WO2017158334A1 - Exfoliation mécanique de matériaux bidimensionnels - Google Patents

Exfoliation mécanique de matériaux bidimensionnels Download PDF

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Publication number
WO2017158334A1
WO2017158334A1 PCT/GB2017/050683 GB2017050683W WO2017158334A1 WO 2017158334 A1 WO2017158334 A1 WO 2017158334A1 GB 2017050683 W GB2017050683 W GB 2017050683W WO 2017158334 A1 WO2017158334 A1 WO 2017158334A1
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graphene
materials
exfoliation
metal
group
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PCT/GB2017/050683
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Amor ABDELKADER
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The University Of Manchester
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    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B35/00—Boron; Compounds thereof
    • C01B35/08—Compounds containing boron and nitrogen, phosphorus, oxygen, sulfur, selenium or tellurium
    • C01B35/14—Compounds containing boron and nitrogen, phosphorus, sulfur, selenium or tellurium
    • C01B35/146—Compounds containing boron and nitrogen, e.g. borazoles
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B21/00—Nitrogen; Compounds thereof
    • C01B21/06—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron
    • C01B21/064—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron with boron
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B32/00—Carbon; Compounds thereof
    • C01B32/15—Nano-sized carbon materials
    • C01B32/182—Graphene
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B32/00—Carbon; Compounds thereof
    • C01B32/15—Nano-sized carbon materials
    • C01B32/182—Graphene
    • C01B32/184—Preparation
    • C01B32/19—Preparation by exfoliation

Definitions

  • 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.
  • 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.
  • 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.
  • Graphene is an atomically thick, two dimensional sheet composed of sp 2 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 m 2 /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 cm 2 /V s".
  • the observed superior properties of graphene introduced it as a potential candidate material for many applications including but not limited to:
  • 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 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.
  • NMP N-methyl pyrrolidone
  • 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.
  • 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.
  • USE ultrasonic exfoliation
  • 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.
  • 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.
  • W014191765A1 describes the production of graphene using electrochemical exfoliation in an organic solvent using double intercalation with both organic and inorganic ions.
  • 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.
  • 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 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:
  • liquid medium comprising a deep eutectic solvent and a Group I or II metal in which a 2-D precursor material has been suspended
  • 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.
  • the 2-D material produced by this process is of better quality than material produced by existing methods.
  • 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.
  • the 2-D material is more stable in liquid suspensions than conventional suspensions.
  • 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.
  • at least 75% by weight of the exfoliated graphene has a lateral size within the range of 5 to 30 microns.
  • 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.
  • the resulting exfoliated material produced according to the invention may have a relatively narrow range of thicknesses.
  • 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.
  • 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.
  • the liquid medium includes particles of an abrasive material.
  • the abrasive material is one more selected from: silica, alumina, boron nitride, diamond, zirconia, clay, zeolite, steel and tungsten carbide, zirconia, clay and zeolite.
  • the abrasive material is alumina.
  • the Group I or II metal serves as a reducing agent.
  • the metal is a Group I metal.
  • the Group I metal is preferably Li, Na, or K, with Li being most preferred.
  • the reducing agent is Li.
  • Group II metals, preferably Ca and Mg, can also advantageously be used, with Ca being most preferred.
  • 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.
  • 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.
  • the mechanical energy is applied by a mill.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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 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.
  • the term 'two dimensional material' refers to nanoplatelets of graphene, TMDCs, layered carbides, and layered nitrides including hBN and mixtures thereof.
  • 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.
  • two-dimensional inorganic compounds are in a form which is single- or few layers thick, i.e.
  • a two- dimensional crystal of a layered material 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.
  • '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.
  • 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.
  • boron-nitride one molecular layer is a single atom thick.
  • 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.
  • transition metal dichalcogenides e.g. M0S2 and WS2
  • a molecular layer is three atoms thick (one transition metal atom and two chalcogen atoms).
  • 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.
  • 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.
  • 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.
  • 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.
  • 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).
  • the TMDC is a compound of one or more of Mo, Ta and W with one or more of S, Se and Te.
  • exemplary TMDCs include NbSe 2 , WS 2 , M0S2, TaS 2 , PtTe 2 , VTe 2 .
  • 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.
  • the measured diameter represents the lateral size of the flakes.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • BNNS h-BN nanosheets
  • the inorganic compound may be a transition metal dichalcogenide (e.g. M0S2, WS2, ⁇ 2, MoSe 2 etc.).
  • M0S2 shows a direct bandgap semiconductor (-1.8 eV) when exfoliated down to a single molecular layer.
  • 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.
  • 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.
  • 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.
  • DESs Deep eutectic solvents
  • IL ionic liquid
  • 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.
  • 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).
  • HBD hydrogen bond donor
  • 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.
  • DES deep eutectic solvents to differentiate them from ionic liquids which contain only discrete anions.
  • 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).
  • z refers to the number of Y molecules that interact with the anion.
  • 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.
  • 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 FeCI 2 , FeC , AgCI, CuCI, CuCI 2 , LiCI, CdCI 2 , SnCI 2 , SnCU, ZnCI 2 , LaC , YCI 3 , AICI 3 .
  • 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).
  • type II DESs hydrated metal halides and choline chloride
  • DES ammonium based deep eutectic solvents
  • alumina spheres which replaces the organic solvents traditionally used in all the mechanical exfoliation of graphite.
  • ammonium-based ionic liquid satisfies high activity of the Et 4 N + ions, which accelerates the kinetic of the intercalation process.
  • 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.
  • 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.
  • 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 elsewhere 1241 .
  • 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
  • XPS X-ray photo-electron spectroscopy
  • 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 ⁇ , 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.
  • 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 ⁇ - ⁇ - 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.
  • the pristine defect-free graphite possessed two bands: one at -1580 cm -1 (the G band) arises from the first order scattering of the E2 9 phonon of sp 2 -bonded carbon atoms; and a band at -2600 cm -1 (the 2D band) corresponding to the double-resonance process.
  • the D Band 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 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.
  • 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.
  • 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.
  • XPS X-ray photoelectron spectroscopy
  • 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.
  • 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.
  • SEM scanning electron microscope
  • the intensity of the 002 peak at 2 theta -26.5 which characterize the ⁇ - ⁇ 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.
  • the Raman spectra of bulk WS2 have two vibration modes of Ai g at ⁇ 420 cm -1 and E 1 2g 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 E 1 2g /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 E 1 2g /Ai g increased to -1.2, suggesting a successful exfoliation.
  • the bands of the exfoliated M0S2 nanosheets are different from that of the bulk material both in terms of Raman frequency and signal intensity.
  • two strong Raman bands deconvoluted by a single Lorentzian centered at 383 cm -1 and 407 cm -1 , were assigned to in-plane E 1 2 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.
  • the Ai g and E 1 2 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 E 1 2 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.
  • the process of the present invention provides an efficient and convenient means for exfoliating 2-D materials.
  • 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.

Abstract

La présente invention décrit un procédé d'exfoliation de composés stratifiés inorganiques pour former des composés inorganiques bidimensionnels (2D). Ces composés bidimensionnels ont le même type de forme physique que le graphène en ce qu'ils peuvent être obtenus sous la forme de cristaux moléculaires simple couche ou d'un petit nombre de couches. Elle concerne également des suspensions de composés bidimensionnels qui sont obtenues à partir du procédé d'exfoliation. La présente invention concerne ainsi un procédé de production de matériaux bidimensionnels comprenant, mais n'étant pas limités, au graphène. Les matériaux bidimensionnels de l'invention sont des matériaux inorganiques et tandis qu'ils présentent des différences chimiques significatives entre les divers types de matériaux, ils partagent la caractéristique commune de posséder de solides liaisons covalentes dans le plan à l'intérieur de chaque couche mais avec des couches qui interagissent l'une avec l'autre par l'intermédiaire des liaisons faibles de van der Waals. Ceci amène les feuilles à s'empiler dans des cristaux massifs en 3 dimensions. Lors de l'exfoliation, les nanofeuilles inorganiques simple couche font preuve d'un nouvel ensemble attrayant de propriétés pour l'utilisation dans la nanoélectronique, l'optoélectronique, la catalyse, les gestions thermiques, etc. Les matériaux bidimensionnels sont produits grâce au procédé de l'invention dans un milieu liquide sous la forme soit de matériaux épais monocouches soit d'un petit nombre de couches. Ces matériaux peuvent être considérés de manière générique comme des structures à nanoplaquettes et sont souvent connus par nanoplaquettes dans un but de brièveté. L'exfoliation du graphène et d'autres matériaux bidimensionnels se produit en résultat d'un délicat équilibre entre les facteurs thermodynamiques et cinétiques.
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CN107697902A (zh) * 2017-09-29 2018-02-16 中国石油大学(北京) 一种硼氮双掺类石墨烯纳米碳材料及其制备方法与应用
CN109231176A (zh) * 2018-11-05 2019-01-18 深圳大学 一种碲化铂纳米片及其制备方法
CN109761226A (zh) * 2019-02-16 2019-05-17 江苏大学 一种液相球磨制备二维纳米石墨片的方法
CN110040722A (zh) * 2019-04-16 2019-07-23 中国科学院物理研究所 一种制备石墨烯气泡的方法
US10414668B1 (en) 2017-11-27 2019-09-17 United States Of America As Represented By The Secretary Of The Air Force Exfoliating layered transition metal dichalcogenides
CN111204753A (zh) * 2020-02-12 2020-05-29 西安工程大学 一种剥离氧化石墨制备氧化石墨烯的方法
CN111517294A (zh) * 2020-06-26 2020-08-11 昆明理工大学 一种金属掺杂纳米黑磷的制备方法
CN112591797A (zh) * 2020-12-23 2021-04-02 陕西科技大学 一种高浓度高分散性的超薄二硫化钼分散液及其制备方法
WO2021204946A1 (fr) 2020-04-09 2021-10-14 2D Fab Ab Fabrication de matière bidimensionnelle
CN113880080A (zh) * 2021-11-15 2022-01-04 中国科学院兰州化学物理研究所 一种超滑石墨烯摩尔结构涂层的制备方法
CN115403037A (zh) * 2022-08-31 2022-11-29 西南交通大学 改性氧化石墨烯及其制备方法，润滑油及其制备方法
WO2024026785A1 (fr) * 2022-08-04 2024-02-08 苏州大学 Nanofeuille bidimensionnelle et son procédé de préparation

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CN107697902A (zh) * 2017-09-29 2018-02-16 中国石油大学(北京) 一种硼氮双掺类石墨烯纳米碳材料及其制备方法与应用
US10414668B1 (en) 2017-11-27 2019-09-17 United States Of America As Represented By The Secretary Of The Air Force Exfoliating layered transition metal dichalcogenides
US11247915B1 (en) 2017-11-27 2022-02-15 United States Of America As Represented By The Secretary Of The Air Force Exfoliating layered transition metal dichalcogenides
CN109231176A (zh) * 2018-11-05 2019-01-18 深圳大学 一种碲化铂纳米片及其制备方法
CN109761226A (zh) * 2019-02-16 2019-05-17 江苏大学 一种液相球磨制备二维纳米石墨片的方法
CN110040722A (zh) * 2019-04-16 2019-07-23 中国科学院物理研究所 一种制备石墨烯气泡的方法
CN110040722B (zh) * 2019-04-16 2023-03-21 中国科学院物理研究所 一种制备石墨烯气泡的方法
CN111204753A (zh) * 2020-02-12 2020-05-29 西安工程大学 一种剥离氧化石墨制备氧化石墨烯的方法
WO2021204946A1 (fr) 2020-04-09 2021-10-14 2D Fab Ab Fabrication de matière bidimensionnelle
CN111517294A (zh) * 2020-06-26 2020-08-11 昆明理工大学 一种金属掺杂纳米黑磷的制备方法
CN112591797A (zh) * 2020-12-23 2021-04-02 陕西科技大学 一种高浓度高分散性的超薄二硫化钼分散液及其制备方法
CN113880080A (zh) * 2021-11-15 2022-01-04 中国科学院兰州化学物理研究所 一种超滑石墨烯摩尔结构涂层的制备方法
CN113880080B (zh) * 2021-11-15 2022-12-02 中国科学院兰州化学物理研究所 一种超滑石墨烯摩尔结构涂层的制备方法
WO2024026785A1 (fr) * 2022-08-04 2024-02-08 苏州大学 Nanofeuille bidimensionnelle et son procédé de préparation
CN115403037A (zh) * 2022-08-31 2022-11-29 西南交通大学 改性氧化石墨烯及其制备方法，润滑油及其制备方法
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