the photocatalyst (Fig. 7b, see the
αhν = A(hν − Eg)n/2
in which α, h, ν, A and Eg are the absorption coefficient,
Planck constant, light frequency, a constant and band gap,
respectively. Also, n is equal to 4 for the indirect band gap
and 1 for the direct band gap. Here n = 4 for both Bi2O3 and
Bi5O7I. Fig. 8b shows the plots of (αhν)n/2 versus hν for Bi2O3
and Bi5O7I. The band gap (ΔEg) is estimated by extrapolating
the straight line to the abscissa axis; therefore, ΔEg (Bi2O3)
and ΔEg (Bi5O7I) are estimated to be 2.26 eV and 2.86 eV,
respectively.
Fig. 7 Cycling times of the photocatalytic degradation of MO (30 mg L−1) in the presence of DS-2 under visible light irradiation after five repeated
cycles and (b) the XRD pattern of DS-2 before and after five repeated cycles.
Fig. 8 (a) UV-vis diffuse reflectance spectra of Bi2O3, Bi5O7I and Bi2O3–Bi5O7I samples, and (b) the plots of (αhν)1/2 vs. hν of Bi2O3, Bi5O7I.
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Fig. 11 EIS Nyquist plots of the prepared photocatalyst samples.
Fig. 9 VB-XPS spectra of Bi2O3, Bi5O7I and DS-2.
degradation. These results demonstrate that ˙O2 and h+ are
−
To determine the relative positions of the conduction
band (CB) and VB edges, the VB-XPS spectra for Bi2O3, Bi5O7I
and DS-2 were measured, as shown in Fig. 9. In contrast to
Bi5O7I, the VB maximum of DS-2 is down-shifted from 1.69 to
1.53 eV. Compared to Bi2O3, the VB maximum of DS-2 is up-
shifted from 1.32 to 1.53 eV. The shift of the VB maximum
position for DS-2 can be attributed to the formation of the
heterojunction, as confirmed by XRD and TEM results. The
VB edges of Bi2O3 and Bi5O7I are 1.32 eV and 1.69 eV, respec-
tively. According to the VB edges, combining with band gaps
of Bi2O3 (2.26 eV) and Bi5O7I (2.80 eV), the CB edge potential
of Bi2O3 and Bi5O7I is −0.94 eV and −1.11 eV, respectively, cal-
culated from the equation of ECB = EVB − Eg.
the main active species in the photocatalytic process under
visible light irradiation.
The typical EIS spectra of pure Bi2O3, Bi2O3–Bi5O7I hetero-
structures and pure Bi5O7I are shown in Fig. 11. As is well-
known, the arc radius in the impedance spectrum can reflect
the reaction rate on the photocatalyst surface, and the
smaller arc radius implies a more efficient separation of
photogenerated electron–hole pairs and a higher efficiency of
charge transfer. The arc radii of the prepared DS-2 sample is
smaller than that of pure Bi2O3 and Bi5O7I, suggesting that
the Bi2O3–Bi5O7I heterostructure could facilitate the separa-
tion and migration of photogenerated electron–hole pairs in
comparison with pure Bi2O3 and Bi5O7I. Therefore, the EIS
measurements demonstrate that the Bi2O3–Bi5O7I hetero-
structure can significantly promote the separation of photo-
generated electron–hole pairs and suppress the recombina-
tion of photogenerated electron–hole pairs in the DS-2
samples, thus improving the photocatalytic activities.
Trapping experiments were performed to detect the active
species generated in the photodegradation process. tert-Butyl
alcohol (TBA), ammonium oxalate (AO) and 1,4-benzoquinone
(BQ) were used as scavengers to trap hydroxyl radicals (˙OH),
−
holes (h+) and superoxide radicals (˙O2 ). It can be observed
from Fig. 10 that the photocatalytic degradation of MO and
phenol molecules is remarkably inhibited with the addition
of AO and BQ. However, the addition of TBA did not cause
significant deactivation of the DS-2 photocatalyst, illustrating
that ˙OH has little contribution to MO and phenol photo-
Based on the results of the trapping experiments and
the VB-XPS data, we readily illustrate the schematic diagram
for the photocatalytic reaction mechanism and charge trans-
fer of the Bi2O3–Bi5O7I heterostructures under visible light
Fig. 10 Active species trapping experiments using DS-2 as the photocatalyst under visible light irradiation, (a) MO (30 mg L−1) and (b) phenol (20
mg L−1).
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Fig. 12 Schematic of (a) the energy bands of Bi2O3 and Bi5O7I, and (b) the formation of Bi2O3–Bi5O7I heterostructures and the possible charge
separation.
irradiation, as shown in Fig. 12. For the p-type Bi2O3, the
Fermi energy level is close to the VB edge, whereas for the
n-type Bi5O7I, the Fermi energy level is close to the CB band.
When the two semiconductors are in contact to form a p–n
junction, there is a diffusion of electrons from Bi5O7I to
Bi2O3 due to their different Fermi energy levels, resulting in
accumulation of negative charges in Bi2O3 close to the junc-
rectional transport and separation of electrons and holes.
−
The h+ and ˙O2 radicals are the main active species for
photocatalytic degradation of phenol and MO over Bi2O3–
Bi5O7I. This work provides a facile and versatile strategy to
fabricate heterojunctions with a high quality interface on a
large scale.
tion. At the same time, the holes transfer from Bi2O3 to Conflicts of interest
Bi5O7I, leaving a positive section in Bi5O7I near the junction.
The authors declare that they have no conflict of interest.
Moreover, the energy bands of Bi2O3 shift upward along the
Fermi level and those of the Bi5O7I shift downward along its
Fermi level (Fig. 12b). With equilibration of Bi2O3 and Bi5O7I
Fermi levels, the diffusion of electrons from Bi5O7I to Bi2O3
stops. Therefore, an equilibrium state is formed and an inter-
built electric field will also be generated at the interface. The
narrow bandgap Bi2O3 is excited to generate photogenerated
electron–hole pairs under visible light irradiation. Due to the
potential difference, the electrons migrate from the conduc-
tion band of Bi2O3 to that of Bi5O7I, and the holes remain on
the valence band of Bi2O3. Similarly, the photogenerated
holes migrate from the valence band of Bi5O7I to that of
Bi2O3. The internal electric field and high interface quality at
the junction will enhance the photogenerated electron and
hole mobility, increasing the photocatalytic activity of the
Bi2O3–Bi5O7I p–n heterojunction.
Acknowledgements
This work is supported by the National Natural Science Foun-
dation of China (21101006), the Natural Science Foundation of
the Colleges and Universities in Anhui province (KJ2017A309).
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