Carbon nanohorns grown from ruthenium nanoparticles
Junfeng Geng,a Caterina Ducati,b Douglas S. Shephard,a Manish Chhowalla,b Brian F. G. Johnson*a
and John Robertsonb
a Department of Chemistry, Cambridge University, Lensfield Road, Cambridge, UK CB2 1EW.
E-mail: bfgj1@cam.ac.uk
b Department of Engineering, Cambridge University, Cambridge, UK CB2 1PZ
Received (in Cambridge, UK) 31st January 2002, Accepted 9th April 2002
First published as an Advance Article on the web 19th April 2002
A nanoscale ruthenium/gold bimetallic cluster of clusters
has been used as a molecular precursor to produce pure
ruthenium nanoparticles (seeds) as catalysts for the growth
of carbon nanohorns (CNHs).
at the top. The cavity diameter is dependent on the size of the
encapsulated metal particle. This type of structure suggests a
‘tip growth mechanism’ under our experimental conditions. Fig.
1(b) shows that the CNHs are not uniform along their central
axis. The lower part of the tube body has larger diameter,
suggesting that a possible growth mechanism may first involve
the core formation of the CNHs, which then acts as a substrate
for subsequent thickening by deposition of secondary carbon
atoms. Fig. 2 shows a HRTEM image for several other CNHs.
It can be seen that the body of the CNHs does not correspond to
a well-ordered graphitic structure such as normal multi-wall
carbon nanotubes, nor of amorphous carbon. Instead they
appear to possess an intermediate structure falling something
between these two. Some well-layered graphitic regions are
observed around the metal particles at the top, giving us an
insight into the activity of the catalytic metal particle in the
growing process.
Both Figs. 1 and 2 indicate that the metal nanoparticles have
a wide size distribution. The smallest metal particle shown in
Fig. 1(a) is about 2 nm in diameter. As the size of a single
precursor cluster is about 8 nm in diameter,9 we consider that
the 2 nm metal particle is possibly formed from the collapse of
a single DAB-Ru96-Au32 molecule after burning off the
organic dendritic core and the periphery CO ligands. The larger
metal nanoparticles are obviously produced by the aggregation
of smaller particles and their subsequent coalescence. Fig. 3
Carbon nanotubes (CNTs) have been one of the most intensely
studied materials since their discovery in 1991.1 Currently, the
most widely adopted method for their synthesis is by chemical
vapour deposition (CVD) of hydrocarbons using Ni, Fe, Co,
Mo, etc., as catalysts.2–4 Single-walled carbon nanohorns were
first observed by Iijima et al. in 1999,5 and recently a synthesis
of nanoscale carbon structures with conical and cylinder-on-
cone shapes was reported.6 Field emission characteristics of
CNTs have been widely studied, often for field emission
displays.7 It has been argued that the CNTs with the highest
length/diameter ratio, hence the largest electrical field enhance-
ment factor, are the best candidates in the carbon nanotube
family for future emission devices. However, a recent publica-
tion indicates that the ‘short and stubby’ nanotubes with
intermediate diameters show the best field emission character-
istics.8 In this communication, we report the use of a Ru/Au
1
1
bimetallic cluster, [DAB-dendr-[N(CH2PPh2)2]16(m:h :h -
Au2Ru6C(CO)16)16] (DAB-Ru96-Au32) to produce metallic
nanoparticles in situ as catalysts for growth of short, stubby and
horn-like carbon nanotubes. Characterisation by high resolution
transmission electron microscopy (HRTEM) reveals that the
carbon structure possesses semi-ordered graphitic features,
which is novel and which should be of considerable potential in
the development of nanoscale electron field emission devices.
The DAB-Ru96-Au32 precursor was prepared by binding the
16[P-N-P] tridentate terminal functionalities of the organic third
generation dendritic core to the di-gold hexa-ruthenium [Au2R-
u6C(CO)16] cluster unit.9 For the growth of CNHs, a TEM grid
coated with a thin carbon film was used as a substrate. It was
first treated with isopropyl alcohol to produce plenty of –OH
groups on the surface, followed by the deposition of the
precursor cluster onto the substrate from a dichloromethane
solution. Growth of the CNHs was performed by CVD. The
catalyst was first annealed at 700 °C under ammonia (3 Torr).
Subsequently acetylene was added (C2H2+NH3 = 75+200
sccm) as the precursor of carbon for the vapour deposition.
Fig. 1(a) shows a TEM image of the CNHs grown in this way.
The CNHs are short and hollow with a ruthenium particle sitting
Fig. 2 TEM image showing a partially ordered graphitic structure of the
CNHs. This image also shows a large dispersion in size for the Ru
nanoparticles.
Fig. 3 High resolution TEM images of Ru particles (single crystals) at the
tip of two CNHs. The left particle has a lattice spacing of 2.05 Å which
corresponds to Ru(101) [2.051 Å], and the right particle has a measured
spacing of 2.14 Å corresponding to Ru(002) [2.138 Å].
Fig. 1 TEM images showing that the CNHs have a hollow core and a
partially ordered graphitic body. Catalytic particles (2–10 nm diameter) can
be seen at the top of the nanohorns.
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CHEM. COMMUN., 2002, 1112–1113
This journal is © The Royal Society of Chemistry 2002