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Carbon Nanotubes: Structure and Properties 35

Chemistry of Carbon Nanotubes

Eduard G. Rakov

D.I. Mendeleev University of Chemical Technology,

Moscow, Russia

CONTENTS

Abstract ............................................................................................................................................38

2.1 Introduction.............................................................................................................................38

2.2 Carbon Nanotube Morphology and Structure........................................................................39

2.3 Synthesis of Carbon Nanotubes .............................................................................................40

2.4 Opening of Carbon Nanotubes ...............................................................................................41

2.5 Functionalization of Carbon Nanotubes.................................................................................42

2.5.1 Attachment of Oxidic Groups ....................................................................................43

2.5.2 Reactions of Carboxylic Groups Attached to Nanotubes...........................................43

2.5.3 Fluorination.................................................................................................................47

2.5.4 Amidation ...................................................................................................................48

2.5.5 Other Types of Covalent Bonding ..............................................................................50

2.5.6 Noncovalent Bonding .................................................................................................53

2.5.7 Dispersions in Oleum .................................................................................................56

2.5.8 Self-Assembly, Film, and Fiber Formation ................................................................56

2.6 Filling the Inner Cavity of Carbon Nanotubes .......................................................................59

2.6.1 In Situ Filling..............................................................................................................60

2.6.2 Post-Processing Filling ...............................................................................................61

2.6.2.1 Filling from Liquid Media ..........................................................................61

2.6.2.2 Filling from Gas Phase................................................................................63

2.6.3 Reactions inside Nanotube .........................................................................................64

2.6.4 The Structure of Crystals inside Nanotubes...............................................................65

2.7 Adsorption and Storage of Gases...........................................................................................66

2.7.1 Hydrogen Problem......................................................................................................67

2.7.2 Carbon Nanotube Gas Sensors...................................................................................69

2.8 Attachment of Biomolecules ..................................................................................................70

2.8.1 Biosensors...................................................................................................................70

2.8.2 Others Fields of Application.......................................................................................72

2.9 Nanotubes as Templates..........................................................................................................72

2.9.1 Substitution of the Carbon Atoms of Nanotubes .......................................................72

2.9.2 Decoration of Carbon Nanotubes...............................................................................73

2.10 Intercalation of “Guest” Moieties...........................................................................................75

2.11 Summary and Conclusions.....................................................................................................77

Acknowledgments ............................................................................................................................77

References ........................................................................................................................................77

ABSTRACT

The main trends and recent achievements in carbon nanotube chemistry are reviewed. Apart from

“traditional” subjects such as opening, filling, and decoration of nanotubes, some new subjects have

also been discussed. A special emphasis has been placed on the functionalization and solubilization

of carbon nanotubes, their self-assembly, film and fiber formation, and sensor and biosensor prepa-

ration. Some basics on carbon nanotubes are introduced.

2.1 INTRODUCTION

Owing to their electronic, mechanical, optical, and chemical characteristics, carbon nanotubes

(NTs) attract a good deal of attention from physicists, chemists, biologists, and scientists from sev-

eral other fields.

1–5

Possible applications in the fields of molecular electronics, nanomechanic

devices, information display, sensoring, energy storage, and composite materials are of interest for

industry.

The perspectives of NT application are greatly dependent on NT chemistry. Chemical behavior

of NTs is very diverse and the processes of NT synthesis, purification, modification, and solubi-

lization

7,8

all contribute to this diversity.

There are different ways to modify NTs:

●

Partial oxidation and decapping (opening) of NTs

●

Attachment of functional groups to the open ends of NTs

●

Attachment of functional groups to the sidewalls of NTs

●

Chemical reactions of functionalities attached to NTs

●

Filling of inner cavities of the NTs with different substances (gaseous, liquid, or solid)

and carrying out a chemical reaction inside the NTs

●

Replacement of the carbon atoms of NTs by atoms of other chemical elements or groups

●

Intercalation (insertion) of “guest” atoms or molecules into the intertubular space of

single-wall NTs (SWNTs) bundles or in between the walls of multiwall NTs (MWNTs)

●

Decoration of outer walls of NTs and using NTs as templates

●

Adsorption of gases

NTs having very large molecular weights cannot form true solutions. Therefore, solubilization

of NT means the formation of their colloid solutions (dispersions). The solubilization can be sub-

divided into the formation of aqueous, organic, or polymeric dispersions.

The geometry and size of NTs allows them to take part in self-assembly and aligning processes.

The directional deposition on certain surfaces, structuring by Langmuir–Blodgett films and by liq-

uid crystals, and formation of colloidal systems are mentioned here.

In its chemical behavior, NTs partially resemble graphite and fullerenes; however, there are also

noticeable differences from both graphite and fullerenes.

8,9

Graphite represents a typical layered

polymeric crystal and each fullerene can be considered as a molecule which can form molecular

crystals (fullerites). However, a great many NTs cannot be classified as either usual molecules or

crystals. SWNTs are similar to polymeric molecules of simple substances and MWNTs resemble

structured nanoparticles. The individual NT can be also assumed as 1-D crystal and well-ordered

NT bundles as 2-D crystals.

Graphite has planar structure corresponding to sp

for

bonds and p for

bond. Fullerenes and

NTs have hybrid bonding orbitals between sp

and sp

. NTs having low percentage of sp

bonds are

in this regard nearer to graphite. All these bonds in straight NTs are concentrated in half-spherical

or conical caps, and in curved NT they are concentrated in the bends. The difference in chemical

activity between NT caps and sidewalls as well as between straight and curved NTs is determined

by the sp

:sp

bonds ratio.

38 Nanotubes and Nanofibers

Specific chemical properties of graphite, fullerenes, and NTs as well as of fullerenes or NTs of

different diameters are also different owing to dissimilar surface curvatures.

The chemistry of new carbon allotropes has been described with a pyramidalization angle

formalism.

For graphite,

⫽ 0°. All carbon atoms in C

have

⫽ 11.6°. Pyramidalization

angle of SWNTs (n, n) of various n (n ⫽ 2 to 10) is calculated to vary between 14–17° and 2°.

Pyramidalization changes the hybridization of atomic orbitals at the C atom so that the

orbital

contains different portions of s and p orbitals, leading to different chemical reactivity. Because of

this hybridization, fullerenes and NTs are known to be more reactive than graphite. The deforma-

tion energy of sp

bond is inversely proportional to the diameter of NT, and therefore tubes with

smaller diameter have greater reactivity. The enthalpy of the reaction decreases with increase in the

diameter. The enthalpy of homolytic reactions of H atoms and methyl radicals with atoms of curved

carbon plane is linearly dependent on the pyramidalization angle and can differ by 1 eV.

The cups of NT, which are often similar to fullerene molecule halves, contain more reactive

C atoms, than the sidewalls. Experimental evidence of the higher chemical reactivity of conforma-

tionally strained carbon sites in MWNTs has been reported.

NTs exhibit more differences owing to various structure and morphology (kinked, branched,

conical NTs, etc.).

The NTs differ from fullerenes by the larger size of internal cavity, and from

graphite by the greater share of accessible surface sites.

Fullerites, graphite, and NTs contain van der Waals gaps, which on being filled can form inter-

calates, or “guest–host” compounds. The dimensionality of such compounds is different: 0-D for

fullerites, 2-D for graphite, and 1-D for NTs. NTs can form “guest–host” compounds of different

types: for instance, the “guests” can exist in the inner cavity of SWNT or MWNT, in the intertube

space of the bundled NTs, or in between the walls of MWNTs having scroll structure.

The development of the chemistry of NTs will determine the fields of their practical applica-

tion. Primarily, NT chemistry is involved with the rational routes of NT purification and sorting.

Chemical modification of NTs can open the way to modify the properties of these materials.

Unlike physical investigation of the solid-state properties of NTs, the study of their chemical

reactivity is still in its infancy. At the same time, there have been very impressive achievements in

the study of the chemistry of NTs in recent years. There are a large number of original publications

and some reviews on the subject.

8,10,16–21

The present review pertains mainly to works in the last five

years.

2.2 CARBON NANOTUBE MORPHOLOGY AND STRUCTURE

Dozens of different morphological varieties of filamentous carbon nanoparticles were revealed in a

remarkably short period of time after the discovery of “classical” NTs. As was mentioned by

Hilding et al.,

NTs can differ by “aspect ratio, NT diameters, surface structures, defect densities,

and physical entanglements.”

The NT of the “classical” type represents a cylindrical particle formed from graphene — a flat

carbon net with the atoms located in the corners of joined hexagons. They may be single-walled

with a diameter between ~0.3 and ~5.0 nm (typical SWNTs have diameters of 1.0 to 1.4 nm, and

length up to 50 to 100 µm) or multi-walled. SWNTs as a rule contain less topological defects and

possess better mechanical and electrophysical properties. The specific surface area of SWNTs is

independent of their diameter and is equal to 1315 m

/g (for outer surface).

MWNTs consist of several (from two to tens) coaxial tubes, with outer diameter of ~1.4 to

100 nm. Their specific surface area depends on the number of walls, and to a lesser extent on the

diameter of the inner tube. The theoretical surface area of double-wall NTs is between 700 and

800 m

/g, and of ten-wall NTs is about 200 m

/g. Nanofibers (NFs) have a diameter from tens to

hundreds of nanometers.

Some MWNTs and NFs have internal cross connections formed by curved graphenes. A mod-

erate relative number (density) of cross connections correspond to a “bamboo-like” structure of

Chemistry of Carbon Nanotubes 39

MWNT, while a high relative number of connections may be attributed to conical structures such

as “herringbone-like” or “cup-stacked” types. The angle of conicity in the conical structures varies

from 15 to 85°. Grafene layers in NFs may also be located perpendicular to the fiber axis.

Nanofibers having regularly changing diameter (“nanobeads”) have been synthesized.

And finally,

there are amorphous NTs or NFs.

SWNTs, MWNTs, and NFs can spontaneously form “secondary” structures. A classification of

carbon nanotubular particles into primary, secondary, and tertiary forms was first introduced by

De Jong and Geus.

The most popular secondary form of the SWNT is a bundle (“rope”), which

consists of tubes in a 2-D triangular lattice, with a lattice constant of 1.7 nm. The bundles usually

have greater lengths as compared to individual NTs. For example, 1-m-long bundles of NTs have

been produced. They can combine to form a structure of higher order. Tangled SWNTs or MWNTs

can form “bucky paper” (“nanopaper” and “nanomats”).

Single-walled nanohorns (nanocones) quite often combine to produce flower- or bud-like structures.

“Secondary” structures formed by MWNTs are more diverse. The formation of bundles of

MWNT is less typical. More often, kinked (L-shaped) and branched (Y-, T-, H-like, “octopus,” tree-

like) forms were synthesized. Tangled MWNTs form aggregates up to 3 mm in diameter (“worms”

and “boiled spaghetti”). The aggregates along with conical “ceder forest,” and “bucky pearl” some

others are the members of “tertiary” structure.

A pyrolytic synthesis sometimes yields coiled NTs, helixes, double helixes, helixes inside NTs,

and even more complex structures, such as brushes, entangled worm-like features, and foil-like

structures at the nanometer scale. Transitional forms, such as nanopeas (fullerenes inside SWNTs),

are also among the structures formed.

The chemistry of different structures must be notably diverse. This diversity has not been stud-

ied yet, and the overwhelming majority of published results pertain to SWNTs, SWNT bundles, and

MWNTs.

2.3 SYNTHESIS OF CARBON NANOTUBES

There are two main methods or groups of processes of NT synthesis: sublimation of graphite with

subsequent desublimation, and decomposition of carbon-containing compounds.

5,7,24

The first

group of processes is associated with high temperatures (up to 4000°C), which can be obtained in

electric arcs (see, for example, Refs. 25–29), by the process of laser ablation,

29–34

by focused solar

radiation,

34,35

or by resistive heating of graphite.

The arc process is remarkable for the larger number of versions, among which some versions

allow the realization of a semicontinuous process in automated facilities. Arching process in liquid

nitrogen,

36,37

in water,

and in aqueous solutions

38,39

have also been developed.

Arching processes in gaseous

or liquid

41,42

hydrocarbons are classified as combined methods,

where the pyrolysis of hydrocarbons takes place along with the sublimation–desublimation of

graphite.

The arc-discharge technique is a popular method. The main disadvantages of the method lies in

the difficulty of organizing a continuous process, the concurrent formation of amorphous carbon,

metal clusters coated with carbon and in some cases fullerenes along with NTs. The total yield of

SWNT, as a rule, does not exceed 20 to 40%.

The second group of methods has its own variations: pyrolysis of gases (chemical vapor depo-

sition, CVD process), solids (e.g., pyrolysis of polymers), aqueous solutions (hydrothermal synthe-

sis),

43,44

or organic solutions (supercritical toluene).

The CVD method can produce NTs in large quantities and can be realized at temperatures of

500 to 1300°C. In accordance with the chemical composition of the carbon source, the method can

be subdivided on the disproportionation of CO,

29,33,46

the pyrolysis of hydrocarbons (CH

, C

, etc., including polymers),

23,47–49

pyrolysis of CH

compounds (for example, alcohols

and pyrolysis of heteroatomic CH

compounds (A, B ⫽ N, O, S, Cl, …, e.g., amines).

40 Nanotubes and Nanofibers

It is also possible to synthesize NTs via a template method, applying, for example, porous

anodic alumina membrane. CVD using hydrocarbons as NT precursors over patterned catalyst

arrays leads to the formation of different complex structures and opens up exciting opportunities in

nanotechnology.

The thermal disproportionation of CO is realized in two main variants: HiPco

2,5,46

and

CoMoCAT.

5,51

These methods are considered by American researchers as the most promising for

the commercial production of SWNTs.

High-value NTs, viz. SWNTs, are produced at temperatures of 900 to 1200°C, mainly using

CO or CH

as a precursor. The availability of precursors, such as C

, methanol, and ethanol,

for SWNT synthesis is also shown. SWNTs in all instances are produced only by catalytic

reactions.

Methods other than the ones mentioned above, for example, thermal decomposition of metal

carbides

7,52,53

or carbonitrides,

chlorination of carbides,

electrolysis of molten salts,

interaction

of cesium metal with microporous carbon,

defoliation of graphite by forming and subsequent

transformation of “guest-host” intercalated compound, transformation of fullerenes C

and C

“rapid thermal processing” of amorphous carbon film containing iron,

sonochemical produc-

tion,

AC plasma processing,

etc., are not widely used.

2.4 OPENING OF CARBON NANOTUBES

Independent of their synthesis method, pristine SWNTs have large aspect ratio (10

to 10

) and

closed ends. Open-ended NTs offer unique possibilities as conduits for flow of low-surface-tension

fluids through their cylindrical pores. Applications of NTs in molecule separation devices, in bio-

catalysis, in molecule detection, and as encapsulation media have been proposed. The end cups on

NTs must be destroyed to make the inner cavity available for filling. The simplest method to open

NTs is their oxidative treatment. The oxidation tends to initiate on the end cups, thus providing a

mechanism for opening the NT. The treatment is accompanied by functionalization of NTs with

oxygen-containing groups (see below).

Liquid or gaseous chemical agents are used as oxidants. Refluxing, sonication, or microwave

(MW) digestion in concentrated acids, such as HNO

, H

and their mixtures, are the most pop-

ular methods of oxidation. Refluxing of MWNTs in concentrated HNO

leads not only to oxidative

opening, but also in the reduction of the NT length and diameter as well as the breaking of entan-

gled MWNTs.

Treatment of SWNTs results in tube cutting and narrowing of diameter distribu-

tion.

60,61

The bundles of SWNTs become disordered and partly exfoliated, when they are immersed

in 70% HNO

for a long period of time.

Nitric acid can be intercalated into SWNT bundles, in

that way disintegrate the tube walls into graphitic flakes and then reforms them into various multi-

shell phases such as MWNTs, cone-shaped phases, and onion-like phases.

A prolonged treatment

of SWNTs leads to amorphization and complete destruction of the tubes.

Bundles in raw SWNT material are significantly thinner (<10 NTs) than those found in soni-

cated and HNO

-oxidized samples (>30 NTs).

64,65

It has been suggested that this thickening is pro-

moted by the H-bonding between –COOH side groups formed during oxidation.

The concentrated H

/HNO

mixture (3:1) is a better agent for cutting SWNTs

60,66

and for

enrichment of large-diameter SWNTs.

67,68

Oxidation of SWNTs with H

is a common tube-opening procedure.

69–71

SWNTs react

exothermically with the H

mixture.

Oxidation in solution (HNO

) has

been found to be effective for opening SWNTs. Aqueous solutions of OsO

, OsO

/NaIO

RuO

KMnO

, H

/KMnO

, H

/(NH

)

have also been successively used to open NTs.

Superacids such as HF–BF

, have been shown to act as an etchant of NTs at room temperature.

The opening of MWNTs can be realized at room temperature electrochemically in H

or in dilute aqueous KCl solutions.

Supercritical water acts on MWNT as an opening and thin-

ning agent.

Chemistry of Carbon Nanotubes 41