Method of manufacturing graphene using metal catalyst

Abstract

The present invention relates to a method for producing graphene on a face-centered cubic metal catalyst having a plane oriented in one direction, and more particularly to a method of producing graphene on a metal catalyst having the (100) or (111) crystal structure and a method of producing graphene using a catalyst metal foil having a single orientation, obtained by electroplating a metal catalyst by a pulse wave current and annealing the metal catalyst. The invention also relates to a method of producing graphene using a metal catalyst, and more particularly to a method of producing graphene, comprising the steps of: alloying a metal catalyst with an alloying element; forming step structures on the metal catalyst substrate in an atmosphere of a gas having a molecular weight of carbon; and supplying hydrocarbon and hydrogen gases to the substrate. On unidirectionally oriented metal catalyst prepared according to the present invention, graphene can be grown uniformly and epitaxially. Moreover, a method for producing graphene according to the present invention can form monolayer graphene by epitaxially growing graphene while increasing the growth rate of graphene.

Claims

1. A method of producing graphene, the method comprising: forming step structures on the surface of a face-centered cubic metal catalyst substrate having a (100) or (111) orientation; and supplying hydrocarbon to the substrate to grow graphene comprising a monolayer graphene thin film thereon, wherein the metal catalyst is allowed with an alloying element and has a cold rolling reduction ratio of 85% or higher.

2. The method of claim 1, wherein the metal catalyst has a thickness of 50 μm or less.

3. The method of claim 1, wherein the metal catalyst is recrystallization-annealed in a hydrogen or reducing atmosphere, and then heated in an atmosphere of methane and hydrogen to grow graphene thereon.

4. The method of claim 1, wherein the metal catalyst is any one selected from the group consisting of copper, silver, gold, and alloys thereof.

5. The method of claim 1, wherein the monolayer graphene thin film 95% or more of the graphene.

6. The method of claim 1, wherein the step structures are formed by annealing at a temperature of 600° C. to 1070° C.

7. The method of claim 1, wherein the alloying element is any one or more selected from among period 2 to period 6 elements among group 3 to group 12 transition metal elements and group 13 to 15 elements.

8. The method of claim 7, wherein the step structures on the metal catalyst are formed in an atmosphere of a gas having a molecular weight higher than the atomic weight of carbon.

9. The method of claim 7, wherein the metal catalyst is aluminum, nickel, austenitic stainless steel, silver, gold or copper.

10. The method of claim 7, wherein the alloying element is any one or more of transition elements which have solid solubility for hydrogen or form carbides at a temperature ranging from 600 to 1060° C.

11. The method of claim 7, wherein the alloying element is any one or more selected from among aluminum, indium, silicon, germanium, tin, antimony and bismuth.

12. The method of claim 8, wherein the gas having a molecular weight higher than the atomic weight of carbon is any one or more selected from among neon, argon, krypton, nitrogen, hydrocarbon, carbon dioxide, carbon monoxide and steam (H2O).

Description

DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a set of SEM photographs showing the rolled state of a copper foil and the state of the copper foil after recrystallization annealing.

(2) FIG. 2(a) shows the state of a copper foil according to the present invention, FIG. 2(b) shows the copper foil annealed at 1000° C. in the hydrogen atmosphere, and FIG. 2(c) shows graphene grown by supplying methane together with hydrogen to the copper foil.

(3) FIG. 3 is a set of graphs showing the rolled state of a copper foil and the results of measuring the orientation of the foil by XRD after growing graphene thereon.

(4) FIG. 4 is a graphic diagram showing the results of measurement of the Raman spectrum of monolayer graphene.

(5) FIG. 5 shows the pulse copper plating state of pulse copper plated specimens and the results of measuring the orientation of the specimens by XRD after growing graphene thereon.

(6) FIG. 6 is a set of optical micrographs showing graphene grown on an electrodeposited copper foil in one direction (CVD, 1050° C. for 10 min; upper: 100× magnification; lower: 1000× magnification).

(7) FIG. 7 shows various steps formed when annealing a metal thin sheet in a gas atmosphere.

(8) FIG. 8 is a set of schematic diagrams showing the arrangement of copper atoms and the state of carbon alloyed with a substitutional element.

(9) FIG. 9 shows the results of synthesizing graphene using copper and a copper alloy at 600° C. for 30 minutes.

(10) FIG. 10 shows the results of synthesizing graphene using copper and a copper alloy by CVD at800° C. for 30 minutes.

(11) FIG. 11 shows the results of synthesizing graphene using copper and various copper alloy catalysts by CVD.

(12) FIG. 12 shows the results of synthesizing graphene by CVD at 1000° C. for 1 second in an atmosphere of a mixed gas of 30 sccm methane and 10 sccm hydrogen.

MODE FOR INVENTION

(13) Hereinafter, the present invention will be described in detail with reference to examples and test examples. It is to be understood, however, that these examples are for illustrative purposes and are not intended to limit the scope of the present invention.

EXAMPLES

Example 1

Fabrication of Graphene Thin Film on Rolled Copper Foil

(14) Tough pitch copper foils (a purity of 99.9% or more, an oxygen content of 0.05% or less) having thicknesses of 0.5 mm and 0.2 mm were annealed, and then cold-rolled to thicknesses of 12 μm, 25 μm, 40 μm, 50 μm and 100 μm. The cold-rolled foils were heated at various annealing temperatures so that 95% or more thereof was oriented in the (100) direction. It was confirmed that a graphene thin film was evenly formed on the annealed foils (see FIG. 1). After recrystallization, only crystal growth occurred on the foils, and thus the orientation of the foils did not change to a new orientation. Table 1 below shows the results of forming graphene layer at various reduction ratios under various heat treatment conditions.

(15) TABLE-US-00001 TABLE 1 Thickness Thickness Recrystallization Formation of raw after annealing of material rolling Reduction (100) plane temperature monolayer (mm) (μm) ratio (%) orientation (° C.) graphene Preparation 0.2 12 94 ◯ 200 ◯ Example 1 Preparation 0.2 25 87.5 ◯ 600 ◯ Example 2 Comparative 0.2 40 80 X 1000 X Example 3 Comparative 0.2 50 75 X 1000 X Example 4 Preparation 0.5 40 92 ◯ 600 ◯ Example 5 Preparation 0.5 50 90 ◯ 800 ◯ Example 6 Comparative 0.5 100 80 X 1000 X Example 7

(16) As can be seen in Table 1 above, when the foils were cold-rolled at a reduction ratio of 85% or more, 95% or more the area thereof was oriented in the (100) direction, and when the foils were recrystallization-annealed, graphene layer could be formed on the copper foils. Also, it can be seen that, when the reduction ratio is high and the thickness of the copper foil is small, the (100) orientation is easily formed. Thus, the rolled copper foil provided according to the present invention has a critical significance when it has a reduction ratio of 85% or more or a thickness of 50 μm or less.

(17) The foils were recrystallization-annealed at various temperatures in an atmosphere of hydrogen having a flow rate of 10 sccm, followed by cooling, and the orientation thereof was measured. Then, the foils were heated to 1000° C. and maintained in an atmosphere of 15 sccm methane and 10 sccm hydrogen for 30 minutes while graphene was grown thereon by CVD. FIG. 2(a) shows the state of a copper foil according to the present invention, FIG. 2(b) shows the copper foil annealed at 1000° C. in a hydrogen atmosphere, and FIG. 2(c) shows graphene grown by supplying methane together with hydrogen to the copper foil. In the present invention, it was found that, even when copper foils are heated at high temperatures, the development of steps thereon differ between an annealing process in which only hydrogen is supplied and an annealing process a mixed gas of methane gas and hydrogen is supplied.

(18) FIG. 3 shows the state of the rolled foil in Preparation Example 5 and the orientation of the foil measured after forming graphene thereon. FIG. 4 shows the results of measuring the Raman spectrum of the foil after growing graphene thereon. As can be seen therein, monolayer graphene was formed on the foil.

Example 2

Fabrication of Graphene Layer on Electrodeposited Copper Plated Tough Pitch Copper Foil

(19) 2-1: Fabrication of Electrodeposited Copper Foil by Pulse-Current Plating and Graphene Layer

(20) Plain tough pitch copper foils were air-stirred in a solution composed of 180-330 g/L of copper sulfate pentahydride (CuSO.sub.4.5H.sub.2O), 40-120 g/L of sulfuric acid and 40-120 ppm of hydrochloric acid at a temperature of 30˜55° C. and a current density of 1-10 A/dm.sup.2, thereby pulse current plating the foils. The results are shown in FIG. 5, and the pulse waveform is expressed as current supply time: rest time.

(21) FIGS. 5(a) and 5(b) show the results of measuring the state of a specimen obtained by plating copper on a copper foil at a ratio of 80:20 at a current density of 4.2-4.3 A/dm.sup.2 and the orientation of a structure obtained by growing graphene on the specimen at 1000° C.; FIGS. 5(c) and 5(d) show the results of measuring the state of a specimen obtained by plating copper on a copper foil at a ratio of 50:50 at a current density of 2.6 A/dm.sup.2 and the orientation of a structure obtained by growing graphene on the specimen at 1000° C.; and FIGS. 5(e) and 5(f) show the results of measuring the state of a specimen obtained by plating copper on a copper foil at a ratio of 20:80 at a current density of 1.6 A/dm.sup.2 and the orientation of a structure obtained by growing graphene on the specimen at 1000° C.

(22) As can be seen therein, in the plated state, the (200) single orientation or a mixed orientation of (111) (200) (220) orientations can appear depending on the plating conditions, but after the growth of graphene at 1000° C., only the (200) orientation appears. Also, when the pulse current plated specimens were annealed, the mixed orientation was arranged to the (200) orientation at 600° C. or higher regardless of the plating conditions. However, even at the above current density range, the mixed orientation appeared even after annealing, when plating was carried out by a PR (pulse-reverse) method or direct current plating.

(23) It appears that the reason why the single orientation is determined depending on the waveform of current even at the same current density is that the pulse wave shows a high atomic packing density compared to other current waves while providing epitaxial electrodeposition, and thus the mixed orientation is arranged to a single orientation by subsequent heating. With respect to another reason, in the manufacturing of general electrodeposited copper foils, plating is carried out at high current density and a high rate of about of 1 m/min, and thus the degree of disorder of deposited copper atoms is high such that the orientation thereof is difficult to rearrange to a single orientation by the introduction of heat energy during annealing.

(24) Thus, the scope of the present invention includes pulse-plating a copper foil by stirring in a solution composed of 180-330 g/L of copper sulfate pentahydride (CuSO.sub.4.5H.sub.2O), 40-120 g/L of sulfuric acid and 40-120 ppm of hydrochloric acid at a temperature of 30˜55° C. and a current density of 1-10 A/dm.sup.2, and annealing the pulse-current plated copper foil at a temperature of 600° C. or higher, and also growing graphene on the plated copper foil.

(25) 2-2: Fabrication of Electrodeposited Copper Foil and Graphene Layer

(26) Copper scrap was dissolved in acid solution, and the solution was supplied into an opening below an anode placed in an electrolysis bath containing 250 g/l of copper sulfate (CuSO.sub.4.H.sub.2O) and 80 g/l of sulfuric acid at 30° C., while an electrolysis reaction (cathode electrode current density: 8 A/dm.sup.2) was induced so that a thin copper foil having a (111) orientation was electrodeposited on a titanium (Ti) rotating drum having a connector to cathode. Herein, the foil side facing the drum side was shiny, and the opposite side was matte. The drum surface was polished to a roughness (Ra) of 0-0.35 μm and anodized to form an oxide layer of 1-20 nm in order to facilitate the separation of an electrodeposited copper foil from the polished drum surface.

(27) Copper was pulse-current plated on the electrodeposited copper foil according to the method of Example 2-1, and graphene was deposited and grown thereon by CVD. The resulting structure was observed with an optical microscope and the orientation thereof was measured by XRD (see FIGS. 5 and 6). As a result, it could be seen that the electrodeposited copper foil after annealing had an unidirectional orientation having the (111) or (200) orientation and that epitaxial graphene was formed on the electrodeposited copper foil produced using the drum having a surface roughness (Ra) of 0.0001-0.35 μm.

Example 3

Formation of Graphene on Copper Alloy Catalyst Substrate

(28) A copper alloy foil containing 140 ppm of silver (see FIG. 9(b)) was heated at 600° C. for 30 minutes in an atmosphere of 70 sccm methane and 10 sccm hydrogen, and whether graphene was formed on the alloy foil was examined. As a control, copper (see FIG. 9(a)) was treated under the same conditions, and whether graphene was formed thereon was examined (see FIG. 9).

(29) It can be seen that, when graphene was formed on copper, graphene islands and carbides were formed, but on the silver-containing copper alloy, graphene was epitaxially formed. However, when copper was previously annealed at 800° C. to form steps, graphene was epitaxially formed thereon.

(30) The above copper alloy and copper had a hexagonal lattice structures having the (111) or (100) orientation after annealing, and these catalyst substrates also had the same orientation in the following examples.

(31) Thus, it could be seen that the addition of a substitutional alloy to copper provides graphene nucleation sites and promotes the development of a step structure to suppress carbide formation and also enables the epitaxial growth of graphene.

Example 4

Examination of Effect of Formation of Step Structure

(32) A 18 μm thick copper alloy foil containing 3.2% nickel, 1.5% silicon and 0.4% magnesium was annealed at 1000° C. for 30 minutes in an atmosphere of a mixed gas of 50 sccm argon and 10 sccm hydrogen to form steps structure, and graphene was synthesized thereon by CVD at 800° C. for 30 minutes in an atmosphere of a mixed gas of 70 sccm methane and 10 sccm hydrogen (see FIG. 10(a)).

(33) As a control, a 25-μm thick copper foil containing no alloy element was annealed at 1000° C. for 30 minutes in an atmosphere of a mixed gas of 50 sccm argon and 10 sccm hydrogen to form a step structure, and graphene was synthesized thereon by CVD at 800° C. for 30 minutes in an atmosphere of a mixed gas of 70 sccm methane and 10 sccm hydrogen (see FIG. 10(b)).

(34) As can be seen in FIG. 10(a), diamond particles were grown on multilayer graphene on the copper alloy foil, and as can be seen in FIG. 10(b), multilayer graphene and diamond particles were grown together on the copper foil. It could be seen that carbon radicals were rapidly produced on the copper alloy foil even at 800° C. lower than 1000-1060° C. at which graphene is conventionally synthesized by CVD. Also, it could be seen that, when a step structure is formed on a copper foil, graphene is grown thereon even at low temperature.

(35) Thus, when a step structure is sufficiently developed, graphene can be synthesized by reducing the concentration of hydrogen gas or shortening the synthesis time.

(36) As can be seen in FIG. 10(a), when the amount of the alloy is excessive, monolayer graphene can be obtained by reducing the alloy amount to atom % or less or reducing the concentration of hydrocarbon and the synthesis time. As can be seen in FIG. 10(b), when the concentration of hydrocarbon gas is excessively high, the production of carbon radicals is faster than the growth of graphene, and thus graphene grows at the nucleation sites while triangular or rectangular plate-like carbon or wire- or particle-like carbon grows. Even in this case, monolayer graphene could be obtained by reducing the concentration of hydrocarbon or increasing the concentration of hydrogen gas during the synthesis of graphene. From this phenomenon, the present inventors could find that the formation of step structures promotes the production of carbon radicals and the growth rate of graphene, thus greatly contributing to forming monolayer graphene.

Example 5

Examination of Growth of Graphene on Catalyst Substrate Containing Various Alloying Elements

(37) As a control for the graphene prepared in Example 4, copper foil was annealed at 1000° C. for 30 minutes in an atmosphere of a mixed gas of 50 sccm argon and 10 sccm hydrogen, and graphene was synthesized thereon by CVD at 1000° C. for 30 minutes in an atmosphere of a mixed gas of 15 sccm methane and 10 sccm hydrogen. The results are shown in FIG. 11(a).

(38) Meanwhile, FIG. 11(b) shows the results obtained by annealing a copper alloy foil containing 80 ppm of silver at 800° C. for 30 minutes in an atmosphere of a mixed gas of argon 20 sccm and 10 sccm hydrogen and then synthesizing graphene thereon by CVD at 1000° C. for 5 minutes in an atmosphere of a mixed gas of 20 sccm methane and 10 sccm hydrogen. FIG. 11(c) shows the results obtained by annealing a copper alloy foil containing 40 ppm of chromium at 1000° C. for 30 minutes in atmosphere of a mixed gas of 50 sccm argon and 10 sccm hydrogen and then synthesizing graphene thereon by CVD at 800° C. for 3 minutes in an atmosphere of a mixed gas of 20 sccm methane and 10 sccm hydrogen; FIG. 11(d) shows the results obtained using a copper alloy foil containing 200 ppm of iron under the conditions as the case of FIG. 11(c); FIG. 11(e) shows the results obtained using a copper alloy foil containing 130 ppm of cobalt under the conditions as the case of FIG. 11(c); FIG. 11(f) shows the results obtained using a copper alloy foil containing 100 ppm of nickel under the conditions as in the case of FIG. 11(c); FIG. 11(g) shows the results obtained using a copper alloy foil containing 140 ppm of silver under the conditions as the case of FIG. 11(c); and FIG. 11(h) shows the results obtained using a copper alloy foil containing 70 ppm of silicon under the conditions as the case of FIG. 11(c). As can be seen therein, the step structures were well developed and graphene was epitaxially grown. Herein, the dark areas in the graphene are separated parts to the catalyst surface, or grain boundaries or twins, which absorbed or irregularly reflected electrons because the step structures below the graphene layer changed.

(39) In addition, it was found that, even when CVD synthesis was carried out at 1000° C. for 1 second in an atmosphere of a mixed gas of 30 sccm methane and 10 sccm hydrogen, well developed step structures and formed graphene nuclei (see FIG. 12).

(40) While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed for carrying out this invention, but that the invention will include all embodiments falling within the scope of the present application.