With the emerging graphene industry growing and industrially manufactured graphene materials being traded worldwide, associated quality regulation issues have also risen. The lack of rapid, reliable, and cost-effective quality control for these materials has become an alarming problem due to the large number of materials traded globally with a significant variation in material properties.
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The benchmark for graphene characterization methods is currently dependent on localized analysis on critical properties such as the number of layers, particle size, and defects. This is only on individual graphene particles and not the mass material traded and used worldwide. However, the development of thermogravimetric analysis addresses and overcomes the limitations presented by the novel graphene material. This article will explore the issues underlying graphene manufacturing and how thermogravimetric analysis can be used to overcome current challenges.
What is Graphene?
Graphene, first discovered in 2004, is a novel material that has disrupted the nanotechnology industry. This allotrope of carbon is known to be the thinnest slice possible of graphite and approximately one atom thick. Its properties consist of but are not limited to tremendous physical strength, flexibility, and good electrical and heat conduction at room temperature.
These outstanding properties have made graphene an attractive candidate for various applications such as electronics, energy storage, protective coatings, sensors, and medicine. However, several challenges have arisen with the mass production of this novel material.
Quality Control
The lack of quality control in the graphene industry has become a challenge for this global market, with many start-up companies trading unregulated graphene materials. This results in a key barrier for the uptake of graphene into more established mass industries.
A surprising result from recent comprehensive studies investigating the quality of industrially produced graphene materials has uncovered that a large volume of materials on the market do not contain graphene. The studies have illustrated that these faux materials are a mixture of few-layer graphene (FLG), consisting of non-exfoliated graphitic and alternative carbon-related particles or containing impurities from the production process.
Current Quality Control Measures
The properties of FLG materials have recently been redefined by the International Organization for Standardization (ISO) due to previous confusion amongst the graphene community. This outline of FLG materials by the ISO consists of these materials being two-dimensional and consisting of three to ten well-defined stacked graphene layers. However, the investigations of the FLG materials found in the global market do not meet these guidelines laid out by the ISO, further outlining the need for quality control measures for the growth and reliability within the graphene industry.
There have been many regulating methods that have attempted to address this global quality issue, such as ISO graphene terminology standards and technical reports on the properties of graphene materials.
The European Graphene Flagship, National Standardization bodies such as the British Standards Institute, and National Metrology Institutes have also created supporting documents and guidance on quality control to aid graphene manufacturers and end-users to meet regulating protocols.
Characterization Techniques for Graphene
The current ISO graphene measurement standards recommend advanced characterization techniques to assess the metrology of graphene materials. These characterization techniques include transmission electron microscopy (TEM), Raman spectroscopy, and atomic force microscopy (AFM), which can assess the properties of graphene materials. However, these localized characterization methods can only probe small areas such as single graphene particles or chemical vapor deposition graphene layers.
Characterization technologies are limited in characterizing graphene powders or measuring key properties at large bulk quantities. Other disadvantages of using these techniques also include being time-consuming, lack of affordability, and requiring skilled personnel to perform the techniques.
Due to typical graphene manufacturers being small start-up companies, the quality control of graphene powders is usually outsourced to academic institutions. These institutions use a small sample and localized testing of a few selected graphene particles which are then used to make up the basis for claims of properties of larger-scale graphene products.
The lack of uniform testing of all particles and the inappropriate association of the properties for a larger scale product translates to this level of testing being unreliable, especially for the properties of graphene powders produced in industrial-scale volumes. This can ultimately lead to a lack of trust and confidence in the claims of graphene manufacturers, which further leads to the requirement of an alternative analytical tool for quality control.
Thermogravimetric Analysis for Determining Fake Graphene
An affordable, simple, and more reliable alternative method of analysis is required to change the quality control measures inappropriately used in the graphene industry.
A research collaboration between The University of Adelaide, Australia, and the National Physical Laboratory has led to the development of a validated analytical tool, thermogravimetric analysis (TGA), for the characterization and quality control of FLG and non-graphene impurities in powder form. TGA is typically used to assess and characterize the thermal properties and impurities of minerals, polymers, and carbon materials. However, its use as a characterization tool for regulating the quality of graphene materials has been overlooked.
The study led by Farzaneh Farivar illustrated the efficacy of utilizing TGA for qualitative and quantitative analysis of graphene materials, which could be the basis of quality control of these materials on the market, further decreasing the spread of ‘fake graphene’.
TGA is a method of thermal analysis that can assess the changes in physical and chemical properties through temperature increases. The research by the Australian team helped establish a baseline of TGA and first derivative (DTG) characteristics of bulk graphene powder forms with specific analytical parameters being set for reliable quality control of graphene materials. The team recognized differences between FLG and ‘fake graphene’ using samples such as FLG, reduced graphene oxide (rGO), and graphene oxide (GO).
This research illustrated that that FLG, graphene oxide, and graphite powders have distinctive graph peaks and have a temperature of maximum mass decomposition rates found to be in specific ranges. The use of TGA enables the structural, chemical, and thermal differences to be quantified between the different powders, which further signifies the efficacy of utilizing this analytical method as a quality control tool.
Using this method, graphene manufacturers could ensure the industrially produced graphene powders they are trading meet the quality regulations of graphene materials rather than ‘fake graphene’, ensuring the product retains the appropriate properties expected of graphene.
A quality control method such as TGA is an affordable and reliable approach used by small start-up companies attempting to disrupt the graphene industry. This regulation method can be added to the current guidance provided by the ISO to ensure ‘fake graphene’ materials do not overtake this novel market, hindering the growth of the graphene industry and the advancement of technology as a whole.
References and Further Reading
Farivar, F., Yap, P., Hassan, K., Tung, T., Tran, D., Pollard, A. and Losic, D., 2021. Unlocking thermogravimetric analysis (TGA) in the fight against “Fake graphene” materials. Carbon, 179, pp.505-513. DOI: https://doi.org/10.1016/j.carbon.2021.04.064
Thomas, S., Thomas, R., Zachariah, A. and Mishra, R., 2017. Thermal and rheological measurement techniques for nanomaterials characterization. Elsevier.
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