BiVO4/Bi2O3 submicrometer sphere composite: Microstructure and photocatalytic activity under visible-light irradiation

Article abstract of DOI:10.1016/j.jallcom.2008.09.083

Bismuth vanadate (BiVO₄) powders were synthesized by homogeneous precipitation method with different surface dispersants. Bi(NO₃)₃·5H₂O and NH₄VO₃ were utilized as starting materials to syn

Full text of DOI:10.1016/j.jallcom.2008.09.083

Journal of Alloys and Compounds 476 (2009) 624–628
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
BiVO4/Bi2O3 submicrometer sphere composite: Microstructure and
photocatalytic activity under visible-light irradiation
Lingzhi Li, Bing Yan∗
Department of Chemistry, Tongji University, Siping Road 1239, Shanghai 200092, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 July 2008
Received in revised form 5 September 2008
Accepted 12 September 2008
Available online 1 November 2008
Bismuth vanadate (BiVO4) powders were synthesized by homogeneous precipitation method with dif-
ferent surface dispersants. Bi(NO3)3·5H2O and NH4VO3 were utilized as starting materials to synthesize
BiVO4 particles with the edge length of about 100–150 nm. Bismuth oxide (Bi2O3) was prepared by decom-
posing Bi(NO3)3·5H2O at 600 ◦C. Techniques of X-ray diffraction (XRD), scanning electron microscope
(SEM), transmission electron microscope (TEM) and ultraviolet–visible diffuse reflectance spectra (UV–vis
DRS) have been employed to characterize the as-synthesized materials. The results showed that the struc-
ture and morphology of BiVO4 were various using different surface dispersant. We have also investigated
the photocatalytic degradation of rhodamin B which showed that BiVO4/Bi2O3 possessed a high photo-
catalytic activity under visible-light irradiation. The photocatalytic activity is in close relation with the
surface dispersant about BiVO4 synthesis. The result was better using N,N-dimethylacetamide (DMA) than
using N-methyl-2-pyrrolidone. The inverse proportion between BiVO4 and Bi2O3 was also important for
the photocatalysis.
Keywords:
Chemical synthesis
Light absorption and reflection
Photocatalysis
Bismuth vanadate
Bismuth oxide
Rhodamin B
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Bi2O3 is a significant metal-oxide semiconductor with a direct
band gap 2.8 eV. It can be excited by visible-light. Bessekhouad
The oxide semiconductor photocatalysis has been broadly inves-
tigated in order to solve existing environmental problems such as
the destruction of inorganic and organic pollutants [1–7] since TiO2
was found to be an effectual photocatalyst [8,9]. TiO2 has various
valuable properties including greatly oxidative, chemically stable,
economical, and non-toxic characteristics. However, it has pho-
toactivity only under ultraviolet (UV) light irradiation (ꢀ < 380 nm)
because of its large band gap energy of 3.2 eV. Doping TiO2 with
non-metal or metal atoms such as N, S, P, F, B, [10–13] V, Cr, Mn,
Fe, Co, Ni and Cu [14,15] were frequently studied recently in order
to transfer the optical absorption of TiO2 from UV to the visible-
light. A great number of novel semiconductor photocatalysts have
been carried out for the development of visible-light sensitive pho-
tocatalysis by using a narrow band gap semiconductor such as CdS
[16], WO3 [17,18], Bi2S3 [19], SnO2 [20], Bi2O3 [21], CaBi2O4 [22],
BiVO4 [23], PbBi2Nb2O9 [24], Bi2WO6 [25], CaIn2O4 [26] etc. in
the recent years. The narrower band gap facilitates excitation of
an electron from the valence band to the conduction band, which
improves the photocatalysts activities. These findings impulse a
new beginning for the design of non-titania based visible-light-
driven semiconductor photocatalyts.
et al. [21] applied Bi2O3 semiconductor to degrade orange II
pollutant in water under visible-light, and BiVO4 is another non-
titania based visible-light-driven semiconductor photocatalyts.
It has attracted extensively interests for water decomposition
and organic photocatalytic degradation due to its narrow band
gap. There are three polymorphisms of BiVO4 which are mon-
oclinic phase, tetragonal phase, and orthorhombic phase; only
monoclinic phase exhibits visible-light photocatalytic properties
[27–29]. However, the photoactivity of pure BiVO4 is low due
to its poor absorption and difficult migration of photogenerated
electron–hole pairs. Moreover, the rapid recombination that occurs
in relation to photoproduced electrons and holes in BiVO4 signif-
icantly diminish the photocatalytic reaction efficiency. We know
that loading a small amount of noble metals or oxidized metals
can greatly improve the photocatalyst activities. Kohtani et al. [30]
reported that the Ag/BiVO4 showed superior visible-light activi-
ties in photocatalytic oxidation of polycyclic aromatic hydrocarbons
and 4-n-alkylphenols. Long et al. [31] found that Co3O4/BiVO4
composite exhibited higher photocatalytic activities with phenol
degradation than the pure BiVO4.
In our study, pure BiVO4, Bi2O3 and BiVO4/Bi2O3 photocata-
lysts were applied to photooxidative degradation of RhB under
visible-light irradiation, respectively. We prepared BiVO4/Bi2O3
heterojunctions by direct mixture of two semiconductors. Sev-
eral techniques were used to synthesize BiVO4, such as
∗
Corresponding author. Tel.: +86 21 65984663; fax: +86 21 65982287.
E-mail address: byan@tongji.edu.cn (B. Yan).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.09.083

L. Li, B. Yan / Journal of Alloys and Compounds 476 (2009) 624–628
625
high-temperature processes [32], sol–gel [33], homogeneous pre-
cipitation [34], hydrothermal [35], and so on. BiVO4 have been
synthesized by a simple homogeneous precipitation method in
our research. Comparing with other synthesis methods, homoge-
neous precipitation method was easier to reach tetragonal phase of
BiVO4. In addition, tetragonal phase has poor photocatalytic activ-
ity under visible-light irradiation. In order to increase its absorptive
performance and conquer its difficult migration of photogenerated
electron–hole pairs, to promote electron–hole separation and effi-
ciency of photocatalytic reaction by mixing Bi2O3. Therefore, we
could obtain superior photocatalytic activity for the decolorization
of RhB under visible-light irradiation.
2. Experimental
2.1. Materials
Bi(NO3)3·5H2O, NH4VO3, rhodamin B (RhB), absolute alcohol, distilled water,
N,N-dimethylacetamide (DMA) and N-methyl-2-pyrrolidone. All chemicals (Shang-
hai Chemical Reagents Company, China) were of analytic grade and used without
treating further for purification.
2.2. Preparation of BiVO4 and Bi2O3
BiVO4 nanoparticles were prepared by
a simple homogeneous precipita-
tion method with different surface dispersant. We mixed aqueous solutions of
Bi(NO3)3·5H2O and NH4VO3 together with the Bi/V molar ratio of 1:1, and added the
mixed compounds into DMA and N-methyl-2-pyrrolidone, respectively. The mixture
was stirred for 4 h at room temperature in air. The resultant precipitates were filtered
and washed with distilled water and absolute alcohol for three times separately in
order to remove inorganic ions and organic surface dispersant possibly remaining
in the final products, then dried at 60 ◦C in air for several hours. Bi2O3 was prepared
by heating Bi(NO3)3·5H2O at 600 ◦C for 6 h and then air cooled to room temperature.
2.3. Characterization
All samples were characterized by X-ray powder diffraction (XRD). The XRD
measurements were performed on
a Bruke/D8-Advance with CuKa radiation
(ꢀ = 1.518 Å). The operation voltage and current were maintained at 40 kV and 40 mA,
respectively. A scan rate of 10◦/min was applied to record the patterns in the 2ꢁ
range 10–70◦. Transmission electron microscopic images were obtained on a JEOL-
2010 microscope with an accelerating voltage of 200 kV. Sample grids were prepared
by ultrasonic powered samples in distilled water for about half an hour and then
dripped the suspension onto a carbon-coated, holey film supported on a copper
grid for TEM. In addition, the morphology and microstructure were characterized
with scanning electronic microscope (SEM, Philps XL-30). Ultraviolet–visible diffuse
reflectance spectra (UV–vis DRS) of the samples were measured by using a spec-
trophotometer (Bws003). BaSO4 was used as a reference standard, and the spectra
were recorded in the range 200–800 nm.
Fig. 2. SEM of BiVO4 (using N-methyl-2-pyrrolidone as surface dispersant) (A);
BiVO4 (using DMA as surface dispersant) (B); Bi2O3 (C).
2.4. Photocatalytic experiments
The photocatalytic activities of the synthesized powders were evaluated by the
degradation of 10 mg/l RhB in pure BiVO4, Bi2O3 and BiVO4/Bi2O3 mixture aqueous
suspension under visible-light irradiation, respectively. The light source for pho-
tocatalysis was a spherical tungsten halogen lamp (Philps 500 W, ꢀ > 380 nm). The
volume of initial RhB solution is 300 ml. All powders contents in the RhB aqueous
solution are 0.12 g/100 ml with different quality ratio of BiVO4 and Bi2O3. Prior to
irradiation, the mixture was sonicated for an hour to ensure adsorption/desorption
equilibrium at room temperature, then the suspension was magnetically during
irradiation. At regular intervals, samples were withdrawn and centrifuged to sepa-
rate solid particles for analysis. The concentration of aqueous RhB was determined
using a UV–vis spectrophotometer (Shanghai UV–722) at 554 nm by measuring its
absorbance. The RhB degradation was calculated by Lambert–Beer equation. Pho-
toactivities for RhB in the dark in the presence of the photocatalyst and under
visible-light irradiation in the absence of the photocatalyst were also evaluated.
Fig. 1. The XRD patterns of BiVO4 (using N-methyl-2-pyrrolidone as surface disper-
sant) (A); BiVO4 (using DMA as surface dispersant) (B); Bi2O3 (C).

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L. Li, B. Yan / Journal of Alloys and Compounds 476 (2009) 624–628
Fig. 3. TEM of BiVO4 (using N-methyl-2-pyrrolidone as surface dispersant) (A); BiVO4 (using DMA as surface dispersant) (B); Bi2O3 (C).
3. Results and discussion
Scherrer’s equation. D = kꢀ/h(l/2)cos ꢁ, where D is the average size
of the crystallites, k is the Scherrer constant, ꢀ is the wavelength of
radiation, h(1/2) is the peak width at half-height and ꢁ corresponds
to the peak position. By fitting various peaks of the profiles to this
formula, we obtained values in the range of 100–150 nanometer
for the particle diameter. The X-ray diffraction peaks in Fig. 1(C)
accorded with the monoclinic phase of Bi2O3 (JCPDF NO. 41-1449,
space group: P21/C, unit cell parameters: a = 5.850 Å, b = 8.170 Å,
c = 7.512 Å and ˇ = 112.98◦).
The morphologies of synthesized materials were ascertained
using scanning electron microscope (SEM) and transmission elec-
tron microscope (TEM). Figs. 2 and 3 showed the morphology
of BiVO4 using N-methyl-2-pyrrolidone as surface dispersant (A);
BiVO4 using DMA as surface dispersant (B) and Bi2O3 (C). It can
be observed clearly that the BiVO4 mainly consist of smaller
To study the phase structures of BiVO4 and Bi2O3 powders,
their X-ray diffraction patterns were measured. Fig. 1 presents the
XRD patterns of the prepared Bi2O3 and BiVO4 powders. The XRD
analysis of the samples showed that all the products were well
crystallized. BiVO4 nanoparticles have been synthesized using N-
methyl-2-pyrrolidone and DMA as surface dispersant, respectively.
From Fig. 1(AandB), we canseethat theobserveddiffraction linesof
the BiVO4 nanoparticles with different surface dispersant showed
similar patterns. The peaks of the resulting BiVO4 powders synthe-
sized at room temperature can be indexed to tetragonal system of
BiVO4 (JCPDS No. 14-0133, space group: I41/amd, unit cell param-
eters: a = 7.300 Å, c = 6.457 Å). And mean size of the crystallite can
be roughly estimated from the broadening of the peaks using the

L. Li, B. Yan / Journal of Alloys and Compounds 476 (2009) 624–628
627
Fig. 4. UV–vis diffuse reflectance spectra of BiVO4 (using N-methyl-2-pyrrolidone
as surface dispersant) (A); BiVO4 (using DMA as surface dispersant) (B); Bi2O3 (C).
Fig. 6. Photocatalytic degradation of RhB using Bi2O3 sensitized BiVO4 (using N-
methyl-2-pyrrolidone as surface dispersant): BiVO4 (A1), Bi2O3 (25 wt. %)/BiVO4
(A2), Bi2O3 (50 wt. %)/BiVO4 (A3), Bi2O3 (75 wt. %)/BiVO4 (A4), Bi2O3 (C), in the dark
(D), the leaching of Bi2O3 (50 wt. %)/BiVO4 (R).
granules, which exist less conglomeration compared with the
Bi2O3. The morphology of BiVO4 was uniform using DMA as sur-
face dispersant than N-methyl-2-pyrrolidone. From this result, we
can conclude that the surface dispersant has great influence on the
structure of the BiVO4, and it is possible to improve photocatalytic
activity through structure design, so we can try to select the right
surface dispersant to obtain homogeneous morphology.
RhB concentration (C/C0) with reaction time over different cata-
lysts are presented in Figs. 6 and 7. We could find that the RhB did
not degrade in the presence of the photocatalyst without visible-
light illumination (Fig. 6D) or in the absence of the photocatalyst
under visible-light irradiation (Fig. 7E). Indeed, we observed that
efficiency of the RhB degradation increased by mixing Bi2O3 and
BiVO4 compared with the pure BiVO4 and Bi2O3. When DMA was
used as surface dispersant, a remarkable performance was shown
by Bi2O3 (50 wt.%)/BiVO4 (Fig. 7B3), and RhB was degraded com-
pletely after 120 min of irradiation. Tetragonal system of BiVO4 had
poor photocatalytic activity under visible-light irradiation because
of its crystal lattice, but the mixture of BiVO4 and Bi2O3 was a kind
of excellent photocatalyst.
The corresponding UV–vis diffuse reflectance spectra for the
as-prepared products are provided in Fig. 4. All the products
exhibit absorption in the visible region; we have measured the
absorption onsets of the samples by linear extrapolation from the
infection point of the curve to the baseline and reached a quan-
titative estimate of the band gap energies. The calculated band
gap between the valence band (VB) and the conduction band (CB)
of the BiVO4 (A), BiVO4 (B) and Bi2O3 (C) can be estimated from
ꢀg(nm) = 1240/Eg(eV) where ꢀg and Eg are absorption band thresh-
old wavelength and band gap. The absorption onsets of the samples
have been measured by linear extrapolation from the infection
point of the curve to the baseline, and quantitative estimates of the
band gap energies have been reached. The absorption band thresh-
old of the BiVO4 (A), BiVO4 (B) and Bi2O3 (C) were approximately
481, 489 and 451 nm, the band gaps of them were estimated to be
2.58, 2.53 and 2.75 eV, respectively. The difference may be due to
the experimental error of UV–vis diffuse reflectance spectra.
Moreover BiVO4 using DMA as surface dispersant showed the
strongest absorption.
We have collected the leaching of Bi2O3 (50 wt.%)/BiVO4 and
evaluated the photocatalytic activities of them. The degradation
of the RhB under visible-light illumination was obvious. We con-
cluded that the reproducibility of photocatalytic activity was good
(Fig. 7R).
RhB (Fig. 5) is famous for its good stability as dye materials. The
photocatalytic activities of the as-prepared samples were evalu-
ated by measuring the degradation of the RhB under visible-light
illumination. We examined its concentration variations in maximal
absorption in UV–vis spectra at 554 nm. The results of variations in
Fig. 7. Photocatalytic degradation of RhB using Bi2O3 sensitized BiVO4 (using
DMA as surface dispersant): BiVO4 (B1), Bi2O3 (25 wt. %)/BiVO4 (B2), Bi2O3 (50 wt.
%)/BiVO4 (B3), Bi2O3 (75 wt. %)/BiVO4 (B4), Bi2O3 (C); without photocatalyst under
visible-light (E).
Fig. 5. Structure of rhodamin B (RhB).

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