Nanomaterials and Fullerenes Notes

5.3 Nanomaterials

Nanomaterials are particles ranging in size from $1$ to $100$ nm.
They can have various geometries such as rod-like, plate-like, or spherical.
Spherical particles smaller than $10$ nm are called clusters.
Two main approaches for manufacturing nanomaterials:
- Top-down: A physical approach involving dividing bulk matter into nanoscale sizes.
- Bottom-up: A chemical approach where nanoscale objects are built from individual atoms or molecules.
Properties like melting point, color, hardness, and magnetic properties change with size below $100$ nm.
Reduced size and compact structures lead to challenging applications in various fields.
Properties differ at the nanoscale due to:
- Increased surface area.
- Quantum effects.
These factors can alter or enhance reactivity, strength, and electrical and magnetic properties.

5.3.2 Properties of Nanomaterials

Nanosized materials exhibit different properties compared to their bulk counterparts.
Opaque substances can become transparent (e.g., Cu).
Inert materials can become catalysts (e.g., Pt, Au).
Insulators can become conductors (e.g., Si).
Nanocrystalline materials have grain sizes in the range of $1$ to $100$ nm.
These materials are exceptionally strong, hard, and ductile at high temperatures.
Nanocrystalline materials are wear-resistant, erosion-resistant, corrosion-resistant, and chemically very active.

5.3.3 Techniques for Synthesis of Nanomaterials

Two general approaches to nanomaterial synthesis:
- Bottom-up Approach
  - Builds materials from the bottom, atom by atom, molecule by molecule, or cluster by cluster.
  - Involves assembly or self-assembly mechanisms to create larger structures from nanometric starting points.
  - Example: Formation of nanoparticles from colloidal dispersion.
  - Another example: Enzymes creating DNA by binding constituent molecules.
  - Results in nanostructures with fewer defects and more homogeneous chemical composition.
- Top-down Approach
  - Involves slicing or cutting a bulk material to obtain nanosized particles.
  - Analogous to making a statue from stone, where material is removed until the desired shape is achieved.
  - Examples include lithographic techniques (electron-beam, photo-ion beam, X-ray lithography), etching, grinding, ball-milling, and sol-gel technique.
  - Can introduce internal stress, surface defects, and contaminations.

5.3.4 Carbon Nano Tubes (CNTs)

Carbon nanotubes are prime examples of nanostructures derived via bottom-up chemical synthesis.
They exhibit extreme diversity in structures and structure-property relationships despite their simple chemical composition and atomic bonding.
Carbon has $4$ valence electrons, enabling it to bond in various ways to create structures with different properties.
Diamond and graphite are natural crystalline forms of pure carbon.
Diamond: $sp^3$ hybridization, where all four electrons are shared equally, creating an isotropically strong structure.
Graphite: $sp^2$ hybridization, where three electrons are shared covalently in a plane, and the fourth is delocalized; results in a hexagonal honeycomb lattice with weak out-of-plane van der Waals bonding.
C-C bond length:
- $sp^3$ hybridization: $1.56$ Å
- $sp^2$ hybridization: $1.42$ Å
Graphitic interplanar spacing: $3.35$ Å
Graphite is thermodynamically stable at high temperatures, whereas diamond is kinetically stable.
Fullerenes and carbon nanotubes are other forms of pure carbon with $sp^2$ hybridization.
CNT significance rose after S. Iijima's discovery in 1991.

5.3.4.1 Structure and Properties of Carbon Nanotubes

CNT arrangements depend on how graphite sheets roll up:
- Non-chiral: Armchair and zigzag arrangements, where the honeycomb lattice is parallel to the tube axis.
  - Armchair: Two C-C bonds on opposite sides of the hexagon are perpendicular to the tube axis.
  - Zigzag: These bonds are parallel to the tube axis.
- Chiral: Helical structures where the C-C bond lies at an angle to the tube axis.
Different types can be identified by following patterns across the tube diameter and analyzing cross-sectional structure.
Armchair arrangements are always metallic, while zigzag and helical tubes can be either metallic or semiconducting.
This impacts their electrical properties, making them useful in electronic industries.
CNTs are tubular forms of carbon, visualized as graphene sheets rolled into cylindrical forms.
They have diameters and lengths up to several micrometers.

5.3.4.1 SWNT and MWNT

Based on the number of co-axial cylinders, CNTs are either single-wall carbon nanotubes (SWNT) or multi-wall carbon nanotubes (MWNT).
SWNT: A single graphene sheet rolled up into a seamless tube with capped ends.
MWNT: More than one graphene sheet rolled up co-axially to form concentric cylinders with an interlayer spacing of $0.34 - 0.36$ nm.
CNT diameter ranges:
- SWNT: $0.4$ to > $3$ nm
- MWNT: ~ $1.4$ to at least $100$ nm
Tube length typically varies from $1-10$ micron.
Density:
- MWNT: $1.75$ gm/cc
- SWNT: $1.4$ gm/cc

Feature	SWNT	MWNT
Layers	Single layer of graphene	Multiple layers of graphene
Synthesis Catalysts	Requires catalysts	Can be produced without catalysts
Bulk Synthesis	Difficult	Easy
Purity	Poor	High
Twistability	Easily twisted	Cannot be easily twisted

Each MWNT exhibits unique conductivity, leading to metallic and semiconducting behavior.
Geometric differences (defects, chirality, diameter) and crystallinity (hexagonal lattice perfection) influence the electronic response.
Bundles of SWNTs behave as metals with resistivities in the range of $0.34 × 10^{-4}$ to $1.0 × 10^{-4}$ Q-cm.
Due to strong C-C bonds, carbon nanotubes possess excellent mechanical properties and stiffness.
MWNTs and SWNTs are flexible and do not break upon bending.
CNTs possess an average Young's modulus of $1.1.8$ TPa, higher than commercially available carbon fibers (800 GPa).
Thermal conductivity along the tube axis may be among the highest compared to other carbon allotropes.
In graphite, photons dominate the specific heat above $20$ K, whereas in MWNT and SWNT, photons dominate at all temperatures.
At room temperature, the thermal conductivity of MWNT is comparable to noncrystalline carbon fiber due to small crystal size in graphitic domains.

5.3.4.2 Synthesis of Carbon Nanotubes

CNTs (SWNT and MWNT) can be produced using various catalytic and noncatalytic synthetic routes.
Different methods yield CNTs with different properties and require different raw materials.
Techniques include:
1. Electric Arc-discharge Process
2. Laser Ablation of Carbon
3. Chemical Vapor Deposition (CVD)
4. High Pressure CO Disproportionation Process (HiPCO)

1. Laser Ablation of Carbon (Laser Vaporization Method)

In 1995, Smalley et al. used a dual-pulsed laser for producing CNTs with $70$ % purity.
Involves using a graphite rod with a 50:50 catalyst mixture of cobalt and nickel at $1200$ °C in flowing argon.
The initial laser pulse vaporizes the target, followed by a second pulse to ensure uniform vaporization and minimize soot deposition.
The second pulse breaks larger particles.
CNTs produced are 10-20 nm in diameter and 100 μm or more in length.
Average nanotube diameter and size distribution depend on growth temperature, catalyst composition, and process parameters.
Arc discharge and laser vaporization are used to obtain high-quality CNTs in small quantities.
Drawbacks:
1. Evaporation of carbon source makes it unclear how to scale up production to industrial standards.
2. CNTs are mixed with carbon residues, making purification, manipulation, and assembly difficult.
Laser technique is not economically advantageous due to high purity graphite, high laser power requirements, and low throughput.

Drawbacks of laser ablation process

This method involves evaporation of carbon source, so it is unclear to scale up the production to industrial level.
Vapourisation methods grow CNT's in highly tangled form, mixed with unwanted forms of C and metal species.
CNT's produced are difficult to purify.

2. Chemical Vapor Deposition (CVD) Method

Promising for large-scale production of carbon nanofiber (CNF), SWNT, and MWNT.
Hydrocarbons (e.g., methane, benzene, acetylene, naphthalene, ethylene, n-hexane) decompose over metal catalysts (e.g., Co, Ni, Fe, Pt, or alloys of these metals) on substrates like silicon, graphite, and silica.
Typical yields for CVD are approximately $30$ %.
Carbon fibers and filaments have been produced using CVD of hydrocarbons with a metal catalyst for the last twenty years.
Large amounts of CNTs are produced by catalytic CVD of acetylene over cobalt and iron.
Carbon/zeolite catalysts can produce fullerenes and bundles of SWNTs along with MWNTs.
Ethylene, supported by iron, cobalt, and nickel catalysts, is used for SWNT/MWNT formation.
Ethylene can be used with reaction temperatures of $545$ °C for Nickel catalyzed CVD and $900$ °C for an uncatalyzed process that produces carbon nanostructures with open ends.
Methane can also be used as a carbon source.
Catalytic decomposition of H₂/CH₄ mixture over cobalt, nickel, and iron yields high amounts of SWNTs at $1000$ °C.
Using H₂/CH₄ atmosphere between a non-reducible oxide (e.g., Al₂O₃ or MgAl₂O₄) and one or more transition metal oxides can produce composite powders containing well-dispersed CNTs.
Higher proportions of SWNTs and lower proportions of MWNTs can be achieved using the decomposition of CH₄ over nanoparticles.
Thermal catalytic decomposition of hydrocarbon is an active research area and a promising route for bulk CNT production.
One issue is the removal of the catalyst support via acid treatment, which can destroy the original structure of the carbon nanotubes.
Alternative water-soluble catalyst supports have proven effective for nanotube growth.

5.3.4.3 Purification of Carbon Nanotubes

During synthesis, byproducts like polyhedral carbon, metal catalyst coated with carbon, amorphous carbon, and fullerenes are also produced.
Purification techniques include filtration, chromatography, centrifugation, gas phase oxidation, and chemical treatment.
Classic chemical techniques are ineffective for removing undesirable impurities from CNTs.
Three basic methodologies used to purify CNTs:
- Gas phase
- Liquid phase
- Intercalation
Microfiltration is performed to remove nanoparticles using membrane filters, removing amorphous carbon and unwanted nanoparticles without chemically modifying the CNTs.
Extended sonication in concentrated acid mixtures cuts CNTs into smaller segments, forming colloidal suspensions in solvents.
These solvents can be deposited on substrates or in solution, providing different functional groups attached to the ends and sides of the CNTs.

5.3.4.4 Properties of Carbon Nanotubes

CNTs are highly conducting. Conductivity depends on chirality, degree of twist, and diameter.
CNTs are expected to be high-strength fibers. SWNTs are stiffer than steel and very resistant to physical forces.
CNTs are elastic, making them useful as probe tips for high-resolution scanning probe microscopy (e.g., AFM).
CNTs show very high thermal conductivity.
Because of the large surface area, CNTs can be used in air, gas, and water filtration.

5.3.4.5 Applications of Carbon Nanotubes

1. Structural materials used in day-to-day life

Combat jackets using CNTs as ultra-strong fibers
Polyethylene with increased elastic modulus by $30$ %
Sports equipment like stronger and lighter tennis rackets, bike parts, golf balls, and baseball bats
In suspension bridges instead of steel

2. Applications in electromagnetic field

Artificial muscles used in robotics for large linear movements
Buckypaper, a thin sheet of nanotubes $250$ times stronger than steel and $10$ times lighter, used for chipboards as a heat sink or a backlight for LCD screens
Chemical nanowires: CNTs can be used to produce nanowires of other metals and chemicals such as gold or zinc oxide.
Computer circuits: CNTs can be used to dissipate heat from tiny computer chips due to their good thermal properties.
Light bulb filament: CNTs are used as an alternative to tungsten filaments in light bulbs.
Magnets: MWCNTs coated with magnetite are used as magnets.
Solar cells: CNTs diode has a photovoltaic effect. Nanotubes can replace ITO in some solar cells.
Superconductor: CNTs have been shown to be superconducting at low temperatures.

3. Applications in Chemical Field

Air pollution filter: Filtering CO₂ from power plant emissions is a likely future application of nanotube membranes.
Biotech container: Nanotubes can be used as a biotech container.
Hydrogen storage: CNTs have the potential to store between $4.2$ and $65$ % hydrogen by weight.
Water filter: Nanotube membranes can be used for filtration of water.

4. Applications in Mechanical Field

Oscillators: The fastest known oscillators (> $50$ GHz) are nanotubes.
Nanotube membrane: In nanotube membrane, liquid flows up to five orders of magnitude faster than predicted by classical fluid dynamics.
Slick surface: CNTs develop materials which are slicker than Teflon and also waterproof.

5. Applications in Electric Circuits

Metallic and Semiconducting nanotubes: CNTs of multi-walled structures with outermost shell diameters exceeding 10 nm can be used as light-emitting semiconductors.
Carbon nanotube interconnects: Very large scale integration (VLSI) interconnects of the future are metallic CNTs because of their desirable properties of high thermal stability, high thermal conductivity and large current-carrying capacity.
Carbon nanotube transistors: Field effect transistors (CNTFFTs) are fabricated using semiconducting CNTs, which show promise due to their superior electrical characteristics over silicon-based MOSFFTs. It is superior in terms of sub-threshold slope which is a very important property for low-power applications.

6. Drug Delivery Vessel

The nanotube's versatile structure is useful for a variety of tasks in and around the human body, apart from cancer treatment.
Allows lower drug dosage by localizing its distribution, which will significantly cut costs to pharmaceutical companies and their consumers.
Common methods for carrying drugs:
1. The drug can be attached to the side of the nanotube or can be attached at the end to trail behind the nanotube.
2. The drug can be placed inside the nanotube for its delivery.

5.4 Fullerene

5.4.1 Introduction

One of the nano-forms of carbon is Buckminster Fullerene (C60).
A fullerene is a molecule of carbon in the form of a hollow sphere, ellipsoid, tube, and many other shapes.
Spherical fullerenes are also called buckyballs or Buckminster Fullerenes C60, and they resemble the balls used in football (soccer).
The first fullerene molecule C60 was prepared in 1985.
The name was a homage to Buckminster Fuller, whose geodesic domes it resembles.
C60, Buckminster Fullerene is the most stable cage cluster structure in which 12 pentagons are present and all the pentagons are separated by hexagons.
The carbon atoms are bonded in an isosahedral structure which is made up of 20 hexagons and 12 pentagons.
After the first discover of Buckminster Fullerene, C60, other types of fullerenes C70, C76, C78, C80, etc. have also been discovered.

5.4.2: Preparation of Fullerenes

A common method used to produce fullerenes is to send a large current between two nearby graphite electrodes in an inert atmosphere of Argon.
The resulting carbon plasma arc between the electrodes cools into sooty residue from which many fullerenes can be isolated.

5.4.3 Properties of C60, Buckminster Fullerene

It is a mustard coloured solid, which appears brown to black with increasing thickness of its film.
On sublimation, it forms translucent magenta face-centered cubic crystals.
It is moderately soluble in aromatic hydrocarbons giving magenta solution.
As a pure solid, it is electrically insulating. With proper impurity addition, it can be made highly conductive and super conductive.
With alkali metals, they form alkali fullerides, M3C60, which are supercondu