The growing demand for graphene has led to an increasing rate of production. However, this increase in graphene production has caused a decrease in product quality. Some products labeled as graphene might actually be fine graphite powder. Identifying these fake graphene products requires a systematic test protocol.
Graphene is the rising star of the 21. century. Countless studies and innovations are based around this two-dimensional wonder material because of its unique abilities. A quick look into the properties of graphene would give you great mechanical strength, chemical stability, high thermal and electrical conductivity, optical transmittance, and high specific surface area. However, these promising properties of graphene are all based on theoretical calculations assuming pristine quality. In reality, all of these properties are greatly affected by the quality of the graphene flakes. The acceptable features of graphene are defined by the International Organization for Standardization (ISO). ISO identifies 2D graphene as a single layer, monocrystal made up of carbon atoms in a hexagonal structure. This version of graphene is accepted as single layered graphene. In addition to single layered graphene; bilayer graphene, few-layer graphene, and multi layered graphene are also commonly accepted by ISO and utilized in researches and product development. The thickness or number of layers is one of the most important factors affecting the quality and acceptability of graphene. The physical properties such as flexibility and tensile strength are especially sensitive to the thickness of graphene. In addition to thickness, carbon content, bond structure, and morphology of graphene products hold an important place in the quality of graphene. The quality and properties of graphene are greatly affected by the production method. Hence, it is important to get accustomed to common production methods and identify appropriate methods for specific requirements. The most common graphene production methods include chemical vapor deposition (CVD), liquid phase exfoliation of graphite, and consecutive oxidation (GO) and reduction of graphite (rGO). Amongst these methods, CVD is classified as a bottom-up approach while the other two are classified as top-down approaches. CVD is widely used to produce continuous graphene films using hydrocarbon gas as feedstock. This method is predicted to provide high quality graphene assuming optimum conditions are met. Liquid phase exfoliation is commonly used due to ease of operation. This method involves mechanical impact through shear stress and sonication. Even though it offers quality graphene, liquid phase exfoliation bears the possibility of unacceptable thickness. The exfoliation process never produces 100% monolayer graphene due to the randomness of the cleaving positions. The oxidation/reduction method provides easier separation of graphite layers leading to acceptable thickness. However, since graphene oxide (GO) shows different qualities than pristine graphene this method often results in impurities on graphene structure. All of these production methods can provide graphene with different thicknesses. Because different properties of graphene are affected on different levels by the layer number it is challenging to define a thickness limit. However, thermodynamic considerations dictate that a maximum of ten layers can behave as graphene crystal at room temperature. This is why producing and sourcing good quality graphene is an important issue especially with the continuously growing demand for graphene.
Read: 60 Uses of Graphene – The Ultimate Guide to Graphene’s (Potential) Applications
Avoiding and identifying “fake” graphene have become an issue as the market has grown and proper quality standards have not been established. The lack of standards for graphene has been affecting the development of graphene applications negatively due to the bad quality of the material sold in the open market. Studies on graphene from different producers show that most graphene products are merely fine graphite powders. Identifying fake graphene requires a systematic test protocol involving several different test methods. This protocol established by Kauling et. al. includes the use of atomic force microscopy (AFM), X-ray photo electron spectroscopy (XPS), Raman spectroscopy, optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). In the development of this protocol, the research group has investigated graphene products of 60 different companies. These technologies allow the investigation of graphene from every aspect. Atomic force microscopy is utilized to get information about the thickness of graphene flakes which is one of the most important features in categorizing the product. On the other hand, the size of the graphene flakes is observed through the use of optical microscopy. Raman spectroscopy takes an important role in identifying structural defects and any presence of GO or rGO providing a good idea about the quality of the product. Scanning electron microscopy and transmission electron microscopy provide further information on the morphology of the graphene product and help to confirm structural integrity. In addition to thickness, carbon content is also an important indicator of quality graphene products. The carbon content can be measure by utilizing X-ray photo electron spectroscopy. As a result of these quality control methods, quality graphene is expected to have a few nanometers of thickness, high carbon content close to 100%, and heavy sp2 bond density since crystalline graphene is expected to have 100% sp2 bonds. An unacceptable number of layers is often caused by production conditions. The low carbon content can be caused by contamination from the chemicals used in the production process. The content of sp3 bonds disturb the hexagonal structure of graphene lattice and result in structural defects. The reason for sp3 bonds could be absorbed hydrogen or the presence of transitional metals. Evidently, there are many factors affecting the quality of graphene products, and acquiring high quality requires a lot of effort, research, and investment. Furthermore, the characterization methods are complex and expensive for ordinary producers since the necessary equipment requires high technology to operate at nano scales. This is why companies can fail to deliver what is promised on the label and it is important to beware of fake graphene. The study of Kauling et. al. reports that none of the samples contained more than 50% of graphene at acceptable thickness. It is important to note that the study of Kauling et. al. does not include GO and rGO samples and a large number of the samples on the market labeled as graphene are actually GO and rGO.In order to reduce the amount of fake graphene out in the market, proper quality control mechanisms and improved production techniques must be established. On the other hand, it is important to realize that regardless of their thickness and impurities different versions of graphene products have useful purposes. The issue is mostly a quality control and labeling problem.
Graphene is the newborn superstar of materials sciences. Hence the characterization, classification, production, and utilization of graphene and graphene based products have been at the center of attention in the last few decades. The potential properties of this wonder material have been theorized and simulated in several different studies. However, achieving these properties has been a challenge due to the problems in achieving pristine graphene flakes. The production process of graphene greatly affects the quality and properties of graphene. Depending on the production method, different impurities or defects are observed in the graphene products. Since achieving optimum production conditions is difficult and expensive, most of the graphene products can fail to deliver what is promised on the label. This is why users should beware of the fake graphene. Standardization and quality control of graphene products hold an important place in this fight against fake graphene. The International Organization for Standardization has recently established the ground rules for the quality of graphene and the categorization of different graphene products. Single layered graphene, bilayer graphene, few-layered graphene, and multi layered graphene up to 10 layers are recognized as acceptable 2D nanomaterials capable of reflecting the theorized properties of graphene. The identification of fake graphene left outside of these categories involves a number of test methods. The structural and chemical integrity of graphene products can be tested by utilizing atomic force microscopy (AFM), X-ray photo electron spectroscopy (XPS), Raman spectroscopy, optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Graphene at desired qualities should have a thickness no more than a few nanometers, carbon content, and sp2 bonds at 100%, and no chemical impurities. It is no secret that current technologies cannot provide graphene products at this quality so it is normal if some of these standards are not met. However, some companies might take the easier route and label graphite powder as graphene for high prices. Using fake graphene can compromise scientific research and lead technological developments down to wrong paths. This is why it is important to identify fake graphene.
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