Bi2MoO6 microstructures: Controllable synthesis, growth mechanism, and visible-light-driven photocatalytic activities

Article abstract of DOI:10.1039/c2ce26260f

The hierarchical flower-like Bi₂MoO₆ microstructures have been prepared via a facile solvothermal approach, employing ethylene glycol as the reaction medium in the presence of ethylenediamine. The flake-like building units of the hierarchical, flower-like Bi₂MoO₆ were constructed by many smaller spherical particles. Reaction temperature, reaction time and the volume of ethylenediamine were studied and found to play crucial roles in the formation of the Bi₂MoO₆ architectures. A possible mechanism in the formation of the microstructures was proposed based on the results of the morphology evolution as a function of reaction time. The hierarchical flower-like Bi₂MoO₆ were found to exhibit highly efficient visible-light driven photocatalytic activities for the degradation of rhodamine B (RhB). Comparisons of three Bi ₂MoO₆ microstructures with different morphologies suggest that the morphologies of the products have a great effect on the photocatalytic activities.

Full text of DOI:10.1039/c2ce26260f

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Bi2MoO6 microstructures: controllable synthesis,
growth mechanism, and visible-light-driven
photocatalytic activities
Cite this: CrystEngComm, 2013, 15,
498
Zhao-Qian Li,a Xue-Tai Chen*a and Zi-Ling Xueb
The hierarchical flower-like Bi2MoO6 microstructures have been prepared via a facile solvothermal
approach, employing ethylene glycol as the reaction medium in the presence of ethylenediamine. The
flake-like building units of the hierarchical, flower-like Bi2MoO6 were constructed by many smaller
spherical particles. Reaction temperature, reaction time and the volume of ethylenediamine were studied
and found to play crucial roles in the formation of the Bi2MoO6 architectures. A possible mechanism in the
formation of the microstructures was proposed based on the results of the morphology evolution as a
function of reaction time. The hierarchical flower-like Bi2MoO6 were found to exhibit highly efficient
visible-light driven photocatalytic activities for the degradation of rhodamine B (RhB). Comparisons of
three Bi2MoO6 microstructures with different morphologies suggest that the morphologies of the
products have a great effect on the photocatalytic activities.
Received 7th August 2012,
Accepted 4th November 2012
DOI: 10.1039/c2ce26260f
www.rsc.org/crystengcomm
spheres,15 cage-like Bi2MoO6 hollow spheres17 and Bi2MoO6
microtubes.25 Design and fabrication of new hierarchically
structured Bi2MoO6 is still a challenge. To the best of our
knowledge, there has been no report of a controllable assembly
of Bi2MoO6 nanoflakes into hierarchical flower-like microstruc-
tures.
In this work, hierarchical flower-like Bi2MoO6 microstruc-
tures were prepared by using ethylene glycol as the reaction
medium in the presence of ethylenediamine. Interestingly, the
flake-like building units of the hierarchical flower-like
Bi2MoO6 were constructed by many smaller spherical particles.
Control experiments showed that reaction temperature, reac-
tion time and the volume of ethylenediamine play crucial roles
in the formation of the Bi2MoO6 architectures. A possible
mechanism of their formation was proposed. Furthermore, the
hierarchical flower-like Bi2MoO6 microstructures exhibit
highly efficient visible-light driven photocatalytic activity for
the degradation of RhB.
1. Introduction
Semiconductor photocatalysis is
a potentially promising
approach to solve current environmental and energy issues.1–6
As a green chemical technology, photocatalysis utilizes solar
energy to decompose harmful organic and inorganic pollu-
tants present in air and aqueous systems7–9 or to split water to
supply clean and recyclable hydrogen energy.10–12 Bi2MoO6 is
an attractive Aurivillius oxide with excellent intrinsic physical
and chemical properties, including ion conduction, dielectric
properties, gas sensing, and catalysis.13–16 Furthermore, as a
n-type semiconductor with a direct narrow bandgap (y2.6 eV),
Bi2MoO6 is also a promising visible-light-driven photocatalyst
for water splitting and degradation of organic contami-
nants.13–19 As the photocatalytic activity is closely related to
the size, morphology and microstructure of the photocatalyst,
hierarchical structures with well-defined morphologies are
expected to optimize performance of a photocatalyst.8,20,21
The creation of hierarchically-structured Bi2MoO6 is thus highly
desirable for practical applications. In contrast to the
22–24
numerous reports of the W analog Bi2WO6
possessing a
2. Experimental section
similar Aurivillius layered structure, there have been only a few
reports of fabrications of well-defined hierarchically structured
Synthesis of flower-like Bi2MoO6 microstructures
13–15,17,25
Bi2MoO6
including hierarchical Bi2MoO6 hollow
All reagents were of analytical grade (Shanghai Chemical
Company) and used without any further purification. In a
typical procedure, a mixture of Bi(NO3)3?5H2O (1.0 mmol) and
Na2MoO4?2H2O (0.5 mmol) in 15 mL of ethylene glycol (EG)
was stirred for 2 h to form a homogeneous transparent
solution. Then, 0.3 ml of ethylenediamine (en) (99 wt%) was
added to form white precipitates immediately. After being
aState Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of
Microstructures, School of Chemistry and Chemical Engineering, Nanjing University,
Nanjing 210093, P. R. China. E-mail: xtchen@netra.nju.edu.cn;
Fax: +86-25-83314502.
bDepartment of Chemistry, The University of Tennessee, Knoxville, TN 37996-1600,
USA
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stirred for another 30 min, the mixture was transferred into a
40 mL Teflon-lined stainless autoclave. The autoclave was
subsequently sealed and heated at 160 uC for 6 h, and then
allowed to cool to room temperature. The product was
collected, washed with deionized water and absolute ethanol
several times, and dried at 80 uC under ambient air for 12 h.
Other experiments were conducted using similar procedures.
Photodegradation experiments
The photocatalytic degradation of rhodamine B (RhB) was
conducted in an XPA-photochemical reactor (Xujiang
Electromechanical Plant, Nanjing, China) equipped with a 400
W metal halide lamp and a cutoff filter (l . 420 nm). An electric
fan and cooling water were used to eliminate the thermal effect.
The photocatalyst (0.02 g) was introduced into a Pyrex reactor
containing 20 ml of RhB aqueous solution (1025 M) at room
temperature in the air. Before the light was turned on, the
solution was continuously stirred for 30 min in the dark to
ensure the establishment of an adsorption–desorption equili-
brium. Then the photocatalytic reaction was initiated. At given
time intervals, a portion of the suspension was removed from
the reactor, centrifuged, and filtered to remove the catalyst
particles before analysis. The RhB concentration was analyzed
through a UV-vis spectrophotometer (Shimadzu, UV-3600). In
order to determine the durability of the photocatalyst, the
photocatalyst was separated from aqueous solution after each
run. The filtered solid was reused in the subsequent recycling
experiment without any treatment.
Materials characterization
The phases of the products were characterized by X-ray powder
diffraction (XRD) with a Shimadzu XRD-6000 powder X-ray
diffractometer using Cu Ka radiation (l = 1.5418 Å), recorded
with 2h ranging from 10 to 70u. Selected area electron
diffraction (SAED) and transmission electron microscopy
(TEM) were carried out on a JEM-2100 high resolution
transmission microscope, employing an accelerating voltage
of 200 kV. Scanning electron microscopy (SEM) images and
energy dispersion X-ray (EDX) spectra of the products were
obtained on a field emission scanning electron microanalyser
(Hitachi S-4800), employing an accelerating voltage of 10 kV or
20 kV by depositing the products on a conductive adhesive tape.
X-ray photoelectron spectroscopy (XPS) was recorded on a PHI
5000 VersaProbe X-ray photoelectron spectrometer, using Al Ka
monochromatic X-ray radiation (1486.6 eV). The peak positions
were calibrated against the reference C 1s peak (285.0 eV) of
contaminating carbon. The UV-vis diffusion reflectance spectra
(DRS) of the samples were analyzed with a UV-vis spectro-
photometer (UV-3600, Shimadzu, Japan) and BaSO4 was used as
reference. Nitrogen adsorption–desorption measurements were
conducted at 77 K on a Micromeritics Tristar 3000 analyzer after
the sample was degassed at 473 K for 6 h. The Brunauer–
Emmett–Teller (BET) surface area was estimated by using
adsorption data in a relative pressure range from 0.05 to 0.3.
3. Results and discussions
Structure and morphology characterization
The crystalline phase of the product was analyzed by powder
X-ray diffraction (XRD). All the peaks in this pattern (Fig. 1a)
can be indexed as pure orthorhombic Bi2MoO6 (JCPDS file
card no. 21-0102, a = 5.506 Å, b = 16.226 Å and c = 5.487 Å). No
characteristic peaks were observed for the impurities.
Additionally, the broad peaks imply small crystalline sizes of
the product, and the crystallite diameter of the nanoparticles
Fig. 1 (a) XRD and (b) Bi 4f, (c) Mo 3d, (d) O 1s XPS patterns of the as-obtained flower-like microstructures prepared at 160 uC with 0.3 ml of en for 6 h.
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was calculated to be y12.3 nm by the Sherrer formula.26 To
investigate the composition of the product and the valence
states of the individual elements, the as-obtained product was
further investigated by XPS. The two peaks (Fig. 1b) at 157.9
uniform flower-like microstructures with a diameter of y1
mm. The high-magnification SEM image (Fig. 2b and 2c)
reveals that the flower-like microstructure is built from many
thin flakes with a thickness of y20 nm. These nanoflakes are
intercrossed with each other and aggregated together, which
was further revealed by TEM images (Fig. 2d and 2e). More
importantly, these flake-like building blocks are constructed
by many smaller spherical particles with a diameter of y15
nm (Fig. 2f), which is close to the value (y12.3 nm) calculated
by the Sherrer formula. This nanoflake morphology con-
structed by smaller spherical particles is different from the
reported structures.15 The high-resolution TEM (HRTEM)
image (Fig. 2f) of a single nanoflake reveals that the smaller
nanoparticles are single crystalline. The interplanar spacings
are 0.377 and 0.35 nm, which correspond to the (111) and
and 163.2 eV can be attributed to Bi 4f7/2 and 4f5/2
,
respectively.16 The two peaks at 231.1 and 234.3 eV (Fig. 1c)
can be assigned to the binding energy of Mo 3d.16 The peak
centered at 529.1 eV (Fig. 1d) corresponds to O 1s.27 The
quantitative analysis of the sample gave Bi, Mo, and O molar
ratio of 2.11 : 0.96 : 5.82, which is close to the stoichiometry in
Bi2MoO6.
The morphology of the sample was further characterized by
scanning electron microscopy (SEM) and transmission elec-
tron microscopy (TEM). Fig. 2a is a typical low-magnification
SEM image of the sample prepared with 0.3 ml of en at 160 uC
for 6 h. It shows that the product is composed of many
¯
(121) crystalline planes of orthorhombic Bi2MoO6 (JCPDS file
Fig. 2 (a–c) SEM, (d, e) TEM, (f) HRTEM, and (g) corresponding fast Fourier transformation (FFT) images of the as-obtained flower-like microstructures prepared at 160
uC with 0.3 ml of en for 6 h.
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card no. 21-0102), respectively. The corresponding fast Fourier
transformation (FFT) image of the product further proves the
single-crystalline nature of these nanoparticles (Fig. 2g).
with the mediation of en. Without the addition of en, the
product was composed of sphere-like hollow microstructures
(Fig. 3a and 3e), similar to those reported by Fu and
coworkers.15 When 0.1 ml (1.49 mmol) of en was added, the
product was Bi2MoO6 nanoflakes (Fig. 3b and 3e). Increasing
the volume of en to 0.2 ml (2.98 mmol), intercrossed Bi2MoO6
microstructures appeared (Fig. 3c and 3e). Further increasing
the volume of en to 0.3 ml (4.47 mmol), well-defined
hierarchical flower-like Bi2MoO6 microstructures were
obtained (Fig. 2a and 1a). If the volume of en is further
increased to 0.4 ml (5.96 mmol), only layered pancake-like
microstructures were obtained and hierarchical flower-like
microstructures can no longer be formed (Fig. 3d). XRD
investigation further confirmed that the product was a mixture
of Bi2MoO6 (JCPDS file card no. 21-0102) and d-Bi2O3 (JCPDS
file card no. 77-2008) (Fig. 3e). The formation of Bi2O3 with a
high content of en can be ascribed to the base effect of en
molecule.
Factors influencing the formation of the flower-like Bi2MoO6
microstructures
In order to get an insight into the formation of flower-like
Bi2MoO6 microstructures, a series of controlled experiments
have been carefully performed whereby one reaction para-
meter (volume of ethylenediamine (en), reaction time, and
temperature) was changed while the other reaction conditions
were kept constant.
EFFECT OF ETHYLENEDIAMINE (EN). The volume of en was
examined to check the effect on the synthesis. En is a common
solvent which could act as strong base, bidentate ligand with
coordination and structure-directing reagent in the formation
of micro/nanomaterials with complex morphology.28–31 In this
work, flower-like Bi2MoO6 microstructures were synthesized
Fig. 3 SEM images of the as-prepared products at 160 uC for 6 h with different volumes of en: (a) 0 ml, (b) 0.1 ml, (c) 0.2 ml and (d) 0.4 ml; (e) their XRD patterns.
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In order to get an insight into the role of en molecules, we
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Based on these observations, we believe that the existence of
en played multiple roles in the formation of the current
hierarchical flower-like microstructures. Although the exact
roles of en in the formation of hierarchical structures of
Bi2MoO6 are still not completely understood, the en molecule
might play at least two roles. Firstly, as a bidentate ligand, en
molecules can cap the facets of the crystals and then mediate
the formation of hierarchical structures through coordination
and H-bonding interactions.31–33 In this respect, the en
molecule could coordinate with Bi or Mo, or interact with O
atom by hydrogen bonding. Besides, one of the –NH2 groups of
en molecule could also extend out and thus attach onto other
growing nanocrystals and further facilitate formation of the
hierarchical microstructures.
conducted a series of control experiments using other amines,
such as butylamine or 1,3-propanediamine instead of en while
keeping other experimental parameters constant. As shown in
Fig. 4, when 2.98 or 5.96 mmol of butylamine was used, only
nanoparticles (Fig. 4a) or nanoflakes (Fig. 4b) were obtained.
The formation of Bi2O3 was found with 5.96 mmol of
butylamine (Fig. 4e). When 1,3-propanediamine was used,
similar hierarchical microstructures could be fabricated
(Fig. 4c and 4d). Furthermore, the amount of 1,3-propanedia-
mine has a great effect on the crystal structure of the final
products. When 5.96 mmol of 1,3-propanediamine was used,
d-Bi2O3 (JCPDS file card no. 77-2008) and Bi2MoO6 (JCPDS file
card no. 21-0102) coexisted in the as-obtained product
(Fig. 4e). These results clearly show that a diamine such as
en and 1,3-propanediamine can really facilitate the formation
of hierarchical structures while monoamine molecule cannot.
EFFECT OF THE REACTION TEMPERATURE. The reaction temperature
is another key parameter affecting the morphology of samples
prepared by hydrothermal processes. Before heat-treatment,
Fig. 4 SEM images of the products obtained at 160 uC for 6 h with different amine sources: (a) 2.98 mmol of butylamine, (b) 5.96 mmol of butylamine, (c) 2.98 mmol
of 1,3-propanediamine, and (d) 5.96 mmol of 1,3-propanediamine; (e) their XRD patterns.
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Fig. 5 (a) SEM image and (b) XRD pattern of the sample obtained from the reaction mixture before heat-treatment.
the reaction mixture was centrifuged and the solid product
was collected. The SEM image and XRD pattern indicates that
the product was composed of Bi2O3 nanoparticles (Fig. 5a and
5b). The formation of d-Bi2O3 at room temperature can be
ascribed to the base effect of en. When the reaction
temperature was controlled at a temperature of 120 uC, the
nanoparticles were the main product (Fig. 6a). The XRD
pattern confirms that these particles are a mixture of d-Bi2O3
(JCPDS file card no. 77-2008) and Bi2O2CO3 (JCPDS file card
no. 84-1752) (Fig. 6d). When the temperature was controlled at
140 uC, intercrossed flower-like microstructures appeared
(Fig. 6b). Apart from the main Bi2MoO6 phase, a weak
Bi2O2CO3 peak also existed in the final product (Fig. 6e).
Further increasing the temperature to 160 uC, the well-defined
hierarchical flower-like Bi2MoO6 was obtained (Fig. 2a and 1a).
When the reaction temperature was kept at 180 uC, the
intercrossed flower-like Bi2MoO6 microstructures, which are
similar to the products obtained at 140 uC, were prepared
(Fig. 6c and 6f). Although the morphology is similar to the
products prepared at 140 uC, the composed flakes are thicker
and the sizes of the microstructures are larger. These above
experimental results indicate that a suitable reaction tempera-
ture is important for the formation of the flower-like
microstructures of Bi2MoO6. At lower temperature (120 uC),
Bi2O3 was easily obtained. There have been some reports of
preparation of Bi2O3 at low temperature. For example, Li
et al.34 synthesized Bi2O3 nanostructures by a citrate-assisted
solution approach at 40 uC. Wang and coworkers35 prepared
Bi2O3 hierarchical nanostructures at 60–80 uC. The existence of
Bi2O2CO3 impurity in some products might be ascribed to the
decomposition of ethylene glycol at high temperature.
Actually, the existence of Bi2O2CO3 impurity in the preparation
of some Bi2MoO6 samples was also found by Fu and
coworkers.15 It is not clear why the Bi2O2CO3 impurity was
produced in these reactions. Ethylene glycol was the only
carbon source in their synthetic approach. Therefore, it is
Bi2WO6 hollow microspheres based on a precipitation trans-
formation reaction using BiOBr as starting material at 160 uC
for 24–48 h. Therefore, we consider that Bi2O2CO3 might
dissolve at a higher temperature and produce the (Bi2O2)2+
monomer or layer-like oligomer, which can react with MoO4
anion to produce the final pure Bi2MoO6.
22
TIME-DEPENDENT EXPERIMENTS AND THE MECHANISM OF FORMATION.
Time-dependent experiments were carried out to understand
the formation of the flower-like microstructures. As mentioned
above, the product was comprised of Bi2O3 nanoparticles
before heat-treatment. After reaction for 1 h, the product was
composed of spherical nanoparticles (Fig. 7a). The main phase
was confirmed as d-Bi2O3 (JCPDS file card no. 77-2008, Fig. 7e)
by XRD. In addition, some weak peaks of Bi2O2CO3 (JCPDS file
card no. 84-1752) could also be found in the XRD pattern
(Fig. 7e). Increasing the reaction time to 2 h, the nanoflakes
were formed (Fig. 7b). When the reaction time was prolonged
to 4 h, the intercrossed microstructures were the dominant
product (Fig. 7c). The main phase of the product obtained at 2
or 4 h was Bi2MoO6 as well as a weak peak of Bi2O2CO3 (Fig. 7f
and 7g). The microstructures then gradually ripen as shown in
Fig. 2a and 7d. After 6 h of the solvothermal growth, well-
defined flower-like Bi2MoO6 microstructures were obtained
(Fig. 2a and 1a). Prolonging the reaction time to 8 h preserved
the morphology (Fig. 7d and 7h).
The studies lead to the following proposed growth mechan-
ism. In the early stage, nanoparticles rapidly formed in the
presence of the base (en). Then a well-known dissolution and
reprecipitation process may occur. Such a process has already
been demonstrated in the formation of micro/nanostruc-
tures.39–42 For example, Zhang et al.42 proposed a general
nonaqueous route for the synthesis of phase-pure InNbO4
nanocrystals based on the one-pot solvothermal reaction of
niobium chloride with indium acetylacetonate in benzyl
alcohol. They believe that amorphous niobium oxide particles
were initially produced in the reaction system. With the
prolonged reaction time, these particles gradually dissolve in
benzyl alcohol to form niobium hydroxyl species, which
instantly react with In(OH)3 on the surface to produce
InNbO4 nuclei. Further growth was then promoted by the
higher thermodynamic stability of InNbO4. In the current
synthesis, Bi2O3 nanoparticles gradually dissolved at a high
22
reasonable to speculate that the formation of CO3 anions
originate from the decomposition of ethylene glycol, which
can also explain the similar phenomenon in our experimental
results. It is well known that both Bi2O2CO3 and Bi2MoO6 are
2n+
22
constructed by alternating (Bi2O2)n
layers and CO3
or
22 36,37
MoO4
.
Recently, Huang et al.38 have reported that
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Fig. 6 SEM images and XRD patterns of the as-prepared products for 6 h at different temperature: (a, d) 120 uC, (b, e) 140 uC, (c, f) 180 uC.
reaction temperature, and react with Na2MoO4 to give
Bi2MoO6 nanoparticles. These nanoparticles then self-assem-
ble to form Bi2MoO6 nanoflakes. The Bi2O2CO3 impurity may
also be dissolved and transformed to Bi2MoO6 in the process.
Hierarchical Bi2MoO6 microstructures finally appeared as a
result of the existence of en. In the growth process, en plays an
important role in the mediation of nanoflakes to the
hierarchical microstructures.
effect. The room temperature UV-vis absorption spectra of
samples A, B and C are shown in Fig. 8a. The samples
exhibited an intense absorption in the visible range, suggest-
ing that they could be photoactive under visible-light irradia-
tion. Additionally, in contrast to A, the intrinsic absorption
edges of B and C show obvious red-shifts and the difference
may be the result of the morphology. Also, the BET surface
areas of A, B and C are 58.4, 31.2 and 15.6 m2 g21, respectively,
from the N2 adsorption–desorption measurements (Fig. 8b). It
can be clearly seen that sample A has a larger BET surface area
than samples B and C. From the XRD patterns of Fig. 3e (0 ml
en) and Fig. 6f, the crystallite sizes of samples B and C were
calculated to be y11.3 nm and y28.7 nm, respectively, by the
Sherrer formula. Although the crystallite size of B is similar to
that A (y12.3 nm), the larger BET surface area of A might be
induced by the intercrossed hierarchical structures. The
results here suggest that the morphology and the crystallite
size have an effect on the physical properties of the micro/
nanomaterials.
Optical properties, BET measurements, and photocatalytic
performances
In this section, for the convenience of description, the samples
synthesized at 160 uC for 6 h with 0.3 ml of en (Fig. 2a), 160 uC
for 6 h with 0 ml of en (Fig. 3a) and 180 uC for 6 h with 0.3 ml
of en (Fig. 6c) were denoted as A, B and C, respectively. It is
well known that the morphology of the products could affect
the physical and chemical properties.8,43 Thus we studied the
optical and photocatalytic properties of A, B, and C with
different morphologies to have a better understanding of the
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Fig. 7 SEM images and XRD patterns of the as-prepared products at 160 uC for different reaction times: (a, e) 1 h, (b, f) 2 h, (c, g) 4 h and (d, h) 8 h.
Visible-light photocatalytic activities of A, B, and C were
studied by monitoring the change in optical absorption of the
RhB solution at y554 nm during its photocatalytic decom-
position process. All three photocatalysts showed photocata-
lytic activities under visible-light irradiation (Fig. 8c and 8d).
More importantly, RhB itself (Fig. 8c, D) was not decomposed
in the absence of the catalyst. A completely degraded RhB in
20 min under visible light irradiation and showed higher
activity than B and C. The significant enhancement of
photocatalytic activities of
A might be ascribed to: (i)
intercrossed hierarchical microstructures in A, which promote
the interfacial electron transfer process and facilitate catalytic
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Fig. 8 (a) Ultraviolet-visible diffusive absorption spectra of samples: A (prepared at 160 uC for 6 h with 0.3 ml of en); B (prepared at 160 uC for 6 h with 0 ml of en); C
(prepared at 180 uC for 6 h with 0.3 ml of en); D RhB photocatalysis. (b) Typical N2 adsorption–desorption isotherm of A, B, and C. (c) Variation of RhB concentration
as a function of irradiation time (with the time of light on set as 0) under visible-light (l . 420 nm) with A, B and C. (d) The absorption variation of RhB dye over A, B
and C. (e) Cycling measurements of RhB photodegradation with A after 20 min visible-light irradiation.
reactions in which electrons are involved.8 (ii) Its higher BET
surface area. The increase of the surface area can provide more
active catalytic sites.15 (iii) A broader bandgap in A (Fig. 8a)
than in B and C, leading to a lower valence band (VB) or higher
conduction band (CB) in A, enhancing its reductive or
oxidative properties.8
The experimental results and analysis show that morphol-
ogy-controlled synthesis of Bi2MoO6 crystals is a feasible
strategy to enhance their photocatalytic activity. The crystallite
size also plays an important role in the photocatalytic
properties. For example, A and B have a smaller crystallite
obviously changed (Fig. 9a), XRD analysis of the solid
materials remaining after 7 photocatalytic cycles showed a
structural change (Fig. 9b and 9c). By comparison with
authentic data, Bi2O2CO3 and Bi2MoO6 coexisted in the
remaining solid materials (Fig. 9c). Formation of carbonate
2+
is known to occur in the case of Bi2O2 upon stirring in
aqueous solution in the presence of CO2 or carbonate.44,45
Kisch et al.45 have used a-Bi2O3 as photocatalyst for decom-
position of organic pollutants. They found that
(BiO)4CO3(OH)2 and (BiO)2CO3 was produced after ten reaction
cycles and the formation of carbonates decrease the photo-
catalytic activity. In this current work, carbon dioxide was
generated as the final oxidation product from the photo-
catalytic decomposition of RhB,46 which lead to the formation
of (BiO)2CO3 after 7 cycles of photocatalytic reactions. It is
known that Bi2O2CO3 has a wider bandgap and has no visible-
light driven photocatalytic activity.36 Therefore, the reason for
the photocatalytic activity drop after 7 cycles might be the
partial transformation from Bi2MoO6 to Bi2O2CO3.
size but
a larger BET surface area than C, and they
demonstrated higher photocatalytic activities (Fig. 8b–8d).
Thus, for C, the main reason for the lower photocatalytic
activity might be its larger crystallite size.
To test the photocatalytic stability, A was used for 10 cycles
in the decomposition of RhB. From Fig. 8e, it can be
concluded that the photocatalytic activity of A decreased
slowly after 7 cycles. Although the morphology of A has not
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Fig. 9 (a) SEM image of the sample A (prepared at prepared at 160 uC for 6 h with 0.3 ml of en) after 7 photocatalytic reaction cycles and corresponding XRD patterns
before (b) and after (c) 7 photocatalytic reaction cycles.
8 H. Tong, S. X. Ouyang, Y. P. Bi, N. Umezawa, M. Oshikiri
and J. H. Ye, Adv. Mater., 2012, 24, 229–251.
4. Conclusions
Hierarchical flower-like Bi2MoO6 microstructures have been
fabricated by using ethylene glycol as the reaction medium in
the presence of ethylenediamine. The formation mechanism
and the effects of reaction conditions, such as reaction
temperature, reaction time and the volume of ethylenedia-
mine, were also investigated. More importantly, hierarchical
flower-like Bi2MoO6 with a larger BET surface area, a larger
bandgap, and better intercrossed hierarchical microstructures
are good photocatalysts for the degradation of RhB by visible
light. This work here shows that morphology-controlled
synthesis of Bi2MoO6 crystals is a very feasible strategy for
enhancing their photocatalytic activity.
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Acknowledgements
This work was supported by the National Basic Research
Program of China (No. 2013CB922102), the Natural Science
Grant of China (No. 21021062) and the US National Science
Foundation (CHE-1012173).
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