Structure and morphology controllable synthesis of Ag/carbon hybrid with ionic liquid as soft-template and their catalytic properties

Article abstract of DOI:10.1016/j.jssc.2008.05.006

Ag/carbon hybrids were fabricated by the redox of glucose and silver nitrate (AgNO₃) in the presence of imidazolium ionic liquid ([C₁₄mim]BF₄) under hydrothermal condition. Monodisperse carbon hollow sub-microspheres encapsulating Ag nanoparticles and Ag/carbon cables were selectively prepared by varying the concentration of ionic liquid. Other reaction parameters, such as reaction temperature, reaction time and the mole ratio of silver nitrate to glucose, play important roles in controlling the structures of the products. The products were characterized by XRD, TEM (HRTEM), SEM, energy-dispersive X-ray spectroscopy (EDX), FTIR spectroscopy and a Raman spectrometer. The possible formation mechanism was proposed. The catalytic property of the hybrid in the oxidation of 1-butanol by H₂O₂ was also investigated.

Full text of DOI:10.1016/j.jssc.2008.05.006

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Journal of Solid State Chemistry 181 (2008) 2171– 2177
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Structure and morphology controllable synthesis of Ag/carbon hybrid with
ionic liquid as soft-template and their catalytic properties
Ã
Shu Ying Wu, Yun Sheng Ding , Xiao Min Zhang, Hai Ou Tang, Long Chen, Bo Xuan Li
School of Chemical Engineering, Hefei University of Technology, Hefei 230009, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Ag/carbon hybrids were fabricated by the redox of glucose and silver nitrate (AgNO3) in the presence of
imidazolium ionic liquid ([C14mim]BF4) under hydrothermal condition. Monodisperse carbon hollow
sub-microspheres encapsulating Ag nanoparticles and Ag/carbon cables were selectively prepared by
varying the concentration of ionic liquid. Other reaction parameters, such as reaction temperature,
reaction time and the mole ratio of silver nitrate to glucose, play important roles in controlling the
structures of the products. The products were characterized by XRD, TEM (HRTEM), SEM, energy-
dispersive X-ray spectroscopy (EDX), FTIR spectroscopy and a Raman spectrometer. The possible
formation mechanism was proposed. The catalytic property of the hybrid in the oxidation of 1-butanol
by H2O2 was also investigated.
Received 17 January 2008
Received in revised form
17 March 2008
Accepted 6 May 2008
Available online 14 May 2008
Keywords:
Ionic liquid
Selective synthesis
Ag/carbon hybrid
Catalysis
& 2008 Elsevier Inc. All rights reserved.
1. Introduction
one-pot hydrothermal technique have been realized [11–17];
polypyrrole/poly(methyl methacrylate) coaxial nanocables have
been synthesized by the sequential polymerization of monomers
within the channels of mesoporous silica, followed by dissolution
of the template [18].
Hybrid core–shell nanoparticle or sub-microparticle represents
a new kind of structure, which has attracted much attention and
has been intensively investigated in recent years. They possess
interesting and special properties which are due to their improved
physical and chemical properties over their single-component
counterparts. Core–shell spheres encapsulating various materials
have drawn considerable attention because of their applications in
many fields such as electrons, magnetism and optics [1]. In
addition, as a new type of one-dimensional nanostructure, the
core–shell nanocables have also attracted broad interest, because
their functions can be further enhanced by fabricating the core
and sheath components from different materials [2–7].
Various strategies have been developed to produce nano-
cables and core–shell spheres. Sastry and co-workers have used
chloroform as a reaction medium to synthesize core–shell Ag–
Au nanoparticles by the replacement reactions between hydro-
phobic spherical Ag nanoparticles and hydrophobized AuCl4 in
toluene [8]. Colloidal carbon spheres and their core/shell
structures with noble-metal nanoparticles have been fabri-
cated by Sun and Li [1]. Meanwhile, the nanocables have also
been developed quickly. They have been synthesized by high-
energy input approaches, including the chemical vapor deposi-
tion route and the carbothermal reduction method [9,10]. In
addition, several mild chemistry methods emerge, for example,
a series of work of synthesis of carbon-based nanocables by the
The noble metals in nanoscales have become a subject of
intense interest in various fields because of their outstanding
properties. It has been experimentally demonstrated that metal
nanoparticles have high catalytic activities for hydrogena-
tion, oxidation, carbonylation, etc. [19–24]. For example, silver
nanoparticles catalyze the reduction of aromatic nitro compounds
[25]. However, they would often congregate during catalytic
processes. The reason is that nano-sized metal particles are active
and prone to coalesce due to van der Waals forces and high
surface energy unless they are protected. The structure of
supporting materials plays an important role in the activity
of the catalyst. Carbon spheres and carbon nanotubes, because of
their interesting properties and large surface area, have been
receiving increasing attention for their application in catalyst
support [26].
Herein, we report a controllable method to prepare Ag/C
nanocables and carbon hollow spheres encapsulating Ag
nanoparticles in their cores. In the experiment, one kind of
imidazolium ionic liquid, 1-n-tetradecyl-3-methylimidazolium
tetrafluoroborate ([C14mim]BF4), was used as the soft-template
to prepare the hybrid. We can alternatively obtain Ag/C nanoc-
ables or carbon hollow spheres encapsulating Ag nanoparticles in
their cores by altering the concentration of [C14mim]BF4, which
realized the controllable synthesis of Ag/C hybrids. Meanwhile,
the catalysis in the oxidation of 1-butanol by H2O2 was also
investigated.
Ã
Corresponding author. Fax: +86 5512901545.
E-mail address: dingys@hfut.edu.cn (Y.S. Ding).
0022-4596/$ - see front matter & 2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2008.05.006

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2. Experimental section
typical TEM image of the Ag/C hybrid, indicating that the products
consist of hollow spheres with Ag nanoparticles encapsulating in
their cava. The diameter of the hollow spheres is about 346 nm
and the shell thickness is 114 nm. The diameter of Ag nanopar-
ticles here is about 25 nm. We could also find that the hollow
spheres are uniform in size. We have taken the photographs of a
single sphere from different directions. The sphere in Fig. 1c is the
same as in Fig. 1d. The two pictures show that the center of
the sphere is hollow and the silver nanoparticle encapsulating in
the cava lies on the wall of the shell.
Fig. 2 shows a typical XRD pattern of the products and the
primary peaks marked with rhomboids can be indexed to the
diffraction of (111), (200) and (220) planes of fcc silver. We can
observe that there are also peaks of AgBr, which are marked with
asterisks in the pattern. The diffraction peak of the (200) plane of
silver metal appeared at 44.301. And its position is so close to the
(220) peak of AgBr (2y, 44.611) that they could not be
differentiated under the experimental conditions. The ionic liquid
we used was synthesized by ionic exchange between [C14mim]Br
and NH4BF4. The residual bromine ions can react with AgNO3,
therefore forming AgBr. Silver bromide might play an important
role in the formation of Ag nanoparticles, which will be illustrated
in the following text. The wide and weak peak around 201 can be
indexed to amorphous carbon.
2.1. Preparation of [C14mim]BF4
All chemical reagents were of analytic purity and they were
used without further purification. The imidazolium ionic liquid, 1-
n-tetradecyl-3-methylimidazolium tetrafluoroborate ([C14mim]
BF4), was synthesized by the ionic exchange reaction between 1-
n-tetradecyl-3-methylimidazolium bromide ([C14mim]Br) and
NH4BF4. The [C14mim]Br was synthesized according to the lite-
rature [27].
2.2. IL-assisted hydrothermal synthesis of Ag/C nano-hybrid
Here we adopted the hydrothermal carbonization co-reduction
process (HCCR) [28] to synthesize Ag/C hybrids. In the process,
0.5 g [C14mim]BF4 and 0.5 g glucose were dissolved in 35 ml
distilled water. Then, 0.5 ml of 0.1 M AgNO3 was added in
dropwise manner under vigorous magnetic stirring. The solution
turned yellow. Followed by 5 min of agitation, the solution was
transferred into a 50 ml Teflon-lined stainless steel autoclave,
which was sealed and maintained at 180 1C for 3 h and then air-
cooled to room temperature. The resulting suspension was
centrifugated at 3000 rpm for 30 min to obtain a black precipitate,
which was then washed by distilled water. The product was re-
dispersed in absolute ethanol for characterization.
2.3. The catalysis in the oxidation of 1-butanol by H2O2
The 0.01 g products prepared by the process described above
were added into a 100 ml three-neck round flask, and then 20 ml
1-butanol was introduced into the flask. The mixture was treated
ultrasonically for about 15 min, which could make the particles
disperse better in the 1-butanol. The 20 ml H2O2 was dripped with
the dropping funnel when the system was heated to 80 1C. The
mixture was stirred for 8 h at 80 1C. After 8 h of reaction, the
mixture was cooled naturally. Then the catalyst was removed by
centrifugation. There are two delaminations and the super
stratum was spilled for characterization.
2.4. Characterization
The particle size and morphology of the resulting products
were characterized using a field emission SEM (X-650 made by
HITACHI), operating at accelerating voltages of 20 kV and trans-
mission electron microscopy (TEM) (H-800) at accelerating
voltages of 100 kV. High-resolution TEM images were obtained
on a JEOL-2010 high-resolution electron microscope at accelerating
voltages of 200 kV. This instrument was equipped with an energy-
disperse X-ray spectroscopy (EDX) system. The powder X-ray
diffraction (XRD) patterns were recorded on a D/Max-rB diffract-
˚
ometer (Rigaku, Japan) with CuKa (l ¼ 1.5406 A). The samples
were also characterized by FTIR spectroscopy (NICOLET MAGNA-
750) and a Raman spectrometer (RAMANLOG6). The oxidative
products were characterized by FTIR spectroscopy (NICOLET
MAGNA-750). The gas chromatograph, which was used to measure
the conversion of the 1-butanol, was GC-9A (SHIMADZU, Japan).
3. Results and discussion
3.1. Characterization of the silver nanoparticles
Fig. 1. SEM and TEM images of the Ag/carbon products prepared at 180 1C for 3 h:
(a) the representative SEM picture; (b) TEM picture of the hollow spheres with Ag
nanoparticles in their cores; (c) and (d) are the TEM pictures of a single sphere.
Fig. 1a shows the scanning electron microscopy (SEM) image,
which shows that the products consist of spheres. Fig. 1b shows a

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♦
♦
♦
*
10
20
30
40
2Theta (deg.)
50
60
70
Fig. 2. XRD pattern of the as-prepared Ag/carbon products.
C
C
d2
d1
To our surprise, we detected a novel structure after detailed
TEM observation. There are some spheres that are linked by thin
wires. Fig. 3a is the picture taken at a relatively low magnification.
From it we can find that there are thin wires, which assemble the
hollow spheres into a novel and interesting structure. In order to
know the structure, we detected it by high-resolution TEM. Fig. 3b
is the image. It can be easily observed that the thin wire is just the
extension of the shell. The contrast between the inner and the
outer of the wire can be observed and it is due to the different
thicknesses.
We used energy-dispersive X-ray analysis (EDX) to investigate
the elemental composition of the noble-metal-loaded hollow
spheres. Fig. 3d1 is the energy-dispersive X-ray spectra taken from
the nanoparticles in the inner side of the spheres. It illustrates that
the nanoparticles are made of metallic silver and carbon. In our
reaction system, the concentration of AgNO3, which was the silver
source, was low and the quantity of glucose was relatively
excessive compared to AgNO3. Therefore, we can find from the
spectra that the content of carbon was much higher. Fig. 3d2
shows the elemental composition of the shell of the hollow
spheres. It is constituted only by element carbon, and the existed
element oxygen derives from glucose.
O
O
CuKa
CuKb
CuKa
CuKb
CuLa
Ag
0
1
2
3
4
5
6
7
8
9 10
0
1
2
3 4 5 6 7 8 9 10
Energy (kev)
Energy (kev)
Fig. 3. TEM images of the Ag/carbon products: (a) panoramic view of the
assembled structure at relative low magnification, (b) high-resolution TEM picture
of an assembled wire, (c) an individual Ag/carbon hollow sphere, (d1) energy-
dispersive X-ray spectrum taken from d1 area in (c) and (d2) energy-dispersive X-
ray spectrum taken from d2 area in (c).
size of the cavity became smaller with time, that is, the shell of
the hollow spheres became thicker. After the reaction time was
extended to 48 h, there were many solid carbon spheres (Fig. 4b).
This indicates that carbonization of glucose takes place in the
inner side of the hollow spheres. There may be pores in the
carbonaceous shell; therefore, glucose molecules can continu-
ously diffuse into the inner side of the hollow spheres via capillary
action [29]. As a result, with time, the hollow spheres gradually
become solid spheres by the continuous carbonizing of glucose.
3.2. The controlled synthesis of Ag/carbon nano-hybrids
3.2.1. Effect of temperature on the structure and morphology of
Ag/carbon hybrid
The typical reaction was carried out at 180 1C for 3 h and we
obtained the hollow sub-microspheres loading Ag nanoparticles
in their cores described as above. We varied the reaction
temperature at 140, 160, 200 and 220 1C. All reactions were
carried out for 3 h. We found that there were no expected
products when the temperature was lower than 160 1C. The
structures were irregular when the reaction was carried out at
temperatures higher than 180 1C. Fig. 4a is the TEM image of the
product synthesized at 200 1C. It shows that the silver nanopar-
ticles in the cores are irregular and the sizes of the hollow spheres
are uneven.
3.2.3. Effect of the mole ratio of AgNO3 to glucose on the structure
and morphology of the Ag/carbon hybrid
The proportion of AgNO3 to glucose plays an important role in
determining the morphology of the product. In
a typical
experiment, 0.5 g glucose and 0.5 ml 0.1 M AgNO3 were added
into the reaction system and we obtained carbon hollow sub-
microspheres in which the silver nanoparticles were encapsu-
lated, and the silver nanoparticles were regular. However, the Ag
particles assembled into triangle, near-cubic or other irregular
silver particles when the mole ratio of AgNO3 to glucose
increased. For example, when the concentration of AgNO3 was
increased to 0.8 M, there were different irregular silver particles in
3.2.2. Effect of reaction time on the structure and morphology of the
Ag/carbon hybrid
We also investigated the influence of the synthesis time on the
shape of the product. Based on the observation, we found that the

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Fig. 4. Representative TEM images of the Ag/carbon products prepared at (a) 200 1C, 3 h; (b) 180 1C, 48 h; (c) the AgNO3 concentration was increased to 0.8 M and (d) the
dosage of [C14mim]BF4 was reduced to 0.1 g while the other reaction parameters were the same as the typical experiment; and (f) electron diffraction pattern taken from an
individual cable (e).
the carbon hollow sub-microspheres as shown in Fig. 4c. Fig. 4c
also shows that the thickness of the shell became thinner (about
47 nm) when the mole ratio of AgNO3 to glucose increased.
the carbonization of glucose took place and produced some
aromatic compounds and oligosaccharides in our experiment. The
FTIR spectrum of the hybrid (Fig. 5a) shows that there are residual
hydroxyls after the hydrothermal treatment. And according to the
spectrum, we find that CQO and CQC groups exist in the
product, which supports the concept of aromatization of glucose
during the hydrothermal reaction. From the FTIR spectrum, we
can also see the weak vibration at 1377 and 1463 cmꢀ1. They can
be attributed to COOꢀ groups, which indicate the occurrence of
redox reactions. The group of CHO of glucose acts as a reductive
group while silver ion plays the role of an oxidant.
We used Raman spectra analysis to observe the structure of the
carbon shell of the hollow spheres. There was a strong peak at
1580 cmꢀ1 (Fig. 5b), which was attributed to the in-plane
vibrations of crystalline graphite. From the spectra we can also
detect that there was a weak peak at 1360 cmꢀ1, which was
attributed to disordered amorphous carbon.
3.2.4. Effect of the ionic liquid’s concentration on the structure and
morphology of the Ag/carbon hybrid
Among the various parameters, the concentration of the
soft-template is particularly crucial for the control of the structure
and morphology of the particles. We gained hollow spheres
when 0.5 g ionic liquids were added in the typical reaction. In
order to explore the function of ionic liquid in determining
the structure and morphology of the hybrids, we reduced the
dosage of [C14mim]BF4 to 0.1 g. It was interesting that we
obtained Ag/C cables with diameters of 360 nm. The silver cores
are straight and uniform in diameter (around 112 nm) throughout
their entire length (shown in Fig. 4d). Fig. 4e is an individual
cable and its electron diffraction pattern is shown in Fig. 4f with
In our approach, glucose acts as the carbon source and
reducing agent, while the noble-metal salt, AgNO3, acts as the
source of the metal to be encapsulated. The silver nitrate has been
confirmed to be the catalyst of the saccharide’s carbonization
under suitable hydrothermal condition [28]. The carbonization of
glucose and the redox between AgNO3 and glucose took place
indeed according to the above discussion.
¯
the electron beam focused perpendicularly on the surface ð022Þ.
The SAED pattern indicates the single-crystalline nature of the
Ag core.
3.3. Formation mechanism of the Ag/carbon hybrids
3.3.1. The reaction in our system
The temperature 180 1C is higher than the normal glycosida-
tion temperature and it leads to aromatization and carbonization
[1]. Under the hydrothermal condition, various chemical reactions
of glucose can take place and result in a complex mixture of
organic compounds [17]. After hydrothermal treatment, the
resulting solution took an orange color. This phenomenon was
consistent with the work of Sun and Li [1]. Thereby, we believed
3.3.2. Mechanism for the formation of the Ag/carbon hybrid
A possible formation mechanism is schematically illustrated in
Fig. 6. The silver ions interact with the glucose molecules and are
deoxidized by glucose to small Ag nanocrystals. The residual
bromine ions react with the silver ions and result in the AgBr
crystal, which offers the nucleation center, and then the small Ag

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nanoparticles, the hydrophobic tails are tilted to the surface to
form micelles in the water solution. And there is an ionic liquid
bilayer on the silver nanoparticles. The bilayer adsorbed on Ag
clusters makes Ag nanoparticles positively charged and the
resulted electrostatic repulsion prevents micelles from aggregat-
ing. Under the hydrothermal condition, glucose can be carbonized
and the micelles serve as the charing core. The carbon spheres
that cap the micelle then form Ag particles that remained in the
core of the spheres.
-
COO
C=O
C=C
The reaction in the absence of ionic liquid only brought about
irregular solid spheres. Hence we believe that ionic liquid acts as a
soft-template, which has the effects of a shape controller and a
stabilizing agent in the reaction. During the reaction, ionic liquid
adsorbs on the silver crystal faces and the adsorption character-
istics depend on its concentration. The result shows that when the
concentration of the surfactant is reduced to one-fifth of that of
the typical reaction, the surfactant would adsorb anisotropically
on the silver crystal faces, which direct the silver metal to grow
into linear structures. While the ionic liquid adsorbs uniformly on
the silver crystal faces in the typical synthesis, the crystal grows
into spherical particles. In the typical experiment, the wires that
linked the spheres were supposed to be formed by carbonation
directed by rod micelles. The majority of [C14mim]BF4 adsorbed
OH
4000
3500
3000
2500
2000
1500
1000
Wavenumber (cm-1)
GC (1580 cm-1)
AC (1360 cm-1)
on the surface of the silver crystal and formed
a bilayer.
Meanwhile, there were also some surfactants that arranged in
rod micelles. The rod micelles offered the carbonation center to
form carbon wires.
3.4. Catalysis of Ag/carbon hybrid in the oxidation of 1-butanol
Since the Ag/C hybrid with a core–shell structure could act as
highly dispersed electrodes, they should help in the electron
transfer between the alcohols and oxidant donors, so they could
have the catalytic effects on the oxidation of alcohols. Here we
selected 1-butanol and H2O2, one kind of green oxygen donors, to
study the catalysis of Ag/C nano-hybrids with the hollow sphere
structure in the reaction of alcohol oxidation.
600
800
1000
1200
1400
1600
1800
2000
Fig. 7 shows the FIIR spectrum of the oxidation product. We
can see that the products are identical irrespective of the silver
catalyst being added or not, and it shows oxidation indeed
happens. When the reaction temperature was 80 1C, and 0.01 g
(0.061 percent of the weight of 1-butanol) Ag/C hybrid was added,
the conversion of 1-butanol in the reaction system was close to
the reaction without a nano-hybrid (Table 1, entries 1 and 2).
However, the silver catalysts showed effective catalytic activity in
the experiment conducted at 110 1C. The conversion increased
from 48.70% to 68.94% when 0.01 g catalysts were used (Table 1,
entries 3 and 4). Increasing the dosage of catalysts to 0.02 g only
brought about an increment of 1.1% (Table 1, entries 4 and 5), so
the Ag/C hybrid has high catalytic efficiency in the oxidated
reaction of 1-butanol by H2O2. The support, carbon shell, could
prevent the silver nanoparticles from congregating. Nevertheless,
it could also reduce the interface of the silver particles and the
reactant, thereby leading to catalyst deactivation. In order to
demonstrate this, we ultrasonically treated the reaction mixture
so as to make the catalyst more dispersive. In the initial
experiment, the ultrasonic treating time was 15 min. And we
found that the conversion increased from 68.94% to 75.78%
(Table 1, entries 4 and 6) when the treating time was extended to
1 h. So the conclusion can be drawn that the pretreatment that
excitated the catalytic activity plays an important role. Other
pretreatment methods that may be more effective to explode
the catalysts’ activity are being studied. The catalytic process
can be explained by an electrochemical mechanism, silver in the
supported carbon spheres serves as an electron relay for an
Raman shift (cm-1)
Fig. 5. (a) FTIR spectra of the Ag/carbon hybrid particles and (b) Raman spectra of
the Ag/carbon hybrid particles.
Fig. 6. The schematic formation of the Ag/carbon structure.
nanocrystals aggregate onto AgBr to form Ag nanoparticles. Once
the Ag nanoparticles appear, they are captured by the organic
cations of the ionic liquid. According to the work of Chen’s group
[30], we suppose that the adsorption and structural configuration
of the ionic liquid on the surface of the Ag nanoparticles are
similar to CTAB (cetyltrimethylammonium bromide) on the
surface of the Ag nanoparticles. The headgroups of the organic
cations of the ionic liquid adsorb on the surface of the

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°
°
80 C blank
80 C Ag catalyze
2818
CHO
1352
1630
C=O
2820
CHO
3427
OH
C=O
1635
1343
3267 OH
4000 3500 3000 2500 2000 1500 1000 500
4000 3500 3000 2500 2000 1500 1000 500
C-O-H
1199
°
°
110 C Ag catalyze
110 C blank
C-O-H 1203
1076
C-O-C
1625
1715
C=O
1623
1715
1063
C-O-C
C=O
2876
2967
2873
2965
OH
3417 OH
3405
4000 3500 3000 2500 2000 1500 1000 500 4000 3500 3000 2500 2000 1500 1000 500
wavenumber (cm-1) wavenumber (cm-1)
Fig. 7. FTIR spectrum of the oxidation products.
Table 1
Acknowledgments
Conversion of the oxidation of 1-butanol
We thank Prof. Jun Zhang and Zhi-gang Wang at Institute of
Chemistry, Chinese Academy of Sciences (CAS) for insightful
discussions on the synthesis of Ionic Liquid and its application,
and also thank Dr. Qing Yang and Dr. Helin Niu at Nanochemistry
and nanomaterials, Hefei National Laboratory for Physical
Sciences at Microscale for their kind help on the characterization.
This research is financially supported by Nature Science Founda-
tion of Anhui Province (5044095), National Science Foundation
of China (10590355), Annual Important Scientific Research Items
of Anhui Province (06022016) and Important Key R&D Projects of
Anhui Province (07010202025).
Entry
Catalyst
Ultrasonic
treating
Reaction
temperature
( 1C)
Conversion
(%)
dosage (g)
time (min)
1
2
3
4
5
6
0.00
0.01
0.00
0.01
0.02
0.01
15
15
15
15
15
60
80
80
37.50
36.70
48.70
68.94
70.04
75.78
110
110
110
110
oxidant and a reductant. The function of metal nanoparticles as a
nanoelectrode makes it possible to catalyze other redox reactions
via an electron transfer-storage process.
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