Page 162 - Carbon Nanotubes

NANBPARTICLES AND FILLED NANOCAPSULES

YAHACHI SAITO
Department of Electrical and Electronic Engineering, Mie University, Tsu 5 14 Japan
(Received 11 October 1994; accepted in revised form 10 February 1995)
Abstract-Encapsulation of foreign materials within a hollow graphitic cage was carried out for rare-earth
and iron-group metals by using an electric arc discharge. The rare-earth metals with low vapor pressures,
Sc, Y, La, Ce, Pr, Nd, Gd, TD, Dy, Ho, Er, Tm, and Lu, were encapsulated in the form of carbides, whereas
volatile Sm, Eu, and Yb metals were not. For iron-group metals, particles in metallic phases (a-Fe, y-Fe;
hcp-Co, fcc-Co; fcc-Ni) and in a carbide phase (M3C, M = Fe, Co, Ni) were wrapped in graphitic car-
bon. The excellent protective nature of the outer graphitic cages against oxidation of the inner materials
was demonstrated. In addition to the wrapped nanoparticles, exotic carbon materials with hollow struc-
tures, such as single-wall nanotubes, bamboo-shaped tubes, and nanochains, were produced by using tran-
sition metals as catalysts.
Key Words-Nanoparticles, nanocapsules, rare-earth elements, iron, cobalt, nickel.

1. INTRODUCTION nanotubes. When a pure graphite rod (anode) is evap-
orated in an atmosphere of noble gas, macroscopic
The carbon-arc plasma of extremely high temperatures quantities of hollow nanoparticles and multi-wall
and the presence of an electric field near the electrodes
play important roles in the formation of nanotubes[ 1,2] nanotubes are produced on the top end of a cathode.
and nanoparticles[3]. A nanoparticle is made up of When a metal-packed graphite anode is evaporated,
filled nanocapsules and other exotic carbon materials
concentric layers of closed graphitic sheets, leaving a with hollow structures (e.g., “bamboo”-shaped tubes,
nanoscale cavity in its center. Nanoparticles are also nanochains, and single-wall (SW) tubes) are also syn-
called nanopolyhedra because of their polyhedral
shape, and are sometimes dubbed as nanoballs be- thesized. Details of the preparation procedures are de-
cause of their hollow structure. scribed elsewhere[&,ll,12].
When metal-loaded graphite is evaporated by arc
discharge under an inactive gas atmosphere, a wide 3. NANOPARTICLES
range of composite materials (e.g., filled nanocapsules, Nanoparticles grow together with multi-wall nano-
single-wall tubes, and metallofullerenes, R@C82, tubes in the inner core of a carbonaceous deposit
where R = La, Y, Sc,[4-6]) are synthesized. Nanocap- formed on the top of the cathode. The size of nano-
sules filled with Lac, crystallites were discovered in particles falls in a range from a few to several tens of
carbonaceous deposits grown on an electrode by nanometers, being roughly the same as the outer di-
Ruoff et a1.[7] and Tomita et a1.[8]. Although rare- ameters of multi-wall nanotubes. High-resolution
earth carbides are hygroscopic and readily hydrolyze TEM (transmission electron microscopy) observations
in air, the carbides nesting in the capsules did not de- reveal that polyhedral particles are made up of con-
grade even after a year of exposure to air. Not only centric graphitic sheets, as shown in Fig. 1. The closed
rare-earth elements but also 3d-transition metals, such polyhedral morphology is brought about by well-de-
as iron, cobalt, and nickel, have been encapsulated by veloped graphitic layers that are flat except at the cor-
the arc method. Elements that are found, so far, to be ners and edges of the polyhedra. When a pentagon is
incapsulated in graphitic cages are shown in Table 1. introduced into a graphene sheet, the sheet curves pos-
In addition to nanocapsules filled with metals and itively and the strain in the network structure is local-
carbides, various exotic carbon materials with hollow ized around the pentagon. The closed graphitic cages
structures, such as single-wall (SW) tubes[9,10], produced by the introduction of 12 pentagons will ex-
bamboo-shaped tubes, and nanachains[l 11, are pro- hibit polyhedral shapes, at the corners of which the
duced by using transition metals as catalysts.
In this paper, our present knowledge and under- pentagons are located. The overall shapes of the poly-
hedra depend on how the 12 pentagons are located.
standing with regard to nanoparticles, filled nanocap- Carbon nanoparticles actually synthesized are multi-
sules, and the related carbon materials are described. layered, like a Russian doll. Consequently, nanopar-
ticles may also be called gigantic multilayered fderenes
2. PREPARATION PROCEDURES or gigantic hyper-fullerenes[l3].
The spacings between the layers (dooz) measured by
Filled nanocapsules, as well as hollow nanoparti- selected area electron diffraction were in a range of
cles, are synthesized by the dc arc-evaporation method 0.34 to 0.35 nm[3]. X-ray diffraction (XRD) of the
that is commonly used to synthesize fullerenes and cathode deposit, including nanoparticles and nano-
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