WO2013010211A1 - Exfoliating laminar material by ultrasonication in surfactant - Google Patents
Exfoliating laminar material by ultrasonication in surfactant Download PDFInfo
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- WO2013010211A1 WO2013010211A1 PCT/AU2012/000847 AU2012000847W WO2013010211A1 WO 2013010211 A1 WO2013010211 A1 WO 2013010211A1 AU 2012000847 W AU2012000847 W AU 2012000847W WO 2013010211 A1 WO2013010211 A1 WO 2013010211A1
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Definitions
- the present invention relates to processes for preparing suspensions of exfoliated materials and to suspensions made thereby.
- Graphene in particular has shown promise due to its interesting electronic, thermal and mechanical properties.
- Graphene comprises sheets of an extended carbocyclic aromatic network and may be regarded as an exfoliated graphite.
- the use of graphene in commercial applications has however been limited due to the relatively small amounts that are produced using conventional techniques.
- graphene sheets have a strong tendency to aggregate even in suspension. This problem increases with increasing concentration of the graphene in the suspension. Similar problems pertain to other materials which consist of platelets or sheets, such as talc, clays etc.
- Aggregation of platelets or sheets in suspension may be inhibited by use of surfactants.
- surfactants can desorb from the surface of exfoliated sheets, leading to aggregation and possibly instability of the suspension.
- the exfoliated sheets are dried, resuspension is generally difficult due to reaggregation. Resuspended materials may require sonication in order to re-exfoliate the material.
- a method for exfoliating a laminar material to form an exfoliated material comprising ultrasonicating said laminar material in a solution of a surfactant for sufficient time to form said exfoliated material.
- concentration of the surfactant in the solution should be maintained sufficient to form a complete monolayer on the surfaces of the laminar material and the exfoliated material in the solution, or sufficient to sterically stabilise the laminar and exfoliated materials against aggregation.
- the monomer is monomelic.
- the process may comprise continuously adding the surfactant so as to maintain the sufficient concentration.
- the surfactant is polymeric.
- the process may comprise continuously or intermittently adding the surfactant so as to maintain the sufficient concentration or it may comprise adding sufficient surfactant before the ultrasonicating that at all times during the ultrasonication the concentration of the surfactant in the solution is maintained sufficient to form a complete monolayer on the surfaces of the laminar material and the exfoliated material in the solution, or sufficient to sterically stabilise the laminar and exfoliated materials against aggregation.
- the surfactant may be polymeric.
- the surfactant may be a non-ionic surfactant. It may be a copolymer of ethylene oxide and propylene oxide. It may have a dy/dc value of less than about 0 Nm " '.mo l .L, or may be from about -0.1 to about -SNm ⁇ -mof L.
- the dy/dc may be in water or in some other solvent in which the exfoliation is performed.
- the concentration of the surfactant in the solution may be sufficient to form a complete monolayer on the surfaces of the laminar material and the exfoliated material in the solution, or sufficient to sterically stabilise the laminar and exfoliated materials against aggregation.
- the concentration of the surfactant in the solution prior to the ultrasonicating is sufficient to form a complete monolayer on the surface of the exfoliated material which is formed from the laminar material in the solution, or sufficient to sterically stabilise the exfoliated material against aggregation.
- the surfactant may be added to the solution during the ultrasonicating at a rate sufficient that at all times during the ultrasonication the concentration of surfactant in the solution is sufficient to form a complete monolayer on the surfaces of the laminar material and the exfoliated material in the solution, or sufficient to sterically stabilise the laminar and exfoliated materials against aggregation.
- the concentration and nature of the surfactant may be such that at all times during the ultrasonication the surface tension of the solution is approximately the same as the interfacial tension of the solution and the exfoliated material.
- the solution may be an aqueous solution. It may have no organic cosolvents therein, or in some instances it may have one or more organic (e.g. alcoholic) cosolvents.
- the laminar material may be graphite.
- the exfoliated material may be graphene.
- the method may additionally comprise drying the solution having the exfoliated material therein. It may further comprise resuspending the exfoliated material in a solvent.
- drying refers to removal of volatile species, in particular solvent, from the solution, commonly by evaporation.
- a method for exfoliating a laminar material, for example graphite, to form an exfoliated material comprising ultrasonicating said laminar material in an aqueous solution of a surfactant which is a copolymer of ethylene oxide and propylene oxide for sufficient time to form said exfoliated material, wherein at all times during the ultrasonication the concentration of the surfactant in the solution is sufficient to form a complete monolayer on the surfaces of the laminar material and the exfoliated material in the solution.
- a surfactant which is a copolymer of ethylene oxide and propylene oxide
- the suspension may resist reaggregation of the exfoliated material, or may resist separation of solids, for at least about 3 months, or at least about 1 year.
- a dried exfoliated material in a third aspect of the invention there is provided a dried exfoliated material, the surface of said exfoliated material being substantially completely coated in a surfactant, optionally a polymeric surfactant.
- the exfoliated material may be suspendable in a liquid without substantial aggregation of the exfoliated material. It may be suspendable without ultrasonication in a liquid without substantial aggregation of the exfoliated material.
- the surfactant may be a non-ionic surfactant. It may be a copolymer of ethylene oxide and propylene oxide. It may have a dy/dc value of less than about 0.
- the exfoliated material may be graphene.
- a surfactant optionally a polymeric surfactant, in the exfoliation, e.g. the ultrasonically promoted exfoliation, of a laminar material to form an exfoliated material.
- the use of the surfactant may be in sufficient quantity to form a complete monolayer on the surfaces of the laminar material and the exfoliated material in the solution. It may be in sufficient quantity to sterically stabilise the laminar and exfoliated materials against aggregation. It may be in a concentration of about 0.5 to about 5 mg surfactant per m 2 of exfoliated material
- the surfactant may be a non-ionic surfactant. It may be a copolymer of ethylene oxide and propylene oxide. It may have a dy/dc value of less than about 0.
- the laminar material may be graphite.
- the exfoliated material may be graphene.
- Figure 1 shows graphene particles size distributions following exfoliation of graphite using CTAB (cetyltrimethylammonium bromide) as surfactant with a solution concentration of 0.6 mM and initial graphite concentration of 1% w/w.
- CTAB cetyltrimethylammonium bromide
- the exfoliated suspension was centrifuged at 1500g for 5 minutes: (a) non-contact mode atomic force microscopy image of a graphene flake deposited onto a mica surface, (b) height variation of graphene and particles and (c) size distribution of graphene particles.
- Figure 2 is a graph showing yield of graphene for different surfactants. The concentration of the surfactants varied depending on the optimum value required for adjusting the surface tension.
- Figure 3 is a graph showing surface tension of the suspension and yield of graphene produced as a function of the ultrasonic exfoliation time.
- CTAB was used as surfactant.
- Figure 4 is a graph showing yield of graphene produced as a function of the addition of F108 Pluronic® (poloxamer 338: HOCC j ⁇ O ⁇ H g O ⁇ C ⁇ C ⁇ H) surfactant. A appropriate amount of surfactant was added after measuring the surface tension to reduce the surface tension to the optimum range for exfoliation.
- F108 Pluronic® polyxamer 338: HOCC j ⁇ O ⁇ H g O ⁇ C ⁇ C ⁇ H
- Figure 5 shows characterisation of exfoliated WS 2 particles by (a) Raman Spectroscopy, (b) TEM, (c) electron diffraction and (d) UV-Vis spectrophotometry.
- Figure 6 shows characterisation of exfoliated talc particles by (a) TEM and (b) electron diffraction
- the method of the present invention enables production of highly concentrated stabilised graphene and other and exfoliated mineral suspensions. It may be applied to any suitable laminar material, for example graphite, molybdenite, tungstite, talc, WS 2 , MoS 2 , BiTe, mica etc. or to mixtures of such materials.
- exfoliation refers to the separation of a laminar material into its constituent laminae. These laminae are referred to variously as “sheets” or “platelets”. In particular, the separated sheets produced by exfoliation of graphite are referred to as graphene.
- graphene Although ideally graphene consists only of monolayer sheets, in the context of the present specification, slightly aggregated material, for example having up to 10 layered sheets, or having mixtures of different aggregation numbers in which the average is up to 10 (or up to 5 or up to 3), is also encompassed by the term “graphene”.
- a "laminar” material is taken to refer to a material comprising particles comprising aggregated laminae and an "exfoliated” material to refer to a material comprising particles of the deaggregated laminae.
- the laminae in a laminar material are commonly adhered together by relatively weak Van der Waals forces. Typically in a laminar material there will be a large number of aggregated sheets.
- An exfoliated material produced by the method of the present invention may be completely exfoliated (i.e. no laminae are aggregated) or may be partially exfoliated (i.e. some laminae are aggregated but the degree of aggregation is reduced relative to the laminar material).
- the degree of aggregation in general depends on the ratio between the initial concentration of graphite to the concentration of surfactant. The inventor has demonstrated greater than 80% conversion of graphite to single and few layer (less than 10 layers) graphene. Commonly a 40% conversion is easily achievable. This conversion may be measured gravimetrically after centrifugation of the exfoliated suspension.
- a known weight of suspension is dried and then re-weighed to determine the percentage conversion. It should be noted that this method entails some minor inaccuracy, although this may be reduced by weighing the material which is not exfoliated, i.e. the graphitic material left over rather than the few layer material exfoliated and stabilised with surfactant.
- the density of the surfactant is about 1.05 (compared to water of about 0.99 and graphene of about 2.25). This second method is the quicker and easier.
- cavitation can lead to transient vapour filled bubbles in the liquid.
- the liquid- vapour interfacial energy should be comparable to the energy required to separate the sheets beyond the range of the Van der Waals forces.
- the interfacial surface area between the laminae and the solution increases due to the separation of laminae from each other (which exposes the separated surfaces to the solution).
- surfactant adsorbs onto the exposed surfaces of the laminae, it becomes depleted from the solution. It is not uncommon for lg of laminar material to produce a surface area once fully exfoliated of about 30000m 2 . The higher the value of dy/dc (i.e.
- dy/dc A suitable value of dy/dc is below about 0 Nm '.mol '.L.
- It may be less than about -0.1, -0.5, -1, -2, -3, -4 or -5 Nm '.mor'.L, or it may be from about 0 to -5 Nmf mol '.L or about -0.1 to -5, -0.5 to -5, -1 to -5, -2 to -5, 0 to -2, 0 to -1 , 0 to 0.5, 0 to 0.1, 0.1 to -5, -0.5 to -2, -0.5 to -1 or -1 to -3, e.g.
- the solution contains dissolved surfactant. It may also contain surfactant micelles, which are aggregates of surfactant molecules. It may also be a suspension, containing suspended laminae and/or aggregates thereof.
- Mineral suspensions have in the past been prepared using monomelic surfactants. These are frequently characterised by a relatively low critical micelle concentration (cmc), resulting in a relatively low concentration of dissolved surfactant. They also are commonly characterised by a relatively high change in aqueous surface tension with concentration (dy/dc).
- polymeric surfactants can be selected to have the appropriate value of dy/dc and also an appropriate cmc for stabilisation throughout the exfoliation process.
- the cmc of the surfactant may be greater than about ImM, or greater than about 1.5, 2, 2.5 or 3mM, or may be about 1 to about 5mM, or about 1 to 3, 1 to 4, 1.5 to 5, 2 to 5, 1.5 to 3 or 2 to 4mM, e.g. about 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5 or 5mM.
- CTAB a monomeric surfactant
- poloxamer 338 a suitable polymeric surfactant
- the term "polymeric" is taken to indicate a degree of polymerisation of greater than 10.
- the use of a polymeric surfactant has several advantages.
- the fact that the surfactant molecule has a large hydrophobic group increases its attractiveness for the surface of the laminae, reducing the chance that the surfactant will desorb from the surface.
- the fact that the surfactant molecule has a large hydrophilic group provides steric stabilisation by preventing close approach of exfoliated laminae in suspension. This reduces the chances of reaggregation of the exfoliated laminae.
- the polymeric surfactant used in the present invention may have a molecular weight (number average or weight average) of about 500 to about 50000, or about 500 to 10000, 500 to 5000, 500 to 1000, 1000 to 50000, 10000 to 50000, 1000 to 10000, 1000 to 5000 or 5000 to 10000, e.g. about 500, 600, 700, 800, 900, 1000, 1500, 2000, 2500, 3000, 3500, 4000, 4500, 5000, 6000, 7000, 8000, 9000, 10000, 15000, 20000, 25000, 30000, 35000, 40000, 45000 or 50000. It may have a narrow molecular weight range or a broad molecular weight range.
- the ratio Mw Mn may be greater than about 1.1 , or greater than about 1.2, 1.3, 1.4, 1.5, 2, 3, 4 or 5, or it may be less than about 5, or less than about 4, 3, 2, 1.5 or 1.2. It may for example be about 1.1, 1.2, 1.3, 1.4, 1.5, 2, 2.5, 3, 3.5, 4, 4.5 or 5. It may have a degree of polymerisation of about 10 to about 1000, or about 10 to 500, 10 to 200, 10 to 100, 10 to 50, 20 to 1000, 50 to 1000, 100 to 1000, 500 to 1000, 20 to 200, 20 to 100 or 100 to 200, e.g.
- surfactants may be used.
- at least one of the surfactants, optionally all, of the surfactants may be polymeric as described above.
- Monomeric surfactants may also be used in some instances - these have lower molecular weights than those described above for polymeric surfactants. They may be capable of reducing the surface tension of water as are the polymeric surfactants.
- the surfactant may be a non-ionic surfactant, or it may be a cationic surfactant or it may be an anionic surfactant or it may be a zwitterionic surfactant.
- Ionic and zwitterionic surfactants have the advantage that they are less temperature sensitive than non-ionic surfactants, which can change their surfactant properties when heated above a critical temperature.
- non-ionic surfactants have the advantage that they are less sensitive to pH and to the presence of ionic species in the liquid. In the present invention, if non-ionic surfactants are used, the method should be conducted at a temperature below that at which the polar head group of the surfactant dehydrates.
- This temperature is characteristic of each individual surfactant and may be readily determined by experiment (or may be found in the open literature for many common surfactants).
- the use of non- ionic surfactants has been shown to be effective in the present invention even in the presence of up to 1M sodium chloride.
- the surfactant may be a copolymer. It may be an ethylene oxide-propylene oxide copolymer. It may have other comonomers or may have no other comonomers. It may be an amine having one or more (optionally 3) ethylene oxide-propylene oxide copolymer substituents on the nitrogen atom. It may be a block copolymer. It may be a triblock copolymer. It may be an ethylene oxide-propylene oxide block copolymer. It may be a poloxamer. It may be an ethylene oxide-propylene oxide-ethylene oxide triblock copolymer. The two ethylene oxide blocks may be the same length or may be different lengths.
- the proportion of ethylene oxide in the polymer may be about 10 to about 90% by weight or mole, or about 10 to 50, 10 to 30, 50 to 90, 70 to 90, 20 to 80, 20 to 50, 50 to 80, 20 to 40 or 60 to 80%, e.g. about 10, 20, 30, 40, 50, 60, 70, 80 or 90%.
- the surfactant may have an HLB (hydrophilic/lipophilic balance) of greater than about 6, or greater than about 7, 8, 10, 12, 15 or 20, or of about 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24 or greater than 24.
- Suitable surfactants which may be used in the present invention include Pluronic® PI 23 (nominally HO(CH 2 CH 2 0) 2 o(CH 2 CH(CH 3 )0)7o(CH 2 CH 2 0) 20 H: HLB about 7), Pluronic® L31 (nominally HO(CH 2 CH 2 0)2(CH 2 CH(CH 3 )0) 16 (CH 2 CH 2 0)2H: HLB about 1-7), Pluronic® F127 (nominally HO(CH 2 CH 2 0)ioi(CH 2 CH(CH 3 )0)56(CH 2 CH 2 0)ioiH: HLB about 22) and Pluronic® F108 HO(C 2 H 4 0), 41 (C
- surfactants having higher HLB also have higher cloud point.
- Commonly surfactants with HLB over about 12 have a cloud point over about 100°C.
- the suspension is in a liquid having a boiling point of 100°C or less (commonly water).
- the surfactant may have an HLB of greater than 12 so as to ensure that the cloud point is not exceeded.
- the surfactant may have HLB of about 1 to about 7, or about 1 to 5, 1 to 3, 3 to 7, 5 to 7 or 3 to 5, e.g. about 1 , 2, 3, 4, 5, 6 or 7.
- a higher HLB is preferable so as to better stabilise the dispersion.
- the surfactant may be a non-foaming surfactant.
- suitable surfactants for use in the present invention include alkyl trimethylammoniurn halides (e.g. bromides) having an alkyl chain length of 12, 14 or 16 and poloxamers having molecular weights from about 1 to about 20kDa and HLB of greater than about 6.
- alkyl trimethylammoniurn halides e.g. bromides
- poloxamers having molecular weights from about 1 to about 20kDa and HLB of greater than about 6.
- the use of a polymeric surfactant improves stability of the prepared suspension, particularly against dilution and changes in solution conditions. This is a significant advantage over monomeric surfactants such as CTAB.
- the continuous addition strategy can be used with any type of surfactant, either monomeric or polymeric.
- the inventor has surprisingly found no surfactant, whether monomeric or polymeric, ionic or non-ionic, that does not function so as to improve the concentration of graphene produced from graphite according to the present invention, provided that a solution of the surfactant can be produced to lower the surface tension to the optimum range.
- all alkylammonium surfactants tested (both single and double chain, irrespective of counterion-ion such as bromide, chloride and acetate) were effective, as were polyethylene oxide alkylethers, a range of Brij® surfactants, Tween® surfactants and polysorbates (polymeric non-ionics e.g. Tween® 80).
- SPAN® 80 (or Arlacel®, which is a sorbitan monooleate) was also effective. All of these surfactants are far more soluble in water than required in order to reach the necessary low surface tension and therefore are able to be used in the continuous addition strategy discussed later in this specification. The inventor considers that the resultant amount of exfoliated material is ultimately limited by the solubility of the surfactant. However, as mentioned in the document, the ability to add a "dry” or neat surfactant (i.e. not in solution) removes this limitation also.
- a solution containing suspended aggregates is ultrasonicated.
- the initial solution (which is also a suspension) may be prepared bydissolving a surfactant in a suitable liquid, commonly an aqueous liquid (e.g. water) and adding the laminar material.
- a surfactant may be sufficient to achieve a surface tension of about 35 to 45mJ.m “2 , or about 35 to 40, 40 to 45, 38 to 43 or 40 to 43 mJ.m “2 , e.g. about 35, 36, 37, 38, 39, 40, 41, 42, 43, 44 or 45mJ.m "2 . It may be sufficient to completely coat the exfoliated material, preferably the completely exfoliated material.
- the concentration will depend on the surfactant, e.g. on one or more of the molecular weight, monomer molecular weight and HLB. It may also depend on the nature and amount of laminar material to be exfoliated. It may be for example from about 0.01 to about 1% w/v, or about 0.05 to 1, 0.1 to 1, 0.5 to 1, 0.01 to 0.5, 0.01 to 0.1, 0.01 to 0.05, 0.1 to 0.5 or 0.05 to 0.5%, e.g. about 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 or l%w/v.
- Agitation e.g. stirring, swirling, shaking etc.
- suspension of the laminar material in the solution This is then ultrasonicated using suitable ultrasonication equipment.
- suitable equipment include stand alone probes and horns with variable tip dimensions that can be inserted into the suspension or flow cell (e.g. QSONICA®-Q700).
- the equipment may be suitable for scale up of the process.
- the ultrasonication may have a power of greater than about 10W, or greater than about 20, 50, 100, 200, 500 or 1000W, or may be about 10 to about 1000W, or about 10 to 500, 10 to 200, 10 to 100, 10 to 50, 50 to 1000, 50 to 100, 100 to 1000, 200 to 1000, 500 to 1000, 100 to 500, 300 to 700 or 500 to 800 W, e.g. about 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150, 200, 250, 300, 350, 400, 450, 500, 600, 700, 800, 900 or 1000W.
- It may have a frequency of greater than about 2kHz, or greater than about 5, 10, 20, 50, 100, 150 or 200kHz, or about 2 to about 200kHz, or about 2 to 100, 2 to 50, 2 to 20, 2 to 10, 10 to 200, 20 to 200, 50 to 200, 100 to 200, 10 to 100, 50 to 100 or 10 to 50kHz, e.g. about 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 120, 140, 160, 180 or 200kHz.
- a suitable ultrasonication condition may be for example about 50-100W at about 10 to 50kHz. The ultrasonication may be continued for sufficient time to achieve the desired degree of exfoliation.
- a suitable time may be for example at least about 0.5 minutes, or at least about 1, 2, 5, 10, 15, 20, 30, 40, 50 to 60 minutes, or about 0.5 to about 60 minutes, or about 0.5 to 30, 0.5 to 10, 0.5 to 2, 0.5 to 1, 1 to 60, 2 to 60, 5 to 60, 10 to 60, 30 to 60, 1 to 30, 1 to 10, 1 to 5, 5 to 30, 10 to 30, 10 to 20 or 5 to 15 minutes, e.g. about 0.5, 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55 or 60 minutes. It may be less than about 30 minutes, or less than about 25, 20 or 15 minutes. In some instances ultrasonication itself may provide the agitation required to prepare a suspension and no separate agitation may be required.
- a surfactant concentration may be used which is sufficient to stabilise the completely exfoliated material, and preferably also to maintain a sufficient concentration of surfactant in solution towards the end of the sonication that the interfacial tension of the solution is comparable to the surface energy of the laminae.
- This may be up to 20 times the concentration of the laminar material prior to sonication on a weight basis, or up to about 15, 10, 5 or 2 times the concentration of the laminar material, for example between about 2 and about 20 times the concentration of the laminar material, or about 2 to 10, 2 to 5, 5 to 20, 10 to 20, 5 to 15, 5 to 10 or 10 to 15 times the concentration of the laminar material, e.g.
- concentration of the laminar material about 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 , 12, 13, 14, 15, 16, 17, 18, 19 or 20 times the concentration of the laminar material.
- the actual ratio will depend on the characteristics of the surfactant, including molecular weight and cross-sectional area, as well as on the nature of the laminar material, in particular its density.
- Monomeric surfactants have in the past commonly been used in low concentration (commonly milimolar range). If one bases a concentration on concentration of monomer units, such concentrations are inadequate for the surfactants and processes of the present invention.
- the concentration of surfactant may be sufficient to to form a complete monolayer on the surfaces of the laminar material and the exfoliated material in the solution.
- the surface area of graphene is typically about 6000 m 2 /g and thus for exfoliation of graphite to graphene, a typical surfactant concentration would be about 3 to about 30g surfactant per gram of graphite.
- the surfactant adsorbed may be expressed as the difference between the mass added to the suspension and that remaining in solution through depletion adsorption techniques.
- the concentration of surfactant in solution can be determined from surface tension isotherms, from the density of solution, or through titration. Stabilisation of the particles through the adsorption of surfactant or polymer can be determined optically from sedimentation experiments.
- surfactant may be fed to the solution during the exfoliation process so that at any time during that process the surfactant concentration is sufficient to stabilise the mixture of laminar material and exfoliated material at that time, and preferably also to maintain a sufficient concentration of surfactant in solution that the interfacial tension of the solution is comparable to the surface energy of the laminae.
- the surfactant may be fed continuously to the solution or it may be fed discontinuously, i.e. in discrete separate additions.
- the surfactant may be fed in neat form or in solution, preferably concentrated solution, so as to reduce the addition rate required.
- a solid surfactant will be fed as a concentrated solution or micellar solution, and a liquid surfactant will be fed neat, however this need not necessarily be the case.
- the solution prior to sonication will in general require some surfactant to be present, so as to stabilise the suspension of the laminar material and promote early exfoliation.
- the addition of the surfactant through the course of the ultrasonication may be such at the completion of the ultrasonication the concentration of surfactant is sufficient to form a complete monolayer on the surfaces of the exfoliated material in the solution.
- the rate of addition of surfactant may typically be at a rate of about 1 to about 10 wt% relative to laminar material per minute, or of about 0.1 to 1 wt%/minute.
- Addition rates may be about 0.1 to 10, 0.1 to 1, 0.1 to 0.5, 0.1 to 0.2, 0.2 to 0.5, 0.5 to 1, 0.5 to 5, 1 to 5, 1 to 2, to to 10, 5 to 10 or 3 to 6wt%/minute, e.g. about 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10wt%/minute.
- the addition may be for about 0.5 to about 5 hours, or about 0.5 to 2, 0.5 to 1, 1 to 5, 2 to 5 or 1 to 3 hours, e.g.
- the addition may be continuous or may be a batchwise equivalent to the above rates. The addition may be continued for sufficient time to achieve the desired degree of exfoliation.
- the addition of surfactant may be accompanied by continuous sonication or by intermittent sonication.
- the rate of addition may be different for different surfactants, and may be based upon the dy/dc value and initial concentration of the surfactant solution added. For example, using Pluronic® F108 with an initial concentration of 10% w/w added to 100 mL of graphite suspension, the rate of addition would be around 1.2 mL/min.
- the threshold value of surface tension is between about 45 and 50mJ/m 2 , and may be about 45 to 48, 47 to 50 or 48 to 50 mJ/m 2 , e.g. about 45, 46, 47, 48, 49 or 50 mJ/m 2 .
- a suitable method for determining the rate of addition of surfactant is as follows.
- the surface tension of the liquid phase is measured as a function of concentration of surfactant and the concentration region identified corresponding to the surface tension of between a lower value (CI) and an expected threshold value (C2e, commonly corresponding to surface tension above about 48-50mJ/m ).
- Surfactant is first added to a suspension of laminar material to produce a liquid of about concentration CI .
- a calibration curve (see for example Fig. 3) is produced form the data obtained in step 3, which shows the surface tension of the solution as a result of surfactant consumed through adsorption to the exfoliated material, as a function of time.
- the time (Tl) at which exfoliation ceases can be determined by observing plateauing of the surface tension/time curve from step 4.
- the concentration at that time is the threshold value C2.
- the lower value CI is less than about 45 mJ/m 2 , or less than about 44,
- the threshold value (C2e, C2) is commonly above 45 mJ/m 2 , or above 46, 47, 48, 49 or 50, or between about 45 and 55, or about 45 to 50, 50 to 55, 48 to 52 to 47 to 40, e.g.
- a method for exfoliating a laminar material to form an exfoliated material comprising: a) preparing a solution of a surfactant in water at a concentration sufficient for the solution to have a first surface tension, said first surface tension being below a threshold surface tension above which ultrasonication does not support exfoliation of the laminar material;
- step c) determining from the results of step c) a threshold surface tension, being the surface tension at which a curve of surface tension against time ceases to trend upwards; e) determining a time T required to achieve the threshold surface tension under the conditions of step c);
- step c ultrasonicating said suspension under the same conditions of ultrasonication as in step c), while adding the surfactant at a rate of (Cl-C2)/T, wherein CI is the concentration of the surfactant required to achieve the first surface tension and C2 is a concentration of the surfactant required to achieve the threshold surface tension.
- Step g) may be conducted for sufficient time to achieve the required degree of exfoliation (e.g. at least about 50%, or at least about 30, 40, 50, 60, 70, 80, 90, 95 or 99%).
- the concentration of the surfactant in the solution may be sufficient to form a complete monolayer on the surfaces of the laminar material and the exfoliated material in the solution, or sufficient to sterically stabilise the laminar and exfoliated materials against aggregation.
- the method may comprise determining the relationship between concentration of the surfactant in an aqueous solution thereof and surface tension of said solution so as to determine CI and C2. It will be noted that the value of the first surface tension may be determined by the experience of a skilled worker, but if necessary may be determined by routine experimentation.
- the process of the invention is commonly conducted under ambient conditions, i.e. atmospheric pressure and about 20 to 25°C, however these conditions may be varied if required. It is known that non-ionic surfactants can change their surfactant properties when heated to sufficient temperature to dehydrate the polar head group. The process is commonly conducted below that temperature. The solution may increase in temperature as the exfoliation proceeds due to input of energy from the ultrasonication. It may therefore be cooled, e.g. by means of a water bath or other suitable means, in order to prevent excess heating..
- the exfoliated material produced by the present invention may resist reaggregation of the exfoliated material for at least about 24 hours, due to the steric stabilisation provided by the surfactant. This effect is particularly noticeable when a polymeric surfactant is used, and is in general increased when high HLB polymeric surfactants are used.
- the exfoliated material may resist reaggregation for at least about 2, 3, 4, 5, 6, 7, 14, 21 or 28 days, or at least about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or 11 months, or at least about 1, 1.25, 1.5, 1.75, 2, 3, 4 or 5 years, or indefinitely.
- the exfoliated material may be isolated from the solution.
- the solvent may be any suitable solvent, for example an aqueous solvent or a non-aqueous solvent. It may be water, an alcohol (e.g. ethanol, methanol, isopropanol etc.), a ketone (e.g. acetone, methyl ethyl ketone), an ether (e.g. diethyl ether) and ester (e.g. ethyl acetate), toluene or any other suitable solvent, or may be a mixture of any two or more of the above suitable solvents that are miscible with each other. It may be a polar or moderately polar solvent or mixture of solvents. It may be an aromatic solvent or a mixture of an aromatic with another solvent as described above.
- an alcohol e.g. ethanol, methanol, isopropanol etc.
- a ketone e.g. acetone, methyl ethyl ketone
- an ether e.g. diethyl ether
- the process described herein may be suitable for preparation of stable suspensions of exfoliated materials at a concentration of up to about 10%, or up to about 5, 2 or 1%, on a w/w or w/v basis.
- stable refers to a suspension that resists physical separation of the solids for at least about 3 months, or at least 6 or 9 months, or at least about 1 , 2, 3, 4 or 5 years.
- the suspension is prepared in an aqueous solution of surfactant and subjected to ultrasonic agitation.
- the surfactant has a dual role. Firstly, it lowers the surface tension of the liquid to match the surface energy of the mineral of choice, so as to facilitate exfoliation. Secondly, it adsorbs to the surface of the particles, providing a repulsive barrier to reaggregation (steric stabilisation).
- surfactant is important in the stabilisation of the exfoliated material.
- Previously published reports use surfactants with low critical micelle concentrations and relatively high changes in aqueous surface tension with concentration (dy/dc).
- the inventor has instead chosen to use, as a representative example, a poloxamer (these are nonionic triblock copolymers having a hydrophobic chain of polyoxypropylene (poly(propylene oxide)) between two hydrophilic chains of polyoxyethylene (poly(ethylene oxide)) e.g. Pluronic® surfactants from BASF) which has a high critical micelle concentration and low dy/dc. This second point is very important.
- the inventor has recognised that single monolayer 2D materials can be continuously 'produced if the surface tension of the solution phase remains in a limit close to the interfacial energy of the mineral during ultrasonic exfoliation.
- continuous addition of surfactant to the suspension as a function of particle production may result in much higher concentrations of exfoliated particles, at least an order of magnitude greater, than using the simple batch based technique. It may result in concentrations 200 to 300 times higher, or at times up to 1000 times higher or even more.
- the concentration of particles can also be increased through simple evaporation of the aqueous solvent. This can be accomplished without substantial reaggregation.
- the poloxamer surfactant sterically stabilises the particles effectively, allowing very high concentrations of the mineral materials to be suspended in water. Furthermore, the particles can be completely dried and redispersed again without significant reaggregation.
- the present invention provides a process suitable for large scale, solution based processing so as to provide high concentrations of graphene and other nanoparticles.
- Possible applications include the production of thin, transparent, conducting films.
- other applications include:
- the first is a batch process where a single addition of surfactant was added prior to the production of graphene using ultrasonic exfoliation.
- the second method involves the continuous addition of surfactant during the exfoliation process. Both of these methods are detailed below.
- a range of surfactants have been studied which are suitable for the reduction of the liquid-vapour interfacial tension to the prescribed range of 40-43 mJm "2 .
- These include the cationic surfactants tetradecyltrimethylammonium chloride (TTAB), hexadecyltrimethylammonium chloride (CTAB), the anionic surfactant sodium dodecylsufate (SDS) and ionic surfactants commonly known as poloxamers, which are block co-polymer surfactants (ABA type) with PEO-PPO-PEO architecture with varying block lengths up to a total molecular mass of 14300 Da. Solutions were prepared in pure water to give the desired surface tension. For example, when using CTAB, the optimum concentration is 6 x 10 "4 molL "1 and for the pluronic F108, a concentration of 0.1% w/w was used.
- a 1% w/w suspension of the graphite powder in the surfactant solution of appropriate concentration to achieve the optimum surface tension range was prepared.
- 0.1% w/w of Pluronic® F 108 or Pluronic®F127 in water provided a surface tension of 41 mJ/m 2 .
- CTAB a concentration of 0.6 mM was used.
- Pluronic®F108 1 g of graphite was added to 98.9 g of water and 0.1 g Pluronic®F 108 surfactant.
- Ultrasonication was carried out using a sonicator "Cell Disruptor”® Model W- 220F from Heat Systems-Ultrasonics Inc. The samples were sonicated for a maximum of 15 minutes (typically about 5 minutes) with a power of 60 W.
- Nano-particulate single layer graphene and multilayer graphene were produced during this sonication.
- the suspension was allowed to cool to room temperature and was subsequently centrifuged to sediment any large particles or non-delaminated graphite.
- Centrifugation rates equivalent to 1500 times the force of gravity (i.e. 1500G) were used for 5 minutes. Under these conditions, a distribution of particles thicknesses was observed. Typically, greater than 95 % of particles consisted of three layers or less. Approximately 40% of these particles were single layer graphene. Higher rates of centrifugation equivalent to 2500G for 30 minutes resulted in a narrower distribution consisting of 90% single layer graphene. However with greater sedimentation, the overall yield was reduced. The lateral dimensions of the particles were typically in the range of 20-60 run irrespective of centrifugation time and rate. The size of the particles was determined form Atomic Force Microscopy imaging and an example distribution is shown in Figure 1.
- the density of the suspension was also used as a method for determining the concentration.
- An Anton-Parr DMA density meter was used.
- surfactant was added during the exfoliation procedure. 1. Initially, after 5 minutes of sonication, the surface tension of the aqueous phase was measured. Surfactant was then added to the suspension in order to reduce the surface tension into the optimum range of 40-43 mJm " . 2. The sonication procedure was continued for a further 5 minutes. The surface tension was once again measured and adjusted back into the optimum range through addition of more surfactant.
- a concentration of graphene of 0.9 % w/w was determined gravimetrically and through density measurements of the suspension.
- This concentration represents a conversion of about 35% of the graphite to single and few layer graphene.
- the concentration is reduced as the surfactant solution effectively dilutes the overall content of graphite/graphene.
- a process where dry surfactant is added continuously is also contemplated. This allows a greater conversion of graphite to graphene to be achieved and furthermore, higher concentrations.
- the stabilised graphene nanoparticles produced using the method described above show remarkable robustness against re-aggregation.
- the polymeric surfactant is important in this respect. Upon dialysing the suspension of graphene nanoparticles, the exfoliated material re-aggregates rapidly (within hours in Milli-Q® water) as the monomeric surfactants tend to desorb from the surface.
- the polymeric non-ionic surfactants do not readily desorb under most normal solution conditions, and resist high salt concentrations (greater than 1M) and pH of 2-13. Concentrated acid or base can lead to desorption or high temperature for those surfactants with cloud points less than 100°C. The concentration of the resulting suspensions can be increased through simply evaporating the solvent (water).
- Suspensions with concentrations greater than 10% w/w can easily be prepared in this way. In practice significantly higher concentrations may be achieved.
- the inventor has reduced the water content of the suspension using a rotary evaporator to less than about 20% with no observable aggregation (determined through dilution and subsequent turbidity measurements). If totally dried, i.e. 0% water, about 90% of the material can be re-dispersed with only a mild shaking or sonication (less than 30s at low powers). It therefore appears that up to 100% solids may be achieveable without significant aggregation.
- the particles can be completely dried and re-dispersed in aqueous suspension. Tests show that more than 90% of the dried graphene particles can be re- dispersed. Furthermore, the dried graphene particles have been added to a number of organic solvents including ethanol and toluene with success. Simple alkanes such as hexane do not allow the particles to be redispersed.
- WS 2 nanoparticle suspensions were prepared using the surfactant assisted ultrasonic exfoliation technique as previously described. A 2% w/w suspension of bulk WS 2 was added to Milli-Q® water. Under continuous sonication at 60 W using a "Cell Disruptor”® Model W-220F (Heat Systems-Ultrasonics Inc.) sonicator, non-ionic surfactant was added from a highly concentrated solution to maintain the concentration of surfactant in water at approximately 0.1% w/w. The exfoliation procedure results in a vast increase in solid- liquid surface area which leads to rapid depletion of the surfactant through adsorption to the particles surface. Thus, continuously adding surfactant during sonication leads to a significantly greater yield of nanoparticulate WS 2 . This concentration of surfactant is suitable for maintaining the optimum surface tension as described previously at ⁇ 40-42 mJ/m 2 .
- the resultant exfoliated suspension was then dialysed in Milli-Q® water for 48 hours to remove any non-adsorbed surfactant from the solution.
- the suspension was then centrifuged at 1500 rpm for 5 minutes to sediment further any large (non-exfoliated) particles.
- the supernatant was collected and was found to be stable for up to 3 months.
- Some sedimentation was observed due to the relatively high density of WS 2 (7.5 g/mL) but the particles could be readily re-dispersed through simple agitation.
- the particles were subsequently characterised in terms of size, charge and spectroscopic properties as described below.
- the yield of single and few layer WS 2 nanoparticles produced using this method was determined gravimetrically to be as high as 0.2-0.25 mg/mL.
- talc Single and few layered talc was produced using the surfactant assisted exfoliation technique.
- Talc (2g) was added to 100 mL of Milli-Q® water.
- Surfactant Pluronic® F108
- a F108 solution in water with a concentration of 10% w/w was used and added at a rate of 1 mL per minute which maintained the concentration of surfactant in the suspension at ⁇ 0.1 % w/w. This concentration reduced the surface tension of the liquid phase to the region of 40-42 mJ/m 2 .
- the suspension was subsequently dialysed for a minimum of 48 hours and centrifuged at 1500 rpm for 5 minutes to remove large particles.
- the single and few layered talc particles were characterised using TEM and electron diffraction as shown in Figure 6.
- the yield of talc was typically 1.2 to 1.5 mg/mL which was measured gravimetrically.
Abstract
Description
Claims
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EP12814402.9A EP2734468B1 (en) | 2011-07-19 | 2012-07-13 | Exfoliating laminar material by ultrasonication in surfactant |
AU2012286515A AU2012286515C1 (en) | 2011-07-19 | 2012-07-13 | Exfoliating laminar material by ultrasonication in surfactant |
ES12814402T ES2835805T3 (en) | 2011-07-19 | 2012-07-13 | Exfoliation of laminar material by ultrasonication in surfactant |
US14/233,312 US9914108B2 (en) | 2011-07-19 | 2012-07-13 | Exfoliating laminar material by ultrasonication in surfactant |
CN201280043964.3A CN103827023B (en) | 2011-07-19 | 2012-07-13 | By supersound process stripped laminar material in surfactant |
JP2014520463A JP6047567B2 (en) | 2011-07-19 | 2012-07-13 | Delamination of layered materials by sonication in surfactants. |
CA2842082A CA2842082A1 (en) | 2011-07-19 | 2012-07-13 | Exfoliating laminar material by ultrasonication in surfactant |
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US20140226429A1 (en) | 2014-08-14 |
EP2734468B1 (en) | 2020-09-09 |
ES2835805T3 (en) | 2021-06-23 |
CN103827023B (en) | 2016-11-09 |
EP2734468A1 (en) | 2014-05-28 |
AU2012286515C1 (en) | 2016-10-27 |
US9914108B2 (en) | 2018-03-13 |
AU2012286515B2 (en) | 2015-07-09 |
AU2012286515A1 (en) | 2014-02-06 |
CN103827023A (en) | 2014-05-28 |
CA2842082A1 (en) | 2013-01-24 |
JP2014529319A (en) | 2014-11-06 |
EP2734468A4 (en) | 2015-07-22 |
JP6047567B2 (en) | 2016-12-21 |
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