A nanosheet-like BiPO4/Bi2O2CO3 heterostructured photocatalyst with enhanced photocatalytic activity

Article abstract of DOI:10.1039/c5ra16146k

A nanosheet-like BiPO₄/Bi₂O₂CO₃ heterostructured photocatalyst with different amounts of BiPO₄ contents was successfully synthesized by the hydrothermal method. The photocatalytic activities of the BiPO₄/Bi₂O₂CO₃ composite, pure Bi₂O₂CO₃ and BiPO₄ were evaluated by the photodegradation of rhodamine B (RhB) solution under UV light irradiation. The results demonstrated that all of the as-prepared BiPO₄/Bi₂O₂CO₃ composites exhibited much higher photocatalytic activity than individual Bi₂O₂CO₃ and BiPO₄. Among the composites, the 3% BiPO₄/Bi₂O₂CO₃ showed the highest photocatalytic activity and the photodegradation efficiency of RhB could reach 92.6% within 40 min, which was about 1.7 and 2.3 times higher than that of pure Bi₂O₂CO₃ and BiPO₄, respectively. Simultaneously, the photostability and recyclability of 3% BiPO₄/Bi₂O₂CO₃ were also investigated by four cycling runs for the photodegradation of RhB and the composite exhibited no significant loss of photocatalytic performance, confirming its long-time reusability. The enhanced photocatalytic activity of the BiPO₄/Bi₂O₂CO₃ composite could be attributed to the formation of a suitable heterojunction structure, which facilitated interfacial charge transfer and improved the separation efficiency of photogenerated electron-hole pairs. Moreover, active species trapping experiments demonstrated that the holes (h⁺) played key roles in RhB degradation. On the basis of experimental and theoretical calculation results, a possible reaction mechanism of the enhanced photocatalytic activity was proposed.

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Nanosheet–like BiPO /Bi O CO heterostructured photocatalyst
4
2
2
3
with enhanced photocatalytic activity
Received 00th January 20xx,
Accepted 00th January 20xx
*ab
ab
ab
ab
ab
ab
Yumin Liu, Peng Zhang, Hua Lv, Jing Guang, Shuang Li and Juhui Jiang
DOI: 10.1039/x0xx00000x
Nanosheet–like BiPO
synthesized by hydrothermal method. The photocatalytic activities of BiPO
were evaluated by the photodegradation of rhodamine B (RhB) solution under UV light irradiation. The results
demonstrated that all of the as-prepared BiPO /Bi CO composites exhibited much higher photocatalytic activity than
individual Bi CO and BiPO . Among the composites, the 3% BiPO /Bi CO showed the highest photocatalytic activity
and the photodegradation efficiency of RhB could reach 92.6% within 40 min, which was about 1.7 and 2.3 times higher
than that of pure Bi CO and BiPO respectively. Simultaneously, the photostability and recyclability of 3%
BiPO /Bi CO were also investigated by four cycling runs for the photodegradation of RhB and the composite exhibited
no significant loss of photocatalytic performance, confirming its long-time reusability. The enhanced photocatalytic activity
of BiPO /Bi CO composite could be attributed to the formation of suitable heterojunction structure, which facilitated
4
/Bi
2
O
2
CO
3
heterostructured photocatalyst with different amounts of BiPO
4
contents was successfully
4
/Bi CO
2
O
2
3 2 2 3 4
composite, pure Bi O CO and BiPO
www.rsc.org/
4
2
O
2
3
2
O
2
3
4
4
O
2 2
3
2
O
2
3
4
,
4
2
O
2
3
4
2
O
2
3
interfacial charge transfer and improved the separation efficiency of photogenerated electron–hole pairs. Moreover,
+
active species trapping experiments demonstrated that the holes (h ) played key roles in RhB degradation. On the basis of
experimental and theoretical calculation results, a possible reaction mechanism of the enhanced photocatalytic activity
was proposed.
photocatalyst, has drawn considerable attention for the promising
application in the photocatalysis field due to its particular layer
structure, suitable band gap, excellent photocatalytic activity and
1
. Introduction
With the emergence of seriously environmental pollution problems
and shortage of energy sources, semiconductorꢀbased photocatalysis
has attracted considerable attention due to its potential application in
environmental remediation and hydrogen production from water
using solar energy. Up to now, much work has been done to develop
semiconductor photocatalysts with enhanced photocatalytic
1
5
longꢀterm stability. Nevertheless, the high recombination rate of
the photogenerated electron–hole pairs is still the main drawback of
16
2 2 3
pure Bi O CO for practical application.
In order to improve the photoꢀquantum efficiency of single
semiconductor, it is generally accepted that the coupling of
semiconductors with appropriate energy band gaps would result in
enhanced photocatalytic activity. In composite semiconductors, the
heterojunction structures are built between the semiconductors with
suitable band positions. Thus, feasible transport of photoinduced
electrons and/or holes from one semiconductor to another can
significantly inhibit recombination rate of the photogenerated
electron–hole pairs, increase the lifetime of charge carriers, and thus
highly improve the photocatalytic activities of the composites. Most
recently, several studies have focused on the fabrication of
1
,
2
3,
4
5,
6
7, 8
performance such as TiO ,
ZnO,
and so on. Among these reported
photocatalysts, Bi O CO is one of the most appealing candidates for
BiVO4,
Bi WO ,
2 6
2
9
,
10
11, 12
Bi O CO ,
BiPO4
2
2
3
2
2
3
the degradation of organic dyes. As a member of Biꢀbased materials,
Bi O CO has been used for medical and healthcare fields for a long
time due to its low mammalian toxicity. But in 2010, for the first
time, Bi O CO has been found to display good photocatalytic
activity under UV light irradiation for the degradation of methyl
orange aqueous solution. Since then, Bi O CO , as a novel
2
2
3
1
3
2
2
3
1
4
2
2
3
Bi
2
Bi
2
Bi
2
O
O
2
2
CO
CO
CO
3
3
3
ꢀbased
composite
Bi CO
CO /Bi
photocatalysts,
such
/Bi WO
as
,
1
7
18
19
/Fe
3
O
4
,
2
O
2
3
/Bi
2
S
3
,
Bi
2
O
2
CO
/MoS
3
2
22
6
2
0
21
O
2
/BiOCl, Bi
2
O
2
3
3
NbO
7
,
Bi
2
O
2
CO
3
2
and so
on.
In spite of these progresses, it is still necessary to further explore
suitable semiconductors to couple with Bi CO and form new
types of heterojunction photocatalysts with remarkably improved
photocatalytic activity. As is known, BiPO is a typical n–type
2
O
2
3
4
semiconductor and has been proved to exhibit powerful
photocatalytic oxidative ability for the decomposition of organic
2
3, 24
pollutants.
What’s more, according to previous reports, the
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conduction band of Bi
2
O
2
CO
3
(ECB = 0.17 eV) is more negative than The crystal structure of the products was investigated by powder X–
(EVB ray diffraction (XRD, Bruker D8–advance). The morphology of the
that of BiPO
4
(ECB = 0.43 eV) while the valence band of BiPO
CO (EVB = 3.55 eV).
Therefore, it can be expected that the formation of JSM–6390–LV, JEOL) and transmission electron microscopy (TEM,
BiPO /Bi CO heterojunction structures might be advantageous JEM2100, JEOL). The optical absorption spectra of the samples
for the transfer and separation of photogenerated charge carriers, and were collected by UV–visible diffuse reflectance
thus results in the improved photocatalytic activity of the spectrophotometer (Cary5000 UV–vis–NIR) using BaSO as a
heterojunction photocatalyst. reference. The Fourier transform infrared spectroscopy (FTIR) of the
In this study, novel nanosheets–like BiPO products was performed on BrukerIFS 66 v/s FTIR
heterojunction photocatalyst was successfully synthesized by a facile spectrophotometer using the KBr disk method. The
hydrothermal method. The photocatalytic activity of photoluminescence (PL) spectra were detected by fluorescence
BiPO /Bi CO catalysts was evaluated by degrading RhB aqueous spectrophotometer (Shimadzu RF–5301PC) at an excitation
solution and the results demonstrated that BiPO /Bi CO wavelength of 300 nm. X–ray photoelectron spectroscopy (XPS)
heterostructured photocatalyst exhibited much higher photocatalytic was performed on ESCALAB MKII X–ray photoelectron
activity than pure Bi CO or BiPO . In addition, a possible spectrometer (VG Scientific, UK) using Mg Kα radiation. The
mechanism of enhanced photocatalytic activity for BiPO /Bi CO Brunauer–Emmett–Teller (BET) surface area of the samples was
heterojunction was proposed on the basis of trapping experiments determined using a volumetric adsorption apparatus (NOVA Surface
and estimated energy band positons. Area Analyzer Station A, USA).
.4 Photodegradation experiments
4
25,
=
4.28 eV) is more positive than that of Bi
2
O
2
3
samples was observed by scanning electron microscopy (SEM,
2
6
4
2
O
2
3
a
4
a
4 2 2 3
/Bi O CO
4
2
O
2
3
4
2
O
2
3
2
O
2
3
4
4
2
O
2
3
2
Photocatalytic activity of the samples was evaluated on degradation
of RhB solution under the irradiation of a 300 W mercury lamp.
Typically, 0.1 g of the asꢀprepared photocatalyst was dispersed in
2
. Experimental
2
.1 Synthesis of BiPO
All chemicals were of analytical grade and used without further
purification. The BiPO nanostructures were synthesized by
hydrothermal method at room temperature. In a typical procedure,
.485 g of Bi(NO ·5H O was dissolved in 1 mL of concentrated
4
nanostructures
–
1
2
00 mL of RhB aqueous solution (5 mg L ). Before illumination,
the above suspension was magnetically stirred in the dark for 30 min
to ensure the establishment of an adsorption–desorption equilibrium
between the photocatalyst powders and RhB molecules. Then the
suspension was exposed to the mercury lamp under magnetic
stirring. At given time intervals, about 5 mL of suspension was taken
out and centrifuged to remove photocatalyst powders. The supernate
was analyzed by T6 UV spectrophotometer at λmax of 552 nm.
4
0
3
)
3
2
nitric acid and magnetically stirred to form a transparent solution.
Afterwards, 45 mL of glycerol/water mixedꢀsolvent (the volume
ratio of glycerol to water is 2:1) was added into the Bi(NO
solution under magnetic stirring . After stirring for 10 min, 0.156 g
of NaH PO ·2H was added to the above solution. After
3 3
)
2
4
2
O
continuous stirring for another 1 h, the mixture was transferred into a
Teflonꢀlined stainless steel autoclave and maintained at 200 C for 1
o
3. Results and discussion
h and then cooled down to room temperature naturally. Finally, the 3.1 Characterization of photocatalysts
resulting precipitate was collected, washed with distilled water and
The phase structure and composition of the samples were
investigated by XRD analysis, as shown in Fig. 1. The diffraction
peaks of pure BiPO and Bi CO samples can be easily indexed to
composite was prepared through a simple the monoclinic phase of BiPO (JCPDS card no. 15–0766) and the
hydrothermal method. In typical procedure, 0.97 of tetragonal phase of Bi CO (JCPDS card no. 41–1488),
Bi(NO ·5H O was dissolved in 2 mL of concentrated nitric acid respectively. NO peaks of any other phases or impurities can be
o
absolute alcohol, and then dried at 80 C for 6 h.
.1 Synthesis of BiPO /Bi CO composite
The BiPO /Bi CO
2
4
2
O
2
3
4
2
O
2
3
4
2
O
2
3
4
a
g
2
O
2
3
3
)
3
2
under magnetic stirring at room temperature. Then, 0.36 g of urea detected, indicating the high purity of the as–prepared samples.
dissolved in 30 mL of distilled water was introduced into the Additional, the intense and sharp diffraction peaks suggest that all of
Bi(NO
prepared BiPO
continuously stirred for another 60 min. After that, the mixture was of Bi
transferred into Teflonꢀlined autoclave and the autoclave was sealed BiPO
3
)
3
solution. After stirring for 60 min, 0.018 g of the asꢀ the asꢀprepared samples are well crystallized. Besides, the XRD
powder was added into the above suspension and patterns of BiPO /Bi CO composites are entirely the same as that
CO and no characteristic diffraction peaks corresponding to
can be detected in the BiPO
and maintained at 180 C for 24 h. After being cooled down to room high BiPO
temperature, the resulting precipitate was collected by does not alter the crystalline structure of Bi
4
4
2
O
2
3
2
O
2
3
4
4
/Bi
2
O
2
CO
3
composites even at
o
4
loadings of 6%, suggesting that modification with BiPO
4
2
O
2
CO
3
catalyst. This
centrifugation, washed with deionized water and absolute alcohol result may be attributed to the relatively small loading contents and
o
several times, and then dried at 80 C for 6 h. The as–prepared high dispersion of BiPO
sample was denoted as 3% BiPO /Bi CO , with a 3% molar was also discovered in the case of Ag
percentage of BiPO to Bi CO . In addition, 1% BiPO
and 6% BiPO /Bi CO composites were also synthesized referring BiPO
4 2 2 3
in Bi O CO catalyst. Similar phenomenon
16
4
2
O
2
3
2
O/Bi
/BiOCl composites. However, the presence of BiPO
/Bi CO composites can be confirmed by TEM, FTIR and
was XPS analyses, as discussed later.
The morphology and microstructure of the as–synthesized
2
O
2
CO
3
and
2
7
4
2
O
2
3
4
/Bi
2
O
2
CO
3
Bi
2
S
3
4
in the
4
2
O
2
3
4
2
O
2
3
to the above procedure. For comparison, pure Bi
prepared by the same method.
2
O
2
CO
3
2.3 Characterization
products were characterized by SEM and TEM techniques. As
2
| J. Name., 2012, 00, 1-3
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ARTICLE
Fig. 1 XRD patterns of (a) BiPO , (b) 6% BiPO /Bi O CO , (c) 3%
2
4
4
2
3
BiPO /Bi O CO , (d) 1% BiPO /Bi O CO and (e) Bi O CO .
2
4
2
2
3
4
2
2
3
2
3
shown in Fig. 2a, pure Bi O CO sample consists of regular and
2
2
3
wellꢀdispersed nanosheets with length of 1–2 ꢁm and thickness of
about 150 nm. It has been reported that Bi O CO , as one of the
2
2
3
typical Aurivillius oxides with the internal layered structure, exhibits
relatively lower crystal growth rate along (001) axis than that along
other axes, which is thus beneficial for the construction of two–
15
dimensional sheet–like/plate–like microstructures. Compared to
Bi O CO , pure BiPO is composed of irregular aggregates of
2
2
3
4
nanoparticles (Fig. 2b). The morphologies of the asꢀprepared
BiPO /Bi O CO composite with various BiPO4 loadings are
4
2
2
3
displayed in Fig. 2c–e. It can be observed that all of the Fig. 2 SEM images of (a) Bi O CO , (b) BiPO , (c) 1%
2
2
3
4
BiPO /Bi O CO composite exhibit a sheet–like microstructure and BiPO /Bi O CO , (d) 3% BiPO /Bi O CO and (e) 6%
4
2
2
3
4
2
2
3
4
2
2
3
these nanosheets tend to interweave with each other to form a porous BiPO /Bi O CO ; (f) and (g) TEM images of 3% BiPO /Bi O CO ;
4
2
2
3
4
2
2
3
structure. Moreover, the thicknesses of BiPO /Bi O CO composite (h) HRTEM image of 3% BiPO /Bi O CO .
4
2
2
3
4
2
2
3
nanosheets are obviously much thinner than that of pure Bi O CO .
2
2
3
This may be attributed to the existence of a small amount of BiPO4,
which will adhere to the Bi O CO nanosheets surface and
effectively inhibit the crystalline growth along the [001] direction, Fig. 3. Two peaks centered at about 3485 and 1630 cm for each
thereby producing much thinner nanosheets. Moreover, the sample can be ascribed to the stretching and deformation vibration of
2
2
3
−
1
1
5
microscopic morphology and crystal structure of the 3% the O−H groups of the adsorbed water molecules, respectively. For
−1
BiPO /Bi O CO composite was further analysed by TEM and highꢀ pure Bi O CO , the peaks at 1468 and 1391 cm are assigned to
4
2
2
3
2
2
3
2
3
−
resolution TEM (HRTEM). The TEM images of the composite (Fig. antiꢀsymmetric vibration
v
3
mode of the CO
group while the peak
mode of the
group. Moreover, the absorption bands for the stretching
−
1
2
f and g) clearly reveal that BiPO
surface of the Bi CO nanosheets. Furthermore, the HRTEM CO
image of the composite (Fig. 2h) clearly shows the lattice fringes of vibration of Bi–O bond can also be detected at around 551 cm ,
4
particles closely adhere to the at 846 cm is ascribed to the outꢀof plane bending v
2
2
−
20
2
O
2
3
3
−1
2
0
different components of BiPO
4
/Bi
2
O
2
CO
3
heterostructured suggesting the formation of Bi
2
O
2
CO
3
.
4
For pure BiPO , two
−
1
photocatalyst. As shown in Fig. 2f, the fringe spacing of 0.306 nm characteristic peaks located at about 535 and 600 cm correspond to
2
8
agrees well with the (111) crystallographic planes of monoclinic the
BiPO , while the fringe spacing of 0.215 nm corresponds to (114) 1023 cm is assigned to
crystallographic planes of tetragonal Bi CO , respectively. The the entire characteristic peaks for Bi
4 4
v (PO ) and δ(O–P–O), respectively. The intensive peak at
−
1
29
4
v
3
(PO
4
). More importantly, in addition to
CO , the characteristic peak
of BiPO (1023 cm ) can also be observed in all of the
BiPO /Bi CO composites, indicating that both BiPO and
3
CO are co–existed in the composite.
2
O
2
3
2
O
2
3
−
1
above HRTEM analysis demonstrates that the BiPO
are intimately coupled in the composite.
4
and Bi
2
O
2
CO
3
4
4
2
O
2
3
4
The chemical composition and chemical bonding of the samples Bi
were characterized by FTIR technique and the results are shown in
2
O
2
To investigate the elemental compositions and chemical state of
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presented in Fig. 4b–e. As shown in Fig. 4b, the peaks located at
58.1 eV and 164.2 eV are attributed to Bi 4f7/2 and Bi 4f5/2
1
,
3+
26
respectively, indicating the existence of Bi ions in the sample. The
peak with binding energy of 530.3 eV (Fig. 4c) corresponds to the
4 2 2 3
lattice oxygen in BiPO /Bi O CO while the O 1s at 532.4 eV is
ascribed to the adsorbed oxygen, including carbonate and hydroxyl
20
groups adsorbed onto the surface of the sample. As for C 1s (Fig.
d), the peaks at a binding energy of 284.4 eV may be attributed to
the adventitious hydrocarbon in the XPS instrument and the peak at
4
30
2
88.5 eV is ascribed to the carbon of carbonate in Bi
2
O
2
CO
3
.
In
addition, the XPS peak of P 2p can be observed at 132.7 eV (Fig.
5+
11
4
e), indicating the existence of P in the composite. Therefore,
combining with the analytic results of TEM, FTIR and XPS, the
BiPO4 and Bi O CO species have been successfully coupled
2
2
3
together to form the heterojunction structure.
The optical absorption properties of semiconductors are closely
related to their electronic band structures and regarded as important
Fig. 3 FTIR spectra of (a) BiPO , (b) 6% BiPO /Bi O CO , (c) 3% factors to evaluate their photocatalytic performance. Fig. 5 shows the
4
4
2
2
3
BiPO /Bi O CO , (d) 1% BiPO /Bi O CO and (e) Bi O CO .
3
UV–vis diffuse reflectance spectra of Bi O CO , BiPO , and
2 2 3 4
4
2
2
3
4
2
2
2
2
3
BiPO /Bi O CO composite. According to Fig 5a, the absorption
4
2
2
3
edge of Bi O CO occurred at about 369 nm while that of BiPO was
2
2
3
4
elements, surface analysis of 3% BiPO /Bi O CO was performed
4
2
2
3
using XPS technique as illustrated in Fig. 4. XPS survey spectrum in
Fig. 4a shows that 3% BiPO /Bi O CO sample contains Bi, O, C
4
2
2
3
and P elements. The corresponding high resolution XPS spectra of
the above four elements in the 3% BiPO /Bi O CO sample are
4
2
2
3
Fig. 4 (a) XPS survey spectrum of 3% BiPO
4
/Bi
2
O
2
CO
3
composite and
the high-resolution XPS spectra of (b) Bi 4f, (c) O 1s, (d) C 1s and (e) Fig. 5 (a) UV–vis diffuse reflectance spectra of the samples and (b)
P 2p. the band gap energies (E ) of Bi CO and BiPO
g
2
O
2
3
4
.
4
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located at about 281 nm because of its relatively large bandgap interaction between N
ARTICLE
2
molecules and the sample. This may be due
energy. All the samples exhibit strong absorption in the UV light to its regular and wellꢀdispersed structure, as revealed by SEM
region, implying the possibility of excellent photocatalytic activity observation. However, the isotherms of BiPO and BiPO /Bi CO
of theses photocatalysts under UV light irradiation. Additionally, composite can be categorized as type IV with a distinct H3
BiPO /Bi CO composite with various BiPO loadings shows hysteresis loop, indicating the presence of porous structures. These
similar absorption characteristics to pure Bi CO due to the low observations are further confirmed by their corresponding pore size
content of BiPO in BiPO /Bi CO sample. Moreover, for BiPO distribution curves (Fig. 6b), where the peaks for both mesopores
4
4
2
O
2
3
4
2
O
2
3
4
2
O
2
3
4
4
2
O
2
3
4
sample, an additional small shoulder tail can be found in the region appeared at around 17 nm and macropores centered at about 60 nm
of 300–500 nm, suggesting the existence of OHꢀrelated defects can be found. The smaller pores may be ascribed to the intraꢀ
3
1, 32
which lead to some localized states into the intrinsic band gap.
The band gap energies (E ) of Bi CO and BiPO were calculated process, while the larger pores may be ascribed to the space between
2 2 3
from the intercept of the tangents to the plots of (αhv O CO ,
aggregated pores formed between the particles during the crystal
g
2
O
2
3
4
1
/2
33
)
versus the intercrossed nanosheets. The BET surface areas of Bi
photon energy (hv). As presented in Fig. 5b, the band gaps of BiPO4, 1% BiPO /Bi O CO , 3% BiPO /Bi O CO and 6%
Bi O CO and BiPO were estimated to be 3.40 eV and 4.32 eV, BiPO /Bi O CO are calculated to be 0.6, 7.1, 1.7, 3.0 and 3.7 cm
respectively.
To determine surface area and porous structure of the asꢀ
11
4
2
2
3
4
2
2
3
3
2
2
3
4
4
2
2
3
–
1
g , respectively.
It is generally considered that the higher the PL intensity is, the
synthesized samples, the nitrogen adsorption–desorption isotherms less probability of the photogenerated charge carriers participates in
and pore size distribution curves were measured. As shown in Fig. the photocatalytic procedure because PL emission mainly comes
6
a, the Bi O CO sample is identified as type III isotherm according from the recombination of free carriers. Fig. 7 shows the PL spectra
2 2 3
to the IUPAC classification, which is the characteristic of the weak of BiPO , Bi O CO and BiPO /Bi O CO composite with an
4
2
2
3
4
2
2
3
excitation wavelength of 300 nm. Compared to BiPO , the low PL
4
intensity of Bi O CO may be attributed to its nanosheetꢀlike
2
2
3
structure, which is beneficial for the separation and transfer of
3
4
photoinduced charge carriers. Moreover, all the asꢀprepared
BiPO /Bi O CO composites exhibit relatively lower PL intensity
4
2
2
3
than that of BiPO4 and Bi O CO , and the 3% BiPO /Bi O CO
3
2
2
3
4
2
2
composite shows the lowest PL intensity, suggesting that the
coupling of BiPO and Bi O CO can suppress the recombination
4
2
2
3
rate of photogenerated electron−hole pairs and thus significantly
enhance the charge separation efficiency.
3
.2 Photocatalytic activity
The photocatalytic activity of BiPO4, Bi O CO
3
and
BiPO /Bi O CO composite with different amount of BiPO contents
2
2
4
2
2
3
4
were evaluated by photodegradation of RhB under UV irradiation.
Fig. 8a shows the photodegradation efficiency of RhB as a function
of time, where C was the initial concentration of RhB and C was the
0
2
Fig. 6 (a) N adsorption/desorption isotherms and (b) pore size
Fig. 7 PL spectra of the as-prepared samples.
distribution curves of the samples.
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Journal Name
as recombination sites at this moment. Subsequently, the interfacial
electron transfer is partially inhibited by excess BiPO , which leads
4
to the decreased amount of photogenerated electron–hole pairs and
photocatalytic activities of heterostructured photocatalysts. Similar
35, 36
results were reported in the previous literatures.
It should be
noted that pure BiPO shows the worst photocatalytic activity of the
4
five samples although it has the largest BET surface area. Generally,
high surface area can provide more active sites for reactants, which
will make the photocatalytic reaction occur more easily and thus
3
7
enhance the photocatalytic activity of the photocatalyst.
Nevertheless, many other influence factors can also affect the
photocatalytic performance of photocatalyst. In our case, the
heterostructure, which can effectively enhance the life of
photogenerated charge carrier, plays
a decisive role here.
The reaction kinetics of RhB photodegradation was studied and
the results are shown in Fig. 8b. It can be seen that the
photodegradation process fits a pseudo–first–order reaction under
UV light irradiation. The reaction rate constant of 3%
ꢀ
1
BiPO /Bi O CO is estimated to be 0.086 min , which is much
4
2
2
3
–1
–1
higher than that of Bi O CO (0.017 min ), BiPO (0.009 min ), 1%
2
2
3
–
4
1
–1
BiPO /Bi O CO (0.021 min ) 6% BiPO /Bi O CO (0.042 min ),
4
2
2
3
,
4
2
2
3
showing the superior photocatalytic activity of 3% BiPO /Bi O CO .
4
2
2
3
Fig. S1 (Supplementary material) presented the temporal UV–vis
absorption spectral changes of RhB over 3% BiPO /Bi O CO
3
4
2
2
photocatalyst. The intensity of the RhB absorption peak at 552 nm
decreases drastically within 40 min and no obvious shift of the
maximum absorption band of RhB can be observed, suggesting that
the aromatic chromophore ring of RhB was directly destroyed by the
attack of hydroxyl radicals (·OH). Similar phenomenon has been
3
8
observed in the BiVO /RhB system reported by Cruz and Pérez.
4
The sharp decrease of the absorption band without apparent
wavelength shift demonstrates that a fast and direct mineralization
of RhB dye is completed by 3% BiPO /Bi O CO , indicating the
Fig. 8 (a) Photodegradation of RhB by different photocatalysts and
(
b) the dynamics of RhB degradation reaction.
4
2
2
3
potential application of this heterostructured photocatalyst in
wastewater treatment. In addition, 3% BiPO /Bi O CO also
4
2
2
3
exhibited excellent stability and reusability for the degradation of
concentration of RhB after irradiation. It can be seen that the RhB (Fig. S2). As shown in Fig. S2, no significant loss of the
degradation efficiency of RhB over 3% BiPO /Bi O CO composite photocatalytic activity can be observed after four successive runs of
4
2
2
3
is 92.6% after 40 min irradiation, about 2.3 and 1.7 times higher than degradation of RhB. XRD (Fig. S3) and SEM (Fig. S4) analyses of
that of pure BiPO (40.8%) and Bi O CO (55.9%), respectively. In the 3% BiPO /Bi O CO after four runs also showed that the crystal
4
2
2
3
4
2
2
3
addition, under identical experimental conditions, all the structure and morphology of the catalyst were not changed during
BiPO /Bi O CO composite photocatalysts exhibit obviously higher the photocatalytic oxidation process. Therefore, these above results
4
2
2
3
photocatalytic activity than both of pure BiPO4 and Bi O CO , imply that 3% BiPO /Bi O CO composite has high stability and less
2
2
3
4
2
2
3
indicating that the photocatalytic activity of BiPO /Bi O CO
photocorrosion during the photocatalytic process, which is critical
heterostructured photocatalyst can be effectively improved by and beneficial for the longꢀterm use in the photodegradation of
4
2
2
3
coupling.
What’s more, the 3% BiPO
percentage of BiPO to Bi
organic contaminants.
sample, with a 3% molar 3.3 Photocatalytic mechanism
exhibits the highest
4
/Bi
2
O
2
CO
CO
3
4
2
O
2
3
,
To understand the reaction mechanism, the trapping experiments
were carried out to identify the active species during the
photocatalytic activity among the composite samples. When the
BiPO content is below or above 3 mol%, the composite samples
show obviously decreased photocatalytic activity. When the loading
amount of BiPO is below its optimum value, BiPO particles loaded
on the Bi CO surface will act as separation centers of
photogenerated charge carriers and theeby improve the interfacial
electron transfer. However, at high BiPO content, overlapping
agglomerates resulted from the excessive BiPO particles can
smother the Bi CO surface and then the BiPO particles will act
4
4 2 2 3
photocatalytic process in the presence of 3% BiPO /Bi O CO
+
catalyst. In this process, hydroxyl radicals (·OH), holes (h ) and
superoxide radical (·O ) can be identified by adding 1.0 mmol
4
4
2−
2
O
2
3
tertbutyl alcohol (TBA), triethanolamine (TEOA) and benzoquinone
39
(
BQ) as scavengers, respectively. It can be observed in Fig. 9 that
4
the degradation efficiency of RhB decreased apparently when TEOA
was used as a hole scavenger, and it was also moderately depressed
4
2
O
2
3
4
6
| J. Name., 2012, 00, 1-3
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PleaseRd So Cn oA t da vd aj un s ct ems argins
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ARTICLE
Fig. 10 Photocatalytic mechanism of BiPO /Bi O CO heterojunction
4
2
2
3
photocatalyst.
Fig. 9 Trapping experiments of active species in the presence of 3%
BiPO /Bi O CO .
4
2
2
3
Bi O CO through the interface in the composite system. In such a
2
2
3
way, the efficient separation of photogenerated electronsꢀhole pairs
by the addition of TBA and BQ. The result demonstrates that on the nꢀn heterojunction can be achieved and the recombination rate
+
photogenerated holes (h ) are the main active species of the of photogenerated charge carriers may be greatly hindered. Then, the
BiPO /Bi O CO system. Meanwhile, hydroxyl radicals (·OH) and transferred electrons in BiPO will react with O molecules adsorbed
4
2
2
3
4
2
2
−
−
2
superoxide radical (·O ) also play an important role in the on the surface of the heterojunction structure to yield ·O .
degradation of RhB over 3% BiPO /Bi O CO .
It is well known that the transfer of photogenerated charge carriers Bi O CO can react with OH or adsorbed H O to give rise to ·OH
Meanwhile, the photoinduced holes accumulated on the surface of
4
2
2
3
−
2
2
3
2
in the heterojunction catalysts is closely related to the band edge radical, achieving the efficient consumption of photoinduced
position of the two semiconductors, which is critical in determining electronsꢀhole pairs. Moreover, the photoinduced holes in Bi O CO
3
2
2
the quantum efficiency and the photocatalytic activity in the are powerful oxidation species and can decompose the adsorbed
photocatalytic process. To further investigate the obtained RhB directly, which is in good agreement with the results of trapping
heterojunctions on the enhanced photocatalytic activity of experiments. Hence, the enhanced photocatalytic activity of the
BiPO /Bi O CO products, the band edge positons of Bi O CO and BiPO /composite is ascribed to the fast separation of photogenerated
4
2
2
3
2
2
4
3
4
0
BiPO can be calculated according to the empirical equation : EVB
=
electronꢀhole pairs due to the hybrid effect of BiPO and Bi O CO .
4
4
2
2
3
X
−
Ee + 0.5Eg, where
X is the absolute electronegativity of the Additional, the nanosheets structure of the as–prepared
1
8
26
semiconductor ( is 6.360 eV and 6.860 eV for Bi O CO and BiPO /Bi O CO composite is favorable for the separation and fast
X
2
2
3
4
2
2
3
BiPO , respectively). Ee is the energy of free electrons on the transfer of photoinduced electron−hole pairs available to participate
4
hydrogen scale (about 4.5 eV) and Eg is the band gap of the in the photocatalytic reaction, which also results in the enhanced
25, 34
semiconductor. The bandgap energies of Bi O CO and BiPO are photocatalytic activity of BiPO /Bi O CO composite.
The main
2
2
3
4
4
2
2
3
3
−
.40 eV and 4.32 eV, respectively, as the results derived from the UV reaction steps during the photocatalytic process are summarized in
vis diffuse reflectance spectra (Fig. 5). Therefore, based on the the following equations:
+
−
equation above, the conduction band (CB) and valence band (VB) Bi O CO + hv → Bi O CO (h ) + Bi O CO (e
potentials of Bi O CO are 0.16 eV and 3.56 eV, and of BiPO are BiPO + hv →BiPO (h ) + BiPO (e
)
(1)
(2)
(3)
(4)
(5)
(6)
2
2
3
2
2
3
vb
2
2
3
cb
+
−
cb
)
2
2
3
4
4
4
vb
−
2
4
−
0
.20 eV and 4.52 eV, respectively.
According to trapping experiments and the calculated energy band Bi
positions, the reaction mechanism of BiPO /Bi CO composite is Bi
illustrated in Fig. 10. Under UV light irradiation, Bi
BiPO can be excited simultaneously and produce electron
pairs. On the one hand, the CB edge potential of Bi CO
is negative than that of BiPO (0.20 eV). As a result, the
photogenerated electron on the surface of Bi CO will easily
migrate to the CB of BiPO when Bi CO and BiPO
semiconductors contact and form the n n type heterojunction
structure. On the other hand, since the VB edge potential of BiPO
4.52 eV) is more positive than that of Bi CO (3.56 eV), the
photogenerated holes on the BiPO surface will transfer to that of
BiPO
4
(ecb ) + O
2
→·O
+
+
2
O
2
CO
CO
3
(hvb ) + H
2
O →·OH + H
+
−
4
2
O
2
3
2
O
2
3
(hvb ) + OH →·OH
−
+
2
O
2
CO
3
and
O
2
/·OH or hvb + RhB → Degradation products
4
−hole
2
O
2
3
(0.16 eV)
4
. Conclusions
4
2
O
2
3
A
series of BiPO
nanosheet–like structures have been successfully synthesized
through a simple hydrothermal process. The 3% BiPO /Bi CO
3
/Bi
O
CO
heterojunction catalysts with
4
2
2
3
4
2
O
2
3
4
−
4
2
O
2
4
composite photocatalyst exhibits obviously improved photocatalytic
activity in degradation of RhB solution under UV light irradiation,
(
2
O
2
3
4
4
which is about 2.3 and 1.7 times higher than that of pure BiPO and
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ARTICLE
CO
Journal Name
Bi
calculation results, the enhanced photocatalytic activity for the
BiPO /Bi CO composite is mainly ascribed to the formation of
heterojunction between BiPO and Bi CO , which can effectively
facilitate interfacial charge transfer and effectively inhibit
2
O
2
3
, respectively. On the basis of experimental and theoretical 16.
N. Liang, M. Wang, L. Jin, S. Huang, W. Chen, M. Xu, Q.
He, J. Zai, N. Fang and X. Qian, ACS Appl Mater
Interfaces, 2014, 6, 11698.
4
2
O
2
3
1
7.
G. Zhu, M. Hojamberdiev, K.ꢀi. Katsumata, X. Cai, N.
Matsushita, K. Okada, P. Liu and J. Zhou, Mater. Chem.
Phys., 2013, 142, 95.
4
2
O
2
3
recombination of photogenerated electron–hole pairs. Such a Bi– 18.
based photocatalytic composite material with excellent
N. Liang, J. Zai, M. Xu, Q. Zhu, X. Wei and X. Qian, J.
Mater. Chem. A, 2014, 2, 4208.
X. Huang and H. Chen, Appl. Surf. Sci., 2013, 284, 843.
H. Lu, L. Xu, B. Wei, M. Zhang, H. Gao and W. Sun,
Appl. Surf. Sci., 2014, 303, 360.
1
2
9.
0.
photocatalysis and stability properties will be promising for potential
applications in water purification and environmental remediation.
2
2
1.
2.
H. Gan, G. Zhang and H. Huang, J. Hazard. Mater., 2013,
2
50, 131.
Acknowledgments
Q. Wang, G. Yun, Y. Bai, N. An, J. Lian, H. Huang and B.
Su, Appl. Surf. Sci., 2014, 313, 537.
C. Pan and Y. Zhu, Environ. Sci. Technol., 2010, 44, 5570.
C. Pan, J. Xu, Y. Wang, D. Li and Y. Zhu, Adv. Funct.
Mater., 2012, 22, 1518.
The authors gratefully acknowledge the financial support from
National Natural Science Foundation of China (Grant No.
U1204503) and Key projects of science and technology of Henan
Educational Committee (Grant No. 14B150049).
2
2
3.
4.
2
2
2
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| J. Name., 2012, 00, 1-3
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Graphical abstract
4 2 2 3
The as-formed BiPO /Bi O CO heterojunction structure can facilitate interfacial charge transfer,
thereby resulting in the enhanced photocatalytic activity.