WO2014112953A1

WO2014112953A1 - Methods of low temperature preparation of one or more layers of graphene on a metallic substrate for anti-corrosion and anti-oxidation applications

Info

Publication number: WO2014112953A1
Application number: PCT/SG2014/000020
Authority: WO
Inventors: Zongyou YIN; Zehui DU; Hua Zhang; Minmin ZHU
Original assignee: Nanyang Technological University
Priority date: 2013-01-18
Filing date: 2014-01-17
Publication date: 2014-07-24

Abstract

Method of preparing one or more layers of graphene on a metallic substrate is provided. The method includes annealing a metallic substrate at a first temperature in a range of about 600 °C to about 800 °C in a hydrogen-containing environment to reduce surface oxides on the metallic substrate; cooling the metallic substrate to a second temperature of around 600 °C or lower for graphene growth; providing a carbon vapor by decomposing a solid carbon source at a temperature in a range of about 200 °C to about 400 °C and contacting the carbon vapor with the metallic substrate to form one or more layers of graphene on the metallic substrate at the second temperature. Metallic substrates comprising one or more graphene layers thus prepared, and use of the method and metallic substrate in anti-corrosion applications, paint, and in anti-oxidation applications for electronic devices and magnetic devices are also provided.

Description

METHODS OF LOW TEMPERATURE PREPARATION OF ONE OR MORE LAYERS OF GRAPHENE ON A METALLIC SUBSTRATE FOR ANTI- CORROSION AND ANTI-OXIDATION APPLICATIONS CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority of US provisional application No. 61/754,229 filed on 18 January 2013, the content of which is incorporated herein by reference in its entirety for all purposes. TECHNICAL FIELD

[0002] The invention relates to methods of preparing one or more graphene layers on a metallic substrate for anti-corrosion and anti-oxidation applications.

BACKGROUND

[0003] According to a survey by the United States (US) Federal Highway Administration, total cost incurred as a result of corrosion-related issues across the entirety of the country was $276 billion, while the total cost of corrosion exceeds $ 1 trillion annually since 2013, according to a widely-cited study (NACE Corrosion Cost Study) from the National Association of Corrosion Engineers (NACE).

[0004] In view of the above, much research efforts have been directed to corrosion prevention, particularly for metallic materials which are prone to corrosion. Amongst the metallic materials, steel is the most important material due to its relatively low cost and widespread use.

[0005] To protect ships and machines against corrosion, for example, a coating of paint may be applied on the metallic materials. In some instances, a thin layer of zinc is formed on metallic materials such as steel and iron using a process known as galvanization. Stainless steel has also been used as a basic building block during fabrication of machines to counter against corrosion. .

[0006] Notwithstanding the above, corrosion prevention using such methods is still very limited, as the anti-corrosion effects may wear off in a short time period of a few months. For example, even though zinc is able to protect steel or iron from corrosion by virtue of its higher reactivity, the protection is lost following depletion of zinc. [0007] Graphene has been shown to be a promising material for anti-corrosion applications for metallic materials such as copper. However, it remains a challenge to apply graphene on materials such as steel and zinc, as they have very low catalytic activity for graphene growth. Use of expensive equipment, such as microwave plasma accessories and high growth temperatures of 850 °C, translates into high costs and difficulties in scaling up of the process for industrial scale applications.

[0008] In view of the above, there remains a need for improved methods to preparing one or more graphene layers on a metallic substrate. SUMMARY

[0009] In a first aspect, the invention relates to a method of preparing one or more layers of graphene on a metallic substrate. The method comprises

a) annealing a metallic substrate at a first temperature in a range of about 600 °C to about 800 °C in a hydrogen-containing environment to reduce surface oxides on the metallic substrate;

b) cooling the metallic substrate to a second temperature of around 600 °C or lower for graphene growth;

c) providing a carbon vapor by decomposing a solid carbon source at a temperature in a range of about 200 °C to about 400 °C and contacting the carbon vapor with the metallic substrate to form one or more layers of graphene on the metallic substrate at the second temperature.

[0010] In a second aspect, the invention relates to a metallic substrate comprising one or more graphene layers prepared by a method according to the first aspect.

[001 1 ] In a third aspect, the invention relates to use of a method according to the first aspect or a metallic substrate according to the second aspect in anti-corrosion applications, paint, electronic devices, and magnetic devices.

[0012] In a fourth aspect, the invention relates to use of a method according to the first aspect or a metallic substrate according to the second aspect in anti-corrosion applications on spacecrafts, submarines, aircrafts, oil tankers, shipping vessels, vehicles, pipelines, sewage pipes, buildings, furniture, decorations, lightings, furnishings, kitchenware, utensils, electronic devices, and in anti-oxidation applications on magnetic devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The invention will be better understood with reference to the detailed description when considered in conjunction with the non-limiting examples and the accompanying drawings, in which:

[0014] FIG. 1 is a schematic diagram depicting chemical vapor deposition (CVD) growth for solid carbon source on substrates according to an embodiment. In the embodiment shown, the CVD process is carried out in a multi-heating-zone CVD system having two heating zones, Heating Zone 1 and Heating Zone 2. The two heating zones may be present in a quartz tube. A solid carbon source is placed at Heating Zone 1, while a metallic substrate such as a metal foil is placed in Heating Zone 2. The metallic substrate is annealed in a hydrogen- containing environment to reduce surface oxides on the metallic substrate. When the metallic substrate is a nanostructured material such as nanowires, the first temperature is not more than 700 °C, as the nanostructured material is not able to withstand temperatures above 700 °C. Upon cooling the metallic substrate to a second temperature that is about 600 °C or lower than about 600 °C for graphene growth, a carbon vapor is provided by heating the solid carbon source and contacting the carbon vapor with the metallic substrate to form one or more layers of graphene on the metallic substrate.

[0015] FIG. 2 A and 2B are graphs showing Raman spectra of (A) graphene grown at 500 °C on steel substrate using polystyrene (PS), polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), and poly(methyl methacrylate) (PMMA) as solid carbon sources, and (B) graphene grown at 500 °C with different layers on steel using PS as carbon source.

[0016] FIG. 3 A is a graph showing Raman spectra of graphene grown on zinc/zinc oxide (Zn/ZnO) nanoparticles at 550 °C, 500 °C, and 450 °C. FIG. 3B is a typical scanning electron microscopy (SEM) image for graphene/Zn/ZnO nanoparticles grown at 550 °C. Scale bar in FIG. 3B denotes 100 nm.

[0017] FIG. 4 A is a graph showing Raman spectra of graphene grown on steel substrate at 600 °C, 550 °C, 500 °C, 480 °C, and 450 °C. FIG. 4B depicts typical Raman mapping of D, G, and 2D peak of graphene grown_, on steel at 450 °C. The scale bar in images represents 3 μηι.

[0018] FIG. 5A is a graph showing Raman spectra of graphene grown on nickel nanowires at 600 °C, 550 °C, 500 °C, 450 °C, and 400 °C. FIG. 5B depicts typical mapping image of D, G, and 2D peak position graphene grown on nickel at 400 °C. The scale bar in images represents 3 μπι. FIG. 5C shows a typical SEM image of nickel nanowires. Scale bar in the image represents 100 nm.

[0019] FIG. 6 is a graph showing typical Raman spectra of graphene film grown on steel or nickel at 550 °C with the carbon source temperature at 320 °C, 360 °C, and 400 °C.

[0020] FIG. 7 is a graph showing typical Raman spectra of graphene film grown on steel or nickel at 550 °C at the gas flow of 400 seem, 300 seem, and 200 seem.

[0021] FIG. 8 shows optical images of (A) steel; and (B) graphene/steel substrate before and after anti-corrosion testing for 24 hours. Scale bar in the figures denote 100 μιη.

[0022] FIG. 9 shows current-voltage curves and corrosion data of graphene grown on steel in a 5 % sea-salt solution. (A) Tafel plots of steel and graphene/steel (G/steel). (B) Corrosion rates of steel and G/steel extracted from Tafel plots. (C) The dependence of the current-voltage curves vs time. Y-axis: current (A/cm²); x-axis: E (V); (D) The corrosion potentials and rate as a function of the time in the G/steel samples.

[0023] FIG. 10 are optical images of (A) Zn/ZnO/ITO; and (B) graphene/Zn ZnO/ITO before and after anticorrosion testing for 24 hours.

[0024] FIG. 11 is (A) a graph showing current- voltage curves test in 5 % sea-salt solution; and (B) a graph showing light transmittance curve for graphene/Zn/ZnO/ITO. For (A), y-axis: current (A/cm²); x-axis: E (V). For (B), y-axis: transmittance (%); x-axis: wavelength (nm).

[0025] FIG. 12 shows magnetization stability from pure Ni nanowires vs graphene/Ni nanowires for (A) as-prepared samples; and (B) after heating in air at 200 °C for 4 hours. Y- axis: magnetization (emu/g); x-axis: magnetic field intensity (Oe).

DETAILED DESCRIPTION

[0026] Methods disclosed herein allow preparation of one or more graphene layers at low temperature on bulk or nanostructured metallic materials. In particular, the method allows preparation of one or more graphene layers on steel and zinc, which are less active, hence posing greater difficulties for depositing graphene thereon. Number of graphene layers on the substrate may be controlled easily by varying process gas flow into the system. In addition, use of low cost and widely available starting materials, such as polystyrene and poly(methyl methacrylate) as solid carbon source, and favorable processing conditions, such as low growth temperatures of 500 °C or lower, translates into reliable, cost-effective, energy-saving, and environment-friendly processes attractive for industrial application. The low processing temperatures also allow formation of graphene on nanostructures, as the nanostructures are not able to withstand processing temperatures above 700 °C.

[0027] Methods disclosed herein may be used as an alternative, or as a supplement, to conventional corrosion protection methods. For example, besides providing additional corrosion protection Iayer(s) hence improved protection against corrosion, in situ coating of graphene on stainless steel as an intermediate layer also enhances adherence between the anti-corrosion coating and steels. This translates into longer term of protection and lifetime of the coated materials. As the graphene coating is at least substantially transparent, appearance of steel is not unduly affected, and may be more cosmetically pleasing compared to coatings using state of the art anti-corrosion paints. Similarly, incorporating graphene-coated nanopowders into paint does not affect original color of the paint.

[0028] With the above in mind, the invention relates in a first aspect, to a method of preparing one or more layers of graphene on a metallic substrate.

[0029] Graphene refers generally to a form of graphitic carbon, in which carbon atoms are covalently bonded to one another to form a two-dimensional sheet of bonded carbon atoms. The carbon atoms may be bonded to one another via sp2 bonds, and may form a 6-membered ring as a repeating unit, and may further include a 5-membered ring and/or a 7-membered ring. In its crystalline form, two or more sheets of graphene may be stacked together to form multiple stacked layers. Generally, the side ends of graphene are saturated with hydrogen atoms.

[0030] As used herein, the term "metallic substrate" refers to a substrate which consists entirely of at least one metal, or has at least one layer consisting of at least one metal on a surface of the substrate, such as a metal-coated polymeric substrate. By the term "metallic", it includes metals in their elemental form, as well as metal alloys. For example, the metallic substrate may comprise or consist of a metal selected from Group 3 to Group 12 of the Periodic Table of Elements, steel, or alloys thereof. In various embodiments, the metallic substrate is selected from the group consisting of zinc, nickel, and steel.

[0031] The metallic substrates may also be 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 substrates 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-304 and grade-316 stainless steels.

[0032] The metallic substrate may be a nanostructured material or a non-nanostructured material. As used herein, the term "nanostructured material" refers to a material with at least one dimension in the nanometer range. In various embodiments, at least one dimension of the nanostructured material is less than 1000 nm, such as a length in the range from about 1 nm to about 1000 nm, about 1 nm to about 500 nm, about 1 nm to about 300 nm, about 1 nm to about 200 nm, about 1 nm to about 100 nm, about 1 nm to about 50 nm

[0033] In various embodiments, the metallic substrate is a nanostructured material. Examples of nanostructured material include nanoparticles, nanopowder, nanorods, nanopillars, nanowires, nanotubes, nanodiscs, nanoflowers, nanoflakes, nanofilms and foams with wall thickness or pore sizes at nano scales. In some embodiments, the metallic substrate is a nanostructured material selected from the group consisting of a nanopowder, a nanoparticle, a nanowire, and combinations thereof. In specific embodiments, the metallic substrate is a nanopowder or a nanowire.

[0034] Accordingly, the term "non-nanostructured material", which is used interchangeably with the term "bulk material", refers to a material that does not have at least one dimension in the nanometer range. Accordingly, the terms "nanostructured material" and "non-nanostructured material" refer to two types of materials that are mutually exclusive to each other.

[0035] In various embodiments, the metallic substrate is a non-nanostructured or bulk material, such as a foil, a plate, a plank, a slab, a rod, a ribbon, or combinations thereof. In some embodiments, the metallic substrate may be commercially available, finished articles such as fasteners, bolts, screws, washers, nuts, latches, threaded rods, threaded inserts, or combinations thereof.

[0036] The graphene is deposited or coated on a metallic substrate, such that the graphene is in contact with the at least one metal comprised in the metallic substrate. In various embodiments, the graphene is deposited or coated on a surface of the substrate having a layer of at least one metal coated thereon, such that the graphene is in contact with the layer consisting of at least one metal. [0037] The method includes annealing a metallic substrate at a first temperature in a range of about 600 °C to about 800 °C in a hydrogen-containing environment to reduce surface oxides on the metallic substrate.

[0038] The term "annealing" as used herein refers to heating or subjecting a material to elevated temperatures for a period of time. The annealing is carried out in a hydrogen- containing environment, meaning that the metal substrate is heated in the presence of hydrogen gas, which may be present alone or in combination with other inert gases such as noble gases, for example helium, neon, argon and krypton; nitrogen, or mixtures thereof. The hydrogen-containing environment is essentially free or completely free of oxygen.

[0039] By heating the metallic substrate at elevated temperatures for a specific time period in a hydrogen-containing environment, metal oxides that may be present on a surface of the metallic substrate are reduced to their corresponding metal(s). Annealing a metallic substrate at a first temperature in a hydrogen-containing environment may be carried out for any suitable temperature that allows reduction of the surface oxides. The first temperature is not more than 700 °C when the metallic substrate is a nanostructured material.

[0040] In embodiments where the metallic substrate is a non-nanostructured material, the first temperature is in a range of about 600 °C to about 800 °C. For example, the first temperature may be in a range of about 650 °C to about 800 °C, about 700 °C to about 800 °C, about 752 °C to about 800 °C, about 600 °C to about 750 °C, about 600 °C to about 700 °C, about 600 °C to about 650 °C, about 650 °C to about 750 °C, or about 650 °C to about 700 °C.

[0041] In embodiments where the metallic substrate is a nanostructured material, the first temperature is in a range of about 600 °C to about 700 °C, such as about 600 °C to about 650 °C, about 650 °C to about 700 °C, about 600 °C, about 650 °C, or about 700 °C.

[0042] Time period for annealing may depend on the temperature and metallic substrate used. In various embodiments, annealing a metallic substrate at a first temperature in a hydrogen-containing environment is carried out for a time period in a range of about 10 minutes to about 20 minutes. For example, when the metallic substrate is a non- nanostructured material, annealing the metallic substrate may be carried out in a hydrogen- containing environment at about 800 °C for about 20 minutes. When the metallic substrate is a nanostructured material, such as nanowires or nanoparticles, for example, annealing the metallic substrate may be carried out in a hydrogen-containing environment at about 700 °C for about 10 minutes.

[0043] In various embodiments, annealing a metallic substrate at a first temperature in a hydrogen-containing environment is carried out in a gas flow comprising or consisting of hydrogen gas. The gas flow may further comprise an inert gas, such as helium, neon, argon and krypton; nitrogen, or mixtures thereof. In various embodiments, the inert gas is argon.

[0044] Advantageously, number of graphene layers on the substrate may be controlled easily by varying process gas flow into the system. One or more layers of graphene, such as 2, 3, 4, 5, or 6 layers of graphene may be prepared on a metallic substrate by varying process gas flow into the system. In various embodiments, number of layers of graphene on the metallic substrate is controlled by varying the volumetric ratio of inert gas to hydrogen gas in the gas flow. In specific embodiments, two layers of graphene are formed on a metallic substrate when argon gas and hydrogen gas flow rates of 200 seem and 15 seem, respectively, are used. When the hydrogen gas flow rate is lowered to 8 seem and below, while keeping the argon gas flow rate at 200 seem, multiple layers of graphene may be formed. In one embodiment, a monolayer of graphene is formed when argon gas and hydrogen gas flow rates of 200 seem and 40 seem, respectively, are used.

[0045] The method of the first aspect includes cooling the metallic substrate to a second temperature of around 600 °C or lower for graphene growth. This also means that the maximum temperature that may be used for graphene growth is around 600 °C.

[0046] In various embodiments, the second temperature is in a range of about 400 °C to about 600 °C. For example, the second temperature may be in a range of about 400 °C to about 550 °C, about 400 °C to about 500 °C, about 400 °C to about 450 °C, about 450 °C to about 600 °C, about 450 °C to about 550 °C, or about 500 °C to about 550 °C. In specific embodiments, the second temperature is in a range of about 400 °C to about 500 °C.

[0047] In some embodiments where graphene is grown on a nanostructured metallic substrate, the first temperature may be equal to the second temperature. This means that both the first temperature and the second temperature are about 600 °C or lower.

[0048] The method of the first aspect includes providing a carbon vapor and contacting the carbon vapor with the metallic substrate to form one or more layers of graphene on the metallic substrate. The one or more layers of graphene is formed on the metallic substrate at the second temperature. [0049] Providing a carbon vapor is carried out by heating a solid carbon source at a suitable temperature in a range of about 200 °C to about 400 °C such that the solid carbon source decomposes to form the carbon vapor. The solid carbon source may be heated in a hydrogen-containing environment, such as in a gas flow comprising or consisting of hydrogen gas, which may further include an inert gas. Examples of inert gases have already been described above. In various embodiments, the inert gas is argon. The solid carbon source may be a carbon-containing material, such as carbon black, amorphous carbon, activated carbon, graphene, and graphene oxide, and polymers.

[0050] In various embodiments, the solid cafbon source comprises or consists essentially of an aromatic polymer. The term "aromatic polymer" as used herein refers to a polymer containing at least one aromatic ring in the monomer unit constituting the polymer. For example, the aromatic polymer may be polystyrene (PS), acrylonitrile butadiene styrene(ABS), styrene acrylonitrile (SAN), styrene-isoprene-styrene (SIS), poly(styrene- butadiene- styrene) (SBS), poly(styrene-ethylene butylene-styrene) (SEBS), and various other styrene copolymers, polybutadiene, polyisoprene, polyethylene-butylene, phenolic resins, phenol formaldehyde resins and various other phenolic copolymers, epoxy resins, nylon, polyurethane and their copolymers and the like. In various embodiments, the solid carbon source is polyethyl glycerol, poly(methyl methacrylate), polyvinylpyrrolidone, combinations thereof, or copolymers thereof.

[0051] The temperature used for heating the solid carbon source may be any suitable temperature in a range of about 200 °C to about 400 °C that allows decomposition of the solid carbon source to form the carbon vapor. Depending on the solid carbon source, different temperatures may be used. In various embodiments, the solid carbon source is heated at about 400 °C to form the carbon vapor. For example, the temperature used for heating the solid carbon source may be in a range of about 200 °C to about 350 °C, about 200 °C to about 300 °C, about 200 °C to about 250 °C, about 250 °C to about 400 °C, about 300 °C to about 400^' °C, about 350 °C to about 400 °C, about 200 °C, about 300 °C, or about 400 °C.

[0052] The carbon vapor is contacted with the metallic substrate to form one or more layers of graphene on the metallic substrate. The carbon vapor may diffuse near or on a surface of the metallic substrate and deposit thereon. In so doing, islands of carbon are formed on the surface of the metallic substrate. As more carbon vapor contacts the metallic substrate, the islands grow and eventually merge to yield a continuous graphene film. [0053] In various embodiments, contacting the carbon vapor with the metallic substrate is carried out in a flow of carbon vapor. The carbon vapor may be formed and allowed to flow over the metallic substrate, such that the carbon vapor diffuses on a surface of the metallic substrate, and precipitates or nucleates on the metallic substrate to form islands of carbon upon contact with the carbon vapor. As more carbon vapor flows over the metallic substrate, the islands grow and eventually merge to yield a continuous graphene film. In some embodiments, the carbon vapor has a flow rate in a range of about 200 seem to about 400 seem. In specific embodiments, the carbon vapor has a flow rate of about 200 seem.

[0054] The one or more layers of graphene is grown or formed on the metallic substrate at the second temperature. Suitable ranges of the second temperature that may be used have already been described above.

[0055] Contacting the carbon vapor with the metallic substrate may be carried out for any suitable time period that allows forming a layer of graphene on the metallic substrate. This may depend, for example, on the temperature used for graphene growth. In various embodiments, contacting the carbon vapor with the metallic substrate is carried out for a time period of about 40 minutes.

[0056] In a second aspect, the invention relates to a metallic substrate comprising a graphene layer prepared by a method according to the first aspect. Examples of metallic substrates that may be used have already been mentioned above. One or more layers of graphene, such as 2, 3, 4, 5, or 6 layers of graphene may be deposited on a surface of the metallic substrate.

[0057] As mentioned above, methods disclosed herein allow preparation of one or more graphene layers on metallic materials, in particular on steel and zinc, which have very low catalytic activity for graphene growth, hence posing greater difficulties for depositing graphene. The methods disclosed herein may advantageously be used to prepare one or more graphene layers on nanostructured materials, given that low processing temperatures of not more than 700 °C, which is below the tolerable or workable temperature ranges of the nanostructured materials, may be used.

[0058] Methods disclosed herein may be used as an alternative, or as a supplement, to conventional corrosion protection methods. Versatility of the methods disclosed herein means that they may be adopted readily and applied across a myriad of areas. [0059] Therefore, the invention relates in a further aspect to use of a method according to the first aspect or a metallic substrate according to the second aspect in anti-corrosion applications, paint, and in anti-oxidation for electronic devices and magnetic devices. In a yet further aspect, the invention relates to use of a method according to the first aspect or a metallic substrate according to the second aspect in anti-corrosion applications on spacecrafts, submarines, aircrafts, oil tankers, shipping vessels, vehicles, pipelines, sewage pipes, buildings, furniture, decorations, lightings, furnishings, kitchenware, utensils, and in anti-oxidation for electronic devices and magnetic devices.

[0060] Hereinafter, the present invention will be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, lengths and sizes of layers and regions may be exaggerated for clarity.

[0061] As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items. It will be understood that, although the terms first, second, third, etc., may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present invention.

[0062] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and/or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. [0063] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0064] The invention 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.

[0065] The invention has been described broadly and generically herein. Each of the narrower species and subgeneric 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.

[0066] 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.

EXPERIMENTAL SECTION

[0067] A general and cost-effective approach towards synthesis of large-area and low- defect graphene on bulk or nanostructured metallic materials, such as steels and any other less active metals, at low temperatures, such as in the range of about 400 °C to about 550 °C, has been developed.

[0068] Exemplary embodiments include use of a multi-heating zone furnace and solid carbon sources e.g. polystyrene (PS) to grow graphene. Based on this strategy, graphene using PS, polyethyl glycerol (PEG), poly(methyl methacrylate) (PMMA), or polyvinylpyrrolidone (PVP) as the solid carbon source, has been prepared.

[0069] Example 1: Multi-heating zone CVD system for graphene growth

[0070] A schematic diagram of the multi-heating-zone CVD system used in the experiments is shown in FIG. 1.

[0071] The multi-heating-zone CVD system includes two heating zones, Heating Zone 1 and Heating Zone 2. Generally, a small ceramic container containing a solid carbon source is placed at Heating Zone 1, while metal substrate/nanopowder is placed in Heating Zone 2. The temperature in Heating Zone 2 is set to a temperature in the range of about 600 °C to about 800 °C and the metal substrates/nanopowders are annealed in a 150 seem hydrogen gas flow for about 20 minutes, to convert surface oxides into their corresponding metals. Following this, the metal substrates/nanopowders are cooled down to the desired graphene growth temperature.

[0072] For graphene growth on nanopowders, the nanopowders were put onto a pre- cleaned glass substrate, having thin layer coverage on the glass substrate. Metal oxides on the surface of the nanopowders are reduced to the corresponding metals by annealing under hydrogen gas. At the proper growth temperature, graphene may be grown onto surface of nanopowders when carbon source vapor arrives after flowing from Heating Zone 1.

[0073] Solid carbon source (e.g. PS) is heated to 400 °C to form amorphous carbon vapor.

Once the active carbon vapor flows over the metallic substrates/templates at Heating Zone 2, it deposits and/or diffuses onto metal surface where it is transformed to graphene. Typical growth time is about 40 minutes. After growth, the furnace cover is opened and cooled down to room temperature.

[0074] Example 2: Low-temperature synthesis of graphene on bulk or nano-template metals

[0075] Steel substrates, zinc or nickel nanostructural templates are rinsed by acetone, alcohol, deionized-water, followed by annealing at a temperature in the range of 600 °C to 800 °C in a 150 seem hydrogen gas (H₂) flow for 20 minutes. [0076] The substrates/templates are then cooled to various growth temperatures of 450 °C, 480 °C, 500 °C, 550 °C, or 600 °C. Subsequently, solid carbon source e.g. PS is heated to form carbon vapor, and graphene growth is initiated when the carbon vapor contacts the metallic templates. Growth time is about 40 minutes.

[0077] FIG. 2A shows Raman spectra of prepared graphene films on steel. By changing flow rate of argon gas and hydrogen gas, for example, different number of layers of graphene on steel may be prepared as shown in FIG. 2B. Method of preparing graphene as disclosed herein may also be grown on Zn metallic nanopowders, as indicated by FIG. 3A and 3B.

[0078] Typical Raman spectra and SEM images of samples grown on steel substrates, zinc nanopowders, and nickel nanowires are shown in FIG. 4 and 5. Corresponding results for zinc powders are presented in FIG. 3.

[0079] From FIG. 4A, it may be seen that deposition of graphene may be achieved on steel even when growth temperature is lowered to 450 °C.

[0080] Example 3; Solid carbon source decomposition temperature

[0081] It has been found herein that decomposition temperature of solid carbon source affects the quality of graphene. To study the effect, three different temperatures of 320 °C, 360 °C, and 400 °C as applied on PS were used.

[0082] FIG. 6 shows typical Raman spectra of the graphene grown on steel or nickel at 550 °C. As temperature increases, the D band gradually reduces, and almost disappears when the carbon source temperature is 400 °C. This indicates that carbon source temperatures at 400 °C or higher are optimal to grow low-defect graphene.

[0083] Example 4: Gas flow rate

[0084] It has also been found herein that another factor affecting graphene quality is gas flow rate. Quality of graphene film grown on steel or nickel depends on the gas flow, which leads to different gas pressure.

[0085] FIG. 7 shows typical Raman spectra of graphene grown on steel or nickel at 550 °C with different gas flow, ranging from 200 seem to 400 seem. As gas flow increases, the D band gradually increases, which indicates the quality of graphene films is degraded. From the results obtained, it may be seen that gas flow rate at about 200 seem is best for graphene growth.

[0086] Example 5: Potential commercial applications [0087] This work has demonstrated a general approach to fabricate large-area and low- defect graphene at low temperatures. The graphenes obtained can offer many potential applications in the following areas.

[0088] Example 5.1 : As replacement for galvanized steels

[0089] Graphene coated steel may replace galvanized steels in the applications of submarines, aircraft carriers, oil tankers/pipelines and sewage pipes. Compared with the zinc anti-corrosion layers, graphene directly grown on steels offers many advantages, such as better adherence and longer lasting effects.

[0090] FIG. 8 and 9 show results of tests carried out on anti-corrosion abilities of the graphene-coated steels. From the results shown in FIG. 8B, it may be seen that graphene- coated steels are almost unchanged after subjecting to 24 h accelerated corrosion tests. Steels without coating, on the other hand, are corroded very seriously as shown in FIG. 8A.

[0091] FIG. 9 further demonstrates enhanced anticorrosion capability of graphene coated steel. Based on the electrochemical analysis, the corrosion rate of the G/Steel samples is 2.02 xlO^"14 (m/s), a reduction of about 9 times compared to the bare steel without graphene protection (1.75 x 10^" (m/s)). Furthermore, the G-grown steel also demonstrate the excellent lifetime for anticorrosion. The graphene coated steel can therefore replace galvanized steels in many applications, such as submarines, aircraft carriers, oil tankers/pipelines, sewage pipes and spacecrafts.

[0092] Example 5.2: As fillers in anti-corrosion paints

[0093] FIG. 10 shows optical images of (A) Zn/ZnO/ITO; and (B) graphene/Zn/ZnO/ITO before and after anticorrosion testing for 24 hours. FIG. 11 is (A) a graph showing current- voltage curves test in 5 % sea-salt solution; and (B) a graph showing light transmittance curve for graphene/Zn/ZnO/ITO.

[0094] From the results obtained, it may be seen that graphene coated Zn nanopowders are more stable than Zn powders. Therefore, these powders may be used in paint to provide long- lasting anti-corrosion ability.

[0095] Further, from FIG. 11B, transmittance (%) of the graphene/Zinc coating is as high as above 88%. Therefore, these paints may be used in areas, such as kitchen wares/utensils, ceilings/windows/walls, vehicles' decorations and portable electronic products, where such transparency is required and/or desirable.

[0096] Example 5.3: In protection of magnetism in Ni metals [0097] FIG. 12 shows magnetization stability from pure Ni nanowires vs graphene Ni nanowires for (A) as-prepared samples; and (B) after heating in air at 200 °C for 4 hours. Y- axis: magnetization (emu/g); x-axis: magnetic field intensity (Oe).

[0098] As seen from the figure, when graphene grows on Ni nanowires, its magnetism demonstrates much better stability after subjecting to harsh condition treatments of heating in air for 4 hours at 200 °C. Magnetism of the graphene-coated Ni nanowires decreases by only

13% from 56 to 49 emu/g after heating, while the decrease is up to 29 % for Ni nanowires without graphene coating due to serious oxidation of Ni. Therefore, graphene coated Ni nanowires may help to enhance stability of electronic and/or magnetic devices from oxidation.

[0099] Methods as disclosed herein allow preparation of one or more graphene layers on metallic materials such as steel and zinc. They provide important advantages over conventional methods for synthesis of large-area graphene. Firstly, the graphene growth process is a generalizable process which may be applied to different types of substrate, especially those less active metals. Specifically, methods developed allow formation of one or more graphene layers on steel and zinc substrates/templates, which are difficult to form graphene on them. As the graphene coating is transparent, appearance of steel is not affected and it is more attractive than those anti-corrosion paints. Consequently, graphene-coated nanopowders, for example, may be incorporated into paint without unduly affecting its transparency.

[00100] Furthermore, methods disclosed herein involve use of low cost starting materials which are widely and easily available. For example, solid carbon sources such as PS, PVP, PEG and PMMA may be used to form the graphene layers. In various embodiments, methods involving very low growth temperatures of 500 °C or lower may be used.

[00101] In-situ coating of graphene on stainless steel enhances adherence between the anti- corrosion coating and steels. Methods of preparing layers of graphene on a metallic substrate disclosed herein may be more effective than state of the art corrosion protection methods in terms of longer term of protection or lifetime of the coated materials. Furthermore, the disclosed methods are reliable, cost-effective, energy-saving, and environment- friendly. Using a method disclosed herein, number of graphene layers on the substrate may be controlled easily. [00102] Methods disclosed herein also provide ability to form graphene on nanostructures, due to low temperature ranges used in processing, in view that nanostructures cannot withstand processing temperatures of above 700 °C.

[00103] While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.

Claims

Method of preparing one or more layers of graphene on a metallic substrate, the method comprising

Method according to claim 1, wherein the first temperature is in a range of about 600 °C to about 800 °C when the metallic substrate is a non-nanostructured material and in a range of about 600 °C to about 700 °C when the metallic substrate is a nanostructured material.

Method according to claim 1 or 2, wherein the second temperature is in a range of about 400 °C to about 600 °C.

Method according to any one of claims 1 to 3, wherein the second temperature is in a range of about 400 °C to about 500 °C.

Method according to any one of claims 1 to 4, wherein the metallic substrate comprises or consists of a metal selected from Group 3 to Group 12 of the Periodic Table of Elements, steel, or alloys thereof.

Method according to any one of claims 1 to 5, wherein the metallic substrate is selected from the group consisting of zinc, nickel, and steel. Method according to any one of claims 1 to 6, wherein the metallic substrate is selected from the group consisting of a foil, a plate, a plank, a slab, a rod, a ribbon, and combinations thereof.

Method according to any one of claims 1 to 7, wherein the metallic substrate is selected from the group consisting of a fastener, a bolt, a screw, a washer, a nut, a latch, a threaded rod, a threaded insert, and combinations thereof.

Method according to any one of claims 1 to 8, wherein the metallic substrate is a nanostructured material selected from the group consisting of a nanopowder, a nanoparticle, a nanowire, and combinations thereof.

Method according to any one of claims 1 to 9, wherein annealing a metallic substrate at a first temperature in a hydrogen-containing environment is carried out in a gas flow comprising or consisting of hydrogen gas.

Method according to claim 10, wherein the gas flow further comprises an inert gas.

Method according to claim 1 1 , wherein number of layers of graphene on the metallic substrate is controlled by varying the volumetric ratio of inert gas to hydrogen gas in the gas flow.

Method according to any one of claims 1 to 12, wherein annealing a metallic substrate at a first temperature in a hydrogen-containing environment is carried out for a time period in a range of about 10 minutes to about 20 minutes.

Method according to any one of claims 1 to 13, wherein providing a carbon vapor comprises heating a solid carbon source at a suitable temperature in a hydrogen- containing environment to form the carbon vapor.

Method according to claim 14, wherein the solid carbon source comprises or consists essentially of an aromatic polymer. Method according to claim 14 or 15, wherein the solid carbon source is selected from the group consisting of polystyrene, acrylonitrile butadiene styrene, styrene acrylonitrile, styrene-isoprene-styrene, poly(styrene-butadiene-styrene), poly(styrene- ethylene/butylene-styrene), styrene copolymers, polybutadiene, polyisoprene, polyethylene-butylene, phenolic resins, phenol formaldehyde resins, phenolic copolymers, epoxy resins, nylon, polyurethane, combinations thereof, and copolymers thereof.

Method according to any one of claims 14 to 16, wherein the solid carbon source is selected from polyethyl glycerol, poly(methyl methacrylate), polyvinylpyrrolidone, combinations thereof, and copolymers thereof.

Method according to any one of claims 1 to 17, wherein the solid carbon source is heated at about 400 °C to form the carbon vapor.

Method according to any one of claims 1 to 18, wherein contacting the carbon vapor with the metallic substrate is carried out for a time period of about 40 minutes.

Method according to any one of claims 1 to 19, wherein contacting the carbon vapor with the metallic substrate is carried out in a flow of carbon vapor.

Method according to claim 20, wherein the carbon vapor has a flow rate in a range of about 200 seem to about 400 seem.

Method according to claim 20 or 21, wherein the carbon vapor has a flow rate of about 200 seem.

Metallic substrate comprising one or more graphene layers prepared by a method according to any one of claims 1 to 22.

Use of a method according to any one of claims 1 to 22 or a metallic substrate according to claim 23 in anti-corrosion applications, paint, and in anti-oxidation for electronic devices and magnetic devices. Use of a method according to any one of claims 1 to 22 or a metallic substrate according to claim 23 in anti-corrosion applications on spacecrafts, submarines, aircrafts, oil tankers, shipping vessels, vehicles, pipelines, sewage pipes, buildings, furniture, decorations, lightings, furnishings, kitchenware, utensils, and in anti- oxidation for electronic devices and magnetic devices.