COMMUNICATION
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Solvent dispersible nanoplatinum–carbon nanotube hybrids
for application in homogeneous catalysisw
Yuhong Chen, Xueyan Zhang and Somenath Mitra*
Received (in Berkeley, CA, USA) 25th September 2009, Accepted 25th January 2010
First published as an Advance Article on the web 5th February 2010
DOI: 10.1039/b919987j
Solvent-dispersible carbon nanotubes/nanoplatinum hybrid
structures are presented, which show excellent catalytic activity
under both heterogeneous and homogeneous conditions.
(o-MWNTs), which were highly dispersible in o-dichlorobenzene
(ODCB), xylene and tetrahydrofuran (THF).
The NP coated unfunctionalized MWNTs, p-MWNTs and
o-MWNTs are referred to as NP-MWNTs, p-NP-MWNTs
and o-NP-MWNTs respectively. Solvent selection was critical
in the reduction of platinum salt during the hybrid formation.
Among the different solvents tested (water, ethanol, THF and
DMF (dimethylformamide)), ethanol was found to be
the most promising one. The controlled decomposition of
1
Carbon nanotubes (CNTs) are effective supports for nano-
2
scale platinum (NP) particles, and together they represent
hybrid structures (NP-CNTs) that combine the unique
3
properties of both. They are expected to find numerous
4
applications ranging from electrochemistry and fuel cells to
5
6
catalysts for chemical reactions. The nanotubes are
platinum dichloride (PtCl
to the formation of relatively uniform nanoparticles that
decorated the MWNTs. The initial concentration of PtCl
2
) under microwave irradiation led
chemically inert and are difficult to disperse in aqueous as
7
,8
well as organic solvents. The dispersibility of NP-CNTs has
not been addressed, and the inherent incompatibility with
solvents and polymers has limited their processibility as
well as applicability. For example, soluble organometallic
compounds containing platinum are routinely used in homo-
2
could be varied to control the final platinum loading, which
was as high as 42% by weight. This approach was also
applicable to other metals, including zirconium, silver,
cobalt, zinc sulfide and cadmium sulfide. In this study, we
demonstrate that the whole process can be carried out
rapidly under microwave induced conditions, leading to the
development of a fast and scalable methodology.
9
geneous catalysis. Dispersible NP-CNTs may very well
represent the next generation of such catalysts. The advantage
of such a system is that the CNTs can be easily separated from
the solution by centrifugation or precipitation. In this paper,
we report for the first time the synthesis of solvent dispersible
NP-CNTs, and their application as catalysts in organic
reactions.
The photographs in Fig.
1 clearly demonstrate the
dispersibility of all the three NP hybrids. The NP-MWNTs
(Fig. 1a and b) showed no evidence of dispersibility in ethanol
or ODCB and sank to the bottom. The p-NP-MWNTs
showed good dispersibility in ethanol due to the presence of
hydrophilic groups on the nanotubes (Fig. 1c). However, its
dispersibility was poor in the organic solvents. Due to the
grafted ODA, the dispersion in the organic phase increased
significantly for o-NP-MWNTs (Fig. 1d). Two colloidal
solutions remained as homogeneous dispersions for several
months without the need for shaking or other forms of
assistance (Fig. 1c and d). The o-NP-MWNTs showed no
aqueous dispersibility as all of the carboxylic groups were
exchanged by ODA. It was evident that the NP-CNT hybrids
retained their respective dispersibility.
Pre-synthesized metal nanoparticles have been deposited on
1
CNTs via self-assembly, a sonochemical process and
0
11
1
2
electron-beam-induced deposition. Many of these methods
require elaborate instrumentation and complex procedures
that limit their real-world applications. Both metals and CNTs
1
3,14
are known to be reactive under microwave heating.
In this
study we developed for the first time a microwave induced
reaction for the synthesis of dispersible NP-CNTs. A detailed
description of the synthetic procedures is reported in the
1
5,16
supplementary information.w
as –COOH were generated via concentrated H SO –HNO
3
Hydrophilic groups such
2
4
treatment under microwave irradiation to render polarity to
the multi-walled carbon nanotubes (p-MWNTs) and make
them dispersible in water and ethanol. The solubility depends
upon the degree of functionalization and can be as high as
Microscopic images of original MWNTs and the dispersible
hybrids are presented to show that the nanotubes did not lose
their structures. Fig. 2a is the original MWNTs and 2 b/d and
c/e are p-NP-MWNTs and o-NP-MWNTs respectively. Scanning
electron microscopic (SEM) images revealed that the NP
were homogeneously deposited on the wall of CNTs. Based
on the SEM and energy dispersive X-ray spectroscopic (EDS)
maps, the distribution of nanoplatinum was quite uniform
with high surface density. The deposition on o-MWNTs
appeared to be more uniform than on p-MWNTs. Trans-
mission electron microscopic (TEM) images clearly showed
that the size of Pt particles ranged from 5 to 50 nm on
p-NP-MWNTs and from 1 to 10 nm on o-NP-MWNTs.
In order to compare the microwave-assisted platinum-salt
decomposition with conventional approaches, the reaction
À1 À1
1
mg mL in the aqueous dispersion and 0.5 mg mL in the
1
5
ethanol. However, the solubility of p-MWNTs in non-polar
solvents was poor. Octadecylamine (ODA) was grafted onto
the nanotube sidewalls to form organic dispersible derivatives
Department of Chemistry and Environmental Science,
New Jersey Institute of Technology, Newark, New Jersey 07102,
USA. E-mail: mitra@njit.edu; Fax: +1 973-596-3586;
Tel: +1 973-596-5611
w Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/b919987j
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652 | Chem. Commun., 2010, 46, 1652–1654
This journal is ꢀc The Royal Society of Chemistry 2010