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Synthesis, functionalization and application of few-walled carbon nanotubes

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Synthesis, Functionalization and Application of Few-Walled Carbon Nanotubes
by
Ye Hou
Department of Chemistry
Duke University
Date:_______________________
Approved:
___________________________
Jie Liu, Supervisor
___________________________
Richard A. MacPhail
___________________________
Tuan Vo-Dinh
___________________________
Thom LaBean
Dissertation submitted in partial fulfillment of
the requirements for the degree of Doctor
of Philosophy in the Department of
Chemistry in the Graduate School
of Duke University
2010
UMI Number: 3413938
All rights reserved
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ABSTRACT
Synthesis, Functionalization and Application of Few-Walled Carbon Nanotubes
by
Ye Hou
Department of Chemistry
Duke University
Date:_______________________
Approved:
___________________________
Jie Liu, Supervisor
___________________________
Richard A. MacPhail
___________________________
Tuan Vo-Dinh
___________________________
Thom LaBean
An abstract of a dissertation submitted in partial
fulfillment of the requirements for the degree
of Doctor of Philosophy in the Department of
Chemistry in the Graduate School
of Duke University
2010
Copyright by
Ye Hou
2010
Abstract
Few-walled carbon nanotubes (FWNTs) have two to three layers of
sidewalls with diameters ranging from 3 to 8 nm and length of around tens of
micrometers; they are unique muti-walled carbon nanotubes (MWNTs) with a
perfect graphitization structure as in single walled carbon nanotubes (SWNTs).
Double-walled carbon nanotubes (DWNTs) are a special type of FWNTs which
are the intermediate between SWNTs and MWNTs. Many of the applications
proposed for DWNTs require the precise control of their diameters.
In this dissertation we describe a simple, scalable approach for the
selective synthesis of high-quality DWNTs by carbon monoxide chemical vapor
deposition (CO-CVD). The inner-tube diameter distribution measured by highresolution transmission electron microscopy (HRTEM) is about 0.7~1.1nm. The
structural correlation and optical properties of high quality DWNTs were also
explored.
To further exploit the excellent properties of FWNTs, it is necessary to
functionalize them in order to disperse them well in solution. FWNTs have been
chemically functionalized via an efficient ultrasound-mediated dissolving metal
method and an ultrasound-mediated oxidation method. Such methods can be
generalized towards other types of carbon nanotubes (CNTs) and the chemically
functionalized CNTs are highly soluble in various organic solvents and aqueous
solutions respectively.
iv
The reported fluorescence from inner shells of double-walled carbon
nanotubes (DWNTs) is an intriguing and potentially useful property. A
combination of bulk and single-molecule methods was used to study the
spectroscopy, chemical quenching, mechanical rigidity, density, and TEM
structures of the near-IR emitters in DWNT samples. It was found that DWNT
inner shell fluorescence is weaker than SWNT fluorescence by a factor of at least
10,000. The near-IR emission from DWNT samples actually arises from SWNT
impurities.
Compared to SWNTs and MWNTs, thin FWNTs are believed to have
extraordinary mechanical properties. The mechanical properties of composite
films (CNTs/PVA) have been investigated. The Young’s modulus of such
composite films with only 0.2wt % functionalized FWNTs show a remarkable
reinforcement, and the Young’s modulus increased steadily with increased
concentration of FWNTs.
In order to tackle the problem of the poor conductivity of the metal oxides,
we have designed a ternary nanocomposite film composed of metal oxide
(MnO2), functionalized FWNTs and a conducting polymer (CP). Each component
in the MnO2/CNT/CP film provides a unique and critical function to achieve
optimized
electrochemical
properties.
Electrodes
prepared
composite materials exhibited excellent electrochemical properties.
v
with
ternary
Contents
Abstract ................................................................................................................iv
List of Tables ........................................................................................................ x
List of Figures .......................................................................................................xi
List of Schemes ...................................................................................................xv
List of Abbreviations ...........................................................................................xvi
Acknowledgements .......................................................................................... xviii
Chapter 1: Introduction ...................................................................................... 1
1.1 Introduction to carbon nanotubes.............................................................. 1
1.1.1 Structure of carbon nanotubes ............................................................. 1
1.1.2 The electronic band structure of SWNTs.............................................. 3
1.2 The synthesis of carbon nanotubes .......................................................... 6
1.3 The functionalization of carbon nanotubes.............................................. 12
1.3.1 Noncovalent functionalization ............................................................. 14
1.3.2 Covalent functionalization................................................................... 15
1.4 The unique physical properties of carbon nanotubes.............................. 17
1.4.1 Mechanical properties of carbon nanotubes ....................................... 17
1.4.2 Electrical properties of carbon nanotubes .......................................... 18
1.4.3 Thermal properties of carbon nanotubes............................................ 18
1.4.4 Optical properties of carbon nanotubes.............................................. 19
1.4.4.1 Optical absorption ........................................................................ 21
1.4.4.2 Photoluminescence...................................................................... 22
vi
1.4.4.3 Raman scattering......................................................................... 26
1.5 The application of carbon nanotubes ...................................................... 28
1.5.1 Carbon nanotube composites............................................................. 29
1.5.2 Energy storage ................................................................................... 30
1.5.3 Field emission..................................................................................... 31
1.5.4 Medical applications ........................................................................... 32
1.5.1 Molecular electronics.......................................................................... 33
Chapter 2: Synthesis of high quality double-walled carbon nanotubes...... 34
2.1 Introduction ............................................................................................. 34
2.2 Experimental methods............................................................................. 36
2.2.1 Catalyst preparation ........................................................................... 36
2.2.2 Synthesis and purification of DWNTs ................................................. 37
2.2.3 Electron diffraction method................................................................. 37
2.2.4 The separation of DWNTs and SWNTs.............................................. 38
2.2.5 The preparation of TEM samples ....................................................... 39
2.3 Results and discussions.......................................................................... 40
2.4 Conclusions............................................................................................. 48
Chapter 3: The functionalization of few-walled carbon nanotubes.............. 50
3.1 Introduction ............................................................................................. 50
3.2 Experimental methods............................................................................. 52
3.2.1 Synthsis of organic solvent soluble FWNTs ....................................... 52
3.2.2 Synthsis of water soluble FWNTs....................................................... 53
3.3 Results and discussions.......................................................................... 54
vii
3.4 Conclusions............................................................................................. 59
Chapter 4: Do inner shells of double-walled carbon nanotubes really
fluoresce? ......................................................................................................... 61
4.1 Introduction ............................................................................................. 61
4.2 Experimental methods............................................................................. 63
4.2.1 Synthsis of high quality DWNTs and SWNTs ..................................... 63
4.2.2 Separation of SWNTs from DWNTs ................................................... 64
4.2.3 Functionalization of DWNTs samples with diazonium salt.................. 64
4.3 Results and discussions.......................................................................... 65
4.4 Conclusions............................................................................................. 83
Chapter 5 Functionalized few-walled carbon nanotubes for mechanical
reinforcement of polymeric composites ........................................................ 86
5.1 Introduction ............................................................................................. 86
5.2 Experimental methods............................................................................. 90
5.2.1 Synthsis and purification of of SWNTs,FWNTs, MWNTs ................... 90
5.2.2 Functionalization of carbon nanotubes by 3M nitric acid .................... 91
5.2.3 Fabrication of functionalized carbon nanotubes and PVA composites91
5.3 Results and discussions.......................................................................... 92
5.4 Conclusions........................................................................................... 109
Chapter 6: Design and Synthesis of Hierarchical MnO2
Nanospheres/Carbon Nanotubes/Conducting Polymer Ternary Composite
for High Performance Electrochemical Electrodes ..................................... 110
6.1 Introduction ........................................................................................... 110
6.2 Experimental methods........................................................................... 115
6.2.1 Synthsis of FWNTs........................................................................... 115
viii
6.2.2 Preparation of hierarchical MnO2 nanosphere and MnO2/fFWNTs
composite .................................................................................................. 116
6.2.3 Functionalization of FWNTs by 6M nitric acid.................................. 117
6.2.4 Caculations....................................................................................... 119
6.2.5 Electrochemical measurement ......................................................... 119
6.2.6 Fabrication of working electrodes with airbrushing technique .......... 119
6.3 Results and discussions........................................................................ 121
6.4 Conclusions........................................................................................... 140
Chapter 7: Conclusions ................................................................................. 142
References ....................................................................................................... 144
Biography ......................................................................................................... 162
ix
List of Tables
Table 2.1: Metallicity of total twenty two DWNTs................................................ 46
Table 5.1: Mechanical properties of 0.2wt% CNTs/PVA composite films ......... 102
Table 5.2: Mechanical properties of different concentration fFWNTs/PVA
composite films................................................................................................. 108
x
List of Figures
Figure 1.1: Schematics of graphene sheet and typical SWNTs structures. .......... 2
Figure 1.2: HRTEM images of (A) SWNTs (B) FWTNs (C) MWNTs .................... 3
Figure 1.3: Three-dimensional view of the grapheme π/π* bands its 2D projection
............................................................................................................................. 4
Figure 1.4: Illustration of allowed wavevector lines leading to semiconducting and
metallic CNTs and examples of bandstructures for semiconducting and metallic
zigzag CNTs. ........................................................................................................ 6
Figure 1.5: Schematics of experimental setup of arc discharge method
8
Figure 1.6: Schematics of experimental setup of laser ablation technique ........... 9
Figure 1.7: Experimental setup of thermal CVD technique................................. 11
Figure 1.8: (A) TEM image of SWNTs ropes (B) HRTEM of a single SWNT rope
made up of ~100 SWNTs. .................................................................................. 13
Figure 1.9: Functionalization possibilities for SWNTs......................................... 14
Figure 1.10: Schematic diagram of electronic density of states for (a) metallic and
(b) semiconducting SWNTs. Arrows indicate the optically allowed interband
transitions. .......................................................................................................... 20
Figure 1.11: Kataura plot showing the different resonance excitations as function
of different energy lasers and diameters ............................................................ 21
Figure 1.12: Schematic density of electronic states for a single nanotube
structure. ............................................................................................................ 23
Figure 1.13: Emission spectrum of individual fullerene nanotubes suspended in
SDS micelles ...................................................................................................... 25
Figure 1.14: A typical Raman spectrum showing the prominent bands that are
characteristic for SWNTs. ................................................................................... 27
Figure 1.15: A prototype 4.5 in FED using a printing method ............................. 32
Figure 2.1: Photographs of catalyst .................................................................... 40
xi
Figure 2.2: Low-magnification TEM images of purified DWNTs ......................... 43
Figure 2.3: A typical high resolution TEM image of the cross section a DWNT
bundle................................................................................................................. 43
Figure 2.4: The Inner-tube diameter distribution of DWNTs measured from
HRTEM images .................................................................................................. 44
Figure 2.5: Raman spectra of raw samples and samples purified at different
temparatures ...................................................................................................... 45
Figure 2.6: Optical propertied of DWNT samples after separation. .................... 47
Figure 2.7: TEM images of density gradient separated DWNTs in different layers
from top to bottom. ............................................................................................. 48
Figure 3.1: The chemical and physical effects of ultrasound .............................. 51
Figure 3.2: The change in nanotube microstructure that occurs under low
intensity ultroasound........................................................................................... 56
Figure 3.3: Photographs of functionalized FWTNs in organic solvents............... 57
Figure 3.4: Low magnification TEM images of PS-FWNTs and HRTEM of PSFWNTs .............................................................................................................. 58
Figure 3.5: Comparison of two different nanotubes before and after
functionalization.................................................................................................. 59
Figure 4.1: Comparative absorption and emission spectra of SWNT and DWNT
samples .............................................................................................................. 69
Figure 4.2: Near-IR fluorescence images of these suspensions, recorded under
the same experimental conditions and displayed on the same false-color intensity
scale ................................................................................................................... 70
Figure 4.3: Spectral properties of individual near-IR emitters in SWNT and DWNT
suspensions........................................................................................................ 72
Figure 4.4: Fluorescence quenching of emitting species in DWNT sample by
covalent functionalization ................................................................................... 74
Figure 4.5: Near-IR fluorescence images of “long” emissive nanotubes in a
DWNT suspension reveal noticeable bending .................................................... 77
xii
Figure 4.6: Spectroscopic analysis of density gradient fractionated DWNT
samples .............................................................................................................. 80
Figure 4.7: HRTEM images of density gradient fractionated DWNT samples .... 83
Figure 5.1: HRTEM images of (a) SWNTs. (b) FWNTs. (c) MWNTs .................. 89
Figure 5.2: Photograph of liquid mixture of PVA and nanotubes placed on glass
slides. ................................................................................................................. 94
Figure 5.3: TEM images of (A) SWNTs used in this study (B) HiPCO SWNTs... 95
Figure 5.4: Normalized Raman spectra of HiPCO SWNTs and SWNT’s used in
this study ............................................................................................................ 96
Figure 5.5: Normalized Raman spectra of different type of CNTs (a) before and
(b) after functionalization .................................................................................... 98
Figure 5.6: Stress-strain curves of composite films containing (a) 0.2 wt % of
different types of CNTs. (b) different concentration of fFWNTs ........................ 100
Figure 5.7: TEM-micrographs of PVA filled with 0.2 wt% functionalized (a)
SWNTs. (b) FWNTs. (c) MWNTs...................................................................... 105
Figure 5.8: HRTEM images of functionalized (a) SWNTs. (b) FWNTs. (c) MWNTs
......................................................................................................................... 107
Figure 6.1: (A) Ragone chart showing energy density vs. power density for
various energy-storage devices. (B) A typical HEV configuration. (C) Market of
supercapacitors ................................................................................................ 112
Figure 6.2: Comparison between different electrode materials......................... 113
Figure 6.3: SEM images of hierarchical MnO2 nanospheres ............................ 117
Figure 6.4: Normalized Raman spectra of FWNTs before and after 6M HNO3
treatment .......................................................................................................... 118
Figure 6.5: Photograph of working electrode after 1000 charge-discharge cycles
......................................................................................................................... 120
Figure 6.6: Sketch of MnO2/fFWNTs/PEDOT:PSS ternary composite ............. 123
Figure 6.7: TEM image of fFWNTs ................................................................... 124
xiii
Figure 6.8: TEM (A) and SEM (B) image of PEDOT:PSS dispersed MnO2
nanospheres in situ grown on fFWNTs............................................................. 125
Figure 6.9: TEM image of direct mixing of MnO2 nanospheres with fFWNTs ... 126
Figure 6.10: XRD pattern of the MnO2/fFWNTs composite .............................. 128
Figure 6.11: XPS spectrum of the MnO2/fFWNTs composite ........................... 129
Figure 6.12: XPS spectra of the composite in Mn 3s (A) and O 1s (B) region.. 130
Figure 6.13: (A) Cyclic voltammograms (scanned from 0-1 V in 1 M Na2SO4), (B)
Galvanostatic charge/discharge curve (at current density of 5mA/cm2) of MnO2
film (black), MnO2 /PEDOT:PSS composite (red) and MnO2/fFWNT/PEDOT:PSS
ternary composite(blue).................................................................................... 133
Figure 6.14: (A) Cyclic voltammograms (scanned from 0-1 V in 1 M Na2SO4), (B)
Galvanostatic charge/discharge curve (at current density of 5mA/cm2) of MnO2
film (black), MnO2 /PEDOT:PSS composite (red) and MnO2/fFWNT/PEDOT:PSS
ternary composite(blue)(down) ......................................................................... 136
Figure 6.15: (A) Typical galvanostatic charge/discharge cycle curves of ternary
composite electrodes obtained at current density of 1.0mA/cm−2, (B) Chargedischarge cycle test .......................................................................................... 138
Figure 6.16: Specific capacitance of MnO2/fFWNT/PEDOT:PSS ternary
composite(blue), MnO2/PEDOT:PSS composite(red), MnO2 film (black) at
different charge/discharge current densities ..................................................... 140
xiv
List of Schemes
Scheme 2.1: Decomposition of methane and carbon monoxide......................... 42
Scheme 3.1: Mechanism of ultrasound-mediated functionalization .................... 53
xv
List of Abbreviations
CNTs
Carbon nanotubes
1D
One-dimension
MWNTs
Multi-Walled Carbon Nanotubes
HRTEM
High Resolution Transmission Electron Microscopy
SWNTs
Single-Walled Carbon Nanotubes
FWNTs
Few-Walled carbon nanotubes
fFWNTs
Functionalized few-Walled carbon nanotubes
DWNTs
Double-walled carbon nanotubes
DOS
Electronic density of states
vHS
Van Hove singularities
RBM
Radial breathing mode
PL
Photoluminescence
CVD
Chemical vapor deposition
CO-CVD
Carbon monoxide chemical vapor deposition
PECVD
Plasma enhanced chemical vapor deposition
HiPco
High-pressure carbon monoxide
FED
Field emission displays
SEM
Scanning electron microscope
TEM
Transmission electron microscope
ITO
Indium tin oxide
xvi
TGA
Thermo gravimetric analysis
XRD
X-ray diffraction
DI
De-ionized
XPS
X-ray photoelectron spectroscopy
CPP
chlorinated polypropylene
SDS
Sodium dodecyl sulfate
SDBS
Sodium dodecylbenzenesulfonate
SC
Sodium cholate
LED
Light-emitting diode
TCF
Transparent conducting film
PVA
Polyvinyl alcohol
SC
Specific capacitance
CP
Conducting polymer
PEDOT-PSS
Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)
EC
Electrochemical capacitor
HEV
Hybrid electric vehicle
FCEV
Fuel cell electric vehicle
PTFE
Polytetrafluoroethylene
CV
Cyclic voltammograms
IR
Internal resistance
xvii
Acknowledgements
I would like to thank my advisor, Professor Jie Liu, for offering me the
opportunity to explore the wonderful nanoscaled world; for his guidance, patience,
understanding, and encouragement during my Ph.D. study. He encouraged me
to not only grow as an experimentalist and a chemist but also as an independent
thinker. His broad and deep academic background and hard working attitude
have been a great inspiration to me.
I am grateful to Professor Richard A. MacPhail, Professor Boris B.
Akhremitchev, Professor Tuan Vo-Dinh, and Professor Thom LaBean for their
interest in my work and being my committee, for spending time reading my
proposals and dissertation, for their guidance over the years
I would also like to thank all my colleagues during my Ph. D. study: Dr.
Cheng Qian, Dr. Qiang Fu, Dr. Lei An, Dr. Chenguang Lu, Dr. Hang Qi, Dr.
Michael Woodson, Dr. Dongning Yuan, Dr. Tom McNicholas, Jianqiu Yang; and
others outside Duke: Dr. Otto Zhou, Dr. Luchang Qin, Dr. R. Bruce Weisman, Dr.
Liwei Chen, Dr. Dmitri A. Tsyboulski, Ru Zhang.
I am indebted to Dr. Todd Woerner, Dr. Mark Wiesner, Dr. Mark Walters
and Mr. Kirk Bryson for their assistance on instruments at Duke University.
Finally, I am grateful to my friends and family. Specifically, my mother
Guifen An and my father Lixun Hou,my husband Fan Yang , my younger sister
Yu Hou, for their long-lasting support on my studies and personal life.
xviii
Chapter 1: Introduction
1.1 Introduction to carbon nanotubes
As early as 1952, LV Radushkevich and collaborators reported about
carbon nanotubes in Russia.[1] In 1976, M Endo from Japan was collaborating
with A Oberlin in France, reported on the observation of carbon nanotubes by
electron microscopy.[2] In 1991, Iijima synthesized a new type of fullerene, quasione-dimensional crystalline structures of carbon atoms, generally referred to as
carbon nanotubes (CNTs).[3] Indeed, as a novel nanostructured material, CNTs
have attracted intense attention because of their amazing mechanical, electrical,
and thermal properties, which when combined with their low density promise a
broad range of potential applications.
Although there has been a substantial amount of work done on CNTs,
challenges still need to be overcome to successfully exploit their superior
properties and various applications.
1.1.1 Structure of carbon naotube
A single-walled carbon nanotube structure can be considered as a strip of
graphitic sheets that is rolled into a seamless cylinder capped with a half
buckyball at each end.[4] Figure 1.1 illustrates the structure of a graphene sheet
and typical single-walled carbon nanotubes (SWNTs). Such a one-dimensional
unit cell can be defined by the vector Ch, where a1 and a2 are unit vectors, and n
1
and m are integers. Rolling up the sheet along one of the symmetry axes gives
either a zig-zag (m=0) tube or an armchair (n=m) tube. Rolling up the sheet in a
direction that differs from a symmetry axis, it is also possible to obtain a chiral
nanotube (n≠m).[5]
SWNTs with structural indices (n,m) can be classified into two categories
based on their electronic properties: metallic and semiconducting tubes. The
(n,m) indices determine two key parameters of SWNT: diameter and chirality. For
a given (n,m) nanotube, if n − m is a multiple of 3, then the nanotube is metallic,
otherwise, the nanotube is a semiconductor. Thus all armchair (n=m) nanotubes
are metallic, and nanotubes (5,0), (6,4), (9,1), etc. are semiconducting.
Figure 1.1 Schematics of graphene sheet and typical SWNT structures. The
images are adapted from ref [5]
2
Up to now, most of the research has focused on two distinct kinds of
CNTs. They are single-walled carbon nanotubes (SWNT) that consist of only
single layer of graphene sheet and multi-walled carbon nanotubes (MWNT) that
consist of multiple layers of sidewalls.[6-7] Recently, unique small diameter
MWNTs, also called few-walled carbon nanotubes (FWNTs) (Figure 1.2B), have
attracted more and more attention due to their unique structure and properties.
FWNTs are nanotubes with sidewalls of 2 to 5 graphenes layers,
diameters ranging from 3 to 8 nm and lengths of around tens of micrometers.[8]
They can be considered as the intermediate between single-walled carbon
nanotube and multiwalled carbon nanotubes and can bridge the gap between
them. My research has been focused on mainly on FWNTs.
Figure 1.2 HRTEM images of (A) SWNTs (B) FWTNs (C) MWNTs. The
images are adapted from (A) ref [6], (B) ref [8] and (C) ref [7]
1.1.2 The electronic band structure of SWNTs
The unique electrical, thermal, and optical properties of carbon nanotubes
3
result from their special electronic structures, which for an individual carbon
nanotube can be metallic, semi-metallic or semiconducting, depending on its
diameter and chirality.[9-10]
Figure 1.3 Three-dimensional view of the graphene π/π* bands and its 2D
projection. The image is adapted from ref [10]
It is important to understand the origins and implications of the graphene
electronic structure before deriving the electronic structure of carbon nanotubes.
In the graphite structure, each carbon atom has 4 electrons in the outmost
shell .Three electrons are dedicated to forming σ bonds due to the sp3
hybridization and one electron is dedicated to form π bonds with other carbon
atoms. This π orbital, perpendicular to the graphene sheet, forms a delocalized π
network across the nanotube, responsible for most important physical properties
of graphene sheet.
Figure 1.3 shows an energy dispersion plot; the upper half of the plot
4
describes the π*-energy anti-bonding band and the lower half describes the πenergy bonding band. The upper π* band and the lower π band are degenerate
at the K points in the Brillouin zone at which the Fermi level passes. The Fermi
surface of an ideal graphite sheet consists of the six corner K points. Since the
density of states at the Fermi level is zero, an isolated sheet of graphite is a zerogap semiconductor. [10]
Once we roll up the graphene sheet into a tube, owing to the periodic
boundary conditions imposed in the circumferential direction, the periodic
boundary conditions imposed by Ch can be used to enumerate the allowed 1D
subbands, the quantized states resulting from radial confinement, as follows: Ch
.
k =2πq (q is an integer).[4] As can be seen from Figure 1.4, in the right panels,
the nanotube will be metallic if one of the cutting lines passes through one of the
K-points. If the lines of quantized wavevectors do not pass through the graphene
Fermi points, then the CNT is semiconducting, with a bandgap determined by the
two lines closest to the Fermi points (left panel). [10]
5
Figure 1.4 Illustration of allowed wavevector lines leading to semiconducting
and metallic CNTs and examples of band structures for semiconducting and
metallic zigzag CNTs. The images are adapted from ref [10]
1.2 The synthesis of carbon nanotubes
Carbon nanotubes can have different individual diameters, chirality,
morphologies length,quality, and properties, all of which are determined by the
method of preparation and further processing.[11] Hence, a wide variety of
synthetic methods have been developed to produce the desired materials and
properties for specific scientific studies or technological applications.
In our earlier research activities, we discovered that simply obtaining
commercial nanotube materials and using them in applications development will
hardly achieve the performance that could have been achieved using the right
kind of nanotubes. We found that even among FWNTs, samples with different
6
diameters and numbers of walls behave very differently in specific applications.
For example, in high current field emission applications, 3-4 wall FWNTs with >5
nm in diameter provide much better long term stability than thinner FWNTs.
However, in flat panel display, where low emission voltage is desired, the FWNTs
with 2 layers of sidewall and 2-4 nm diameters are the best choice. The
realization that specific applications need specific kinds of nanotubes is an
important concept for achieving high performance in any application. It is clear
that future developments in nanotube-based science and technology will
continue to rely on further improvements in controlling the synthesis of nanotubes.
So far, arc discharge, laser ablation, and chemical vapor deposition (CVD)
are the three main methods for SWNT production.[11-12]
The arc discharge method for synthesizing carbon nanotubes, initially
used to produce fullerene, was first introduced by S. Iijima in 1991 (Figure 1.5).
[3] Typically, in this technique,[13] the arc is usually operated with a high direct
current (100A) flow, two carbon rods placed end to end, separated by
approximately 1mm, in an enclosure that is usually filled with inert gas at low
pressure. By adjusting the pressure and catalyst carefully, carbon nanotubes can
be obtained by arc discharge between the two carbon electrodes with catalyst.
CNTs synthesized by the arc discharge method usually possess high structural
perfection due to the high growth temperature. A metal catalyst is needed for
synthesis of SWNTs in the arc discharge method, while MWNTs can be
synthesized without catalyst.
7
Figure 1.5 Schematics of experimental setup of arc discharge method. The
images are adapted from ref [13]
The laser ablation method (Figure 1.6) was developed by Smalley et al. in
1995.[14] Graphite target sitting in a furnace at high temperature (1200℃) was
evaporated by a beam of high power laser pulses. A noble gas flow passed the
growth chamber and carried the CNTs downstream during laser ablation. Carbon
nanotubes can be found in the soot at cold end. Similar to the arc discharge
method, a metal catalyst is also needed to synthesize SWNTs in the laser
ablation method.[13]
Arc discharge and laser ablation methods have been developed to
produce both high quality MWNTs and SWNTs.[15] The as-produced carbon
nanotubes have few defects such as pentagons or heptagons on the sidewalls of
the carbon, indicating high crystallinity due to the relatively higher temperatures
8
at the reaction zone. However, the equipment requirements and the large
amount of energy consumed make the cost associated with these methods
relatively high and less favorable for mass production of carbon nanotubes. [11]
Figure 1.6 Schematics of experimental setup of laser ablation technique. The
image is adapted from ref [13]
The second issue is that CNTs grown by these methods are highly tangled
and, mixed with unwanted forms of carbon and/or metal species. They are
difficult to purify for practical applications.
Chemical vapor deposition (CVD) is the term used to describe
heterogeneous reactions in which both solid and volatile products are formed
from a volatile precursor through chemical reactions, and the solid products are
deposited on a substrate.[11] Chemical vapor deposition of hydrocarbons over a
metal catalyst is a classical method that has been used to produce various
9
carbon materials such as carbon fibers and filaments since 1970s. [16] It involves
the decomposition of a gaseous or volatile compound of carbon, catalyzed by
metallic nanoparticles, which also serve as nucleation sites for the initiation of
carbon-nanotube growth (Figure 1.7). CVD carbon nanotube synthesis is
essentially a two-step process.[16] The first step is preparation of catalyst and is
critical since the diameter of carbon nanotubes depends on the size of catalyst
clusters. Most commonly used catalysts are Ni, Fe and Co clusters. The metal
nanoparticles are usually mixed with a catalyst support such as MgO or Al2O3 to
increase the surface area for higher yield of the catalytic reaction of the carbon
feedstock with the metal particles. The catalyst on substrate is then heated to a
desired temperature, and then gaseous carbon molecules, such as methane,
carbon monoxide or acetylene are feed through the catalyst particles. After a
period of growth, the carbon feeding gas is turned off and the product is cooled
and collected.
10
Figure 1.7 Experimental setup of thermal CVD technique.
Compared with the arc discharge and laser ablation methods, the CVD
method is suitable for producing CNTs not only in bulk volumes, but also on flat
substrate surfaces, hence provides a convenient way for further device
fabrications. The CVD method also allows for more control over the morphology
and structure of the produced nanotubes through tuning of parameters such as
temperature, carbon feeding and catalyst.[17] Moreover, CVD is considered to be
the most efficient method for large scale production of CNTs with low cost. So far,
most of the CNTs synthesized by the CVD method possess more defects than
those synthesized by the arc discharge and laser ablation methods. This is
probably due to the relatively low CNT growth temperature which is normally no
11
more than 1100
o
C. However, the degree of structural perfection can be
enhanced by annealing the samples above 1200℃ in vacuum.[18]
In this dissertation, the CVD method is exploited to synthesize bulk
samples of all types of CNTs. Bulk synthesis of CNTs by CVD method is mainly
discussed in details in chapters two.
1.3 The functionalization of carbon nanotubes
As-produced carbon nanotubes are typically agglomerated bundles of
various species (Figure 1.8).[19] The extremely high aspect ratios in combination
with the high flexibilities, and the fact that the tube surfaces attracted to each
other by strong van der Waals interactions, all dramatically increase the
possibilities for entanglements of CNTs. Entangled CNTs are insoluble in all
solvents since they are held together tightly in bundles, resulting in, for example,
composites systems with only modest improvements in mechanical or other
properties. [20] Therefore, aggregates have imposed great limitations to most
carbon nanotube applications.
12
A
B
10nm
Figure 1.8 (A) TEM image of SWNTs ropes (B) HRTEM of a single SWNT
rope made up of ~100 SWNTs. The images are adapted from ref [19]
To make nanotubes more easily dispersible in liquids, it is necessary to
physically or chemically attach certain molecules, or functional groups, to their
smooth sidewalls without significantly changing the nanotubes’ desirable
properties. This process is called functionalization.[21] It has been demonstrated
functionalization is an efficient way to enhance the solubility of CNTs in various
solvents and to produce novel hybrid materials potentially suitable for
applications.
In recent years, several approaches to the functionalization of CNTs have
been developed. These approaches include two main categories: (a) attaching
functional groups onto the conjugated skeleton of CNT through covalent
chemical reactions; (b) adsorption or wrapping of various functional molecules
via noncovalent interaction. Specifically, these approaches include: defect-group
13
functionalization, covalent sidewall functionalization, noncovalent exohedral
functionalization with surfactants, noncovalent exohedral functionalization with
polymer and endohedral functionalization with for example C60. [22]
Figure 1.9 Functionalization possibilities for SWNTs: A) defect-group
functionalization, B) covalent sidewall functionalization, C) noncovalent
exohedral functionalization with surfactants, D) noncovalent exohedral
functionalization with polymers, and E) endohedral functionalization with, for
example, C60 . For methods B-E, the tubes are drawn in idealized fashion, but
defects are found in real situations. The images are adapted from ref [22]
1.3.1 Noncovalent Functionalization
In the past few years, the noncovalent surface treatment of CNTs by
surfactants or macromolecules was widely used in the preparation of both
aqueous and organic solutions, to obtain highly stable suspensions. Noncovalent
14
methods are particularly interesting because CNTs are shown to be dispersed
with high efficiency without disturbing their characteristic π system.[23]
A simple but widely used noncolvalent method for solubilizing CNTs is
based on noncovalent interactions between amphiphilic molecules (surfactants)
and CNTs, where the hydrophilic parts of such molecules interact with the
solvent and hydrophobic parts are adsorbed onto the nanotube surface, thus
solubilizing CNTs and preventing them from the aggregation into bundles and
ropes.[24] Typical examples include charged surfactants such as sodium dodecyl
sulfate (SDS), tetraalkylammonium bromide, or cetyltrimethyl ammonium
bromide (CTAB), as well as neutral surfactants, such as Triton. [25-27]
Additionally, CNTs can be dissolved in aqueous and organic media by
various aromatic derivatives through π-π interactions between the aromatic
moieties and the CNT sidewalls. The sp2 bonded carbon structures at the
sidewalls form highly delocalized π-π electrons. These highly delocalized π
electrons can be used to form functionalized carbon nanotubes with other π
electron rich compounds through π-π interaction. It has been shown that various
conjugated molecules such as pyrene,[28] porphyrin,[29] and π conjugated
polymers[30-31] can form a supramolecular complex with nanotubes, and the
resulting complex materials showed good solubility.
1.3.2 Covalent Functionalization
Functional groups can be covalently attached by oxidizing defects or by
creating covalent bound to carbon nanotubes.
15
In general, oxidation of CNTs yields defects such as vacancies or
pentagon-heptagon pairs (Stone-Wales defects) that result in a locally enhanced
chemical reactivity of the graphitic nanostructures. Oxygen-containing functional
groups such as carboxylic acid are created and these groups can then be used
as chemical anchors for further derivatization. Among various defect site
oxidation reactions, liquid-phase oxidations mainly involved acidic etching by hot
nitric acid, sulfuric/nitric acid mixtures, piranha (H2SO4/H2O2) solutions or by
hydrogen peroxide.[32-34]
The creation of covalent bound to carbon nanotubes can be realized
through alkylation, arylation, fluorination, nitrene addition and so on.[35-37]
These approaches will convert sp2 carbon atoms to sp3 hybridization, and
therefore destroy the extensive π-conjugation systems which may lead to the
loss of structures and properties of SWNTs. FWNTs are special kinds of
nanotubes that we have developed over the last few years and introduced to the
research field. Such nanotubes can have a level of structural perfection as good
as single walled carbon nanotubes and can be highly purified. The nanotubes
have 2 to 5 layers of sidewalls with diameter ranging from 2 to 8 nm. Unlike
single walled carbon nanotubes, even after covalent functionalization, the fewwalled nanotubes (FWNTs) keep their straight shape as an indication of
structural integrity of the inner tubes.[38]
In this dissertation, we developed a simple and efficient ultrasoundmediated covalent functionalization method to attach chemical groups on any
16
kind of CNTs at room temperature. Acidic potassium permanganate solution was
used to obtain CNTs that with highly soluble in water. CNTs in organic solvents
can be fabricated by a dissolving metal reduction method under low intensity
ultrasound. It turned out that these functinalized CNTs are highly soluble in either
aqueous solution or various organic solvents such as THF, DMF, and chloroform.
1.4 The unique physical properties of carbon nanotubes and
their applications
1.4.1 Mechanical properties of carbon nanotubes
The strength of the sp² carbon-carbon bonds in a graphene layer is
probably the strongest chemical bond known in nature. This strength combined
with the low mass density gives carbon nanotubes amazing mechanical
properties. The mechanical properties (such as strength) of nanotubes greatly
exceed those of other fibres.
The mechanical properties of carbon nanotubes are usually characterized
by two main measurable parameters, the Young’s Modulus and tensile strength.
Theoretically, the tensile strength and young’s modulus of carbon nanotubes has
been found to be upwards of ~140GPa and ~1TPa respectively. The Young's
modulus of the best nanotubes can be as high as 1000 GPa which is
approximately 5 times higher than steel. The tensile strength or breaking strain of
nanotubes can be up to 63 GPa, around 50 times higher than steel.[39] These
properties, coupled with the lightness of carbon nanotubes, give them great
potential in applications such as composite reinforcement, lubrication or
17
aerospace. It has been suggested that nanotubes could be used in the “space
elevator”.[40]
1.4.2 Electrical properties of carbon nanotubes
The electrical properties of carbon nanotubes are also extraordinary. Their
small sizes offer exciting possibilities for smaller and faster devices with better
performance. The applications of carbon nanotubes could find successful uses
from sophisticated applications to everyday electronics.
Depending on their structure, CNTs can be metallic or semiconducting.
Their conductivity has been shown to be a function of their chirality, the degree of
twist and their diameter.[12] Theoretically, metallic nanotubes can carry an
electrical current density of 4 × 109 A/cm2 more than 1,000 times greater than
metals such as copper, while other CNTs behave more like silicon.[41]
Conductivity in MWNTs is quite complex. In a word, carbon nanotubes have
been proposed as the basis for everything from elevator cables that could lift
payloads into earth orbit[40] to computers smaller than human cells.
1.4.3 Thermal properties of carbon nanotubes
As the size of the device reduces, the thermal management in nanosize
devices becomes increasingly important.[42] Therefore, the thermal conduction
of nanometer materials is fundamentally critical in controlling the performance
and stability of nanosize devices.
Preliminary experiments and simulation studies on the thermal properties
of CNTs show high thermal conductivity,[43] exhibiting a property known as
18
"ballistic conduction". The thermal properties of carbon nanotubes display a wide
range of behaviors which are related both to their graphitic nature and their onedimensional structure, diameter and length. Measurements show that thermal
conductivity of a SWNT can reach to about 3500 W·m−1·K−1 at room-temperature;
it is almost 10 times higher compared this to that of copper, a metal well-known
for its good thermal conductivity, which transmits 385 W·m−1·K−1.[43] In some
experiments, carbon nanotubes have been shown to double the thermal
conductivity in the resin at only a 1% loading of CNTs. It is therefore expected
that nanotube reinforcements in polymeric materials may significantly improve
the thermal and thermomechanical properties of the composites.
1.4.4 Optical propertieds of carbon nanotubes
The optical properties of carbon nanotubes usually refer specifically to the
absorption, photoluminescence, and Raman spectroscopy of carbon nanotubes.
Optical properties of carbon nanotubes originate from electronic transitions within
one-dimensional density of states (DOS). The density of states (DOS) of a
system describes the number of states at each energy level that are available to
be occupied.[44] A high DOS at a specific energy level means that there are
many states available for occupation. A DOS of zero means that no states can
be occupied at that energy level. Typically, DOS of one-dimensional
nanomaterials is not a continuous function of energy, but it descends gradually
and then increases in a discontinuous sharp spike. These sharp peaks found in
one-dimensional materials are called Van Hove singularities. In contrast, three19
dimensional materials have continuous DOS. The electronic density of states
(DOS) for both metallic and semiconducting nanotubes is schematically shown in
Figure 1.10.[45]
Figure 1.10 Schematic diagram of electronic density of states for (a) metallic
and (b) semiconducting SWNTs. Arrows indicate the optically allowed
interband transitions. The images are adapted from ref [45]
Optical transitions occur between the v1 − c1, v2 − c2, etc., states of
semiconducting or metallic nanotubes and are traditionally labeled as S11, S22,
M11, etc. A Kataura plot (Figure 1.11)[46] is a graph relating the energy of the
band gaps in a carbon nanotube to its diameter. Each point in the plot shows one
optical transition energy Eii, which determines the energy of the light absorption
by the nanotube.
20
Figure 1.11 Kataura plot showing the different resonance excitations as
function of different energy lasers and diameters. The image is adapted from
ref [46]
1.4.4.1 Optical absorption
Since all types of nanotubes are active in the UV-vis-NIR region. Optical
absorption spectroscopy has been widely used for evaluating the purity of the
sample. The absorption intensity provides information for the resonant peaks
originating from van Hove singularities at visible region and is proportional to the
amount of CNTs that are solubilized in the solution. Optical absorption in carbon
nanotubes present sharp peaks originating from electronic transitions from the v2
to c2 (energy E22) or v1 to c1 (E11) levels, etc. Since these resonant peak positions
are chirality- and diameter-dependent, this method has been also used to
21
estimate the relative composition of metallic to semiconducting CNTs by
comparing the intensity of metallic peaks with that of semiconducting
nanotubes.[47] It should be noted that the sharpness deteriorates with increasing
energy. Since many nanotubes have very similar E22 or E11 energies, significant
overlap occurs in absorption spectra.[48] Additionally, the strong inter-tube
coupling, combined with the large inhomogeneity in nanotube structures present
in nanotubes samples, absolutely obscures any fine structure in the absorption
spectra. Absorption spectra of SWNT bundles contain three broad absorption
regions corresponding to the first and second allowed transitions for
semiconductor SWNTs, and the first allowed transition for metallic SWNTs,
respectively. Because strong electronic coupling mixes the energy states from
the different SWNTs in bundles, absorption spectra do not exhibit any features
identified with a particular nanotube structure. [49]
1.4.4.2 Photoluminescence (PL)
Photoluminescence (PL) is one of the important tools for nanotube
characterization. With two thirds of single walled carbon nanotubes (SWNTs)
predicted to be direct bandgap semiconductors, photoluminescence (PL) from
the recombination of electron-hole pairs at the bandgap is expected. As shown in
the Figure 1.12, the excitation of PL usually occurs as follows: an electron in a
nanotube absorbs excitation light via photon energy E22 transition, creating an
electron-hole pair (excitons). Both electron and hole rapidly relax (via phononassisted processes) from c2 to c1 and from v2 to v1 states, respectively. Then they
22
recombine through a c1 − v1 transition resulting in fluorescence emission near E11.
The values of E11 and E22 will vary with tube structure.[50]
Figure 1.12 Schematic density of electronic states for a single nanotube
structure. Solid arrows depict the optical excitation and emission transitions of
interest; dashed arrows denote nonradiative relaxation of the electron (in the
conduction band) and hole (in the valence band) before emission. The image is
adapted from ref [50]
Excitonic luminescence can not be produced in metallic tubes — an
electron can be excited, thus resulting in optical absorption, but excited states will
decay non-radiatively through these states, leading to no fluorescence in metallic
nannotubes.
23
The potential for a material to fluoresce depends strongly on its intrinsic
band structure, but also some other internal and external factors. Examples of
internal factors include surface modification, dislocations, amount of dopants,
and defect concentration on surface. Some external factors include the dielectric
environment, electric fields, magnetic fields, and hydrostatic pressure, and any
external interactions in bundles. These factors may deplete or fill existing bands,
or change the band structure completely.[51] Resulting in the shifted PL emission
energies, or altered PL intensities and maybe even destroy the luminescence
altogether. Therefore, the key is to isolate the nanotubes, minimizing their
interaction with the environment. With most SWNT diameters being within 0.8-2.0
nm, the emission wavelengths falls in the 800 to 1600nm region in the near
infrared (Figure 1.13).[52]
24
Figure 1.13 Emission spectrum (red) of individual fullerene nanotubes
suspended in SDS micelles in D2O excited by 8 ns, 532-nm laser pulses,
overlaid with the absorption spectrum (blue) of the sample in this region of first
van Hove band gap transitions. The detailed correspondence of absorption and
emission features indicates that the emission is band gap photoluminescence.
from a variety of semiconducting nanotube structures. The image is adapted
from ref [52]
Investigations show that the fluorescence spectrum of semiconducting
tubes is dependent on tube diameter and (n,m) structural indices. The powerful
technique of two dimensional spectro-fluorimetry can immediately dissect the
SWNT spectra into clearly separated peaks arising from distinct (n,m) species,
enabling a successful spectral assignment. [53]
25
1.4.4.3 Raman scattering
Raman mapping is widely used to obtain information about the structure
and electronic properties of both semiconducting and metallic SWNTs and is less
sensitive to nanotube bundling than PL. It has good spatial resolution (~0.5
micrometers) and sensitivity (single nanotubes) and requires only small amount
of samples. As shown in Figure 1.14,[54] the Raman spectra of SWNTs have
many characteristic features. Herein, we will discuss three of them, the radial
breathing mode (RBM), the D mode and the G mode, because these three will be
used in the following chapters and are the primary modes by which nanotube
samples are characterized.
26
Figure 1.14 A typical Raman spectrum showing the prominent bands that are
characteristic for SWNTs. The peaks with stars are from silicon substrate. The
image is adapted from ref [54].
The radial breathing mode (RBM) corresponds to radial expansioncontraction of the nanotube and is dependent on the environment around the
tubes such as the substrate, surfactant, outer-force, etc. A typical RBM
frequencies range is 100–350 cm−1 and is one of the most important regions in
SWNT Raman spectra. Calculations from the RBM are the most accurate
method to measure the diameter of a SWNT. The RBM frequency νRBM (in cm−1)
depends on the nanotube diameter d (in nanometers) and can be estimated as
νRBM = 234/d + 10,[55] where the dt is the tube diameter and ωRBM is the
frequency of breathing mode. The Raman response for this mode depends
27
strongly upon the laser excitation energy; thus, several laser lines are required to
estimate the mean diameter and diameter distribution of a bulk SWNT samples.
Another very important mode is the G mode (G from graphite). This mode
corresponds to planar vibrations of carbon atoms and is present in most graphitelike materials. The G mode is derived from the G band in graphite at a singlepeak of 1582 cm-1. The G band in a SWNT falls in the range between 1565 and
1590 cm-1 because of the tube curvature. The G band can be used, though with
much lower accuracy compared to RBM mode, to estimate whether the tube is
metallic or semiconducting. A metallic tube generally has a broader shoulder.
This feature can be used to distinguish the tube type.
The D mode appears in the range between 1350 and 1370 cm-1 and
relates to the disordered sp2 bonding such as sp1 and sp3 carbon. Amorphous
carbon and MWNTs often show strong D band, and the D band is present in all
graphite-like carbons and indicates the defect and impurity level of a SWNT
sample. Therefore, the ratio of the G/D modes is conventionally used to quantify
the structural quality of carbon nanotubes. High-quality nanotubes have this ratio
significantly higher than 100.
1.5 The application of carbon nanotubes
The remarkable physical properties of carbon nanotubes make them a
very unique material with a whole range of promising applications. Some
commercial products on the market today utilizing CNTs include stain resistant
textiles, CNT reinforced tennis rackets and baseball bats. Samsung already has
28
CNT based flat panel displays on the market. Many companies are also looking
forward to producing transparent conductive coatings and replacing ITO coatings.
In this dissertation we only describe some of the important materials science
applications of carbon nanotubes. Specifically we discuss nanotubes as
mechanical reinforcements in high performance composites, the electrochemical
applications of nanotubes, nanotube-based field emitters, and their use in
medical investigations.
1.5.1 Carbon nanotube composites
It has been shown that carbon nanotubes could behave as the ultimate
one-dimensional material with remarkable mechanical properties owing to the
strong carbon–carbon covalent bond. CNTs are known to have an extremely high
Young’s modulus of up to 1TPa and tensile strength approaching 180 GPa,
which is 19 and 56 times that of steel.[39] Therefore, embedding carbon
nanotubes in polymeric matrices for various nanocomposite materials has been a
popular subject. They have been used as reinforcements in high strength, light
weight, high performance composites[56]; one can typically find these in a range
of products ranging from expensive tennis rackets to spacecraft and aircraft body
parts.
However, in the practice CNTs have shown only limited enhancements to
the mechanical properties of polymer matrix and the composite processing is still
limited to bench-top one. Indeed, there are still many well-known issues [57] that
need to be resolved both theoretically and experimentally to take the maximum
29
benefits from carbon nanotubes to polymer composite systems. The biggest
challenge is to fully disperse individual nanotubes with ultrahigh loading into the
high viscous polymer matrix (e.g. epoxy resin). Due to their high aspect ratio and
the strong van der Waals forces between them, carbon nanotubes naturally exist
in the form of micron-sized aggregates that are generally insoluble; theses
agglomerates would create cracks in composites rather than reinforce them.
Moreover, the very high aspect ratio of nanotubes creates extremely high
viscosity in melt or dissolved polymers and therefore limits their loading in matrix
(typically less than 5%).
Due to the high electrical and thermal conductivity, CNTs can also be
applied
as
electrically
and
thermal
conducting
additives
in
polymer
composites.[58-59]
1.5.2 Energy storage
The outstanding electrical conductivity, mechanical properties and the
high surface-to-volume ratio make carbon nanotubes potentially useful as
electrodes in batteries and capacitors.
Their small diameter and porous nature makes it possible to distribute the
nanotubes homogeneously in an electrode material and to introduce high
electrochemically accessible surface area to react with the electrolyte. It also
improved penetration of the electrolyte due to the uniform distribution. Their high
electrical conductivity also provides the electrode with increased electrical
conductivity, and the function of the electrical bridge between other electroactive
30
particles. The excellent mechanical properties of CNTs increase the stability of
electrodes upon cycling. [60]
Research has shown that CNTs have the highest reversible capacity of
any carbon material for use in lithium ion batteries.[61] In addition, CNTs are
outstanding materials for supercapacitor electrodes and are now being marketed
for this application. CNTs also have applications in a variety of fuel cell
components. [62]
1.5.3 Field emission
CNTs have been considered as preferred field emitters due to their low
threshold voltages, good emission stability and long emitter lifetime.[63] The high
electrical conductivity and incredible sharpness of their tip means that CNTs emit
at especially low voltage, an important feature for building low-power electrical
devices. Furthermore, the current is extremely stable.
These characteristics make them useful in flatpanel displays. Samsung
already released the fabrication of a 4.5 in diodetype flat panel display using arcbased SWNTs (Figure 1.15).[7] Even though SWNTs were shown to have
excellent emitting performance, they readily degraded at high emission
current.[64] In this sense, FWNTs have been examined as the best field emitting
materials because they were shown to have a low threshold voltage comparable
to SWNTs and a better structural stability compared to MWNTs.[8] Other
applications utilizing the field-emission characteristics of CNTs include general
types of low-voltage cold-cathode lighting sources, lightning arrestors, and
31
electron microscope sources.
Figure 1.15 A prototype 4.5 in FED using a printing method. The image is
adapted from ref [7].
1.5.4 Medical applications of carbon nanotubes
Since a large part of the human body consists of carbon, CNTs are
generally though of as a biocompatible material. Therefore, combining carbon
nanotubes with biological systems may significantly improve medical science —
especially diagnostics and disease treatment. The exploration of CNTs in
biomedical applications is just underway, but has significant potential. Nothing
has been fully developed and finalized yet, but we see progress every day.
As an example, gene therapy could also be improved by using carbon
nanotubes. In general, a damaged or missing gene could be replaced with
another one from outside. But it is a complicated process because DNA can’t
pass through the cell membrane. It has been shown that a single strand of DNA
32
can be bonded to a modified nanotube, which can then be successfully inserted
into a cell. [65]
Another application of carbon nanotubes in medicine is for sensing
molecules or species. Due to their high electrical conductivity, carbon nanotubes
can promote the electron transfer in proteins. For example, in heme containing
proteins carbon nanotubes are able to access the heme centre of biomolecules
that is generally not sensed by the glass electrodes.[66]
1.5.5 Molecular electronics
In any nanoscale electronic circuit, the interconnections between switches
and other active devices become increasingly important. Owning to their
geometry, electrical conductivity, and ability to be precisely derived, there is great
interest in the possibility of constructing nanoscale electronic devices from
nanotubes, and some progress is being made in this area. There are several
areas of technology where carbon nanotubes are already being used, including
flat-panel displays, scanning probe microscopes and sensing devices. A lot of
research is being done to design CNT based transistors as well. The unique
properties of carbon nanotubes will undoubtedly lead to more applications.
33
Chapter 2: Synthesis of High Quality Double-Walled
Carbon Nanotubes.
2.1 Introduction
Few-walled carbon nanotubes have 2 to 5 layers of sidewalls with
diameters ranging from 3 to 8 nm and length of around tens of micrometers; they
are unique MWNTs with a graphitization structure as perfect as SWNTs. Highly
pure FWNTs had been synthesized in our group by thermal chemical vapor
deposition methods using different carbon sources, such as methane and
alcohols.[8, 67-68] Most of FWNTs obtained were a mixture of double-walled
carbon nanotubes (DWNTs), triple-walled carbon nanotubes (TWNTs) and
quadruple-walled carbon nanotubes (QWNTs). To further exploit the excellent
properties of FWNTs, it is necessary to understand the role that each component
of FWNTs plays. Thus, preparing high quality single component of FWNTs with
controlled structures and diameters would be very important not only for the
basic studies but also the real applications of FWNTs in industry.
Double-walled carbon naotubes — the thinnest multi-walled carbon
nanotubes
retaining the excellent graphitization of single walled carbon
nanotubes —must be considered as a new classes of carbon nanophases, since
numerous advances hinge on the unique characteristics of their two concentric
tubes. They combine the superior mechanical strength of MWNTs with the
outstanding electronic properties of SWNTs and are believed to replace SWNTs
34
or MWNTs in various applications eventually. They also open a possibility of
functionalizing the outer walls, which will ensure connections with the external
environment, while retaining the remarkable mechanical and electronic properties
of the inner nanotube. Last but not least, DWNTs offer an ideal system for
investigating the unique properties, e.g. the correlation between two concentric
shells during growth, the optical properties of coaxial nanotubes.
These promising applications of DWNTs have accelerated the research
activities on the synthesis of DWNTs during the past several years. However,
the synthesis of DWNTs with high quality is not straightforward. To date, all
processes[69-72] except for the peapod method [73-75] usually provide very low
yield mixtures containing DWNTs with random diameter distributions (from 2 to 6
nm for outer diameters). Recently, Endo et al[76] reported that more than 95% of
DWNT bundles were synthesized by methane CVD method with a conditioning
catalyst and a two-step purification process, nevertheless, the challenge posed
by controlling the diameter distribution is still daunting. Since the electronic
properties of CNTs depend strongly on their diameter and chirality, DWNTs with
a narrow diameters distribution are highly desirable for the exploration of their
properties and future application.
In this direction, we have recently opened up a simple carbon monoxide
CVD (CO-CVD) method[77-79] for the efficient growth of high quality DWNTs.
This work aims at increasing both the selectivity and the yield. It represents a
breakthrough in growth of monodisperse DWNTs in bulk materials, enabling a
35
rich understanding of their exceptional properties and diverse applications. Since
it is still unclear as to whether there is a definite relationship between the
intershell coupling, the helicity of the constituent tubes, and the electronic
properties of these systems, etc.. In this study, we also explored the structural
correlation between two adjacent graphitic layers in DWNTs[80-83] when it grows.
Moreover, although photoluminescence (PL) properties of SWNTs are well
established, PL from double-walled carbon nanotubes has become a
controversial topic in current research. Some researchers believe that the
emission is exclusively from DWNTs,[84-85] while others report the opposite.[8688] It should be noted that CVD-grown DWNT samples are usually a mixture of
DWNTs and a certain amount of SWNTs after purification. The SWNTs could be
trapped inside DWNTs bundles and then released during the sonication process.
Therefore, it is unclear whether the detected PL signal originated from the few
SWNTs or truly from DWNTs. In this study, SWNTs were isolated from DWTNs
using density-gradient ultracentrifugation.[89-90] The results of optical properties
together with TEM analysis of each separated section by density-gradient
ultracentrifugation are reported.
2.2 Experimental Methods
2.2.1 Catalyst preparation
In this work, bimetallic catalysts Co/Mo supported on MgO support were
prepared as reported previously. Typically, catalyst precursors were made by
dissolving
desired
amounts
of
Mg
36
(NO3)2⋅6H2O,
Co
(NO3)2⋅6H2O,
(NH4)6Mo7O24⋅4H2O, glycine and citric acid in de-ionized water followed by
evaporation at 120oC. The catalyst powders were obtained by combusting the
precursors over 300oC quickly and then annealing at 500oC for 1h. The catalysts
produced are fine powder with high surface area (100~200 m2/g) which allows
catalyst nanoparticles well distributed on and the high yield.
2.2.2 Synthesis and purification of DWNTs
DWNTs were synthesized in a large CVD setup made up of a horizontal
tube furnace and gas flow control units. In a typical growth experiment, 5g
catalysts were put into a 3-inch quartz tube and the system was flushed with Ar
while the catalysts were heated to 850oC. Carbon monoxide was then introduced.
After running the reaction for 30mins, carbon monoxide flow was turned off and
the system cooled in Ar atmosphere.
The high-purity DWNTs were obtained after a two-step purification
process. The as-synthesized products were first oxidized at 525oC in 20% air in
Ar for 1h to burn the carbonaceous impurities and chemically active SWNTs,
followed by refluxing in 3N HCl water solution to remove the support MgO and
the catalyst Co/Mo. A flexible dark buckypaper can be obtained after filtration.
2.2.3 Electron diffraction method
1. Measure accurately the layer line spacings for each layer line seen in a
diffraction pattern.
2. Measure the diameter of each wall as accurately as possible from its high
resolution TEM image using the line intensity profiles.
37
3. Identify the layer lines corresponding to each helicity. The total number of lines
should be 3 for each helicity other than 0o or 30o. Use the complementary
relations like D1 = D2 + D3 if necessary.
4. Calculate the ratio of the chiral indices for each helicity observed from the
diffraction pattern using the equation below:
v/u=(2D2-D1)/ (2D1-D2)
5. List all possible chiral indices that give the observed v/u ratio experimentally.
6. Select the chiral indices that give the diameters matching closely the ones
measured from the high resolution images and the graphite c-axis spacing
(c/2≈0.335 nm).
Once the chiral index assignment for each layer is complete, the true
diameter, the chiral angle and the metallicity of each layer can be determined.
2.2.4 The separation of DWNTs and SWNTs
A 2% SC (SC Aldrich >99% ) solution was mixed with 0.4mg/ml purified
DWNTs following by high intensity ultrasonication at 75% amplitude (VCX 130
ultrasonics processor from Sonics & Materials Inc.) for 1h. The product was
centrifuged for 2h with speed 13500g (Centrifuge 5417C from Eppendorf North
America Inc).
OptiPrep Density Gradient Medium (60% (w/v) solution of iodixanol in
water (sterile)) was purchased from Sigma-Aldrich. This gradient medium
solution is diluted with distilled water, so we have samples with different densities:
10%, 20%, 30%, 40%, 50% and 60%. SC is added to these solutions to make
38
the SC concentration of 2%, which is the same with in the SC/DWNT suspension.
After that the solutions with six different densities were gently added into the
ultracentrifuge tube (purchased from Beckman Coulter): the 1.5ml the highest
density 60% solution at the bottom, then 1ml 50% solution, 1ml 40% solution,
2ml 30% solution, 1ml 20% solution, and 1ml 10% at the toppest layer. Then the
7.5ml liquid gradient was held still for 72h to allow the formation of a linear liquid
gradient.
0.4ml SC/DWNT suspension was diluted with 0.4ml 2% SC/60% gradient
medium, therefore the tube suspension is of 30% density. Then this 0.8ml
suspension was added in the liquid grident at about 30% layer position.
Finally, the above solution was ultracentrifuged. The Optima L-90K
Ultracentrifuge (purchased from Beckman Coulter) worked for 40h with speed
350000rpm. After ultracentrifugation, we obtained SC/DWNTs with different
layers: tubes were located at 20%-40% density positions. At the very top layer, it
forms three distinguished layers which are purple, green and yellow.
2.2.5 The preparation of TEM samples
Samples for TEM analysis were prepared by mixing the suspended
DWNTs with ethanol using bath sonication, dropping the resulting solution onto a
Cu grid coated with a lacey carbon film, and finally air-drying the sample for
analysis. HRTEM imaging was performed using a Hitachi HF2000 microscope
operating at an accelerating voltage of 200 kV.
39
2.3 Results and Discussions
Catalyst engineering is the key factor for CNT synthesis; we have
developed a mild combustion technique to prepare catalysts nanoparticles on
supporting material (Figure 2.1). This technique involves the exothermic
decomposition reaction of a redox mixture containing metal nitrates and fuels
(organic compound) in a short time. This method allows us to control the
homogeneity and stoichiometry of catalyst nanoparticles and prepare fineparticulate (Co/Mo) on high surface area materials (MgO) simply and
economically. The voluminous structure also reduces the aggregation of catalyst
nanoparticles. It is worth noting that by controlling the ratio between fuel
materials, both fine powder (Figure 2.1A) and “pop-corn” like catalyst (Figure
2.1B) can be obtained. However, for real applications, “pop-corn” like catalyst is
preferred, because the smaller the catalyst particles, the more chance people
could inhale them.
Figure 2.1 Photograof (A) powder catalyst (B) pop-corn catalyst.
40
In our CVD system, in order to prepare high quality DWNTs in bulk,
carbon
monoxide
was
employed.
Comparing
the
mechanism
for
the
decomposition of CH4 and CO,[77] When hydrocarbon compounds are used as
carbon source, such as methane, they will decompose to carbon and hydrogen
by multiple steps with a lot of active carbon intermediates, and high temperature
favors the decomposition process, so that the raw CNT products have lots of
carbon impurities. In contrast, the thermal decomposition of CO is a bi-molecular
reaction, and only C and CO2 are produced providing pure carbon as a carbon
source in synthesizing CNTs. Since CO2 is a very stable gas and desorbs from
the surface without further reaction, the carbon feeding rate is much slower than
the single molecular reaction, and thus the structure of the product can be
controlled. Moreover, at lower temperatures the reaction equilibrium is on the
exothermic carbon dioxide side and at higher temperatures the endothermic
formation of carbon monoxide is the dominant product; therefore, high
temperature favors carbon consumption and less carbon impurities would be
produced. (Scheme 2.1) We have found that the amount of amorphous-carbon in
the raw product was significantly reduced; even bulk amounts of catalysts were
used.
41
Scheme 2.1 Decomposition of methane and carbon monoxide
Transmission electron microscopy (TEM) revealed that the percentage of
DWNTs in purified materials is extremely high (more than 95%) and most of
products are arranged in bundles (Figure 2.2).
42
Figure 2.2 Low-magnification TEM images of purified DWNTs.
Figure 2.3 A typical high resolution TEM image of the cross section a DWNT
43
From the cross-sectional HRTEM image, (Figure 2.3) we observed that
our nanotubes consist of homogeneous-sized and concentric layers. The inner
diameters of DWNTs are mainly 0.76nm and 0.90nm measured by HRTEM
images (established from 85 individual nanotubes).
Figure 2.4 The Inner-tube diameter distribution of DWNTs measured from
HRTEM images (established from 85 individual nanotubes)
This result is confirmed by Raman spectroscopy. As shown in Figure 2.4,
the peaks at 268 cm-1 and 312 cm-1 are correspond to diameters of 0.91nm and
0.77nm using the equation ωRBM = 234/dt + 10,[55] where the dt is the tube
diameter in nm and ωRBM is the frequency of radial breathing mode in cm-1. It is
well known that SWNTs with smaller diameters are more reactive toward
oxidation than thicker tubes, and DWNTs have much higher chemical stability
44
than SWNTs, Therefore, the peaks between 206 cm-1 and 236 cm-1 are
considered to come from the few SWNTs in the raw materials since the
intensities of those peaks decrease and even disappear after oxidation. It is
believed that DWNTs are the only products after purification and the peak at 268
cm-1 and 312 cm-1 are from the inner shell of DWNTs because their intensities
are not affected after purification.
Figure 2.5 Raman spectra of raw samples (blue line) and the sample
purified at 450 oC (red line) and DWNTs purified at 525 oC (black line),
revealing the corresponding RBMs (radial breathing modes) at 160, 176,
268 and 312 cm-1.
Structural correlation between two adjacent graphitic layers in 22 DWNTs
was statistically examined by using electron diffraction. As can be seen from
Table 2.1, the structure of DWNTs is a mixture of semiconducting-metallic (S-M),
45
S-S and M-M tubes; there were nineteen metallic walls out of 44 shells, 7 M-S
(inner-outer), 7 S-S, 4 S-M and 4 M-M nanotubes. For the inner shell, 11 were
metallic and 11 were semiconducting. For the outer shells, 8 were metallic and
14
were
semiconducting.
Our
experimental
study
on
the
structural
characterization of DWNTs shows that graphitic layers in DWNTs exhibit no
strong tendency towards the metallicity of the constituent shells.
Table 2.1: Metallicity of 22 DWNTs
Figure 2.6B is a photograph of distinct groups of density gradient sorted
DWNT suspension. Visually, the separation is evident by the formation of two
groups. The first group consisted of isolated nanotubes that had split into three
sharp bands of different colors, from small to large densities, these bands
appeared purple, green, and orange. The second group was black and
fractionated to five bands as density increased. The optical behavior of the eight
bands was measured. As can be seen from Figures 2.6A and 2.6C, strong
absorption and fluorescence peaks are detected from the top three bands.
However, despite the high concentration of nanotubes in the second group, no
optical signal could be detected. Since the majority of the nanotubes are DWNTs,
we estimate that nanotubes on the bottom are all DWNTs, and those on the top
46
group should have only single walled structures. This estimation is consistent
with our TEM observations.
.
A
B
C
Figure 2.6 A.Optical absorbance spectra of SWNTs after seperation as a
function of faction.B.Photograph after separation using density gradient
ultracentrifugation. C.Optical fluorescence spectra of SWNTs after separation
as a function of fractions.
The HRTEM images in Figure 2.7 confirmed that nanotubes in the top
three bands are all SWNTs and their diameter increased with the increase of
density, while nanotubes in the other bands have double-walled structure with no
PL characteristic. These results indicate that the fluorescence signals arise from
SWNT impurities rather than from the DWNTs. SWNTs trapped inside bundles
are released during high intensity ultrasonication process and then fluoresce.
The absence of fluorescence from DWNTs implies that the efficient and nonradiative relaxation paths exist between the Van Hove States, resulting in the
severely quenched fluorescence.
47
Figure 2.7 TEM images of density gradient separated DWNTs in different
layers from top to bottom
These results reveal that the emission is effectively quenched due to their
interlayer interaction and the fluorescence and absorption peaks only arise from
the SWNTs impurites.
2.4 Conclusions
In conclusion, DWNTs with narrow diameter distribution have been
synthesized by a simple carbon monoxide CVD process. Our results represent a
breakthrough in the efficient and scalable synthesis of highly quality DWNTs.
Further studies are now in progress. More work is under way to use a fluidized
bed reactor to overcome the limits caused by catalyst packing. Additionally, a
structural correlation between two adjacent graphitic layers in 22 DWNTs was
examined statistically, and the results that graphitic layers in DWNTs exhibit no
strong tendency towards the metallicity of the constituent shells. Moreover,
48
optical behavior of DWNTs suspension is investigated. Even though the optical
absorption and the optical photoluminescence spectra show the characteristic
features of SWNTs, the PL signals originating from DWNTs are severely
suppressed. This suppression may be a consequence of an interlayer interaction.
49
Chapter 3: Functionalization of High Quality Few-Walled
Carbon Nanotubes
3.1 Introduction
Carbon nanotubes have attracted extensive attention due to their unique
properties and potential applications. However, realistic applications have been
hindered by the difficulty of the preparation of highly dispersible nanotubes. In
this sense, extensive studies on functionalization of carbon nanotubes have been
carried out in recent years to obtain well dispersed CNTs suspension.
Mickelson et al first reported the extensive sidewall functionalization of
SWNTs by fluorination in 1998.[91] The authors showed that fluorinated SWNTs
could be further functionalized with other functional groups. Tour and co-workers
proposed the covalent sidewall modification of CNTs via in situ generated aryl
diazonium salts, providing highly functionalized and well-dispersed hybrid
materials in organic solvents and water.[92] Sidewall functionalization via a
variety of other methods[93-103] have also been reported.
Although SWNTs with a high degree of sidewall functionalization may
overcome this problem, one must be aware that chemical functionalization may
dramatically change SWNTs’ structure and thus their properties owing to their
single layer of graphene sheet.[94, 104] Few walled carbon nanotube, a unique
novel family of small diameter multiwalled carbon nanotubes with 2–5 layers of
graphene sheets and high structural uniformity and perfection, has been
50
synthesized recently. This material is believed to hold the promise to solve the
problem related to the dispersion and structural damage during functionalization.
Among various techniques, functionalization of carbon nanotubes via
lithium in liquid ammonia has been considered as a convenient route.[105-106]
Despite the impressive results by using this method, this reaction is not easy to
manipulate due to the condensation of liquid ammonia.
In the past decades, sonication has been widely used in the dispersal of
carbon nanotubes (CNTs) in various liquids. However, they are seldom employed
in the functionalization of CNTs. Indeed, a number of common reactions used in
synthetic organic chemistry can be carried out more efficiently using ultrasound;
[107-108]because it can create extraordinary physical(hot spot, microjets, shear
forces, shock waves) and chemical conditions (T roughly 5000K and pressures
about 1000 atmospheres) in extremely small and transient regions with very
short lifetimes when cavitation occurs (Figure 3.1).[109-110]
Bulk Solution:
Room Temperature
Core:
≈ 5000 K
≈ 1000 atm
Interfacial Zone:
≈ 1900 K
Figure 3.1 The chemical and physical effects of ultrasound.
51
We report here an accommodating, easy manipulating, efficient, scalable
way to functionalize few walled carbon nanotubes, using lithium and naphthalene
in THF under mild sonication. This method can be used for attaching various
functional groups on CNTs surface. Additionally, FWNTs have also been
chemically functionalized via efficient ultrasound-mediated oxidation method. It
turned out that any kind of CNTs can be modified by these methods and the
chemically functionalized CNTs are highly soluble in various organic solvents and
aqueous solution respectively.
3.2 Experimental Methods
3.2.1 Synthesis of organic solvent soluble FWNTs
Functional groups such as alkyl, chlorinated polypropylene (CPP),
polystyrene (PS) were successfully grafted to the sidewalls of FWNTs according
to the reaction shown in Scheme 3.1.
52
Li
THF
)))))))
+
Li
Scheme 3.1. Mechanism of ultrasound-mediated functionalization.
In a typical experiment, The reactions were carried out by adding
FWNT(20mg), Lithium(0.12g), naphthalene(0.33g) followed by the addition of
THF(50ml) under an atmosphere of argon to a dry 100 mL, three-neck, roundbottomed flask. The reaction mixture was then sonicated at rt. for 4h. And then
add 0.5g CPP/2ml Styrene monomer. Sonicate at rt. for another 3h. The reaction
mixture was quenched by slow addition of ethanol followed by water. and then
the mixture was acidified with 10% HCl, filtered through a 0. 45 μ m PTFE
membrane, and washed successively with water and ethanol. The functionalized
SWNTs were dried overnight under vacuum for the characterization use.
3.2.1 Synthesis of water soluble FWNTs
Highly water soluble FWNTs were successfully obtained according to the
following reaction:
53
In a typical experiment, the surface treatment process is as follows.
FWNTs (10.0 mg) were weighed and placed into a 30 mL glass bottle with 50ml
and sonicated in the ultrasonic bath for 30-60mins. 0.5M H2SO4; 0.1g KMnO4
was dissolved in 20ml 0.5M H2SO4. The acidic KMnO4 solution was then added
to the FWNTs solution bottle. This makes an acidicsolution of 8.0 M HNO3 and
8.0 M H2SO4 with a total volume of about 18.0 mL. The reaction was carried out
for desired time to allow the functionalization and dispersion of the carbon
nanotubes. After the reaction, the carbon nanotubes were filtered through a 0. 45
μm PTFE membrane, and washed thoroughly with DI water until the PH=7. The
sonochemically treated carbon nanotubes were then ready for further use or
modification.
3.3 Results and Discussion
Ultrasound can cause physical phenomena- hot spots, microjets, shear
forces, and shock waves in liquids that create extreme conditions to disentangle
carbon nanotubes and drive chemical reactions. Ultrasound forms hot spots, in
the mixture of SWNTs and organic liquids, where the temperature and pressure
can instantaneously exceed 1000 atmospheres and 5000 K, respectively.
Organic molecules, such as alkyl iodide, are decomposed at the hot spots, and
reactive species are formed. Besides the hot spots phenomena, microjets, shear
54
forces, and shock waves will help greatly not only for the formation of carbanion
complex, intercalation of lithium cation, but also the exfoliation of FWNT bundles.
During the reaction, we hypothesize that the mild sonication assisted
chemical reaction allows continuous functionalization and exfoliation proceeds
from the outer nanotubes to inner nanotubes gradually in a small nanotube
bundle. Small bundles were firstly obtained in solution under ultrasonic irradiation.
The outer nanotubes in the small bundles are functionalized while the formation
of small bundles followed by an exfoliation with the aid of sonication;
consequently, inner nanotubes are exposed and functionalized further. Ideally,
nanotube bundles can be totally broke into individual tubes and all the individual
nanotubes are functionalized, resulting in a “super” homogeneous solution
(Figure 3.2).
55
OH
COOH
COOH
HO
COOH
HO
COOH
HO
COOH
HO
COOH
HO
O
H
COOH
COOH
H
O
COOH
O
O
H
H
COOH
COOH
H
O
COOH
O
COOH
H
COOH
H
O
COOH
O
H
COOH
COOH
H
COOH
O
COOH
COOH
H
COOH O
O
H
H
COOH
O
COOH
H
O
H COOH
))))))
COOH
COOH
O
H
COOH
OH
COOH
OH
COOH
))))))
O
H O
H
COOH
COOH
HO
OH
COOH
HO
COOH
HO
O
H
O
H
COOH
O
H
COOH
O
H
O
COOH
O
COOH OH O
COOH
H
O
H H
H
COOH
OH
COOH
COOH
OH
COOH
OH
COOH
COOH
OH
OH
OH
OH
COOH
O
H
O
COOH
H
COOH
H
COOH
O
HO
COOH
COOH
OH
COOH
O
H
O
COOH
⌒
⌒
⌒
⌒
⌒
))))))
OH
COOH
OH
COOH
OH
COOH
OH
COOH
OH
COOH
COOH
COOH
OH
COOHCOOH
OH
OH
OH OH
))))))))))) = Low intensity ultrasound (sonication bath)
= Cross section of carbon naontubes
Figure 3.2. The change in nanotube microstructure that occurs under low
intensity ultroasound.
56
As can be seen from Figure 3.3, the products polystyrene grafted FWNTs
(PS-FWNTs) have high solubility in common organic solvents, such as,
chloroform, THF and DMF.
Figure 3.3 Photographs of (left) PS-FWNTs and pure FWNTs in THF (right)
Functionalized PS-FWNTs dispersions in (A) THF (B) DMF (C) CHCl3.
TEM images reveal that FWNTs are coated with functionnalized groups
and the HRTEM clearly shows the integration of FWNT structure even after
functionalization. (Figure 3.4) These highly soluble FWNTs are very promising for
both increasing the loading of CNTs in composites and improving the load
transfer between adjacent nanotues or CNTs and polymers. Our results indicate
that sonication significantly increased the reaction efficiency and yield, resulting
in big improvement in the dispersal of FWNTs
57
A
B
5nm
100nm
Figure 3.4. Low magnification TEM images of PS-FWNTs (Left) and HRTEM
of PS-FWNTs (Right).
What’s more, highly water soluble FWNTs has also been obtained via
acidic KMnO4 oxidation by mild sonication. More importantly, the high proportion
of carboxylic group functionalized FWNTs can be further modified with various
organic groups, which provide an alternative way to functionalize CNTs and
broaden the chemical reactivity of the carbon nanostructures. However, SWNTs
are seriously shortened and lost their structure after functionalization while
FWNTs still keep their structural integrity under the same functionalizaion
condition (Figure 3.5). Therefore, FWNTs are chemically stable than SWNTs and
are better candidate as reinforcement filler in the fabrication of high performance
composites.
58
B
A
200nm
100nm
D
C
100nm
100nm
Figure 3.5. Comparison of two different nanotubes before and after
functionalization.
3.4 Conclusions
In summary, we provide a new, simple, and effective method for
functionalization of CNTs with the assistance of ultrasound.water soluble FWNTs
are prepared by a simple chemical reaction. Aryl, polymer etc. groups and
59
oxygen-based functional groups were introduced to the nanotubes through
covalent bonds. Well-dispersed FWNTs were obtained. A most interesting
feature of the method is that it affords predominantly very small bundles or
individualized CNTs because the lithium intercollates between the CNTs of the
bundles.
FWNTs were more soluble in polar solvents such as THF. The covalent
polymer functionalized FWNTs are expected to be more compatible with the
composite system; the water soluble FWTNs can be further functionalized with
other chemical groups. Our method allows for functionalization of nanotubes to
be performed in solution phase at low temperatures and the synthesis is facile.
60
Chapter 4: Do Inner Shells of Double-Walled Carbon
Nanotubes Really Fluoresce?
(published partially in Nano Lett. 2009, 9, 3282.)
4.1 Introduction
Double-walled
carbon
nanotubes
(DWNTs)
are
unusual
artificial
nanomaterials that are intermediate in structure between single-walled and multiwalled nanotubes (SWNTs and MWNTs).[111] In these structures, SWNTs are
concentrically nested with typical interwall separations of approximately 0.37 ±
0.04 nm.[111-114] DWNTs can be prepared directly in nanotube growth
reactors,[113, 115] or indirectly by thermal annealing of SWNTs that have been
internally loaded with fullerene molecules to form “peapods”.[112] DWNT
samples produced by either production method contain some residual SWNTs
that are typically removed through thermal oxidation and acid treatment to
achieve purity levels above 90%.[76, 85] Even when such purification treatments
do not totally eliminate SWNT impurities, they are expected to quench effectively
the characteristic near-IR fluorescence of residual SWNTs,[52] because minimal
sidewall chemical derivatization (~1 per 10,000 C atoms) significantly suppresses
SWNT fluorescence.[116-117] Several groups have reported that aqueous
suspensions of chemically purified DWNT samples show considerable near-IR
emission at wavelengths consistent with fluorescence from DWNT inner
shells.[48, 84-86, 118-119] High-resolution transmission electron microscopy
61
(HRTEM) further confirmed a match between the diameter distributions of the
inner shells of those DWNTs and diameters of emitting nanotube species
deduced from spectral analysis.[84-85] These experiments along with additional
observations led to the conclusion that inner shells of double-walled nanotubes
fluoresce intensely in the near-IR.[48, 84-86, 118-121] By contrast, a study of
DWNTs synthesized through peapod annealing found severe quenching of inner
shell fluorescence.[87] It was suggested that this quenching results from stronger
intershell electronic coupling related to slightly smaller interwall spacings (<0.346
nm)[87] in peapod-derived samples. A recent study of CVD-grown DWNTs
purified
by
density
gradient
centrifugation
found
apparent
inner
shell
fluorescence that was susceptible to acid quenching and approximately 6 times
weaker than SWNT fluorescence.[122] It was accordingly assigned to SWNT
impurities. DWNT inner shell emission thus remains controversial.
In view of the well-known fluorescence quenching in nanotube bundles,
which have intertube spacings of ∼0.32 nm,[19, 123] and measurements showing
efficient exoergic energy transfer from smaller to larger diameter SWNTs
separated by more than 1 nm, [124]one would expect inner shell emission to be
severely quenched in all DWNTs. Until fluorescence from DWNT inner shells is
directly detected, one must consider the possibility that emission from
catalytically produced DWNT samples may arise from residual fluorescent SWNT
impurities that remain even after chemical purification. Such purification is
typically performed on dry nanotube samples containing large aggregates.[48,
62
84-85, 118-119] One can imagine that residual SWNTs inside aggregates might
be shielded from chemical damage and then act as emissive impurities after
being released during subsequent dispersion.
4.2 Experimental Methods
4.2.1 Synthsis of high quality DWNTs and SWNTs
In an attempt to resolve these issues and properly understand DWNT
inner-tube fluorescence, we prepared samples of purified CVD-grown DWNTs.
We then performed a set of bulk and single-particle measurements designed to
reveal whether the emission was intrinsic to the DWNTs in the sample, or instead
arose from SWNT impurities. We measured the single-emitter brightness,
spectra, abundance, mechanical stiffness, chemical quenchability, and buoyant
densities of emitting species in the samples. Our results clearly indicate that
residual SWNTs are the only significant source of near-IR fluorescence
previously attributed to inner shells of DWNTs.
DWNTs were synthesized at Duke University by carbon monoxide
chemical vapor deposition (CO-CVD) using a binary Co/Mo catalyst supported on
MgO powder. The nanotube product was oxidized at 525 °C in Ar containing 20%
air for 1 h and then refluxed in 3 N HCl solution to remove residual SWNCTs and
obtain DWNT purities estimated at ~95% from TEM imaging. As a SWNT
reference, we used raw nanotubes grown by the HiPco process (Rice University
batch 162.8). These SWNTs were purified using a milder form of the air
oxidation/HCl reflux process used to treat DWNTs.
63
4.2.2 Separation of SWNTs from DWNTs
Suspensions were prepared by placing ~0.1 – 1.0 mg of solid nanotube
material in 2 mL of aqueous 2% sodium deoxycholate (NaDOC) solution and
sonicating the mixture for 2 h with a tip sonicator (Misonix XL-2000) at 8 W input
power. During sonication, the sample temperature was stabilized with an external
ice bath. Dispersions were mildly centrifuged for 30 min at 10,000 ×g to remove
large nanotube bundles. This centrifugation step was omitted for density gradient
separation experiments. To prepare samples containing higher abundances of
long (> 3 μm) individual CNTs, we used milder dispersion conditions in which
several micrograms of CNTs in 2 mL of aqueous surfactant solution was exposed
either to 30 min of bath sonication (Fisher Scientific FS 14), or to intense but brief
tip sonication (input power of ~ 5 – 7 W for 5 s).[125-126] Our methodology and
apparatus for capturing images and spectra of freely moving or immobilized
individual nanotubes have been presented in prior publicaiton.[117, 125-126]The
procedure for measuring intrinsic fluorescence action cross-sections of SWNTs,
i.e. the product of absorption cross-section σ and fluorescence quantum yield
ΦFL , has also been previously described.[126]
4.2.3 Functionalization of DWNTs samples with diazonium salt
We investigated the susceptibility of DWNT sample fluorescence to
chemical derivatization by exposing suspended nanotubes to solutions of aryl
diazonium salts known to react with nanotube sidewalls.[92] The 4bromobenzenediazonium tetrafluoroborate reactant (Fisher Scientific) was used
64
without further purification to prepare an aqueous 1 mg/mL solution. Near-IR
fluorescence microscope images of individual nanotubes immobilized in agarose
gel were then recorded as ~10 μL of this diazonium salt solution was deposited
at the open edge of the sample slide and allowed to diffuse through the gel. We
studied fluorescence quenching in bulk samples by adding 5 μL aliquots of the 4bromobenzene-diazonium tetrafluoroborate solution to 1 mL of nanotube
suspension in a model NS1 NanoSpectralyzer (Applied NanoFluorescence, LLC).
4.3 Results and Discussion
Persistence lengths of individual long nanotubes present in weakly
sonicated DWNT suspensions were deduced from their bending amplitudes, as
observed in near-IR fluorescence videomicroscopy using 659 nm laser excitation
(0.2 to 1 kW/cm2 intensity), 90× magnification, and a 50 ms frame acquisition
time. The (n,m) identities of individual emissive nanotubes were deduced from
their emission spectra.[50] As described in detail elsewhere,[127-128] the
bending analysis began with finding each nanotube’s backbone shape from nearIR images through a custom procedure based on an intensity-weighted center of
mass method. The shape was then decomposed into Fourier modes according to
∞
(
)
the method of Gittes, et al.[129] using the relation θ ( s) = 2 ⋅ ∑ an cos nπ s .
L n =0
L
Here θ(s) is the angle tangent to the nanotube at contour position s, L is the total
nanotube length, n is the mode number, and an is the mode amplitude. The
amplitude of each mode was extracted by inverse Fourier transformation of this
65
equation. At thermal equilibrium, the nanotube bending stiffness, χ, is found from
the following inverse proportionality to variance of bending mode amplitude:
χ=
kBT
L2
.
⋅
2
an 2 (nπ )
Here kB is the Boltzmann constant, T is the ambient temperature, and
angular brackets denote an ensemble average.
Fractionation of DWNT samples was performed using an iodine-based
density gradient medium (OptiPrep, a 60 wt% iodixanol /water mixture). Density
gradients were formed in a 14 mm diameter ultracentrifuge tube by layering
715 μL volumes of premixed iodixanol / 2% aqueous NaDOC solutions having
iodixanol contents ranging from 7.5 to 35% in 2.5% steps. The tube was then
held at an angle of ~10 degrees from horizontal for 1 h to allow diffusional
formation of a linear density gradient. Undiluted OptiPrep was added to the
DWNT sample to raise its density to 1.173 g/cm3 (32.5% iodixanol content). Then
1.5 mL of this sample solution was injected into the section of the centrifuge tube
with similar density. The remaining volume in the tube was filled with a 2%
NaDOC aqueous solution to within ~3 mm from the top. Samples were
centrifuged for 16 h at 288,000 ×g on a Sorvall Discovery 100 SE centrifuge
equipped with a Beckman SW41-Ti rotor. After centrifugation, the sample was
separated into ~280 μL fractions using a Biocomp 152 piston gradient
fractionator.
66
We prepared samples for TEM analysis by mixing the suspended DWNTs
with ethanol using bath sonication, dropping the resulting solution onto a Cu grid
coated with a lacey carbon film, and air-drying the sample. HRTEM imaging was
performed using a Hitachi HF2000 microscope operating at an accelerating
voltage of 200 kV.
Following conventional ultrasonic dispersion and centrifugation in aqueous
surfactants, the DWNT samples showed weak but clear near-IR fluorescence in
the 950 to 1200 nm region characteristic of small diameter SWNTs.[50, 52]
Photoluminescence maps revealed diameter distributions consistent with the
inner shell diameters measured for the samples by high resolution transmission
electron microscopy (HRTEM) Careful examination showed that the emission
peak positions differed from those of pristine SWNTs by ~1 to 4 nm, consistent
with earlier observations.[84, 119, 121] Because the optical resonances of
SWNTs are somewhat sensitive to environment and chemical history,[130-133] it
was unclear whether this red-shifted emission arose from DWNT inner shells
inside the special dielectric environment of their outer shells,[84, 119, 121] or
instead from residual SWNTs spectrally perturbed by sample processing.[133] To
investigate this point, we compared emission spectra of pristine SWNTs and
SWNTs that had been subjected to a milder form of the same purification
procedures used for the DWNT samples. We found that purification-processed
SWNTs showed emission peak positions very similar to those observed from
DWNT samples We therefore infer that the small red shifts in E11 fluorescence
67
from chemically purified DWNT samples do not provide secure evidence of inner
shell emission.
If a DWNT inner shell is electronically perturbed by the adjacent outer
shell, it should show a different emissive quantum yield than a SWNT of the
same structure. Changes in spectral line width, reflecting environmental effects
on exciton dynamics,[134-135] should also be evident when comparing DWNT
inner shells to equivalent SWNTs. Measurements of fluorescence emission
efficiencies and line widths may thus be useful in distinguishing SWNT from
DWNT emitters.
We first prepared SWNT and DWNT dispersions that were
matched in surfactant, preparation method, and absorbance. As illustrated in
Figure 4.1, the DWNT bulk suspensions showed near-IR emission that was
significantly weaker, by a factor of ~5 in the case shown. However, it is difficult to
draw definitive conclusions from comparative measurements on bulk samples
because of possible differences in (n,m) distributions, unknown (n,m)-dependent
molar absorptivities, overlapping absorption features, and possible impurity
effects.
68
Figure 4.1 Comparative absorption and emission spectra of SWNT and DWNT
samples suspended in 2% NaDOC/H2O. The two samples were adjusted for
similar absorbance values. (excitation wavelength 659 nm, excitation intensity
~ 800 W/cm2, frame acquisition time 50 ms)
A much more direct approach is to use near-IR fluorescence microscopy
to measure the relative abundance and emissive brightness of individual
nanotubes in the samples. Figure 4.2 shows typical near-IR fluorescence images
of these suspensions recorded under identical experimental conditions and
displayed on the same intensity scale.[125] It appears that the DWNT sample
69
contains a low concentration of emitters that are individually similar in brightness
to those in the SWNT sample.
Figure 4.2 Near-IR fluorescence images of these suspensions, recorded under
the same experimental conditions (excitation wavelength 659 nm, excitation
intensity ~ 800 W/cm2, frame acquisition time 50 ms) and displayed on the
same false-color intensity scale.
Quantifying the brightness of an individual nanotube requires care,
however, because emission intensity will depend not only on quantum yield, but
70
also on the nanotube length, its orientation relative to the excitation beam
polarization, its (n,m) identity, and the difference between its E22 absorption peak
and the excitation wavelength. To control for these variables, we restricted
observations to nanotubes that had optically resolvable lengths (greater than
2 μm) and could be identified from their emission spectra as (8,3), (7,5), or (7,6).
These species have E22 peaks close to our 659 nm excitation wavelength.[126]
Figure 4.3a displays overlaid normalized emission spectra from individual (7,5)
emitters in DWNT and SWNT samples, and the inset shows the near-IR
fluorescence images of those nanotubes on a matched intensity scale. The two
nanotubes are nearly identical in spectrum and emissive brightness per unit
length. Figure 4.3b displays measured (7,5) line widths (full-width at halfmaximum) and peak wavelengths for 8 SWNTs and 12 emitters from a DWNT
sample. The data reveal no systematic differences between the samples in line
widths, peak wavelengths, or their correlations. Using calibrated conditions for
excitation and detection, we also measured their spectrally integrated emission
signals per unit length. Figure 4.3c shows these fluorescence action crosssections for 14 SWNTs and 16 emitters from the DWNT sample. Note that the
experimental values represent the product of fluorescence quantum yield and
absorption cross-section per carbon atom at 659 nm (not at the E22 peak, as in
our previous report).[126] Although the measured fluorescence action crosssections vary systematically with (n,m) species, no significant differences are
seen between emitters from SWNT and DWNT samples. We thus find that
71
individual fluorescent nanotubes of the same (n,m) species show equivalent
emission peak wavelengths, emission line widths, and emissive brightness
whether observed in SWNT or in DWNT samples.
Figure 4.3 Spectral properties of individual near-IR emitters in SWNT
and DWNT suspensions. a, Examples of emission spectra and Lorentzian
fits for “long” individual (7,5) nanotubes found in weakly sonicated SWNT and
DWNT suspensions. Their fluorescence images are shown in the inset on the
same false-color intensity scale. b, Spectral linewidths (as full widths at halfmaximum) of (7,5) nanotubes vs. peak emission wavelength for 8 individual
SWNTs and 12 individual emitters in a DWNT sample. c, Fluorescence
action cross-sections vs. emission wavelength for 14 SWNTs and 16
individual emitters in a DWNT sample. Clusters of data points are labeled
with their (n,m) identities.
Another approach to distinguishing SWNT from DWNT emission is to
monitor fluorescence quenching caused by sidewall covalent functionalization.
We have previously shown that exposure to diazonium salts causes individual
SWNTs observed by near-IR fluorescence microscopy to display irreversible
stepwise decreases in emission intensity reflecting single-molecule reactions with
the nanotube.[117] It is expected that emission from a DWNT inner shell would
be far more resistant to such chemical quenching because the outer shell would
72
protect against chemical attack. Indeed, there are indications that inner wall
electronic structure remains intact while optical resonances of DWNT outer shells
are destroyed by covalent functionalization or electrochemical doping.[86, 118]
To ensure that the observed nanotubes were directly exposed to the
functionalization reactant rather than trapped in the interior of nanotube bundles,
we performed measurements only on well dispersed samples.
We first monitored near-IR fluorescence from bulk DWNT dispersions after
adding aliquots of a diazonium salt solution known to functionalize carbon
nanotubes.[136] Figure 4.4a shows emission spectra before and after two such
additions. Fluorescence quenching was rapid and nearly complete for the more
reactive large band gap species, as was observed in samples of SWNTs.[137]
We also studied this process by preparing immobilized DWNT dispersions in
agarose gels and then monitoring individual emitting centers using near-IR
fluorescence microscopy. Each emissive object showed irreversible diazonium
quenching. Figure 4.4b plots the emission intensity from two different segments
of a long emissive nanotube in a DWNT sample as a function of time after
exposure to the diazonium solution. Emission from the DWNT sample was
quenched in locally stepwise patterns, and approximately 7 to 13 single-molecule
reaction events were needed to quench 90% of the fluorescence from a 1 μm
segment. These step sizes closely match those found earlier for SWNTs,[117]
implying that the exciton quenching efficiency of individual derivatization sites
was not smaller for DWNT emitters than for SWNTs. We also observed that
73
fluorescence from the DWNT suspensions was readily quenched by exposure to
acid or potassium permanganate. Our results show that the chemical quenching
behavior of DWNT emitters is qualitatively and quantitatively similar to that of
SWNTs and indicate that the emitting centers in DWNT and SWNT samples
have comparable exposure to the surrounding medium.
Figure 4.4 Fluorescence quenching of emitting species in DWCNT
sample by covalent functionalization. a, Fluorescence spectra of a 1 mL
DWCNT
suspension
after
addition
of
5
μL
portions
of
bromobenzenediazonium salt solution. The initial spectrum has been scaled
down by a factor of 10 for clarity. b, Stepwise fluorescence quenching
observed by plotting intensities from different segments of an immobilized
individual long emitter in a DWCNT sample as a function of time after
exposure to bromobenzenediazonium salt solution. The inset shows
locations of the segments whose emission intensities are plotted in the main
frame.
74
Another property that can be probed by near-IR fluorescence microscopy
is the mechanical stiffness of emissive long nanotubes. In aqueous suspension,
individual SWNTs with lengths greater than 3 μm show noticeable bending
induced by Brownian forces.[127-128, 138] Mechanically, SWNTs can be
modeled as inextensible elastic beams with in-plane bending stiffness χ = EI ,
where E is the elastic modulus and I is the area moment of inertia about the tube
axis. The ratio of bending stiffness to thermal energy gives a characteristic
persistence length Lp = χ kBT , where kB is the Boltzmann constant and T is the
sample temperature. Lp represents the length scale over which a nanotube
shows significant curvature induced by thermal fluctuations. In a parallel
experimental study of SWNTs in aqueous surfactant suspension, we have
confirmed the theoretical expectation that Lp varies as the cube of nanotube
diameter.[128] The measured persistence lengths of SWNTs with diameters
between 0.77 and 1.15 nm ranged between 30 and 100 μm. Because the
bending stiffness of a bundle of elastic rods is the sum of its components’ χ
values, the persistence length of a DWNT ( LDW
P ) can be estimated by adding its
inner and outer shell persistence lengths. DWNT outer shells have diameters at
least ~0.66 nm greater than the inner shell. This implies that the persistence
lengths of DWNTs with inner shell diameters of 0.7 to 1.2 nm should exceed
those of SWNTs with the same diameters by factors of approximately 5 to 9.
Thus, bending
measurements
on
long
75
emissive nanotubes in DWNT
suspensions should clearly reveal whether the emitters have single- or doublewalled structures.
We captured near-IR fluorescence spectra and images of 17 randomly
selected long nanotubes from DWNT samples in aqueous suspension. Three of
these images are displayed in Figure 4.5A. The solid symbols in Figure 4.5B
show measured persistence lengths of the 17 DWNT emitters as a function of
spectroscopically deduced diameter. Also plotted on this graph (as open symbols)
are Lp measurements of nanotubes in a SWNT sample and two smooth curves
representing the d3 dependence of the SWNT data and the much higher Lp
values predicted for DWNTs. The persistence lengths found for the emitters in
the DWNT sample are in excellent agreement with those of SWNTs and fall far
below the values expected for DWNTs.
76
Figure 4.5 Near-IR fluorescence images of “long” emissive nanotubes in a
DWCNT suspension reveal noticeable bending.
77
Our final approach to identifying the source of near-IR emission from
DWNT samples was purification by density gradient ultracentrifugation. This is a
bulk method that can sort SWNTs based on the structure dependence of their
buoyant densities in surfactant suspensions.[90] Calculations and a recent
experimental report show that density gradient ultracentrifugation can be
effective in separating SWNTs from DWNTs.[122] We performed density gradient
ultracentrifugation on a stock DWNT suspension and then collected fractions at
various depths corresponding to different densities. Figure 4.6a shows a photo of
the centrifuged sample along with a scale identifying the fraction numbers. We
analyzed
fractions
1
to
16
by
absorption
spectroscopy,
fluorescence
spectroscopy, and HRTEM. The topmost fractions 1 to 4 (corresponding to
densities of 1.046, 1.049, 1.053, and 1.057 g/cm3) had distinct pink, green and
yellow colors. They showed sharp spectral absorption and emission peaks
characteristic of SWNTs with diameters in the range of 0.7 to 1.0 nm (Figure
4.6b). These first four fractions accounted for more than 95% of the sample’s
total near-IR emission. Absorption profiles of fractions 5 to 8 (corresponding to
densities of 1.061, 1.066, 1.071, and 1.074 g/cm3) showed broad peaks near 750
and 1100 nm that shifted to longer wavelengths as the fraction number increased.
Because the HRTEM results described below revealed an absence of DWNTs in
these fractions, the features are assigned to E11 transitions of metallic SWNTs
and E22 transitions of semiconducting SWNTs with significantly larger diameters
(e.g. ~1.75 nm for fraction 7). Some near-IR fluorescence was detectable from
78
each of these collected fractions. In Figure 4.6d we plot the emission intensity at
966 nm (a distinct and intense spectral feature of (8,3) nanotubes) and the
fractions’ absorption at 966 nm as a function of depth in the centrifuged sample
tube. The fluorescence signal peaks strongly at fraction 2 (in a low density region
near 27 mm) whereas the absorption reaches a peak near fraction 9 in a higher
density region. This indicates that the (8,3) emitting species are physically
separable from the major absorbers in the sample. Similar results were found for
other emitting species.
79
Figure 4.6 Spectroscopic analysis of density gradient fractionated
DWCNT samples. a, Image of the DWCNT suspension after density gradient
centrifugation, showing numbering of the collected fractions. b, Absorption
and fluorescence spectra (excited at 660 nm) of fractions 1 to 8 collected
from the tube shown in a. c, Image of a sample containing fractions 7 to 10
from the first separation after a second step of density gradient
centrifugation. d, Absorbance (circles) and relative emission intensity
(triangles), measured at 966 nm, of fractions from the first separation step.
e, Absorbance (circles) and relative emission intensity (triangles), measured
at 966 nm, of fractions collected from the tube shown in c after second step
processing of fractions 7 through 10 (marked as solid circles in d) from the
first separation step.
The tail seen in the fluorescence profile suggests cross-contamination
between fractions, possibly arising from mixing of layers during collection. We
therefore repeated the density gradient centrifugation procedure on combined
80
fractions 7 to 10 (filled circles in Figure 4.6d), which had very strong absorption
and weak emission. Figure 4.6c shows a photo of this re-centrifuged sample.
Fluorescence and absorption data were then measured on re-centrifuged
fractions to give the results plotted in Figure 4.6e. It can be seen that the residual
emissive component in these fractions became separated more completely from
the strongly absorbing component, with ~80% of the emission arising from the
top 6 fractions. From the data we estimate that the fluorescence / absorption ratio
for combined fractions 7 to 10 in Figure 4.6a is at least a factor of 10,000 lower
than for fraction 2. These results suggest that the early, emissive fractions of
lower density contain SWNT impurities from the original DWNT sample, whereas
the later, nonemissive fractions of higher density contain the pure DWNTs.
To check this interpretation, we analyzed the composition of each density
gradient fraction using HRTEM. Figure 6 shows representative images. In
fractions 1 to 4 we found only SWNTs, with typical diameters progressively
increasing from approximately 0.7 nm in fraction 1 to 1.0 nm in fraction 4. These
findings are consistent with the spectrofluorimetric data in Figure 4.6b. Fractions
5 to 7 showed SWNTs with larger diameters, from 1.0 to 1.6 nm. DWNTs were
not detected in these first seven fractions, but were observed as a small
proportion of the nanotubes in fraction 8. The DWNTs seen in HRTEM images of
fractions 8 and 9 have outer and inner shell diameters of ~1.35 ± 0.1 and 0.6 ±
0.1 nm, respectively. Significant emission signals at wavelengths corresponding
to these inner shell diameters were detected only from the upper fractions. We
81
found that that DWNT abundance and DWNT diameters increase further in later
fractions of increasing density. These later fractions also contain some SWNTs
with very large diameters. The results of the HRTEM analysis confirm that
density gradient centrifugation of DWNT samples separates small diameter
residual SWNTs from DWNTs with comparable inner-tube diameters. Our
combined spectrometric and HRTEM measurements on purified fractions provide
evidence that the quantum yield of near-IR emission from DWNT inner shells is
at least 10,000 times lower than for SWNTs of comparable diameter. We note
that Green and Hersam recently reported a milder suppression of fluorescence
from DWNT suspensions purified by density gradient treatment and similarly
deduced that the emission of their DWNT samples came from SWNT impurities
rather than from DWNT inner shells.[122]
82
Figure 4.7 HRTEM images of density gradient fractionated DWCNT
samples. The number in the upper right hand corner of each frame shows
the fraction number as collected from the tube shown in Figure 5a. Fractions
1 – 7 show exclusively SWNTs; fractions 10 – 14 show predominantly
DWCNTs.
4.4 Coclusions
In summary, we have applied a set of complementary experimental
methods to clarify the source of near-IR fluorescence from samples of directly
grown DWNTs. Previous reports have attributed this emission to DWNT inner
shells. Our fluorimetry of bulk DWNT samples shows emission that is weaker
than but spectroscopically very similar to emission from similarly processed
SWNTs. Measurements on individual nanotubes reveal that the DWNT sample
contains a low relative concentration of emitters that individually match SWNTs in
spectral position, spectral width, and absolute fluorimetric brightness per carbon
atom. We find that near-IR fluorescence from DWNT bulk samples is quickly and
efficiently quenched by addition of a reactant that chemically derivatizes
83
nanotube side walls. In this process, individual emitters from the DWNT sample
show stepwise fluorescence quenching from single-molecule reactions that is
qualitatively and quantitatively similar to quenching found previously for SWNTs.
All of these findings are consistent with near-IR emission from SWNT impurities
rather than from DWNT inner shells. This interpretation is supported by
measurements of thermally induced bending amplitudes in long emissive
nanotubes in DWNCT suspensions. Each observed nanotube has a stiffness
value characteristic of a SWNT but far lower than expected for a double-walled
structure. Finally, spectroscopic and HRTEM analysis of DWNT samples
processed by density gradient centrifugation shows that fractions with emissive
SWNT impurities can be separated from nearly nonemissive fractions containing
DWNTs. These data allow us to estimate that the fluorescence quantum yield of
DWNT inner shells in our samples is at least 4 orders of magnitude below that of
SWNTs of the same diameter. We found very similar results using samples
independently prepared and purified (to >95% DWNT content) by the M. Endo
group.[118] Our findings contradict previous reports of inner shell emission from
DWNTs,[48, 84-86, 118-121] including some studies on samples purified by
procedures expected to suppress fluorescence of any SWNT impurities. It may
be that residual SWNTs trapped within nanotube bundles can evade chemical
reaction during such processing and then be freed as the sample is dispersed
into surfactant solution. Alternatively, it seems conceivable that a small fraction of
inner shells may be exposed or released from DWNTs during extensive chemical
84
and physical treatment. Whatever the origin of the emissive SWNT impurities,
our study finds that they are the source of near-IR fluorescence previously
attributed to DWNT inner shells.
85
Chapter 5: Functionalized Few-Walled Carbon
Nanotubes for Mechanical Reinforcement of Polymeric
Composites
(published partially in ACS Nano 2009;3:1057–62.)
5.1 Introduction
To date, carbon nanotubes (CNTs) are the strongest single molecules
measured and are known to have an extremely high Young’s modulus of up to
1TPa and tensile strength approaching 180 GPa.[139-141] They are lighter than
existing fibers and believed to increase the performance of structural composites,
including those used in airplanes, space vehicles, and various leisure goods etc.
However, CNTs have provided only limited enhancements to the mechanical
properties of polymer matrix and the composite processing is still limited to
bench-top scale. Indeed, many well-known issues[57, 142-143] are still need to
be resolved in order to take the maximum benefits from carbon nanotubes to
polymer host. Among them, the main problems[144] are: 1) the dispersion of
nanotubes in matrix; 2) the load transfers between the nanotubes and polymer
matrix; 3) the structure integrity (defect density) of the nanotubes; and 4) the
purity and cost of the nanotubes. It is generally believed that high quality SWNTs
are the best reinforcing filler because of their small size and high Young’s
modules. However, it is extremely hard to separate SWNT[20] from bundles into
individual nanotubes in real applications. Additionally, the difficulty in purification
and the high cost limited them in large scale applications. For CVD-grown multi86
walled carbon nanotubes (MWNTs), the cost is much lower and it was recently
discovered that MWNTs under certain conditions can provide better mechanical
reinforcement of polymer composites.[145] However, CVD-grown MWNTs still
have limited use in the application of composites because of their low structural
perfection and uniformity.[146-147] Fortunately, few walled carbon nanotubes
(FWNTs)[8, 67-68] provide a direction towards solving all these problems.
FWNTs-defined as nanotubes with sidewalls of 2 to 5 layers, diameters ranging
from 3 to 8 nm and lengths around tens of micrometers are unique small
diameter MWNTs with near perfect graphitization structure (Figure 5.1b).[8] Here,
we have investigated the application of such materials as reinforcing fillers in
polyvinyl alcohol (PVA)-based composites. The results have demonstrated that
such material is the best compromise as structural reinforcing fillers that
combined the easiness in synthesis and purification, high structural perfection
and good tolerance to surface functionalization.
Recently, Coleman et al reviewed the published data using dY/dVf as a
benchmark for composite reinforcement.[56, 148] In this paper, we also used
dY/dVf value to compare the reinforcing property of different CNTs and the data
collected from a review of current literatures.[145-147, 149-160] In order to
eliminate the effect of the quality of the nanotube samples made from different
methods, different types of nanotubes (SWNTs, FWNTs, MWNTs) grown by
similar CVD methods were produced (Figure 5.1), functionalized and
incorporated in a PVA matrix. All of the samples were prepared using the same
87
thermal CVD in a horizontal tube furnace. Free standing CNT/PVA films with
similar thickness were fabricated. The mechanical properties of the composite
films were investigated with a DMA 2980 tensile tester from Thermal Instruments.
It shows that FWNTs are easier to produce, easier to purify than SWNTs and
have better structural integrity than MWNTs. These properties, plus the
significantly improved reinforcing property measured in this study, allow us to
predict that FWNTs are highly suitable, if not the best reinforcing filler, for the
next generation of composite materials. The consistent better performances from
FWNTs in our study also confirm the previous analysis.[147]
a
88
b
c
Figure 5.1 HRTEM images of (a) SWNTs. (b) FWNTs. (c) MWNTs.
89
5.2 Experimental Methods
5.2.1 The synthesis and purification of SWNTs,FWNTs,MWNTs
In this work, all types of carbon nanotubes were prepared by bimetallic
catalysts Co/Mo with different molar ratio supported on MgO support. Typically,
desired amount of Co(NO3)2 · 6H2O, Mg(NO3)2 · 6H2O, (NH4) 6Mo7O24 · 4H2O,
glycine, and citric acid dissolved into deionized water to make a clear solution,
then the solution was slowly heated at 120oC for 12 h to obtain a viscous
precursor. When increasing the temperature to over 300 oC quickly, the precursor
combusted suddenly to produce large amount of porous powder. The catalyst for
CNT growth was finally obtained after annealing the porous powder at 550 oC to
remove any organic residues.
FWNTs / MWNTs were synthesized in a simple CVD setup made of a
horizontal tube furnace and gas flow control units. Methane was employed as
carbon source and hydrogen was also added with certain ratio to control
methane decomposition rate. In a typical growth experiment, Co/Mo supported
MgO catalyst was put into a quartz tube and was flushed with hydrogen, while
the catalyst was heated to growth temperature. Methane was then introduced.
After reacting for desired time (10–30 min), methane flow was turned off and
hydrogen flow was turned on while the system is being cooled down.
90
Pure FWNTs were obtained after a simple two-step purification process.
The as-synthesized products were first oxidized at 600 oC in 20% air/ 80%Ar for
1h, followed by refluxing in 3N HCl water solution.
MWNTs were purified by refluxing the as-synthesized materials in 3N HCl
water solution
SWNTs were synthesized in the same CVD setup. In a typical SWNTs
growth experiment, catalysts were put into a quartz tube and the system was
flushed with Ar while the catalysts were heated to 750 oC. Carbon monoxide was
then introduced. After the reaction lasted for 30min, carbon monoxide flow was
turned off and the system was cooled in Ar. The as-synthesized SWNTs were
first oxidized at 300 oC in 20% air/ 80% Ar for 1h, followed by refluxing in 3N HCl
water solution.
5.2.2 The functionalization of carbon nanotubes by 3M nitric acid
Desired amount of CNTs were dispersed in 3M HNO3 in a vial and
sonicated in a bath for 5h. The resulting suspension was filtered using a 0.2 μm
PTFE filter paper and washed with DI water.
5.2.3 The fabrication of functionalized carbon nanotubes and PVA
composites
PVA was used as a prototype matrix to investigate the effect of different
types of nanotubes on the enhancement of mechanical properties. It (from
Aldrich, molecular weight 86000 g/mol, 99+% hydrolyzed) was dissolved in
distilled water at 90 °C and subsequently cooled to room temperature. The
91
nanotubes used were purified SWNTs, FWNTs, MWNTs (Figure 5.1), prepared
by our thermal CVD method. Water soluble CNTs were obtained via mild
sonication-mediated oxidation in 3M HNO3 . The composites were prepared with
PVA and a functionalized carbon nanotube solution. To fabricate free-standing
composite films, an appropriate amount of functionalized CNTs was mixed with a
PVA aqueous solution at room temperature via tip sonicator. Highly uniform free
standing films were fabricated by casting this mixture solution in molds and the
evaporation of excess water. The thickness of the resulting film was about 40µm.
The films were peeled off the substrates and cut into strips to facilitate
mechanical properties testing. Prior to testing, all specimens underwent an
additional drying procedure for 1 h at 60 °C, to ensure the evaporation of any
remaining water. The volume fraction of CNT in each film was calculated from
the mass fraction using the densities, 1) 1300kg/m3 for PVA, 2) 1500kg/m3 for
SWNTs, and 3 ) 2150kg/m3 for MWNTs and FWNTs. Tensile testing was carried
out using a DMA 2980 tensile tester. A speed of 1N/min was used to obtain the
tensile modulus, Y. In all cases, at least four strips were measured and the mean
and standard deviation of Y were calculated.
5.3 Results and Discussions
It is known that poor dispersion of nanotubes in solvents and polymer
matrices is a key problem in making high performance CNT composite materials.
Covalent functionalization of the nanotubes is a promising strategy to not only
improve nanotube dispersion but also provide a means for creating microscopic
92
interlinks between nanotubes and polymer. However, too much defect under
extreme chemical treatment can compromise the mechanical properties of the
nanotubes. Therefore, in this study, a moderate functionalization method was
employed to obtain good dispersion of CNTs in polymer matrix without creating
too high defect density on nanotubes. The method was chosen to be the mildest
treatment that will prepare a stable water suspension to prepare composite films
without noticeable aggregations of nanotubes.
Figure 5.2 shows a photograph of liquid mixtures containing PVA/ FWNTs
and PVA/functionalized FWNTs on two glass slides. The large FWNT
agglomerates can be clearly seen from the liquid on left slide, while the
composite suspension with functionalized FWNTs on right slide are very smooth.
The results clearly show that CNTs has been successfully functionalized
by the moderate functionalization method. Indeed, the functionalization of the
nanotubes is critical to obtain sufficient microscopic interlinkings and load
transfer to obtain homogeneous and stable nanotubes dispersion in composite
film.
93
Figure 5.2 Photograph of liquid mixture of PVA and nanotubes placed on
glass slides. Left: purified but unfunctionalized FWNTs. Right: functionalized
FWNTs.
Transmission electron microscopy (TEM) revealed that SWNTs used in
this study(Figure 5.3A) are highly pure (more than 90%) and most of products
are arranged in bundles similar to HiPCO SWNTs in Figure 5.3B. No metal
catalysts are observed in our SWNTs samples.
94
A
B
Figure 5.3 TEM images of (A) SWNTs used in this study (B) HiPCO SWNTs
Figure 5.4 shows normalized resonance Raman spectra of SWWTs used
in this study and HiPCO SWNTs. The defect concentration of those SWNTs can
be estimated by the intensity of the D-band (ID) at ~1300 cm-1 relative to the
intensity of the G-band (IG) at ~1590 cm-1. As can been seen from Figure 5.4, the
SWNTs used in this study have better structural uniformity compared to that of
HiPCO SWNTs. Therefore, the SWNTs used in this study are representative of
high quality CVD-grown SWNTs.
95
Figure 5.4 Normalized Raman spectra of HiPCO SWNTs and SWNT’s used
in this study.
Raman
spectroscopy
(Figure
5.5)
clearly
indicates
the
covalent
functionalization of the CNTs. The defect concentration of CNTs before and after
functionalization can be estimated by the intensity of the D-band (ID) at ~1300
cm-1 relative to the intensity of the G-band (IG) at ~1590 cm-1. As can be seen
from Figure 5.5, after functionalization, the relative intensity of D bands of all type
of CNTs are higher as the functional groups are attached to the sidewalls. We
also noticed that the suspension of functionalized CNT and PVA mixture is very
96
uniform without any visible aggregations, while large agglomerates still exit with
unfunctionalized CNTs are used.
97
a
b
Figure 5.5 Normalized Raman spectra of different type of CNTs (a)
before and (b) after functionalization
98
The mechanical properties of different types of CNTs (SWNTs, FWNTs,
MWNTs) in PVA composite have been investigated. The typical stress-strain
curves for the PVA based composite are given in Figure 5.6 and their mechanical
properties are summarized in Table 1. Results indicate that all of composite films
with as little as 0.2wt % nanotube show higher Young’s modulus and higher
tensile strength than those of pure PVA materials. In particular, the improvement
is more pronounced with functionalized FWNT (fFWNT) films. Composite films
with 0.2 wt% fFWNT exhibit the highest Young’s modulus of 6.33GPa, which is
1.99GPa higher than that of pure PVA film, coupled with a 42 MPa increase in
tensile strength (Figure 5.6a, Table 5.1). This represents a reinforcement of
dY/dVf = 1658 GPa which is ~400 Gpa higher than the highest value of dY/dVf =
1244 GPa previously reported in PVA-CNTs composites. In the case of SWNTs
and MWNTs, the reinforcement (dY/dVf = 741 GPa and 767 respectively) is
much lower than that of FWNTs.
99
a
b
Figure 5.6 Stress-strain curves of composite films containing (a) 0.2 wt % of
different types of CNTs. (b) different concentration of fFWNTs (functionalized
FWNTs).
100
It should be noted that nanotube diameters, lengths along with the amount
of impurities and structural defects may vary among samples from different
batches and different laboratories. Thus, experimental results using the same
type of nanotubes from different researchers may show a discrepancy. In this
study, our aim is to investigate the optimum type of nanotubes as reinforcing filler
in practical applications. The fact that higher value of dY/dVf (741 Gpa) were
observed with the SWNTs used here compared with the highest reported value
( 305 Gpa) in PVA/SWNTs composite[152] verified that the SWNTs used here
are of high quality and our experimental comparison is appropriate. The same is
true for MWNTs used in this study.
We found that FWNTs shows a remarkable reinforcement value of dY/dVf
= 1658 GPa (Figure 5.6, Table 5.1), much better than the results from SWNTs
and MWNTs.
It is known that dispersion of CNTs in polymer is the most
fundamental issue in composite systems. Ideally, individual nanotubes must be
uniformly dispersed in polymer matrix in order to achieve uniform stress
distribution and efficient load transfer. Theoretically, in order to maximize
reinforcement, we will need nanotubes with small diameters to obtain greater
surface area and maximized interaction with matrix. Therefore, if we could
routinely produce individually dispersed high quality SWNT in polymer matrix,
they should show best performance in the composite material compare with other
types of CNTs with same defect density, length, etc,.
However, in real
applications, a true solvent for any unfunctionalized individual carbon nanotubes
101
are yet to be achieved. The fabrication of uniform polymer composite with
SWNTs is more challenging due to severe bundling of the nanotubes. Although
any type of carbon nanotubes tend to form bundles, the strong intrinsic van der
Waals attractions in SWNTs (~0.5ev/nm for SWNT-SWNT contact)[20] produce
larger bundles, making them more difficult to be dispersed in any solvents or
polymer matrices. Although chemical modification can improve the dispersion of
SWNTs in matrix materials, covalent functionalization can severely disrupt their
single layer structure. Additionally, modulus can also be considerably reduced by
even small nanotube curvature[161-162] and SWNTs tend to be "wavier" than
others.
Table 5.1 Mechanical Properties of 0.2wt% CNTs/PVA Composite Films
102
In the case of MWNTs, they have diameters of more than 8nm and can be
well dispersed in polymer matrix (Figure 5.7c) after functionalization. However,
the interfacial area is compromised with the increase of diameter compared with
that of SWNTs. Moreover, the biggest ID/IG ratio in RAMAN spectrum (Figure 5.5)
of pristine MWNTs suggested their high defect density and low structural
uniformity, which may also contribute to limit their mechanical improvement in
composites.
FWNTs is a unique small diameter (3-8nm) MWNTs in terms of their
morphology. The length of FWNTs is as long as 20 micrometers and their aspect
ratio can be reached as high as ~6600 (Table 5.1). The bigger diameter and
thicker wall made FWNTs much easier to be individually dispersed in solvent or
polymer than SWNTs. The quality of the CNTs dispersion in composite films has
been evaluated by TEM micrograph (Figure 5.7).
103
104
c
Figure 5.7 TEM-micrographs of PVA filled with 0.2 wt%
functionalized (a) SWNTs. (b) FWNTs. (c) MWNTs.
Figure 5.7b shows the uniform FWNTs distribution in PVA matrix. It is
believed that the good dispersion of functionalized FWNTs in PVA provides huge
surface area and more uniform stress distribution, minimizes the presence of
stress concentration centers. Meanwhile, SWNTs are extremely difficult to be debundled even after functionalization compared to FWNTs and MWNTs (Figure
5.7a). Therefore, the effective modulus for SWNTs bundles are far below those
theoretically expected for their individuals and the large agglomerates may also
originate cracks in composites rather than reinforce their mechanical strength.
Additionally, SWNTs are more chemically unstable. As can be seen from Figure
5.8a, some of SWNTs are shortened and their conjugated single-layer structures
105
are destroyed under the same reaction condition, which may compromise their
mechanical properties to certain degree. In the case of FWNTs, the very low ID/IG
ratio indicates that FWNTs have the perfect structure compare with all other
CVD-grown carbon nanotubes. They should be the most robust in load transfer
from matrix to nanotubes. Moreover, as can be seen from Figure 5.8, FWNTs are
chemically stable than SWNTs and MWNTs, they can retain straight morphology
and structural integrity even after covalently functionalization. We believe that all
of these factors, the easiness in synthesis and purification, the tolerance against
chemical functionalization, the formation of stable and uniform dispersion in
polymer and good structural perfection of the materials, have contributed to the
observed higher reinforcing capability of the FWNTs, making them an optimum, if
not the best, choice as structural filler in composite materials.
106
Figure 5.8 HRTEM images of functionalized (a) SWNTs. (b)
FWNTs. (c) MWNTs
107
The mechanical properties of composites as they correlate to the
concentration of fFWNTs have also been investigated (Figure 5.6b, Table 5.2). In
general, the Young’s modulus and tensile strength increased steadily with the
concentration of functionalized FWNTs (ranging from 0.2wt % to 1wt %),
indicating that the PVA films become more resistant to deformation. Additionally,
as can be seen from Figure 5.6b, 0.2wt % functionalized FWNTs have
reinforcement of dY/dVf = 1658 GPa which is notably higher than that of pure
FWNTs/PVA composite. This suggests that the unfunctionalized FWNTs have
large nanotube agglomerates, while the modified FWNTs have improved
dispersion and higher interface area and therefore possess higher mechanical
enhancement of the composite materials. Results also demonstrate that the rate
of increase of Young’s modulus slows with the increase of fFWNT concentration,
indicating more functional groups are needed to further improve the dispersion
and avoid the aggregation at higher fFWNTs loading. This could be resolved by
carefully modifying the condition of the chemical reaction.
Table 5.2 Mechanical Properties of Different Concentration fFWNTs/PVA
Composite Films
108
5.4 Conclusions
In summary, this research explored the question of optimum reinforcing
filler for polymeric composites materials, based on different CNTs/PVA
composite and the multitude of factors that affect their mechanical properties.
We conclude that the mechanical properties of CNTs/PVA composite vary with
CNTs materials. Variations in defect density, diameter and length, the aspect
ratio, surface-functionalization, dispersion state, nanotube loading and the
interfacial adhesion between the nanotubes and the polymer matrix are all
factors that could change the mechanical properties of the composite. From our
research, it is shown that the optimum candidates, if not the best, for polymer
reinforcement are functionalized FWNTs. Compared with other type of CNTs,
they can be produced and purified more easily;[8] they can keep the structure
uniformity after functionalization; their smaller diameters give them better
dispersion in the matrix materials and therefore larger interfacial area, stronger
interfacial adhesion and more efficient load transfer between the nanotubes and
the polymer host. It is foreseeable that our investigation of the extraordinary
mechanical properties of FWNTs will encourage more exploration of their diverse
applications, especially in composite materials.
109
Chapter 6: Design and Synthesis of Hierarchical MnO2
Nanospheres/Carbon Nanotubes/Conducting Polymer
Ternary Composite for High Performance
Electrochemical Electrodes
(published partially in Nano Lett. 2010, 10, 2727.)
6.1 Introduction
As the limited availability of fossil fuel and the environmental impacts of a
society based on such energy sources becoming more obvious, the need for
renewable energy sources has attracted attentions of the world. Systems for
electrochemical energy storage and conversion include batteries, fuel cells, and
supercapacitors. Among them, supercapacitors, also known as electrical double
layer capacitor, ultracapacitor, or electrochemical capacitor (EC), have attracted
much attention because of their high power density, long cycle life (>100 000
cycles), and rapid charging–discharging rates.[163] They can be applied in a
large variety of applications, including consumer electronics, memory back-up
systems, industrial power, energy management, public transportation, and
military devices. More importantly, supercapacitors are critical components in the
next generation all-electric cars and cars based on fuel cells that use hydrogen or
alcohol as clean and renewable energy media. Such future transportation
alternatives can significantly reduce our need on fossil fuel (Figure 6.1B).
Supercapacitors can provide energy densities (1~10Wh/kg) higher than those of
conventional electrolytic capacitors (<0.05Wh/kg) and higher power densities
110
(1~2kW/kg), than batteries (<0.1kW/kg) (Figure 6.1A). They also have longer life
cycle, up to 1,000,000 charge-discharge cycles (appr. 15-20 years) compared to
that of batteries (500~2000 cycles).[164] The market for supercapacitors is
expected to reach $2 billion in 2012 with electric vehicles being the major
application (Figure 6.1C). However, the limited energy density of supercapacitors
restricts their application to power delivery over only few second.[165] Most of
available commercial devices have a specific energy below 10 Wh/kg, whereas
the lowest value for batteries is 35–40 Wh/kg in lead-acid batteries, and as high
as 150Wh/kg for lithium ion batteries.[164] Therefore, performance (energy and
power densities, safety, and cycle life) of the electrochemical capacitors need to
be improved significantly in order to satisfy the rapidly increasing demands for
these applications.
111
A
C
B
HEV
Figure 6.1 (A) Ragone chart showing energy density vs. power density for
various energy-storage devices. (B) A typical HEV configuration. (C) Market
of supercapacitors
Various materials have been investigated as the electrodes in ECs,
including carboneous materials,[166-168] conducting polymers[169-170] and
transition-metal oxides.[171-172] Conducting polymers are highly redox active
materials and are reported to store charge of up to 250 F/g (Figure 6.2[173]).
However, conducting polymers suffer from poor cycleability and impedes them
from commercial use.[174] This is attributed to their poor mechanical property for
repeated redox processes. High surface area carbon material, such as activated
carbon (ACs), is currently widely used in commercial supercapacitors. While ACs
is widely used because of their high surface area (1,000–3,000 m2/g) and their
moderate cost, they exhibit a limited capacitance and lower energy density.[175]
In addition to high-surface-area carbons, higher capacitance can also be
112
achieved by using transition metal oxides. They typically exhibit a capacitance of
the electrode material that is more than 10 times that of carbon based materials
(Figure 6.2). Hydrous ruthenium oxides have been insensitively studied because
of their high theoretical specific capacitance (1358 F/g).[176] However, its high
cost excludes it from wide application.
Figure 6.2. Comparison between different electrode materials. The images
are adapted from ref [173]
MnO2 is generally considered to be the most promising transition metal
oxides for the next generation of supercapacitors because of its high energy
density, low cost, environmental friendliness, and natural abundance.[177-178]
The published results thus far established that the electrochemical performance
of MnO2 depended on their morphology, porosity, specific surface area, electrical
conductivity and ionic transport within the pores.[179-180] In this context, layered
mesoporous birnessite-type manganese oxide materials are attracting great
interest due to their high surface area, low density, and good permeation.[181185] However, the fabrication of birnessite-type manganese oxide architectures
113
remains a significant challenge due to the fast and uncontrolled growth process.
[186-187]
Literature reviews of electrodes made from MnO2 showed that high
specific capacitance and rate capability should be obtained in principle.[163-164,
188-189] However, a key weakness of the metal oxide material is its limited
electric conductivity.[190] To effectively utilize MnO2 materials, binary composites
of hydrous MnO2 with either carbon nanotubes (CNTs)[191-196] or conducting
polymers[60, 197-198] have been explored and demonstrated improvement in
the electrochemical performance. However, conducting polymers and metaloxides both suffer from mechanical instability.[190] Electrodes made of hydrous
MnO2 with conducting polymers showed mechanical instability and poor
cycleability. For composites with carbonaceous materials, including CNTs, the
reported enhancement of electrochemical performance is more pronounced
when only a small amount of metal oxide is incorporated in the electrode.[199]
However, for practical applications, particularly for large capacitor applications,
such as power sources for the HEV or FECV, high metal oxide concentration in
electrodes and high mass loading of total active materials are needed.
Unfortunately, due to the dense morphology and the intrinsically poor electrical
conductivity of MnO2, its electrochemical performance is unsatisfactory if the
loading of MnO2 is high. When the weight percentage is increased, MnO2
becomes densely packed with limited accessible surface area, only a very thin
layer of the oxide material participated in the charge storage process, resulting in
114
both high resistance and a comparatively low specific capacitance. These lead to
a significant degradation from the theoretical advantages of the material and
eventually reduces its attractiveness in future application. Therefore, the low
energy density caused by the limited loading of hydrous MnO2 still remains a
major problem. Extensive efforts are still needed to improve the electrochemical
utilization of MnO2, especially in the cases where high metal oxide loading is
needed.
In the present paper, we report a novel design and synthesis of a ternary
MnO2/CNT/CP composite for high performance electrochemical electrodes. New
insights on the design of an ideal electrochemical capacitor with both high energy
and power density are provided. In our approaches, we focus on the synergistic
effects from the combination of MnO2, functionalized few walled carbon
nanotubes (fFWNTs) grown by CVD method[8] and commercial PEDOT:PSS
conducting polymer in the composites to effectively utilize the full potential of all
the desired functions of each component. Hence, this composite provide a
direction towards solving the potential problems and is very promising for the
next generation high performance electrochemical electrodes.
6.2 Experimental Methods
6.2.1 The synthesis of FWNTs
In this work, few walled carbon nanotubes were prepared by bimetallic
catalysts Co/Mo supported on MgO support following our published process.
Typically, desired amount of Co(NO3)2 · 6H2O, Mg(NO3)2 · 6H2O, (NH4)
115
6Mo7O24 · 4H2O, glycine, and citric acid dissolved into deionized water to make a
clear solution, then the solution was slowly heated at 120oC for 12 h to obtain a
viscous precursor. When increasing the temperature to over 300 oC quickly, the
precursor combusted suddenly to produce large amount of porous powder. The
catalyst for CNT growth was finally obtained after annealing the porous powder
at 550 oC to remove any organic residues.
FWNTs were synthesized in a simple CVD setup made of a horizontal
tube furnace and gas flow control units. Methane was employed as carbon
source and hydrogen was also added with certain ratio to control methane
decomposition rate. In a typical growth experiment, Co/Mo supported MgO
catalyst was put into a quartz tube and was flushed with hydrogen, while the
catalyst was heated to growth temperature. Methane was then introduced. After
reacting for desired time (10–30 min), methane flow was turned off and hydrogen
flow was turned on while the system is being cooled down.
6.2.2 The Preparation of hierarchical MnO2 nanosphere and
MnO2/fFWNTs composite
The growth of MnO2 is based on the redox reaction between MnSO4 · H2O
(Aldrich) and KMnO4 (Aldrich) on functionalized few walled carbon nanotubes
(fFWNTs) in water according to the following Equation:
2MnO4 - + 3Mn 2+ + 2H2O
5MnO2 + 4H+
MnO2/fFWNTs composite has been synthesized via sonochemical
coprecipitation methods. Desired amount of KMnO4 was dissolved in PH=2
116
aqueous solution. Meanwhile, a slightly more than calculated amount of
MnSO4·H2O and fFWNTs were dispersed in PH=2 aqueous solution and
sonicated for 1hour. After 1 hour, add the KMnO4 solution to the fFWNTs/MnSO4
solution
and
continue
to
sonicate
for
another
2hours.
The
resulting
MnO2/fFWNTs composite suspension was filtered using a 0.2 μm PTFE filter
paper and washed with DI water. If fFWNTs is not added in the process,
hierarchical MnO2 nanospheres are produced. As can be seen from Figure 6.1,
the produced MnO2 nanospheres using this simple method are extremely uniform,
and the size is around 150nm.
Figure 6.3 SEM images of hierarchical MnO2 nanospheres.
6.2.3 The Functionalization of few-walled carbon nanotubes by 6M
nitric acid
Desired amount of pure FWNTs were dispersed in 6M HNO3 in a vial and
irradiated in a low intensity sonication bath for 5h. The resulting suspension was
filtered using a 0.2 μm PTFE filter paper and washed with DI water.
117
Figure 6.4 Normalized Raman spectra of FWNTs before and after 6M
HNO3 treatment.
Normalized Raman spectroscopy in Figure 6.2 clearly indicates the
covalent functionalization of the FWNTs. The relative intensity of D bands of
FWNTs is increased as the functional groups are attached to the sidewalls, while
the slight changed intensity of D bands indicate FWNTs still kept their structure
uniformity after functionalization. We also noticed that the nitric acid treated
FWNTs under mild sonication showed improved “solubility” and can be
homogeneously dispersed in aqueous solution.
118
6.2.4 Calculations
Specific capacitance Cspec=I/(dV/dt)me, I is the applied current, dV/dt is the
potential change rate determined from galvanostatic charge/discharge curve, me
is the mass of electrochemically active materials.
6.2.5 Electrochemical measurement
The electrochemical measurements were carried out using Gamry
Reference 600 potentiostat/galvanostat. In order to calculate specific capacitance,
galvanostatic charge/discharge test were performed by cycling potential from 0 to
1 V at different current densities. Cyclic voltammetry was performed in a potential
range between 0 and 1 V at different scan rates in 1 M Na2SO4. All electrode
potentials were measured relative to an Ag/AgCl reference electrode using a Pt
foil as a counter electrode.
6.2.6 The fabrication of the working electrodes with airbrushing
technique
Inspired by the achievement of electrodes design of other groups, we
employ airbrushing technique to prepare working electrode, which allows the
printing of colloidal inks with arbitrary viscosities and allows quick controlled
deposition of electrode films with high homogeneity on various substrates.
Calculated amount of MnO2 or MnO2 and PEDOT:PSS (Aldrich) or
MnO2/fFWNTs and PEDOT:PSS were mixed in DI water solution and then
sonicated for 10 mins use the tip sonicator to prepare the colloidal inks. The
working electrode was then fabricated by airbrushing the colloidal inks onto the
119
gold substrate with area of 1 cm2. The gold substrate was heated on hot plate at
120 oC while the preparation of electrodes. Finally, electrodes were annealed at
120 oC in the oven for 2 hours and are ready for future characterization.
Figure 6.5 Photograph of working electrode after 1000 charge-discharge cycles.
Figure 6.3 shows the electrode after 1000 charge-discharge cycles
prepared with this method. The Photograph indicates a uniform ternary
composite film and also further confirms the stability of the electrode film after
charge-discharge cycles.
120
6.3 Results and discussions
Looking carefully at the charge storage process, a main existing problem
is that the underlying bulk oxide materials remain as dead volume, resulting
significant reduction in specific capacitance at high mass loading.[200]
Additionally, in conventional methods of preparation of these metal oxide based
nanocomposites films, binder material such as polytetrafluoroethylene(PTFE)
needs to be added in order to improve the film stability. However, PTFE is
insulating and reduces the electrical conductivity of the film. Therefore, if a binder
material is highly conductive and can further disperse the densely packed
MnO2/CNTs bundles and stabilize them in solutions; it could solve the potential
problem. PEDOT:PSS conducting polymer, which is water soluble and can
disperse carbon nanotubes and other nanomaterials in water, could be a good
candidate for such a binder. In general, our strategy is shown schematically in
Figure 6.4f, where the MnO2 nanospheres are directly grown on CNTs. In such a
composite, MnO2 offers the desired high specific capacitance and CNTs
framework provides the improved electrical conductivity and mechanical stability.
However, the controlled growth of the right MnO2 nanostructures on CNTs is not
trivial. The synthesis processes are complicated when grow MnO2 nanophase on
CNTs surface, since the interfacial chemistry may affect the nature of the
deposited MnO2. Fortunately, sonochemical processing provided a method for
such synthesis. It has proven to be a useful method for not only disperse and
functionalize CNTs,[39, 201] but also prepare mesoporous metal oxide.[202-203]
121
In the present study, we have explored a simple but efficient ultrasound mediated
co-precipitation method to prepare hierarchical MnO2 nanosphere directly on well
dispersed fFWNTs.
Furthermore, PEDOT:PSS is added in the composite and provides much
needed functions. It can not only act as dispersant to stabilize the composite
suspension to facilitate the electrode film fabrication, but also offer good interparticle connectivity between the oxide material and the CNTs. Indeed, both
CNTs and PEDOT:PSS are also involved in the charge storage process as
conducting additives, both of which can contribute to the energy storage of the
entire film. Thus, such a ternary composite is expected to offer much improved
performance of the electrode film, which is often difficult to achieve from either
the pristine hydrous MnO2 or its binary composites such as PEDOT:PSS/MnO2.
122
Figure 6.6 Sketch of MnO2/fFWNTs/PEDOT:PSS ternary composite.
In this work, the MnO2/fFWNTs composites were synthesized using an
efficient approach. Briefly, FWNTs with high surface area[38] were first
functionalized by acid treatment to attach carboxylic groups or hydroxyl groups
on the sidewalls of the outer shell. Since the inner tube is protected by the outer
shell, the electrical conductivity and structural integrity of the inner tubes remains
superior. As can be seen from Fig. 2A, the functionalized FWNTs still keep their
intact structure after nitric acid treatment. MnO2 precursors (KMnO4 and MnSO4)
were exposed to ultrasonic irradiation in the presence of modified FWNTs
afterwards at room temperature, leading to the formation of the hierarchical MnO2
123
nanophere on fFWNTs mesoporous fFWNTs network (Figure 6.5) within short
time. The MnO2 loading on fFWNTs can be controlled by tuning the ratio
between fFWNTs and the MnO2 precursor.
Figure 6.7 TEM image of fFWNTs.
Figure 6.6 shows the morphology and microstructure of a representative
composite with 60 wt% MnO2, 30 wt% fFWNTs and 10 wt% PEODT:PSS.
124
Figure 6.8 TEM (A) and SEM (B) image of PEDOT:PSS
dispersed MnO2 nanospheres in situ grown on fFWNTs
125
As can be seen in the figures, a unique hierarchical MnO2 architecture has
been successfully grown on a continuous functionalized FWNTs network.
Evidently, MnO2 nanosphere showes a tendency to have strong interaction with
fFWNTs (Figure 6.6A) compared with the direct mixing of MnO2 nanosphere with
fFWNTs (Figure 6.7).
Figure 6.9 TEM image of direct mixing of MnO2 nanospheres with
fFWNTs
126
No aggregations of the MnO2 nanoparticles off fFWNTs scaffold are
observed in the composite, indicating that the nucleation is predominantly on the
exterior surfaces of fFWNTs. Although the exact growth mechanism has not
been completely understood, we suggest that oxygen-containing functional
groups on fFWNTs can act as anchoring sites or nucleation sites for the growth
of MnO2. The Mn2+ ions in the solution are preferentially adsorbed on these sites
due to electrostatic force between Mn2+ ions and polar oxygen functional groups
introduced by acid treatment. Subsequently, the Mn2+ ions are oxidized by
KMnO4 to form amorphous MnO2 particles under ultrasonic irradiation.
Additionally, transmission electron microscopy (TEM) image (Figure 2C) reveals
that the MnO2 nanospheres have uniform “crumpled paper ball” morphology
(Figure 6.6A inset). SEM further confirms that MnO2 exhibited an architecture
with lots of small “wormhole- like” pores size of 2–50nm. (Figure 6.6B inset).
Noticeably, those mesopores provide huge surface areas, enabling effective
electrolyte transport and active-site accessibility. Additionally, the MnO2
nanospheres are intertwined with both highly conductive fFWNTs and
PEDOT:PSS, facilitating efficient electron transport. We also observed that
electrodes prepared without PEDOT:PSS have large aggregations and can be
easily peeled off from the current collector, indicating PEDOT:PSS works as not
only the additional current collector, but also the binder material.
The two characteristic peaks at 37.1◦ and 66.3◦ in XRD analysis marked
by arrow in Figure 6.8 indicates the presence of MnO2 and the weak, broad
127
signals suggest that MnO2 is in nanocrystalline nature, which is favorable for
supercapacitor applications.[204]
Figure 6.10 XRD pattern of the MnO2/fFWNTs composite.
The XPS spectrum (Figure 6.9) acquired from MnO2/fFWNTs composite
shows only signals from Mn, C and O. The average manganese oxidation state
was determined from the Mn 3s and O 1s core level spectra. On the basis of the
analysis of Mn 3s spectrum,[205-206] the manganese oxidation state is around
3.84 and the one obtained based on the analysis of O 1s spectrum[207] is 3.78.
128
Figure 6.11 XPS spectrum of the MnO2/fFWNTs composite.
The average manganese oxidation state was determined from the Mn 3s
and O 1s core level XPS spectra (Figure 6.10). As reported previously, the Mn
oxidation state can be determined from the binding energy width (ΔE) between
the separated Mn 3s peaks caused by multiplet splitting. By reference to ΔE data
of 5.79, 5.50, 5.41 and 4.78eV acquired from genuine samples of MnO, Mn3O4,
Mn2O3 and MnO2, respectively, the possible oxidation state of Mn contained in
the composite was estimated as 3.84. Another effective way of determining the
129
Mn oxidation state is from the O 1s core level spectrum. By using the equation
that suggested in this work, the possible valence of Mn was 3.78.
Figure 6.12 XPS spectra of the composite in Mn 3s (A) and O 1s (B) region.
The raw data are represented by the black lines, and the fitted data are
represented by the red and blue lines. The peaks at 531.6eV and 533.2eV are
originated from Mn-O-Mn bond and Mn-O-H bond.
As we discussed earlier, CNTs serve both as electroactive material and as
the scaffold for the deposition of the porous MnO2 nanospheres, thus both of their
surface area and electrical conductivity are critical for obtaining high performance.
The ideal CNTs should have high surface area as well as high conductivity. Most
of the times, these two properties do not exist together. Normally, in order to
obtain high surface area, CNTs are functionalized with functional groups to
improve suspension stability and to reduce the bundle size before the fabrication
130
of composites.[208] However, electrical conductivity of CNTs decreases if too
much functional groups are created on the sidewall of nanotubes. Therefore, in
order to have both high capacitance and good rate performance, a proper
balance between the specific area and the electrical conductivity must be
achieved through well controlled functionalization steps. To investigate the
chemical functionalization effect of carbon nanotubes on the electrochemical
performance of electrodes by our functionalization method, composite film
electrodes with both purified and 6M nitric acid functionalized nanotubes have
been fabricated. Cyclic voltammograms (CV) of those composites in 1MNa2SO4
solution at very high scan rate of 500mVs−1 are depicted in Figure 6.11A. As can
be seen from Figure 6.11A, composites with both purified FWNT and further
functionalized FWNTs show rectangular CV curve, indicating both composite
films are highly reversible as ideal capacitors. Calculations based on
glavanostatic charge/discharge curves (Figure 6.11B) indicated that films
fabricated with functionalized FWNTs exhibit much higher specific capacitance
(427F/g) compared with the specific capacitance (381F/g) of the film prepared
with unfunctionalized FWNTs. It is believed that the good specific capacitance of
electrodes prepared with functionalized FWNTs is mainly due to the increased
surface area and consistently high electrical conductivity. Furthermore, the
enhanced hydrophilicity by functionalization not only facilitate the access of the
electrolyte ions onto the fFWNTs surface, but also improved the interaction
between MnO2 nanoparticles and fFWNTs making electron transport between
131
fFWNTs and MnO2 easier. Additionally, the high specific capacitance value
confirms that the combination of all three types of the materials allows
maximizing the utilization of manganese oxide.
132
Figure 6.13 (A) Cyclic voltammograms (scanned from 0-1 V in 1 M Na2SO4),
(B) Galvanostatic charge/discharge curve (at current density of 5mA/cm2) of
MnO2
film
(black),
MnO2
/PEDOT:PSS
composite
(red)
and
MnO2/fFWNT/PEDOT:PSS ternary composite(blue).
133
In order to further explore the advantages of this novel design for real
applications, we investigated the electrochemical properties of composite
electrodes with high content of MnO2 (60%) and fairly high mass loading
(~1.5mg/cm2). As comparison, specific capacitance values of pure MnO2 film,
MnO2/PEDOT:PSS composite film have also been tested. The MnO2
nanospheres used for the comparison were synthesized with a similar method
but without the addition of fFWNTs. As can be seen from Figure 6.12A, the
severely distorted CV shape of pure MnO2 film indicates its intrinsically poor
electric conductivity. MnO2/PEDOT:PSS binary composite show improved current
than that of pure MnO2 electrode under the same sweep speed, suggesting that
conductive PEDOT:PSS facilitates the electron transport in the film. Surprisingly,
the CV shape of the ternary composite film of with even 60 wt% MnO2 is nearly
rectangular and its current value is much higher than that of pure MnO2 film and
MnO2/PEDOT:PSS electrode. It implies that even at high concentration of MnO2
and fairly high mass loading, the ternary composite exhibit the behavior closer to
an ideal capacitor and the electrochemical utilization of MnO2 has been greatly
improved by the introduction of fFWNTs and PEDOT:PSS as “inner” and “outer”
current collector. We are also glad to see the trend in specific capacitances
calculated based on the glavanostatic charge/discharge curves in Figure 6.12B.
As can be seen from the constant current charge–discharge curves, ternary
composite shows ideal capacitive behavior with very sharp responses and small
internal resistance (IR) drop. Specific capacitance of the ternary composite film
134
reaches
200F/g
even
at
high
concentration
of
MnO2
(MnO2:60wt%,
fFWNTs:30wt%, PEDOT:PSS:10wt%) at current density of 5mA/cm2. Much lower
specific capacitance is obtained for MnO2 (129F/g), MnO2/PEDOT:PSS (132F/g)
respectively. The improved high capacitance can be attributed to the optimized
structure of the ternary composite. In a word, the combination of MnO2, fFWNTs
and PEDOT:PSS into a single electrode showed excellent electrochemical for
energy storage applications.
135
Figure 6.14 (A) Cyclic voltammograms (scanned from 0-1 V in 1 M
Na2SO4), (B) Galvanostatic charge/discharge curve (at current density
of 5mA/cm2) of MnO2 film (black), MnO2 /PEDOT:PSS composite (red)
and MnO2/fFWNT/PEDOT:PSS ternary composite(blue)(down).
136
Long cycling life is an important requirement for supercapacitor electrodes.
The cycling life test over 1000 cycles for the ternary composite electrode was
carried out. Figure 6.13A demonstrates the very stable charge-discharge cycles.
Figure 6.13B illustrates that the nanocomposite electrodes showed only less than
1% decay in available specific capacity after 1000 cycles. Charge-discharge
cycle test of the ternary composite film suggests that the synergetic interaction
among fFWNTs, MnO2 and PEDOT: PSS significantly improved the electrical
properties and the mechanical stability of the electrode.
137
Figure 6.15 (A) Typical galvanostatic charge/discharge cycle curves
of ternary composite electrodes obtained at current density of
1.0mA/cm−2, (B) Charge-discharge cycle test.
Rate capability is another important factor for the use of supercapacitors in
power applications. A good electrochemical energy storage device is required to
138
provide its high energy density (or specific capacitance) at a high chargedischarge rate. The variation of specific capacitance of different MnO2 electrodes
with
an
increase
in
current
density
is
shown
in
Figure
6.14
MnO2/fFWNTs/PEDOT:PSS ternary composite not only exhibit high specific
capacitance values but also maintain them well at high current density compared
to other electrodes. As shown in Figure 5, the ternary composite preserved 85%
of its specific capacitance (from 200 to 168 F/g) as the current density increases
from 5 to 25 mA/cm2. However, The specific capacitance values of MnO2 film
(129 F/g) are not only much lower than the ternary composite but also decreased
significantly with increased current densities (e.g., from 129 to 20 F/g at current
density of 5-25 mA/cm2). Similar result was observed in MnO2/PEDOT:PSS
binary composite (e.g., from 132 to 37 F/g at current density of 5-25 mA/cm2).
The superior rate capability in the ternary composites electrode can be attributed
to the reduced short diffusion path of ions, high surface area and increased
electrical conductivity. Due to the synergetic contribution from fFWNTs and
PEDOT:PSS, the high-surface area and porous network structure allows a higher
rate of solution infiltration and facilitate the ions insertion/extraction and electrons
transport in the electrode film. On the contrary, the severer aggregation, lower
conductivity and poor mechanical stability in MnO2 film and MnO2/PEDOT:PSS
composite increased the ion diffusion and electron transport resistance,
compromising their electrochemical performances.
139
Figure 6.16 Specific capacitance of MnO2/fFWNT/PEDOT:PSS ternary
composite(blue), MnO2/PEDOT:PSS composite(red), MnO2 film (black) at
different charge/discharge current densities.
6.4 Conclusions
In summary, a simple and cost-effective approach is developed to
fabricate outstanding MnO2/CNT/CP ternary nano-composite. In such a
composite, each component provide much needed critical function for efficient
use of metal oxide for energy storage;fFWNTs not only provide high surface for
the deposition of hierarchical MnO2 porous nanospheres but also improve the
electrical conductivity and the mechanical stability of the composite;PEDOT:PSS
functions as an effective dispersant for MnO2/fFWNTs structures and as binder
140
material in improving the adhesion to the substrate and the connection among
MnO2/fFWNTs particles in the film; the highly porous MnO2 nanospheres provide
high surface area for improved specific capacitances. Working together, these
components assemble into a mesoporous, interpenetrating network structure,
which offers the composite with high specific capacitance, excellent rate
capability and long cycling life stability. We believe this design concept can be
generalized towards other electrochemical materials containing metal oxides,
such as RuO2, Co3O4 and NiO, opening a new avenue for a large spectrum of
device applications.
141
Chapter 7: Conclusions
Up to now, most of the fundamental studies and engineering applications
are mainly performed on two kinds of CNTs: single-walled carbon nanotubes and
multi-walled carbon nanotubes. FWNTs are special kinds of nanotubes that we
have developed over the last few years and introduced to the research field.
Such nanotubes can have a level of structural perfection as good as SWNTs and
can be synthesized and purified much easier than SWNTs.
A double-walled carbon nanotube is one component of FWNTs. We have
opened up a simple carbon monoxide CVD method for the efficient growth of
high quality DWNTs. DWNTs with small diameter distribution has been controlled
synthesized and the structural correlation between two adjacent graphitic layers
in 22 DWNTs was statistically examined by using electron diffraction. It turned
out that there is no strong tendency towards the metallicity of the concentric
layers. Additionally, our combined spectrometric and HRTEM measurements on
each fraction provide evidence that the quantum yield of near-IR emission from
DWNT inner shells is at least 10,000 times lower than for SWNTs of comparable
diameter and the emission of DWNT samples came from SWNT impurities rather
than from DWNT inner shells.
Ultrasonic cavitation can create extraordinary physical chemical conditions
when occurs (a temperatures of roughly 5000 Kelvin and pressures about 1000
atmospheres in extremely small region within very short lifetimes). Such extreme
142
conditions are very effective in breaking C=C double bond, permitting the
attachment
of
functional
groups.
Acoustic
cavitation
also
allows
the
functionalization and exfoliation to proceed gradually from the outer layer to inner
layer in bundles. By ultrasound-mediated covalent functionalization, highly water
soluble FWNTs can be obtained via acidic potassium permanganate KMnO4
oxidation or concentrated nitric acid treatment; highly organic solvent soluble
FWNTs can be achieved by a dissolving metal reduction method.
The mechanical properties of composites with functionalized FWNTs and
PVA have been investigated. Due to the easiness in synthesis and purification,
the tolerance against chemical functionalization, the formation of stable and
uniform dispersion in polymer and good structural perfection of FWNTs, we
observed higher reinforcing capability of the FWNTs, making them an optimum, if
not the best, choice as structural filler in composite materials. It is foreseeable
that our investigation of the extraordinary mechanical properties of FWNTs will
encourage the real applications of them in high performance composite materials.
We have also designed and synthesized a novel ternary MnO2/CNT/CP
composite for high performance electrochemical electrodes. The electrochemical
properties of composite electrodes with high content of MnO2 (60%) and fairly
high mass loading (~1.5mg/cm2) showed high specific capacitance, excellent rate
capability and long cycling life stability. Such design opens a new avenue for a
large spectrum of device applications.
143
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Biography
Place and Date of Birth
Changchun, Jilin, People’s Republic of China
Apr. 3rd, 1977
Education
Ph.D. (Chemistry)
Duke University, Durham, NC
Advisor:
Professor Jie Liu,
2005–2010
M.S. (Chemistry)
Texas Tech University, Lubbock, TX
Advisor:
Professor Dominick J. Casadonte, Jr., 2003 – 2005
M.S. (Chemistry) Northeast Normal University, Changhun, China
Advisor:
Professor Qiang Fu,
2001 – 2003
B.S. (Chemistry)
Jilin Normal University, Siping, China
Advisor:
Professor Chuanbi Li,
1997 – 2001
Publications
1. Functionalized Few-Walled Carbon Nanotubes for Mechanical Reinforcement
of Polymeric Composites
Hou Y, Tang J, Zhang HB, et al. ACS Nano, 2009, 3, 1057
2. Do Inner Shells of Double-Walled Carbon Nanotubes Fluoresce?
Tsyboulski DA, Hou Y, Fakhri N, et al. Nano Lett., 9, 3282 (co-first author)
3. Design and Synthesis of Hierarchical MnO2 Nanospheres/Carbon
Nanotubes/Conducting Polymer Ternary Composite for High Performance
Electrochemical Electrodes
Hou Y, Cheng YW, Hobson T, et al. Nano Lett., 10, 2727
4. Room Temperature Purification of Few-Walled Carbon Nanotubes with High
Yield
Feng, Y.; Zhang, H.; Hou, Y.; McNicholas, T. P.; Yuan, D.; Yang, S.; Ding, L.;
Feng, W.; Liu, J. ACS Nano, 2008, 2, 1634.
162
Conference Activities
1. Cheng Qian, Hang Qi, Ye Hou, Jie Liu
Dispersing Few-Walled Carbon Nanotube in Water and Common Organic
Solvents
ACS Fall National Meeting, 2006, San Francisco, CA, Poster
2. Ye Hou, Cheng Qian, Jie Liu
Synthesis of Highly quality Double-Walled Carbon nanotubes
MRS Fall Meeting, 2007, Boston, MA, Oral
3. Dmitri A. Tsyboulski, Ye Hou, Nikta Fakhri, Matteo Pasquali, Jie Liu, and R.
Bruce Weisman
Elusive fluorescence from double-walled carbon nanotubes: New experimental
approaches and results
MRS Fall Meeting, 2007, Boston, MA, Oral
4. Ye Hou, Cheng Qian, Jie Liu
Few walled carbon nanotubes in the mechanical reinforcement of poly vinyl
alcohol
ACS, Fall Meeting, 2008, Philadelphia, PA, Poster
5. Ye Hou, Ru Zhang, Cheng Qian, Jie Liu
Photoluminescence quenching in double-wall carbon nanotubes
ACS Spring Meeting, 2009,Salt Lake City, UT, Poster
6. D. Tsyboulski, Y. Hou, N. Fakhri, S. Gosh, R. Zhang, S. Bachilo, L. Chen, M.
Pasquali R. Weisman
Do Inner Shells of Double-Walled Carbon Nanotubes Really Fluoresce?
ECS Meeting, 2009, San Francisco, CA, Oral
163
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