Pierson Hugh O. Handbook of carbon, graphite, diamond and fullerene. New Jersey / carbon

366 Carbon, Graphite, Diamond, and Fullerenes

3.0

FULLERENES IN THE CONDENSED STATE

In

the

previous

section,

the

formation

and

characteristics

of

single

fullerene

molecules

were

reviewed.

In this

and

the

following

sections,

the

formation,

characteristics,

and properties

of solid

aggregates

of fullerenes

are examined.

Fullerenes

aggregates

are a new discovery:

their character-

ization

and the

determination

of their

properties

is still

at an early

stage

and

much

work

remains

to be done.

Yet

what

has

been

accomplished

so far

shows

the

striking

potential

of these

materials.

3.1 Crystal Structure of Fullerenes Aggregates

It was

originally

assumed

that thesolid

aggregate

of C,fullerenes

had

a hexagonal

close-packed

structure.

However,

recent

x-ray

diffraction

studies

have

shown

unambiguously

that

it adopts

the

face-centered

cubic

(fee)

structure

(providing

that

all

solvent

molecules

are

eliminated).

Synchroton

x-ray

powder

profile gives

a lattice constant a = 1.417

nm. This

value implies the close packing

of

pseudospheres

having

a diameter

of

1.002

nm.

This is consistent

with

the

fitted radius

of the

C,

skeleton

of

0.353

nm

and

a carbon

van

der

Waals

diameter

of

0.294

nm, which is

slightly

smaller

than that

of

graphite

(0.335

nm).

The

intermolecular

bonding

is dominated

by van

der Waals

forces,

as confirmed

by measure-

ments

of the

isothermal

compressibility.t14]

The

C,

aggregates,

grown

from

solution,

are shiny

and

black

and

reach 300 pm

in size.

They display

a tenfold

symmetry.t15]

The Cr,-, aggregates

are

different

as they have

a hexagonal

structure

with

lattice

parameters

a = 1.063

nm and c = 1.739

nm.n4]

Mechanical Properties. C,, aggregates

are considered

the softest of

the solid

phases

of carbon. El41However,

calculations

showthat,

under

high

pressure

when

compressed

to less

than

70%

of its original

volume,

they

could

be

harder

than

diamond.t3]

They have high impact strength

and

resilience.

They should

also have

high

lubricity

since

the

molecules

are

bonded

by van

der

Waals

forces

in

all planes

which

should

allow

the

molecules to slip

readily

over

each other

in a manner

similar to the ab planes

of the

graphite crystal.

The Fullerene

Molecules

367

The aggregates

have a constricted

micropore

structure

with a

micropore width of 1 .19 - 1.26 nm and a relatively

high internal surface area

(131.9 m2/g)Pl

Semiconductor

Properties.

Calculations

indicate that C, could be

a direct band-gap semiconductor

similar to gallium arsenide. However, the

semiconductor properties

have yet to be determined.

Fullerene-Diamond

Transformation.

The rapid compression

of C,

the fullerenesand

the formation of a shining and transparent

material which

was identified as

a

polycrystalline diamond

in an amorphous carbon

matrix.t’fl

Thus the

fullerenes are the first known phase

of carbon that

transforms

into diamond at room temperature.

Graphite

also transforms

Most of the

studies of the

chemical

reactivity of the fullerenes

have

been done with C,,

aggregates.

Although the molecule is stable from the

physical standpoint,

it has a high electron affinity and is reactive chemically,

especially with free

radicals.f1a]-[20)

Fullerenes

are aromatic structures

and dissolve readily in the arche-

typal aromatic compound, i.e., benzene

and in other aromatic solvents.

They oxidize

slowly in a mixture of concentrated

sulfuric and nitric acids at

temperatures

above 50°C. In pure oxygen, C,,

begins to sublime at 350°C

and ignites at 365°C;

in air, it oxidizes rapidly to CO and CO, and is

more

reactive than carbon

black or any other form of graphite.n6]

368 Carbon,

Graphite,

Diamond,

and

Fullerenes

The C,

has been

described

as a “radical sponge” since it can be

considered

as having thirty carbon-carbon

double bonds,

where

free

radicals can

attach

themselves

covalently

on the

outside

of the

carbon

framework without destroying the kinetic and thermodynamic

stability of the

molecule. Thestable compoundsthat

are thus formed are called “exohedral”,

that is, formed outside the C,

shell.

Fullerene-Organic

Compounds.

Exohedral

organic

compounds

investigated

so far include the C,-benzyl

radicals (CsH5CH2-), the &,-ally1

group (CH,:CHCH,-),

and the &-methyl

group (CH,-) .t1Q)t20)

Fullerene-Organometallics.

Also recently

investigated

are

the

organo-metallic

exohedral complexes of osmium, ruthenium,

and platinum

which are readily attached to the external framework of the C,

molecule by

solution chemistry. [211[23)An osmylated-C,,

compound is shown

in Fig.

15 .8 .t2’]

370 Carbon,

Graphite,

Diamond,

and Fullerenes

Superconductivity.

Some

intercalation

compounds

are

supercon-

ductors, such

as C&K,,

with an onset

of superconductivity

at 17

K and zero

resistance at

5 K.t5)t251

Other

compounds

have

even

higher critical

temperatures,

i.e., 30

K for rubidium-doped

C, and

43

K for

rubidium-

thallium-doped

C,.t31

Different

from

the

exohedral

materials

described

in

the

preceding

section

are the “endohedral”

compounds,

where

the

foreign

atoms

are

located

within

the fullerene

cage instead of outside.

These

compounds

are

designated

(MB&),

the symbol

“@”indicating

that

the

M atom

is located

within

the

C,

cage,

“n”representing

the

number

of

carbon

atoms of the

fullerene

(60, 70, 76, etc.).

Composition

such

as

(Y@C,)

and

(Y2@Cs2)

(as

identified

by spectral

analysis)

are readily

produced

by laser

vaporiza-

tion

of a graphite-yttria

rod;

other

endohedral

compounds

with

lanthanum

and

uranium

have

also

been

synthesized.t2)t3tt16)

The

C,

and

the

higher

fullerenes

are

produced

from condensing

carbon vapors,

providing

that the

condensation

is sufficiently

slow

and

the

temperature

is sufficiently

high

(>2000”C).t2)

These

conditions

exist in the

basic apparatus

shown

in Fig.

15.1 0.t26) An arc is generated

between

two

rod-electrodes

made of high-purity

graphite,

in

an

atmosphere

of pure

helium.

The

distance

between

the

electrode

tips

is maintained

constant.

The fullerenes

are collected

by solvent

extraction

of the

resulting

soot,

with

a solvent

such

as N-methyl-2-pyrrolidinone.

Yield

of dissolvable

fullerenes

as high as 94% are reported.t27)

In

such

a system,

hydrogen,

water

vapor,

and

other

contaminants

must be totally

excluded

since

they

would

tend

to form

dangling

bonds

and

prevent

closure

of the

fullerene

molecule.t3)

The

helium

atmosphere

is

necessary

because

it slows

migration

of the carbon

chains

away

from

the

graphite

rod and

gives them

sufficient

time

to form

the

initial cluster

radicals

to rearrange

into

pentagons

before

the radicals

start

to

polymerize.t13)

High fullerene

yields are also

reported

in a benzene-oxygen

flame

with

optimum

conditions

of 9 kPa,

a C/O

ratio

of

0.989

and

a dilution

of 25%

helium.t26)

372

Carbon,

Graphite, Diamond, and

Fullerenes

REFERENCES

1.

Kroto, H. W., Heath,

J. R.,

O’Brien,

S. C.,

Curl, Ft. F., and Smalley,

R. E., Nature, 318:162-163

(1985)

2.

Smalley,

R. E., Act.

Chem.

Res., 2598-I

05 (1992)

16.

Ismail, I. M. K., Carbon, 30(2):229-239 (1992)

24.

Nakajima, T. and Matsuo, Y.,

Carbon, 30(7):1119-l

120

(1992)

25.

Hebard,

A. F., et al., Nature,

350:600-601

(April 18,

1991)

26

Parker,

D. H.,

et al., J. Am. Chem. Sot.,

113:7499-7503

(1991)

27.

Parker,

D. H.,

et al.,

Carbon,

30(3):1167-l

182

(1992)

28.

Howard,

J. B., et al.,

Carbon,

30(8):1183-1201

(1992)

Glossary 375

Aqua Regia: A mixture of

concentrated nitric acid and concentrated

hydrochloric acid in the ratio

1:3 respectively.

Atomic Number: The number

of protons

in the nucleus

of an atom

which

is equal

to

the

number

of electrons

revolving

around

the

nucleus.

The

atomic number

determines

the location

of the element

in the Periodic

Table.

Avogadro Constant (NJ:

(Formally

Avogadro’s

number)

The number

of

atoms or molecules

contained in one

mole

of any substance

(6.0221367

x

102”). Avogadro

proposed

that each

distinct

substance

in the gaseous

state

consists

of

characteristic

discrete

particles

called

molecules:

a

specified

volume

of

any

gas

measured

at

a

uniform

pressure

and

temperature

contains

the same

number

of molecules.

Bandgap:

The energy-distance

electrons

have

to

move

to

go

from

the

valence

band to the

conductor

band.

Benzene

Ring:

The structure

of

the hydrocarbon

compound

benzene,

C&H,.

It

is

a

six-carbon

ring

where

all C-C bonds

are

equivalent

and

intermediate

in length between

single

and double

bonds,

One

electron

per

carbon

atom (for

a total

of six)

is delocalized.

These

six

electrons

have

equal probability

of being

found

anywhere

around

the

ring.

They

reside

in

pi bonds

which

are perpendicular

to the plane

of the molecule.

Benzene

is

the archetypal

aromatic

compound.