chemistry

Graphane

Graphane is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Graphane rather than just read about it. In short: Graphane is a two-dimensional polymer of carbon and hydrogen with the formula unit (CH)n where n is large. Partial hydrogenation results in hydrogenated graphene, which was reported by Elias et al. in 2009 by a TEM study to be "direct evidence for a new graphene-based derivative".

Key takeaways

Graphane belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Graphane to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Graphane from memory before moving on to harder problems.

Reference excerpt

Graphane is a two-dimensional polymer of carbon and hydrogen with the formula unit (CH)n where n is large. Partial hydrogenation results in hydrogenated graphene, which was reported by Elias et al. in 2009 by a TEM study to be "direct evidence for a new graphene-based derivative". The authors viewed the panorama as "a whole range of new two-dimensional crystals with designed electronic and other properties". With the band gap ranges from 0 to 0.8 eV

Synthesis Its preparation was reported in 2009. Graphane can be formed by electrolytic hydrogenation of graphene, few-layer graphene or high-oriented pyrolytic graphite. In the last case mechanical exfoliation of hydrogenated top layers can be used.

Structure The first theoretical description of graphane was reported in 2003. The structure was found, using a cluster expansion method, to be the most stable of all the possible hydrogenation ratios of graphene. In 2007, researchers found that the compound is more stable than other compounds containing carbon and hydrogen, such as benzene, cyclohexane and polyethylene. This group named the predicted compound graphane, because it is the fully saturated version of graphene.

Graphane is effectively made up of cyclohexane units, and, in parallel to cyclohexane, the most stable structural conformation is not planar, but an out-of-plane structure, including the chair and boat conformers, in order to minimize ring strain and allow for the ideal tetrahedral bond angle of 109.5° for sp3-bonded atoms. However, in contrast to cyclohexane, graphane cannot interconvert between these different conformers because not only are they topologically different, but they are also different structural isomers with different configurations. The chair conformer has the hydrogens alternating above or below the plane from carbon to neighboring carbon, while the boat conformer has the hydrogen atoms alternating in pairs above and below the plane. There are also other possible conformational isomers, including the twist-boat and twist-boat-chair. As with cyclohexane, the most stable conformer for graphane is the chair, followed by the twist-boat structure. While the buckling of the chair conformer would imply lattice shrinkage, calculations show the lattice actually expands by approximately 30% due to the opposing effect on the lattice spacing of the longer carbon-carbon (C-C) bonds, as the sp3-bonding of graphane yields longer C-C bonds of 1.52 Å compared to the sp2-bonding of graphene which yields shorter C-C bonds of 1.42 Å. As just established, theoretically if graphane was perfect and everywhere in its stable chair conformer, the lattice would expand; however, the existence of domains where the locally stable twist-boat conformer dominates "contribute to the experimentally observed lattice contraction." When experimentalists have characterized graphane, they have found a distribution of lattice spacings, corresponding to different domains exhibiting different conformers. Any disorder in hydrogenation conformation tends to contract the lattice constant by about 2.0%. Graphane is an insulator. Chemical functionalization of graphene with hydrogen may be a suitable method to open a band gap in graphene. P-doped graphane is proposed to be a high-temperature BCS theory superconductor with a Tc above 90 K.

Variants Partial hydrogenation leads to hydrogenated graphene rather than (fully hydrogenated) graphane. Such compounds are usually named as "graphane-like" structures. Graphane and graphane-like structures can be formed by electrolytic hydrogenation of graphene or few-layer graphene or high-oriented pyrolytic graphite. In the last case mechanical exfoliation of hydrogenated top layers can be used. Hydrogenation of graphene on substrate affects only one side, preserving hexagonal symmetry. One-sided hydrogenation of graphene is possible due to the existence of ripplings. Because the latter are distributed randomly, the obtained material is disordered in contrast to two-sided graphane. Annealing allows the hydrogen to disperse, reverting to graphene. Simulations revealed the underlying kinetic mechanism.

Potential applications p-Doped graphane is postulated to be a high-temperature BCS theory superconductor with a Tc above 90 K. Graphane has been proposed for hydrogen storage. Hydrogenation decreases the dependence of the lattice constant on temperature, which indicates a possible application in precision instruments.

Illustrations

Worked examples

Example 1 — a first encounter with Graphane

Start with the simplest possible case. Write down what Graphane claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Graphane before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Graphane ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Graphane

In research: Graphane appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Graphane in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Graphane is common in secondary-school and first-year university syllabi. It links to neighbouring topics Hydrocarbons, Polymers, Superconductors, so understanding it makes those chapters shorter.
In everyday life: Look for Graphane outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Graphane in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Graphane means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Graphane out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Graphane in simple terms?

Why does Graphane matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Graphane?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Graphane.