Metal nanoparticles related materials supported on carbon nanotubes: methods and applications.

1. Functionalization
DOI: 10.1002/smll.200500324
Metal Nanoparticles and Related Materials Supported
on Carbon Nanotubes: Methods and Applications
Gregory G. Wildgoose, Craig E. Banks, and Richard G. Compton*
From the Contents
1. Introduction.............183
2. Methods of
Functionalizing CNTs
with Metal
Nanoparticles...........183
3. Applications of CNT-
Supported
Nanoparticles...........188
4. Summary and Outlook
................................191
Keywords:
· carbon nanotubes
· catalysis
· functionalization
· nanoparticles
· sensors
Functionalizing nanotubes: MoO2 is deposited at the end of multiwalled carbon nanotubes.
182 www.small-journal.com 2006 Wiley-VCH Verlag GmbH Co. KGaA, D-69451 Weinheim small 2006, 2, No. 2, 182 – 193
reviews R. G. Compton et al.

2. Carbon nanotubes are one of the most intensively explored nanostructured
materials. In particular, carbon nanotubes are unique and ideal templates
onto which to immobilize nanoparticles allowing the construction of
designed nanoarchitectures that are extremely attractive as supports for
heterogeneous catalysts, for use in fuel cells, and in related technologies that
exploit the inherent smallness and hollow characteristics of the nano-
particles. Here we overview the recent developments in this area by exploring
the various techniques in which nanotubes can be functionalized with metals
and other nanoparticles and explore the diverse applications of the resulting
materials.
1. Introduction
The applications and scope of carbon nanotubes (CNTs)
have dramatically increased and continue to expand since
their rediscovery in 1991. Oberlin and Endo[1]
reported in
1976 that carbon fibers had been prepared with various ex-
ternal shapes that contained a hollow tube with diameters
ranging from 20 to more than 500 Š along the fiber axis.
They observed stacks of carbon layers, parallel to the fiber
axis, which where arranged in concentric sheets. They also
noted that very small cementite crystals, typically about
100 Š in diameter, were formed at the tip of the central
tube of each fiber.[1]
In 1978, Wiles and Abrahamson first
mentioned carbon fibers down to 4 nm in diameter (namely,
carbon nanotubes, although the term “nanotubes” originates
from the 1990s) found on a graphite electrode.[2]
In their
publication, Wiles and Abrahamson described a thick mat
of fine fibers and crystallites which they found on graphite
and carbon anodes following low-current arc operation in
nitrogen at atmospheric pressure. They observed fibers
ranging in diameter from %4 nm up to 100 nm with lengths
up to 15 micrometers, which also held many small crystalline
particles. Further details of the structure of the carbon
fibers were presented at a conference in 1979,[3]
which were
then republished elsewhere.[4]
From their electron diffrac-
tion study Abrahamson et al. reported that the fibers con-
sisted of wrapped graphitic basal layers with a hollow core.
Also they noted that the basal layer spacing was distorted
from the normal graphitic spacing, and larger. Carbon nano-
tubes were then reported by Iijima in 1991[5]
and the
number of publications each year utilizing carbon nanotubes
escalates at a hugely increasing rate.
Carbon nanotubes have intrinsic properties, which in-
clude, high surface area, unique physical properties and
morphology, high electrical conductivity, and their inherent
size and hollow geometry can make them extremely attrac-
tive as supports for heterogeneous catalysts.[6–10]
A plethora of literature therefore exists on the function-
alization of carbon nanotubes with metal particles. In this
Review, we critically overview literature reports of metal
nanoparticles supported on carbon nanotubes and selective-
ly explore their applications.
2. Methods of Functionalizing CNTs with Metal
Nanoparticles
There are many ingenious methods of depositing metal
nanoparticles onto CNT substrates in the literature, each of-
fering varying degrees of control of particle size and distri-
bution along the CNT. In this section, we first review exam-
ples where electrodeposition techniques have been used to
great advantage, before illustrating some interesting chemi-
cal deposition methods where the resulting materials are
used in electrochemical applications. Throughout, the sheer
volume of material has required us to be highly selective in
the choice of illustrative material presented.
2.1. Electrochemical Methods
Considering the recent interest in the use of CNTs from
both an electrochemical point of view and as metal nanopar-
ticle catalyst supports, it is somewhat surprising to find so
few examples of electrochemically controlled metal nano-
particle deposition on CNTs in the literature, and these are
limited to the noble metals Pd,[11–13]
Pt,[11,14–17]
Au,[11]
Ag,[16,18]
and bimetallic Pt–Ru.[19]
Nonetheless, electrochemistry is a
powerful technique for the deposition of many metals and/
or the surface modification of CNTs, being both rapid and
facile, and thus allowing the chemist to easily control the
nucleation and growth of metal nanoparticles.[12,13,16,17]
By
varying the deposition potential, substrate, and deposition
time one can control the size and distribution of metal
nanoparticles. In particular, many of the imaginative meth-
ods of decorating CNTs with metal nanoparticles involve te-
dious and time-consuming treatments that allow impurities
in the bath solutions to be incorporated either into the
[*] G. G. Wildgoose, Dr. C. E. Banks, Prof. R. G. Compton
Physical and Theoretical Chemistry Laboratory
University of Oxford, South Parks Road
Oxford, OX1 3QZ (UK)
Fax: (+44)1865-275-410
E-mail: richard.compton@chem.ox.ac.uk
small 2006, 2, No. 2, 182 – 193 2006 Wiley-VCH Verlag GmbH Co. KGaA, D-69451 Weinheim 183
Functionalized Carbon Nanotubes

3. nanoparticles or onto the walls of the CNTs themselves,
thus impairing the optical or catalytic properties of the
nanoparticles.[17]
Electrochemically deposited nanoparticles,
particularly noble metals such as Pt or Pd, are often of very
high purity, are formed rapidly, and have good adhesion to
the CNT substrate.[17,19]
One particularly elegant advantage of electrochemical
deposition is illustrated by the recent work of Guo and
Li.[13,17]
Often for successful deposition of nanoparticles
onto CNT substrates the CNTs have to be first oxidatively
treated to introduce oxygen-containing functionalities such
as quinonyl, carboxyl, or hydroxyl groups to which the
metal nanoparticle precursors can bind. This usually in-
volves harsh treatments with concentrated mixtures of
strong oxidizing acids, such as sulfuric and nitric acid, and/
or ozonolysis, which may lead to severe damage to the
CNTs. In their efforts to develop electrochemical methods
for the deposition of Pt[17]
and Pd[13]
nanoparticles onto
CNTs, Guo and Li carried out a much gentler electrochemi-
cal pretreatment to oxidize single-walled carbon nanotubes
(SWCNTs), thus introducing the required functionality to
the SWCNTs without damaging them. Their simple three-
step electrochemical method for depositing Pt or Pd nano-
particles is shown schematically in Figure 1, and produces
metal nanoparticles %5 nm in diameter.[13,17]
He et al. took advantage of the high purity of electro-
chemically deposited nanoparticles to decorate CNTs with
bimetallic Pt–Ru nanoparticles with diameters of 60–
80 nm.[19]
Again the multiwalled carbon nanotubes
(MWCNTs) underwent an electrochemical oxidation pre-
treatment before the deposition took place potentiostatical-
ly at À0.25 V from a solution containing various concentra-
tion ratios of ruthenium chloride and chloroplatinic acid in
0.5m H2SO4. This facile deposition method then allowed the
authors to investigate the effect of varying amounts of Ru
in the Pt–Ru nanoparticles on their electrocatalytic activity
towards methanol oxidation. Ru is known to inhibit the poi-
soning of Pt catalysts by adsorbed CO, formed as an inter-
mediate in the methanol oxidation process, following a bi-
functional mechanistic pathway as shown below.[19]
Pt þ CH3OH ! PtÀCOads þ 4Hþ
þ 4eÀ
ð1Þ
Ru þ H2O ! RuÀOH þ Hþ
þ eÀ
ð2Þ
RuÀOH þ PtÀCOads ! Ru þ Pt þ CO2 þ Hþ
þ eÀ
ð3Þ
The authors found that the optimal ratio of Pt:Ru was
4:3, giving greater catalytic activity and stability than Pt
nanoparticles alone, with important potential applications in
direct methanol oxidation fuel cells (DMFCs).[19]
Qu et al. made an interesting hybrid thin-film electrode
of carbon nanotubes modified with Pt nanoparticles and
[tetrakis(N-methylpyridyl)porphyrinato]cobalt (CoTMPyP).
The Pt nanoparticles are formed by first adsorbing [PtCl6]2À
onto the CoTMPyP-modified CNTs through electrostatic in-
teraction with the CoTMPyP, and then reducing the plati-
num complex potentiostatically at À0.7 V.[14]
Whilst the easiest method of forming a CNT-modified
electrode is to cast the CNTs onto the surface of an elec-
trode material such as glassy carbon, several authors have
made use of interesting electrode constructions where the
CNTs are grown directly on a substrate, which is electrically
connected to a potentiostat, and then metal nanoparticles
are electrodeposited onto the CNTs. Dekker and co-work-
ers grew CNTs on a SiO2 substrate and connected the CNTs
using Ti wires so that they act as both a 1D template for the
deposition process and as nanowires electrically contacting
the nanoparticles.[11]
Metal complexes, such as H[AuCl4],
K2[PtCl4], and (NH4)2[PdCl4] were used to deposit Au, Pt,
and Pd potentiostatically. The size of the metal nanoparticle
was controlled by the concentration of the metal salt and
the electrochemical deposition parameters, and the coverage
of the nanoparticles on the sidewalls of the SWCNTs was
controlled by the nucleation potential. Day et al. also devel-
oped a similar approach for depositing either Ag or Pt
nanoparticles onto as-grown CNTs.[16]
They used this
method to study the nucleation and growth processes of
metal nanoparticles deposited onto CNTs under an electro-
chemical regime, and determined that both metal nanoparti-
cles and nanowires could be grown on CNTs depending on
the conditions employed. They also found that the density
of nanoparticles on the CNTs was affected by the distance
along the tube from the gold contact electrode used to
“wire” the nanotubes to the electrochemical circuit, with a
higher density being formed nearest the gold contact and a
much lower density towards the furthest end of the tube
(Figure 2). They speculated that this may be due to the driv-
ing potential decreasing with distance along the nanotube.
The distribution and density of nanoparticles along the
SWCNT appears to be controlled by many factors, such as
Craig E. Banks recently completed his DPhil in the group of Prof.
R. G. Compton and is currently a postdoctoral research associate at
the University of Oxford. His research interests are diverse encom-
passing all aspects of electroanalysis, sonoelectrochemistry, and
nanotechnology.
Gregory G. Wildgoose was born in Derbyshire (UK) in 1980. He com-
pleted his MChem at the University of Oxford in 2003, where he car-
ried out his undergraduate research in the group of Prof. R. G. Comp-
ton. He has pursued further research in the same group as a DPhil
student. His research interests in electrochemistry are broad but are
currently focused on modified carbon electrodes and derivatized
carbon nanotubes, with particular emphasis on the fundamental as-
pects of derivatized carbon electrodes, micro- and nanoscale archi-
tectures on electrode surfaces, and their practical applications.
Richard Compton was born in Scunthorpe (UK) in 1955. He is a Pro-
fessor of Chemistry at Oxford University, a Fellow of St John’s Col-
lege, and Honorary Professor at Sichuan University (China). He is
Editor-in-Chief of the journal Electrochemical Communications and re-
ceived the Alexandro Volta Medal from the Electrochemical Society in
2004. He will deliver the Royal Society of Chemistry Tilden lectures
in 2005-6 and will receive the Breyer Medal from the Royal Australian
Chemical Institute in 2006. He has published over 650 papers on di-
verse areas of electrochemistry.
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reviews R. G. Compton et al.

4. the type of pretreatment the CNTs undergo, whether this is
chemical or electrochemical, as well as the method of manu-
facture and type of CNT formed, that is, SWCNT versus
MWCNT, bamboo-like MWCNTs versus hollow-tube-like
MWCNTs.[20]
For example,
using an electrochemical
oxidative pretreatment,
Guo and Li observed that
Pd nanoparticles tend to
be preferentially attached
at the ends, kinks, and
connecting regions of
SWCNTs, an example of
the “aggregate effect”,
where there is likely to be
a higher number of carbox-
yl moieties and therefore a
higher likelihood of com-
plexation.[13]
This is in
agreement with current sci-
entific thought that the
ends of the CNTs where
the most number of car-
boxyl moieties are likely to
be found are also the re-
gions of highest electro-
chemical activity being
rather more like edge-
plane graphite than the
tube walls, which are more
like basal-plane graph-
ite.[21–23]
In order to demon-
strate this effect, we have
shown that, by analogy to
graphite surfaces where
MoO2 is only deposited along edge-plane steps on the surfa-
ces to form nanowires,[24,25]
MoO2 is only electrodeposited at
the edge-plane-like ends of MWCNTs as quasi-spherical
“nanoplugs” (Figure 2).[26]
In contrast, other authors have reported that metal
nanoparticle clusters can be evenly deposited on both the
CNT sidewalls and the ends of the tubes, as shown in
Figure 3.[11,16,19]
Cui et al. electrodeposited Pt nanoparticles
onto a CNT “forest”, or vertical array of aligned CNTs
grown on a Ta plate, and found that when the CNTs were
grown in a dense array the Pt nanoparticles were mainly de-
posited on the tube ends, as expected.[15]
However this
effect may not be simply due to the tube ends being the re-
gions of highest electroactivity, as when the CNT forest was
grown more sparsely, the authors found that the sidewalls of
the CNTs could also be decorated.[15]
One possible explana-
tion for sidewall decoration is that in some cases the CNTs
were chemically pretreated with acids to oxidize them or to
remove the metal catalysts used in their manufacture. As
mentioned above, this harsh treatment damages the CNT
sidewalls and introduces holes and defects into the tube
walls, which correspond to edge-plane-like sites similar to
those at the ends of the tubes.
An ingenious approach to controlling the dispersion of
nanoparticles on CNTs and depositing nanoparticles evenly
and with a very narrow size distribution comes again from
the work of Guo and Li.[12]
They first modified the CNT sur-
face with a monolayer of 4-aminobenzene molecules cova-
Figure 1. A schematic mechanism for the electrochemical deposition of Pt or Pd nanoparticles onto SWCNTs
(adapted from Refs. [12,13]).
Figure 2. a) Schematic representation of the electrochemical deposi-
tion of MoO2 as nanoplugs at the ends and edge-plane-like defect
sites on MWCNTs. Also shown are HRTEM images of a MoO2 nano-
plug at the end of a MWCNT showing b) the MoO2 lattice spacings
and c) the graphite sheets at the end of the MWCNT around the
nanoplug. Adapted from Ref. [26].
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Functionalized Carbon Nanotubes

5. lently attached to the CNT via the direct electrochemical re-
duction of the corresponding diazonium salt of nitroben-
zene. Thus the grafting of the aryl moieties to the CNT sur-
face and reduction of the nitro groups to the corresponding
amines is carried out in one simple step, as shown in
Figure 4.[12]
The palladium salt [PdCl6]2À
is then adsorbed
onto the aminobenzene monolayer via electrostatic interac-
tions and Pd nanoparticles (%2.5 nm in diameter) are easily
produced via further potentiostatic reduction.[12]
2.2. Chemical and Physical Methods
The main disadvantage of using electrochemical meth-
ods to deposit metal nanoparticles onto CNTs is that bulk
production, that is, producing gram
quantities of modified CNTs, is
very difficult. Here we briefly con-
sider a wide variety of different
methods of producing metal (and
other) nanoparticles on CNTs that
do not involve electrochemical dep-
osition but are used in some form
of electrochemical application dis-
cussed in the next section.
One approach is to deposit the
metal nanoparticles from a metal
vapor. This metal vapor can be pro-
duced in a number of ways. Deposi-
tion occurs either at the same time
as CNT formation using: 1) chem-
ical vapor deposition (CVD) tech-
niques to deposit Au,[27]
Co,[28]
or
Ni;[29,30]
2) incorporating the metal
nanoparticle catalyst used to grow
the CNTs into the CNTs them-
selves, as in the case of the Ni-, Fe-,
or Co-decorated CNTs produced
by Hiraki et al.,[31]
or by depositing
the nanoparticles onto preformed
CNTs in the case of Co[32]
or Pt nanoparticles.[33,34]
One ex-
ample by Bera et al. involves the formation of Pd nanoparti-
cles on CNTs where both are simultaneously grown via arc
discharge methods.[35]
Of particular interest here is the work
of Kukovitsky et al. who grew CNTs from Ni catalysts and
discovered that when the Ni catalyst melted it then traveled
through the growing CNTs and then the Ni vapor deposited
on neighboring CNT sidewalls as Ni nanoparticles![29]
Other
methods involving metal(-based) vapors include: laser abla-
tion of a copper sheet under a He atmosphere to deposit
quasi-spherical oxidized Cu particles, between 100 nm and
2 mm in diameter, onto ultrasonically pretreated
MWCNTs;[36]
electron-beam induced deposition of RuO2
onto MWCNTs using W(CO)6 as a precursor.[37]
Thermal
decomposition methods were used by Xue et al.[38]
to depos-
it Pd, Pt, Au, and Ag nanoparticles
and by Chen et al.[39]
to deposit Cu
nanoparticles from their metal salts.
Govindaraj et al. also noticed some
formation of Au, Ag, Pt, and Pd
nanoparticles when they were
trying to make metal nanowires by
thermal decomposition methods
inside nanotubes and in the inter-
laminar spacings between the
graphite sheets forming the nano-
tube walls.[40]
Another common method of
deposition is to adsorb a metal salt
onto the CNT surface and then to
reduce the salt to the metal at high
temperatures under an atmosphere
of H2 or Ar. This method has been
applied to the decoration of CNTs
Figure 3. FE-SEM images showing a) Pt deposited on SWNTs from a solution containing 2 mm K2PtCl6 in
0.5m perchloric acid. A deposition potential and time of À0.4 V (versus Ag/AgCl) and 30 s were used. The
SWNT network density was 6.8 mmSWNT mmÀ2
. In the bottom right of the image is the Au contact electrode,
which was also exposed to solution. b–d) are higher resolution FE-SEM images taken from 25 mm (b),
15 mm (c), and very close (actual distance not indicated in original paper) to the contact electrode. Reprint-
ed with permission from Ref. [26].
Figure 4. An illustration of how Pt nanoparticles can be uniformly dispersed along CNTs modified with
aminobenzene, as used by Guo et al. Adapted from Ref. [12].
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reviews R. G. Compton et al.

6. with Pt,[41–45]
Pt–Ru,[46]
and Pd.[47]
Che et al. used this
method to deposit both Pt, Ru, and Pt-Ru nanoparticles
onto forests of CNTs.[48,49]
Fukunaga et al. demonstrated
that often, when one uses these methods, some modicum of
patience is a prerequisite. When they sealed MWCNTs to-
gether with either Ti, Nb, or Ta and a small amount of I2,
into a quartz ampoule and heated it at 10008C for up to
215 h, CNTs decorated with TiC, TaC, and NbC nanoparti-
cles were formed.[50]
A collaboration between the research
groups of Lin and Wai modified the high-temperature re-
duction method by carrying out the decoration of CNTs
with Pt, Pd, Rh, and Ru nanoparticles via the hydrogen re-
duction of the metal–b-diketone complex precursors using
supercritical CO2 as a green solvent.[51–54]
Hydrophobic and hydrogen-bonding interactions have
been successfully employed to attach nanoparticles consist-
ing of TiO2 and complexes of Ru as well as phthalocyanine
and polymethine dyes.[55]
Surfactants such as sodium do-
decylsulfate allowed Wei et al. to decorate MWCNTs with
nanoparticles of EuF3 and TbF3,[56]
Lee et al. to deposit Pt
nanoparticles,[57]
and Chaudhary et al. to disperse thiol-en-
capsulated nanocrystals of CdSe/ZnS to form quantum dots
on the nanotube surface.[58]
Gold nanoparticles capped with
monolayers of alkanethiols were attached to either the
outer walls of CNTs using hydrophobic and hydrogen-bond-
ing interactions,[59]
or to the ends of CNT forests where the
sidewalls had been protected by coating them in polysty-
rene.[60]
This elegant method developed by Chopra et al.
allows the CNTs to be selectively derivatized with different
nanoparticles at different ends of the CNTs, as shown in
Figure 5.[60]
The inner walls and holes in the sidewalls of
acid-oxidized SWCNTs were decorated with nanoparticles
of Gd(OAc)3 via hydrophobically driven insertion reac-
tions.[61]
Other metal nanoparticles deposited through non-
bonding interactions include Pt,[62,63]
Sn–Ru,[64]
Ni,[65,66]
and
Pt–Ru, as shown in Figure 6.[64,67]
Quantum dots, that is,
nanoparticles of sufficiently small size as to exhibit quantum
confinement in respect of at least one of their properties
(such as electronic or optical properties), have been pre-
pared as CdSe from thio-
glycolic acid[68]
or amino-
ethanethiol,[69]
whilst Baner-
jee and Wong formed
in situ CdSe and CdTe
quantum dots on the car-
boxyl moieties located on
oxidized SWCNTs via ozo-
nolysis of the SWCNTs in
the case of CdSe[70]
and
without ozonolysis for
CdTe.[71]
The “polyol” method,
normally used to prepare
colloidal suspensions of
metal nanoparticles, can be
adapted by adding a sup-
port material such as
CNTs to capture the de-
positing nanoparticles. The
Figure 5. The method of Chopra et al. for the selective modification and/or metal-nanoparticle deposition at
different ends of an aligned CNT forest coated in a polymer membrane. Adapted from Ref. [60].
Figure 6. a) HRTEM image of nanoparticles derived from
[Ru5CÀ(CO)14Pt(COD)] on carbon nanotubes, showing the nano-
particles at the tips. b) Schematic representation. Reprinted with
permission from Ref. [64].
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Functionalized Carbon Nanotubes

7. method involves refluxing a solution of the metal salt pre-
cursor at 393–443 K in a polyol solvent (normally ethylene
glycol) where the polyol homogeneously decomposes to re-
lease the reducing agent for metal-ion reduction.[72]
Chen
et al. used this method under microwave-assisted heating to
decorate CNTs with both Pt and Pt–Ru nanoparticles with
diameters of %3 nm with direct applications in fuel-cell
technology.[72,73]
A further adaptation of the polyol method
found in the literature combines the well-known benefits of
power of ultrasound (solvent cavitation, localized heating)
to produce Pt[74,75]
and nanocrystalline Sn[76]
nanoparticles
on CNTs with uniform diameters less than 5 nm.
Electroless deposition methods rely on a chemical, as
opposed to electrochemical reduction process, whereby a
chemical species whose redox potential is suitably lower
than that of the metal species being reduced provides the
driving force for the reaction. Such methods have been used
principally to form Pt nanoparticles on CNTs,[77–81]
with
more adventurous choices of metals being deposited includ-
ing Au and Ag,[81]
Ni and Pd,[77]
and Cu and Ni,[82]
with the
latter two examples activated via a preceding step to deposit
Pd–Sn catalyst particles onto the CNT.[77,82]
Of particular
mention here is the work of Qu and Dai, who have devel-
oped a reductant-free electroless deposition method called
substrate-enhanced electroless deposition, or SEED.[83,84]
They discovered that by supporting the CNTs on a metal
substrate with a lower redox potential than that of the
metal ion to be reduced, spontaneous deposition onto the
CNTs could occur. In this way they deposited metal nano-
particles of Cu, Ag, Au, Pt, and Pd onto MWCNTs and
SWCNTs, some of which are shown in Figure 7.[84]
Finally one of the most imaginative methods of reducing
metals to form nanoparticles on CNTs involves the reduc-
tion of Ag, Pd, and Pt–Ru alloy onto poly(vinylpyrrolidone)
to form a colloidal suspension, or onto SWCNTs using
60 kGy (dose rate: 6.48”105
GyhÀ1
) Co-60 g-irradiation to
bring about the reduction! Nanoparticles with diameters of
15–25 nm were produced using this method.[85]
In the next section we discuss the applications that
CNT-supported metal nanoparticles have found, with partic-
ular emphasis on their electrochemical applications.
3. Applications of CNT-Supported Nanoparticles
Considering that the vast majority of studies involving
metal-nanoparticle deposition on CNTs are predominantly
focused on the transition metals Pt, Pd, Ru, Ag, and Au, it
is hardly surprising that the most obvious applications of
CNT-supported metal nanoparticles are in catalysis. In par-
ticular, with natural hydrocarbon-based fuel reserves such
as oil and gas rapidly running out, and possible fears over
the long-term safety and economic viability of nuclear tech-
nologies and their associated short-term risks, developing
effective catalysts for hydrogen, ethanol, or methanol
oxidation or oxygen reduction catalysis for fuel cell
and battery technologies is a very active area of re-
search.[12–15,17,19,41,46,48,49,51,57,62,67,72–75,80,86,87]
We shall therefore
briefly discuss fuel cell and battery applications of CNT-sup-
ported metal nanoparticles catalysts separately in sec-
tion 3.5.
3.1. Catalysts
Selective hydrogenation catalysts are crucial in the pet-
rochemical and fine chemical industries for the synthesis of
a wide variety of chemicals.[47]
Zhang et al. found that CNTs
decorated with Co nanoparticles were catalytic for the for-
mylation of 1-octene with a high activity and excellent re-
gioselectivity.[32]
Tessonier et al. describe the interesting cat-
alytic properties of Pd nanoparticles deposited entirely on
the interior walls of MWCNTs for the selective hydrogena-
tion of cinnamaldehyde to hydrocinnamaldehyde.[47]
Cinna-
maldehyde contains both a C=C and a C=O bond in an a,
b-unsaturated arrangement; the possible reaction pathways
are shown in Figure 8. Compared to commercial Pd dis-
persed on a high-surface-area carbon substrate, the authors
reported only a slightly higher catalytic hydrogenation rate
for the Pd–MWCNTs, however, the Pd–MWCNTs showed a
dramatic increase in selectivity with hydrocinnamaldehyde
being the major (80%) product. The Pd supported on ac-
Figure 7. SEM images of a) pristine SWNTs, and of Cu-supported
SWNTs after being immersed in an aqueous solution of b) H[AuCl4]
(3.8 mm), c) K2[PtCl4] (4.8 mm), and d) (NH4)2[PdCl4] (7.0 mm) for 5 s.
e–h) SEM images of SWNTs (e,f) and MWNTs (g,h) supported by a
zinc foil after being immersed in an aqueous solution of Cu(NO3)2
(3.8 mm) and [Ag(NH3)2]+
(3.8 mm), respectively, for 5 s. Reprinted
with permission from Ref. [83].
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reviews R. G. Compton et al.

8. tivated carbon showed almost no selectivity for hydrocinna-
maldehyde over phenyl propanol, and cinnamyl alcohol was
never observed in either case.[47]
The authors propose that
an unusual interaction between the Pd nanoparticles and
the inner walls of the MWCNTs coupled with the relative
lack of micropores and of oxygenated surface groups on the
MWCNTs might explain these results.[47]
In their work on
the deposition of Pd nanoparticles on MWCNTs in super-
critical CO2, Ye et al. also demonstrated that the Pd-
MWCNTs were excellent catalysts for the hydrogenation of
olefins, including the conversion of stilbene into 1,2-diphe-
nylethane, with conversions of 96% after 10 min expo-
sure.[52–54]
Other metal nanoparticles used for catalysis in-
clude: Ru–Sn and Ru–Pt alloys for heterogeneous cataly-
sis,[64]
Au,[88]
Pt-catalyzed hydrogen peroxide oxidation,[34]
gold–thiol monocapped nanoparticles,[59]
and Pd, Pt, Au,
and Ag for environmental catalysis.[38]
3.2. Hydrogen Storage and Sensing
A few authors have studied H2 storage on Ni[65,66,89]
and
Pt[79]
nanoparticle-decorated CNTs, and in the case of CNT
supports decorated with 6 wt.% Ni nanoparticles,
%2.8 wt.% of hydrogen was reversibly chemisorbed, which
is much higher than for CNTs alone.[65]
Kong et al. used Pd
nanoparticles supported on CNTs for hydrogen sensing
rather than storage, achieving a limit of detection of around
400 ppm,[90]
whilst a theoretical paper by Yildirim and Ciraci
claimed that Ti adatoms adsorbed onto a CNT surface
could feasibly bind up to four molecules of hydrogen, the
first molecule being bound dissociatively with no energy
barrier and the remaining three adsorptions with significant-
ly elongated HÀH bonds.[91]
Such a material could store up
to 8 wt.% H2 leading to better storage and catalyst materi-
als.
3.3. Optical and Electronic Applications
The optical and electronic properties of nanoparticles,
and in particular quantum dots, are often different from
that of the bulk material and can be tuned to a certain
degree by varying the particle size and the interactions with
the substrate or support surface upon which the nanoparti-
cles are dispersed.[11,69–71]
For instance, gold nanoparticles
appear blue, unlike the lustrous yellow of the bulk material,
because the dielectric coupling between the nanometer-
sized particles red-shifts the surface plasmon adsorption.[11]
Photovoltaic devices using TiO2,[55]
CdSe,[69,70]
and CdTe[71]
nanoparticle “heterojunctions” have been investigated.
Figure 9 shows schematically how a polymer/CdSe quantum
dot/SWCNT composite undergoes electronic excitation
upon optical adsorption, leading to exciton dissociation and
carrier transport via a cascade process.[69]
Such devices can
have potentially high conversion rates by tailoring the metal
nanoparticles electronic properties such as electronic struc-
ture (bandgap), carrier trapping, and delocalization, leading
to better charge separation and more efficient light harvest-
ing than is currently possible with conventional cells using
bulk material properties.[71,78]
Another interesting applica-
tion of the optical properties of nanoparticles is given by
Chaudhary et al. who used the fluorescence properties of
CdSe/ZnS core–shell quantum dots to visualize the size and
manipulate clusters of CNTs using optical detection.[58]
Nanoparticle and quantum dot heterojunctions are also
of interest in nanoelectronic devices for forming controlled
electronic contacts between conducting CNTs.[78]
To this end
Jurkschat et al. have deposited conductive nanoparticles of
MoO2 solely at the ends of MWCNTs forming hemispheri-
cal “nanoplugs”, which it is envisaged could be used as
nanoscale electronic contacts (Figure 2).[26]
In addition Ni,
Fe, Co,[31]
Au,[27]
and RuO2/W(CO)6
[37]
decorated CNTs for
use as field emitters with low threshold voltages and high
amplification factors compared to CNT field emitters alone,
have been investigated for possible applications in, for ex-
ample, field-emitter displays.
Figure 8. The various possible products of cinnamaldehyde hydroge-
nation and the ratio of these products formed when using Pd nano-
particles supported on either activated carbon or MWCNTs. Adapted
from Ref. [47].
Figure 9. A schematic illustrating efficient “cascade” light harvesting
via a p-type polymer j CdSe quantum dot j SWCNT composite photo-
cell equilibrated at the Fermi level. Adapted from Ref. [69].
small 2006, 2, No. 2, 182 – 193 2006 Wiley-VCH Verlag GmbH Co. KGaA, D-69451 Weinheim www.small-journal.com 189
Functionalized Carbon Nanotubes

9. 3.4. Other Applications
Lu et al. have immobilized palladium nanoparticles on
SWCNTs and applied this in the detection of methane. The
methodology showed advantages over conventional catalytic
beads and metal oxide sensors with a tenfold increase in
sensitivity and reduced power consumption by a factor of
100.[92]
Both Shi et al.[63]
and Qiaocui et al.[44]
used CNT-sup-
ported Pt nanoparticles to fabricate a cholesterol biosensor
involving the immobilization of the Pt-modified CNTs and
the cholesterase enzyme in a sol–gel matrix on an electrode.
The detection of cholesterol occurred indirectly using the
hydrogen peroxide produced by the action of the enzyme
and allowed the sensor to have a low detection limit
(1.4 mm) over a good linear range (4–100 mm). The Pt–CNTs
also limited the effect of interferents compared to bulk ma-
terials or CNTs alone.[63]
Also of note is the Pt–CNT-sup-
ported catalysis of both hydrogen peroxide and cysteine for
the bioanalytical sensing of both species.[43]
Other biological
applications and developments in the use of nanoparticles in
general, as well as CNT-supported nanoparticles, are re-
viewed in great depth by Penn et al.[93]
and will therefore
not be discussed further here.
In general, most applications of nanomaterials are for
constructive purposes to try to solve problems of the day,
such as finding alternative power sources to hydrocarbon-
based fuels using “green” fuel-cell technologies, discussed in
the next section, or solar-powered cells.
3.5. Energy Storage and Related Applications
Dong et al.[95]
prepared a hybrid thin film containing
platinum nanoparticles and [tetrakis(N-methylpyridyl)por-
phyrinato]cobalt (CoTMPyP) modified MWCNTs support-
ed on a glassy carbon electrode (GC/MWCNT/CoTMPyP/
Pt) to develop a new electrode material to exploit in fuel
cells. The procedure involves physically adsorbing CoTM-
PyP onto a MWCNT by simply immersing into an aqueous
solution containing CoTMPyP for 90 min. The electrode is
placed into another aqueous solution containing K2PtCl6,
which allows [PtCl6]2À
to be adsorbed onto the surface of
the modified glassy carbon electrode. Finally the electrode
is placed into an electrolyte solution to electrochemically
reduce [PtCl6]2À
to form platinum nanoparticles in situ. This
procedure produces a single layer of platinum but multilay-
ers can be achieved by repeating the above procedure.
The electrochemical reduction of oxygen in aqueous so-
lution at this modified electrode showed excellent electroca-
talysis, which was verified to be solely due to the presence
of the platinum nanoparticles. The authors established that
the electrochemical process was occurring through a four-
electron process that indicated that the final product was
water. If any of these studies are to be commercially used,
such as in the fuel-cell industry, the process should ideally
produce water as the final product since the formation of
hydrogen peroxide is an unwanted side product that can
attack polymeric membranes and also significantly reduce
the power efficiency.
CNTs have potential for use as membrane materials
within fuel cells, batteries, and related technologies. Howev-
er the majority of these applications will require alignment
and aggregation to form such a membrane since CNTs com-
monly form bundles, which have the disadvantage of de-
creasing their surface area available for supporting nanopar-
ticles.[96,97]
Ajayan et al.[98]
has reported a simple technique for
alignment that produces aligned arrays of carbon nanotubes,
which involves cutting thin slices (50–200 nm) of a nano-
tube–polymer composite. However, the degree of orienta-
tion of the nanotubes in the composite was affected by the
thickness of the slices, and the aligning effect becomes less
pronounced with increasing slice thickness.
Fisher, Martin, and co-workers[99]
have reported tem-
plate-synthesized carbon tubules, which can be fabricated as
free-standing nanoporous carbon membranes. This novel
concept is realized via a CVD-based synthesis within the
pores of an alumina template membrane, which produces a
ensemble of uniform, open-ended hollow tubes. They noted
that carbon was deposited in-between the template pores,
which serves to hold the nanotube template together when
the underlying membrane is dissolved in a HF solution, thus
affording an elegant free-standing carbon-tubule membrane.
Fisher and co-workers have filled these nanotube ensem-
bles with the electrocatalytic metals of platinum and ruthe-
nium and with alloys of platinum and ruthenium via immer-
sion in the appropriate metal salt(s) solution, which are
then chemically reduced to the corresponding metal. Inter-
estingly, capillary action results in platinum nanoclusters
that are formed exclusively on the inner walls of the CNTs
ensemble.
These nanoclusters of metals supported on the carbon
nanotubes were then attached to a glassy carbon electrode
by using Nafion as an adhesive and were found to electro-
catalyze oxidation reduction and methanol oxidation; these
are highly important reactions in fuel-cell technology. Possi-
ble applications in heterogeneous catalysis are also conceiv-
able.[100]
Further work from this group has shown excellent elec-
trocatalysis using nanoparticle-modified nanotubes toward
oxygen reduction in aqueous solution observing a large re-
duction in the overpotential while noting that both the
outer and inner tubules were electrochemically active for
the intercalation of lithium ions, suggesting applications in
lithium-ion batteries.[101]
Girishkumar et al. have reported a membrane electrode
assembly for hydrogen fuel cells.[102]
The method involves
electrophoretic deposition to form a film of functionalized
CNTs on a carbon-fiber electrode. A schematic representa-
tion of the proton-exchange membrane assembly is shown
in Figure 10. The benefits of this technique include uniform
deposition of charged particles with control of film mor-
phology performed by modulating the applied electric field.
The catalyst particles, when suspended in a solvent, become
charged under the influence of a dc electric field and mi-
grate toward the oppositely charged electrode. The film cast
on the electrode surface is robust, and the amount of depo-
sition can be controlled by changing the duration of the ap-
190 www.small-journal.com 2006 Wiley-VCH Verlag GmbH Co. KGaA, D-69451 Weinheim small 2006, 2, No. 2, 182 – 193
reviews R. G. Compton et al.

10. plied field. The authors dispersed SWCNTs in THF, which
they subjected to a low dc field that induces the CNTs to
move toward the positive electrode and assemble in the
form of a film.[102]
Electrophoretic deposition is then employed to cast
films of Pt black on the CNTs where Nafion solution is
added to the THF solution containing the Pt black, which
gives the suspended particles a net negative charge. Thus
the Pt black particles migrate under the influence of an ap-
plied dc field toward the positive electrode and are deposit-
ed on the electrode surface. Control of the amount of metal
catalyst and carbon support is achieved by controlling the
dc voltage and time of deposition.[102]
Zhao et al. have investigated Pt–Sn nanoparticles on
MWCNTs and have explored their use as an anode catalyst
for a direct ethanol fuel cell, which may find use as a poten-
tial mobile power source.[103]
They compared the functionali-
zation of the CNTs with that of carbon black and observed
a better performance in the former, which they attributed to
the structural difference of CNTs over carbon black, higher
electric properties, and lower organic impurities of CNTs as
compared to their carbon black sample.
Last, Nakamura and co-workers have explored the
effect of platinum loadings on carbon nanotubes.[104]
They
report that 12 wt.% Pt deposited onto CNTs results in 10%
higher voltages than 29 wt.% Pt deposited on carbon black;
this ultimately reduces the Pt wastage by 60% in polymer–
electrolyte fuel cells. The higher performance of the Pt-dec-
orated nanotubes was attributed to 1) well-dispersed plati-
num nanoparticles on the CNTs allowing more triple-phase
boundaries (gas j electrolyte j electrode), 2) high electric
conductivity, and 3) networks and interiors of the CNTs,
which might consist of spaces for gas diffusion.
4. Summary and Outlook
Carbon nanotubes make excellent nanoparticle supports
due to their unique geometry and properties. It is envisaged
that the future will see a whole range of new metals and
alloys immobilized on carbon nanotubes. The application of
carbon nanotubes functionalized with metal, alloy, metal
oxide, compound semiconductor, and other nanoparticles, is
likely to expand greatly in the future.
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