Method for manufacturing metal/polymer hybrid nanoparticles with narrow size distribution by miniemulsion polymerisation

Abstract

Method for manufacturing nanoparticles comprising a metallic core coated with a layer of polymer material comprising the following steps: a) preparing a water-in-oil emulsion comprising droplets of an aqueous phase, dispersed in an organic phase, b) adding nanoparticles comprising a metallic core coated with a shell of carbonaceous material, whereby nanoparticles trapped in the droplets are obtained, c) adding precursor monomers of the polymer material, and d) adding a polymerisation initiator, adding the precursor monomers and the polymerisation initiator resulting in polymerisation of the monomers, whereby nanoparticles coated with a layer of polymer material dispersed in the organic phase are obtained.

Claims

1. A method for manufacturing nanoparticles comprising a metallic core coated with a layer of polymer material comprising the following steps: a) preparing a water-in-oil emulsion comprising droplets of an aqueous phase, dispersed in an organic phase, b) adding nanoparticles comprising a metallic core comprising cobalt iron, nickel, copper, silver, gold, or one of the alloys thereof coated with a continuous shell of graphene, whereby nanoparticles trapped in the droplets are obtained, c) adding precursor monomers of the polymer material, and d) adding a polymerisation initiator, adding the precursor monomers of the polymer material and the polymerisation initiator resulting in polymerisation of the monomers, whereby nanoparticles coated with a layer of polymer material, dispersed in the organic phase, are obtained.

2. The method according to claim 1, wherein the polymer material is chosen among polystyrene, poly(methyl methacrylate), polyurethane, a polyacrylic, polypropylene, a polyimide, polyetherimide and a polymer having a pyrene group.

3. The method according to claim 1, wherein the thickness of the layer of polymer material ranges from 1 nm to 100 nm.

4. The method according to claim 3, wherein the thickness of the layer of polymer material ranges from 2 nm to 50 nm.

5. The method according to claim 1, wherein the diameter of the droplets ranges from 20 nm to 1 μm.

6. The method according to claim 5, wherein the diameter of the droplets ranges from 30 nm to 100 nm.

7. The method according to claim 1, wherein electrically insulating nanoparticles are added to the emulsion.

8. The method according to claim 7, wherein the electrically insulating nanoparticles are made of metal oxide.

9. The method according to claim 7, wherein the electrically insulating elements are hydrophobic and in the organic phase.

10. The method according to claim 7, wherein the electrically insulating elements are at least one selected from the group consisting of silica nanoparticles, barium titanate (BaTiO.sub.3), strontium titanate (SrTiO.sub.3), and diamond nanoparticles.

11. The method according to claim 1, wherein in a), the water-in-oil emulsion comprises droplets of the aqueous phase dispersed in the organic phase, wherein the droplets form micelles, wherein the size of the micelles is 2 to 3 times greater than the size of the nanoparticles.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will be understood more clearly on reading the description of embodiment examples given merely by way of indication and not restriction with reference to the appended drawings wherein:

(2) FIGS. 1, 2, 3, 4 and 5 represent different steps of a particular embodiment of the method according to the invention,

(3) FIG. 6 is a graph representing the mean micelle size obtained by dynamic light scattering (DLS) according to the percentage (by weight) of additives in the aqueous phase,

(4) FIGS. 7 and 8 represent, schematically, and as a sectional view, nanoparticles before and after coating with a polymer layer, according to particular embodiments of the invention,

(5) FIG. 9 is a photographic image showing a dispersion comprising a solvent and metal/C nanoparticles (on left) and a stabilised emulsion (on right) according to a particular embodiment of the method according to the invention,

(6) FIG. 10 is a photographic image showing a uniform suspension of metal/C nanoparticles coated with a thin layer of polymer, obtained according to a particular embodiment of the invention,

(7) FIG. 11 is an image obtained by transmission electron microscopy of core/shell structure nanoparticles obtained according to a particular embodiment of the method according to the invention,

(8) FIGS. 12a and 12b are Raman spectra centred, respectively, on the main line of graphene and on the main line of cobalt oxide, obtained on Co/C nanoparticles, Co/C nanoparticles coated with a layer of polymer according to a non-covalent grafting method in homogeneous medium with pre-existing polymer (designated by ‘ex-situ’), Co/C nanoparticles coated with a layer of polymer according to a covalent grafting method with in-situ polymerisation in homogeneous medium by sonochemistry (designated by ‘sonochemistry’), Co/C nanoparticles coated with a layer of polymer according to a particular embodiment of the method according to the invention, i.e. a covalent grafting method with in-situ polymerisation in heterogeneous medium or miniemulsion (designated by ‘emulsion’).

(9) The different parts represented in the figures are not necessarily represented according to a uniform scale, to render the figures more legible.

(10) The different possibilities (alternative embodiments and embodiments) should be understood as not being mutually exclusive and may be combined with one another.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

(11) Reference is made, firstly, to FIGS. 1 to 5 which represent the different steps of a method for coating nanoparticles 20 with a layer of polymer 23 of controlled thickness. The method comprises the following steps:

(12) a) preparing a water-in-oil emulsion 10 comprising droplets 11 of an aqueous phase, dispersed in an organic phase 12 (FIG. 1),

(13) b) adding nanoparticles 20 comprising a metallic core 21 coated with a shell of carbonaceous material 22, whereby nanoparticles 20 trapped in the droplets 11 are obtained (FIG. 2),

(14) c) adding precursor monomers of the polymer material, and

(15) d) adding a polymerisation initiator.

(16) Contacting the monomers and the polymerisation initiator results in polymerisation of the monomers, whereby nanoparticles 20 coated with a layer of polymer material 23, dispersed in the organic phase 12, are obtained (FIGS. 3, 4 and 5).

(17) During step a), an aqueous phase and an organic phase are contacted so as to obtain a biphasic mixture, then an emulsification of the biphasic mixture in the presence of a surfactant (or emulsifier) is carried out, whereby a water-in-oil emulsion 10, formed of droplets 11 of the aqueous phase dispersed in the organic phase 12 is obtained. The droplets 11 form micelles (hydrophilic core—hydrophobic tails).

(18) The emulsification is, for example, formed by stirring (sonication). The mixture remains stable thanks to the addition of emulsifier. The velocity or evolution kinetics of the mixture is quasi-nil, which makes it a confined reaction medium that is particularly stable and favourable for polymer synthesis by monomer polymerisation.

(19) The emulsion 10 may contain further, non-reactive, ingredients, but necessary for emulsion stabilisation.

(20) As represented in FIG. 6, the size of the micelles 11 varies according to the quantity of surfactants (additives) present in the emulsion. On the other hand, the size of the micelles 11 does not change with the addition of the nanoparticles. Size denotes herein and hereinafter the mean size.

(21) Preferably, the emulsion 10 is a miniemulsion, i.e. the droplets 11 dispersed, in the organic phase 12, have a size ranging from 20 nm to 1 μm, preferably from 30 nm to 100 nm, and more preferentially from 30 nm to 60 nm. The size of the droplets 11 will be chosen according to the size of the nanoparticles 20 and the thickness of the layer of polymer 23 sought.

(22) During step b), nanoparticles 20 are added to the previously formed emulsion.

(23) Nanoparticles denote elements of submicronic size (typically less than 1 μm) of spherical, elongated, ovoid shape, for example. Preferably, they consist of spherical particles. The greatest dimension thereof is referred to as diameter or size.

(24) They may have a wide size distribution, for example have a diameter ranging from 5 nm to 1 μm. This size may be determined by photon correlation spectroscopy.

(25) As represented in FIG. 7, the particles 20 have a core-shell structure. The shell 22 is rigidly connected to the core 21 of the particle.

(26) The core 21, or kernel, is a metallic material. Metallic material denotes a metal or a metal alloy. Preferably, it consists of a metal. Preferably, it consists of cobalt, nickel, iron, copper, silver or gold. It may also consist of one of the alloys thereof, such as alloys of cobalt and iron (CoFe), of cobalt, iron and nickel (CoFeNi), of nickel and iron (NiFe) or one of the nitrides thereof, such as iron nitride Fe.sub.4N or Fe.sub.16N.sub.2.

(27) The core 21 is coated with a coating 22 or shell. The coating 22 is made of an organic or inorganic carbonaceous material. Preferably, it consists of an inorganic coating 22.

(28) The coating is an organised 2D carbon coating on a non-planar surface (for example on the surface of a nanoparticle).

(29) Preferably, the coating 22 is made of graphene. It may comprise one layer or a plurality (two, three, four, etc.) layers of graphene. For example, it comprises from 1 to 50 lamellae of graphene, preferably from 2 to 10, for example from 2 to 5, and even more preferentially from 3 to 10.

(30) Preferably, the carbonaceous shell 22 is continuous so as to fully cover the core 21 of the particle 20 to protect the core of the nanoparticles from oxidation, and render same more hydrophilic.

(31) The nanoparticles 20 added in step b) are annotated as metal/C nanoparticles.

(32) The nanoparticles 20 may be manufactured by flame, laser or plasma spray pyrolysis (SP), or by chemical vapour deposition (CVD). This type of powder has a substantial size distribution.

(33) The carbonaceous shell 22 may be manufactured by decomposition of a precursor gas containing carbon, for example acetylene, by SP or CVD.

(34) The emulsion will make it possible to sort these nanoparticles 20 according to the size thereof. For example, for a powder wherein the mean diameter of the nanoparticles 20 is of the order of 30 nm, the size dispersion is wide and can range from less than 5 nm to more than 300 nm. This is detrimental for the manufacture of a nanocomposite material with controlled properties.

(35) Preferably, the size of the micelles 11 is 2 to 3 times greater than the mean size of the nanoparticles 20 (for example between 60 and 90 nm for 30 nm). Only the nanoparticles 20 of mean size of the order of 30 nm will be trapped in the emulsion and only these nanoparticles 20 will be subsequently coated with a layer of polymer 23. In this way, the largest nanoparticles 20 (for example 100 nm or 300 nm) will form a sediment not coated with polymer which will be readily subsequently removed. The smallest nanoparticles 20 (for example of 10 nm and less) imperfectly coated with carbonaceous coating 22 are oxidised rapidly (formation of carboxyls and/or metal oxides), and therefore more hydrophobic, remain in the organic phase (FIGS. 1 to 4).

(36) As represented in FIG. 2, about one third of the nanoparticles 20 of diameter of interest are stabilised in the micelles 11. The other nanoparticles 20 are suspended in the organic phase or in the sediment, and are not involved in polymerisation.

(37) According to the size of the nanoparticles 20, a micelle 11 may contain a single nanoparticle 20 or a plurality of nanoparticles 20, for example in aggregate form. Advantageously, a micelle 11 contains a single nanoparticle 20.

(38) According to an alternative embodiment of the method, not shown, the emulsion 10 further includes hydrophobic elements. The hydrophobic elements are dispersed in the dispersing phase (i.e. the organic phase). Preferably, the elements are electrically insulating. Electrically insulating denotes an intrinsic electrical resistivity greater than 10.sup.12 ohm.Math.cm.

(39) They may consist of mineral nanoparticles, for example silica nanoparticles, nanoparticles of complex oxides, for example of barium titanate (BaTiO.sub.3) or/and strontium titanate (SrTiO.sub.3), diamond nanoparticles and/or silicon carbide (SiC) nanoparticles.

(40) They may consist of tubular or lamellar nanoparticles.

(41) Tubular or lamellar nanoparticles denote particles wherein one of the dimensions is substantially less than the two others. Such tubular or lamellar particles most frequently have a thickness e (or a diameter d) substantially less than the length L or width I thereof. Preferably, the ratio e/L (or d/L) and e/I (or d/I) is less than or equal to 0.5 and preferably less than or equal to 0.1 or 0.01.

(42) Advantageously, the tubular or lamellar nanoparticles are made of hexagonal boron nitride (h-BN). They may also consist of graphene oxide GO.

(43) The lamellar nanoparticles may, for example, be exfoliated. Exfoliated denotes that lamellae or sheets of the stack forming the lamellar nanoparticles are removed so as to obtain lamellar particles formed from one or a few sheets (2, 3, 4 or 5 for example). The tubular nanoparticles may be manufactured by precursor gas decomposition by SP or by CVD.

(44) In the emulsion 10, formed in step b), the polymerisation initiator and the monomers, precursors of the polymer, are added. Advantageously, quantities of initiator and monomer are chosen so as to obtain a low polymerisation yield in order to create a very thin layer 23 of polymer on the surface of the nanoparticles 20 (for example from 5 nm to 10 nm). Low denotes a polymerisation yield less than 50%, and preferably less than 25%, preferentially less than 20%, for example of the order of 10%. The quantity of monomers consumed is determined by the polymerisation yield.

(45) Advantageously, the polymer is chosen among polystyrene (PS), poly(methyl methacrylate) (PMMA), polyurethane (PU), a polyacrylic (PAA), polypropylene (PP), a polyimide (PI) and polyetherimide (PEI). The polymer may also be a polymer functionalised by a conjugated pi group, such as pyrene. It consists, for example, of polystyrene functionalised by a pyrene group (Py-PS) or indeed a polyacrylic functionalised by a pyrene group (Py-PAA).

(46) According to a first alternative embodiment, steps c) and d) are carried out simultaneously.

(47) According to a further alternative embodiment, the method successively includes steps c), d) and c).

(48) According to a further embodiment, step c) is carried out before step b).

(49) When the polymer precursors are contacted with the polymerisation initiator, polymerisation is initiated (FIG. 3).

(50) The polymerisation is a radical polymerisation. This is initiated by the entry into the micelle 11 of a (hydrophilic) oligo-radical previously formed in aqueous phase which will induce progressive consumption of the (hydrophobic) monomers stored in the dispersing phase (herein oil), until the micelle is saturated.

(51) Conditions suitable for reacting the polymerisation primer are set up, typically by raising the temperature and/or by sonication.

(52) For example, the polymerisation is performed by heating the emulsion to a temperature from 40° C. to 80° C., preferably from 50° C. to 80° C., and preferentially from 60° C. to 70° C. These temperature ranges may be adapted according to the temperature at which the polymerisation primer becomes reactive.

(53) The polymerisation step generally lasts from some minutes to some tens of minutes, for example about 20 minutes. This step may be performed under ultrasound using a sonication probe.

(54) Following the polymerisation step (FIG. 4), the droplets 11 of the emulsion 10 are converted into solid elements dispersed in the organic phase. “Dispersion” denotes a stable suspension of solid elements, preferably individualised and not agglomerated, in a liquid continuous phase. The elements have a mean size equivalent to the mean size of the droplets 11 of the emulsion 10 from which they originate.

(55) The solid elements may be beads 30 of polymer material, obtained in the case where the micelles do not contain, at the time of polymerisation, metal/C nanoparticles 20. Alternatively, as represented in FIG. 8, the solid elements may be metal/C nanoparticles 20 coated with a layer of polymer 23, annotated as metal/C/polymer, in the case where the micelles contain, at the time of polymerisation, metal/C nanoparticles 20.

(56) In the case where a micelle 11 includes, at the start of the polymerisation step, a plurality of nanoparticles 20, the layer of polymer 23 coats all of these nanoparticles 20.

(57) The method makes it possible to graft a layer 23 of polymer covalently to the surface of the nanoparticles 20.

(58) The method is carried out at ambient pressure (1 bar) in a hermetic chamber, for example made of glass. The method is, advantageously, carried out with nitrogen bubbling to deoxygenate the reaction medium.

(59) Following the polymerisation step, a “wash” (precipitation/dilution sequence) is advantageously carried out to remove the unused reaction products and retrieve the latex. The latex, similar to a cohesive powder, consists of metal/C/polymer nanoparticles 20, of very homogeneous size (for example 40 nm±2 nm), and of polymer beads 30 (FIG. 4).

(60) A centrifugation separation step may then be carried out to separate the metal/C/polymer nanoparticles from the polymer beads (FIG. 5). About one third of the nanoparticles 20 introduced may thus be retrieved.

(61) The nanoparticles 20 obtained with the method form a powder. The nanoparticles are monodispersed, i.e. they have a size distribution between a maximum diameter and a minimum diameter such that the ratio thereof is less than or equal to 5, 3 or 2 and advantageously less than or equal to 1.5 for example 1.3 or 1.2 or 1.1. The characteristics of such a powder (ratio 1.1) are, for example, a mean nanoparticle diameter of 40 nm, a maximum diameter of 42 nm and a minimum diameter of 38 nm. The diameter of the nanoparticles 20 may be measured with a laser granulometer or by dynamic light scattering (DLS) in solution.

(62) All the dimensional characteristics mentioned above and hereinafter may also be measured using the following techniques: SEM (scanning electron microscope) and TEM (transmission electron microscope), ellipsometry and spectrophotometry.

(63) The metal/C/polymer nanoparticles 20 may subsequently be used to produce another material. For example, they may be dispersed in a solvent and form a stable colloidal solution. They may be dispersed in a mineral matrix or in a polymer matrix, the polymer being identical to or different from that of the shell. It may consist of a thermoplastic polymer or of a photosensitive resin. By way of illustration, they may be coated in a polymer matrix (for example made of polystyrene, epoxy or polyimide) so as to form a uniform metal-polymer nanocomposite film.

(64) The polymer matrix, for example made of PI or epoxide, may contain photosensitive cross-linking agents, preferably to ultraviolet (UV).

(65) This film may be formed, on a substrate, for example made of silicon, by spin coating. Further deposition techniques may be envisaged, such as dip coating, screen printing, or ink jet. A deposition technique wherein the temperatures involved do not exceed the melting point of the polymer shell of the nanoparticle will be chosen.

(66) The film may be exposed to a light source, preferably to an ultraviolet (UV) source. The film may be hot-pressed, preferably in the vicinity of the glass transition temperature.

(67) Such a film contains a homogeneous dispersion of metallic nanoparticles with a narrow size dispersion. Furthermore, as the nanoparticles are solidly coated with an even thin layer of dielectric polymer, the separating distance between the nanoparticles is controlled. For example, the edge-to-edge separating distance between two metal/C particles of diameter ϕ is 1×ϕ, ½×ϕ or ⅔×ϕ and advantageously less than or equal to ⅓×ϕ. For example, such a film is characterised by a homogeneous dispersion of Co/C nanoparticles of mean diameter 30 nm with a mean intergranular distance of 10 nm, up to 5 nm.

(68) The volume percentage of nanoparticles in the composite material ranges, for example, from 0.01 to 30%.

(69) Advantageously, to form a capacitor, the volume percentage ranges for example from 0.01% to 10%, for example from 0.01% to 5%, and preferably from 0.1% to 2%.

(70) Advantageously, for inductors or filters, the volume percentage is greater than 10%, for example from 10% to 30%, or greater than 30%. The mass percentage of nanoparticles in the composite material ranges, for example, from 0.5% to 90%, and preferably from 0.5% to 10% for capacitors and from 50% to 90% for inductors or filters.

(71) Such a film has a high resistivity (for example 10.sup.12-10.sup.15 μOhm.Math.cm).

(72) Such a composite material may, for example, form an electronic device element such as an inductor, a filter or a capacitor, and more particularly an inductor or a filter made of thin layers on silicon or on ferrite or a capacitor made of thin layers on silicon. Such an inductor and such a filter may be used for the integration of RF filtering or power conversion modules, for example for 5G telephony. Such a capacitor may be used for the integration of power conversion modules, for example for electric vehicles.

Illustrative and Non-Limiting Examples of an Embodiment

(73) Nanoparticles used: Co/C, diameter 30-50 nm

(74) Products and conditions used for in-situ polymerisation: Monomer: Styrene 99%, Surfactant: sodium deoxycholate (DOC) 97%, Initiator: 2,2′-azobis (2-methyl propionitrile) (AIBN) 98%, Proportion: DOC+AIBN=1% Styrene (by weight), Reaction temperature=65° C., N.sub.2 bubbling, Sonication: 500 W at 20 kHz, Duration: 20 to 30 min.

(75) Products and conditions used to retrieve latex (precipitation/washing): Emulsion product (raw latex, monomer residues, initiator, surfactant), Precipitation solvent: methanol 98%, Dilution solvent (Styrene): toluene 98%, Precipitation/dilution sequence repeated 4 times, Vacuum air drying.

(76) Products and conditions used to retrieve polymer-coated nanoparticles: Washed latex (coated nanoparticles and polymer nano-beads), Solvent: chloroform 98%, Stirring+Centrifugation at 12 10.sup.3 rpm for 20 min, Retrieval of the supernatant, Sequence repeated 4 times,

(77) Products and conditions used for deposition of a film comprising the polymer-coated nanoparticles: Coated particles, Solvent: chloroform 98%, Addition of substrate polymer: Polystyrene 0.75 g/mL (35 kg/mol), Sonication: 500 W at 20 kHz, Duration: 20 min, Spin-coating: 1000 rpm, Drying on hot plate at 65° C. for 10 min.

(78) The grafted polymer thickness on the surface of the Co/C nanoparticles is of the order of 5 nm (FIG. 11).

(79) The Raman spectra obtained on these nanoparticles 20 show that the carbon layer is not damaged and/or that the oxidation is contained, unlike other techniques routinely used for coating nanoparticles such as in-situ polymerisation in homogeneous medium (or sonochemistry), or such as a technique consisting of coating nanoparticles 20 with a previously separately (or ex-situ) polymerised polymer (FIGS. 12a and 12b).

(80) The same results were observed for Ni/C nanoparticles coated with a polystyrene layer.