Guozhong Cao. Nanostructures & Nanomaterials: Synthesis, Properties & Applications
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Chapter
6
Special Nanomaterials
6.1.
Introduction
In the previous chapters, we have introduced the fundamentals and
general methods for the synthesis and fabrication of various nanostruc-
tures and nanomaterials including nanoparticles, nanowires and thin
films. However, there are a number of important nanomaterials not
included in these discussions, since their syntheses are unique and diffi-
cult to group into previous chapters. Examples of such nanomaterials are
carbon fullerenes and nanotubes, ordered mesoporous materials, organic-
inorganic hybrids, intercalation compounds, and oxide-metal core-shell
structures. In addition, bulk materials with nanosized building blocks,
such as nanograined ceramics and nanocomposites, have not been
discussed
so
far. In this chapter, we will discuss the synthesis of these
special nanomaterials. Most of these nanomaterials are unique, do not
exist in nature, and are truly “man-made” relatively recently, therefore,
a brief introduction to the materials, such as their peculiar structures and
properties, has also been included in the discussion. All the discussion
has been kept very general, but detailed references are given
so
that the
readers can easily find literature to gain more insight to those subjects
when needed.
229
230
Nanostructures and Nanomaterials
6.2.
Carbon Fullerenes and Nanotubes
Carbon is a unique material, and can be a good metallic conductor in the
form of graphite, a wide band gap semiconductor in the form of diamond,
or a polymer when reacted with hydrogen. Carbon provides examples of
materials showing the entire range of intrinsic nanometer scaled structures
from fbllerenes, which are zero-dimensional nanoparticles, to carbon
nanotubes, one-dimensional nanowires to graphite, a two-dimensional
layered anisotropic material, to fullerene solids, a three-dimensional bulk
materials with the fullerene molecules as the fundamental building block
of the crystalline phase. In this section, we will briefly discuss the syn-
thesis and some properties of fullerenes, fullerene crystals and carbon
nanotubes. For more general research information about carbon science or
detailed information on carbon fullerenes and carbon nanotubes, the read-
ers are referred to excellent review articles and books, such as that by
Dresselhaus'*2 and references therein.
6.2.1.
Carbon fu//erenes
Carbon fullerene commonly refers to a molecule with
60
carbon atoms, c60,
and with an icosahedral ~ymmetry,~ but also includes larger molecular
weight fullerenes C,(n
>
60).
Examples
of
larger molecular weight
fbllerenes are C70, c76, C7*, Cso, and higher mass fbllerenes, which possess
different geometric structure,M e.g. C70 has a rugby ball-shaped symmetry.
Figure
6.1
shows the structure and geometry of some fullerene
molecule^.^
The name of fbllerene was given to this family of carbon molecules because
of
the resemblance of these molecules to the geodesic dome designed and
built by
R.
Buckminster Fuller,* whereas the name of buckminster fbllerene
or buckyball was specifically given to the
c60
molecules, which are the most
widely studied in the fbllerene family and deserve a little more discussion on
its structure and properties.
The
60
carbon atoms in
c60
are located at the vertices of a regular trun-
cated icosahedron and every carbon site on
c60
is equivalent to every other
site. The average nearest neighbor C-C distant in
c60
(1.44
is almost
identical to that in graphite
(1.42
A).
Each carbon atom in
c60
is trigonally
bonded to other carbon atoms, the same as that in graphite, and most
of
the faces on the regular truncated icosahedron are hexagons. There are
20
hexagonal faces and
12
additional pentagonal faces
in
each
c60
molecule,.
which has a molecule diameter of
7.10A.3>10
Although each carbon atom
in
c60
is equivalent to every other carbon atom, the three bonds emanating
Special Nanomaterials
23
1
Fig.
6.1.
(a) The icosahedral
C,o
molecule. (b) The
C,,
molecule as a rugby ball-shaped
molecule. (c) The
C8,
molecule as an extended rugby ball-shaped molecule. (d) The
Cs0
molecule as an icosahedron.
[M.S.
Dresselhaus and
G.
Gresselhaus,
Ann.
Rev.
Muter.
Sci.
25,
487
(I
995).]
from each atom are not equivalent. Each carbon atom has four valence
electrons for the formation of three chemical bonds, and there will be two
single bonds and one double bond. The hexagonal faces consist of alter-
nating single and double bonds, whereas the pentagonal faces are defined
by single bonds. In addition, the length of single bonds is 1.46
A,
longer
than the average bond length, 1.44
A,
while the double bonds are shorter,
1.40
A.
The structures of other fullerene molecules can be considered
as
a
modification of
C60
by varying the number of hexagonal faces as far
as the Euler’s theorem is not violated, which states that a closed surface
consisting of hexagons and pentagons has exactly 12 pentagons and an
arbitrary number of hexagons.13 For example,
C7,,
structure can be envi-
sioned by adding a belt
of
five hexagons around the equatorial plane of the
c60
molecule normal to one of the five-fold axis.
Fullerenes are usually synthesized by using an arc discharge between
graphite electrodes in approximately 200torr of He gas, first demon-
strated in 1990 by Kratschmer and coworkers.
l4
The heat generated at the
contact point between the electrodes evaporates carbon to form soot and
fullerenes, which condense on the water-cooled walls of the reactor. This
discharge produces a carbon
soot
that can contain up
to
-
15%
fullerenes:
c60
(-13%) and
C,,
(-2%).
The fullerenes are next separated from the
soot, according to their mass, by
use
of
liquid chromatography and using
232
Nanostructures and Nanomaterials
a solvent such as a toluene. However, there is no definite understanding of
the growth mechanism of the fullerenes. Fullerene chemistry has been a
very active research field, because of the uniqueness of the
c60
molecule
and its ability to have a variety of chemical reaction~.'~J~
6.2.2.
FMerene-derived crystals
In a solid state, fullerene molecules crystallize into crystal structures
through weak intermolecular forces and each fullerene molecule serves as
a fundamental building block
of
the crystalline phase. For example, the
c60
molecules crystallize into a face-centered cubic (FCC) structure with
lattice constant of 14.17A and a
C60-C60
distance of lO.O2A.I7 The mol-
ecules are almost freely rotating with three degree of rotation at room
temperature, as shown by nuclear magnetic resonance methods. The crys-
talline forms of fullerenes are often called fullerites.'* Single crystals can
be grown either from solution using solvents such as
CS2
and toluene or
by vacuum sublimation, though the sublimation yields better crystals and
is generally the method of choice.'9
6.2.3.
Carbon nanofubes
There are excellent reviews and books on the synthesis and the physical
properties of carbon nanot~bes,2@-~~ and therefore, in this section, only a
brief summary of the fundamentals and general approaches for the syn-
thesis of carbon nanotubes is presented. There are single-wall carbon nan-
otube or SWCNT, and multi-wall carbon nanotube or MWCNT. The
fundamental carbon nanotube
is
a single-wall structure and can be under-
stood by referring to Fig. 6.2.' In this figure we see that points
0
and A
are crystallographically equivalent on the graphene sheet, where X-axis
is placed parallel to one side of the honeycomb lattice. The points
0
and
A can be connected by a vector
Ch
=
rial
+
ma2,
where
al
and
a2
are unit
vectors for the honeycomb lattice of the graphene sheet. Next we can draw
normals to
ch at points
0
and
A
to obtain lines OB and AB'. If we now
superimpose
OB
onto AB', we obtain a cylinder of carbon atoms that con-
stitutes a carbon nanotube when properly capped at both ends with half of
a fullerene. Such a single-wall carbon nanotube is uniquely determined by
the integers
(n,m).
However, from an experimental standpoint, it is more
convenient to denote each carbon nanotube by its diameter
dt
=
Chh
and
the chiral angle
8.
Depending on the chiral angle, a single-wall carbon
Special
Nanomaterials
233
Fig.
6.2.
The chiral vector
OA
or
C,
=
nu,
+
mu2
is defined on the honeycomb lattice of car-
bon atoms by unit vectors a1 and a2 and the chiral angle
0
with respect to the zigzag axis.
Along the zigzag axis,
0
=
0".
Also shown is the lattice vector
OB
=
Tof the one-dimensional
nanotube unit cell. The rotation angle
$
and the translation
T
(not shown) constitute the basic
symmetry operation
R
=
($17)
for the carbon nanotube. The diagram is constructed for
(n,m)
=
(4,2). The area defined by the rectangle
(OAB'B)
is the area of the one-dimensional
unit cell of the nanotube.
[M.S.
Dresselhaus,
Ann.
Rev.
Muter:
Sci.
27,
1
(1997).]
nanotube can have three basic geometries-armchair with
8
=
30", zigzag
with
0
=
0",
and chiral with 0<0<30", as shown in Fig. 6.3.24
Multi-wall carbon nanotube consists of several nested coaxial single-
wall tubules. The arrangement of the carbon atoms in the hexagonal net-
work of the multi-wall carbon nanotube is often helicoidal, resulting in the
formation of chiral tubes.31 However, there appears to be no particular
ordering between individual cylindrical planes forming the multi-wall car-
bon nanotube such as can be found in graphite where the planes are
stacked relative to each other in an
ABAB
configuration. In other words,
a given multi-wall carbon nanotube will typically be composed
of
a mix-
ture of cylindrical tubes having different helicity or no hecility, thereby
resembling turbostratic graphite. Typical dimensions of multi-wall carbon
nanotube are outer diameter:
2-20
nm, inner diameter: 1-3 nm, and
length: 1-100 km. The intertubular distance is 0.340nm, which is slightly
larger than the interplanar distance in graphite.
Carbon nanotubes can be prepared by arc e~aporation,~~ laser
ablation,26 pyrolysis,27
PECVD,28
and electrochemical
method^.^^?^^
Carbon nanotubes were first synthesized by Iijima in 1991 in the carbon
cathode by arc di~charge.~] However, the experimental discovery of sin-
gle-wall carbon nanotubes came in 1993,32,33 whereas the discovery in
1996 of a much more efficient synthesis route, involving laser vaporiza-
tion of graphite to prepare arrays of ordered single-wall nan~tubes
offered major new opportunities for quantitative experimental studies
of
carbon nanotubes.
234
Nanostructures and Nanomaterials
Fig.
6.3.
Schematic models for single-wall carbon nanotubes with the nanotube axis
normal to (a) the
0
=
30”
direction (an armchair
(an)
nanotube); (b) the
8
=
0”
direction
(a zigzag
(n,O)
nanotube); and (c) a general direction
OB
(see Fig.
6.2)
with
0”
<
0
C
30”
[a chiral
(n,rn)
nanotube]. The actual nanotubes shown in the figure correspond to
(n,m)
values
of
(a)
(5,5),
(b)
(9,0),
and (c)
(10,5).
[M.S.
Dresselhaus, G. Dresselhaus, and
R.
Saito,
Carbon
33,
883
(1995).]
The formation of the carbon nanotubes in most cases requires an “open
end” where the carbon atoms arriving from the gas phase could coherently
land and incorporate into the structure. Growth of nested multi-wall nano-
tubes can be stabilized by the strained “liplip” bonding between the
coaxial edges, highly fluctuating and, therefore, accessible for new atoms.
In general the open end can be maintained either by a high electric field,
by the entropy opposing the orderly cap termination, or by the presence of
a metal cluster.
The presence
of
an electric field in the arc-discharge is believed to pro-
mote the growth of carbon nan~tubes.~~,~~ Nanotubes form only where the
current flows, on the larger negative electrodes. Typical rate of the cath-
ode deposition is about a millimeter per minute, with the current and volt-
age in the range of
100
A
and
20
V
respectively, which maintains a high
temperature of
2000-3000°C.
For example, Ebbesen and Aja~an~~ used
carbon arc evaporation to produce carbon nanotubes in high yields. In
their experiment, an arc plasma is generated between the two carbon elec-
trodes in an inert atmosphere, e.g. He, by applying a
DC
current density
of
-
150
A/cm2 with a voltage of
-20
F?
The extremely high growth tem-
peratures required in the arc discharge experiments can cause the grown
carbon nanotubes
to
sinter, and the sintering of carbon nanotubes is
believed to
be
the predominant source
of
defects.37
Special Nunomaterials
235
An addition of a small amount of transition metal powder such as Co,
Ni or Fe, favors a growth of single-wall nan~tubes.~*!~~ Thess
et
al.34
grew
uniform diameter
(10,lO)
nanotubes with high yield, by condensation of
laser-vaporized carbon catalyst mixture at a lower temperature of
-
1200°C. It is believed that the alloy cluster anneals all unfavorable stmc-
tures into hexagons, which in turn welcome the newcomers and promote
the continuous growth of straight nanotubes. Figure
6.4
illustrates the ener-
getics of growth, the relative binding energies in nanotubes, graphite and
the feedstock components.I8
Growth
of
aligned carbon nanotubes was first demonstrated by CVD
directly on Fe nanoparticles embedded in mesoporous silica.38 The diam-
eter, growth rate and density of vertically aligned carbon nanotubes are
found to be dependent on the size of the catalyst.39 Plasma induced well-
aligned carbon nanotubes can be grown on contoured surfaces and with a
growth direction always perpendicular to the local substrate surface as
shown in Fig.
6.5.40
The alignment is primarily induced by the electrical
self-bias field imposed on the substrate surface from the plasma environ-
ment. It was found that switching the plasma off effectively turns the
alignment mechanism off, leading to a smooth transition between the
plasma-grown straight nanotubes and the thermally grown “curly” nan-
otubes as shown in Fig.
6.6.40
DC-bias has found to enhance the nucle-
ation and growth of aligned carbon nanot~bes.~~
AC
Energy,
eV/atom
I
atoms
in
thc
gas
I
I
-0.180
Graphcne sheet
Fig.
6.4.
“Food chain” illustrates how the metal (Ni/Co) cluster is able to eat essentially any
carbon material it encounters and feed the digested carbon bits to the growing end of the
nanotube. The vertical axis shows the cohesive energy per atom for the different forms of
carbon consumed in nanotube growth. The energy cost for curving the graphene sheet into
the cylinder of the (1 0,lO) tube is only 0.045 eV,
or
0.08 eV nm2/d for a tube
of
any diam-
eter
d.
The elastic stretching of a tube by
15%
adds approximately 0.66 eV per atom above
the graphene.
[R.E.
Smalley and
B.I.
Yakobson,
Solid
State
Commun.
107,
597 (1998).]