WO2013066269A1

WO2013066269A1 - Method of forming od, id, or 3d graphene and use thereof

Info

Publication number: WO2013066269A1
Application number: PCT/SG2012/000412
Authority: WO
Inventors: Zehui DU; Zongyou YIN; Hua Zhang
Original assignee: Nanyang Technological University
Priority date: 2011-11-02
Filing date: 2012-11-01
Publication date: 2013-05-10

Abstract

The present invention relates to a method of forming graphene on a OD (dimensional), ID or 3D metallic nanostructure, comprising coating the metallic nanostructure with a layer of liquid carbon precursor, carbonizing the layer of liquid carbon precursor coated on the metallic nanostructure at a carbonizing temperature to convert the layer of liquid carbon precursor to a layer of amorphous carbon, crystallizing the layer of amorphous carbon at a crystallizing temperature to convert the layer of amorphous carbon to a layer of crystallized graphene, and quenching the layer of crystallized graphene. The so-formed OD, I D or 3D graphene can be processed further to produce high-performance electrical conductors for electronics, batteries, supercapacitors and hydrogen storage, and nanocomposites for heat spreading.

Description

METHOD OF FORMING 0D, ID, OR 3D GRAPHENE AND USE THEREOF

Cross-Reference to Related Application

[001] This application claims the benefit of priority of United States of America

Provisional Patent Application No. 61/554,620, filed 02 November 2011, the contents of which being hereby incorporated by reference in its entirety for all purposes.

Technical Field

[002] The invention relates to methods of forming graphene, and in particular, to forming graphene in zero dimension (0D), one dimension (ID), and three dimensions (3D). The invention further relates to the use of 0D, ID, or 3D graphene in high performance electrical conductors for electronics, batteries, supercapacitors and hydrogen storage, and in nanocomposites for heat spreading.

Background

[003] Graphene is a one-atom-thick planar sheet of sp² -bonded carbon atoms that are densely packed in a honeycomb crystal lattice. Graphene initially is a two-dimensional , (2D) carbon material. From the 2D configuration, graphene can be formed into other dimensions. For example, graphene can be wrapped up into a 0D buckyball, rolled into a ID nanofiber or crosslinked to form a 3D foam or aerogel. Compared to the 2D graphene which only exhibits attractive in-plane electrical, thermal, mechanical and structural properties, the remaining, multidimensional graphene materials (i.e. configurations other than 2D) can extend the respective properties to many different directions and hence demonstrate greater potential in applications such as electronics, batteries, supercapacitors, hydrogen storage and heat spreading, for example. [004] While there exist technologies to prepare graphene as such, most of the approaches focus on forming 2D graphene sheets.

[005] Therefore, there is a need to provide for methods of forming graphene directly in dimensions in addition to 2D, such as in 0D, ID, or 3D.

Summary

[006] The present invention is based on the inventors' surprising discovery that graphene in dimensions other than 2D, such as in 0D, ID, or 3D, can be directly formed without being converted or processed from 2D graphene. The inventors have further found a general and common method to form graphene in these alternative or additional dimensions.

[007] Accordingly, in a first aspect of the invention, there is provided a method of forming graphene on a metallic nanostructure, comprising:

(i) coating the metallic nanostructure with a layer of liquid carbon precursor;

(ii) carbonizing the layer of liquid carbon precursor coated on the metallic nanostructure at a carbonizing temperature to convert the layer of liquid carbon precursor to a layer of amorphous carbon;

(iii) crystallizing the layer of amorphous carbon at a crystallizing temperature to convert the layer of amorphous carbon to a layer of crystallized graphene; and

(iv) quenching the layer of crystallized graphene.

[008] The metallic nanostructure can be a 0D nanoparticle such that 0D graphene can be formed by the method of the first aspect. [009] Alternatively, the metallic nanostructure can be a ID nano fiber such that ID graphene can be formed by the method of the first aspect.

[010] Yet alternatively, the metallic nanostructure can be a 3D foam or aerogel such that 3D graphene can be formed by the method of the first aspect.

[011] In another aspect of the invention, there is provided a method of forming an electrode comprising graphene, the method comprising:

(a) forming at least one layer of metallic nanostructures on a substrate;

(b) coating the metallic nanostructures with a layer of liquid carbon precursor;

(c) carbonizing the layer of liquid carbon precursor coated on the metallic nanostructure at a carbonizing temperature to convert the layer of liquid carbon precursor to a layer of amorphous carbon;

(d) crystallizing the layer of amorphous carbon at a crystallizing temperature to convert the layer of amorphous carbon to a layer of crystallized graphene; and

(e) quenching the layer of crystallized graphene.

[012] The substrate can be rigid or flexible such that rigid or flexible electrodes comprising graphene can be formed.

[013] Accordingly, in yet another aspect, there is provided a method of forming a flexible electrode comprising graphene, the method comprising:

(i) coating metallic nanostructures with a layer of liquid carbon precursor; (ii) carbonizing the layer of liquid carbon precursor coated on the metallic nanostructures at a carbonizing temperature to convert the layer of liquid carbon precursor to a layer of amorphous carbon;

(iii) crystallizing the layer of amorphous carbon at a crystallizing temperature to convert the layer of amorphous carbon to a layer of crystallized graphene;

(iv) quenching the layer of crystallized graphene to obtain metallic nanostructures coated with graphene;

(v) coating a polymer solution on the metallic nanostructures coated with graphene, wherein the polymer solution comprises the polymer of a flexible substrate; and

(vi) curing the polymer solution to form a polymer film having the metallic nanostructures coated with graphene.

[014] Further, transparency of the electrodes can be tuned from being transparent to non- transparent by tuning the thickness and/or number of layers of graphene formed on the electrodes.

[015] Thus, in a further aspect of the invention, there is provided a method of forming a transparent electrode comprising graphene, the method comprising:

(i) coating metallic nanostructures with a layer of liquid carbon precursor;

(ii) carbonizing the layer of liquid carbon precursor coated on the metallic nanostructure at a carbonizing temperature to convert the layer of liquid carbon precursor to a layer of amorphous carbon; (iii) crystallizing the layer of amorphous carbon at a crystallizing temperature to convert the layer of amorphous carbon to a layer of crystallized graphene;

(v) dispersing the metallic nanostructures coated with graphene in a solution;

(vi) coating the solution of (v) onto a substrate; and

(vii) drying the coating of (vi).

[016] In yet another aspect of the invention, there is provided a method of forming a heat spreader comprising graphene nanocomposite, the method comprising:

(i) coating metallic nanostructures with a layer of liquid carbon precursor;

(v) mixing the metallic nanostructures coated with graphene with a metal powder;

(vi) processing the mixture of (v) to form the graphene nanocomposite. [017] The mixture of metal nanostructures coated with graphene and the metal powder may be processed by hot pressing, moulding, or sintering.

Brief Description of the Drawings

[018] In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily drawn to scale, emphasis instead generally being placed upon illustrating the principles of various embodiments. In the following description, various embodiments of the invention are described with reference to the following drawings.

[019] Fig. 1 shows the field emission scanning electronic microscope (FESEM) image of Cu nanofibers prepared by an electrospinning technique, followed by pyrolysis and H₂ reduction.

[020] Fig. 2A and Fig. 2B show the FESEM and transmission electronic microscope (TEM) images, respectively, of core-shell Cu nanofibers with C in the core. The carbon core can improve the resistance of the Cu nanofibers to thermal impact during the growth of graphene.

[021] Fig. 3 shows a schematic illustration of the formation of graphene nanofiber films for rigid transparent conductor.

[022] Fig. 4A and Fig. 4B show FESEM image and Raman spectra, respectively, of the 0D graphene with Cu nanoparticle as the template. The G and 2D peaks are the characteristic peaks for graphene.

[023] Fig. 5A and Fig. 5B show FESEM image and Raman spectra, respectively, of graphene nanofibers with Ni in core. The G and 2D peaks are the characteristic peaks for graphene. The D peak with extremely low intensity indicates the obtained graphene has few defects.

[024] Fig. 6A and Fig. 6B shows FESEM image and Raman spectra, respectively, of 3D graphene with Ni foam as the template. The G and 2D peaks are the characteristic peaks for graphene. The D peak with extremely low intensity indicates the obtained graphene has few defects.

[025] Fig. 7 shows transmission spectra of the graphene nanofiber films with Ni in core.

[026] Fig. 8 shows field emission scanning electronic microscope (FESEM) image of parallel graphene nanofibers with Ni in the core.

[027] Fig. 9 shows FESEM image of crossed graphene nanofibers with Ni in the core.

Description

[028] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practised. These embodiments are described in sufficient detail to enable those skilled in the art to practise the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.

[029] The present invention is based on the inventors' surprising discovery that graphene in dimensions other than 2D, such as in 0D, ID, or 3D, can be directly formed without being converted or processed from 2D graphene. The inventors have further found a general and common method to form graphene in these alternative or additional dimensions. [030] Accordingly, a method of forming graphene on a metallic nanostructure is disclosed. The method comprises coating the metallic nanostructure with a layer of liquid carbon precursor and carbonizing the layer of liquid carbon precursor coated on the metallic nanostructures at a carbonizing temperature to convert the layer of liquid carbon precursor to a layer of amorphous carbon. The layer of amorphous carbon is next crystallized at a crystallizing temperature to convert the layer of amorphous carbon to a layer of crystallized graphene, followed by quenching the layer of crystallized graphene. The quenched crystallized graphene results in graphene being formed on the metallic nanostructure.

[031] Graphene is a substance made of pure carbon, with atoms arranged in a regular hexagonal pattern similar to graphite, but in a one-atom thick sheet. It is an allotrope of carbon whose structure is a single planar sheet of sp -bonded carbon atoms that are densely packed in a honeycomb crystal lattice.

[032] A nanostructure is a structure or object that can have any form and has dimensions typically ranging from 1 to 100 nm (nanometre). More specifically, a nanostructure has at least one dimension being less than 100 nm. Nanostructures can be classified, for example, into the following dimensional types: zero dimensional (0D) including, but not limited to, nanospherical particles (also called nanoparticles or nanospheres); one dimensional (ID) including, but not limited to, nanorods, nanowires (also called nanofibers) and nanotubes; two dimensional (2D) including, but not limited to, nanoflakes, nanodiscs, nanocubes and nanofilms; three dimensional (3D) including, but not limited to, porous foam or aerogel having nanosized pores therein. "0D graphene" refers to the graphene growing on the surface of the nanoparticles. "ID graphene" refers to the graphene nanofibers with graphene deposited on the surface of the nanofibers. "3D graphene" refers to the micro/nanoporous graphene foams or graphene aerogels. In such 3D graphere structures, the graphene grows on the metallic walls interconnected with nanopores in the 3D metallic foam or aerogel. The metallic portions can be etched off to form pure graphene

nanoparticles (0D), nanofibers (ID), foams and aerogels (3D). The thickness of the graphene can be one to a few atomic layers. In the present context, while the following description and Examples describe in detail nanostructures in 0D, ID, or 3D form, it is to be understood and appreciated that the form of the nanostructures is not limited to such only and accordingly, forms such as 2D nanostructures are also included. Further, a mixture of different dimensional nanostructures can be used for the formation of graphene, such as but not limited to, a mixture of 0D and ID nanostructures, a mixture of 0D and 3D nanostructures, a mixture of ID and 3D nanostructures, or a mixture of 0D, ID, and 3D nanostructures.

[033] In the present context, the nanostructures are metallic. The metallic nanostructures advantageously and simultaneously act as substrates for the growth of graphene thereon and as catalysts for the formation and growth of graphene. The nanostructures can be formed of pure metals, mixtures of metals, alloys, or metal-carbon mixtures. Examples of metallic nanostructures formed of pure metals include, but are not limited to, Ni, Co, Fe, Cu, Zn, Mn, Ga, Ge, As, Se, In, Sn, Sb, Te, Al, Pd, Pt, Au, Ag, Nb, Mo, Ta, W, Rh, Re, Ti, V, Cr, Zr, Hf, Ru, Os, Ir, or a mixture thereof. Examples of metallic nanostructures formed of alloys include, but are not limited to, at least one metal selected from the group consisting of Ni, Co, Fe, Cu, Zn, Mn, Ga, Ge, As, Se, In, Sn, Sb, Te, Al, Pd, Pt, Au, Ag, and/or at least one refractory metal selected from the group consisting of Nb, Mo, Ta, W, Rh, Re, Ti, V, Cr, Zr, Hf, Ru, Os, and Ir. The molar percentage of the refractory metals in the alloy based on total content of the allow can vary from 0 to 40%. In addition, the metallic

nanostructures can also be made of steels containing different types of alloying elements such as carbon steels, nickel steels, nickel-chromium steels, molybdenum steels, chromium steels, chromium-vanadium steels, tungsten steels, nickel-chromium-vanadium steels, or silicon-manganese steels. For example, the metallic nanostructures can be made of nickel- containing steels such as the steels with the SAE (Society of Automotive Engineers) grade 200 series, 300 series, and 800 series, such as, grade-310 and grade-316 stainless steels.

[034] In the present context, 0D graphene refers to graphene wrapped up into buckyball- shape particles continuously or discontinuously. The nanoparticles are substantially spherical, though not necessarily always the case. The "buckyball" is not limited to a spherical shape. 0D graphene can also be cylindrical, discoidal, tabular, ellipsoidal, equant or irregular in shape. The nanoparticles can be prepared by chemical reduction of metal precursors with surfactant and/or nano templates. The reduction can be carried out under hydrothermal conditions. Further, commercially available metallic nanoparticles are also suitable for use in the present methods. The nanoparticles can be used as such with or without further surface treatment.

[035] In the present context, ID graphene refers to graphene rolled into nanofiber shape continuously or discontinuously. The nanofibers can be less than 1 μιη in diameter and above 5μιη in length, with an aspect ratio of length/diameter of 5 or more. The cross- section of the nanofibers can be spherical, cylindrical, discoidal, tabular, ellipsoidal, equant or irregular shape. The core of the nanofibers can be hollow (i.e. nanotubes) or filled (i.e. nanorods). In various embodiments, the nanofibers are parallelly aligned, gridded (i.e. cross aligned), or randomly grown with or without a substrate support. The nanofibers can be prepared by chemical reduction of metal precursors with surfactant and nano templates under hydrothermal conditions. The nanofibers can also be prepared by electrospinning, pyrolysis followed by H₂ reduction. In an exemplary embodiment, Fig. 1 shows one example of Cu nanofibers prepared by electrospinning. The nanofibers can also be commercially available metallic nanofibers. The nanofibers can be used as such with or without further surface treatment.

[036] In various embodiments, the nanofibers are formed of core-shell structure. The core-shell structure can be formed by a metal comprised in the shell and a refractory metal, oxide or carbon based material comprised in the core. Examples of suitable metals, refractory metals, oxide or carbon based materials for forming the core-shell structure are described in above paragraphs. In certain embodiments, the core-shell nanofibers with a refractory metal in the core are prepared by a coaxial electrospinning method. Two solutions containing different metal precursors are electrospun at the same time through a coaxial needle. A first solution passes through an inner needle while a second solution passes through an outer needle. The thus-obtained wet nanofibers are then dried and annealed at 300 to 600°C in atmosphere for 0.5 to 4 hours to obtain the metal oxide nanofibers. The metal oxide nanofibers are further reduced in a H₂ atmosphere for 0.5 to 3 hours to obtain metallic nanofibers.

[037] In further embodiments, core-shell nanofibers with oxide in the core are prepared by the electrospinning of a solution containing the metal precursor and at least one polymer selected from poly(vinyl alcohol) (PVA), polyaniline (PANI), poly( vinyl pyrrolidone) (PVP), polyethylene oxide (PEO), polycarbonate (PC), polymethylmethacrylate (PMMA), nylon, polyurethane, polylactic acid, polystylene (PS), polyimide (PI), polyvinyl chloride (PVC), polyvinyl phenol, polypropylene (PP), polyethylene (PE), polyethylene naphthalate (PEN), polyethylene terephthalate (PET), cellulose acetate, collagen, proteins, sugar, natural polymers, modified natural polymers, synthetic polymers, or their copolymers. The wet composite nanofibers are heated at 150 to 550°C for about 1 hour in atmosphere to obtain the oxide nanofibers. The nanofibers are then heated at 300 to 750°C in argon and H₂ mixed gases environment for a short period ranging from 0.5 to 60 min to reduce the surface layer of the nanofibers to metal while the core is kept to be oxide.

[038] In yet further embodiments, the core-shell nanofibers with metal in the shell and carbon in the core are prepared by the electrospinning of a solution containing the metal precursor and at least one polymer selected from PVA, PANI, PVP, PEO, PC, PMMA, PS, PI, PVC, PP, PE, PEN, PET, nylon, polyurethane, polylactic acid, polyvinyl phenol, cellulose acetate, collagen, proteins, sugar, natural polymers, modified natural polymers, synthetic polymers, or their copolymers. The wet composite nanofibers are heated to 100 to 350°C at the ramping rate of 0.1 to 10°C/min and dwelled for 1 to 2 hour in atmosphere. The samples are then ramped up to 300 to 600°C in argon and dwelled for another 0.5 to 3 hours. Finally the nanofibers are reduced in H₂-containing atmosphere for 0.5 to 30 min. Fig. 2A and Fig. 2B show examples of the core-shell nanofibers with copper in the shell and the carbon in the core.

[039] Porous foams are 3D metallic material containing large volume fraction of pores that are connected to each other to form an interconnected network. The porosity of the foams is defined to be more than 30 vol%. The pore size is in the range of several nanometers to hundreds of micrometers. The foams can be prepared by directly foaming the metal melts with gas injection, or adding blowing agents such as metal hydride and CaC0₃ followed by solidification. The foams can also be commercially available foams. For example, commercially available foams include Ni or Cu based foams supplied by Lyrun Metal Foam (China), Mitshubshi Materials corporation (Japan), Reade Advanced Materials (USA) or RECEMAT Internationals (Netherlands).

[040] In the present context, 3D graphene refers to graphene crosslinked into porous foams or aerogels. The pores inside the foams or aerogels are varied in a few nanometers to tens of micrometers in size. Porous aerogels are similar to the porous foams except that the pores and the walls between the pores are less than 100 nm. The porosity of the aerogels is defined to be more than 80 vol%. The aerogels can be prepared by a sol-gel processing method in which, the metal alkoxides or metal inorganic salt are first mixed in a solvent with chelating agents, surfactant and/ or polymers, followed by crosslinking them into M- O-M or M-OH-M networks (where M: metal; O: oxygen; H: hydrogen) by hydrolysis and condensation reactions. When the crosslinking is stopped, the solution turns into a gel. After driving off the solvent by supercritical drying, freeze drying or vacuum drying, the gel is annealed and then reduced to aerogel. The aerogels can also be commercially available aerogels.

[041] The method includes coating the metallic nanostructure with a layer of liquid carbon precursor. By "coating" is meant that a layer of the liquid carbon precursor is applied onto the metallic nanostructure surface. The metallic nanostructure surface may be coated entirely or partially with the layer of the liquid carbon precursor. The coating step may be carried out once, twice, thrice, or more. The thickness of the coating layer may be less than 100 nm, such as 90 nm, 80 nm, 70 nm, 60 nm, 50 nm, 40 nm, 30 nm, 20 nm, 10 nm, or less. [042] In certain embodiments, the coating step comprises dip coating. For example, in the dip coating step, the metallic nanostructure is dipped into a liquid carbon precursor coating solution and then is withdrawn from the solution at a controlled speed. Coating thickness generally increases with faster withdrawal speed. A faster withdrawal speed pulls more fluid up onto the surface of the metallic nanostructure before it has time to flow back down into the solution. The coating thickness is primarily affected by fluid viscosity, fluid density, and surface tension. The applied coating may remain wet for several minutes until the solvent evaporates. Once the layer is formed, another layer may be applied on top of it with another dip coating step.

[043] In alternative embodiments, the coating step comprises spin coating. For example, in the spin coating step, an excess amount of the liquid carbon precursor solution is placed on a substrate including the metallic nanostructures, which is then rotated at high speed in order to spread the fluid by centrifugal force. Rotation is continued while the fluid spins off the edges of the substrate, until the desired thickness of the coating is achieved. The higher the angular speed of spinning, the thinner the coating.

[044] In yet further embodiments, the coating step comprises spray coating. For example, in the spray coating step, a spraying equipment is used to apply the liquid carbon precursor solution by pressurizing the solution and atomizing the solution into small droplets depositing onto the surface of the metallic nanostructures.

[045] In various embodiments, the liquid carbon precursor comprises a hydrocarbon compound in liquid phase at room temperature (i.e. 25°C) and atmospheric pressure (i.e. 101.325 kPa). The liquid carbon precursor wets the surface of the metallic nanostructure with a contact angle of less than 90°. Examples of the liquid carbon precursors include, but are not limited to, Cs-Ci₂ alkanes (e.g. pentane, octane, dodecane), C₁₈-C6o alkenes and their polymers and copolymers, alkyne and their polymers and copolymers, polyol compounds and their polymers and copolymers (e.g. ethylene glycol, propylene glycol, dipropylene glycol, glycerol, poly(ethylene glycol), polypropylene glycol, ethers, polyethers, esters, carboxylic acids, aromatic compounds, mineral oils (e.g. paraffinic oils, naphthenic oils, aromatic oils), vegetation oils (e.g. olive oil, palm oil, soybean oil, canola oil, pumpkin seed oil, corn oil, sunflower oil, safflower oil, peanut oil, grape seed oil, sesame oil, argan oil), animal oil (e.g. fish oil), synthetic oil and their mixtures. Further exemplary liquid carbon precursors can also be polymer containing solutions. The weight percentage of the polymers in the solutions should be less than 40%. The polymers can have -C(-C)_n-C- as the main chains. Examples include, but are not limited to, PVA, PANI, PVP, PEO, PMMA, PE, PP, PS, PEN, nylon, polyvinyl phenol, natural polymers, modified natural polymers, synthetic polymers and their copolymers.

[046] The method further includes carbonizing the layer of liquid carbon precursor coated on the metallic nanostructure at a carbonizing temperature to convert the layer of liquid carbon precursor to a layer of amorphous carbon. By "carbonizing" is meant converting the liquid carbon precursor into carbon residue through pyrolysis, and in the present context, the carbon residue is amorphous carbon, which is a carbon material without long-range crystalline order.

[047] In various embodiments, the carbonizing step includes heating the layer of liquid carbon precursor coated on the metallic nanostructure at a carbonizing temperature of 180 to 600 °C, such as 200 to 550 °C, 200 to 500 °C, 200 to 450 °C, 200 to 400 °C, 200 to 350 °C, or 200 to 300 °C. The carbonizing temperature can be appropriately determined by a person skilled in the art once the composition/nature of the liquid carbon precursor used in the coating step is known. In one exemplified embodiment, liquid carbon precursor used in the coating step is polyethylene glycol (PEG200) and the carbonizing temperature selected for the carbonizing step can be from 200 to 300 °C, such as about 250 °C.

[048] In further embodiments, the carbonizing step includes heating the layer of liquid carbon precursor coated on the metallic nanostructure for a period of 1 to 120 min, such as 10 to 110 min, 10 to 100 min, 10 to 90 min, 10 to 80 min, 10 to 70 min, 10 to 60 min, 10 to 50 min, or 10 to 40 min. The period for the carbonizing step can be appropriately determined by a person skilled in the art once the composition/nature of the liquid carbon precursor used in the coating step and therefore the carbonizing temperature are known. In one exemplified embodiment, liquid carbon precursor used in the coating step is polyethylene glycol (PEG200), the carbonizing temperature selected for the carbonizing step is about 250 °C, and the period for carbonizing at this temperature can be from 20 to 40 min, such as 30 min.

[049] The carbonizing step may be carried out in an enclosed environment or heating chamber, such as a furnace. The furnace can be pre-heated to the carbonizing temperature of between 180 and 600 °C such that the dwelling time is kept to a minimum. The coated metallic nanostructure may be loaded into the furnace immediately after the coating step so that the maximum thickness of the layer of the liquid carbon precursor is maintained at the surface and therefore converted to the layer of amorphous carbon. Alternatively, if a thinner layer of graphene is desired to be formed (for example, when the graphene is used to form transparent electrodes to be described in later paragraphs), the coated metallic nanostructure may be allowed to dry off or otherwise reduce the thickness of the coating of the liquid carbon precursor before loading into the furnace for carbonization.

[050] In various embodiments, the carbonizing step is carried out in a non-oxidising environment, such as an inert or a reductive environment. For example, the carbonizing step is carried out in an inert environment filled with argon (Ar). In another example, the carbonizing step is carried out in a reductive environment filled with a gas mixture of Ar and hydrogen (H₂).

[051] In further embodiments, the carbonizing step is carried out in an inert or a reductive environment with a total gas flow rate of 200 to 500 seem, such as 200 to 450 seem, 200 to 400 seem, 200 to 350 seem, 200 to 300 seem, or 200 to 250 seem. In one embodiment, the carbonizing step is carried out in a reductive environment with a total gas flow rate of about 200 seem, and the gas mixture comprises a molar ratio of 1 :5 (H₂:Ar).

[052] After forming the layer of amorphous carbon on the metallic nanostructures, the method next includes crystallizing the layer of amorphous carbon at a crystallizing temperature to convert the layer of amorphous carbon to a layer of crystallized graphene. In this crystallizing step, the non-crystalline ordered amorphous carbon is heated to a point where the carbon atoms are re-arranged to form a one- atom-thick planar sheet of sp²- bonded carbon atoms that are densely packed in a honeycomb crystal lattice, i.e. graphene.

[053] The crystallizing step is carried out in an enclosed environment. In various embodiments, the crystallizing step includes increasing the temperature to a crystallizing temperature of 450 to 1,000 °C, such as 450 to 650 °C, 450 to 750 °C, 450 to 850 °C, 450 to 950 °C, 450 to 1000 °C. In illustrative embodiments, the crystallizing step includes increasing the temperature to a crystallizing temperature of about 750 or 800 °C. [054] In certain embodiments, the crystallizing step is carried out in the same furnace where the carbonizing step is carried out. In alternative embodiments, the crystallizing step is carried out in another enclosed environment or heating chamber. In the embodiments where both the carbonizing and crystallizing steps are carried out in the same furnace, after carbonizing the metallic nanostructures at the carbonizing temperature for a dwelling period, the furnace is quickly brought to the crystalling temperature without removing the carbonized metallic nanostructures from the furnace. In various embodiments, the temperature of the furnace is quickly brought up to the crystallizing temperature at a ramping rate of 0.5 to 200 °C/min, such as 5 to 180 °C/min, 5 to 160 °C/min, 5 to 140 °C/min, 5 to 120 °C/min, 5 to 100 °C/min, or 10 to 100 °C/min. After ramping up the temperature to the crystallizing temperature, the crystallizing temperature is maintained for a period of time. For example, the furnace is maintained at the crystallizing temperature for 1 sec to 30 min, such as 5 sec to 25 min, 1 to 20 min, 5 to 20 min, or 5 to 15 min. In exemplified embodiments, the crystallizing temperature is maintained for about 5 min, or 8 min, or 15 min.

[055] In various embodiments, the crystallizing step is carried out in a non-oxidising environment, such as an inert or a reductive environment. For example, the crystallizing step is carried out in an inert environment filled with argon (Ar). In another example, the crystallizing step is carried out in a reductive environment filled with a gas mixture of Ar and hydrogen (H₂).

[056] In further embodiments, the crystallizing step is carried out in an inert or a reductive environment with a total gas flow rate of 200 to 500 seem, such as 200 to 450 seem, 200 to 400 seem, 200 to 350 seem, 200 to 300 seem, or 200 to 250 seem. In one embodiment, the crystallizing step is carried out in a reductive environment with a total gas flow rate of about 200 seem, and the gas mixture comprises a molar ratio of 1:5 (H₂:Ar).

[057] In either one of the carbonizing step or crystallizing step, or both, the heating environment may be provided by, for example, induction heating, radiant heating, infrared, ultraviolet, microwave, laser, or plasma. Other heating sources or means may likewise be used.

[058] The method further includes quenching the layer of crystallized graphene to form the graphene. During the crystallizing step, the carbon atoms rearrange their order and arrangement. Once the crystal lattice of graphene is formed, the crystallized graphene is quenched to fix the orderly arrangement permanently. By "quenching" is meant rapid cooling of the heated and crystallized graphene layer on the metallic nanostructure.

[059] In various embodiments, the quenching step includes reducing the temperature of the layer of crystallized graphene to room temperature in less than a few minutes, such as less than 10 min, 9 min, 8 min, 7 min, 6 min, 5 min, 4 min, 3 min, 2 min, 1 min, or less. For example, the quenching step can be carried out in an inert environment with a total gas flow rate of 50 to 200 seem to rapidly cool the layer of crystallized graphene. The inert environment may be provided by Ar or a mixture of Ar and H₂. Alternatively, the quenching step may be carried out by immersing the heated and crystallized graphene layer on the metallic nanostructure in a bath of quenching medium. The bath of quenching medium may be provided by ice water, for example.

[060] In certain embodiments, the metallic nanostructures may be etched away to obtain pure 0D, ID, or 3D graphene. For example, the metallic nanostructures may be etched with an etching solution such as, but is not limited to, hydrochloride acid, nitric acid, hydrofluoric acid, sulfate acid, or a mixture thereof.

[061] The so-formed graphene, with or without the metallic nanostructures, may be used to further form electrodes or nanocomposite heat spreader.

[062] Thus, in another aspect, a method of forming an electrode comprising graphene is provided. The method comprises:

(a) forming at least one layer of metallic nanostructures on a substrate;

(c) carbonizing the layer of liquid carbon precursor coated on the metallic nanostructures at a carbonizing temperature to convert the layer of liquid carbon precursor to a layer of amorphous carbon;

(e) quenching.the layer of crystallized graphene.

[063] The coating, carbonizing, crystallizing, and quenching steps in forming graphene have been described in detail in previous paragraphs corresponding to the respective step in the first aspect and are not repeated hereinafter. In addition, it is to be understood and appreciated that the forming of at least one layer of metallic nanostructures on a substrate (i.e. step (a)) can be carried out before or after forming the graphene.

[064] The substrate can be rigid or flexible such that rigid or flexible electrodes comprising graphene can be formed. Further, transparency of the electrodes can be tuned from being transparent to non-transparent by tuning the thickness and/or number of layers of graphene formed on the electrodes.

[065] Transparent conductive electrodes comprising graphene formed herein possess the following characteristics: (1) large surface area, (2) high electronic conductivity and thermal conductivity, (3) excellent chemical stability, (4) high flexibility, and (5) work function at about 4.6 eV, close to that of tin indium oxide (ITO, 4.7 eV). Further, the transparency of the transparent conductive electrodes can be in the range of 50 to 99.9% and the sheet resistance in the range of 1 to 1000 O/sq. The transparent electrodes may be employed for touch screens, thin-film solar cell, flexible displays, or solid-state lighting especially organic light emitting diodes.

[066] While the graphene described herein relates mostly to OD or ID graphene, 3D graphene are equally suitable depending on the intended use or applications. The ID nanofibers can be processed to form parallel, crossed, or random patterns. The transparency of the graphene films can be adjusted by controlling the graphene nanofiber or nanoparticle density and coating thickness. Nanofiber or nanoparticle density here refers to the number of the nanofibers or nanoparticle per cm². The substrates used may be rigid transparent substrates, such as but are not limited to, glass, BK7 (a kind of borosilicate crown optical glass), fused silica, crystal quartz, CaF₂, ZnSe, sapphire, MgF₂, MgO, Germanium (Ge), and calcite (CaC0₃). The substrates used may also be flexible transparent substrates such as, but are not limited to, polydimethylsiloxane (PDMS), PET, PEN, PC, PVA, polyethersulfone (PES), PVC, PI, PMMA, or various commercial available acrylic glass.

[067] In various embodiments where rigid electrodes are desired, the forming step comprises electrospinning the metallic nanostructures onto a rigid substrate and annealing the rigid substrate to form the at least one layer of metallic nanostructures thereon. For example, to prepare transparent conductive electrodes on a rigid substrate, graphene nanofibers may be grown on the rigid transparent substrates such as quartz, fused silica, or sapphire.

[068] In an illustrative embodiment, the growth process is schematically illustrated in Fig. 3. Firstly, the nanofibers are formed by electrospinning, followed by pyrolysis and H₂ reduction. The electrospinning solution consists of at least one metal precursor and one polymer. The metal precursors may be, but are not limited to, metal acetate, ethoxide, propoxide, butoxide, acetylacetonate, nitrate, chloride, fluoride, suphate, or phosphate. At least 10% of the metal precursors are easily ionisable, such as metal nitrate, chloride, fluoride, suphate, or phosphate. The polymers may be PVA, PANI, PVP, PEO, or PC. After electrospinning, the obtained electrospun composite nanofibers are then dried and annealed at 300 to 600 °C in atmosphere for 0.5 to 4 hours to form the pure metal oxide nanofibers. The metal oxide nanofibers are next reduced to metal nanofibers in H₂ atmosphere for 0.5 to 3 hours. The prepared metal nanofibers are then coated with a liquid carbon precursor and loaded into a tube furnace which was pre-filled with a 1:5 mixture of H₂ and Ar at a total gas flow rate of 200 to 500 seem. The liquid carbon precursor is first carbonized to amorphous carbon at a carbonizing temperature of 300 to 600 °C for 1 to 40 minutes, followed by crystallizing into graphene at a crystallizing temperature of 450 to 900 °C. During the carbonizing and/or crystallizing step, the nanofibers simultaneously are annealed and attached to the rigid substrate. The carbonizing time and/or crystallizing time for graphene formation is typically in the range of tens of seconds to a few minutes, which determines the number of layers of graphene finally grown on the nano fiber and also serves to avoid the deformation of the metallic nanofibers. The crystallized graphene is then quenched to room temperature within a few minutes in a flow of 200 seem Ar to form graphene nanofibers with a metal in the core, and thus the transparent conductive electrodes on a rigid substrate.

[069] In various embodiments where flexible electrodes are desired, such as transparent conductive electrodes on flexible substrates, the forming step comprises coating metallic nanostructures with a layer of liquid carbon precursor, carbonizing the layer of liquid carbon precursor coated on the metallic nanostructures at a carbonizing temperature to convert the layer of liquid carbon precursor to a layer of amorphous carbon, crystallizing the layer of amorphous carbon at a crystallizing temperature to convert the layer of amorphous carbon to a layer of crystallized graphene, quenching the layer of crystallized graphene to obtain metallic nanostructures coated with graphene, coating a polymer solution on the metallic nanostructures coated with graphene, wherein the polymer solution comprises the polymer of a flexible substrate, and curing the polymer solution to form a polymer film having the metallic nanostructures coated with graphene. The cured polymer film may then be peeled off to form free-standing flexible electrode. In one illustrative embodiment, metallic nanofibers are first prepared. For example, parallel or cross aligned (i.e. gridded) Ni nanofibers are prepared by electrospinning using a rotating, electrically- grounded wheel as the receptor. The above described graphene formation steps are then carried out on the Ni nanofibers. After growing graphene, the graphene nanofibers are transferred onto flexible substrates such as PDMS, PES, PET, PEN, PC, PVA, PVC, PI, PMMA. In this step, the polymer solution of the substrate is spin coated or slowly poured onto the graphene nanofibers and cured at 50 to 100 °C. Next, the polymer layers buried with the parallel or cross patterned graphene nanofibers are peeled off to form a self-stand electrode.

[070] Non-transparent conductive electrodes are the electrodes with transparency less than 50%, such as less than 40%, 30%, 20%, 10%, or less, and sheet resistance in the range of 1 to 500 Q/sq, such as 1 to 400 0/sq, 1 to 300 0/sq, 1 to 200 0/sq, or 1 to 100 Q/sq. The non- transparent conductive electrodes referred herein can be used as electrodes for various - electronics, batteries, supercapacitors and hydrogen storage, RFID (radio frequency identification) antennas, flexible circuits, LED strings/arrays and for EMI (electromagnetic) shielding.

[071] In various embodiments, more than one layer of metallic nanostructures is formed on the substrate. The substrate can be rigid or flexible. By forming more than one layer of metallic nanostructures, such as two layers, three layers, or more, transparency of the thus- formed conductive electrode is gradually reduced. In certain embodiments, the transparency of the conductive electrode may tuned from being transparent to non-transparent state. For example, the non-transparent electrode is a graphene paper consisting of ID graphene with or without the metallic nanostructure template. The ID graphene may be grown on nanofibers formed by electrospinning followed by pyrolysis and H₂ reduction described above. The nanofibers may be interlaced with each other to form a layer with a total thickness of more than 1 μπι, for example. After coating with the liquid carbon precursor, the nanofiber layers are then carbonized, crystallized, and quenched to form the graphene paper. The graphene paper may be etched to remove the metallic nanostructure template. The advantage of the graphene paper produced by this procedure is that there is no polymer binder or surfactant contained in the papers. Further, by controlling the number of nanostructure layers and therefore the total thickness of the resultant graphene paper, transparency of the electrode can be tuned.

[072] In alternative embodiments, 3D graphene are used to form non-transparent electrodes. For example, free-standing 3D graphene porous foams or aerogels with or without the metallic nanostructure template may be used. The pore size in the electrodes may be less than 1 μηι and porosity may be higher than 50%. The 3D graphene porous foam or aerogel electrodes have the following features: (1) ultralarge surface area; (2) flexible; (3) highly conductive; (4) excellent chemical stability; and (5) with a work function at about 4.6 eV, close to that of tin indium oxide (ITO, 4.7 eV).

[073] In another aspect, a method of forming an electrode comprising graphene is provided. The method comprises:

(i) coating metallic nanostructures with a layer of liquid carbon precursor;

(ii) carbonizing the layer of liquid carbon precursor coated on the metallic nanostructures at a carbonizing temperature to convert the layer of liquid carbon precursor to a layer of amorphous carbon;

(v) dispersing the metallic nanostructures coated with graphene in a solution;

(vi) coating the solution of (v) onto a substrate; and

(vii) drying the coating of (vi). [074] The coating, carbonizing, crystallizing, and quenching steps in forming graphene have been described in detail in previous paragraphs corresponding to the respective step in the first aspect and are not repeated hereinafter.

[075] The electrodes formed can be transparent or non-transparent.

[076] In various embodiments, the transparent conductive electrodes are prepared by coating a graphene-containing solution on either rigid or flexible transparent substrates. The graphene used herein can be nanoparticles or nanofibers. The graphene nanoparticles or nanofibers may be ultrasonically dispersed to form a graphene solution. The graphene solution may contain 0.1 to 60 wt% of the graphene nanoparticles and/or nanofibers, 0 to 10 wt% of surfactants and 0 to 30 wt% polymeric binders. The surfactant used may include, but is not limited to, sodium dodecylbenzene sulfonate (SDBS), alkylazides, 1 1- azidoundecanol (AUO), 1 1-azidoundecanoic acid,sodium cholate, cetyltrimethylammonium bromide, hexadecyl trimethyl ammonium bromide, triton X-100, or PVP. The polymer binder may include, but is not limited to, polyurethane resin, polyester resin, alkyd resin, butyral resin, acetal resin, polyamide resin, acrylic resin, styrene-acrylic resin, styrene resin, nitrocellulose, benzyl cellulose, styrene-maleic anhydride resin, polybutadiene resin, poly(vinyl chloride) resin, poly( vinyl acetate) resin, fiuororesin, silicone resin, epoxy resin, phenol resin, maleic acid resin, urea resin, melamine resin, benzoguanamine resin, ketone resin, rosin, chlorinated polyolefin resin, or chlorinated polyurethane resin. The type of the polymer binder used can be determined by the criteria of the coating method and the types of substrates used. The solvent of the graphene solution may include, but is not limited to, de-ionized water, organic solvent such as methanol, ethanol, iso-propanol, n-butanol, i- butanol, neopentyl butanol, hexanol, octanol, ethylene glycol, benzyl alcohol, chloroform, N-methyl-pyrrolidone, N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), glycidyl ether such as n-butyl glycidyl ether (BGE), isopropyl glycidyl ether (IGE) and phenyl glycidyl ether (PGE), aromatic solvent such as benzene, toluene, xylene, ethyl benzene, diethyl benzene, C5-C20 alkyl benzene, chlorobenzene, or dichlorobenzene. The graphene solution may be coated onto the substrates by spray coating, dip coating, spin coating or mayer-rod coating methods. The obtained wet coatings are then hot pressed by a punch laminator to improve the adherence of the coatings. The temperature for the laminating may be set at 25 to 180 °C. The graphene solutions may also form patterned electrodes by screen printing or inkjet printing.

[077] In various embodiments, the graphene used for forming non-transparent electrodes is 0D and/or ID graphene with or without metallic nanostructure template. The 0D and/or ID graphene is firstly ultrasonically dispersed in a solution to form an ink and then coated onto a substrate. The substrates can be rigid or flexible materials such as, but are not limited to, non-transparent materials including ceramics (for example, A1₂0₃ and Zr0₂), metals, oxides, semiconductors (for example, Si, GaN, GaAs, InAs and InP) and polymers. The substrates may also be ceramic, metal, oxide, semiconductor, or polymer foams. Compared to the graphene solutions for forming transparent electrodes, the graphene solutions in such cases contain a higher content of graphene, for example, 30 to 80 wt% of graphene, 0 to 10 wt% of surfactants and 0 to 20 wt% polymeric binders. The surfactant may include, but is not limited to, SDBS, alkylazides, AUO, 11 -azidoundecanoic acid, sodium cholate, cetyltrimethylammonium bromide, hexadecyl trimethyl ammonium bromide, triton X-100, or PVP. The polymer binder may include, but is not limited to, thermosetting polymer such as polyurethane resin, polyester resin, alkyd resin, butyral resin, acetal resin, polyamide resin, acrylic resin, styrene-acrylic resin, styrene resin, nitrocellulose, benzyl cellulose, styrene-maleic anhydride resin, polybutadiene resin, poly( vinyl chloride) resin, poly(vinyl acetate) resin, fluororesin, silicone resin, epoxy resin, phenol resin, maleic acid resin, urea resin, melamine resin, benzoguanamine resin, ketone resin, rosin, chlorinated polyolefin resin, or chlorinated polyurethane resin. The solvent of the graphene solution may include, but is not limited to, de-ionized water, organic solvent such as methanol, ethanol, iso- propanol, n-butanol, i-butanol, neopentyl butanol, hexanol, octanol, ethylene glycol, propylene glycol, benzyl alcohol, chloroform, N-methyl-pyrrolidone, DMF, glycidyl ether such as BGE, IGE and PGE, and aromatic solvent such as benzene, toluene, xylene, ethyl benzene, diethyl benzene, C5-C20 alkyl benzene, chlorobenzene, or dichlorobenzene. The inks may be coated onto the substrates by mayer-rod coating, spray coating, dip coating, or spin coating. The coatings are then dried by a hot plate or oven with a temperature of 80 to 150 °C. The graphene inks may also form the patterned electrodes by screen printing or inkjet printing.

[078] In yet further aspect, a method of forming a heat spreader comprising graphene nanocomposite is disclosed. The method comprises:

(i) coating metallic nanostructures with a layer of liquid carbon precursor;

(iii) crystallizing the layer of amorphous carbon at a crystallizing temperature to convert the layer of amorphous carbon to a layer of crystallized graphene; (iv) quenching the layer of crystallized graphene to obtain metallic nanostructures coated with graphene;

(vi) processing the mixture of (v) to form the graphene nanocomposite .

[079] The coating, carbonizing, crystallizing, and quenching steps in forming graphene have been described in detail in previous paragraphs corresponding to the respective step in the first aspect and are not repeated hereinafter.

[080] In various embodiments, the processing step comprises hot pressing, moulding, or sintering the mixture of the metallic nanostructures coated with graphene and the metal powder.

[081] The step of processing the graphene nanocomposite may further include sawing, machining and polishing the nanocomposite article to the desired shape or size.

[082] In illustrative embodiments, a nanocomposite comprising 0.1 to 80 wt% of graphene prepared herein and 20 to 99.9 wt% of metal matrix is developed for heat spreading. The graphene used may be 0D, ID and/or 3D graphene, such as ID and 3D graphene with or without the metallic nanostructure template. The metal matrix may include metals with high thermal conductivity (more than 100 W/mK), such as but are not limited to, copper, aluminum, silver, gold, molybdenum, or alloy thereof. The

nanocomposite containing the as-prepared graphene has an improved thermal conductivity compared to the pure metal matrix. Due to this improved and superior thermal conductivity, the nanocomposite containing graphene may be used to form thermal conducting components such as heat spreaders and heat sinks for high power electronic packaging, integrated circuits (for example, CPUs or chipsets), display device (for example, plasma display panel or liquid crystal display) and other thermal management applications.

[083] In various embodiments, nanoparticles with copper as the nanostrucrure template are used to form nanocomposites. The nanoparticles may be used alone or mixed with other metal powders having high thermal conductivity to form the nanocomposite. The nanoparticles are firstly mixed and then placed in a graphite die to form a pellet. The pellet is then hot pressed at a temperature of 650 to 850 °C and a pressure of 30 to 120 MPa for 1 to 45 minutes to form a nanocomposite with relative density higher than 90% compared to pure copper density. Alternatively, the pellet may be sintered by spark plasma sintering to form the nanocomposite. The sintering may be performed at a temperature ranging from 500 to 700 °C and a working pressure of 50 to 100 Mpa for 1 to 15 minutes. Finally, processing steps such as sawing, machining and polishing may be performed to

manufacture a plate having a desired shape and size. The plate may be used as a heat spreader or heat sink.

[084] In alternative embodiments, nanofibers with copper as the nanostructure template are used to form the nanocomposite. The nanofibers may be dispersed in a solvent with a low evaporation temperature such as alcohol, acetone or isopropanol. Then metal nanopowders are added into the solution with mechanically stirring. The average size of the metal nanopowders may be less than 1 μπι. The mixture is then centrifuged, dried and hot pressed or spark plasma sintered at 500 to 1,100 °C to form the nanocomposite.

Alternatively, the mixture may also be cold pressed at room temperature and sintered at 650 to 1,200 °C under vacuum or by microwave. Finally, processing steps such as sawing, machining and polishing may be performed to manufacture a plate having a desired shape and size. The plate may be used as a heat spreader or heat sink.

[085] In yet further embodiments, ID or 3D graphene in dry form is used. The ID or 3D graphene is added into a hot melting metal liquid with mechanical stirring. The mixed solution may then be poured into a mold and gradually cooled down to form the nanocomposite with various desired shape and size.

[086] In order that the invention may be readily understood and put into practical effect, particular embodiments will now be described by way of the following non-limiting examples.

[087] Examples

[088] Example 1 : Preparation of 0D graphene

[089] A sample of copper nanoparticles purchased from Alfa Aesar (MA, USA) were dip coated with a thin layer of polyethylene glycol (PEG) with molecular weight of 200 (PEG200) which were then dissolved in ethanol. The PEG coated copper nanoparticles were then transferred to a tube furnace pre-filled with Ar and heated at 250 °C for 30 min. The nanoparticles were then rapidly heated to 800 °C and dwelled for 5 min. After that, the nanoparticles were quenched to room temperature within a few minutes in a flow of 200 seem Ar.

[090] Fig. 4A and Fig. 4B show FESEM image and Raman spectra, respectively, of the 0D graphene with Cu nanoparticle as the template.

[091] Example 2: Preparation of ID graphene

[092] A sample of nickel nanofibers was prepared by electrospinning a solution containing 10 wt% nickel acetate, 4 wt% nickel chloride, 12 wt% polyacrylonitrile (PAN) polymer and 74 wt% N, N dimethylformamide, followed by pyrolysis at 500 °C for 1 hour. The nanofibers were then reduced in H₂ at 500 °C for 30 min. The nickel nanofibers are then dip coated with a thin layer of PEG200 solution. The PEG200 coated Ni nanofibers were then transferred to a tube furnace pre-filled with Ar and heated at 250 °C for 30min. The nanofibers were then rapidly heated to 800 °C and dwelled for 5 minute. After that, the nanofibers were quenched to room temperature within a few minutes in a flow of 200 seem Ar.

[093] Fig. 5A and Fig. 5B show FESEM image and Raman spectra, respectively, of ID graphene with Ni nanofibers as the template. The G and 2D peaks are the characteristic peaks for graphene. The D peak with extremely low intensity indicates the obtained graphene has few defects.

[094] Example 3: Preparation of 3D graphene

[095] A sample of nickel foam with pore size of a few micrometers (purchased from Reade Advanced Materials, USA) was dip coated with a thin layer of PEG200 solution. The PEG200 coated nickel foam was then transferred to a tube furnace pre-filled with Ar and heated at 250 °C for 30 min. The foam was then rapidly heated to 800 °C and dwelled for 5 minute. After that, the foam was quenched to room temperature within a few minutes in a flow of 200 seem Ar.

[096] Fig. 6A and Fig. 6B shows FESEM image and Raman spectra, respectively, of 3D graphene with Ni foam as the template. The G and 2D peaks are the characteristic peaks for graphene. The D peak with extremely low intensity indicates the obtained graphene has few defects.

[097] Example 4: Preparation of rigid transparent electrode [098] A sample of a thin layer of nickel composite nanofiber was prepared by

electrospinning a solution containing 10 wt% nickel acetate, 12 wt% PAN polymer, and 78 wt% N,N dimethylformamide on a PAN coated quartz substrate with a size of 2 cm ^χ 7.5 cm. The sample was then pyrolyzed at 500 °C for 30 minutes and then reduced by H₂ for 10 min. The nickel nanofibers were then spin coated with a thin layer of PEG200. The PEG200 coated nanofibers were then transferred to a tube furnace pre- filled with Ar and heated at 250 °C for 30 min. The nanofibers were then rapidly heated to 750 °C and dwelled for 8 minute. After that, the nanofibers were quenched to room temperature within a few minutes in a flow of 50 seem Ar.

[099] The rigid transparent electrodes formed thereof have a transparency of about 80- 90% at a wavelength of 300-1 100 nm and possess a sheet resistivity of about 50 0/sq.

[0100] Fig. 7 shows the transmission spectra of the graphene nanofiber films with Ni in core.

[0101] Example 5: Preparation of flexible transparent electrode

[0102] Graphene nanofibers prepared in Example 2 were ultrasonically dispersed in a solution of 3 mg/ml polymer binder, polyvinyl pyrroridol (PVP). The solution was then mayer-rod coated on an A4-sized polyethylene terephthalate (PET) sheet.

[0103] The flexible transparent electrodes formed thereof have a transparency of about 85-

90% at a wavelength of 300-1 lOOnm and possess a sheet resistivity of about 70 Ω/sq.

[0104] Example 6: Preparation of patterned, flexible and transparent electrodes

[0105] Parallel Ni nanofibers and cross aligned Ni nanofibers were separately prepared by electrospinning using a rotating, electrically-grounded wheel as a receptor. In each case, after growing graphene on the Ni nanofibers, the nanofibers were transferred onto flexible substrates, such as polydimethylsiloxane (PDMS). In this step, a Sylgard 184 silicone elastomer kit produced by Dow Corning Corporation was used to produce the PDMS substrate. The base and its curing agent was mixed in 10: 1 mass ratio and the solution was then slowly poured on the graphene nanofibers and cured at 70 °C for 12 hours. Finally, the films were peeled off to form a self-standing substrate with the parallel or cross patterned graphene electrodes.

[0106] Fig. 8 shows field emission scanning electronic microscope (FESEM) image of parallel graphene nanofibers with Ni in the core.

[0107] Fig. 9 shows FESEM image of crossed graphene nanofibers with Ni in the core.

[0108] Example 7: Preparation of non-transparent electrode with graphene foam

[0109] 3D graphene foam prepared in the Example 3 was cut into a thin layer with a thickness of about 100 μιη. It was then bound to a dense copper foil to form an electrode with a high surface area.

[0110] Example 8: Preparation of non- transparent electrode-graphene paper

[0111] A layer of nickel composite nanofibers with a thickness of about 100 μηι was prepared by electrospinning a solution containing 14 wt% nickel acetate, 12 wt% PAN polymer, and 74 wt% N,N dimethylformamide on an Al foil. The layer of nanofibers was then dried at 60 °C for 24 hours in vacuum to remove the solvent. The composite nanofiber layer was then peeled off to form a free-standing membrane. It was pyrolyzed at 500 °C for 30 minutes and then reduced by H₂ for 10 min. The layer of nickel nanofibers was then dip coated with a thin layer of PEG200. The layer of PEG200 coated Ni nanofibers was then transferred to a tube furnace pre-filled with Ar and heated at 250 °C for 30 min. The layer of nanofibers was then rapidly heated to 750 °C and dwelled for 15 min. After that, the layer of nanofibers was quenched to room temperature within a few minutes in a flow of 50 seem Ar. The graphene paper obtained thereof possesses a sheet resistivity of about 20 Ω/sq.

[0112] Example 9: Preparation of nanocomposite for heat spreading

[0113] The 0D graphene nanoparticles prepared in Example 1 were put in a graphite die. The die was then put in a chamber of a high vacuum furnace and was hot pressed at 750 °C and 50 MPa for 15 min. The sample was then polished by 400 grit sand paper. The final dimension of the sample size was 13 mm (diameter) x 2 mm (height). Thermal conductivity of the sample was tested by LFA 447 NanoFlash (Netzsch, Germany) at room temperature. The measured thermal conductivity reached 392.6 W/mK, which was higher than that of pure copper prepared and tested under the same conditions (333.2 W/mK).

[0114] In summary, compared to conventional gaseous growth method, the present method of using liquid carbon source enables the growth of graphene on all external and internal surfaces for the 0D, ID and 3D nanostructures. On the other hand, conventional gaseous growth method only enables the growth of graphene on portions of the nanostructure's surface which are in contact with the gaseous carbon source during chemical vapour deposition growth. Therefore, graphene obtained by the present method using liquid carbon source is more continuous and uniform. The present forming method is easily applicable and adopted in manufacturing industries to produce graphene in large scale, for example in mass production of electrodes or heat spreaders, by simply coating the large amount of the metallic nanostructure catalyst with the liquid carbon source and then crystallizing the coated nanostructures. [0115] Further, compared to graphene obtained by conventional gaseous growth methods, the present graphene layer is more continuous and uniform. The better graphene quality leads to enhanced properties and thus makes presently obtained graphene-based electrodes more attractive over ITO-based electrodes, such as highly flexible, better chemical stability, lower cost and rich in resource. Compared with Cu or Ag nanofiber-based conductors, the present graphene films have better chemical stability since graphene can protect the metal cores from oxidation/corrosion. Furthermore, the present graphene films have a work function of 4.6eV, close to that of ITO (about 4.7eV), while for metal fibers, the work function is lower. As such, the present graphene-based electrodes can directly replace ITO- based electrodes for existing ITO-based devices. Further compared with pure graphene films, the present graphene films is expected to possess better conductivity since the metal cores can connect those area where the graphene lattice network might break or meet with defects.

[0116] By "comprising" it is meant including, but not limited to, whatever follows the word "comprising". Thus, use of the term "comprising" indicates that the listed elements are required or mandatory, but that other elements are optional and may or may not be present.

[0117] By "consisting of is meant including, and limited to, whatever follows the phrase "consisting of. Thus, the phrase "consisting of indicates that the listed elements are required or mandatory, and that no other elements may be present.

[0118] The inventions illustratively described herein may suitably be practiced in the absence of any element or elements, limitation or limitations, not specifically disclosed herein. Thus, for example, the terms "comprising", "including", "containing", etc. shall be read expansively and without limitation. Additionally, the terms and expressions employed herein have been used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the inventions embodied therein herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention.

[0119] By "about" in relation to a given numberical value, such as for temperature and period of time, it is meant to include numerical values within 10% of the specified value.

[0120] The invention has been described broadly and genetically herein. Each of the narrower species and sub-generic groupings falling within the generic disclosure also form part of the invention. This includes the generic description of the invention with a proviso or negative limitation removing any subject matter from the genus, regardless of whether or not the excised material is specifically recited herein.

[0121] Other embodiments are within the following claims and non- limiting examples. In addition, where features or aspects of the invention are described in terms of Markush groups, those skilled in the art will recognize that the invention is also thereby described in terms of any individual member or subgroup of members of the Markush group.

Claims

A method of forming graphene on a metallic nanostructure, comprising:

(i) coating the metallic nanostructure with a layer of liquid carbon precursor;

(iv) quenching the layer of crystallized graphene.

The method of claim 1, wherein the metallic nanostructure comprises a metal, an alloy, or a metal-carbon mixture.

The method of claim 2, wherein the alloy comprises at least one metal selected from the group consisting of Ni, Co, Fe, Cu, Zn, Mn, Ga, Ge, As, Se, In, Sn, Sb, Te, Al, Pd, Pt, Au, and Ag, and/or at least one refractory metal selected from the group consisting of Nb, Mo, Ta, W, Rh, Re, Ti, V, Cr, Zr, Hf, Ru, Os and Ir.

The method of claim 3, wherein the molar percentage of the at least one refractory metal in the alloy based on total content of the alloy is between 0 and 40%.

The method of claim 2, wherein the metal-carbon mixture is selected from the group consisting of carbon steel, nickel steel, nickel-chromium steel, molybdenum steel, chromium steel, chromium-vanadium steel, tungsten steel, nickel-chromium- vanadium steel, silicon-manganese steel, and a mixture thereof.

6. The method of any one of claims 1 to 5, wherein the liquid carbon precursor comprises a hydrocarbon compound in liquid phase at room temperature (i.e. 25°C)- and atmospheric pressure (i.e. 101.325 kPa).

7. The method of any one of claims 1 to 6, wherein coating in step_(i) comprises dip coating, spray coating or spin coating.

8. The method of any one of claims 1 to 7, wherein carbonizing in step (ii) comprises heating the layer of liquid carbon precursor coated on the metallic nanostructure at a carbonizing temperature of 180 to 600 °C.

9. The method of claim 8, wherein carbonizing in step (ii) comprises heating the layer of liquid carbon precursor coated on the metallic nanostructure at a carbonizing temperature of 200 to 300 °C.

10. The method of claim 8 or 9, wherein carbonizing in step (ii) further comprises heating the layer of liquid carbon precursor coated on the metallic nanostructure for a period of 1 to 120 min.

11. The method of claim 10, wherein carbonizing in step (ii) further comprises heating the layer of liquid carbon precursor coated on the metallic nanostructure for a period of 20 to 40 min.

12. The method of any one of claims 1 to 1 1, wherein crystallizing in step (iii) comprises increasing the temperature to a crystallizing temperature of 450 to 1,000 °C.

13. The method of claim 12, wherein crystallizing in step (iii) comprises increasing the temperature to a crystallizing temperature of 750 to 850 °C.

14. The method of claim 12 or 13, wherein crystallizing in step (iii) further comprises increasing the temperature to the crystallizing temperature at a ramping rate of 0.5 to 200 °C/min.

15. The method of claim 14, wherein crystallizing in step (iii) further comprises

increasing the temperature to the crystallizing temperature at a ramping rate of 10 to 100 °C/min.

16. The method of any one of claim 12 to 15, wherein crystallizing in step (iii) further comprises maintaining the crystallizing temperature for a period of 1 sec to 30 min.

17. The method of claim 16, wherein crystallizing in step (iii) further comprises

maintaining the crystallizing temperature for a period of 5 to 15 min.

18. The method of any one of claims 1 to 17, wherein the carbonizing step (ii) and/or the crystallizing step (iii) is carried out in an inert or reductive environment.

19. The method of claim 18, wherein the carbonizing step (ii) and/or the crystallizing step (iii) is carried out in an inert environment with a total gas flow rate of 200 to 500 seem.

20. The method of claim 19, wherein the carbonizing step (ii) and/or the crystallizing step (iii) is carried out in an inert environment with a total gas flow rate of 200 to 300 seem.

21. The method of any one of claims 18 to 20, wherein the inert environment comprises argon, or argon and hydrogen.

22. The method of any one of claims 1 to 21 , wherein quenching in step (iv) comprises reducing the temperature of the layer of crystallized graphene to room temperature in less than 10 min.

23. The method of claim 22, wherein the quenching step (iv) is carried out in an inert environment with a total gas flow rate of 50 to 200 seem.

24. The method of claim 23, wherein the inert environment comprises argon, or argon and hydrogen.

25. The method of any one of claims 1 to 24, wherein the metallic nanostrucrure is a zero-dimension nanoparticle, one-dimension nanofiber, or three-dimension foam or aerogel.

26. A method of forming an electrode comprising graphene, the method comprising:

(a) forming at least one layer of metallic nanostructures on a substrate;

(c) carbonizing the layer of liquid carbon precursor coated on the metallic nanostructures at a carbonizing temperature to convert the layer of liquid carbon precursor to a layer of amorphous carbon; (d) crystallizing the layer of amorphous carbon at a crystallizing temperature to convert the layer of amorphous carbon to a layer of crystallized graphene; and

(e) quenching the layer of crystallized graphene.

27. The method of claim 26, wherein forming in step (a) comprises electrospinning, pyrolysis and H₂ reduction to form the at least one layer of metallic nanostructures on a rigid substrate.

28. The method of claim 26 or 27, wherein more than one layer of metallic

nanostructures is formed on the substrate.

29. A method of forming a flexible electrode comprising graphene, the method

comprising:

(i) coating metallic nanostructures with a layer of liquid carbon precursor;

(iv) quenching the layer of crystallized graphene to obtain metallic nanostructures coated with graphene; (v) coating a polymer solution on the metallic nanostructures coated with graphene, wherein the polymer solution comprises the polymer of a flexible substrate; and

A method of forming an electrode comprising graphene, the method comprising:

(i) coating metallic nanostructures with a layer of liquid carbon precursor;

(iv) quenching the layer of crystallized graphene to obtain metallic

nanostructures coated with graphene;

(v) dispersing the metallic nanostructures coated with graphene in a solution;

(vi) coating the solution of (v) onto a substrate; and

(vii) drying the coating of (vi).

A method of forming a heat spreader comprising graphene nanocomposite, the method comprising:

(vi) processing the mixture of (v) to form the graphene nanocomposite.

32. The method of claim 31, wherein processing in step (vi) comprises hot pressing, moulding, or sintering the mixture of metal nanostructures coated with graphene and the metal powder.

33. Graphene formed on metallic nanostructures according to a method of any one of claims 1 to 25.

34. An electrode comprising graphene formed according to a method of any one of claims 26 to 30.

35. A heat spreader comprising graphene nanocomposites formed according to a method of claim 31 or 32.