CHEM. RES. CHINESE UNIVERSITIES 2011, 27(2), 181—184
Coating Multi-walled Carbon Nanotubes with Uniform
Silica Shells: Independent of Surface Chemistry
LI Ling1, DENG Xiao-yong1*, BAI Ming-kun1, WU Ming-hong1,2* and LIU Yuan-fang1,3
1. Institute of Nanochemistry and Nanobiology, 2. School of Environmental and
Chemical Engineering, Shanghai University, Shanghai 200444, P. R. China;
3. Beijing National Laboratory for Molecular Sciences, Department of Chemical Biology, College of Chemistry
and Molecular Engineering, Peking University, Beijing 100871, P. R. China
Abstract A facile and general method was described to coat six types of multi-walled carbon nanotubes, functionalized by either noncovalent or covalent way, with smooth silica shells. 3-Aminopropyltriethoxysilane(APTES) and
pH value play important roles in the coating process and the thickness of silica shell could be controlled by the added
amount of silicon alkoxides. After the removal of multi-walled carbon nanotubes by calcination, the silica nanotubes
were successfully prepared.
Keywords Carbon nanotube; Silica; Core-shell structure
Article ID 1005-9040(2011)-02-181-04
1
Introduction
Since the discovery of carbon nanotubes(CNTs), studies
on one-dimensional nanomaterials have been a hot research
topic in the interdisciplinary fields[1]. Among them, the structures containing core/shell and hierarchical nanostructures have
attracted intense attention due to their enhanced properties or
multifunctionality for the application in catalysis, bioanalysis,
molecular separation, pollutant decomposition, and the preparation of hydrogen fuel, gas sensors, biosensors, and solar
energy conversion devices[2]. Silica-derivative nanostructures
are of special interest because of their hydrophilic nature, the
ease of functionallization and more widely potential application[3]. In the past decade, smooth, uniform silica shells have
been successfully deposited on a variety of colloidal particles
of metals[4], metal oxides[5], and semiconductor quantum dots[6]
based on the well-known Stöber sol-gel method. A range of
methods have been also developed to coat CNTs with silica,
such as plasma-enhanced chemical vapor deposition[7], precipitation[8], colloidal method[9], catalytic gas-flow reaction method[10], vapor-phase method[11], reverse micelle method[12] and
other solution-based methods[13]. However, some of those
methods require specific decoration before coating, or some of
them need special equipments, while some of them are comparatively complicated.
Herein, we proposed a general method to coat soluble
CNTs, independent of their surface chemistry, with uniform
silica shells. The procedure comprises successive acid- and
base-catalyzed hydrolysis and condensation of silicon alkoxides, and yields homogeneous silica shells on the CNTs with
controlled thickness.
2
Experimental
The raw multi-walled CNTs(MWCNTs) were purchased
from Shenzhen Nanoharbor Company(China), with a diameter
of 10―20 nm, lengths ranging from several microns to tens of
microns, and the purity more than 98%.
Six types of MWCNTs were used in this study, three of
which were covalently functionalized MWCNTs, and the others
were noncovalently functionalized MWCNTs. Through acyl
chloride-active reaction, acid-oxidized MWCNTs were covelently modified with taurine[14], polyethylene glycol(PEG,
Mw=20000)[15] and polyethylenimine(PEI, Mw=25000)[16], respectively, named Tau-MWCNTs, PEG-MWCNTs and
PEI-MWCNTs. The origin and properties of MWCNTs, and
detailed modification procedure were described in our previous
papers[14,15]. The other three kinds of CNTs were from surfactant-wrapped MWCNTs. There were three kinds of surfactants:
cationic, anionic and nonionic surfactants. The representative
and common surfactants in each surfactant group, such as cetyltrimethylammonium bromide(CTAB), sodium dodecyl benzyl sufonate(SDBS) and polysorbate 80(usually called Tween
80) were chosen to solubilize CNTs[17]. Briefly, pristine
MWCNTs were firstly dispersed in a 1%(mass fraction)
aqueous solution of CTAB, SDBS or Tween 80, to give a concentration of 1 mg/mL under sonication for 1 h. Excess CTAB,
SDBS or Tween 80 was removed by several centrifugation and
redispersion cycles. The CTAB-, SDBS- or Tween 80-wrapped
MWCNTs were named CTAB-MWCNTs, SDBS-MWCNTs
———————————
*Corresponding author. E-mail: xydeng@shu.edu.cn; mhwu@staff.shu.edu.cn
Received January 25, 2010; accepted March 15, 2010.
Supported by the National Basic Research Program of China(No.2006CB705604), the National Natural Science Foundation
of China(No.20907028), the Project of Science and Technology Commission of Shanghai Municipality, China(Nos.09XD1401800,
09530501200) and the Project of Shanghai Leading Academic Disciplines, China(No.S30109).
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CHEM. RES. CHINESE UNIVERSITIES
and Tween 80-MWCNTs, respectively.
All the above six kinds of MWCNTs are easily watersoluble and the surface chemistry of them was assessed by zeta
potential measurement, which was performed on a ZetaSizer
3000HSA(Malvern Instruments Ltd., Worcestershire, UK). As
shown in Table 1, the surface potentials of PEI-MWCNTs and
CTAB-MWCNTs are positive. Modification with taurine or
Table 1
Material
Zeta potential/mV
wrap by SDBS generated negatively-charged MWCNTs. Functionallization with PEG or wrap by Tween 80 generated nearly
neutrally, but little negatively charged MWCNTs, which might
come from the small amount of noncovalent carboxyl groups.
The different zeta potential reflects the successful surface modification of MWCNTs.
Zeta potential of MWCNTs
SDBS-MWCNTs
CTAB-MWCNTs
Tween 80-MWCNTs
PEI-MWCNTs
Tau-MWCNTs
PEG-MWCNTs
–62.9
+60.9
–1.7
+4.8
–13.2
–3.5
Then, with these six kinds of MWCNTs as the templates,
the silica coated MWCNTs were synthesized. Typically, ethanol
(100 mL) containing 3-aminopropyltrimethoxysilane(APTES)
and tetraethoxysilane(TEOS) in a molar ratio of 1:9(typically 5
μL of APTES and 45 μL of TEOS) was added to 25 mL of an
aqueous dispersion of MWCNTs(ca. 5 mg) containing citric
acid(1.85 mmol/L, pH=3) under sonication. The mixture was
stirred for 1 h, and sonicated every 20 min. Afterwards, a solution of NH4OH(1 mol/L) was added drop-wisely until a pH
value of 8―9 was reached. Subsequently, the solution was
stirred for 12 h at room temperature. After the centrifugation,
the sediment was collected and washed three times with ethanol
by centrifugation and redispersion cycles. Finally, the darkish
product was obtained and dried at 60 °C in a vacuum oven,
yielding silica-coated MWCNTs.
3
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Results and Discussion
were examined by transmission electron microscopy(TEM,
JEM-200CX) and high-resolution TEM(HRTEM, JEM 2010F).
As shown in Fig.1, independent of the functionallization and
surface chemistry, all the MWCNTs are homogeneously coated
with a uniform layer of silica with a shell thickness of ca.
15―20 nm. The result demonstrates that this method needs no
specific functionallization before coating and appears to be
feasible for preparing a variety of MWCNTs. Moreover, the
thickness of silica shell can be controlled by the amount of
APTES and TEOS(1:9, molar ratio). When 20, 30, 40 and 50
μL of APTES and TEOS were added to the system for preparing silica-coated Tau-MWCNTs, the thickness of silica shells
increased from ca. 2.5 nm to 15 nm(Fig.2). Under the investigation by HRTEM, the silica coated on MWCNTs is amorphous. Furthermore, after calcination at 550 °C, the silica
nanotubes with the smooth inner and outer walls were prepared
[Fig.3(B)].
The morphology and structure of silica-coated MWCNTs
Fig.1
TEM images of Tween 80-MWCNTs(A), SDBS-MWCNTs(B), CTAB-MWCNTs(C), Tau-MWCNTs(D),
PEI-MWCNTs(E) and PEG-MWCNTs(F) after coating with silica shells
Fig.2
Effect of total amount of APTES and TEOS on the thickness of silica shells
As for coating Tau-MWCNTs, the thickness increases from ca. 2.5 nm(A, 20 μL), 5 nm(B, 30 μL) , 10 nm(C, 40 μL) to 15 nm(D, 50 μL).
No.2
LI Ling et al.
Fig.3
183
HRTEM images of silica-coated Tau-MWCNTs(A) and the silica nanotubes after calcinations(B)
There were two steps in the coating procedure. Under
acid-catalyzed conditions, it was likely that the sol-gel-derived
silicon oxide networks yielded primarily linear or randomly
branched polymers, which entangled and formed additional
branches, resulting in the formation of an initial thin and uniform silica shell on MWCNTs[18]. When the pH value increased,
Fig.4
leading to basic condition, more silicon alkoxides hydrolyzed
on MWCNTs, which increased the total silica shell thickness[19].
However, the pH value of alkali-catalyzed condensation was
perfect at 8―9. Once the pH value reached 10 or 11, part of the
silicon alkoxides could form monodispersed spherical silica
nanoparticles in solution(Fig.4).
Effects of pH on silica coating
The pH value in the step of alkali-catalyzed condensation is perfect at 8―9. PEI-MWCNTs are used as the templates.
(A) pH=8; (B) pH=9; (C) pH=10; (D) pH=11.
The crucial factor in the synthetic strategy is the addition
of APTES. In the controlled experiment to grow silica on
MWCNTs without APTES, for example, there are super thin
and rough silica shells on PEI-MWCNTs(Fig.5). The result
shows that APTES plays an important role in the coating
process. First, since APTES easily interacts with the sidewall
and functional groups of nanotubes[20], the APTES molecules
may be firstly absorbed on MWCNT sidewalls for polymerization, and then the surface would become more favorable for the
growth of silica by the hydrolysis of TEOS[21]. Second, APTES
could also catalyze the hydrolysis of TEOS[22,23]. Riegel et al.[22]
indicated that the existence of APTES accelerated the rate of
hydrolysis of 3-glycidoxypropyl-trimethoxysilane. Kim et al.[23]
also reported that the grafted amine groups of a silane coupling
agent containing amine groups(2-aminoethyl-3-aminopropyltrimethoxysilane) on MWCNTs could activate the formation of
silica shell by acid-base interaction.
4
Fig.5
TEM image of super thin and rough silica
shells coated on PEI-MWCNTs without
addition of APTES
Conclusions
In summary, a facile and general liquid process has been
successfully employed to prepare silica shells on either noncovalently or covalently functionallized MWCNTs, independent
of their surface chemistry. These silica-coated MWCNTs can be
further calcined to form hollow silica nanotubes. This new
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CHEM. RES. CHINESE UNIVERSITIES
Vol.27
coating method is expected to prepare other inorganic core/
shell nanostructures with silica shells.
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Fan W., Gao L., Chem. Lett., 2005, 34, 954
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Bottini M., Tautz L., Huynh H., Monosov E., Bottini N., Dawson
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