Nanotubes Review. Tiene El Intro y Los Metodos

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ISSN:1369 7021 Elsevier Ltd 2007JAN-FEB 2007 | VOLUME 10 | NUMBER 1-228

Carbon nanotubes becoming clean

Carbon nanotubes (CNTs) are now well into their teenage years. Early

on, theoretical predictions and experimental data showed that CNTs

possess chemical and mechanical properties that exceed those of manyother materials. This has triggered intense research into CNTs. A variety

of production methods for CNTs have been developed; chemical

modification, functionalization, filling, and doping have been achieved; and

manipulation, separation, and characterization of individual CNTs is now

possible. Today, products containing CNTs range from tennis rackets

and golf clubs to vehicle fenders, X-ray tubes, and Li ion batteries.

Breakthroughs for CNT-based technologies are anticipated in the areas

of nanoelectronics, biotechnology, and materials science. In this article,

I review the current situation in CNT production and highlight the

importance of clean CNT material for the success of future applications.

Nicole Grobert

Department of Materials, University of Oxford, Parks Road, Oxford, OX1 3PH, UK

E-mail: [email protected]

Carbon fibers and filaments have been studied for over 100 years.

Hughes and Chambers1, and Schtzenberger and Schtzenberger2

reported the growth of filamentous carbon in 1889 and 1890,

respectively. In the early 1950s, Radushkevich and Lukyanovich3

published a report on hollow carbon fibers (Fig. 1). Since then,

the demand by the space and aerospace industry for stronger,

lightweight materials with improved mechanical properties has led

to substantial progress in the production and characterization of

carbon filaments and hollow carbon fibers.

History of carbon nanotubesEarly on, it was realized that generating highly crystalline carbon fibers

resistant to crack propagation would require further development of

growth methods such as chemical vapor deposition. In the 1970s, Endo

et al.4 showed a viewgraph of what is now called a single- or double-

walled carbon nanotube (SWNT or DWNT) (Fig. 2). At that time,

although electron microscopes were powerful enough to study the

structure of the carbon filaments in detail, the images did not reveal

the number of walls clearly. For example, it cannot be determined
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JAN-FEB 2007 | VOLUME 10 | NUMBER 1-2

whether the nanotube inFig. 2 consists of one or two concentric

graphene cylinders (Fig. 3)4,5. The carbon fiber community then was

familiar with filament-like structures and did not pay further attention

to smaller-scale objects like the CNTs observed by Endo et al.4. Instead,

research focused on generating carbon fibers from polymer-based

precursors using rayon, polyacrylonitrile (PAN), or mesophase pitch.

Graphitic nanofibers were also encountered by the catalysis

community and steel industry. Here, carbon nanofibers were, and still

are, considered unwanted byproducts causing poisoning6 of metal

catalyst particles or damage to reactor walls in furnaces7. This process

is also known as metal dusting or corrosion (Fig. 4)8-10.

Yet the paper entitled Helical microtubules of graphitic carbon

published by Iijima in 199111 caused an unprecedented change in

carbon science. Iijima was fascinated by the Krtschmer-Huffman paper

on the lab-scale production of C6012. He examined soot produced

by the Krtschmer-Huffman method provided by Ando12,13 using a

high-resolution transmission electron microscope (HRTEM). In 1990,

Iijima presented images of carbon nanotubes (Fig. 5) at a meeting

in Richmond, Virginia and one month later he published his paper in

Nature11. A combination of factors was responsible for the tremendous

impact of Iijimas paper and for the attention that these graphitic

nanofibers continue to attract today14. 15 years later, the number of

publications related to CNTs is still increasing exponentially.

Motivation carbon nanotube propertiesIdeal CNTs may be described as nanoscale graphene cylinders that are

closed at each end by half a fullerene. Structures comprising only one

cylinder are termed SWNTs, whereas multiwalled nanotubes (MWNTs)

contain two or more concentric graphene cylinders. Ideal SWNTs are

classified according to three possible crystallographic configurations,

zigzag, armchair, and chiral, depending on how the graphene sheet

is rolled up. In the zigzag conformation, two opposite C-C bonds of

each hexagon are parallel to the tube axis, whereas in the armchair

conformation the C-C bonds are perpendicular to the axis. In all other

arrangements, the opposite C-C bonds lie at an angle to the tube axis,

resulting in a so-called helical nanotube that is chiral (Fig. 6).

Fig. 1 Low-resolution transmission electron micrograph depicting three hollowcarbon fibers at MAG 20 000. These fibers were first reported by Radushkevichand Lukyanovich in 1950. (Adapted and reprinted with permission from 3.)

Fig. 2 First viewgraph of what today is called SWNT or DWNT. Based on thisimage, it is difficult to determine the exact number of walls. (Adapted andreprinted with permission from4. 1976 Elsevier.)

Fig. 3 Simulated images of a DWNT in a SWNT bundle and a SWNT in aDWNT bundle. To avoid misinterpretation of such complicated transmissionelectron micrograph images, the authors suggest analyzing the cross sectionof the nanotubes and bundles. If possible, TEM images should be combinedwith image simulations and other characterization methods, e.g. Raman

spectroscopy. (Adapted and reprinted with permission from5

. 2006 Elsevier.)

Carbon nanotubes becoming clean REVIEW

Fig. 4 Typical scanning electron micrograph of the carbon filaments oftenobserved during metal dusting processes. (Scale bar = 2 m.)


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CNTs may exhibit extraordinary aspect ratios. SWNTs are found to

grow up to several centimeters long15, i.e. 109 times their diameter.MWNTs exhibit lengths of up to a centimeter16and have diameters

from 5-100 nm. CNT properties are strongly dependent on their

structure, e.g. for typical diameters, all armchair SWNTs and one-third

of all zigzag nanotubes are metallic, the rest are semiconducting17.

Extraordinary mechanical properties of SWNTs were predicted

shortly after their discovery18. Doping of CNTs can be used to tune

their electronic response19. Several accounts have shown that CNTs can

act as field-effect transistors20 or that vertically aligned CNTs are ideal

candidates for low-resistance interconnects21,22 (Figs. 7aand b).

These properties make CNTs attractive for applications in

nanoelectronics20 and quantum computing23, as gas sensors24-26, or

fillers in polymer27, ceramic28,29, or metal30 composites31. Today,

numerous spin-off companies offer CNTs at competitive prices, and

more and more products containing CNTs are becoming available.

However, no currently available CNT-based application makes use

of the properties of individual nanotubes. To date, all commercial CNT-

based applications rely on the bulk properties of CNT ensembles. The

much anticipated breakthrough ofindividual CNTs, e.g. in electronic

devices, has yet to be achieved. One of the key limiting factors is the

unavailability of clean materials. As discussed in the following sections,

current growth methods are insufficient for the controlled production

of CNTs with uniform structure and uniform properties.

Properties of individual carbon nanotubesThe data reported for mechanical and electrical properties of CNTs

vary significantly.Table 1 shows the variation in experimentally

determined values of the Youngs modulus and tensile strength. Often,

measurements have to be carried out on several tubes until expected

results are found and confirmed, because of the variation in structure

exhibited by different nanotubes. This is disconcerting because

applications require uniform properties, yet it is self-evident because

current synthesis methods cannot yet generate monochiral nanotubes,

i.e. nanotubes of the same atomic structure and hence properties. The

inconsistency of results can be understood as follows. First, nanotubes

contained in a typical sample differ in length, diameter, chirality, and

number of walls, or, more generally, in their atomic structure. Second,

nanotubes produced via different methods contain different levels of

defects and byproducts. While some labs specialize in the production

of CNTs, others carry out physical measurements, such as spectroscopy

and microscopy, on samples obtained through collaborations or from

Fig. 5 Iijimas TEM micrographs of three MWNTs consisting of three, five, andseven concentric graphene shells. (Adapted and reprinted with permissionfrom11. 1991 Nature Publishing Group.)

Fig. 6 Models of three atomically perfect SWNT structures.

REVIEW Carbon nanotubes becoming clean

(b)(a) (c)

Table 1 Variation of Youngs modulus and tensile strength found for different carbon nanotubes.

Author Youngs modulus[TPa]

Tensile strength[TPa]

Nanotube type Method

Treacy et al.102 1.8 MWNTARC thermal vibrations (TEM)

Krishnan et al.103 1.25 SWNTLSR thermal vibrations (TEM)

Wong et al.104 1.28 MWNTARC AFM

Salvetat et al.105 0.81 MWNTARC AFM

Salvetat et al.106 0.01-0.05 MWNTCVD AFM

Yu et al.107 0.27-0.95 0.01-0.06 MWNTARC AFM (dual cantilevers)

Demczyk et al.108 0.8 0.15 MWNTARC TEM (tension)

Bacon109 0.8 Carbon whiskers

Edie et al.110 0.2-0.9 0.002-0.007 Carbon fibers


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commercial suppliers. Therefore, a detailed characterization of the CNT

material is not available, because companies and labs specializing in

CNT synthesis often do not have access to or cannot afford detailed

and time consuming structural characterization of each CNT batch.

Often, only the first samples produced using a new process are studied

in detail. Subsequent batches are assumed to exhibit similar features.

Unfortunately, this approach is unsuitable for CNT evaluation because

small fluctuations in synthesis conditions can change the structure of

CNTs dramatically.

Carbon nanotube synthesisThe principle of CNT production is simple. All known production

techniques involve a carbon feedstock, a metal catalyst, and heat. All

methods for producing SWNTs require a metal catalyst, while in the

production of MWNTs, carbon arc discharge is the one exception that

does not require any metal catalyst. Several books and reviews13,32-35

describe CNT production methods, hence I will focus on the properties

of MWNTs and SWNTs obtained through different methods.

Carbon arc discharge

In 1992, Ebbesen and Ajayan36 showed that MWNTs can be produced

in a carbon arc discharge (Figs.8a, 8b, 9a, and 9b). The carbon arc

discharge uses two graphite electrodes through which a direct current

is passed in an inert He atmosphere. The anode is consumed and a

cigar-like deposit forms on the cathode. The outer shell of this deposit

is gray and hard with a black soft inner core that contains MWNTs,

polyhedral particles, and amorphous carbon36. SWNTs may also be

obtained but require mixed metal catalysts, such as Fe:Co, Ni:Y37, that

are inserted into the anode. After arcing, SWNTs are found distributed

in the chamber as a fluffy web-like material37. Ando13 showed that the

arc evaporation of a graphite rod with a pure Fe catalyst in conjunction

with a hydrogen and inert gas mixture may be used for growing

macroscopic SWNT nets of up to 20-30 cm in length. Replacing He by

H2 results in MWNTs with a very thin innermost tube of


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as ropes or bundles consisting of several individual SWNTs. Byproducts

such as amorphous carbon or encapsulated metal catalyst particles are

also present. The laser-ablation technique favors the growth of SWNTs;

MWNTs are usually not generated with this method. The quality,

length, diameter, and chirality distribution of the material are believed

to be comparable with those of SWNTs grown by arc discharge.

Chemical vapor deposition (CVD)

The catalyzed decomposition of hydrocarbons has been known to

generate carbon fibers since filamentous carbon was produced by

passing cyanogen over red-hot porcelain in 18902. In the 1980s, Endo48

developed the floating catalyst reactor using catalyst particles 10 nm

in diameter. This method is a precursor to the aerosol-based CNT

production widely used today49-55, where pyrolysis of hydrocarbons

in the presence of a transition metal catalyst (Fe, Ni, Co, etc .)

generates MWNTs and SWNTs. MWNTs are mainly produced at lower

temperatures (300-800C) in an inert gas atmosphere, whereas SWNTs

require higher temperatures (600-1150C)45,56-58 and a mixture of H2

and an inert gas such as Ar. DWNT generation using CVD techniques

involves a more complicated catalyst preparation procedure. DWNT

samples generally also contain SWNTs and triple-walled CNTs59.

The decomposition of hydrocarbons is aided by a plasma in plasma-

enhanced CVD (PECVD) or plasma-assisted CVD (PACVD). Park et

al.60 used a combination of CVD and PECVD to create CNTs from an

acetylene and H2 gas mixture on stainless steel plates. CVD and PECVD

are commonly used to grow aligned MWNTs (Figs.8c and d), and

SWNTs on various substrates including Ni61, Si62, SiO249,53,54,63,64,

Cu/Ti/Si(100)65, stainless steel60,66, glass67, etc. The structure of

MWNTs produced by CVD, in particular, differs significantly from

MWNTs generated by arc discharge (MWNTARC). For example, CVD-

grown MWNTs (MWNTCVD) are usually less crystalline and exhibitmany more defects than MWNTARC. Therefore, MWNTCVD are less

straight than MWNTARC(Figs. 9c and d). In some instances, depending

on the catalyst, spiral growth can be observed (Fig. 10) 64,68-70.

CVD is probably the most versatile production method for

CNTs, especially for generating doped CNTs, e.g. with B, N, or

both19,54,63,64,68,71-76. MWNTCVD can reach a centimeter in length and

usually possess larger diameters of up to 100 nm. The number of walls

in MWNTCVD can vary from three to >100. MWNTCVD usually grow

perpendicular to the substrate; therefore their length is relatively easy

to measure (Figs. 8c and 11). The CNT and carbon-fiber communities

are still debating at what diameter CNTs should still be called CNTs

and from what diameter the structures are carbon nanofibers.

Electrolysis

Electrolysis is a less common method for CNT production. To date,

it is the only condensed phase method for generating CNTs, and was

developed by Hsu et al. in 199577,78. MWNTs are formed when a

current is passed through two graphite electrodes immersed in molten


Fig. 9 TEM images reveal the structural differences of MWNTs produced via CVDtechniques and by arc discharge. (a) TEM image of raw MWNTARCrevealing the

presence of polyhedral carbon particles. (b) TEM image of purified MWNTARC.(c) TEM images of nanotubes generated by pyrolysing 2-amino-4,6-dichloro-s-triazine over laser-etched Co substrates. (d) MWNTCVD exhibiting metalparticles (~40 nm OD) at their ends; inset showing a close-up of the particlecontaining nanotube tips. (Parts (a),(b) adapted and reprinted with permissionfrom101. Parts (c),(d) adapted and reprinted with permission from 64. 1999and 2000 Springer.)

Fig. 10 SEM image of carbon nanocoils generated by pyrolysis of melamine overCo-oxide substrates.

Fig. 11 SEM image of a bundle of MWNTCVD with a human hair for comparison.Here, the length of the MWNTCVD is roughly the same as the diameter of thehuman hair, 80 m.

(b)

(a) (c)

(d)


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ionic salts, such as LiCl at 600C. After electrolysis, the carbonaceous

material is extracted by dissolving the ionic salt in distilled water and

separating the dispersion by filtration. The material produced contains

MWNTs, carbon-encapsulated metal particles from the salt ions,amorphous carbon, and carbon filaments (Fig. 12)77. Adding less than

1 wt.% of other salts, such as SnCl2 or PbCl2, results in the formation

of metal nanowires coated with amorphous carbon77,79,80. So far,

only the generation of MWNTs (MWNTELE) has been reported. The

formation of SWNTs via electrolysis has not been observed. MWNTELE

possess diameters of 10-20 nm, consist of only a few walls, e.g. 10-15,

and are estimated to be >500 nm long. MWNTELE occur predominantly

in entangled bundles also containing amorphous carbon, spheroidal

carbon particles, and metal-encapsulated particles.

Several other production methods for CNTs have been reported.

These processes are often based on earlier concepts and the number of

publications constitutes a minor contribution to the literature.

To summarize, the structure and properties of CNTs are highly

sensitive to the production method and synthesis parameters such as

temperature, reactor size, gas flow and pressure, precursors, etc. For

example, MWNTARC are highly crystalline, have few defects, and are

straight, whereas MWNTCVD are longer, exhibit larger diameters, are

highly defective, and are not straight. Defects influence not only the

electronic structure but also the mechanical properties81. This may

help to understand why the Youngs modulus of MWNTCVD is orders of

magnitude smaller than the Youngs modulus of MWNTARC (Table1).

Characterization and purificationMany groups specialize in the analytical characterization of specific

properties, such as field emission, Raman spectroscopy, conductivity

measurements, and mechanical testing. Other labs use CNTs to

develop products, such as composite materials, flat-panel displays, or

atomic force microscopy tips. Often these research groups do not have

expertise in the production of CNT materials themselves, but purchase

CNTs that are readily available from numerous spin-off companies.

In order to test the quality of commercial CNT material, I randomly

chose several companies and tried to compare their CNT materials

using the information provided.Fig. 13shows images of MWNT

samples from the webpages of four different companies. While all four

samples are sold as MWNTs, it is obvious that the morphology of the

CNT material is very different. In addition, the micrographs are taken

using different microscopy techniques, e.g. TEM and scanning electronmicroscopy (SEM), and at different magnifications.

To illustrate that a single electron microscopy image is not enough

for CNT characterization, I show TEM and SEM images of one sample

labeled SWNTCVD at a range of magnifications (Figs.14 and 15).

At higher magnification, the TEM image depicts double- and triple-

walled nanotubes. Thus, the sample, although labeled SWNTCVD,

is in fact a collection of SWNTs, DWNTs, MWNTs, metal catalyst

particles, and soot, as can be seen clearly at lower magnification.

SEM images taken at lower magnification reveal that the SWNTCVD

material occurs as large agglomerates rather than individual nanotubes.

Therefore, providing only a single micrograph at a given magnification


Fig. 12 Typical TEM viewgraph of an electrolytically produced MWNT sample.(Adapted and reprinted with permission from78. 1995 Nature PublishingGroup.)

Fig. 13 Four randomly chosen images of different nanotube samples labeledMWNTs from supporting information provided by the supplier.

Fig. 14 TEM images of a CNT sample taken at different magnifications. Thesample was labeled as SWNTs by the supplier. At lower magnification, it can beseen that the material contains byproducts such as amorphous carbon, metalencapsulated particles, and graphitic carbon particles. Higher magnificationshows the absence of SWNTs and the presence of double- and triple-walledcarbon nanotubes.


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is insufficient to characterize the material. It is not possible to judge

the quality of the commercial material discussed above from the

information provided by the companies. Structural specifications areoften descriptive rather than factual. Samples, for instance, are named

split MWNTs, short MWNTs, long MWNTs, curly MWNTs, or

similar. Furthermore, the information provided for different samples

varies. Sometimes TEM, SEM, and Raman data is available, other

samples are characterized by different techniques, such as atomic force

microscopy, and BET surface area, or field-emission measurements.

As a result of poor sample quality and lack of consistent

characterization information, a number of labs have developed

purification and separation techniques to obtain clean nanotube

material and to select nanotubes of specific characteristics82,83.

However, such purification methods often also remove large parts of

the nanotubes themselves. For example, oxidation of MWNTs grown

by arc discharge reduces the sample by 99%84. Other methods, such

as acid treatment or surfactants may damage or functionalize the

nanotube surface, and hence may also alter their properties85.

Therefore, in order to be able to provide researchers with specific

CNT material, one first has to characterize the samples carefully. A

minimum characterization must include the electron microscopic

analysis of material at different length scales including SEM, TEM, and

analytical methods. The production method, including the catalyst

employed, needs to be stated. From the micrographs, approximate CNT

content, typical length and diameter, as well as distribution of length

and diameter should be specified. For SWNTs, chirality distributions

should be made available using optical spectroscopy methods86.

Oxidation resistance data would provide information on the

crystallinity of CNTs, e.g. the higher their oxidation resistance, the less

defects are present. Therefore, a standardized characterization protocol

is vital in order to compare samples objectively87.

Carbon nanotube growth systematicsApplications that take advantage of individual CNT properties require

precise classification of the CNT material. Production methods have

advanced significantly and the quality of catalytically grown CNTs has

improved steadily so that large quantities of fairly clean CNTs can now

be produced. However, structural control has yet to be achieved. One

needs to be able to reproduce CNT material independent of the person

carrying out the experiment and the specifics of the setup.This can only be achieved by understanding the growth of CNTs

as a function of the precursor and catalyst materials, experimental

parameters such as gas flow, pressure, and temperature, and production

method. High-throughput synthesis using a combinatorial approach to

catalyst selection88-90, in conjunction with structural characterization

of the catalyst particles and in situ electron microscopy studies91-97, are

essential to create a better understanding of CNT growth.

Although in situ investigations combined with density functional

theory calculations have suggested that CNT growth catalyzed by Ni

particles is a surface-diffusion-based process94,96, there is still a lack

of consistent atomic-scale data98,99 on nanotube formation for other

transition metals commonly used for nanotube growth. Furthermore,it is still unclear whether carbon dissolves and diffuses through the

metal catalyst particle and then precipitates as a carbon filament,

or whether carbon diffuses on the surface of the catalyst particle, or

whether bulk and surface diffusion compete. Therefore, synergetic

experimental-theoretical studies are essential for the clarification of

growth mechanisms that currently are still based on models originally

postulated for carbon fibers in the 1970s5,100. The findings of classical

catalysis and metal corrosion studies ought to be revisited as a focus

for current investigations. At the same time, unspecific descriptions

of CNT samples need to be replaced by a standardized nomenclature.

The abbreviation MWNT alone is insufficient to describe sample

quality. Since there is a clear difference in CNT samples produced

via different methods, the description ought to at least include the

production method, e.g. MWNTARC for MWNTs grown by arc discharge,

or MWNTCVD for those generated using CVD.

SummaryThe demand for uniform CNTs entails two major challenges that need

to be overcome. One is the reproducibility of specific CNT material

and the second is the transition from lab- to large-scale production.

For this, it is indispensable to establish clear characterization patterns 87

and a standard description of real CNT material.

Acknowledgments

I would like to thank the Royal Society for financial support.

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Fig. 15 SEM images of a CNT sample taken at different magnifications. Thesample was labelled as SWNTs by the supplier. At lower magnification itbecomes apparent that the nanotube material occurs is large agglomeratesrather than individual nanotubes.


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