Synthesis of CuO@NiO core-shell nanoparticles by homogeneous precipitation method

Article abstract of DOI:10.1016/j.jallcom.2012.05.086

Core-shell CuO-NiO mixed metal oxide nanoparticles in which CuO is the core and NiO is the shell have been successfully synthesized using homogeneous precipitation method. This is a simple synthetic method which produces first a layered double hydroxide precursor with core-shell morphology which on calcination at 350 °C yields the mixed metal oxide nanoparticles with the retention of core-shell morphology. The CuO-NiO mixed metal oxide precursor and the core-shell nanoparticles were characterized by powder X-ray diffraction, FT-IR spectroscopy, thermal gravimetric analysis, elemental analysis, scanning electron microscopy, transmission electron microscopy, and diffuse reflectance spectroscopy. The chemical reactivity of the core-shell nanoparticles was tested using catalytic reduction of 4-nitrophenol with NaBH₄. The possible growth mechanism of the particles with core-shell morphology has also been investigated.

Full text of DOI:10.1016/j.jallcom.2012.05.086

Journal of Alloys and Compounds 537 (2012) 232–241
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Synthesis of CuO@NiO core-shell nanoparticles by homogeneous
precipitation method
⇑
Nisha Bayal, P. Jeevanandam
Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247 667, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 March 2012
Received in revised form 18 May 2012
Accepted 22 May 2012
Available online 29 May 2012
Core-shell CuO–NiO mixed metal oxide nanoparticles in which CuO is the core and NiO is the shell have
been successfully synthesized using homogeneous precipitation method. This is a simple synthetic
method which produces first a layered double hydroxide precursor with core-shell morphology which
on calcination at 350 °C yields the mixed metal oxide nanoparticles with the retention of core-shell mor-
phology. The CuO–NiO mixed metal oxide precursor and the core-shell nanoparticles were characterized
by powder X-ray diffraction, FT-IR spectroscopy, thermal gravimetric analysis, elemental analysis, scan-
ning electron microscopy, transmission electron microscopy, and diffuse reflectance spectroscopy. The
chemical reactivity of the core-shell nanoparticles was tested using catalytic reduction of 4-nitrophenol
Keywords:
Core-shell nanoparticles
Homogeneous precipitation
Mixed metal oxide nanoparticles
with NaBH
4
. The possible growth mechanism of the particles with core-shell morphology has also been
investigated.
Ó 2012 Elsevier B.V. All rights reserved.
1
. Introduction
When two or more metal oxides are mixed together either by
in turn depend on the synthetic methods that were used for their
preparation. For the synthesis of mixed metal oxide nanoparticles,
different methods such as homogenous precipitation [15–17], co-
precipitation [18,19], impregnation [20], sol-gel [21,22], sono-
chemical [23], micro-emulsion [24], hydrothermal [25], thermal
decomposition [26] and chemical vapor deposition [27] have been
employed. Among the reported preparation methods, solution
based ones possess some advantages; they lead to materials with
fine particle size, high purity and good homogeneity. Among the
solution based methods, homogeneous precipitation has been
widely used since one can easily control the particle size with
the production of powders with uniform size [28–30]. In the
homogenous precipitation method, the chemical composition and
homogeneity of the produced mixed metal oxide nanoparticles de-
pend on concentration of the reagents, pH, aging conditions and
calcination temperature of the precursors [31,32].
physical or by chemical means to produce mixed metal oxide
nanoparticles, a new set of physicochemical properties may be ob-
tained that can be completely different from that of the constituent
metal oxides [1,2]. Sometimes, the combination of two metals in a
common oxide matrix can result in materials with novel structures
such as hollow spheres, nanorods decorated with nanoparticles,
etc. [3,4]. Homogeneity of the synthesized mixed metal oxide
nanoparticles affects the physicochemical properties of the nano-
particles. Mixed metal oxide nanoparticles are well known as phot-
ocatalysts for reactions such as oxidation of organic pollutants in
water and hydrogen generation [5–7]. Some mixed metal oxide
nanoparticles show antibacterial activity. For example, Gordon
et al. have reported antibacterial activity of ZnO–Fe
cles against Escherichia coli and Stapyloccus aureus [8]. Hu et al.
have reported that NiO/SrBi destroys pathogenic bacteria under
2 3
O nanoparti-
Layered metal hydroxides are divalent metal hydroxides of gen-
II
nꢀ
2
O
4
eral formula [M (OH)2-x]A x/n yH
2
O in which some of the missing
visible light irradiation [9]. CuO–NiO mixed metal oxide nanopar-
ticles are well known as catalysts for various organic reactions
such as methanol synthesis, steam reforming and decomposition
hydroxyl groups are compensated by the presence of anions such
ꢀ
ꢀ
3ꢀ
2ꢀ
4
as Cl , Br , NO , or SO
ions in the interlayer space for charge bal-
ance [33,34]. Layered metal hydroxides have been used as precur-
sors for the preparation of mixed metal oxide nanoparticles; the
precursors on calcination lead to the formation of mixed metal oxide
nanoparticles [35–37]. Homogeneous precipitation method has
been widely used for the preparation of layered metal hydroxides.
For example, Jobbagy et al. have synthesized copper containing
[
10–12]. CuO–NiO nanoparticles have been used in humidity sens-
ing [13] and CuO doped NiO fibers have been used as non-enzy-
matic glucose sensors [14].
The physicochemical properties of nanoscale metal oxides de-
pend on particle size, shape, surface area and homogeneity which
2
a-Ni(OH) by homogeneous precipitation using urea as the precipi-
⇑
Corresponding author. Tel.: +91 1332 285444; fax: +91 1332 286202.
E-mail address: jeevafcy@iitr.ernet.in (P. Jeevanandam).
tating agent [38]. Systems such as hollow glass microsphere-
CoFe O , Y Al O12:Er -SiO , MnO /Fe O , TiO /Fe O and Gd/TiO
2 4 3 5 2 2 3 4 2 3 4 2
3
+
0
925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2012.05.086

N. Bayal, P. Jeevanandam / Journal of Alloys and Compounds 537 (2012) 232–241
233
5
0 ml aqueous solution of 4-nitrophenol (5 mmol) and 50 ml aqueous solution of
NaBH (0.529 mol/L) were taken in a 250 ml beaker. Then, 10 mg of the catalyst
CuO/NiO/CuO@NiO) was added and the mixture was stirred at room temperature.
with core-shell morphology have been synthesized using homoge-
neous precipitation method [39–41,17,42,43]. In the literature,
core-shell nanoparticles have been reported to be useful in various
applications such as photocatalysis, antimicrobial activity and tar-
geted drug delivery [44–48]. In the present study, the synthesis of
CuO–NiOmixed metal oxide nanoparticles with core-shell morphol-
ogy using homogeneous precipitation for the first time has been re-
ported. First, a precursor with core-shell morphology is synthesized
which is then calcined to produce the core-shell nanoparticles. The
4
(
The time required for decolorization of 4-nitrophenol solution indicating the com-
pletion of reduction was noted for each sample. A blank reaction was also done
without using any catalyst.
3
. Results and discussion
Fig. 1 shows the powder XRD patterns of NiO, CuO and CuO–NiO
2 3 3
precursors, copper hydroxy nitrate (Cu (OH) NO ) and nickel
precursors. All the diffraction peaks in the XRD pattern of the NiO
precursor (Fig. 1(a)) could be indexed to a-Ni(OH) (JCPDS file No.
2
hydroxide ( -Ni(OH) ), possess layered structures [49,50].
a
2
2
. Experimental
The chemicals used were nickel nitrate hexahydrate (Rankem), copper nitrate
*
Cu
2
(OH)
3
NO
3
Ò
*
# CuO
trihydrate (Himedia), urea (Himedia) and Millipore water. All the chemicals were
analytical grade and used without further purification. More synthetic details are as
follows.
For the synthesis of CuO–NiO mixed metal oxide nanoparticles, 50 ml aqueous
solution of nickel nitrate (50 mmol), 50 ml aqueous solution of copper nitrate
(
50 mmol) and 20 g urea were taken in a 250 ml beaker. The temperature of the
solution was increased to 80 °C and the contents were stirred at this temperature
for 6 h. For the synthesis of pure CuO and NiO nanoparticles, the homogenous pre-
cipitation reactions were carried out using 50 mmol aqueous solutions of copper ni-
trate or nickel nitrate and the precipitation time periods were 2 h (CuO precursor)
and 6 h (NiO precursor). In the case of CuO precursor, synthesis at longer precipita-
tion times (e.g., 6 h) led to the formation of CuO along with the precursor of interest
*
(
2 3 3
Cu (OH) NO , see the XRD results discussed later). The precipitates were washed
many times with water to remove the impurities and they were dried inside an
oven at 80 °C. Finally, calcination of the precursor powders were carried out at
* #
#
#
*
6 hours
hours
*
5
*
*
* *
#
*
3
50 °C inside a muffle furnace (Nabertherm, heating rate = 1 °C/min) to obtain
CuO–NiO mixed metal oxide nanoparticles.
2
Powder X-ray diffraction patterns were recorded using a Bruker AXS-D8 diffrac-
o
tometer (Cu-K
a
radiation) with a step size of 2h = 0.02 . Elemental analysis (CHN)
was done with the help of a Vario ELIII elemental analyzer. Thermal gravimetric
1
30
45
60
75
90
analysis was done in air using a Perkin-Elmer (pyris diamond) instrument at a heat-
2
θ
o
ing rate of 5 /min. Diffuse reflectance spectra were recorded using a Shimadzu UV-
3
600 UV–Vis–NIR spectrometer using BaSO
4
as the reference. FT-IR spectra were re-
Fig. 2. XRD patterns of CuO precursors prepared at two different precipitation
times.
ꢀ
1
corded using a NEXUS Thermo Nicolet IR spectrometer in the range 4000–400 cm
using KBr pellets. Field emission SEM images of the pure and mixed metal oxide
nanoparticles were recorded with the help of a FEI Quanta 200F microscope oper-
ating at an accelerating voltage of 20 kV. Transmission electron micrographs were
recorded with a FEI TECNAI G2 electron microscope operating at an accelerating
voltage of 200 kV. For the TEM measurements, the powder samples were dispersed
in ethanol with the help of a low power sonicator and a drop each from the disper-
sions was allowed to dry on carbon coated copper grids.
#
NiO
*
CuO
*
*
The chemical reactivity of the metal oxide nanoparticles was tested using the
catalytic reduction of p-nitrophenol at room temperature. This test has been used
in the literature to examine the reactivity of metal oxide nanoparticles. About
(001)
#
#
*
*
*
#
*
*
*
*
*
*
*
*
(c)
(
002)
(b)
(a)
(
c)
(
120)
(
b)
(003)
(006)
25
(110)
(a)
10
15
20
30
35
2θ
1
5
30
45
2
60
75
90
θ
(
c)
Fig. 3. XRD patterns of (a) pure NiO, (b) pure CuO and (c) CuO–NiO nanoparticles.
(
b)
a)
(
101
)
(
110
)
(
003
)
(006)
(
Table 1
CHN analysis data for NiO, CuO and CuO–NiO precursors.
1
5
30
45
60
75
90
2θ
Sample
% Carbon
% Nitrogen
% Hydrogen
NiO precursor
CuO precursor
CuO–NiO precursor
5.78
1.05
3.45
6.60
5.83
6.15
1.48
1.26
1.10
Fig. 1. XRD patterns of (a) NiO precursor, (b) CuO precursor and (c) CuO–NiO
precursor. The inset shows the enlarged portion of the XRD pattern in the 2h range,
o
1
0–35 .

2
34
N. Bayal, P. Jeevanandam / Journal of Alloys and Compounds 537 (2012) 232–241
100
100
90
80
70
60
90
80
70
6 hours
2
hours
800
200
400
600
800
200
400
Temperature (°C)
(b)
600
Temperature (°C)
(
a)
100
90
80
70
200
400
600
800
Temperature (°C)
(
c)
Fig. 4. Thermal gravimetric analysis patterns of (a) NiO precursor, (b) CuO precursors prepared at two different precipitation times and (c) CuO–NiO precursor.
o
3
8-0715). The (101) reflection at 2h = 33.17 shows an asymmetry
synthesized at shorter precipitation time (2 h) consists of XRD
peaks only due to Cu (OH) NO and no peaks due to CuO are no-
on the higher angle side and this is attributed to the formation of
2
3
3
turbostratic phase in
tern of CuO precursor (Fig. 1(b)) matches with that of copper hy-
droxy nitrate Cu (OH) NO (JCPDS File No. 74-1749). In the case
of XRD pattern of the mixed metal oxide precursor (Fig. 1(c)), the
a
-type hydroxides [54]. The diffraction pat-
ticed in the pattern. The XRD patterns of all the precursors calcined
at 350 °C are shown in Fig. 3. The peaks in the patterns can be in-
dexed to CuO (monoclinic) and NiO (cubic) phases. The crystallite
size of NiO, CuO in pure and the mixed metal oxide nanoparticles
was calculated using Debye-Scherrer equation and the crystallite
size of pure NiO and CuO are 5.8 and 13.6 nm, respectively. The
crystallite size of NiO and CuO in the mixed metal oxide nanopar-
ticles are 4.7 and 18.4 nm, respectively. The elemental analysis
data for all the precursors are given in Table 1 and the thermal
gravimetric analysis plots are shown in Fig. 4. In the TGA pattern,
the precursor for nickel oxide shows a two step weight loss
(Fig. 4(a)). The first step that occurs at about 130 °C (weight loss
ꢁ8.89%) is attributed to the loss of intercalated water molecules
and the second weight loss step observed at about 300 °C is due
2
3
3
o
(
(
(
003) reflection of
001) reflection of Cu
001) reflection in the case of mixed metal oxide precursor at
a
-Ni(OH)
2
at 2h = 11.91 overlaps with the
2
(OH)
3
NO and hence the most intense
3
o
2
h = 12.62 becomes broader compared to that of pure CuO precur-
sor (See the inset in Fig. 1). Similarly, the (006) and (101) reflec-
o
o
tions of
2
a-Ni(OH) at 2h = 24.29 and 33.17 , respectively, are
close to the peaks due to (002) and (120) planes of the mixed metal
o
oxide precursor at 2h = 25.39 and 33.60 . Thus these peaks in the
mixed metal oxide precursor also look broader compared to those
peaks due to the pure CuO precursor.
Fig. 2 shows the XRD patterns of CuO precursors synthesized at
two different precipitation times (2 and 6 h). The XRD pattern of
CuO precursor synthesized at longer reaction time (6 h) consists
of peaks due to copper hydroxy nitrate and CuO. The precursor
ꢀ
2ꢀ
to the removal of intercalated anions such as NO
OCN and the decomposition of nickel hydroxide to NiO. The total
observed weight loss is about 37.2% which is close to that reported
3
, CO
3
and
ꢀ
2
in the literature for a-Ni(OH) [51,52].

N. Bayal, P. Jeevanandam / Journal of Alloys and Compounds 537 (2012) 232–241
235
1.0
1.0
0.8
0.9
0.6
0.8
0.4
0.7
0.2
0.6
0.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
3.0
3.5
4.0
4.5
5.0
5.5
6.0
hυ (eV)
hυ (eV)
(
a)
(b)
1.0
0.8
0.6
0.4
0.2
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
hυ (eV)
(c)
Fig. 5. Diffuse reflectance spectra of (a) NiO nanoparticles, (b) CuO and (c) CuO–NiO nanoparticles.
Table 2
FT-IR band positions (cmꢀ1) and assignments for NiO, CuO and CuO–NiO precursors.
(
c)
NiO precursor
CuO precursor
CuO–NiO precursor
Assignments
3640
3450
2256
1630
1375
–
3525
3450
–
1630
–
1418, 1340
1043
875
750, 700
511
3640
3490
2250
1630
–
1404, 1340
1043
860
t(OH)
t(OH)
t(CN)
2
d(H O)
ꢀ
t
t
t
t
t
t
t
t
t
s
(COO )
3(NO
1(NO
4(NO
2(NO
3
3
3
3
)
)
)
)
(
b)
–
–
–
–
–
638
745, 695
503
426
(Cu–O–H)
(Cu–O)
(Ni–O–H)
(Ni–O)
426
(
a)
475
–
–
On the basis of thermal gravimetric and CHN analyses, the pro-
posed stoichiometry of the NiO precursor is Ni(OH)1.27(OCN)0.45
4000
3500
3000
2500
2000
1500
1000
500
(CO
)
0.07(NO
)
0.12.0.53H
O. It may be recalled that the NiO precur-
-Ni(OH) . The TGA curves for the CuO
3
3
2
-1
sor is a hydroxy deficient
a
2
Wavenumber (cm )
precursors synthesized with two different precipitation times are
Fig. 6. FT-IR spectra of (a) NiO, (b) CuO and (c) CuO–NiO precursors.
shown in Fig. 4(b). The overall percent weight loss for the CuO

2
36
N. Bayal, P. Jeevanandam / Journal of Alloys and Compounds 537 (2012) 232–241
Fig. 7. FE-SEM images of (a) NiO precursor, (b) CuO precursor and (c) CuO–NiO precursor.
Fig. 8. FE-SEM images of (a) NiO nanoparticles, (b) CuO and (c) CuO–NiO nanoparticles.

N. Bayal, P. Jeevanandam / Journal of Alloys and Compounds 537 (2012) 232–241
237
Fig. 9. EDXA plots of (a) NiO nanoparticles, (b) CuO and (c) CuO–NiO nanoparticles.
Fig. 10. TEM images of (a) CuO–NiO precursor and (b) CuO–NiO nanoparticles.
precursor synthesized at 2 h is about 33.9% which is close to the
theoretical weight loss expected for Cu (OH) NO (33.74%) [53].
Whereas, the precursor synthesized at 6 h precipitation time
shows an overall weight loss of only about 12%. The lower percent
weight loss is attributed to the formation of CuO along with copper
hydroxy nitrate at longer precipitation times, as indicated by the
2
3
3

2
38
N. Bayal, P. Jeevanandam / Journal of Alloys and Compounds 537 (2012) 232–241
Table 3
CuO and the second at 4.17 eV due to NiO. The increase of band gap
in the case of pure CuO and NiO nanoparticles in comparison to the
bulk values is attributed to the quantum size effect owing to the
smaller particle size. The band gap values of NiO and CuO in the
mixed metal oxide is similar to those in the pure metal oxide nano-
particles. In the literature, the increase of band gap compared to
the bulk NiO and CuO has been reported by many authors. For
example, Shi et al. have reported a band gap of about 3.9 eV for
NiO nanotubes and Song et al. have reported a band gap of about
3.7 eV for NiO nanorods [54,55]. One would like to compare the
band gap of CuO nanoparticles prepared in the present study with
the values reported for CuO nanoparticles. For example, Wang et al.
have reported a band gap of 2.43 eV (size = 4 nm), Zhang et al. have
reported a band gap of 2.75 eV (size = 10 nm), and Kumar et al.
have reported a band gap of 2.1 eV (size = 10 nm) [59–61]. In the
present study, the CuO nanoparticles show a band gap of 2.1 eV (si-
ze = 15 nm) whereas in the case of CuO@NiO nanoparticles, the ob-
served band gap for CuO is 1.9 eV (size = 18.4 nm).
Time required for the decolorization of 4-nitrophenol in the presence of different
metal oxide nanoparticles as catalysts.
S.
No.
Reaction condition
Time required for
decolorization (min)
1
2
.
.
4-Nitrophenol + NaBH
4-Nitrophenol + NaBH
nanoparticles
4-Nitrophenol + NaBH
nanoparticles
4
4
No decolorization
16
+ CuO
3
4
.
.
4
4
+ NiO
34
2
4-Nitrophenol + NaBH
nanoparticles
+ CuO@NiO
7.5
7.0
6.5
6.0
5.5
5.0
4.5
The FT-IR spectra of pure precursors (
NO ) and the mixed metal oxide precursor are shown in Fig. 6 and
the IR band assignments are given in Table 2. The broad bands at
2 2
a-Ni(OH) and Cu (OH)3-
3
ꢀ1
about 3450 and 3490 cm are attributed to hydrogen bonded hy-
droxyl groups stretching. Along with these broad bands, there are
ꢀ1
sharp bands observed at about 3640 and 3525 cm which are
attributed to the presence of isolated hydroxyl groups [62]. The
ꢀ1
band at about 1630 cm , present in the spectra of all the precur-
sors, is assigned as the bending vibration of water molecule [63].
ꢀ1
The IR spectrum of
2
a-Ni(OH) shows a band at 1375 cm attrib-
NiO precursor
uted to the presence of carbonate ions in the interlayer. The band
CuO precursor
NiO-CuO precursor
ꢀ1
ꢀ1
at 2256 cm is attributed to OCN group. The band at 638 cm
ꢀ1
is due to Ni–O–H bending and the band at 475 cm is due to
Ni–O stretching [64,65]. Free nitrate ion shows two active vibra-
tion modes, where as the nitrate ion present in the interlayers
shows four vibrational modes [66]. These four vibrations have been
observed in the IR spectrum of CuO precursor; the peak at
0
1
2
3
4
5
6
Time (hours)
Fig. 11. pH changes versus time during the precipitation of NiO precursor, CuO
precursor and mixed CuO–NiO precursor.
ꢀ
ꢀ
1
1
ꢀ1
1
1
043 cm
,
one singlet at 875 cm
340 cm and another doublet at 750 and 700 cm [67]. The IR
,
doublet at 1418 and
ꢀ1
XRD results (Fig. 2). The TGA pattern of the mixed metal oxide pre-
cursor is shown in Fig. 4(c). It shows a two step weight loss; the
first one at 250 °C (weight loss ꢁ21.6%) is due to the decomposi-
spectral features observed in the case of the mixed metal oxide
precursor consist of a combination of bands due to copper hydroxy
nitrate and nickel hydroxide with minor changes in the band
positions.
2 3 3
tion of Cu (OH) NO and the second at about 310 °C (weight loss
ꢁ
12.5%) is due to the decomposition of nickel hydroxide. From
the observed weight loss values and the XRD results (Fig. 1 and
), it can be concluded that the mixed metal oxide precursor is
Cu (OH) NO rich. The percentage of Cu (OH) NO and Ni(O-
H)1.27(OCN)0.45(CO 0.12 0.53 H O in the mixed metal
The FE-SEM image of pure NiO precursor shows petal-like struc-
tures (Fig. 7(a)). The FE-SEM image of CuO precursor (Cu
2
(OH)3-
2
NO ) shows large plate-like morphology (Fig. 7(b)). On the other
3
hand, the FE-SEM image of CuO–NiO precursor (Fig. 7(c)) shows
particles with core-shell morphology. The core consists of large,
elongated plates and at the edges of these plates, petal-like struc-
tures can be noticed. The FE-SEM images of pure and mixed metal
oxide nanoparticles which were obtained by calcination of the cor-
responding precursors are shown in Fig. 8. From Figs. 8(a) and (b),
it can be seen that the morphology of pure NiO nanoparticles is pe-
tal-like as that of its precursor, whereas the plate-like morphology
of the CuO precursor is lost on calcination. The core-shell morphol-
ogy is retained on heat treatment in the case of mixed metal oxide
nanoparticles (Fig. 8(c)). In the case of copper hydroxy nitrate with
plate-like morphology, the thermal decomposition starts from the
edges and during the decomposition, the sudden release of interca-
2
3
3
2
3
3
3
)
0.07(NO
3
)
2
oxide precursor was calculated to be 66.3% and 33.7%, respectively.
Fig. 5 shows the diffuse reflectance spectra for pure and mixed
metal oxide nanoparticles. NiO and CuO are both p-type semicon-
ductors with bulk band gap of about 3.9–4 and 1.2 eV, respectively
[
54–56]. The band gap of nanoparticles depends on the particle
size which is same as the grain size. According to the Brus equa-
tion, the band gap of semiconductor nanoparticles is given by
[
57,58]:
ꢀ
ꢁ
2
2
1:786e2
e R
2
ꢀ
h
p
1
1
EðRÞ ¼ E
g
þ
ꢀ
þ 0:248E
R
ð1Þ
2
ꢂ
ꢂ
2
R
m
e
m
h
ꢀ
2ꢀ
ꢀ
lated species (NO
the structure. But when Cu
Ni(OH) , the sudden collapse of the structure is prevented and
3
, CO
3
, OCN , etc.) as gases leads to collapse of
where E
g
is the band gap of bulk sample, R is the radius of the par-
2
(OH) NO is covered with a shell of a-
3
3
⁄
⁄
ticle, m
e
is the effective mass of electron, m
h
is the effective mass
2
of hole, e is the electronic charge and E
R
is the Rydberg energy.
hence the morphology of the core-shell particles is not lost on
calcination.
Fig. 9 shows the EDXA plots of NiO, CuO and CuO@NiO nanopar-
ticles. The EDX analysis of elements such as Cu, Ni, O showed that
the core as well as the edge contains nickel and copper; the mean
atomic percent of Cu and Ni were found to be about 19.8 and 4.8 in
Fig. 5(a) shows the diffuse reflectance spectrum of pure NiO
nanoparticles and the band gap is estimated as 4.18 eV. The DRS
spectrum of pure CuO nanoparticles shows a band gap absorption
at about 2.1 eV (Fig. 5(b)). CuO–NiO mixed metal oxide nanoparti-
cles show two band gap absorptions (Fig. 5(c)); one at 1.9 eV due to

N. Bayal, P. Jeevanandam / Journal of Alloys and Compounds 537 (2012) 232–241
239
Fig. 12. FE-SEM images recorded at different time intervals during the precipitation of CuO–NiO precursor: (a) 30 min, (b) 1 h, (c) 2 h, (d) 4 h and (e) 6 h.
the core, where as the mean atomic percent of Cu and Ni in the
edge were about 18.8 and 6.8, respectively. The source of carbon
in the EDX patterns is from the carbon conducting tape used for
sprinkling the powder samples during the measurements. Fig. 10
represents the TEM images of CuO–NiO mixed metal oxide precur-
sor and the mixed metal oxide nanoparticles. One can clearly no-
tice the core-shell morphology of the precursor as well as the
mixed metal oxide nanoparticles.
due to the reduction of 4-nitrophenolate ions (yellow) to 4-amino-
phenolate ions (colorless) [70]. The results on the catalytic reduc-
tion of 4-nitrophenol using the metal oxide nanoparticles (CuO,
NiO and CuO@NiO) are given in Table 3. From Table 3, it can be no-
ticed that CuO@NiO core-shell nanoparticles exhibit better cata-
lytic reactivity compared to the NiO and CuO nanoparticles. The
mechanism of formation of particles with core-shell morphology
is discussed below.
The catalytic reactivity of core-shell and pure metal oxide nano-
particles was studied by the reduction of 4-nitrophenol with
To understand the mechanism, the pH changes as a function of
precipitation time for all the three precursors were measured and
2
+
2+
NaBH
oxide nanoparticles [68,69]. An aqueous solution of 4-nitrophenol
is yellow colored which on reduction with NaBH become colorless
4
. This reaction has been used to test the reactivity of metal
are shown in Fig. 11. Precipitation of Cu and Ni ions from a
solution containing both the ions are independent processes
2
+
4
2 3 3
[38,71]. First, Cu ions precipitate as Cu (OH) NO in the pH range

2
40
N. Bayal, P. Jeevanandam / Journal of Alloys and Compounds 537 (2012) 232–241
(
a) Synthesis of copper hydroxy nitrate
[
120]
7
0oC, 6h
Cu(NO ) .3H O + Urea
[001]
3
2
2
Growing particle
of copper hydroxy nitrate
Copper hydroxy nitrate platelets
Nickel hydroxide
(
b) Synthesis of CuO@NiO nanoparticles
Precipitation of
nickel hydroxide
0oC, 6h
7
Cu(NO ) .3H O + Ni(NO ) .6H O + Urea
3
2
2
3 2
2
Calcination at
o
3
50 C
NiO
CuO
CuO@NiO
core-shell nanoparticles
Fig. 13. Schematic illustration of the formation of (a) copper hydroxy nitrate and (b) CuO@NiO core-shell nanoparticles.
5
–6. As the pH exceeds six, dissolution of Cu
2
(OH)
3
NO
3
starts and it
reactivity studies on the core-shell nanoparticles using the cata-
lytic reduction of 4-nitrophenol indicate that the core-shell nano-
particles exhibit better catalytic performance compared to pure
CuO and NiO nanoparticles.
produces CuO [32]. It is proposed that an interplay of urea hydro-
lysis and base consumption due to the precipitation of metal ions
leads to the pH changes [38,72]. It can be noticed in Fig. 11 that
the change in pH observed during the precipitation of pure CuO
precursor is similar to the change in pH observed during the pre-
cipitation of CuO–NiO mixed precursor; in the mixed system, the
pH remains comparatively lower. This is because when both Ni
and Cu ions are present during the precipitation, the base con-
sumption is more. Since 50 mmol of Cu and 50 mmol of Ni (to-
tal = 100 mmol) have to precipitated with the same amount of urea
Acknowledgments
2
+
2
+
The award of research fellowship (JRF/SRF) to Ms. Nisha Bayal
by the Council of Scientific and Industrial Research, Government
of India is gratefully acknowledged. Thanks are due to the Institute
Instrumentation Centre, IIT Roorkee for providing the facilities.
2+
2+
(
20 g), the pH change is lower when the same amount of urea is
used to precipitate these metal ions separately.
The FE-SEM images recorded at different stages of precipitation
References
(
Fig. 12) prove that the Cu2 ions precipitate before Ni ions and
+
2+
[
1] L. Nakka, J.E. Molinari, I.E. Wachs, J. Am. Chem. Soc. 131 (2009) 15544.
particles with plate-like morphology appear first. After the precip-
2
+
2+
[2] J.A. Rodriguez, D. Stacchiola, Phys. Chem. Chem. Phys. 12 (2010) 9557.
itation of Cu ions, Ni ions start precipitating on edges as well as
on core of the plates to produce CuO@NiO precursor core-shell par-
[
[
3] C. Hofmann, I. Rusakova, T.O. Ely, D.P. Centurion, K.B. Hartman, A.T. Kelly, A.
Luttge, K.H. Whitmire, Adv. Funct. Mater. 18 (2008) 1661.
4] D.P. Wang, H.C. Zeng, Chem. Mater. 21 (2009) 4811.
ticles. A schematic illustrating the formation of Cu
CuO@NiO core-shell nanoparticles is shown in Fig. 13. The Ni
2 3 3
(OH) NO and
2
+
[5] R. Sasikala, A. Shirole, V. Sudarsan, T. Sakuntala, C. Sudakar, R. Naik, S.R.
Bharadwaj, Int. J. Hydrogen Energy 34 (2009) 3621.
ions start precipitating first on the edges because the edges are
more reactive compared to the core. The precursor particles on cal-
cination at 350 °C produce CuO@NiO core-shell particles with the
retention of morphology.
[6] S. Chen, W. Zhao, W. Liu, S. Zhang, J. Sol-Gel Sci. Technol. 50 (2009) 387.
[7] Z. Zou, H. Arakawa, J. Photochem. Photobiol., A 158 (2003) 145.
[
8] T. Gordon, B. Perlstein, O. Houbara, I. Felner, E. Banin, S. Margel, Colloids Surf.,
A 374 (2011) 1.
[
9] C. Hu, X.X. Xu, J. Guo, J.H. Qu, Environ. Sci. Technol. 40 (2006) 5508.
[
[
[
10] D. Meloni, R. Monaci, V. Solinas, A. Auroux, E. Dumitriu, Appl. Catal., A 350
2008) 86.
11] L.D. Rogatis, T. Montini, A. Cognigni, L. Olivi, P. Fornasiero, Catal. Today 145
2009) 176.
12] V. Narayanan, S.A. Ananthan, Int. J. Chem. 3 (2011) 55.
(
4
. Conclusions
(
CuO@NiO nanoparticles with core-shell morphology have been
[13] V. Jeseentharani, B. Jeyaraj, J. Pragasam, A. Dayalan, K.S. Nagaraja, Sens.
Transducers J. 113 (2010) 48.
14] F. Cao, S. Guo, H. Ma, G. Yang, S. Yang, J. Gong, Talanta 86 (2011) 214.
15] F. Ribot, S. Kratohvil, E. Matijevic, J. Mater. Res. 5 (1989) 1123.
[16] M. Jobbagy, M.A. Blesa, A.E. Regazzoni, J. Colloid Interface Sci. 309 (2007) 72.
[17] J. Chen, Y. Qian, X. Wei, J. Mater. Sci. 45 (2010) 6018.
successfully synthesized using homogeneous precipitation. This
method provides a good opportunity to synthesize nanoparticles
with controlled morphology. Characterization of the precursors
and the oxides by various analytical techniques indicate the forma-
tion of core-shell nanoparticles. The FE-SEM and TEM images of
CuO–NiO nanoparticles show core-shell morphology and the EDX
analysis indicated that both core and shell consist of copper and
nickel but the shell is nickel rich compared to the core. The present
homogeneous precipitation method may be extended for the prep-
aration of other core-shell metal oxide nanoparticles. Chemical
[
[
[
18] P.C.M. Van Stiphout, D.E. Stobbe, F.T.V.D. Scheur, J.W. Geus, Appl. Catal. 40
1988) 219.
19] M.N. Kopylovich, A.M. Kirillov, A.K. Baev, Russ. J. Appl. Chem. 74 (2001) 12.
(
[
[20] H.G. El-Shobaky, Appl. Catal., A 278 (2004) 1.
[
[
[
21] W. Qin, X. Li, J. Qi, Langmuir 25 (2009) 8001.
22] A.M. Shobe, S.K. Gill, L.J. Hope-Weeks, J. Non-Cryst. Solids 356 (2010) 1337.
23] Z. Zhong, Y. Mastai, Y. Koltypin, Y. Zhao, A. Gedanken, Chem. Mater. 11 (1999)
2350.

N. Bayal, P. Jeevanandam / Journal of Alloys and Compounds 537 (2012) 232–241
241
[
[
24] T. Anukunprasert, C. Saiwan, E.D. Bartolomeo, E. Traversa, J. Electrochem. 18
2007) 295.
25] G.A. El-Shobaky, N.R.E. Radwan, M.S. El-Shall, A.M. Turky, H.M.A. Hassan,
Colloids Surf., A 311 (2007) 161.
26] F. Melo, N. Morlanes, Catal. Today 133–135 (2008) 374.
27] S.G. Shyu, J.S. Wu, S.H. Chuang, K.M. Chi, Y.S. Sung, Chem. Commun. 19 (1996)
[48] E.B. Santos, J.M. Souza e Silva, F.A.A. Sigoli, I.O. Mazali, J. Nanopart. Res. 13
(2011) 5909.
[49] J.W. Lee, J.M. Ko, J.D. Kim, J. Phys. Chem. C 115 (2011) 19445.
[50] S. Pillet, M. Souhassou, C. Lecomte, P. Rabu, M. Drillon, C. Massobrio, Phys. Rev.
B 73 (2006) 115116/1.
(
[
[
[51] Y.L. Zhao, J.M. Wang, H. Chen, T. Pan, J.Q. Zhang, C.N. Cao, Int. J. Hydrogen
Energy 29 (2004) 889.
2
239.
[
[
28] M. Zhang, G. Sheng, J. Fu, T. An, X. Wang, X. Hu, Mater. Lett. 59 (2005) 3641.
29] G.J.A.A. Soler-Illia, R.J. Candal, A.E. Regazzoni, M.A. Blesa, Chem. Mater. 9
[52] G.R. Fu, Z.A. Hu, L.J. Xie, X.Q. Jin, Y.L. Xie, Y.X. Wang, Z.Y. Zhang, Y.Y. Yang, H.Y.
Wu, Int. J. Electrochem. Sci. 4 (2009) 1052.
(
1997) 184.
[53] L. Ilcheva, M. Maneva, P. Bozadziev, J. Therm. Anal. 16 (1979) 205.
[54] C. Shi, G. Wang, N. Zhao, X. Du, J. Li, Chem. Phys. Lett. 454 (2008) 75.
[55] X. Song, L. Gao, J. Am. Ceram. Soc. 91 (2008) 3465.
[
[
[
30] C.K. Srikanth, P. Jeevanandam, J. Alloys Compd. 486 (2009) 677.
31] V. Ranga Rao Pulimi, P. Jeevanandam, J. Magn. Magn. Mater. 321 (2009) 2556.
32] C. Henrist, K. Traina, C. Hubert, G. Toussaint, A. Rulmont, R. Cloots, J. Cryst.
Growth 254 (2003) 176.
33] J. Rocha, M. del Arco, V. Rives, M.A. Ulibarri, J. Mater. Chem. 9 (1999) 2499.
34] R. Rojas, C. Barriga, M.A. Ulibarri, P. Malet, V. Rives, J. Mater. Chem. 12 (2001)
[56] Z.H. Liang, Y.J. Zhu, Chem. Lett. 33 (2004) 1314.
[57] K. Bargohain, S. Mahamuni, Mater. Res. Soc. 17 (2002) 1220.
[58] K.M. Shrestha, C.M. Sorensen, K.J. Klabunde, J. Phys. Chem. C 114 (2010) 14368.
[59] H. Wang, J.Z. Xu, J.J. Zhu, H.Y. Chen, J. Cryst. Growth 244 (2002) 88.
[60] X. Zhang, D. Zhang, X. Ni, H. Zheng, Solid State Electron. 52 (2008) 245.
[61] R.V. Kumar, R. Elgamiel, Y. Diamant, A. Gedanken, Langmuir 17 (2001) 1406.
[62] M.Z.B. Hussein, Z. Zainal, C.Y. Ming, J. Mater. Sci. Lett. 19 (2000) 879.
[63] L. Xu, Y.S. Ding, C.H. Chen, L. Zhao, C. Rimkus, R. Joesten, S.L. Suib, Chem. Mater.
20 (2008) 308.
[
[
1
071.
[
[
[
[
35] C. Li, L. Wang, M. Wei, D.G. Evans, X. Duan, J. Mater. Chem. 18 (2008) 2666.
36] J.P. Ramirez, G. Mul, F. Kapteijn, J.A. Moulijn, J. Mater. Chem. 11 (2001) 821.
37] W. Luo, J.R. Dahn, Chem. Mater. 21 (2009) 56.
38] M. Jobbagy, G.J. De, A.A. Soler-Illia, A.E. Regazzoni, M.A. Blesa, Chem. Mater. 10
(
1998) 1632.
[64] D. Sun, J. Zhang, H. Ren, Z. Cui, D. Sun, J. Phys Chem. C 114 (2010) 12110.
[65] P. Jeevanandam, Y. Koltypin, A. Gedanken, Nano lett. 1 (2001) 263.
[66] H. Niu, Q. Yang, K. Tang, Mater. Sci. Eng., B 135 (2006) 172.
[67] N. Zotov, K. Petrov, M.D. Pankova, J. Phys. Chem. Solids 51 (1990) 1199.
[68] S. Saha, A. Pal, S. Kundu, S. Basu, T. Pal, Langmuir 26 (2010) 2885.
[69] D. Jiang, J. Xie, M. Chen, D. Li, J. Zhu, H. Qiu, J. Alloys Compd. 509 (2011) 1975.
[70] S. Wunder, F. Polzer, Y. Lu, Y. Mei, M. Ballauff, J. Phys. Chem. C 114 (2010)
8814.
[
[
[
[
[
39] C.N. Xie, Z.M. Yang, J. Nanopart. Res. 13 (2011) 347.
40] J. Liu, D. You, M. Yu, S. Li, Mater. Lett. 65 (2011) 929.
41] S. Zhang, S. Wang, S. Li, J. Nanosci. Nanotechnol. 10 (2010) 5612.
42] Q. He, Z. Zhang, J. Xiong, Y. Xiong, H. Xiao, Opt. Mater. 31 (2008) 380.
43] H. Morimoto, M. Minato, T. Nakagama, M. Sato, Y. Kobayashi, K. Gonda, M.
Takeda, N. Ohuchi, N. Suzuki, J. Sol-Gel Sci. Technol. 59 (2011) 650.
44] W. Yan, H. Fan, C. Yang, Mater. Lett. 65 (2011) 1595.
[
[
[
[
45] X. Yan, C. Zou, X. Guo, W. Guo, J. Mater. Chem. 22 (2012) 5629.
46] S. Rana, J. Rawat, R.K.D. Misra, Acta Biomater. 1 (2005) 691.
47] F.H. Chen, Q. Gao, J.Z. Ni, Nanotechnology 19 (2008) 165103.
[71] M. Jayalakashmi, N. Venugopal, B.R. Reddy, M.M. Rao, J Power Sources 150
(2005) 272.
[72] S.K. Meher, P. Justin, G.R. Rao, Appl. Mater. Interfaces 3 (2011) 2063–2073.