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

Doc Brown's Chemistry KS4 science GCSE/IGCSE/O level/A Level Chemistry Revision Notes

NANOCHEMISTRY Nanoscience Nanotechnology Nanostructures

Why is graphene so strong? What is the structure of graphene and fluorographene?

See also allotropes of carbon - diamond, graphite, fullerenes etc.

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 5. Graphene and Fluorographene

• Graphene - structure and properties

  • What is graphene? What is graphene's molecular structure? What might we use graphene for?

  • Graphene is a smart material, but can also be considered as a 2D nanomaterial because it is only one atom thick.

  • Graphene is essentially a single layer of carbon in the form of graphite, with its layered structure of hexagonal rings of carbon atoms (structure of graphite).

  • It is possible to synthesise graphite in individual layers just one atom thick and the product is known as graphene whose 'honeycombed' lattice is shown below.

    • Each graphene molecule is a fully planar shape.

    • The C-C bond length is 0.142 nm, as in graphite, its mid-way between 0.154 nm for C-C bond and 0.134 for a C=C bond.

    • The C-C-C bond angle is 120^o, what you expect for the internal angle of a perfectly symmetrical planar hexagon.

    • The C-C bonds are strong, giving a strong 2D lattice of carbon atoms.

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  • Each layer consists of hexagonal rings of carbon atoms linked together in a planar lattice, the formula is C_n where n is a very large number BUT a graphene layer is just a single atom in thickness!

  • another version of the skeletal formula representation of graphene.

    • This image shows how the structure of graphene relates to polyaromatic hydrocarbons like naphthalene C₁₀H₈ and anthracene C₁₄H₁₀ consisting of 2 and 3 fused benzene rings respectively. If you take this 'aromatic fusion' to its extreme in two dimensions, the hydrogens go and the result is a graphene molecule of almost 100% carbon atoms.

  • Graphene is spatially considered a 2D material.

    • Note on 'dimension descriptions' at the atomic or molecular level

      • zero dimensional, 0D e.g. an individual atom, or 'quantum dot'

      • one dimensional, 1D e.g. nanowires, carbon nanotubes, long polymer molecules like poly(ethene) - though not as neatly aligned as nanotubes can be. Linear plastic molecules tend to be a bit jumbled and branched in reality.

      • two dimensional, 2D e.g. the layers of graphite (previously described) or hexagonal boron nitride (later section), the polymer Kevlar.

      • three dimensional, 3D e.g. the giant covalent lattices of diamond and silica, cubic boron nitride (see further down), cross-linked polymers like Bakelite. Thermosetting resins that set hard due to cross-linking e.g. those used in fibreglass constructions. In all these cases you are dealing with a continuous covalent bond network in all directions.

  • In graphene each carbon atom forms three sigma bonds with other carbon atoms (sp² hybridisation) via three of its four valency electrons, but the 4th electron is 'delocalised' i.e. shared in common with other carbon atoms giving extra bonding (pi electrons, pi bonding) ...

    • ... this gives a very tightly packed and strongly bonded network of carbon atoms within the layer, the carbon - carbon bonds are short and strong (three per carbon atom) giving it a very high tensile strength (300 times that of steel) and yet is very light low density material.

    • ... AND because these pi or delocalised electrons are free to move through the layer, electrical conduction readily occurs if a potential difference is applied.

  • This delocalised system means that it is unsaturated and theoretically atoms can add to it, BUT it is chemically relatively inert apart from combustion!

    • It is this 'large scale' delocalisation that gives graphene its chemical stability.

  • Graphene has a high thermal stability (like graphite, up to 3000^oC if no oxygen present), high electrical conductivity, a high optical transparency and is chemically relatively inert.

    • All four of these physical and chemical properties make it a potentially really useful substance for materials scientists to use in a variety of quite different applications e.g.

  • Graphene, this remarkable material made of sheets of carbon just one atom thick, has been shown to undergoes a self-repairing process to correct holes when exposed to loose carbon atoms.

    • It would appear that when holes were punched in it by a beam of atoms, if carbon atoms where near the surface of the graphene sheets, complete hexagonal rings would reform making the two-dimensional (2D) sheets complete.

    • Graphene has outstanding mechanical strength with respect to its thinness and combined with its electronic properties, it is a promising material for a wide range of future applications.

    • However, graphene's very thinness makes it easily damaged when working with it, so any 'self-healing' property is most welcome!

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  • USES of graphene - potential applications of graphene

    • So, what can we use graphene for?

    • Graphene can be used to manufacture excellent transistors for the electronics industry.

    • Conduction can be made to depend on an ambient electric field, making it a very sensitive surface and metal films cannot be made as thin, so less sensitive.

    • Graphene could be used in highly sensitive gas sensors - molecules 'landing' on the surface cause measurable electronic changes and perhaps even individual molecules can generate a signal.

      • For the same reason, its applications may include use as a touch screen surface.

    • Graphene is 100x stronger than steel in bulk so has the potential to be used for small scale BUT strong components in devices.

      • Graphene fibres maybe stronger than the current carbon fibres, but unlikely to replace them on production cost grounds.

    • Graphene is highly resistant to attack by strong acids (e.g. nitric, hydrochloric, sulfuric, hydrofluoric acids) or strong alkalis (e.g. sodium hydroxide, potassium hydroxide) and so can be used to give surfaces an ultra-thin protective layer which is transparent.

    • It can be used as support membrane for transmission electron microscopy used to study the molecular detail of materials such as fragile DNA molecules and nanoparticles themselves!

      • A graphene support membrane can be made relatively strong but very thin and so transparent that it does not interfere with the 'picture' generated by the electron microscope.

    • New techniques have been developed to produces highly selective filter materials based on graphene that could lead to more efficient desalination. Scientists have succeeded in creating subnanoscale pores in a sheet of the one-atom-thick materials including graphene, which is one of the strongest materials known.

      • Therefore sheets of graphene, with these subnanoscale pores, can behave like a semi-permeable membranes.

      • So, one possible use of these is the desalination of seawater, very important in regions of the world where fresh water is scarce e.g. desert regions of countries in Africa and the Middle-East.

      • Pure water can be extracted as the water molecules can diffuse through the graphene membrane, leaving behind the larger hydrated ions from the salts found in seawater e.g. the sodium ions and chloride ions from sodium chloride.

      • The idea is to get these subnanoscale pores small enough to stop larger molecules or ions passing through the graphene membrane filter, but large enough to allow water molecules to diffuse through.

  • -

  • NOT on some syllabuses yet, but very interesting!

    • Technically graphene is an allotrope of carbon (like diamond and graphite) though it is, in reality, a single layer of graphite.

    • Graphene is a two-dimensional (2D) allotrope of carbon (diamond is 3D).

    • However, since carbon has a valency of four, there are other possible linear one-dimensional (1D) forms based on single, double or triple carbon-carbon covalent bonding systems.

      • So, again, technically, these are also allotropes of carbon.

    • So, research is going into trying to synthesise and investigating the properties of linear structures based on:

      • (i) C=C=C=C=C=C=C= etc. i.e. consecutive carbon-carbon double bonds

        • This is an example of a cumulene (3 or more consecutive carbon-carbon double bonds), so it is effectively a polycumulene.

      • (ii) C–CΞC–CΞC–CΞC– etc. i.e. alternating single and triple carbon-carbon bonds.

        • This material is called 'linear acetylenic carbon', an example of a polyyne (–CΞC–)_n, or carbyne (though the name carbyne is applied to other chemical species).

        • Carbyne is predicted to be an extraordinarily strong material, stronger than diamond, but it is difficult to synthesise and is very unstable.

  • -

• Fluorographene (also a smart material)

  • What is fluorographene? What is fluorographene's molecular structure? What might we use fluorographene for?

  • In fluorographene, the fourth valency electron of carbon is paired with one from fluorine to form a 4th single covalent bond between carbon and fluorine.

  • -

  • 

  • Each carbon atoms forms four single bonds directed to the corners of a tetrahedron.

  • The C-C-F and C-C-C bond angles will al be around 109^o (the symmetrical tetrahedral angle, as in methane)

  • The three strong C-C carbon-carbon bonds from each carbon atom produce a strong giant 2D network of links, but the fourth bond (carbon-fluorine C-F, also very strong) 'pokes' up and down from this 'rippled' 2D array of carbon atoms.

  • This means, unlike in graphene, there is no 'spare' 4th electron from carbon to form a delocalised electron system that allows conduction of electricity, so it is an electrical insulator.

  • Fluorographene is a saturated organic compound of (CF)_n where n is a very large number (empirical formula CF).

  • It is, in a simplified way, a 2D version of TEFLON/PTFE which consists of long 1D poly(tetrafluoroethene) polymer molecules.

    • PTFE is -(C₂F₄)_n-, empirical formula CF₂, fluorographene's empirical formula is CF.

  • Like PTFE, fluorographene has a very high thermal stability and excellent 'non-stick' properties.

  • Also like PTFE, it is chemically very stable, i.e. very inert, so, combined with its great thermal stability, it can be used as a very stable surfacing material over a wide temperature range.

  • Uses of fluorographene

    • It is a high quality super-thin insulator.

    • Its electronic applications include use in LEDs (light emitting diodes)

    • Like TEFLON, it can used as an inert protective 'non-stick' surface, but I think Teflon is lot cheaper to produce!

    • Self-cleaning windows.

    • Self-cleaning ovens.

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• Graphene oxide - use as a water filter

  • Research is on going to develop water filters based on graphene e.g. to convert sea water or brackish fresh water into potable drinking water.

  • This could aid millions of people, especially in the developing world like Africa, to have access to clean drinkable water.

  • A graphene oxide sieve has been shown to filter out salts but it is not as efficient as desired and rather costly at the moment.

  • The idea is to just letter water through and act as a barrier to salts like sodium chloride, a process called desalination.

  • The graphene oxide layers would act like a semi-permeable membrane.

See also allotropes of carbon - diamond, graphite, fullerenes etc.

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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