Properties of Nanomaterials

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Types of Nanotubes and Their Properties

Types of Nanotubes

  • Inherent issue in the synthesis of carbon nanotubes (CNTs):
    • 30-40 different types of nanotubes per batch.
    • Metallic vs Semiconducting.
    • Singled-walled versus multi-walled.
    • Different lengths and diameters.
  • Very difficult to separate them.
  • Need a better process to produce the specific type wanted.

Types of Nanotubes

  • Think about a nanotube as a piece of rolled up graphene.
    • This isn't how they are made, but an easy way to visualize them.
  • Depending on the section “cut out” to roll up, the carbon atoms are going to have different orientations or “twists” across the tube. 
    • These have been named zig-zag, armchair, or chiral based on the pattern of the carbon rings.

Conductance:

  • In general, the “twist” of the nanotube is called the chirality.
  • The chirality affects the conductance of the nanotube, it's density, it's lattice structure, and other properties.
  • Nanotubes are either metallic or semi-conducting based on their chirality.

Metallic v Semi-conducting

Conductance and Chirality

  • Chirality is represented as a vector (n,m).
    • Armchair: n=m; (n, n).
    • Zig-zag: m=0 (n, 0).
    • Chiral: (n, m).
  • A SWCNT is considered metallic if the value n - m is divisible by three. Otherwise, the nanotube is semiconducting. 

vector: (10,0)

vector: (7,3)

vector: (10,7)

Semi-conducting & zig-zag

Semi-conducting & chiral

Metallic & chiral

Surface-to-Volume Ratio

Where Surface is Important

Many technologies are based on surface interactions, including:

  • Filtration.
    • Air and water purification.
  • Heat transfer.
  • Catalysis.
  • Sensors technologies.
  • Super-batteries.

Very large surfaces for little material are needed!!

Surface to Volume Ratio (S/V)

Definition: Amount of surface area per unit volume of an object

surface area:volume     or

surface area

volume

Ex: S/V of a cube (with side a = 1 cm)

Visualize S/V Ratios

  • A large cube made up of smaller cubes has a much greater S/V ratio.
  • Each tiny cube contributes surface area to the overall area, where the volume remains the same. 

Smaller Item, More Surface Atoms

Nanomaterials have a large number of surface atoms and thus a large surface-to-volume ratio.

  • The larger the surface-to-volume ratio, the more atoms are located on the surface.
  • Surface atoms behave very differently than inner atoms.

Fraction of the atoms on the surface

Carbon-based Nanomaterials

EVERY ATOM IS ON THE SURFACE!

Comparing Materials: Carbon

Compare S/V of carbon nanomaterials to other forms of carbon
For example: Diamond vs Carbon Nanotubes.

  • Diamond: C bound to 4 other C's; arranged in a tetrahedral geometry. 
  • Nanotubes: C bound to 3 other C's; arranged in a tubular shape. 
  • A diamond has a crystalline unit cell structure throughout the entire material, while nanotubes are 1 carbon atom thick tube-like materials with ALL ATOMS ON THE SURFACE. 

Carbon Nanotube

“Dissolved” Nanoparticles

Gravity, Buoyancy, Sinking, Floating

An object will float when placed in a fluid of higher density, it will sink when the fluid has lower density.

Gravity, Density, and Floating

Let’s observe a piece of Styrofoam and a chunk of gold in a glass of water.

LiCl: Floating or Sinking?

Lithium Chloride (LiCl)
Density LiCl = 2.1 g/mL
Li+ = 7 g/mol
Cl- = 35.45 g/mol

Water
Density H2O = 1 g/mL
H2O = 18 g/mol

 

Solvation

Solvation forces are much stronger than gravity and buoyancy for an ion.

In between

  • Nanopartilces behave like larger atoms/ions.
    • They do not "feel" gravity.
    • They have strong solvation effects.
  • Gold nanoparticles act like something in between a chunk of gold and gold ions (Au3+).
  • Gold nanoparticles act as a suspension in water, not  a solution.

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