Fahlman B.D. Materials Chemistry

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Table 6.6. Speciﬁc Tensile Strengths of Various Materials

187

Material Tensile

strength (MPa)

Density

(kg dm

3

)

Speciﬁc strength

(kN m kg

1

)

Breaking

length

(km)

Concrete 10 2.30 4.34 0.44

Rubber 15 0.92 16.3 1.66

Brass 580 8.55 67.8 6.91

Nylon 75 1.15 97.3 9.92

Polypropylene 80 0.90 88.9 9.06

Aluminum 600 2.70 222 22.7

Steel 2,000 7.86 254 25.9

Titanium 1,300 4.51 288 29.4

Silicon carbide 3,440 3.16 1,088 110

Glass ﬁber 3,400 2.60 1,307 133

Graphite 4,300 1.75 2,457 250

Kevlar

3,620 1.44 5,246 534

CNTs 62,000 1.34 46,268 4,716

The strength of a material divided by its density.

The length beyond which a strip of the material, of uniform width, would break under its own weight if

suspended from both ends.

A synthetic ﬁber used in bullet-proof vests, comprised of poly(paraphenylene terephthalamide).

Figure 6.68. Illustration of the deformation modes of SWNTs, resulting in a high elasticity. This behavior

is likely an artifact of in-plane ﬂexibility of a graphene sheet, and facile rehybridization of carbon atoms

from sp

to sp

geometries. Reproduced with permission from Ajayan, P. M. Chem. Rev. 1999, 99, 1787.

538 6 Nanomaterials

traditional composite materials used for applications that require both high-strength

and lightweight materials. There are currently plenty of examples of carbon ﬁber

composites – aircraft and spacecraft parts, racing car bodies, golf club shafts, bicycle

frames, ﬁshing rods, automobile springs, sailboat masts, and many others – all

incorporating bulk carbon ﬁbers, ca. 5–10 mm in diameter. Though this technology

has been around sinc e the 1950s, the much larger aspect ratios of CNTs translates to

an even greater impact on the future of materials reinforcement.

In order to broaden the scope of CNTs for textile applications, it is most desirable

to convert the as-formed powders into useful ﬁbers and yarns. An intriguing recent

precedent consists of shrinking the yarn-spinning process, used by the earliest

civilizations, to the nanoregime. In this process, MWNTs from a bamboo-like forest

array were drawn into ﬁbers, and weaved into yarns, that were both strong and

highly ﬂexible (Figure 6.70). One possible application for these advanced nanotex-

tiles is the design of a “supersuit” for the next generation of soldier. This uniform

will feature a number of functionalities that will react appro priately to its surround-

ings (e.g., deactivation of gaseous warfa re agents, “kicking in” artiﬁcial muscles,

climate control, etc. – Figure 6.70, bottom).

Figure 6.69. High-resolution TEM image of a bundle/rope of single-wall carbon nanotubes, formed

spontaneously through the self-assembly of individual SWNTs (scale bar is 10 nm). Reproduced with

permission from Thess, A.; Lee, R.; Nikolaev, P.; Dai, H.; Petit, P.; Robert, J.; Xu, C.; Lee, Y. H.; Kim, S. G.;

Rinzler, A. G.; Colbert, D. T.; Scuseria, G. E.; Tomanek, D.; Fischer, J. E.; Smalley, R. E. Science 1996,

6.3. Nanoscale Building Blocks and Applications 539

200 μm

50 μm

20 μm

50 μm

20 μm

2 μm

Advanced display

Night vision, 360°

field of view, and

other imaging

technology would

project video data

on a visor display.

Breathing air

Respiratory protection and

climate control would be

built into the suit.

Chem and bio protection

Smart fibers would fend off

chemical and biological agents,

self-decontaminate, and become

waterproof.

Multifunctional fabric

Flexible most of the time, the battle suit

fabric would stiffen automatically

in response to ballistic danger or

on demand in a medical

emergency.

Biosensors

Vital signs would

be measured and

relayed to medics

automatically.

Artificial muscles

Connected to an

exoskeleton, these

would provide

extra strength.

Figure 6.70. SEM images of spinning MWNT arrays into ﬁbers, and subsequent images of MWNT yarns.

Shown below is an image of a futuristic uniform to equip the next generation of warﬁghter. The SEM

AAAS. The image of the futuristic soldier was furnished by the Army Natick Soldier Center (http://nsc.

natick.army.mil) – refer to the Aug. 11, 2003 issue of Chemical and Engineering News for more details.

540 6 Nanomaterials

In addition to increasing strength and stiffness, the incorporation of CNTs also

imparts conductivity to the polymer matrix. The addition of CNTs also enhances the

thermal conductivity/stability, solvent resistance, and glass transition temperature of

the native polymer. Further, due to the high aspect ratio of CNTs, much lower

dopant levels are required to yield the desired properties, relative to standard

additives such as carbon black and larger graphitic ﬁbers. There are an endless

number of potential applications for CNT composites – imagine future ﬂat-panel

displays that are ﬂexible, exhibiting the consistency of common fabrics; or, shirts

that are capable of monitoring the external temperature and automatically heating /

cooling. The automotive industry currently uses CNT composites for a number of

applications, such as increasing the strength of side mirr ors. In addition, CNT-doped

nylon is being used for fuel lines to reduce the buildup of static electrical charges,

and prevent fuel line rupturing during an accident.

It should be noted that reinforcement applications are not limited to organic

polymers. Th ough ceramics are already hard and chemically/thermally resistant,

these materials may also be doped with CNTs to im prove their inherent brittleness.

This has already been proven for the incorporation of 5–10% of CNTs within an

alumina matrix. The resultant material exhibits ﬁve times the fracture toughness and

seven times greater electrical conductivity relative to undoped Al

, with an added

property of unidirectional heat conductivity.

[187]

In order to continue the development of intriguing CNT-based composites, three

main challenges are being addressed:

(i) Ensuring a homogeneous dispersion of nanotubes throughout the polymer

matrix

(ii) Controlling the direction/orientation of the CNTs in the matrix

(iii) Separating individual SWNTs from bundles/ropes (exfoliation)

In order for efﬁcient load delocalization and strength enhancement of a composite,

there must be a strong interaction between the dopant ﬁbers and polymer matrix.

However, since CNTs are allotropes of carbon, they are inherently insoluble in any

solvent. Hence, there are a number of strategies that have been used to modify the

surface of CNTs to facilitate their interactions with the surrounding matrix. Not

only are these methods essential for the dispersion and self-alignment of nanotubes

throughout a polymer host (or on a surface for molecular devices), but also for the

puriﬁcation of CNTs (e.g., separation of metallic and semiconducting CNTs through

selective functionalization, followed by traditional HPLC).

There are two types of surface modiﬁcations for CNTs: noncovalent interactions

(Figure 6.71) and covalent sidewall (Figure 6.72)

[188]

or defect-site

[189]

(Figure 6.73)

functionalization.

[190]

Both methods, as well as physical techniques such as sonica-

tion, are successful in separating individual SWNTs from bundles – a process known

as exfoliation. In general, it is most desirable to incorporate isolated SWNTs in a

composite rather than bundles, since the latter features poor intertube interactions

resulting in a lower overall strength – especially at low CNT concentrations.

As illustrated in Figure 6.71, noncovalent modiﬁcations consist of either

wrapping the CNT with polymers or biological macromolecules,

[191]

or placement

6.3. Nanoscale Building Blocks and Applications 541

of conjugated macromolecules on the surface through p-stacking interactions.

As such, these types of interactions have the advantage of not altering the electronic

properties of the CNT (unlike covalent modiﬁcations). However, since covalent

functionalization places speciﬁc chemical groups onto the CNT surface, this route

offers a greater potential for the selective tunability of CNT properties as well as

molecular control over the organization of CNTs for device fabrication.

The driving force behind the endcap and sidewall functionalization of SWNTs is

the strain brought about from the high degree of surface curvature. The deviation of

the carbon geometry from planarity is referred to as the pyramidalization angle, y

which is deﬁned as 0



for the sp

carbon atoms in C

, and 19.5



(i.e., 109.5



–90



)

for the sp

carbon in CH

(Figure 6.74). Since the y

of the carbon atoms in C

11.6



, the desired geometry is closer to tetrahedral rather than trigonal planar. As a

result, fullerenes readily undergo addition reactions to relieve this high degree of

surface strain. Since the endcap of a SWNT may be considered as a half-fullerene

Figure 6.71. Examples of noncovalent interactions with a macromolecule solubilizing agent and CNTs.

Shown is (a) the solubilization of SWNTs with PolyT30 single-strand DNA, followed by the displacement

of the biomolecule through preferred p–p stacking between the conjugated dye methylene blue and the

SWNT surface – causing the controlled precipitation of SWNTs from solution. Also shown, (b), is another

method for solubilization/precipitation of SWNTs through use of complimentary single-strand DNA.

American Chemical Society.

542 6 Nanomaterials

Figure 6.72. Schematic of the various methods to perform sidewall functionalization of SWNTs.

Reproduced with permission from Banerjee, S.; Hemraj-Benny, T.; Wong, S. S. Adv. Mater. 2005,

6.3. Nanoscale Building Blocks and Applications 543

structure, with a y

 9.7



regardless of the nanotube diameter,

[192]

these points are

highly reactive toward electrophilic addition.

In contrast, the sidewall pyramidalization angle is much less than the endcap, on

the order of 3–6



, depend ing on the diameter/chirality of the SWNT. For an

armchair (5,5) SWNT, two types of C—C bonds are present along the sidewalls –

either parallel or perpendicular to the nanotube axis. Each of these bonds exhibits a

different degree of p orbital overlap with adjacent carbon atoms, as quantiﬁed by the

p orbital misalignment angle, f (Figure 6.74). Wher eas f ¼ 0



for fullerenes,

the misalignment of neighboring p orbitals in SWNTs may be signiﬁcant – as

large as 21.4



for (5,5) species. In general, a SWNT of smaller diameter has a

greater sidewall reactivity than larger analogues due to relatively large p orbital

misalignment angles (much more inﬂuential than y

). It should also be noted that the

overall curvature of CNTs is less than fullerenes, which make SWNTs signiﬁcantly

less reactive than fullerenes.

HOOC

COOH

Pristine

SWNTs

Oxidized

SWNTs

1) HNO

2) H

3) KMnO

4) K

5) O

−

-R

ROH

pyridine

R=(CH

)

Metal

Colloid

SOCI

EDC / DCC

N-R

Figure 6.73. Illustration of possible defect sites on the surface of a SWNT, along with methods used for

covalent functionalization of SWNTs at defect sites. The types of defects shown are (A) 5- or 7-membered

rings in the framework, which results in bending, (B) sp

-hybridized defects (R ¼ —OH, —H),

SWNT shown with – COOH capping groups (may also be terminated with —OH, ═O, —NO

, and —H

groups). Reproduced with permission from Hirsch, A. Angew. Chem. Int. Ed. 2002, 41, 1853

Wiley-VCH.

544 6 Nanomaterials

Synthesis of carbon nanotubes

Now that you are familiar with the properties and applications of CNTs, we must

now consider the techniques used for their synthesis. In addition to the experimental

details of CNT growth, this section will also provide mechanistic details on how

these interesting nanostructures form. Fortunately, recent studies have shown that

the growth mechanism of CNTs is the same for other 1-D nanostructures such as

nanowires.

There are three primary methods that are used to grow CNTs: laser evaporation of

graphitic targets, arc discharge methods, and chemical vapor deposition (CVD). You

might recognize the ﬁrst two methods as also being used for to generat e fullerenes;

not surprising, sinc e CNTs are esse ntially 1-D extensions of fullerene clusters. Since

both of thes e methods involve the sublimation of graphite followed by condensation

of carbon atoms, growth occurs at an extremely high temperature – close to the m.p.

of graphite (3,000–4,000



C). To synthesize SWNTs, a catalytic amount of a nano-

sized species (e.g., metals: Co, Ni, Fe, Y, Mo, Au, Ag, Cu, Pd, Rh, Mg, Mn, Cr,

Sn, Al; semiconductors: Si, Ge; carbides: SiC, Fe

C; oxides: SiO

,Al

, TiO

lanthanide oxides)

[193]

must be pres ent to prevent the leading edge from closing,

which would generate spheroidal fullerenes. For MWNT growth, a catalyst is not

Figure 6.74. Schematic of (a) a metallic (5,5) SWNT, (b) the pyramidalization angles of typical sp

and

carbon atoms, (c) the p orbital misalignment angles along C1–C4 in the (5,5) SWNT and its capping

fullerene, C

. Reproduced with permission from Niyogi, S.; Hamon, M. A.; Hu, H.; Zhao, B.; Bhowmik,

Chemical Society.

6.3. Nanoscale Building Blocks and Applications 545

required; however, the products will contain a large number of other carbonaceous

products such as fullerenes and amorphous carbon.

[194]

Whereas arc-based methods generally contain a mixture of products, with

SWNTs/MWNTs of varying diameters/morphologies, amorphous soot, etc., a

dual-pulsed laser approach, ﬁrst used in the mid-1990s, results in SWNT ropes

with >70% purity.

[195]

Though both of these methods may be used to generate small

quantities of CNTs, they are relatively expensive and not easily scalable to generate

industrial quantities. Further, the CNTs arising from vaporization methods are

typically in a tangled array, with other forms of carbon intermixed with remaining

catalytic metal (esp. affecting subsequent electronic applications).

Consequently, CVD is now the method-of-choice for the synthesis of CNTs.

As discussed in Chapter 4, this method consists of the decomposition (typically

thermal) of a hydrocarbon precursor on the surface of catalytic metal nanostructures.

Methane and acetylene have been used most extensively as precursors; other alter-

natives now include CO, C

, and methanol/ethanol. As with any CVD approach,

this method is easily scalable, and is used to generate kilogram quant ities of CNTs

for an ever-increasing laundry list of applications.

Most importantly, CVD is most amenable for the facile growth of vertically-

aligned CNTs known as “forests” from surface-immobilized catalyst nanoclusters.

This strategy, pioneered by Dai and coworkers at Stanford, has been used to grow

ordered arrays of both MWNTs and SWNTs using a variety of experimental

modiﬁcations.

[196]

The vertical alignment of the CNTs takes place through strong

intratube/intertube van der Waals interactions (e.g., MWNT arrays, Figure 6.75a),

tube-patterned substrate interactions (e.g., SWNT arrays, Figure 6.75b), or induced

by electric ﬁelds (e.g., SWNT arrays, Figure 6.75c). These strategies set an impor-

tant precedent of growing nanostructures along speciﬁc growth directions from

speciﬁc sites – essential for the fabrication of future integrated circuits and other

advanced electronic devices. Vertically-aligned or suitably-patterned 1-D nanos-

tructures (not limited to CNTs only) are also of use for the design of superhydro-

phobic coatings, which are able to levitate water droplets above a surface to preclude

wetting (Figure 6.76).

[197]

Beyond useful societal applications, nanotube forests may

also be grown in the shape of popular media icons or even Presidential ‘NanOba-

mas’ (Figure 6.77) – a very cool artistic application for nanomaterials, indeed!

Carbon nanotube growth emanates from catalyst nanoparticles that have been

deposited onto a substrate surface (Figure 6.78), with resultant tube diameters

related to the size of the seed catalyst. Interestingly, whe reas a number of carbon

sources such as methane and various alcohols may be used for SWNT growth from

catalyst powders, acetylene (C

) is a key precursor for vertically-aligned SWNT

growth.

[198]

Often, a metallic underlayer (e.g., Al/Al

, Cr, Ir, W, Ta, Ti) is used as

a diffusion barrier in order to facilitate and control the growth of SWNTs on

substrates such as Si, SiO

, or metals (e.g., Au, Ag, W, steel alloys).

[199]

The

morphology of this underlayer plays a crucial role in the overall structure (i.e.,

random 2-D vs. 3-D vertically-aligned arrays) of the resultant SWNTs.

196

546 6 Nanomaterials

There are also reports of the one-step synthesis of aligned MWNT bundles and

Y-junction CNTs using a controlled mixture of metallocene-based precursors

(M(C

)

,M¼ Fe, Co, Ni) – precluding the use of surface-immobilized catalyst

nanoclusters.

[200]

Since the precursor acts as both the carbon source and catalyst,

Figure 6.75. Examples of the ordered growth of carbon nanotubes. Shown are (a) MWNT arrays grown

from squared regions of iron nanoclusters; (b) side-view SEM image of a “SWNT power line on Si posts”;

6.3. Nanoscale Building Blocks and Applications 547