Enhanced visible photocatalytic activity of a BiVO4-AgVO 3 composite synthesized by an in situ growth method

Article abstract of DOI:10.1039/c4ra02110j

β-AgVO₃ nanowires were synthesized by a simple hydrothermal process. With β-AgVO₃ providing a V source, BiVO ₄-AgVO₃ composites were obtained through an in situ growth process. BiVO₄-AgVO₃ composites exhibited enhanced photocatalytic properties. We discussed the possible mechanism of enhanced photocatalytic activities. The effect of content and size of BiVO₄ was also studied specifically. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra02110j

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Enhanced visible photocatalytic activity of a
BiVO4@b-AgVO3 composite synthesized by an
in situ growth method†
Cite this: RSC Adv., 2014, 4, 20058
a
a
Yanmei Yang, Yuanyuan Liu, Baibiao Huang, a Rui Zhang,b Ying Dai,b Xiaoyan Qina
Received 11th March 2014
Accepted 17th April 2014
*
*
and Xiaoyang Zhanga
DOI: 10.1039/c4ra02110j
www.rsc.org/advances
b-AgVO3 nanowires were synthesized by a simple hydrothermal separation of charge carriers for b-AgVO3 are of great impor-
process. With b-AgVO3 providing source, BiVO4@b-AgVO3 tance and value to break through its limited application in
composites were obtained through an in situ growth process. photocatalysis.
a
V
BiVO4@b-AgVO3 composites exhibited enhanced photocatalytic
BiVO4 is a good choice to form heterojunction structure with
properties. We discussed the possible mechanism of enhanced pho- b-AgVO3. The matched energy band levels between the two
tocatalytic activities. The effect of content and size of BiVO4 was also materials can enhance the separation of photoinduced elec-
studied specifically.
trons and holes. Moreover, as b-AgVO3 can be used as a V
source, in situ growth of BiVO4 at b-AgVO3 can be realized. And
via in situ growth, BiVO4 nanoparticles can be distributed and
xed on the surface of b-AgVO3, which would suppress the easy
aggregation of BiVO4. In addition, the closely touched interfaces
between BiVO4 and b-AgVO3 are favorable for charge carries
separation. BiVO4 materials are chemical stable, low cost and
non-toxicity, and are extensively applied as an environmentally
friendly photocatalyst to produce O2 from water and decompose
organic pollutants.20,21 However, its photocatalytic activity is still
limited by the low quantum yield, even though BiVO4 possesses
good absorption in visible light range. Thus, several methods
are developed to improve its photocatalytic activity. And
coupling with other semiconductors was proved to be an
effective way and various BiVO4 based composites were devel-
oped, such as Bi2S3/BiVO4, BiVO4/WO3.11,22 However, to the best
of our knowledge, no report was found on BiVO4@b-AgVO3
composite and its photocatalytic activity.
It is well known that photocatalysis is becoming an effective
technology to solve the energy shortage and environmental
problems.1–3 Besides traditional TiO2, hundreds of photo-
catalysts were developed over the past decades.4–7 However,
limited absorption in the visible light range and high rate of
charge carrier recombination are still two basic problems
existing in photocatalytic research. To overcome these limita-
tions, lots of methods have been developed, including noble
metal modication, semiconductor coupling, element doping,
dye sensitization, etc.2,8–10 And among these methods, compos-
ited photocatalysts exhibit excellent advantages in improving
the separation efficiency of photoinduced electrons and holes
due to the interface electric eld. Therefore, various composites
were prepared and well studied, such as Bi2S3/BiOCl, Bi2S3/
BiVO4 and AgI/BiOI etc.1,11,12
b-AgVO3 is a widely used material in cathode for lithium
batteries, bacterial inactivation and H2S sensor.13–15 Also, it is a
semiconductor with excellent optical absorption in visible light
region. However, its studies in photocatalytic property are very
limited. Only several reports are focused on the degradation of
organic pollutants over b-AgVO3,16,17 compared to other famous
Ag-based materials such as AgCl, Ag3PO4.18,19 The main reason
can be attributed to the rapid recombination of photoinduced
charge carriers. Therefore, researches on enhancing the
Herein, we prepared BiVO4@b-AgVO3 composite by an in situ
growth process. b-AgVO3 can not only provide a V source but
also immobilize the generated BiVO4 nanoparticles. The
composite displayed enhanced photocatalytic activity when
compared with single BiVO4 or b-AgVO3 under visible light
irradiation. We discussed the mechanism of enhanced prop-
erties from aspects of morphology, optical absorption property,
BET surface area and energy band levels between the two
materials. Morphology and matched energy band levels are the
major factors for improved photocatalytic activity of BiVO4@b-
AgVO3 composite.
aState Key Lab of Crystal Materials, Shandong University, Jinan, 250100, China.
E-mail: bbhuang@sdu.edu.cn; yyliu@sdu.edu.cn
bSchool of Physics, Shandong university, Jinan, 250100, China
† Electronic supplementary information (ESI) available: Experimental details,
Fig. S1–S3. See DOI: 10.1039/c4ra02110j
Fig. 1a shows the XRD patterns of as-prepared b-AgVO3,
BiVO4, and BiVO4@b-AgVO3 composites. The diffraction peak
positions of the prepared b-AgVO3 are identical to the standard
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lattice fringes with interplanar spacing of 0.775 nm corre-
sponding to the (ꢀ1 0 1) plane are measured, while the growth
direction is perpendicular to plane (ꢀ1 0 1). That is to say the
preferential orientation of b-AgVO3 nanowires are along (0 1 0)
plane which is consistent with the previous research.14 For the
composite (Fig. 2b–e), when BiVO4 is 5%, several BiVO4 nano-
particles with the size of about 80 nm are dispersed on the
surface of b-AgVO3 by the in situ growth process (Fig. 2b). With
increasing of BiVO4 content, more BiVO4 nanoparticles are
formed and the size has a small tendency to be larger (Table 1),
which is in agreement with the enhanced intensity of XRD
peaks. In Fig. 2e, when BiVO4 content reaches to 20%, b-AgVO3
nanowires are covered with BiVO4 nanoparticles with the size of
about 175 nm. The BiVO4 prepared for comparison shows
irregular board-like morphology with a size of 1–2 mm (Fig. S3†).
Therefore, the in situ growth method is favorable for the
formation of nanoparticles and prevents the aggregation of
BiVO4.
To illustrate the optical absorption properties of BiVO4@b-
AgVO3 composites, DRS spectra were measured, as can be seen
in Fig. 3a. For comparison, the spectra of pure b-AgVO3 and
BiVO4 are also given. It shows that both b-AgVO3 and BiVO4
have good optical absorption in visible light, with a clear
absorption edge at around 551 nm and 506 nm, respectively.
The composite has a small difference with a little shi of
absorption edge from 551 nm to 506 nm. To obtain the band
gaps of b-AgVO3 and BiVO4, we cited the formula:12 ahn ¼ A(hn
ꢀ Eg)n/2, where a is the absorption coefficient, hn is the discrete
photon energy, A is a constant, and Eg is the band gap. The
value of n depends on whether the transition is direct (n ¼ 1)
or indirect (n ¼ 4) in a semiconductor. b-AgVO3 and BiVO4 are
direct transition semiconductors, thus n is 1.16,22 The curve of
(ahn)2 versus hn is given in Fig. 3b. When (ahn)2 is set as 0, we
can get the band gap Eg of b-AgVO3 and BiVO4 to be 2.25 eV
and 2.45 eV, respectively.
Fig. 1 XRD patterns of as-prepared products (a) and the intensity ratio
of typical peaks of (ꢀ1 2 1) for BiVO4 to (5 0 1) for b-AgVO3.
cards (JCPDS 29-1154), which suggests that the prepared b-
AgVO3 has a monoclinic structure (Fig. S1†). However, the peak
intensities are different, and the peaks for (5 0 1) facets in the
prepared b-AgVO3 is much stronger, while that for (ꢀ2 1 1) and
(ꢀ4 1 1) facets are much weaker. The results indicate the pref-
erential orientation of b-AgVO3. BiVO4 shows a monoclinic
scheelite structure (JCPDS 14–0688) which displays the best
photocatalytic activity among the three tetragonal, ortho-
rhombic, and monoclinic phases (Fig. S2†).23 For the composite,
BiVO4@b-AgVO3-5% has very weak diffraction intensity of
BiVO4 because of the low content. And with increasing of BiVO4
content, its diffraction intensity becomes stronger gradually.
Meanwhile, the peaks of b-AgVO3 do not shi, indicating that
the introduction of BiVO4 does not change the crystal structure
of b-AgVO3. In Fig. 1b, the intensity ratio of typical peak of (ꢀ1 2
1) for BiVO4 to that of (5 0 1) for b-AgVO3 is given. It clearly
shows that with increasing BiVO4 content from 5% to 20%, this
ratio increases steadily, which further proves the increasing
amounts of BiVO4 in BiVO4@b-AgVO3 composites.
The SEM images of pure b-AgVO3 and BiVO4@b-AgVO3
composites with different contents of BiVO4 are presented in
Fig. 2. From Fig. 2a we can see that b-AgVO3 shows nanowire
morphology with a size of about 200 nm in width and more than
10 mm in length, and the surface is very smooth. In addition, the
Furthermore, the position of CB minimum and VB
maximum are determined. For a semiconductor at the point of
zero charge, its VB edge can be calculated by the empirical
1
2
equation EVB ¼ c ꢀ Ee þ Eg, in which EVB is the valence band-
edge potential, c is the electronegativity of the semiconductor,
expressed as the geometric mean of the absolute electronega-
tivity of the constituent atoms, Ee is the energy of free electrons
on hydrogen scale with the value of about 4.5 eV, Eg is the
bandgap energy of a semiconductor, and ECB can be obtained by
the equation ECB ¼ EVB ꢀ Eg.24 For b-AgVO3, c is about 5.88 eV
and Eg is 2.25 eV, so its top of VB is calculated to be 2.50 eV.
Accordingly, its bottom of CB is calculated to be 0.25 eV. In the
same way, c value of BiVO4 is 6.07 eV, its EVB and ECB are 2.8 eV
and 0.35 eV, respectively.
The photocatalytic activities of as-prepared samples were
evaluated by photodegradation of RhB under visible light irra-
diation at room temperature. Fig. 4a displays the average
degradation rates of BiVO4@b-AgVO3 composites (0, 5%, 10%,
15%, 20%) under visible light irradiation for 4 h. For compar-
ison, the photocatalytic activities of pure b-AgVO3 and BiVO4
were also measured under the same condition. Obviously,
Fig. 2 SEM and HR-TEM images of b-AgVO3 (a), SEM images of
BiVO4@b-AgVO3-5% (b), BiVO4@b-AgVO3-10% (c), BiVO4@b-AgVO3-
15% (d), BiVO4@b-AgVO3-20% (e).
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Table 1 Average size of BiVO4, BET values and average photodegradation rate of RhB for b-AgVO3, BiVO4, and b-AgVO3@BiVO4 composites
Sample code
a
b
c
d
e
f
BiVO4 molar ratio (%)
0
100
5
10
15
20
Average size of BiVO4 (nm)
—
2.8042
0.9%
—
1.2115
5.3%
80
4.6333
7.8%
100
6.2214
11.1%
140
3.8801
17.1%
175
7.1321
10.8%
BET (m2 gꢀ1
)
Average photodegradation rate of RhB
DC/C0 (hꢀ1
)
areas, optical absorption properties, morphologies and energy
band structure of composite are discussed below. The BET
specic surface areas and average photodegradation rate of
pure b-AgVO3, BiVO4, and BiVO4@b-AgVO3 composites are
presented in Table 1. It shows that the specic BET surface
areas of b-AgVO3 and BiVO4 are very small and the introduction
of different BiVO4 contents does not have large effect. In addi-
tion, the change of BET surface areas does not show the same
tendency with that of average photodegradation rate. Therefore,
it can be concluded that the BET surface areas make little
contribution to the enhanced and various photocatalytic activ-
ities of BiVO4@b-AgVO3 composites. In terms of the optical
absorption properties, from the previous analysis it can be seen
both of b-AgVO3 and BiVO4 have good optical absorption in
visible light region. And the coupling between them does not
make large difference, only leading to a little shi of absorption
edge. So it is not the major factor in the photodegradation
process. The main factors that affect the activities of as-
prepared products lie in the energy band structure of b-AgVO3
and BiVO4 and the varied amounts and morphologies with
increasing of BiVO4 content, which will be illustrated speci-
cally in the following.
Fig. 3 DRS spectra of as-prepared products (a) and the plots of (ahn)2
versus hn of b-AgVO3 and BiVO4 (b).
Fig. 4 The average photodegradation rates of as-prepared products
(a) and the temporal evolution of the absorption spectra of RhB
solution by BiVO4@b-AgVO3-15% (b).
Fig. 5 illustrates the band energy levels of b-AgVO3 and BiVO4
and the possible charge separation process, According to above
calculation. Under visible light irradiation, both of b-AgVO3 and
BiVO4 can be excited and generate electrons and holes. As the
CB of b-AgVO3 (0.25 eV) is more negative than that of BiVO4
(0.35 eV), while the VB of BiVO4 (2.80 eV) is more positive than
that of b-AgVO3 (2.50 eV), the photoinduced electrons on the CB
of b-AgVO3 can transfer to the CB of BiVO4 and the holes will
pass from the VB of BiVO4 to the VB of b-AgVO3. In this process,
the simultaneously occurred transfer of electrons and holes
BiVO4@b-AgVO3 composites can degrade RhB more effectively
than pure b-AgVO3 and BiVO4 and the photocatalytic activities
of composites are affected dramatically by the content of BiVO4.
The best activity comes from BiVO4@b-AgVO3-15% with 17.1%
hꢀ1, while the rates are 0.9% hꢀ1 and 7.8% hꢀ1 over pure b-
AgVO3 and BiVO4@b-AgVO3-5%, respectively. In addition, the
BiVO4@b-AgVO3-10% and BiVO4@b-AgVO3-20% show nearly
the same degradation rates, 11.1% hꢀ1 and 10.8% hꢀ1. As a
result, it can be concluded that the optimal BiVO4 content is
15% for the BiVO4@b-AgVO3 composite. Fig. 4b shows the
temporal evolution of the absorption spectra of RhB solution
with BiVO4@b-AgVO3-15% composite. We can see the absorp-
tion at 554 nm decreases gradually under visible light irradia-
tion, which indicates the effective degradation of RhB. The TOC
(total organic carbon) before and aer photocatalysis by
BiVO4@b-AgVO3-15% was evaluated with the value of 8.793 mg
Lꢀ1 and 5.779 mg Lꢀ1, respectively. We can conrm aer 4 h
under visible light irradiation, TOC decreased by 34.3%. Thus,
we concluded that it is possible for Rhodamine B to be con-
verted to CO2 eventually.
To explain the photocatalytic mechanism of different pho-
Fig. 5 Schematic diagram for energy band of b-AgVO3 and BiVO4 and
tocatalytic activities, the possible factors including surface the possible charge separation mechanism.
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decreases charge carriers' recombination possibility, increases
their quantum yield and lifetime. So, electrons on CB of BiVO4
can be consumed by reducing absorbed O2 molecules to OHꢀ,
considering the more negative potential (0.35 eV) compared to
the reduction potential of O2/OHꢀ (0.401 eV), while the holes
can be consumed by two pathways. (I) Can be used for the direct
degradation of RhB.25 (II) Oxidize the surface OHꢀ to $OH due to
the more positive potential than the standard oxidation
potential of OHꢀ/$OH (1.99 eV),26 which plays an important role
in oxidative degradation of RhB.27 As a result, charge carriers
can be separated effectively and enhance the photocatalytic
activities of BiVO4@b-AgVO3 composites.
With increasing content of BiVO4, the activities of compos-
ites increase rstly and then decrease, the reason of which is
attributed to the different contents and morphologies of BiVO4.
Firstly, with increase of BiVO4 content (5–15%), more BiVO4
nanoparticles (about 100–150 nm) are formed on the surface of
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Acknowledgements
This work is nancially supported by a research Grant from the
National Basic Research Program of China (the 973 Program;
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