Supramolecular self-assembly of amphiphiles on carbon nanotubes: A versatile strategy for the construction of CNT/metal nanohybrids, application to electrocatalysis

Article abstract of DOI:10.1021/ja8026373

Homogeneous coating of carbon nanotubes with metallic nanoparticles was achieved using supramolecular auto-organization of amphiphilic molecules as template. The resulting Pd nanoparticles/carbon nanotube nanohybrids were then evaluated in electrocatalysis experiments, showing superior activity in ethanol oxidation compared to analogous systems. Copyright

Full text of DOI:10.1021/ja8026373

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Supramolecular Self-Assembly of Amphiphiles on Carbon Nanotubes: A
Versatile Strategy for the Construction of CNT/Metal Nanohybrids, Application
to Electrocatalysis
Nicolas Mackiewicz,§ Geetarani Surendran,‡ Hynd Remita,‡ Bineta Keita,‡ Guangjin Zhang,‡
Louis Nadjo,‡ Agne`s Hage`ge,# Eric Doris,*,§ and Charles Mioskowski§,†
CEA, iBiTecS, SerVice de Chimie Bioorganique et de Marquage, F-91191 Gif sur YVette, France, Laboratoire de
Chimie Physique, UniVersite´ Paris XI, F-91405 Orsay, France, and Institut Pluridisciplinaire Hubert Curien-DSA,
F-67087 Strasbourg, France
Received January 31, 2008; E-mail: eric.doris@cea.fr
Since the discovery of carbon nanotubes (CNTs) by Iijima in
1991,1 intensive studies of their properties have been carried out,
suggesting great potential for applications in nanotechnology. Due
to their high specific surface area, electric conductivity, and
chemical stability, carbon nanotube nanohybrids are receiving
increasing interest for catalytic applications from fuel/electrochemi-
cal cells to heterogeneous catalysis. The “bottom-up” construction
of such nanodevices requires the assembly of noble metal nano-
Figure 1. General scheme of amphiphile self-assembly onto CNTs.
particles (NP) on carbon nanotubes, and several routes have been
developed to link the metallic particles to their surface.2 Examples
include electroless plating,3 electrodeposition,4 chemical deposi-
tion,5 and the direct assembly of NPs.6 Strategies have also been
directed toward the introduction of functional groups on CNTs to
create preferred sites for nucleation of metal ions (impregnation
process), prior to reduction.7 These strategies include chemical or
Figure 2. Chemical structure of amphiphiles used in this study.
electrochemical oxidation at defect sites of carbon nanotubes,8
wrapping of CNT with polymers,9 and grafting of tethers such as
pyridinium was chosen for its ability to interact through overlapping
of its π and π* orbitals with those of the metal atoms. The synthesis
dendrons.10
In this paper, we report an alternative strategy for the anchoring
of metallic NP on carbon nanotubes. Our approach is based on the
work previously reported by our group on the self-assembly of
surfactants and analogous systems on the nanotube surface.11 This
leads to supramolecular structures made of rolled-up half-cylinders
orientated perpendicular to the nanotube lattice (Figure 1). When
amphiphilic surfactants are mixed with CNT at a concentration
greater than the critical micellar concentration (CMC), they self-
organize as hemimicelles on the nanotube surface. While the
hydrophobic portion of the amphiphile is adsorbed on CNT walls
by van der Waals interactions, its hydrophilic headgroup is oriented
toward the aqueous phase. This produces half-cylinders that can
be visualized by transmission electron microscopy (TEM). Negative
staining TEM pictures of coated CNTs showed regular striations
on their entire surface. Adsorption of amphiphilic molecules on
the nanotubes creates a distribution of ionic species that prevents
aggregation and induces stable nanotube suspensions in water. The
objective of this study was to take advantage of the high density
of potential binding motifs (ionic species) distributed along the
nanotube lattice, to anchor and grow the metallic nanoparticles.
Two different amphiphiles were used (Figure 2). Both contained
a long alkyl chain for interaction with the nanotube surface and a
polar head made of either nitrilotriacetic acid (NTA) or pyridinium,
for the binding of the metallic NPs. While the choice of NTA was
inspired by the presence of the three chelating carboxylate groups,
of the NTA-based amphiphile 1 is described in the Supporting
Information. The pyridinium-based amphiphile 2 was purchased
from a commercial source.
With the above amphiphiles 1 and 2 in hand, we induced their
self-assembly on the nanotube surface. In a typical experiment, 10
mg of the surfactant in 2 mL of 100 mM Tris buffer was first
sonicated using an ultrasonic tip to stimulate micelle formation.
This preliminary step is crucial for efficient assembly of the
surfactants on CNTs. Multiwalled carbon nanotubes (MWNTs) (50
mg from n-TEC) in 3 mL of the same buffer were sonicated in
parallel before being added to the micelle solution. The mixture
was further sonicated for 5 min and then centrifuged at 15 000 rpm
to remove the excess surfactant molecules. At the end of the process,
the nanotubes were dispersed in pure water to reach a final
concentration of 0.2 mg/mL (as determined by spectrophotometric
absorption). TEM observation of the aqueous dispersions showed,
in the two cases studied, the expected self-assembled regular motifs
on the graphitic side walls of the nanotubes (Figure 3a,b). The
nanotubes fully covered with ionic polar heads were thus exploited,
in the following step, for complexation to the metallic NP. The
coated carbon nanotubes were subsequently treated with a metal
salt. We chose, as model metal, palladium derived from two
different complexes: Pd(NH3)4Cl2 or Pd(CH3CN)2Cl2. The salts
were introduced to the reaction mixture to give a final concentration
of 0.2 mg/mL. The carbon nanotube/metal salt solution was then
allowed to stand for 1 day prior to reduction. This delay provides
enough time for the interactions between the coated CNT and the
metallic salt to develop. The solutions were deoxygenated under
argon for 10 min and the samples submitted to electron beam
§ CEA, iBiTecS, Service de Chimie Bioorganique et de Marquage.
‡ Universite´ Paris XI.
# Institut Pluridisciplinaire Hubert Curien-DSA.
† Deceased June 2, 2007.
9
8110 J. AM. CHEM. SOC. 2008, 130, 8110–8111
10.1021/ja8026373 CCC: $40.75
2008 American Chemical Society

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C O M M U N I C A T I O N S
between the carbonous template and the metallic NP. While several
washings with water induce progressive release of the NPs from
the pristine CNT surface, constant NP loading was measured in
the case of surfactant-mediated deposition.
With the Pd-coated nanotubes in hand, their performances toward
electrocatalytic oxidation of 1 M EtOH in 1 M KOH were evaluated
by cyclic voltammetry at a scan rate of 50 mV ·s-1. Three different
samples were tested: CNT-1-Pd, CNT-2-Pd, and pristine
CNT-Pd. The following parameters, including the faradaic onset
potential (Eonset), the forward peak potential (Ef), the backward peak
potential (Eb), the forward peak current intensity (If) expressed in
mA·cm-2 ·mg-1 of Pd are shown in Table S1 and the cyclic
voltammograms are provided in Figures S4-S6 (see Supporting
Information). Among these parameters, the most striking differences
appear with respect to current intensities. The large superiority of
CNT-1-Pd in terms of current intensity is obvious, being nearly
25 times higher (3540 mA ·cm-2 ·mg-1) than that of previously
reported MWCNT-Pd assemblies.13 The electrodes were also
tested for their durability under continuous cycling between -1.02
and -0.220 V. Comparisons were made after 200 cycles. It is
remarkable that 94 and ∼100% of the initial current was still
observed for CNT-1-Pd and CNT-2-Pd, respectively. These
characteristics designate our nanohybrids among the best systems
for ethanol oxidation.
Figure 3. TEM pictures CNT functionalized with amphiphile 2 (a) and 1
(b) (negative staining); TEM pictures of Pd nanoparticles on CNT-2 (c)
and CNT-1 (d). Scale bar ) 20 nm.
irradiation (mean dose rate 2200 Gy ·s-1). Pd salts were reduced
in situ by solvated electrons and by radicals originating from solvent
radiolysis. Electron beam irradiation has the advantage, over
chemical methods, of inducing homogeneous reduction and nucle-
ation which leads to small and monodisperse metal nanoparticles.12
At the end of the electron beam reduction, the nanotubes were
collected by centrifugation, analyzed by TEM, and the metal content
was determined by inductively coupled plasma-mass spectrometry
(ICP-MS). Similar results were obtained regardless of the palladium
complex employed. As shown in Figure 3c,d, TEM pictures
confirmed the successful anchoring of the metallic nanoparticles
on the CNT side walls. Nanotubes are decorated with a dense and
uniform loading of NPs. Their size distribution was evaluated by
statistical diameter measurement using selected TEM images. The
latter indicated a NP size mainly between 1 and 3 nm with a mean
diameter of ca. 2 nm. Noteworthy is that no NP aggregation was
observed neither on the nanotube surface nor in solution.
In conclusion, we have shown that the hemimicelle self-assembly
of amphiphilic molecules provided an effective template for the
homogeneous and dense deposition of noble metal nanoparticles
on carbon nanotubes. Electrocatalytic applications of the resulting
nanohybrids were evaluated, and superior activity in certain
oxidation reactions, compared to analogous systems, was observed.
Hence, CNT/amphiphile/NP nanohybrids are emerging as promising
systems for fuel cell applications.
Acknowledgment. Dr. Alexander Yuen is gratefully acknowl-
edged for helpful discussion.
Supporting Information Available: Experimental procedures,
isotopic profile of Pd, SEAD, Table S1, Figures S4-S6. This material
is available free of charge via the Internet at http://pubs.acs.org.
References
ICP-MS measurements indicated a metal content of 12 wt %
for CNTs covered with amphiphile 1 and of 21% for CNTs covered
with amphiphile 2. This technique also permitted the unambiguous
characterization of the NP as palladium by isotopic profile
comparison; furthermore, selected area diffraction confirmed the
metallic nature of Pd (see Supporting Information). Control
experiments were run in parallel using the overall same sequence,
without initial coating of the nanotubes. At the end of the reduction
process, the samples were again analyzed by TEM and ICP-MS.
TEM images of pristine nanotubes that were reacted with Pd salts
showed sparse nanoparticle distribution. This trend was further
confirmed by ICP-MS, which indicated low metal content on carbon
nanotubes (4% by mass). This result was anticipated since the
control sample was composed of “naked” nanotubes that were not
covered with the amphiphilic binding motifs. In addition, the
nanotubes have not been oxidatively treated and therefore do not
incorporate carboxylic groups which could have served as anchoring
moieties for the metallic NP. These experiments demonstrate that
the initial self-assembly of the surfactants on the nanotube is a key
element for successful metallic nanoparticle deposition. The sur-
factant coating of the nanotube also strengthens the interaction
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