Luận văn high yield synthesis of multi walled carbon nanotubed from caco3 supported iron iii nitrate catalyst

VIETNAM BATIONAL UNIVERSITY HANOI COLLEGE OF TECHNOLOGY Nguyen Dac Dung HIGH YIELD SYNTHESIS OF MULTI-WALLED CARBON NANOTUBES FROM CaCO; SUPPORTED TRON IIT NITRATE CATALYST MASTER T

VIETNAM BATIONAL UNIVERSITY HANOI

COLLEGE OF TECHNOLOGY

Nguyen Dac Dung

HIGH YIELD SYNTHESIS OF MULTI-WALLED

CARBON NANOTUBES FROM CaCO; SUPPORTED

TRON (IIT) NITRATE CATALYST

MASTER TIIESIS

Hanoi - 2006

VIETNAM BATIONAL UNIVERSITY HANOI

COLLEGE OF TECHNOLOGY

Nguyen Dac Dung

HIGH YIELD SYNTHESIS OF MULTI-WALLED

CARBON NANOTUBES FROM CaCO; SUPPORTED

TRON (IIT) NITRATE CATALYST

Speciality: Nano Materials and Devices

MASTER THESIS

Advisor: Dr Phan Ngoc Minh

Hanoi - 2006

Content

Abbreviations

Preface and target af the work

Chapter 1 Introduction to carbon nanotubes material

1.1 Brief history of carbon canotubes

1.2 Geometry of carbon nanotubes

1.3 Syntheses of carbon canotubes

1.3.1 Are discharge

1.3.2 Laser ablation

1.3.3 Chemical vapar deposition

1.4 Growth mechanism of carbon nanotubes

2.1.1, Description of the CVD system for growing carbon nanotubes

2.1.2 Synthesis of carbon nanotubes

2.2.2, Raman spectroscopy of carbon nanotubes

2.2.3 Xray diffraction of carbon nanotubes

2.2.4 Thermogravimetric analysis

Chapter 3 Results and discussien

3.1 Catalytic Fe nanoparticles in the CNTs growth process

3.1.1 Effect of supported iron salts on the CVD produels

3.1.2, Formation of catalytic Fe nanoparticles nucleating CN'I's

3.2 Liffect of erowth temperature

Chemical Vapor Deposition

Differential Thermo-Gravimetric Ananlysis

Llectro-Chemical Double Layer Energy Dispersive X-ray spectroscopy

Fourier Transform Infrared

Lligh Resolution Transmission Lectron microscope Multi-Walled Carbon Nanotubes

Plasma Enhanced Chemical Vapor Deposition Standard Cubic Centimeters per Minute

Scanning Electron Microscope Scanning Transmission Electron Microscope Scanning Tunneling Microscope

Single-Walled Carbon Nanotubes

Transtnission Electron microscope

Thermo-Gravimetric Analysis X-Ray Diffraction

Preface and target of the work

Carbon nanotubes were identified for the first time in 1991 by Sumic Iijima at

the NUC Research Laboratory By using high resolution transmission electron microscope (HRTEM) he clearly observed the tiny tubes called multi-walled carbon nanotubes (MWCN'1ls) in the soot made from by-product obtained in the synthesis

of fullerenes The MWCNTs comprise carbon atoms amanged in a graphitic structure rolled up to form concentric cylinders [38] Twa years later, single-walled carbon nanotubes (SWCNIS) were synthesized by adding metal particles to the carbon electrodes [9, 36]

Their small diameter (of the order of a nanometer) and their long length (of the

order of microns) Iead to aspect ratios so large that the carbon nanotubes possibly reach to ideal one-dimensional (ID) systems Depending on the chirality of their slomic structurc, they can be excellent metals or semiconductors with a band gap that is inversely proportional to their diameter Theoretical and experimental results have shown extremely high elastic modulus, greater than 1 TPa and strengths 10-

106 times higher than strongest sleel [77] Tn atkdition to exceptional mechanical properties, they also possess superior thermal properties: thermally stable up to 2800°C in vacuum, thennal conductivity about twice as high as diamond [16] The above properties make carbon nanolubes (CNTs) the object of widespread studies in both basic science and technology ‘'hey can be applied in many fields: fabrication

of nano sized electronic devices, energy storage equipments, field emission display, nano probes, nano composites,

‘There are many methods (mentioned in detail in seotion 1.3) for synthesizing carbon nanotubes having different performance from diverse material sources The

arc discharge method relates to conuceling two graphile rods to a power supply, placing them millimeters apart, and vaporizing carban by a hot plasma Hs product can be SWCNTs and MWCNTs with few structural defects Tubes tend to be short wilh random sizes and directions, This method can produce Jarge scale production

of CNTs but its typical yield of about 30% is not high Laser ablation method was firstly used in 1996 by Smalley at al using intense laser pulses blasting graphite to

form primanly SWCNTs The diameters of SWCNTs can be controlled in a large

range by varying the reaction temperature Although the yield of laser ablation method can reach to 70%, it has never been candidate for large-scale production because of requiring expensive lasers and the limitation of a laser spol area Emerging, as the best method for industrial production of CN‘I's is chemical vapor deposition (CVD) Carbon feedstocks are hydrocarbons in gaseous and liquid

phases, alcohol, ele., deconposed ai 600-1200°C inlo carbon aloms recombining Lo

nanotubes over metal nanoparticles Carbon nanotubes produced by CVD having the yield probably up to 100% are usually long MWCNTs with quite high defects Thus, the investigation of suitable technologies to syhesive large-seale production

of carbon nanotubes with high yield and purity to reduce cost satisfying for industrial demands is an opening solution until now

The most cormmon and oplimal method for large-scale production of CNTs

"

catalytic chemical vapor deposition (CCVD) (discussed in section 1.3.3) In the CCVD process, catalyst supports are the essential ingredients such as, MgO, Al,O3,

S102, CaCQs ctc., duc to their high surtiace area for CCVD reaction The choice of

CaCO, as catalyst support was reported in Ref [18] ‘The advantages of this technique are

* CaCO; support is easily dissolved in a dilute acid, thus the CNTs purification is a one-siep procedure, simple and harmless lo CNTs structure

CaCO; and Fe salts from which catalysts synthesized are available in anarket and low cost

© This is the simplest CVD method for large scale production of CNTs

By supplying catalysts and collecting CVD product continuously, the production yield is significantly increased

With the aim of large scale and low cost production and the idea using CaCO,

support, this thesis investigates the technological aspects that relate to synthesis of

carbon nanotubes We develop a simple method for making catalyst only by grinding CaCO and Me salts, therefore, neglect the impregnating and drying steps, that reduce stages in CNTs synthesis The addition of H, gas in CVD process is believed not only to form Ke nanoparticles enhancing catalytic activity but also to improve the CNTs yield By varying growth temperature, another role of CaCQ; as the faclor contributing to the [formation of Fe nanoparticles necessary to (he CNTs growth is studied in this thesis Furthermore, Fe salt radicals are found significant to the creation of Fe nanoparticles on the support (CaCO; or CaO) At last, the more dilute acid (HCI 10%) is used for purification process

‘the arrangement of the thesis:

In addition to the “Preface and target of the work” and “Conclusion” parts the thesis is organized into three chapters as follow

Chapter | shows an overview of carbon nanotubes material, the CNTs

synthesizing methods and ability in industrial applications

Chapter 2 ists the experimental process for synthesis of carbon nanotubes

‘This chapter also introduces investigation methods mainly used during this thesis

Chapter 3 indicates the effect of Fe content in catalysts The formation of Fe nanoparticles necessary to CNTs growth is studicd The structural characteristics of the CNTs depend on the growth temperature are characterized ‘the optimal ghemigal vapor dopusition process is established for the aim of large-scale production of carbon nanotubes it is confirmed that by using the presented

technique we can produce 97.9 % purity, 78.6 % yield CNTs with mass of 50

grams/day.

Chapter 1 Introduction to carbon nanotubes material

1.1 Brief history of carbon nanotubes

In 1970's and 1980's, small diameter carbon filaments were produced through

the synthesis of carbon fibers by the decomposition of hydrocarbons at high

temperature in the presence of transition metal catalyst nanoparticles [57, 78]

However, there was not any detailed systematic study on such small filaments until

the observation of carbon nanotubes by Iijima in 1991 [38] These tubes (called

multiwall carbon nanotubes) contained at least two layers, often many more, and ranged in outer diameter from about 3 nm to 30 nm with both closed ends

A new class of carbon nanotubes with only single layer was discovered in

1993 [9, 36] These single-walled nanotubes with diameters typically in the range 1-

2 nm are generally narrower than the multiwalled nanotubes and tend to be curved

rather than straight Since these pioneering works, the study of carbon nanotubes

has developed rapidly

Fig 1.1: Multi-walled CNTs observed in 1991 [38].

1.2 Geometry of carbon nanotubes

‘The structure of carbon nanotubes has been characterized by High Resolution

Transmission Electron Microscope (IRTEM) and Scanning Tunneling Microscope

(STM) These lechniques directly confirmed thal the carbon nanotubes are cylinders derived from the honeycomb lattice representing a single atomic layer of crystalline graphite (a graphen sheet) Most important structures are single walled carbon tamotubes (SWCNTs) and multuwalled carbon nanotubes (MWCNTs) A SWCNT

is considered as a cylinder with only one wrapped graphene sheet Multi walled carbon nanotubes (MWCNTs) are similar to a set of concentric SWNTs The structure of a single walled carbon nanotube is cxplained in terms of its LD uni ceil, defined by the vectors C and 7’ in Fig 1.2a [20]

The circumference of any carbon nanotube is expressed in terms of the chiral vector Cy — nay + m&, which connects two crystallographically equivalent sites ona

2D graphene sheet (see Itig 1.2a) ‘he construction in Fig 1.2a depends uniquely

on the pair of iutegers (n, m1) which specify the chiral vector Fig 1.2a shows the chiral angle § between the chiral vector and the “zigzag” direction (6 = 0) and the unit vectors 4, and A, of the hexagonal honeycomb lattice of the graphene sheet Three distinct types of carbon nanotube structures can be generated by rolling up the graphene sheet into a cylinder as discribe below and shown in Fig 1.3 ‘The zigzag, and armchair nanotubes, respectively, correspond ta chiral angles of 0 = 0

and 30”, and chiral nanotubes correspond to 0 < @ < 30° The intersection of the

vector OB (which is normal la C,) with the first lattice poinl deicnnines the

fundamental one dimension (1D) translation vector T The unit cell of the 1D lattice

is the reclangle defined by the vectors C, and T (Fig 12a)

®:metal e:semiconductor armchair

Fig 1.2: (a) The chiral vector OA or CỤ = nâ; + mâ› is defined on the honeycomb

lattice of carbon atoms by unit vector @, and 4; and the chiral angle @ with respect to the zigzag axis Along the zigzag axis 0 = 0° Also shown are the lattice vector OB = T of the 1D nanotube unit cell and the rotation angle y and the translation t which constitute the basic symmetry operation R = ( yt)

for the carbon nanotube The diagram is constructed for (n, m) = (4, 2) (b)

Possible vectors specified by the pair of integers (n, m) for general carbon nanotubes, including zigzag, armchair, and chiral nanotubes Below each pair

of integers (n, m) is listed the number of distinct caps that can be joined

continuously to the carbon nanotube denoted by (n, m), The encircled dots denote metallic nanotubes while the small dots are for semiconducting

nanotubes.

Fig 1.3: Schematic models for single-wall carbon nanotubes with the nanotube axis

normal to the chiral vector which, inturn, is along: (a) the @ = 30° direction

[an armchair (n, n) nanotube], (b) the @ = @ direction [a zigzag (n, 0)

nanotube] and (c) a general 6 direction with 0 < @ < 30° [a chiral (n, m)

nanotube] The actual nanotubes shown here correspond to (n,m) values of :

(a) (5, 5), (b) (9, 0), and (c) (10, 5)

The cylinder connecting the two hemispherical caps of the carbon nanotube

(see Fig 1.3) is formed by superimposing the two ends of the vector C,, and the cylinder joint is made along the two lines OB and AB’ in Fig 1.2a The lines OB

and 4B’ are both perpendicular to the vector C at each end of Cy, In the (n, m)

notation for Cy = nf + mA, the vectors (n, 0) or (0, m) denote zigzag nanotubes

and the vectors (n, n) denote armchair nanotubes All other vectors (n, m)

correspond to chiral nanotubes The nanotube diameter d, is given by

where Cụ is the length of C,, ac.c is the C — C bond length (1.42 A), The chiral angle 6 is given by

10

achiral Differences i the nanolube diameter d, and chiral angle @ give mse Lo

differences ini the properties of various carbon nanotubes The symmetry vector R = (w/t) of the symmetry group for the nanotubes is indicated in I'ig 1.2a, where both

the translalion unit or pitch t and the rolaliom angle y are shown, The mumber of

hexagons, N, per unit cell of a chiral nanotube, specified by integers (n, m) is given

by

where d,=difn mis not a multiple of 3d or d, = 3d, ifm is a multiple of 3d

and d is defined as the largest common divisor of (n, m) Each hexagon in the honeycomb latiice (Fig 1.24) conlains two carbon atoms The unil cell area of the carbon nanotube is N times larger than that for a graphen layer and consequently the unit cell area for the nanotube in reciprocal space is corresponding 1/N times

amaller Table 1.1 [21{ provides a summary of relation useful for describing the

structure of single wall nanotubes Hig 1.2b indicates the nanotubes that are

semiconducting and those that are metallic, and shows the number of distinct

fullerene caps that can be used Lo close the end of an (n, 1) nanotube, such that the

fullerene cap satisfies the isolated pentagon rule

11

Table 1.1 Structural parameters for carbon nanotubes

coordinate

C, — Chữalvector €,— nai — ma (6 <In|<m)

L Length of Cy L— V3a_ y(n? + nm+m’)

"In this table n, m, ti, to, p, q are integers and d, dz, N and M are integer

functions of these integers

») ged (n, m) denotes the greatest common divisor of the two integers n and m

1.3 Syntheses of carbon nanotubes

1.3.1 Are discharge

The are discharge was the first available method for the production of both

MWCNTs and SWCNTs This technique has been in use for a long time for the

production of carbon fibers It is a worthy note that carbon nanotubes were probably observed before 1991 but not recognized

In the are-discharge technique, two graphite rods were used as the anode and

cathode, and placed inside a growth chamber filled with helium atmosphere When

a high current is passed through the opposing graphite anode and cathode, helium gas plasma evaporates carbon atoms in the anode, which deposits to form carbon

nanotubes on the cathode

To produce isolated SWCNT catalysts such as Co, Ni, Fe, Y are used Mixed

catalysts such as Fe/Ni, Co/Ni and Co/Pt are used to grow bundles of SWNTs For the synthesis of MWCNTs no catalyst is necessary and with the use of halide

(potassium chloride) as a promoter in hydrogen atmosphere large scale production

would be expected [34] The nanotubes are found in the inner region of the cathode

deposit and they are surrounded by a hard shell consisting of nanoparticles, fullerenes and amorphous carbon [1, 37],

1.3.2, Laser ablation

An efficient route for synthesis of CNTs with a narrow diameter distribution is

a laser ablation of a graphite target In the laser ablation technique, an intense laser

beam is utilized to vaporize a graphite target doped with metal catalysts in a tube

furnace at temperatures near 1200°C, while an inert gas jet passing through the

deposition chamber carries nanotubes onto a metal collector

In the carly report [30], the laser beam scanned across the target surface under

computer control to maintain the smooth, uniform face for vaporization The target

was supporled by grapbrite poles ia 1 ich quartz tube evacuated to 10 mTorr and then filled with 500 Torr argon flowing at SO seem The flow tube was mounted in a high temperature furnace with a maximum temperature of 1200°C The soot produced by the laser vaporization was swepl by the Mowing Ar gas [rom the high temperature zone, aud deposit on a water-cooled copper collector positioned downstream, just outside the fumace

MWCNTs are produced if the targel is made of pure graphite [33] bul in case

of the target composed of graphite and metal [76] SWCN's are synthesized

1.3.3 Chemical vapor deposition (CVD)

Chemical vapor deposition (CVD) is a process whereby a solid material is deposited from a vapor by a chemical reaction occurring on or in.a surrounding area

of a vonnally healed substrate surface The solid maternal is oblaimed as a coating, a

powder, or as single crystals By varying the experimental conditions: substrate

material, substrale lemperalure, composilion of the reaction gas mixture, total

pressure gas flows, ctc., materials with difforent properties can be grown

In the CVD synthesis of CN'I's different hydrocarbons such as acetylene

(C;H;), methane (CHạ), benzene (CsHg), ete and also carbon monoxide (CO) are

decomposed over different metals (Fe, Co, Ni ) at temperatures between 400°C

and 1200°C There are some other carbon allotropes except CNTs in a CVD product

such as fibers [31, 33], planar nano-graphenes [63], amorphous, etc depending on the CVD conditions ‘his is why technological research on synthesizing CM's is necessary beside basic research

Tt can be classified CVD methods for synthests of CNTs

cording Lo

techniques making catalyst as described below

CVD method on flat substrates

For the CVD synthesis of CNTs on lal subsirates, a quartz tube is offer

horizontally placed in a thermal or electrical furnace The substrate (e.g, silicon,

aluminium, graphite ) is coated with a metal catalyst The metal catalyst on the

substrate is derived from coating metal oxalate, nitrate solution [39, 54] or

evaporating the metal film [91] The substrate and the metal are then exposed to an atmosphere containing a hydrocarbon gas and a carrier gas (Ar, N3) for a certain

tume to grow CNTs

An aided CVD method for flat substrates is plasma enhanced chemical vapor depostion (PECVD) In the PECVD method, carbon nanotubes are also deposited

on a metal coated flat substrate but the substrate is located inside a plasma, Usually

one of the following plasmas is used: RF- (radio frequency), MW-(microwave) or a

DC-(direct current) plasma Often a methane (CH,) and hydrogen (H,) gas mixture

with a ratio of 1% CH, to 99% H) is used at a total pressure between 1 and 40 mbar and a temperature of 900 °C [43]

CVD methad on catalytic substrates

Catalytic substrates are usually 3d metallic substrates such as Ni containing

tcl grid and plate,

alloy, stainless Is Melallie substrate surface prior to CVD

reaction is important because of conditions for forming nanoparticles ‘the

oxidation-reduction treatment is oxidation of the metallic surface by airflow at

elevated lermporatures and then by Dow of hydrogen for reduction of the melal oxide [81] ‘Ihe other for treating the surface is introducing hydrogen flow in the whole CVD process [32] Oxidation in air can result in the fragment of the metallic surface

to form fine nanoparlicle siructures The reduction by hydrogen is necessary for forming metal nanoparticles enhancing catalytic activity ‘Ihe two treatments also create granular structures increasing the catalytic surface area Most of CVD

produols in cases using bulk metalhe substrates were MWONTs (32, 49, 71, 81]

Floating catalytic CVD

Floating catalytic CVD is a process where un-supported catalysts are formed

in sift by injecting volalile organomelallie molecules, such as metallocenes or iron pentacarbonyl, into a reactor with a carbon feedstock [2, 13] The schematic diagrams of two conyenUonal apparatuses for Moaling calalyst method are shown in Fig 1.7 74] The carbon feedstock can be in liquid or gaseous phase The fumace has two heating zone, one with the sublimation temperature of precursor (90-110°C

fer ferrocen), the other with lamperature of CVD reaction (900-1100°C) [74] The

precursor (ferrocen) vapor is carried into the reactor by argon flux that includes a hydrocarbon flow (Fig 1.7) or the vapor of ethanol that is generated by bubbling

Ar gas via ethanol liquid (Fig 1.7a)

‘This method allows a three-dimensional dispersion of carbon feedstock with

the catalytic particles so that the ability for continuous production of high purity

CNTs with low cost is preferable for industrial applications [13, 50]

17

Fig 1.7: Schematic diagram of floating CVD system with (a) gaseous and (b) liquid

carbon feedstock

Catalytic CVD (CCVD)

This is the most promising method for synthesis of large-scale CNTs with low

cost because of the ability to control the reaction parameters [52, 58] By this process, a variety of liquid, solid, or gaseous carbon sources are introduced into the

reaction zone over a catalyst at a controlled temperature [52, 58] Most of catalysts

employed are related to Fe, Co, Ni and other transition metals (or combination of

them) dispersed on different supports such as SiO, AhO3, MgO or zeolites [11, 17,

52, 70] For synthesizing these supported catalysts, the impregnant method is

usually used Metal salts or mixture of metal salts are dissolved in distilled water,

ethanol or other suitable solvents to form solution which is then added the apposite

support amount and evaporated Fig 1.8 [41] shows a simple scheme for a CCVD

system According to the scheme, a catalyst is sprayed on the quartz boat and is

placed in the furnace center After a CCVD process, CNTs deposited on the quartz

boat are taken out.

Ges Outlet

Inlet of HatQpHty 8

Fig 1.8: A simple schematic CCVD system: (A) the furnace, (B) a quarts tube with

gas inlet and outlet, (C) a quartz boat for carrying the catalyst during the

synthesis, (D) the thermocouples

1.4, Growth mechanism of CNTs

‘The growth mechanism of carbon nanotubes is not exactly known, There may

‘be different mechanisms because the formation of carbon nanotubes depends on

which synthesis method is used A metal catalyst is necessary for growing MWCNT

im the CVD method while not in the arc discharge and laser ablation method

Tlowever, for the growth of SWCNT a catalyst 1s needed for all three methods

anenttioned above

‘The general CNT growth mechanism in a CVD process involves the dissociation of hydrocarbon molecules catalyzed by the transition metal, and

dissolution and saturation of carbon aloms in the metal nanoparticle [22] The

precipitation of carbon from the safuwated metal particle leads to formation of

tubular carbon solids in sp” structure

One theory hypothesizes thal metal catslysl particles are [loating or are supported on a substrate [68] Catalyst particles are considered as spherical or pear- shape, in which case the deposition will take place on only one half of the surface

19

Fig 1.9: Visualization of a possible CNT growth mechanism

The carbon diffuses along the concentration gradient and precipates on the opposite

half, around and below the bisecting diameter However, it does not precipate from

the apex of the hemisphere, which accounts for the hollow core that is characteristic

for CNTs For supported metal particle (see Fig 1.9), CNT grows upwards from

metal particle remaining attached to the substrate (strong catalyst-support

interaction) or the particle detaches and moves at the head of growing CNT (weak

catalyst-support interaction),

1.5 Purification

A major problem with applications of carbon nanotubes beside large-scale

synthesis is their purity, Most of the synthesis methods were based on the use of

metal catalyst The as-synthesized product contains several kinds of carbonaceous

and metal particles that could considerably affect the properties of CNTs and the

performance of any application built of them Therefore, it is necessary to purify

raw product of CNTs to have detailed characterization and application

There are several purification techniques used for mass production of CNTs as described below:

20

1.8.1 Oxidalion

Gas phase oxidation [25], thermal annealing in air, oxygen atmosphere [60] are effectiveness Lo climinate carbonaccous particles The main disadvantages ol” these are not only the impurities oxidized but also the CN, so the control of the temperature and time requires accurately In addition, metal catalysts encapsulated

in tip and walll structure could not be eliminated by oxidation and affect the function

of practical applications

1.5.2 Acid treatment

In general, the acid treatment will remove the metal catalyst Mirstly, the surface of the metal must be exposed by oxidation or sonication The metal catalyst

is then exposed to acid and solvated The CNTs remain im suspended form Wher

using a treatment in HINO,, the acid only has an effect on the metal catalyst It has

no effect on the CNTs and other carbon particles [10] If a treatment in HC! is used,

A special form of filtration is cross flow filtration In cross flow filtration the membrane is a hollow fiber ‘'he membrane is permeable to the solution ‘The filtrate

is pumped down the bore of the fiber at some head pressure from a reservoir and the inajor Fraction of the fast flowing solution that does nol permeale oul the sides of

the fiber is fed back into the same reservoir to be cycled through the fiber

repeatedly A fast hydrodynamic flow down the fiber bore (cross flow) sweeps the imembrine surface preventing the build-up of a filter cake [10, 28]

suspension liquid magnetic stirring bar

membrane filter

SWF caught

on Bitor

filteret quid, nanospheres

Fig 1.10: Schematic diagram of micro filtration cell [5]

1.6 Physical properties

1.6.1 Electronic properties

The electronic structure of carbon nanotubes is derived by a simple tight-

binding calculation for the n-electrons of carbon atoms The two-dimensional

graphite electronic structure is considered as the origin of that of single-wall carbon

nanotube A simple approximation of the 1 and s-band electronic structure of a

graphene layer is [65]:

(1.4)

where t is the nearest-neighbor tight binding overlap energy

The wave vector associated with the C, direction becomes quantized (circumferential direction), while the wave vector associated with the direction of the translation vector T (along the tube axes) remains continuous for nanotubes with

an infinite length

22

Tor (ín, n) armehair nanotubcs the pcriodie bouadary condition permit only wave vectors in the eireuroferential đirection which sabsfy the relatiơn qÀ = ad

where 2 2mk,„ is the De Broglie wavelengti

Substitution of discrete allowed values for ky, q given by (1.5) into Kg, (1.4)

yields the energy dispersion relations 1," (k) for armchair nanotubes [over phys 1]

Ba Recause of the degencragy point between the valance and conduction bands al the band crossing, the (5, 5) nanotube is thus a zero-gap semiconductor, which will

exhibit metallic conduction at finite temperature

All @, ) annchair nanolubes yield 4n sub bands with 2n conduction and 2n valance bands, and of these bands, two are non-degenerate and (n-I) are doubly degenerate They all have a band degeneracy between the highest valence band and the lowest conduction band at k — 42n/3a, where the bands cross the Fermi level All armchair nanotubes are expected 1o exhibit metallic conduction, similar to the behavior of 2D graphene sheets

The energy bands for the C, (nm, 0) zigzag nanotube ean be oblained by the

‘boundary conditions for ky, , are written as:

nk,,a—2ag (q“

Substiluting equation (1.7) milo equation (1.4) yields the energy dispersion

relation E,*(k) for zigzag nanotubes | 65 |

<1 As On -1.0 5 BI - 1 AWS

Fig 1.11: One-dimensional energy dispersion relation of (5, 5) armchair, (9, 0}

and (10, 0) zigzag nanotubes Doubly degenerated bands are in bold lines

= 43/31 or k = 0 for armchair or zigzag nanotubes, respectively The same k values also denote the energy gaps (including zero energy gap) for the general case

of chiral tubes The (n, m) chiral tanoluibes are metals if'n-an is @ multiple of 3

‘The density of states of (9, 0) and (10, 0) zigzag nanotubes are shown in Fig, 1.12 [65] The density of states near the Fermi level located at I = 0 is zero for the semiconducting (10, 0) nanotube and is non-vero for the metallic (9, 0) nanotube

Fig 1.12: Electronic ID density of states per unit cell for a (9, 0) and (10 0) zigsag

nanotube Dotted lines correspond to the density of states of a 2D graphene sheet [65]

‘The energy gap of semiconducting nanotubes depends on the reciprocal

nanotube diameter dy:

strongest chemical bonds In nanotubes the overall density of defects can be

extremely low depending on the synthesizing method and synthesizing conditions This has led lo predictions of a very high axial strength and ultra-low friction

between the shells

Theoretical calculations of axial Young’s modulus for individual SWCNTs

give results around 1 TPa or slightly higher [35, 51] Most of the theoretical allention has been on SWCNTs because the interlube imlerachons arc weak m

MIWCNTs and thus less important in estimating axial mechanical properties In Ref

[51] T.u presented Young’s modulus values for MWCNTs as well as SWCNTs and

tạ a

obtains values from 0.97 TPa to 1.1] TPa with the valuc mercasing slighfly with the

number of layers

The first Young’s modulus measurement [79] relaled thermal vibration

amplitudes of MWCN'ls to their Young’s modulus and obtained an average value

of 1.8 TPa with a large spread After that, with AIM techniques, values such as

1.28 + 0.59 TPa for are-discharge produced tubes have been obtained [86] Measuring SWCN1's is more complicated than measuring MWCN'Ts due to their

small diameters and the tendency to bundle Krishnan et al reported in Ref [42] a

ineasurement of individual SWCNTs using the thermal vibration method of Ref

[79] and they obtain an average value of 1.25 Pa Although the cument

measurements suffer from inaccuracies due to vibration amplitude measurement and

assumptions made on AFM tip charaeleristies, the current agreement is that both

SWCNTs and MWCNTs have a Young’s modulus value around 1 TPa

Theoretical tensile strength of nanombes is high [7, 83, 84, 88] The

aneasuroments fall short of the theoretical predictions that may result [rom limitations in the theoretical description or from the presence of imperfections in the

structure Measurements of tensile strength, thal is, the strength corresponding to

failure, suffer from difficultics and inaceuracies related to the small size of the

measured object ‘Tensile strength has been measured for MWCN'I's in Ref [89]

where the conlaet is ortly to the outmost layer of the multi-walled tube A sword and

sheath type of failure of this layer was reported I'he tensile strength values ranging,

from 11 GPa to 63 GPa were reported [89] For individual SWCNTs the experimental value of tensile strength is still an open question but for bundles of SWCN1's tensile strength values ranging between a few GPa and several tens of GPa depending on the bundle and measurement characteristics have been reported

]45 82, 90]

The lugh axial Young’s modulus and tensile strength combined with the low

weight and fiber-like form make nanotubes enticing candidates for composite

reinforecment Composites of nanotubes are based on dispersing nanotubes into a matrix of material that acts as the main body of the composite

1.7 Application of carbon nanotubes

1.7.L Energy storage

Electrochemical supercapacitors

At any electrode in contact with an electrolyte, there exists a dipole layer, the

so-called electrochemical double layer (ECDI.) Typically 0.4 mm thick, the ECDT

acts as a capacitor if the potential difference across it is less than that needed to dissociate the electrolyte (into hydrogen or oxygen in the case of water), The

capacitance is given by @ eA/d, where ¢ is the diclectric constant, A is the

electrode surface area, and d is the HCDL thickness, ‘Ihis capacitance can be huge for the case of CNTs because theoretically SWCNT have an outer specific surface

area of 1314 7g” [67] Fleclrodes can be made of carbon nanotubes from aligned

carbon nanotubes directly grown on an aluminum substrate or a paste of nanotubes with an organic binder applied on aluminum foils A model of an electrode with

qwmotubes directly grown on an electrode is shown in Fig 1.13 [27]

Liydrogen storage

The advanlage of hydrogen used for energy source is thal its combustion product is water Two common means to store hydrogen are gas phase and electrochemical adsorption Because of their cylindrical and hollow geometry, and nanometer-scale diameters, it has been predicted thal the carbon nanotubes can store liguid and gas in the inner cores through a capillary effect A Temperature- Programmed Desorption (TPD) study on SWCNT-containing material (0.1-0.2wi% SWNT) estimates a gravimetric storage density of 5 10wt? SWCNT when H, exposures were carried out at 300 torr for 10 min at 277K followed by 3 min at

133K [24] If all the hydrogen molecules are assumed to be inside the nanotubes,

the reported density would imply a much higher packing density of Hp inside the

tubes than expected from the normal H-H, distance

An even higher hydrogen uptake, up to 14-20 wt %, at 20 — 400°C under ambient

pressure was reported in alkali-metal intercalated carbon nanotubes It is believed

that in the intercalated systems, the alkali metal ions act as a catalytic center for H;

dissociative adsorption FTIR measurements show strong alkali-H and C-H

stretching modes An electrochemical absorption and desorption of hydrogen

experiment performed on SWNT-containing materials (MER Co, containing a few

percent of SWNTs) reported a capacity of 110mAl/g at low discharge currents The

experiment was done in a half-cell configuration in 6M KOH electrolyte and using a

nickel counter electrode Experiments have also been performed on SWNTs

synthesized by a hydrogen are-discharge method Measurements performed on

relatively large amount materials (~ 50% purity, 500mg) showed a hydrogen

storage capacity of 4 2wt% when the samples were exposed to 10MPa hydrogen at room temperature About 80% of the absorbed H, could be released at room temperature

@® solvatised ions separator electrolyte electrode

Fig 1.13: Model of an electrochemical double layer capacitor with nanotubes

directly grown on the metal substrate [27]

28

1.7.2 Camposite materials

‘The high aspect ratio (length to radius ratio) and high conductivity of CNTs inakes Ihem excellent for conducting eomposiles A mixture of conducling and insulating phases becomes conducting when the volume fraction of conducting phase exceeds a ‘percolation threshold’ of 16%, the minimum amount to give a continuous path across the whole sample This threshold is independent of the size and shape of the conducting phase, as long as its particles are equiaxed, When the conducting phase consists of long thin particles, the chance of contact increases, which reduces the percolation threshold so thal conduction occurs at much lower loading,

Existing conducting composites are made of carbon black in polymers This should require aboul 16% of carbon, but iL has been reduced by clever processmg Tig 1.14 shows an example of composites of MWCNTs in epoxy resin achieving percolation at loadings of 0.01% and even 0.004% with careful processing [66]

Those are extremely low loadings The first use of such composiles was by Hyperion for electrostatically applying paint onto car components [6] Nanotube

composites have a much betier surface finish than the previous carbon black or

carbon fiber composites

Another application of conducting composites is as a transparent conductor

Thore is a huge markel for transparent clectroric conductors such as indium lin oxide (LO) in displays In this field, there is a drive towards flexible displays on plastic substrates ITO is less good for this situation as it is brittle and has poor adhesion to plastic Conducting composites of SWCNTs can be transparent if thin enough [59] ‘They have the huge advantage of being truly flexible and compatible with polymer substrates This is a large potential market with few competitors

A novel method is to draw and spin a carbon manolube fiber directly from a

CVD reaction chamber, as done by Li ct al |48] and shown schematically in Fig 1.15 and SEM image of a produced fiber The resulting fiber can then be post impregnaled with epoxy to make @ composite Thess fibers presonlly have failure

strengths of up to 1 GPa, compared with an expected ultimate strength of 30-50 GPa for a single nanotube In fact, fibers made by each method creep before failure,

suggesting that a great deal of straightening and aligning is occurring before failure

It also shows that much optimization is needed to share the load better across the

Fig 1.14: Conductivity versus carbon content for three different carbon/epoxy

composite systems showing the rapid increase in conductivity corresponding

to the percolation threshold [66]

Fig, 1.15: Schematic representation of carbon nanotube made fiber apparatuses

(left) and its product (right) [48].

Chapter 2 Experimental and investigation methods

2.1, Experimental

2.1.1 Description of the CVD system for growing CNI's

Tn 2002, researchers at Taboratory of Physics and Technology of Electronic

Devices — Institute of Materials Science constructed the thermal CVD system as

shown in ig 2.1 ‘his apparatus includes: i) a thermal furnace, a quartz reactor and

a thermocouple; ii) gas tanks (CyH;, Hy, No, NH), gas-guiding tubes and mass controlling flowmeters

Thermal furnace and quartz reactor: ‘he fumace was coiled from resistive

wires It has 300 mm heating zone in length and a maximum temperature to 1200°C

‘The quartz reactor is horizontally placed inside the tabe furnace and has diameter of

40 mm, length of 1500 mm with two ends connecting to in-out gas-guide tubes The

temperature of the furnace can be controlled with error of 1°C by using a K

themnocouple and a temperature control system

Gas system: Gases (C:ll, Ih, N2, ) are introduced to the tube through corresponding mass Mowmrelers and are mixed before going inlo the reactor The exhaust gas is guided via vacuum oil to avoid air refluxing into the reactor that

causes explosive reactions

2.1.2 Synthesis of carbon nanotubes

As mentioned in section 1.3.3, we here used the CCVD method for producing CNTs with large scale The synthesizing, process has three steps: (1) making catalyst for CNTs growth, (2) depositing chemical vapor on catalyst and (3) purifying the CVD product

Fig 2.1: Four sides view of the CVD apparatus for growing CNTs in Lab of

Physics and Technology of Electronic Devices — Institute of Materials Science

Experimental set up

A catalyst sample was uniformly dispersed in a stainless steel boat laying on a rack (holder bar) The boat-rack was then inserted in the reactor at the center of the heating zone of the furnace Fig 2.2 shows the schematic illustration for

experimental set up for growing CNTs

wa a