KR20210128177A - Method for Preparing Graphene - Google Patents

Abstract

The present invention relates to a method for preparing graphene and provides a method for preparing graphene in a high production yield with a specific surface area of 1000 m^2/g or more without a separate classification process by performing a repeated oxidation process in which graphite is primarily oxidized using an acid and an oxidizing agent, and then the same is secondary oxidized using the acid and the oxidizing agent, followed by pulverization and reduction.

Description

Method for preparing graphene {Method for Preparing Graphene}

The present invention relates to a method for producing graphene, and more particularly, to a method for producing graphene having a specific surface area of ^{1,000 m 2 /g or more with a high production yield without a separate classification process.}

Graphene has a structure in which planes in which carbons are arranged like a honeycomb hexagonal network are stacked in layers, and due to these structural properties, physical and chemical stability is very high. In addition, graphene is attracting attention as a key new material in the material industry as a material with very high application potential in various fields due to its excellent electrical conductivity and thermal conductivity.

Graphene has existed only in theory, but in 2004 Andre Geim and Konstantin Novoselov were the first to separate it by attaching and peeling glass tape to the pencil lead. .

To date, graphene manufacturing methods are largely classified into two types. A bottom-up method of chemically synthesizing graphene from carbon and a top-down method of exfoliating graphene from graphite. As a bottom-up method, there is a chemical vapor deposition method or an epitaxial growth method, and these methods can control the number of graphene layers and growth factors using various types of substrates. In the case of chemical vapor deposition (CVD), large-area, high-purity graphene can be produced, but there is an issue with mass production due to high manufacturing cost. As a top-down method, there are a method of mechanically separating from graphite and a method of obtaining graphene oxide by separating it in a solution phase through oxidation. Graphene oxide in powder form obtained through the oxidation of graphite has excellent crystallinity, but has a potential for organic impurity contamination, and has a disadvantage in that physical properties are slightly lowered compared to graphene made by chemical vapor deposition (CVD), which has fewer structural defects. It is the most advantageous manufacturing method for production and industrialization [refer to Korean Patent Registration No. 10-1109961].

In general, graphene oxide obtained through oxidation of graphite has a multilayer structure having a size of 5 to 10 μm. Such graphene oxide is reduced after an exfoliation process through mechanical grinding to increase the specific surface area, but there is a problem that the ^{specific surface area is greatly limited to 700 m 2 /g or less.} In order to obtain reduced graphene having a specific surface area of 1,000 m ² /g or more, graphene oxide powder having a specific particle size or less must be separated and used through a classification process after mechanically pulverizing graphene oxide. However, ^{the particle size range that can provide such a specific surface area of 1,000 m 2} /g or more is small, so that the production yield does not even reach 10%, and there is also a cumbersome process according to classification.

Therefore, it is required to develop a technology capable of producing graphene having a specific surface area of ^{1,000 m 2} /g or more with a high production yield without a separate classification process.

Republic of Korea Patent No. 10-1109961

As a result of intensive research and examination to solve the above-described problems in the manufacture of graphene having a high specific surface area, the present inventors repeated primary oxidation of graphite using an acid and an oxidizing agent and then secondary oxidation using an acid and an oxidizing agent. ^{It was found that graphene having a specific surface area of 1,000 m 2} /g or more can be efficiently and economically manufactured with a high production yield without a separate classification process by grinding and reducing after the oxidation process, and the present invention has been completed. .

Accordingly, an object of the present invention is to provide a method capable of producing graphene having a specific surface area of ^{1,000 m 2 /g or more with a high production yield without a separate classification process.}

On the other hand, the present invention comprises the steps of (i) primary oxidation of graphite using sulfuric acid, potassium permanganate and hydrogen peroxide to obtain graphene oxide; (ii) secondary oxidation of the graphene oxide using sulfuric acid, potassium permanganate and hydrogen peroxide to obtain nano-oxide graphene; (iii) pulverizing the nano-oxide graphene; And (iv) provides a method for producing reduced graphene comprising the step of reducing the pulverized nano-oxide graphene.

In one embodiment of the present invention, the reduced graphene may have a BET specific surface area of 1,000 m 2 /g or more.

In one embodiment of the present invention, the oxidation in steps (i) and (ii) may be carried out at less than 55 °C.

In one embodiment of the present invention, potassium permanganate in step (ii) may be used in an amount of 1 to 6 times the weight of graphene oxide.

In one embodiment of the present invention, the grinding in step (iii) may be performed using a mixer.

In one embodiment of the present invention, the grinding in step (iii) may be performed at 2,000 to 3,000 rpm.

In an embodiment of the present invention, the grinding in step (iii) may be performed for 15 to 30 seconds and the process of resting for 1 to 3 minutes may be repeated 20 to 40 times.

In one embodiment of the present invention, the nano graphene oxide pulverized in step (iii) may be obtained in the form of an aggregate having a particle size of 100 μm or less.

In one embodiment of the present invention, the reduction in step (iv) may be performed by heating reduction.

In one embodiment of the present invention, the heat reduction may be performed by primary heat treatment at 300 to 400° C., and then secondary heat treatment at 900 to 1,000° C. under a hydrogen gas atmosphere.

According to the present invention, graphite is first oxidized using an acid and an oxidizing agent, then subjected to a repeated oxidation process of secondary oxidation using an acid and an oxidizing agent, and then pulverized and reduced to achieve a specific surface area of ^{1,000 m 2 /g or more without a separate classification process} Graphene having a high production yield can be efficiently and economically produced. Graphene prepared according to the present invention has a high specific surface area, so it can be advantageously used in environmental fields such as water treatment, odor and toxic gas removal by utilizing porous adsorption properties as well as electrode materials for energy storage devices.

1 is graphene oxide (GO) (A) obtained by the primary oxidation reaction, and KMnO ₄ is put in an amount of 1 times (B) and 2 times (C) relative to the weight of graphene oxide, respectively, and the secondary oxidation reaction is performed It is an SEM image of the obtained nano graphene oxide (Nano-GO).
2 is graphene oxide (GO) obtained by the primary oxidation reaction, and KMnO ₄ Nano obtained by secondary oxidation reaction by putting 0.1 times, 1 times, 2 times and 3 times the weight of graphene oxide, respectively. UV-visible spectrum of graphene oxide (Nano-GO).
3 is a graph showing the relationship between the ratio and electrical conductivity of the specific surface area of 1,000 m 2 /g or more of the reduced graphene obtained in Comparative Examples 1 to 3 and Examples 1 to 4, respectively.

Hereinafter, the present invention will be described in more detail.

One embodiment of the present invention relates to a method for producing graphene.

A method for producing graphene according to an embodiment of the present invention comprises: (i) primary oxidation of graphite using sulfuric acid, potassium permanganate and hydrogen peroxide to obtain graphene oxide; (ii) secondary oxidation of the graphene oxide using sulfuric acid, potassium permanganate and hydrogen peroxide to obtain nano-oxide graphene; (iii) pulverizing the nano-oxide graphene; and (iv) reducing the pulverized nano graphene oxide.

The graphene prepared according to an embodiment of the present invention may have a BET specific surface area of 1,000 m 2 /g or more, such as 1,000 to 1,500 m 2 /g, preferably 1,100 to 1,500 m 2 /g.

The BET specific surface area is nitrogen according to ASTMD 3663-78 standard based on the Brunauer-Emmet-Teller method described in the publication The Journal of the American Chemical Society, 60, 309 (1938). It refers to the specific surface area determined by adsorption.

In one embodiment of the present invention, the step (i) is _{by primary oxidation of graphite (graphite) with sulfuric acid, potassium permanganate (KMnO 4} ) and hydrogen peroxide (H ₂ O ₂ ) as an oxidizing agent to graphene oxide This is the step to obtain a pin.

The sulfuric acid may be used in an amount of 10 to 100 ml per 1 g of graphite. If the sulfuric acid is used in an amount of less than 10 ml, the primary oxidation may not proceed sufficiently, and if used in an amount of more than 100 ml, the amount of acid waste may increase, and cleaning may be difficult.

The potassium permanganate may be used in an amount of 1 to 10 times the weight of graphite. When the potassium permanganate is used in an amount of less than one time, the primary oxidation may not proceed sufficiently, and when used in an amount of more than ten times, the amount of potassium permanganate used increases and the production cost may increase.

The hydrogen peroxide may be used in an amount of 1 to 10 ml per 1 g of graphite. When the hydrogen peroxide is used in an amount of less than 1 ml, the primary oxidation may not proceed sufficiently.

The primary oxidation may be carried out at less than 55 ℃. If the oxidation temperature is more than 55 ℃, there is a risk of explosion by sulfuric acid and the graphite may be converted into an amorphous form.

The primary oxidation may be performed by sequentially treating graphite with sulfuric acid, potassium permanganate and hydrogen peroxide.

Specifically, in the primary oxidation, graphite is stirred in sulfuric acid for 1 to 2 hours, potassium permanganate is added in an ice bath, stirred for 1 to 2 hours, and then reacted for 5 to 24 hours, and then hydrogen peroxide is added for 10 minutes It can be carried out by reacting for 1 hour.

Through the primary oxidation, oxygen-containing functional groups are introduced into graphite to increase the interlayer distance, so that a single layer of graphene oxide can be prepared.

In one embodiment of the present invention, in the step (ii), the graphene oxide obtained through the primary oxidation is secondary oxidized using sulfuric acid and potassium permanganate and hydrogen peroxide as oxidizing agents to obtain nano graphene oxide am.

The sulfuric acid may be used in an amount of 10 to 100 ml per 1 g of graphene oxide. If the sulfuric acid is used in an amount of less than 10 ml, secondary oxidation may not proceed sufficiently, and when used in an amount of more than 100 ml, oxidation proceeds too much and is not filtered through a filter after purification to obtain nano-oxide graphene It can be difficult.

The potassium permanganate may be used in an amount of 1 to 6 times, preferably 1 to 4 times, more preferably 1 to 3 times the weight of graphene oxide. If the potassium permanganate is used in an amount of less than 1 times, secondary oxidation may not proceed sufficiently and the BET specific surface area may be reduced, and if used in an amount of more than 6 times, the electrical conductivity of the finally obtained graphene may decrease It may be difficult to obtain nano-oxide graphene because it proceeds too much and is not filtered through a filter after purification.

The hydrogen peroxide may be used in an amount of 1 to 10 ml per 1 g of graphene oxide. If the hydrogen peroxide is used in an amount of less than 1 ml, secondary oxidation may not proceed sufficiently.

The secondary oxidation may be carried out at less than 55 ℃. If the oxidation temperature is 55° C. or higher, there is a risk of explosion by sulfuric acid and graphene oxide may be converted into an amorphous form.

The secondary oxidation may be performed by sequentially treating graphene oxide with sulfuric acid, potassium permanganate and hydrogen peroxide.

Specifically, in the secondary oxidation, graphene oxide is dispersed in sulfuric acid for 1 to 2 hours, potassium permanganate is added in an ice bath to react for 4 to 24 hours, and then hydrogen peroxide is added to react for 10 minutes to 1 hour. It can be done by

Through the secondary oxidation, nano-scale graphene oxide having a particle size of, for example, 300 to 500 nm can be obtained. In the present invention, nano-oxide graphene having the above particle size range can be obtained simply and easily through the above-described primary and secondary repeated oxidation process, and then by going through a grinding process, 1,000 m ^{2 without a separate classification process} Graphene having a specific surface area of /g or more can be obtained.

In one embodiment of the present invention, step (iii) is a step of pulverizing nano graphene oxide in order to improve the specific surface area.

Grinding in step (iii) may be performed using a mixer, and may be performed at 2,000 to 3,000 rpm. If the pulverization is performed at less than 2,000 rpm, the pulverization may not be performed smoothly and classification may not be performed.

In order to prevent a decrease in specific surface area due to partial reduction of graphene oxide in the grinding process, the grinding may be performed for 15 to 30 seconds and then the process of resting for 1 to 3 minutes may be repeated 20 to 40 times. have.

Through the pulverization, agglomeration occurs due to the functional groups present on the surface of the nano graphene oxide, and thus it can be obtained in the form of an aggregate having a particle size of 100 μm or less, for example, 20 to 100 μm.

In one embodiment of the present invention, the step (iv) is a step of reducing the pulverized nano graphene oxide to obtain reduced graphene.

The reduction may be performed by heating reduction.

The heat reduction may be performed in a hydrogen gas atmosphere at a temperature range of 300°C to 1,000°C. Preferably, the heat reduction may be performed by primary heat treatment at 300 to 400° C., and then secondary heat treatment at 900 to 1,000° C. under a hydrogen gas atmosphere. Defects can be minimized by removing oxygen functional groups and impurities through the two-step heat treatment as described above.

In the manufacturing method according to an embodiment of the present invention, graphite is first oxidized with sulfuric acid, potassium permanganate and hydrogen peroxide, and then undergoes an iterative oxidation process of secondary oxidation with sulfuric acid, potassium permanganate and hydrogen peroxide, followed by pulverization and reduction processes ^{Thus, graphene having a specific surface area of 1,000 m 2} /g or more, preferably 1,300 m ² /g or more, without a separate classification process, can be produced in high yield, for example, 80% or more, preferably 90% or more, more preferably It can be manufactured efficiently and economically up to 100%. That is, the manufacturing method according to an embodiment of the present invention can manufacture ^{graphene having a specific surface area of 1,000 m 2} /g or more with a high production yield without a separate classification process.

In addition, the manufacturing method according to an embodiment of the present invention can manufacture graphene having high electrical conductivity, for example, electrical conductivity of 90 to 400 S/m with high production yield.

Graphene prepared according to the present invention has a high specific surface area, so it can be advantageously used in environmental fields such as water treatment, odor and toxic gas removal by utilizing porous adsorption properties as well as electrode materials for energy storage devices.

Hereinafter, the present invention will be described in more detail by way of Examples, Comparative Examples and Experimental Examples. These Examples, Comparative Examples, and Experimental Examples are only for illustrating the present invention, and it is apparent to those skilled in the art that the scope of the present invention is not limited thereto.

Example 1: Preparation of reduced graphene

1) primary oxidation step

Add 10 g of graphite (particle size: 200 μm or less) to 400 mL of sulfuric acid (SA), stir for 1 to 2 hours, and then add 50 g of _{potassium permanganate (KMnO 4 ) in an ice bath to 1} to 2 hours. Graphite prepared by stirring in the ice bath for 1 to 2 hours was subjected to oxidation reaction at less than 55° C. for 5 to 24 hours. _{Afterwards, sulfuric acid and KMnO 4} were recovered using a reduced pressure filter to reuse the used oxidizing agent and used in the subsequent secondary oxidation reaction. After dilution by adding 400 to 800 g of ice to the reaction mixture, _{50 mL of hydrogen peroxide (H 2} O ₂ ) was added and reacted for 1 hour to complete the primary oxidation reaction. After the oxidation reaction was completed, the reactants were filtered using a ceramic filter, distilled water was added, and repeated washing was performed by centrifugation. After washing, it is possible to obtain graphene oxide powder through a drying process.

2) Secondary oxidation step

400 mL of sulfuric acid was added to 10 g of the powdery graphene oxide (GO), and dispersed for 1 to 2 hours using an ultrasonic bath (sonic bath). After that, KMnO ₄ was added in an amount of 1 times the weight of graphene oxide in an ice bath. Then, oxidation reaction was carried out at less than 55 °C for 4 to 24 hours. Then, it was diluted with 400 to 800 g of ice, and _{50 mL of H 2} O ₂ was added to react for 1 hour to complete the secondary oxidation reaction. After dilution in distilled water, the acid was purified and separated using an electrodialysis device (ED), and then dried to obtain nano-graphene oxide (Nano-GO).

3) crushing step

The nano graphene oxide was pulverized at 2,500 rpm for 15 to 30 seconds using a mixer, and the process of resting for 2 minutes was repeated 30 times to pulverize while controlling the temperature of the graphene oxide not to increase.

4) reduction step

The pulverized nano-oxide graphene was subjected to a primary heat treatment at 300 to 400 ° C using a rotary drying incinerator, and then secondary heat treatment at 900 to 1,000 ° C in a hydrogen gas atmosphere using a tube furnace to prepare reduced graphene.

Example 2: Preparation of reduced graphene

When the secondary oxidation reaction was performed, reduced graphene was prepared in the same manner as in Example 1, except that _{KMnO 4 was added in an amount twice the weight of graphene oxide.}

Example 3: Preparation of reduced graphene

When the secondary oxidation reaction was performed, reduced graphene was prepared in the same manner as in Example 1, except that _{KMnO 4 was added in an amount three times the weight of graphene oxide.}

Example 4: Preparation of reduced graphene

When performing the secondary oxidation reaction, reduced graphene was prepared in the same manner as in Example 1, except that _{KMnO 4 was added in an amount of 4 times the weight of graphene oxide.}

Comparative Example 1: Preparation of reduced graphene

Reduced graphene was prepared in the same manner as in Example 1, except that the secondary oxidation step was not performed.

Comparative Example 2: Preparation of reduced graphene

When performing the secondary oxidation reaction, reduced graphene was prepared in the same manner as in Example 1, except that _{KMnO 4 was added in an amount of 0.1 times the weight of graphene oxide.}

Comparative Example 3: Preparation of reduced graphene

When performing the secondary oxidation reaction, reduced graphene was prepared in the same manner as in Example 1, except that _{KMnO 4 was added in an amount of 0.5 times the weight of graphene oxide.}

Experimental Example 1: SEM analysis of graphene oxide

Nano obtained by secondary oxidation reaction by putting graphene oxide (GO) (A) obtained by the primary oxidation reaction and KMnO ₄ in an amount of 1 times (B) and 2 times (C), respectively, relative to the weight of graphene oxide Scanning electron microscope (SEM) analysis was performed for particle size analysis of graphene oxide (Nano-GO).

The results are shown in FIG. 1 .

1, it can be seen that the particle size of the graphene nano-oxide (Nano-GO) subjected to the secondary oxidation reaction is distributed in the range of 300 nm to 500 nm. It can be seen that the particle size of nano-graphene oxide (Nano-GO) is smaller than that of graphene oxide (GO) that has undergone the primary oxidation reaction.

Experimental Example 2: Measurement of UV-Visible Light Spectrum of Graphene Oxide

Graphene oxide (GO) obtained by the primary oxidation reaction and KMnO ₄ Nano graphene oxide obtained by the secondary oxidation reaction by adding 0.1 times, 1 times, 2 times and 3 times the weight of graphene oxide, respectively The UV-visible spectrum of (Nano-GO) was measured.

The results are shown in FIG. 2 .

2, it can be confirmed that the peaks of the UV-visible spectrum of graphene oxide (Nano-GO) that has undergone secondary oxidation and graphene oxide (GO) that have undergone primary oxidation are different from each other. Graphene exhibits a π-π* transition peak at 230 nm, which is a phenomenon arising from the aromatic structure of graphene. However, in the case of nano-graphene oxide (Nano-GO), it can be seen that the π-π* transition peak at 230 nm is reduced compared to graphene oxide (GO). This is believed to be because the size of the nano-graphene oxide (Nano-GO) is reduced and the aromatic properties are reduced.

Experimental Example 3: Analysis of specific surface area and electrical conductivity of classified reduced graphene

Before reducing the pulverized nano graphene oxide obtained in the Examples and Comparative Examples, classifying for 4 to 8 hours using a sieve shaker, 45 μm or less, 45 μm or more, 75 μm or less, 75 μm or more 100 μm Hereinafter, it was classified into a range of more than 100 µm and less than 150 µm and more than 150 µm and less than or equal to 200 µm. The yield for each section is shown in Table 1 below (unit: %).

More than 150㎛ 200㎛ or less More than 100㎛ 150㎛ or less More than 75㎛ 100㎛ or less More than 45㎛ 75㎛ or less 45 μm or less Comparative Example 1 20 50 15 10 5 Comparative Example 2 - 30 35 25 10 Comparative Example 3 - 15 40 30 15 Example 1 - - 35 40 25 Example 2 - - 25 45 30 Example 3 - - 25 45 30 Example 4 - - 25 45 30

From Table 1, it can be seen that the nano-oxide graphene of the example has a higher yield of aggregates having a lower particle size than the nano-oxide graphene of the comparative example. In addition, _{it can be seen that when KMnO 4} is used one or more times, there is no aggregate of nano-oxide graphene having a size of more than 100 μm.

The classified nano graphene oxide was reduced under the same conditions as in Example 1 to measure the BET specific surface area for each section. The results are shown in Table 2 below (unit: m 2 /g).

More than 150㎛ 200㎛ or less More than 100㎛ 150㎛ or less More than 75㎛ 100㎛ or less More than 45㎛ 75㎛ or less 45 μm or less Comparative Example 1 712.36 740.81 870.86 925.37 1080.4 Comparative Example 2 - 910.56 993.36 1105.6 1188.7 Comparative Example 3 - 920.89 1048.24 1175.9 1269.4 Example 1 - - 1331.5 1334.6 1335.7 Example 2 - - 1320.2 1326.4 1327.9 Example 3 - - 1321.7 1328.6 1330.1 Example 4 - - 1328.4 1330.1 1330.5

From Table 2, it can be confirmed that the reduced graphene prepared according to Examples 1 to 4 has a specific surface area of 1,000 m 2 /g or more, particularly 1,300 m 2 /g or more in all aggregate particle size ranges. Therefore, according to the manufacturing method of the present invention ^{, it can be seen that graphene having a specific surface area of 1,000 m 2} /g or more can be efficiently and economically manufactured with a high production yield without a separate classification process. On the other hand, the reduced graphene of Comparative Example 1 that did not undergo the secondary oxidation reaction and the reduced graphene _{of Comparative Examples 2 and 3 using KMnO 4} in an amount of 0.1 times and 0.5 times the weight of graphene oxide, respectively, exceeded 45 μm, respectively. , it can be confirmed that the specific surface area is less than 1,000 m 2 /g in the aggregate particle size range of more than 75 μm and more than 100 μm.

In addition, taking a look at Tables 1 and 2, it can be seen that the graphene oxide agglomerates nano of Examples 1 to 4 have a higher specific surface area even though they have the same particle size as the graphene oxide aggregates of Comparative Examples 1 to 3 have.

In addition, the reduced graphene obtained in Comparative Examples 1 to 3 and Examples 1 to 4, that is, reduced graphene in _{which the ratio of KMnO 4} used is 0 times, 0.1 times, 0.5 times, 1 times, 2 times, 3 times, and 4 times, respectively The ratio between the specific surface area of the pin and the electrical conductivity of 1,000 m 2 /g or more was measured, and the relationship between them is shown in FIG. 3 . Electrical conductivity was measured using a 4-point probe. Each reduced graphene powder was mixed with a polytetrafluoroethylene (PTFE) binder to prepare a film, and measured using a sample compressed using a roll press.

3, _{it can be confirmed that the reduced graphene of Examples 1 to 4 in which the ratio of KMnO 4} used is one-fold or more is 100% in the ratio of the specific surface area of 1,000 m 2 /g or more. Therefore, it can be seen that it is preferable in terms of the specific surface area of the reduced graphene to control the ratio of _{KMnO 4 to be used by one or more times.} However, since the electrical conductivity _{decreases as the KMnO 4} usage ratio increases, it can be seen that it is advantageous to control the _{KMnO 4 usage ratio to 3 times or less.}

As the specific parts of the present invention have been described in detail above, for those of ordinary skill in the art to which the present invention pertains, it is clear that these specific techniques are only preferred embodiments, and the scope of the present invention is not limited thereto. do. Those of ordinary skill in the art to which the present invention pertains will be able to make various applications and modifications within the scope of the present invention based on the above contents.

Accordingly, the substantial scope of the present invention will be defined by the appended claims and their equivalents.

Claims (10)

(i) primary oxidation of graphite using sulfuric acid, potassium permanganate and hydrogen peroxide to obtain graphene oxide;
(ii) secondary oxidation of the graphene oxide using sulfuric acid, potassium permanganate and hydrogen peroxide to obtain nano-oxide graphene;
(iii) pulverizing the nano-oxide graphene; and
(iv) a method for producing graphene comprising the step of reducing the pulverized nano graphene oxide. The method according to claim 1, wherein the reduced graphene has a BET specific surface area of 1,000 m 2 /g or more. The method according to claim 1, wherein the oxidation in steps (i) and (ii) is performed at less than 55°C. The method according to claim 1, wherein in step (ii), potassium permanganate is used in an amount of 1 to 6 times the weight of graphene oxide. The method according to claim 1, wherein the pulverization in step (iii) is performed using a mixer. The method according to claim 5, wherein the grinding in step (iii) is performed at 2,000 to 3,000 rpm. The method according to claim 5, wherein the grinding in step (iii) is performed for 15 to 30 seconds and the process of resting for 1 to 3 minutes is repeated 20 to 40 times. The method according to claim 1, wherein the nano graphene oxide pulverized in step (iii) is obtained in the form of an aggregate having a particle size of 100 μm or less. The method according to claim 1, wherein the reduction in step (iv) is performed by heating reduction. The method according to claim 9, wherein the heat reduction is performed by performing a primary heat treatment at 300 to 400°C, and then performing a secondary heat treatment at 900 to 1000°C in a hydrogen gas atmosphere.

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