JP5582464B2 - Method for producing metal nanosheet, and metal nanosheet - Google Patents

Description

  The present invention relates to a method for producing a metal nanosheet, and a metal nanosheet obtained by the production method.

  A nanosheet is a two-dimensional material having a high anisotropy such that the lateral size is several tens to several hundreds or more, while the thickness is usually on the order of nanometers. Such nanosheets not only inherit the functionality of the matrix (conductivity, semiconducting properties, dielectric properties, etc.) but also have a large specific surface area required for catalytic reactions and the like. In addition, the nanosheet may exhibit outstanding physical properties such as a one-dimensional quantum size effect. Furthermore, nanosheets can be reconfigured in three dimensions as functional blocks, once separated nanosheets, to construct physical superlattice-like nanostructured materials that cannot be achieved thermodynamically. It has the potential to be controlled. Therefore, for nanosheets, the superlattice approach, which has so far been the mainstream of gas-phase synthesis techniques such as molecular beam epitaxy, can be developed in the liquid phase. It has attracted a lot of interest from industry, especially in the field.

Regarding the method of synthesizing such nanosheets,
Synthesis method (1): Method of growing from molecules, ions, etc. Synthesis method (2): A method of separating a layered compound into a single layer has been proposed.

  These synthesis methods (1) and (2) have been studied for the synthesis of nanosheets of metal compounds, but a technique for producing nanosheets of noble metals such as platinum and gold using a liquid phase reaction. Has also been proposed (see Patent Documents 1 and 2).

JP 2006-228450 A JP 2008-57023 A

  For the application development described above, the lineup and expansion of nanosheets is indispensable. In particular, the birth of nanosheets with completely different properties from existing nanosheets is the key to opening up the application development of new nanosheet materials, so pioneering search including their synthesis methods is an important issue. Yes.

  However, it has only been reported that metal nanosheets have been realized for a very small portion of metal, and this is insufficient for the lineup and expansion of nanosheets. On the other hand, in the above synthesis method (2), since it is necessary to infinitely swell the host layer by inserting different ions or solvent molecules between the layers of the host layer, conventionally, the starting material of the nanosheet has been synthesized thermodynamically. Limited to synthesizing crystalline layered compounds (sulfide-based, oxide-based, hydroxide-based, carbon-based) or metal compound nanosheets with clay minerals existing in nature as precursors, At present, no attention has been paid to methods for obtaining metal nanosheets.

  In view of the above-described problems, the object of the present invention is to produce a metal nanosheet capable of producing various metal nanosheets by using a layered compound of a metal compound as a precursor, and produced by such a method. It is to provide a metal nanosheet.

  In order to solve the above problems, the inventors of the present invention have made extensive studies and, as a result, by proceeding the reduction reaction of each of the host layers while suppressing atomic diffusion, the lamellar structure of the layered compound is not greatly broken, It has been found that a metal nanosheet or a laminate thereof can be obtained. The present invention has been made on the basis of these series of findings, and provides a metal nanosheet having a thickness of 1 nm or less and a two-dimensional anisotropy having a dimension in the sheet plane direction of 30 nm or more, and a method for producing the metal nanosheet. Is.

First, the method for producing a metal nanosheet according to the present invention includes at least a layered compound preparing step of preparing a layered compound in which the metal compound is layered, and the above-described structural phase transition via a topological structure phase transition depending on conditions for suppressing atomic diffusion. Reduction to obtain a metal nanosheet having a thickness of 1 nm or less and a dimension in the sheet plane direction of 30 nm or more reflecting the atomic arrangement of the metal compound of the layered compound by reducing the metal compound as a precursor a step, was perforated, the metal compound is ruthenium, titanium, niobium, vanadium, manganese, cobalt, and any one of a sulfide of molybdenum, characterized in that an oxide or hydroxide.

  According to this invention, it is a new technique of manufacturing the metal nanosheet which makes a layered compound a precursor by reducing the metal compound which comprises a layered compound, and various metal nanosheets can be manufactured. Therefore, it is possible to greatly realize the nanosheet lineup and expansion necessary for application to various fields such as catalysts, thin film forming materials, and coating films. In addition, according to the present invention, since the reduction reaction of each host layer can proceed while suppressing atomic diffusion, a metal nanosheet or a laminate thereof can be obtained without greatly destroying the lamellar structure of the layered compound. be able to. Therefore, a metal nanosheet having a thickness of 1 nm or less comparable to the existing metal cluster size and a size of 30 nm or more, for example, several μm in the sheet plane direction can be realized.

  In the present invention, in the reduction step, for example, the metal compound is reduced in the state of a metal compound nanosheet obtained by peeling the layered compound in a single layer, or in the state of the layered compound, so that the single layer or the multilayer is formed. A metal nanosheet is obtained.

  In the present invention, when the former method is employed, after performing the layered compound preparation step, before the reduction step, a single layer peeling step is performed in which the layered compound is peeled off to obtain the metal compound nanosheet. In the reduction step, the metal compound is reduced in the state of the metal compound nanosheet.

  In this case, after the single layer peeling step and before the reduction step, a deposition step of depositing the metal compound nanosheet to form a thin film is performed, and in the reduction step, the metal compound nanosheet is in a thin film state. The metal compound may be reduced.

  In the present invention, when the latter method is adopted, in the reduction step, the metal compound is reduced in the state of the layered compound to obtain a metal nanosheet structure in which a plurality of the metal nanosheets are laminated. In this case, after the reduction step, a single layer peeling step of peeling the nanosheet structure into a single layer of the metal nanosheet may be performed.

  In the present invention, it is preferable to use a baking treatment in a reducing atmosphere in the reduction step. Such a reducing atmosphere can be realized by, for example, a hydrogen-containing atmosphere.

In the present invention, a wet process using a reducing solution may be used in the reduction step. Such a reducing solution can be realized by a solution such as sodium borohydride (NaBH ₄ ).

  In the present invention, the metal compound is, for example, a sulfide, metal oxide, or metal hydroxide of a metal such as ruthenium, titanium, niobium, vanadium, manganese, cobalt, or molybdenum. The metal compound may be a clay mineral.

  According to this invention, it is a new technique of manufacturing the metal nanosheet which makes a layered compound a precursor by reducing the metal compound which comprises a layered compound, and various metal nanosheets can be manufactured. Therefore, it is possible to greatly realize the nanosheet lineup and expansion necessary for application to various fields such as catalysts, thin film forming materials, and coating films. In addition, according to the present invention, since the reduction reaction of each host layer can proceed while suppressing atomic diffusion, a metal nanosheet or a laminate thereof can be obtained without greatly destroying the lamellar structure of the layered compound. be able to. Therefore, a metal nanosheet having a thickness of 1 nm or less comparable to the existing metal cluster size and a size of 30 nm or more, for example, several μm in the sheet plane direction can be realized. Such two-dimensional anisotropy can be expected not only to inherit the functionality of the matrix (conductivity, semiconducting properties, dielectric properties, etc.), but also to exhibit outstanding physical properties such as a one-dimensional quantum size effect. Moreover, since the metal nanosheet to which the present invention is applied has a huge specific surface area, it can be used as a catalyst, or as a thin film material or a coating material. Furthermore, since a metal nanosheet is excellent in functionality as an industrially important metal material, a nanostructure produced using the metal nanosheet is useful as an electrode or a catalyst material.

It is process drawing which shows the manufacturing method of the metal nanosheet which concerns on Embodiment 1 of this invention. It is process drawing which shows the manufacturing method of the metal nanosheet which concerns on Embodiment 2 of this invention. In the manufacturing method of the metal nanosheet which concerns on Example 1 of this invention, it is explanatory drawing of the layered compound preparation process at the time of using a K-type layered ruthenium oxide, and a single layer peeling process. In the manufacturing method of the metal nanosheet which concerns on Example 1 of this invention, it is explanatory drawing of the layered compound preparation process at the time of using Na type layered ruthenium oxide, and a single layer peeling process. It is explanatory drawing of the deposition process of the ruthenium oxide nanosheet performed with the manufacturing method of the metal nanosheet which concerns on Example 1 of this invention. In the manufacturing method of the metal nanosheet which concerns on Example 1 of this invention, it is explanatory drawing which shows a mode that the mode after depositing a ruthenium oxide nanosheet was observed by AFM. It is explanatory drawing of the reduction process performed with respect to the ruthenium oxide nanosheet (K induction | guidance | derivation type) in the manufacturing method of the metal nanosheet which concerns on Example 1 of this invention. In the manufacturing method of the metal nanosheet which concerns on Example 1 of this invention, it is explanatory drawing which shows a mode that the metal ruthenium nanosheet obtained by performing a reduction | restoration process to a ruthenium oxide nanosheet (K induction type) was observed by AFM. In the manufacturing method of the metal nanosheet which concerns on the reference example of Example 1 of this invention, it is explanatory drawing which shows the X-ray-diffraction pattern after baking to a ruthenium oxide nanosheet (K induction | guidance | derivation type). It is explanatory drawing of the reduction | restoration process performed with respect to the ruthenium oxide nanosheet (Na induction | guidance | derivation type) in the manufacturing method of the metal nanosheet which concerns on Example 1 of this invention. It is explanatory drawing of the reduction | restoration process performed with the manufacturing method of the metal nanosheet which concerns on Example 2 of this invention. It is explanatory drawing of the reduction process performed with the manufacturing method of the metal nanosheet which concerns on Example 3 of this invention.

  Embodiments of the present invention will be described below with reference to the drawings.

<Embodiment 1>
FIG. 1 is a process diagram illustrating a method for producing a metal nanosheet according to Embodiment 1 of the present invention, and FIGS. 1 (a) and 1 (b) each illustrate production of a metal nanosheet according to Embodiment 1 of the present invention. In the method, there are a process diagram in the case where a deposition process for depositing a metal compound by means such as self-organization is not performed, and a process chart in a case where a deposition process is performed.

  As shown in FIG. 1A, in the method for producing a metal nanosheet of this embodiment, at least a layered compound for preparing a layered compound in which a metal compound such as a metal sulfide, a metal oxide, or a metal hydroxide is layered is prepared. Preparation step ST10 and reduction step ST40 for reducing the metal compound to obtain a metal nanosheet having a layered compound as a precursor are performed. Clay minerals may be used as the layered compound. In the reduction process ST40, a firing process in a reducing atmosphere such as a hydrogen-containing atmosphere or a wet process using a reducing solution containing sodium borohydride or the like is used.

  In this embodiment, in the reduction step ST40, the metal compound is reduced in the state of the metal compound nanosheet to obtain a metal nanosheet. For this reason, after performing layered compound preparatory process ST10, before layering process ST40, single layer peeling process ST20 which makes a layered compound peel a single layer, and obtains a metal compound nanosheet is performed.

  Further, as shown in FIG. 1 (b), after the single layer peeling step ST20, and before the reduction step ST40, a deposition step ST30 is performed in which a metal compound nanosheet is deposited in a self-organized manner to form a thin film. In step ST40, the metal compound may be reduced in the state of a thin film of the metal compound nanosheet to obtain a metal nanosheet.

  In any of these cases, in the monolayer peeling step ST20, an infinite swelling process (soft chemical reaction) may be used in which different ions or solvent molecules are inserted between layers of the layered compound to expand the layers.

<Embodiment 2>
FIG. 2 is a process diagram showing a method for producing a metal nanosheet according to Embodiment 2 of the present invention. As shown in FIG. 2, in this embodiment, as in the first embodiment, a layered compound preparation step of preparing a layered compound in which at least a metal compound such as a metal sulfide, a metal oxide, or a metal hydroxide is layered is prepared. ST10 and reduction step ST40 for obtaining a metal nanosheet using a layered compound as a precursor by reducing the metal compound are performed. Clay minerals may be used as the layered compound. In the reduction process ST40, a firing process in a reducing atmosphere such as a hydrogen-containing atmosphere or a wet process using a reducing solution containing sodium borohydride or the like is used.

  In this embodiment, in the reduction step ST40, the metal compound is reduced in the state of a layered compound to obtain a metal nanosheet structure in which a plurality of metal nanosheets are laminated. For this reason, in this embodiment, a single-layer metal nanosheet can be obtained by performing a single-layer peeling step ST50 in which the nanosheet structure is separated into a single-layer metal nanosheet after the reduction step ST40.

  In addition, for the purpose of facilitating the single-layer peeling step ST50, an infinite swelling that expands the layer by inserting different ions or solvent molecules between the layers of the layered compound after the layered compound preparation step ST10 and before the reduction step ST40. It is preferable to perform the conversion process. Such infinite swelling may be performed after the reduction step ST40. In this case, different ions or solvent molecules are inserted between the layers of the metal nanosheet structure via ultrasonic treatment or the like, and the layers of the metal nanosheet structure are inserted. Will spread.

<Example 1>
As Example 1 of the present invention, a method for producing a metal ruthenium nanosheet (metal nanosheet) from a layered compound of ruthenium oxide (metal compound) by the method according to Embodiment 1 described with reference to FIG. explain.

[Layered Compound Preparation Step ST10 and Single Layer Stripping Step ST20]
FIG. 3 is an explanatory diagram of a layered compound preparation step ST10 and a single layer peeling step ST20 when K-type layered ruthenium oxide is used in the method for manufacturing a metal nanosheet according to Example 1 of the present invention. FIG. 4 is an explanatory diagram of a layered compound preparation step ST10 and a single layer peeling step ST20 when Na-type layered ruthenium oxide is used in the method for producing a metal nanosheet according to Example 1 of the present invention.

As shown in FIG. 3, in the manufacturing method of the metal nanosheet which concerns on Example 1 of this invention, K-type layered ruthenium oxide (layered compound) is used. For this reason, in the layered compound preparation step ST10, ruthenium oxide and potassium carbonate are weighed out in a molar ratio of 8: 5 and wet-mixed in acetone for 1 hour using an agate mortar. Next, after the mixed powder is pelletized using a tablet molding machine, the pellet is placed on an alumina boat and baked at 850 ° C. for 12 hours in a tubular furnace under argon flow. Next, after pulverizing the pellet, the pellet is washed with ion-exchanged distilled water, and the supernatant is removed. What repeated this operation until a supernatant liquid becomes neutral is set as K type | mold layered ruthenium oxide (layered compound). Next, 1M HCl is added to K-type layered ruthenium oxide, and acid treatment is performed in a water bath at 60 ° C. for 72 hours. Next, the supernatant is removed by washing with ion exchange distilled water. This operation is repeated until the supernatant becomes neutral, and the powder obtained after filtration is converted to K-derived hydrogen-type layered ruthenium oxide (H _0.2 RuO _2.1 · 0.9H ₂ O).

Next, in the single layer peeling step ST20, a 10% TBAOH aqueous solution was added to the hydrogen-type layered ruthenium oxide at a ratio of TBA ⁺ / H ⁺ = 5 or TBA ⁺ / H ⁺ = 10 and shaken for 10 days. The supernatant recovered by centrifugation (2000 rpm, 30 min) is used as an aqueous dispersion of ruthenium oxide nanosheets. That is, in this embodiment, without reacting hydrogen-type layered ruthenium oxide with ethylamine, the hydrogen-type layered ruthenium oxide is directly reacted with protons of hydrogen-type layered ruthenium oxide, the protons are replaced with tetrabutylammonium ions, and the layers of layered ruthenium oxide are expanded. Thus, the layered ruthenium oxide is infinitely swollen (infinite swelling treatment). As a result, the ruthenium oxide nanosheet (metal compound nanosheet) is peeled off as a single layer, so that the ruthenium oxide nanosheet can be obtained.

(Example using Na-type layered ruthenium oxide)
In the method for producing a metal nanosheet according to Example 1 of the present invention, when Na-type layered ruthenium oxide is used, as shown in FIG. 4, in the layered compound preparation step ST10, first, Ru, RuO ₂ , Na ₂ CO ₃ are added. Weigh in a molar ratio of 1: 3: 2, respectively. Next, Ru and RuO ₂ are pulverized and mixed in an agate mortar, and then Na ₂ CO ₃ is added and pulverized and mixed again. The obtained mixed powder is pelletized using a tablet molding machine and calcined at 700 ° C. for 1 hour in an Ar atmosphere. Next, the pellet is pulverized and mixed in the glove box using an agate mortar. The obtained powder is again pelletized by using a tablet molding machine, and the main fired material at 900 ° C. for 12 hours in an Ar atmosphere is defined as Na-type layered ruthenium oxide (layered compound). Next, sodium persulfate is added to layered ruthenium oxide (sodium type) so that NaRuO ₂ : Na ₂ S ₂ O ₈ = 1: 1 (molar ratio), and 1 g (NaRuO ₂ + Na ₂ S ₂ O _8). ) / Add water at a rate of 200ml to oxidize.

Next, after collecting the powder by suction filtration, 1M hydrochloric acid is added, and acid treatment is performed in a water bath at 60 ° C. for 24 hours. Thereafter, the powder obtained by washing with ion-exchanged distilled water and removing the supernatant is used as Na-derived hydrogen-type layered ruthenium oxide (H _0.2 RuO ₂ .0.5H ₂ O).

Next, in the single layer peeling step ST20, a 10% TBAOH aqueous solution is added to the hydrogen-type layered ruthenium oxide at a ratio of TBA ⁺ / H ⁺ = 1 to 10 and shaken for 10 days, followed by centrifugation (2000 rpm, 30 The supernatant recovered by min) was used as an aqueous dispersion of ruthenium oxide nanosheets (metal compound nanosheets) (infinite swelling treatment).

[Deposition process ST30 by self-organization]
Self-organization method, Langmuir-Blodgett method, electrophoresis method, etc. have been devised as the deposition method of metal compound nanosheets, but it can be easily taken out from the industrial viewpoint with low environmental load. From the viewpoint, it is desirable to adopt the self-organization method. FIG. 5 is an explanatory diagram of a ruthenium oxide nanosheet deposition step ST30 performed by the metal nanosheet manufacturing method according to Example 1 of the present invention. FIG. 6 is an explanatory view showing a state after depositing a ruthenium oxide nanosheet in the method for producing a metal nanosheet according to Example 1 of the present invention, and FIGS. 6 (a) and 6 (b) each show a K induction. Of an AFM (Atomic Force Microscope) after depositing a ruthenium oxide nanosheet of a type, and AFM after depositing a Na-derived ruthenium oxide nanosheet It is explanatory drawing which shows a mode observed by these.

First, three types of substrates, a silicon substrate, a quartz substrate, and a glass substrate, were immersed in HCl / methanol (30 minutes), ultrapure water (30 minutes), and H ₂ SO ₄ / methanol (30 minutes). Wash the substrate with water until it is neutral, and store the substrate in ultrapure water.

  Next, deposition is carried out on the above substrates (silicon substrate, quartz substrate, glass substrate) by self-organization of hydrogen-type ruthenium oxide nanosheets (K-derivative type and Na-derivative type) by the alternate lamination method (LBL method). Step ST30 is performed.

  In this embodiment, as shown in FIGS. 5A and 5B, after the substrate is immersed in an aqueous polycation solution (an aqueous solution of an amphiphilic cationic polymer) to form a positive charge layer, The substrate is washed in three steps, and water drops on the substrate are blown with a nitrogen gun.

  Next, after immersing the substrate in a colloidal solution of metal compound nanosheets (an aqueous dispersion of ruthenium oxide nanosheets), the substrate is washed three times with ultrapure water, and then water droplets on the substrate are blown off with a nitrogen gun.

  As a result, a monolayer film (thin film) of ruthenium oxide nanosheets is formed on the substrate. When producing a ruthenium oxide nanosheet multilayer film (thin film), the above laminating operation is repeated.

  More specifically, a 1 wt% aqueous solution of a copolymer of polyvinylamine 14% and polyvinyl alcohol 86% (hereinafter referred to as a PVA copolymer (polyvinohol) copolymer) is used as a polycation. First, a substrate having a hydrophilic surface (silicon substrate, quartz substrate, glass substrate) is immersed in an aqueous solution of PVA copolymer for 10 minutes to coat the substrate surface with polycation.

  Next, the ruthenium oxide nanosheet is adsorbed on the substrate in a self-organized manner by immersing the substrate in a colloidal solution of ruthenium oxide nanosheets that are anionic to produce a ruthenium oxide nanosheet monolayer film. According to this method, the ruthenium oxide nanosheet can be deposited on the substrate under the condition that the nanosheet coverage is 85% or more.

In the deposition step ST30, the concentration of the ruthenium oxide nanosheet (K-derived type) in the aqueous dispersion is 0.3 g · L ⁻¹ , the pH is 11, and the immersion time is 20 minutes. Under such conditions, as shown in FIG. 6A, the ruthenium oxide nanosheet having a lateral size of submicron to several microns and a thickness of about 1.2 to 1.4 nm is adsorbed on the substrate in the form of a single layer thin film. The

On the other hand, when the ruthenium oxide nanosheet (Na-derived type) is used, in the deposition step ST30, the concentration of the ruthenium oxide nanosheet aqueous dispersion is 0.3 g · L ⁻¹ , the pH is 11, and the immersion time is 20 minutes. Under such conditions, as shown in FIG. 6B, the ruthenium oxide nanosheet having a lateral size of submicron to several microns and a thickness of about 1.2 to 1.5 nm is adsorbed on the substrate in the form of a single layer thin film. The

As described above, in this embodiment, when ruthenium oxide nanosheets (anionic nanosheets) are adsorbed by a self-assembly reaction, the positive charge layer formed on the substrate surface is made of a PVA copolymer, and the PVA copolymer is , Amphiphilic with a cationic part (NH ₂ ⁺ ), a hydrophilic part (OH) and a hydrophobic part (—CH ₂ —CH ₂ —). For this reason, the composite thin film of a nanosheet and a polymer can be formed on a hydrophilic surface such as a quartz substrate surface or a glass substrate surface. Moreover, the composite thin film of a nanosheet and a polymer can also be formed on a hydrophobic surface such as a PET (Polyethylene terephthalate) substrate surface or a PE (Polyethylene) film surface. Further, since a copolymer polymer (PVA copolymer) having a cation part, a hydrophilic part and a hydrophobic part is used, the charge density of the positive charge layer is adjusted by changing the ratio of the cation part, the hydrophilic part and the hydrophobic part. be able to.

[Reduction Step ST40]
FIG. 7 is an explanatory diagram of a reduction step ST40 performed on a ruthenium oxide nanosheet (K-derived type) in the method for producing a metal nanosheet according to Example 1 of the present invention, and FIGS. It is explanatory drawing which shows the X-ray-diffraction pattern of a ruthenium oxide nanosheet before a reduction process, respectively, and explanatory drawing which shows the X-ray-diffraction pattern after performing a reduction | restoration process at various temperatures to a ruthenium oxide nanosheet. FIG. 8 is an explanatory view showing a state in which a metal ruthenium nanosheet obtained by performing a reduction process on a ruthenium oxide nanosheet (K-derived type) is observed by AFM in the method for producing a metal nanosheet according to Example 1 of the present invention. FIGS. 8A and 8B are an explanatory diagram showing a state where a 5 μm square portion of a metal ruthenium nanosheet formed on a substrate is observed, and an explanatory diagram showing a state where a 3 μm square portion is observed. .

  FIG. 9 is an explanatory diagram showing an X-ray diffraction pattern after firing the ruthenium oxide nanosheet (K induction type) in the metal nanosheet manufacturing method according to the reference example of Example 1 of the present invention. 9A and 9B are an explanatory diagram when the ruthenium oxide nanosheet is fired in an oxygen-containing atmosphere, and an explanatory diagram when the ruthenium oxide nanosheet is fired in an inert atmosphere.

  First, the in-plane diffraction pattern (see FIG. 7A) of a single layer film of unfired ruthenium oxide nanosheets (K induction type) could be attributed by a two-dimensional oblique lattice. When the lattice constant was refined, a = 0.5610 (8) nm, b = 0.5121 (6) nm, and γ = 109.4 (2) °.

  Next, the monolayer film of the ruthenium oxide nanosheet was heated to 200, 300, 400, 500, 600, 700, 800, and 900 ° C., respectively, at a temperature rising rate of 5 ° C./min, and held at the reached temperature for 2 hours. After that, it was naturally cooled. At that time, firing was performed in a reducing atmosphere (Example), an oxygen-containing atmosphere (Reference Example), and an inert atmosphere (Reference Example).

(Firing under reducing atmosphere / Example)
First, when a monolayer film of a ruthenium oxide nanosheet (K-derived type) was fired under a hydrogen gas flow as a reducing atmosphere, the diffraction line of the nanosheet disappeared at 200 ° C. as shown in FIG. Two peaks were detected. Since these two peaks can be attributed to (1 0 0) and (1 1 0) of the metal ruthenium (hexagonal crystal a ≈ 0.27 nm), the a axis is oriented in the horizontal direction with respect to the substrate. In other words, it was suggested that metal ruthenium with the c-axis oriented perpendicular to the substrate was generated.

  When the heating temperature was further increased, the diffraction line of c-axis-oriented metal ruthenium became stronger, indicating that the crystallinity was improved. At this time, the diffraction line of (1 0 1) came to be detected, and it was found that a small amount of polycrystalline metal ruthenium was also generated.

  Further, from the shape image of the thin film metallized at 200 ° C. shown in FIGS. 8A and 8B by AFM, it was found that a small sheet-like substance was generated inside the original nanosheet. From the observed image, it was found that the metal ruthenium nanocrystal (metal ruthenium nanosheet) had a sheet size of 50 nm or less and a thickness of about 0.8 nm. This observed thickness is much thinner than that of various conventionally known oxide nanosheets and can be interpreted as reflecting the release of oxygen and metallization.

  The reason why small anisotropic nanocrystals are formed is that a ruthenium oxide nanosheet having a two-dimensional lattice constant of a = 0.5610 (8) nm, b = 0.5121 (6) nm, and γ = 109.4 (2) ° This is presumably because c-axis-oriented metal ruthenium having a hexagonal lattice of ≈0.27 nm is accompanied by severe sheet size shrinkage and breakage.

(Baking in an oxygen-containing atmosphere / Reference example)
On the other hand, when the monolayer film of ruthenium oxide nanosheet (K induction type) was baked under air flow, no change was seen up to 200 ° C. as shown in FIG. In addition to the nanosheet diffraction lines, diffraction lines belonging to polycrystalline rutile RuO ₂ were observed. The intensity of the rutile RuO ₂ diffraction line increased with increasing heating temperature (≦ 700 ° C.), and the nanosheet diffraction line disappeared. Furthermore, when heated to 800 ° C. or higher, the rutile RuO ₂ diffraction line became extremely weak, and when reaching 900 ° C., the diffraction line disappeared completely. This can be considered that volatile RuO ₄ was generated by oxidation in the atmosphere. This result suggests that the nanosheet once undergoes phase transition to polycrystalline rutile-type RuO ₂ during firing in the atmosphere, and then RuO ₂ is oxidized to produce volatile RuO ₄ .

(Baking in an inert atmosphere / reference example)
In addition, when a monolayer film of ruthenium oxide nanosheet (K induction type) was baked under an argon gas flow as an inert atmosphere, no change was seen up to 200 ° C. as shown in FIG. 9B. When heated at 300 ° C., the diffraction lines of the nanosheet almost disappeared. Furthermore, when heated above 400 ° C, diffraction lines attributed to polycrystalline rutile RuO ₂ were observed, and the intensity increased until the temperature reached 800 ° C. When the heating temperature reached 900 ° C., a diffraction pattern attributed to polycrystalline metal ruthenium was observed. In other words, it was found that the nanosheet once transformed into polycrystalline rutile-type RuO ₂ , and then RuO ₂ was reduced to produce polycrystalline metal ruthenium.

[Main effects of Example 1]
As described above, the reduction temperature of the ruthenium oxide nanosheet in Example 1 is very low compared to 900 ° C. in the case of an inert atmosphere, and is interesting in that the produced metal ruthenium is c-axis oriented. Titanium oxide nanosheet monolayer film in which atoms are constrained in two dimensions has the stability of maintaining the structure up to 800 ° C when heated in the atmosphere, and also generates c-axis oriented anatase when heated above 900 ° C It is known to show a good crystal growth. This is explained by atomic thermal diffusion.

  Compared to such a case, since the temperature at which the ruthenium oxide nanosheet undergoes structural change in any atmosphere is extremely low, 200 ° C. to 300 ° C., it can be considered that the diffusion of atoms due to heat is small. For this reason, it is considered that metal nanosheets reflecting the original sheet structure or layered structure are generated without significant movement of atoms even when the sheet size shrinks or breaks due to the release of oxygen. At the same time, this shows that even the starting material of the same element type can change the lateral size of the resulting metal nanosheet due to the difference in its two-dimensional structure.

  In this example, low-temperature reduction firing was performed on ruthenium oxide, but by softening a nanosheet of oxide, hydroxide, sulfide, etc. under soft conditions using a chemical reducing agent, It is thought that a topological reaction can be induced and a new metal nanosheet can be produced. In addition, if a nanosheet that can only be made of hydroxide is metallized to produce a metal nanosheet, then the metal nanosheet is re-oxidized, whereby a new oxide nanosheet can be synthesized.

[Reduction Step ST40 for Ruthenium Oxide Nanosheet (Na-Induced Type)]
FIG. 10 is an explanatory diagram of a reduction process performed on a ruthenium oxide nanosheet (Na-derived type) in the method for producing a metal nanosheet according to Example 1 of the present invention, and FIGS. 10 (a), 10 (b), and 10 (c). ) Is an explanatory view showing an X-ray diffraction pattern of ruthenium oxide before the reduction step, an explanatory view showing a state where a 5 μm square portion of the metal ruthenium nanosheet obtained by performing the reduction step on ruthenium oxide is observed by AFM, and It is explanatory drawing which shows a mode that the part of 3 micrometers square was observed by AFM.

  In the above embodiment, the reduction step ST40 for the K-derived ruthenium oxide nanosheet has been described, but substantially the same results can be obtained even when the reduction step ST40 is performed on the Na-derived ruthenium oxide nanosheet. When firing a monolayer film of Na-derived ruthenium oxide nanosheets, it is heated to 200, 300, 400, 500, 600, 700, 800, and 900 ° C, respectively, at a heating rate of 5 ° C / min in a hydrogen gas atmosphere. The mixture was kept at the reached temperature for 2 hours and then naturally cooled.

  First, the in-plane diffraction pattern of the unbaked ruthenium oxide nanosheet (Na-derived type) single-layer film was as shown in FIG. 10A, and could be attributed to a two-dimensional hexagonal lattice. When the lattice constant was refined, a = 0.929 (6) nm was obtained. This was baked under a hydrogen gas flow serving as a reducing atmosphere, and the results of AFM observation are shown in FIGS. 10 (b) and 10 (c).

  As shown in FIGS. 10B and 10C, it is suggested that after firing, a metal ruthenium nanosheet having a thickness of about 0.8 nm maintaining the original shape of the nanosheet is formed. This decrease in thickness is also observed when a single-layer film of a K-induced ruthenium oxide nanosheet is heated, and it can be considered that oxygen is released by reduction.

  Here, when starting from a single-layer film of Na-derived ruthenium oxide nanosheets compared to a case of starting from a single-layer film of K-derived ruthenium oxide nanosheets, significant shrinkage / breakage within the nanosheets occurs. Not seen. This is because the two-dimensional hexagonal lattice of Na-derived ruthenium oxide nanosheets is relatively close to the a-axis lattice constant of metal ruthenium (a≈0.27 nm), so topological metallization (production of μm metal nanosheets) ) Was caused.

<Example 2>
As Example 2 of the present invention, a method for producing a metal ruthenium nanosheet (metal nanosheet) by the method according to Embodiment 1 described with reference to FIG. 1A will be described with reference to FIG. FIG. 11 is an explanatory diagram of a reduction process performed in the method for producing a metal nanosheet according to Example 2 of the present invention, and FIGS. 11A and 11B are each an X-ray diffraction pattern of the metal ruthenium nanosheet after the reduction process. It is explanatory drawing which shows a pattern, and the explanatory view which shows a mode that the metal ruthenium nanosheet was observed by AFM. In this example as well, the layered compound preparation step ST10 and the single layer peeling step ST20 are the same as those in Example 1, and thus the description thereof is omitted.

  In this example, a colloidal solution of K-derived ruthenium oxide nanosheets was dropped on glass and dried to produce a multi-layered ruthenium oxide nanosheet restacked thin film of oriented ruthenium oxide nanosheets on the substrate. . When this was baked at 200 ° C. under a hydrogen gas flow serving as a reducing atmosphere, a thin film sample having a metallic luster was produced (reducing step ST40).

  In the XRD pattern of the thin film sample after firing shown in FIG. 11A, a glass halo used as a substrate and a single peak at around 42 ° were observed. The 42 ° peak corresponds to (0 0 2) of the metal ruthenium, suggesting that the c-axis is oriented in the normal direction of the substrate. When SEM observation of this c-axis oriented metal ruthenium thin film sample was performed, as shown in FIG. 11B, a large number of nanoscale-thick plate materials (metal ruthenium sheets) were observed. As described above, it is difficult to estimate the exact thickness of a single metal ruthenium plate produced by SEM observation, but the thickness was about several tens of nanometers when estimated directly from the observation field of view.

  As described above, from these results, ruthenium oxide nanosheets obtained by dropping a colloidal solution of ruthenium oxide nanosheets onto glass and drying can also become oriented metal ruthenium sheets layered when reduced at low temperatures. It became clear.

<Example 3>
As Example 3 of the present invention, a method for producing a metal ruthenium nanosheet (metal nanosheet) by the method according to Embodiment 2 described with reference to FIG. 2 will be described with reference to FIG. FIG. 12 is an explanatory diagram of a reduction process performed in the method for producing a metal nanosheet according to Example 3 of the present invention. FIGS. 12A and 12B are each a layered compound (layered ruthenium oxide) after the reduction process. It is explanatory drawing which shows X-ray diffraction pattern of this, and explanatory drawing which shows a mode that the metal ruthenium nanosheet structure obtained by performing a reduction | restoration process to a layered compound was observed with AFM. Also in this example, the layered compound preparation step ST10 is the same as that in Example 1, and therefore the description thereof is omitted.

  In this example, a layered compound of a hydrogen-type ruthenium oxide nanosheet (K-derived type) is fired at 200 ° C. under a hydrogen gas flow serving as a reducing atmosphere (reducing step ST40). As a result, a powder sample having a metallic luster was produced. The XRD pattern of the fired sample shown in FIG. 12A was attributed to a single phase of metal ruthenium.

  When SEM observation of this powder was performed (see FIG. 12B), plate-like ruthenium (metal ruthenium nanosheet structure) reflecting the original layered structure was observed. SEM observations suggest that the metal ruthenium nanosheet structure has a thickness of about several tens to several hundreds of nm.

  From these results, it became clear that a layered metal ruthenium sheet can also be obtained by reducing a layered compound of a hydrogen-type ruthenium oxide nanosheet (K-derived type) at a low temperature of about 200 ° C.

  In addition, with respect to the metal ruthenium nanosheet structure, when ultrasonic waves are applied in the liquid, water molecules (solvent molecules) are inserted into each layer of the metal ruthenium nanosheet structure. As a result, the metal ruthenium nanosheet structure has a single layer of ruthenium. A nanosheet will be manufactured (single layer peeling process ST50).

(Other examples)
In the above embodiment, the case where a metal ruthenium oxide nanosheet is used as a metal compound nanosheet to obtain a metal ruthenium nanosheet has been described. However, titanium oxide, niobium oxide, vanadium oxide, manganese oxide, cobalt oxide, and molybdenum oxide. The present invention may be applied to obtain metal nanosheets from other metal oxide nanosheets. In addition, the present invention may be applied to obtain metal nanosheets from layered compounds of metal compounds such as metal sulfides and metal hydroxides, not limited to metal oxide nanosheets. The present invention may be applied to obtain metal nanosheets from the above.

ST10 Layered compound preparation step ST20, ST50 Single layer peeling step ST30 Deposition step ST40 Reduction step

Claims (10)

at least,
A layered compound preparation step of preparing a layered compound in which the metal compound is layered;
The metal compound is reduced via a topotactic structural phase transition under the condition of suppressing atomic diffusion, the layered compound is used as a precursor, and the thickness is 1 nm or less reflecting the layered structure of the layered compound. A reduction step of obtaining a metal nanosheet having a dimension in the sheet plane direction of 30 nm or more;
I have a,
The metal compound is a sulfide, oxide, or hydroxide of any one of ruthenium, titanium, niobium, vanadium, manganese, cobalt, and molybdenum .   2. The metal nanosheet production according to claim 1, wherein in the reduction step, the metal compound is reduced in a state of a metal compound nanosheet obtained by exfoliating the layered compound or a state of the layered compound. Method. After performing the layered compound preparation step, before the reduction step, perform a single layer peeling step to peel the layered compound to obtain the metal compound nanosheet,
The method for producing a metal nanosheet according to claim 2, wherein the metal compound is reduced in the state of the metal compound nanosheet in the reduction step. After the single layer peeling step, before the reduction step, perform a deposition step of depositing the metal compound nanosheet to form a thin film,
The method for producing a metal nanosheet according to claim 3, wherein, in the reduction step, the metal compound is reduced in a state of a thin film of the metal compound nanosheet.   3. The method for producing a metal nanosheet according to claim 2, wherein in the reduction step, the metal compound is reduced in a state of the layered compound to obtain a metal nanosheet structure in which a plurality of the metal nanosheets are laminated. The method for producing a metal nanosheet according to claim 5, wherein after the reduction step, a single-layer peeling step of peeling the metal nanosheet structure into a single-layer metal nanosheet is performed.   The method for producing a metal nanosheet according to any one of claims 1 to 6, wherein in the reduction step, a firing treatment is performed in a reducing atmosphere.   The method for producing a metal nanosheet according to any one of claims 1 to 6, wherein in the reduction step, wet processing using a reducing solution is performed. The metal compounds, sulfides ruthenium, method for producing a metal nanosheet according to any one of claims 1 to 8, characterized in that an oxide or hydroxide. A metal nanosheet comprising a reduction product of a metal compound constituting the layered compound used as a precursor,
The metal compound is a sulfide, oxide or hydroxide of any one of ruthenium, titanium, niobium, vanadium, manganese, cobalt, and molybdenum;
The metal compound is reduced via a topological structural phase transition under conditions that suppress atomic diffusion, thereby reflecting the layered structure of the layered compound and having a thickness of 1 nm or less and a sheet plane. A metal nanosheet having a dimension in a direction of 30 nm or more.