WO2012118434A1

WO2012118434A1 - Methods to produce metallized carbon nano particles

Info

Publication number: WO2012118434A1
Application number: PCT/SE2012/050221
Authority: WO
Inventors: Joachim KARTHÄUSER
Original assignee: Karthaeuser Joachim
Priority date: 2011-03-01
Filing date: 2012-02-28
Publication date: 2012-09-07
Also published as: EP2681154A1; SE1100140A1; EP2681154A4

Abstract

Methods to modify dispersed carbon nano particles using electrochemistry are disclosed. First, dispersions of CNT, graphene, graphite or the like in water or organic solvents are prepared. Secondly, said dispersions are brought in contact with a solution of ionic compounds in a liquid, such as dissolved metal salts in water, whereby the dispersion of carbon nano particles is in electrical connection with one electrode, typically the minus pole, and the second solution is in electrical connection with a second electrode, typically the plus pole. The useful voltage for converting metal salts to the respective metal is between 0 and 10 V, and the voltage may be applied continuously or in intervals, such as every millisecond with a pause of one millisecond. Using the method, metal is precipitated onto or close to the carbon nano particles. A useful method is to pump the dispersion of nano particles and to let it enter the second liquid in the form of growing drops, similar to a dropping mercury electrode. Following the electrochemical metal deposition, the metalized carbon nano particles can be separated and used in various products including composites, coatings, capacitors, cables and other products.

Description

Title: Methods to produce metallized carbon nano part

Field of the invention and background literature:

This invention relates to chemical modification, in particular electrochemical modification of carbon nano particles such as CNT, graphene, carbon black and the like. The products formed are metalized carbon nano particles. No directly related disclosures were found in the literature.

US 2010/0052223 by Yong Hyup KIM discloses methods for producing CNT/metai composite cables. The method involves precipitating CNT and metal ions (which are reduced to metals) from a mixture of CNT and metal ions such as CuS04 onto a metal tip or wire while applying a voltage.

Similarly, US 2010/0122910 by Quanfang Chen discloses an electrochemical co-deposition method for forming CNT/metal nano-composites. A dispersion or solution of CNT and metal salts is used to deposit CNT and metal ions simultaneously onto an electrode which turns into a CNT reinforced metal during the co-deposition of CNT and metal. As will be apparent from the disclosure below, this invention uses two different liquids, i.e. one first liquid dispersion of CNT or other CNP and a second, different liquid containing metal salts, and the purpose of the method is to alter the properties of the nano particles as such.

Electrical fields are used in the literature for modifying the CNT production process, see US

2008/0061477 by Peter David Capizzo as example. Metallization of CNP has been reported by way of electrodeless deposition (e.g. T.W. Ebbesen et al., Advanced Materials 1996, 8, No.2, 155-157) or by way of direct mixing with metals such as lithium, see e.g. US 2007/0190422 by Robert Scott Morris, "CNT lithium metal powder battery". CNT's have been deposited onto solid metal films (which constituted electrodes) from dispersed phases using traditional electrochemistry. The purpose was to produce CNT-containing metal films, see Xiao-xin Qjn et al, Journal of Electroanalytical Chemistry, 651 (2011), 233-236 (online 30.11.2010) or Susuma Aral et al, Electrochemical Communications 5 (2003),797-799.

Abbreviations used in the text:

Carbon nano particles (CNP): carbon nano tubes (CNT), single or multi wall, graphene, cones, discs, graphite, carbon black

Ionic compounds: acids such as HCI or H2S04, metal compounds such as CuS04, AgN03, salts of metals including zinc, sodium, potassium, calcium, palladium, platinum, rhodium, iron etc.

Dispersions: CNP which have been distributed in a solvent such as water, isopropanol, heptane, silicone oil. Typically, CNT are sonicated or treated mechanically in order to break up agglomerates, i.e. bundles of CNT, into smaller fragments, ideally into single CNT. Graphitic cones, graphite and carbon black are more easily dispersed using ball mill or mechanical shear. These dispersions show electrical conductivity, typically like a semiconductor. Metallzation and metalized CNP (MCNP): metallization is understood as reduction of metal cations to the respective metal, MCNP means substances where metal has been deposited onto CNP whereby the metals may be be attached firmly by chemical bonds or loosely by physical forces to the CNP. Also metal fragments such as isolated metal nano wires (not bonded to CNP) are included in the definition.

Invention:

In this invention, methods are disclosed which allow the facile production of MCNP by

electrochemical treatment of dispersions of CNP in the presence of a second liquid containing ionic compounds such as metal salts, comprising the following steps:

a. an electrically conductive dispersion of CNP is prepared and provided,

b. said dispersion is brought into contact with an electrode to allow transport of electrical current,

c. said dispersion is also brought in contact with a second liquid, namely a solution of ionic compounds whereby said solution is in contact with the opposite electrode, and whereby the two electrodes are not in direct contact with each other through a homogeneous phase, d. an optionally variable current is passed between the electrodes such that the dissolved ionic compounds can discharge, in particular such that metal cations can discharge and be converted to pure metal in proximity to the conductive CNP dispersion, products including MCNP are separated from the mixture of dispersion and solution of ionic compounds for purification, isolation for further use, and any other further processing including recycling to the electrochemical treatment.

The products obtained using this method are metalized CNP or MCNP. Not being bound by theory, the products may be metal nano wires, metalized CNP, CNP which carry metal clusters on the tips of CNT, or mixtures of various conceivable products comprising CNP and pure metal. They may also comprise functionaiized CNP such as oxidized or chlorinated CNP, especially if the conductive dispersion has been connected to the plus pole of the voltage supply, i.e. if anions such as sulfate or chloride have been discharged near the CNP dispersion.

A simple embodiment includes the dispersion of CNP in e.g. paraffin oil or silicone oil, and electrochemical treatment using a second liquid, e.g. Copper sulfate (CuS04) dissolved in water. The oil and water phase are separated by a boundary. Passing an electrical current through the electrodes placed in the respective phases leads to the deposition of copper metal close to the CNP or CNT. Gentle stirring of the oil phase or both phases allows removal of metalized CNP (MCNP) into the bulk of the oil phase. MCNP are separated and isolated or further processed after deposition of the desired amount of metal.

A preferred embodiment includes pumping the conductive dispersion of CNP through hollow needles made of stainless steel. The outside of the needles is coated with an electrically insulating material. The dispersion is in contact with typically the minus pole of a voltage supply, typically 1-10 V DC. The voltage may be applied at a frequency, e.g. ON for 1 millisecond (ms), off for 1 ms, and so forth (see below). Drops of the electrically conductive dispersion of CNP form at the tip of the needles, and due to the pumping of fresh dispersion the surface is constantly renewed. The dispersion is pumped into a second liquid containing ionic compounds (e.g. copper sulfate CuS04 in water), and the second liquid is in contact with the opposite electrode, typically the plus pole of said voltage source. In this arrangement, using CuS04 as example, copper metal is formed as copper ions are discharged at the interface between the dispersion and the second liquid. At the same time, oxygen is formed by oxidation at the counter-electrode. At the tips of CNT, the high field strength leads to preferred deposition, and growth of copper is the preferred consecutive reaction. To achieve a more uniform growth or deposition of copper on all CNP, both the surface is renewed at a certain rate leading to un-modified CNP reaching the interface, and secondly the voltage supply can be switched ON and OFF at certain intervals, e.g. at 1000 Hz or any other suitable frequency. Eventually, drops of the dispersion will fall off from the needle - or rise from the needle if a solvent lighter than the second liquid is used - upon which they no longer are in contact with the voltage supply, hence

electrochemical deposition of metal is stopped. These drops can be separated from the second liquid by different means, depending on the nature of the solvents used. Suitable methods include phase separation, centrifuging, decanting, filtering, solvent removal by membranes, washing of filter residues, drying and so forth. A part of the product may be recycled to the metallization process.

The products obtained {MCNP) differ from CNP in that they contain a certain amount of metal which may vary, e.g. in the order of ppm levels in weight or less to 100% and more. Small loadings of metal (here especially platinum, rhodium, silver, nickel and the like) are typically useful for applications such as catalysis. Intermediate loadings of metal (such as copper, zinc, tin, lithium and the like), such as in the % range, already increase the bulk conductivity and alter the electronic nature of the CNP. Different metal exhibit different electro-negativity, therefore combinations of MCNP with different metals exhibit properties of pn-junctions. High loadings of metal (as examples copper), such as many %, considerably increase the bulk electrical conductivity, therefore all those applications using the high electrical conductivity of CNP will benefit from use of MCNP instead of CNP. These products include heating films, current conductors, both DC (direct current) but also and in particular AC (alternating current), electrical junctions and interconnects, cables, sensors, batteries, capacitor foils and super-capacitors.

Examples and preferred embodiments:

1. Preparation of dispersions: electrically conductive dispersions of CNP can be prepared elegantly by applying high power ultrasound to CNP agglomerates in a liquid such as water, alcohol, ketone, paraffin, silicon oil, epoxy, amine, ester and the like. 0,5-1,5% by weight of CNT, such as obtainable from Bayer under the tradename Baytubes™ are dispersed with ultrasound within 5-30 minutes such that a resistance of 100-1000 Ohm (two tips of a standard Ohm-Meter, 1 cm distance) is achieved. These dispersions may contain additives as the case may be, e.g. for regulation of viscosity, for prevention of re-agglomeration or for preparation of products at a later stage after metallization. Fumed silica increases the viscosity. Cellulose, especially in micronized or nano form, stabilizes CNP and prevents re-agglomeration but reduces the electrical conductivity of especially CNT somewhat. 2, Different reactors, two phases stirring, or needles

In a simple embodiment, the dispersion of CNP and the solution of ionic compounds may constitute two different and incompatible phases. Electrochemical deposition of metal will hence occur at the phase boundary from which MCIS3P may be isolated in dispersed form or as a film. The two phases may be stirred. In a more complex embodiment, the CNP dispersion is pumped into the second liquid e.g. through one hollow needle or a plurality of hollow needles or channels. If incompatible phases are used, e.g. CNP in silicone oil and CuS04 in water, it is very practical to pump the dispersion upward to form growing droplets where the surface will be renewed constantly. As long as the droplets are in contact with the dispersion, electrochemical deposition will occur, but over- deposition is prevented as the surface is renewed. Eventually, the droplet will detach from the dispersion and move upward (due to its lower density compared with the aqueous phase) upon which no more electrochemical metal deposition will occur. The treatment may be carried out with simultaneous pumping of the aqueous solution, a.o. to avoid depletion of ions. The dispersion may also be provided in the same base fluid as the second solution containing ionic compounds. In this case, mixing at the phase boundary (here understood as boundary characterized by different concentrations of CNP and ionic compounds, respectively) will occur, but metal deposition is nonetheless efficient. It is also possible to pump the second solution into the CNP dispersion such that the second solution forms growing droplets, however, in many practical cases, the above mentioned approach is preferred.

3. Different solvents, different metals

The following solvents are very practical for CNP dispersion: silicone oil, e.g. AK 50 from Wacker, paraffin oil (paraffins containing especially between 4 and 20 carbon atoms), esters, e.g. alkylacetate, glycols, e.g. Carbitol, amines and amine-esters, e.g. Desmophen NH 1420 from Bayer, alcohols such as isopropanol, ketones such as acetone or MEK, epoxides and diepoxides such as raw materials for epoxy composites, and water. If water is used as dispersion medium for CNP, the two different phases of CNT and dissolved metal salts will mix at the interface, however, during the mixing process there is still a high concentration of CNT in the original CNT phase, and a high concentration of metal salt in the original metal salt phase. At the boundary it is thus possible to carry out electrochemical reduction of metal salt to metal on the electron-conducting CNP surfaces. This mode of operation requires that the resulting mixture of CNT, metal salt and water (and e.g. alcohol) is treated by appropriate separation techniques, such as centrifugation to separate the CNP from liquid. The partially metaiized MCNP may be treated a couple of times.

The second liquid is preferably water containing water-soluble metal salts whereby the metals include especially copper, zinc, iron, chromium, noble metals such as gold, nickel, platinum etc., but also alkali, earth alkali metals as well as transition metals. Copper in general is a very suitable metal to improve the electrical conductivity of all CNP. Inert gas coverage is required as small copper particles have a strong tendency to oxide formation. Nickel, platinum, rhodium, silver etc are metals useful for catalytic purposes. Zinc and copper are very useful for conductive coatings and coatings for anti-corrosion purposes. In some cases, it is desirable to use other solvents than water. 4, Dispersion on plus - for oxidation

In one embodiment, the CNP dispersion is connected to the plus pole of the voltage supply, and the second liquid, connected to the minus pole of the voltage supply, contains sulfate or chloride anions. The electrochemical discharge leads to the formation of oxygen or chlorine in highly reactive states, Thus, the CNP are oxidized or halogenated, respectively,

5. Dispersion on minus, H2S04 or other acids as medium, H formation, reduction

In one embodiment, an acid such as sulfuric acid is used as ionic compound in the second solution. The CNP dispersion is in electrical connection with the minus pole. Electrochemical treatment leads to the formation of hydrogen atoms in proximity to the CNP, hence to the chemical reduction of said CNP. This chemical reduction improves the electrical conductivity of CNP, and the products are therefore useful for the manufacture of a.o. transparent electrodes for photovoltaic purposes. Especially graphene is often available as oxidized graphene, and the electrochemical treatment according to this invention is a highly suitable technique to increase the conductivity of graphene in oxide form.

6. Voltage supply

For the deposition of metals, voltages between zero (0) and ten (10) volts are especially useful. A voltage higher than the theoretically needed electrochemical potential difference is useful in order to overcome over-voltages and in order to increase the metal deposition rate. A constant voltage may lead to local metallization, probably due to lower resistance in areas where already metals have been deposited, this despite the changing surface of the dispersion, if e.g. the droplets formed in needles are used. In these cases, it is very preferable to apply the voltage in intervals, e.g ON for one millisecond and OFF for one millisecond. The useful frequency and duration of ON and OFF periods which may be very different, is governed by the solvents used, metal salts used, the required polydispersity and other parameters.

7. Separation

Following the metallization or other electrochemical modification (e.g. reduction, oxidation, halogenation), MCNP need to be purified for further processing or conversion to final products. If incompatible phases are used, it is practical to phase-separate the e.g. oil and aqueous phase, and to concentrate MCNP e.g. by centrifuging, continuous decanting, solvent washing or the like. If the phases are compatible, e.g. dispersion and second liquid as water-based liquids, centrifuging, decanting, filtering etc are useful methods. Further washing with pure water is suitable for removing metal salts or acids. Some MCNP may be recycled for further electrochemical processing. Products may be dried, washed, concentrated by removal of solvents by distillation, dispersed in different solvents, or processed in different and well-known ways known to the expert.

8. Product examples

In one embodiment, MCNP (metal preferably being copper) are used as conductive components in electrically powered heating films whereby preferably silicone rubber is used as surrounding matrix. A useful composition for a preferred application, the de-icing of wind power blades or wings, is 30-90 % by weight silicone rubber, 5-50% by weight carbon black, 0,1-10% by weight MC!MP and 5-30% by weight fillers such as fumed silica. MCNP enable higher volume conductivity than CNP, therefore the heating tape can be made thinner than it is possible not using MCNP. The heating tape is preferably covered with a thin (such as 100 micron) gelcoat based on epoxy or polyurethane on the top

(towards the outside), and a thermally insulating layer such as a foamed gelcoat on the bottom. The heating foil may contain holes such that the two gelcoat layers can crosslink chemically. The lower gelcoat layer may be covered with an adhesive tape to allow retro-fitting of existing wind power or similar installations.

In one embodiment, MCNP based on CNP dispersions which contain cellulose, especially micronized or nano-cellulosa, are used as feedstocks for conductive paper-like sheets. For this, 2 parts of cellulose and optionally some fumed silica and 1 part of CNT, graphene, cones and discs or the like are dispersed in water or alcohol. This dispersion is electrochemicaliy treated in order to deposit metal such as copper, silver, zinc, nickel or the like on the dispersed and preferably cellulose-coated CNP. The MCNP dispersion is washed, excess metal salt is removed, and the dispersion is filtered and spread out for drying to form sheets of any length and thickness e.g. between 10 and 50 micrometer. The dried paper is useful as capacitor foil, incorporation of different metals, such as lithium, also opens the application as battery component.

In one embodiment, hydrogenated CNP, i.e. CNP which have been exposed to hydrogen atoms formed by electrochemical reduction of H+ or H30+ (acid in water) are used as feedstock for transparent electrodes on a photovoltaic device.

In one embodiment, two different MCNP, e.g. MCNP based on zinc and MCNP based on copper, are combined in thin layers. Such a combination shows properties of p-n-junctions, e.g. the resistance across the two layers to a DC voltage is dependent on the polarity.

In one embodiment, MCNP are used for transport of electrical currents. MCNP show lower percolation thresholds than the CNP they are based upon. The resistance is further highly pressure dependent if measured in an elastic matrix such as rubber. Therefore, very low resistance is obtained if a rubber/MCNP system is kept compressed. Also, the resistance to high frequency alternating current, especially with frequencies exceeding 1 MHz, is extremely low. The electrical conductivity can be used in analytical chemistry as the dropping mercury electrode used in potentiometry can be replaced by dropping MCNP or dropping CNP electrodes. These are even useful in less common solvents. The CNP dispersion can be used in synthetic electrochemistry as replacement for liquid mercury, e.g. in a modification of the chlor-alkaii electrolysis. Also, both CNP and MCNP are useful directly as catalysts in electrochemical reactions. Electron transport through the tips of CNT leads to a high local current density, therefore reactions requiring such high current density, such as the formation of peroxodisulfate (S2082-) from two sulfate ions, can occur at the boundary of the CNP (especially CNT) dispersion. Also, MCNP can be integrated into gas diffusion electrodes.

In one embodiment, MCNP are used as components in coatings and paints. Zinc-MCNP and Cu- MCNP are both useful, the former for anti-corrosion effects, the latter as anti-fouling agent e.g. in marine paints. Using MCNP in general, slightly conductive paints can be formulated with the advantage of improved corrosion protection by better connectivity to the sacrificial anode. MCNP are also protecting against radiation.

In one embodiment, MCNP are used in composites as re-inforcing agents. MCNP show significantly- stronger adhesion or compatibility with common matrix materials such as epoxy, polyurethane, PET, and other preferably polar plastics and thermosets, therefore composites incorporating MCNP show higher tensile strength and improved fatigue resistance than comparable composites based on the respective CNP. Similarly, the pure MCNP can be compressed and heated with the purpose of melting the metal phases. The resultant product is a metal / CNP nano-composite with mechanical properties between pure metal and single CNP. Applications within ballistics protection and similar tasks are opened using these composites. Similarly, the pure MCNP, preferably carrying metals such as nickel, silver, platinum, rhodium, iron, manganese and the like, can be shaped into larger structures such as a web, optionally using a support, and can be used as catalysts or catalyst carriers for chemical reactions.

In one embodiment, relatively large amounts of metal, e.g. twice the weight of the dispersed CNP, are deposited on or near the CNP. At the same time, a low pumping speed is used such that growth of metal by electrochemical deposition is favoured at the sites where such groth has already started. The resulting products are heavily metalized CNP and can be used in similar applications where pure metal nano wires are used, e.g. as sensors or in applications described above.

This list is not meant as an exhaustive or limiting list of all conceivable applications.

Advantages of the invention:

The disclosed methods allow the facile production of MCNP using a versatile, safe and efficient technology. In addition, the production process can be up-scaled easily. The products have a range of potential applications from nano-technology, electronics, catalysis, photovoitaics, composite materials, coatings, energy storage and others, in comparison to CNP currently used in such applications, MCNP show, in comparison to the respective CNP, higher electrical conductivity, higher heat conductivity, less tendency to re-agglomeration, higher compatibility to most plastics and thermosets and therefore higher capability to re-inforce composite systems.

Due to their metal content, MCNP enable certain new functions, such as the formation of p-n- junctions upon combination of two different MCNP, or such as catalysis.

The method allows very high production rates and specific deposition of desired metals if compared with the electrodeiess deposition (or electroless plating).

The solvent used to disperse CNP and additives can be chosen with only few limitations such that the eiectrochemically treated dispersion can be used directly, after washing and removal of e.g.

undesired salts and water, in final products such as described above.

In comparison to US 2010/0122910, this invention avoids the undesirable co-deposition of metals and CNT onto an electrode. Here, the objective is to metalize nano particles as such.

Claims

Claims:

1. A method which allows the facile production of MCNP (metalized carbon nano particles) by electrochemical treatment of dispersions of CNP (carbon nano particles such as carbon nano tubes, graphene cones and discs and sheets, carbon black, graphite) in the presence of a second liquid containing ionic compounds such as metal salts, comprising the following steps:

a. an electrically conductive dispersion of CNP is prepared and provided,

c. said dispersion is also brought in contact with a second liquid, namely a solution of ionic compounds whereby said solution is in contact with the opposite electrode, and whereby the two electrodes are not in direct contact with each other through a homogeneous phase, d. an optionally variable current is passed between the electrodes such that the dissolved ionic compounds can discharge, in particular such that metal cations can discharge and be converted to pure metal in proximity to the conductive CNP dispersion,

e. products including MCNP are separated from the mixture of dispersion and solution of ionic compounds for purification, isolation for further use, and any other further processing including recycling to the electrochemical treatment.

2. A method according to the previous claim characterized in that the CNP dispersion is prepared using silicone oil, paraffin or ester oil, organic solvents such as isopropanol, liquid raw materials for epoxy or polyurethane polymers including epoxies and amines and amine-esters, or water, or mixtures thereof, and that optionally other additives including fumed silica, cellulose, glycols, Ti02 powder and others are co-dispersed with CNP in said liquids.

3. A method according to the previous claims characterized in that the second liquid according to claim 1 c) is preferably water, and that the ionic compounds are water-soluble salts of metals including copper, zinc, sodium, potassium, lithium, calcium, palladium, platinum, rhodium, iron, nickel, manganese, rhodium, silver, or where the ionic compounds are mineral acids such as sulfuric acid.

4. A method according to any of the previous claims whereby either the dispersion or said second liquid is pumped into the other liquid, preferably through at least one hollow needle or a plurality of needles, such that the contact surface between the dispersion and the second liquid is constantly renewed and brought to a very high level.

5. A method according to any of the previous claims whereby the dispersion is in contact with the minus pole of a direct current (DC) voltage supply and whereby the second liquid is in contact with the plus pole of said voltage supply whereby the preferred voltage is between 0 and 20 V, more preferably between 0 and 10 V, and whereby the voltage may be switched ON and OFF at a certain frequency, e.g. at 100 or 1000 Hz, and whereby said arrangement is used to deposit metals on or near the CNP, or it is used for producing reactive hydrogen atoms for chemical reduction of CNP.

6. A method according to any of the preceding claims whereby the dispersion is in contact with the plus pole of the said voltage supply (referring to claim 5), and whereby this arrangement is used to form reactive oxygen or halogen atoms on or near the CNP in order to accomplish an electrochemical oxidation, haiogenation or further functiona!isation of said CNP whereby functionaiisation includes consecutive chemical derivatization using alcohols, amines, acids, epoxides and isocyanates.

7. A method according to any of the preceding claims whereby the e!ectrochemieaily treated dispersion of CNP is separated by filtration, de-canting, centrifugation, solvent distillation or other classical means from the second liquid, and whereby the isolated CNP are used for further and repeated electrochemical treatment until the metallization level is sufficient, and where after sufficient electrochemical treatment the MCNP are incorporated into technical products.

8. Use of MCNP prepared according to one of the preceding claims in products including capacitors, batteries, transparent electrodes, electronic interconnects, electrical heating films, cables, catalysts, especially in gas diffusion electrodes and catalyst carriers, photo-vo!taics, coatings and paints, composites containing either epoxy / amine systems or polyurethanes other thermosets, ballistic protection materials whereby MCNP are treated at high temperature in order to fuse metallic regions.