Few-layer graphene: what it is, how it is made, and what it is good for

From graphite to graphene

Graphite is a stack of flat sheets of carbon atoms arranged in hexagons, held together by weak forces between the layers. One sheet on its own is graphene. In 2004 researchers in Manchester isolated single sheets by peeling graphite with adhesive tape and showed their unusual electronic properties (Novoselov et al., 2004). A single layer is extremely strong, conducts heat and electricity very well and is almost transparent.

Single sheets are slow and expensive to make in quantity. Most graphene used in industry is therefore few-layer graphene: thin stacks of a handful of sheets. The ISO vocabulary for graphene notes that graphene layers behave as a two-dimensional material up to about ten layers thick for electrical measurements; beyond that, the properties are no longer distinct from bulk graphite (ISO/TS 80004-13).

How layers are counted

The number of layers changes graphene's electronic structure, and that change shows up in its Raman spectrum. The so-called 2D peak, near 2,700 cm⁻¹ for green laser light, is a single sharp peak for one layer and becomes broader and splits into components as layers are added. This makes Raman spectroscopy a fast, non-destructive way to identify single, double and few-layer graphene (Ferrari et al., 2006). The D peak, near 1,350 cm⁻¹, indicates defects and edges, which matters for quality.

Other measurements complete the picture: atomic force and electron microscopy for thickness and flake size, X-ray photoelectron spectroscopy for oxygen content, and sheet resistance or conductivity of a pressed film or coating.

How it is made

• Chemical vapour deposition grows continuous films on metal foils, mainly for electronics. Excellent quality, but small volumes and high cost.
• Graphene oxide and reduced graphene oxide come from oxidising graphite with strong acids and oxidants, separating the layers, then reducing them again. Scalable, but the sheets keep many defects and oxygen groups and conduct far less well.
• Liquid-phase exfoliation shears or sonicates graphite in a liquid to peel off flakes. Scalable, but flakes tend to be small and thick.
• Electrochemical exfoliation uses graphite as an electrode. An applied potential drives ions and gas between the layers, which push them apart. In aqueous salt solutions this has given yields above 85% of sheets with three layers or fewer, flakes up to 44 micrometres across and little oxidation (Parvez et al., 2014). The method is fast, works in water at mild conditions and keeps the carbon lattice largely intact.

What it is good for

Used in small amounts, few-layer graphene improves the materials it is added to.

Conductive additives in batteries. Electrodes need a conductive network so electrons reach every active particle. Thin, flat sheets can form that network at lower loadings than carbon black, leaving more room for active material and improving rate performance.

Inks and printed electronics. Concentrated graphene inks can be painted or printed into conductive films for heaters, sensors, antennas and electrodes. Electrochemically exfoliated graphene has been brushed into films with a sheet resistance of 11 ohms per square (Parvez et al., 2014).

Barrier and anticorrosion coatings. Overlapping flat sheets make the path through a coating longer for water and oxygen.

Composites and thermal materials. A small percentage in plastics, rubber or concrete can raise stiffness, strength, thermal conductivity or make the material antistatic, and graphene fillers and films spread heat in electronics and battery packs.

What it isn't

• Not a replacement for graphite in anodes. Anodes need graphite particles that store lithium between their layers. Graphene acts as a helper.
• Not better in any amount. Properties depend on dispersion and on bonding to the surrounding material. Agglomerated graphene can make a product worse, and benefits often level off at low loadings.
• Not one material. The word covers everything from single sheets to thick platelets with different sizes, oxygen contents and defect levels. Results from one product say little about another.

Questions to ask a supplier

• How many layers, and was that measured by Raman, microscopy or both?
• What is the lateral flake size distribution?
• What is the oxygen content and the D to G peak ratio?
• What conductivity does a pressed film or coating reach?
• In what form is it supplied: powder, paste or dispersion, and in which medium?

Where Aeroborn fits

Aeroborn makes few-layer graphene by electrochemical exfoliation of graphite recovered from spent batteries, the second step of our Battery Recycling and Upgrading platform. The material has 1 to 10 layers, flakes up to 15 micrometres and a conductivity of about 1,200 S/cm. Request the data sheet.