In chemistry, exfoliation is a process that separates layered materials into nanomaterials by breaking the bonds between layers using mechanical, chemical, or thermal procedures.
While exfoliation has historical roots dating back centuries, significant advances and widespread research gained momentum after Novoselov and Geim's discovery of graphene using Scotch tape in 2004. Their Nobel Prize-winning research primarily relied on mechanical exfoliation for the production of graphene which sparked an immediate interest in the exfoliation process. Today, exfoliation is regarded as the most widely used nanomaterial production technique.
Exfoliation typically involves breaking weak bonds called van der Waals bonds to create two-dimensional materials, such as graphene or transition metal dichalcogenide monolayers. While various reversible chemical processes, such as intercalation can disrupt the weak bonds in a lamellar structure and introduce guest species, many of them fail to produce single-sheet materials as the processes are not strong enough to cancel the interlayer attractions. However, during exfoliation, the high energy input leads to an extreme bond-breaking process that irreversibly separates the layers into single sheets. Lately, it has been shown that if the energy input is substantial enough, the procedure can even break much stronger, bonds such as metallic or ionic bonds to create non-van der Waals materials like hematene or other nanoplatelets.
Exfoliation has practical applications in fields such as electronics, biomedicine, and aerospace engineering. It can be used to create materials with properties tailored for specific uses such as efficient energy storage or lightweight and durable materials.
History While the use of exfoliation can be traced back to ancient Chinese and Mayan pottery, the earliest scientific work involving exfoliation was the production of vermiculite by Thomas H. Webb, in 1824. However, during this early period, no substantial research was conducted to understand the nature of the mechanisms that facilitated these reactions. Arguably, the first research that delved into the mechanism of the process rather than its usage was Brodie's work, which revealed that certain acids produced lamellar carbon structures in 1855. Despite this discovery, extensive research on the topic did not immediately follow. The exfoliation concepts we have today were not developed until the realization that graphite absorbed alkali metals in 1926. This discovery laid the groundwork for a more solid theoretical framework, enabling scientists to apply the method in their production processes. One method that made use of this theoretical background and eventually led to further development of the process as a production technique was Rüdorff and Hoffman's work, which introduced an electrochemical method for exfoliation in 1938. The development of electrochemical exfoliation piqued the interest of more researchers and more people started regarding the process as a production technique. One of the most notable examples of the success of the method as a mass production technique was the invention of the first commercial lithium carbon fluoride batteries in 1973.
The real turning point for exfoliation research came in 2004 when Novoselov and Geim isolated graphene through mechanical exfoliation using Scotch tape. This innovative research earned them the Nobel Prize in Physics in 2010, reigniting the interest in exfoliation methods. In subsequent years, numerous processes were developed for more precise manufacturing and higher yields. While most of the exfoliation research focused on graphite and graphene during the last few decades, recently, the rather difficult processing of graphene and its lack of an obvious band structure led many research groups to begin working on different elements to utilize exfoliation for the production of other nanomaterials. One of the most significant breakthroughs of this new wave of research has been the discovery of non-van der Waals nanoplatelets. This discovery demonstrated that exfoliation could occur without relying on weak bonds, which opened up new and promising applications in the industry.
Types The exfoliation process is typically applied to lamellar structures with weak bonds. While these bonds are weak enough to be easily broken by an external force, they are strong enough to not separate into single layers. In order to separate the material into single layers, the attraction between consecutive layers must be overcome. As the interest in exfoliated material research surged many researchers started to develop new and better ways to overcome these interlayer attractions. Despite the high number of methods it is possible to classify them into three distinct categories based on the source of energy used in the process: mechanical, chemical, and thermal exfoliation.
Mechanical exfoliation In mechanical exfoliation, external forces act upon the material, breaking the bonds due to the stress accumulated within the material. Depending on the intensity and the specific nature of the phenomenon, these forces break the van der Waals forces, separating materials into 2D nanostructures. Sometimes, a solvent is introduced to the material to facilitate complete breakdown, as liquid environments significantly reduce bond strength compared to vacuum conditions.
While mechanical exfoliation is effective in separating the layers, it lacks predictability and systematic results. The process requires repetition until individual layers are achieved. To obtain consistent nanomaterials with specific properties, experimentation and fine-tuning of conditions based on the results is required. Therefore, mechanical exfoliation techniques are rather empirical, and most mathematical models rely on empirical results rather than the ab-initio calculations.
The original method proposed by Novoselov and Geim, micromechanical cleavage, was essentially a mechanical exfoliation method. Consequently, mechanical exfoliation methods were developed more rapidly than the others. Major mechanical exfoliation methods include micromechanical cleavage and liquid phase separation.
Micromechanical cleavage Micromechanical cleavage is the original graphene production method proposed by Novoselov and Geim. This process involves using sticky tape to get graphite samples and separating the layers until getting a single layer. Although the process yielded high-purity single-layered materials, it fell out of favor quickly as it required several repetitions for a single layer of graphene and was likely to damage the graphene layers during the process.
Liquid phase separation Liquid phase separation is one of the most widely used exfoliation methods. Its high yields, high purity, and scalability make it one of the most preferred exfoliation methods. It works by providing a liquid medium for the mechanical exfoliation methods. The liquid medium significantly reduces the strength of the bonds in the material compared to vacuum conditions, making it easier for mechanical forces to break the weak bonds in the material. However, due to the interfacial tension forces, liquid phase separation does not always yield uniform results. When the tension forces do not balance out, graphene single layers may break due to the tension forces. To achieve relatively uniform results, the overall energy of the system must be minimized. The best way to optimize this condition is to use solvents with similar surface tensions to the material of interest. Liquid phase separation utilizes various external forces to break the van der Waals forces. The most widely used liquid phase separation methods include sonication, which uses sound waves, and shearing, which uses shear forces.
Sonication The sonication method utilizes ultrasonic sound waves to create micrometer-sized bubbles in liquid environments. When these bubbles reach a critical size, they collapse with an instantaneous temperature of 5000 K, a local pressure of 20MPa, and a heating/cooling rate up to 109 K s−1.These sudden physical differences create shock waves that can act on lamellar materials and break the weak forces in between the layers. Although sonication is a long-known laboratory technique, its implementation into graphene exfoliation was in 2008 and it led to liquid exfoliation becoming the predominant technique.
While sonication is generally used as an exfoliation method on its own, it is also used as a further processing method to perfect the nanoflakes created with other exfoliation methods. Therefore it is a common technique used in combination with the other methods. One disadvantage of sonication is the reaction time though. A complete exfoliation reaction may take days to finish. However, prolonged exfoliation times allow the creation of more stable solutions, making long sonication times favorable for obtaining purer, defect-free products. Nanomaterials created with sonication yield 1.5 times larger unperturbed particle size.
Shearing The shearing method makes use of lab mixers to exfoliate lamellar structures into single-layered nanomaterials. Lab mixers create a sufficient shear force that allows consecutive layers of the material to slip over each other. Which produces massive quantities of highly pure material. Although the shearing method was widely used as a further processing method to break up relatively larger clusters of nanomaterials into single layers, before, in 2010, it was introduced as a direct method to exfoliate graphite into graphene. Later studies confirmed the applicability of the method to other lamellar materials such as h-BN, MoS2, WS2, MoSe2, and MoTe2.
While this method has a high yield and purity among the other exfoliation methods, its known linear relations with concentration, mixing time, rotor speed, rotor diameter, and inverse relation with liquid volume, gives one of the best controllability out of all the exfoliation methods. This innovative procedure has been adapted for household kitchen mixers, significantly reducing the costs and complexity of the exfoliation methods, thereby sparking another wave of research in layered structures.
Chemical exfoliation Chemical exfoliation employs the intercalation process to separate material layers. During intercalation, guest ions or free electrons are introduced to the layers, disrupting the bond structure and forming new bonds. For example, in the case of van der Waals forces, which are common in chemical exfoliation, positive and negative regions are induced, attracting ions. Given that the bonds between layers are weak, they tend to break, forming new, stronger bonds with these ions. Typically, these stronger bonds lead to the creation of functional groups that significantly reduce interlayer attractions. At this stage, the interlayer attraction becomes low, and thanks to the ability of the functional groups to decompose with further processing, the layers can be easily separated.
Chemical exfoliation's scalability advantages over other production methods have made it one of the most widely used techniques. In addition to its scalability, the variety of chemicals available plays an important role in encouraging researchers to explore various production methods. Chemical exfoliation is also commonly used in combination with mechanical and thermal exfoliation methods to obtain purer results. The most widely used chemical exfoliation methods are chemical vapor deposition, graphite oxide reduction, and electrochemical exfoliation.
Chemical vapor deposition