An introduction to graphene dispersions and powders

Introduction

Graphene is the name given to a single, pristine, isolated atomic layer of graphite. When stacked in an orderly manner, graphene forms it parent material graphite, found in the tip of every pencil and in nature.

As with many nanomaterials, graphene can by fabricated using either bottom-up or top-down techniques. A widely used bottom-up method for graphene synthesis involves catalytically decomposing a gaseous carbon source, typically a hydrocarbon, onto a metallic surface at elevated temperatures. The resulting material is normally a polycrystalline film, which can be separated from the metal using various techniques beyond the scope of this white paper. The top-down technique, which we will discuss here, consists of exfoliating the stacked graphene layers from within the graphite, preserving the excellent properties provided by the crystalline structure of the parent material.

Exfoliation techniques

There are many techniques that enable the exfoliation of graphene from graphite. The most commonly used techniques within the scientific community were sonication directly from graphite and the production of graphene through reduction of graphene oxide. Chemical intercalation provides an alternative, but although usually associated with higher typically has detrimental effects on the quality of graphene. In this section we will discuss some of these techniques.

Reduction of graphene oxide

Graphene oxide is normally synthesised using Hummers' method. The resulting graphene oxide dispersions and powders are, however, not conducting and as such require a de-oxygenation step, known as reduction. The relatively routine way to obtain gram quantities of graphene oxide from Hummers' method enables laboratories around the world to investigate graphene materials. Graphene oxide has its own set of interesting properties and is thoroughly discussed in another BGT Materials white paper entitled "An introduction to graphene oxide and its derivatives".

Sonication

Sonication was the first technique where gram quantities of pristine graphene were successfully isolated from graphite ¹. Sonication applies shear forces to graphite and by carefully selecting the solvent, the graphene-solvent interaction enables the exfoliation of graphene layers.  Frequently used solvents include n-methyl-2-pyrrolidone (NMP) and dimethylformamide (DMF), with the former being the most common. Sonication produces high quality material with no visible oxidation. However, the high power consumption coupled with the low yield means this method is unsuitable for industrial production.

Chemical intercalation

Figure 1. BGT Materials graphene suspensions in liquid (left) and example of Grat-G1 flakes deposited on an oxidized silicon substrate.

Another popular technique for exfoliating graphene from graphite is chemical intercalation. By inserting ions in between the graphene layers in graphite, the interlayer spacing increases greatly, forming a graphite intercalation compound (GIC). The unusual optical, electrical and magnetic properties of GICs have been studied for decades, and certain GICs have been found to show superconductivity at low temperatures.

Applications

Once graphene is isolated from graphite there are many possible applications that can be envisaged, ranging from water filtration, through energy storage in batteries and supercapacitors, providing fillers to mechanically reinforce plastics or improve their electrical or thermal conductivity, to fabricating flexible and transparent electrodes. Because of the large field of possible applications, there is not a specific graphene material that can enable all of them. In contrast to a "one product fits all" approach, in reality it is a often quite a challenging process to find the correct graphene material for the intended application.

Composites

Electrically conductive polymer

Making polymers conductive first of all requires an electrically conductive graphene additive. As such, graphene oxide is not suited since it is electrically insulating. Reduced graphene oxide and pristine graphene materials may instead be used. The size of the flakes needs to be as large as possible in order to minimise the contact resistance, so that electrical current can easily travel from one flake to the next. Typically, adding up to 10 wt% graphene to a polymer exceeds the electrical percolation threshold, increasing the polymer's electrical conductivity by orders of magnitude².

Mechanically reinforcement of polymers

Mechanical reinforcement of polymers challenges graphene materials in a different manner. Effective stress transfer between the graphene filler and polymer matrix is required, necessitating an even distribution of graphene flakes throughout the polymer, as agglomeration will hinder the performance of the composite. Meanwhile, the viscosity of a polymer/graphene mixture increases with the graphene loading and flake size3. Since the composites must offer low enough viscosities to remain processable using existing tooling,

Figure 2 300% increase of Young modulus of PVA reinforced by BGT Materials Grat-G1.

a balance must be reached between the stress transfer and viscous requirements.One promising technique to achieve this balance is by decorating a pristine graphene layer with chemical groups (functionalisation), which can anchor into the polymer matrix, greatly aiding the interfacial stress transfer. These chemical groups can either be applied to graphene flakes during a separate treatment stage or be intrinsic to the exfoliation technique. The presence of small amounts of O containing groups ( ＜10at% ) in our graphene Grat-G powder series means that these material are ideal for plastic reinforcement.

Conductive inks

Another area where graphene can make a significant impact is conductive ink. Conductive ink comprises a liquid containing a conductive material, normally in the form of small metallic particles, which can be printed and then cured or heated to create electrically conductive patterns. Graphene-based conductive inks typically use a graphene loading between 5 wt% - 15 wt %, and can be air-dried, eliminating the need for the extra processing steps required by metallic-based conductive inks (e.g. lithographic and etching steps). Note that the graphene used in such systems is intrinsically different than that used for mechanically reinforcing polymers. Here, multilayer graphene with short lateral dimensions is preferred, in order to achieve the desired electrical performance whilst providing viscosities suitable for printing.

Figure 3. BGT Materials graphene ink (Product shown: Grat-Ink 101B)

 

The sheet resistance, an important figure of merit for these printed materials, can be under 10 Ohms per square for films 25 μm thick. Both water-based and waterproof inks are manufactured by BGT Materials for applications including preparing conductive papers, EMI shielding, and thermal dissipation among others. Our work in collaboration with University of California, Riverside showed that coating PET with graphene ink increases the thermal conductivity by a factor of 600⁴.

Energy storage

Graphene materials are ideal for energy storage. They have a  high surface area, are very light and at the same time enjoy very high electrical conductivity. Particularly important is the role of graphene in Si-enhanced graphene anodes where graphene acts as a structurally rigid network to prevent silicon from  swelling. At the same time it provides  an excellent electrically conducting network for energy storage. Because graphene is also very light and enjoys a large surface area, it is an important material in electrochemical capacitors, or supercapacitors⁵

Bibliography
1. Hernandez, Y. et al. High-yield production of graphene by liquid-phase exfoliation of graphite. Nat. Nanotechnol. 3, 563–8 (2008).
2. Stankovich, S. et al. Graphene-based composite materials. Nature 442, 282–6 (2006).
3. Corcione, C. E., Freuli, F. & Maffezzoli, A. The aspect ratio of epoxy matrix nanocomposites reinforced with graphene stacks. Polym. Eng. Sci. 53, 531–539 (2013).
4. Malekpour, H. et al. Thermal conductivity of graphene laminate. Nano Lett. 14, 5155–61 (2014).
5. Bonaccorso, F. et al. Graphene, related two-dimensional crystals, and hybrid systems for energy conversion and storage. Science (80-. ). 347, 1246501–1246501 (2015).