Growth Mechanism and Structure of Carbon Nanotubesfaculty.kfupm.edu.sa/CHE/motazali/files/very important thesis for my Ph.d.pdf · Aus dem Departement für Physik Universität Freiburg

Aus dem Departement für Physik Universität Freiburg (Schweiz)

Growth Mechanism and Structure of Carbon Nanotubes

Inaugural-Dissertation

zur Erlangung der Würde eines Doctor rerum naturalium der Mathematisch-Naturwissenschaftlichen Fakultät

der Universität Freiburg in der Schweiz

vorgelegt von

Philippe Mauron

aus Ependes (FR)

Dissertation Nr. XXXX Hausdruckerei Universität Freiburg

2003

Von der Mathematisch-Naturwissenschaftlichen Fakultät der Universität Freiburg in der Schweiz angenommen, auf Antrag der Herren Prof. Dr. Peter Schurtenberger, Universität Freiburg (Präsident der Jury) Prof. Dr. Andreas Züttel, Universität Freiburg (Referent) Prof. Dr. Peter Oelhafen, Universität Basel (Koreferent) Prof. Dr. Louis Schlapbach, Universität Freiburg (Koreferent) Der Leiter der Dissertation Der Dekan Prof. Dr. Andreas Züttel Prof. Dr. Dionys Baeriswyl

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Abstract

With the revolutionary discovery of so-called fullerenes and carbon nanotubes different research fields in the domain of carbon experienced an enormous boom. Fullerenes are spherical molecules, the smallest kinds of which are composed of 60 carbon atoms that are arranged like the edges of the hexagons and pentagons on a football. Nanotubes can be described as cylindrical rolled up graphene sheets. Due to their special one-dimensional form they have interesting physical properties. They e.g. depending on the chirality, have a metallic or semiconducting electrical conductivity. Nanotubes have a large geometric aspect ratio and they are the first nanocavities. This and other prop-erties one would like to use in different applications e.g. as electrode material in super capacitors and hydrogen storage material for the energy storage or as field emitters in flat panel displays.

Until now nanotubes could only be synthesized in small quantities or with complicated methods. A promising method is the chemical vapor deposition (CVD) of nanotubes in which typically hydrocarbons are dissociated on metal-coated substrates at temperatures from 600 to 1200°C. The growth mechanism of nanotubes with this method and in general is not well known up to now.

In a first part of this thesis the synthesis of oriented nanotube films was studied. They were produced by the CVD method. Nanotube films with a thickness of 20 to 35 µm could be synthesized in an acetylene/nitrogen atmos-phere. Iron oxide clusters that were produced with a thermal decomposition of iron nitrate films were used for the synthesis. It turned out that the density of the nanotube film depends on the number of iron oxide clusters on the substrate, which in the reduced state serve as nucleation centres for the nano-tube growth. By rising the temperature and lowering the concentration of the solution the density of clusters can be increased. A perpendicular orientation in respect to the surface could be ascribed to a high density of nanotubes.

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In the second part of this thesis a fluidised-bed CVD reactor was built with the goal to produce large quantities of nanotubes. With this method a substrate powder with a high specific surface area (100-300 m2·g-1) is fluidised in the gas stream. Due to the high specific surface area and a large number of metal oxide clusters, which are formed by the thermal decomposition and the good convection in the gas stream, large quantities of nanotubes can be synthesised.

For the synthesis with iso-pentan and acetylene different synthesis parameters like the iron concentration in the magnesium substrate powder (mass ratio of Fe:MgO 1.25-15%), the temperature (450-850°C) and the reaction time (0.5-40 min.) were investigated. For temperatures of 700°C and an iron concentration of 15% the yield of nanotubes (rel. mass of the nano-tubes on the substrate) was 35%. By lowering the iron concentration the structure of the nanotubes could be changed to smaller diameters, which leads to a higher specific surface area (max. 1200 m2·g-1). Such nanotubes are an advantage in the application of super capacitors. As a function of the synthesis temperature the structure of the nanotubes also changed. For the synthesis with acetylene for low temperatures (500-650°C) we got multi walled nano-tubes, which have larger diameters (5-30 nm). For higher temperatures (750-850°C) the majority was bundles of single walled nanotubes with diameters in the range of 1-3 nm. In contrast to the synthesis with acetylene there is an activation time necessary for methane until the growth of nanotubes begins. It was shown that the activation time is dependant on the synthesis temperature and that it decreases with increasing temperature. It was therefore possible for the first time to synthesise single walled nanotubes at a low temperature of 600°C with methane.

The synthesised nanotubes were investigated by scanning- and transmis-sion electron microscopy, Raman spectroscopy, X-ray diffraction and thermal gravimetric analysis. In order to analyse the evolution of the reaction the gas at the outlet of the reactor was analysed with a mass spectrometer. The complex multistage synthesis could be described for the first time.

iii

Zusammenfassung

Mit der revolutionären Entdeckung von so genannten Fullerenen und Kohlen-stoff-Nanotubes erlebten verschiedene Forschungsgebiete, die Kohlenstoff unterschiedlicher Art untersuchten, einen riesigen Aufschwung. Fullerene sind sphärische Moleküle deren kleinste Sorte aus 60 Kohlenstoffatomen besteht, die so angeordnet sind, wie die Ecken der Hexagone und Pentagone auf einem Fussball. Nanotubes können theoretisch als zylindrisch aufgerollte Graphit-schicht beschrieben werden. Wegen ihrer speziellen eindimensionalen Form haben sie interessante physikalische Eigenschaften. Sie haben z.B. als Funktion ihrer Chiralität eine metallische oder halbleitende elektrische Leit-fähigkeit. Sie haben ein sehr grosses geometrisches Länge- zu Breite-Verhältnis und sie sind die ersten Nanokavitäten. Diese und weitere Eigenschaften möchte man in verschiedenen Anwendungen nutzen z.B. als Elektrodenmaterial in Superkondensatoren und Wasserstoffspeichern zur Energiespeicherung oder als Feldemitter in Flachbildschirmen.

Nanotubes konnten lange Zeit nur in sehr kleinen Mengen oder mit auf-wendigen Methoden hergestellt werden. Eine viel versprechende Methode ist die chemische Gasphasenabscheidung (CVD1), bei welcher typischerweise Kohlenwasserstoffe bei Temperaturen von 600 bis 1200°C auf metallbe-schichteten Substraten zersetzt werden, wobei sich Nanotubes bilden. Der Pro-zess des Wachstums von Nanotubes war bis anhin mit dieser Methode und auch im Allgemeinen noch nicht genau im Detail bekannt.

In einem ersten Teil dieser Doktorarbeit wurde das Wachstum von orien-tierten Nanotubes untersucht, welche mit der CVD Methode synthetisiert wurden. In einer Acetylen/Stickstoff Atmosphäre konnten Nanotubefilme hergestellt werden, die eine Dicke von 20 bis 35 µm haben. Für die Synthese

1 Chemical Vapor Deposition

iv

wurden Eisenoxidcluster verwendet, die durch thermische Zersetzung eines Eisennitratfilms gebildet wurden. Es stellte sich heraus, dass die Dichte des Nanotubefilms von der Anzahl der Eisenoxidcluster auf dem Substrat abhängt, die im reduzierten Zustand Nukleationszentren für das Wachstum von Nano-tubes dienen. Die Dichte und Grösse der Cluster kann durch die Konzentration der Eisennitratlösung und die Synthesetemperatur variiert werden. Durch Erhöhung der Temperatur und Verringerung der Konzentration der Lösung kann die Clusterdichte vergrössert werden. Die senkrechte Ausrichtung der Nanotubes bezüglich der Oberfläche kann auf eine hohe Nanotubedichte zurückgeführt werden.

Im zweiten Teil dieser Arbeit wurde eine Wirbelschicht CVD-Anlage gebaut mit dem Ziel grosse Mengen von Nanotubes herzustellen zu können. In dieser Methode wird ein Substatpulver mit einer hohen spezifischen Oberfläche (100-300 m2·g-1) im Gasstrom aufgewirbelt. Wegen der hohen Oberfläche, der grossen Anzahl von Metalloxidclustern, die bei der thermischen Zersetzung von Eisennitrat gebildet werden und der guten Durchmischung im reaktiven Gasstrom, können grosse Mengen von Nano-tubes hergestellt werden.

Für die CVD-Synthese mit Iso-Pentan und Acetylen wurden verschiedene Syntheseparameter wie die Eisenkonzentration auf dem Magnesiumoxidpulver (Massenverhältnis Fe:MgO 1.25-15%), die Temperatur (450-850°C) und Synthesezeit (0.5-40 Min.) untersucht. Für Temperaturen von 700°C und einer Eisenkonzentration von 15% konnte eine Nanotubeausbeute (rel. Gewichtsan-teil von Nanotubes auf dem Substrat) von 35% erreicht werden. Durch Verringerung der Eisenkonzentration konnte die Struktur der Nanotubes in Richtung kleinerer Durchmesser verändert werden, was eine höhere spezifi-sche Oberfläche zur Folge hat (max. 1200 m2·g-1), die zum Beispiel in der Anwendung von Superkondensatoren von Vorteil ist. Als Funktion der Synthesetemperatur änderte sich die Struktur der Nanotubes auch. Für die Synthese mit Acetylen erhielten wir für niedrige Temperaturen (500-650°C) mehrwandige Nanotubes, welche einen grösseren Durchmesser haben (5-30 nm). Für höhere Temperaturen (750-850°C) hingegen mehrheitlich Bündel von einwandigen Nanotubes mit Durchmessern im Bereich von 1-3 nm. Im Gegensatz zur Synthese mit Acetylen ist mit Methan eine Aktivierungszeit

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erforderlich, bevor das Wachstum von Nanotubes einsetzt. Es zeigte sich, dass diese Aktivierungszeit abhängig ist von der Synthesetemperatur und exponen-tiell mit steigender Temperatur abnimmt. Es war daher möglich mit langen Synthesezeiten zum ersten Mal einwandige Nanotubes auch bei tiefen Tem-peraturen von 600°C mit Methan herzustellen.

Die synthetisierten Nanotubes wurden mit dem Raster- und Transmis-sionselektronenmikroskop, mit Ramanspektroskopie, Röntgendiffraktometrie und gravimetrischer Thermoanalyse untersucht. Um den Reaktionsverlauf während der Synthese zu analysieren, wurde am Ausgang des Reaktors das Gas mit einem Massenspektrometer untersucht. Die mehrstufige komplexe Synthese konnte so zum erten Mal beschrieben werden.

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Contents

CHAPTER 1 INTRODUCTION ........................................................ 3 1.1 CARBON ALLOTROPES .................................................................... 4 1.2 SYNTHESIS OF CARBON NANOTUBES ............................................. 13 1.3 GROWTH MECHANISM OF CARBON NANOTUBES............................. 20 1.4 PHYSICAL PROPERTIES OF CARBON NANOTUBES............................ 22 1.5 APPLICATIONS OF CARBON NANOTUBES........................................ 27 1.6 REFERENCES ................................................................................ 36

CHAPTER 2 INVESTIGATION METHODS................................. 43 2.1 ELECTRON MICROSCOPY............................................................... 43 2.2 RAMAN SPECTROSCOPY................................................................ 48 2.3 GAS ADSORPTION FOR THE DETERMINATION OF SURFACE AREA .... 55 2.4 X-RAY DIFFRACTION .................................................................... 60 2.5 REFERENCES ................................................................................ 63

CHAPTER 3 SYNTHESIS OF ORIENTED NANOTUBE FILMS BY CHEMICAL VAPOR DEPOSITION.......................................... 65

PH. MAURON, CH. EMMENEGGER, A. ZÜTTEL, CH. NÜTZENADEL, P. SUDAN, L. SCHLAPBACH

CARBON 40 (2002) 1339–1344

CHAPTER 4 FLUIDISED-BED CVD SYNTHESIS OF CARBON NANOTUBES ON FE2O3/MGO......................................................... 79

PH. MAURON, CH. EMMENEGGER, P. SUDAN, P. WENGER, S. RENTSCH, A. ZÜTTEL

Diamond and Related Materials 12 (2003) 781–786

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CHAPTER 5 CARBON NANOTUBES SYNTHESISED BY FLUIDISED-BED CVD ...................................................................... 93

PH. MAURON, CH. EMMENEGGER, P. SUDAN, P. WENGER, S. RENTSCH, A. ZÜTTEL

PROCESSING AND FABRICATION OF ADVANCED MATERIALS XI, EDITED BY T.S. SRIVATSAN AND R.A. VARIN, ASM INTERNATIONAL

(COLUMBUS, OHIO USA) 2003, P. 93–104

CHAPTER 6 CARBON NANOTUBES SYNTHESISED WITH METHANE ON FE/MGO AND CONI/MGO SUBSTRATES IN A FLUIDISED BED..................................................................... 107

PH. MAURON, P. SUDAN, P. WENGER, R. GREMAUD, A. ZÜTTEL SUBMITTED TO CARBON

DANKSAGUNG ................................................................................ 121

CURRICULUM VITAE ................................................................... 123

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Chapter 1

Introduction

In the recent years several breakthroughs stimulated the research in the field of carbon. The most influential was the identification of the structure of the fullerenes in 1985 by Kroto and his co-workers [1]. A further breakthrough was the discovery of multi walled carbon nanotubes by Iijima [2] in 1991 and single walled carbon nanotubes discovered independently by Iijima [3] and Bethune et al. [4] in 1993 (Fig. 1). Prior to the discovery of nanotubes the relevance of the investigations made on carbon fibres with diameters bigger than 7 nm was not realised until the connection between fullerenes and nanotubes was made. At least two articles raise the question about pre Iijima nanotubes. In an article reporting about the decade of discovery of nanotubes [6] it is e.g. stated that nanotubes could have been unknowingly produced in the late nineteenth century by chemists experimenting on methane [7]. In 1960 Bacon [8] produced nanoscale scrolls of graphite. The fact, that Iijima who was using a procedure that generates a mixture of scrolls and tubes [9-11], suggests that Bacon may also have done this. In 1979 Wiles et al. [12] found

Fig. 1: Number of articles found in “SciFinder Scholar” [5] with the keyword nanotube

Chapter 1 Introduction

4

“mats of small fibres” on one electrode when sparks were passed between two graphite electrodes.

In another article from Gibson [13], Davis is proposed as the one who saw the first nanotube in 1953 [14]. He described thread-like structures obtained from the reaction of CO and Fe3O4 at 450°C on surfaces of firebricks exhibiting “iron spots”.

The scientists [15] who investigated the growth of vapor grown carbon fibres may also have produced nanotubes because they used the same technique with which nanotubes are produced also today.

Carbon allotropes The chemical element carbon can combine with itself and other elements in three types of hybridisations. This gives the rich diversity of structural forms of solid carbon and is the basis of organic chemistry and life.

Carbon has the electronic ground state configuration 1s22s22p2 and can form sp3, sp2 and sp1 hybrid bonds. In the sp3 hybridisation four equivalent 2sp3 hybrid orbitals are tetrahedrally oriented around the atom (Fig. 2) and can form four equivalent σ bonds by an overlap with orbitals of other atoms. An example is the ethane molecule (C2H6) where a Csp3-Csp3 σ bond (or C-C) is formed between two carbon atoms by the overlap of sp3 orbitals, and three Csp3-H1s σ bonds are formed on each C atom.

In the sp2 hybridisation three equivalent 2sp2 orbitals are formed and one unhybridised 2p orbital is left. They are coplanar and oriented at 120° to each other and form σ bonds by an overlap with orbitals of neighbouring atoms as e.g. in ethane (C2H4). The remaining p orbitals on each C atom form a π bond by the overlap with the π orbital from the neighbouring C atom. Such bonds formed between two C atoms are represented as Csp2=Csp2 (or C=C).

Fig. 2: The different hybridisations of carbon a) sp1, b) sp2, c) sp3

Chapter 1

5

In the sp1 hybridisation two linear 2sp1 orbitals are formed and two 2p orbitals are left. Linear σ bonds are formed by the overlap of the 2sp1 hydride orbitals of neighbouring atoms as for example in the ethyne molecule (acetylene). Two π bonds are formed with the overlapping unhybridised π orbitals of the two C atoms. These bonds are represented as Csp≡Csp (or C≡C).

In the aromatic carbon-carbon bond exemplified by the aromatic molecule benzene (C6H6) the carbon atoms are bonded with sp2 σ bonds in a regular hexagon. The ground state π orbitals are all bonding orbitals and are fully occupied; there is a large delocalisation energy that contributes to the stability of the molecule. The aromatic carbon-carbon bond is denoted as Car Car. 1.1.1 Diamond Diamond exists in a cubic and hexagonal form (Lonsdaleite). In the most frequent cubic form each carbon atom is linked with four other carbon atoms by four sp3 σ bonds in a tetrahedral array with a C–C bond length of 1.544 Å [16]. This is nearly 10% larger than in graphite. However the atomic density (1.77·1023 cm-3) is 56% higher than in graphite. The crystal structure is zinc blend type (FCC) with a diatomic basis. The second carbon atom is at position (¼, ¼, ¼) in the unit cell and the lattice constant is a0 = 3.567 Å (Fig. 3 left).

The physical properties of diamond are given by its structure. Diamond is a wide-gap semiconductor (5.47 eV), the hardest material in nature (Mohs

Fig. 3: Diamond in the cubic form (left) and the hexagonal Lonsdaleite (right).


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hardness 10) and has the highest atomic density. Diamond, as also graphite (in-plane) have the highest thermal conductivity (~25 W·cm-1·K-1) and the highest melting point (4500 K).

The hexagonal diamond (Lonsdaleite) has a wurtzite crystal structure (Fig. 3 right) and a C–C bond length of 1.52 Å [17]. The gravimetric density of both types of diamond is 3.52 g·cm-3. 1.1.2 Graphite In graphite the atoms are arranged in layers of a honeycomb network in which the carbon atoms are bonded with sp2 σ bonds and a delocalised π bond. In the most common hexagonal crystal form of graphite the layers are stacked in an ABAB… sequence (called Bernal stacking) (Fig. 4). The in-plane nearest neighbour distance aC-C is 1.421 Å [16] and the lattice constant is a0 = 2.461 Å. The c-axis lattice constant is c0 = 6.708 Å and the interplanar distance c0/2.

A minor component of well-crystallised graphite is the rhombohedral form of graphite in which the graphene (single layer of crystalline graphite) layers are stacked in the ABCABC… sequence. The lattice constant is also a0 = 2.456 Å and c0 = 3· (3.438) = 10.044 Å. The Bernal AB stacking of graphite is more stable than the ABC stacking. The density of both forms of graphite is 2.26 g·cm-3 [17].

The weak interlayer bonding of graphite originates from the small overlap of the π-orbitals between atoms of adjacent layers and not to Van der Waals bonding [18].

Fig. 4: hexagonal graphite (ABAB stacking) with unit cell

Chapter 1

7

1.1.3 Buckminsterfullerenes1 Experimental and theoretical work has shown that the most stable form of carbon clusters form linear chains [19] for clusters of up to about 10 atoms. For clusters that have 10 to 30 carbon atoms the ring is the most stable form [20]. Carbon clusters between 30 and 40 carbon atoms are unlikely and clusters above 40 atoms form cage structures. Especially stable structures are the C60 (Fig. 5), whose structure was identified the first time by Kroto et al. in 1985 [21]. The carbon atoms are located at the 60 vertices of a truncated icosahedron that has 90 edges and 32 faces of which 12 are pentagons and 20 hexagons, consistent with Euler’s theorem for polyhedra:

2f v e+ = + (1)

where f, v and e are the number of faces, vertices, and edges of the polyhedra. The average nearest-neighbour C–C distance is with aC-C = 1.44 Å almost

equal to that in graphite. Each carbon atom is trigonally bonded to three other carbon atoms in an sp2-derived bonding configuration. The curvature of the trigonal bonds in C60 leads to some admixture of sp3 bonding, characteristic for tetrahedrally bonded diamond, but absent in graphite [23].

Further stable fullerenes are C70, C78, C80…

1 in honor of Buckminster Fuller.

Fig. 5: C60 Buckminsterfullerene


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1.1.4 Carbon nanotubes A single walled carbon nanotube (SWNT) can be described as a rolled up graphene sheet that is closed at each end with half of a fullerene. A nanotube is usually characterised by its diameter dt and the chiral angle θ (0 ≤ |θ| ≤ 30°) (Fig. 6). The chiral vector Ch is defined with the two integers (n, m) and the basis vectors of the graphene sheet [22]:

1 2 ( , )h n m n m= + ≡C a a (2)

The chiral angle θ is the angle between the chiral vector Ch and the so-called “zigzag” direction (n, 0). The integers (n, m) determine dt and θ:

2 21td n m nm a

π= + + ,

nmmn

m

++=

222

3sinθ (3)

The graphene sheet is then rolled up in the direction of the chiral vector Ch and one gets a (n, m) nanotube. Special classes of nanotubes are the so-called “armchair” nanotubes (n, n) and the “zigzag” nanotubes (n, 0). All the others are “chiral” nanotubes (n, m) with n ≠ m and m ≠ 0 (Fig. 7). The rectangle that is formed with the below defined translation vector T and the chiral vector Ch define the unit cell of a nanotube that can be translated in only one direction.

Fig. 6: Definition of the unit cell of a carbon nanotube [23].

Chapter 1

9

1 1 2 2t t= +T a a , Rd

nmt +=

21 ,

Rdmnt +

−=2

2 (4)

where dR is the biggest common divider of (2n + m, 2m + n). With a good experimental agreement the thinnest SWNT that can be

closed with half of a C60 has a diameter dt of 6.78 Å. Nevertheless thinner tubes with a diameter of 4 Å were reported [24-26]. In Fig. 8 different chiral vectors Ch are shown with the corresponding number of different Fullerencaps. Whether a nanotube is a conductor or a semiconductor is determined by its chirality. If (n-m)/3 is an integer then the nanotube is a metal and otherwise a semiconductor. Further geometric parameters of SWNT are given in table 1.

Fig. 7: a) (5, 5) armchair, b) (9, 0) zig-zag and c) (10, 5) chiral nanotube [23].

Fig. 8: Different chiral vectors and the number of possible fullerencaps [23].


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SWNT are often found in bundles that are formed by a triangular arrangement of individual SWNT. The nanotubes are held together by week Van der Waals forces.

Multi walled carbon nanotubes (MWNT) are nanotubes with more than

one graphene cylinder nested one into another (Fig. 9). A AB or ABC stacking is not possible in MWNT because of the radius of curvature. In this sense MWNT are turbostratic. Nevertheless there is some correlation between the layers so that they are not completely turbostratic. According to theoretical calculations the distance between two layers is d = 3.39 Å, slightly bigger than in graphite. Based on TEM lattice fringe images, the interlayer separation of d = 3.4 Å is commonly reported for MWNT [27]. This value is close to the value of turbostratic graphite (d = 3.44 Å [16]).

1.1.5 HOPG Highly oriented pyrolytic graphite (HOPG) is prepared by the pyrolysis of hydrocarbons at temperatures above 2000°C [16] and heat treated to higher temperatures. Stress annealed (above 3300°C) HOPG exhibits electronic, transport, thermal and mechanical properties close to those of single-crystal graphite with a very high degree of c-axis alignment. In stress annealed HOPG the crystalline order extends to about 1µm within the basal plane and 0.1 µm along the c-direction with a c-axis alignment much better than 1°.

Fig. 9: A MWNT with three walls

Chapter 1

11

Symbol Name Formula Value

aC-C carbon-carbon distance 1.421 Å (graphite)

a length of unit vector 3 C Ca − 2.46 Å

a1, a2 unit vectors 3 1 3 1, , ,

2 2 2 2a a

−

in (x, y) coordinates

b1, b2 reciprocal lattice vectors 1 2 1 2, 1 , , 13 3a a

π π −

in (x, y) coordinates

Ch chiral vector 1 2 ( , )h n m n m= + ≡C a a n, m: integers

L circumference of nanotube 2 2hL a n m nm= = + +C 0 ≤ |m| ≤ n

dt diameter of nanotube 2 2

tL n m nmd aπ π

+ += =

θ chiral angle

2 2

2 2

3sin2

2cos2

3tan2

m

n m nmn m

n m nm

mn m

θ

θ

θ

=+ +

+=

+ +

=+

0 ≤ |θ| ≤ 30°

d the highest common divisor of (n, m)

dR the highest common divisor of (2n+m, 2m+n)

if not a multiple of 3d

3 if a multipleof 3dR

d n md

d n m

− = −

T translation vector of 1D unit cell

1 1 2 2

1

2

2

2R

R

t tm ntdn mtd

= ++

=

+= −

T a a

t1, t2: integers

T length of T 3R

LTd

=

N number of hexagons per 1D unit cell ( )2 22

R

n m nmN

d

+ += 2N ≡ nC/unit cell

R symmetry vector‡ 1 2 ( , )

, 0 / , 0 /

p q p q

d mp nq p n d q m d

= + ≡

= − ≤ ≤ ≤ ≤

R a a p, q: integers†

M number of 2π revolutions ( ) ( )2 2 / RM n m p m n q d = + + + M: integer

R Basic symmetry operation‡ hN M d= +R C T

ψ rotation operation R ψτ

= ψ: radians

τ translation operation dN

=Tτ τ, χ: length

† (p,q) are uniquely determined by d = mp – nq, subject to conditions stated in table, except for zigzag tubes which Ch = (n, 0), and we define p = 1, q = –1, which gives M = 1.

‡ R and R refer to the same symmetry operation.

Table 1: Different parameters of carbon nanotubes [23].


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1.1.6 Glassy carbon Another common carbon material is glass carbon (GC), which is produced by degradation of polymers at temperatures in the order of 900 to 1000°C [16]. GC is a family of disordered carbon with graphene ribbons in which the carbon atoms are ordered in the honeycomb in-plane structure of graphene layers but with a poor 3D ordering. The ribbons form a turbostratic structure. 1.1.7 Carbon Fibres Carbon fibres can be classified into two classes. The first are polymer based carbon fibres consisting primarily of polyacrylonitrile fibres and pitch fibres. The second class are vapor grown carbon fibres (VGCF). In carbon nanotube research VGCF are of particular interest because their morphology and microstructure are closely related to that of carbon nanotubes [16]. Polymer based fibres are synthesised by stabilisation of the polymeric precursor fibre at 200 to 350°C and a heat treatment at ~1000°C to carbonise the filaments by removal of impurities such as H, O, N, … A further heat treatment at temperatures from 1300 to 3000°C is used to change the mechanical properties. VGCF are synthesised by the decomposition of hydrocarbons at temperatures between 700 to 2500°C preferably in a hydrogen atmosphere.

1.1.8 Amorphous carbon Amorphous carbon (a-C) is a highly disordered network of carbon atoms with predominantly sp2 bonds, with only approximately 10 % sp3 bonds and no sp1 bonds [16]. a-C has no long-range order but only some short-range order (~10Å) that depends on the carbon bonding type (sp2/sp3) and the hydrogen content.

Chapter 1

13

1.2 Synthesis of carbon nanotubes 1.2.1 Arc-discharge technique This technique provides the high temperature needed for the evaporation of carbon atoms into a plasma (>3000°C). With the carbon arc method both multi- and single walled carbon nanotubes can be produced [29]. Also other carbonaceous products as e.g. carbon whiskers [30], soot and fullerenes were synthesised with this method [31]. The type of product synthesised is deter-mined by the pressure and type of gas used. A view of an apparatus is shown in Fig. 10. It consists of two carbon electrodes, the thicker cathode on which the deposit forms is separated from the thinner anode by ~1 mm. During the deposition the anode is consumed. A voltage of 20-25 V is applied between the electrodes and the current is between 50-120 A. The optimal pressure for producing nanotubes is around 500 torr of He (fullerenes are efficiently pro-duced at a pressure below 100 torr). For the synthesis of MWNT no catalyst is necessary. 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 [32-34]. To produce isolated SWNT catalysts such as Co, Ni, Fe, Y and Gd are used. Mixed catalysts such as Fe/Ni, Co/Ni and Co/Pt are used to grow bundles of SWNT. Gram quantities of bundles of SWNT can be produce with the carbon arc method [29]. A hole is drilled in

Fig. 10: Illustration of an arc-discharge apparatus for the production of fullerenes andnanotubes [28].


14

the anode and filled with the metal powder [4]. The SWNT are found in a web-like structure in the chamber and not on the cathode.

Nanotubes and a number of impurities can be prepared in a high yield with this method. The best ratio of nanotubes to nanoparticles is in the order of 2:1 [35] and the nanotubes have to be purified after the synthesis [36]. The quality of the nanotubes is very good and they have a high graphitisation degree. 1.2.2 Laser ablation In the laser ablation method a laser vaporises a graphite target which is placed in an electrical furnace (Fig. 11) heated at 1200°C. Flowing argon gas (~500 torr) sweeps the nanotubes from the high temperature zone to the water-cooled copper collector outside the furnace [37, 38]. If a pure graphite target is used MWNT are produced [37] like in the arc-discharge process but if the target is composed e.g. of 1.2 atom% Co/Ni with equal amounts of Co and Ni added to the graphite then SWNT are synthesised [38]. High yields with >70-90% conversion of graphite to SWNT were reported in the condensing vapour of the heated flow tube.

The produced material consists of ropes of SWNT with a diameter between 10 and 20 nm and up to 100 µm or more in length. The average nanotube diameter and the diameter distribution can be adapted by varying the synthesis temperature and the composition of the catalyst [74]. The diameters of the SWNT have strongly peaked distributions.

Fig. 11: Illustration of a laser ablation apparatus. According to [39].

Chapter 1

15

1.2.3 CVD-method In the CVD-method different hydrocarbons as e.g. benzene (C6H6), pentane (C5H12) acetylene (C2H2), methane (CH4) etc. and also carbon monoxide are decomposed over different metals (Fe, Co, Ni…) at temperatures between 500 and 1200°C. This method was used for a long time for the synthesis of carbon fibres [15] but there were no indications that it could also be used for the synthesis of carbon nanotubes until Yacamán et al. [40] reported this method the first time for the production of nanotubes.

The difference between nanotubes and vapor grown carbon fibres is not always evident. Carbon fibres are often much thicker (100 nm to hundreds of micrometers), have a lower graphitisation degree and sometimes also have other structures than the “tubular” structure of nanotubes e.g. the so-called “herringbone” or “platelet” structure [41, 42]. As the fibre diameter decreases, the characteristics of vapor grown carbon fibres bear a close connection to carbon nanotubes [16].

Different modifications of the CVD-method exist and they are explained more in detail below.

1.2.3.1 CVD-method on flat substrates

For the CVD synthesis of nanotubes on flat substrates a horizontal quartz tube is often used which is located in an electrical furnace (Fig. 12). The substrate (e.g. silicon, aluminium, graphite…) is coated with a metal catalyst. The substrate is either coated with a metal oxalate or nitrate solution [40, 44], a metal film is evaporated [45] or preformed metal clusters are applied onto the substrate [46] etc. The substrate and the metal is then exposed to a atmosphere

Fig. 12: Illustration of a horizontal CVD furnace used to grow carbon nanotubes [43].


16

containing e.g. 10 % acetylene and 90% nitrogen for ~15 min. up to several hours. If the metal coating is correctly applied to the substrate oriented nano-tube films are obtained (Fig. 13). For a more detailed discussion see Chapter 3 and refs. [47, 48].

1.2.3.2 Fluidised-bed CVD-method

The goal of the fluidised-bed CVD-method [50-55] is the synthesis of nanotubes in a large-scale; therefore nanotubes are grown on a fluidised high surface area precursor powder (~100-300 m2g-1).

The build-up and a photo of the system are shown in Fig. 14. The system is basically composed of a quartz tube with a filter in the middle on which the substrate is fluidised by the gas flow and an electrical furnace. Two different reaction gases can be mixed and on the outlet a probe is connected to a mass spectrometer (MS) in order to control the evolution of the reaction.

Before the synthesis is started the precursor powder is impregnated in a metal nitrate solution (Fig. 15). Large quantities of nanotubes (~1g) can be deposited because on the high surface area precursor a lot of metal clusters that are nucleation centres for the growth of nanotubes are exposed to the reaction gas. Furthermore the substrate is in good contact with the gas because it is fluidised in the quartz tube. After the synthesis the substrate can be removed with an adequate solution. MgO is very well suited as substrate because it is easy to remove in a HCl solution. Details on the synthesis are discussed in Chapter 4, 5 and 6.

Fig. 13: Dense film of parallel oriented nanotubes grown on a silicon wafer [49].

Chapter 1

17

Fig. 14: Schema of the fluidised-bed reactor (left) and a photo of the apparatus (right)

Fig. 15: Schema of the precursor powder preparation and nanotube growth on thepowder grains (Fig. 16).


18

1.2.3.3 CVD in the gas phase

In the gas phase method no substrate is used. The catalyst is introduced in the flowing gas stream e.g. in form of volatile organometallic molecules. The gas phase method was also used for the synthesis of carbon fibres. Tibbetts et al. used a mixture of methane or hexane with organometallics such as iron pentacarbonyl (Fe(CO)5) [56] and ferrocene (Fe(C5H5)2) [57]. It is also possible to produce carbon nanotubes with this method by the decomposition of hydrocarbons [58] or carbon monoxide [59] in presence of metallocenes or iron pentacarbonyl. With this process Zhu et al. could synthesise centimetre long strands of SWNT by catalytic pyrolysis of n-hexane [60]. A disadvantage of this method is the big amount of encapsulated metal clusters [61]. 1.2.3.4 Plasma assisted chemical vapor deposition (PACVD)

In the PACVD-method the nanotubes are also deposited on a flat substrate that is coated with a metal prior to the deposition but the substrate is located inside a plasma. Usually one of the following plasmas is used: RF- (radio frequency), MW- (micro wave) or a DC-plasma (direct current). Often a methane (CH4) and hydrogen (H2) gas mixture with a ratio of 1% CH4 to 99% H2 is used at a total pressure between 1 and 40 mbar and a temperature of 900 °C [62]. The type of nanotubes are often much thicker than nanotubes produced by a thermal CVD. Thick tubes can very well be aligned perpendicular to the surface (Fig. 17).

Fig. 16: Nanotubes grown on a low surface area Al2O3 grain.

Chapter 1

19

Fig. 17: Parallel oriented nanotubes produced with the PACVD method [63].


20

1.3 Growth mechanism of carbon nanotubes The growth mechanism of nanotubes may be different depending on which method is used because with the arc-discharge and laser-ablation method MWNT can be grown without a metal catalyst contrary to carbon nanotubes synthesised with the CVD method, where it is known that metal particles are necessary. In contrast, for the growth of SWNT metals are necessary for all three methods mentioned above.

The growth mechanism of nanotubes is not well understood; different models exist but some of them cannot unambiguously explain the mechanism. The metal or carbide particles seem to be necessary for the growth because they are often found at the tip inside the nanotube or also somewhere in the middle of the tube (Fig. 18 left). In 1972 Baker et al. [15] made a model of the growth of carbon fibres, which is shown in Fig. 18 on the right side. It is supposed that acetylene decomposes at 600°C on the top of a nickel cluster on the support. The dissolved carbon diffuses in the cluster, precipitates on the rear side and forms a fibre. The carbon diffuses through the cluster due to a thermal gradient formed by the heat release of the exothermic decomposition of acetylene. The activation energies for filament growth were in agreement to those for diffusion of carbon through the corresponding metal (Fe, Co, Cr) [64, 65]. Whether the metal cluster moves away from the substrate (tip growth) or whether it is stays on the substrate (base growth) is explained by a weaker or

Fig. 18: TEM image of a MWNT with a metal cluster at the tip (left), growth model ofvapor grown carbon fibres (right), according to [66].

Chapter 1

21

stronger metal-support interaction respectively [66]. As stated by Baker the model has a number of shortcomings. It e.g. cannot explain the formation of fibres produced from the metal catalysed decomposition of methane, which is an endothermic process [66].

Oberlin et al. [67] proposed a variation of this model. The fibre is formed by a catalytic process involving the surface diffusion of carbon around the metal particle, rather than by bulk diffusion of carbon through the catalytic cluster. In this model the cluster corresponds to a seed for the fibre nucleation.

Amelinckx et al. [68] adaptated the growth model of Baker et al. to explain the growth of carbon nanotubes.

For the synthesis of nanotubes the metal clusters have to be present in form of nanosized particles (see Chapter 3). Furthermore it is supposed that the metal cluster can have two roles: 1) It acts as a catalyst for the dissociation of the carbon-bearing gas species, 2) carbon diffuses on the surface of the metal cluster or through the metal to form a nanotube. The most active metals are Fe, Co and Ni, which are good solvents for carbon [69].

The exact chemical composition of the catalyst particles during the synthesis was unknown. Lepora et al. [70] investigated the evolution of an iron-based catalyst in an acetylene atmosphere with an in-situ X-ray diffractometer. The Fe2O3 is reduced over Fe3O4 to FeO and finally to Fe3C without a reduction to pure iron. It was shown that this reduction involves a volume reduction of 44.5%. This could explain the growth of nanotubes on aluminium substrates [71]. For SWNT it is supposed that the nanoparticles have to be smaller than for MWNT [72] but this is in contradiction to the arc-discharge in which SWNT grow radial from one bigger metal cluster [73].

Fig. 19: Alternative growth model of vapor grown carbon fibres where the metal clus-ter acts as a seed for the growth [67].


22

1.4 Physical properties of carbon nanotubes In this chapter the electronic and vibrational properties of nanotubes are discussed because they are relevant in the Raman spectroscopy. Since nanotubes are a one-dimensional material they show special quantum proper-ties e.g. the electronic density of states show numerous Van Hove singularities that are of importance in the resonant Raman spectra of nanotubes.

1.4.1 Electronic properties of nanotubes By using periodic boundary conditions the electronic structure of SWNT can be obtained from that of a graphene sheet (two-dimensional graphite). A simple approximation of the π and π*-band electronic structure of a graphene layer is [74]:

22 0

3( , ) 1 4cos cos 4cos2 2 2

y yxg D x y

k a k ak aE k k γ = ± + +

(5)

where γ0 is the nearest-neighbour tight binding overlap energy. The wave vector associated with the Ch 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.

For (n, n) armchair nanotubes the periodic boundary condition permit only wave vectors in the circumferential direction which satisfy the relation qλ = πdt where λ = 2π/kx,q is the de Broglie wavelength:

,23

x qqk

naπ

= , (q = 1, 2,…N) (6)

where N (= 2n for armchair tubes) is the number of hexagons per 1D unit cell. Substitution of the discrete values for kx,q into equation (5) yields the energy dispersion relation Eq

a(k) for armchair nanotubes [74]

20( ) 1 4cos cos 4cos

2 2a

qq ka kaE knπγ = ± + +

,

(- < < )( =1, 2, ... )

kaq Nπ π

(7)

Chapter 1

23

The calculated dispersion relations for a (5, 5) nanotube are shown in Fig. 20 where ten bands are in the conduction band and again ten in the valance band. In each case there are four bands doubly degenerated (bold lines) and two bands non-degenerated. The conduction and valance band cross at the Fermi level at k = 2/3·π/a. Because of the degeneracy point between the valance and conduction bands at the band crossing, the (5, 5) nanotube is thus a zero-gap semiconductor, which will exhibit metallic conduction at finite temperature.

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

For a (n, 0) zigzag nanotube the boundary conditions for ky,q are written as:

,2

y qqk

naπ

= , (q = 1, 2,…N) (8)

Fig. 20: One-dimensional energy dispersion relation of (5, 5) armchair, (9, 0) and (10, 0) zigzag nanotubes. Doubly degenerated bands are in bold lines.


24

Substitution into equation (5) yields the energy dispersion relation Eqz(k) for

zigzag nanotubes [74]:

20

3( ) 1 4cos cos 4cos2 2 2

zq

ka q qE k π πγ = ± + +

(9)

3 3kaπ π

− < <

, (q = 1, 2, …N)

The calculated dispersion relations for a (9, 0) and a (10, 0) nanotube are shown in Fig. 20. There is no energy gap for the (9, 0) zigzag nanotube, whereas the (10, 0) nanotube shows an energy gap. For a general (n, 0) zigzag nanotube the energy gap at k = 0 becomes zero when n is a multiple of 3 otherwise there is an energy gap at k = 0.

The k values for the band degeneracies for metallic tubes are k = ±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. 21. 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. Of special interest are the singularities in the 1D density of states corresponding to extrema in the E(k) relations. The energy gap of semicon-

Fig. 21: 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 [75].

Chapter 1

25

ducting nanotubes depends on the reciprocal nanotube diameter dt:

0 C Cg

t

aE

dγ −= (10)

and is independent of the chiral angle of the semiconducting nanotube. Density of states measurements by scanning tunnelling spectroscopy (STS) confirm that some nanotubes (about 1/3) are conducting, yet most (about 2/3) are semiconducting (Fig. 22) [76,77]. Measurements confirm that the band gap is proportional to 1/dt.

Fig. 22: dI/dV of current voltage curves obtained by STM on individual nanotubes. Tubes no. 1 to 6 are chiral and no. 7 is zigzag. Nanotubes no. 1 to 4 are semicon-ducting and no. 5 to 7 are metallic (left), density of states (dI/dV)/(I/V) of a semicon-ducting tube with a diameter of 1.3 nm. The left inset displays the raw dI/dV data. The right inset is the calculated density of states for a (16, 0) nanotube (right) [77].


26

1.4.2 Vibrational properties of carbon nanotubes The phonon dispersion of a SWNT can be calculated by folding the phonon dispersion curves of a two-dimensional graphene layer analogous as for the case of the 2D electronic states. There are 2N carbon atoms in the unit cell of a carbon nanotube; therefore we have 6N phonon dispersion relations [74]. This model is applicable for almost all phonon modes but at low frequencies it does not always give the correct dispersion relation. Calculated phonon densities of states of a (10, 10) nanotube and a 2D graphene sheet are shown in Fig. 23. They were calculated by solving the three-dimensional carbon nanotube dynamic matrix [74]. One difference between the phonon density of states of a graphen sheet and a nanotube appears in the small peaks due to Van Hove singularities.

3D graphite has 6 normal phonon modes. The irreducible representation is given by [16]:

2 2 22 2g u u gE E A BΓ = + + + (11)

The phonon dispersion relation of 3D graphite is similar to that of a 2D graphene sheet due to the week interplanar coupling in graphite [78].

Fig. 23: 2D phonon dispersion relation (a) of a (10, 10) armchair nanotube (left) and ofa 2D graphene sheet (right) and the corresponding density of states (b) [74].

Chapter 1

27

1.5 Applications of carbon nanotubes 1.5.1 Electrochemical double layer (ECDL) capacitors ECDL capacitors consist of a pair of polarizable electrodes, a separator and an electrolyte. The tension varies between 1 and 4 V depending on the electrolyte. The capacity is in the order of 2000 F/l, the energy density varies between 5 and 10 Wh/kg and the power density lies between 1 and 10 kW/kg [79]. The ECDL capacitors are a compromise between batteries (high energy density) and conventional capacitors (high power density) (Fig. 24).

The equivalent circuit of a capacitor is shown in Fig. 25. It consists of a capacitor C in parallel with RP responsible for the self-discharge and a serial resistance RS representing the internal resistance. The ideal capacitor should have an infinite parallel resistance RP and a serial resistance RS that is zero.

1.5.1.1 The electrochemical double layer

If a metal electrode is immersed in a solution with the corresponding metal ions Mez+ then the following reaction can occur [80]:

-Me e Mez z+ + (12)

Fig. 24: Ragone plot of different energy storagesystems [43].

Fig. 25: schema of an ECDL capacitor.


28

Depending on which reaction is energetically favoured the forward or back reaction is more important. If the forward reaction is favoured the electrode will be charged positively. These positive surface charges attract negative anions so that a charge double layer is formed. The inside of the electrode and the solution have a different potential φMe≠φS.

The established equilibrium can be distorted when a constant tension is applied between the considered electrode and a counter electrode. If the electrode is in an inert solution only the double layer is charged or discharged depending on the sign of the tension, provided the potential is below the decomposition potential.

In the simplest consideration the distance between the layers is half the diameter a/2 of the solvated excess ions. This simple model is called the Helmholtz “rigid” double layer which can be compared with a plate capacitor with a plate distance of a/2 and a capacity of

( )

0

2r AC

aε ε

= (13)

where A is the surface of the electrode and εr the dielectric constant of the solvent. Within the rigid double layer the potential φ drops linearly as a function of the distance x:

d constdxϕ

= (14)

The assumption of the rigid double layer is an insufficient description of the distribution of the charges in front of the electrode because of the thermal movement. According to Gouy-Chapman [81, 82] the space charge gets smaller with increasing distance to the electrode (diffuse double layer) whereas, according to Stern the part in front of the electrode can be considered as a rigid double layer. The potential drop inside the diffuse double layer is

( ) ( )-

o.H. S Seξχϕ ξ ϕ ϕ ϕ= − + (15)

where ξ = x-a/2. The potential inside the diffuse double layer decreases exponentially from φ = φo.H. at ξ = 0 to φ = φS for ξ→ ∞. χ is the distance where

Chapter 1

29

the potential of the outer Helmholtz layer is dropped to a factor of 1/e and is considered to be the thickness of the diffuse double layer. The galvanic potential ∆φ between the electrode and the solution is composed of two parts:

( ) ( ) ( )Me o.H. o.H. S rigid diffuseϕ ϕ ϕ ϕ ϕ ϕ ϕ∆ = − + − = ∆ + ∆ (16)

The potential difference φo.H.- φS is also called the zeta-potential ζ (Fig. 26). For solutions with a concentration of 0.1 mol/L or higher χ already decreases to the order of magnitude of the thickness of the rigid layer. At a high enough ionic strength the whole double layer can be considered as rigid.

Fig. 26: Potential in an electrolytic double layer. Me: metal of the electrode, o.H.: outer Helmholtz surface, S: solvent, a/2: radius of the solvated ions, χ “thickness of the double layer”, ∆φ: galvanic potential, ζ: zeta potential


30

1.5.1.2 The double layer capacity

In the simples case the electrolytic double layer corresponds to a charged plate capacitor. For a capacitor there is a linear correlation between the charge on the plates Q and the tension between the plates U. The constant of proportion-ality is the capacity C

Q CU= (17)

In the electrolytic double layer the charge corresponds to the excess charge in the barrier layer and the tension U corresponds to the potential difference electrode-solution (galvanic potential ∆φ) i.e. C = Q/∆φ = Q/(φ-φS). This equation does not account for the difference in potential between the electrode and solution that could exist even without the presence of excess charges. It is better to use

zero

QCϕ ϕ

=−

(18)

where φ = φzero for Q = 0. In general an exact proportionality between Q and (φ-φzero) cannot be expected because a change in potential has the effect of a reorientation of the dipole layer (e.g. in aqueous solutions); furthermore the excess ions in the diffuse double layer are partially screened. In general a change in dQ by a change of dφ will be a function of the potential φ (dQ = dQ(φ)). The differential quotient dQ/dφ is therefore better to describe the charging process. It is termed differential double layer capacity Cd. With i = dQ/dt we have for the charging current iC

( )d d d dd d d dC d

Q Qi Ct t tϕ ϕ φ

ϕ= = = (19)

To determine Cd it is sufficient to increase the potential of an electrode linearly.

The capacity of a double layer is finally given by the serial connection of the rigid and diffuse double layer capacity:

rigid diffuse

1 1 1C C C

= + (20)

Chapter 1

31

1.5.1.3 Carbon nanotubes as active material in ECDL capacitors

Carbon nanotubes are used as an electrode material for electrochemical double layer capacitors because of their high specific surface area, their electrical conductibility and electrochemical inertness. Theoretically SWNT have an outer specific surface area of 1314 m2g-1, according to equation (13) such high surface areas are good requirements for high capacities. Two types of electrodes can be made of carbon nanotubes. The first consists of aligned nanotubes directly grown on an aluminium substrate or a paste of nanotubes with an organic binder is applied on aluminium foils. A model of an electrode with nanotubes directly grown on an electrode is shown in Fig. 27. In practice two electrodes are connected in series to get a real capacitor. The electrolyte is either aqueous or organic as e.g. 1 M Et4NBF4 dissolved in acetonitrile.

Fig. 27: Model of an electrochemical double layer capacitor with nanotubes directlygrown on the metal substrate.

Electrode Type of material

Ax directly grown CVD nanotubes

Bx paste of CVD nanotubes

Cx paste of fluidised-bed nanotubes

Dx paste of fluidised-bed nanotubes

Ex activated carbon

Table 2: Different type of electrodes made with different carbonous material


32

The capacities of different electrode materials based on carbon nanotubes and activated carbon have been investigated [83]. In table 2 the material of the five different types of electrodes is numerated. The capacity per geometrical electrode surface [F/cm2] increases linearly with the thickness of the active material on the aluminium electrode (Fig. 28). The specific capacity of elec-trodes with nanotubes directly grown on aluminium (electrodes Ax) is very low because the density (mass of active material per area [mg/cm2]) is low. It was shown that the density and thickness of the nanotubes on the aluminium electrode could not be further increased [84]. The capacity increases linearly as a function of the density of the active material (Fig. 29). According to equa-tion (13) the capacity should increase linearly with an increasing BET surface area but saturation is reached for high specific surface areas as can be seen in Fig. 30. This is due to the fact that not the whole surface area measured with the BET method is accessible for the solvated ions that have a diameter of 12 Å.

Fig. 28: specific capacity vs. the electrode thickness for electrodes Ax, Bx, Cx and Dx

Fig. 29: specific capacity vs. the electrode density for electrodes Ax, Bx, Cx and Dx

Chapter 1

33

1.5.2 Hydrogen storage in carbon nanotubes Much work on reversible hydrogen sorption by carbon nanotubes was stimulated by the findings published in an article from A.C. Dillon et al. [85]. They estimated the hydrogen storage capacity of carbon nanotubes at that time to be 5 to 10 mass%. The investigation was carried out on a sample containing 0.1 to 0.2 mass% SWNT, the rest of the more than 99% residual carbonaceous material was assumed to be inert contrary to the fact that activated carbon is an excellent adsorber. Hirscher et al. [86] showed that the desorption of hydrogen originates from Ti-alloy particle, introduced during the ultrasonic treatment, in the sample rather than from the carbon nanotubes.

The main motivation for the investigation of the hydrogen interaction with carbon nanotubes lies in the main difference between carbon nanotubes and high surface area graphite. In nanotubes the graphene sheets are curved and there is a cavity inside the tube. In capillaries which have a width not exceeding a few molecular diameters, the potential fields from opposite walls will overlap so that the attractive force acting on adsorbate molecules will be increased as compared with that on a flat carbon surface [87].

Assuming that hydrogen behaves similar to nitrogen, hydrogen would only form one monolayer of liquid at the surface of carbon at temperatures above the boiling point. With geometrical considerations the maximum amount of condensed hydrogen of a monolayer on the surface of a nanotube can be calculated [88]. The maximum amount of adsorbed hydrogen for a

Fig. 30: specific capacity vs. the BET surface area of electrodes Ax, Bx,Cx,Dx and Ex


34

8

6

4

2

0

H/(

H+C

) mas

s [%

]

3.53.02.52.01.51.00.50.0d [nm]

1086420Ns

Ns = 1

Ns = 2

Ns = 5

Ns = 10

SWNT with an outer specific surface area of 1315 m2g-1 is 3.3 mass% (H/C = 0.4) (Fig. 31 left) [89, 90]. The amount of adsorbed hydrogen decreases as a function of the number of shells in the nanotube. The bulk absorption in the cavity is proportional to the diameter of the nanotubes and is also highest for SWNT (Fig. 31 right). It is supposed that the density inside the nanotube is the same as for liquid hydrogen.

The electrochemical hydrogen adsorption of nanotubes is reversible [91-94]. The maximal discharge capacity measured at 298 K is 2 mass%. The electrochemically reversibly stored hydrogen as a function of the BET specific surface area is shown in Fig. 32 (round markers together with the fitted line). The quadratic markers correspond to measurements of other nanostructured carbon samples [95]. The measurements of the hydrogen uptake in the gas phase at 77 K exhibit the same quantities as the electrochemical measurements at room temperature (298 K).

Fig. 31: Calculated amount of adsorbed hydrogen on nanotubes assuming condensation of one monolayer. Hydrogen adsorbed at the outer surface of nanotubes as a function of the number of shells (Ns) (left, solid line), hydrogen absorbed inside the tube as a function of the nanotube diameter (right, dotted lines).

Chapter 1

35

The reversible hydrogen sorption process on nanostructured carbon samples is based on the physisorption; the amount of adsorbed hydrogen is proportional to the BET specific surface area. The amount of adsorbed hydrogen from the gas phase at 77 K and electrochemically at room temperature is 1.5 mass%/1000 m2/g-1. There is no evidence for the geometric structure of the nanostructured carbon. All attempts to open the nanotubes and absorb hydrogen inside the tubes did not show an increased absorption.

Fig. 32: Reversible amount of hydrogen vs. the BET surface area of carbon nanotubesamples and two high surface area graphite samples measured electrochemically at 298K (round markers and linear fit). The quadratic markers correspond to other nanostructured carbon measured with the gas adsorption at 77K [95]. The dotted line corresponds to the model of a monolayer adsorption of hydrogen.


36

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Chapter 1

37

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38

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[49] Ph. Mauron, Wachstum von CVD (Chemical Vapor Deposition) Nanotubeschichten, Diplomarbeit, Universität Freiburg, Schweiz (1999).

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Chapter 1

41

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43

Chapter 2

Investigation methods

2.1 Electron microscopy When a high-energy (~keV) electron beam interacts with a specimen then a wide range of secondary signals are formed which can give interesting information of the sample (Fig. 1). In a scanning electron microscope (SEM) mainly the secondary electrons (SE), backscattered electrons (BSE) and characteristic X-rays are analysed (EDX). If the sample is thin enough the elastically scattered electrons can be investigated in a transmission electron microscope (TEM).

Fig. 1: Signal generated when a high-energy electron beam interacts with a thinspecimen. Most signals can be detected in a SEM or TEM.

Chapter 2 Investigation methods

44

2.1.1 Scanning electron microscopy (SEM) In a SEM an electron beam generated by field- or thermo emission is scanned over the specimen. The backscattered or secondary electrons are counted with an appropriate detector and are imaged on a screen.

If the electrons are scattered inelastically then they are scattered at the electrons of the target material. Compared to impacts with the nucleus of the atom the change in direction of the electrons can be neglected provided the electron energy is not to low. The electrons are loosing their energy continuously through the interaction with the electrons of the target. The loss in energy dE per distance ds can be described by the Bethe equation [1]:

40

20

d lnd 8 2

eE NZ e Es E Jπε

− =

(1)

where N is the number of atoms, Z the atomic number of the target material, J the ionisation energy of the target and E the Energy of the incident electron. Through this interaction the excited secondary electrons can escape the outermost layer of the target material and they can be detected.

The energy loss of elastically back-scattered electrons on the target nucleus can be neglected because of the big mass difference of electrons and target nucleus. Considering the screening of the nuclear charge by the electron shells, the angular scattering of electrons by the coulomb field of the atomic nucleus can be described by the screened Rutherford scattering cross-section:

( )( ) ( ) ( )

4 20

2 2 2220 0

d 1d 16 sin 2 2

e ZqE

σθπε θ θ

= =Ω +

(2)

where θ is the scattering angle, Ω is the solid angle and θ0 the screening angle given by the ratio of the reduced wave length of the electron λ/2π and the radius of the nucleus. The probability for a scattering at a small angle is much higher than for big angles.

The different properties of the secondary and backscattered electrons are taken advantage of in the SEM. Since the energy of the secondary electrons is low they are attracted with a positively charged grid on the SE-detector. The

Chapter 2

45

energy of the backscattered electrons is too big to be attracted by the grid. The secondary electrons give particularly topographic information of the sample.

The energy of the backscattered electrons is high enough to create electron-hole pairs in a semiconductor detector. They give information of the chemical composition of the target material because of the Z2 dependence of the cross section.

2.1.2 Transmission electron microscopy (TEM) The imaging ray path in a TEM is analogous to that of an optical microscope. The operation of the objective lens allows the imaging of the object in the corresponding image plain by focusing of electrons starting with different directions from individual object points to the corresponding image points (Fig. 2). Additionally in the back focal plane of the objective all those elec-

Fig. 2: The two basic operations of a TEM: projecting the diffraction pattern on theviewing screen (left) or projecting the image on the screen (right). The intermediatelens selects the back focal- or the image plane of the objective lens as its object [2].


46

trons are focused to one point, which started in the same direction from any point in the object (Fig. 2) leading to the diffraction pattern of the object. A bright field image contrast can be obtained by arranging a contrast aperture in the back focal plain in order to eliminate all electrons, which are scattered and diffracted, respectively into large angles. Depending on the setting of the intermediate lens one may image either the back focal plane or the image plane of the objective lens in the image plane of the projector lens. Thus one obtains either the diffraction pattern or the image of the specimen on the viewing screen.

The relation between the accelerating voltage U and the electron velocity v is given by [1]:

( )

20 2

0 02/

m ce U m ca v c

= −−

(3)

where m0 is the rest mass of the electron and c the velocity of light in vacuum. The wavelength λ attributed to an electron with velocity v is given by the de Broglies relation:

( )2

01 /h v c

m vλ = − (4)

where h is Planck’s constant. An electron with a kinetic energy of Ekin = 100 keV has a velocity v of ≈ 55% light velocity and a wavelength of λ = 3.7 pm.

The contrast in TEM arises mainly form the elastic scattering process.

The inelastically scattered electrons are not focused in the image plane because of the chromatic aberration of the objective lens.

There are mainly three different ways to form the contrast [1]:

scattering-absorption contrast: Only the unscattered electrons or the electrons scattered through extremely small angles contribute to the image. The scattered electrons are removed form the imaging beam by insertion of a contrast aperture in the back focal plane of the objective lens or even unintentionally due to the effect of the spherical aberration of the objective lens. This image mode leads to the so-called bright field (BF) image (Fig. 3

Chapter 2

47

left). The removal of the scattered electrons from the beam by the objective aperture produces the same effect as absorption within the specimen. At low resolution the image can be described to a good approximation as two-dimensional projection of the mass density into the plane normal to the beam direction. diffraction contrast: Only the scattered electrons or part of them are used for the image formation. The unscattered electrons are e.g. stopped by the displacement of the contrast aperture. An image formed in this way is called a dark field (DF) image (Fig. 3 right). phase contrast: In high resolution TEM the image contrast is mainly caused by the interference of the primary beam (unscattered electrons) with the scattered electrons in the image plane leading to a bright field image. In TEM the phase contrast is formed by the phase shift relationship introduced by the spherical aberration and the aberration of defocusing. The refractive power of the outer zones of the objective lens is higher than that of the inner ones, thereby causing a phase shift between the undiffracted beam and the diffracted beams. The phase shift can be changed by an appropriate defocus in order to adjust the image phase contrast.

Fig. 3: Comparison of the use of an objective aperture in TEM to select the direct (left)or the scattered electrons (right) forming bright field (BF) and dark field (DF) imagesrespectively [2].


48

2.2 Raman spectroscopy 2.2.1 Introduction If a light quantum hν0 hits e.g. a molecule it is scattered with a high probability elastically with the energy hν0 in the so-called Rayleigh scattering process (Fig. 4). With a much lower probability (~10-5) it is scattered inelasti-cally with an energy hν0±hνs in the Raman scattering process in which the vibrational energy hνs is exchanged. According to the Botzmann’s law most molecules are in their vibrational ground state at ambient temperature. Therefore the Raman process that transfers energy to the molecule and leaves a quantum with lower energy hν0-hνs has a higher probability than the reverse process. The corresponding Raman lines are called Stokes and anti-Stokes lines, respectively [3].

In the classical theory the Raman scattering is explained as follows: If a

diatomic molecule is irradiated with an alternative electric field E:

( ) ( )0 0cos 2t tπν=E E (5)

an electric dipole moment P is induced:

α=P Ε (6)

Fig. 4: Polarized molecule in an electric field (left), energy diagram of the different light scattering processes. The elastic Rayleigh scattering and the inelastic Raman scattering (Stokes/Anti-Stokes) (right)

Chapter 2

49

where α is the polarisability (in general a tensor). If the molecule vibrates with a frequency νs, the nuclear displacement is written as

( )0 cos 2 sq q tπν= (7)

For a small amplitude of vibration, α is a linear function of q

00

..qqαα α ∂

= + + ∂ (8)

where α0 corresponds to the polarisability at the equilibrium position. By combining the equations we get [4]:

( ) ( ) ( ) 0 0 0 0 0 0 00

1cos 2 cos 2 cos 22 s st q t t

qαα πν π ν ν π ν ν ∂ = + + + − ∂

P E E (9)

The first term corresponds to an oscillation of a dipole with frequency ν0 (Rayleigh scattering), the second and third term corresponds to the Raman scattering with frequency ν0+νs (anti-Strokes) and ν0-νs (Strokes). A vibration is only Raman active when (∂α/∂q)0 is not zero. The classical equation does not describe the intensities of the Stokes and anti-Stokes lines.

The intensity ratio of the anti-Stokes and Stokes lines of a Raman active vibration νs is given by [5]:

4 1

0

0expanti Stokes s s

Stokes s

I hcI kT

ν ν νν ν

−− + = −

(10)

where T is the specimen Temperature. For crystals the polarizability α is replaced by the susceptibility tensor χ:

0 Eε=P χ (11)

Whether a vibration is Raman active or not is determined by the symme-try of the crystal described in group theory. When the frequency of the incident radiation approaches an electronic transition frequency (electronic level r in Fig. 4), the intensity of the Raman bands is strongly enhanced.


50

Raman spectra obtained with exciting frequencies close to absorption bands are called resonance Raman spectra.

For crystals transparent to the incident and scattered light the conserva-tion of energy and momentum can be written as [6]:

0 1 sh h h hν ν ν ν= − = ± (12)

0 1 s= − = ±k k k q (13)

where νs and qs are the frequency and wave vector of a crystal excitation. For typical light scattering experiments in or near the visible the range of scattering wave vectors is 0≤kd106 cm-1. For first-order scattering processes the accessible range |qs|, is small compared to a reciprocal lattice vector. In high order processes the range of the individual wave vectors of the excitation can be from zero to a reciprocal lattice vector since k = Σqi.

In the following cases the wave vector conservation (13) breaks down: 1) The scattering medium has no translation symmetry i.e. in crystals with defects, solid solutions and in amorphous solids. The absence of translation symmetry allows scattering by modes with qs ≠ k. 2) The scattering volume is small. In this case light scattering is due to excitations with wave vectors in a range ∆q º 2π/d where d is a characteristic length in the scattering volume. 3) The incident and scattered waves are damped inside the scattering volume. Under this conditions (metals and small gap semiconductors that are opaque to the light) k0 and k1 are complex.

Chapter 2

51

2.2.2 Raman spectra of carbon 2.2.2.1 Graphite

Among the six vibrational modes in graphite (equation 11, Chapter 1) only the two E2g vibrational modes are Raman active. The E2g1 mode is at 42 cm-1 and the E2g2 mode at 1582 cm-1. The first order Raman spectrum of HOPG is shown in Fig. 5 (left). The E2g2 mode is also called “G-band”, whereas G means graphite. The peak around 1350 cm-1 is the so-called “D-band” (disorder). In perfect graphite this mode is forbidden and becomes only active in presence of disorder where the wave vector conservation (equation 13) breaks down and other modes become active. The ratio of the integrated D-and G-band is inversely proportional to the crystallite size of graphite (Fig. 5 (right)). For glassy carbon the D-band is very intense due to the high disorder in this material.

Fig. 5: Normalised raman spectra of HOPG, glassy carbon, CVD MWNT and CVDSWNT. The laser wavelength was λ = 514.5 nm (left). The inverse crystallite size vs.the ratio of the D- and G-band; according to [7] (right).


52

2.2.2.2 Nanotubes

Among the 6N calculated phonon dispersion relations for SWNT only a few modes are Raman or infrared active. The number of the Raman- (A1g, E1g, E2g symmetries) and IR-active (A2u, E1u symmetries) modes for SWNT can be predicted by group theory. The number of Raman and IR active modes do not depend on the nanotube diameter and chirality. There are only 15 to 16 Raman active modes and 6 to 9 IR-active modes (table 1). There are only six or seven intense Raman-active modes for any nanotube chirality. Some Raman modes for a (10, 10) SWNT are displayed in Fig. 6. The A1g mode, which occurs at about 165 cm-1 for an isolated (10, 10) nanotube, is strongly dependant on the nanotube diameter, while the modes near 1580 cm-1 are not (Fig. 7). The power law predicted for the A1g radial breathing mode frequency ω0

RBM (dt) for an isolated SWNT valid in the range 0.6 nm ≤ dt ≤1.4 nm is [8]:

( )1.0017 0.0007

(10,10)0 0(10,10)RBM t

t

ddd

ω ω±

= (14)

where ω0(10,10) = 169 cm-1

and d(10,10) = 1.375 nm are the mode frequency and diameter of an isolated (10, 10) armchair nanotube. When the SWNTs are in bundles, inter-tube interactions (Van der Waals) become important and the dispersion relation for the diameter dependence of the radial breathing mode is

( ) ( )0 0(10,10) (10,10) /RBM t RBM RBM t RBM td d d dω ω ω ω ω= ∆ + = ∆ + (15)

Nanotube structure Point group Raman-active IR-active

armachair (n, n) n even Dnh 4A1g+4E1g+8E2g A2u+7E1u

armachair (n, n) n odd Dnd 3A1g+6E1g+8E2g 2A2u+5E1u

zigzag (n, 0) n even Dnh 3A1g+6E1g+8E2g 2A2u+5E1u

zigzag (n, 0) n odd Dnd 3A1g+6E1g+8E2g 2A2u+5E1u

chiral (n, m) n≠m≠0 CN 4A+5E1+8E2 4A+5E1

Table 1: Raman-and IR-active modes for SWNT with different symmetries [8].

Chapter 2

53

where ω0RBM (dt) is given by equation (13) and ∆ωRBM is a frequency upshift

which is constant for nanotube diameters near a (10, 10) tube. Some typical values are ∆ωRBM = 14 cm-1 [11], 6.5 cm-1 [12] and 6cm-1 [13].

One-dimensional quantum effects are observed in the Raman spectra of SWNT through the resonant Raman enhancement effect in nanotubes between

Fig. 6: Atomic displacements of calculated Raman modes, frequencies and symmetriesof a (10, 10) nanotube. Only one displacement of the two doubly degenerate E1g and E2g modes are shown [10].

Fig. 7: Diameter dependence of the first order Raman-active mode frequencies for armchair (left) and zigzag nanotubes (right) [14].


54

incident or scattered photons and the electronic transitions between the Van Hove singularities in the 1D density of states in the valence and conduction bands. This resonant effect can be seen experimentally by measuring spectra with different laser excitation energies (Fig. 8). An example of a spectrum of CVD SWNT synthesised in the fluidised-bed is shown in Fig. 5.

The CVD MWNT shown in Fig. 5 have a strong peak at 1576 cm-1 that is strongly shifted down relative to the 1582 cm-1 mode observed for HOPG. This downshift is also reported in the first Raman study of MWNT [16]. A second weaker mode is found at 1348 cm-1.

Fig. 8: Raman spectra for purified SWNTs exited at five different laser excitation wavelengths [15]. 1320 nm → 0.94 eV; 1064 nm → 1.17 eV; 780 nm → 1.58 eV; 647.1 nm → 1.92 eV; 514.5 nm → 2.41 eV.

Chapter 2

55

2.3 Gas adsorption for the determination of surface area The physical gas adsorption of finely divided and porous solids can be used to determine their specific surface area. The Brunauer-Emmett-Teller (BET) method is the most common procedure for the determination of the specific surface area. 2.3.1 Adsorption isotherms The quantity of gas adsorbed by a sample of solid is proportional to its mass m and it depends also on the temperature T, the pressure p of the gas and the nature of both the solid and the gas. The relationship between the amount of gas n (moles per gram) and the relative pressure p/p0 are expressed in the adsorption isotherm f [17]:

0 , ,( / )T gas solidn f p p= (16)

at a constant temperature, for a given gas adsorbed on a particular solid, where p0 is the saturation pressure of the adsorptive.

The majority of isotherms due to physisorption can be grouped into five classes (I-V) proposed by Brunauer, Deming, Deming and Teller [18] (Fig. 9).

Fig. 9: adsorption isotherms: type (I-V) and the stepped VI isotherm [17].


56

2.3.2 Adsorption forces The physical adsorption (physisorption) of a gas by a solid is due to the forces of attraction between the individual molecules of the gas and the atoms or ions composing the solid. The overall interaction energy ( )zφ of a molecule at

distance z from the surface can be represented by the following expression [17-20]:

( ) D R P F FQz µφ φ φ φ φ φ= + + + + (17)

The two first terms are always present for adsorption phenomena. Dφ are the

“dispersion” forces, also called “Van der Waals” forces. By omitting the dipole-quadrupole and quadrupole-quadrupole interactions the remaining dipole-dipole interaction can be represented as [21]:

61( )D z C zφ −= − (18)

where C1 is the dispersion constant associated with instantaneous dipole-dipole interaction. The second term Rφ is the short-range repulsive force (due

to the interpenetration of the electron clouds of the two atoms) and can be represented in a simplified form as [21]:

122( )R z C zφ −= − (19)

where C2 is an empirical constant. The electrostatic energies Pφ , Fµφ and FQφ

may or may not be present depending on the nature of the adsorbent and the adsorptive. If the solid is polar, dipoles are induced in the gas molecules that lead to the interaction energy Pφ [17]. Fµφ is due to the contribution of

permanent dipoles in the molecule and FQφ accounts for a gas molecule that

possesses a quadrupole moment. Adsorption can be distinguished between “non-specific” adsorption where

only Dφ and Rφ are involved and “specific” adsorption where Pφ , Fµφ or FQφ are

present in addition [22, 23].

Chapter 2

57

2.3.3 The BET model The BET model is based on the kinetic model of gas adsorption like the model described by Langmuir in which the surface consists of an array of adsorption sites. It is supposed that the rate of adsorption and desorption is in a dynamic equilibrium. When θ0 is the fraction of bare sites and θ1 the fraction of occupied sites then θ0+ θ1 = 1. The rate of condensation on a unit area of surface is rc = a1pκθ0 where p is the pressure, a1 the condensation coefficient and κ = 1/2 NA/(MRT)1/2 is given by the kinetic gas theory. The desorption of a condensed molecule is an activated process in which the activation energy is equated to the isosteric heat of adsorption q1. The rate of evaporation is given by [17]:

1 /1 1e q RT

e mr z θ ν −= (20)

where zm is the number of sites per unit area and ν1 the frequency of oscillation of the molecule normal to the surface. At equilibrium rc is equal to re and with θ1 = n/nm where n [mol] is the amount of adsorbed molecules and nm the monolayer capacity the Langmuir equation [24] is given by:

1m

n Bpn Bp

=+

with 11 /

1eq RT

m

aBz

κν

= (21)

In practice B is an empirical constant. The BET isotherm is the extension of the Langmuir mechanism to a

multilayer adsorption. In the second layer the rate of condensation onto the molecules in the first layer is equal to the rate of evaporation from the second layer and so on for further layers. The amount of adsorbed molecules can be summed up under the assumptions that for all layers except the first one: 1) the head of adsorption is equal to the molar heat of condensation qL; 2) the evaporation-condensation conditions are identical (ν2 = ν3 =…= νi and a2 = a3 =… = ai); 3) when p = p0 the adsorptive condenses to a bulk liquid i.e. the number of layers becomes infinite (p0: saturation vapour pressure). This leads to the BET equation [25]:

( )( ) ( ) ( )

0

0 0

/1 / 1 1 /m

c p pnn p p c p p

= − + −

with ( )1 /1 2

2 1e Lq q RTac

aνν

−= (22)


58

In practice ( )1 /e Lq q RTc −= is often used where (q1-qL) is the net heat of ad-sorption. If c exceeds 2 the curve corresponds to a type II isotherm (Fig. 10).

The point on the isotherm at which the linear portion begins “point PB” or the point where the linear extrapolation cuts the adsorption axis “point PA” were suggested to indicate the completion of the monolayer (Fig. 11). The specific surface area A per unit mass is calculated with:

m m AA n a N= (23)

where am (= 16.2 Å2 for nitrogen) is the area occupied by one molecule of adsorbate in the monolayer, NA the Avogadro number and nm are the number of moles of adsorbate per gram of adsorbent.

In practice an adsorption experiment is often performed with nitrogen as the adsorbate at the temperature of liquid nitrogen (77K). At a constant flux of e.g. 0.5 sccm (standard cubic centimetres) of nitrogen. During the experiment the volume and the reduced pressure are recorded (Fig. 11). “Point PA” is determined with a linear fit and the specific surface area A of the adsorbent is calculated with the following formula:

2

23

[ ] [Å ] 6.02[ / ][ ] 22.4 10

m mV sccm aA m gm mg −

⋅= ⋅

⋅ (24)

Fig. 10: n/nm versus p/p0 calculated with the BET equation for different values of c.

Chapter 2

59

2.3.4 Pore sizes: micro-, macro- and mesopores Within a given solid, and between one solid and another the individual pores may vary greatly both in size and in shape. Of special interest is the width w of the pores, e.g. the diameter of a cylindrical pore or the distance between the sides of a slit-pore. A convenient classification of pores according to their average width is given in table 2. The classification corresponds to character-istic effects in the adsorption isotherm. Micropores: the interaction potential is significantly higher than in wider pores therefore the amount of gas adsorbed (at a given relative pressure) is enhanced. Mesopores: capillary condensation, with its characteristic hysteresis loop, takes place. Macropores: the pores are so wide that it is impossible to map out the isotherm in detail because the relative pressures are so close to unity.

Fig. 11: Adsorbed volume V versus p/p0 for a MWNT sample

Width

Micropores less than ~20 Å

Mesopores between ~20 and ~500 Å

Macropores more than ~500 Å

Table 2: Classification of pores to their width [26]


60

2.4 X-ray diffraction (XRD) 2.4.1 Introduction According to Bragg’s law there is constructive interference between waves reflected from successive lattice planes with distance dhkl when the path differ-ence between the waves is a multiple n of the wavelength λ of the radiation:

( )2 sinhkln dλ θ= (25)

When the crystal size decreases, as for example in powders, the diffrac-tion lines become broader. The crystallite size can be calculated according to the Scherrer formula [27]:

( ) ( )0

0.92cosL

λθθ

⋅∆ = (26)

where L[Å] is the apparent size of the crystallite and ∆(2θ) is the FWHM of the diffraction line at θ0. 2.4.2 XRD of carbon nanotubes MWNT also show a diffraction peak around 2θ = 25.9° which is near the (002) peak of crystalline graphite that is located at 2θ = 26.6°. The distance between the layers in MWNT is d ≈ 3.43 Å which is slightly higher than the interlayer

Fig. 12: Normalised XRD spectra of graphite and MWNT for different iron ratios in the precursor powder. Crystallite size L vs. iron concentration c calculated according to equation (26) (inset).

Chapter 2

61

distance in graphite which is d = 3.354 Å (Fig. 12). In the spectra it is clearly seen that the width of the peaks get larger for lower iron concentrations, this means that the nanotubes are thinner for lower concentrations (inset Fig. 12).

If SWNT with the same diameter are arranged in well-defined triangular bundles (Fig. 13) they show peaks that correspond to the lattice planes of the triangular arrangement. The distance from the center of a tube to the center of a nearest neighbor is a = 2·R+g, where R is the nanotube radius and g the Van der Waals gap, which was found to be 3.15 Å [28] like in solid C60. The dis-tance d between two layers is given by d = (√3/2)·a. In Fig. 14 the XRD spec-trum of Buckytubes is shown. The nanotube diameters are calculated to be d ≈ 1.0 nm.

Fig. 13: Illustration of a cross section of a SWNT bundle; a is the distance from the centre of a tube to the nearest neighbour and d the distance between two SWNT layers.

Fig. 14: XRD of HiPco Buckytubes and simulation of a (13, 0) SWNT bundle with 61nanotubes with a diameter of d(13, 0) = 1.02 nm and length of l = 9.9 nm; the first diffraction peak is at 2θ ≈ 7.7°


62

2.5 Mass spectrometry In a quadrupole mass spectrometer electric fields are used to separate ions according to mass. The ions are separated as they pass along the central axis of four parallel, equidistant rods that have DC and alternating (radio frequency, RF) voltages applied to them (Fig. 15). One opposed pair of rods has a poten-tial of +(U+V·cos(ωt)) applied to it and the other pair has -(U+V·cos(ωt)) applied [29]. U is a fixed potential and V·cos(ωt) represents a RF field. Depending on the ion mass, the voltages (U, V) and the frequency ω the ions will oscillate in a complex fashion in the transverse direction of the quadru-poles. In z-direction the ions have to be accelerated through a potential before they enter the quadrupolar field. By suitable choices of U, V and ω only ions of one mass (m/z) will oscillate stably about the central axis; all other ions will oscillate with an increasing amplitude, strike the poles and will be lost.

In practice, the frequency ω is fixed. Typical values are in the range of 1-2 MHz. The DC voltage U may be 1000 V, and the maximum RF voltage V is 6000 V.

Fig. 15: Illustration of a quadrupole mass spectrometer (left) and the different poten-tials applied to the four rods (right), the dotted lines correspond to planes with zero electric field [29].

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63

References

[1] H. Bethge, J. Heydenreich, Electron Microscopy in Solid State Physics, Elsevier (Amsterdam, Oxford, New York) 1987.

[2] D.B. Williams, C.B. Carter, Transmission Electron Microscopy; Basics I, Plenum Press (New York, London), 1996.

[3] Infrared and Raman Spectroscopy, edited by B. Schrader, VCH (Weinheim, New York, Basel, Cambridge, Tokyo), 1995.

[4] J.R. Ferraro, K. Nakamoto, Introductory Raman Spectroscopy, Academic Press (Boston, San Diego, New York, London, Sydney, Tokyo, Toronto) 1994.

[5] P.V. Huong, R. Cavagnat, Phys. Rev. B 51, 10048 (1995). [6] Light Scattering in Solids I, edited by M. Cardona, Springer-Verlag

(Berlin, Heidelberg, New York), 1983. [7] F. Tuinstra, J.L. Koenig, J. Chem. Phys, 53, 1126 (1970). [8] M.S. Dresselhaus, P.C. Eklund, Advances in Physics 49, 705 (2000). [9] P.C. Eklund, J.M. Holden, R.A. Jishi, Carbon 33, 959 (1995). [10] R. Saito, G. Dresselhaus, M.S. Dresselhaus, Physical properties of

Carbon Nanotubes, Imperial College Press (London), 1998. [11] U.D. Venkateswaran, A.M. Rao, E. Richter, M. Menon, A. Rinzler,

R.E. Smalley, P.C. Eklund, Phys. Rev. B 59, 10928 (1999). [12] L. Alvarez, A. Righi, T. Guillard, S. Rols, E. Anglaret, D. Laplaze, J.L.

Sauvajol, Chem. Phys. Lett. 326, 186 (2000). [13] D. Kahn, J.P. Lu, Phys. Rev. B 60, 6535 (1999). [14] P.C. Eklund, J.M. Holden, R.A. Jishi, Carbon 33, 959 (1995). [15] A.M. Rao, E. Richter, S. Bandow, B. Chase, P.C. Eklund, K.A.

Williams, S. Fang, K.R. Subbaswamy, M. Menon, A. Thess, R.E. Smalley, G. Dresselhaus, M.S. Dresselhaus, Science 275, 187 (1997).

[16] H. Hiura, W. Ebbesen, T.W. Tanigaki, Chem. Phys. Lett 202, 509, (1993).

[17] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press (London, New York, Paris, San Diego, San Francisco, Sao Paulo, Sydney, Tokyo, Toronto) 1982.


64

[18] S. Brunauer, L.S. Deming, W.S. Deming, E. Teller, J. Amer. Chem. Soc. 62, 1723 (1940).

[19] R.M. Barrer, J. Colloid Interface Sci. 21, 414 (1966). [20] R.M. Barrer, Zeolites and Clay Minerals as Sorbents and Molecular

Sieves, Academic Press (London, New York) 1973, p. 174. [21] D.M. Young, A.D. Crowell, Physical Adsorption of Gases, p. 9, 18,

Butterworths (London) 1962. [22] A.V. Kiselev, Discuss. Faraday Soc. 40, 205 (1965). [23] A.V. Kiselev, N.V. Kovaleva, Y.S. Nikitin, J. Chromatography, 58, 19

(1971). [24] I. Langmuir, J. Amer. Chem. Soc. 38, 2221 (1916). [25] S. Brunauer, P.H. Emmett, E. Teller, J. Amer. Chem. Soc. 60, 309

(1938). [26] IUPAC Manual of Symbols and Terminology, Appendix 2, Pt. 1,

Colloid and Surface Chemistry. Pure Appl. Chem. 31, 578 (1972). [27] A. Guinier, X-Ray diffraction, Dover Publications Inc., (New York)

1994. [28] A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu, Y.H.

Lee, S.G. Kim, A.G. Rinzler, D.T. Colbert, G.E. Scuseria, D. Tomanek, J.E. Fischer, R.E. Smalley, Scince 273, 483 (1996).

[29] Ch.G. Herbert, R.A.W. Johnstone, Mass Spectrometry Basics, CRC Press (Boca Raton, London, New York, Washington D.C.) 2003.

65

Chapter 3

Synthesis of oriented nanotube films by chemical vapor deposition

Ph. Mauron, Ch. Emmenegger, A. Züttel, Ch. Nützenadel, P. Sudan, L. Schlapbach

Université de Fribourg, Chemin du Musée 3, CH-1700 Fribourg, Switzerland

Carbon 40 (2002) 1339–1344

Abstract

Oriented nanotube films (20–35 µm thick) were synthesised on flat silicon substrates by chemical vapor deposition (CVD) of a gas mixture of acetylene and nitrogen. For the CVD we used metal oxide clusters formed by spin coating an iron(III) nitrate ethanol solution onto a silicon substrate and subsequent heating.

The cluster density and its effects on the nanotube density were investigated as a function of the iron(III) nitrate concentration and the synthesis temperature. A high nanotube density was achieved with a high density of iron oxide clusters as nucleation centres for the growth of nanotubes. The cluster density was controlled by the iron(III) concentration of the ethanolic coating solution and by the synthesis temperature. The perpendicular orientation of the nanotubes with respect to the substrate surface is attributed to a high density of nanotubes.

Chapter 3 Synthesis of oriented nanotube films by chemical vapor deposition

66

3.1 Introduction Carbon nanotubes were discovered by Iijima [1] in 1991. Carbon nanotubes are cylindrical hollow structures consisting of sp2-bonded carbon atoms. Theoretically, single-walled nanotubes (SWNT) can be described as a graphite sheet rolled up to a cylinder and closed at each end with a half of a fullerene molecule [2]. Each SWNT is characterised by its diameter and chiral angle [3]. Multi-walled nanotubes (MWNT) consist of several graphite cylinders nested one into another.

The most widely used technique to produce nanotubes is the arc discharge evaporation method [1,4,5] also used for fullerene synthesis [2]. An electric arc discharge is produced between two carbon electrodes in an inert helium or argon atmosphere. The temperature of > 3000°C between the electrodes is high enough to sublime the carbon.

A second approach is the laser ablation method [6]. A piece of graphite is vaporised by laser irradiation in an inert gas. With the arc discharge and the laser ablation method both SWNT and MWNT can be produced. MWNT are synthesised with pure graphite; for SWNT however, metal particles are required. In the arc discharge method a drilled carbon rod is filled with a metal powder (Ni, Co, Pt, Cu, Co/Ni, Co/Pt, Fe/Ni…), and in the laser ablation method a transition-metal/graphite composite is used.

These synthesis methods have both the advantage to produce high quality nanotubes. A disadvantage of the vaporisation methods is the high temperature (> 3000°C) required, this limits a scale-up of the processes. Furthermore, the diverse by-products e.g. amorphous carbon and diverse nanoparticles have to be removed by subsequent purification [7].

A more efficient approach to the nanotube synthesis without the above-mentioned drawbacks is the CVD method [8], earlier used for the synthesis of carbon filaments. In this method a carbon-containing gas or vapor (C2H2, CH4, CO, pentane…) is dissociated over supported metal clusters at 500 to 1200°C for some minutes up to several hours. The metal clusters on the substrate serve as nucleation centres for the nanotube growth. It is suggested that for the carbon filament as well as for the nanotube growth the carbon-containing gas decomposes on the metal surface and that the carbon diffuses from one side

Chapter 3

67

through the metal particle and precipitates on the other side [9]. The CVD method has the advantage to produce lower quantities of by-products. The graphitization degree of such nanotubes as compared to those produced by arc discharge or laser ablation [4] is lower because of the lower synthesis temperatures.

Many publications on the synthesis of oriented nanotube films have been reported in recent years where different preparation methods of the metal clusters and variations of the CVD synthesis were developed.

Modifications to the CVD method are for example the plasma-enhanced CVD [10, 11] or the plasma-enhanced hot-filament CVD [12,13]. It is claimed that the alignment of the nanotubes is primarily induced by the electrical self-bias field imposed on the substrate surface from the plasma environment [11]. There exist several methods for the preparation of the metal clusters. Starting with a thin metal film, e.g. a deposited cobalt, nickel or iron film on a flat [14, 15] or porous substrate [16], clusters can be formed by subsequent heating. Different methods exist where pre-formed clusters are used e.g. casting on a substrate a solution of metal clusters produced by the reverse micelle method [17], using porous substrates such as iron particles embedded in mesoporous silica [18, 19] or an iron/silica nanocomposite [20]. In contrast, gas phase synthesis is a method to grow nanotube films without pre-formed substrates [21-23]. In this method e.g. metallocenes or Fe(CO)5 vapors and acetylene are decomposed in an argon flow at 1100°C [21]. Starting with a liquid precursor as in microcontact printing, a patterned elastomeric stamp is inked for example in an iron(III) nitrate solution and then pressed onto a silicon substrate [24].

For the CVD synthesis of nanotubes we use metal clusters formed by spin coating of an iron(III) nitrate ethanol solution on a silicon substrate followed by subsequent heating. In this paper we investigate the metal cluster density as a function of the iron(III) nitrate concentration in ethanol and the synthesis temperature and its effects on the nanotube density. In addition, the influence of the nanotube density on their orientation is determined.


68

3.2 Experimental 3.2.1 Description of the CVD apparatus The CVD apparatus used in our experiments is basically composed of a quartz glass tube into which we introduced a sample holder and a gas mixture (2 sccm C2H2, 98 sccm N2) for the reaction. For the pyrolysis of acetylene (C2H2), the sample was heated (650 - 750°C) with a wound tantalum filament situated in the bottom of the sample holder.

3.2.2 Synthesis of nanotubes The deposition of nanotubes was carried out in three steps as shown in Fig. 1.

Step 1 (coating): In order to deposit a very homogeneous iron(III) nitrate film on the silicon substrate, iron(III) nitrate ethanol solutions of different concentrations (7.5 - 60 mmol/l) were applied with a spin-coater. The thickness of the deposited film can be changed by varying the concentrations of the iron(III) nitrate solution.

Step 2 (heating): Previously to the deposition of the nanotubes the quartz glass tube was evacuated to a pressure of 10-6 mbar, then the coated substrate was heated to the desired temperature (650 - 750°C). It took about 8 minutes

Fig. 1. Deposition of nanotubes in three steps: (1) spin-coating of iron(III) nitrate solution on the silicon substrate. (2) formation of Fe2O3 clusters by heating to 650 -750°C. (3) pyrolysis of acetylene at 650 - 750°C

Chapter 3

69

to reach the final temperature. The temperature was maintained constant until a thermal equilibrium in the tube was obtained after 10 minutes. Subsequently a constant flux of nitrogen (98 sccm) was introduced to increase the pressure to 500 mbar. Ironoxide clusters were formed by heating the iron(III) nitrate-coated substrate according to the reaction:

4 Fe(NO3)3 Ø 2 Fe2O3 + 12 NO2 + 3 O2 (1)

Step 3 (synthesis): The synthesis of nanotubes was performed by adding a

constant flux of acetylene (2 sccm) to the nitrogen carrier gas during 30 minutes.

The objective of our experiments was to synthesise a dense film of nanotubes. We investigated the influence of the iron(III) nitrate concentration and the synthesis temperature on the density of the nanotubes onto the substrate. A first series of samples was analysed after the second step to observe the cluster formation, and a second series after the third step to observe the nanotube growth.

The obtained samples were analysed by high resolution scanning electron microscopy (HRSEM) with a Zeiss DSM 982 Gemini instrument.

3.3 Results 3.3.1 Experiment I: Distribution of clusters Experiments with iron(III) nitrate solutions with concentrations of 7.5, 15, 30 and 60 mmol/l and synthesis temperatures of 650, 700 and 750°C were carried out (Table 1). In a first series the samples were only heated without introduction of acetylene to observe the cluster formation.

With high iron(III) nitrate solution concentrations of 60 mmol/l, the coating on the silicon substrate was unchanged after heating and no clusters were observed at all temperatures. We observed the same results for a concentration of 30 mmol/l and low temperatures of 650 and 700°C. At higher temperatures (750°C) formation of clusters was observed. For lower concentrations (7.5 and 15 mmol/l) clusters are formed also at lower


70

temperatures (650°C) and high densities of clusters (number of clusters per area) could be achieved (Fig. 2a-d).

Detailed SEM images showed that the clusters have a cubic shape. AFM investigations indicated that the heights are in the same order of magnitude as the diameters determined by SEM. X-Ray Photoelectron Spectroscopy (XPS) and X-Ray Diffraction measurements have shown that the iron(III) nitrate on the silicon substrate decomposes to Fe2O3 during the heating process. No nitrogen was found on the heated substrates by means of XPS.

Distributions of the cluster diameters and densities were determined by processing the SEM images with an image analysis program [25]. The value of the maxima of the size distributions are listed in Table 1. Obviously the diameter of the clusters depends on the iron(III) nitrate concentration as well as on the temperature. The diameter of the clusters decreases with decreasing concentration on the iron(III) nitrate solution coated onto the substrate at a

d [nm] c = 7.5 mmol/l c = 15 mmol/l c = 30 mmol/l

T = 650°C 40 120 ---

T = 700°C 30 90 ---

T = 750°C 20 30 45

Table 1: Maximum of the diameter distribution of Fe2O3 clusters formed on silicon. Different concentrations of iron(III) nitrate solutions and various temperatures were used.

N [µm-2] c = 7.5 mmol/l c = 15 mmol/l c = 30 mmol/l

T = 650°C 115 2 ---

T = 700°C 130 9 ---

T = 750°C 257 38 4

Table 2: Density (number of clusters per µm2) of Fe2O3-clusters formed on silicon. Different concentrations of iron(III) nitrate solutions and various temperatures were used.

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71

constant synthesis temperature. Additionally the diameter of the clusters decreases with increasing synthesis temperature at a constant iron(III) nitrate concentration of the coating solution.

Table 2 shows the cluster density evaluated from the SEM images. The cluster density increases from <10 to >200 µm-2 with increasing synthesis temperature and decreasing iron(III) nitrate concentration. For a given Fe(NO3)3 film thickness the total quantity of Fe2O3 formed on the substrate is constant, and therefore the density of clusters has to increase when the diameter decreases.

Fig. 2. SEM images (back scattered electrons) of Fe2O3 clusters formed on silicon substrates. The substrates were coated with iron(III) nitrate ethanol solutions ofdifferent concentrations by use of a spin-coater and then heated to different temperatures: (a) c = 15 mmol/l, T = 700°C, (b) c = 15 mmol/l, T = 750°C, (c) c = 7.5 mmol/l, T = 650°C, (d) c = 7.5 mmol/l, T = 750°C (see Table 1).


72

We observed that the diameter of the clusters decreases with decreasing iron(III) nitrate concentration at a fixed final temperature. On heating the iron nitrate film the salt melts at 47.2°C. At higher temperatures the iron(III) nitrate decomposes to Fe2O3 according to the reaction (1) described above. NO2 and O2 will leave the reaction zone through gas diffusion and clusters are formed because of the cohesion forces. A thick film favours the formation of large clusters because of the large amount of material available. Consequently, thick coatings will form few large clusters and thin coatings lead to a large number of small ones. Furthermore, we observed that the diameter of the clusters decreases with increasing temperature at a constant iron(III) nitrate concentration of the coating solution. The wetting properties of the coating may change depending on the temperature, and therefore the wetting of the film is better at higher temperatures and consequently leads to smaller clusters. An other explanation could be a different wetting of the film because of a different temporal change in the chemical composition of the decomposing film at different synthesis temperatures.

Kinetic considerations lead to the conclusion that the formation of the iron oxide clusters depends not only on the synthesis temperature but also on time. If the cluster formation would not be time-dependent, the coating on the silicon heated to a higher temperature would pass the cluster distribution at a lower temperature. Considering Fig. 2a and b, it is improbable that a distribution synthesised at a high temperature like that in Fig. 2b (high cluster density and no large area without clusters) can be reached once the system has passed a distribution (low temperature) as shown in Fig. 2a. (low cluster density and large areas with no clusters). The rate-determining factor may be the diffusion of NO2 and O2 out of the reaction zone.

3.3.2 Experiment II: Synthesis of nanotubes In a second experiment we grew nanotubes on the clusters. This was done with subsequent addition of acetylene after the heating process.

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73

The nanotubes synthesised with this method have diameters between 20 and 28 nm and TEM investigations show that well-graphitized MWNT are obtained. The highest nanotube densities and film thicknesses of about 22 and 35 µm were observed with the lowest iron(III) nitrate concentration (7.5 mmol/l) at high temperatures (700 and 750°C) (Fig. 3a). These are exactly the conditions with the highest cluster densities. In this case the nanotubes are oriented perpendicular to the substrate surface although each single nanotube

Fig. 3. SEM images (secondary electrons) of nanotube films on a silicon substrate. The samples were cut and are shown from the side. The nanotubes were synthesized by pyrolysis of acetylene over an iron(III) nitrate coated silicon substrate. (a) Film thickness d≈33µm, diameter of the nanotubes 23 - 28 nm, concentration of the Fe(NO3)3 solution c = 7.5 mmol/l, synthesis temperature T = 700°C. (b) Film thickness d ≈ 5 µm, diameter of the nanotubes 20 - 25 nm, concentration of the Fe(NO3)3 solution c = 15 mmol/l, synthesis temperature T = 700°C.


74

has a curly shape. For conditions with a lower cluster density the nanotube density is also lower and the nanotubes are not strictly oriented vertically (Fig. 3b). These observations lead to two conclusions: (1) The higher the cluster density, the higher is the density of nanotubes. (2) An oriented film of nanotubes can be attributed to a sufficiently high density of nanotubes (≈ 30 - 40 nanotubes per µm2). At high densities the nanotubes have spatially only one degree of freedom in which they can grow, namely perpendicular to the surface. The distribution of the nanotube diameters is narrow (20 - 28 nm) although the cluster distributions have broader dispersions.

SEM investigations of nanotubes deposited during only 30 - 60 seconds showed that more than two-thirds of the clusters do not act as seeds of nucleation for a nanotube. We counted 30 - 40 nanotubes per square micrometer for synthesis parameters for which we get more than 120 clusters per square micrometer. The number of active clusters does not change during the synthesis time (30 minutes) because all nanotubes on a determinated surface range have a narrow length distribution. The density of nanotubes could be further increased if all clusters would be nucleation centres.

Similar densities of nanotubes could be synthesized on aluminium substrates that exhibit a much better adhesion of the nanotubes as compared to silicon because the synthesis temperature of 650°C is near the melting point of aluminium (660°C). At this temperature surface melting of the aluminium substrate joins the nanotubes with the substrate. Such samples can be used as electrodes for high-capacity electrochemical double-layer capacitors [26].

3.4 Discussion Yudasaka et al. [14] observed that the size of the nickel particles can be controlled by the original thickness of the nickel thin film which was evaporated onto a quartz glass substrate by an electron beam as well as by the heat-treatment temperature. We determined that both the diameter of the Fe2O3 clusters and the cluster density can be controlled by the synthesis temperature and the concentration of the iron(III) nitrate solution coated onto the substrate.

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Ren et al. [12] used the plasma-enhanced hot-filament CVD method with a gas mixture of acetylene, nitrogen and ammonia to synthesize nanotube films. They obtained nanotubes with diameters ranging between 20 to 400 nm and lengths up to 50 µm that were perpendicular to the surface and only the thicker nanotubes were very straight. Our nanotubes with diameters ranging from 20 to 28 nm and lenghts up to 33 µm synthesized with the CVD method have a more curved shape but also have a preferential direction perpendicular to the surface when the nanotube density is sufficiently high (≈ 30 - 40 nanotubes per µm2). The nanotube density of our films is higher than that observed with the films described in Ref. [12]. At lower nanotube diameters their density is also increasing and the nanotubes get also curved shapes.

Bower et al. [11] claimed that in the plasma-enhanced CVD the alignment of the nanotubes is primarily induced by the electrical self-bias field imposed on the substrate surface from the plasma environment, but in our CVD process without an electric field the perpendicular orientation is caused by the increased nucleation density.

3.5 Conclusion Carbon nanotubes films were synthesized by the CVD method. The metal clusters needed for the nucleation of nanotubes were formed by spin coating an iron(III) nitrate ethanol solution on a silicon substrate and subsequent heating. We showed that the nanotubes are oriented perpendicularly to the substrate surface for high densities of nucleation centres. A high nanotube density can be achieved with a high density of iron oxide clusters that are nucleation centres for the growth of nanotubes. Furthermore, the cluster density can be varied with the iron(III) nitrate concentration of the coating solution and the synthesis temperature.


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3.6 References

[1] Iijima S. Nature 1991;354(6348):56-58. [2] Kroto HW, Heath JR, O’Brien SC, Curl RF, Smalley RE. Nature

1985;318(6042):162-163. [3] Dresselhaus MS, Dresselhaus G, Saito R. In: Endo M, Iijima S,

Dresselhaus MS, editors. Carbon nanotubes. Great Britain: Pergamon, 1996:27-35.

[4] Bethune DS, Kiang CH, de Vries MS, Gorman G, Savoy R, Vazquez J, et al. Nature 1993;363(6430):605-607.

[5] Journet C, Bernier P. Appl Phys A 1998;67(1):1-9. [6] Guo T, Nikolaev P, Thess A, Colbert DT, Smalley REChem Phys Lett

1995;243(1-2):49-54. [7] Rinzler AG, Liu J, Dai H, Nikolaev P, Huffman CB, Rodriguez-Macias

FJ, et al. Appl Phys A 1998;67(1):29-37. [8] José-Yacamán M, Miki-Yoshida M, Rendón L, Santiesteban JG. Appl

Phys Lett 1993;62(6):657-659. [9] Baker RTK, Barber MA, Harris PS, Feates FS, Waite RJ. Nucleation

and growth of carbon deposits from the nickel catalyzed decomposition of acetylene. J Catal 1972;26(1):51-62.

[10] Küttel OM, Groening O, Emmenegger Ch, Schlapbach L. Appl Phys Lett 1998;73(15):2113-2115.

[11] Bower Ch, Zhu W, Jin S, Zhou O. Appl Phys Lett 2000;77(6):830-832. [12] Ren ZF, Huang ZP, Xu JW, Wang JH, Bush P, Siegal MP, et al.

Science 1998;282(5391):1105-1107. [13] Chen Y, Patel S, Ye Y, Shaw DT, Guo L. Appl Phys Lett

1998;73(15):2119-2121. [14] Yudasaka M, Kikuchi R, Ohki Y, Ota E, Yoshimura S. Appl Phys Lett

1997;70(14):1817-1818. [15] Terrones M, Grobert N, Olivares J, Zhang JP, Terrones H, Kordatos K,

et al. Nature 1997;388(6637):52-55. [16] Fan S, Chapline MG, Franklin NR, Tombler TW, Cassell AM, Dai H.

Science 1999;283(5401):512-514.

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[17] Ago H, Komatsu T, Ohshima S, Kuriki Y, Yumura M. Appl Phys Lett 2000;77(1):79-81.

[18] Li WZ, Xie SS, Qian LX, Chang BH, Zou BS, Zhou WY, et al. Science 1996;274(5293):1701-1703.

[19] Nath M, Satishkumar BC, Govindaraj A, Vinod CP, Rao CNR. Production of bundles of aligned carbon and carbon-nitrogen nanotubes by the pyrolysis of precursors on silica-supported iron and cobalt catalysts. Chem Phys Lett 2000;322(5):333-340.

[20] Pan ZW, Xie SS, Chang BH, Sun LF, Zhou WY, Wang G. Chem Phys Lett 1999;299(1):97-102.

[21] Satishkumar BC, Govindaraj A, Sen R, Rao CNR. Chem Phys Lett 1998;293(1-2):47-52.

[22] Andrews R, Jacques D, Rao AM, Derbyshire F, Qian D, Fan X, et al. Chem Phys Lett 1999;303(5-6):467-474.

[23] Mayne M, Grobert N, Terrones M, Kamalakaran R, Rühle M, Kroto HW, et al. Chem Phys Lett 2001;338(2-3):101-107.

[24] Kind H, Bonard JM, Emmenegger Ch, Nilsson LO, Hernadi K, Maillard-Schaller E, et al. Adv Mater 1999;11(15):1285-1289.

[25] Barrett S. Image SXM. 1995. [26] Emmenegger Ch, Mauron P, Züttel A, Nützenadel Ch, Schneuwly A,

Gallay R, et al. Appl Surf Sci 2000;162-163(1-4):452-456.

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Chapter 4

Fluidised-bed CVD synthesis of carbon nanotubes on Fe2O3/MgO

Ph. Mauron, Ch. Emmenegger, P. Sudan, P. Wenger, S. Rentsch, A. Züttel

Université de Fribourg, Chemin du Musée 3, CH-1700 Fribourg, Switzerland

Diamond and Related Materials 12 (2003) 781-786

Abstract

Carbon nanotubes were synthesised by the fluidised-bed CVD synthesis of iso-pentane (C5H12) and acetylene (C2H2) on a magnesium oxide (MgO) powder impregnated with an iron nitrate (Fe(NO3)3·9H2O) solution. Large quantities of nanotubes can be produced by this technique due to the high specific surface area (100 m2·g-1) of the precursor powder and the good convection in the fluidised bed. The main advantage of MgO is its high solubility in hydrochloric acid. Different synthesis parameters such as the iron content in the precursor (2.5–15%), the synthesis temperature (450–850°C) and the synthesis time (0.5–40 min.) were investigated. The synthesised material was characterised by scanning- and transmission electron microscopy, X-ray diffraction, Raman spectroscopy and BET. The purified samples consist of mostly single and multi walled carbon nanotubes.

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4.1 Introduction Carbon nanotubes are regarded as a promising material for different potential applications due to their unique physical and chemical properties. They could be used for example as electrodes for electrochemical double-layer capacitors [1–3], field emitters [4,5] in flat panel displays, as electron source in x-ray tubes [6] and in nanoelectronic devices [7,8]. More then ten years after the discovery of nanotubes by Iijima [9], a cheap and if possible continuous large-scale synthesis of multi (MWNT) and single walled nanotubes (SWNT) is still a challenge. The arc discharge evaporation [9–11] and laser ablation [12] were the first methods by which nanotubes of both types were produced. For these two methods, the scale up procedure is not evident because of the elaborate and costly system. A technically more simple and cheaper method is the synthesis of nanotubes by CVD. First MWNT were produced by the CVD of acetylene [13] and afterwards also SWNT by disproportionation of carbon monoxide [14]. There exist basically two different types of CVD methods: in the first metal particles are deposited on a substrate (on a powder [13,14] or on a flat substrate [15,16]) and in the second a metal containing vapour is used for the synthesis in the gas phase [17,18]. When a precursor powder with a high specific surface area is used, the amount of metal particles exposed to the gas is much larger as compared to a flat substrate. In the fluidised-bed synthesis a large quantity of a high surface area precursor powder is in good contact with the gas due to the fluidisation of the powder [19], therefore large quantities of nanotubes can be produced.

4.2 Experimental 4.2.1 Description of the fluidised-bed CVD reactor The CVD apparatus consists of a vertical furnace and a quartz glass tube with a diameter of 3 cm in which in the middle a quartz filter of porosity 3 (pores of 20 to 40 µm) is mounted (Fig. 1). For a given substrate powder a certain gas flow is necessary in order to fluidise the bed. In our case a flow of 410 scm3 (standard cm3) was necessary. For the reaction for example 42 scm3 acetylene or 10 scm3 argon that passes through a bubbler filled with iso-pentane is

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mixed with the carrier gas (368 resp. 400 scm3 argon). 10 scm3 argon that passes through the bubbler with iso-pentane evaporate the same quantity of carbon as 42 scm3 acetylene.

4.2.2 Synthesis of nanotubes A magnesium oxide (100 m2·g-1) supported iron oxide powder produced by impregnation in an iron nitrate ethanol solution is used as precursor powder. To get a precursor with a MgO to Fe weight ratio of 5% 25 g of MgO were suspended in 100 ml ethanol and 9.19 g of iron nitrate (Fe(NO3)3·9H2O) previously dissolved in 100 ml ethanol was stirred together and sonicated for 20 min. in order to homogenise the mixture. Afterwards the precursor was dried and grinded into a fine powder.

Fig. 1. Fluidised bed reactor.


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For one deposition, typically 1.5 g of precursor powder was filled in the quartz tube and the atmosphere was purged with 410 scm3 argon for 5 min. Then the furnace was heated to the synthesis temperature. By heating up the precursor powder iron oxide clusters were formed [16] due to the thermal decomposition of the iron nitrate at 125°C [20]. The synthesis was started with the introduction of 42 scm3 acetylene mixed with 368 scm3 argon for 20 min.

One gram of the “as produced” nanotubes was stirred in 100 ml 10% HCl for 15 hours at a temperature of 75°C in order to remove the MgO substrate. Afterwards the nanotubes were filtered, thoroughly washed with distilled water and dried in air.

In this paper, we describe the influence of different synthesis parameters such as the iron ratio in the precursor (1.25–15%), the synthesis temperature (450–850°C), the synthesis time (0.5–40 min.) and the type of carbon source on the yield, the specific surface area and the graphitisation degree of the purified nanotubes. In order to follow the evolution of the synthesis process the gas was sampled at the outlet with a probe connected to a mass spectrometer. The synthesised material was characterised by scanning- and transmission electron microscopy (SEM/TEM), X-ray diffraction (XRD), micro Raman spectroscopy (RS) and the specific surface area was determined by BET [21].

Fig. 2. Carbon yield as a function of the iron ratio in the precursor powder fornanotubes synthesised at 700°C with iso-pentane for 20 min.

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4.3 Results The influence of the iron content in the precursor powder on the morphology and type of nanotubes was studied by varying the iron content from 1.25 to 15% at a fixed temperature of 700°C, a synthesis time of 20 min. and iso-pentane as carbon source (10 scm3 argon that passes through bubbler with iso-pentane). The yield of carbon was determined by the ratio of the weight of the “as produced” nanotubes oxidised at 800°C and the weight before the oxida-tion. The yield increases linearly as a function of the iron content (Fig. 2).

Fig. 3. (a) TEM images of MWNT synthesised for 20 min. at 700°C with iso-pentane as carbon source, Fe ratio: 5%; (b) Fe ratio: 10%; (c) and (d) SWNT bundlessynthesised at 800°C with acetylene; (e) SEM images of MWNT produced with iso-pentane at 700°C; (f) SWNT bundles synthesised with acetylene at 850°C.


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TEM investigations showed that the material consists of MWNT (Fig. 3a, b & e) even though the iron content was varied in a rather large domain. The diameters of the nanotubes in the samples were not monodisperse but the tubes are more regular for higher iron ratios. The specific surface area increases remarkably with decreasing iron content form about 260 m2·g-1 at 15% to 1200 m2·g-1 at 2.5%. This is an indication that the characteristic length of the structures decreases for a smaller iron content. The reduction of the characteristic length was confirmed by means of Raman. In the first order spectra two major features are seen one is the so-called D-band (disorder) around 1350 cm-1 and the other is the G-band (graphite) around 1580 cm-1. Pure graphite only shows the E2g Raman mode at 1582 cm-1 in this range. The D-band only becomes active in presence of disorder [22,23] and for graphite the ratio of the integrated D- and G-band varies inversely with the crystallite size [22]. The ratio of the integrated intensities of the D- and G-band is increasing for lower iron ratios (Fig. 4) this means that the structures are a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a

Fig. 4. First order Raman spectra of nanotubes synthesised at 700°C for 20 min. withdifferent iron ratios in the precursor powder and iso-pentane as carbon source. The spectra are normalised and shifted.

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smaller or less ordered. XRD of the synthesised nanotubes show that there are residual iron carbide (Fe3C) particles that are not solved in the acid purification step (Fig. 5). Since iron carbide is soluble in acids it follows that the iron carbide particles are enclosed in the hollow channel of the nanotubes or that they are encapsulated in other carbonous material. The diffraction peaks where the vertical lines are indicated correspond to the three most intense peaks of iron carbide. They were calculated to be at 37.65° (121), 45.00° (031) and 49.13° (221) respectively. The carbide peaks are more intense for higher iron ratios in the precursor powder. The peak at ~43° corresponds to the (101) diffraction peak of graphite and is also present in nanotubes. The more intense carbide peaks at high iron ratios could be due to carbide inside MWNT which diameter is bigger for high iron ratios. The fact of reducing the iron content that reduces the size of the Fe2O3 particles, which serve as nucleation centres for the nanotube growth, did not result in the synthesis of SWNT.

Fig. 5. Normalised XRD pattern of nanotubes around the (101) graphite peak. Thetubes are synthesised on precursor powders with different Fe ratios with iso-pentane at 700°C. The vertical lines indicate the positions of the three major iron carbide (Fe3C) peaks.


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In Fig. 6 the nanotube yield is plotted as a function of the synthesis time at a fixed temperature of 700°C with an iron ratio of 5% and 42 scm3 acetylene as carbon source. The dotted line corresponds to the calculated yield supposed all the acetylene is deposited. At a temperature of 700°C the deposition rate is decreasing fast as a function of time and after 40 minutes the rate is almost zero. For a typical synthesis the temporal conversion of the acetylene and other gases that were measured with the mass spectrometer during the synthesis are plotted in Fig. 7. Following the purging step a flow of 42 scm3 acetylene and 368 scm3 argon (t = 0–5 min.) were passed through the fluidised bed in order to get a reference of the acetylene amount. Afterwards the reaction chamber was purged again with 410 scm3 argon (t = 5–10 min.) before the furnace was switched on. During the heating process up to 650°C (t = 10–25 min.) different peaks appeared, for example CO2, H2O, CH4, N2/CO, O2 and C2H2, they are due to adsorbed CO2 and H2O on the MgO and the decomposition of the Fe(NO3)3·9H2O at 125°C [20]. New peaks appear when the acetylene is introduced (t = 25–45 min.): N2/CO, H2, CH4, CO2 and H2O. They are due to the decomposition of the acetylene and reduction of the Fe2O3 formed during the decomposition of the iron nitrate [16,24]. After the introduction of acetylene the different gas peaks decrease with different rates.

Fig. 6. Carbon yield as a function of synthesis time. The line termed “max. yield” isthe yield supposed all acetylene is converted.

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The slowest is the H2 due to the continuous decomposition of acetylene. As a function of the synthesis time, the conversion rate of acetylene is decreasing. After 5 min. synthesis (t = 30 min.) ~60% of the acetylene is consumed whereas at the end (t = 45 min.) only ~30%. Furthermore not all the acetylene is used for the nanotube growth a part is for example converted in CO, CH4, CO2 and possibly also in other hydrocarbons. By varying the synthesis temperature form 450 to 850°C at a fixed iron content of 5%, a synthesis time of 20 min. and acetylene as carbon source the yield increased exponentially (Fig. 8).

TEM investigations showed that at the lowest temperature of 450°C no nanotubes were synthesised. In the temperature range of 500 to 650°C MWNT were formed and at temperatures from 700 up to 850°C SWNT were synthesised (Fig. 3c, d & f). It is interesting to remark that no SWNT were deposited at 700°C with iso-pentane as the carbon source with the same iron ratio of 5%. This shows that the type of carbon source plays also a role in the synthesis of SWNT. The change of the nanotube type as a function of temperature is also clearly seen in the Raman spectra. In Fig. 9, the normalised D- and G-bands of the samples are plotted. From 450 to 650°C the appearance

Fig. 7. Partial pressure of acetylene and other gases as a function of the synthesis timemeasured with a probe connected to a mass spectrometer. The right axis indicates thetemperature in the furnace.


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of the lines is more or less the same. At 650°C the G-band is more intense than the D-band. At temperatures of 700 to 800°C the intensity of the G-band becomes higher and is decreasing again at 850°C. The absolute intensities of

Fig. 8. Carbon yield vs. synthesis temperature for acetylene as carbon source with aniron ratio of 5%.

Fig. 9. First order Raman spectra of nanotubes synthesised at different temperaturesfor 20 min. with a precursor powder of an iron ratio of 5%. The spectra are normalisedand shifted.

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the bands are considerably increased at temperatures between 700 and 850°C with a maximum at 800°C compared to lower temperatures. The spectra of MWNT in Fig. 4 and that of SWNT (Fig. 9) show a clear difference at the G-band. For MWNT we have a shoulder at the right side of the band and for SWNT the shoulder is on the left side. For SWNT different Raman modes at lower wavenumbers than 1590 cm-1 were calculated for different SWNT types [25].

XRD patterns of the nanotubes synthesised at different temperatures are shown in Fig. 10 around the (101) graphite peak at 2θ = 44.6°. The three major carbide peaks are most intense at a temperature of 850°C and are decreasing for lower temperatures until they have disappeared at 450°C where no nanotubes were synthesised. Despite the fact that in the TEM almost only SWNT bundles were found the (002) graphite peak at around 2θ = 25° for nanotubes is still present. For pure SWNT the (002) peak should be absent and

Fig. 10. Normalised XRD pattern of nanotubes around the (101) graphite peak. Thetubes are synthesised at different temperatures with acetylene on a precursor powderwith an iron ratio of 5%. The vertical lines indicate the positions of the three majoriron carbide (Fe3C) peaks.


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peaks at lower angles should appear due to the hexagonal arrangement of the SWNT in bundles [26]. The specific surface area increases form ~500 m2·g-1 at a synthesis temperature of 450°C to ~680 m2·g-1 at 600°C where a maximum is reached and decreases again to ~280 m2·g-1 at 850°C.

4.4 Discussion Usually SWNT are synthesised with methane (CH4) [27] or CO dispropor-tionation at rather high temperatures up to 1200°C [14]. Recently Hornyak et al. reported the synthesis of SWNT with methane at temperatures from 680 to 850°C with an iron molybdenum impregnated Al2O3 catalyst [28]. At temperatures equal or less than 670°C they got only poorly organised carbon and a yield of ~3% whereas at a temperature of 680°C they got SWNT with a yield of ~25%. In our experiments both SWNT and MWNT could be synthesised with acetylene. The temperature range where SWNT were synthesised is in the same range (700–850°C) (we do not have made depositions at temperatures higher than 850°C) but the difference lies in the lower temperature range (500–650°C) where we got MWNT.

4.5 Conclusion We have shown that the fluidised-bed CVD synthesis of carbon nanotubes on a Fe2O3/MgO precursor powder is very well suited to produce large quantities of nanotubes (~0.5 g). The Fe2O3/MgO combination has the advantage that the substrate is easy to remove with hydrochloric acid. Depending on the synthesis temperature both MWNT and SWNT could be synthesised with acetylene as the carbon source. For temperatures of 500 to 650°C MWNT and for higher temperatures of 700 to 850°C SWNT were synthesised. With iso-pentane MWNT were synthesised at 700°C whereas the use of acetylene resulted in SWNT with the same iron ratio of 5% in the precursor powder. This work was financially supported by the EU5 project CARBEN.

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4.6 References

[1] C. Emmenegger, P. Mauron, P. Sudan, P. Wenger, V. Hermann, R. Gallay, A. Züttel, to be published in: T.S. Srivatsan, R.A. Varin (Ed.), 11th International Symposium on Processing and Fabrication of Advanced Materials, Columbus, Ohio-USA, 2002.

[2] K.H. An, W.S. Kim, Y.S. Park, J.M. Moon, D.J. Bae, S.C. Lim, Y.S. Lee, Y.H. Lee, Adv. Funct. Mat. 11 (2001) 387.

[3] J.H. Chen, W.Z. Li, D.Z. Wang, S.X. Yang, J.G. Wen, Z.F. Ren, Carbon 40 (2002) 1193.

[4] L.O. Nilsson, O. Groening, C. Emmenegger, O. Kuettel, E. Schaller, L. Schlapbach, H. Kind, J.M. Bonard, K. Kern, Appl. Phys. Lett. 76 (2000) 2071.

[5] J.M. Bonard, H. Kind, T Stockli, L.O. Nilsson, Solid-State Electronics 45 (2001) 893.

[6] H. Sugie, M. Tanemura, V. Filip, K. Iwata, K. Takahashi, F. Okuyama, Appl. Phys. Lett. 78 (2001) 2578.

[7] K. Tsukagoshi, N. Yoneya, S. Uryu, Y. Aoyagi, A. Kanda, Y. Ootuka, B.W. Alphenaar, Physica B 323 (2002) 107.

[8] F. Leonard, J. Tersoff, Phys. Rev. Lett. 88 (2002) 258302-1 [9] S. Iijima, Nature 354 (1991) 56. [10] D.S. Bethune, C.H. Kiang, M.S. de Vries, G. Gorman, R. Savoy, J.

Vazquez, R. Beyers, Nature 363 (1993) 605. [11] C. Journet, P. Bernier, Appl. Phys. A 67 (1998) 1. [12] T. Guo, P. Nikolaev, A. Thess, D.T. Colbert, R.E. Smalley, Chem.

Phys. Lett. 243 (1995) 49. [13] M. José-Yacamán, M. Miki-Yoshida, L. Rendón, J.G. Santiesteban.

Appl. Phys. Lett. 62 (1993) 657. [14] H. Dai, A.G. Rinzler, P. Nikolaev, A. Thess, D.T. Colbert, R.E.

Smalley, Chem. Phys. Lett. 260 (1996) 471. [15] M. Yudasaka, R. Kikuchi, Y. Ohki, E. Ota, S. Yoshimura, Appl. Phys.

Lett. 70 (1997) 1817. [16] P. Mauron, C. Emmenegger, A. Züttel, C. Nützenadel, P. Sudan, L.

Schlapbach, Carbon 40 (2002) 1339.


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[17] P. Nikolaev, M.J. Bronikowski, R.K. Bradley, F. Rohmund, D.T. Colbert, K.A. Smith, R.E. Smalley, Chem. Phys. Lett. 313 (1999) 91.

[18] H.W. Zhu, C.L. Xu, D.H. Wu, B.Q. Wei, R. Vajtai, P.M. Ajayan, Science 296 (2002) 884.

[19] A. Weidenkaff, S.G. Ebbinghaus, P. Mauron, A. Reller, Y. Zhang, A. Züttel, Mat. Sci. & Eng. C, C19 (2002) 119.

[20] T.T. Kodas, M.H. Hampden-Smith, Aerosol processing of materials, p 190, Wiley-VCH, New York, 1999.

[21] S. Brunauer, P.H. Emmett, E. Teller, J. Amer. Chem. Soc. 60 (1938) 309.

[22] F. Tuinstra, J.L. Koenig, J. Chem. Phys. 53 (1970) 1126. [23] A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (2000) 14095. [24] A. Lepora, C. Métraux, B. Grobety, C. Emmenegger, A. Züttel, (in

preparation). [25] A.M. Rao, E. Richter, S. Bandow, B. Chase, P.C. Eklund, K.A.

Williams, S. Fang, K.R. Subbaswamy, M. Menon, A. Thess, R.E. Smalley, G. Dresselhaus, M.S. Dresselhaus, Science 275 (1997) 187.

[26] A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu, Y.H. Lee, S.G. Kim, A.G. Rinzler, D.T. Colbert, G.E. Scuseria, D. Tomanek, J.E. Fischer, R.E. Smalley, Science 273 (1996) 483.

[27] J. Kong, A.M. Cassell, H. Dai, Chem. Phys. Lett. 292 (1998) 567. [28] G.L. Hornyak, L. Grigorian, A.C. Dillon, P.A. Parilla, K.M. Jones, M.J.

Heben, J. Phys. Chem. B 106 (2002) 2821.

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Chapter 5

Carbon nanotubes synthesised by fluidised-bed CVD

Ph. Mauron, Ch. Emmenegger, P. Sudan, P. Wenger, S. Rentsch, A. Züttel

University of Fribourg, Physics Department, Pérolles, CH-1700 Fribourg, Switzerland

Processing and Fabrication of Advanced Materials XI, Edited by T.S. Srivatsan and R.A. Varin, ASM International (Columbus, Ohio USA) 2003,

p. 93-104

Abstract

Large quantities of nanotubes can be produced by the fluidised bed technique due to the high specific surface area of the precursor powder and the excellent convection in the fluidised bed reactor. Nanotubes were synthesised by chemical vapour deposition (CVD) of iso-pentane (C5H12) or acetylene (C2H2) on a metal oxide supported precursor e.g. magnesium oxide (MgO) powder with a specific surface area of A = 100 m2/g impregnated with an iron nitrate (Fe(NO3)3) solution.

The influence of the iron content in the precursor (1.25-15 %), the synthesis temperature (450-850 °C) and the synthesis time (0.5-40 min.) on the synthesised material was investigated. The yield of the deposited carbon nanotubes reaches 32 wt% (T = 700 °C, t = 40 min.) and after removing the magnesium the samples consists of ≈ 98 % carbonous material mostly multi and single walled carbon nanotubes.

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5.1 Introduction Carbon nanotubes are a new form of carbon discovered by Iijima [1] in 1991 six years after the discovery of an other new form of carbon namely the fullerenes by Kroto et al. [2]. The smallest fullerene is a C60 that is composed of 12 pentagons and 20 hexagons ordered in the form of a football. The other so far well known modifications of carbon are sp2 bonded graphite and sp3 bonded diamond. Carbon nanotubes are cylindrical hollow structures consisting of sp2 bonded carbon atoms. A single walled nanotube (SWNT) can be described as a rolled up graphite sheet that is closed at each end with a half of a fullerene. There exists different directions in which the graphite sheet can be rolled up to form a cylinder of various diameters dt. The chirality is described by the chiral angle θ [3].The graphite sheet is rolled up in the direction of the chiral vector C h that is the sum of the basis vectors a 1 and a 2 of the unit cell of graphite multiplied by two integers n and m ( C h = n ⋅ a 1 + m ⋅ a 2 ). The chiral angle θ and the tube diameter dt are uniquely related to the integers n and m by:

( )mnm += 2/3tanθ , π/122 amnmndt ⋅⋅++=

Each SWNT can therefore be characterised by the integers n and m. Multi walled nanotubes (MWNT) consist of several graphite cylinders nested one into another. Carbon nanotubes are regarded as a promising material for different potential applications due to their unique physical and chemical properties e.g. as electrode for electrochemical double-layer capacitors [4,5] or as hydrogen storage material [6,7]. Due to the high specific surface area we use nanotubes as electrode material for electrochemical double-layer capacitors. Nanotubes are directly grown on aluminium substrates or a paste of nanotubes with an organic binder is applied on the aluminium electrodes. For such applications large quantities of nanotubes are necessary. Although nanotubes have attracted a lot of interest in the last ten years a cheap and if possible continuous large scale synthesis process of MWNT and SWNT is still a challenge. In the last years a lot of work has been done in the field of CVD

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synthesis of nanotubes [8-11] because of some important advantages compared to the arc-discharge evaporation [1,12,13] and laser-ablation [14] methods used before. Due to the simpler components and lower temperatures required for the synthesis (<3000 °C) a scale-up of the CVD-method is technically easier and cheaper to realise than with the two other methods.

5.2 Experimental 5.2.1 Description of the fluidised-bed CVD reactor The CVD apparatus used in our experiments is composed of a vertical furnace and a quartz glass tube with a length of 40 cm and a diameter of 3 cm. In the middle of the tube a quartz filter of porosity 3 (pores of 20 to 40 µm) is mounted (Fig. 1). The precursor powder is filled in the tube on the quartz filter and is fluidised by the gas flow. For a given substrate powder a certain gas flow is necessary that the bed is fluidised. In our case a flow of 410 sccm is needed.

Fig. 1: fluidised-bed CVD reactor


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For the reaction e.g. 42 sccm of C2H2 or 10 sccm of Ar that pass through a bubbler filled with the liquid iso-pentane (C5H12) is mixed with 360 respectively 400 sccm Ar witch is the carrier gas. 10 sccm Ar that pass through the bubbler with iso-pentane evaporate the same quantity of carbon as 42 sccm of acetylene.

5.2.2 Synthesis of nanotubes The precursor powder is a magnesium oxide (MgO, A = 100 m2/g) supported iron oxide powder produced by impregnation in an iron nitrate ethanol solution. To get a precursor with e.g. a MgO to Fe weight ratio of 5 %, 25 g of MgO were suspended in 100 ml ethanol and 9.19 g of iron nitrate (Fe(NO3)3·9H2O) previously dissolved in 100 ml ethanol and stirred together and sonicated for 20 min. in order to get a homogenous mixture. Afterwards the precursor was dried and grinded into a fine powder.

For one deposition typically 1.5 g of precursor powder was filled in the quartz tube and the atmosphere was purged with 410 sccm of argon for 5 min. Then the furnace was heated to the synthesis temperature. By heating up the precursor powder iron oxide clusters were formed [15,16] due to the thermal decomposition of the iron nitrate at 125 °C [17]. The synthesis was started with introduction of e.g. 42 sccm C2H2 mixed with 368 sccm argon for 20 min.

In order to remove the MgO substrate one gram of the “as produced” nanotubes was stirred in 100 ml of 10 % HCl for 15 hours at a temperature of 75 °C. Afterwards the nanotubes were filtered and thoroughly washed with distilled water and dried.

In this paper we describe the influence of different synthesis parameters like the synthesis temperature (450-850 °C), the iron content in the precursor (1.25-15 %), the synthesis time (0.5-40 min.) and the type of carbon source on the specific BET surface area, the yield and the graphitisation degree of the purified nanotubes. The synthesised material was characterised by scanning- and transmission electron microscopy (SEM/ TEM), X-ray diffraction (XRD), micro Raman spectroscopy (RS) and the specific surface area was determined by BET [18].

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5.3 Results In order to study the influence of the iron content in the precursor powder on the morphology and type of nanotubes the iron ratio in the precursor powder was varied from 1.25 to 15.0 % at a fixed temperature of T = 700 °C, a synthesis time of 20 min. and 10 sccm iso-pentane as carbon source. The yield of carbon was determined by the ratio of the weight of the “as produced” nanotubes oxidised at 800 °C and the weight before the oxidation. Fig. 2 shows the yield linearly increasing as a function of the iron content. TEM investigations showed that the nanotubes are mostly multi walled tubes (Fig. 3a), b) & e)) even though the iron content was varied in a rather large range. The MWNT have diameters in the range of 5 to 30 nm. The tube diameters are not monodisperse in each sample but it turned out that the tubes synthesised at higher Fe ratios are more regular.

The BET surface area increases remarkably with decreasing iron content form about 260 m2/g at 15 % to 1200 m2/g at 2.5 % where a maximum is reached (Fig. 4). This behaviour indicates that the structures in the sample e.g. the diameter of the nanotubes are getting smaller. This fact is also seen with micro raman spectroscopy. Raman is an excellent method to investigate carbonous materials because the different bond types (sp2 and sp3) can be distinguished.

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30

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20

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10

yiel

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Fig. 2: Carbon yield vs. iron ratio in the precursor powder for nanotubes synthesised at700 °C with iso-pentane for 20 min.


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The spectra were acquired with an Ar ion laser with a wavelength of λ = 514.5 nm. In the first order raman spectra of the nanotubes two major peaks are seen (Fig 5). The so called D peak (disorder) at ~1350 cm-1 and the G peak (graphite) at ~1580 cm-1. Additional features are present at ~1100 cm-1 [19] and ~1500 cm-1 [20]. In this domain pure graphite only shows the E2g raman mode at 1582 cm-1. The D peak is forbidden in perfect graphite and only becomes active in presence of disorder [21,22]. For graphite the ratio of the integrated D and G peak varies inversely with the crystallite size [22]. In

Fig. 3: a) TEM images of MWNT synthesised for 20 min. at 700 °C with iso-pentaneas carbon source, Fe ratio: 5 %, b) Fe ratio: 10 %, c) & d) SWNT bundles synthesisedat 800 °C with acetylene, e) SEM images of MWNT produced with iso-pentane at700 °C, f) SWNT bundles synthesised with acetylene at 850 °C

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our case the ratio of the D and G peak is increasing for lower Fe ratios (Fig. 5) this means that the structures are smaller or less ordered. Compared to arc-discharge MWNT [23] the ratio of the D and G peak is relatively low but comparable to other CVD nanotubes [24,25]. This could be due to the relatively low synthesis temperature. The fact of reducing the iron content that reduces the size of the Fe2O3 particles [16] that serve as nucleation centres for the nanotube growth did not result in a synthesis of SWNT.

1200

1000

800

600

400

200

BET

surf

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]

151050Fe ratio [%]

Fig. 4: BET surface as a function of the iron ratio in the precursor powder fornanotubes synthesised at 700 °C with iso-pentane.

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nsity

[a.u

.]

18001600140012001000raman shift [cm-1]

1.25 % 2.5 % 5 % 7.5 % 10 % 15 %

Fig. 5: First order raman spectra of nanotubes synthesised at 700 °C for 20 min. withdifferent iron ratios in the precursor powder and iso-pentane as carbon source. Thespectra are normalised and shifted.


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The nanotube quantity synthesised as a function of time is represented in Fig. 6. As one can see the growth of nanotubes by decomposition of acetylene in the fluidised bed is almost stopped after 40 min. at 700 °C. The straight line would correspond to the yield when all acetylene is consumed.

By varying the synthesis temperature from 450 to 850°C at a fixed iron ratio of 5 %, a synthesis time of 20 min. and 42 sccm acetylene as carbon source the yield increased exponentially (Fig. 7).

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max. yield - yield

yield

Fig. 6: Carbon yield as a function of time. The line termed “max. yield” is the yieldsupposed all acetylene is converted.

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yiel

d [w

t%]

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Fig. 7: Carbon yield vs. synthesis temperature for acetylene as carbon source with aniron ratio of 5 %.

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TEM investigations showed that at the lowest temperature of 450 °C no nanotubes were synthesised. In the temperature range of 500 to 650 °C MWNT were grown and at temperatures from 700 °C up to 850 °C SWNT were synthesised (Fig. 3c) d) & f)). The MWNT have again diameters from 5 to 30 nm. The SWNT are grouped into bundles of different diameters typically 10 to 30 nm. XRD showed that the (002) diffraction peak of graphite at 2θ≈26° for the nanotubes is still present, indicating that MWNT or another graphitic phase is present in the sample. Pure SWNT do not show a (002) peak but rather peaks at lower angles due to the periodic hexagonal order of the SWNT in bundles [26]. It is interesting to remark that no SWNT bundles were deposited at 700 °C with iso-pentane as the carbon source whereas with acetylene SWNT were present with the same iron ratio of 5 % in the precursor powder. This means for the growth of SWNT the type of gas used plays also an important role. The change of the nanotube type as a function of the temperature is clearly seen in the raman spectra. In Fig. 8 the D and G peaks of the samples are plotted. They are normalised to 100 % and up shifted with decreasing temperature. From 450 to 650 °C the appearance of the lines are more or less the same. The intensity of the D peak gets smaller compared to the G peak as the temperature is increased. At temperatures of 700 °C to

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nsity

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.]

18001600140012001000raman shift [cm-1]

450 °C 500 °C 550 °C 600 °C 650 °C 700 °C 750 °C 800 °C 850 °C

Fig 8: First order raman spectra of nanotubes synthesised at different temperatures for20 min. with an iron ratio in the precursor powder of 5 % and acetylene as carbonsource. The spectra are normalised and shifted.


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800 °C the intensity of the G-peak becomes higher and is decreasing again at 850 °C. The absolute intensities of the lines are considerably increased at temperatures between 700 and 850 °C with a maximum at 800 °C compared to lower temperatures. The spectra of MWNT in Fig. 5 and that of SWNT (Fig. 8) show a clear difference at the G peak. For MWNT we have a shoulder at the right side of the peak and for SWNT the shoulder is on the left side. For SWNT different raman modes at lower wavenumbers than 1590 cm-1 were calculated for different SWNT types [27]. The BET surface shows a maximum of 680 m2/g at 600 °C and decreases to 280 m2/g at 850 °C (Fig. 9). The maximum is reached at low temperatures where the ratio of the D and G peak is high in the raman spectra. SWNT theoretically have a surface of 2628 m2/g when the inner and outer surface of the tube are taken into consideration. But when the tubes are closed at one end with a fullerene cap and at the other side by a carbide particle that was not removed the surface is divided by a factor of two. Furthermore SWNT are in the ideal case bundled in a hexagonal lattice therefore not the total surface can be reached by the nitrogen molecules.

700

600

500

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surf

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]

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Fig. 9: BET surface of nanotubes synthesised at different temperatures with acetylenefor 20 min. with an iron ratio in the precursor powder of 5 %.

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5.4 Summary and Conclusion With the fluidised-bed CVD synthesis large quantities (~0.5 g) of carbon nanotubes could be produced because of the high specific surface area of the precursor powder on which the nanotubes were grown. A precursor powder of the type Fe2O3/MgO has the advantage that it is easily removed with a diluted hydrochloric acid. Different synthesis parameters like the synthesis time, synthesis temperature, the iron ratio in the precursor powder were investigated. As carbon source acetylene and iso-pentane were used. Depending on the synthesis temperature either MWNT or SWNT can be synthesised with acetylene as the carbon source. For Temperatures of 500 to 650 °C MWNT and for higher temperatures of 700 to 850 °C also SWNT were synthesised. When iso-pentane was used as the carbon source only MWNT were grown at 700 °C with the same iron ratio of 5 % that resulted in SWNT with acetylene at the same temperature. The BET surface of the product is dependant on the iron ratio in the precursor powder and the synthesis temperature. It reaches a maximum of 1200 m2/g for iron ratios of 2.5 % at 700 °C with iso-pentane and 680 m2/g at 600 °C for iron ratios of 5 % with acetylene. The BET surface is high for samples where the ratio of the D and G peak is high.


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5.5 References

[1] S. Iijima, Nature, 354, 56-8 (1991) [2] H.W. Kroto, J.R. Heath, S.C. O’Brien, R.F. Curl, R.E. Smalley, Nature,

318, 162-3 (1985) [3] M.S. Dresselhaus, G. Dresselhaus, R. Saito, Edited by M. Endo, S.

Iijima, M.S. Dresselhaus, Carbon nanotubes, p 27–35, Pergamon, Great Britain (1996)

[4] Ch. Emmenegger, Ph. Mauron, P. Sudan, P. Wenger, V. Hermann, R. Gallay, A. Züttel, to be published in the 11th International Symposium on Processing and Fabrication of Advanced Materials, Edited by T.S. Srivatsan and R.A. Varin, Columbus, Ohio-USA (2002)

[5] K.H. An, W.S. Kim, Y.S. Park, J.M. Moon, D.J. Bae, S.C. Lim, Y.S. Lee, Y.H. Lee, Adv. Funct. Mat., 11, 387-92 (2001)

[6] L. Schlapbach, A. Züttel, Nature, 414, 23-8 (2001) [7] A. Züttel, P. Sudan, Ph. Mauron, T. Kiyobayashi, Ch. Emmenegger, L.

Schlapbach, Int. J. Hydrogen Energy, 27, 203-212 (2002) [8] M. José-Yacamán, M. Miki-Yoshida, L. Rendón, J.G, Santiesteban.

Appl. Phys. Lett., 62, 657-9 (1993) [9] H.W. Zhu, C.L. Xu, D.H. Wu, B.Q. Wei, R. Vajtai, P.M. Ajayan,

Science, 296, 884-6 (2002) [10] J. Kong, A.M. Cassell, H. Dai, Chem. Phys. Lett., 292, 567-74 (1998) [11] P. Nikolaev, M.J. Bronikowski, R.K. Bradley, F. Rohmund, D.T.

Colbert, K.A. Smith, R.E. Smalley, Chem. Phys. Lett., 313, 91-7 (1999) [12] D.S. Bethune, C.H. Kiang, M.S. de Vries, G. Gorman, R. Savoy, J.

Vazquez, R. Beyers, Nature, 363, 605-7 (1993) [13] C. Journet, P. Bernier, Appl. Phys. A, 67, 1- 9 (1998) [14] T. Guo, P. Nikolaev, A. Thess, D.T. Colbert, R.E. Smalley, Chem.

Phys. Lett., 243, 49-54 (1995) [15] M. Yudasaka, R. Kikuchi, Y. Ohki, E. Ota, S. Yoshimura, Appl. Phys.

Lett., 70, 1817-8 (1997) [16] Ph. Mauron, Ch. Emmenegger, A. Züttel, Ch. Nützenadel, P. Sudan, L.

Schlapbach, Carbon, 40, 1339-44 (2002)

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[17] T.T. Kodas, M.H. Hampden-Smith, Aerosol processing of materials, p 190, Wiley-VCH, New York (1999)

[18] S. Brunauer, P.H. Emmett, E. Teller, J. Amer. Chem. Soc., 60, 309-19 (1938)

[19] R.J. Nemanich, S.A. Solin, Phys. Rev. B, 20, 392-401 (1979) [20] T.C. Chieu, M.S. Dresselhaus, M. Endo, Phys. Rev. B, 26, 5867-77

(1982) [21] F. Tuinstra, J.L. Koenig, J. Chem. Phys, 53, 1126-30 (1970) [22] A.C. Ferrari, J. Robertson, Phys. Rev. B, 61, 14095-107 (2000) [23] H. Hiura, T.W. Ebbesen, K. Tanigaki, Chem. Phys. Lett., 202, 509-12

(1993) [24] W. Li, H. Zhang, C. Wang, Y. Zhang, L. Xu, K. Zhu, S. Xie, Appl.

Phys. Lett., 70, 2684-6 (1997) [25] M. Sveningsson, R.-E. Morjan, O.A. Nerushev, Y. Sato, J. Bäckström,

E.E.B. Campbell, F. Rohmund, Appl. Phys. A, 73, 409-18 (2001) [26] A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu, Y.H.

Lee, S.G. Kim, A.G. Rinzler, D.T. Colbert, G.E. Scuseria, D. Tomanek, J.E. Fischer, R.E. Smalley, Science, 273, 483-7 (1996)

[27] A.M. Rao, E. Richter, S. Bandow, B. Chase, P.C. Eklund, K.A. Williams, S. Fang, K.R. Subbaswamy, M. Menon, A. Thess, R.E. Smalley, G. Dresselhaus, M.S. Dresselhaus, Science, 275, 187-91 (1997)

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Chapter 6

Carbon nanotubes synthesised with methane on Fe/MgO and CoNi/MgO substrates in a fluidised bed

Ph. Mauron, P. Sudan, P. Wenger, R. Gremaud, A. Züttel

University of Fribourg, Physics Department, Pérolles, CH-1700 Fribourg, Switzerland

submitted to Carbon

Abstract

Carbon nanotubes were synthesised on two different substrates (Fe/MgO, CoNi/MgO) by chemical vapour deposition (CVD) of methane (CH4) in a fluidised bed reactor with a yield of up to 15% for the Fe/MgO substrate at 750°C. For the synthesis with methane there is an activation time necessary until the growth of nanotubes begins, which decreases exponentially with the synthesis temperature. It was found that the activation time depends also on the substrate type. It is lower for the Fe/MgO substrate. With a long synthesis time of > 45 min, single walled nanotubes could also be synthesised at a low temperature of 600°C on the Fe/MgO substrate. The synthesised nanotubes were characterised by scanning- and transmission electron microscopy, Raman spectroscopy and thermo gravimetric analysis.

Chapter 6 Carbon nanotubes synthesised with methane on Fe/MgO and CoNi/MgO…

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6.1 Introduction The chemical vapour deposition (CVD) method is a technically simple and cost-effective manner to synthesise single- and multi-walled carbon nanotubes [1,2] (SWNT/MWNT). The arc discharge [3-5] and laser ablation [6] methods have the disadvantage that they are operated at higher temperatures, that they need expensive components and that a scale-up is complicated to carry out. In the CVD method the amount of nanotubes synthesised on a flat substrate is small because only a small surface is exposed to the reaction gas. The synthesis on a substrate powder in a fixed bed set-up is inefficient because for large powder quantities the diffusion of the gas through the powder is low. Promising for a large-scale synthesis in the industrial production are the following relatively new CVD-variations, the synthesis in the gas phase [7, 8] and the synthesis in a fluidised bed reactor [9-15]. The synthesis in the gas phase has the disadvantage that a rather large amount of encapsulated nanoparticles are produced [16]. With the fluidised bed method a large quantity of a high surface area precursor powder is in good contact with the reaction gas due to the fluidisation of the powder. A scale-up of this system is much easier than with the other methods. We have [15] shown that depending on the temperature, SWNT and MWNT could be synthesised with C2H2 on a Fe/MgO substrate, which is easy to remove with an HCl solution.

Methane has the advantage that the self-pyrolysis is much lower because it is chemically more stable as compared e.g. to acetylene. This leads to purer nanotubes with less amorphous carbon impurities. A further advantage of methane is its cheap availability.

Hornyak et al. [17] reported the synthesis of SWNT with methane in a temperature window from 680 to 850°C with an iron molybdenum impreg-nated Al2O3 catalyst. They claimed that the turn-on at the low-temperature side appears to be controlled by the thermodynamics of SWNT growth.

In this paper we show that SWNT can be synthesised with the fluidised bed method on Fe/MgO and CoNi/MgO substrates, that the temperature win-dow is due to the activation of the reaction that is temperature dependant and that the low temperature side of the temperature window can be pushed down to even lower temperatures (at least down to 600°C) for longer synthesis times.

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6.2 Experimental The fluidised bed CVD apparatus used in these experiments consists of a vertical furnace and a quartz glass tube in which in the middle a quartz filter is mounted. For more details see ref. [15].

The two investigated substrates were produced by impregnation of a MgO powder (100 m2·g-1) in a metal nitrate ethanol solution. For both substrates a MgO to metal weight ratio of 5% was chosen [15] and for the CoNi/MgO substrate the Co:Ni atom ratio was 1:1. 25g of MgO were suspended in 100 ml ethanol and the metal nitrate previously dissolved in 100 ml ethanol were stirred together and sonicated for 20 min in order to homogenise the mixture. Afterwards the precursor was dried and grinded into a fine powder.

For each deposition 1.5 g of precursor powder was filled into the quartz tube and the atmosphere was purged with 410 sccm argon prior to the heating-up process. By heating up the precursor powder metal oxide clusters were formed [18] due to the thermal decomposition of the metal nitrate. For the reaction 72 sccm methane was mixed with the carrier gas (338 sccm argon) to a total gas flow of 410 sccm in order to fluidise the substrate powder. The synthesis time was fixed to 45 min and all the experiments were performed at atmospheric pressure.

After the synthesis the MgO substrate was removed by stirring one gram of the “as produced” nanotubes in 10% HCl for 15 hours at 75°C. Then the nanotubes were filtered, washed with distilled water and dried in air.

The yield of carbon was determined by thermogravimetric analysis (TGA). The nanotubes were heated in an oxygen/nitrogen atmosphere with a ramp of 5°C/min. The yield was defined as the relative weight loss due to the oxidation of the carbon.

In this paper, we report the influence of the synthesis temperature (600–900°C) and sort of metal (Fe or CoNi) on the MgO substrate on the yield and type of carbonaceous material. The outlet of the reactor was connected to a mass spectrometer in order to determine the evolution of the reaction. The products were characterised by scanning- and transmission electron microscopy (SEM/TEM), Raman spectroscopy (RS) and thermogravimetric analysis (TGA).


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6.3 Results 6.3.1 Synthesis of nanotubes on the Fe/MgO substrate The influence of the synthesis temperature on the yield and type of carbona-ceous material was studied by varying the temperature between 600 and 900°C. For low temperatures of 600 and 620°C there was no carbon deposition (Fig.1) but at 650°C nanotubes were deposited and the yield increased consid-erably from zero to about 8%. A maximum of 14.6% was reached at 750°C and for higher temperatures the yield decreased again to 4.2% at 900°C.

A SEM image of nanotubes synthesised at 800°C is shown in Fig. 2a and a TEM image of nanotubes produced at 650°C is shown in Fig. 2c. The produced material is dense and is composed mostly of bundles of SWNT. The bundle diameter varies between 6 and 60 nm for nanotubes synthesised at 800°C (Fig. 2a). On Fig. 2c an irregular branched SWNT bundle is shown. The round patterns correspond to SWNT perpendicular to the image plane.

The Raman spectra of the nanotubes in the range of the G-band as a function of the synthesis temperature are shown in Fig. 3. The spectra were taken with an argon laser with a wavelength of 514.5 nm. It is clearly seen that the G-bands have a shoulder at the left side. This split into multiple modes of the G-band is typical for SWNT [19]. SWNT are present for all temperatures in the sample. Furthermore the D-band decreases for higher temperatures.

Fig. 1: Carbon yield as a function of the synthesis temperature on Fe/MgO withmethane as carbon source for a synthesis time of 45 min.

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During the synthesis the gas that passed the fluidised bed was analysed with a mass spectrometer at the outlet of the system. In Fig. 4 the amount of methane is given as a function of the synthesis time. The dotted line corresponds to the time where 72 sccm of methane were introduced in the reactor. The solid lines correspond to the methane measured at the outlet of the reactor as a function of the synthesis time for three different synthesis temperatures. At e.g. a synthesis temperature of 680°C for approximately 11 minutes the methane pressure is constant and then it decreases. After passing a minimum the pressure decreases again and is almost constant at the end of the synthesis. It seems that there is some activation needed before the reaction

Fig. 2: SEM images of nanotubes synthesised with CH4 for 45 min on Fe/MgO at 800°C (a) and on CoNi/MgO at 750°C (b). TEM image of SWNT bundles synthesisedat 650°C on Fe/MgO (c), on CoNi/MgO at 800°C (d) and on Fe/MgO at 600°C (t > 45min) (e).


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Fig. 3: Raman spectra of nanotubes as a function of the synthesis temperature synthesised with CH4 on the Fe/MgO substrate. The spectra are normalised and shifted. (λ = 514.5 nm)

begins. The minimum corresponds to the maximum of methane consumption. Exactly the inverse behaviour is seen for the hydrogen pressure (Fig. 5), it is constant for 11 minutes, reaches a maximum after 15 minutes and decreases again. As defined in Fig. 5 the activation time ta and the time to reach the maximum in methane consumption tm are plotted in Fig. 6 as a function of the synthesis temperature. It is clearly seen that both times decrease exponentially

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with increasing temperature. With an exponential fit of the measured points the lowest temperature where we can expect to get nanotubes within 45 min was calculated to be at 625°C. The decrease of methane consumption is faster for higher temperatures. For synthesis times longer than 45 min SWNT could

Fig. 5: H2 pressure measured for three different synthesis temperatures as a function of the synthesis time. ta corresponds to the activation time of the reaction and tm to the maximum of H2 pressure. Between the two dotted lines, 72 sccm of CH4 were introduced in the reactor.

Fig. 4: CH4 pressure measured at the outlet of the furnace for three different synthesistemperatures as a function of the synthesis time. Between the two dotted lines 72 sccmof CH4 were introduced in the reactor.


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also be synthesised at a low temperature of 600°C (Fig. 2e) but with a longer activation time of ta > 45 min. The corresponding Raman spectrum is given in Fig. 7.

Fig. 6: Activation time ta and maximum of H2 pressure tm as a function of the synthesis temperature for the synthesis of nanotubes on Fe/MgO with CH4.

Fig. 7: Raman spectrum of nanotubes synthesised at a low temperature of 600°C withCH4 on the Fe/MgO substrate for a synthesis time > 45 min. The spectrum is normalised. (λ = 514.5 nm)

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6.3.2 Synthesis of nanotubes on the CoNi/MgO substrate The yield on the CoNi/MgO substrate was much lower than on the Fe/MgO. The onset of nanotubes was at 670°C and reached a maximal yield of only 3% at 750°C.

Fig. 2b shows a SEM image of SWNT bundles synthesised at 750°C on CoNi/MgO. A SWNT bundle with a diameter varying between 30 and 45 nm is seen in the TEM image in Fig. 2d. The products consist mostly of SWNT bundles.

The Raman spectra of nanotubes synthesised at different temperatures are given in Fig. 8. All the spectra show typical fingerprints of SWNT at the left side of the G-band. In contrast to nanotubes grown on Fe/MgO the D-band has a minimum at 700°C.

Fig. 8: Raman spectra of nanotubes as a function of the synthesis temperaturesynthesised with CH4 on the CoNi/MgO substrate. The spectra are normalised andshifted. (λ = 514.5 nm)


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The activation time ta as a function of the temperature is shown in Fig. 9. It also decreases with temperature as with the Fe/MgO substrate but it is higher. With the exponential fit of the measured points the lowest temperature where we can expect to get nanotubes within 45 min was calculated to be at 662°C. In contrast to Fe/MgO, ta and tm are very close. This means that when the reaction is once activated its deactivation begins shortly afterwards.

6.4 Discussion Compared to the synthesis of nanotubes with methane we did not observe an activation time for the synthesis of nanotubes with acetylene. For this carbon source the conversion is maximal at the beginning and decreases as a function of time. The yield increases exponentially as a function of temperature [15] whereas with methane the yield decreases at temperatures above 750°C. For higher temperatures the deactivation is much faster as can be seen in Fig. 4 and 5. The peaks are more pronounced for higher temperatures and the integral of the H2 peaks follow the same type of curve as in Fig. 1.

Fig. 9: Activation time ta and maximum of H2 pressure tm as a function of the synthesis temperature for the synthesis of nanotubes on CoNi/MgO with CH4.

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Like Hornyak et al. [17] we also have found a temperature window (625 to 900°C) but the low-temperature side seems not to be controlled by the thermodynamics of the SWNT growth. The temperature window can be pushed down to lower temperatures (at least down to 600°C) and seems to be given by the activation of the reaction, which depends on the temperature. The assumption that the low-temperature side is not controlled by the thermodynamics of the SWNT growth is also supported by Maruyama et al. [20] who synthesised SWNT at low temperatures of 550°C with alcohol.

In order to understand better this activation time we tried to synthesise nanotubes in a hydrogen atmosphere. For a temperature of 680°C on Fe/MgO for 45 min three experiments were performed. 1) heating up and synthesis in CH4/H2 mixture (72/338 sccm). 2) heating up in argon and also synthesis in CH4/H2 mixture (72/338 sccm) but no nanotubes were grown with these parameters. 3) heating up in hydrogen and synthesis in CH4/Ar mixture (72/338 sccm) with the rather unexpected result that the activation time ta and the maximum of methane conversion tm were shifted up by approximately 5min. It is expected that the presence of hydrogen would facilitate the reduction of the iron oxide particles and therefore the activation time would be shifted down. By changing the methane concentration in the gas mixture the activation time could also be lowered with a higher methane concentration.

6.5 Conclusion Carbon nanotubes were synthesised in a fluidised bed reactor on two different substrates (Fe/MgO, CoNi/MgO) by chemical vapour deposition of methane with a yield up to 15% for the Fe/MgO substrate at 750°C. We have shown that there is an activation time necessary for the synthesis of carbon nanotubes with methane as the carbon source. This activation time decreases exponentially with the synthesis temperature. It was found that the activation time is lower for the Fe/MgO substrate than for CoNi/MgO. With a long synthesis time > 45 min, SWNT could also be synthesised at a low temperature of 600°C on Fe/MgO. The synthesis time, the type of substrate and the methane concentration in the gas mixture give the low-temperature onset for the nanotube growth.


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6.6 References

[1] José-Yacamán M, Miki-Yoshida M, Rendón L, Santiesteban JG. Appl Phys Lett 1993;62(6):657-9.

[2] Dai H, Rinzler AG, Nikolaev P, Thess A, Colbert DT, Smalley RE. Chem Phys Lett 1996;260(3-4):471-5.

[3] Iijima S. Nature 1991;354(6348):56-8. [4] Bethune DS, Kiang CH, de Vries MS, Gorman G, Savoy R, Vazquez J,

et al. Nature 1993;363(6430):605-7. [5] Journet C, Bernier P. Appl Phys A 1998;67(1):1-9. [6] Guo T, Nikolaev P, Thess A, Colbert DT, Smalley RE. Chem Phys Lett

1995;243(1-2):49-54. [7] Nikolaev P, Bronikowski MJ, Bradley RK, Rohmund F, Colbert DT,

Smith KA, et al. Chem Phys Lett 1999;313(1-2):91-7. [8] Zhu HW, Xu CL, Wu DH, Wei BQ, Vajtai R, Ajayan P. Science

2002;296(5569):884-6. [9] Liu BC, Liang Q , Tang SH , Gao LZ, Zhang BL, Qu MZ, et al. Chin

Chem Lett 2000;11(11):1031-4. [10] Weidenkaff A, Ebbinghaus SG, Mauron Ph, Reller A, Zhang Y, Zuttel

A. Mat Sci Eng C 2002;19(1-3):119-23. [11] Wang Y, Wei F, Luo G, Yu H, Gu G. Chem Phys Lett 2002;364(5-

6):568-72. [12] Wang Y, Wei F, Gu G, Yu H. Physica B 2002;323(1-4):327-9. [13] Venegoni D, Serp Ph, Feurer R, Kihn Y, Vahlas C, Kalck Ph. Carbon

2002; 40(10):1799-807. [14] Qian W, Yu H, Wei F, Zhang Q, Wang Z. Carbon 2002;40(15):2968-

70. [15] Mauron Ph, Emmenegger Ch, Sudan P, Wenger P, Rentsch S, Züttel A.

Diamond Relat Mater 2003;12(3-7):780-5. [16] Bladh K, Falk LKL, Rohmund F. Appl Phys A 2000;70(3):317-322. [17] Hornyak GL, Grigorian L, Dillon AC, Parilla PA, Jones KM, Heben

MJ. J Phys Chem B 2002;106(11):2821-5. [18] Mauron Ph, Emmenegger Ch, Züttel A, Nützenadel Ch, Sudan P,

Schlapbach L. Carbon 2002;40(8):1339-44.

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[19] Rao AM, Richter E, Bandow S, Chase B, Eklund PC, Williams KA, et al. Science 1997;275(5297):187-91.

[20] Maruyama S, Kojima R, Miyauchi Y, Chiashi S, Kohno M. Chem Phys Lett 2002; 360(3-4):229-34.

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Danksagung

Louis Schlapbach für die Gelegenheit in seiner Forschungsgruppe eine Doktorarbeit gemacht haben zu können.

Andreas Züttel für die Gelegenheit unter seiner Leitung die Doktorarbeit geschrieben haben zu können und für die Unterstützung bei der Doktor-arbeit.

Die Mitglieder der Metallhydrid Gruppe: Christophe Emmenegger, Patrik Sudan, Pascal Wenger, Tiana Nicolet, Konstantin Siegmann, Daniel Chartouni, Christoph Nützenadel, Samuel Rentsch, R. Gremaud und Daniela Zbinden für die gelungene Zusammenarbeit und die freund-schaftliche Unterstützung.

Die Mineralogie: Bernhard Grobety, Anna Lepora und Cédric Mettraux für die interessanten Diskussionen und gute Zusammenarbeit.

Massoud Dadras für die Einführung und Unterstützung im Umgang mit dem TEM.

Jean-Nicolas Aebischer für die Geduld mit mir beim Gebrauch des Ramanspektrometers und der TGA.

Anke Weidenkaff für die konstruktiven Diskussionen. Prof. Peter Oelhafen für die Übernahme des Koreferats. Die Werkstat und Elektronik: Elmar Moser, Oswald Raetzo, Roger

Vonlanthen, Francis Bourqui und Christophe Neururer für die hervor-ragende Arbeit.

Die Mitglieder der Forschungsgruppe Festkörperphysik: Pascal Ruffieux, Michael Bielmann, Lars-Ola Nilson, Oliver Gröning, Pierre-Angelo Gröning, Carine Calli, Richard Clergereaux, Patrick Schwaller, Eliane Maillard Schaller, Martin Collaud-Coen, Olivier Küttel, Philippe Aebi, Thorsten Pillo, Joseph Hayoz, Marc Bovet, Christian Koitzsch, Florian Clerc, Dusanka Aebi-Naumovic, Elisabeth François für die gute Zusam-menarbeit.

Meine Eltern und meine Schwester für die moralische Unterstützung.

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Curriculum vitae

Persönliche Daten Name Mauron Vorname Philippe Geburtstag 06.10.1973 Heimatort Ependes (FR) Nationalität Schweiz

Ausbildung 1980 – 1986 Primarschule, Düdingen (FR) 1986 – 1988 Sekundarschule, Düdingen (FR) 1988 – 1989 Sekundarschule, Freiburg (FR) 1989 – 1993 Gymnasium, St. Michael, Freiburg (FR) 1993 – 1994 Zwischenjahr 1994 – 1999 Physikstudium, Universität Freiburg (FR) 1999 – 2003 Doktorat, Universität Freiburg (FR)

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Publications Synthesis and Application of C-Nanotubes A. Züttel, Ch. Nützenadel, Ph. Mauron, Ch. Emmenegger, P. Sudan, L. Schlapbach, A. Weidenkaff, T. Kiyobayashi and S. Orimo Proceedings of EuroCarbon 2000, Berlin, p. 1083 (2000). Carbon Nanotubes Synthesized on Metallic Substrates Ch. Emmenegger, P. Mauron, A. Züttel, Ch. Nützenadel, A. Schneuwly, R. Galley and L. Schlapbach Applied Surface Science 162, 452 (2000).

Metal nanoparticles for the production of carbon nanotubes by decomposi-tion of different carbon sources A. Weidenkaff, S.G. Ebbinghaus, A. Reller, Ph. Mauron, Y. Zhang, A. Züttel Materials Science & Engineering, C: Biomimetic and Supramolecular Systems 19, 119 (2001). Carbon nanostructures: growth, electron emission, interactions with hydrogen Schlapbach, Louis; Groning, Oliver; Nilsson, Lars-Ola; Ruffieux, Pascal; Sudan, Patrick; Mauron, Philippe; Emmenegger, Christophe; Groning, Pierangelo; Zuttel, Andreas AIP Conference Proceedings 591(Electronic Properties of Molecular Nanostructures), 609 (2001). Hydrogen Interaction with carbon nanostructures A. Züttel, P. Sudan, Ph. Mauron, Ch. Emmenegger, T. Kiyobayashi, L. Schlapbach Electrochemical and Chemical Reactivity of Amorphous and Nanocrystalline Materials 377, 95 (2001).

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Synthesis of oriented nanotube films by chemical vapor deposition Ph. Mauron , Ch. Emmenegger, A. Züttel, Ch. Nützenadel, P. Sudan, L. Schlapbach Carbon 40, 1339 (2002). Hydrogen sorption by carbon nanotubes and other carbon nanostructures A. Züttel, Ch. Nützenadel, P. Sudan, Ph. Mauron, Ch. Emmenegger, S. Rentsch, L. Schlapbach, A. Weidenkaff and T. Kiyobayashi Journal of Alloys and Compounds 330, 676 (2002). Hydrogen Storage in Carbon Nanostructures A. Züttel, P. Sudan, Ph. Mauron, T. Kyiobaiashi, Ch. Emmenegger L. Schlapbach, International Journal of Hydrogen Energie 27, 203 (2002). Hydrogen adsorption on sp2-bonded carbon: Influence of the local curvature P. Ruffieux, O. Gröning, M. Bielmann, P. Mauron, L. Schlapbach, P. Gröning Physical Review B 66, 245416 (2002). Hydrogen desorption from Lithiumtetrahydroboride (LiBH4) A. Züttel, P. Wenger, S. Rentsch, P. Sudan, P. Fischer, Ph. Mauron, Ch. Emmenegger, Proceedings of WHEC (2002). Carbon nanotubes synthesised by fluidised-bed CVD Ph. Mauron, Ch. Emmenegger, P. Sudan, P. Wenger, S. Rentsch, A. Züttel Processing and Fabrication of Advanced Materials XI, Edited by T.S. Srivatsan and R.A. Varin, ASM International (Columbus, Ohio USA) 2003, p. 93-104.

Curriculum vitae

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Hydrogen Storage Materials: Metals, Carbon and Complexes A. Züttel, S. Rentsch, P. Wenger, P. Sudan, P. Mauron, C. Emmenegger Processing and Fabrication of Advanced Materials XI, Edited by T.S. Srivatsan and R.A. Varin, ASM International (Columbus, Ohio USA) 2003, p. 107-122. Carbon nanotubes as active electrode material Ch. Emmenegger, Ph. Mauron, P. Sudan, P. Wenger, V. Hermann, R. Gallay, A. Zuettel Processing and Fabrication of Advanced Materials XI, Edited by T.S. Srivatsan and R.A. Varin, ASM International (Columbus, Ohio USA) 2003, p. 480-491. Fluidised-bed CVD synthesis of carbon nanotubes on Fe2O3/MgO Ph. Mauron, Ch. Emmenegger, P. Sudan, P. Wenger, S. Rentsch, A. Züttel; Diamond and Related Materials 12, 780 (2003). Synthesis of carbon nanotubes over Fe catalyst on aluminium and suggested growth mechanism C. Emmenegger, J.-M. Bonard, P. Mauron, P. Sudan , A. Lepora, B. Grobety, A. Zuttel, L. Schlapbach Carbon 41, 539 (2003). LiBH4 a New Hydrogen Storage Material A. Züttel, P. Wenger, S. Rentsch, P. Sudan, P. Mauron, C. Emmenegger Journal of Power Sources 118, 1 (2003). Investigations of Electrochemical Double Layer (ECDL) Capacitors Electrodes based on Carbon Nanotubes and Activated Carbon Materials Ch. Emmenegger, Ph. Mauron, P. Sudan, P. Wenger, V. Hermann, R. Gallay, A. Züttel, accepted in Journal of Power Sources.

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Physisorption of Hydrogen in Single-Walled Carbon Nanotubes P. Sudan, A. Züttel, Ph. Mauron, Ch. Emmenegger, P. Wenger, L. Schlapbach accepted in Carbon. Sorption of Hydrogen in Single-Walled Carbon Nanotubes at High Temperatures P. Sudan, A. Züttel, Ph. Mauron, P. Wenger, L. Schlapbach submitted to Carbon. Carbon nanotubes synthesised with methane on Fe/MgO and CoNi/MgO substrates in a fluidised bed Ph. Mauron, P. Sudan, P. Wenger, R. Gremaud, A. Züttel submitted to Carbon.

Documents

Growth Mechanism and Structure of Carbon Nanotubesfaculty.kfupm.edu.sa/CHE/motazali/files/very important thesis for my Ph.d.pdf · Aus dem Departement für Physik Universität Freiburg