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UNIVERSITY OF PISA
Stabilization of nanostructures and their
applications
Phd thesis of the XXIX cycle
Candidate: Francesco Criscitiello
Internal supervisor: prof. Andrea Pucci
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Acknowledgements
This work is based on the collaboration between the Department of Chemistry and Industrial Chemistry of the University of Pisa (DCCI) and ENI SpA (San Donato Milanese, Milan, Italy), under the research contract n° 3500027239, entitled "Nuovi oligomeri anfifilici come additivi per carburanti e lubrificanti". The main outcomes reported in the PhD thesis are the result of different research projects carried out at the "Smart Polymers Laboratory" at DCCI in Pisa and that lie outside the topics of the research contract n° 3500027239.
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Abstract
The dispersion of nanostructured materials into liquid and solid matrices is an old challenge in the field of material sciences. In order to exploit the properties of nanoparticles or nanotubular structures they must be homogeneously distributed into the final material.
The present thesis is part of a significant work devoted to the preparation of new surfactants and functional polymers for the dispersion in different media of carbon-based and metal-based functional and nanostructured materials.
More specifically, chemical modification of species able to stabilize nanostructures is carried out for the realization of nanocomposites with smart features.
To this aim, different organic low and high molecular weight surfactants are investigated. The first part of this work aims to investigate the use of a functionalized ethylene-propylene rubber (EPR) as a polymeric dispersant for pristine or chemically-modified multi-walled carbon nanotubes (MWCNTs). The soft covalent approach allows higher compatibility with the polymer matrix and superior properties in terms of electric conductance. Spectroscopic, thermo-mechanical and morphological studies are carried out to evaluate the nanocomposite properties and their final applications as innovative stress-strain sensors.
The second part of this work investigates the synthesis of low molecular weight surfactants for MWCNTs dispersion. In particular, two different perylene bisimides derivatives are synthetized and their dispersions with MWCNTs in different solvents investigated by Raman spectroscopy and
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thermogravimetric analysis in order to explore the ability of π-π stacking interactions and effective electrostatic repulsion to promote nanotubes exfoliation and stabilization in different media. Solid mixtures are eventually studied as nanostructured and cost-effective temperature sensors. The final part is devoted to the synthesis and the characterization of polyisobutene succinic anhydrides (PIBSAs) and their succinimide (PIBSI) derivatives as capping of silver nanoparticles prepared by bottom-up approaches. PIBSAs and PIBSI supramolecular assemblies are characterized by new spectroscopic procedures and their utilization is eventually extended as dispersants of MWCNTs.
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Summary
Acknowledgements ... 3
Abstract ... 4
An insight into nanomaterials ... 13
1.1. Nanoparticles, the 0D materials ... 15
1.1.1. Noble metals nanoparticles ... 16
1.1.1.1. The surface plasmon resonance... 17
1.1.2. Production of noble metal nanoparticles... 24
1.1.2.1. Brust method32... 25
1.1.2.2. Turkevich method15-17,37 ... 27
1.1.2.3. Hyramatsu-Osterloh method30... 28
1.1.3. The problem of stabilising nanoparticles ... 29
1.1.3.1. Kinetic stabilization... 30
1.1.3.2. Thermodynamic stabilization ... 32
1.1.4. Current applications of noble metal nanoparticles ... 33
1.2. Nanotubes and nanowires, 1D materials ... 34
1.2.1. Carbon nanotubes... 35
1.2.2. Electrical properties ... 40
1.2.3. Mechanical properties ... 45
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1.2.5. Preparation techniques ...48
1.2.6. The importance of stabilization of carbon nanotubes ...52
1.2.6.1. Non-covalent functionalization ...54
1.2.6.2. Covalent functionalization ...56
1.2.7. Current application of carbon nanotubes ...59
1.3. Aim of the work ...60
Ethylene-propylene rubber (EPR) dispersants of MWCNTs in polymer nanocomposites for sensing applications* ... 62
2.1. Preparation of CNT/polymer nanocomposites for stress-strain sensors ...62
2.2. Preparation of MWCNT/EPR-g-FA nanocomposite by Diels-Alder crosslinking for stress-strain sensors ...71
2.2.1. Synthesis of the succinimide EPR-FA ...72
2.2.2. Reversible crosslinking of EPR-g-FA with BM and successive dispersion of MWCNTs ...75
2.2.3. Evaluation of the crosslinking density ...76
2.2.4. Thermogravimetric characterization of MWCNT/EPR-g-FA nanocomposites ...79
2.2.5. Microscopic characterization of MWCNT/EPR-g-FA by scanning electron microscopy ...80
2.2.6. Mechanical properties of MWCNT/EPR-g-FA nanocomposites ...82
2.2.7. Electrical properties of MWCNT/EPR-g-FA: determination of the percolation threshold and study of piezoelectric behaviour ...85
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2.2.8. Recycling of MWCNT/EPR-g-FA nanocomposites ... 90 2.3. Covalent functionalization of carbon nanotubes with furfurylamine via Diels-Alder reaction ... 92
2.3.1. Thermogravimetric characterization of MWCNT -FA ... 96 2.3.2. Characterization of MWCNT-FA by X-ray photoelectronic spectroscopy (XPS) ... 100 2.3.3. Characterization of functionalised MWCNT-FA via Raman spectroscopy ... 103 2.3.4. Microscopic characterization of MWCNT-FA by scanning electron microscopy ... 105 2.3.5 Preparation of elastomeric nanocomposite MWCNT/EPR-g-FA by Diels-Alder crosslinking for mechanical deformation sensors .... 106 2.3.6. Evaluation of the crosslinking density ... 108 2.3.7. Thermogravimetric characterization of EPRgMA/MWCNT -FA composites ... 109 2.3.8. Microscopic characterization of EPR-g-MA/MWCNT-FA by scanning electron microscopy ... 110 2.3.9. Mechanical properties of EPR-g-MA/MWCNT-FA
nanocomposites ... 112 2.3.10. Electrical properties of EPR-g-MA/MWCNT-FA:
determination of the percolation threshold and study of piezoelectric behaviour ... 114 2.4. Experimental section ... 117 2.4.1 Materials ... 118
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2.4.2. Preparation of elastomeric nanocomposite
EPR-g-FA/MWCNTs by crosslinking via Diels-Alder with BM: succinimide synthesis ...119 2.4.3. Reversible crosslinking of EPR-g-FA with BM and dispersion of MWCNTs ...121 2.4.4. Functionalization of MWCNTs with furfurylamine (FA) in the bulk ...121 2.4.5. Functionalization of MWCNTs with furfurylamine (FA) in solution ...122 2.4.6. Cross-linking of the polymer matrix with functionalized
MWCNTs ...123 Low molecular weight polycyclic aromatic compounds as dispersants of carbon nanotubes for temperature sensors ... 125
3.1. Preparation of MWCNT/PZPERY water dispersions for
miniaturized temperature sensors ...127 3.1.1. Thermogravimetric characterization of MWCNT/PZPERY dispersion ...128 3.1.2. Spectroscopic characterization of MWCNT/PZPERY
dispersion ...129 3.1.3. Characterization of MWCNT/PZPERY dispersion via Raman spectroscopy ...133 3.1.4. Microscopic characterization of MWCNT/PZPERY dispersion by transmission electron microscopy (TEM) ...135 3.1.5. Resistance sensitivity to temperature changes ...136
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3.2. Perylene bisimide metal complexes as new MWCNTs dispersants
for the realization of temperature sensors ... 141
3.2.1. Spectroscopic characterization of PeryC dyes ... 142
3.2.2. Metal complex formation studies ... 145
3.2.3. Thermogravimetric characterization of MWCNT/PeryC_M dispersions ... 148
3.2.4. Characterization of MWCNT/PeryCs dispersion via Raman spectroscopy ... 150
3.2.5. Microscopic characterization of MWCNT/PeryC and MWCNT/PeryC_M dispersions by field emission scanning electron microscopy (FE-SEM) ... 152
3.2.6. Resistance sensitivity to temperature changes ... 153
3.3. Experimental section ... 158
3.3.1 Materials ... 158
3.3.2. Synthesis of N,N’-bis(2-(1-piperazinyl)ethyl)-3,4,9,10-perylenetetracarboxylic acid diimide dichloride (PZPERY) ... 158
3.3.3. Preparation of the MWCNT/PZPERY dispersion ... 159
3.3.4. Synthesis of N,N’-bis(2-aminomethyl-15-crown-5)-3,4,9,10-perylenetetracarboxylic acid diimide (PeryC) ... 160
3.3.5. Preparation of stock solution of PeryC and metal binding experiments ... 161
3.3.6. Preparation of the MWCNT dispersions ... 161
Preparation and characterization of polyisobutene succinic anhydride (PIBSA) derivatives and their potential use in the nanoscience field ... 163
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4.1. Preparation and characterization of polyisobutene succinic
anhydrides (PIBSAs) and their succinimide (PIBSI) derivatives ...166
4.2. Estimated CMC for PIBSI systems via emission spectroscopy ....177
4.3. Preliminarily investigation on the application of PIBSA derivatives as dispersants for nanostructures ...182
4.3.1. Preparation of silver nanoparticles by Hyramatsu-Osterloh30 method for the development organo-soluble metal derivatives ...182
4.3.1.1. Thermogravimetric quantification of silver nanoparticles in solution ...184
4.3.1.2. Characterization of silver nanoparticles in solution by dynamic light scattering (DLS) ...185
4.3.1.3. Spectroscopic characterization of silver nanoparticles in solution by Uv-vis absorption ...187
4.3.2. Dispersion of MWCNTs with PIBSI-H-HMW and evaluation of the dispersion efficiency in miniaturized temperature sensors ...188
4.3.2.1. Weight percentage of MWCNTs in MWCNT/PIBSI-H-LMW dispersions by thermogravimetric and spectroscopic analysis ...189
4.3.2.2. Atomic force microscopies (AFM) of MWCNTs/MWCNT/PIBSI-H-2300 dispersions ...192
4.3.2.3. Resistance sensitivity to temperature changes ...193
4.4. Experimental section ...196
4.4.1 Materials ...196
4.4.2. Synthesis of PIBSA-LMW ...196
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4.4.4. Synthesis of PIBSA-HMW ... 198 4.4.5. Synthesis of PIBSI-H-HMW ... 198 4.4.6. Preparation of silver nanoparticles via Hyramatsu-Osterloh method with PIBSI-T-LMW ... 199 4.4.7. Dispersion of MWCNTs with PIBSI-H-HMW ... 199 Instruments ... 201
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An insight into
nanomaterials
Nanotechnology is design, fabrication and application of nanostructures or nanomaterials, and the fundamental understanding of the relationships between physical properties or phenomena and material dimensions. Nanotechnology deals with materials or structures in nanometer scales, typically ranging from subnanometers to several hundred nanometers.1 One nanometer is 10-3 micrometer or 10-9 meter. Nanotechnology is a new scientific domain. Similar to quantum mechanics, on nanometer scale, materials or structures may possess new physical properties or exhibit new physical phenomena. Some of these properties are already known. For example, band gaps of semiconductors can be tuned by varying material dimension. There may be many more unique physical properties not known to us yet. It is possible to assume that nanotechnology has changed our everyday lives. In the past few years, nanostructures (chemical species with at least one dimension in the range from 1 to 100 nm) has attracted a huge interest2.Suitable control of the properties of nanoscale structures can lead to new science fields as well as new miniaturized devices and technologies. The importance of nanotechnology was pointed out by Feynman in late fifties, in one of his lecture entitled “There is plenty of room at the bottom”3.
Recently there has been a gargantuan growth of nanotechnologies and complex nanosystems because of new strategies for the synthesis of nanomaterials and new tools for their characterization4-6.
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The new physical properties or phenomena will not only satisfy everlasting human curiosity, but also promise new advancement in technology. For example, ultra-strong ultra-light multifunctional materials may be made from hierarchical nanostructures. Nanotechnology also promises the possibility of creating nanostructures of metastable phases with non-conventional properties including superconductivity and magnetism. By decreasing the dimensions of particles, all the aspect related to the surface will assume a great importance due to the fact that the total surface will grow of several degrees of magnitude from a bulk material to a nanomaterial1. Yet another very important aspect of nanotechnology is the miniaturization of current and new instruments, sensors and machines that will greatly impact the world we live in.
So, there can be many fields of application of nanotechnologies: Medicine: hyper thermal treatments
Pharmacology: drug delivery Catalysis: highly efficient catalysts Sensors: miniaturized sensors
Lubricating systems: tribological systems Informatics: quantum computers
Other examples of possible miniaturization are: computers with infinitely great power that compute algorithms to mimic human brains, biosensors that warn us at the early stage of the onset of disease and preferably at the molecular level and target specific drugs that automatically attack the diseased cells on site, nanorobots that can repair internal damage and remove chemical toxins in human bodies, and nanoscaled electronics that constantly monitor our local environment.
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Nanomaterials are divided in four main families depending on their shape and dimensions:
0D: nanoparticles that present all the three dimensions in the nanoscale (1-100 nm);
1D: nanotubes and nanowires that have only one dimension beyond this range;
2D: graphene and nanolayers that have one dimension in nanoscale and the other above;
3D: zeolites that present bulk structure with nanopores.
For the purpose of this thesis, 0D and 1D materials properties are investigated. In particular, metal nanoparticles and carbon nanotubes are exploited for the preparation of smart nanomaterials.
1.1. Nanoparticles, the 0D materials
Noble metal or silica nanoparticles, quantum dots, fullerenes belong to the nanostructured 0D family. Their employment ranges from biomedicine7, for example the new hyper-thermal treatment to selectively destroy cancerous cells8, to the use of peculiar optical properties of PbS or PbSe quantum dots in devices9. In biosensors and drug delivery fields of application, silica nanoparticles have been investigated in the last ten years10. As far as 0D materials are concerned, this thesis is focused on the application of noble metal nanoparticles, i.e. based on silver and gold.
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1.1.1.
Noble metal nanoparticles
Nanoparticles are known since 4000 B.C, and, for example, in China and Egypt, colloidal solution of gold and silver were used as medicals. Roman industry had reached quite a proficiency in producing dichroic glass by adding two fundamental additives: silver nanoparticles and gold nanoparticles. The famous Lycurgus cup (Figure 1) is one example of how this glass can interact with light in different way, i.e., transmission, reflection and even scattering.
Figure 1: Lycurgus cup appear to be red if the light is transmitted or green id it is reflected
Around 1600 Paracelsus described the preparation of "Aurum Potable, oleum auri: quinta essentia auri" for reducing tetrachloride auric acid with an alcoholic plant extract11.
If the expression "colloidal silver" is from the late nineteenth century, the bactericidal properties of silver as metal are known since ancient times. For example, Mesopotamians, Greeks and Romans used silver containers to serve beverages to the kings and aristocracy (the silver container disinfects the liquid, killing bacteria and pathogenic microorganisms12).
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Even the cutlery, forks and spoons were made with silver for reasons of hygiene: a tradition that is still present today.
The first documented synthesis of metal nanostructures dates back to 1856 when Michael Faraday prepared an aqueous colloidal gold solution by reduction with white phosphorus in CS2 in a two-phase system13. His
samples showed an intense red colour, stable to the point that some are still preserved in the Faraday Museum in London.
The nature of the colour of the nanoparticles of noble metals, such as gold or silver, is to be found in the nanostructure of metal clusters.
1.1.1.1. The surface plasmon resonance
Metals, at a macroscopic level, are characterized by a good reflectance at almost all frequencies14 (Figure 2). In fact, above the Fermi energy almost all energy levels are empty and can accept excited electrons. This means that all wavelengths, independently of their frequency, can be absorbed. After this premise, it is possible to think that metals are black, but the electrons, once excited, tend to return to the ground state by emitting exactly the same amount of energy absorbed, making the surface of a metal glows shiny.
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Figure 2: Gold and silver reflectance compared to aluminium. Aluminium presents a high reflectance in the visible while gold reflected at wavelengths between 400 and 500 nm, corresponding in the spectrum visible to the yellow colour and silver around 300 nm which is not
seen by human eye
Going down to the nanoscale dimensions (10-9 m) significant effects are produced on the optical properties of noble metals, such as gold and silver
15-18. The noble metal nanoparticles often have a different colour than the one
shown by the corresponding macroscopic materials. For example, gold nanoparticles having a diameter of the order of 5 nm have a red-brown colour.
This physical property is derived from a particular phenomenon that takes the name of localized surface plasmon resonance (LSPR)19-21.
When the nanoparticles are invested by an electromagnetic wave with a wavelength greater than their size, the free electrons on the metal surface will begin to swing in line with the electromagnetic field generated by the incident wave (for example, visible light or infrared). The oscillation of the
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field induces a polarization of the electrons on the surface of the nanoparticle due to a redistribution of positive and negative charges, which generates a restoring force that tries to return the system to the equilibrium state. As a consequence, an oscillating dipole is created and characterized by a precise frequency of oscillation ω (Figure3).
Figure 3: Oscillation of the electrons on the surface of metal nanoparticles induced by an electric field of an incident electromagnetic wave22
The plasmon resonance frequency is influenced by the electron density of the metal, the dielectric constant of the environment and the size of the particle.
The first who noticed this feature was the German physicist Gustav Mie that, at the beginning of '900, presented a theoretical study on the plasmon resonance of gold nanoparticles using electromagnetic theory of Maxwell19,20,23. His research laid the theoretical foundations that have allowed us to understand the origin of the colour of the noble metals in nanostructured form.
Being the surface plasmon resonance of a very small particle of metal a phenomenon related to the oscillation of a cloud of electrons on a sphere, it is possible to assume its dependence on the diameter. By solving the
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Maxwell equation for an electromagnetic wave that interacts with a small metal ball, Mie came to express a series of dipolar oscillations on a cross section. However, by decreasing the size of the particles much below the radius of the incident wavelength, the theory presented limits. In fact, for 2R << γ the equation proposed by Mie simplifies to:
Cext =24π 2R3ε m 3 2 ⁄ γ ε′′ (ε′+ 2ε m)2+ ε′′2 (1.1)
where Cext is the coefficient of cross section, or the interaction area of the
particle with radius R, ε' and ε’’ are the real and imaginary parts of the dielectric constant in the material mass, εm constant in the medium. In this
equation two terms multiplied together are present: the first is due to the contribution of diffusion and the second to absorption contribution.
Knowing that I(x) is the amount of light transmitted from one x layer of a solution of nanoparticles, it follows:
dI(x)
dx = −NCextI(x) (1.2)
Where N is the number of particles hit by the incident light. By integrating and switching to a logarithmic system, it is possible to recover the absorbance A: A = log10 I0 I(x) = NCextx 2,303 (1.3)
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A =NCextx
2,303 = [M]εx (1.4)
where ε is the molar extinction coefficient in M-1cm-1. Remembering that the coefficient of cross section per surface unit (Qext) is:
Qext = Cext πR2 (1.5) It is possible to write: ε = NCext 2,303[M]= NQextπR2 2,303[M] (1.6)
by dividing and multiplying with the volume of a single particle, assumed spherical, the equation becomes:
ε = 3 4πR3 QextπR2 2,303[M]= 3 4 QextVm (2,303)R (1.7)
and substituting it in the first (1.1)
4 3 (2,303)πR3 Vm = 24π2R3εm3⁄2 γ ε′′ (ε′+ 2ε m)2+ ε′′2 (1.8)
It shows that there is no dependence of the absorption position of the radius R particle. In fact, Mie assumed that the electronic structure and the
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dielectric constant of a nanoparticle was the same as the macroscopic material. Since experimentally there are evidences of a dependence of the value of the absorption frequency from the size of nanoparticles, the Mie theory were modified considering the quantum effect for particles of nanometric size1.
For a very small particle (R << λincident),the diffusion of the surface electrons
in a metallic particle becomes dramatically important when their mean free path is smaller than the diameter of the particle itself. In general, the smaller the particle, the more easily and quickly electrons reach the surface where they are spread, losing their coherence. All this leads to a broadening of the resonance band, and this phenomenon is inversely proportional to the diameter of the particle.
To better understand the dependence of the plasmon resonance frequency on the size the nanoparticle a damping constant γ has been introduced:
γ =γ0+ AυF
R (1.9)
where γ0 is the damping constant of the "bulk", A is a constant, υF is the
speed of electrons corresponding to the Fermi energy, and R is the radius of the nanoparticle. This effect of size is considered intrinsic, since the dielectric function of the material is considered dependent on the size. Then the wavelength of absorption increases with the size but decreases the amplitude of the peak.
One of the most effective explanations of the reason why the absorption frequency of nanoparticles depend on their diameter lies in considering energy levels of the metal as quantized. In fact, it has been shown that if the dimensions of the nanoparticles are quite small, continuous density of
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electronic states which characterizes a metal disappears in favour of the formation of discrete energy levels1.
So, for clusters of a few atoms, the distance between the energy levels (δ) is given:
δ =4EF
3N (1.10)
where EF is the Fermi energy and N is the number of electrons in the cluster.
So, with decreasing number of electrons (i.e. with smaller clusters), the energy gap between the bands of those of valence and conduction will be greater (shorter wavelengths). This is in agreement with the experimental data, which show bathochromic shift of the resonance peak with the growth of the nanoparticles diameter.
Another phenomenon, which influences the position of the peak, is the shape of the cluster. For example, gold nanoparticles of size of about 5 nm have a resonance frequency around 520 nm and silver nanoparticles of the same diameter around 400 nm. If, however, one observes the absorption spectrum of the elongated nanocrystals (nanorods), two frequencies of resonance become visible, that is attributed to two separate oscillating dipoles: one transverse and one longitudinal to the axis of the nanostructure (Figure 4)
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Figure 4: Surface plasmon resonance of the transverse and longitudinal oscillating dipoles in the metal particles with different elongation1
The first give origin to the band centred around 520/400 nm, while the second, the longitudinal one, will move to higher wavelengths in function of the rod lenght24,25.
1.1.2.
Production of noble metal nanoparticles
Two main methods can be used to produce nanoparticles: top-down;
bottom-up.
The top-down methods are typically considered physical methods which start from bulk material. An example of top-down methods are the atomic layer deposition26 (ALD), which consists in the deposition of thin nanometric layers of the metal or the laser ablation synthesis in solution27 (LASIS), which is based on irradiating with a laser a gold matrix of high purity.
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In general, the top-down methods have the advantage of being easier to be implemented than the bottom-up methods, but have the defect of resulting in nanosystems with high degree of imperfection. In fact, the material is reduced in size to obtain ultrafine powders with a very wide distribution of diameters and shapes, not necessarily equal to those desired.
On the other side, the term "bottom up" refers to all methods of preparation of nanostructured systems that start from atoms or metal complexes and go on assembling them to the desired particle size. Compared to the "top-down" method, the products of bottom-up techniques result in much more regular structures, free from surface defects as the "driving force" of the process is the Gibbs free energy reduction. The so-produced nanostructures are in a state as similar as possible to that of equilibrium. The "bottom-up" methods are basically chemical methods in the classic meaning of the term and, in the case of the noble metal particles such as gold or silver, are based on the reduction of the metal ion precursor in solution28-31. Three methods of preparation are worth noticing:
Brust method32
Turkevich method33
Hyramatsu-Osterloh method30
1.1.2.1. Brust method
32Until the early 90's the only way for the preparation of gold nanoparticles34 in organic solvent was the use of trifenilphosphine as stabilizers then, in early nineties was reported for the first time the use of thiols of different chain length as stabilizers35, but the removal of the solvent at the end of
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In 1994 Brust and Schiffrin solved the problem by providing a method for the preparation of stable gold nanoparticles in a biphasic system, drawing from the preparation in two stages of Faraday13. This method has had a great impact in the literature due to easy implementation, reproducibility and the ability to obtain nanoparticles with a narrow distribution of diameters between 1,5 and 5,2 nm. Moreover, the metal cluster so obtained can be isolated and dispersed again in an organic solvent without causing changes in the shape or morphology. The dispersibility of the nanoparticles is in fact directly related to the solubility of the stabilizer in the solvent medium as will be disclosed late.
The Brust method involves the dissolution of HAuCl4 in water and adding
an organic phase of toluene. The transfer process takes place with the aid of a surfactant dissolved in toluene, typically the tetraoctylammonium bromide (TOAB) capable of complexing the auric ion, thus withdrawing it from the aqueous phase. As reducing agent is employed the sodium borohydride (NaBH4) dissolved in water and the reaction needs to occur under vigorous
stirring in order to ensure the best contact between the two phases. The stabilizing agent is present in the organic phase during reduction and is generally a long chain thiol.
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Figure 5: Reaction scheme proposed for the Brust method. The reduction happens due to the presence of sodium borohydride, while the stabilization takes place thanks to the thiol36
Au nanoparticles with new electrical, optical, or thermal properties can be also obtained using thiol modified with specific functional groups.
1.1.2.2. Turkevich method
15-17,37Typically, the reduction of the metal precursor is accompanied by the presence of chemical species and stabilizers, which prevent a subsequent aggregation of the formed particles. Such stabilizing agents may be introduced during the reduction or may be themselves reducing agents as in method proposed by Turkevich in 195116,17,33. It simply consists in the reduction of HAuCl4 with citrate sodium in excess in hot aqueous solution
to obtain spherical particles of about 20 nm in diameter.
Part of the citrate dissolved in solution is oxidized (the hydroxyl group is transformed into a carbonyl group) allowing the reduction of Au3+. The first metal clusters formed in the growing process are stabilized by the excess of citrate.
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Figure 6: Reaction scheme proposed by Turkevich. The reduction and stabilization are carried out by the citrate ion
In 1973 it was proposed a modification of the method by changing the ratio between the reducing stabilizer and the Au precursor in order to modulate the diameter of the obtained particles38.
1.1.2.3. Hyramatsu-Osterloh method
30The synthesis is fast, very reproducible, and simple (only three reagents in its first appearance, tetrachloroauric acid or silver acetate, alkylamine, and a solvent). The reducing equivalents in the reaction are provided by the amine, which can undergo metal-ion-induced oxidation to nitriles39,40.
Figure 7: Reaction mechanism proposed by Hyramatsu-Osterloh
As it is possible to observe from Figure 7, the carbon double bond in oleylamine does not play a significant role in this process since other aliphatic amines, for example 1-dodecylamine, work equally in the reaction. Even in this method, the stabilizer works also as reducing agent. The metal
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(M) particles are stable in dried form and they can be eventually modified with hydrophobic and hydrophilic surfactants to afford nanoparticles that are soluble in organic solvents or in water. The versatility of this mechanism can provide unlimited examples, i.e. starting from any functional, yet primary amine.
1.1.3.
The problem of stabilising nanoparticles
The stabilization of nanoparticles is a fundamental process for their realization and utilization as it prevents their agglomeration, i.e. by limiting the thermodynamically favoured processes of growth. From a thermodynamic point of view, in fact, particles having a larger diameter are more stable than smaller particles (Ostwald ripening41,42).
Two kind of stabilization are possible: sterically or thermodynamically; kinetic or electrostatic.
Usually, organic molecules are employed as stabilizers with both low and high molecular weight:
ionic surfactants; non-ionic surfactants; alkylthioles;
alkylamines;
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1.1.3.1. Kinetic stabilization
This stabilization is named kinetic as it is based on the electrostatic repulsion of two charges of the same sign. The model includes two layers. The first layer corresponds to the particle having a positive charge on its surface. The stabilizing layer is characterized by an equal and opposite charge making the total effective charge on the particle surface neutral (Figure 8). The forces that come into play are coulombic forces, Van der Waals forces and Brownian movements of the solvent and the stability is to be found in their balance.
Figure 8: Electrostatic stabilization of two nanoparticles. The electrostatic repulsion between charges of the same sign, avoids the aggregation to bigger adducts
In fact, up to certain separation distances, the electrostatic repulsion between the shells around the particle prevails, while when a minimum distance is reached Van der Waals attractive forces prevail. This theory is called DLVO theory (Figure 9) on the stability of a particle in solution by the name of creators Derjaguin, Landau, Verwey and Overbeek43-49.
If we call d the thickness of the layer around the particle and S0, the distance
between the surfaces of the particles we can affirm that: for S0 >> d there is no interaction between the particles
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for S0 = d is maximum repulsion due to the maximum overlap of the
electric potentials of the stabilizers layers
to S0 <d the attractive forces of Van der Waals forces prevail over
the other.
The disadvantages of this stabilization are attributable to the fact that is applicable only to diluted systems, and that the obtained particles are not redispersible in a medium once aggregated because, once dried, electrostatic repulsion is overwhelmed by the distance between particles into the crystal as it is shown in Figure 9.
Figure 9: Representative diagram of the DLVO theory: the particles, surpassed a barrier of potential energy placed at a certain distance between them, begin to feel the effects of repulsive forces. For further approaches starts the aggregation towards prevail of the Van der Waals forces
A typical example of electrostatic stabilization of nanoparticles is the one proposed in the Turkevich method.
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1.1.3.2. Thermodynamic stabilization
The thermodynamic stabilization offers numerous advantages regarding the dimensional stability of the nanoparticles and their dispersion in various organic or aqueous systems.
The stabilizing layer can be adsorbed or anchored to the surface of the particles during the growth, and acts through a mechanism ruled by entropy. In fact, the aggregation between nanoparticles is inhibited by the fact that, as the distance between two particles decrease, the entropy proportionally decreases with the degrees of freedom of the stabilizing molecules.
If we call H the distance between the surfaces of two nanoparticles and L the thickness of the stabilizing layer thickness (Figure 10) it is possible to affirm that:
for H = 2L begins the interaction between the stabilizing layer; if the coating is optimal for H < 2L it is ΔG > 0 with ΔH ≈ 0; if the coating is not sufficient for L < H < 2L it is possible to have a
certain degree of mixing of the stabilizing chains, but when it is H ≤ L interaction is still favoured.
Figure 10: Interaction between the stabilizing chains (left) and dependence of ΔG from the distance in a high or optimal coated systems, and insufficient or low coated systems (right)1
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A wide variety of molecules can be used for this stabilization as shown in Figure 11. A class of stabilizing agents widely used for the steric stabilization of nanoparticles is made of polymers. For metal nanoparticles, the use of polymers functionalized with thiol groups, or other capping functionalities, is essential for the control of their size and for the modulation of their optoelectronic properties.
Figure 11: An example of several chemical species able to interact with a metal nanoparticle in order to stabilise its growth50
1.1.4.
Current applications of noble metal nanoparticles
Fascinating aspects of metal NPCs are the optoelectronic properties, which are associated to the surface plasmon absorption, a feature related to quantum size effects. Non-linear optics (NLO) applications for laser production employing metal nanoparticles are also a huge field of study
51-53. The combination of this photonics discipline with biology and medicine
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such as labelling, detection, and transfer of drugs, including genetic materials54-56. Excellent sensor devices are becoming available by exploiting the optical and electrochemical characteristics of metal nanoclusters on substrates like biological systems57,58 or polymeric matrices58. Moreover, although bulk gold is well known for being inert, the reactivity of the gold cores in Au-NPCs has proven to be very efficient in catalytic applications59, such as CO and methanol oxidation60,61 or conjugated functional groups
hydrogenation62. On the other hand, silver nanoparticles have attracted high
interest for their antibacterial properties, also once embedded into a polymer matrix63. Recently, they have been employed for the production of conductive inks, exploiting their metallic conductivity once coalesced on a substrate64.
1.2. Nanotubes and nanowires, 1D materials
In the field of 1D materials, nanotubes and nanowires can be found.
In particular silicon nanowires have attracted interest in the field of nanolithography65,66 for the fabrication of advanced electronic devices67 or highly selective nanosensors68.
The surface plasmon resonance of silver nanowires is also well exploited in the development of optical devices69-71. The study of heterogeneous catalysis led to the fabrication of metal nanotubes with a high total surface, able to interact with reactive molecules and to create a path through the synthesis reactors72.
Nevertheless, the most important and attractive 1D species are represented by the carbon nanotubes, focus of the present thesis. Their employment is
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wide as it ranges from solar cells73 to sensors74 and from conductive inks75 to nanomedicine76.
1.2.1.
Carbon nanotubes
Since 1952, when Radushkevich and his staff published pictures of aligned tubular structures of carbon, nanotubes are known, unfortunately this publication occurred in Russia and never came to the attention of the world77,78. Even in 1972 Endo, producing carbon fibres, was aware of the presence of such wires of carbon79, and only in the 1991 S. IIjima80-82 officialised the discovery of helical microtubules of graphitic carbon. During an experiment for producing fullerene, he found needle-like structures in the reactor and started to study them instead. From that moment, there has been a vertiginous increment in studies and publications throughout the countries on these structures and their fabulous properties. This new allotropic form of carbon, called nanotube, appears to be ideally consisting of one or more sheets of graphene rolled up on themselves83. There are essentially three different types of carbon nanotubes depending on the quantity of graphene sheets involved in their structure:
Single-Walled Carbon Nanotube, SWCNT; Double-Walled Carbon Nanotube, DWCNT; Multi-Walled Carbon Nanotube, MWCNT.
The last kind was the first to be discovered in 1991, the others was discovered later and their structures are shown in Figure 12.
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Figure 12: Structure of the three main families of carbon nanotubes classified as a) single wall, b) multiple wall, c) double wall
In reality, the body of the cylinder free from defects is made up of merely carbon hexagons as graphene, while the caps are formed from both hexagons and pentagons as the C6084.
Table 1: Important dimensions and aspect ratio of CNTs
SWCNT MWCNT
Dext 0,7 – 3 nm 5 - 50 nm
L 5 - 30 µm 1 - 10 µm
L/D 104 - 105 100 - 1000
Their diameter varies from a minimum of 0.7 up to maximum of 50 nm and because of their shape, the aspect ratio (ratio of length to diameter) is high. Consequently, CNTs fall in the class of one-dimensional structures (1D).
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Figure 13: Structure of a SWCNT. The extremities of a carbon nanotube are capped by two semi spheres that resemble the structure of fullerenes.
The SWCNT are mainly characterized by their diameter and by their chiral vector Ch or "helicity", that is practically the direction of rolling of the
graphene sheet on the tube axis. Ch can be represented in terms of two
integer numbers (n, m) corresponding to the vectors a1 and a2 of the graphite.
Ch
⃗⃗⃗⃗ = na⃗ 1+ ma⃗ 2 (1.11)
The diameter of the generic nanotube (n, m) is given by the expression
d =|Ch| π = a
(n2+ nm + m2)12
π
(1.12)
Where a = |a1| = |a2| and it is the lattice constant of graphite. The angle
between a1 and Ch is defined as chiral angle (θ), which can assume values
between 0° and 30°. The structures in which a1 and a2 possess values (n, 0)
and (n, n), corresponding respectively to angles of 0 ° and 30 °, take the name of zig-zag and armchair while the rest of the structures are called chiral85. The SWCNT armchair have metallic character as will be disclosing later, the others are all semiconductor with a band gap proportional to the diameter of the tube.
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From the chiral vector is taken a perpendicular vector of traslation (T) which is oriented in the direction of the axis of the nanotube. These pair of vectors, identifies the unitary cell of the nanotube.
Figure 14: Unitary cell of a graphene sheet individuated by chiral and translational vectors. The angle (θ) can assume values between 0° and 30°. The structures in which angles are 0 ° and 30 °,
take the name of zig-zag and armchair while the rest of the structures are named chiral
This is true for both single and multiple walls nanotubes, but the second ones have another peculiarity to deal with, the so-called lip to lip interaction (Figure 15).
Figure 15:Schematic representation of the section of a multi wall carbon nanotube (MWCNT) without bridges and, more realistically, with bridges that cause the so-called lip-lip interactions
These interactions are due to the carbon bridges between the walls that are supposed to favour the growth of the structure during their preparation. These interactions give rise to CNT with narrower diameters and, above all, prevent the formation of structural defects86. During the preparation of the
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CNTs, it can happen that their structure deviates from theory giving rise to imperfections. Possible defects are many87, but can be classified into:
topological (presence of non-hexagonal rings);
of hybridization (change of hybridization from sp2 to sp3);
of doping (in the presence of other structure in addition to carbon atoms).
These defects, in addition to causing a small local deformation of the structure of the CNTs, can generate a significant variation in its chirality with a considerable deviation from the typical properties of the CNTs, in particular electric properties88. In Figure 16 a typical defect called
Stone-Wales defect can be observed, which is a transformation occurring in an armchair nanotube under axial tension as will be shown later89.
Figure 16: Atomic arrangement of the Stone–Wales (SW) model. A) The SW transformation leading to the 5–7–7–5 defect, generated by rotating a C–C bond in a hexagonal network, B) HR-TEM image obtained for the atomic arrangement of the SW model, C) Simulated HR-TEM image for the
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This change in molecular junction reflects on conductivity properties. Comparing two nanotubes with and without defects, by looking at the density of states of the CNT, it is possible to see how this seemingly small defect greatly changes the electrical properties in the two parts by causing one to behave as a semiconductor (8.0) and the other as conductor (7,1) as will be clearly shown later.
Figure 17: Changes in electrical conductivity of an undefected CNT (8.0) and a defected CNT (7.1) and the relative bad gap changes, from conductor to semiconductor88
1.2.2.
Electrical properties
The study of the electrical properties of carbon nanotubes starts from the analysis of the properties of the two-dimensional sheet of graphene90-92. As stated before Ch and T are the helicity and the translational vectors that
identify the unitary cell. By bending the cell to form the wall of a nanotube, a planar wave is generated93 and, as established by the Floquet theorem, the wave functions of the Schrödinger equation solutions of a system with periodic potential (the surface of a nanotube) remain equal94. By applying
the Bloch theorem is clear that the wave is quantized as shown by equation95:
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K
⃗⃗ C⃗⃗⃗⃗ = 2πq h (1.13)
Where K is the wave vector and q an integer96. From this equation it is possible to determine the distance between the allowed wave vectors.
∆K =q + 1 r −
q
r (1.14)
In figure 18 it is possible to understand how the wave vectors are disposed, depending on the θ angle, in the Brillouin zone of the three varieties of nanotubes.
Figure 18: Brillouin zone for a) armchair b) zig zag c) chiral and the disposition of allowed wave vectors in compare to the hexagon formed by carbon atoms
The Brillouin zone is defined as the smallest part identified by the primitive cell of the reciprocal lattice of a graphene sheet that maintains all the symmetrical properties of the crystal.
When the K vectors pass through one of the nodal points of this zone, the nanotubes are metallic and this can happen only for those having indices (n, n), i.e. the armchair. For all the other possibilities the CNTs are semiconductors97,98.
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Another way to recognize if a nanotube is conductive or semiconducting is by analysing the density of states (DOS)99.
The DOS of a one-dimensional system describes the number of states available to be occupied by electrons at various energies. A DOS highest value at a certain energy outlines the availability of more states to become occupied by electrons.
Figure 19: At Fermi level a metallic CNT (on the right) presents a discrete value of the DOS with an energy gap of zero while the semiconductors (on the left) have a DOS value of zero at the and an
energy gap of about 0.7 eV88
The graphs show the DOS with symmetric singularity compared to the energy Fermi level (0 eV). These are the Van Hove singularities and correspond to the permitted critical points of the Brillouin zone. As shown in Figure 19, at Fermi level, a metallic CNT presents a discrete value of the DOS with an energy gap of zero, while the semiconductors have a DOS
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value of zero at the and an energy gap of about 0.7 eV. The CNTs are interesting materials for the ability to transport electrons. It is known from the literature that, when the length of the conductor is smaller than the mean free path of the electron, the transportation becomes ballistic. In the electrical conductivity model of classical physics, the electrons are seen as free particles that are able to pass their energy to other electrons and in a conventional electric conductor move toward the positive pole under the influence of an external electric field and internal resistance. In quantum physics they experience a resistance due to scatter on lattice defects and the conduction is observed by introducing the existence of phonons. They are quasiparticles (or collective lattice excitations) representing normal modes of vibration of electrons. The conductivity of the material decreases with rising temperature because the scatter on phonons becomes more efficient due to thermal excitation. Electrons scatter on lattice defects too, but this effect is temperature-independent. However, the effect becomes important at low temperatures because phonon scatter ceases under these conditions (in fact, the specific residual resistance is a measure for material’s purity). Despite Ohm’s law, the resistance of a nanowire is independent of its length because charge transport is achieved by so called conduction channels. To understand the electric properties of carbon nanotubes, it is important to be aware that the conductance is quantized. This means that raising the external electric field applied does not lead to a continuous, but to a stepwise increase in the current intensity. An elucidating experiment has been reported by W. de Heer and co-workers in 1998100. Assuming a conventional
mechanism of conduction, the lattice interactions would have heated the system during experiments until damages by superheating would have been inevitable. So there is strong evidence that the electrons travel long distances through the nanotube without interacting with the carbon framework. This
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kind of electron movement is called ballistic transport because the electrons do not experience any interaction on their free paths and it is due to this mechanism that the conductor does not heat much. No interaction with phonons means no excitation of lattice and so, no vibrations. This is of special interest for the development of new electronic devices. Conventional materials are limited by their tolerance to the heat generated by the current passage.
Measurements on single nanotubes fixed between two contacts revealed that the transport of electrons through the contact sites is achieved by tunnelling and that the electronic wave function extends from one contact to the other. So, a nanotube could be a real quantum wire.
It is important to notice that this conductibility is influenced by temperature variations. As said, above heating upon a certain value of temperature an increase in resistance is induced, but also, upon cooling resistance will increase. Strangely, after cooling to very low temperatures, the resistance of carbon nanotubes has been observed to decrease not continually until approximating the specific residual value.
As stated before, the conductivity a carbon nanotubes depends on intrinsic structure and on quantum effects, so it must be introduced a very simplified version of the Landauer-Buttiker formalism101,102 which shows the effective resistance composition of a CNT:
R = Rq+ Ra+ Rb (1.15)
Where R is the total resistance, Rq is the quantized resistance of the
nanotube, Ra is the resistance generated by diffusion processes, Rb is the
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So, the resistance of the nanotube is of a quite complex nature and it is dependent on its morphology and in particular from its chiral vector and its length.
Also, in a certain range of temperatures (for example from 20 °C to 40 °C), the resistivity measured for the CNTs having semiconducting behaviour, decreases with the temperature through an exponential decay so it is possible to apply the Steinhart-Hart equation103 for thermistors with negative
temperature coefficient (NTC thermistors):
1 T= 1 T0 +ln (R R⁄ 0 ) B (1.16)
With temperature expressed in Kelvin grades, B is one of the Steinhart-Hart parameters and R0 is the resistance at a reference temperature T0. By
resolving the equation in R one obtains:
R = R0eB( 1 T− 1 T0) (1.17)
1.2.3.
Mechanical properties
An early motivation to study carbon nanotubes has been the expectance of extraordinary mechanical properties for both single and multiwall species. The perspective to obtain highly strain-resistant composites or fibrous materials creates lot of researches and expectations in scientists around the year 2000.104
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Figure 20:Measuring the mechanical properties of a single nanotube by means of two atomic force microscopy tips; scheme (a) and microscope image (b). The torn and markedly elongated nanotube is clearly to
be seen96
When the first suitable samples and techniques became available (Figure 20), measurements revealed stupendous values for Young’s modulus and tensile strength indeed. A “superelastic” nanotube stood an elongation of 280% without the tubular structure collapsing. It’s important to say that CNTs do follow the Hook’s law
𝜎 = 𝐸𝜀 (1.18)
where σ is the stress applied, ε the deformation that occurs and E is the modulus or Young’s modulus. Its mean value was around 1 TPawhich is the highest ever reported for any material (steel as a modulus of about 0,20 TPa).
A peculiarity of nanotubes under tensile stress is shown especially at very high loads. At first, in the Hookeian range, the answer of the system is perfectly elastic and reversible. Exceeding this range, however, will lead to irreversible changes to the tube structure. The migration of carbon atoms or defects that occurs during stress may affect the chirality and the electronic properties of the CNT. Metallic nanotubes may become semiconducting, and a tube structure with varying diameters is formed. The whole effect
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results from the formation of pentagon – heptagon defects that tend to occur pair - wise (Stone–Wales defects) as its show in Figure 21.
Figure 21: The effect of mechanical strain on carbon nanotubes: (a) evolution of undulated defects on the concave side of a bent MWNT, (b) migration of Stone – Wales defects upon tensile stress96
1.2.4.
Thermal properties
The thermal properties of carbon nanotubes are dominated by phonons. The measured specific heat of SWNTs closely matches calculations based on the phonon band structure of isolated nanotubes, showing direct evidence of 1D quantization of the phonon band structure. Theoretical work predicts a room-temperature thermal conductivity of 6600 W/m K for individual nanotubes.88 Measurements show a room temperature thermal conductivity over 3000 W/m K for individual multiwall nanotubes, which is about an order of magnitude higher than that of copper. It was also found that addition of nanotubes to epoxy resin can double the thermal conductivity for a loading of only 1%, leading the path to a larger use in polymer composites105-107.
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1.2.5.
Preparation techniques
There are various preparation techniques for the production of CNTs108 but the most common are:
electric arc; laser vaporization;
chemical vapour deposition (CVD).
These techniques need a source of carbon and often require the use of a metal catalyst. The electric arc and the laser vaporization provide a vaporization of a graphite block at very high temperatures producing nanotubes, with yields of about 70% at low cost, but with an inhomogeneous distribution of the dimensions109.
Today, the most widely used technique for the production of carbon nanotubes is the CVD110 method, commonly used in the semiconductor material industry for the deposition of thin films111.
This process consists in a pyrolysis of carbon volatile sources like methane or other hydrocarbons112, and their deposition on nanoparticles of a metal catalyst supported on a substrate in order to promote the growth of CNT. The substrate is constituted by materials such as silicon, quartz, magnesium oxide, alumina or metals such as titanium, tungsten and molybdenum113. The catalytic species, usually based on iron, nickel, platinum, palladium or their organometallic derivatives, are generated directly on the surface of the substrate by electrodeposition. A density of nanostructured catalytic sites for the growth of nanotubes, which governs the diameter of the final product, is thus formed on the substrate. The precursors are transported on the catalytic site by a flow of inert gas such as Argon. They are often diluted with hydrogen, ammonia or water, in order to avoid the co-deposition of
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amorphous carbon on the catalyst, which will be limiting the activity of the catalyst itself.
Figure 22:A CVD system for the preparation of carbon nanotubes
The required decomposition temperatures for this process are comprised between 600 and 800 ºC, for the formation of MWCNTs, and between 900 and 1200 ºC for SWCNT. The typical yield of this process ranges from 20% to 100% depending on the type of catalyst employed, the operating temperature, the pressure of the gaseous mixture supplies and the species used.
This synthesis is a continuous method, as the carbon source that is used is a gas, which is reintroduced into the reaction chamber continuously, without the need to interrupt the process. There are many variations of this process, which is now the standard method of CNT industrial production. A schematic of a reactor of this type is shown in Figure 22.
Furthermore, the CVD technique allows to vary different parameters and, in particular, it is useful to control the geometry of the nanotubes, the diameter and the number of walls of MWCNTs.
To promote the decomposition of the precursor gases during the growth, the CVD process may be assisted by a plasma or by a gas formed by a significant
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percentage of ionized atoms or molecules, like the Plasma-enhanced chemical vapour deposition (PECVD114,115). This method differs from classic CVD because the precursor gas molecules are activated by the presence of a plasma, generated by applying a current between two electrodes.
Figure 23: A plasma enhanced chemical vapour deposition (PECVD) system for the production of CNTs
Considerable attention in the literature is currently dedicated to processes to obtain selectively high purity SWCNT. Two main processes have been developed:
HiPco®116
CoMoCAT®117.
Both preparation techniques are based on the reaction of a catalytic disproportion of CO:
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The processes are carried out by injecting the catalyst into the flow of CO at a temperature around 950 °C and pressure up to 10 atm. In particular, a precursor of the metal catalyst decomposes at high temperatures generating metal clusters in the vapour phase, which catalyse the disproportion reaction of the CO and the production of SWCNT.
The difference between these methods is the catalyst. For the first process Fe(CO)5, while in the second a catalyst based on Co-Mo is employed.
Through these techniques, it is possible to obtain SWCNT with purities > 90% with diameters of about 1 nm.
It is important to obtain high purities because the purification of CNTs after their formation is quite difficult. Regardless of the method of synthesis, the nanotubes always need a step of purification before their use, due to the presence of carbonaceous species such as amorphous carbon, graphite, fullerenes and carbon particles or impurities brought from the metal catalyst. The purification techniques can be divided into:
chemical methods; physical methods.
The chemical techniques are oxidative paths based on the assumption that the carbonaceous impurities, residuals of the catalyst and transition metals react faster than the carbon nanotubes. Although the method allows to obtain a good degree of purification of CNT, oxidative technique can greatly reduce the yield of the final product.
Nowadays, the oxidative processes involve two steps:
a high-temperature (600 – 1500 °C) annealing in an inert atmosphere, and a subsequent oxidation with mineral acids. During the heat treatment, there is a rearrangement of the structure of nanotubes that provides a graphitization of structural defects by
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reducing the chemical reactivity of the material. Practically, the heat treatment makes the tubes the most resistant to successive oxidation oxidation is generally carried out with nitric acid (HNO3) or sulfuric
acid (H2SO4) or a combination of the two with hydrochloric acid
(HCl) at a temperature comprised between 100 and 150 °C. The oxidation can be supported with ultrasonication that help the dispersion of the nanotubes in the oxidising mixture and allow the removal of the catalyst particles contained within the CNT bundles. If the oxidative purification is conducted in drastic conditions and not controlled; this can generate a partial rupture structure of the nanotube, with consequent loss of properties.
The most used physical method is the microfiltration, assisted by ultrasonication. Applying ultrasound to a sample of carbon nanotubes during the filtration process cause the formation of a suspension in the solvent, typically CS2 that has the role to solubilise the impurities, and keep
at the bottom the CNTs.
All of these techniques aren’t able to purify the nanotubes at 100%, and the pursuit of a compromise between purity and undamaged structures is always needed118.
1.2.6.
The importance of stabilization of carbon nanotubes
As mentioned before, nanotubes tend to form solid bundles (Figure 24) that have not the peculiar properties of single nanotubes. An important step in the path of utilizing these nanostructures into devices is the expholiation of those bulk bundles.
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Figure 24: CNTs stack on each other in macrostructures called bundles that must be exfoliated to exploit all their properties of electrical and thermal conduction119
Physical methods like ultrasonication are the most used today. Ultrasounds are able to debundle nanotubes and keep separating one from each other for a given time. Nevertheless, it is important to evidence that higher time of sonication can cause a fracture in the graphitic structure (Figure 25)120. In the TEM images (Figure 25, inset) single (unbundled) nanotubes are the dominant species. However, after 40 min of sonication, the histograms show a clear decrease in the number of longer nanotubes and an increase in the number of shorter nanotubes, with the average length reduced by about 40%. As a consequence, sonication time must be carefully controlled and limited. Moreover, the low solubility in most solvents is an important issue that can hamper the application range of this class of materials.
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Figure 25: Comparison between carbon nanotubes after 5 minutes of sonication and after 40 minutes120.
A way to solubilise CNTs after their exfoliation, is to stabilise them with molecules that can impart their own solubility to the system stabilizer/CNT through:
Non-covalent interactions; Covalent functionalizations.
1.2.6.1. Non-covalent functionalization
The non-covalent functionalization relies on the absorption on the surface of the CNT, via secondary interactions, of molecules such as aromatic compounds121,122 (π-π stacking), surfactants123 (induced dipoles) and polymers124 (van der Waals forces).
This method offers the advantage of preserving the conjugated system of the sidewalls of the CNTs thus not influencing the properties, while improving the dispersibility. In Figure 26 is shown an example of π-π stacking between an aromatic compound like a pyrene derivative and the carbon nanotube. The interaction between the electronic cloud of the aromatic dye and that of
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the graphitic surface is quite effective and it is reported in literature for many systems and also recently reviewed.125-127
Figure 26: Non-covalent functionalization of a SWCNT by π-π interaction with a pyrene dye.
Notably, non-covalent functionalization by surfactants123 has attracted
considerable attention as it is highly scalable and reversible and, in this case, the extremities of the surfactant molecule grant to all the system a great dispersability. A mechanism of nanotube isolation from a bundle, with the combined assistance of ultrasonication and surfactant adsorption, was proposed. 127 The role of ultrasonic treatment is likely to provide high local
shear, particularly to the nanotube bundle end. Once spaces or gaps at the bundle ends are formed, they are propagated by surfactant adsorption, ultimately separating the individual nanotubes from the bundle. Generally, ionic surfactants, due to their nature, are preferable for CNT/water (or other polar solvents) soluble solutions. Alternatively, non-ionic surfactants are proposed when organic solvents have to be used. The adsorption mechanism on nanotube walls was suggested to produce specific self-organization of surfactant molecules (Figure 26 and Figure 27).
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Figure 27: Schematic representations of the mechanism by which surfactants help to disperse CNT. (a) CNT encapsulated in a cylindrical surfactant micelle; (b) hemimicellar adsorption of surfactant
molecules on a CNT; (c) random adsorption of surfactant molecules on a CNT128
1.2.6.2. Covalent functionalization
There are two kind of covalent functionalization76,129:
Soft functionalization of walls;
Hard functionalization of edges and defects.
The covalent functionalization, also called "soft" functionalization can be carried out through the reaction with molecules that promote the reactivity of the wall of the carbon nanotubes due to disalignment of the π orbitals. This type of reaction therefore involves only the π bond of the C=C of the nanotube and often the mechanism appears to be identical to the chemistry of alkenes. In fact, one of the first reaction of the CNTs was the fluorination. The reaction proved interesting because as a result the fluorine may be
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replaced with another functional group such as a hydrocarbon chain130. There are also other many types of soft reaction as it is shown in Figure 28.
Figure 28: Versatility of CNTs soft functionalization131
The second type of chemical reaction, the hard functionalization, exploits the defects present on the CNT, as heptagons, pentagons and gaps in the atomic structure and other defects caused by oxidative purification processes. The use of strong acids leads to the formation of oxygenated functional groups. In particular, the treatment of CNTs with strong acids
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such as HNO3, H2SO4 (or a mixture of them), or with the acids together with
H2O2, or with strong oxidizing agents such as KMnO4, promotes the
reactivity of the double bond C=C of the CNT and the consequent formation of oxygenated functional groups such as carboxylic acids, ketones and alcohols.
Depending on the oxidant, the removal of the caps and the reduction of the original length of the nanotubes might occur.
The oxidized nanotubes can be used as precursors for further chemical reactions to form amide or ester bonds after treatment of the CNT with thionyl chloride (SOCl2) as shown in Figure 29.
Figure 29: Hard functionalization by the use of acids takes place at imperfections131
This method is very versatile since the R moiety can be of very different nature, from apolar alkyl chain to water soluble poly(ethylene oxide) oligomers, from aromatic to polymeric chains.
