Graphene and related 2D materials have the potential to disrupt technologies such as energy storage devices, composites, and electronics, through their exceptional material properties. Dependant on the material these can include properties such as high electrical conductivity, high mechanical strength, high thermal conductivity, etc.
However, when 2D materials are produced at industrial scale, their properties can differ from materials produced in a laboratory.
Liquid phase exfoliation (LPE) is one of the most widely used methods for producing 2D materials at scale. With this method, bulk powders, such as graphite, are perturbed in an organic solvent to generate shear forces and break the 2D layers apart into smaller nanomaterials – known as exfoliation. However, nanoplatelet dispersions produced with this method often contain unexfoliated bulk particles, exhibit a large variation in particle size, and the surface chemistry can vary with the addition of functional chemical groups introduced during synthesis.
These properties can all affect the performance of these materials when they are embedded in final products. This means that to optimise the performance of 2D materials and enable their commercialisation, we need to be able to measure the chemical and structural properties of nanoplatelets in dispersions.
Currently, several analysis techniques can be used to measure critical material properties of 2D materials, with several of these methods now standardised. However, these techniques can often be lengthy, expensive, measure only a limited sample population, require specialist knowledge to operate, and require dry samples, which make them unsuitable for use in a manufacturing facility.
There is a strong need for the development of rapid and cost-effective tools to efficiently measure 2D materials properties in liquid dispersions and allow for their optimisation. Ideally, the measurement should be performed at the production line to enable rapid feedback and optimisation of exfoliation parameters for product development.
To address this need, NPL developed a new method for the characterisation of 2D materials in dispersion using Nuclear Magnetic Resonance (NMR) proton relaxation. This technique measures the relaxation time of protons in solvent molecules, which we have demonstrated can be correlated to changes in specific surface area or surface chemistry of 2D materials in dispersion. The advantages of NMR proton relaxation are the relatively low cost, fast analysis time (in the order of seconds), and the fact that materials can be measured directly in a liquid, allowing it to be operated at the production line.
NPL validated this method by comparing graphene properties measured with NMR proton relaxation to several more accurate characterisation techniques, as detailed in two publications focussed on the structural properties and chemical properties. During a recent visit at the Graphene Engineering Innovation Centre (GEIC) at the University of Manchester, NPL demonstrated the use of this technique to several 2D material manufacturers with a practical demonstration of the potential of this technique for the characterisation of industrial materials. A short video was filmed during this visit: YouTube Link.
The use of a benchtop NMR relaxation instrument with real-world industrial samples, demonstrated the feasibility to use this method for the rapid quality control of 2D materials in liquid dispersions. This method is relatively low-cost and offers companies a solution to monitor the degree of exfoliation of their dispersions without the need for more advanced, expensive characterisation tools. This solution could help in tuning the exfoliation parameters employed, and ultimately optimise the material for selected applications. The optimisation of 2D materials for advanced applications, such as the reduction of carbon emissions, will ultimately lead to huge savings for companies and the development of more efficient real-world products.
NPL is now working with instrument manufacturers, as well as material producers, to optimise the method and move it out of the laboratory to become part of the industrial process itself.
Sofia Marchesini, Higher Research Scientist, NPL said: “I am excited about the potential of this technique as a quality control process to monitor the manufacturing of nanomaterials and product formulations. These measurements are fast to perform and the setup is compatible with flow-through experiments, which means it could be integrated into an industrial production line.”