Bi2O3/CuCr2O4-CuO core/shell structured nanocomposites: Facile synthesis and catalysis characterization

Article abstract of DOI:10.1016/j.jallcom.2006.10.081

In this paper, spherical monodisperse Bi₂O₃ nanoparticles were first prepared. Then, Bi₂O₃/CuCr₂O₄-CuO nanocomposites with a homogeneous core/shell nanostructure have been further synthesized by encapsulating CuCr₂O₄-CuO on the surfaces of the as-prepared Bi₂O₃ nanoparticles via an in situ precipitation method. Techniques of XRD, TEM, EDS, and FTIR have been employed to characterize the as-synthesized materials. The formation mechanism of the core/shell structured Bi₂O₃/CuCr₂O₄-CuO nanoparticles was also briefly discussed. The results indicate that the pre-coating of NH₄⁺ on the surfaces of Bi₂O₃ nanoparticles plays an essential role in the formation of homogeneous Bi₂O₃/CuCr₂O₄-CuO core/shell nanoparticles. Meantime, the catalytic properties of the as-synthesized Bi₂O₃/CuCr₂O₄-CuO core/shell nanoparticles have been readily evaluated. The results show that the resultant Bi₂O₃/CuCr₂O₄ core/shell nanomaterials demonstrate high catalytic activities towards the oxidation of CO.

Full text of DOI:10.1016/j.jallcom.2006.10.081

Journal of Alloys and Compounds 448 (2008) 287–292
Bi O /CuCr O –CuO core/shell structured nanocomposites:
2
3
2 4
Facile synthesis and catalysis characterization
a,b,c,∗
, Hua Chengb
Wei Li
a
State Key Laboratory for Powder Metallurgy, Central South University, Changsha, Hunan 410083, PR China
b
School of Chemistry and Chemical Engineering, Central South University, Changsha, Hunan 410083, PR China
School of Materials Science and Engineering, Central South University, Changsha, Hunan 410083, PR China
c
Received 16 July 2006; received in revised form 11 October 2006; accepted 12 October 2006
Available online 20 November 2006
Abstract
In this paper, spherical monodisperse Bi
2
O
3
nanoparticles were first prepared. Then, Bi
2
O
3
/CuCr
2
O
4
–CuO nanocomposites with a homogeneous
nanoparticles
core/shell nanostructure have been further synthesized by encapsulating CuCr
via an in situ precipitation method. Techniques of XRD, TEM, EDS, and FTIR have been employed to characterize the as-synthesized materials.
The formation mechanism of the core/shell structured Bi /CuCr –CuO nanoparticles was also briefly discussed. The results indicate that
nanoparticles plays an essential role in the formation of homogeneous Bi /CuCr –CuO
core/shell nanoparticles. Meantime, the catalytic properties of the as-synthesized Bi /CuCr –CuO core/shell nanoparticles have been readily
core/shell nanomaterials demonstrate high catalytic activities towards the oxidation
2
O
4
–CuO on the surfaces of the as-prepared Bi O
2 3
2
O
3
2 4
O
+
the pre-coating of NH
4
on the surfaces of Bi
2
O
3
2
O
3
2 4
O
2
O
3
2 4
O
evaluated. The results show that the resultant Bi
of CO.
2 3 2 4
O /CuCr O
©
2006 Elsevier B.V. All rights reserved.
Keywords: Bismuth oxides; Copper/chromium composite oxides; Nanomaterials; Core/shell structures; Spherical monodisperse nanoparticles; Solution-based
chemical method; Catalysis
1
. Introduction
trostatic attraction [1]. Sometimes, many kinds of catalysts
may be integrated into one core/shell composite system [7].
Bi2O3 and CuCr2O4–CuO composite oxides have long been
widely used in catalysis, solid-state propellant addictives,
electroceramics, and solid-state electrolytes [8–13]. There-
fore, it is of great importance to exploit a novel kind of
Bi2O3/CuCr2O4–CuO composite nanomaterials with core/shell
structures.
In recent years, core/shell structured nanomaterials have
gained extensive attentions due to their high stability, high
catalytic activities, homogeneous morphologies and size, and
controllable composition and surface structures, which can be
fine tuned to satisfy specific applications [1–6]. Generally,
core/shell nanoparticles are composed of two or more than
two types of matters, in which one behaves as the “core”
and the other acts as the “shell”. When hollow core/shell
nanoparticles made of complexed organic/inorganic compos-
ites are used as catalysts, their catalytic activities can be
enhanced noticeably, and selective catalysis may be achieved
in virtue of the steric hindrance, enantiotropy, and elec-
In this paper, monodisperse spherical Bi2O3 colloidal par-
+
ticles were firstly prepared and NH4 group was introduced
to their surfaces. Then, a [Cu(NH3)4]2 layer was encapsu-
+
lated on Bi2O3 nanoparticles via the complexing interaction
+
2+
3+
between NH4 and Cu . Next, Cr was further deposited on
the surfaces of Bi2O3 by virtue of the electrostatic attraction
between [Cu(NH3)4]2 and [Cr(OH)4] . Finally, monodisperse
spherical Bi2O3/CuCr2O4–CuO core/shell structured nanopar-
ticles were obtained by a following appropriate annealing
process. In addition, the catalytic activities for the oxida-
tion of CO of the as-prepared core/shell nanoparticles were
studied.
+
−
∗
Corresponding author at: Section of Science and Technology, Central South
University, Yuelu Campus, Changsha 410083, PR China. Tel.: +86 731 8830768;
fax: +86 731 8830768.
E-mail address: 973@mail.csu.edu.cn (W. Li).
0
925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2006.10.081

2
88
W. Li, H. Cheng / Journal of Alloys and Compounds 448 (2008) 287–292
◦
2
. Experimental
tor temperature was increased from room temperature to 260 C with a rate of
3
◦
C/min. The variation of CO2 concentration in the final gases was detected by
a SQ-206 gas chromatograph equipped with a Porapak-Q column.
2
.1. Materials synthesis
2
.1.1. Preparation of monodisperse Bi2O3 nanoparticles
A typical synthetic procedure the monodisperse spherical Bi2O3 nanoparti-
3. Results and discussion
cles is described in detail in Refs. [14,15], and briefly shown as follows: 0.01 mol
3
.1. Analysis of phase structures
Bi(NO3)3·6H2O was dissolved in 30 mL of 0.005 M HNO3 to form a transpar-
3
+
ent solution. The 20 mL PEG8000 was slowly added into the Bi solution in
◦
a 90 C water bath under thorough stirring. After being further stirred for ca.
Fig. 1 presents XRD patterns of the as-synthesized core/shell
3
0 min, 50 mL of 4 M NaOH solution was poured into the above solution, in
nanoparticles before and after calcinations. It can be seen that the
precursors before calcinations are mainly composed of Bi2O3
which a light yellow suspension would be formed immediately. After being vig-
◦
orously stirred for 2 h in a 90 C water bath, the resulting light yellow precipitates
(JCPDS: 712274 and 501088) as core and copper ammonium
were collected by centrifugation at a rotation rate of 24,000 r/min, followed by
successive washing with absolute ethanol, acetone, and deionized water for sev-
eral times. The final products were obtained after dried in a vacuum oven for
chromate as shell, and the chemical formula of the shell mate-
rials was found to be (NH4)0.78Cu1.04(NH ) CrO (OH)
according to JCPDS-420072 (Fig. 1a). The XRD pattern of the
3 0.14
4
0.86
5
h.
◦
samples after calcined at 500 C for 5 h in argon atmosphere is
2
.1.2. Preparation of CuCr2O4–CuO nanocomposites
shown in Fig. 1b, indicating that only diffraction peaks for Bi2O3
can be observed. This may be due to the thermal decomposition
of (NH4)0.78Cu1.04(NH3)0.14CrO4(OH)0.86, resulting in the for-
mation of amorphous or poorly crystallized CuCr O inorganic
In comparison, the CuCr2O4–CuO nanocomposites were also prepared by a
complexing-coprecipitation method as described in the following: ammonia was
added dropwise into 100 mL of 0.5 M Cu(NO3)2 under strongly stirring until a
◦
dark blue solution was obtained. The resulting solution was put in a 90 C water
2
4
bathandstirredfor30 min. Then, 100 mLof0.5 MCr(NO3)3 solutionwasslowly
dropped into the above solution under stirring, in which brown precipitates were
gained. Next, ammonia was used to adjust the pH value of the solution to be
around 8. After being stirring for another 6 h, the dark brown precipitates were
filtered out and dried in an oven for 24 h to obtain the dark brown precursors.
Some of the precursors were used for DTA analysis to investigate the thermal
decomposition procedure. Black CuCr2O4–CuO composite nanomaterials can
component. The diffractions of peaks referring to CuCr2O4 are
too weak to be seen as compared with Bi2O3. When the anneal-
ing temperature is elevated to 800 C, the XRD pattern indicates
◦
that the as-prepared samples mainly consist of Bi2O3 and spinel
CuCr O , accompanying by trace of CuO as shown in Fig. 1c.
2
4
◦
If the calcinations temperature is further increased to 1000 C,
reflections referring to CuCrO2, Cr2O3, and CuO phases can be
seen in the XRD pattern in Fig. 1d, in addition to Bi2O3 and
CuCr2O4. As reported, CuCr2O4 will decompose to CuCrO2
and Cr2O3 at high temperature [16]. As a result, although the
XRD results are fairly ambiguous, the aforementioned observa-
tions imply that the copper chromite component is successfully
coated on the surfaces of Bi2O3 core materials, which will be
further discussed in detail in the following sections.
◦
be obtained after the precursors are annealed at 800 C for 5 h.
2
.1.3. Fabrication of monodisperse Bi2O3/CuCr2O4–CuO core/shell
nanoparticles
A certain amount of the as-prepared monodisperse Bi2O3 nanoparticles were
washed with deionized water several times, then placed into 100 mL isopropyl
alcohol and sonicated for 1 h, resulting in a light yellow colloidal suspensions.
Next, 20 mL of 5% ammonia and 20 mL deionized water were added into the
◦
mixed suspensions under vigorous stirring under a 60 C water bath to adjust
its pH value to be ca. 8. The suspension was further stirred for 1 h at constant
temperature, followed by aging for 5 h at room temperature. Thereafter, a cer-
tain amount of 0.05 M Cu(NO3)2 solution was dropped slowly into the above
3.2. Fourier transformation infrared (FTIR) analyses
◦
suspension under continuous stirring in a 60 C water bath, during which 5%
ammonia was used to keep the pH of the solution to be about 8. After the addi-
tion of Cu(NO3)2 solution, the resultant suspension was mildly stirring in a
bath for 5 h and then allowed to age for 5 h at room temperature. Next, an equal
amount of 0.05 M Cr(NO3)3 solution was added into the suspensions in a similar
way to that of Cu(NO3)2 solution except that the aging time is 36 h. Finally, the
brown black precursors were collected by filtering. The dark black samples were
obtained by calcining the precursors at 300 C for 2 h and then 800 C for 5 h in
air. The molar ratio of CuCr2O4 to Bi2O3 in the whole preparation process was
controlled to be 1:4.
The FTIR spectrum of monodisperse spherical Bi2O3
nanoparticles employed as core materials (templates) is pre-
sented in Fig. 2a. Four characteristic bands can be clearly seen
−
1
from Fig. 2a, in which the band at ∼3449 cm corresponds to
−1
the stretching vibration of OH , while the band appearing at
1633 cm represents the stretching mode of absorbed H2O.
◦
◦
−
1
∼
−1
In addition, the band located at ∼1381 cm can be ascribed to
−
−1
the bending vibration of NO3 , and the band at ∼444 cm is
2
.2. Materials characterization
attributed to the stretching mode of Bi–O. The FTIR data reveal
−
thatthereareabundantOH groupslocatedonthesurfacesofthe
The phase structures of the as-prepared samples were characterized by X-
ray diffraction (XRD) taken on a Germany Bruker D-8 Diffractometer (Cu K␣
radiation, λ = 1.5418 A˚ ). The surface structures were analyzed by Fourier trans-
Bi2O3 nanoparticles [17,18], which would play a critical role in
the following reactions for the formation of core/shell structures
2+
3+
via the interactions between Bi2O3 particles and Cu /Cr [19].
Fig. 2b depicts the FTIR spectrum of the CuCr2O4–CuO com-
posites obtained by complexing-coprecipitation method. Two
formation infrared spectra (FTIR). The microscopic morphologies and grain
sizes were observed with a Japan Hitachi H-800 transmission electron micro-
scope operated at an accelerating voltage of 200 kV. The catalytic activities of
the products on the conversion of CO were evaluated with a fixed-bed and quartz
glass reactor. The 0.2 g of the as-prepared products was used as the catalysts.
The volume percentages of CO and O2 in the mixed gases were 1.0% and 16%,
respectively. N2 was adopted as balanced gas in the catalytic process. The veloc-
ity of gas was controlled with a D07-7A mass flowmeter, and the total flux of
the mixed gases was 100 mL/min. During the whole reaction process, the reac-
−
1
−1
strong absorption bands at ∼615 cm and ∼515 cm can be
seen in the short-wave region, which corresponds to the vibra-
tion absorption of Cu–O–Cr in CuCr2O4 [20,21]. As to the
samples after coating, the characteristic absorption bands for
−
1
−1
−1
)
Bi–O (∼444 cm ) and Cu–O–Cr (∼605 cm , ∼515 cm

W. Li, H. Cheng / Journal of Alloys and Compounds 448 (2008) 287–292
289
◦
◦
◦
Fig. 1. XRD patterns of the as-synthesized Bi2O3-based core/shell nanoparticles before (a) and after calcined at 500 C (b), 800 C (c) and 1000 C (d), respectively
for 5 h.
−
can be found easily. Meanwhile, the stronger OH vibration
−
1
−1
−1
(
at ∼3469 cm , ∼3298 cm ), absorbed H2O (∼1624 cm ),
−
−1
+
−1
NO3 (∼1402 cm ), and NH4 (∼1271 cm ) groups are also
observed. These results are consistent with the aforementioned
XRD analysis in Fig. 1a, implying that an inorganic layer com-
posed of Cu and Cr components have been successfully coated
on the surfaces of Bi2O3 nanoparticles by means of complexing
with ammonia. Fig. 2d presents the FTIR pattern of the sam-
◦
ples in Fig. 2c after being calcined at 800 C for 5 h. As shown
−1
in Fig. 2d, only the characteristic bands for Bi–O (∼444 cm
)
−
1
−1
and Cu–O–Cr (∼605 cm , ∼515 cm ) are retained compared
with that of Fig. 2c, indicating that the products after annealing
are composite oxides consisted of Bi2O3 and CuCr2O4. This
result also agrees with the previous XRD data in Fig. 1c.
3.3. Microscopic morphologies
Fig. 3a shows the typical TEM image of Bi2O3 nanopar-
ticles used as templates (core materials) for the synthe-
sis of Bi2O3/CuCr2O4–CuO core/shell nanoparticles. It can
be seen that all the Bi2O3 samples are of well-dispersed
Fig. 2. FTIR spectra of the as-synthesized monodisperse Bi2O3 nanoparticles
a), CuCr2O4–CuO nanocomposites (b), and the core–shell structured Bi2O3-
based nanoparticles before (c) and after (d) calcination.
(

2
90
W. Li, H. Cheng / Journal of Alloys and Compounds 448 (2008) 287–292
Fig. 3. (a) Typical TEM image of the as-synthesized Bi2O3 nanoparticles. Low (b) and high (c) magnification TEM images of the as-synthesized Bi2O3/CuCr2O4–CuO
core/shell nanoparticles. (d) Selected area electron diffraction (SAED) pattern of the particle as marked in (b).
spherical nanoparticles. No agglomerates are observed in the
TEM image. The average particle diameter is ca. 60 nm.
The low-magnification TEM image of the as-synthesized
Bi2O3/CuCr2O4–CuO nanocomposites is shown in Fig. 3b. It
is apparent that the samples are composed of monodisperse
spherical core/shell structured nanoparticles with an average
particle size of about 78 nm. Fig. 3c gives further insight into
the microstructural morphology of the composite nanoparticles
with a high-magnification TEM image, revealing that the com-
posites are of core/shell structures with core diameter of 60 nm
and shell thickness of about 18 nm. Although HRTEM is needed
to delicately examine the grain size and crystallinity of the thin
CuCr2O4–CuO encapsulating layer, from Fig. 3c we can observe
that the grain size of the shell CuCr2O4–CuO is in the range of a
few nanometers. Fig. 3d shows the electron diffraction pattern of
the core/shell nanoparticle as marked in Fig. 3b. The diffraction
pattern is ambiguous and therefore hard to be indexed but it is
composed of bright points implying good crystallinity of the as-
synthesizedBi2O3/CuCr2O4–CuOcore/shellnanoparticles. The
EDS analysis of randomly selected grains in Fig. 3b elucidates
that the nanocrystallites comprise Cu, Cr, and Bi elements, as
shown in Fig. 4, which further demonstrates that the as-prepared
samples are Bi2O3/CuCr2O4–CuO nanocomposites.
3.4. Formation mechanism of Bi2O3/CuCr2O4–CuO
core/shell nanoparticles
Based on the previous results and analyses, the
Bi2O3/CuCr2O4–CuO core/shell nanoparticles in the present
experiments are proposed to be formed via the following
+
way (Fig. 5). Firstly, NH4 was coated on the surfaces of
+
Bi2O3 by virtue of the electrostatic attraction of NH4 and
Bi2O3 nanoparticles due to their high specific surface area
−
2+
3+
and abundant OH on the surfaces. Secondly, Cu and Cr
were preferentially deposited on the surfaces of Bi2O3. It
Fig. 4. EDS spectrum of the as-synthesized Bi2O3/CuCr2O4–CuO core/shell
nanoparticles.
is recognized that Cu2 and Cr would exist in forms of
+
3+

W. Li, H. Cheng / Journal of Alloys and Compounds 448 (2008) 287–292
291
Fig. 5. Schematic illustration of the formation mechanism of Bi2O3/CuCr2O4–CuO core/shell nanoparticles.
−
CuCr2O4–CuO, or mechanically mixture of CuCr2O4–CuO and
Bi2O3 (with a molar ratio of 1:4), and CuCr2O4–CuO loading
on Bi2O3 have also been studied. The results are also presented
in Fig. 7 and Table 1. It is found that the catalytic perfor-
mances of the other four materials are all poorer than that of
the Bi2O3/CuCr2O4–CuO core/shell nanoparticles.
Monodisperse spherical Bi2O3/CuCr2O4–CuO core/shell
nanoparticles may exhibit several advantages compared with
other materials when they are used as catalysts. First, high
[
Cu(NH3)4]2+ and [Cr(OH)4] [3] in a solution with excess
+
ammonia. In addition, the NH4 on the Bi2O3 particles has a
strong complexing interaction with Cu . Therefore, Cu can
2+
2+
2+
be absorbed on the surfaces of Bi2O3 in forms of [Cu(NH3)4] .
Likewise, Cr3 may be further coated on the surfaces of Bi2O3
+
2+
particles via the electrostatic attraction between [Cu(NH3)4]
−
2+
2+
and [Cr(OH)4] . Henceforth, Cu and Cr will be deposited
on the Bi2O3 surfaces continuously, resulting in the for-
mation of Bi2O3/(NH4)0.78Cu1.04(NH3)0.14CrO4(OH)0.86
core/shell nanoparticles. Finally, monodisperse spher-
ical
were obtained by the thermal decomposition of
core/shell
Bi2O3/CuCr2O4–CuO
core/shell
nanoparticles
Bi2O3/(NH4)0.78Cu1.04(NH3)0.14CrO4(OH)0.86
nanoparticles in air. To substantiate this hypothesis, NaOH
instead of ammonia was used to adjust the pH value of the
reaction solution while keeping other parameters unchanged
in a parallel experiment for comparison. The typical TEM
image of the resultant products is shown in Fig. 6. As seen
in Fig. 6, the samples are nanoparticles of Bi2O3 loaded with
CuCr2O4–CuO on their surfaces. No homogeneous core/shell
nanoparticles could be observed. Hence, the surface pre-
+
functionalization of Bi2O3 particles with NH4 before coating
is a key step for the fabrication of homogeneous monodisperse
Bi2O3/CuCr2O4–CuO core/shell nanoparticles.
3
.5. Catalytic performances on the oxidation of CO
The relationship between temperatures and conversion per-
centages of CO with the as-prepared Bi2O3/CuCr2O4–CuO
composites as catalysts is summarized in Table 1 and Fig. 7. It
is clear that the conversion percentages of CO to CO2 increases
◦
◦
rapidlyfromlessthan5%at120 Ctoabove90%at160 Cwhen
thehomogeneousCuCr2O4–CuOencapsulatedBi2O3 core/shell
nanoparticles are used as catalysts. The reaction of CO to CO2
◦
is nearly complete when the temperature reaches 180 C. For
Fig. 6. TEM images of the sample when using NaOH instead of NH4OH as
precipitation agents.
comparison, the catalytic properties of the merely Bi2O3 and

2
92
W. Li, H. Cheng / Journal of Alloys and Compounds 448 (2008) 287–292
Table 1
Microstructural morphology, particle size, and catalytic properties for the oxidation of CO over various samples
Samples
Remarks
Temperature dependence of CO → CO2
conversion percentage
◦
◦
5
0% ( C)
100% ( C)
Bi2O3
CuCr2O4–CuO
Bi2O3/CuCr2O4–CuO
Bi2O3/CuCr2O4–CuO
Bi2O3 + CuCr2O4–CuO
Monodisperse nanospheres
Nanocomposites
Core/shell nanoparticles
CuCr2O4–CuO loaded on Bi2O3 nanoparticles
Simply mechanical mixtures
163
200
150
180
190
200
255
180
202
215
core/shell structure have been successfully synthesized by using
monodisperseBi2O3 nanospheresastemplates. Theresultsshow
+
that the pre-coating of NH4 on the surfaces of Bi2O3 nanopar-
ticles plays an essential role in the formation of homogeneous
Bi2O3/CuCr2O4–CuO core/shell nanoparticles. In addition, the
as-prepared core/shell nanoparticles exhibited promising cat-
alytic activities toward the oxidation of CO. Therefore, the
Bi2O3/CuCr2O4–CuO core/shell nanomaterials synthesized in
the present experiment provide promising for pollution treat-
ment in car exhaust purification.
Acknowledgement
Supporting from the postdoctoral funding of Central South
University is grateful acknowledged.
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Fig. 7. Conversion percentage as a function of temperature for the oxidation of
CO over various samples.
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1
[
In conclusion, monodisperse spherical CuCr2O4–CuO
(
encapsulated Bi2O3 nanoparticles with a homogeneous