INTRODUCTION
Since their discovery by Iijima, carbon nanotubes (CNTs) have garnered significant interest due to their remarkable mechanical, thermal, and electrical properties, leading to various applications in biomaterials, multifunctional composites, and electronic components However, their high aspect ratio and tendency to form bundles complicate the processes of disentanglement and dispersion, which limits their commercial use Effective dispersion often requires chemical treatments to promote debundling and enhance interactions with solvents Covalent methods involve binding organic groups to the nanotube surface, but this disrupts the essential sp² hybridized network that contributes to their unique properties Alternatively, non-covalent methods utilize non-disruptive interactions, such as π–π stacking and Coulomb interactions, through chemical bridging agents, which help maintain the integrity of the nanotubes' delocalized π-electron network and preserve their sensitive electrical and thermal characteristics Surfactants and polymers are commonly used as these bridging agents.
In Vietnam, significant research on nanomaterials, particularly carbon nanotubes (CNTs), has been conducted, with the Institute of Materials Science being one of the pioneers in successfully producing CNTs in 2002 Over the years, various institutions, including the International Training Institute for Materials Science (ITIMS) and the Institute of Engineering Physics at Hanoi University of Technology, along with the R&D center of Saigon High Tech Park, have engaged in large-scale production of multi-walled carbon nanotubes (MWNTs), although the quality of these products has often been limited Additionally, the National Key Laboratory of Materials and Electronic Devices at the Materials Science Institute of the Vietnam Science and Technology Institute has also contributed to this field.
In 2009, Prof Phan Hong Khoi and Phan Ngoc Minh successfully utilized multi-walled carbon nanotubes (MWNTs) in tungsten tips for field emission devices, as well as in composite plating films of Ni-MWNTs and Cr-MWNTs That same year, Bui Van Ga and colleagues published research on the super-hydrophobic properties of PS/MWNTs composites, highlighting their potential for biogas storage applications Recent studies at Ho Chi Minh University of Technology have focused on the synthesis and application of carbon nanotubes (CNTs), yielding promising results Notably, Cao Duy Vinh's research on the functionalization of MWNTs using a sulfuric and nitric acid mixture demonstrated significant advancements Additionally, blending modified MWNTs with polyvinyl alcohol (PVA) showed enhanced electrical conductivity Prof Nguyen Huu Nieu's work further explored the use of modified MWNTs as a support in the fabrication of magnetic iron (III) oxide.
To date, there has been no research on the dispersion of multi-walled carbon nanotubes (MWNTs) using surfactants Recognizing the critical importance of the dispersion process, this thesis will concentrate on addressing key issues related to this topic.
Purifying and evaluating the effectiveness of purification process
The role of purification in the dispersion of carbon nanotubes
Providing new method for fast dispersing carbon nanotubes in aqueous solution using different surfactants (Sodium Dodecyl Sulfate (SDS) and Triton X – 100)
Determining the extinction coefficient – a very important parameter for quantitative assessment of carbon nanotube dispersions
Comparing the dispersing power of two surfactants: SDS and Triton X –
Determining the optimum CNT-to-surfactant ratio for each surfactant
Influence of pH value on the stability of MWNTs’ suspension
OVERVIEW
CARBON NANOTUBES
The carbon phase diagram at high pressure (>1 GPa) is shown in Figure 2.1 below The phase diagram presents several main features:
Solid lines indicate equilibrium phase boundaries in the transformation of graphite to diamond Point A illustrates the catalytic synthesis of diamond from graphite, while point B marks the pressure and temperature threshold for rapid solid-solid transformation from graphite to diamond Conversely, point C identifies the threshold for the quick conversion of diamond back to graphite Point D demonstrates the transformation of single crystal hexagonal graphite into retrievable hexagonal-type diamond Finally, point E highlights the upper limits of shock compression and quench cycles that facilitate the conversion of hexagonal graphite particles into hexagonal diamond.
20 compression/quench cycles that convert hex-type graphite to cubic-type diamond;
B, F, G: threshold of fast P/T cycles, however generated, that convert either type of graphite or hexagonal diamond into cubic-type diamond; H, I, J: path along which a single crystal hex-type graphite compressed in the c-direction at room temperature
The transition line, the boundary between the graphite and stable diamond regions, runs from 1.7GPa/ 0°K to the graphite/diamond/liquid triple point I at 12GPa/5000°K
The graphite/liquid/vapor triple point, the graphite/vapor phase boundary and the liquid/vapor phase boundary occur at pressures too low for scale of diagram (not presented here)
The melting line of graphite extending from the graphite/liquid/vapor triple point at 0.011 GPa/ 5000°K to the graphite/ diamond/ liquid triple point at 12 GPa/5000°K
– The dotted line (diamond GFB) represents the graphite-diamond kinetic transformation under shock compression and quenches cycles
– The diamond melting line runs at high P and T , above the triple point
In the previous sections, we explored how carbon atoms bond to create solids known as allotropes or polymorphs Despite sharing the same fundamental building block, these allotropes differ in their atomic hybrid configurations, which can be categorized as sp3 (tetragonal), sp2 (trigonal), or sp (digonal).
Figure 2.2 Model of carbon allotropies
These allotropic solids can be classified into three major categories (Figure 2.2):
• The sp 2 structures include graphite, the graphitic materials, amorphous carbon, and other carbon materials
• The sp 3 structures involve diamond and lonsdaleite (a form detected in meteorites)
• The fullerenes and nanotubes are the third and fourth allotropes of carbon and consist of a family of spheroidal or cylindrical molecules with all the carbon atom sp 2 hybridized
Allotropes of carbon, including diamond-like carbon (DLC) materials, can occur in combination, often comprising microcrystalline diamond and graphite produced through low-pressure synthesis Recent research has identified various diamond polytypes, such as the 6-H hexagonal diamond, and a carbon phase characterized by a three-dimensional network with sp² bonds This newly discovered phase may theoretically possess hardness surpassing that of traditional diamond.
To comprehend the structure and characteristics of nanotubes, it's essential to first examine the bonding structure and properties of carbon atoms Each carbon atom consists of six electrons, with two occupying the 1s orbital The remaining four electrons participate in bonding through sp3, sp2, and sp hybrid orbitals, which are crucial for forming various structures such as diamond, graphite, nanotubes, and fullerenes.
Diamond's structure consists of carbon atoms with four valence electrons forming sp³ hybrid orbitals, resulting in four equivalent σ covalent bonds that connect to other carbon atoms in a tetrahedral arrangement This unique three-dimensional interlocking structure renders diamond the hardest known material Due to the absence of delocalized π bonds and the presence of σ bonds, diamond acts as an electrical insulator The tightly held electrons within the carbon bonds absorb ultraviolet light, making pure diamond appear clear to the human eye, while its high index of refraction enhances the beauty of large diamond crystals Additionally, diamond exhibits exceptional thermal conductivity.
Graphite consists of carbon atoms where three outer-shell electrons occupy planar sp² hybrid orbitals, creating three in-plane σ bonds and an out-of-plane π bond, resulting in a hexagonal network The sheets of this hexagonal structure are held together by Van der Waals forces, with a spacing of 0.34 nm between them The σ bond length in sp² configuration is 0.14 nm and has a strength of 420 kcal/mol, while in sp³ configuration, it measures 0.15 nm and is 360 kcal/mol strong Consequently, graphite exhibits greater strength in-plane compared to diamond.
Graphite's unique properties are attributed to its out-of-plane π orbitals, which enhance its thermal and electrical conductivity The presence of these loose π electrons also gives graphite its characteristic black color when interacting with light Additionally, the weak Van der Waals forces between graphite sheets contribute to its softness, making it an excellent lubricant as the sheets can easily slide over one another.
Carbon nanotubes (CNTs) are hollow cylinders formed by rolling graphite sheets, exhibiting sp² bonding The unique circular curvature of CNTs leads to quantum confinement and σ–π rehybridization, resulting in slightly out-of-plane σ bonds and a more delocalized π orbital outside the tube This structural configuration enhances their mechanical strength, electrical and thermal conductivity, and chemical and biological reactivity compared to graphite Additionally, CNTs can incorporate topological defects like pentagons and heptagons into their hexagonal network, allowing for the formation of various shapes such as capped, bent, toroidal, and helical nanotubes Electrons become localized in these defects due to the redistribution of π electrons A nanotube is considered defect-free if it consists solely of a hexagonal network, while it is termed defective if it contains pentagonal, heptagonal, or other structural and chemical defects.
When a graphite sheet is rolled into a nanotube, the sp² hybrid orbital undergoes deformation, leading to rehybridization toward an sp³ orbital or σ-π bond mixing This structural rehybridization, combined with the confinement of π electrons, imparts nanotubes with exceptional electronic, mechanical, chemical, thermal, magnetic, and optical properties.
Fullerenes (C60), composed of 20 hexagons and 12 pentagons, exhibit unique bonding characteristics that combine sp2 and sp3 hybridization due to their high curvature These distinctive structures lead to remarkable phenomena such as metal-insulator transitions, unusual magnetic correlations, and diverse electronic and optical properties As a result, fullerenes have gained significant attention for their potential applications across various fields, including electronics, magnetism, optics, chemistry, biology, and medicine.
Iijima was the first to identify carbon nanotubes (CNTs) as concentrically rolled graphene sheets with various helicities and chiralities, rather than as a simple scroll of graphene as previously suggested Initially observing multi-walled carbon nanotubes (MWNTs) with 2 to 20 layers, he later confirmed the existence and structure of single-walled carbon nanotubes (SWNTs) in a 1993 publication The properties of CNTs are highly sensitive to their graphitization degree, diameter, and whether they are single-walled or multi-walled SWNTs, which are seamless cylinders made from a single graphene sheet, were first reported in 1993, while MWNTs, composed of multiple concentric graphene cylinders, were discovered two years earlier.
Figure 2.4 Computer-generated images of carbon nanotubes [17]
In order to describe fundamental characteristic of the nanotubes, two vectors, C h and T, are introduced in Figure 2.5 [15]
Figure 2.5 Chiral vector and unit cell of CNT
Ch is the vector that defines the circumference on the surface of the nanotube connecting two equivalent carbon atoms
The chiral angle is used to separate carbon nanotubes into three different classes by their electronic
Where: and are two basic vectors of graphite n and m are integers n and m are also called indexes and determine the chiral angle
The chiral angle is used to separate carbon nanotubes into three different classes by their electronic properties
Armchair: n=m and θ0 0 Zigzag: m=0, n>0 and θ=0 0 Chiral: 0