5790
J. Phys. Chem. C 2008, 112, 5790-5794
Synthesis of Tin (II or IV) Oxide Coated Multiwall Carbon Nanotubes with
Controlled Morphology
Hai-Tao Fang,*,† Xue Sun,‡ Li-Hua Qian,§ Da-Wei Wang,§ Feng Li,§ Yi Chu,‡
Fu-Ping Wang,‡ and Hui-Ming Cheng§
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China,
School of Science, Harbin Institute of Technology, Harbin 150001, China, and Shenyang National Laboratory
for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
ReceiVed: September 10, 2007; In Final Form: February 9, 2008
How to obtain tin oxide (SnO or SnO2) coated multiwall carbon nanotubes (MWNTs) by a simple SnCl2
solution method and finely control the coverage rate and particle size of SnO2 deposits is reported. We find
that the amount of HCl added plays a key role in the synthesis. Without adding HCl, SnO but not SnO2
coated MWNTs is obtained due to the hydrolysis of the SnCl2 in water. In the case of adding the minimum
amount of HCl, which can completely suppress the hydrolysis, a continuous SnO2 coating is formed on MWNTs
with abundant surface graphitic edges. Due to the inhibition effect of HCl on SnO2 growth, increasing the
amount of HCl added leads to the formation of uniformly dispersed and discontinuous SnO2 nanoparticles
with smaller sizes of 2-4 nm and higher crystallinities on the MWNTs. Surface graphitic edges on MWNT
side walls causing high surface energy facilitate the deposition of SnO2 nanoparticles. For MWNTs with
perfect surface structure, few SnO2 nanoparticles are deposited on the nanotubes and the agglomeration of
SnO2 nanoparticles is prone to happen.
1. Introduction
Due to the one-dimensional nanostructure and unique electrical, mechanical, and chemical properties of multiwall carbon
nanotubes (MWNTs), various nanosized metal oxides (such as
SnO2,1-11 SnO,12 TiO2,13,14 ZnO,15,16 MnO2,17,18 RuO2,19,20
Co3O4,21 Eu2O3,22 V2O5,23 ZrO2,24 Cu2O,25 etc.) are coated on
MWNTs to avoid the aggregation and increase the apparent
conductivity of these nanosized metal oxides, thus improving
their various performances. Among these metal oxides, tin oxide
(SnO or SnO2) coated MWNTs are attractive because tin oxides
are very important functional materials in many application
fields. Applications of tin oxide coated MWNTs as sensitive
materials for gas sensors1-5 and biosensors,6 as catalysts for
fuel cells,7,8 as electrode materials for Li-ion batteries,9,12 for
supercapacitors10 and for electrocatalytic decomposition of
organic wastewater,11 have been reported.
Several strategies have been adopted to prepare tin oxide
coated MWNTs. A simple SnCl2 solution method based on the
oxidation of Sn2+ cations by dissolved oxygen26 or by added
HNO327 to form SnO2 at room temperature is used in several
publications.1-3,5,8,10 Besides a chemical precipitation process
using SnCl4 solution,9,11,28,29 a microwave-polyol process30 using
SnCl2 and diethylene glycol, a chemical vapor deposition
method31 using SnH4, a hydrothermal method32 and a vaporphase method33 using SnCl4 have been reported. Among the
above-mentioned methods, the SnCl2 solution method is sim* Corresponding author. Tel: 86-451-86413921. Fax: 86-451-86413922.
E-mail: [email protected].
† School of Materials Science and Engineering, Harbin Institute of
Technology.
‡ School of Science, Harbin Institute of Technology. E-mail: X.S.,
[email protected];Y.C.,[email protected];F.-P.W.,[email protected].
§ Chinese Academy of Sciences. E-mail: L.-H.Q., [email protected]; D.W.W, [email protected]; F.L., [email protected]; H.-M.C., [email protected].
Figure 1. XRD patterns of bare A-MWNTs, SnO/A-MWNT and SnO2/
A-MWNT nanocomposites obtained with different amounts of HCl
added.
plest, as no heating or special equipment is needed. However,
it is still a challenge to effectively and finely control the
oxidation state and morphology of tin oxides by this simple
solution method. In this paper, how to obtain SnO or SnO2
coated MWNTs and control the coverage rate and size of SnO2
nanoparitcles using the SnCl2 solution method is described.
2. Experimental Section
Two types of purified MWNT samples (A-MWNTs and
L-MWNTs), purchased from Shenzhen Nanotech Port Co. Ltd.,
were used in this study. The claimed purity of both MWNTs is
95%. The diameter of A-MWNTs and L-MWNTs is 25-40
and 4-20 nm, respectively.
A simple solution method similar to that reported by Zettl26
was adapted to prepare SnO/MWNT and SnO2/MWNT nanocomposites. The as-received MWNTs were first treated by
refluxing in nitric acid (40%) at 110 °C for 2 h to improve
10.1021/jp077261g CCC: $40.75 © 2008 American Chemical Society
Published on Web 03/21/2008
Tin Oxide Coated Multiwall Carbon Nanotubes
J. Phys. Chem. C, Vol. 112, No. 15, 2008 5791
Figure 3. TGA results of bare A-MWNTs, SnO2/A-MWNTs(0.13 mLHCl) and SnO2/A-MWNTs(0.7 mL-HCl).
JCPDS 06-0395) is identified. Clearly, with 0.13 or 0.7 mL HCl
addition, a SnO2 phase (indexed with JCPDS 41-1445) is
identified. Diffraction peaks from both SnO and SnO2 are
evidently broad, indicating that they are in nanosize.
The effect of the HCl addition on the oxidation state of
tin oxide is associated with the hydrolysis of SnCl2 in
water. Donaldson et al. confirm that if the pH value of SnCl2
solution is in the range of 1.24-4.13, Sn4(OH)2Cl6 colloidal
particles are formed as a result of the hydrolysis of SnCl2
reaction:34
4SnCl2 + 2H2O T Sn4(OH)2Cl6 + 2HCl
Figure 2. TEM images of SnO coated A-MWNT prepared without
adding HCl: (a) low magnification; (b) high magnification.
their dispersibility in aqueous solution by forming oxygencontaining functional groups on their side walls. Acid treated
MWNTs (10 mg) were added into 40 mL distilled H2O
following ultrasonication for 5 min. HCl (38%, 0.13 or 0.7 mL)
was added during the ultrasonication. Then 1 g of hydrous
SnCl2‚2H2O was added, and the solution was stirred for 2 h at
room temperature. Finally, the sediments were filtrated and
washed completely by distilled water and then dried in air at
90 °C for 6 h. SnO2/A-MWNT nanocomposites prepared by
adding 0.13 or 0.7 mL of HCl is denoted as SnO2/A-MWNT(0.13 mL-HCl) and SnO2/A-MWNT(0.7 mL-HCl), respectively.
In the process of preparing SnO/A-MWNT nanocomposite, no
HCl was added during the process.
The bare MWNTs, SnO2/MWNT and SnO/MWNT nanocomposites were characterized by X-ray diffraction (XRD) with
Cu KR radiation, Raman spectroscopy and transmission electron
microscopy (TEM). The content of SnO2 in SnO2/MWNT
nanocomposites was measured by thermal gravimetric analysis
(TGA).
3. Results and Discussion
3.1. Effect of HCl Addition. Figure 1 shows the XRD
patterns of the bare A-MWNTs and SnO/A-MWNT and SnO2/
A-MWNT nanocomposites obtained with different amounts of
HCl added. Without adding HCl, a SnO phase (indexed with
(1)
The pH value of the SnCl2 solution (1 g of SnCl2 in 40 mL
of H2O) is measured to be 1.90. Therefore, the Sn4(OH)2Cl6
colloidal particles are formed once the SnCl2‚2H2O is added
into distilled H2O. Carbon nantoubes dispersed in the solution
supply nucleation sites for the deposition of Sn4(OH)2Cl6
colloidal particles. No Sn4(OH)2Cl6 (JCPDS 15-0676) is found
but the SnO phase is identified in the sample prepared without
adding HCl, indicating that the Sn4(OH)2Cl6 converts into SnO
during the drying process at 90 °C. A possible thermal
decomposition reaction for the formation of SnO is
Sn4(OH)6Cl2 f 4SnO + 2HCl + 2H2O
(2)
Figure 2 shows the TEM images of SnO/A-MWNT nanocomposite. The deposition of SnO nanoparticles on A-MWNTs
is very evident. Therefore, the SnCl2 solution method can be
modified to prepare SnO coated MWNTs.
With adding 0.13 or 0.7 mL HCl, SnO2 but not SnO is
formed. According to reaction 1, adding HCl into the SnCl2
solution can suppress the hydrolysis of SnCl2. The minimum
amount of HCl (38%) added to completely suppress the
hydrolysis of the SnCl2 solution (1 g of SnCl2 dissolved in 40
mL of H2O) is found to be 0.13 mL. The SnCl2 solution is
transparent after adding more than 0.13 mL of HCl, indicating
that no Sn4(OH)2Cl6 colloidal particle exists, and tin composition
exists as Sn2+ cations in the SnCl2 solution. The formation of
SnO2 phase can be ascribed to the reaction between Sn2+ cations
and dissolved oxygen in water:18
2SnCl2 + O2 + 2H2O T 2SnO2 + 4HCl
(3)
Apparently, the existence of Sn2+ cations in the solution is
necessary for the formation of SnO2. If no HCl is added in the
SnCl2 solution, the transformation of Sn2+ cations into Sn4(OH)2Cl6 colloidal particles by the hydrolysis reaction is detrimental
to the formation of SnO2. This is why no SnO2 phase is
identified in the sample prepared without adding HCl.
5792 J. Phys. Chem. C, Vol. 112, No. 15, 2008
Fang et al.
Figure 4. TEM image (a) and HRTEM image (b) of SnO2/A-MWNTs(0.7 mL-HCl). TEM image (c) and HRTEM image (d) of SnO2/A-MWNTs(0.13 mL-HCl).
Figure 5. Raman spectra of A-MWNTs and L-MWNTs after refluxing
in nitric acid.
We find that the amount of HCl added determines the content
of SnO2 in SnO2/A-MWNT nanocomposites. The content of
SnO2 in SnO2/A-MWNT(0.13 mL-HCl) and SnO2/A-MWNT(0.7 mL-HCl), measured by the TG method, as shown in Figure
3, is 64 and 25 wt %, respectively.
The morphology of SnO2/A-MWNT nanocomposites is also
determined by the amount of HCl added. For SnO2/A-MWNT(0.7 mL-HCl), dispersed and discontinuous SnO2 nanoparticles
with sizes of 2-4 nm were deposited on A-MWNTs, as shown
in Figure 4a,b. When the HCl added is decreased to 0.13 mL,
the content of SnO2 increases and a continuous SnO2 coating
is formed on A-MWNTs, as shown in Figure 4c. The reason
why SnO2 deposition on A-MWNTs rather than SnO2 agglomeration happens is discussed in the next section. A HRTEM
image of the SnO2 coating on a nanotube (Figure 4d) clearly
indicates its polycrystalline structure. Some nanoparticles
(marked with circles in Figure 4d) with sizes of 5 nm are
observed. The thickness of the SnO2 coating is about 7 nm.
Because of the higher content of SnO2, one X-ray diffraction
peak of A-MWNT at 2θ of 43° is absent for SnO2/A-MWNT(0.13 mL-HCl), as shown in Figure 1.
The effect of HCl addition on the content and morphology
of the SnO2 deposit in SnO2/A-MWNT nanocomposites is due
to the inhibition effect of HCl on SnO2 growth. According to
reaction 3, excess HCl addition retards the formation of SnO2.
Moreover, H+ ions from HCl can combine with O2- in SnO2
lattice forming H2O, thus decreasing the growth rate of SnO2.
More HCl addition leads to slower SnO2 growth. Therefore,
fewer SnO2 nanoparticles with dispersed and discontinuous
morphology are deposited on A-MWNTs when the amount of
HCl added is as high as 0.7 mL.
The size of SnO2 nanoparticles in SnO2/A-MWNT(0.7 mLHCl) is 2-4 nm, smaller than that in SnO2/A-MWNT(0.13 mLHCl). This phenomenon is also because of the inhibition effect
of HCl on SnO2 growth. Although SnO2/A-MWNT(0.13 mLHCl) has a higher content of SnO2 and larger SnO2 nanoparticles, its XRD peak at 2θ of 26° from SnO2 (110) is less sharp
in comparison with that of SnO2/A-MWNT(0.7 mL-HCl). More
HCl addition leads to lower SnO2 growth rate, which is
advantageous to the ordered growth of SnO2 in atomic scale,
thereby finally obtaining SnO2 nanoparticles with higher crystallinity. Because the crystallinity of SnO2 nanoparticles in SnO2/
A-MWNT(0.13 mL-HCl) is lower than that in SnO2/A-MWNT(0.7 mL-HCl), the XRD peaks of SnO2 for SnO2/A-MWNT(0.13
mL-HCl) are less sharp. Some low ordered structures (marked
with a square in Figure 4d) in a SnO2 nanoparticle of SnO2/
A-MWNT(0.13 mL-HCl) are found using HRTEM.
Tin Oxide Coated Multiwall Carbon Nanotubes
J. Phys. Chem. C, Vol. 112, No. 15, 2008 5793
Figure 6. HRTEM images of A-MWNTs (a, b) and L-MWNTs (c, d).
Figure 7. TEM images of SnO2/L-MWNT nanocomposite.
3.2. Effect of Nanotube Microstructure. To investigate the
effect of nanotube microstructures on the deposition of SnO2,
L-MWNTs with microstructure different from A-MWNTs were
used to prepare SnO2/L-MWNT nanocomposite by the same
process for SnO2/A-MWNT(0.7 mL-HCl). Figure 5 shows the
Raman spectra of A-MWNTs and L-MWNTs after refluxing
in nitric acid. Evidently, the D-band/G-band intensity ratio of
L-MWNTs is much lower than that of A-MWNTs, indicating
that the defect density in L-MWNTs is lower than that in
A-MWNTs. Moreover, radial breathing mode (RBM) peaks are
present in the Raman spectrum of L-MWNTs but not in
A-MWNTs.
Figure 6 shows HRTEM images of these two MWNTs.
Normal nanotubes with graphene layers parallel to its tube axis
(Figure 6a) and herringbone-type nanotubes with graphene layers
oblique to its tube axis (Figure 6b) are found in A-MWNTs.
For the normal nanotubes, many broken graphene layers with
surface graphitic edge structure appear at the surface region.
These broken graphene layers are developed from defects35
(such as topological defects and carbon dangling bond) on the
side walls during the purification process and the nitric acid
treatment. For the herringbone-type nanotubes, abundant surface
graphitic edges exist as well. In comparison with A-MWNTs,
L-MWNTs have more perfect graphene layer structure and fewer
surface graphitic edges, as shown in Figure 6c. Triple wall
MWNTs are sometimes found in L-MWNTs (Figure 6d), and
this observation is consistent with RBM peaks in the Raman
spectrum of L-MWNTs.
Figure 7 shows TEM images of SnO2/L-MWNT nanocomposite prepared by adding 0.7 mL of HCl. The agglomeration
of SnO2 nanoparticles is very serious and very few SnO2
nanoparticles are deposited on L-MWNTs. By comparing the
5794 J. Phys. Chem. C, Vol. 112, No. 15, 2008
microstructure of SnO2/L-MWNT with SnO2/A-MWNT nanocomposites, one can confirm the necessity of abundant surface
graphitic edges on nanotubes for the deposition of SnO2
nanoparticles.
A-MWNTs with many surface graphitic edges have higher
specific surface energies than L-MWNTs with perfect surface
structures. For the SnO2/A-MWNT nanocomposites, to decrease
the surface energy of the whole reaction system much more,
the deposition of SnO2 nanoparticles on surface graphitic edges
happens. But for the SnO2/L-MWNT nanocomposite, due to
the absence of surface graphitic edges and their lower specific
surface energy, the surface energy of the whole system is lower
when SnO2 nanoparticles agglomerate. As a result, SnO2
agglomeration rather than uniform deposition on L-MWNTs is
prone to happen. Surface graphitic edges actually act as
nucleation sites for SnO2 deposition. We believe that surface
graphitic edges on the side walls facilitate the deposition of
various metal oxide nanoparticles by other solution processes.
4. Conclusions
Tin oxide (SnO or SnO2) coated MWNTs with abundant
surface graphitic edges were synthesized by a simple solution
method using SnCl2 solution. For MWNTs with perfect
structure, few SnO2 nanoparticles are deposited on the nanotubes
and the agglomeration of SnO2 nanoparticles is prone to happen.
Because surface graphitic edges bring higher specific surface
energy and serve as nucleation sites for tin oxide deposition,
MWNTs with abundant surface graphitic edges are superior to
MWNTs with perfect surface structure for coating tin oxides
or other metal oxides by solution methods.
Moreover, we find that the HCl addition not only affects the
oxidation state of the coated tin oxide but also determines the
content and the morphology of SnO2 deposit in SnO2/A-MWNT
nanocomposites. Without adding HCl, SnO but not SnO2 coated
MWNTs is obtained due to the hydrolysis of the SnCl2 in water.
Adding the minimum amount of HCl, which can completely
suppress the hydrolysis, synthesizes a continuous SnO2 coating
on A-MWNTs with abundant surface graphitic edges. Increasing
the HCl addition leads to the formation of uniformly dispersed
and discontinuous SnO2 nanoparticles with smaller sizes of 2-4
nm and higher crystallinities on A-MWNTs. Benefiting from
the uniform dispersion of SnO2 nanoparticles with sizes less
than 4 nm on high conductive MWNTs, we believe that SnO2/
A-MWNT(0.7 mL-HCl) can show high performances as catalysts in fuel cells and as electrode materials for the electrocatalytic decomposition of organic wastewater.
Acknowledgment. This work was financially supported by
the National Science Foundation of China (Grant No. 50472084
and 50602011), the Postdoctoral Science Foundation of
Heilongjiang Province, and the Science Foundation of Heilongjiang
Province (E200517).
Fang et al.
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