First demonstrated in 2008, [1] liquid-phase exfoliation (LPE) is a solution-processing method which is used to convert layered crystals into 2-dimensional nanosheets in large quantities. [2] It is currently one of the pillar methods for producing 2D nanosheets. [3] According to IDTechEx, the family of exfoliation techniques which are directly or indirectly descended from LPE now make up over 60% of global graphene production capacity. [4]
This method involves adding powdered layered crystals, for example of graphite, to appropriate solvents and inserting energy, often by ultrasonication, although high-shear mixing [5] is often commonly used. The addition of energy causes a combination of fragmentation and exfoliation resulting in the removal of small nanosheets from the layered crystals. [6] In this way graphite can be converted into large quantities of graphene nanosheets. [7] In general, these nanosheets tend to be a few monolayers thick and of lateral sizes ranging from tens of nanometers to many microns. [8] These dispersed nanosheets form quasi stable suspensions so long as solvents used have surface energies similar to that of the nanosheets. Dispersed concentrations of order 1 gram per litre can be achieved. In addition to solvents, it is also possible to use molecular stabilizers, for example surfactants or polymers to coat the nanosheets and stabilise them against regaggregation. [9] This has the advantage that it allows nanosheets to be suspended in water.
Although this method was first applied to exfoliate graphite to yield graphene nanosheets, it has since been used to produce a wide range of 2D materials including molybdenum disulfide, tungsten diselenide, boron nitride, nickel(II) hydroxide, germanium monosulfide, SnP3, and black phosphorus. The liquid suspensions produced by liquid phase exfoliation can be used to create a range of functional structures. For example, they can be printed into thin films and networks using standard techniques such as inkjet printing. [10]
Printed structures have been used in a range of applications in areas included printed electronics, sensors and nanocomposites. Related methods include exfoliation by wet ball milling, homogenization, microfluidization and wet jet milling. [11] Liquid phase exfoliation is different from other liquid exfoliation methods, for example the production of graphene oxide, because it is much less destructive, leaving minimal defects in the basal planes of the nanosheets. It has recently emerged that LPE can also be used to convert non-layered crystals into quasi-2D nanoplatelets. [12]
Liquid phase exfoliation was first described in detail in a paper by a research team in Ireland in 2008, [14] although a very short description of a similar process was published by the Manchester group around the same time. [15] While other papers had previously described methods to exfoliate layered crystals in liquids, [16] these papers were the first to describe exfoliation in liquids without any previous ion intercalation or chemical treatment.
LPE involves inserting layered crystals into appropriate stabilizing liquids and then adding energy to remove nanosheets from the layered crystals. A number of different methods have been used to supply energy to the liquid. The earliest and most common is ultrasonication. [17] In order to scaleup the process, high shear mixing was introduced in 2014. [18] This method proved extremely useful and inspired a number of other methods of generating shear in the suspension, including wet ball milling, homogenization, microfluidization and wet jet milling. [19]
The simplest stabilizing liquids are solvents with surface energy close to the layered crystal being exfoliated. In practice, liquids with surface tensions close to 70 mJ/m2 are used. [20] In addition aqueous surfactant solutions are often used. [21] Less common, but useful for certain applications, is using molecular or polymeric additives to stabilise the exfoliated nanosheets. [22] [23] [24]
A very wide range of 2D materials have been produced by LPE. The first material to be exfoliated was graphene in 2008. This was followed in 2011 by the exfoliation of BN, MoS2 and WS2. [25] Since, the a wide range of 2D materials have been exfoliated including molybdenum diselenide, tungsten diselenide, gallium sulphide, molybdemum trioxide, nickel(II) hydroxide, germanium monosulfide, SnP3, black phosphorus etc. [26]
Recent work has shown that liquid phase exfoliation can be used to produce 2D-nanoplatelets from non-layered 3D-strongly bonded bulk materials. [27] This is intuitively unexpected as these 3D-solid bulk crystals consists of strong bonds in all the three-directions. Nevertheless, many non-layered materials such as boron, silicon, germanium, iron disulfide, iron oxide, iron trifluoride, manganese telluride, have been converted to 2D nanoplatelets when sonicated in appropriate solvents. [28] This raises many open questions on the mechanism of liquid-phase exfoliation process. [29] For layered materials, the energy required to break inter-plane (perdominately van der Waals) bonds forces is small compared to that required to break in-plane ionic or covalent bonds. Then, the exfoliation procedure results in the formation of 2D-nanosheets. [30] However, for non-layered 3D-strongly bonded materials, with minimal difference in bonding between different atomic planes, there is no "easily exfoliated" direction and sonication should yield quasi spherical particles. [31] Nevertheless, near isotropic materials such as silicon have been exfoliated to give high-aspect ratio platelets. [32] Therefore, developing an understanding of the mechanisms by which non-layered materials are exfoliated will be important, in particular because the application scope of such nonlayered 2D-nanoplatelets is broad, ranging from biomedical applications to energy storage to opto-electronics. [33]
The mineral pyrite ( PY-ryte), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula FeS2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral.
Molybdenum disulfide is an inorganic compound composed of molybdenum and sulfur. Its chemical formula is MoS
2.
A bilayer is a double layer of closely packed atoms or molecules.
Graphene is an allotrope of carbon consisting of a single layer of atoms arranged in a hexagonal lattice nanostructure. The name is derived from "graphite" and the suffix -ene, reflecting the fact that the graphite allotrope of carbon contains numerous double bonds.
Phosphorene is a two-dimensional material consisting of phosphorus. It consists of a single layer of black phosphorus, the most stable allotrope of phosphorus. Phosphorene is analogous to graphene. Among two-dimensional materials, phosphorene is a competitor to graphene because it has a nonzero fundamental band gap that can be modulated by strain and the number of layers in a stack. Phosphorene was first isolated in 2014 by mechanical exfoliation. Liquid exfoliation is a promising method for scalable phosphorene production.
Tungsten disulfide is an inorganic chemical compound composed of tungsten and sulfur with the chemical formula WS2. This compound is part of the group of materials called the transition metal dichalcogenides. It occurs naturally as the rare mineral tungstenite. This material is a component of certain catalysts used for hydrodesulfurization and hydrodenitrification.
A two-dimensional polymer (2DP) is a sheet-like monomolecular macromolecule consisting of laterally connected repeat units with end groups along all edges. This recent definition of 2DP is based on Hermann Staudinger's polymer concept from the 1920s. According to this, covalent long chain molecules ("Makromoleküle") do exist and are composed of a sequence of linearly connected repeat units and end groups at both termini.
A nanosheet is a two-dimensional nanostructure with thickness in a scale ranging from 1 to 100 nm.
Jonathan Coleman is the Erasmus Smith's Professor of Natural and Experimental Philosophy in the School of Physics and a Principal Investigator in CRANN at Trinity College Dublin. Coleman's research focuses on solution-processing of nanomaterials and their use in applications. He is most well known for the development of liquid phase exfoliation, a widely used method for preparing two-dimensional nanosheets.
Molybdenum diselenide is an inorganic compound of molybdenum and selenium. Its structure is similar to that of MoS
2. Compounds of this category are known as transition metal dichalcogenides, abbreviated TMDCs. These compounds, as the name suggests, are made up of a transition metals and elements of group 16 on the periodic table of the elements. Compared to MoS
2, MoSe
2 exhibits higher electrical conductivity.
Transition-metal dichalcogenide (TMD or TMDC) monolayers are atomically thin semiconductors of the type MX2, with M a transition-metal atom (Mo, W, etc.) and X a chalcogen atom (S, Se, or Te). One layer of M atoms is sandwiched between two layers of X atoms. They are part of the large family of so-called 2D materials, named so to emphasize their extraordinary thinness. For example, a MoS2 monolayer is only 6.5 Å thick. The key feature of these materials is the interaction of large atoms in the 2D structure as compared with first-row transition-metal dichalcogenides, e.g., WTe2 exhibits anomalous giant magnetoresistance and superconductivity.
In materials science, the term single-layer materials or 2D materials refers to crystalline solids consisting of a single layer of atoms. These materials are promising for some applications but remain the focus of research. Single-layer materials derived from single elements generally carry the -ene suffix in their names, e.g. graphene. Single-layer materials that are compounds of two or more elements have -ane or -ide suffixes. 2D materials can generally be categorized as either 2D allotropes of various elements or as compounds.
A two-dimensional semiconductor is a type of natural semiconductor with thicknesses on the atomic scale. Geim and Novoselov et al. initiated the field in 2004 when they reported a new semiconducting material graphene, a flat monolayer of carbon atoms arranged in a 2D honeycomb lattice. A 2D monolayer semiconductor is significant because it exhibits stronger piezoelectric coupling than traditionally employed bulk forms. This coupling could enable applications. One research focus is on designing nanoelectronic components by the use of graphene as electrical conductor, hexagonal boron nitride as electrical insulator, and a transition metal dichalcogenide as semiconductor.
A rapidly increasing list of graphene production techniques have been developed to enable graphene's use in commercial applications.
Boron nitride nanosheet is a two-dimensional crystalline form of the hexagonal boron nitride (h-BN), which has a thickness of one to few atomic layers. It is similar in geometry as well as physical and thermal properties to its all-carbon analog graphene, but has very different chemical and electronic properties – contrary to the black and highly conducting graphene, BN nanosheets are electrical insulators with a band gap of ~5.9 eV, and therefore appear white in color.
Two dimensional hexagonal boron nitride is a material of comparable structure to graphene with potential applications in e.g. photonics., fuel cells and as a substrate for two-dimensional heterostructures. 2D h-BN is isostructural to graphene, but where graphene is conductive, 2D h-BN is a wide-gap insulator.
Andrea Carlo Ferrari is a professor of nanotechnology at the University of Cambridge.
In material science, layered materials are solids with highly anisotropic bonding, in which two-dimensional sheets are internally strongly bonded, but only weakly bonded to adjacent layers. Owing to their distinctive structures, layered materials are often suitable for intercalation reactions.
Jaime C. Grunlan is a material scientist and academic. He is a Professor of Mechanical Engineering, and Leland T. Jordan ’29 Chair Professor at Texas A&M University.
Exfoliation is a process that separates layered materials into nanomaterials by breaking the bonds between layers using mechanical, chemical, or thermal procedures.