Liquid Phase Exfoliation of Graphite to Obtain Pure Graphene
Although graphene has some fascinating properties that have real-life applications, pure forms of this ‘wonder material’ is elusive. Although a multitude of manufacturing techniques are available, many of them have poor control over the shapes, sizes or quantity of graphene sheets. Liquid phase exfoliation is a method that has the potential to produce pure, singular sheets of graphene at industrial scales.
Table of Contents
Graphene From Graphite
Why is Graphene so Special?
Graphene is a simple network of carbon atoms that are arranged in a hexagonal pattern, forming a single, two-dimensional sheet. Since its isolation in 2004, there have been tremendous research efforts to understand and manufacture graphene for use in ‘real world’ applications1. Its unique electronic properties make it an ideal material for semiconductors, considered to be a future replacement for silicon. Electronic devices based on graphene have the potential to be smaller, faster, more efficient and even more flexible than current counterparts.
The reason for its great electric conductivity is due to electrons moving very easily and very quickly in graphene – it is known as a ‘zero-energy band gap semiconductor’2. However, the big challenge that stands in the way of its commercialization is its manufacturing. Graphene is a fickle molecule, with tiny changes in its structure changing the way it behaves in a sometimes unpredictable way. If we want to harness its power, we must first be able to produce pure graphene consistently.
There are two general approaches to making graphene – bottom-up and top-down. Bottom-up methods make use of molecular building blocks to control its synthesis, carefully piecing smaller pieces together to obtain a final structure. On the other hand, top-down approaches involve whittling down its bulk material – graphite – into single sheets of graphene.
In principle, bottom-up techniques provide high-quality sheets of graphene through controlled assembly processes. However, these methods are usually difficult to scale up to industrially useful quantities. They are also generally limited to smaller pieces of graphene, as large sheets of graphene become less soluble with increasing molecular weight3. Once molecules become too large and insoluble, they are more difficult to react.
Using top-down methods to obtain graphene circumvents many of these issues, with the added advantage that bulk graphite is cheap to obtain. The downside to this is that there is less control over the products that are formed, as they are governed by mechanical forces rather than chemical synthesis. However, certain classes of top-down methods have been shown to be useful in obtaining pure, size-consistent graphene flakes.
Exfoliation of Graphite
Shearing away sheets of graphene from bulk graphite is a process known as ‘exfoliation’, much like how we scrape dead skin off our bodies with scratchy rocks. In the graphene world, exfoliation first came to light when some adhesive tape was used to peel graphene layers away from some pencil smudges! Of course, there are many more sophisticated methods available today that accomplish graphene exfoliation under a variety of conditions.
Simple shearing techniques such as sonication of the bulk material provide more control over the mechanical forces that separate individual sheets of graphene. Exfoliating graphite oxides (sticking oxygen groups to the end of the carbon) turns out to be much more efficient than just graphite on its own. Oxides of graphite – carboxyls, hydroxyls, phenols and epoxides – disrupt the flat, planar structure of graphene, making the separation between the layers easier through electrostatic repulsion.
Liquid Phase Exfoliation (LPE)
Liquid phase exfoliation (LPE) processes are another improvement upon existing exfoliation technologies. They encompass a range of versatile exfoliation methods, including solvothermal assisted LPE (SALPE), electrochemical LPE (ELPE), high shear mixing (HSM) and ultrasound induced LPE (UILPE). LPE techniques are generally cost-effective and can be easily scaled up, hence they are of great interest. High yield, high-quality production of graphene is crucial for both the study of its properties as well as for future ‘real world’ commercial applications.
How Liquid Phase Exfoliation Works
The principle of LPE lies in assisting the separation between graphene layers. In graphite, they are held together by strong electrostatic attractions that require a large amount of mechanical force to separate. One way to reduce this energy input is to disrupt the attractive forces holding layers of graphene together. This is done by first immersing the bulk material in a special liquid, followed by exfoliation, hence LPE.
The liquids that work best in LPE are aromatic molecules with electron-withdrawing groups, making the structure electron deficient. They can then slip between the layers of graphene and disrupt the electrostatic forces holding them together. The mechanism of this insertion is likely to be charge transfer through π-π stacking.
These organic molecules do not act strictly as ‘exfoliating agents’ as they are not involved in the actual separation of the graphene flakes. Rather, they adsorb onto the graphene surfaces during and after the flakes have been exfoliated, preventing re-aggregation of the sheets. LPE is extremely versatile in that other 2D materials can also be separated in this manner, such as transition metals and other ‘graphene-like’ structures.
Solvent Choice in LPE
Since surface tension is the key factor in maintaining separation of the single sheets, solvent choice plays a big role in LPE. The most common solvents currently in use for exfoliation are N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF) and ortho-dichlorobenzene (o-DCB) with high surface tensions (of around 40 mJ m-2)5. However, these organic solvents can cause irritation and toxicity to organs and hence the search for alternative solvents is an area of active research. One promising alternative that has been investigated is the use of hexafluorobenzene and benzene in a 1:1 ratio, producing graphene dispersions at high concentrations (50 mg ml-1)6.
It is worth noting that graphene dispersion concentration is the standard of benchmarking different graphene products. However, since the lateral flake size, initial concentration, solvent volume and other experimental conditions differ from case to case, they must also be taken into account when assessing the overall viability of the technique.
In addition to quantitative descriptions, it is worth looking at qualitative representations of the graphene flakes. This can be done using electron microscopy, atomic force microscopy or Raman spectroscopy. In high-resolution transmission electron microscopy (HR-TEM) for example, both the number of layers as well as information on the electron diffraction patterns can be elucidated. Raman spectroscopy has also been found to be useful in identifying structural defects and electronic structure, including chemical modifications7.
Solvothermal assisted LPE (SALPE)
In some cases, energy in the form of heat can improve the efficiency of liquid phase exfoliation. Monolayer and bilayer graphene sheets have been produced using solvothermal-assisted LPE (SALPE) techniques8. The graphite was first heated at 1000 °C to expand the space between the layers, before mixing with acetonitrile and heated again, so that the solvent penetrates between the graphene interlayer spaces. The mixture was then sonicated, producing both monolayer and bilayer graphene sheets, showing that the acetonitrile was able to induce dipole interactions within graphene sheets in bulk graphite.
The introduction of liquid phase exfoliation of graphene from bulk graphite presents an extremely useful and versatile top-down approach to producing high quality samples. LPE techniques have several advantages, including a relatively low cost for high yield and the ease of scaling up. High yield and high-quality graphene production methods are crucial for harnessing of graphene’s properties for future applications as a material with various applications.
Cover image by Adrien Nicolaï/RPI (Source)
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