Microwave-assisted chemical modification of carbon nanohorns: Oxidation and Pt deposition

Article abstract of DOI:10.1016/j.cplett.2006.09.074

Minimum irradiances necessary to chemically modify carbon nanohorns by microwave radiation were investigated using XPS, TEM, and Raman spectroscopy. Only 150 W min, equivalent of operating a home-use microwave oven for less than 10 s, is sufficient to oxidize nanohorns, with the result that nearly all materials used for this study become dispersible in polar solvents. Also, irradiation of less than 250 W min is enough to deposit Pt particles on the nanohorns by the polyol method, with a unique property that Pt particles are deposited mostly at the horn ends.

Full text of DOI:10.1016/j.cplett.2006.09.074

Chemical Physics Letters 433 (2006) 97–100
www.elsevier.com/locate/cplett
Microwave-assisted chemical modification of carbon
nanohorns: Oxidation and Pt deposition
*
Suguru Yoshida, Masahito Sano
Department of Polymer Science and Engineering, Yamagata University, 4-3-16 Jyonan, Yonezawa, Yamagata 992-8510, Japan
Received 2 August 2006
Available online 28 September 2006
Abstract
Minimum irradiances necessary to chemically modify carbon nanohorns by microwave radiation were investigated using XPS, TEM,
and Raman spectroscopy. Only 150 W min, equivalent of operating a home-use microwave oven for less than 10 s, is sufficient to oxidize
nanohorns, with the result that nearly all materials used for this study become dispersible in polar solvents. Also, irradiation of less than
250 W min is enough to deposit Pt particles on the nanohorns by the polyol method, with a unique property that Pt particles are depos-
ited mostly at the horn ends.
Ó 2006 Elsevier B.V. All rights reserved.
1. Introduction
materials. Therefore, it is necessary for many applications
to control surface properties of SWNHs.
A single-walled carbon nanohorn (SWNH) is made of a
single graphene sheet rolled to a conical shape, with poly-
gons of carbon atoms other than hexagons to close its
pointing end [1]. Its diameter ranges from 2 to 5 nm and
the length is ꢀ40 nm. SWNHs typically form a 100 nm
wide spherical aggregate of the secondary structure as if
each cone radiates from the central region. The structure
of the part where cones are united is not known. Laser
ablation of graphite produces uniformly sized SWNHs
with a trace amount of graphitic impurities without using
metal catalyst [2]. High purity of the as-grown sample
and the secondary structure make SWNHs better suited
for applications such as catalyst supports [3–5] and drug
delivery [6,7] than single-walled carbon nanotubes
(SWCNTs). Yet, its chemical characteristics are similar to
SWCNTs because both materials are based on a graphene
sheet. Highly extended uniform p-electron surfaces provide
no pores for other molecules to enter inside; they have low
chemical reactivity and show poor wettability against most
In contrast to SWCNTs, where varieties of chemical
methods are studied to modify their surface properties, oxi-
dation seems to be the only reaction reported so far for
SWNHs except for a recent report on azomethine ylides
[8]. SWNHs have been oxidized by either treating with con-
centrated HNO3 or heating in CO2 or O2 at high tempera-
tures in order to increase pore volumes or support metal
catalysts [9–11]. The former method leaves unwanted acids
that require removal and the latter costs energy. We have
previously shown that SWCNTs are heated efficiently by
microwave radiation and demonstrated that microwave
irradiation is a practical technique for chemical and
mechanical modification [12,13].
Being encouraged by chemical similarity of SWNHs and
SWCNTs, we have examined in the present study if the
same methodology is applicable to SWNHs. In particular,
we focus on the minimum irradiances necessary to modify
the surface properties of SWNHs, because prolonged acid
treatments or higher temperature annealing are known to
open large pores and corrugate the wall [14]. Such a severe
modification obviously changes surface properties and may
cause significant damages on the structure. As we expected
that extensive microwave irradiation causes irreversible
*
Corresponding author. Fax: +81 238 26 3072.
E-mail address: mass@yz.yamagata-u.ac.jp (M. Sano).
0009-2614/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2006.09.074

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S. Yoshida, M. Sano / Chemical Physics Letters 433 (2006) 97–100
structural changes on SWNHs, we examined two indepen-
dent chemical modification schemes in this paper. The first
microwave oxidation of SWNHs showed that only
150 W min was sufficient to improve dispersibility up to
25 times that of the as-grown sample. Then we examined
the second example, Pt nanoparticles directly synthesized
with SWNHs by polyol decomposition. It was found in this
case that less than 250 W min was sufficient and that most
Pt particles were formed at the pointing ends of SWNHs.
2. Experimental
High purity SWNHs were granted from NEC Corpora-
tion. In a typical experiment, 10 mL glass tube containing
1.5 mg of the as-grown SWNH was gently placed in a
microwave reactor (Discover System, CEM) and
2455 MHz radiation was applied for a controlled time at
a given microwave power. After allowing the sample to
cool down to room temperature, 0.3 mg of the irradiated
SWNHs was taken out and dispersed in 4 mL of tetrahy-
drofuran (THF). The mixture was sonicated in a bath soni-
cator for 1 min and was centrifuged at 3000g for 10 min.
The absorbance at 400 nm of the supernatant was used
as a measure of the dispersed amount. A small amount
of the irradiated SWNHs was also examined by X-ray pho-
toelectron spectroscopy (XPS) (Shimazu ESCA-1000, Mg
X-rays). The Raman spectra (Nicolet AlmegaXR) were
taken on the powdered SWNHs using 532 nm excitation.
For the Pt synthesis, 40 mg of SWNHs was added to a
25 mL ethylene glycol solution containing 0.4 mL of
0.4 M KOH and 1.0 mL of 0.05 M H2PtCl6 Æ 6H2O and
sonicated for 30 min. A 2.0 mL of the mixture was trans-
ferred to a glass tube and the microwave at 250 W was
applied for 1 or 2 min. After irradiation, the SWNHs were
collected on a Teflon filter, washed thoroughly with water,
and dried for further examinations.
Fig. 1. (a) Relative dispersed amount of SWNHs in THF (closed circles)
and oxygen concentration (open circles) for various irradiation times at
80 W. (b) Relative dispersed amount (closed circles) and the ratio of the
band intensities (open circles) for various irradiation times at 50 W. (c)
Relative dispersed amount plotted against irradiance for all conditions.
3. Results and discussion
ples gave reproducible values (within 1.5%) on repetitive
measurements. This means that irradiation at the initial
stage effectively cleans the surface of SWNHs. The amount
of additional oxygen introduced by the microwave radia-
tion is estimated to be ꢀ5%. Another point is a drop of
the oxygen concentration at the longest irradiation. In
the case of SWCNTs, carboxylic acids are known to
decompose at lower temperatures than ether or quinone
groups [15]. We speculate that the prolonged irradiation
raises the temperature of SWNHs high enough to decom-
pose carboxylic acids, resulting in a reduced oxygen con-
tent. The remaining oxygen-containing groups, such as
ether and quinone, are expected to provide sufficient polar-
ity to make SWNHs still dispersible in THF.
The relative dispersed amount at 80 W irradiation is
plotted against the irradiation time in Fig. 1a. Also plotted
is the oxygen concentration of irradiated SWNHs mea-
sured by XPS. The dispersibility is clearly improved as
the irradiation time is extended. The dispersed amount is
increased nearly 25 times in comparison with the as-grown
sample. The dispersed amount appears to saturate, because
it corresponds to nearly complete dispersion of the used
material. Starting with a larger amount of SWNHs may
give a still higher saturated amount.
This improved dispersibility is a result of oxidation, as
evidenced by a similar trend exhibited by the oxygen con-
centration curve. Oxygen-containing groups make SWNHs
more polar and lead to better dispersibility in THF. There
are two points to be noted: the oxygen concentration of the
as-grown sample fluctuated significantly by each measure-
ment (from 3% to 9%), which indicated the presence of
organic contaminants. On the contrary, the irradiated sam-
The extent of reaction is also studied by Raman spec-
troscopy. The Raman spectra of SWNHs generally consist
of a G-band around 1580 cmÀ1 and a D-band around
1325 cmÀ1 in similar intensities. The G-band is assigned

S. Yoshida, M. Sano / Chemical Physics Letters 433 (2006) 97–100
99
to axial vibration of the sp2 carbons comprising graphitic
hexagons, and the D-band is considered to be the sp3 car-
bons as well as amorphous carbons. Thus, the ratio of D-
to G-band intensities, ID/IG, gives a measure of the extent
of oxidation. Fig. 1b shows the dispersibility and the ID/IG
ratio at 50 W irradiation for various time intervals. Similar
dependences indicate that the dispersibility is enhanced due
to oxidation, in agreement with the XPS results.
A close inspection of the dispersibility curves shown in
Figs. 1a and b reveals that it takes a shorter time by
80 W irradiation to improve the dispersibility than 50 W.
In fact, previous SWCNT studies [12,13] indicate that the
irradiance (microwave power times irradiation time) is a
relevant quantity in microwave chemistry. Fig. 1c summa-
rizes all data as a function of the microwave irradiance. As
expected, the dispersed amounts at different powers fall
onto a single curve. This suggests that the temperature dur-
ing irradiation is an important factor [12]. Technically, the
temperature of SWNHs during irradiation depends not
only on irradiance but on the amount and packing of the
SWNHs in the glass tube as well. In the present experi-
ment, the latter factor was kept constant. Regardless of
the microwave power, approximately 150 W min is suffi-
cient to improve dispersibility. Irradiation made nearly
all irradiated SWNHs dispersible, demonstrating high effi-
ciency of the microwave process.
One of the expected uses of SWNHs is a metal catalyst
support for fuel cell applications. Microwave radiations
have been used to deposit Pt nanoparticles on SWCNTs
by the polyol method [16]. Thermal decomposition of polyol
(ethylene glycol in the present case) produces species that
reduces Pt salt to afford metallic Pt colloids. We applied
250 W radiation for 2 min to the aqueous reaction mixture
containing polyol, Pt salt and SWNHs, with concentrations
adjusted for 20 wt% loading. We also obtained a similar
result with 1 min irradiation, so the minimum irradiance is
probably smaller. Transmission electron microscopy
(TEM) (Fig. 2a) shows that metal nanoparticles are depos-
ited on SWNHs. XPS (Fig. 2b) reveals that the nanoparticles
are metallic Pt (70.9 and 74.4 eV). The average particle diam-
eter is 3.6 nm and most of the particles fall within 1 nm range
(Fig. 2c). These size and distribution are typical for the Pt
nanoparticles synthesized by the polyol method [16].
Fig. 2d is a histogram of a number of particles vs. the dis-
tance from an estimated center as measured on TEM micro-
graphs. Since TEM images is a 2D projection of a 3D object,
the particle distribution should be parabolic if Pt nanoparti-
cles are distributed only at the pointing ends of a SWNH
Fig. 2. (a) TEM image of Pt supporting SWNHs. (b) XPS spectrum of Pt supporting SWNHs. (c) Diameter distribution of Pt particles on SWNH. (d) A
histogram of a number of Pt particles against the distance from the center of the SWNH imaged in TEM. The solid line is a parabolic function of the
distance.

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S. Yoshida, M. Sano / Chemical Physics Letters 433 (2006) 97–100
sphere. The histogram, which fits well with a parabolic curve,
indicates that the Pt nanoparticles are located mostly at the
horn ends. Microwave radiation heats both ethylene glycol
and SWNHs. In the present short time intervals of irradia-
tion, in which convection and thermal diffusion are insignif-
icant, SWNHs are heated faster than ethylene glycol in bulk.
Although a SWNH is smaller than the wavelength, the near-
field radiation can still be focused at the pointing end. In this
case, thermal decomposition of ethylene glycol is enhanced
near the horn end, where Pt synthesis should occur faster
than other regions. Furthermore, the same focusing effect
oxidizes the horn end faster than other parts of the SWNH,
with additional reactivity gained by non-hexagonal carbons
at the horn end. These two enhancement factors explain the
observed particle distribution.
Industrial
Technology
Development
Organization
(NEDO) as a part of the Nano Carbon Technology
(NCT) project.
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4. Conclusion
Only 150 W min of irradiance has a significant effect on
dispersibility, and less than 250 W min is sufficient to deposit
Pt nanoparticles. The oxidation involves no unwanted acid
or high power. The present study demonstrates that micro-
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We thank NEC Corporation for generous offer of
SWNHs. This work was supported by New Energy and