Part 4. From fullerenes - bucky balls to carbon nanotubes - structure, properties and uses

Doc Brown's Chemistry Revision Notes

NANOCHEMISTRY Nanoscience Nanotechnology Nanostructures

Alphabetical keyword index for the nanoscience pages :  Index of nanoscience pages : boron nitride * Buckminsterfullerenes-bucky balls * carbon nanotubes * fat nanoparticles * fluorographene * fullerenes * graphene * health and safety issues * liposomes * nanochemistry * nanomaterials * nanoparticles * nanoscale * nanoscience * nanosize-nanosized-particles * nanostructures * nanotechnology * nanotubes * problems in nanomaterial use * silver nanoparticles * safety issues * sunscreens-sunblockers * titanium dioxide

Part 4. Fullerenes, buckyballs and carbon nanotubes

Many of the basic ideas described here are ok for GCSE level chemistry, but, beware GCSE students, there are some quite advanced details on the uses and properties of fullerenes and nanotubes described as well.

• STRUCTURE, PROPERTIES and USES
• What are fullerenes? What is the formula and structure of Buckminsterfullerene? What are fullerenes used for? What is a 'bucky ball'? What is a nanotube?
  • Fullerenes and nanotubes are another allotrope of carbon.
  • Allotropes are different atomic/molecular forms of the same element in the same physical state - in this case three solid allotropes of the element carbon.
• Apart from the carbon allotropes of diamond and graphite, a 3rd form of carbon exists as fullerenes or 'bucky balls'. [see table of diagrams]
  • There is also amorphous carbon e.g. soot or charcoal, but this is mainly chemically modified impure graphite.
  • Allotropes are different atomic, molecular arrangements of the same element in the same physical state.
• They consist of hexagonal rings of carbon atoms (like in graphite or graphene) and alternating pentagonal or heptagonal carbon rings to allow curvature of the surface (see diagram further down) producing molecules that have a complete hollow shape.
  • They are a sort of a hollow 'cage' or 'ball' or 'closed tube' shaped molecules of pure carbon atoms. [see table of diagrams]
  • The carbon atoms still form three bonds per carbon atom, and most of their carbon atom rings are hexagonal, but some are five and seven membered rings.
  • Fullerenes can be classed as nanoparticles BUT they are smaller molecular versions equating to sections of the tiny molecular carbon tubes called carbon nanotubes, which are nanoparticles.
    • AND they are very interesting molecules in themselves and provide a way into studying carbon nanotubes in terms of their molecular structure and applications in nanotechnology.
    • These fullerenes (and carbon nanotubes) are quite different from other forms of carbon e.g. in the form of soot, graphite or diamond.
• The carbon-carbon bonds in Buckminster Fullerene C₆₀ (shown on right) form a pattern like a soccer ball and this fullerene is a brownish-reddish-magenta colour when dissolved in organic solvents. It is a black? solid insoluble in water.
  • The first fullerene to be discovered (C₆₀) was named Buckminsterfullerene (fullerene-60), is derived from the American architect R. Buckminster Fuller who invented the geodesic dome design in building construction.
• Other typical fullerenes have formulae such as ...
  • C₂₈, C₃₂, C₅₀, and C₇₀ which is red in solution, rugby ball shape - US American football shape
  • In these fullerenes the carbon atoms lie at vertices of a polyhedron with 12 pentagonal faces with a minimum of two hexagonal faces. [see table of diagrams]
• They are NOT considered giant covalent structures and are classed as relatively small simple molecules, even though fullerenes have the general formula C_n!
  • They do dissolve in organic solvents giving coloured solutions.
  • The colour depends on the solvent ranging from red to deep purple and violet.
  • They are the only soluble allotrope of carbon.
  • Although solid, their melting points are not that high.
  • These small molecules of nanoparticle size have very different properties compared to 'lumps' of graphite and diamond.
• What are the uses of fullerenes?
  • Fullerene molecules can be used for drug delivery into the body, as lubricants, as catalysts and in the form of carbon nanotubes can be used for reinforcing composite materials, eg sports equipment like tennis rackets (see further down the page).
  • They have many chemical synthetic and pharmaceutical applications.

    • The may be use as vehicles to carry drugs into cells, the cage like fullerene molecules could contain a drug, and the combination can pass easily through the wall of a target cell.

  • Chemical derivatives of fullerenes have fascinating complex electrical and magnetic behaviour including superconductivity and ferromagnetism. (nano nature?, beyond the scope of this page?)
  • C₆₀ is an optical limiter.
    • When light is shone on it, a solution of fullerene-60 turns darker instantly and the more intense the light, the darker it gets, so the intensity of transmitted light is limited to a maximum value.
    • This limiting light transmittance property can be used in the design of safety goggles in intense light situations e.g. people working with laser beams.
  • Fullerenes may used in certain medical applications - an example of nanomedicine
    • The idea is to use the very small fullerene molecules to easily deliver drugs directly into cells in a highly controlled manner.
      • This is possible because the extremely small diameter of the nanoparticle fullerenes (which act like a cage to hold the drug molecules) allows them to readily pass through cell membranes and readily absorbed into the body.
    • Fullerene molecules have very a high surface area / volume ratio and may be used in the development of new types of catalysts in the chemical industry, perhaps catalyst molecules can be attached to fullerene structure.
  • Fullerenes are being developed that have excellent lubricating properties (maybe superior to lubrication oils) and these lubricants significantly reduce friction in moving metal parts of machines from cog wheels to ball bearings and maybe artificial joints after orthopaedic operations on hips and knees!
    • Perhaps the nanoparticles behave like tiny ball bearings.
• Fullerenes are mentioned here to illustrate the different forms of carbon AND they can be formed into continuous tubes to give very strong fibres of 'pipe like' molecules called 'nanotubes'.
  • These nanotube molecules-particles behave differently compared to bulk carbon materials like graphite and the much smaller fullerene molecules.
• What are Nanotubes and their properties and uses? -
• What is the molecular structure of carbon nanotubes? (sometimes called 'buckytubes') and of what use are they in carbon nanotechnology?

  • Carbon nanotubes are essentially long cylindrical fullerenes.

  • 

  • Carbon nanotubes are one of the most intensively studied and characterised used nanomaterials, consisting of tiny cylinders of made carbon atoms, no wider than a strand of DNA with a wide range of properties of great use to materials scientists.

    • You can think of them as stretched out fullerenes, but using many more carbon atoms. [see table of diagrams]

    • Despite being composed of so many carbon atoms, they are still considered nanomaterials because their diameter is of nanoscale proportions.

  • In other words, lots of varieties of carbon nanotubes, differing in size and atomic arrangement can have very different properties.

    • You can also fabricate multiple layered carbon nanotubes like an elongated 'Russian doll'!
      • These presumably would make a stronger fabricated material.

  • Uses of carbon nanotubes - long fullerene molecules, one basis of the relatively new nanotechnology

  • Carbon nanotubes have a very high tensile strength, very good electrical conductivity and a relatively high thermal conductivity - good conductors of electricity and heat.
    • They are used as a component in strong composite materials.
      • This is partly due to carbon nanotubes have a high length to diameter ratio and they don't easily break when stretched.

  • Some carbon nanotubes are excellent insulators, semiconductors or conduct electricity as well as copper!

    • Nanotubes can conduct electricity and will find technological applications tiny electrical circuits in computer chips and electronic instruments (see last section on type A and B carbon nanotubes).
    • They can be used as semiconductors or 'miniature wires' in electrical circuits and of great use in miniature electronic circuitry in computers and other electronic devices.
  • They act as a component of industrial catalysts for certain reactions whose economic efficiency is of great importance (time = money in business!).
    • The catalyst can be attached to the nanotubes which have a huge surface are per mass of catalyst 'bed'.
    • They large surface combined with the catalyst ensure two rates of reaction factors work in harmony to increase the speed of an industrial reaction so making the process more efficient and more economic.
  • Carbon nanotube fibres are very strong and so they are used in 'composite materials' e.g. reinforcing graphite and plastics with very fine carbon fibres in sports equipment like tennis rackets and golf clubs.
    • This makes the sports equipment much stronger and durable, but not increasing the mass significantly.

    • Nanotubes can be stronger than steel with only ¹/₆th the weight, so adding strength without adding weight to sports equipment.

    • Bundles of the nanotubes, processed into fibres, have very high tensile strength and can be much stronger than steel on a weight for weight basis.
    • Note that pure the carbon allotrope graphite is a soft slippery solid with low physical strength.

  • Carbon nanotubes could be used to make tiny mechanical devices, molecular computers as well as extremely strong materials.

  • Carbon nanotubes are an important additive in other oil based lubricants to enhance their performance.
    • Additives are added to lubricating oils to improve their effectiveness in reducing friction and as a chemical stabiliser eg to inhibit thermal degradation of the oil in high temperature situation, but I'm not sure what the function of carbon nanotubes is in this case? I suspect the reasons involve some complex physics of viscosity well beyond the scope of these notes!
  • The structure and properties of carbon nanotubes [see table of diagrams]
    • The main cylinder or tube is made only from carbon hexagons (essentially graphite layers curved into a 'molecular pipe').
    • However pentagons are needed to close the structure at the ends or form spherical or rugby football shaped molecules.
    • The carbon nanotube molecule is held together by strong covalent carbon-carbon bonds which extends all along the nanotube or all round the smaller 'bucky ball' molecules as they are sometimes called.
    • Single or multiple-walled carbon nanotubes tubes, made from concentric nanotubes (i.e. one tube inside a larger nanotube), can be formed.
    • Note that graphite is soft and malleable.
    • The behaviour of electrons depends on the length of the carbon nanotube, so some forms are excellent conductors and others
      are semiconductors.
      • This is a typical nanoscale (quantum) effect,
      • i.e. there are major differences between the properties of the bulk material the size-dependent properties on the nanoscale (silver is another good example).
• Diagrams of the molecular structure of buckminster fullerenes ('fullerenes') and nanotubes
  • (graphite shown for comparison, one isolated layer is the same as graphene)

A section of multi-layered graphite	One of the simplest 'buckyballs'  C60	A longer buckminsterfullerene which is 'rugby ball' or 'sausage' shaped, C72 etc.
A section of a carbon nanotube e.g. 6 x 100 nm, the ends would be like those of the 'sausage' above right. All images © doc brown

• Some further discussion on molecular structure

• One possible skeletal formula representation of a layer of graphite or a molecule of graphene (graphene described in next section) is shown above in Kekule style as in aromatic compounds.

  • The C-C bond length in graphite or graphene is 0.142 nm, midway between a single C-C carbon-carbon bond length of 0.154 nm and a double C=C carbon-carbon bond of 0.134 nm.

  • The carbon-carbon bond order in graphite/graphene is 1.33, which follows from 4 valency electrons overlapping from each carbon atom BUT each carbon atom forms three C-C bonds.

    • The bond order is 1.5 in benzene, the average of a carbon-carbon single bond (bond order 1) and carbon=carbon double bond (bond order 2), but there is a C-H bond too.

    • The orbitals of three of carbon's electrons will overlap with each other to form three discrete sigma bonds in a trigonal planar arrangement, but the fourth electron from each carbon contributes to a pi bonding orbital that extends throughout the layer, above and below it, just like in benzene!

  • The C-C-C bond angle is exactly 120^o, what you would expect for the planar carbon hexagons.

  • In graphite the planar hexagonal ring layers of carbon atoms are 0.335 nm apart. This intermolecular bonding distance is a much greater distance than covalent, metallic or ionic bond lengths.

    • To understand this point, you must be clear in you mind about the difference in nature between a weak intermolecular bond force and the strong bonding between atoms in covalent, ionic and metallic structures.

• How you get the curvature in the molecular shape of fullerenes and the ends of nanotubes. The C-C-C bond angles for a planar carbon pentagon will be ~108^o and for a planar carbon hexagon ~120^o.

• Note the analogous structure of carbon nanotubes and graphite layers or the graphene molecule.

  • Nanotubes are essentially a single layer of graphite (or a molecule of graphene) wrapped around to form an elongated tube-like molecule, they only consist of hexagonal rings throughout the sides of the nanotubes.

• An example of the versatility of carbon nanotubes based on two possible fabrications, giving subtle differences in molecular structure and properties is described below.

  • The two diagrams below illustrate a short section of long carbon nanotubes displaying the two principal symmetries of hexagonal carbon ring orientation with respect to the central axis of a carbon nanotube.

    • These two types of single walled nanotubes can be made by folding graphenes/graphite sheets in different directions (but at 90^o with respect to each form).

  • etc. and etc.

  • In A the longest axis of the carbon hexagons is aligned at 90^o to the principal axis of the carbon nanotube.

    • A is sometimes referred to as the 'armchair' form and is electrically semiconducting and can be used as a semi-conductor transistor in microelectronic circuits.

  • In B the longest axis of the carbon hexagons is aligned in the same direction as the principal axis of the carbon nanotube.

    • B is sometimes called the zig-zag form and has metal like electrical conductivity and can be used in micro-circuitry i.e. one of the tiniest possible electrical circuit dimensions.

WHERE NEXT?

NANOSCIENCE - NANOCHEMISTRY INDEX

Part 1. Introduction to nanoscience, nanoparticles, commonly used terms explained

Part 2. Nanochemistry - introduction, uses & potential applications described

Part 3. Uses of Nanoparticles of titanium(IV) oxide (e.g. sun cream), fat (e.g. cosmetics), silver (e.g. medical applications)

Part 4. From fullerenes & bucky balls to carbon nanotubes - structure, properties, uses

Part 5. Graphene, graphene oxide and fluorographene - structure, properties, uses

Part 6. Cubic and hexagonal boron nitride BN

Part 7. Problems, issues and implications associated with using nanomaterials

see also INDEX of Smart materials pages

and A general survey of materials - natural & synthetic, their properties & uses

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