WO2017158334A1

WO2017158334A1 - Mechanical exfoliation of 2-d materials

Info

Publication number: WO2017158334A1
Application number: PCT/GB2017/050683
Authority: WO
Inventors: Amor ABDELKADER
Original assignee: The University Of Manchester
Priority date: 2016-03-15
Filing date: 2017-03-13
Publication date: 2017-09-21
Also published as: GB201604408D0

Abstract

This invention relates to a method for exfoliating inorganic layered compounds to form two- dimensional (2D) inorganic compounds. These two-dimensional compounds have the same sort of physical form as graphene in that they can be obtained as single-or few-layered molecular crystals. It also relates to suspensions of two-dimensional compounds which arise from the exfoliation method. The present invention thus relates to a process for the production of 2-D materials including, but not limited to, graphene. The 2-D materials of the invention are inorganic materials and whilst there are significant chemical differences between the various types of materials, they share the common feature of having strong in-plane covalent bonds within each layer but with layers which interact with each other via weak van der Waals bonds. This leads the sheets to stack into 3-dimensional bulk crystals. Upon exfoliation, single-layer inorganic nanosheets exhibit attractive new set of properties for use in nanoelectronics, optoelectronics, catalysis, thermal managements, etc. The 2-D materials are produced by the process of the invention in a liquid medium in the form of either a monolayer or few-layer thick materials. These materials can be regarded generically as nanoplatelet structures and are often known as nanoplatelets for brevity. The exfoliation of graphene and other 2-D materials occurs as the result of a delicate balance between thermodynamic and kinetic factors.

Description

MECHANICAL EXFOLIATION OF 2-D MATERIALS

FIELD OF INVENTION

This invention relates to a method for exfoliating inorganic layered compounds to form two- dimensional (2D) inorganic compounds. These two-dimensional compounds have the same sort of physical form as graphene in that they can be obtained as single- or few-layered molecular crystals. It also relates to suspensions of two-dimensional compounds which arise from the exfoliation method.

The present invention thus relates to a process for the production of 2-D materials including, but not limited to, graphene. The 2-D materials of the invention are inorganic materials and whilst there are significant chemical differences between the various types of materials, they share the common feature of having strong in-plane covalent bonds within each layer but with layers which interact with each other via weak van der Waals bonds. This leads the sheets to stack into 3-dimensional bulk crystals. Upon exfoliation, single-layer inorganic nanosheets exhibit attractive new set of properties for use in nanoelectronics, optoelectronics, catalysis, thermal managements, etc. The 2-D materials are produced by the process of the invention in a liquid medium in the form of either a monolayer or few-layer thick materials. These materials can be regarded generically as nanoplatelet structures and are often known as nanoplatelets for brevity. The exfoliation of graphene and other 2-D materials occurs as the result of a delicate balance between thermodynamic and kinetic factors.

The source of the energy used in an exfoliation process can have a significant effect on the viability and outcome of the process. Changing the source of energy or the driving force is analogous with the concept of changing the nature of a reactant in a chemical reaction to satisfy the thermodynamic requirements of a reaction process. It is the thermodynamic requirements which determine whether or not a process is possible. However, kinetic factors are equally important because the kinetics of a process determine whether or not a process can be conducted within a practicable timescale. An efficient exfoliation process therefore needs to balance both of these factors in such a way as to make the process both thermodynamically favoured and kinetically practical. It is not possible to predict ab initio where such a sweet spot might occur. BACKGROUND

Graphene is an atomically thick, two dimensional sheet composed of sp² carbons in a honeycomb structure. It can be viewed as the building block for all the other graphitic carbon allotropes. Graphite (3-D) is made by stacking several layers on top of each other, with an interlayer spacing of -3.4 A and carbon nanotubes (1 -D) are a graphene tube. Graphene is hydrogenated graphene, the carbons of the C-H groups being sp3 carbons.

Single-layer graphene is one of the strongest materials ever measured, with a tensile strength of -130 GPa and possesses a modulus of ~1 TPa. Graphene's theoretical surface area is - 2630 m²/g and the layers are gas impermeable. It has very high thermal conductivity (5000+ W/mK) and the theoretically calculated charge carrier mobility is higher than 200000 cm²/V s". The observed superior properties of graphene introduced it as a potential candidate material for many applications including but not limited to:

(a) additive for mechanical, electrical, thermal, barrier and fire resistant properties of a polymer;

(b) surface area component of an electrode for applications such as fuel cells, super- capacitors and lithium ion batteries;

(c) conductive, transparent coating for the replacement of indium tin oxide; and

(d) components in electronics.

Thus, inorganic single or few layer compounds can be used either alone or in combination with other such materials and/or with graphene to form ultrathin electronic devices with astonishing properties. BN and M0S2 have been used in conjunction with graphene to form quantum tunnelling transistor heterostructures (WO2012/127245) while M0S2 and WS2 have been used in conjunction with graphene to form photovoltaic heterostructures (WO2013/140181).

Graphene was first isolated in 2004 by Professor Geim's group at the University of Manchester. Graphene research since then has increased rapidly. Much of the "graphene" literature is not on true monolayer graphene but rather two closely related structures:

(i) "few layer graphene", which is typically up to 10 graphene layers thick. Thus the graphene may have 2, 3, 4, 5, 6, 7, 8, 9 or 10 layers. The unique properties of graphene are lost as more layers are added to the monolayer and above 10 layers the material becomes effectively bulk graphite; and (ii) Graphene oxide (GO), which is a graphene layer which has been heavily oxidised in the exfoliation process used to make it and has typically 30at.% oxygen content. This material has inferior mechanical properties, poor electrical conductivity and is hydrophilic (hence a poor water barrier).

There are now a variety of methods to produce graphene but graphene was first isolated by the mechanical exfoliation of graphite by using an adhesive tape to isolate individual layers. It has been shown subsequently that graphite can also be exfoliated by using ultrasonic energy to separate the layers when in an appropriate solvent, such as NMP (N-methyl pyrrolidone) as described in Coleman 2008 & 2009, Y. Hernandez, et al, Nat. Nanotechnol., 2008, 3, 563; M. Lotya, et al, J. Am. Chem. Soc, 2009, 131 , 36 1.

Graphite is an allotrope of carbon, the structure of which consists of graphene layers stacked along the c-axis in a staggered array usually denoted as ABAB. The layers are held together by weak van der Waals forces so that the separation between layers is 0.335 nm. Graphite is a cheap and abundant natural material, which makes it an excellent raw material for inexpensive production of graphene.

As noted above, graphite has been used to make graphene via exfoliation, wherein the stacked layers of graphite are separated to produce graphene. This has been achieved by using mechanical forces such as ultrasound (ultrasonic exfoliation, USE) and also by intercalating compounds into the graphite interlayer structure so as to weaken the interlayer bonding and promote layer separation. There are two routes that have been reported to intercalate compounds into graphite structure: chemical and electrochemical. The chemical method is based on the direct reaction of solid graphite materials with the intercalation species (usually in liquid or vapour phase). This process is kinetically slow and usually assisted by sonication or heating. The second route, the electrochemical approach, involves generating the intercalated species through an electrochemical reaction on a graphite cathode or on a graphite anode.

A number of workers have investigated ways of obtaining 2D materials such as graphene and there are a number of different approaches. Most of the work to date has focused on the production of graphene but there are reports of methods of producing other 2-D materials which meet with varying degrees of success. Despite a great deal of work there are still problems in obtaining a reliable, efficient and convenient large scale supply of graphene and other 2-D materials. In "Mechanochemical delamination of graphite in the presence of various inorganic salts & formation of graphene by its subsequent liquid exfoliation" Theoretical & Experimental Chemistry Vol 50 No2 May 2014, the authors describe a process in which solid state ball milling in conjunction with an inorganic salt is used to exfoliate graphene. This process is a two-step process in which graphene is exfoliated in the second step which is separate from the first step. It appears that the inorganic salts work as a surfactant to facilitate the exfoliation in a subsequent step by reducing the surface energy of the graphite.

Dry milling of a mixture of NaCI and MoS2 followed by exfoliation of the mixture in a second step using ultrasound disintegration is described by the authors in J Mater Chem. C 2013 Vol 1 pp641 1-6415 "Improved dispersant— free liquid exfoliation down to the graphene like state of solvent free mechanochemically delaminated bulk MoS2". This process is quite similar to the process described in the reference but with MoS2 being exfoliated instead of graphite. In this case the functionalizing salt is limited to NaCI only.

J Mater Chem A 2015 (3) pp1682-1687 "High - N content holey few layered graphene electrocatalysts: scalable solvent-less production" describes the production of nitrogen- doped graphene rather than pure pristine graphene. The process is conducted as a dry ball milling process and the product of the milling is in the powder form. The process requires a heat treatment step at 800° C in order to effect exfoliation due to the very slow kinetics of the exfoliation process.

Cent. Eur.J. Chem 2011 Vol 9 (1) pp47-51 "Preparation & examination of multilayer graphene nanosheets by exfoliation of graphite in high efficient attritor mill" describes a process which is based on the exfoliation of graphene by the mechanical forces only. There is no chemical reaction and/or chemical surface modification involved in this particular process and the method described in this paper maybe comparable with the ultrasonication or shear mixer methods.

W014191765A1 describes the production of graphene using electrochemical exfoliation in an organic solvent using double intercalation with both organic and inorganic ions. In this particular case an electrochemical potential is used as the driving force for the intercalation reaction between graphite and an inorganic salt which is used as the source of inorganic ions. The process requires the use of a cathodic current to protect the graphene from any oxidation. WO15019093A1 describes a process for producing functionalised graphene and graphene hybrids. Again, and electrochemical potential is used to drive the intercalation process and the process takes place in an electrolyte which is a solvent free ionic liquid, DES, or molten inorganic salt. The electrochemical process consumes a significant part of the electrolyte due to the fast decomposition and forms gases without reacting with the graphite which can be problematic. This means it can be necessary to add fresh electrolyte during the process to replenish lost electrolyte.

WO15075455A1 describes the production of graphene oxide by the exfoliation of graphite in an oxidative electrochemical process. The process involves intercalating oxide groups between the graphene layers and then exfoliating the layers in the form of graphene oxide in aqueous solution.

JP2015036361A describes a process for the production of BN for thermal management applications by grinding in water or organic solvents. There are no chemical reactions involved in the production of the BN although the surface of the BN is modified by titanate, zirconate, etc. to improve the thermal conductivity of the composite.

Similar processes involving electrochemical energy and the use of inorganic salts to those described above are also referred to in Abdelkadar A M et al, Applied Materials & Interfaces, September 2014 "Continuous electrochemical exfoliation of micrometer sized graphene using synergistic ion intercalations and organic solvents"

Although each of the above processes are capable of producing graphene and other 2-D materials on a research scale, there remains a need for a reliable and efficient method for producing 2-D materials which is capable of being scaled up to meet future commercial requirements. To date, this need has not been satisfied.

Another aim is to produce exfoliated graphene and other 2-D materials which have greater uniformity in size and/or shape than exfoliated graphene (or other 2-D materials) which have been prepared by conventional techniques. It is desired that the particle sizes should reside within a relatively narrow size range. Similarly, it is a further aim to produce exfoliated 2-D material in which the majority of the material falls within a narrow range of thicknesses. In context, this may refer to both atomic thickness and molecular thickness.

A further aim is to produce an exfoliated 2-D product which has a lower number of defects compared with a 2-D material produced in a known method. This is important in the case of a 2-D material such as graphene because defects can have a negative impact on the bulk properties of the 2-D material. In the case of graphene, defects can have a negative impact on both the electrical properties and also on the membrane properties.

The process of the invention satisfies some of all of the above aims.

According to the present invention provides a method for the production in a liquid medium of a 2-D material in the form of nanoplatelets having a thickness of less than 100 nm, wherein the process comprises:

(a) providing liquid medium comprising a deep eutectic solvent and a Group I or II metal in which a 2-D precursor material has been suspended;

(b) providing a source of mechanical energy;

(c) applying mechanical energy to the liquid media for a period of time to generate a suspension of 2-D material;

wherein the source of energy is operative to provide energy to the system by the action of shear force and friction a and wherein the period of time for which the mechanical energy is applied to the liquid media is 30 hours or less. This process can be described as a process for the exfoliation of 2-D materials.

The 2-D material produced by this process is of better quality than material produced by existing methods. In this respect, the material produced by the process of the present invention is actually physically distinct from that produced by prior art processes. This leads to some useful properties. For example, the 2-D material is more stable in liquid suspensions than conventional suspensions.

Thus, according to another aspect of the present invention, there is provided a 2-D material produced by exfoliation of a 2-D material precursor material in a deep eutectic solvent. Yet another aspect of the invention relates to a suspension of a 2-D material in a deep eutectic solvent. In either of the above aspects, this suspension may also contain a Group I or Group II metal.

The resulting exfoliated material produced according to the invention may have a relatively narrow lateral size range. In the case of graphene, for example, at least 75% by weight of the exfoliated graphene has a lateral size within the range of 5 to 30 microns. In certain cases, it is possible for at least 95% by weight of the exfoliated graphene to have a lateral size within the range of 5 to 20 microns. With regard to lateral size, in the case of a material such as hBN, the exfoliated flakes are typically smaller than those of graphene and have sizes ranging in the 100s of nm to about 1000nm. In this case, it is possible to ensure that at least 75% by weight of the exfoliated hBN has a size less than 500nm.

Independently, the resulting exfoliated material produced according to the invention may have a relatively narrow range of thicknesses. Again, in the case of graphene, it is possible to produce exfoliated graphene in which at least 75% by weight of the exfoliated graphene has a thickness of from 2 to 7 layers. In certain cases, it is possible that at least 95% by weight of the exfoliated graphene has a thickness of from 2 to 5 layers. In the case of other 2-D materials such as hBN or TMDCs, it is possible to achieve correspondingly narrow ranges for the lateral size, and separately for the thickness of the exfoliated flakes. Thus, in the case of hBN and TMDCs, it is similarly possible to obtain exfoliated material in which at least 75% by weight of the exfoliated material has a thickness of from 2 to 7 layers (these being either atomic layers or molecular layers, as appropriate).

Embodiments described in relation to the exfoliation process are equally applicable to these aspects of the invention relating to the material per se and a suspension of the material in the deep eutectic solvent.

In an embodiment, the liquid medium includes particles of an abrasive material. Preferably, the abrasive material is one more selected from: silica, alumina, boron nitride, diamond, zirconia, clay, zeolite, steel and tungsten carbide, zirconia, clay and zeolite. Most preferably, the abrasive material is alumina.

The Group I or II metal serves as a reducing agent. In a one, the metal is a Group I metal. The Group I metal is preferably Li, Na, or K, with Li being most preferred. In a particularly preferred embodiment, the reducing agent is Li. Group II metals, preferably Ca and Mg, can also advantageously be used, with Ca being most preferred.

In an embodiment, the mechanical energy is applied for a period of 24 hours, and more preferably 20 hours or less, and even more preferably for a period of 18 hours or less. Ideally, the minimum time period for applying the mechanical energy should be at least 6 hours and more preferably at least 12 hours or even 15 hours. The exact time period used will depend on the nature of the particular 2-D material.

In an embodiment, the mechanical energy is applied by a mill. However any mechanical mixer capable of imparting both a shear force and friction to the liquid medium will be suitable for exfoliation of the 2-D material. The mill is advantageously a ball mill. However, other milling techniques (particularly mortar and pastel) may provide as good or even better mechanical force transfer in certain systems. In each case, the mill provides energy to the system by the action of shear force and friction.

The rate of rotation of the ball mill is preferably in the range of from 20rpm to 2000 rpm, and more preferably is in the range of 400rpm to l OOOrpm, and even more preferably in the range 400 to 600rpm. It is possible to vary the rate of rotation of the ball milling during performance of the process. In one embodiment, however, it is preferred to keep the rate of rotation constant throughout the entire process.

The process of the present invention may be carried out under ambient conditions. Ideally, the process is carried out at room temperature though in principle the process can be conducted at any temperature between the freezing point of the deep eutectic solvent and the boiling point of the solvent. Normally, a suitable solvent system will be chosen such that the freezing point is at or below room temperature so that the medium is liquid under ambient conditions. However, in certain circumstances it is possible to warm the medium either to render it liquid if the freezing point is higher, or simply to provide additional energy to the system during ball milling. The upper limit of the temperature to which the medium is warmed is ultimately determined by the evaporation and boiling of the solvent. However for practical purposes, the temperature is ideally lower than this. In this instance the temperature might be raised to 30°C, 40°C, 50°C, 60°C or even 80°C but ideally the temperature should be kept below 60°C in the interests of energy efficiency.

In an embodiment, the mass ratio of the 2-D material precursor relative to the Group I metal can range from: 1 part 2-D material precursor to 4 parts Group I metal, to 4 parts 2-D material precursor to 1 part Group I metal. More preferably the mass ratio is in the range from 1 :2 to 2:1. Most preferably, there is usually an excess of the Group I metal relative to the 2-D material precursor i.e. the ratio is in the range from 1 part 2-D material precursor to between 1.1 and 1.9 parts of the Group I metal with an optimum ratio being somewhere around 1.5 parts of the Group I metal.

In an embodiment, the mass ratio of the 2-D material precursor relative to the Group II metal can range from: 1 part 2-D material precursor to 4 parts Group II metal, to 4 parts 2-D material precursor to 1 part Group I metal. More preferably the mass ratio is in the range from 1 :2 to 2: 1. Most preferably, there is usually an excess of the Group II metal relative to the 2-D material precursor i.e. the ratio is in the range from 1 part 2-D material precursor to between 1.1 and 1.9 parts of the Group II metal with an optimum ratio being somewhere around 1.5 parts of the Group II metal.

Brief Description of the Drawings

Embodiments of the invention are further described hereinafter with reference to the accompanying drawings:

Figure 1 : XRD pattern of the initial graphite and result product after ball-milling in urea- choline chloride DES using different rotating speed.

Figure 2: AFM image and height profile for 2 flakes of the graphene produced after 24 hour of milling.

Figure 3: XPS wide scan analysis of the graphene sample obtained from the mechanochemical process.

Figure 4: XPS C1 peak comparison of graphite and the exfoliated materials after 20 hours of milling at 500 rpm showing insignificant amount of oxygen after the mechanochemical process.

Figure 5: Comparison between the bulk hBN and the exfoliated nanosheets (a) XRD, (b) Raman

Figure 6: BN nanosheets produced by the mechanochemical process, (a) SEM image, (b) tapping mode AFM image, (c) statistic of the flakes lateral size measured by the SEM, (d) statistic of the flakes thickness measured by the AFM.

Detailed description

Throughout this specification, the term 'two dimensional material' refers to nanoplatelets of graphene, TMDCs, layered carbides, and layered nitrides including hBN and mixtures thereof. Thus, the term 'two dimensional material' encompasses mixtures of 2-D materials. Nanoplatelets may be single or few layered particles of the respective inorganic layered material. The term 'two-dimensional' may mean a compound in a form which is so thin that it exhibits different properties than the same compound when in bulk. Not all of the properties of the compound will differ between a few-layered particle and a bulk compound but one or more properties are likely to be different. Typically, two-dimensional inorganic compounds are in a form which is single- or few layers thick, i.e. up to 10 molecular layers thick. A two- dimensional crystal of a layered material (e.g. an inorganic compound or graphene) is a single or few layered particle of that material. The terms 'two-dimensional' and 'single or few layered' are used interchangeably throughout this specification. Two-dimensional materials are not truly two dimensional, but they exist in the form of particles which have a thickness that is significantly smaller than their other dimensions. The term 'two-dimensional' has become customary in the art.

The term 'few-layered particle' may mean a particle which is so thin that it exhibits different properties than the same compound when in bulk. Not all of the properties of the compound will differ between a few-layered particle and a bulk compound but one or more properties are likely to be different.

A more convenient definition would be that the term 'few layered' refers to a crystal that is from 2 to 10 molecular layers thick (e.g. 2 to 5 layers thick). A molecular layer is the minimum thickness chemically possible for that compound. In the case of boron-nitride one molecular layer is a single atom thick. The same is true of graphene though in this case it is correct to refer to atomic layers rather than molecular layers and the minimum thickness is a single atomic layer. Few layer graphene may contain up to 10 atomic layers. A useful definition is from 2 to 10 layers, and more preferably this will be 2 to 5 layers.

In the case of the transition metal dichalcogenides (e.g. M0S2 and WS2), a molecular layer is three atoms thick (one transition metal atom and two chalcogen atoms). Thus, few-layer particles crystals are generally less than 50 nm thick, depending on the compound and are preferably less than 20 nm thick, e.g. less than 10 or 5 nm thick.

The term 'multi-layered particle' refers to a particle which exhibits similar properties to the same compound when in bulk. A more convenient definition would be that the term 'multi- layered particle' refers to a particle that is 10 or more molecular layers thick.

The term "2-D precursor material" will be readily understood by the person skilled in the art as a material which can be exfoliated to form a 2D material. Many compounds which are capable of forming 2-D materials exist in a number of allotropic forms, some of which are layered and some of which are not. For example boron nitride can exist in a layered graphite-like structure (hexagonal boron nitride or hBN) or as a diamond-like structure in which the boron and nitrogen atoms are tetrahedrally orientated. For the absence of doubt, it is hexagonal boron nitride that is referred to throughout this specification.

TMDCs are structured such that each layer of the compound consists of a three atomic planes: a layer of transition metal atoms (for example Mo, Ta, W...) sandwiched between two layers of chalcogen atoms (for example S, Se or Te). Thus in one embodiment, the TMDC is a compound of one or more of Mo, Ta and W with one or more of S, Se and Te. There is strong covalent bonding between the atoms within each layer of the transition metal chalcogenide and predominantly weak Van der Waals bonding between adjacent layers. Exemplary TMDCs include NbSe₂, WS₂, M0S2, TaS₂, PtTe₂, VTe₂.

It may be that greater than 75% by weight (e.g. greater than 90% or greater than 98%) of the particles of the 2-D material formed by the exfoliation process of the present invention has a diameter between 50 and 750 nm; this is particularly the case for hBN. Here, the measured diameter represents the lateral size of the flakes. It may be that greater than 75% by weight (e.g. greater than 90% or greater than 98%) of the particles have a diameter i.e. lateral size of less than 500 nm. Thus, it may be that greater than 75% by weight (e.g. greater than 90% or greater than 98%) of the particles have a diameter between 100 and 500 nm.

In general terms, for any 2D material the size distribution of flakes is preferably such that at least 90% by weight, and more preferably 98% or 99% by weight, of the exfoliated 2D flakes fall within a single order of magnitude.

It may be that greater than 75% by weight (e.g. greater than 90% or greater than 98%) of the particles of the 2-D material have a thickness of from 1 to 10 molecular layers. It may be that greater than 75% by weight (e.g. greater than 90% or greater than 98%) of the 2-D material has a thickness of from 1 to 5 molecular layers. Thus, it may be that greater than 75% by weight (e.g. greater than 90% or greater than 98%) of the 2-D material has a thickness of from 1 to 3 molecular layers. These statements apply particularly equally to particles of any of the possible 2-D materials. For the avoidance of doubt, for material such as graphene where the individual layers are atomic layers then the term "molecular layer" should be read as meaning "atomic layer". It may be that greater than 75% by weight (e.g. greater than 90% or greater than 98%) of the 2-D material has a thickness of from 3 to 8 molecular layers. Thus, it may be that greater than 75% by weight (e.g. greater than 90% or greater than 98%) of the 2-D material has a thickness of from 4 to 6 molecular layers. As above, these statements apply to 2-D materials in general and the term "atomic layer" should be understood in place of the term "molecular layer" where the context requires it for materials such as graphene.

Graphene is of especially large interest because of its ability to exhibit an ambipolar electric field effect, ballistic conduction of charge carriers, and the quantum Hall effect at room temperature, as well as significant barrier properties. Increasingly, because it is recognised that there are possible applications outside of the field of graphene, research is being performed on other inorganic 2-D materials which it is hoped will provide technology solutions for the future. The study of inorganic 2D materials was originally dominated by research into clays and other layered oxides but now extends to materials such as hexagonal boron nitride (hBN), transition metal dichalcogenides (TMDCs), transition metal carbides (MAXen), and black phosphorous.

The 2-D material may be h-BN. Single layer h-BN is structurally similar to graphene, but unlike its carbon analogue, it is an insulator with a large band gap (~6eV). This, added to unique features such as excellent chemical, mechanical properties, and thermal stability, allows using h-BN nanosheets (BNNS) in a variety of applications, such as component in nanodevices, solid lubricant, UV-light emitter and as insulating thermo-conductive filler in composites.

The inorganic compound may be a transition metal dichalcogenide (e.g. M0S2, WS2, ΜοΤβ2, MoSe₂ etc.).

Upon exfoliation, single-layer inorganic nanosheets of these 2-D materials exhibit attractive properties such as modified band gaps etc. For example, M0S2 shows a direct bandgap semiconductor (-1.8 eV) when exfoliated down to a single molecular layer.

Existing "bottom up" production techniques such as CVD and epitaxial growth have failed to provide the kilogram quantities required for applications such as composites, conductive inks, energy storage, and water treatment. "Top-down" techniques such as sonication and mechanical delamination, in which energy is used to exfoliate graphite and other bulk inorganic layered materials particles, have proved most promising for large quantities of 2D materials. However, these methods tend to produce either very small and/or highly defective flakes due to the energy input.

For decades, graphite was known to be able to host a number of different atoms and small molecules between its graphene layers with this particular feature making graphite an ideal electrode for rechargeable batteries. These graphite intercalation compounds decompose readily in water producing metal/organic hydroxides and free standing graphene sheets. This principle has been proposed as a possible route for scalable production of graphene and could theoretically be used to produce other inorganic 2D materials. However, due to the slow kinetic nature of the intercalation process, the intercalante was only hosted in regions of the graphite close to the edges of the grains. This does not lead to reliable exfoliation. Upon exfoliation in water, graphite with expanded edges was produced and further intercalation, water decomposition or sonication steeps were needed to achieve full exfoliation.

In other cases, methods have been developed in which lithium is electrochemically intercalated into graphite and then treated with ammonia in a process involving two separate steps. However, due to the expanding nature of the cathode, the initial distance between the electrodes had to be large, hence a high voltage had to be applied to overcome the high IR drop. As a result, the organic solvent used as the electrolyte dissociated at later stage of the process and hindered the intercalation process. Therefore, an additional sonication step was necessary to achieve reasonable exfoliation.

The process of the present invention improves on existing methodology for a number of reasons. One significant advantage is that because electrochemical intercalation methods need conductive electrodes, existing methodologies only work for conductive materials such as graphite unless conductive additives are also used. Most of the inorganic layered materials are insulating and hence conductive additives are required to assemble the electrode, which may contaminate the final product of the 2-D materials. The process of the present invention is able to produce 2-D particles in the form of nanoplatelets for any 2-D material irrespective of their electrical properties and without the addition of conductive additives. This is because the process does not rely on electrochemistry. The process of the present invention uses a mill to provide mechanical energy to the bulk material from which the 2-D material is ultimately obtained. Surprisingly, exfoliation is possible under these conditions even though the thermodynamics of the process are completely different from that of the traditional electrochemical methods. Graphite and many of the inorganic layered materials such as TMDCs are known to be good lubricants and it is hard to achieve its mechanical delamination in a ball mill down to graphene or graphene-like flakes. This is mainly because the random ordination of the graphite grain makes the applied forces by the milling balls a mixture of shearing and compressing forces. The two forces can cancel each other and even where there is some exfoliated graphene flakes they have a great tendency to restack together via van der Waal bond. Intercalating these layered materials with different chemical species is known to reduce the van der Waal forces that hold the graphene-like flakes together. Another problem with the traditional intercalation process is that the slow kinetics of the intercalation process usually means that the process has to be accelerated by applying an electrode potential in order to force the intercalation. This has consequences on the integrity of the resulting 2-D material.

The resulting 2-D product is of better chemical purity than materials obtained by conventional processes. The resulting product also has a reduced incidence of defects compared with materials obtained by conventional processes. Individually, each of these advantages appears to be counterintuitive given the relatively aggressive mechanical nature of the ball milling process. This switch of energy source also has the additional benefit that it makes the route applicable to non-conductive inorganic layered materials. Another advantage is that the process of the invention is scalable; the same cannot be said of electrochemical methods. A further unexpected advantage of the process is that the liquid formulations of 2-D material are more stable than conventional liquid formulations of 2-D material. The reason for this is not presently clear though it may be due to the better structural integrity of the 2-D material that is produced in the process of the present invention.

The milling process is conducted in a liquid medium which is based on deep eutectic solvents. Deep eutectic solvents (DESs) are now widely acknowledged as a new class of ionic liquid (IL) analogues because they share many characteristics and properties with ILs. The terms DES and IL have been used interchangeably in the literature though it is necessary to point out that these are actually two different types of solvent. DESs are systems formed from a eutectic mixture of Lewis or Br0nsted acids and bases which can contain a variety of anionic and/or cationic species; in contrast, ILs are formed from systems composed primarily of one type of discrete anion and cation. Although the physical properties of DESs are similar to other ILs, their chemical properties suggest application areas which are significantly different. DESs contain large, nonsymmetric ions that have low lattice energy and hence low melting points. They are usually obtained by the complexation of a quaternary ammonium salt with a metal salt or hydrogen bond donor (HBD). The charge derealization occurring through hydrogen bonding between for example a halide ion and the hydrogen-donor moiety is responsible for the decrease in the melting point of the mixture relative to the melting points of the individual components. It is known in the art that if a range of quaternary ammonium salts are heated with ZnC a deep eutectic solvent system can be produced. For example, a melting point of 23-25 °C can be obtained when choline chloride is used as the ammonium salt.

A range of liquids formed from eutectic mixtures of salts and hydrogen bond donors have now been developed in the art and such solvents are well known to the skilled person. Indeed, any of these liquids can be used in the process of the present invention. The reader is specifically directed towards a review in Chem Rev 2014, 114, 11060-11082, the content of which is specifically intended to be incorporated herein by reference. In particular, solvents explicitly mentioned in that document are to be understood as forming part of the preferred embodiments of this invention. In other words, deep eutectic solvents mentioned in that review are incorporated herein as preferred solvents in the process of the present invention.

The novel liquids described in that review were termed deep eutectic solvents to differentiate them from ionic liquids which contain only discrete anions. The term DES refers to liquids close to the eutectic composition of the mixtures, i.e., the molar ratio of the components which gives the lowest melting point.

Deep eutectic solvents can be described by the general formula Cat⁺ X zY where Cat+ is in principle any ammonium, phosphonium, or sulfonium cation, and X is a Lewis base, generally a halide anion. The complex anionic species are formed between X- and either a Lewis or Br0nsted acid Y (z refers to the number of Y molecules that interact with the anion). The majority of studies have focused on quaternary ammonium and imidazolium cations with particular emphasis being placed on more practical systems using choline chloride and this is what we use in our experiments to demonstrate proof of concept.

DESs are largely classified depending on the nature of the complexing agent used and the process of the invention can use any of Type I, Type II, Type III and Type IV solvents based on the classification used in the above quoted review article.

DESs formed from MCI_X and quaternary ammonium salts, type I, can be considered to be an analogous type to the well-studied metal halide/ imidazolium salt systems. Examples of type I eutectics include chloroaluminate/imidazolium salt melts and less common ionic liquids such as those formed with imidazolium salts and various metal halides including FeC . Various metal halides can be employed including, but not limited to: one or more of FeCI₂, FeC , AgCI, CuCI, CuCI₂, LiCI, CdCI₂, SnCI₂, SnCU, ZnCI₂, LaC , YCI₃, AICI₃.

The range of nonhydrated metal halides which have a suitably low melting point to form type I DESs is limited; however, the scope of deep eutectic solvents can be increased by using hydrated metal halides and choline chloride (type II DESs). The relatively low cost of many hydrated metal salts coupled with their inherent air/moisture insensitivity makes their use in large scale industrial processes viable.

We used ammonium based deep eutectic solvents (DES) as the source of ammonium and the wet grinding media in combination with alumina spheres, which replaces the organic solvents traditionally used in all the mechanical exfoliation of graphite. The use of ammonium-based ionic liquid satisfies high activity of the Et₄N⁺ ions, which accelerates the kinetic of the intercalation process. In addition, DES are known as green solvents having the advantages of non-flammability, high thermal stability, wide liquid phase range, negligible vapor pressure and easy recycling; which will eliminate the safety problems associated with the organic solvents and also promote to an eco-friendly process. The present work has selected choline chloride -urea system due to its ability to dissolve reasonable amount of lithium ions (2.5 wt. % for LiCI) and also its bio-compatibility, which allows the produced 2D nanosheets to be used in bio-applications.

The kinetic energy provided by the high-speed rotation of the ceramic balls during ball milling is able to open the edges of the graphite grains. In the presence of ammonium ions and another strong reducing agent such as a Group I metal such as Li or Na metals, active carbon species (mostly carboradicals, carbocations and carbanions) formed at the broken edges. This caused further opening at the edge of the graphite gallery and facilitate the intercalation of both lithium and ammonium ions to a deeper extended on the graphite grain. The reaction between Li, ammonium ions, and graphene result in forming a charged graphene sheets that significantly weaken the van der Waal forces and, coupling with the ammonia and hydrogen gases bubbles produced from the interlayer reaction, the graphene sheets can no longer hold together and become dispersed as individual thin sheets in the solution. The same effects will occur with other 2-D materials.

Experimental details

Choline chloride was recrystallized from ethanol absolute. The crystals were then filtered out from the solution and dried over night at 70 °C under vacuum. Urea was separately dried under vacuum prior to mixing. The dry salts were then mixed in their eutectic composition (2: 1 mole ratio of urea: choline chloride) and the deep eutectic solvent was synthesized as described elsewhere¹²⁴¹. The grinding process was conducted in vertical laboratory attrition mill (PE 075, Netzsch Feinmahltechnik GmbH, Selb, Germany). The grinding chamber is a double wall 750 ml tank with silicon nitride inner wall. Wear resistant commercially available yttria stabilized zirconia milling media with diameters of 2mm were charged with 2g of natural graphite and 4g of battery grad lithium chips into the grinding chamber. The DES was then poured into the chamber until the liquid covered the grinding media. The ball milling was run with different rotation speed for the required duration.

The exfoliation product was washed with DMSO several times. In the final step, the sample was sonicated for 15 minutes and then centrifuge at 1500 rpm to remove the thick graphite flakes. The samples for SEM, Raman and AFM were dropped from the DMSO suspension. Dry powder and membrane were obtained by filtration using Anodisc^® alumina membranes with 100nm pore size, and then dried at 100 °C under vacuum.

Characterisation of the resulting powder can be achieved as follows. X-ray photo-electron spectroscopy (XPS) were collected using a Kratos Axis Ultra X-ray photoelectron spectrometer, equipped with an aluminium/magnesium dual anode and a monochromated aluminium X-ray sources. Fourier-transform IR (FTIR) spectroscopy was performed at room temperature using a Varian 3100 FTIR spectrometer. The samples were ground with potassium bromide and then pressed into disks. Raman spectra were obtained using a Renishaw system 1000 spectrometer coupled to a He-Ne laser. The laser spot size was -1-2 μηι, and the power was about 1 mW when the laser is focused on the sample using an Olympus BH-1 microscope. The Raman bands were fitted using Lorentzian function. Atomic force microscope (AFM) images were obtained using a Multimode Nanoscope V scanning probe microscopy (SPM) system (Veeco, USA) with Picoscan v5.3.3 software. Tapping mode was used to obtain the images under ambient conditions. TGA was performed in air using a Jupiter Netzsch STA 449 C instrument. The sample was placed into alumina crucible and heated with a rate of 10°C/min from 30°C up to 800°C. Scanning electron microscopy (SEM) was performed using a Philips XL30 FEG SEM, operating at an accelerating voltage of 5 kV. The TEM analysis used a FEI Tecnai F20 microscope. The samples were supported on a 3 nm ultrathin carbon film-supported Cu TEM grids (G3347N, Agar Scientific). The XRD analysis was conducted using a Philips XPERT APD powder X-ray diffractometer (λ = 1.54 A, CUKQ radiation). Samples prepared according to the invention showed a narrow lateral size range and/or a narrow range of thicknesses for the exfoliated flakes. The following examples indicate some of the results achieved.

Example 1 - the exfoliation of graphite to produce graphene.

Graphite was mixed with 1.5 times its weight of Li and the mixture was suspended in choline chloride-urea DES by ball milling. The solid exfoliation product was recovered from the DES by a series of washing and filtration steps. The produced powder was first subjected to XRD analysis. It is clear from the XRD pattern (see figure 1) that the transversal dimension of the graphite grains as well as the number of graphene sheets alongside the c-axis are sharply decreased as a result of the mechanochemical treatment in DES. The intensity of the (002) peak decreases and the fwhm of the (002) peak increases as a result of Scherrer broadening. The reduction of the (002) intensity indicates that there are a sharp weakening in the ττ-ττ- stacked layers and reflects the decrease of the graphite particle size in direction perpendicular to the basal plan. Also, the intensity of the 002 peak decreased with increasing the rotation speed of the ball mill indicating that the exfoliation is critically depend on the energy provided by the milling process. Significantly, it was found that the pattern of the product obtained by milling using pure DES without the presence of Li, was almost identical to that of the initial source graphite. This result indicates clearly the essential role of the Group I metal in the exfoliation process.

Raman spectroscopy is a conventional technique to evaluate carbon materials. It is also a well-known technique to estimate the level of defects of the produced graphene. For that purpose, a couple of thin films of graphene were made via vacuum filtration of dispersions prepared from the graphene powder produced at different milling rotation speed. Approximately 30 Raman spectra were recorded at different spots of the film. The Raman spectrum of the original natural graphite was also recorded for comparison. In general, the pristine defect-free graphite possessed two bands: one at -1580 cm^-1 (the G band) arises from the first order scattering of the E2₉ phonon of sp²-bonded carbon atoms; and a band at -2600 cm^-1 (the 2D band) corresponding to the double-resonance process. Upon introducing defects either edges or topological defects in the sheet, another band (the D Band) arose at about 1350 cm^-1. The Raman spectrum obtained from the edge area of pristine graphite shows a very weak D band indicating a highly ordered structure with low defects and a flake size larger than the Raman spot (a few microns diameter). The Raman spectra recorded for the graphene samples after mechanochemical treatment at 500 rpm for 20h indicate that the obtained materials is not homogeneous in term of the flakes thickness and defect disruption. Comparing the 2D regions of different represented spectra, it is clear that the wide asymmetrical peak of graphite at 2680 cm^-1 replaced by a more symmetrical peaks at positions ranged between 2645-2660 cm^-1 , which can be attributed to 2-5 layers of graphene.

From the defects formation point of view, the appearance of a shoulder on the G band (usually called the D' band) and the increase in the intensity of the D band both suggested the presence of structural disorder in the mechanochemically exfoliated graphene. The ratio of ID/IG is often used to evaluate density of defects of graphene sheets. For more than 70% of the flakes the ID/IG is -0.60, much smaller than that of chemically produced GO and reduced graphene and comparable with that produced via the electrochemical exfoliation.

Interestingly, the ID/IG value decreased with decreasing the rotation speed of the ball mill. Increasing the time of the milling to 48 hours resulted in a more homogeneous thickness of the obtained flakes, more than 85% of the spectra recorded were similar to that of natural graphite. Thus, contrary to expectations, longer milling times are not effective in ensuring the reliable formation of 2-D nanoplatelets. The value ID/IG significantly increased with increasing the milling time, possibly due to decreasing flake size.

To confirm the thickness of the produced graphene, atomic force microscopy (AFM) analysis was used. The flakes were deposited from a diluted ethanol suspension on a Si substrate. Figure 2 shows representative examples of the flakes produced after 20 hours of mechanochemical treatment at 500 rpm rotating speed. The height profile of the flakes indicated a thickness of -0.9 and -2.5 nm. The statistical analysis of 15 flakes also showed the thickness range for more than 95% of the flakes was between 2-5 layers, which is in a good agreement with the Raman results. The lateral particles size as indicated by the AFM ranged from 5 to 20 micron. This is a particular advantage of the present work as the majority of the literature showed that ball milling of graphite, either in dry or wet conditions, produce small diameter flakes with lateral sizes of few tens of nm to a few microns. In other words, the process of the present invention is able to produce a relatively narrow size distribution for the nanoplatelets.

It is also important to evaluate the chemical purity of the exfoliated graphene. Hence X-ray photoelectron spectroscopy (XPS) was used to probe the chemical composition of the mechanochemically exfoliated graphene.

Figure 3 shows the XPS survey scan spectrum of the graphene product after 20 hours of milling at 500 rpm. The spectrum shows a strong C1 s peak at 284.5 eV, a small 01 s peak at 532.6 eV and a weak OKLL Auger band between 955-985 eV. The exfoliated graphene spectrum showed also a small N1 s peak at -400 eV, no other elements such as CI, or Li, are found in the sample. The concentration of elements N and O in graphene is calculated to be about 2.8 % and 4.6 at%, respectively. This value of oxygen is very close to the oxygen content value of the starting materials (3.6 at%) suggesting that the exfoliation process is not oxidative. This was further confirmed by the high-resolution scan of the C1s of graphene (see Figure 4), which was almost identical to that of the starting graphite flakes. The thermal gravimetric analysis in air (TGA) also confirmed the absence of any functional group bonded to the graphene sheets with the freshly exfoliated graphene was almost featureless before the decomposition. Interestingly the decomposition temperature of graphene is about 150 °C lower than that of graphite, which is also seen for other exfoliated system. We believe this low thermal stability is a result of the high reactivity of graphene and the presence of defects. Transmission electron microscopy provided further evidence for graphene exfoliation. The diffraction pattern shown in Figure 5 showed that the intensity ratio of the inner spots (i.e. - 1010) to that of the outer spots (i.e. -2110) is > 1 indicating monolayer graphene.

Example 2 - synthesis of hBN Nano sheets

The route we followed to exfoliate inorganic layered materials is similar to that of exfoliating graphite. Typically 1 g of hBN is mixed with 1.5 g lithium and ball milled in the same DES for 24 hours.

Figure 6 shows a series of scanning electron microscope (SEM) images of the obtained hBN nanosheets, which possess different lateral sizes varying from hundreds to thousands of nanometers. This lateral size is 3 to 10 orders of magnitude larger than the hBN nanosheets obtained by the sonication method.

Considering the initial particles size of the bulk hBN, less than 2 micron, the non-distractive nature of the present process is clearly demonstrated. Undisputable evidence for the effective exfoliation arose from the AFM analysis. Tapping mood image revealed flakes with thickness ~ 1 nm, close to the theoretical thickness value of single layer hBN. Most of the produced hBN have a thickness less than 3 nm corresponding to flakes of less than 10 layers.

Further evidence of the exfoliation was obtained from the Raman spectroscopy and XRD analysis. Bulk hBN exhibits a characteristic Raman peak from E2₉ phonon mode (B-N vibration mode), which is analogous to E2₉ mode (G band) in graphene. The typical E2₉ band position of bulk h-BN is around 1365 cm^-1. After exfoliation, the E2₉ band shifted to 1362 cm^-1 and the intensity of the peak reduced significantly which is consistent with previously reported results for hBN exfoliation. XRD spectrum of the BN after ball milling indicated the materials retrains it hexagonal in plane structure. However, the intensity of the 002 peak at 2 theta -26.5, which characterize the ττ-π stacking of the BN sheets in the bulk gallery, has significantly decreased and the peak clearly widened. This is analogous to what is observed when graphite is exfoliated to graphene.

Example 3 - exfoliation of TMDCs

We performed a further experiment in order to further validate the process of the present invention for use with TMDCs. Again, we employed a process using ball milling in a deep eutectic solvent to exfoliate TMDCs. The exfoliation of both M0S2 and WS2 into nanosheets was confirmed by the Raman analysis.

Generally, the Raman spectra of bulk WS2 have two vibration modes of Ai_g at ~ 420 cm^-1 and E¹2g at -354 cm^-1 , representing the out-of-plane W-S phonon mode and the in-plane W- S phonon mode, respectively. The ratio between the intensity of the two peaks E¹2g /Ai_g are in the range of 0.55 for the bulk sample and it increases with reducing the number of layers. From figure 5, it is clear that the ratio E¹2g /Ai_g increased to -1.2, suggesting a successful exfoliation.

Similarly, the bands of the exfoliated M0S2 nanosheets are different from that of the bulk material both in terms of Raman frequency and signal intensity. For the exfoliated nanosheets two strong Raman bands, deconvoluted by a single Lorentzian centered at 383 cm^-1 and 407 cm^-1 , were assigned to in-plane E¹2_g and out-of-plane Ai_g vibrational modes. There was no sign of structural distortion in the Raman spectra, suggesting the absence of structure damage and/or covalent bond formation upon the mechanochemical exfoliation. Unlike the bulk M0S2, the Ai_g and E¹2_g modes for exfoliated M0S2 appeared with equal intensities, indicating weaker coupling between the electronic transition at the K point with the Ai_g phonon existing in M0S2 nanosheets. The peak positions difference between Ai_g and E¹2_g was measured to be 24 cm^-1. The value of this difference for the bulk material was found to be 27 cm^-1 , signifying the success of exfoliation and the existence of 2H-M0S2 in few layers. The AFM images confirmed the exfoliation down to few layers.

It can therefore be seen that the process of the present invention provides an efficient and convenient means for exfoliating 2-D materials. In conclusion, we have shown that deep eutectic solvents can provide an excellent medium for the exfoliation of graphene and other 2-D materials. This can be achieved relatively easily in a ball mill such as a plenary ball-mill. The resulting 2-D nanoplatelets are of good quality, being relatively defect free and relatively pure compared with existing materials. The use of the ball mill give alternative and potentially easier routes for scale-up, which maintaining a reasonable flake diameter.

Claims

1. A method for the production in a liquid medium of a 2-D material in the form of nanoplatelets having a thickness of less than 100 nm, wherein the process comprises:

(b) providing a source of mechanical energy;

wherein the source of energy is operative to provide energy to the system by the action of shear force and friction and wherein the period of time for which the mechanical energy is applied to the liquid media is 30 hours or less.

2. A process as claimed in claim 1 , wherein the liquid medium includes particles of an abrasive material; preferably, the abrasive material is one more selected from: silica, alumina, boron nitride, diamond, zirconia, clay, zeolite, steel and tungsten carbide.

3. A process as claimed in claim 1 or 2, wherein the metal is a Group I metal, preferably Li.

4. A process as claimed in claim 1 or 2, wherein the metal is a Group II metal, preferably Ca.

5. A process as claimed in any of claims 1 , 2, 3 or 4, wherein the mechanical energy is applied for a period of 24 hours, and more preferably 20 hours or less.

6. A process as claimed in any preceding claim, wherein the mechanical energy is applied by a mill, preferably a ball mill.

7. A process as claimed in claim 6, wherein the rate of rotation of the ball mill is in the range of from 20rpm to 2000 rpm, and more preferably is in the range of 400rpm to lOOOrpm.

8. A process as claimed in any preceding claim, wherein the process of the present invention is carried out at a temperature between ambient conditions and 60°C.

9. A process as claimed in any preceding claim, wherein the mass ratio of the 2-D material precursor relative to the Group I metal can range from: 1 part 2-D material precursor to 4 parts Group I metal, to 4 parts 2-D material precursor to 1 part Group I metal, and more preferably the mass ratio is in the range from 1 :2 to 2: 1.

10. A process as claimed in any preceding claim, wherein the deep eutectic solvent has the formula Cat+ X- zY where Cat+ is an ammonium, phosphonium, or sulfonium cation, X is a Lewis base, Y is either a Lewis or Br0nsted acid Y, and z refers to the number of Y molecules that interact with the anion.

1 1. A process as claimed in claim 10, wherein the deep eutectic solvent includes one or more quaternary ammonium cations, and preferably wherein the deep eutectic solvent comprises choline chloride and / or an imidazolinium salt.

12. A process as claimed in claim 11 , wherein the deep eutectic solvent is

formed from MClx and one or more quaternary ammonium salts, where MClx is a metal halide.

13. A process as claimed in claim 12, wherein the metal halide is one or more of FeCI₂, FeC , AgCI, CuCI, CuCI₂, LiCI, CdCI₂, SnCI₂, SnCU, ZnCI₂, LaC , YCI₃, AlC .

14. A process as claimed in any preceding claim, wherein the 2-D material is one or more of: graphene, TMDCs, layered carbides, and layered nitrides including hBN.

15. A 2-D material produced by exfoliation of a 2-D material precursor material in a deep eutectic solvent.

16. A 2-D material as claimed in claim 14 produced by exfoliation of a 2-D material precursor material in a deep eutectic solvent according to the process of any of claims 1 to 14.

17. A suspension of a 2-D material in a deep eutectic solvent.

18. A suspension according to claim 17, wherein the deep eutectic solvent has the formula Cat+ X- zY where Cat+ is an ammonium, phosphonium, or sulfonium cation, X is a Lewis base, Y is either a Lewis or Br0nsted acid Y, and z refers to the number of Y molecules that interact with the anion.

9. A suspension according to claim 17 or 18, further comprising a Group I or Group II