Fahlman B.D. Materials Chemistry

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laser and arc plasma systems), but may also result in lowering the cost due to a more

straightforward industrial scale-up.

Buckyballs represent the smallest fullerene that obeys the Isolated Pentagon Rule

(IPR) – i.e., an energetic requirement that pentagons be surrounded by hexagons, so

that adjacent pentagons do not share an edge. Calculations show that p bonds shared

between six-membered rings have large positive bond resonance energies (BREs)

Carbon Chains/

Rings

100

×10

0.7 nm

100 120

Cluster Size (Atoms)

Ion Signal

Fullerenes

“Forbidden

Zone”

Figure 6.24. Mass spectrum of carbon clusters in a supersonic beam, originally observed by Nobel

Laureates Smalley and Curl. Reproduced with permission from J. Chem. Phys. 1984, 81, 3322. Also

shown is the molecular structure of Buckminsterfullerene, C

, containing alternating six- and ﬁve-

membered rings of sp

hybridized carbon atoms. This is only one isomer for C

, out of a staggering

total of 1,812 possible structures.

[100]

488 6 Nanomaterials

and bond orders, indicating a high degree of aromaticity and stability/unreactivity.

However, p bonds between adjacent ﬁve-membered rings have large negative BREs

with very small bond orders, indicating a much lower thermodynamic stability.

Most likely, this difference is due to the increased ring strain that would be imposed

in the fullerene structure as a result of two smaller rings directly adjacent to one

another. Theoretical calculations indicate that the strain energy of the icosohedral

Buckminsterfullerene structure is at least 2 eV lower than any other non-IPR isomer,

of which there are over 1,800 possibi lities.

Interestingly, it has recently been reported that adjacent pentagons containing at

least one N atom instead of C (e.g., C

rather than C

), may actually be more

stable than C

(Figure 6.27).

[102]

This apparent anomaly is a direct contradiction of

the IPR. The most plausible explanation is the reduction of ring strain due to sp

hybridization of N, as well as the addition of p-electron density (from the N lone-pair

electrons) to the pentagons, resulting in an enhanced aromaticity/stability. To date,

only short-lived azafullerenes C

NandC

–

have been identiﬁed experimentally;

the search continues for stable structures, since these will likely result in dramatically

different properties and associated applications relative to their C-only analogues.

Vaporization laser

He Gas Pulse

He inlet

Rotating Graphite

Disk

Supersonic Nozzle

Mass

Spectrometer

"Integrating

Cup"

To Vacuum

Electric Arc

Graphite

Rod

Electrodes

Current Source

Figure 6.25. Schematic of apparati ﬁrst used to synthesize fullerenes. Illustrated are (a) the Smalley/Curl

supersonic laser evaporation system, and (b) the Huffman/Kratschmer electric arc apparatus.

6.3. Nanoscale Building Blocks and Applications 489

to pump

Watercooled Prode

Top Plate

Viewport

Burner

Bottom Plate

Oxygen Oxygen

Fuel/Argon

Vacuum Ports

VACUUM

PUMP

Auxiliary Ports

HELIUM

CYLINDER

WATER

OUT

MERCURY

MANOMETER

flow control

valves

25 mm

rubber

tubing

rubber

septum

oil

manometer

copper

mounting rod

arc

graphite

electrode

1 L

Pyrex

reactor

vessel

SCALE: 5 cm

DC ARC WELDER

AC METER

220V/50 A

AC OUTLET

Pyrex

guide

arm

SUBMERSIBLE

PUMP

Figure 6.26. Cross-section schematics of reactors used for fullerene synthesis. Shown are (a) a reduced-

pressure fuel-rich pyrolytic chamber, and (b) a benchtop modiﬁed arc evaporation system. Reproduced

with permission from (a) Hebgen, P.; Goel, A.; Howard, J. B.; Rainey, L. C.; Vander Sande, J. B. Proc.

Although fullerenes have been actively investigated for more than two decades,

there is an ongoing debate regarding the growth mechanism of these nanoclusters.

Since the formation of fullerene s via laser/arc/combustion techniques occurs too

rapidly to isolate intermediate species, most of the mechanistic proposals are based

on theoretical techniques (quantum mechanical and molecular dynamics). It was

once thought that fullerenes were formed from the folding of preformed graphitic

sheets that emanated from the target following laser ablation. However, a variety of

experiments have shown that the growth process likely initiates from small linear

chains of carbon atoms. As the number of carbon atoms increases, the chains

preferentially connect into ring structures due to their greater stabilities. In particu-

lar, for C

where n < 10 (with the exception of C

as discussed below), linear

species are the preferred morphology rather than rings (Figure 6.28). The preference

for ring formation for n ¼ 6, and n  10 (especially for C

, etc.) is due to

the enhanced aromaticity/stability of planar rings when there are 4n +2p electrons

(where n ¼ 1, 2, 3, ... – the H

€

uckel rule).

Figure 6.29 illustrates a proposed mechanism for the subsequent steps of fullerene

growth, involving the self-assembly of carbon rings. When n  30 or so, the

monocyclic rings are proposed to form graphitic sheets. The “pentagon road”

mechanism proposed by Smalley

[103]

assumes that the graphitic sheets contain

both hexagon and pentagon units. Closure of the sheets to form Buckyballs effec-

tively results in growth termination. In contrast, the “fullerene road” model assumes

the initial formation of smaller non-IPR fullerenes, which undergo thermal

rearrangement to yield C

and higher fullerenes.

[104]

As discussed earlier, pentagon units are essential to the fullerene structure, sinc e

they allow the planar graphitic sheet to curl. The driving force for this rearrangement

is likely the C—C bonding of edge carbons that satisﬁes their unﬁlled valences.

As noted in Figure 6.29, adequate annealing is required in order to incorporate a

Figure 6.27. Illustration of C

that (a) satisﬁes the IPR and (b) violates the IPR with adjacent

pentagons. The structure with adjacent pentagons, (b), is more stable than (a) by 12.5 kcal mol

1

Chemical Society.

6.3. Nanoscale Building Blocks and Applications 491

monocyclic rings

linear chains

Number of Carbon Atoms

Relative Stability [kcal/mol]

−20

−40

−60

−80

−100

2 4 6 8 10 12 14 16 18 20

Figure 6.28. The relative stabilities of linear-chain carbon clusters vs. monocyclic rings with changing

cluster size. Reproduced with permission from Hutter, J.; Luthi, H. P.; Diederich, F. J. Am. Chem. Soc.

Figure 6.29. Proposed mechanism for fullerene growth. Reprinted from Yamaguchi, Y.; Maruyama, S.

492 6 Nanomaterials

sufﬁcient number of pentagons. For instance, if the cooling rate is too high,

amorphous soot particles will be preferentially formed rather than fullerenes.

In addition, an overall low growth temperature will not be sufﬁcient to cause cage

formation, yielding planar graphitic fragments instead of fullerenes.

Regardless of the proposed mechanism, a ﬁnal thermal annealing step is likely

required to organize the hexagon and pentagon subunits into the lowest-energy IPR

arrangement. This rearrangement step is known as the Stone–Wales (SW) transfor-

mation, and involves a concerted reorganization of the hexagon/pentagon units.

We already saw an example of a rar e SW transformation where the non-IPR

N-containing species was actually lowest in energy (Figure 6.27). However, most

often this rearrangement occurs in the opposite direction – transforming adjacent

pentagons into a hexagon-isolated structure. It should be noted that the Stone–Wales

transformation is actually thermally forbidden via the Woodward–Hoffman rules;

calculations show an energy barrier of at least 5 eV for this pathway. However, it has

been shown that this rearrangement may likely be catalyzed by additional carbon

and/or hydrogen atoms that are present during laser/arc or thermal combustion

syntheses (Figure 6.30).

Interestingly, a fullerene structure may serve as a nucleation site for additional

layers of graphite en route toward multishell fullerenes (Figure 6.31). These are

denoted as “C

240

” where the @ symbol represents the encapsulated species.

There are even triple-layered structures such as “C

240

560

.”

[105]

Though

very small quantities (<0.01%) of multilayered fullerenes are found in the soot

resulting from laser vaporization, the yield may be improved by in vacuo sub-

limation of the vapor phase at a high temperature (ca. 1,200



C).

Although the “brute force” methods of laser/arc and high-temperature pyrolysis

represent the most common techniques for generating fullerenes, a goal of the

synthetic organic chemist has long been the solution-phase, stepwise synthesis of

. In 1999, a promising step in that direction was accomplished with the ﬁrst

nonpyrolytic synthesis of “buckybowls.”

[106]

These structures had a bowl-shaped

structure, and consisted of the hexagon-isolated pentagon backbone exhibited by

fullerenes (Figure 6.32). In early 2002, a chlo rinated C

precursor was reported

using a traditional 12-step organic synthesis. This compound was subsequently

converted to Buckyballs using high-temperature vacuum pyrolysis (Figure 6.33).

The yield of C

was <1% – certainly not useful for commercial production of

Buckyballs! However, the novelty of this approach was that pyrolysis did not

decompose the precursor into smaller units, but rather served to stitch together

adjacent arms of the molecular precursor. Hence, this method provides a targeted

route toward individual fullerenes based on the structure of the precursor, rather than

high-energy methods that always result in a mixture of fullerene products.

In addition to pristine fullerene structures, it has been discovered that various

metal ions may be encapsulated inside the caged structure to yield endohedral

fullerenes. Thus far, a variety of alkali and lanthanide metals, Group V atoms,

noble gases, and neutral molecules such as H

[107]

CO, and H

O have been

sequestered inside the C

structure. Calculations have shown that the encapsulation

6.3. Nanoscale Building Blocks and Applications 493

of noble gas atoms and small ions (e.g., Li



,Cl



) actually stabilize the fullerene

cage, whereas larger species (e.g., Rb

,Br



) dest abilize the cage.

[108]

Metallo-

fullerenes (M@C

) are typically grown by either laser ablation of metal-doped

graphite disks at high temperature (ca. 1,200



C), or carbon arc techniques with

metal-doped graphite rods. An example of a metallofullerene (Gd

) was

shown in Figure 6.22a; these structures are commonly employed as MRI contrast

agents.

Figure 6.30. The potential energy surface of the Stone–Wales transformation before/after the addition

of catalyzing moieties such as (a) carbon, and (b) hydrogen atoms. Reproduced with permission from

(a) Eggen, B. R.; Heggie, M. I.; Jungnickel, G.; Latham, C. D.; Jones, R.; Briddon, P. R. Science 1996,

494 6 Nanomaterials

You may be wondering “how does the ion get inside the cage?” That is, does this

occur during the growth of the fullerene structure itself, or does the metal ion insert

after the cage is already formed? It has been shown that the latter likely occurs, with

the exact entrance pathway dependent on the size of the dopant species. Small

dopants such as He or H

may directly pass through either hexagon or pentagon

units of the cage toward the vacant core. However, for larger ions/atoms, some

framework C—C bonds must be reversibly broken in order to accommodate the

incoming species – aptly referred to as a window mechanism (Figure 6.34). Since

non-IPR fullerenes have relatively large strain ener gies due to fused pentagons, this

process should occur readily for these struct ures. Indeed, there has been much recent

interest in synthesizing “unconventional” metallofullerenes such as Sc

[109]

Figure 6.31. Proposed scheme for the formation of the multishell fullerene C

240

. Reproduced from

Mordkovich, V. Z.; Shiratori, Y.; Hiraoka, H.; Takeuchi, Y. Synthesis of Multishell Fullerenes by Laser

Vaporization of Composite Carbon Targets, found online at http://www.ioffe.rssi.ru/journals/ftt/2002/04/

p581–584.pdf.

Figure 6.32. Backbone structure of semibuckminsterfullerene, C

6.3. Nanoscale Building Blocks and Applications 495

Unlike the empty C

counterpart, these structures are stable since the incoming

metal atom donates electron density to the C—C bond between adjacent pentagons,

causing a decrease in the local bond strain.

As you might imagine, relatively large ions such as Cs

,orSc

are likely not

encapsulated through a simple reversible windowing mechanism. In order for this to

occur, more than one C—C bond would need to be broken (Figure 6.34b), which

increases the energetic barrier for this to occur. More recently, a “hole-repairing

mechanism” was proposed for Y@C

metallofullerenes, in which calculations

predict the combina tion of a large C

open-cage fullerene and a smaller C

fragment that effective ly repairs the framework hole (Figure 6.35).

[110]

Vapor-phase syntheses of 0-D nanostructures

In addition to fullerenes, a variety of metallic and compound (oxides, nitrides,

chalcogenides) nanoclusters/nanoparticles may be synthesized using vapor techni-

ques. The primary beneﬁts of this approach relative to solution-phase methods

Figure 6.33. Synthesis of a C

precursor. Reproduced with permission from Scott, L. T.; Boorum, M. M.;

McMahon, B. J.; Hagen, S.; Mack, J.; Blank, J.; Wegner, H.; de Meijere, A. Science 2002, 295, 1500.

496 6 Nanomaterials

discussed in the next section is a much higher rate of production and a greater purity/

compositional control. That is, gas-phase generated nanoparticles will be free of

impurities acquired from solvents and added reagents such as surfactants, reducing

agents, etc. Further, complex compositions and (meta)stable phases (e.g., high-

temperature supercondu ctors, intermetallics, etc.) are often more easily synthesized

using gas-phase decomposition techniques rather than low-temperature solvent

processing.

[111]

Figure 6.35. Images of the calculated structures involved in the “hole-repairing” mechanism for

endohedral fullerene growth. Shown from left to right are: the C

open cage, top and side views of the

Y fragment, and the ﬁnal Y@C

metallofullerene. Reproduced with permission from Gan, L.-H.;

Figure 6.34. Theoretical intermediates during endohedral fullerene formation. Image (a) shows the nine-

membered ring formed by a single pentagon–hexagon bond. By comparison, image (b) illustrates the

formation of a larger 13-membered ring by breaking two framework bonds. Reproduced with permission

6.3. Nanoscale Building Blocks and Applications 497