Enhanced visible light photocatalytic degradation of Rhodamine B by Bi/Bi2MoO6 hollow microsphere composites

Article abstract of DOI:10.1039/c4ra09830g

Bi/Bi₂MoO₆ hollow microsphere (BMO-HMS) composites were successfully synthesized via a microwave-assisted reaction of a Bi₂MoO₆ precursor in an ethylene glycol solution using a microwave synthesis system and subsequent annealing in a nitrogen atmosphere. The morphology, structure and photocatalytic performance of the composites in the degradation of Rhodamine B (RhB) were characterized by scanning electron microscopy, X-ray diffraction, electrochemical impedance spectra, UV-vis absorption spectroscopy, Raman spectroscopy and nitrogen adsorption-desorption, respectively. The results show that the BMO-HMS composites exhibit enhanced photocatalytic performance in the degradation of RhB with a maximum degradation rate of 91% under visible light irradiation compared with the pure Bi₂MoO₆. The improved photocatalytic performance is ascribed to the enhanced light absorption and the reduced electron-hole pair recombination with the presence of Bi in the composites.

Full text of DOI:10.1039/c4ra09830g

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Enhanced visible light photocatalytic degradation
of Rhodamine B by Bi/Bi MoO hollow microsphere
2
6
Cite this: RSC Adv., 2014, 4, 62387
composites†
a
b
a
a
a
Jinliang Li, Xinjuan Liu,* Likun Pan,* Wei Qin and Zhuo Sun
Bi/Bi
2
MoO
6
hollow microsphere (BMO-HMS) composites were successfully synthesized via a microwave-
MoO precursor in an ethylene glycol solution using a microwave synthesis system
assisted reaction of a Bi
2
6
and subsequent annealing in a nitrogen atmosphere. The morphology, structure and photocatalytic
performance of the composites in the degradation of Rhodamine B (RhB) were characterized by
scanning electron microscopy, X-ray diffraction, electrochemical impedance spectra, UV-vis absorption
spectroscopy, Raman spectroscopy and nitrogen adsorption–desorption, respectively. The results show
that the BMO-HMS composites exhibit enhanced photocatalytic performance in the degradation of RhB
Received 5th September 2014
Accepted 12th November 2014
2 6
with a maximum degradation rate of 91% under visible light irradiation compared with the pure Bi MoO .
DOI: 10.1039/c4ra09830g
The improved photocatalytic performance is ascribed to the enhanced light absorption and the reduced
www.rsc.org/advances
electron–hole pair recombination with the presence of Bi in the composites.
showed different photocatalytic performances in the degradation
of organic pollutants under visible light irradiation. Among these
1
. Introduction
Over the past few decades, environmental pollution and energy structures, hollow microsphere (HMS) have attracted great
1,2
shortages have become the focus of world attention. As one of interest due to its high surface area, low density and effective
the most promising solutions for these issues, semiconductor light-harvesting property for wide application in supercapacitors,
24–28
29
photocatalysis has attracted much attention, because it is a lithium ion batteries and photocatalysis.
Miao et al.
green technology for the degradation of organic pollutants, synthesized Bi MoO HMS by aerosol-spraying and found that it
2
6
reduction of heavy metal ions, air purication and hydrogen exhibited high photocatalytic activity in the degradation of
3
–10
23
production.
TiO only exhibit high photocatalytic activity under ultraviolet and Yin et al. prepared Bi
However, the traditional photocatalysts such as Rhodamine B (RhB) under visible light irradiation. Tian et al.
30
MoO
6
HMS using solvothermal and
2
2
11–13
light irradiation, which limits their practical applications.
To better utilize solar light which is composed of about 45% catalytic performance in the degradation of RhB and phenol
visible light, it is desirable to exploit novel visible light-sensitive under visible light irradiation than solid Bi MoO . However, the
semiconductor photocatalysts.
quick recombination of photo-generated charge carriers still
Bi MoO
, as an aurivillius oxide semiconductor with a narrow exists in Bi MoO HMS, which decreases its photocatalytic
band gap of 2.66 eV, has attracted considerable attention because performance.
hard-template method, which showed much better photo-
2
6
2
6
2
6
14
of its non-toxicity and high quantum yield for photocatalysis
under visible light irradiation.
Bi, as a semi-metal with high carrier mobility, has attracted
It is known that the photo- extensive interest as good thermoelectric material and cata-
14–17
3
1–34
catalytic performance depends on the morphology and micro- lyst.
Recently, many promising studies have been reported
photocatalysts with to combine Bi with Bi or TiO for photocatalysis, and the
incorporation of Bi was found to enhance the photocatalytic
microboxes and microspheres have been synthesized, which performance of Bi or TiO obviously due to the enhanced
light absorption and reduced recombination of photo-
18,19
structure of photocatalysts.
Bi
2
MoO
6
2
O
3
2
20
21
various morphologies such as nanosheets, nanoparticles,
22
23
O
2
3
2
3
2,35,36
a
generated electron–hole pairs.
composite material based on Bi and Bi
Therefore, development of
MoO should be a
Engineering Research Center for Nanophotonics & Advanced Instrument, Ministry of
Education, Shanghai Key Laboratory of Magnetic Resonance, Department of Physics,
2
6
East China Normal University, Shanghai 200062, China. E-mail: lkpan@phy.ecnu. promising method for more effective photocatalysis. Unfortu-
edu.cn; Fax: +86 21 62234321; Tel: +86 21 62234132
nately, so far little attention has been focused on the application
b
Center for Coordination Bond and Electronic Engineering, College of Materials
of Bi/Bi MoO (BMO) composites in photocatalysis.
2
6
Science and Engineering, China Jiliang University, Hangzhou 310018, China.
E-mail: lxj669635@126.com
In this work, BMO-HMS composites were synthesized
through microwave-assisted reaction of Bi MoO precursor in
2
6
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c4ra09830g
an ethylene glycol (EG) solution and subsequent annealing in
1
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nitrogen atmosphere. The possible mechanism for the forma- 2.3 Photocatalytic experiments
tion of BMO-HMS composites was discussed. Due to special
The photocatalytic performance of the samples was evaluated
through a photochemical reactor (Bilon Co., Ltd). The distance
between metal halogen lamp and quartz tube is 60 mm, and the
HMS composite structure of the BMO-HMS composites exhibit
excellent photocatalytic performance under visible light
irradiation.

lter size (l > 400 nm) is 30 mm ꢂ 100 mm. The samples
(
(
80 mg) were dispersed in 80 mL RhB aqueous solutions
10 mg L ). The mixed suspensions were rst magnetically
ꢁ
1
2. Experimental
stirred in the dark for 30 min to reach the adsorption–desorp-
tion equilibrium. Under ambient conditions and stirring, the
mixed suspension was exposed to the visible irradiation
produced by a 400 W metal halogen lamp (l > 400 nm) and the
UV irradiation produced by a 500 W high pressure Hg lamp with
the main wave crest at 365 nm (no lter), respectively. At certain
time intervals, 2 mL of the mixed suspensions were extracted
and centrifuged to remove the photocatalysts. The ltrates were
analysed by recording the UV-vis spectra of RhB using Hitachi
U-3900 UV-vis spectrophotometer.
2.1 Synthesis of BMO-HMS
All the reagents were analytical grade and used without
further purication. 0.121 g Na MoO $2H O was dissolved in
2
4
2
1
0 mL EG to obtain solution A. A certain amount of glucose
and 0.485 g Bi(NO ) $5H O were dissolved in 10 mL EG to
3
3
2
obtain solution B. Then solution A was added into solution B
under vigorous stirring and stirred for 30 min to produce a
uniform transparent mixture solution. Subsequently, the
mixture solution was placed in a 35 mL microwave tube and
then put into an automated focused microwave system
ꢀ
(
Explorer-48, CEM Co.) and treated at 160 C with a micro-
3
. Results and discussion
wave irradiation power of 150 W for 10 min. The samples
synthesized using 0, 0.02, 0.1, 0.2 and 0.4 mmol glucose,
The XRD patterns of Bi MoO , BMO-1, BMO-2 and BMO-3 are
shown in Fig. 1. Bi MoO exhibits several diffraction peaks at
2
6
named as Bi
2 6
MoO , BMO-1, BMO-2, BMO-3 and BMO-4 were
2
6
isolated by washing three times with distilled water, dried in
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
10.9 , 23.5 , 28.3 , 32.7 , 33.2 , 36.1 , 46.7 , 47.2 , 55.5 , 56.3
ꢀ
ꢀ
a vacuum oven at 80 C for 24 h, and then annealed at 450 C
in nitrogen atmosphere for 120 min. For the electrochemical
impedance spectra (EIS) testing, the as-synthesized
composites with 5 wt% cellulose binder were homoge-
nously mixed in terpineol to form a slurry. Then, the resul-
tant slurries were coated on the FTO using a screen-printing
ꢀ
and 58.5 , indexed to (020), (111), (131), (200), (060), (151), (202),
(062), (133), (191) and (262) crystal planes of Bi
2
MoO
6
(JCPDS
37,38
21-0102).
Compared with Bi MoO , the new diffraction
2 6
ꢀ
ꢀ
ꢀ
peaks at 27.8 , 38.0 and 39.6 , corresponding to the (012), (104)
and (110) planes of Bi (JPCDS 44-1246), appear in XRD patterns
of BMO-1, BMO-2, BMO-3 and BMO-4, which conrms the
existence of Bi in the BMO composites. It should be noticed that
the diffraction peaks of Bi enhance with the increase of glucose
in precursor solution, which indicates that more Bi was
approach. Finally, these prepared electrodes were dried at
ꢀ
1
00 C for 30 min.
31,32
reduced. Similar result is also reported by others.
Moreover,
2.2 Characterization
the XRD pattern of BMO-2 before annealing is also provided in
Fig. 1 for comparison. It is observed that there is no diffraction
peak of Bi in BMO-2 before annealing, indicating that the Bi was
generated aer annealing.
The surface morphology, structure and composition of the
samples were characterized by eld-emission scanning elec-
tron microscopy (FESEM, Hitachi S-4800), X-ray diffraction
(
XRD, Holland Panalytical PRO PW3040/60) with Cu-Ka
Fig. 2(a)–(e) show FESEM images of Bi MoO , BMO-1, BMO-2,
2
6
radiation (V ¼ 30 kV, I ¼ 25 mA), and energy dispersive X-ray
2 6
BMO-3 and BMO-4. Bi MoO nanosheets show a self-assembly
spectroscopy (EDS, JEM-2100), and Raman spectroscopy
(
(
Renishaw inVia), respectively. The Brunauer–Emmett–Teller
BET) specic surface areas (SBET) of the samples were eval-
uated on the basis of nitrogen adsorption isotherms
measured at 77 K using a BELSORP-max nitrogen adsorption
apparatus (Micrometitics, Norcross, GA). The diffuse
absorption spectra of the samples were recorded on a UV-vis
spectrophotometer (Hitachi U-3900) equipped with an inte-
grated sphere attachment. EIS measurement was carried out
on an electrochemical workstation (AUTOLAB PGSTAT302N)
under dark condition using a three electrode conguration
with the as-prepared lms as working electrodes, a Pt foil as
counter electrode and a standard calomel electrode as
ꢁ
1
reference electrode. The electrolyte was 10 mg L
RhB
aqueous solution. EIS were recorded in the frequency range
of 0.1 Hz–1 MHz, and the applied bias voltage and ac
amplitude were set at open-circuit voltage and 10 mV.
Fig. 1 XRD patterns of (a) Bi
and unannealed), (d) BMO-3 and (e) BMO-4.
2 6
MoO , (b) BMO-1, (c) BMO-2 (annealed
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(b) show the FESEM images of the samples prepared via
microwave treatment for 1 min and 10 min, respectively. It can
be found that the morphology is dependent on the microwave
reaction time. Aer microwave treatment for 1 min, micro-
spheres with a smooth surface and a diameter of about 300 nm
were obtained, as shown in Fig. 3(a). When the microwave
reaction time was prolonged to 10 min, hierarchical ower-like
BMO-HMS was obtained, as shown in Fig. 3(b). The ower-like
BMO-HMS would further crystallize in annealing process.
Finally, the BMO-HMS with cage-like pores on the outer shell
consisting of nanoparticles was formed aer annealing in
nitrogen (Fig. 2(c)).
The possible growth mechanism of BMO-HMS is proposed
6
+
3+
in Scheme 1. The Mo and Bi were co-coordinated to be
23,29
complex compounds with the EG.
At the initial reaction
stage (1 min), the Bi MoO nanoparticles tended to assemble
2
6
into spherical agglomerates driven by reducing the surface
energy of nanoparticles. With increased reaction time (10 min),
the formed microspheres dissolved and recrystallized, and the
hierarchical ower-like BMO-HMS was formed through the
39
mass diffusion and Ostwald ripening process. Moreover, the
3
+
chelation occurred between glucose and Bi ions, which led to
32
a stable ve-membered ring coordinated cation. During the
following process of calcination, the ve-membered ring coor-
dinated cation could be decomposed and the Bi was reduced
2 6
Fig. 2 FESEM images of (a) Bi MoO , (b) BMO-1, (c) BMO-2, (d)
BMO-3 and (e) BMO-4, (f) EDS spectrum of BMO-2.
3+
to Bi, leading to the formation of BMO-HMS with cage-like
29
pores on the outer shell.
coralline structure. It can be observed from the Fig. 2(b)–(e) that
the BMO samples are entirely composed of spheres with an
average diameter of about 500 nm. The BMO spheres display
hollow structure with cage-like pores on the outer shell. The
walls of BMO-HMS consist of nanoparticles with size of
approximately 50 nm. The result indicates that the glucose plays
a key role in determining the morphology of BMO samples.
Meanwhile, the shell thickness was reported to increase with
the increase of glucose content in precursor solution because of
the enhanced density of condensed glucose and bimetallic
29
complexes. The BMO-2 sample was identied by EDS linked to
FESEM, as shown in Fig. 2(f). The peaks of Bi, Mo and O in EDS
spectrum prove the existence of Bi MoO . The atom ratio of Bi
2
6
and Mo is 2.2, indicating the presence of excessive Bi in the Scheme 1 The formation mechanism of BMO-HMS composite.
BMO-2 sample, which is in agreement with the XRD result.
To well understand the formation mechanism of BMO-HMS,
the sample synthesized using 0.1 mmol glucose at different
microwave reaction times were examined by SEM. Fig. 3(a) and
Fig. 3 FESEM images of the samples obtained at different microwave
treatment times: (a) 1 min, (b) 10 min.
Fig. 4 Raman spectrum of BMO-2.
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Fig. 4 shows the Raman spectrum of BMO-2. The A1g peak at
ꢁ
1
8
02 cm is assigned to the symmetric stretch of a MoO octa-
6
ꢁ1
hedron. The A1g mode at 845 cm shows a characteristic of
orthorhombic distortion of MoO octahedron in Bi MoO with
aurivillius structure of layered slabs (Bi
through the corner sharing of distorted MoO
peaks at 145, 283, 354 cm are attributed to the bending modes
6
2
6
2
+
2
6
O
2
) connected
octahedra. The
40
ꢁ1
40
6
of MoO octahedral unit. The weaker peak located at about
ꢁ
1
1
84 cm corresponds to second-order Raman scattering peaks
41
of Bi. No characteristic peak of carbon is found in the Raman
spectrum, indicating that carbon impurity was not generated
aer washing and annealing.
Nitrogen adsorption–desorption isotherms and pore size
2 6
Fig. 6 UV-vis diffuse absorption spectra of (a) Bi MoO , (b) BMO-1, (c)
BMO-2, (d) BMO-3 and (e) BMO-4. Inset shows a schematic illustration
of multiple reflections within the hollow structure.
distribution curves (inset) of Bi
2 6
MoO and BMO-2 are displayed
in Fig. 5, and the ones of other samples are shown in Fig. S1
ESI.† All of the samples show type IV isotherms with H
3
42,43
hysteresis loops.
This behavior may be caused by the exis-
44
tence of non-rigid aggregates. The specic surface areas of
The charge transfer and recombination behaviors of the
samples were studied by analysing the EIS spectra under dark
condition. Fig. 7 shows the typical Nyquist plots of Bi MoO ,
2 6
Bi MoO , BMO-1, BMO-2, BMO-3 and BMO-4 are 9.85, 19.6,
2
6
2
ꢁ1
1
7.8, 13.2 and 9.16 m g , respectively, which means that the
glucose is helpful to increase the specic surface area of
BMO-HMS. However, excessive glucose can decrease the specic
surface area of BMO-HMS. The reason may be that the excessive
BMO-1, BMO-2, BMO-3 and BMO-4. The semicircle in the EIS
spectra is ascribed to the contribution from the charge transfer
resistance (Rct) and constant phase element (CPE) at the
3
+
glucose could reduce more Bi to be Bi, which changes the
46
photocatalyst/electrolyte interface. The inclined line, resulting
35
porous structure of BMO-HMS. The broad pore size distribu-
tion, as shown in the insets of Fig. 5 and S1,† indicates the
existence of mesopores and macropores in these samples.
The UV-vis diffuse absorption spectra of Bi MoO , BMO-1,
from the Warburg impedance (Z ), corresponds to the ion
W
diffusion process in the electrolyte. The corresponding equiva-
lent circuit is shown in the inset of Fig. 7. It is clearly observed
that the Rct decreases with the increase of glucose content in
precursor solution, possibly because more Bi in the BMO-HMS
composite with the increase of the glucose content favors the
2
6
BMO-2, BMO-3 and BMO-4 were measured using a UV-vis
spectrophotometer with an integrating sphere (Hitachi
U-3900), as shown in Fig. 6. The hollow structure is benecial to
the absorbance of BMO due to the multiple reections, as
35
electron transfer and suppresses the charge recombination.
Such an effect can be explained from the view of stepwise
structure of energy levels constructed in BMO-HMS composites,
as shown in Fig. 8. The conduction band (CB) and valence band
45
shown in the inset of Fig. 6. Therefore, the BMO-HMS
composites exhibit higher absorbance than Bi MoO . Further-
2
6
more, the absorbance of the BMO-HMS composites increases
especially in visible light region with the increase of glucose
content in precursor solution, possibly due to more Bi reduced
1
6,17
(
VB) of Bi
2
MoO
6
is ꢁ0.32 V and 2.34 V (vs. NHE), respectively.
47
The work function of Bi is ꢁ0.28 V (vs. NHE). Such energy levels
are benecial for photo-induced electrons to transfer from the
31
in the BMO-HMS composites. Such an enhancement in the
2 6
Bi MoO CB to the Bi, which could efficiently separate the photo-
induced electrons and hinder the charge recombination in
electron-transfer processes, thus enhance the photocatalytic
visible light absorbance of BMO-HMS is benecial to the
12
increase of photo-generated electrons and holes.
2 6
Fig. 5 Nitrogen adsorption–desorption isotherms and corresponding Fig. 7 Nyquist plots of (a) Bi MoO , (b) BMO-1, (c) BMO-2, (d) BMO-3
pore size distribution curves (inset) of (a) Bi
2
MoO
6
and (b) BMO-2.
and (e) BMO-4. Inset is the corresponding equivalent circuit model.
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content in precursor solution. The degradation rate of RhB for
Bi MoO is 66%. When the glucose is introduced into Bi MoO
2
6
2
6
precursor solution, the degradation rate of the sample (BMO-1)
is increased to 73%, and reaches a maximum value of 91% for
BMO-2. The enhancement of the photocatalytic performance
should be mainly ascribed to the reduction in electron–hole pair
recombination due to the stepwise energy level structure in the
BMO-HMS composites and the increase in the light absorption
with the presence of Bi, which have been conrmed by the EIS
32,35
measurement and UV-vis absorption spectra.
However, when
the glucose content in precursor solution is further increased,
the degradation rate is decreased to 79% and 53% for BMO-3
and BMO-4, which may be due to its increased recombination
of photo-generated electron–hole pairs in Bi. Furthermore, the
high degradation rate of RhB for BMO-2 under UV light irradi-
2 6
Fig. 8 Schematic diagram of energy levels of Bi MoO and Bi.
48–50
performance.
precursor solution is further increased (BMO-3 and BMO-4), the photocatalyst.
ct increases, which is possibly because excessive Bi in the
composite acts as recombination centre and promotes the
However, when the glucose content in ation indicates that BMO-2 is also an excellent UV light
R
32
4. Conclusions
recombination of photo-generated electron–hole pairs in Bi.
Photocatalytic degradation of RhB by Bi MoO , BMO-1,
2
6
BMO-HMS composites were successfully synthesized via
microwave-assisted method and subsequent annealing in
nitrogen atmosphere, and their photocatalytic performances
were investigated. The results show that (i) BMO-HMS
composites exhibit better photocatalytic performance than
BMO-2, BMO-3, BMO-4 and commercial TiO2 (P25, Degussa)
were performed under visible light irradiation, as shown in
Fig. 9. Photocatalytic degradation of RhB by BMO-2 was also
performed under UV light irradiation for comparison. The
normalized temporal concentration changes (C/C
during the photocatalytic process are proportional to the
normalized maximum absorbance (A/A ), which can be derived
0
) of RhB
2 6
Bi MoO , and their photocatalytic performances are related to
the glucose content in precursor solution; (ii) BMO-2 achieves a
highest RhB degradation rate of 91% at 120 min; (iii) the
enhanced photocatalytic performance is ascribed to the
increased light adsorption and reduced electron–hole pair
recombination with the presence of Bi in the composites.
0
from the change in the RhB absorption prole at a given time
interval. The results of corresponding adsorption experiments
before photocatalysis were shown in Fig. S2 ESI,† in which no
signicant difference between adsorption capacities of the
2 6
Bi MoO and BMO-HMS composites was observed. In Fig. 9, it
can be seen that the concentration of RhB is hardly reduced Acknowledgements
under visible light irradiation in the absence of the photo-
Financial support from the National Natural Science Founda-
tion of China (no. 21401180) is gratefully acknowledged.
catalysts and BMO-HMS composites exhibit better photo-
catalytic performance than Bi MoO and P25. The photocatalytic
performance of BMO-HMS composites depends on the glucose
2
6
Notes and references
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2 6
MoO , (b) BMO-1,
(
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