Facile synthesis of porous Bi2O3-BiVO4p-n heterojunction composite microrods with highly efficient photocatalytic degradation of phenol

Article abstract of DOI:10.1016/j.jallcom.2016.07.128

In the present work, high-quality porous Bi₂O₃-BiVO₄composite microrods (CMRs) with a uniform size distribution have been successfully prepared through a facile solvothermal method followed by an annealing treatment, in which Bi(OH)C₂O₄-BiVO₄precursors were first synthesized using Na₂C₂O₄and NaVO₃as starting materials and then thermally decomposed to produce porous Bi₂O₃-BiVO₄p-n heterojunction CMRs. The as-prepared Bi₂O₃-BiVO₄product was characterized by UV–Vis diffuse reflectance spectroscopy (DRS) and valence-band X-ray photoelectron spectroscopy (XPS). Significantly enhanced photocatalytic activity in degrading colorless organic phenol under visible-light illumination (λ?>?420?nm) was observed with the porous CMRs, more than 48 times higher than that of the mixture of BiVO₄and Bi₂O₃, and 192 and 160 times higher than pure BiVO₄and Bi₂O₃, respectively. A direct Z-scheme mechanism was employed to describe the transfer of photogenerated electrons and holes in the Bi₂O₃-BiVO₄CMR system, and the dramatically enhancement of the observed photoactivity may be attributed to the high separation efficiency of electron-hole pairs, resulting from the p-n heterojunction formed in the porous structure.

Full text of DOI:10.1016/j.jallcom.2016.07.128

Journal of Alloys and Compounds 688 (2016) 1080e1087
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Facile synthesis of porous Bi2O3-BiVO4 p-n heterojunction composite
microrods with highly efficient photocatalytic degradation of phenol
,
Mengmeng Mao a, Fang Chen a, Changcheng Zheng b e, Jiqiang Ning c, Yijun Zhong a,
, d, *
Yong Hu a
a Institute of Physical Chemistry, Zhejiang Normal University, Jinhua, 321004, PR China
b Mathematics and Physics Centre, Department of Mathematical Sciences, Xi'an Jiaotong-Liverpool University, Suzhou, 215123, PR China
c Vacuum Interconnected Nanotech Workstation, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, 215123, PR China
d State Key Lab of Silicon Materials, College of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, PR China
e State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, CAS, 865 Changning Road,
Shanghai, 200050, China
a r t i c l e i n f o
a b s t r a c t
Article history:
In the present work, high-quality porous Bi2O3-BiVO4 composite microrods (CMRs) with a uniform size
distribution have been successfully prepared through a facile solvothermal method followed by an
annealing treatment, in which Bi(OH)C2O4-BiVO4 precursors were first synthesized using Na2C2O4 and
NaVO3 as starting materials and then thermally decomposed to produce porous Bi2O3-BiVO4 p-n het-
erojunction CMRs. The as-prepared Bi2O3-BiVO4 product was characterized by UVeVis diffuse reflectance
spectroscopy (DRS) and valence-band X-ray photoelectron spectroscopy (XPS). Significantly enhanced
photocatalytic activity in degrading colorless organic phenol under visible-light illumination
Received 26 March 2016
Received in revised form
22 June 2016
Accepted 14 July 2016
Available online 15 July 2016
Keywords:
Composite microrods
Bi2O3-BiVO4
Porous structure
p-n heterojunction
Solvothermal method
Z-scheme mechanism
(l > 420 nm) was observed with the porous CMRs, more than 48 times higher than that of the mixture of
BiVO4 and Bi2O3, and 192 and 160 times higher than pure BiVO4 and Bi2O3, respectively. A direct Z-
scheme mechanism was employed to describe the transfer of photogenerated electrons and holes in the
Bi2O3-BiVO4 CMR system, and the dramatically enhancement of the observed photoactivity may be
attributed to the high separation efficiency of electron-hole pairs, resulting from the p-n heterojunction
formed in the porous structure.
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
photocatalysis performances, resulting from expanded visible-light
absorption and improved charge separation [15,16]. The internal
Nowadays, semiconductor photocatalysis has been regarded as
a cost-effective and sustainable green chemical technology to
eliminate organic pollutants by utilizing solar energy [1e7]. Un-
fortunately, most semiconductors have weak photocatalytic activ-
ity because of their poor photo-quantum efficiency [8,9]. It has
been widely accepted that heterostructured semiconductors could
improve visible light responsive efficiency due to the hybrid nature
of the composite semiconductors which can effectively reduce the
recombination rate of photogenerated electrons and holes [10e14].
Among various heterogeneous photocatalysts, those with p-n
heterojunctions generally exhibit significantly enhanced
electric field formed within the p-n junction is believed to play a
decisive role [17,18], which has been thoroughly studied in the field
of modern semiconductor devices, including diodes, photodiodes,
and solar cells [19e21]. There can be two types of underlying
mechanisms that govern carrier separation and transfer processes
with considering the effect of a p-n junction (Type II). One is a
double-charge transfer model, in which the photogenerated elec-
trons transfer to the conductor with a more positive conduction
band (CB) while holes transfer to the other one with a more
negative valence band (VB). The other model is Z-scheme type, in
which the charges recombine directly between the weaker oxida-
tive holes and reductive electrons, leaving more oxidative holes and
more reductive electrons in the heterostructure system [22,23].
Recently, bismuth-based photocatalysts have attracted
tremendous attention because of their unique crystalline structures
and superior optical properties [24e26]. For example, monoclinic
* Corresponding author. Institute of Physical Chemistry, Zhejiang Normal Uni-
versity, Jinhua, 321004, PR China.
E-mail address: yonghu@zjnu.edu.cn (Y. Hu).
http://dx.doi.org/10.1016/j.jallcom.2016.07.128
0925-8388/© 2016 Elsevier B.V. All rights reserved.

M. Mao et al. / Journal of Alloys and Compounds 688 (2016) 1080e1087
1081
scheelite bismuth vanadate (m-BiVO4), with a narrow band gap of
2.2. Characterizations
about 2.40 eV, is an outstanding visible-light responsive photo-
catalyst, and has been widely applied to photodegradation of
organic contaminants and investigation of photocatalytic evolution
of O2 [27,28]. However, despite the advantage of visible light
response, the photocatalytic ability of BiVO4 can not meet practical
application needs due to its poor migration of photogenerated
electrons and holes [29]. To overcome this shortcoming, the strat-
egy of combining BiVO4 with another semiconductor to form a p-n
heterojunction has been proposed, aiming at improving electron-
hole pair separation and interfacial charge transfer efficiency, to
promote the photocatalytic activity [30]. Up to now, a series of
BiVO4-based p-n heterojunction photocatalysts with various mor-
phologies have been reported, including BiOCl-BiVO4 [31], Cu2O-
BiVO4 [32,33], CuO-BiVO4 [34], Co3O4-BiVO4 [35], as well as Bi2O3-
BiVO4 [36,37], which were prepared via different methods and
exhibited enhanced photocatalytic performance.
In this study, we demonstrate a facile and novel synthetic
strategy developed for controlled fabrication of a particular type of
bismuth-based p-n heterojunction system made of porous Bi2O3-
BiVO4 composite microrods (CMRs). The as-prepared porous Bi2O3-
BiVO4 CMRs exhibit significantly enhanced photocatalytic activity
in degrading phenol under visible-light illumination. As an
example, after degrading phenol for 1 h, a rate of 96.3% was ach-
ieved, which is more than 48 times greater than what was pro-
duced by the mechanical mixture of BiVO4 and Bi2O3, and 192 and
160 times better than pure BiVO4 and Bi2O3, respectively. The
dramatically enhanced photocatalytic activity can be attributed to
the unique porous p-n heterojunction structure which is expected
to have high separation efficiency of electron-hole pairs. The band
structure of the Bi2O3-BiVO4 composite system is characterized by
UVeVis diffuse reflectance spectra (DRS) and valence-band X-ray
photoelectron spectroscopy (XPS), and a direct Z-scheme for elec-
trons and holes transfer mechanism is proposed to explain the
enhanced photocatalytic activity. Our experiments demonstrate
that the as-prepared porous Bi2O3-BiVO4 CMR is a promising
photocatalyst which can be applied to water pollution.
Powder X-ray diffraction (XRD) measurements were performed
with a Philips PW3040/60 X-ray diffractometer using Cu K radi-
a
ation with a scanning rate of 0.06 deg sꢁ1. Scanning electron mi-
croscopy (SEM) was carried out for morphology characterization,
on a Hitachi S-4800 scanning electron micro-analyzer with an
accelerating voltage of 15 kV. The attached energy dispersive X-ray
spectrometry (EDS) was employed for elemental analysis. Trans-
mission electron microscopy (TEM) and high-resolution trans-
mission electron microscopy (HRTEM) were also conducted by
using a JEM-2100F field-emission TEM with samples prepared by
dispersing the products in ethanol and dropping the suspension on
a holey carbon net supported on copper grids. UVeVis diffuse
reflectance spectra (UVeVis DRS) of the as-prepared samples were
measured by using a Thermo Nicolet Evolution 500 UVeVis spec-
trophotometer in an absorption mode over the spectral range of
200e800 nm and absorption spectra were also measured at room
temperature with a PerkinElmer Lambda 900 UVeVis spectro-
photometer. N2 adsorption-desorption isotherms were obtained at
77 K on a Micrometrics ASAP 2020 surface area and porosity
analyzer. And the samples were degassed in vacuum at 160 ꢀC for
4 h before the measurement. Liqui TOCII (ELEMENTAR Corporation)
system was used for the measurement of total organic carbon (TOC)
concentration. X-ray photoelectron spectroscopy (XPS) measure-
ments were carried out at room temperature on a ESCALAB 250Xi
X-ray photoelectron spectrometer. The photoluminescence (PL)
spectra were measured using an excitation wavelength of 325 nm
on a FLS 920 fluorescence spectrophotometer. The GC-MS spectra
were measured using a GCMS-QP2020. The experiments were
carried out under room conditions.
2.3. Photocatalytic test
The photocatalytic activities of the as-prepared samples were
evaluated by the degradation of phenol under visible light irradi-
ation which was provided by a 500 W Xe lamp with a 420 nm cut-
off filter. The reaction cell was placed in a sealed black box with the
top open. In a typical process, 15 mg of the as-prepared sample, as
the photocatalyst, was added into 20 mL of phenol solution (con-
centration: 25 mg/L). The solution was first dispersed in an ultra-
sonic bath for 5 min, then stirred for 1 h in the dark to reach
adsorption equilibrium between the catalyst and the solution, and
finally exposed to visible light irradiation. The resultant suspension
was collected by centrifugation at a series of given times, and the
phenol degradation concentrations were measured by the method
of UVeVis spectroscopy.
2. Experimental section
All the reagents employed in this work were of analytical grade,
purchased from the Shanghai Chemical Reagent Factory and used
as received, without further purification.
2.1. Synthesis of porous Bi2O3-BiVO4 CMRs
To prepare Bi(OH)C2O4-BiVO4 CMR precursors, 0.35 mmol of
Bi(NO3)3$5H2O and 1 g of polyvinylpyrrolidone (PVP, MW~ 58 K)
were first dissolved in 15 mL of ethylene glycol (EG) under stirring
to achieve mixture A. 0.28 mmol of Na2C2O4 and 0.07 mmol of
NaVO3 were added into 25 mL of distilled water, to obtain solution
B. Solution B was then added dropwise into solution A and kept
stirring for 20 min at room temperature. The resulting mixture was
sealed in a 50 mL PTFE-lined stainless-steel autoclave and heated at
180 ꢀC for 10 h. The products were collected by centrifugation and
washed with ethanol and distilled water, respectively, for three
times before dried at 80 ꢀC for 4 h to obtain Bi(OH)C2O4-BiVO4
CMRs. The CMR precursors were heated to 350 ꢀC in an air flow
with a heating rate of 1.5 ꢀC minꢁ1 and maintained for 4 h for the
aim of calcination which finally leads to the formation of porous
Bi2O3-BiVO4 CMRs. For comparison, pure BiVO4 product was also
prepared under the same solvothermal condition except that
Na2C2O4 was not added. In addition, pure Bi2O3 product was also
prepared and shown in Supporting Information.
2.4. Radical-trapping experiment
In order to identify the major active species in degrading phenol,
radical-trapping experiments were conducted by using three
chemicals, benzoquinone (a superoxide anion radical scavenger,
Oꢁ2 ), Na2C2O4 (a hole scavenger) and tert-butanol (an $OH radical
scavenger). Similar to photocatalytic tests,15 mg of the as-prepared
porous Bi2O3-BiVO4 CMRs and scavengers were added into 20 mL of
phenol solution (concentration: 25 mg/L). Dispersed in an ultra-
sonic bath for 5 min, the mixture was also stirred for 1 h in the dark
to reach adsorption equilibrium between the catalyst and the so-
lution. After irradiated with visible light, the suspensions were
collected by centrifugation at given time intervals and then the
concentration of phenol was measured by the UVeVis absorption
method.

1082
M. Mao et al. / Journal of Alloys and Compounds 688 (2016) 1080e1087
3. Results and discussion
The fabrication of porous Bi2O3-BiVO4 CMRs is schematically
illustrated in Fig. 1. The porous Bi2O3-BiVO4 CMRs are obtained via a
two-step process, in which Bi(OH)C2O4-BiVO4 CMRs are first pre-
pared, and then the CMR precursors are transformed into porous
Bi2O3-BiVO4 CMRs by thermal treatment. The Bi(OH)C2O4-BiVO4
CMR precursors are produced via a simple solvothermal method,
using Bi(NO3)3$5H2O, Na2C2O4 and NaVO3 as the starting materials.
They are mixed with EG and water and maintained at 180 ꢀC for
10 h to form Bi(OH)C2O4-BiVO4 CMRs. The Bi(OH)C2O4-BiVO4 CMR
product is kept at 350 ꢀC for 4 h so that thermal decomposition
occurs, releasing CO2 to produce Bi2O3, and porous Bi2O3-BiVO4
CMRs are therefore obtained. The morphology of the precursors is
completely retained and the final products are highly porous due to
the creation and release of CO2.
The crystallographic structure and phase purity of the as-
prepared BiVO4 and porous Bi2O3-BiVO4 CMR were examined by
XRD analysis, Fig. 2 shows the XRD pattern of the as-prepared
porous Bi2O3-BiVO4 CMRs, which obviously consists of two sets of
diffractions. In addition to the obvious body-centered monoclinic
phase of BiVO4 (a ¼ 5.195 Å, b ¼ 11.70 Å, c ¼ 5.092 Å, JCPDS card no.
14-0688) diffraction peaks, the other peaks marked by triangles are
Fig. 2. XRD pattern of the as-prepared porous Bi2O3-BiVO4 CMRs.
due to the release of CO2 in thermal decomposition process. The
geometrical structure of the porous Bi2O3-BiVO4 CMRs is further
elucidated by TEM measurement (Fig. 3e), revealing a porous
structure of the final product, in a good agreement with the SEM
result. The associated EDS pattern (inset in Fig. 3e) further confirms
the existence of Bi, O and V elements in the Bi2O3-BiVO4 product.
Furthermore, the element contents show in Supporting Informa-
tion (see Fig. S2). Fig. 3f shows a high-resolution TEM (HRTEM)
image of the edge section of one porous Bi2O3-BiVO4 CMR, which
clearly reveals the heterojunction region between BiVO4 and Bi2O3.
The corresponding interplanar spacings are about 0.32 and
0.31 nm, which correspond to the (201) plane of tetragonal Bi2O3
and (121) plane of monoclinic BiVO4, respectively. Interestingly, it
has been also found that the mixed solvent (EG and water) has an
important effect on the formation of microrod morphology. No
intact rod-structure was observed in our experiments when only
pure EG or H2O is used as solvent (SEM images, Fig. S3, see Sup-
porting Information). EG is one of the most frequently used organic
solvents because of its amphiphilic aggregation due to its high
cohesive energy, dielectric constant, and considerable hydrogen
bonding ability, and EG is also a reducing agent with a relatively
high boiling point [40,41]. In this case, only Bi spheres can be ob-
tained in pure EG reaction system. Thus, the mixed solvent (EG and
water) was chosen as the reaction solvent.
well matched with the tetragonal phase of
b
-Bi2O3 (a ¼ b ¼ 7.742 Å,
c ¼ 5.631 Å, JCPDS card no. 27-0050) [38], indicating the complete
transformation of Bi2O3-BiVO4 hetero-nanostructures from Bi(OH)
C2O4-BiVO4 CMRs precursor. Additionally, the XRD patterns of the
as-prepared pure BiVO4 and Bi(OH)C2O4-BiVO4 CMRs precursor are
shown in Fig. S1 (see Supporting Information) [39].
The morphology and microstructure of the porous Bi2O3-BiVO4
CMRs, as well as the as-prepared Bi(OH)C2O4-BiVO4 CMRs precur-
sor, are examined by SEM as shown in Fig. 3. The low-magnification
SEM image (Fig. 3a) reveals that the as-obtained Bi(OH)C2O4-BiVO4
precursor is completely composed of numerous uniform microrods
(3.5e4.5 mm in length and about 1 mm in diameter), and the high-
magnification image (Fig. 3b) exhibits that the surface of the Bi(OH)
C2O4-BiVO4 precursor is more smooth than that of Bi2O3-BiVO4
CMR. The SEM images of the as-obtained Bi2O3-BiVO4 CMRs (Fig. 3c
and d) show that the product well maintains the shape of the
Bi(OH)C2O4-BiVO4 precursor, except that the surface of the micro-
rods has been significantly modified. In contrast to the precursor
microrods, the Bi2O3-BiVO4 CMRs contain a lot of wormhole-like
pores. This is a result of the chemical transformation of Bi2O3-
BiVO4 hetero-microstructures from Bi(OH)C2O4-BiVO4 precursor
UVeVis DRS is used to characterize the optical response of the
as-obtained samples. As shown in Fig. 4a, all the Bi2O3-BiVO4 CMRs,
pure BiVO4 and Bi2O3 samples exhibit light absorption in the
visible-light range, whose absorption edges are located at ca.570,
580 and 520 nm, respectively. Compared with pure BiVO4, the as-
prepared porous Bi2O3-BiVO4 CMRs exhibit a little increased pho-
toresponse in the whole visible-light region, suggesting that the
product can utilize solar energy to obtain enhanced photocatalytic
activity [42,43]. The band gap energy of a semiconductor can be
evaluated by the following equation [44]:
À
Á
n=2
a
h
n
¼ A h
n
ꢁ Eg
(1)
where
a, h,
n
, A and Eg represent the absorption coefficient, Planck's
constant, light frequency, energy-independent constant, and band
gap, respectively. The value of the index n depends on the charac-
teristics of the materials: for direct band-gap semiconductors,
n ¼ 1; for indirect band-gap semiconductors, n ¼ 4. For pure BiVO4,
Fig. 1. Schematic illustration of the formation of porous Bi2O3-BiVO4 CMRs via a solid-
phase reaction process.

M. Mao et al. / Journal of Alloys and Compounds 688 (2016) 1080e1087
1083
Fig. 3. (a, b) SEM images of the as-prepared Bi(OH)C2O4-BiVO4 CMRs; (c, d) SEM, (e) TEM and (f) HRTEM images of the as-prepared porous Bi2O3-BiVO4 CMRs. The inset in (e) shows
an EDS spectrum.
n is 4, for Bi2O3, n is 1 [38]. So, the Eg values of BiVO4 and
b
-Bi2O3 are
heterostructured catalyst, which may greatly improve the photo-
catalytic activity of the hybrid semiconductor system.
about 2.15 and 2.50 eV (Fig. 4b), respectively. Furthermore, valence
band XPS (VB-XPS) spectra were measured to study the band
structure of the two components, which were shown in Fig. 4c. The
positions of the VB edge of BiVO4 and Bi2O3 are located at 2.78 eV
and 2.04 eV, respectively. According to the VB edge of them, and
combined with the band gap derived from DRS, the band positions
of the BiVO4 and Bi2O3 can be calculated by the following empirical
formula: [45]
The photocatalytic activity of the as-obtained porous Bi2O3-
BiVO4 CMR sample is investigated by degrading the refractory
organic pollutant of phenol, and it is found that the porous Bi2O3-
BiVO4 CMRs also exhibit superior catalytic performance. Fig. 5a
shows the absorption evolution of the degraded phenol at a series
of reaction time. After irradiated with visible light for 60 min,
nearly 96.3% phenol is degraded by the porous Bi2O3-BiVO4 CMRs,
while pure BiVO4, Bi2O3, the mechanical mixture of BiVO4 and
Bi2O3, exhibit lower degradation rates of about 0.5%, 0.6%, 2.06%
(Fig. 5b), respectively. To further studied reusability of the as-
prepared porous Bi2O3-BiVO4 CMR photocatalyst, we collected
and reused the photocatalyst for 4 cycles, and the results are shown
in Fig. S4 (in the supporting information). Only a small amount loss
in photocatalytic activity is observed, which might be partly caused
by incomplete collection of the photocatalyst during each cycle.
Additionally, the porous Bi2O3-BiVO4 CMR also exhibits highly
photocatalytic efficiency for the degradation of an organic dye
methyl orange (MO) (Fig. S5, in the supporting information). After
irradiated for 60 min, nearly 93.2% of MO is degraded.
ECB ¼ EVB ꢁ Eg
(2)
where EVB is the VB potential and ECB is the CB potential. The po-
sition of the CB edges of BiVO4 and Bi2O3 are located at 0.63 eV
and ꢁ0.46 eV, respectively. N2 adsorption-desorption measure-
ment was employed to further characterize the porous structure of
the as-prepared porous Bi2O3-BiVO4 CMRs. The N2 adsorption-
desorption isotherm result (Fig. 4d) can be classified as type IV
isotherm with a distinct hysteresis loop, indicating the existence of
abundant mesopores in the products [46,47]. The Barrett-Joyner-
Halenda (BJH) pore size distribution curve (inset in Fig. 4d) shows
that the average size of the pores is centered around 30 nm, and the
BET surface area and pore volume are calculated to be about
16.7 m2 gꢁ1 and 0.063 cm3 gꢁ1, respectively. These porous channels
are anticipated to facilitate the diffusion of pollutants inside the
In order to identify the major active species produced in the
photodegradation process, radical-trapping measurements were
also performed. Fig. 6a shows the photodegradation rate of phenol
reduced by the addition of 1 mM tert-butanol under visible-light

1084
M. Mao et al. / Journal of Alloys and Compounds 688 (2016) 1080e1087
n
Fig. 4. (a) UVeVis DRS of different samples, (b) the plots of (
ah
n)
vs. (hn) of BiVO4, Bi2O3, (c) VB-XPS spectra of BiVO4 and Bi2O3, and (d) N2 adsorption-desorption isotherm and
pore-size distribution curve of the as-obtained Bi2O3-BiVO4 CMRs.
Fig. 5. (a) UVeVis absorption spectra of phenol using porous Bi2O3-BiVO4 CMRs under visible-light irradiation. (b) Photodegradation of phenol in the presence of different samples
under visible-light irradiation for 60 min.
irradiation [48], indicating that $OH radicals are not the main active
oxidizing species in the photoreaction process. At the meantime,
the phenol concentration remains almost similar with the use of
Na2C2O4 and benzoquinone. We can therefore conclude that the
respectively, degraded by Bi2O3-BiVO4 CMR photocatalyst. At first,
phenol occupied the main position. After 30 min of photocatalysis,
the phenol was partly degraded to acetone, acetic acid, 1,2-etha-
nediol and so on. After 60 min, phenol was almost degraded
completely, leaving the intermediate products.
ꢂꢁ
photogenerated holes and O2 are the dominant reactive species
contributing to the oxidative degradation of phenol. A TOC test was
also performed to further investigate the activity of the porous
Bi2O3-BiVO4 CMRs. Fig. 6b shows that the mineralization yield
reaches a value of 51.6% after irradiating for 60 min. Obviously, the
rate of TOC reduction of porous Bi2O3-BiVO4 CMRs is slower than
that of the degradation, indicating that the phenol is partially
decomposed into H2O and CO2. The GC-MS chromatogram was used
to identify the intermediate products of degradation process
[49,50]. Fig. S6 (see Supporting Information) shows the GC-MS of
different sample with reaction time of 0, 30 and 60 min,
To further understand the mechanism of the enhanced photo-
catalytic performance, PL and electrochemical impedance spectra
(EIS) were investigated for the as-obtained samples. Generally, a
lower PL intensity reveals a lower recombination rate of electron-
hole pairs, resulting in a higher photocatalytic performance [7,51].
From Fig. S7 (see Supporting Information), it can be seen that all the
samples feature a broad emission at around 432 nm under an
excitation wavelength of 320 nm. The PL intensity of Bi2O3-BiVO4
CMRs is lower than that of the pure BiVO4 and Bi2O3, indicating that
the as-obtained Bi2O3-BiVO4 CMR shows higher photo-induced

M. Mao et al. / Journal of Alloys and Compounds 688 (2016) 1080e1087
1085
Fig. 6. (a) Plots of active species trapped in the system for the photodegradation of phenol using porous Bi2O3-BiVO4 CMRs as photocatalyst, and (b) TOC degradation as a function of
time for the decomposition of phenol using porous Bi2O3-BiVO4 CMRs as photocatalyst.
charge separation efficiency than that of pure samples. Fig. S8 (see
Supporting Information) shows the arc radius of the Nyquist plot of
the Bi2O3-BiVO4 CMRs electrode is smaller than that of the BiVO4
electrode, which suggests a better charge transfer efficiency of the
Bi2O3-BiVO4 CMRs. The enhanced separation and transfer efficiency
of the photogenerated carriers may account for the high
Fig. 7. Schematic diagram representing the (a) double-charge transfer mode, and (b) Z-scheme type mode in Bi2O3-BiVO4 p-n heterojunction CMRs.

1086
M. Mao et al. / Journal of Alloys and Compounds 688 (2016) 1080e1087
degradation activity of Bi2O3-BiVO4 CMRs [42]. Furthermore, the
photocurrent transient response measurements of pure BiVO4,
pure Bi2O3 and Bi2O3-BiVO4 CMRs were also carried out in a 0.1 M
Na2SO4 solution under visible-light irradiation. Fig. S9 (in Sup-
porting Information) displays the rapid and consistent photocur-
rent responses for each switch-on and -off event in multiple 40 s
on-off cycles. It is worth to note that the photocurrent density of the
strategy is not restricted to the specific materials discussed in this
work and can be extended to the preparation of a wide range of
porous microstructures for various applications.
Acknowledgements
Financial support from the Natural Science Foundation of China
(21171146, 21371152), Zhejiang Provincial Natural Science Foun-
dation of China (LR14B010001), the Zhejiang Provincial Public
Welfare Project (2016C31015) and the State Key Laboratory of Sil-
icon Materials at Zhejiang University (2015-10) are gratefully
acknowledged. CCZ acknowledges partial support by an open
project from State Key Laboratory of Functional Materials for
Informatics, Shanghai Institute of Microsystem and Information
Technology.
Bi2O3-BiVO4 CMRs electrode (ca. 0.12e0.15
that of the pure BiVO4, Bi2O3 electrode (ca. 0.04e0.05
m
A cmꢁ2) is higher than
m
A cmꢁ2). The
enhanced photocurrent response of the as-prepared Bi2O3-BiVO4
CMRs also indicates higher separation efficiency of the photo-
induced electron-hole pairs in such CMR structures [6,28].
Based on the measured bandgap structures of BiVO4 and b-Bi2O3
and the trapping experiments, a possible direct Z-scheme mecha-
nism is proposed to explain the enhanced photocatalytic perfor-
mance of the Bi2O3-BiVO4 CMRs, which is schematically illustrated
in Fig. 7b. BiVO4 is known as an intrinsic n-type semiconductor
with a band gap of 2.15 eV, and its CB and VB energy levels are
positioned at ca. 0.63 and 2.78 eV (vs standard hydrogen electrode
(NHE)). And Bi2O3 is an intrinsic p-type semiconductor with a band
gap of 2.50 eV, with the CB at ca. ꢁ0.46 eV and VB at ca. 2.04 eV.
After the two materials get in contact, a p-n heterojunction will
form when their Fermi levels reach equilibrium state [18,52], and
an inner electric field, pointing from the BiVO4 side to Bi2O3, is built
across the interface. The Fermi level of p-type Bi2O3 moves up, at
the meantime, that of n-type BiVO4 moves down until the equi-
librium state is established, resulting in that the whole energy band
of Bi2O3 is raised up and BiVO4 is descended. Under visible light
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jallcom.2016.07.128.
References
[1] Y.X. Niu, M.Y. Xing, B.Z. Tian, J.L. Zhang, Appl. Catal. B 115e116 (2012) 253.
[2] R.C. Pawar, Y. Pyo, S.H. Ahn, C.S. Lee, Appl. Catal. B 176e177 (2015) 654.
[3] S. Pal, S. Maiti, U.N. Maiti, K.K. Chattopadhyay, CrystEngComm 17 (2015) 1464.
[4] Y.R. Wang, J.Q. Ning, E.L. Hu, C.C. Zheng, Y.J. Zhong, Y. Hu, J. Alloy Compd. 637
(2015) 301.
[5] E.L. Hu, X.H. Gao, A. Etogo, Y.L. Xie, Y.J. Zhong, Y. Hu, J. Alloy Compd. 611
(2014) 335.
[6] Q.Z. Wang, Y.B. Shi, L.L. Pu, Y.T. Ta, J.J. He, S.L. Zhang, J.B. Zhong, J.Z. Li, B.T. Su,
Appl. Surf. Sci. 367 (2016) 109.
[7] Q.Z. Wang, D.H. Jiao, J.H. Lian, Q. Ma, J. Yu, H.H. Huang, J.B. Zhong, J.Z. Li, J. Alloy
Compd. 649 (2015) 474.
[8] H.W. Huang, X.W. Li, X. Han, N. Tian, Y.H. Zhang, T.R. Zhang, Phys. Chem.
Chem. Phys. 17 (2015) 3673.
[9] H.L. Lin, H.F. Ye, S.F. Chen, Y. Chen, RSC Adv. 4 (2014) 10968.
[10] T. Sano, E. Puzenat, C. Guillard, C. Geantet, S. Matsuzawa, N. Negishi, J. Phys.
Chem. C 113 (2009) 5535.
[11] N. Liang, M. Wang, L. Jin, S.S. Huang, W.L. Chen, M. Xu, Q.Q. He, J.T. Zai,
N.H. Fang, X.F. Qian, ACS Appl. Mater. Interfaces 6 (2014) 11698.
[12] J. Kang, Q. Kuang, Z.X. Xie, L.S. Zheng, J. Phys. Chem. C 115 (2011) 7874.
[13] H.H. Niu, S.W. Zhang, R.B. Wang, Z.Q. Guo, X. Shang, W. Gan, S.X. Qin, L. Wan,
J.Z. Xu, J. Phys. Chem. C 118 (2014) 3504.
[14] T. Morimoto, K. Suzuki, M. Torikoshi, T. Kawahara, H. Tada, Chem. Commun.
(2007) 4291.
[15] J. Debgupta, R. Devarapalli, S. Rahman, M.V. Shelke, V.K. Pillai, Nanoscale 6
(2014) 9148.
[16] F.K. Meng, J.T. Li, S.K. Cushing, M.J. Zhi, N.Q. Wu, J. Am. Chem. Soc. 135 (2013)
10286.
[17] C.S. Tan, C.H. Hsiao, S.C. Wang, P.H. Liu, M.Y. Lu, M.H. Huang, H. Ouyang,
L.J. Chen, ACS Nano 8 (2014) 9422.
[18] L. Chen, Q. Zhang, R. Huang, S.F. Yin, S.L. Luo, C.T. Au, Dalton Trans. 41 (2012)
9513.
[19] Y.X. Deng, Z. Luo, N.J. Conrad, H. Liu, Y.J. Gong, S. Najmaei, P.M. Ajayan, J. Lou,
X.F. Xu, P.D. Ye, ACS Nano 8 (2014) 8292.
irradiation (l > 420 nm), both BiVO4 and Bi2O3 could be excited,
creating photogenerated electron and hole pairs. Thus, the photo-
excited electrons on the CB of Bi2O3 possess a higher reducibility
and photoexcited holes on the VB of BiVO4 have a stronger oxi-
dizability in this heterostructure system. According to the con-
ventional double-charge transfer mode, the photoinduced
electrons on the CB of Bi2O3 transfer to the CB of BiVO4, while holes
ꢂꢁ
on the VB of BiVO4 transfer to the VB of Bi2O3. However, since O2
is one of the main active species, the CB potential of BiVO4 is lower
than the standard reduction potential of Oꢁ2 /O2 (O2 þ eꢁ/Oꢁ2 , E(O2/
O2 ꢁ) ¼ ꢁ0.33 eV). It is impossible to produce the dominant reactive
ꢂ
ꢂꢁ
species O2 through double-charge transfer mechanism in this
photocatalytic process (Fig. 7a). As a result, a direct Z-scheme
electrons and holes transfer mode is proposed to explain the
enhanced photocatalytic activity [22,53]. In this hetero-structure
system, both BiVO4 and Bi2O3 can be excited under visible-light
irradiation, the holes on the VB of Bi2O3 and the electrons on the
CB of BiVO4 recombine directly at their interfacial region. The
electrons on the CB of Bi2O3 are captured by the surface chem-
isorbed O2 to give O2 ꢁ, and the remaining holes in BiVO4 can
ꢂ
directly oxidize phenol. In this case, the Bi2O3-BiVO4 CMRs are a
type of direct Z-scheme photocatalysts under our experimental
conditions and the photoactivity is dramatically improved based on
this charge transfer mode.
[20] R. Cheng, D.H. Li, H.L. Zhou, C. Wang, A.X. Yin, S. Jiang, Y. Liu, Y. Chen,
Y. Huang, X.F. Duan, Nano Lett. 14 (2014) 5590.
[21] S.W. Choi, A. Katoch, J.H. Kim, S.S. Kim, ACS Appl. Mater. Interfaces 6 (2014)
17723.
[22] J.C. Wang, L. Zhang, W.X. Fang, J. Ren, Y.Y. Li, H.C. Yao, J.S. Wang, Z.J. Li, ACS
Appl. Mater. Interfaces 7 (2015) 8631.
[23] P. Zhou, J.G. Yu, M. Jaroniec, Adv. Mater 26 (2014) 4920.
[24] R.X. Jin, Y. Yang, Y. Xing, L. Chen, S.Y. Song, R.C. Jin, ACS Nano 8 (2014) 3664.
[25] X. Xiao, C. Liu, R.P. Hu, X.X. Zuo, J.M. Nan, L.S. Lia, L.S. Wang, J. Mater. Chem. 22
(2012) 22840.
[26] A. Etogo, E.L. Hu, C.M. Zhou, Y.J. Zhong, Y. Hu, Z.L. Hong, J. Mater. Chem. A 3
(2015) 22413.
[27] D.L. Jiang, L.L. Chen, J.J. Zhu, M. Chen, W.D. Shi, J.M. Xie, Dalton Trans. 42
(2013) 15726.
[28] X.H. Gao, H.B. Wu, L.X. Zheng, Y.J. Zhong, Y. Hu, X.W. Lou, Angew. Chem. Int.
Ed. 53 (2014) 5917.
4. Conclusions
In summary, we have demonstrated a facile solvothermal
method followed by an annealing process to synthesize high-
quality porous Bi2O3-BiVO4 composite microrods (CMRs) with
uniform size distribution. Due to high separation efficiency of
electron-hole pairs caused by the p-n heterojunction, intrinsically
porous structure nature, the as-prepared Bi2O3-BiVO4 CMRs exhibit
excellent photocatalytic activity for the degradation of colorless
organic phenol under visible-light irradiation. A Z-scheme transfer
mechanism is proposed and further confirmed by the reactive
species trapping experiments. We believe that this new synthesis
[29] P.M. Rao, L.L. Cai, C. Liu, I.S. Cho, C.H. Lee, J.M. Weisse, P.D. Yang, X.L. Zheng,
Nano Lett. 14 (2014) 1099.
[30] J. Cao, C.C. Zhou, H.L. Lin, B.Y. Xu, S.F. Chen, Appl. Surf. Sci. 284 (2013) 263.
[31] Z.Q. He, Y.Q. Shi, C. Gao, L.N. Wen, J.M. Chen, S. Song, J. Phys. Chem. C 118
(2014) 389.
ꢀ
ꢀ
[32] E.A. Ruiz, U.M. García-Pereza, M.D.L.G. Galvana, P.Z. Robledo, B.B. Reyes,

M. Mao et al. / Journal of Alloys and Compounds 688 (2016) 1080e1087
1087
J. Peral, Appl. Surf. Sci. 328 (2015) 361.
117 (2013) 5894.
[33] W.Z. Wang, X.W. Huang, S. Wu, Y.X. Zhou, L.J. Wang, H.L. Shi, Y.J. Liang, B. Zou,
Appl. Catal. B 134e135 (2013) 293.
[44] C.H. Ding, F.P. Cao, L.Q. Ye, K.C. Liu, H.Q. Xie, X.L. Jin, Y.R. Su, Phys. Chem.
Chem. Phys. 17 (2015) 23489.
[34] W.Z. Wang, J. Wang, Z.Z. Wang, X.Z. Wei, L. Liu, Q.S. Ren, W.L. Gao, Y.J. Liang,
H.L. Shi, Dalton Trans. 43 (2014) 6735.
[45] S.N. Gu, W.J. Li, F.Z. Wang, S.Y. Wang, H.L. Zhou, H.D. Li, Appl. Catal. B 170e171
(2015) 186.
[35] X.M. Dang, X.F. Zhang, X.L. Dong, W.Q. Ruan, H.C. Ma, M. Xue, RSC Adv. 4
(2014) 54655.
[36] Y.F. Cheng, H. Wang, Y. Zhu, F. Liao, Z.P. Li, J.K. Li, J. Mater Sci. Mater Electron
26 (2015) 1268.
[46] J.G. Yu, H.T. Guo, S.A. Davis, S. Mann, Adv. Funct. Mater 16 (2006) 2035.
[47] D.V. Bavykin, V.N. Parmon, A.A. Lapkin, F.C. Walsh, J. Mater. Chem. 14 (2004)
3370.
[48] J.T. Tang, Y.H. Liu, H.Z. Li, Z. Tan, D.T. Li, Chem. Commun. 49 (2013) 5498.
[49] J. Zhang, X.J. Wang, X. Wang, J.K. Song, J.Y. Huang, B. Louangsouphoma,
J.F. Zhao, RSC Adv. 5 (2015) 71922.
[37] L.Z. Li, B. Yan, J. Alloy Compd. 476 (2009) 624.
[38] H.Y. Jiang, P. Li, G.G. Liu, J.H. Ye, J. Lin, J. Mater. Chem. A 3 (2015) 5119.
[39] M. Muruganandham, R. Amutha, G.J. Lee, S.H. Hsieh, J.J. Wu, M. Sillanpa,
J. Phys. Chem. C 116 (2012) 12906.
ꢀ
ꢀ
ꢀ
[50] A.M. Peiro, J.A. Ayllon, J. Peral, X. Domenech, Appl. Catal. B 30 (2001) 359.
[51] N. Tian, H.W. Huang, Y. He, Y.X. Guo, T.R. Zhang, Y.H. Zhang, Dalton Trans. 44
[40] G.X. Tong, Y. Liu, T. Wu, C.L. Tong, F.F. Du, J. Mater. Chem. C 3 (2015) 5506.
[41] S. Ashoka, G. Nagaraju, C.N. Tharamani, G.T. Chandrappa, Mater. Lett. 63
(2009) 873.
(2015) 4297.
[52] H. Liu, W.R. Cao, Y. Su, Y. Wang, X.H. Wang, Appl. Catal. B 111e112 (2012)
271.
[42] C. Lv, G. Chen, J.X. Sun, C.S. Yan, H.J. Dong, C.M. Li, RSC Adv. 5 (2015) 3767.
[43] Q. Zhu, W.S. Wang, L. Lin, G.Q. Gao, H.L. Guo, H. Du, A.W. Xu, J. Phys. Chem. C
[53] R. Qiao, M.M. Mao, E.L. Hu, Y.J. Zhong, J.Q. Ning, Y. Hu, Inorg. Chem. 54 (2015)
9033.