JP6430754B2 - Method for producing graphene film, graphene film and laminate using the same - Google Patents

Description

  The present invention relates to a method for producing a graphene film, a graphene film, and a laminate using the same.

Graphene is a sheet-like two-dimensional material composed of carbon atoms forming sp ² bonds with a thickness of 1 carbon atom, and is often manufactured by chemical vapor deposition. However, this manufacturing method requires a high temperature of 1000 ° C. and a catalyst, and also requires a transfer process. On the other hand, as a simpler method, a method for producing graphene or a graphene film (a graphene thin film or a graphene self-supporting film) by reducing graphene oxide obtained by oxidation of graphite has been studied (for example, non-processing) Patent Documents 1 and 2). Graphene obtained by this production method is sometimes referred to as reduced graphene oxide. In the reduced graphene oxide, part of the oxygen-containing functional group that the graphene oxide has often remains. Therefore, reduced graphene oxide can be positioned as a material positioned between graphene and graphene oxide.

  The graphene oxide reduction method is capable of controlling the electrical conductivity of the graphene film over a wide range by adjusting the degree of reduction, etc., and producing a graphene film with a high degree of freedom such as the thickness of the graphene film. It is superior to the chemical vapor deposition method in that it is possible.

Advanced Materials Research, 2014, 1004-1005, 1013-1016 Nature Communications 1, Article number: 73, doi: 10.1038 / ncomms1067

  However, the electrical conductivity of the graphene film obtained by the reduction of graphene oxide is relatively low, and it is desired that a wide range of electrical conductivity including higher electrical conductivity can be realized.

  Therefore, a main object of the present invention is to enable the production of graphene films having a wide range of electric conductivity including high electric conductivity by reduction of graphene oxide.

  The present inventor has intensively studied to solve the above problems, and by combining alcohol and hydrogen iodide, manufactures a graphene film having a very high electric conductivity compared with a conventional reduced graphene oxide film. The present inventors have found that this can be done and have completed the present invention.

  That is, according to the present invention, a part or all of the oxygen-containing functional group in the graphene oxide film is obtained by contacting a graphene oxide film containing graphene oxide having an oxygen-containing functional group with a fluid containing hydrogen iodide and alcohol. Provided is a method for producing a graphene film, which includes a reduction step.

  The ratio of the volume of hydrogen iodide to the volume of alcohol in the fluid may be less than 0.4. Thus, even with a small amount of hydrogen iodide, a graphene film can be efficiently produced according to the above method.

  The pressure of the fluid may be lower than the critical pressure of alcohol. Even under such mild conditions, the graphene film can be efficiently produced according to the above method.

In another aspect, the present invention provides a graphene film including graphene oxide having an oxygen-containing functional group. The graphene film according to one aspect of the present invention has a sheet resistance Rs (kΩ / □) that satisfies the following formula (1).
Rs ≦ 0.15 × T + 1.44 × 10 ⁷ × (103−T) ^−4.3 (1)
(T: light transmittance (%) in the thickness direction of the graphene film)

  In still another aspect, the present invention provides a laminate including an insulating substrate and the graphene film provided on the insulating substrate. The graphene film according to the present invention can be easily formed even on an insulating substrate.

  According to the present invention, the reduction of graphene oxide can make it possible to produce a graphene film having a wide range of electrical conductivity including high electrical conductivity. Hydrogen iodide used in the method of the present invention is safer and easier to handle than other reducing agents such as hydrazine. In addition, it is possible to reduce the amount of hydrogen iodide used, and it can be expected that the environmental load will be reduced.

2 is an XRD spectrum of the graphene film and the graphene oxide film of Example 1. FIG. The horizontal axis indicates the diffraction angle (2θ), and the vertical axis indicates the peak intensity. It is a thermogravimetric change spectrum of the graphene film and graphene oxide film of Example 1. The horizontal axis represents temperature, and the vertical axis represents mass residual ratio. It is a Raman spectrum of the graphene film of Example 1 and graphite. The horizontal axis indicates the Raman shift, and the vertical axis indicates the peak intensity. It is a graph which shows the sheet resistance with respect to the light transmittance of the graphene film of Example 2 and Comparative Example 3. The horizontal axis represents light transmittance (T), and the vertical axis represents sheet resistance (Rs). A broken line shows the curve of Rs = 0.15 * T + 1.44 * 10 < ⁷ > * (103-T) ^-4.3 .

  Hereinafter, preferred embodiments of the present invention will be described. However, the present invention is not limited to the following embodiment.

  In this specification, “graphene oxide film” means a film formed when a graphene oxide formed by oxidation of graphite is formed. The “graphene film” means a film formed by reducing part or all of the oxygen-containing functional group in the graphene oxide film, and may be referred to as “reduced graphene oxide film”. The graphene film may include graphene oxide having oxygen-containing functional groups that remain without being reduced. The graphene oxide film and the graphene film can be a thin film (graphene oxide thin film or graphene thin film) or a self-supporting film (graphene oxide self-supporting film or graphene self-supporting film). The “thin film” means a film formed on a substrate in a mode in which the film cannot be maintained by itself. The “self-holding film” means a film that can hold the film form by itself without a substrate. The graphene self-holding film may be a laminate composed of a plurality of graphene thin films.

A graphene film manufacturing method according to an embodiment includes a step of oxidizing graphite to obtain graphene oxide having an oxygen-containing functional group, a step of forming a graphene oxide film containing graphene oxide, and an oxidation And a step of reducing some or all of the oxygen-containing functional groups in the graphene film.

  Graphene oxide can be obtained, for example, by oxidation of graphite by the Hammers method described in Non-Patent Document 1. The Hammers method generally involves pre-treating graphite by reaction with a compound selected from ammonium persulfate, diphosphorus pentoxide and sulfuric acid (first stage), and treating the pre-treated graphite with sulfuric acid and a strong oxidizing agent (eg, A step of oxidizing with potassium permanganate) (second stage). Usually, the pretreated graphite is washed with water and dried before being subjected to the second stage. The graphite oxidized in the second stage (graphene oxide) may be washed with hydrogen peroxide, hydrochloric acid, water, or the like.

  Graphene oxide has an oxygen-containing functional group generated by oxidizing graphite. The oxygen-containing functional group can include, for example, at least one functional group selected from an epoxy group (—COC—), a hydroxyl group (—OH), a carbonyl group (—CO—), and a carboxy group (—COOH).

  By forming graphene oxide, a graphene oxide self-supporting film can be formed. The graphene oxide self-supporting film can be obtained, for example, by preparing a dispersion of graphene oxide, filtering the dispersion through a filtration membrane, and drying the membrane formed on the filtration membrane. Examples of the solvent for the dispersion include water and ethanol.

  Alternatively, a graphene oxide thin film may be formed over the substrate. Thereby, the laminated body which has a board | substrate and the graphene oxide thin film provided on this board | substrate can be manufactured. The graphene oxide thin film on the substrate can be obtained, for example, by a method of applying a dispersion or solution containing graphene oxide to the substrate and drying the applied dispersion or solution. Examples of the method for applying the dispersion or solution to the substrate include spin coating, dip coating, and spraying.

  The substrate is not particularly limited, but may be selected from insulating substrates such as a glass substrate, a quartz substrate, and a plastic substrate. The method according to the present embodiment does not necessarily require processing at a high temperature such as 400 ° C. or higher (usually 1000 ° C.) in the chemical vapor deposition method, and therefore a substrate having a relatively low heat resistance such as a plastic substrate. Can be used. The surface of the substrate may be hydrophilized in advance using a UV ozone cleaner or the like.

  The thickness of the graphene oxide film is not particularly limited, but may be, for example, 1 to 500 μm in the case of a graphene oxide self-supporting film, and may be, for example, 0.001 μm or more and less than 1 μm in the case of a graphene oxide thin film. Good.

By reducing the oxygen-containing functional group of the graphene oxide in the graphene oxide film, the sp ³ -bonded carbon is converted to the sp ² -bonded carbon, which is higher than the non-conductive graphene oxide film before reduction. A conductive graphene film is formed. Prior to the reduction step, the graphene oxide film may be dried. Drying can be performed, for example, by heating at 45 ° C. or lower under reduced pressure or vacuum.

  The reduction of the oxygen-containing functional group can be performed by bringing the graphene oxide film into contact with a fluid containing hydrogen iodide and alcohol. When the graphene oxide film is provided over the substrate (in the case of a graphene oxide thin film), the graphene oxide film may be subjected to a reduction process together with the substrate. The fluid used in the reduction process can be a gas, a liquid, a supercritical fluid, or a combination thereof. The alcohol may be, for example, ethanol.

  For example, after a graphene oxide film, hydrogen iodide, and alcohol are placed in a reaction vessel and the reaction vessel is sealed, the temperature and pressure in the reaction vessel can be adjusted to conditions such that the reduction reaction proceeds.

  The fluid pressure (absolute pressure) in the reduction step may be not more than the critical pressure of alcohol (in the case of ethanol, not more than 6.14 MPa). In the method according to the present embodiment, since hydrogen iodide and alcohol are combined, the reduction reaction can proceed sufficiently efficiently even under high pressure conditions such that a supercritical fluid of alcohol is formed. it can. By reducing the pressure of the fluid to a critical pressure or less, a container for high pressure resistance (for example, 9 MPa) becomes unnecessary, and thus the cost can be suppressed. From the same viewpoint, the absolute pressure of the fluid may be 2 MPa or less or 1 MPa or less. The lower limit value of the absolute pressure of the fluid is not particularly limited, but may be, for example, 0.1 MPa (atmospheric pressure) or more from the viewpoint of an appropriate reaction rate and the like.

  The temperature of the fluid in the reduction step may be, for example, 400 ° C. or lower or 300 ° C. or lower, or 150 ° C. or higher or 200 ° C. or higher from the viewpoint of an appropriate reaction rate.

  The reaction time for the reduction is not particularly limited, but may be, for example, 1 to 8 hours or 2 to 5 hours.

  The reaction vessel is not particularly limited as long as it can withstand the pressure and temperature of the fluid during the reduction process. The reaction vessel may be, for example, a stainless steel reaction tube.

  The fluid in the reduction process may contain air. The reduction process according to the present embodiment can be performed in an air atmosphere without particularly substituting with an inert gas such as nitrogen or argon.

  The ratio of the volume of hydrogen iodide to the volume of alcohol in the fluid may be less than 0.4, 0.2 or less, 0.1 or less, or 0.08 or less. Thus, even when the ratio of hydrogen iodide is small, a graphene film with sufficiently high electrical conductivity can be obtained. From the viewpoint of electrical conductivity and the like, the volume ratio of hydrogen iodide may be, for example, 0.0005 or more or 0.005 or more.

  After the reduction reaction, the formed graphene film may be washed with ethanol or the like.

  The graphene film obtained by the method described above is a reduced graphene oxide film. This graphene film may or may not substantially contain graphene oxide having an oxygen-containing functional group. A graphene film containing graphene oxide can have multiple functions as compared to a substantially pure graphene film formed by a chemical vapor deposition method or the like. By adjusting the degree of reduction and the like, characteristics such as hydrophilicity (or hydrophobicity), electrical conductivity, and proton conductivity of the obtained graphene film can be controlled in a wide range. In addition, it is expected that various functions can be imparted to the graphene film using oxygen-containing functional groups.

Further, the reduced graphene oxide film (graphene film, particularly graphene thin film) may have high electrical conductivity such that the sheet resistance Rs (kΩ / □) satisfies the following formula (1) or (1) ′. . The light transmittance is a transmittance of light having a wavelength of 550 nm. In general, a high light transmittance value corresponds to a thin film thickness of the graphene film.
Rs ≦ 0.15 × T + 1.44 × 10 ⁷ × (103−T) ^−4.3 (1)
Rs ≦ 0.12 × T + 1.2 × 10 ⁷ × (103−T) ^−4.6 (1) ′
(T: light transmittance (%) in the thickness direction of the graphene film)

  The lower limit of the sheet resistance of the graphene film is not particularly limited, and may be, for example, 0.001 kΩ / □ or more, 0.05 kΩ / □ or more, or 1.0 kΩ / □ or more.

  The reduced graphene oxide film may satisfy Formula (1) or Formula (1) ′ when the light transmittance T is in the range of 50 to 100%.

  The film thickness of the graphene film is not particularly limited, but may be, for example, 1 to 500 μm in the case of a graphene self-holding film, and may be, for example, 0.001 μm or more and less than 1 μm in the case of a graphene thin film.

  Hereinafter, the contents of the present invention will be described in more detail using examples and comparative examples. However, the present invention is not limited to the following examples.

Example 1
1. Preparation of graphene oxide self-supporting film (oxidation of graphite and film formation of graphene oxide)
A dispersion of graphene oxide (dispersion medium: water) was prepared by the Hammers method according to the method described in Non-Patent Document 1. The obtained graphene oxide dispersion was filtered under reduced pressure to form a graphene oxide film on the filtration membrane. This film was dried at room temperature to obtain a self-supporting graphene oxide film (graphene oxide self-supporting film).

2. Preparation of graphene film (reduction of graphene oxide self-supporting film)
A graphene oxide self-supporting membrane and ethanol (50 mL) were added to a stainless steel reaction tube having a capacity of 75 mL equipped with a heater and a pressure gauge, hydrogen iodide (0.3 mL) was further added, and the reaction tube was sealed. At this time, the air in the reaction tube was not replaced. The graphene oxide self-supporting film was previously dried by heating at 45 ° C. under reduced pressure. The pressure in the reaction tube was adjusted to 1 MPa, and the reduction reaction was advanced by heating at 300 ° C. for 4 hours. The reaction tube was cooled to room temperature with an ice bath to obtain the graphene self-supporting film of Example 1. The film thickness of the graphene film was 7.5 μm. The film thickness of the graphene film was measured using a cross-sectional image obtained by SEM (scanning electron microscope).

3. Evaluation (1) Electrical conductivity The sheet resistance of the graphene film was measured at room temperature using a 4-probe resistivity meter Σ-5 + (manufactured by NP Corporation).
The sheet resistance of the graphene film was 8.3Ω / □. The electric conductivity of the graphene film calculated from this and the film thickness was 16064 S / m.

(2) X-ray diffraction FIG. 1 shows XRD spectra of a graphene film and a graphene oxide film before reduction at a diffraction angle of 5 to 60 °. The XRD spectrum was measured using a RINT Ultimate III thin film XRD (manufactured by Rigaku Corporation). In the graphene oxide film, a peak was observed at 2θ = 10.12 ° (interlayer distance 0.873 nm), whereas in the graphene film, a peak was observed at 2θ = 24.60 ° (interlayer distance 0.362 nm). It was done. This result suggests that most of the oxygen-containing functional groups present in the graphene oxide film were reduced.

(3) Thermogravimetric analysis FIG. 2 shows the thermogravimetric change spectra of the graphene film and the graphene oxide film. The thermogravimetric change spectrum was measured using a DTG-60A thermal analyzer (manufactured by Shimadzu Corporation). In the graphene oxide film, a large mass decrease is observed around 200 ° C., which indicates that the graphene oxide film has an oxygen-containing functional group formed by oxidation of graphite. The mass reduction rate of the graphene oxide film at 500 ° C. was about 37%. In contrast, the mass reduction rate of the graphene film of Example 1 was about 10%, and no significant mass reduction was observed. This also suggests that most of the oxygen-containing functional groups in the graphene oxide film have been reduced.

(4) Raman spectrum FIG. 3 shows the Raman spectra of the graphene film and graphite. The Raman spectrum was measured using NRS-7200 (manufactured by JASCO Corporation). In the Raman spectrum of the graphene film, the intensity ratio of G-band (1580 cm ^-1 vicinity) D band to the intensity of (1350 cm around ^-1) was less than 1 like the graphite, D band was markedly observed . This is presumably due to the fact that, when synthesizing graphene oxide from graphite, a crystal defect caused by the collapse of a part of the crystal structure of graphite remains in the graphene film. That is, it is suggested that a part of the oxygen-containing functional group remains in the graphene film.

Comparative Example 1
The graphene oxide self-supporting film obtained by the same method as in Example 1 is processed by a method according to the method using supercritical ethanol described in Non-Patent Document 1, thereby obtaining the graphene film of Comparative Example 1 It was. That is, a graphene oxide self-supporting film and ethanol (65 mL) were added to a stainless steel reaction tube, and the reduction reaction was allowed to proceed by heating for 4 hours at a pressure of 9 MPa and 300 ° C. in a sealed reaction tube. The thickness of the obtained graphene film was 14.4 μm, the sheet resistance was 21.6 Ω / □, and the calculated electrical conductivity of the graphene film was 3215 S / m.

Comparative Example 2
The graphene oxide self-supporting film obtained by the same method as in Example 1 was processed by a method according to the method using hydrogen iodide and acetic acid described in Non-Patent Document 2 to obtain the graphene film of Comparative Example 2 It was. That is, a graphene oxide self-supporting film, hydrogen iodide (2 mL) and acetic acid (5 mL) were added to a reaction tube, and heated in a sealed reactor at 40 ° C. under vacuum pressure (10 ⁻¹ Pa) for 24 hours. The reduction reaction was allowed to proceed. The thickness of the obtained graphene film was 6.5 μm, the sheet resistance was 19.6 Ω / □, and the electric conductivity of the graphene film calculated from this was 7850 S / m.

  Table 1 shows the manufacturing method, film thickness, sheet resistance, and electrical conductivity of the graphene films of Example 1 and Comparative Examples 1 and 2.

Example 2
1. Preparation of graphene oxide thin film (oxidation of graphite and film formation of graphene oxide)
A dispersion of graphene oxide was prepared by the Hammers method in the same manner as in Example 1. A graphene oxide dispersion was applied on a glass substrate subjected to UV ozone hydrophilization treatment by a spray method to form a graphene oxide thin film.

2. Preparation of graphene thin film (reduction of graphene oxide thin film)
Add a glass substrate with graphene oxide thin film and ethanol (50 mL) to a stainless steel reaction tube with a capacity of 75 mL equipped with a heater and pressure gauge, and then add hydrogen iodide (0.3 mL). Sealed. At this time, the air in the reaction tube was not replaced. The substrate provided with the graphene oxide film was previously dried by heating at 45 ° C. under reduced pressure. The pressure in the reaction tube was adjusted to 1 MPa, and the reduction reaction was advanced by heating at 300 ° C. for 4 hours. The reaction tube was cooled to room temperature with an ice bath to obtain the graphene thin film of Example 2. As a light transmittance of the graphene thin film, a light transmittance of 550 nm (= (intensity of transmitted light / intensity of incident light) × 100) was measured using a spectrophotometer.

Comparative Example 3
The graphene oxide thin film obtained by the same method as in Example 2 was processed by a method according to the method using supercritical ethanol described in Non-Patent Document 1 to obtain a graphene film of Comparative Example 3. That is, a glass substrate provided with a graphene oxide thin film and ethanol (65 mL) were added to a stainless steel reaction tube, and the reduction reaction was advanced by heating at a pressure of 9 MPa and 300 ° C. for 4 hours in a sealed reaction tube.

A graphene oxide thin film and a graphene thin film having different light transmittances were prepared by the same method as in Example 2, and the light transmittance and sheet resistance were measured. FIG. 4 is a graph showing the relationship between the sheet resistance Rs and the light transmittance T of the graphene film. The sheet resistance tends to increase as the light transmittance increases, that is, as the film thickness decreases. However, when compared with the same light transmittance (film thickness), the graphene film of Example 2 is the same as that of Comparative Example 3. It was confirmed that the sheet resistance was low (electric conductivity was high) compared to the graphene film. FIG. 4 also shows a curve (broken line) corresponding to Rs = 0.15 × T + 1.44 × 10 ⁷ × (103−T) ^−4.3 , and the sheet resistance Rs of the graphene film of Example 2 is Further, it was also confirmed that the formula (1) was satisfied. FIG. 4 shows a curve corresponding to Rs = 0.12 × T + 1.2 × 10 ⁷ × (103−T) ^−4.6 (two-dot chain line) and Rs = 0.08 × T + 3.2 × 10 ⁶ × A curve corresponding to (103-T) ^-4.6 (dashed line) is also shown.

Examples 3-7
The graphene thin films of Examples 3 to 7 were obtained in the same manner as in Example 1 except that the amounts of ethanol and hydrogen iodide were changed to those shown in Table 2. Table 2 shows the sheet resistance at 80% light transmittance of the graphene thin films of Examples 2 to 7.

  The graphene film obtained by the method according to the present invention is, for example, a transparent electrode utilizing light transmission, electrical conductivity, etc., application to a foldable large touch panel or laser member, wiring material utilizing high current density resistance, etc. Application to chemical sensors or hydrogen storage materials using chemical sensitivity and chemical reaction, application to mechanical sensors or MEMS (Micro Electro Mechanical Systems) devices using mechanical strength and light element materials, etc. Application to high-speed transistors, spin devices, single-electron transistors, or semiconductor memories using high mobility and two-dimensionality can be expected.

  Furthermore, the combination of graphene film and various substrate materials includes heat conductive material, high-performance catalyst material, hybrid conductive material, solar cell, various thin display devices, supercapacitor electrode, oxygen reduction electrode catalyst, antistatic film, and Possibility of oxidation prevention (protection) of nanoparticles such as metal particles, and a complete gas barrier material is assumed.

  In addition, in consideration of the fact that the graphene film material itself is extremely inexpensive, it is expected to reduce the amount of metal used or as a noble metal-free conductive material.

Claims (3)

  Contacting a graphene oxide film containing graphene oxide having an oxygen-containing functional group with a fluid containing hydrogen iodide and alcohol to reduce a part or all of the oxygen-containing functional group in the graphene oxide film, Manufacturing method of graphene film.   The manufacturing method according to claim 1, wherein a ratio of a volume of the hydrogen iodide to a volume of the alcohol in the fluid is less than 0.4.   The manufacturing method of Claim 1 or 2 whose pressure of the said fluid is below the critical pressure of the said alcohol.

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