PROCESS FOR THE CONTINUOUS PRODUCTION OF SUB-MICRON TWO-DIMENSIONAL MATERIALS SUCH AS GRAPHENE

Abstract

A system and a method of continuously separating submicron thickness laminar solid particles from a solid suspension, segregating the suspension into a submicron thickness particle fraction suspension and a residual particle fraction suspension, the method comprising the steps of; providing a continuous centrifuge apparatus; providing a suspension of submicron thickness laminar solid particles in a solid suspension; wherein the solid suspension comprises the submicron thickness solid particles in a liquid continuous phase; separating the solid suspension in the apparatus.

Claims

1. A method of continuously separating a solid suspension containing submicron thickness laminar solid particles into a submicron thickness particle fraction suspension and a residual particle fraction suspension, the method comprising the steps of: providing a continuous centrifuge apparatus; providing a solid suspension of submicron thickness laminar solid particles; separating the solid suspension in the apparatus; wherein the solid suspension comprises the submicron thickness laminar solid particles in a liquid continuous phase.

2. The method of claim 1 wherein the continuous centrifuge apparatus is a disc stack centrifuge.

3. The method of claim 1, wherein the submicron scale laminar solid particles comprise particles of a material having a crystalline structure comprising atomically thin layers, which have been partially delaminated into atomically thin nano-platelets.

4. The method of claim 1, wherein the submicron laminar solid particles comprise particles of partially delaminated graphite, hexagonal boron nitride, molybdenum disulphide, tungsten diselenide or other transition metal dichalcogenides.

5. The method of claim 1, wherein the submicron laminar solid particles comprise particles of partially delaminated graphite or hexagonal boron nitride.

6. The method of claim 1, wherein the submicron laminar solid particles comprise particles of partially delaminated molybdenum disulphide, molybdenum diselenide, molybdenum ditelluride, tungsten disulphide and tungsten diselenide.

7. The method of claim 1, wherein the submicron particles are in the particle size range of 1 to 100 nm.

8. The method of claim 1, in which the suspension is a solid suspension in water as the continuous phase.

9. A system for the continuous separation of delaminated submicron thickness particles from a suspension of laminar particles, the system comprising: a continuous centrifuge apparatus; wherein the suspension of laminar particles comprises a solid suspension comprising submicron thickness solid particles in a liquid continuous phase; the system being configured to continuously feed the solid suspension to the centrifuge which separates the solid suspension into a suspension of submicron thickness laminar particles and a solid suspension residue.

10. The system of claim 9, wherein the continuous centrifuge apparatus is a disc stack centrifuge.

11. The system of claim 9, wherein the submicron laminar solid particles comprise particles of a material having a crystalline structure comprising atomically thin layers, which have been partially delaminated into atomically thin nano-platelets.

12. The system of claim 9, wherein the submicron thickness laminar solid particles comprise particles wherein the suspension is a suspension of partially delaminated graphite, hexagonal boron nitride, molybdenum disulphide, molybdenum diselenide, molybdenum ditelluride, tungsten disulphide and tungsten diselenide.

13. The system of claim 10, wherein the submicron thickness laminar solid particles comprise particles of partially delaminated graphite or hexagonal boron nitride.

14. The system of claim 10, wherein the submicron thickness laminar solid particles comprise particles of partially delaminated molybdenum disulphide, tungsten diselenide or other transition metal dichalcogenides.

15. The system of claim 9, in which the suspension is a solid suspension in water as the continuous phase.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0070] FIG. 1 shows a section through a typical disc stack centrifuge as used in the present invention.

[0071] FIG. 2 shows a section through a typical disc stack centrifuge with the solids discharge gap open.

[0072] FIG. 3 shows a chart of particle size distribution of a graphene dispersion produced by the current invention.

[0073] FIG. 4 shows a chart of particles size distribution of a boron nitride dispersion as produced by the current invention.

[0074] FIG. 5 shows the Raman spectra of a sampled of graphene produced by the current invention.

[0075] FIG. 6 shows a chart of the particle size distribution of a molybdenum disulphide suspension after the use of the method of the present invention.

[0076] FIG. 7 shows a chart of particle size distribution of a graphene dispersion produced by the present invention compared with a dispersion produced by an alternative process.

DETAILED DESCRIPTION OF THE INVENTION

[0077] FIG. 1 and FIG. 2 show a cross section through a disc stack centrifuge (100). Separation takes place inside a rotating vessel (110). During operation, the vessel rotates about axis (170), generating centrifugal force in a radial direction. The suspension is introduced to the rotating centrifuge vessel (110) from the bottom via an inlet pipe (120), and is accelerated in the distributor (130), which may be configured to provide smooth acceleration of the input suspension. Leaving the distributor, the suspension enters the disc stack (140). The separation takes place between the discs, with the liquid continuous phase moving radially through the disc stack towards the centre. During this movement, the suspended solids that are denser than the liquid continuous phase are differentially accelerated outwards in the opposite direction to the liquid movement. When the liquid reaches the centre, it is discharged through the exit (150) where it is collected or may be recirculated for further centrifugation. The particles separated from the suspension move to the periphery (160).

[0078] These particles collect in the periphery where they may be discharged. The solid may be discharged by means of a gap (180) between the top (190) and bottom (200) of the rotating vessel at suitable pre-set intervals, the gap being opened by mechanical means for example a hydraulic system, or the gap can be set at a permanent suitable width to enable a continuous discharge of solids.

[0079] In the present invention, the disc stack centrifuge may be used to separate graphene nano-platelets according to their thickness or particle size. In some embodiments, the disc stack centrifuge can enable a sorting of the graphene nano-platelets according to the number of atomic layers present.

[0080] In the present invention, the disc stack centrifuge may be used to separate hexagonal boron nitride nano-platelets according to their thickness or particle size.

[0081] In the present invention, the disc stack centrifuge may be used to separate molybdenum disulphide nano-platelets according to their thickness or particle size.

[0082] Molybdenum diselenide, molybdenum ditelluride, tungsten disulphide, tungsten diselenide are further materials which have shown some success with the separation process described by the present invention.

[0083] For example, a disc stack centrifuge may be used to concentrate nano-platelets having at least one common characteristic (e.g., sheet resistance, Raman spectra, number of atomic layers).

[0084] The present invention may exclude graphene and hexagonal boron nitride.

[0085] With a 2-dimensional material such as graphene the rate of separation of particles depends on both the effective density and the size of each particle, and as the particles are not spherical the separation rate may also depend on the orientation of the platelets.

[0086] FIG. 3 shows an example of the particle size distribution of a graphene suspension, before and after the use of the present invention. The particle size was measured using a Malvern Mastersizer™. The solid line (500) shows the range of particle sizes in the first suspension before the disc stack centrifuge process was carried out. The peak has a maximum at approximately 13.5 μm. The dashed line (510) shows the range of particle sizes in the suspension after the disc stack centrifuge process. The peak has a maximum at approximately 0.05 μm. This demonstrates that the disc stack centrifuge has surprisingly (?) separated the graphene nano-platelets very effectively compared with prior art techniques.

[0087] We have demonstrated that this technique using the equipment and configuration we have identified can separate nano-platelets that are particularly suitable for end uses such as conductive inks, in a continuous flow process that can process from 50 litres per hour to 4000 litres per hour of suspension, compared with prior art techniques that require many hours to process a few millilitres of suspension.

[0088] FIG. 4 shows an example of the particle size distribution of a boron nitride suspension before and after the use of the present invention. The solid line (520) shows the range of particle sizes in the first suspension before the disc stack centrifuge process was carried out. The peak has a maximum at approximately 0.5 μm. The dashed line (530) shows the range of particle sizes in the suspension after the disc stack centrifuge process. The peak has a maximum at approximately 0.26 μm. This demonstrates that the disc stack centrifuge has also separated the boron nitride nano-platelets very effectively.

[0089] FIG. 5 shows an example of Raman spectroscopy of a sample of graphene nano-platelets prepared using the present invention. The G peak (600), the D peak (610) and the 2D peak (620) are used to indicate the number of defects and sheet thickness. In this case the ratio D/G=0.49 which is higher than some samples obtained using other methods, and is perhaps consistent with smaller flakes.

[0090] The symmetric shape of the 2D peak indicates that the sample has good quality in terms of percentage of monolayer graphene.

[0091] The sheet resistance of this sample was 8.26 Ohm/Sq, which is advantageously lower than many samples produced using prior art techniques.

[0092] FIG. 6 shows an example of the particle size distribution of a molybdenum disulphide suspension after the use of the method of the present invention. The solid line shows the range of particle sizes in the suspension after the disc stack centrifuge process. The peak has a maximum centered at approximately 0.72 μm. The material entering the disc stack separator (not shown) had a mean particle size of 100 μm. This demonstrates that the disc stack centrifuge has also separated the molybdenum disulphide nano-platelets very effectively into a sub-micron fraction.

[0093] FIG. 7 shows a chart of particle size distribution of a graphene dispersion produced by the present invention compared with a dispersion produced using a hydrocyclone. A hydrocyclone is another continuous separation process that is used to separate particles in suspension into two streams according to particle size and/or mass. The dotted trace on the chart shows the starting suspension which in this case is a graphene dispersion produced using a homogeniser. The peak particle size is around 7 microns. The dashed trace shows the suspension resulting from the overflow of a hydrocyclone, which has a similar distribution to the starting suspension. The solid line then shows the particle size distribution of the overflow from a disc stack separator, showing a clear reduction in peak particle size by volume, to around 0.1 micron.

[0094] The process of the invention allows the selection of desirable properties of the material, which may be classified in the following ways.

[0095] As an example, the nano-platelets may be classified by the lateral size of each platelet. A range of methods have been used to give an indication of the particle size distribution of the graphene nano-platelets. This ranges from Transmission Electron Microscope (TEM) analysis for an “idealised” system of discrete particles to a laser scattering testing and sieve analysis for powders and suspensions of nano-platelets that will be more indicative of the end product.

[0096] Lateral flake size may be measured by scanning or transmission electron microscopy. This technique comprises preparing a sampled of either the powder or a dispersion of the material, producing an electron microscope image, and then measuring the perpendicular length and width of the flakes.

[0097] TEM samples are drop-casted onto holey carbon grids and allowed to dry at 60° C. for 72 hours under vacuum. Bright field and energy filtered TEM micrographs are taken at random locations across the grids, to ensure a non-biased representation of the level of exfoliation.

[0098] The samples may be characterised using low resolution TEM. The aim of this is twofold: to assess the nature and quality of the exfoliated flakes; and in some cases, to measure the lateral flake dimensions. Samples are prepared by drop-casting and imaging the grids on a Jeol 2100™ TEM operating with an LaB6 electron gun at 200 kV.

[0099] The thickness of samples prepared in the same way can also be measured using atomic force microscopy (AFM). This technique can both provide estimates of lateral size as well as thickness of nano-platelets.

[0100] In practice electron microscope techniques are quite time-consuming to perform. Therefore, a quicker method of size classification is used during testing and production, which is compared to the electron microscope imagery for calibration.

[0101] Samples of the products have been analysed using a Malvern Mastersizer 3000™. This uses the technique of laser diffraction to measure the size of particles, by measuring the intensity of light scattered as a laser beam passes through a dispersed particulate sample. This data is then analysed to calculate the size of the particles that created the scattering pattern. The Malvern Mastersizer™ settings were “non-spherical particle mode”, using red and blue light, and water as the medium. A stirrer was fitted to ensure the samples were uniform.

[0102] Sieve testing has been used to give an indicative size distribution by weight, which is analogous to the measurements obtained at the Malvern Mastersizer™ by volume. The results are in a similar range to that obtained in the Malvern Mastersizer™ which gives an independent cross check of the latter's measurements.

[0103] Two sieve sizes are used, a 150 μm and a 38 μm hole size. The sieves are weighed and stacked and then placed on the sieve shaker. 5.29 g of powder produced by drying the product of the present invention is added in the top. An initial time of 5 minutes is used at full shaker capacity (3 mm/g). The material is then weighed and the sieves are reassembled with the material still in the sieves. The sieves are then put back on the shaker for another 5 minutes.

[0104] Raman spectroscopy is used to classify the quality of the platelets. In the case of graphene, the G peak, the D peak and the 2D peak are used to indicate the number of defects and sheet thickness. The ratio of intensity of D/G bands is a measure of the defects present on graphene structure. The G band is a result of in-plane vibrations of SP2 bonded carbon atoms whereas the D band is due to out of plane vibrations attributed to the presence of structural defects. All platelets have edges, which are defects in the crystal structure and so the D band is never zero as it would be in a perfect, infinite plane. If there are some randomly distributed impurities or surface charges in the graphene, the G-peak can split into two peaks, G-peak (1583 cm-1) and D′-peak (1620 cm-1). The main reason is that the localized vibrational modes of the impurities can interact with the extended phonon modes of graphene resulting in the observed splitting. The D peak usually appears around 1350 cm-1 and the G peak usually appears around 1570 cm-1. The 2D peak sometimes labelled as G′ corresponds to the same vibrations as the D band, but can be used to assess the number of atomic layers in a sample. Combination peaks D+D′ and D+D″ may appear around 2460 cm.sup.−1 and 2930 cm.sup.−1.

[0105] The Raman spectra is measured using a Horiba XploRA™ Raman Microscope. Samples are supplied as a thin film on a filter membrane. After baseline removal, the peaks D, G, D′, D+D′ and 2D are manually identified using the manufacturer's analysis software. The D/G ratio is calculated by dividing the peak intensity of the D peak by the peak intensity of the G peak. At least five spectra are analysed for each sample, and the D/G and D/D′ ratio is calculated for each, which are then averaged.

[0106] In the present analysis, the interpretation of the 2D peak is not as straightforward as in pure graphene. Due to a large number of flakes in each sample, each flake gives a 2D band, the intensity and shape of which is dependent on the number of layers and stacking of those layers. In single layer graphene, the 2D peak is a single peak with a 2D/G ratio around 4, whereas for bilayer graphene the peak splits and the intensity reduces. The peak changes shape with each additional layer.

[0107] Without wishing to be bound by theory, when Raman spectroscopy is carried out on a sample of nano-platelets with many overlapping flakes a straightforward characterization is not possible. In general, the 2D peak is less clearly defined than for single layer graphene, but a well-rounded peak is indicative of more single layer graphene in the sample than the uneven peaks caused by multilayer samples.

[0108] An important parameter is the sheet resistance, measured in ohms/square. The measurement using a four-point probe is carried out on a sample of the product applied to a substrate, such as a graphene film on a nylon membrane.

[0109] The sample is weighed, and measurement is taken according to instructions from the manufacturer of the four-point probe. In this case, we used a Jandel™ RM3000 set to 10 mA measuring current. Six measurements are taken at different positions of each sample and an average taken. The measurement is normalized based on the measured weight to an equivalent 30 mg sample.

[0110] For both Raman and Sheet resistance measurements, the samples were prepared by the following steps:

[0111] Obtain a dispersion with nominal concentration of 100 g/L.

[0112] Obtain Nylon 66 membrane and plastic petri dish for storage.

[0113] Weigh nylon membrane in precision scales and note weight on petri dish.

[0114] Obtain vacuum pump and place in fume cupboard.

[0115] Place weighed membrane on glass filter ensuring the centre point of the membrane is aligned with the centre point of the glass filter.

[0116] Wet the membrane with a few ml of de-ionised water until membrane is soaked with water.

[0117] Place glass funnel on glass filter and clamp in place with metal clamp

[0118] Turn on vacuum pump, until nylon membrane rests firmly and uniformly on glass filter and all water is filtered through the membrane.

[0119] Using a micro-pipette measure 0.3 ml of graphene dispersion in a volumetric cylinder and top up dispersion with de-ionised water up to a total volume of approximately 20 ml.

[0120] Turn vacuum pump on and pour dispersion in glass funnel.

[0121] When all water is filtered through pour 20 ml of de-ionised water in funnel and allow this to filter through.

[0122] Turn off vacuum pump.

[0123] Remove clasp and glass funnel.

[0124] Remove membrane from glass filter.

[0125] Place membrane on petri dish

[0126] Place petri dish in oven for at least 2 hours at 50° C.

[0127] An indication of the surface area of the product may be obtained by the BET (Brunauer-Emmett-Teller) gas adsorption method, supported by the electron microscopy described above. Surface areas of the samples are analysed by N2 physisorption at the liquid nitrogen temperature using a Micromeritics™ ASAP® 2020 instrument. Before the analysis, the samples are de-gassed at 200° C. for 12 hours at a pressure lower than 10.sup.−3 mmHg. The surface area of the samples is then calculated applying the BET equation to the collected data. The eventual presence of micropores is assessed by t-plot analysis. The pore size distributions and cumulative pore volumes for both adsorption and desorption branches are calculated following the BJH (Barrett-Joyner-Halenda) method. All the calculations are performed following the IUPAC recommendations.

[0128] In one example, using graphite particles delaminated into graphene, the measurements before and after separation were taken using the techniques described above and produced the following results:

TABLE-US-00001 Sample L5443 Before separation After separation Sheet resistance Ω/Sq. 11.7 5.9 Raman D/G  0.08 0.22 Modal particle size. 4.8 micron 0.12 micron

[0129] Factors which may be controlled in the process to select the quality of graphene produced are as follows.

[0130] The loading of solid material in the liquid phase, which affects the viscosity of the suspension, and the productivity of delamination process prior to the separation. Although the separation process may be modelled as a Stokes settlement process, it is in fact far more complex as a suspension of mixed particles and platelets with a wide variation in size will not behave like a uniform suspension of spheres.

[0131] The solid content is preferably between 1% and 20% by weight, more preferably between 1% and 10% by weight.

[0132] The range of particle size in the input suspension ranges from 0.01 μm to 100 μm as measured by particle size distribution analysis.

[0133] The range of thickness of the particles in the input suspension is between 0.2 nm and 100 μm, preferably less than 100 nm, most preferably in the range one atom thick up to 30 layers thick.

[0134] The temperature of the suspension may affect the viscosity. An advantageous effect is that while it is known that the viscosity of water reduces with increasing temperature, the bulk viscosity of a graphene suspension has been discovered to increase with temperature. This enables the separation of graphene from a mixed suspension to be fine-tuned by careful selection of the operating temperature during the centrifugation stage in order to optimise the viscosity ratio between the sediment and the supernatant. Advantageously we have found the most efficient separation rate to take place at a temperature of between 5° C. and 50° C., for example between 20° C. and 40° C. preferably at 35° C.

[0135] The viscosity of the continuous phase may be between 0.0001 Pa.Math.s and 10 Pa.Math.s. Preferably the viscosity is between 0.0001 Pa.Math.s and 0.1 Pa.Math.s. Most preferably the viscosity is between 0.0004 Pa.Math.s and 0.001 Pa.Math.s.

[0136] A surfactant may be added to the suspension which will also vary the viscosity and the stability of the suspension. The preferred surfactant is sodium cholate.

[0137] The density of the continuous liquid phase may be varied to improve the degree of separation, although this will reduce the speed of separation.

[0138] In the case of nano-platelets, the effective density of the surfactant coated platelet in suspension varies with the number of layers in the platelet. For example, monolayer graphene in sodium cholate has an effective density around 1.16 g/cc.

[0139] The density of the input suspension may be between 0.3 g/cc and 5 g/cc, preferably between 1 g/cc and 1.5 g/cc.

[0140] The density of the continuous phase liquid used in the suspension may be between 0.3 g/cc and 1.5 g/cc, preferably between 0.9 and 1.4 g/cc, most preferably 1.1 g/cc.

[0141] By selecting the appropriate combination of these factors, the disc stack separator can be used in combination with an exfoliation process to select the desired properties of the nano-platelets in a continuous process, enabling efficient industrial scale production.

[0142] Measured Effectiveness of Separation.

TABLE-US-00002 Average Particle size Average Particle size (D4, 3) before (D4, 3) after separation Material separation (μm) (μm) Graphite/Graphene 13.5 0.05 Hexagonal Boron 0.5 0.26 Nitride Molybdenum between 15 and 100 0.72 disulphide Molybdenum 15 0.8 diselenide Molybdenum 50 Sub-micron ditelluride Tungsten disulphide 20 0.8 Tungsten diselenide 25 Sub-micron

[0143] The last four sets of figures are subject to error due to small sample size.

[0144] Other materials considered suitable for use in the present invention are niobium diselenide, vanadium telluride, manganese oxide and molybdenum trioxide.

[0145] These sizes are as given by a Malvern Mastersizer™ using the standard settings. These are indicative of the dimensions of the nano-platelets, using the sphere equivalent diameter. As we are operating on the limit of the operating range, using non-spherical samples, these figures present evidence of the successful size separation by the continuous centrifuge, but are not indicative of the actual flake dimensions. However, the flake dimensions have been verified to be in a similar range using electron microscopy and other methods described above.

[0146] The results and conditions in this document are taken at 25° C. unless mentioned otherwise.