Ag2O-Bi2O3 composites: Synthesis, characterization and high efficient photocatalytic activities

Article abstract of DOI:10.1039/c2ce25301a

Ag₂O nanoparticle modified α-Bi₂O₃ composite photocatalysts were synthesized and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV-vis diffuse reflectance spectroscopy. XRD patterns confir

Full text of DOI:10.1039/c2ce25301a

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Ag O–Bi O composites: synthesis, characterization and high efficient
2
2
3
photocatalytic activities{
a
Lin Zhu, Bo Wei, Lingling Xu,* Zhe L u¨ , Hailin Zhang, Hong Gao and Jixin Che
b
a
b
a
a
c
Received 1st March 2012, Accepted 13th June 2012
DOI: 10.1039/c2ce25301a
Ag O nanoparticle modified a-Bi O composite photocatalysts
In recent years, there have been some methods based on band
gap tuning of photocatalysts, including metal/non-metal ion
impregnation, size reduction, sensitization, metal ion implantation
2
2 3
were synthesized and characterized by X-ray diffraction
XRD), scanning electron microscopy (SEM) and UV-vis
diffuse reflectance spectroscopy. XRD patterns confirmed that
the composites consist of cubic phase Ag O and monoclinic
a-Bi phase. The Ag O loading was greatly increased due to
the rough surface of the acid etched Bi microrods.
Photocatalytic activity of the Ag O–Bi composites was
evaluated using the degradation of methyl orange (MO). The
correlation between the Ag O content and photocatalytic
activity was estimated. The composite with Ag to Bi ratio of
: 1 as raw material showed the highest visible light activity
(
6–8
etc. In fact, composite semiconductors reveal improved activities
in the decomposition of organic dyes by reducing the recombina-
tion probability of photoinduced electrons and holes. Recently,
2
2
O
3
2
Ag O has been reported to be a good sensitizer by compositing
2
2 3
O
with TiO
2 2 2
and Ag O–TiO presented enhanced activity under both
2
2 3
O
9–12
UV and visible light excitation.
Considering the UV excitation
response of TiO , it is reasonable to develop visible light responsive
2
2
catalysts combined with Ag O. a-Bi O with a band gap of 2.84 eV
2
2 3
13–16
has been used as visible light photocatalyst.
have studied the Bi –Bi 42x nanocomposites by UV-vis
radiation on Bi O and found excellent visible light activity.
Hameed et al.
3
2
O
3
2
O
and the enhancement mechanism was attributed to the
separation of photoinduced electron and holes by the carrier
transfer.
17
2
3
Further, they have also investigated the nanocomposites NiO–
Bi and ZnO–Bi , which showed superior photocatalytic
activity compared with that of pure Bi O .
2
O
3
2 3
O
18,19
Heterojunction
WO composite
2
3
semiconductors Bi
2
O
3
–BaTiO
3
and Bi
2
O
3
–Bi
2
6
Introduction
microspheres has been fabricated and exhibited high photocata-
lytic activity for the degradation of rhodamine B, methyl orange
2
Since the photocatalytic splitting of water using TiO was first
20,21
(
MO) and methylene blue.
reported on the Ag O–Bi
irradiation.
In this paper, novel Ag O–Bi O composites were fabricated
However, few studies have been
reported by Fujishima and Honda, the transformation and
utilization of solar energy through such means has been studied
2
2 3
O composite catalyst under visible light
1–3
intensively.
Subsequently, the investigation of photo-decom-
2
2 3
position of water contaminants by semiconductor photocatalysts
has become an area of growing interest and different kinds of
through a coprecipitation method. The composite materials with
narrowed band gaps exhibited higher catalytic activities towards
the degradation of MO than the pure Bi O under visible light
2
photocatalysts have been studied. TiO is one of the most
2
3
commonly used photocatalysts for its low cost and radiation
stability. However, its catalytic process must be performed under
UV light due to its wide band gap (3.2 eV), as only a small portion
2
irradiation. The relationship between the Ag O loading and the
photocatalytic activities was discussed and the optimization of the
Ag O–Bi O composite was obtained. The enhanced mechanism
2
2 3
4,5
of the sunlight can be utilized. Therefore, exploration of visible
light-responsive photocatalysts with good photocatalytic properties
and proper band gap are greatly needed for environmental
remediation applications.
was also discussed.
Experiment section
Preparation of Bi O microrods
2
3
a
Key Laboratory of Photonic and Electric Bandgap Materials, Ministry
of Education, School of Physics and Electronic Engineering, Harbin
Normal University, Harbin, 150025, P. R. China.
E-mail: xulingling_hit@163.com
2 3
Bi O microrods were successfully synthesized through a copreci-
pitation method with PEG-8000 as surfactant at room tempera-
ture. 10 g PEG-8000 (analytical grade) was firstly dissolved in
40 mL deionized water to form a homogenous solution. Then,
b
Center for Condensed Matter Science and Technology, Department of
Physics, Harbin Institute of Technology, Harbin, 150080, P. R. China.
E-mail: bowei@hit.edu.cn
The Aviation University of Air Force, Changchun, 130022, P. R. China
1.94 g of Bi(NO
3
)
3
?5H
2
O (99%) was added into the above solution,
(1 M, analytical grade) to
c
followed by adding 20 mL of HNO
3
{
Electronic supplementary information (ESI) available: Change of MO
inhibit the hydrolysis process. The whole mixture together with
solution concentration in darkness, XRD, SEM and XPS results. See
DOI: 10.1039/c2ce25301a
200 mL deionized water was stirred vigorously at 50 uC until a
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homogeneous, clear solution was obtained. Then the precursor
mixture was heated to 80 uC, 60 mL of NaOH (4 M, analytical
grade) was added and stirred continuously for 45 min. After
cooling to room temperature, the precipitates were collected by
filtering and followed by washing with deionized water several
times. After a drying treatment at 80 uC for 2 h in air, Bi O
from the reaction beaker at given times during the process. The
suspensions were centrifuged to separate the powder, and the MO
concentration of the filtrate was determined using a Perkin-Elmer
Lambda-35 UV-vis spectrometer.
2
3
Results and discussion
microrods were finally obtained by annealing the precipitates at
The structure and phase purity of the Ag
were examined by XRD and the patterns are shown in Fig. 1. The
diffraction peaks of the Ag O–Bi complexes consist of two sets
2 2 3
O–Bi O composites
6
50 uC for 2 h in a muffle furnace.
2
2 3
O
2 2 3
Ag O–Bi O composite
of strong diffraction peaks, indicating that the synthesized
products are composite materials with high crystallinity. Those
can be mainly indexed as monoclinic a phase Bi O (JCPDS card
Before the deposition of Ag
out on the Bi microrods. The Bi
0 mL of HCl (36–38%) for 1 h and then washed repeatedly with
deionized water, resulting in etched Bi O microrods with rough
2
O, an acid etching process was carried
2
O
3
2 3
O
powders were immersed in
2
3
2
No. 76-1730) and cubic Ag
2
O (JCPDS card No. 76-1393). The
2
3
+
peaks of Ag O gradually increase with increasing Ag concentra-
2
surfaces. Ag O–Bi
2
2 3
O composites with different ion ratios
tion, which can also be confirmed by the SEM images of Ag
2
O–
+ 3+
Ag : Bi ) ranging from 1 : 3 to 5 : 1 were prepared by a simple
(
Bi O composites (Fig. S2{). In addition, slight b phase Bi O
2
3
2 3
chemical precipitation method. Taking the ion ratio of 1 : 1 for
example, 0.466 g etched Bi O microrods and 2 g PEG-8000 were
(JCPDS card No. 29-0236) can also be found, which might be
2
3
resulting from the annealing process of the precursors.
dissolved in 50 mL water and stirred for 5 min. Then 0.312 g of
AgNO
(¢99.8%) was added into the suspension and the pH of
the mixture was adjusted to 14 by dropwise addition of NaOH
4 M) with vigorous stirring for 10 min. The composite was
filtered, washed, and dried at 90 uC for 2 h, forming nano-Ag O
In order to obtain detailed information about the morphology
3
of Bi
out and the results are given in Fig. 2a–e. Fig. 2 shows the low-
magnification image of the as-obtained Bi sample without acid
2 3
O and composite samples, SEM observation were carried
(
2 3
O
2
etching, which displays a rod-like morphology with relatively
uniform size. The average diameter ranges from 1.5 to 2 mm and
the length is in the range of 20–30 mm. For comparison, the
decorated Bi
were synthesized by the same conditions without etched Bi
comparison, Bi microrods without etching were also deposited
by similar steps.
2
O
3
2
microrods. Similarly, pure Ag O nanoparticles
2 3
O . For
2 3
O
deposition process was also carried out directly on the Bi
microrods without acid etching and the morphology of Ag
Bi (3 : 1) composites is shown in Fig. 2b. It can be seen that the
Ag O nanoparticles are sparsely deposited on the non-etched
Bi microrods due to the smooth surface. Meanwhile, the acid
etching treatment induced a coarser surface, as confirmed in
Fig. 2c, which is very beneficial for Ag O loading. As expected, an
obvious increase of Ag O nanoparticles loading was observed on
acid etched Bi microrods, as shown in Fig. 2d. A distinct
2 3
O
2
O–
2 3
O
Characterization techniques
2
The morphology, particle size and chemical composition of the
product were examined by scanning electron microscopy (SEM,
Hitachi S-4800) equipped with an energy dispersive X-ray
spectrometer (EDX). X-ray diffraction (XRD) measurement was
carried out by a Rigaku-D/max 2500 diffractometer with Cu-Ka
radiation (l = 0.15418 nm) for phase identification. The UV-vis
diffuse reflectance spectra (DRS) were measured by a Shimadzu
UV-2550 spectrophotometer. Surface areas were determined from
nitrogen adsorption–desorption isotherms at liquid nitrogen
temperature using a Quantachrome NOVA 2000e surface area
analyzer. The Brunauer–Emmett–Teller (BET) method was used
2 3
O
2
2
2 3
O
comparison between Fig. 2b and Fig. 2e suggests that the acid
etching is an effective way to provide a rough surface platform to
2
load Ag O nanoparticles in high capacity. Therefore, all the
microrods were treated by acid etching before the deposition
process in order to obtain the well deposited composites. Typical
2
21
for the surface area calculation (1.42–3.13 m
g ). X-ray
photoelectron spectroscopy (XPS) experiments were measured
with a Thermofisher K-Alpha X-ray photoelectron spectrometer
using Al-Ka radiation (12 kV, 6 mA). The binding energies of
elements were calibrated using C 1s (285.06 eV) as reference.
Photocatalytic activity of the composites
2 2 3
In a typical experiment, 0.02 g of Ag O–Bi O composite was
dispersed into 60 mL MO aqueous solution with a concentration
25
of 6.15 6 10 M. The suspension was irradiated using a 500 W
Xe lamp (10 cm away from the suspension) at room temperature,
and the illuminant wavelength started from 400 nm, filtered by an
optical filter. Before irradiation, the mixture was magnetically
stirred for 30 min in darkness to ensure the establishment of an
adsorption–desorption equilibrium between the photocatalysts and
dyes (Fig. S1{). Then the solution was exposed to the visible light
irradiation under vigorous stirring. 5 mL of solutions was taken
Fig. 1 XRD patterns of Ag O–Bi O composites with different Ag to Bi
2 2 3
2
ratios. Diffraction peaks of cubic Ag O (JCPDS No. 76-1393) are given.
5
706 | CrystEngComm, 2012, 14, 5705–5709
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2 3 2 2 3
diffuse reflectance spectra (DRS) of Bi O and typical Ag O–Bi O
(1 : 3), (3 : 1) and (5 : 1) composites were recorded and used for
determination of the band gaps. The absorption edges can be
derived from Fig. 3a. The results indicate that the composites
exhibit improved absorption ability in the visible range with the
elevated amount of Ag
activity would be greatly increased by the effective absorption of
visible light compared with pure Bi . The band gap (E ) can be
2
O, which suggests that the photocatalytic
2
O
3
g
determined by the extrapolation of the curves in Fig. 3b by
2
plotting (ahn) versus hn, where a and hn are the absorption
23,24
coefficient and incident photon energy, respectively.
gap of Bi microrods is estimated to be 2.82 eV, which agrees
well with the previously reported values.
obvious decrease of the band gap was observed for Ag
1 : 3), (3 : 1), (5 : 1) composites, with the estimated values of 2.60,
.30 and 2.28eV, respectively.
The correlation between photocatalytic activity and the content
of Ag O loading was studied by the decolorization of MO
solution. The comparative results of the MO degradation on Bi
with different Ag O contents are presented in Fig. 4. It can be seen
that Bi microrods have a low photocatalytic activity under
visible light and the degradation is only 7% in 60 min. Meanwhile,
the loading of Ag O in composites had a crucial influence on
photocatalytic activity. A small amount of Ag O on Bi surface
could lead to an obvious increase of decomposition efficiency.
Ag O acts as a visible light sensitizer that enlarges the visible light
absorption of the composites, leading to the improvement of
photocatalytic activity. Meanwhile, Ag O–Bi (4 : 1) and (5 : 1)
show a decreased photoactivity, which can be attributed to the
The band
2 3
O
1
3–16
Meanwhile, an
2 3
O–Bi O
2
(
2
2
2 3
O
2
2 3
O
2
Fig. 2 (a) Low-magnification SEM image of Bi
typical image of Ag O–Bi (3 : 1) composite by non-etched Bi
microrods; (c) Bi microrod treated with acid etching; (d, e) low and
high magnification SEM images of Ag O–Bi composite (3 : 1) with
acid etching, respectively; (f) the EDX spectrum of Ag O–Bi composite
3 : 1) with acid etching.
2
O
3
microrods; (b) a
2 3
O
2
2
2
O
3
2 3
O
2 3
O
2
2
2 3
O
2
2 3
O
2
2 3
O
(
inhibition of separation of electron and holes for the excess Ag
2
O
chemical compositions of the Ag
further analyzed by EDX. From the corresponding EDX spectra
Fig. 2f), it can be seen that the elements in the composite are Bi,
2 2 3
O–Bi O (3 : 1) composite were
loading.
These results also suggest that the Ag
effective way to increase the photocatalytic activity of the Bi
microrods. An enhanced photocatalytic activity of the Ag
Bi composites was observed with an elevated Ag content from
: 3 to 5 : 1. Especially, Ag O–Bi (3 : 1) reveals the highest
2
O deposition is an
(
2 3
O
Ag and O. Moreover, this composite was also examined by XPS
analysis, which verifies the presence of Bi, Ag and O elements in
the desired valence states (Fig. S3{). The results suggest that the
2
O–
2 3
O
1
2
2 3
O
2 3 2
composite is composed of Bi O and Ag O, which is consistent
photocatalytic activity, at which 78% of MO was decomposed
after 60 min under visible light irradiation. The degradation rates
with XRD studies.
The efficiency of photocatalytic activity is greatly determined by
the energy band structure feature of a semiconductor. Thus, it is a
promising way to combine two kinds of semiconductor photo-
catalysts to adjust the band gap and absorb visible light in a wider
0 0
for MO were calculated by plotting ln(C /C), where C is the initial
concentration and C is the concentration at time t, versus
21
irradiation time. The slope k (min ) denotes the degradation
2 3
rate, which was calculated and the results are: 0.001 (pure Bi O ),
range. Ag
2
O with a band gap of 1.3 eV shows strong absorption of
22
both UV and visible light. Here, the deposition of Ag O
2
2 3
nanoparticles on the Bi O microrods can hence enlarge the light
absorption range of the composite photocatalysts. UV-visible
2
Fig. 3 UV-vis diffuse reflectance spectra (a) and plots of (ahn) versus the
photon energy (hn) (b) of the pure Bi
composites.
2
O
3
and typical Ag O–Bi
2
2
O
3
Fig. 4 Photocatalytic degradation of MO over the Ag
2 2 3
O–Bi O
composites with different ion ratios of Ag and Bi under visible light.
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The results in this paper shows that Ag
exhibit higher photocatalytic activity than that of pure Bi
microrods, which is mostly attributed to the composite interface
and energy band match of Ag O and Bi , inhibiting the
recombination of photoinduced electrons and holes. However,
with a large amount of Ag O loading on the composites, the
2 2 3
O–Bi O composites
2 3
O
2
2 3
O
2
photoinduced electrons and holes would recombine easily on the
Ag O, leading to decreasing of photocatalytic activity in the
2
Ag O–Bi O (4 : 1 and 5 : 1) composites. Therefore, the optimized
2 2 3
ratio for Ag O and Bi O is obtained at 3 : 1, which exhibits the
2 2 3
highest photocatalytic activity.
Fig. 5 Schematic diagram for electron–hole separations at the interface
Conclusion
and in both semiconductors.
2 2 3
In conclusion, Ag O–Bi O composites were successfully synthe-
0
.002 (1 : 3), 0.008 (1 : 2), 0.011(1 : 1), 0.043 (2 : 1), 0.051 (3 : 1),
.029 (4 : 1) and 0.026 (5 : 1). It was found that the degradation
rate of MO decomposition is dependent on the amount of Ag
loading and the highest decomposition rate is obtained by Ag O–
Bi (3 : 1), which is 5 times that of pure Bi . After the
photocatalytic reaction, the typical composite Ag O–Bi O (3 : 1)
sized through a chemical precipitation method and high photo-
catalytic activities were observed in a wide range of composition
ratios. The XRD results confirmed that crystal is composed of
cubic Ag O and monoclinic a-Bi O . UV-vis diffuse reflectance
0
2
O
2
2
2 3
2
O
3
2 3
O
spectra revealed that the Ag O–Bi O composites could absorb
2 2 3
2
2
3
more visible light than Bi O . The photocatalytic properties of
2 3
was further examined by XRD (Fig. S4{) and SEM (Fig. S5{),
which indicate that photocatalyst shows reasonable stability both
in crystallinity and morphology.
2 2 3
Ag O–Bi O composites estimated by the degradation of MO
show that the composite catalysts present the higher activity than
2 3
that of pure Ag O and Bi O . The optimized ratio of Ag O–Bi O
2
2
3
2
As shown in Fig. 5, the band structure of the synthesized Ag O–
2
composites was estimated at 3 : 1 for Ag to Bi and the
photocatalytic mechanism has been proposed based on the
relative band position of these two semiconductors.
2 3
Bi O composite was proposed to discuss the possible process of
the photocatalytic degradation of MO. It has been reported that
Ag O can be photoactivated under both blue and green excitation
2
2
5
(
below 550 nm). Meanwhile, Bi O (E = 2.8 eV) can absorb
2 3 g
Acknowledgements
visible light with wavelength shorter than 440 nm. Therefore, when
the catalysts are excited by visible light with photon energy higher
2
This work was supported by the National Science Foundation
of China (51102069, 11074060). This work was partly supported
by Innovative Talents Fund of Harbin (2010RFQXG034).
or equal to the band gap of Ag O and Bi O , electrons (e ) in the
2
2 3
valence band (VB) can be excited to the conduction band (CB)
+
with simultaneous generation of the same amount of holes (h ) in
the VB, as demonstrated in eqn 1 and 2. The photoactivated
electrons and holes in the Ag O–Bi O composite could inject into
Notes and references
2
2 3
1
2
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process. The photoinduced holes (trapped by H O) produce
2
?
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