High yield synthesis of multi-walled carbon nanotubed from CaCO3 supported iron (III) nitrate catalyst

Two years later, single-walled carbon nanotubes SWCNTs were synthesized by adding metal particles to the carbon electrodes [9, 36].. Thus, the investigation of suitable technologies to s

VIETNAM NATIONAL UNIVERSITY HANOI

COLLEGE OF TECHNOLOGY

Nguyen Duc Dung

HIGH YIELD SYNTHESIS OF MULTI-WALLED

IRON (III) NITRATE CATALYST

MASTER THESIS

VIETNAM NATIONAL UNIVERSITY HANOI

COLLEGE OF TECHNOLOGY

Nguyen Duc Dung

HIGH YIELD SYNTHESIS OF MULTI-WALLED

IRON (III) NITRATE CATALYST

Speciality: Nano Materials and Devices

Content

2.2.2 Raman spectroscopy of carbon nanotubes 38

Abbreviations

STEM Scanning Transmission Electron Microscope

Preface and target of the work

Carbon nanotubes were identified for the first time in 1991 by Sumio Iijima at the NEC Research Laboratory By using high resolution transmission electron microscope (HRTEM) he clearly observed the tiny tubes called multi-walled carbon nanotubes (MWCNTs) in the soot made from by-product obtained in the synthesis

of fullerenes The MWCNTs comprise carbon atoms arranged in a graphitic structure rolled up to form concentric cylinders [38] Two years later, single-walled carbon nanotubes (SWCNTs) 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) lead to aspect ratios so large that the carbon nanotubes possibly reach to ideal one-dimensional (1D) systems Depending on the chirality of their atomic structure, 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-

100 times higher than strongest steel [77] In addition to exceptional mechanical properties, they also possess superior thermal properties: thermally stable up to

above properties make carbon nanotubes (CNTs) the object of widespread studies in both basic science and technology They 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 section 1.3) for synthesizing carbon nanotubes having different performance from diverse material sources The arc discharge method relates to connecting two graphite rods to a power supply, placing them millimeters apart, and vaporizing carbon by a hot plasma Its product can be SWCNTs and MWCNTs with few structural defects Tubes tend to be short with random sizes and directions This method can produce large 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 primarily 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 spot area Emerging as the best method for industrial production of CNTs is chemical vapor deposition (CVD) Carbon feedstocks are hydrocarbons in gaseous and liquid

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 synthesize large-scale production

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

The most common and optimal method for large-scale production of CNTs is catalytic chemical vapor deposition (CCVD) (discussed in section 1.3.3) In the

technique are:

purification is a one-step procedure, simple and harmless to CNTs structure

market and low cost

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

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

carbon nanotubes We develop a simple method for making catalyst only by

that reduce stages in CNTs synthesis The addition of H2 gas in CVD process is believed not only to form Fe nanoparticles enhancing catalytic activity but also to

the factor contributing to the formation of Fe nanoparticles necessary to the CNTs growth is studied in this thesis Furthermore, Fe salt radicals are found significant to

dilute acid (HCl 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 1 shows an overview of carbon nanotubes material, the CNTs synthesizing methods and ability in industrial applications

Chapter 2 lists 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 studied The structural characteristics of the CNTs depend on the growth temperature are characterized The optimal chemical vapor deposition 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 (HRTEM) and Scanning Tunneling Microscope (STM) These techniques directly confirmed that 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 nanotubes (SWCNTs) and multiwalled 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 explained in terms of its 1D unit

The circumference of any carbon nanotube is expressed in terms of the chiral

2D graphene sheet (see Fig 1.2a) The construction in Fig 1.2a depends uniquely

on the pair of integers (n, m) which specify the chiral vector Fig 1.2a shows the chiral angle θ between the chiral vector and the “zigzag” direction (θ = 0) and the

unit vectors â1 and â2 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 to chiral angles of θ = 0 and 30o, and chiral nanotubes correspond to 0 < θ < 30o The intersection of the vector OB

(which is normal to C h) with the first lattice point determines the

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

Fig 1.2: (a) The chiral vector OA or C h = nâ 1 + mâ 2 is defined on the honeycomb

lattice of carbon atoms by unit vector â 1 and â 2 and the chiral angle θ with respect to the zigzag axis Along the zigzag axis θ = 0 o

Also shown are the

lattice vector OB = T of the 1D nanotube unit cell and the rotation angle ψ and the translation τ which constitute the basic symmetry operation R = ( ψ/τ) 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

(a)

(b)

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 θ = 30 o direction

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

nanotube] and (c) a general θ direction with 0 < θ < 30 o [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 h and the

cylinder joint is made along the two lines OB

and AB'

in Fig 1.2a The lines OBand AB'

are both perpendicular to the vector C h at each end of C h In the (n, m)

notation for C h = nâ1 + mâ2, the vectors (n, 0) or (0, m) denote zigzag nanotubes and the vectors (n, n) denote armchair nanotubes All other vectors (n, m)

d  a  m ma m  C  (1.1) where Ch is the length of C h, aC-C is the C – C bond length (1.42 Å) The chiral angle θ is given by

1  

tan 3 / 2n m n

      (1.2)

be between 0 < θ < 30o, then by symmetry θ = 0 for a zigzag nanotube Both armchair and zigzag nanotubes have a mirror plane and thus are considered as achiral Differences in the nanotube diameter dt and chiral angle θ give rise to

differences in the properties of various carbon nanotubes The symmetry vector R =

(ψ/τ) of the symmetry group for the nanotubes is indicated in Fig 1.2a, where both the translation unit or pitch τ and the rotation angle ψ are shown The number of hexagons, N, per unit cell of a chiral nanotube, specified by integers (n, m) is given

by

N = 2(m2 + mn + n2)/dR (1.3)

and d is defined as the largest common divisor of (n, m) Each hexagon in the honeycomb lattice (Fig 1.2a) contains two carbon atoms The unit 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 smaller Table 1.1 [21] provides a summary of relation useful for describing the structure of single wall nanotubes Fig 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 to close the end of an (n, m) nanotube, such that the fullerene cap satisfies the isolated pentagon rule

Table 1.1 Structural parameters for carbon nanotubes

C Ca

a

aa

x, y coordinate

1π21a

1π2

2

ab

x, y coordinate

1

12

2

m nm n

m n a

C

a C h

.cos

2 2 1

1.t t tt

Rd

mn

Rd

nm

)mnmn

(N

2 2

a) In this table n, m, t 1 , t 2 , p, q are integers and d, d R , N and M are integer functions of these integers

b)

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

1.3 Syntheses of carbon nanotubes

1.3.1 Arc discharge

The arc 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 arc-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

Fig 1.4: An arc discharge apparatus produced the first CNTs [78]

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

deposition chamber carries nanotubes onto a metal collector

Fig 1.5: Laser ablation system for growing CNTs [87]

In the early report [30], the laser beam scanned across the target surface under computer control to maintain the smooth, uniform face for vaporization The target was supported by graphite poles in a 1 inch quartz tube evacuated to 10 mTorr and then filled with 500 Torr argon flowing at 50 sccm The flow tube was mounted in a

produced by the laser vaporization was swept by the flowing Ar gas from the high temperature zone, and deposit on a water-cooled copper collector positioned downstream, just outside the furnace

MWCNTs are produced if the target is made of pure graphite [33] but in case

of the target composed of graphite and metal [76] SWCNTs 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 normally heated substrate surface The solid material is obtained as a coating, a powder, or as single crystals By varying the experimental conditions: substrate material, substrate temperature, composition of the reaction gas mixture, total pressure gas flows, etc., materials with different properties can be grown

In the CVD synthesis of CNTs different hydrocarbons such as acetylene (C2H2), methane (CH4), benzene (C6H6), etc and also carbon monoxide (CO) are

and 1200°C There are some other carbon allotropes except CNTs in a CVD product such as fibers [31, 53], planar nano-graphenes [63], amorphous, etc depending on the CVD conditions This is why technological research on synthesizing CNTs is necessary beside basic research

It can be classified CVD methods for synthesis of CNTs according to techniques making catalyst as described below:

CVD method on flat substrates

For the CVD synthesis of CNTs on flat substrates, a quartz tube is often

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, N2) for a certain time 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

and a temperature of 900 °C [43]

Fig 1.6: Schematic diagram of PECVD system

CVD method on catalytic substrates

Catalytic substrates are usually 3d metallic substrates such as Ni containing alloy, stainless steel grid and plate, etc Metallic 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 temperatures and then by flow of hydrogen for reduction of the metal oxide [81] The 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 nanoparticle structures The reduction by hydrogen is necessary for forming metal nanoparticles enhancing catalytic activity The two treatments also create granular structures increasing the catalytic surface area Most of CVD products in cases using bulk metallic substrates were MWCNTs [32, 49, 71, 81]

Floating catalytic CVD

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

in situ by injecting volatile organometallic molecules, such as metallocenes or iron

pentacarbonyl, into a reactor with a carbon feedstock [2, 13] The schematic diagrams of two conventional apparatuses for floating catalyst method are shown in Fig 1.7 [74] The carbon feedstock can be in liquid or gaseous phase The furnace

precursor (ferrocen) vapor is carried into the reactor by argon flux that includes a hydrocarbon flow (Fig 1.7b) 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]

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

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

(b)

(a)

Fig 1.8: A simple schematic CCVD system: (A) the furnace, (B) a quartz 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

in the CVD method while not in the arc discharge and laser ablation method However, for the growth of SWCNT a catalyst is needed for all three methods mentioned 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 atoms in the metal nanoparticle [22] The precipitation of carbon from the saturated metal particle leads to formation of tubular carbon solids in sp2 structure

One theory hypothesizes that metal catalyst particles are floating 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

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:

1.5.1 Oxidation

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

in tip and wall 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 Firstly, 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 in suspended form When

no effect on the CNTs and other carbon particles [10] If a treatment in HCl is used, the acid has also a little effect on the SWNTs and other carbon particles [14, 15, 55]

1.5.3 Micro filtration

Micro filtration is based on size or particle separation CNTs and a small amount of carbon nanoparticles are trapped in a filter The other nanoparticles (catalyst metal, fullerenes and carbon nanoparticles) are passing through the filter [5, 14, 55] Figure 1.10 shows a schematic diagram of a micro filtration cell

A special form of filtration is cross flow filtration In cross flow filtration the membrane is a hollow fiber The 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 major fraction of the fast flowing solution that does not permeate out 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 membrane surface preventing the build-up of a filter cake [10, 28]

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 binding calculation for the л-electrons of carbon atoms The two-dimensional graphite electronic structure is considered as the origin of that of single-wall carbon

graphene layer is [65]:

2 2

where t is the nearest-neighbor tight binding overlap energy

(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

For (n, n) armchair nanotubes the periodic boundary condition permit only wave vectors in the circumferential direction which satisfy the relation qλ = πdt

,

3 x q 2

n k a q, (q = 1, 2, …, 2n) (1.5) Substitution of discrete allowed values for kx, q given by (1.5) into Eq (1.4)

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

y q

nk a q (q = 1,…, 2n) (1.7) Substituting equation (1.7) into equation (1.4) yields the energy dispersion relation Eqz (k) for zigzag nanotubes [65]:

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

= ±2π/3T 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 nanotubes are metals if n-m is a 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 E = 0 is zero for the semiconducting (10, 0) nanotube and is non-zero for the metallic (9, 0) nanotube

Fig 1.12: Electronic 1D density of states per unit cell for a (9, 0) and (10, 0) zigzag

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

d



 , (1.9) and is independent of the chiral angle of the semiconducting nanotube

1.6.2 Mechanical properties

The mechanical properties of carbon nanotubes are closely related to the

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 to 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 attention has been on SWCNTs because the intertube interactions are weak in MWCNTs and thus less important in estimating axial mechanical properties In Ref [51] Lu presented Young‟s modulus values for MWCNTs as well as SWCNTs and

obtains values from 0.97 TPa to 1.11 TPa with the value increasing slightly with the number of layers

The first Young‟s modulus measurement [79] related thermal vibration amplitudes of MWCNTs to their Young‟s modulus and obtained an average value

of 1.8 TPa with a large spread After that, with AFM techniques, values such as 1.28 ± 0.59 TPa for arc-discharge produced tubes have been obtained [86].Measuring SWCNTs is more complicated than measuring MWCNTs due to their small diameters and the tendency to bundle Krishnan et al reported in Ref [42] a measurement of individual SWCNTs using the thermal vibration method of Ref [79] and they obtain an average value of 1.25 TPa Although the current measurements suffer from inaccuracies due to vibration amplitude measurement and assumptions made on AFM tip characteristics, the current agreement is that both SWCNTs and MWCNTs have a Young‟s modulus value around 1 TPa

Theoretical tensile strength of nanotubes is high [7, 83, 84, 88] The measurements fall short of the theoretical predictions that may result from limitations in the theoretical description or from the presence of imperfections in the structure Measurements of tensile strength, that is, the strength corresponding to failure, suffer from difficulties and inaccuracies related to the small size of the measured object Tensile strength has been measured for MWCNTs in Ref [89] where the contact is only to the outmost layer of the multi-walled tube A sword and sheath type of failure of this layer was reported The 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 SWCNTs 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 high axial Young‟s modulus and tensile strength combined with the low weight and fiber-like form make nanotubes enticing candidates for composite

reinforcement 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

1.7.1 Energy storage

Electrochemical supercapacitors

At any electrode in contact with an electrolyte, there exists a dipole layer, the so-called electrochemical double layer (ECDL) Typically ~0.4 nm thick, the ECDL 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 C = εA/d, where ε is the dielectric constant, A is the electrode surface area, and d is the ECDL thickness This capacitance can be huge for the case of CNTs because theoretically SWCNT have an outer specific surface

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

Hydrogen storage

The advantage of hydrogen used for energy source is that 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 that the carbon nanotubes can store liquid and gas in the inner cores through a capillary effect A Temperature-Programmed Desorption (TPD) study on SWCNT-containing material (0.1–0.2wt%

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 H2 inside the

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

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 110mAh/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 arc-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

temperature

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

directly grown on the metal substrate [27]

1.7.2 Composite materials

The high aspect ratio (length to radius ratio) and high conductivity of CNTs makes them excellent for conducting composites A mixture of conducting 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 that conduction occurs at much lower loading

Existing conducting composites are made of carbon black in polymers This should require about 16% of carbon, but it has been reduced by clever processing Fig 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] These are extremely low loadings The first use of such composites was by Hyperion for electrostatically applying paint onto car components [6] Nanotube composites have a much better surface finish than the previous carbon black or carbon fiber composites

Another application of conducting composites is as a transparent conductor There is a huge market for transparent electronic conductors such as indium tin oxide (ITO) 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 nanotube fiber directly from a CVD reaction chamber, as done by Li et al [48] and shown schematically in Fig 1.15 and SEM image of a produced fiber The resulting fiber can then be post impregnated with epoxy to make a composite These fibers presently 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 many individual CNTs

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 CNTs

In 2002, researchers at Laboratory of Physics and Technology of Electronic Devices – Institute of Materials Science constructed the thermal CVD system as shown in Fig 2.1 This apparatus includes: i) a thermal furnace, a quartz reactor and

a thermocouple; ii) gas tanks (C2H2, H2, N2, NH3), gas-guiding tubes and mass controlling flowmeters

Thermal furnace and quartz reactor: The furnace was coiled from resistive

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

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

thermocouple and a temperature control system

Gas system: Gases (C2H2, H2, N2,…) are introduced to the tube through corresponding mass flowmeters and are mixed before going into 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

Making catalysts

various weight contents

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