From hollow olive-shaped BiVO4 to n-p core-shell BiVO 4@Bi2O3 microspheres: Controlled synthesis and enhanced visible-light-responsive photocatalytic properties

Article abstract of DOI:10.1021/ic101961z

In this study, hollow olive-shaped BiVO₄ and n-p core-shell BiVO₄@Bi₂O3 microspheres were synthesized by a novel sodium bis(2-ethyl hexyl)sulfosuccinate (AOT)-assisted mixed solvothermal route and a thermal solution of NaOH etching process under hydrothermal conditions for the first time, respectively. The as-obtained products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy, Brunauer-Emmett-Teller surface area, and UV-vis diffuse-reflectance spectroscopy in detail. The influence of AOT and solvent ratios on the final products was studied. On the basis of SEM observations and XRD analyses of the samples synthesized at different reaction stages, the formation mechanism of hollow olive-shaped BiVO4 micro-spheres was proposed. The photocatalytic activities of hollow olive-shaped BiVO₄ and core-shell BiVO₄@Bi₂O₃ microspheres were evaluated on the degradation of rhodamine B under visible-light irradiation (λ > 400 nm). The results indicated that core-shell BiVO ₄@Bi₂O₃ exhibited much higher photocatalytic activities than pure olive-shaped BiVO₄. The mechanism of enhanced photocatalytic activity of core-shell BiVO₄@Bi₂O ₃ microspheres was discussed on the basis of the calculated energy band positions as well. The present study provides a new strategy to enhancing the photocatalytic activity of visible-light-responsive Bi-based photocatalysts by p-n heterojunction.

Full text of DOI:10.1021/ic101961z

800 Inorg. Chem. 2011, 50, 800–805
DOI: 10.1021/ic101961z
From Hollow Olive-Shaped BiVO4 to n-p Core-Shell BiVO4@Bi2O3 Microspheres:
Controlled Synthesis and Enhanced Visible-Light-Responsive Photocatalytic
Properties
Mei-Li Guan, De-Kun Ma,* Sheng-Wei Hu, Yan-Jun Chen, and Shao-Ming Huang*
Nanomaterials and Chemistry Key Laboratory, Wenzhou University, Wenzhou, Zhejiang 325027, People’s
Republic of China
Received July 8, 2010
In this study, hollow olive-shaped BiVO4 and n-p core-shell BiVO4@Bi2O3 microspheres were synthesized by a
novel sodium bis(2-ethylhexyl)sulfosuccinate (AOT)-assisted mixed solvothermal route and a thermal solution of
NaOH etching process under hydrothermal conditions for the first time, respectively. The as-obtained products were
characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy,
Brunauer-Emmett-Teller surface area, and UV-vis diffuse-reflectance spectroscopy in detail. The influence of AOT
and solvent ratios on the final products was studied. On the basis of SEM observations and XRD analyses of the
samples synthesized at different reaction stages, the formation mechanism of hollow olive-shaped BiVO4 micro-
spheres was proposed. The photocatalytic activities of hollow olive-shaped BiVO4 and core-shell BiVO4@Bi2O3
microspheres were evaluated on the degradation of rhodamine B under visible-light irradiation (λ > 400 nm). The
results indicated that core-shell BiVO4@Bi2O3 exhibited much higher photocatalytic activities than pure olive-shaped
BiVO4. The mechanism of enhanced photocatalytic activity of core-shell BiVO4@Bi2O3 microspheres was discussed
on the basis of the calculated energy band positions as well. The present study provides a new strategy to enhancing
the photocatalytic activity of visible-light-responsive Bi-based photocatalysts by p-n heterojunction.
Introduction
template-free methods.5 Compared to spherical counter-
parts, however, the synthesis of hollow crystals with non-
spherical shapes remains a significant challenge to material
scientists because of the difficulties in employing template
technologies.6 There are only limited successes in the synthe-
sis of nonspherical hollow crystals through the template-free
route as well. For example, hollow spindle-shaped hematite
crystals could be fabricated by the selective dissolution of the
interiors under hydrothermal conditions with the exterior
protected by adsorbed phosphate anions.7
Materials with a hollow interior have attracted much atten-
tion compared to solid counterparts because they can be used to
lower the density, enhance the specific surface area, modulate
the refractive index, withstand cyclic changes in volume, and
widely perform as drug delivery, catalysis, sensors, lightweight
materials, and various new applications.1 Up to now, hollow
spherical structures have easily been achieved through conven-
tional hard-template,2 soft-template,3 sacrificial-template,4 and
BiVO4 has stimulated scientists’ tremendous interest be-
cause of its unique properties, such as ferroelasticity,8 ionic
conductivity,9 gas sensing,10 and coloristic properties.11
Moreover, it has also been considered as one of the excellent
visible-light-responsive photocatalysts and widely applied in
*To whom correspondence should be addressed. E-mail: dkma@wzu.edu.cn
(D.-K.M.), smhuang@wzu.edu.cn (S.-M.H.). Fax:þ86 577 88373064. Tel: þ86
577 88373031.
€
(1) (a) Caruso, F.; Caruso, R. A.; Mohwald, H. Science 1998, 282, 1111.
(b) Mathiowitz, E.; Jacob, J. S.; Jong, Y. S.; Carino, G. P.; Chickering, D. E.;
Chaturvedi, P.; Santos, C. A.; Vijayaraghavan, K.; Montgomery, S. Nature 1997,
386, 410. (c) Lou, X. W.; Wang, Y.; Yuan, C.; Lee, J. Y.; Archer, L. A. Adv. Mater.
2006, 18, 2325. (d) Li, H. X.; Bian, Z. F.; Zhu, J.; Zhang, D. Q.; Li, G. S.; Huo,
Y. N.; Li, H.; Lu, Y. F. J. Am. Chem. Soc. 2007, 129, 8406. (e) Kim, S. W.; Kim,
M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc. 2002, 124, 7642. (f) Zhang, H. G.;
Zhu, Q. S.; Zhang, Y.; Wang, Y.; Zhao, L.; Yu, B. Adv. Funct. Mater. 2007, 17,
2766. (g) Chen, J.; Saeki, F. B.; Wiley, J.; Cang, H.; Cobb, M. J.; Li, Z. Y.; Au, L.;
Zhang, H.; Kimmey, M. B.; Li, X. D.; Xia, Y. N. Nano Lett. 2005, 5, 473.
(2) Wolosiuk, A.; Armagan, O.; Braun, P. V. J. Am. Chem. Soc. 2005, 127,
16356.
(5) Li, J.; Zeng, H. C. Angew. Chem., Int. Ed. 2005, 44, 4342.
(6) Lou, X. W.; Archer, L. A.; Yang, Z. C. Adv. Mater. 2008, 20, 1.
(7) Jia, C. J.; Sun, L. D.; Yan, Z. G.; You, L. P.; Luo, F.; Han, X. D.;
Pang, Y. C.; Zhang, Z.; Yan, C. H. Angew. Chem., Int. Ed. 2005, 44, 4328.
(8) David, W. I. F.; Wood, I. G. J. Phys. C: Solid State Phys. 1983, 16,
5149.
(9) Hirota, K.; Komatsu, G.; Yamashita, M.; Takemura, H.; Yamaguchi,
O. Mater. Res. Bull. 1992, 27, 823.
(3) Zhang, D. B.; Qi, L. M.; Ma, J. M.; Cheng, H. M. Adv. Mater. 2002,
14, 1499.
(4) Jeong, U. Y.; Xia, Y. N. Angew. Chem., Int. Ed. 2005, 44, 3099.
(10) Zhao, Y.; Xie, Y.; Zhu, X.; Yan, S.; Wang, S. X. Chem.—Eur. J.
2008, 14, 1601.
(11) Zhang, L.; Chen, D. R.; Jiao, X. L. J. Phys. Chem. B 2006, 110, 2668.
r
pubs.acs.org/IC
Published on Web 12/20/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 50, No. 3, 2011 801
the photocatalytic evolution of O2 and degradation of or-
ganic pollutants.12 The photocatalytic properties of BiVO4
are strongly dependent on its morphologies and micro-
structures.13 Therefore, many efforts have been made to
synthesize BiVO4 with various morphologies, such as three-
dimensional mesocrystals,14 hyperbranched structures,10
hierarchical microspheres,15 nanotubes,16 and nanoplates.17
In addition, the photocatalytic properties of BiVO4 are closely
related to the phase structures.18 There are three crystalline
phases reported for synthetic BiVO4, namely, a tetragonal
zircon structure, a monoclinic scheelite structure, and a
tetragonal scheelite structure. Among these phase structures,
the monoclinic scheelite structure of BiVO4 possesses the best
photocatalytic performance under visible-light irradiation.
However, its photocatalytic activity is not ideal yet because
of the difficult migration of photogenerated electrons and
holes and thus needs to be further improved for practical
application.
Composite semiconductors can reduce the recombination
probability of photogenerated electrons and holes and thus
have been widely employed to improve the photocatalytic
activity of materials.19 Among various composite photocata-
lysts, the fabrication of a p-n heterojunction has been dem-
onstrated as a very efficient means of separating electron-
hole pairs because of the internal electric field with the direction
from an n-type semiconductor to a p-type semiconductor.20
BiVO4 is intrinsic n-type semiconductor and Bi2O3 is intrinsic
p-type semiconductor with good photocatalytic activity.
Theoretically, a p-n heterojunction will form when n-type
BiVO4 and p-type Bi2O3 semiconductors combine together.
However, up to now, there are few reports on p-n Bi2O3/
BiVO4 composite photocatalysts. Especially, a core-shell
heterojunction of the composite photocatalysts, to the best of
our knowledge, has never been reported. Recently, Li et al.
reported that Bi2O3/BiVO4 submicrometer spheres prepared
by the direct mixture of Bi2O3 and BiVO4 could show much
higher photocatalytic activities than pure Bi2O3 and BiVO4.21
However, it is difficult to form an efficient and sound hetero-
junction with the physical mixture, which is crucial to im-
proving the photocatalytic efficiency of the materials.20
In this study, we represent a mixed solvothermal route to
synthesize hollow olive-shaped BiVO4 with a monoclinic
scheelite structure through a sodium bis(2-ethylhexyl)sulfo-
succinate (AOT)-induced aggregate and Ostwald ripening
process, free of templates, seeds, and catalysts. Subsequently,
n-p core-shell BiVO4@Bi2O3 was fabricated by a thermal
aqueous solution etching process under hydrothermal con-
ditions, employing olive-shaped BiVO4 as a precursor. Fur-
thermore, the photocatalytic activities of hollow olive-shaped
BiVO4 and n-p core-shell BiVO4@Bi2O3 microspheres
were contrastively studied.
Experimental Section
I. Synthesis of Hollow Olive-Shaped BiVO4. A total of
1 mmol of Bi(NO3)3 5H2O was added into 40 mL of mixed
3
solvents (ultrapure water and ethylene glycol with a volume
ratio of 1:1) under agitation. Then 1 mmol of NaVO3 2H2O and
3
3 mmol of AOT were introduced into the solution in sequence.
After more agitation for 5 min, the solution was poured into a
stainless steel autoclave with a Teflon liner of 50 mL capability
and heated at 160 °C for 18 h. After the autoclave was cooled to
room temperature, the products were separated centrifugally and
washed with ultrapure water and absolute ethanol three times.
Then the products were dried under vacuum at 60 °C for 4 h.
II. Synthesis of Core-Shell BiVO4@Bi2O3. A total of 0.5
mmol of the as-obtained olive-shaped BiVO4 powders was dis-
persed in 40 mL of ultrapure water under vigorous stirring. Then
5 mmol of NaOH was added into the solution. After stirring
for a while, the solution was poured into a stainless steel auto-
clave with a Teflon liner of 50 mL capacity, sealed, and heated
at 180 °C for 12 h. The post-treatment process of the sample is
the same as that of olive-shaped BiVO4.
III. Characterization. Powder X-ray diffraction (XRD) was
carried out with a Bruker D8 Advance X-ray diffractometer
using Cu KR radiation (λ = 0.154 18 nm) at a scanning rate of
8°/min in the 2θ range of 10-70°. Field-emission scanning
electron microscopy (FE-SEM) images were taken on a Nova
NanoSEM 200 scanning electron microscope. Transmission elec-
tron microscopy (TEM) observations, energy-dispersive X-ray
(EDX) spectroscopy, and high-resolution TEM (HRTEM)
images were performed with a JEOL JEM 2010 high-resolution
transmission electron microscope, using an accelerating voltage
of 200 kV. The optical diffuse-reflectance spectra were recorded
on a UV2501PC (Shimadzu) using BaSO4 as a reference. The
Brunauer-Emmett-Teller (BET) surface area was measured
with a ASAP2020 specific surface area and porosity analyzer.
Photoelectrochemistry measurementswerecarried outin a three-
electrode cell with a flat quartz window using an electrochemical
system (CH1660C). The BiVO4 films prepared by olive-shaped
BiVO4 and the Bi2O3 films prepared by Bi2O3 nanoplates
(obtained by hydrothermal complete etching of BiVO4 with
20 mmol of NaOH at 220 °C) were used as working electrodes. A
platinum wire and saturated calomel electrode were used as
counter and reference electrodes, respectively. A 0.1 M Na2SO4
aqueous solution was used as an electrolyte. A 500 W Xe light
with a 400 nm cutoff filter was used as the visible-light source.
IV. Photocatalytic Properties Study. The photocatalytic ac-
tivities of olive-shaped BiVO4 and core-shell BiVO4@Bi2O3
were evaluated by degradation of rhodamine B (RhB) under
visible-light irradiation from 500 W Xe light (CHF-XM500,
purchased from Beijing Lituo Science and Technology Co. Ltd.)
equipped with a 400 nm cutoff filter. In every experiment,
100 mg of photocatalysts were added to 100 mL of a RhB solu-
tion (10-5 mol/L). Before illumination, the solution was mag-
netically stirred in the dark for 2 h to ensure the establishment of
an adsorption-desorption equilibrium between the photocata-
lystsandRhB. After that, the solutionwas exposedtovisible-light
irradiation under magnetic stirring. At given time intervals, 3 mL
aliquots were sampled and centrifuged to remove the photo-
catalyst particles. Then, the filtrates were analyzed by record-
ing variations of the absorption band maximum (553 nm) in
the UV-vis spectra of RhB by using a Shimadzu UV2501PC
spectrophotometer.
(12) (a) Kudo, A.; Omori, K.; Kato, H. J. Am. Chem. Soc. 1999, 121,
11459. (b) Li, G. S.; Zhang, D. Q.; Yu, J. C. Chem. Mater. 2008, 20, 3983.
(13) (a) Tokunaga, S.; Kato, H.; Kudo, A. Chem. Mater. 2001, 13, 4624.
(b) Zhou, L.; Wang, W. Z.; Liu, S. W.; Zhang, L. S.; Xu, H. L.; Zhu, W. J. Mol.
Catal. A 2006, 252, 120.
(14) Zhou, L.; Wang, W. Z.; Xu, H. L. Cryst. Growth Des. 2008, 8, 728.
(15) Ma, D. K.; Wang, S.; Cai, P.; Jiang, J. L.; Yang, D. P.; Huang, S. M.
Chem. Lett. 2009, 38, 962.
(16) Ren, L.; Jin, L.; Wang, J. B.; Yang, F.; Qiu, M. Q.; Yu, Y.
Nanotechnology 2009, 20, 115603.
(17) Xi, G. C.; Ye, J. H. Chem. Commun. 2010, 46, 1893.
(18) Zhang, X.; Ai, Z. H.; Jia, F. L.; Zhang, L. Z.; Fan, X. X.; Zou, Z. G.
Mater. Chem. Phys. 2007, 103, 162.
(19) Ni, M.; Leung, M. K. H.; Leung, D. Y. C.; Sumathy, K. Renewable
Sustainable Energy Rev. 2007, 11, 401.
(20) (a) Kim, H. G..; Pramod, H. B.; Lee, J. S. Angew. Chem., Int. Ed.
2005, 44, 4585. (b) Long, M. C.; Cai, W. M.; Cai, J.; Zhou, B. X.; Chai, X. Y.; Wu,
Y. H. J. Phys. Chem. B 2006, 110, 20211. (c) Ahn, K. S.; Yan, Y. F.; Kang, M. S.;
Kim, J. Y.; Shet, S.; Wang, H. L.; Turner, J.; Al-Jassim, M. Appl. Phys. Lett.
2009, 95, 022116.
(21) Li, L. Z.; Yan, B. J. Alloys Compd. 2009, 476, 624.

802 Inorganic Chemistry, Vol. 50, No. 3, 2011
Guan et al.
Figure 1. Powder XRD patterns of the products synthesized by a mixed
solvothermal route (a) and a thermal solution of NaOH etching process (b).
Figure 2. Panoramic (a) and high-magnification (b) FE-SEM images
and TEM image (c) of the as-obtained BiVO4 products. (d) Panoramic
TEM image of the products synthesized by a thermal solution of NaOH
etching process under hydrothermal conditions. The inset is a magnified
TEM image performed on the rim of an individual microsphere. (e) HRTEM
image taken from the edge of a single core-shell microsphere.
Results and Discussion
The crystallinity and phase purity of the as-obtained prod-
ucts were examined by XRD measurements. As can be seen
from Figure 1a, all of the reflections of the products obtained
by an AOT-assisted mixed solvothermal route can be readily
indexed to a pure monoclinic scheelite structure of BiVO4 [space
group I2/b], which is in good agreement with the reported data
(JCPDS No. 75-2480). After being etched by a thermal solution
of NaOH under hydrothermal conditions, monoclinic BiVO4
was partially changed into cubic Bi2O3 (asterisks denote Bi2O3;
JCPDS No. 27-0052), as shown in Figure 1b.
Panels a and b of Figure 2 represent panoramic and high-
magnification FE-SEM images of the as-obtained BiVO4
products, respectively. From Figure 2a, it can be seen that the
products consist of large numbers of uniform olive-shaped
particles. The diameters of the individual olivealong long and
short axis directions are 1-1.5 and 0.6-0.9 μm, respectively.
The magnified FE-SEM image shows that the products have
hollow interiors (Figure 2b), judged from broken particles.
TEM images were further used to confirm this point. As
shown in Figure 2c, the obvious contrast of brightness in
olive-shaped microspheres assuredly means the formation of
cavities in the light region. Careful FE-SEM observations of a
total of 500 olive-shaped microspheres indicate that 54 micro-
spheres (ca. 10%) are open. After the olive-shaped BiVO4
microspheres were etched by a thermal solution of NaOH,
core-shell microspheres were formed, as shown in the TEM
images in Figure 2d. The magnified TEM image (inset in the
upper-right corner) more clearly shows distinct bright/dark
contrast between the shell and core. HRTEM was further
used to characterize the as-synthesized core-shell structures.
Figure 2e is a HRTEM image taken from the edge of an
individual microsphere. The spacings of the adjacent lattice
unchanged, in the absence of AOT, the resultant products
consist of predominant bone-shaped microcrystals (Figure
S2a in the Supporting Information). It was also found that the
volume ratios of water to ethylene glycol had an obvious
influence on the morphologies of the final products. When the
volume ratio of ethylene glycol to water was increased from
1:3, to 1:1, to 3:1, the resultant products took on star-shaped
(Figure S2b in the Supporting Information), uniform olive-
shaped (Figure S2c in the Supporting Information), and
irregular olive-shaped morphologies (Figure S2d in the Sup-
porting Information) in sequence. Ethylene glycol has been
successfully used to control the morphologies of various metal
oxides by a coordination action with transition-metal ions.22
In the experimental process, we observed that the aqueous
solution became white turbid when Bi(NO3)3 5H2O was added
3
in water. This was due to the formation of slightly soluble
BiONO3 as a result of Bi(NO3)3 hydrolysis.23 With the intro-
duction of ethylene glycol into the solution, the turbid solu-
tion gradually became clear under agitation. This phenom-
enon indicates that BiONO3 was dissolved and a coordination
action between Bi3þ ions with ethylene glycol molecules took
place. On the other hand, the solubilities of the reactants and
products in water and ethylene glycol were different. There-
fore, the nucleation and growth rate of the crystals could be
adjusted by modulating the volume ratios of mixed solvents.
As a result, as in some previous reports,24 different morphol-
ogies of BiVO4 could be produced under mixed solvothermal
conditions.
˚
planes are ca. 3.20 and 2.79 A, which are consistent with the
In order to understand the morphological evolution pro-
cess of hollow olive-shaped BiVO4, XRD analyses and SEM
interplanar spacings of (111) and (200) planes of cubic Bi2O3,
respectively. The EDX spectra recorded on the individual
microspheres were carried out as well (see the Supporting
Information, Figure S1). The ratio of Bi to V is ca. 65.58:34.42
on the basis of the results of calculation. This further shows
that the microsphere consists of Bi2O3 and BiVO4. According
to the above-mentioned results, it is known that the core and
shell correspond to BiVO4 and Bi2O3, respectively.
(22) (a) Jiang, X. C.; Wang, Y. L.; Herricks, T.; Xia, Y. N. J. Mater.
Chem. 2004, 14, 695. (b) Scott, R. W. J.; Coombs, N.; Ozin, G. A. J. Mater.
Chem. 2003, 13, 969.
(23) Ma, D. K.; Huang, S. M.; Chen, W. X.; Hu, S. W.; Shi, F. F.; Fan,
K. L. J. Phys. Chem. C 2009, 113, 4369.
(24) (a) Liu, Z. P.; Peng, S.; Xie, Q.; Hu, Z. K.; Yang, Y.; Zhang, S. Y.;
Qian, Y. T. Adv. Mater. 2003, 15, 936. (b) Yao, W. T.; Yu, S. H.; Pan, L.; Li, J.;
Wu, Q. S.; Zhang, L.; Jiang, J. Small 2005, 1, 320. (c) Yao, W. T.; Yu, S. H.;
Huang, X. Y.; Jiang, J.; Zhao, L. Q.; Pan, L.; Li, J. Adv. Mater. 2005, 17, 2799.
In the present system, AOT plays a key role in the forma-
tion process of olive-shaped BiVO4. Keeping other conditions

Article
Inorganic Chemistry, Vol. 50, No. 3, 2011 803
Scheme 1. Possible Formation Process of Hollow Olive-Shaped
BiVO4 Microspheres
regions. The steep absorption edge of the two spectra indi-
cates that the visible-light adsorption is not due to the tran-
sition from the impurity level but to the band-gap transition.
Their band gaps couldbe determined with theformulaRhν =
A(hν - Eg)n/2, where R, h, ν, A, Eg, and n are the absorption
coefficient, Planck’s constant, the incident light frequency, a
constant, the band-gap energy, and an integer, respectively.
Among them, n depends on the characteristics of the optical
transition in a semiconductor, i.e., direct transition (n=1)
or indirect transition (n=4). For BiVO4 and Bi2O3, both of
them pertain to direct transition and the values of n are
1.13b,25 The band-gap energy (Eg value) of BiVO4 can be thus
estimated from a plot of (Rhν)2 versus the photon energy (hν).
The intercept of the tangent to the x axis will give a good
approximation of the band-gap energy for the olive-shaped
BiVO4 and core-shell BiVO4@Bi2O3, as shown in Figure 5b.
The estimated band-gap energies of the as-obtained BiVO4
and core-shell BiVO4@Bi2O3 were about 2.43 and 2.52 eV
from the absorption onsets, respectively. The band-gap
energy of olive-shaped BiVO4 microspheres is close to that
of BiVO4 microtubes reported previously (2.36 eV).26 Ac-
cording to previous reports, Bi2O3 nanoparticles with sizes of
40-100 nm have a band-gap energy of ca. 2.85 eV.27 The
Bi2O3 shell has a thickness of ca. 90 nm. Therefore, its band-
gap energy can be approximately estimated to be ca. 2.85 eV
as well. As a result, the core-shell BiVO4@Bi2O3 has a larger
band-gap energy than single-phase olive-shaped BiVO4 and a
smaller one than Bi2O3.
In order to evaluate the photocatalytic activities of the as-
synthesized olive-shaped BiVO4 and core-shell BiVO4@-
Bi2O3, the degradation ability of RhB dye in water under
visible-light irradiation was studied. Parts a and b of Figure 6
show the temporal evolution of the UV-vis spectra during
photodegradation of RhB on olive-shaped BiVO4 and core-
shell BiVO4@Bi2O3 microspheres under visible-light irradia-
tion (λ > 400 nm). As can be seen from the above two
spectra, absorption of both RhB/olive-shaped BiVO4 and
RhB/core-shell BiVO4 suspensions gradually decreased dur-
ing photodegradation. In addition, the major absorption
peak corresponding to RhB was shifted from 553 to 496 nm
step by step, indicating the removal of ethyl groups one by
one, which was in agreement with the literature.28 The cor-
responding plots for the concentration changes of RhB deter-
mined from its characteristic absorption peak (at 553 nm) are
shown in Figure 6c. It is obvious that core-shell BiVO4@-
Bi2O3 shows higher photocatalytic activities than olive-shaped
BiVO4 under identical experimental conditions.
Figure 3. Powder XRD patterns of the products synthesized at different
reaction stages: (a) 30 min; (b) 1 h; (c) 3 h.
Figure 4. FE-SEM images of the products synthesized at different
reaction stages: (a) 30 min; (b) 1 h; (c) 3 h.
observations at different reaction stages were carried out.
Figures 3 and 4 represent XRD patterns and corresponding
FE-SEM images of the samples synthesized for 30 min and
1 and 3 h, respectively. As can be seen from these patterns and
FE-SEM images, the products are poor-crystalline nanopar-
ticles within the initial 30 min. With the reaction time ex-
tended to 1 h, these amorphous nanoparticles have begun to
aggregate together and form some olive-shaped spheres. As
the reaction time was prolonged to 3 h, the products have
grown into well-defined and crystalline olive-shaped micro-
spheres. On the basis of the above results, the possible forma-
tion process of olive-shaped BiVO4 spheres was proposed as
3-
follows. At the initial stage of the reaction, Bi3þ with VO4
ions precipitated quickly under the driving force of low-
solubility products of BiVO4. Subsequently, the newborn
nanoparticles aggregated into loose olive-shaped microspheres
under the inducement action of AOT molecules absorbed on
nanoparticles’ surfaces through intermolecular interaction.
Finally, inside-out Ostwald ripening among these amorphous
nanoparticles took place because the surfaces of the nanopar-
ticles were in contact with the surrounding solution. As a
result, the interior nanoparticles have a tendency to dissolve,
which provides the driving force for spontaneous inside-out
Ostwald ripening.1c This dissolution process could initiate at
regions either near the surface or around the center of the
olive-shaped microspheres, presumably depending on the
packing of primary nanoparticles and ripening characteristics,
to produce olive-shaped hollow spheres. A simple schematic
illustration for formation of the process is given in Scheme 1.
The energy band structure feature of a semiconductor is
considered the key factor in determining its photocatalytic
activity. Figure 5a shows the UV-vis diffuse-reflectance
spectra of the as-synthesized olive-shaped BiVO4 and core-
shell BiVO4@Bi2O3 microspheres, respectively. Both of the
samples show strong absorption in the UV- and visible-light
(25) Zhou, L.; Wang, W. Z.; Xu, H. L.; Sun, S. M.; Shang, M. Chem.—
Eur. J. 2009, 15, 1776.
(26) Zhou, L.; Wang, W. Z.; Zhang, L. S.; Xu, H. L.; Zhu, W. J. Phys.
Chem. C 2007, 111, 13659.
(27) Zhang, L. S.; Wang, W. Z.; Yang, J.; Chen, Z. G.; Zhang, W. Q.;
Zhou, L.; Liu, S. W. Appl. Catal., A 2006, 308, 105.
(28) (a) Zhao, W.; Chen, C.; Li, X.; Zhao, J. C. J. Phys. Chem. B 2002,
106, 5022. (b) Zhang, C.; Zhu, Y. F. Chem. Mater. 2005, 17, 3537.

804 Inorganic Chemistry, Vol. 50, No. 3, 2011
Guan et al.
Figure 5. UV-vis diffuse-reflectance spectra (a) and plots of (Rhν)2 versus the photon energy (hν) (b) of the as-synthesized olive-shaped BiVO4 and
core-shell BiVO4@Bi2O3 microspheres.
Figure 6. Changes of UV-vis spectraofolive-shapedBiVO4 (a) andcore-shell BiVO4@Bi2O3 microspheres(b)suspendedin aRhB solutionasa function
of the irradiation time and RhB concentration changes over olive-shaped BiVO4 and core-shell BiVO4@Bi2O3 microspheres (c), respectively.
Why does core-shell BiVO4@Bi2O3 exhibit superior pho-
tocatalytic activity over olive-shaped BiVO4? It is well-known
that the activities of photocatalysts mostly depend on the
specific surface area, light energy utilization ratio, and quan-
tum efficiency. The larger the surface area of the materials,
the higher the photocatalytic activity they show. On the basis
of the N2 adsorption-desorption isotherms, the calculated
BET surface areas of olive-shaped BiVO4 and core-shell
BiVO4@Bi2O3 are 1.4451 and 1.4810 m2/g, respectively. They
show almost the same surface area. The responsive range of
visible light for BiVO4@Bi2O3 is narrower than that of olive-
shaped BiVO4. The control experiment also reveals that Bi2O3
hierarchical architectures composed of nanosheets with much
higher surface area (33.1 m2/g) exhibit an photocatalytic
performance inferior to that of core-shell BiVO4@Bi2O3
under the same measurement conditions (see the Supporting
Information, experimental part and Figure S3-S5). There-
fore, the enhanced photocatalytic activity of BiVO4@Bi2O3 is
most possibly related to the core-shell heterojunction be-
tween BiVO4 and Bi2O3. According to photoelectrochemical

Article
Inorganic Chemistry, Vol. 50, No. 3, 2011 805
Scheme 2. Diagram for Band Levels of Core-Shell BiVO4@Bi2O3
Composites and the Possible Charge-Separation Process
electrons on the surfaces of Bi2O3 would easily transfer to
BiVO4 under the inducement action of the internal p-n
electric field, leaving holes on the Bi2O3 valence band. In such
a way, the photoinduced electrons and holes could be effec-
tively separated. Therefore, according to the above results, the
higher photocatalytic activity of core-shell BiVO4@Bi2O3
than of single-phase BiVO4 and Bi2O3 could be mostly as-
cribed to an enhanced quantum efficiency, i.e., efficient sepa-
ration of photogenerated carriers via a p-n heterojunction.
The detailed reasons need to be clarified, and further study is
underway.
Conclusions
In summary, hollow olive-shaped BiVO4 and core-shell
BiVO4@Bi2O3 microspheres have been successfully synthe-
sized by an AOT-assisted mixed solvothermal route and
thermal solution etching method under hydrothermal condi-
tions, respectively. In order to obtain well-defined olive-shaped
BiVO4 microspheres, the use of AOT and the selection of
appropriate solvent ratios are indispensable. On the basis of
analysis of the temporal evolution process, the formation of
olive-shaped BiVO4 probably went through AOT-molecule-
induced aggregation and an inside-out Ostwald ripening pro-
cess. Both olive-shaped BiVO4 and core-shell BiVO4@Bi2O3
microspheres exhibited visible-light-responsive photocatalytic
activities, while the latter showed much higher activity under
the same experimental conditions. According to the calcu-
lated band-edge positions of the two semiconductors, the
enhanced activity of core-shell BiVO4@Bi2O3 microspheres
can be mostly attributed to the p-n heterojunction structure
and thus the reduced recombination probability of photo-
generated hole-electron carriers. The present core-shell
BiVO4@Bi2O3 microspheres are promising applications in
the field of environmental remediation for good visible-light-
responsive photocatalytic activity and easy separation.
measurements (see Figures S6 and S7 in the Supporting
Information), olive-shaped BiVO4 and Bi2O3 nanoplates
respectively show anodic and cathodic photocurrents when
the light is on, indicating that BiVO4 is an n-type semicon-
ductor and Bi2O3 is a p-type semiconductor. When they were
combined, a p-n-type heterojunction would be produced. In
recent years, this kind of p-n composite photocatalyst has
been used to reduce the recombination probability of photo-
generated electrons and holes via the internal electric field and
thus lead to a high quantum yield.20 However, whether it is
valid to separate photogenerated charge carriers strongly
depends on the band-edge positions of the two semiconductors.
The conduction band edge of a semiconductor at the point of
zero charge can be calculated by the empirical equation EVB
=
X - Ee þ 0.5Eg,29 where EVB is the valence band-edge poten-
tial, X is theelectronegativity of thesemiconductor, expressed
as the geometric mean of the absolute electronegativity of the
constituent atoms, Ee is the energy of free electrons on the
hydrogen scale (about 4.5 eV), Eg is the band-gap energy
of the semiconductor, and ECB can be determined by ECB
=
Acknowledgment. The authors appreciate partial financial
support from NSFZJ (Grants Y4090118 and R4090137), the
ZJED Innovative Team, and NSFC (Grants 51002107 and
50772076).
EVB - Eg. The X values for BiVO4 and Bi2O3 are ca. 6.04 and
5.95 eV. The band-gap energies of BiVO4 and Bi2O3 adopt
2.43 and 2.85 eV, respectively. Given the equation above, the
top of the valence band and the bottom of the conduc-
tion band of BiVO4 and Bi2O3 are calculated to be 0.32 and
2.75 eV and 0.03 and 2.88 eV, respectively. According to the
above results, Scheme 2 was proposed to illustrate a possible
charge-separation process. As shown in Scheme 2, both BiVO4
and Bi2O3 could be easily excited by visible light (λ > 400 nm)
and induce the generation of electron and hole pairs. However,
the conduction band-edge potential of Bi2O3 (0.03 eV) is more
active than that of BiVO4 (0.32 eV). Therefore, photoinduced
Supporting Information Available: EDX spectroscopy re-
corded on an individual microsphere, FE-SEM images of the
products under different synthetic conditions, experimental part
for the synthesis and photocatalytic activity testing of Bi2O3 with
hierarchical architectures, FE-SEM image of the as-obtained
Bi2O3, UV-vis spectra of Bi2O3 hierarchical architectures sus-
pended in a RhB solution as a function of the irradiation time,
plots for RhB concentration changes over olive-shaped BiVO4,
core-shell BiVO4@Bi2O3 microspheres, and Bi2O3 hierarchical
architectures, and measured I-V curves for pure BiVO4 and
Bi2O3, respectively. This material is available free of charge via
the Internet at http://pubs.acs.org.
(29) (a) Bulter, M. A.; Ginley, D. S. J. Electrochem. Soc. 1978, 125, 228.
(b) Cheng, H. F.; Huang, B. B.; Dai, Y.; Qin, X. Y.; Zhang, X. Y. Langmuir 2010,
26, 6618.