Graphene: A Complete Chemical History

Graphene is often introduced as a physics story — the "miracle material" peeled off with sticky tape that won a Nobel Prize. But its real origin is chemical, and it begins almost a century and a half before 2004. This is the story of graphene told through its chemistry: how oxidizing graphite in 1859 accidentally opened the door, how the single carbon sheet was named and first measured, and how the modern toolbox of oxidation, reduction, exfoliation and vapor growth turns graphite into the thinnest material we know.

What graphene actually is

Graphene is a single, two-dimensional sheet of carbon atoms bonded into a honeycomb lattice. Each carbon is sp²-hybridized: three of its four valence electrons form strong in-plane σ bonds to three neighbors at 120°, giving the famous hexagonal "chicken-wire" network with a carbon–carbon distance of about 0.142 nm. The fourth electron sits in a p orbital perpendicular to the plane, and these overlap into a delocalized π system that spreads across the entire sheet. In chemical terms, graphene can be viewed as a polycyclic aromatic hydrocarbon extended toward infinite size; that aromatic π system is one useful lens for understanding its reactivity and electronic behavior.

That π cloud is also why graphene is the conceptual "mother" of the other carbon nanomaterials. Roll a sheet into a cylinder and you have a carbon nanotube; wrap it into a ball by inserting twelve pentagons and you have a fullerene; stack many sheets, held together only by weak van der Waals forces about 0.335 nm apart, and you have ordinary graphite. Defects such as five- or seven-membered rings curve and warp the plane, which is exactly how more exotic shapes are built. Understanding graphene as one flat aromatic sheet — and its relatives as that sheet rolled, wrapped or stacked — makes the whole carbon family click into place.

The accidental beginning: graphite oxide (1859)

The chemical history of graphene starts with a British chemist trying to measure the atomic weight of graphite. In 1859 Benjamin C. Brodie treated graphite with potassium chlorate (KClO₃) and fuming nitric acid (HNO₃), and found that the product had swelled, gained mass, and now contained carbon, hydrogen and oxygen.¹ He had made what we now call graphite oxide — graphite whose sheets are decorated with oxygen-containing groups — without realizing he had taken the first step toward isolating a single carbon layer. Crucially, he noticed the material could be dispersed in water, a hint that the oxidized sheets were being pushed apart.

In 1898 Ludwig Staudenmaier refined the recipe, adding concentrated sulfuric acid (H₂SO₄) and dosing the chlorate in small portions so that a more highly oxidized product could be made in a single flask.² These nineteenth-century oxidations were slow and dangerous — the chlorate route releases toxic and explosive gases — but they established the central chemical trick that graphene science still relies on: oxidation forces graphite layers apart by inserting oxygen between them.

Naming the sheet, and the theory that came first

For a long time the single layer existed only on paper. In 1947 the physicist P. R. Wallace worked out the electronic band structure of a single graphite layer as a stepping stone to understanding bulk graphite, deriving the now-iconic linear (cone-shaped) energy–momentum relationship near the corners of the Brillouin zone.³ He never used the word "graphene," but his calculation predicted the unusual electronic behavior that would make the real material famous decades later.

The first person to actually make and measure something close to a single sheet was the chemist Hanns-Peter Boehm. In a 1962 paper memorably titled "the thinnest carbon films," Boehm and his co-workers chemically reduced graphite oxide — using hydrazine and thermal treatment — and used electron microscopy and X-ray methods to identify lamellae down to a single carbon layer.⁴ In modern terms, the earliest experimental graphene-like single layers were produced through reduced graphite oxide chemistry, by chemists. Boehm later gave the material its name: in a 1986 nomenclature recommendation he and his colleagues proposed "graphene," combining graph- from graphite with the -ene suffix used for fused aromatic hydrocarbons.⁵ The International Union of Pure and Applied Chemistry (IUPAC) formally adopted the definition in the mid-1990s,²⁴ stipulating that "graphene" should refer specifically to a single layer, never to the three-dimensional stack. (For a detailed scholarly account of this history, see the review by Dreyer, Ruoff and Bielawski.²³)

The Hummers revolution and modern graphene oxide

If the nineteenth-century recipes opened the door, a 1958 method threw it wide open. Working at the U.S. National Lead Company, William Hummers and Richard Offeman replaced the slow chlorate oxidation with a fast one: graphite stirred into a mixture of potassium permanganate (KMnO₄), sodium nitrate (NaNO₃) and concentrated sulfuric acid produced graphite oxide in a few hours rather than days.⁶ The Hummers method is still the most widely used route to graphene oxide today, and almost every modern protocol is a variation on it.

Those variations exist because the classic recipe has drawbacks: it gives off toxic nitrogen gases (NO₂ and N₂O₄), leaves nitrate residues, and incompletely oxidizes large flakes. In 2010 the Tour group at Rice University introduced an "improved" method that removes the sodium nitrate entirely, uses extra permanganate together with a sulfuric/phosphoric acid mixture, and produces a more uniform, more oxidized product with no toxic gas evolution and a higher yield of intact sheets.⁷ The chemistry of how graphite turns into graphite oxide was itself clarified later: the oxidation proceeds through distinct, isolable stages — first a sulfuric-acid graphite intercalation compound, then a "pristine" graphite oxide, and only on contact with water the familiar graphene oxide — with the diffusion of oxidant between the layers as the rate-limiting step.⁸

What graphene oxide really is

Graphite oxide and its single-layer form, graphene oxide (GO), are not simple, neat molecules — they are non-stoichiometric and somewhat disordered, which is why their structure was debated for over a century. The most widely accepted picture is the Lerf–Klinowski model, developed using solid-state NMR, in which the carbon plane carries hydroxyl (–OH) and epoxide (bridging –O–) groups on its basal surface, while carboxyl (–COOH) and carbonyl groups sit mainly at the sheet edges.⁹ Oxidation converts much of the flat sp² network into puckered sp³ carbon, which is why GO is an electrical insulator while graphene is a superb conductor.¹⁰

Those oxygen groups are also what make GO so chemically useful. They are hydrophilic, so a flake of graphite oxide readily exfoliates in water into individual, single-layer GO sheets that form stable dispersions — something pristine graphene will not do. Because the sheet has hydrophilic edges and a partly hydrophobic basal plane, GO behaves like a two-dimensional amphiphile, and the abundant –OH, epoxide and –COOH groups are convenient chemical "handles" for attaching polymers, biomolecules or nanoparticles.¹⁰ In short, graphene oxide is the form of graphene you can pour, print and functionalize. ACS Material supplies both graphene oxide and reduced graphene oxide alongside its broader graphene product line.

From graphene oxide back to graphene: reduction

Because GO is easy to make and disperse, one of the most scalable routes to graphene-like material is to make GO and then strip the oxygen back off, recovering the conjugated π network. This "reduced graphene oxide" (rGO) can be produced chemically — classically with hydrazine, or more benignly with ascorbic acid (vitamin C) — as well as thermally or electrochemically.¹¹ Reduction restores much of graphene's electrical conductivity and removes most of the oxygen, which is why rGO is attractive for composites, electrodes and coatings made at industrial scale.¹⁰

It is important to be honest about what reduction can and cannot do. The aggressive oxidation that made the sheet dispersible also punched permanent holes and defects into the lattice, and reduction never fully repairs them: rGO retains residual oxygen and structural disorder, so its conductivity and strength fall short of pristine, defect-free graphene. The trade-off — easy processing and low cost versus the highest electronic quality — runs through every decision about how to make graphene, and is exactly the theme of the interactive tool below.

2004: the isolation of a true 2D crystal

For decades, theory suggested that a strictly two-dimensional crystal would be unstable as an isolated, free-standing object at finite temperature — thermal fluctuations were expected to make it crumple. That expectation was challenged experimentally in 2004, when Andre Geim, Konstantin Novoselov and their colleagues at the University of Manchester used a disarmingly simple method — repeatedly peeling highly oriented graphite with adhesive tape — to isolate flakes just one atom thick on oxidized silicon wafers, and showed that they were stable, crystalline, and electrically active.¹² A key practical trick was that a single graphene layer becomes faintly visible under an optical microscope when placed on silicon with a carefully chosen oxide thickness, thanks to interference.

The following year settled any remaining doubt. The Manchester group and, independently, Philip Kim's group at Columbia showed that charge carriers in graphene behave like massless relativistic particles (Dirac fermions) and exhibit an unusual "half-integer" quantum Hall effect together with a measurable Berry's phase — precisely the kind of physics implied by Wallace's 1947 band structure.¹³¹⁴ The work set off an explosion of research that the discoverers themselves chronicled in their widely read overview "The rise of graphene,"¹⁵ and in 2010 Geim and Novoselov received the Nobel Prize in Physics. Their achievement was not inventing graphene — chemists had made it decades earlier — but unambiguously isolating, identifying and measuring the single sheet.

How graphene is made today: four routes

There is no single best way to make graphene; the right method depends entirely on what you need it for. Four broad families dominate:

Mechanical exfoliation — the "Scotch tape" method — peels layers apart with adhesive force.¹² It produces the highest-quality, most defect-free flakes, which makes it indispensable for fundamental research, but the flakes are tiny and the process cannot be scaled.

Liquid-phase exfoliation uses ultrasound or shear to split graphite directly into few-layer graphene suspended in a solvent (such as N-methylpyrrolidone) or a surfactant/water mixture whose surface energy matches that of graphene.¹⁶ It yields defect-free, oxygen-free sheets in printable inks and is well suited to composites and coatings, at the cost of modest monolayer yields.

The graphene-oxide route — oxidize, exfoliate, then reduce — is the most chemically scalable path to large quantities of graphene-like powder, trading some electronic quality for throughput and low cost.⁷¹¹

Chemical vapor deposition (CVD) grows graphene from a carbon gas such as methane on a hot metal foil. In a landmark 2009 result, large-area, predominantly single-layer films were grown on copper foil in a conveniently self-limiting way, because carbon barely dissolves in copper.¹⁷ CVD graphene must then be transferred off the metal — usually with a sacrificial polymer support — onto the substrate where it will be used. By 2010 this had been pushed to roll-to-roll production of 30-inch films for flexible transparent electrodes, with sheet resistance and transparency competitive with commercial indium tin oxide in those demonstrations.¹⁸ CVD gives the large, continuous, high-quality films that electronics and transparent-conductor applications demand. ACS Material offers CVD graphene on a range of substrates, polymer-supported films in its Trivial Transfer series for easy transfer, and single-layer graphene for the most demanding work.

Why the chemistry decides the properties

Graphene's celebrated properties are direct consequences of its bonding. The in-plane σ framework makes it extraordinarily strong and stiff: a careful nanoindentation experiment measured an intrinsic strength of roughly 130 gigapascals and a Young's modulus near 1 terapascal, establishing graphene as one of the strongest materials ever tested.¹⁹ (This is the precise statement behind the popular shorthand that graphene is "stronger than steel"; what matters is its exceptional strength per unit weight in an atomically thin sheet.) The same rigid lattice carries heat through lattice vibrations with remarkable efficiency — suspended single-layer graphene showed a room-temperature thermal conductivity of several thousand watts per meter-kelvin, higher than most bulk crystals.²⁰

The delocalized π electrons govern how graphene interacts with light and electricity. A single sheet absorbs a fixed fraction of visible light — about 2.3%, a value set by fundamental constants — which is why graphene is nearly transparent yet still optically detectable, and why layer counting by optical contrast works at all.²¹ Those same π electrons behave as massless Dirac fermions, giving graphene very high charge-carrier mobility.¹²¹³ Chemistry even gives us a fast, non-destructive way to read graphene's quality: Raman spectroscopy fingerprints the number of layers and the defect density through the shapes and ratios of its characteristic bands, a technique that became a workhorse of the field after a key 2006 study.²²

Functionalization and the wider graphene family

Because graphene is one giant aromatic molecule, it can be modified by the full repertoire of organic and surface chemistry. Covalent functionalization attaches groups directly to carbon atoms — easiest at reactive edges and defects, or via the abundant oxygen groups of GO — which improves dispersibility and lets chemists tune the material, at the price of disrupting some of the conjugation.¹⁰ Non-covalent functionalization instead relies on π–π stacking and van der Waals interactions to decorate the sheet without breaking bonds, preserving its electronic structure. Doping with heteroatoms such as nitrogen or boron shifts its electronic behavior for catalysis and energy storage.

"Graphene" is therefore best understood as a family. It includes pristine monolayer and bilayer graphene, few-layer graphene, graphene oxide and reduced graphene oxide, graphene quantum dots, fully hydrogenated "graphane," and a range of graphene-like two-dimensional materials. Each member trades off quality, processability and cost differently, which is why a real laboratory keeps several of them on the shelf.

Applications, tied back to the chemistry

Every major application traces back to a property we have already met. The strong, conductive sheet reinforces and adds conductivity to composites; the huge surface area and conductivity make graphene and rGO valuable in batteries and supercapacitors; high carrier mobility drives sensors and flexible electronics; transparency plus conductivity targets transparent conductive films and touch panels;¹⁸ and the oxygen chemistry of GO underpins membranes for filtration and biomedical uses such as drug delivery and biosensing.¹⁰ The thread running through all of them is the same aromatic carbon sheet that Brodie first disturbed in 1859.

If you would like to discuss which form of graphene best fits your application, contact ACS Material.

Related ACS Material products

• Graphene Series — graphene, graphene oxide and reduced graphene oxide powders and dispersions
• CVD Graphene — large-area films on metal and target substrates
• Single Layer Graphene
• Trivial Transfer Graphene — polymer-supported films for easy transfer
• Graphene-like Materials

Frequently asked questions

References

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