WO2013027899A1

WO2013027899A1 - Method for manufacturing a large-surface area graphene film

Info

Publication number: WO2013027899A1
Application number: PCT/KR2011/009919
Authority: WO
Inventors: 이해신; 류성우; 홍순형; 이빈
Original assignee: 한국과학기술원
Priority date: 2011-08-22
Filing date: 2011-12-21
Publication date: 2013-02-28
Also published as: KR20130021051A; KR101294223B1

Abstract

The present invention relates to a method for manufacturing a large-surface area graphene film, and more particularly, to a method for manufacturing a large-surface area graphene film in a very simple, fast, and inexpensive manner. According to the present invention, the method for manufacturing the large-surface area graphene film comprises the steps of: forming a graphene oxide by oxidizing graphene; forming a graphene oxide dispersion solution by dispersing the graphene oxide into water; forming a reduced graphene oxide dispersion solution by reducing the graphene oxide; and forming a graphene film at an interface between air and water by inputting the reduced graphene oxide dispersion solution into a salt solution.

Description

【Specification】

[Name of invention]

Manufacturing method of large area graphene film

Technical Field

The present invention relates to a method for producing a graphene film, and more particularly to a method for producing a large area graphene film in a very simple, fast and cheap method. Background Art

Graphene starts with graphite, which we are familiar with. By removing a layer from a layer of carbon atoms stacked in layers and analyzing its properties with physical principles, graphene is the next generation because it has amazing electrical and thermal conductivity and transparent and flexible properties. As a new material, it began to attract attention. In 2010, two Russian scientists who stripped graphene from graphite and studied its physical properties clearly won the Nobel Prize in Physics, further accelerating the study of graphene's properties and properties (Geim, AK, et al., Nature (2007) 6, 183).

Research published to date suggests that there are several ways to make graphene. Chemical exfoliation using scotch tape to further exfoliate from graphite, chemical exfoliation, epitaxial growth, chemical vapor deposition (CVD) and HOPGOiighly ordered pyrolytic graphite (chemical exfoliation) method (Novoselov, KS, et al., Proc. Natl. Acad. Sci. USA (2005) 102, 10451; Novoselov, KS, et al., Science (2004) 306, 666; Rrishnan , A. et al. Nature (1997) 388, 451; Dresselhaus, MS & Dresselhaus, G. Adv. Phys. (2002) 51, 1). In the case of the mechanical peeling method performed in the early days, since only a small amount of graphene could be manufactured by manual labor, it is difficult to commercialize. When manufactured using chemical vapor deposition, it is known that graphene having excellent properties having a high conductivity of a single layer can be manufactured. However, in the case of chemical vapor deposition, graphene needs to be grown to a large area in order to be practically used in electronic materials. At this time, several tons of Cu and metal substrates used as catalysts are required, which makes it difficult to commercialize them at an expensive manufacturing cost. . Currently, Sungkyunkwan University lab in Korea has used a roll-to-roll method to produce transparent 30-inch ² size graphene. Commercialization of chemical oxidation / reduction process that can produce graphene in bulk at low cost It has the biggest advantage in doing so. However, due to the nature of the oxidation / reduction process, graphenes of arbitrary size and type are manufactured, and there is a difficulty in complete reduction during reduction, so there is a limitation in conductivity characteristics in actual use such as a film such as a uniform high conductivity transparent electrode. (Geng, J. et al. J. Phys. Chem. C (2010) 114, 14433).

[Detailed Description of the Invention]

[Technical problem]

Accordingly, an object of the present invention is to provide a method for producing a large-area graphene film in a very simple, fast and cheap method.

[Technical Solution]

According to an aspect of the invention, the step of oxidizing graphite to form graphene oxide; Dispersing the graphene oxide in water to form a dispersion of graphene oxide; Reducing the graphene oxide to form a dispersion of reduced graphene oxide; And a graphene film is formed at the air / water interface by injecting the dispersion of the reduced graphene oxide into an aqueous salt solution.

According to another aspect of the invention, providing a dispersion of oxide graphene whose edge portion is modified with a hydrophilic group; And it is provided with a method for producing a graphene film comprising the step of allowing the graphene to self-assemble at the air / water interface by reacting the dispersion and the aqueous salt solution of graphene.

According to another aspect of the invention, there is provided a graphene film produced by the above-described method.

Advantageous Effects

The present invention provides a graphene film formed by self-assembly between a solution and an air interface using a salt ing out method used for protein separation and purification, thereby limiting the mass production and solution process of the conventional CVD process. By not using low-conductivity graphene oxide, a high-conductivity transparent electrode can be mass-produced.

[Brief Description of Drawings]

1 is a process flow diagram showing a method for manufacturing a large-area graphene film according to an embodiment of the present invention. .

FIG. 2 is a view illustrating a process of forming graphene film by self-assembly where reduced graphene oxide meets metal ions. Figure 3 is a photograph showing a large area graphene film prepared by the manufacturing method according to an embodiment of the present invention.

4 is a graph showing an atomic force microscope (AFM) photograph and size histogram of graphene filtered by vacuum filtration.

FIG. 5 shows an AFM image and size histogram of graphene transmitted by vacuum filtration.

6 is a photograph showing an example of forming a graphene film using various types of salt aqueous solutions.

7 is a scanning electron micrograph showing a cross section of a graphene film transferred on a silicon substrate.

8 is a transmission electron microscope (TEM) photograph of the graphene films prepared in Preparation Example 3.

9 is a graph showing the results of measuring the surface conductivity of the graphene film produced.

[Form for implementation of invention]

Hereinafter, with reference to the drawings will be described the present invention in more detail. 1 is a process flow diagram showing a method for manufacturing a large-area graphene film according to an embodiment of the present invention.

Referring to FIG. 1, in step S100, graphite is oxidized to form graphene oxide. Graphite can be treated with an acid solution for oxidation. Graphite is a carbon-only material that is difficult to disperse in aqueous solution. Therefore, the graphite can be treated with an acid solution to introduce a hydrophilic group containing oxygen on the surface thereof to increase the water dispersibility. The hydrophilic group may be a carboxyl group, a carbonyl group, an epoxy group, or a hydroxyl group. An acid solution is preferably used as an acid solution.

H ₂ S0 ₄ , KMn0 ₄ , HC1 or HN0 ₃ are representative. These may be used alone or in combination, and for example, H ₂ SO ₄ and KMn0 ₄ may be used together to obtain very good oxidation power. As the graphite is oxidized by the above-described oxidation treatment, oxygen atoms are bonded between the layers of the graphite.

In step S110, the additive graphene oxide is dispersed in water to form a dispersion of graphene oxide. When graphene oxide is dispersed in water as it is, it is not uniformly dispersed even if it is shaken vigorously, because it is not yet peeled off, and the particles are large and dispersed in various sizes. Therefore, the graphene oxide did not peel off When the dispersion is treated with an ultrasonic wave generator in order to make pins into a single layer of graphene oxide, the graphene oxide particles are separated and become a graphene oxide dispersion uniformly dispersed. In this case, if the ultrasonic treatment time is too long, the carbon bond may be broken at the same time as the graphene oxide particles are peeled off, and thus the particle size may be reduced.

In step S120, the graphene oxide is reduced to form a dispersion of reduced graphene oxide. Graphene oxide has a stable dispersion in aqueous solution, but due to the oxidative reaction has a lot of _S p2 bonds between the carbon is not conductive. Therefore, in order to restore the conductivity, it is desirable to reduce and remove the functional groups produced by the oxidation reaction. At this time, hydrazine, NaBH ₄ (sodium borohydride), HKhydrogen iodide), hydroquinone, etc. may be used as a reducing agent for the reduction. Alternatively, various reduction methods may be included as a reduction method such as heat treatment using hydrogen and argon at high temperatures.

Graphene oxide is well dispersed in water because it is a hydrophilic particle. However, reduced graphene oxide (RG0), which reduces graphene oxide, can aggregate into particles and become hydrophobic due to the interaction due to the interlayer pi-pi bond as oxygen-containing functional groups are removed. Thus, a solvent to prevent agglomeration can be added to the graphene oxide dispersion. The solvent is N-methylpyrlidone (N-methylpyrrol idone), ethylene glycol (ethylene glycol), glycerin (glycerin), dimethylpyridone

(dimethylpyrrol idone), acetone, tetrahydrofuran, acetonitrile, dimethyl formamide, dimethyl sulphoxide, amine or alcohol ( water-combustible solvents such as alcohol). The graphene oxide dispersion has a brown or yellow color depending on the concentration, and may change from brown or yellow to black when the graphene oxide is reduced.

The concentration of the reduced graphene oxide may be 0.001 to 5 tng / ml, preferably 0.01 to 2 mg / ml. When the concentration is less than 0.001 mg / ml, delayed self-assembly may occur due to the low concentration of graphene, and in the case of 5 mg / ml second fruits, uneven self-assembly may occur due to the aggregation of graphene (aggregat ion). Can be.

A large-area graphene film may be prepared by adding the reduced graphene oxide dispersion to the aqueous salt solution in step S130 to form a graphene film at an air / water interface. The salt aqueous solution is at least one selected from the group consisting of organic salts and inorganic salts Merchant salts. For example, the salt aqueous solution is alkaline earth metals such as Li, Na, K, Be, Ca, Mg, Au. Ions of a metal selected from transition metals such as Ag, Fe, Cu, Νί, Co, and post-transition metals such as A1, Ga, In, and metalloids such as B, Si, Ge, and As. The concentration of the aqueous salt solution may be 10mM to 10M, preferably 100mM to 1M depending on the concentration of the reduced graphene oxide dispersion. When the dispersion of the reduced graphene oxide is added to the aqueous salt solution, it is preferable to dropwise add dropwise so that the graphene film is widely spread at the air / water interface.

FIG. 2 is a view illustrating a process of forming graphene film by self-assembly where reduced graphene oxide meets metal ions. The reduced graphene oxide may remain with a hydrophilic group at the edge part even after the reduction. Referring to FIG. 2, the edge of the reduced graphene oxide may be negatively charged due to the loss of protons by the basicity of the aqueous salt solution. On the other hand, when positively charged metal ions (Metal ^{n +} in the figure) are adsorbed on the edges of the reduced graphene oxide and become electrically neutral, the graphene is entangled by the salting-out effect. At the same time, the reduced graphene oxide is located on the surface of the aqueous salt solution, ie, the air / water interface, due to its hydrophobicity. Due to the amphipathic nature of the reduced graphene oxide, very thin films can be formed by self-assembly. As a result, a large-area graphene film can be easily manufactured by a self-assembly method. The average thickness of the graphene film may be lnm to l, 000nm. Transparency and conductivity may be selectively controlled according to the thickness of the graphene film.

Figure 3 is a photograph showing a large area graphene film prepared by the manufacturing method according to an embodiment of the present invention. Referring to Figure 3, a film of 20cm X 20cm size was produced, the size of the manufacturing mechanism of the present invention can be substantially increased without limitation.

According to one embodiment of the invention, the graphene film self-assembled on the surface of the salt solution can be transferred onto the substrate. For example, the graphene film may be transferred onto the substrate by dipping the substrate into the salt solution or removing the aqueous salt solution. As the type of substrate, a polymer substrate such as PET, PE, PEN, or PDMS, or a substrate which is made of glass, quartz, or silicon, which can be thermally treated, can be used.

If necessary, the functional groups of the graphene that are not reduced may be reduced by heat treating the substrate on which the graphene film is formed in a reducing gas atmosphere to improve the conductivity of the graphene film. The heat treatment can be carried out for 30 seconds to 48 hours at a temperature of 200 to 800 ^° C. When temperature rises fast-Rapid increase of residual moisture in graphene film It is preferable to increase the temperature at a rate of 5 to 50 ^° C / min since there may be a loss of the graphene film according to the foot.

Thereafter, for recrystallization of exfoliated graphene, the temperature may be heat-treated in a vacuum at a temperature of 900 to 2,000 ^° C at a rate of 5 to 50 ^° C / min for 30 seconds to 48 hours. . By recrystallizing the defect of the graphene generated during the oxidation process by the heat treatment for recrystallization it is possible to maintain a uniform conductivity over the entire surface of the film. The final manufactured graphene film may have a multilayer structure, may have an average of 1 to 60 layers, preferably 1 to 30 layers. Graphene having a multilayer structure having 30 or more layers has a structure close to graphite in the production of a large-area film, which may have a transparency of 60% or less in the visible light region (550 nm), thereby degrading performance as a transparent film. .

According to an embodiment of the present invention, a transparent conductive film may be prepared by transferring the graphene film on a transparent substrate. The transparent conductive film may be used in solar cells or electronic paper, transparent electronic devices, flexible devices, and the like.

The graphene film manufactured by the above-described manufacturing method may be manufactured inexpensively and easily and has excellent conductivity, and thus may be used in next-generation future electronic devices such as transparent display materials, next-generation semiconductor materials, and transparent electrode materials. Hereinafter, the present invention will be described in more detail with reference to examples, but the following examples are presented to aid understanding, and the spirit of the present invention is not limited to the following examples.

Synthesis of Graphite Oxide

Place a reaction vessel with 45 mL of concentrated sulfuric acid in an ice bath and lower the temperature to 5-7 ^° C. Graphite powder lg was slowly added to the reaction vessel and stirred. Next, when adding 3.5 g of KMn0 _{4, the} mixture was stirred very slowly to prevent the reduction of graphite due to heat. The temperature was then raised to 35-37 ^° C. and stirred for 2 hours at a speed of 280 rpm. After the stirring was completed, the reaction vessel was placed in an ice bath again to lower the temperature of the reaction water to 5-7 ^° C. 50 mL of tertiary distilled water was dropped dropwise very slowly over 30 minutes into the semi-aqueous water, and then 350 mL of tertiary distilled water was further added slowly. 20 mL of hydrogen peroxide was slowly added dropwise to the reaction solution. At this time, the color of the half water that was dark green It turned to yellow and observed bubble formation. Hydrogen peroxide was added until the bubble disappeared.

The reaction vessel was allowed to settle for one day to allow the product to sink below the vessel, and then the excess acid solvents were removed to remove excess metal ions. 10 ¾> aqueous hydrochloric acid solution was added to the bottom of the submerged product and shaken vigorously to remove the remaining metal ions. Then solvent and metal transfers were removed by vacuum filtration under the filter paper (pore size: 0.2 pm). After repeating this process three times, the product filtered on the filter paper was mixed with about 100 mL of 10% hydrochloric acid aqueous solution, and evenly divided into four membrane bags (MWC0: 12-14,000). The membrane bag was placed in a 5L beaker full of tertiary distilled water and dialyzed five times for three hours to remove almost all metal ions in the product. After the dialysis process was completed, the product was vacuum filtered again to remove the solvent and air-dried to prepare graphite powder.

Graphene Oxide Dispersion Preparation

Graphite oxide was dissolved in tertiary distilled water by 1 mg / mL to form a graphite oxide dispersion. Next, to disperse the graphite oxide into a single layer of graphene oxide, a dispersion was sonicated for 1 minute to prepare a uniform graphene oxide dispersion.

Preparation of Reduced Graphene Oxide Dispersion

The graphene oxide was dispersed in a solvent in which water and dimethylformamide were mixed at a volume ratio of 1 _{: 9} , and hydrazine monohydrate, a representative reducing agent, was added to the graphene oxide dispersion as much as 3 ig / mg by mass ratio of graphene oxide. Dropped. Hydrazine was added as slowly as possible to prevent coagulation between particles. The reduced graphene oxide dispersion added with hydrazine was reacted for 24 hours using a shaker. After reaction, the color of the dispersion gradually changed from brown (graphene oxide) to black (reduced graphene oxide).

<Production example 4> Size Category Classification of Oxidized Graphite

The synthesized graphite oxide was dispersed in water to make a graphene oxide dispersion. Next, vacuum filtration was performed using an aluminum oxide film having a pore size of 3.0 / mi. The graphene oxide filtered on the membrane was dispersed in a small amount of water, and then dried to obtain graphene oxide larger than 3.0 _{m in} size.

4 is a graph showing an atomic force microscope (AFM) photograph and size histogram of graphene filtered by vacuum filtration. Referring to FIG. 4, it was confirmed by AFM that the graphene having a size of 3 or more and graphene having a size of 3 IM or less were separated by vacuum filtration.

FIG. 5 shows an AFM image and size histogram of graphene transmitted by vacuum filtration. Referring to FIG. 5, the graphene oxide having a size smaller than 0.8 _m may be identified by AFM by passing the graphene oxide dispersion formed in a syringe filter having a hole size of 0.8 _μm and drying the dispersion passed through the filter. By separating the size of the graphene, which is arbitrarily prepared, a high-conductivity film was manufactured by minimizing the contact resistance generated between the graphenes generated when the large-area film was manufactured using only graphene having a size of 3 or more.

Graphene formation

Calcium chloride (CaCl ₂ ), iron chloride (I II) (FeCl ₃ ), copper chloride (II) (CuCl ₂ ), sodium chloride

(NaCl), aluminum chloride (A1C1 ₃ ), magnesium chloride (MgCl ₂ ), potassium chloride (KC1), barium chloride

(BaCl ₂ ) was dissolved in water at a concentration of 100 mM, respectively, to form an aqueous salt solution. Each of the above aqueous salt solution was placed in a wide beaker, and the reduced graphene oxide dispersion was dropped one by one to spontaneously spread at the interface between the aqueous salt solution and air to form a graphene film. Figure 6 is a photograph showing an example of forming a graphene film using various types of salt aqueous solution.

Transfer of Graphene Film to Substrate

The graphene film self-assembled on the surface of the solution was transferred to a silicon substrate through dipping, washed with distilled water spray, and dried at room temperature. The thickness of the film Was measured by scanning electron microscopy (SEM). 7 is a scanning electron micrograph showing a cross section of a graphene film transferred on a silicon substrate. Referring to Figure 7, it can be seen that the graphene film having a thickness of about 60 nm was prepared.

Example 3

Heat treatment salt treatment and recrystallization salt treatment of graphene

The graphene film prepared in Example 2 was heated to 400 at a rate of 20 ?? / min and heat treated for 4 hours in an Ar and ¾ atmosphere. If the temperature increase rate is fast, because the loss of the graphene film due to the rapid evaporation of the moisture remaining in the graphene film was raised slowly. The heat treatment of the graphene film reduced some of the remaining functional groups of graphene without being reduced. After the exfoliation (exfol iated) graphene was re-crystallized in a vacuum atmosphere at a rate of 20 ℃ / min to 1,000 ？? and then heat treated for 4 hours.

Analysis of Microstructure and Composition of Graphene

The microstructure of the graphene films prepared in Preparation Example 3 was analyzed. 8 is a transmission electron microscope (TEM) photograph of the graphene films prepared in Preparation Example 3. Referring to FIG. 8, it can be seen that graphene having an average of 1 to 7 layers is manufactured. 8 is a TEM picture of single layer graphene, and the right picture is a TEM picture of graphene having a multilayer structure of 7-10 layers.

Conductivity Characterization of Graphene Film

The resulting graphene film was measured for surface conductivity using a 4-point probe device and the transmittance was measured using a UV-vis spectrometer device. Referring to FIG. 9, the self-assembled graphene film reduced by Examples 2 and 3 may have a surface resistance value of 800 to 9 kΩ / cm ² in a transparency range of 75-85%.

Although described above with reference to the drawings and embodiments, those skilled in the art will be free from the technical spirit of the present invention described in the claims below. It will be appreciated that various modifications and changes can be made to the disclosed embodiments without departing from the scope thereof.

Industrial Applicability

The present invention can be applied and used in a wide range of fields such as displays, touch screens, light emitting diodes, optoelectronic devices such as solar cells, and the like.

Claims

[Range of request]

[Claim 1]

Oxidizing the graphite to form graphene oxide;

Dispersing the graphene oxide in water to form a dispersion of graphene oxide; Reducing the graphene oxide to form a dispersion of reduced graphene oxide; And

A method for producing a graphene film comprising the step of forming a graphene film at the air / water interface by introducing the dispersion of the reduced graphene oxide in an aqueous salt solution.

[Claim 2]

The method according to claim 1,

The salt solution is a method for producing a graphene film comprising a salt of at least one selected from the group consisting of organic salts and inorganic salts.

[Claim 3]

The method according to claim 1,

The salt aqueous solution is a method for producing a graphene film containing ions of at least one metal selected from the group consisting of alkali metals, alkaline earth metals, transition metals, post-transition metals, and metalloids.

[Claim 4]

The method according to claim 1,

The concentration of the aqueous salt solution is a method of producing a graphene film of 10mM to 10M.

[Claim 5]

The method according to claim 1,

Method for producing a graphene film of the average thickness of the graphene film is lnm to l, 000nm.

[Claim 6]

Providing a dispersion of graphene oxide with edge portions modified with hydrophilic groups; The method for producing a graphene film comprising the step of allowing the graphene to self-assemble at the air / water interface by contacting the dispersion solution of the graphene with an aqueous salt solution.

[Claim 7]

The method according to claim 6,

After self-assembly, the substrate may be removed by backing the substrate or by removing an aqueous salt solution. Method of producing a graphene film further comprising the step of transferring the graphene film.

[Claim 8]

The method according to claim 7,

The method of producing a graphene film further comprises the step of reducing the functional groups of the remaining graphene is not reduced by heat treatment of the substrate to which the graphene film is transferred in a reducing gas atmosphere.

[Claim 9]

The method according to claim 8,

The method of manufacturing a graphene film further comprising the step of performing a heat treatment for recrystallization of the exfoliated graphene after the heat treatment for the reduction.

[Claim 10]

The method according to claim 7,

Method of producing a graphene film comprising the step of transferring the graphene film produced by the self-assembly to a transparent substrate.

[Claim 11]

Graphene film prepared by the manufacturing method according to any one of claims 1 to 10.

[Claim 12]

The method according to claim 11,

Graphene film having a surface resistance value of 800 to 9,000 Ω / η ^{2 in} the range of 75-85% transparency.