FIELD OF THE INVENTION

The present invention relates to the exfoliation of crystalline layered materials to produce nanosheet monolayers thereof.

BACKGROUND OF THE INVENTION

The exfoliation of crystalline layered materials to produce so-called "atomically" thin nanosheets of the layered materials has been the subject of much research interest at least in part due to an ever increasing realisation of the unique advantageous properties of the so formed nanosheets.

Owing to their thin sheet-like character, such nanosheets are often referred to in the art as "two- dimensional (2D) materials" .

While conventional exfoliation techniques can certainly produce atomically thin nanosheets from crystalline layered materials, those nanosheets are typically made up of multiple (3-10) stacked monolayer nanosheets. It still remains a challenge to exfoliate crystalline layered materials into monolayers.

Those skilled in the art will appreciate that by definition a "monolayer" nanosheet represents a layer of the crystalline layered material with a thickness of only one atom. For example, a graphene monolayer nanosheet can be described as a one atom thick sheet of sp2 carbon atoms arranged in a honeycomb crystal lattice structure.

Although two-dimensional (2D) materials have (i) grown into an extended family that now accommodate hundreds of members, and (ii) demonstrated promising advantages in many fields, their practical application still remains hindered by the lack of scalable high-yield production of monolayer products.

An opportunity therefore remains to develop a process for exfoliating crystalline layered materials on scale to provide for nanosheet monolayers in good yield. SUMMARY OF THE INVENTION

The present invention provides a process for producing nanosheet monolayers from a crystalline source material having a layered structure, the process comprising ball milling the crystalline source material with a liquid branched polymer having a viscosity at 20 °C of at least 7000 mPas.

It has now been found nanosheet monolayers can be produced in excellent yield from a crystalline source material having a layered structure in a relatively simple and scalable process employing ball milling and liquid branched polymer. Yields of over 90% of nanosheet monolayers can advantageously be achieved.

Without wishing to be limited by theory, it is believed the liquid branched polymer used in accordance with the invention interacts uniquely with both the ball mill/milling balls and the crystalline source material to not only protect the integrity of the nanosheet monolayers during exfoliation, but also to promote a high adherent environment within the ball mill to facilitate selective peeling of the nanosheet monolayers from the crystalline source material.

In one embodiment, the crystalline source material is selected from graphite, boron nitride, carbon nitride, covalent organic framework, zeolitic imidazolate framework, transition metal dichalcogenide and layered double hydroxides.

In a further embodiment, the liquid branched polymer is a polyamine, polyesteramide, polyimide, polyester, polyepoxide or polyether.

Further aspects and/or embodiments of the invention are discussed in more detail below.

BRIEF DESCRIPTION OF DRAWINGS

Figure 1 illustrates a photograph showing graphene produced by a lab mill at a capacity of hundred gram. The obtained product can be stored in powder form;

Figure 2 illustrates the obtained graphene show a thickness measured by Atomic Force Microscopy (AFM) analysis showing a sheet thickness in the range of 0.5 to 0.8 nm, indicating most of the nanosheets are monolayers;

Figure 3 illustrates the TEM diffraction patterns of the obtained graphene is in line with theoretical diffraction pattern of monolayer graphene, further confirming the graphene nanosheets are monolayers;

Figure 4 illustrates the intensity ratio of Raman I(2D)/I(G) is at 3.1, corresponding to monolayer graphene;

Figure 5 illustrates the powder graphene product can be dispersed in several types of solvents (From left to right are water, acetone, ethanol, dimethylacetamide);

Figure 6 illustrates the powder graphene product can be dried into powder and re-dispersed in solvent and still be monolayers without significant re stacking (i.e. the re -dispersed product presents as monolayers in the solvent);

Figure 7 illustrates monolayer nanosheets could not be identified when low-viscosity PEI is used as the exfoliation medium (Control 1);

Figure 8 illustrates monolayer nanosheets could not be identified when linear PEG-400 is used as the exfoliation medium (Right, Control 3);

Figure 9 illustrates only nanoparticles (not monolayer nanosheets) could be identified when ball milling was performed without the liquid branched polymer used in accordance with the invention (Control 4);

Figure 10 illustrates the method can be used for the exfoliation of various layered materials (from left to right, covalent organic framework TAPB-PDA (COF), metal organic framework (MOF) zeolitic imidazolate framework (ZIF-L), porous graphitic carbon nitride (g-C N^, and hexagonal boron nitride (h-BN)). All of the nanosheets produced comprised monolayers and could be dispersed in water;

Figure 11 illustrates a TEM image of covalent organic framework TAPB-PDA (COF) nanosheets comprising monolayers produced in accordance with the invention; Figure 12 illustrates a TEM image of metal organic framework (MOF) zeolitic imidazolate framework (ZIF-L) nanosheets comprising monolayers produced in accordance with the invention;

Figure 13 illustrates a TEM image of graphitic carbon nitride (g-CNN-t) nanosheets comprising monolayers produced in accordance with the invention; and

Figure 14 illustrates a TEM image of graphitic carbon nitride (g-C-Na) nanosheets comprising monolayers produced in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a process for producing nanosheet monolayers from a crystalline source material having a layered structure.

As used herein, a "nanosheet monolayer(s)" is intended to mean a layer of a crystalline layered material having a thickness of only one atom. Nanosheet monolayers typically present a thickness ranging from about 0.3 to 1 nm. Such materials are considered to be two-dimensional in the sense they present a width and length, but no practical height. While there is no particular limitation on the width and length dimensions the nanosheet monolayer(s) can have, they will typically range in the order of about 100 nm to about 2000nm.

For avoidance of any doubt, the nanosheet monolayers produced in accordance with the present invention are not nanosheet materials made up of multiple stacked monolayers. Rather, the nanosheet monolayers produced in accordance with the invention are the individual monolayers per se that make up such multi -monolayer materials.

Those skilled in the art will be familiar with using conventional analytical techniques to assess the dimensions of nanosheets, including nanosheet monolayers. For example, the dimensions of nanosheet monolayers in accordance with the invention can be readily determined using atomic force microscopy (AFM) and/or assessing diffraction patterns of samples obtained using Transmission electron microscopy (TEM).

The nanosheet monolayers produced in accordance with the present invention are derived from a crystalline source material having a layered structure. Such materials are made up of stacked nanosheet monolayers. Those skilled in the art will be familiar with suitable crystalline source materials. The present invention can advantageously be applied using all such crystalline source materials.

Examples of crystalline source materials having a layered structure that may be used in accordance with the invention include, but are not limited to, graphite, boron nitride, carbon nitride, covalent organic frameworks, metal organic frameworks, transition metal dichalcogenide and layered double hydroxides.

Examples of suitable layered covalent organic frameworks include, but are not limited to, TAPB-PDA COF, TFB-DAB COF, TAPB-BPDA COF, and TFPB-HZ COF.

Examples of suitable metal organic frameworks (MOF) include, but are not limited to, zeolitic imidazolate framework (ZIF-L), Cu(BDC) MOF and Aluminum tetra-(4 -carboxyphenyl) porphyrin framework (A1-M0F).

Examples of suitable transition metal dichalcogenide include, but are not limited to, molybdenum disulphide, tungsten disulphide, molybdenum diselenide, tungsten diselenide and molybdenum ditelluride.

Examples of suitable layered double hydroxides include, but are not limited to, Mg-Al, Ca-Al, Zn-Al and Cu-Al LDHs.

Provided the crystalline source material used in accordance with the invention can be subjected to ball milling, there is no particular limitation on the physical size which it may present. Generally, the crystalline source material used in accordance with the invention will be in particulate form having an average size ranging from about 1pm to about 30pm.

The process in accordance with the invention comprises ball milling the crystalline source material. Conventional ball milling equipment and techniques can advantageously be employed in the present invention.

Suitable ball milling equipment includes, but is not limited to, planetary ball mills, horizontal ball mills and vertical ball mills. Those skilled in the art will appreciate ball milling equipment will include a milling container/jar and milling balls. There is no particular limitation on the material from which the milling container and balls are made provided they can withstand operating parameters and are substantially inert toward the materials being processed.

Suitable milling containers include, but are not limited to, those made from stainless steel, zirconia (zirconium dioxide) and agate.

Suitable milling balls include, but are not limited to, those made from silicon carbide, stainless steel, zirconia (zirconium dioxide) and agate.

In one embodiment, both the milling container and milling balls are made from zirconia.

The size of the milling containers and balls used can be readily adjusted by those skilled in the art to suit the scale of the operation. The invention can advantageously be performed at lab, pilot and industrial scale.

In one embodiment, the milling container has a volume ranging from about 50ml to about 100L, or 50ml to about 10L, or 50ml to about IL.

Those skilled in the art will be able to readily select the size of the milling balls to suit the task at hand, including taking into account the volume of the milling container and the required ratio of milling balls to crystalline source material.

In one embodiment, the milling balls have a diameter ranging from about 1 mm to about 20 cm.

In one embodiment, ball milling is conducted using milling balls having at least two different diameters.

The weight ratio of the milling balls to the crystalline source material will generally range from about 0.5:5.

There is no particular limitation on the speed at which ball milling is to be conducted.

In one embodiment, ball milling is conducted at a speed ranging from about 300 to about 600 rpm, or from about 450 to about 550 rpm, or at about 500 rpm.

The timeframe in which ball milling is undertaken will vary depending upon, for example, the nature of the crystalline source material, the amount of crystalline source material and the ball milling equipment. Those skilled in the art will be able to readily select a suitable time taking into account such parameters.

Generally, ball milling will be conducted for at least about 3 hours, or at least about 4 hours, or at least about 5 hours. For example, ball milling may be conducted for a time ranging from about 3 hours to about 25 hours, or about 4 hours to about 25 hours, or about 5 hours to about 25 hours.

Ball milling can advantageously be performed at room temperature and atmospheric pressure.

If desired, ball milling can be conducted at temperatures below and/or higher than at room temperature, for example at temperatures ranging from about 10°C to about 100°C.

In one embodiment, ball milling is conducted at ambient temperature and pressure.

Ambient temperatures are considered to range from about 10° C to about 45° C.

Ambient pressure is considered to be about one standard atmosphere.

An important feature of the process in accordance with the invention is that the crystalline source material is subjected to ball milling with a liquid branched polymer having a viscosity at 20° C of at least 7000 and mPas.

Without wishing to be limited by theory, it is believed the liquid branched polymer used in accordance with the invention interacts uniquely with both the ball mill/milling balls and the crystalline source material to not only protect the integrity of the nanosheet monolayers during exfoliation (i.e. it prevents or minimises fracturing of the nanosheets), but also to promote a high adherent environment within the ball mill to facilitate selective peeling of the nanosheet monolayers from the crystalline source material.

By the branched polymer being in "liquid" form is meant that the branched polymer per se has a liquid composition at the temperature at which ball milling is conducted. In other words, the branched polymer to be used does not require the use of solvent to facilitate its liquid formation at the temperature at which ball milling is conducted.

The liquid branched polymer used in accordance with the invention will generally have a liquid state within a temperature ranging from at least about 10° C to about 100° C. As those skilled in the art will appreciate, some liquid branched polymers may only reside in a liquid state (irrespective of temperature).

Accordingly, ball milling undertaken in accordance with the invention may be performed using the liquid branched polymer without the presence of any solvent to facilitate liquefaction of the branched polymer. In other words, ball milling performed in accordance with the invention may be conducted using liquid branched polymer that has not being diluted with solvent.

In one embodiment, the liquid branched polymer does not comprise solvent.

In a further embodiment, ball milling is performed using the liquid branched polymer in the absence of solvent.

In another embodiment, ball milling is conducted using the liquid branched polymer that is not diluted in a solvent.

The presence of solvent can reduce the viscosity of the liquid branched polymer and consequently reduce the adherent or sticky nature of the polymer. That in turn can reduce the exfoliation efficiency of the process and hinder formation of the nanosheet monolayers.

Those skilled in the art will be familiar with the molecular structural requirements of branched polymers relative to linear polymers.

The liquid branched polymer used in accordance with the invention has a viscosity at 20° C of at least 7000 mPas.

The viscosity of the liquid branched polymer referenced herein is that determined at 20° C using a rotational viscometer. The viscosity at 20° C of the liquid branched polymer used in accordance with the invention may range from at least 7000 mPas to about 200,000 mPas, for example from about 100,000 mPas to about 160,000 mPas.

The process in accordance with the invention can advantageously be performed using a diverse range of liquid branched polymers.

Examples of suitable liquid branched polymers include, but are not limited to, polyamine, polyesteramide, polyester, polyimide, polyepoxide and polyethther.

In one embodiment, the liquid branched polymer is selected from polyamine, polyesteramide, polyester, polyimide, polyepoxide, polyethther and combinations thereof.

A further embodiment, the liquid branched polymer comprises a functional group selected from carboxylic acid, hydroxyl, amino and epoxy.

Such functional groups may be present as a terminal end group of the liquid branched polymer.

Without wishing to be limited by theory it is believed at least some functional groups present on the liquid branched polymer may undergo reaction during ball milling with the crystalline source material to become bound to the surface thereof. That interaction is not only believed to facilitate exfoliation of the crystalline source material into nanosheet monolayers, but also minimise or prevent re-stacking of the nanosheet monolayers after they are formed.

Examples of suitable branched polyamines include, but are not limited to, polyethylenimine, such as polyethylenimine- 100000 and polyAziridine PZE-1000 and polyaniline.

Examples of suitable branched polyesteramides (PEA) include, but are not limited to, tetraaniline-grafted PEAs and PEAs with pendant C-C double bonds.

Examples of suitable branched polyesters include, but are not limited to, terminal hydroxylhyperbranched polyesters, and terminal carboxyl-hyperbranched polyesters.

Examples of suitable branched polyimides include, but are not limited to, hydroxyl polyimides and polyamide amines. Examples of suitable polyepoxides include, but are not limited to, bisphenol A epoxy resin, bis(4-glycidyloxyphenyl)methane and bisphenol F diglycidyl ether (DGEBF).

Examples of suitable polyeththers include, but are not limited to, polypropylene glycol.

The molecular weight of liquid branched polymers used in accordance with the invention will generally range from about 1000 to about 1,000,000.

Reference herein to molecular weight of the liquid branched polymer is intended to mean a number average molecular weight (Mn) as determined by gel permeation chromatography (GPC).

Provided the nanosheet monolayers are formed, there is no particular limitation on the weight ratio of liquid branched polymer to crystalline source material that can be used. The liquid branched polymer will typically be used in accordance with the invention in a weight ratio to the crystalline source material ranging from about 0. 1 to 6, for example from about 0.5 to about 5.0 or from about 3.5 to about 4.5.

The process in accordance with the invention advantageously promotes a high degree of exfoliation of the crystalline source material into nanosheet monolayers.

In one embodiment, at least 40 wt. %, or at least 50 wt. %, or at least 60 wt. %, or at least 70 wt. %, of the crystalline source material used is converted into nanosheet products. Of the nanosheet product produced, at least 50 %, or at least 60 %, or at least 70 %, or at least 75 % by number is present in the form of nanosheet monolayers.

In one embodiment, at least 60 %, at least 70 %, at least 80 %, or at least 90 %, or at least 95 % by number of nanosheet material produced is in the form of nanosheet monolayers.

The number of nanosheet monolayers within the nanosheet material produced is determined by a statistic atomic force microscopy (AFM) characterisation.

On completion of the ball milling the liquid branched polymer will comprise dispersed therethrough the so formed nanosheet monolayers. Depending upon the degree of conversion, the liquid branched polymer may also comprise a percentage of partially exfoliated crystalline source material.

The nanosheet monolayers produced in accordance with the invention can be readily isolated by applying a conventional washing and collection regime.

For example, the liquid branched polymer comprising the nanosheet monolayers maybe washed with a suitable liquid. That liquid will typically be a solvent for the liquid branched polymer. Those skilled in the art will be able to readily select a suitable washing liquid for that purpose.

Examples of suitable solvents for washing include, but are not limited to, water, methanol, ethanol and combinations thereof.

The washed nanosheet monolayers may be collected using conventional techniques. For example, they may be collected by centrifugation or filtration.

Once isolated, the produced nanosheet monolayers may be dried. Surprisingly, isolated nanosheet monolayers produced in accordance with the invention are not prone to agglomeration. Furthermore, the resulting dried nanosheet monolayers can be readily redispersed in a liquid.

Examples of suitable liquids into which nanosheet monolayers can be dispersed include, but are not limited to, water, acetone, ethanol, methanol, dimethylacetamide and combinations thereof.

Without wishing to be limited by theory, it is believed the ability of the so formed nanosheet monolayers to resist aggregation/re stacking upon being isolated and dried and/or being able to be readily disbursed in a liquid having been isolated and dried at least in part may be due to a residual amount of the liquid branched polymer being bound to the surface of the nanosheet monolayers. That residual liquid branched polymer is believed to potentially function as a steric stabiliser that advantageously can prevent aggregation/re stacking and dispersion within a liquid.

Where the nanosheet monolayers are produced having residual liquid branched polymer bound to the surface thereof, if desired, that residual liquid branched polymer can be removed. The technique employed to remove such residual liquid branched polymer may vary depending upon the nature of the nanosheet monolayers and/or the liquid branched polymer. Those skilled in the art will be able to select a suitable technique having an understanding of the composition of both the nanosheet monolayers and the liquid branched polymer. For example, the nanosheet monolayers may be subjected to acid washing and/or a heat treatment such as being calcined.

EXAMPLES

Materials and equipment

Polyethyleneimine (PEI) with average Mn at -10,000, branched PEI with average Mn at -600, PEG Poly (ethylene glycol) with average Mn at -10,000, and Graphite (0.5 g, particle size 30 pm) were bought from Sigma- Aldrich. Planetary ball mill (mill (ZQM-P2, Changsha Mitrcn Instrument Equipment Co., Ltd) and ZrO2 balls (diameters of d=10 mm, d=5 mm and d=0. 1 mm) was applied as exfoliation equipment.

It is to be understood that in the following examples, the addition of branched polymer, the viscosity of the polymer, and the weight ratio of polymer to layered material are important to achieve a high monolayer percentage. The present examples are therefore considered to demonstrate the role of branched polymer to the present invention.

To demonstrate the importance of the polymer assistance, four types of polymer were used as exfoliation medium. (1) High-molecular weight branched Polyethyleneimine (PEI) with average Mn at -10,000 by GPC, average Mw at -25,000 by LS (coded as high-viscosity PEI) (Sigma- Aldrich, 408727); (2) Low-molecular weight branched PEI with average Mn at -600 by GPC, average Mw at -800 by LS (coded as low -viscosity PEI) (Sigma-Aldrich, 408719); (3) Liner Polyethyleneimine with average Mn at -10,000 by GPC (coded as liner PEI) (Sigma-Aldrich, 765090); (4) Poly (ethylene glycol) with average Mn at -10,000 by GPC (coded as liner PEG- 400) (Sigma-Aldrich, 202398).

A first experimental run was conducted for the production of graphene using a planetary ball mill (mill (ZQM-P2, Changsha Mitrcn Instrument Equipment Co., Ltd). Three kinds of ZrO2 balls with different weights and diameters (100g o d=10 mm, 200g o d=5 mm and 20g o d=0. 1 mm) were loaded in the milling jar. Graphite (0.5 g, particle size 30 pm) and 2g high-viscosity PEI was charged into a 250-mL milling jar. The milling jar was loaded in the ball mill with its revolution radius at 10 cm and rotation radius equal to the radius of the milling jar at 39 mm. The rotation speed was set as 250 rpm. for revolution and 500 rpm. for rotation. The whole process was conducted in the ambient environment. After the milling process lasted for 10 hours, 100g milli-Q water was added to the milling jar. Then the milling jar was loaded into the mill again for 30 min at a lower speed (revolution: 150 rpm, rotation: 300 rpm) to disperse the nanosheets in water. A fdtration rinsing process was followed to remove excess PEI. Subsequently, the dispersion was centrifugated at 1,500 rpm (Sigma 2-16P) for 20 min to remove thick flakes. The supernatant was transferred to give a final dispersion product.

Three control experimental runs were undertaken to demonstrate the importance of the polymer type. The control experimental runs used the same conditions except for the added polymer type. Control experimental run 1 used low-viscosity PEI. Control experimental run 2 used linear PEI. Control 3 used linear PEG. Other conditions remain unchanged, i.e. starting material graphite, polymer addition weight 2g, rotation speed 50 rpm, milling time lOh, and milling atmosphere of air.

Other two control experimental runs were further undertaken to demonstrate the importance of the weight ratio (0.5 to 4) of the added polymer to the layer materials. The control experimental runs used the same conditions except for the added weight ratio. Control 4 used all the same conditions except for without adding polymer. Other conditions remaining unchanged, i.e. starting material graphite, rotation speed 50 rpm, milling time lOh, and milling atmosphere of air.

Other types of nanosheets including hexagonal boron nitride (h-BN), graphic carbon nitride (g- C3N4), TAPB-PDA covalent organic framework (TAPB-PDA COF), and zeolitic imidazolate framework (ZIF-L) were produced using high-viscosity PEI with addition weight of 2g. Other conditions remain unchanged, i.e. rotation speed 50 rpm, milling time lOh, and milling atmosphere of air.

Results

The results show that graphene nanosheets produced from the inventive experimental run with suitable parameters (polymer type high-viscosity PEI, polymer addition weight 2g, rotation speed 50 rpm, milling time lOh, and milling atmosphere of air) are mostly monolayer graphene with monolayer percentage up to 91.2 (Figure 2, Figure 3, and Figure 4). The produced graphene nanosheets have a lateral size from lOOnm to 1 pm while their thickness measured by AFM characterization is in the range of 0.5 nm to 0.8 nm. The exfoliation yield was 78.3% by weighting the total obtained graphene products from 2-g graphite.

When low-viscosity PEI was used as the exfoliation medium (Control 1, Figure 7), the thickness of graphite was only exfoliated to several tens of nanometers and the exfoliation yield was lower than 20%. When liner PEI or liner PEG was used as exfoliation medium (Control 2 and 3, Figure 8), no efficient exfoliation was achieved. These results demonstrated that adding branched polymer is important for producing high-percentage monolayer graphene products.

When milling graphite alone (without adding polymer, control 4, Figure 9), graphene nanosheets cannot be produced. Only small nanosized particles were obtained. These results demonstrated that varying the amount of branched polymer plays a role in producing the nanosheet monolayers.

Other experiment runs using layered starting materials other than graphite also show successful exfoliation. All the obtained nanosheets can be dispersed in water at a concentration of 1 mg/mL (Figure 10). TEM characterizations ofTAPB-PDA COF (Figure 11), ZIF-L (Figure 12), g- C3N4 (Figure 13), and h-BN (Figure 14) show typical nanosheets properties of the obtained products, demonstrating a successful exfoliation.

The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.