Method for synthesizing non-layered colloidal single-crystalline oxide nanosheet

Abstract

The present invention addresses the problem of newly providing a method for synthesizing a non-layered colloidal single-crystalline oxide nanosheet. Provided is a method for synthesizing a non-layered colloidal single-crystalline oxide nanosheet.

Description

Synthesis of non-layered colloidal single-crystalline oxide nanosheets

[0001] The present invention relates to a method for synthesizing non-layered colloidal single-crystalline oxide nanosheets.

[0002] Non-Patent Document 1 discloses the self-assembly of two-dimensional (2D) transition metal oxide nanosheets.

[0003] NATURE COMMUNICATIONS | 5:3813 | DOI: 10.1038 / ncomms4813 |www.nature.com / naturecommunications

[0004] An object of the present invention is to provide a novel method for synthesizing non-layered colloidal single-crystalline oxide nanosheets.

[0005] The present inventors have conducted extensive research to achieve the above object.

[0006] That is, the present invention relates to the following method for synthesizing a non-layered colloidal single-crystalline oxide nanosheet.

[0007] The present invention encompasses the following non-layered colloidal single-crystalline oxide nanosheets:

[0008] Item 1. An oxide nanosheet, wherein the oxide is a compound having a crystalline structure that does not have a layered structure, and the nanosheet is a single crystal.

[0009] Item 2. The oxide nanosheet according to Item 1, wherein the oxide is a simple oxide.

[0010] Item 3. The oxide nanosheet according to Item 1, wherein the oxide is an oxide of a divalent, trivalent, and / or tetravalent metal.

[0011] Item 4. The oxide nanosheet according to Item 1, wherein the oxide is an oxide that can be synthesized from metal ions that can enter between the layers of the anionic surfactant when forming an anionic surfactant-oxide nanosheet complex.

[0012] Item 5. The oxide nanosheet according to Item 1, wherein the oxide nanosheet is free-standing.

[0013] Item 6. The oxide nanosheet according to Item 1, wherein the oxide nanosheet has a thickness of approximately 1 nm or less.

[0014] Item 7. The oxide nanosheet according to Item 1, wherein the oxide nanosheet has a lateral size of 100 nm to 5 μm.

[0015] Item 8. The oxide nanosheet has a theoretical specific surface area of ​​100 m 2 / g~500m 2 3. The oxide nanosheet according to item 1, wherein the surface area of ​​the oxide nanosheet is 1 / g.

[0016] Item 9. A colloidal solution comprising the oxide nanosheet according to Item 1, wherein the oxide nanosheet exists individually as a colloidal dispersion in a solvent.

[0017] Item 10. A method for producing an oxide nanosheet, comprising: (1) mixing anionic surfactant (anionic surfactant)-metal ion complex crystals and a salt composed of a chaotropic anion and a monovalent or divalent cation that does not react with water to hydrolyze in water containing a trace amount of an amide solvent to produce an anionic surfactant-oxide nanosheet complex; and (2) removing the surfactant from the anionic surfactant-oxide nanosheet complex obtained in step (1).

[0018] Item 11. The method for producing an oxide nanosheet according to Item 10, wherein the step (1) is a step of mixing anionic surfactant (anionic surfactant)-metal ion complex crystals, a chaotropic anion, and a salt composed of a monovalent or divalent cation that does not react with water to hydrolyze in water containing a trace amount of an amide solvent, and producing an anionic surfactant-oxide nanosheet complex by hydrothermal synthesis.

[0019] Item 12. The method for producing an oxide nanosheet according to Item 10, wherein the method for producing an oxide nanosheet comprises obtaining a colloidal solution of an oxide nanosheet.

[0020] The present invention can provide a novel method for synthesizing non-layered colloidal single-crystalline oxide nanosheets.

[0021] FIG. 1 shows an example of a synthesis technique for the non-layered colloidal single-crystalline oxide nanosheet of the present invention. In the synthesis method for the non-layered colloidal single-crystalline oxide nanosheet, a solid phase of a surfactant (L β The oxide nanosheets are synthesized using the cerium oxide phase. First, (1) metal salts (raw materials for oxides (e.g., cerium (III)), such as Ce2(SO4)3·nH2O and Ce(NO3)3·6H2O) are mixed with an anionic surfactant (e.g., SOS) to synthesize surfactant-cerium complex crystals. Next, (2) surfactant-cerium complex crystals and chaotropic anions (NO3 - etc.) and monovalent cations (NH4 +(e.g., NH4NO3) and a salt (e.g., NH4NO3) are mixed and hydrothermally synthesized to produce an anionic surfactant-oxide nanosheet composite. (3) Next, the anionic surfactant-oxide nanosheet composite is sonicated in a solvent to exfoliate it, producing a colloidal solution of oxide nanosheets (e.g., cerium(IV) oxide (CeO2)). Figure 2 shows low-accelerating-voltage atomic image observation (1) (SEM image) of ceria nanosheets (non-layered colloidal single-crystalline oxide nanosheets) synthesized in this invention. Ceria nanosheets with exposed 111 planes and a fluorite structure were formed. Figure 3 shows low-accelerating-voltage atomic image observation (2) (SEM image) of ceria nanosheets (non-layered colloidal single-crystalline oxide nanosheets) synthesized in this invention. The contrast matched the thickness (0.95 nm) of the ceria. A ceria nanosheet containing four cerium layers was formed. Figure 4 shows the stability evaluation (AFM image) of the ceria nanosheet (non-layered colloidal single-crystalline oxide nanosheet) synthesized in the present invention. After calcination, the ceria nanosheet was confirmed to have thermal stability that contributes to ionic conductivity measurement. Figure 5 shows the temperature-variable environment control (1) (Raman spectroscopy measurement) of the ceria nanosheet (non-layered colloidal single-crystalline oxide nanosheet) synthesized in the present invention. The formation of ceria with exposed 111 planes, which have many surface defects, was confirmed in the ceria nanosheet. Figure 6 shows the temperature-variable environment control (2) (Raman spectroscopy measurement) of the ceria nanosheet (non-layered colloidal single-crystalline oxide nanosheet) synthesized in the present invention. At medium to low temperatures, the ceria nanosheet exhibits Ce. 3+It was confirmed that the surface defects due to the formation of ions were maintained. Figure 7 shows the introduction of multiple metal species into surfactant crystals in the ceria nanosheet (non-layered colloidal single-crystalline oxide nanosheet) synthesized in this invention. (1) Precursors: Combination of Ce(NO3)3·6H2O and Gd(NO3)3·nH2O (gadolinium(III) nitrate n-hydrate) Combination of Ce(NO3)3·6H2O and Sm(NO3)3·6H2O (samarium nitrate hexahydrate) (2) Synthesis of Intermediates: In N-methylformamide (NMF), ammonium nitrate (NH4NO3) was added, and in the presence of sodium octadecylsulfonate (SOS), (i) a combination of Ce(NO3)3·6H2O and Ga(NO3)3·6H2O, or (ii) a combination of Ce(NO3)3·6H2O and Sm(NO3)3·6H2O was used for hydrothermal synthesis to produce the intermediates (anionic surfactant-oxide nanosheet composites). (Ce 3+ , and Gd 3+ , or Sm 3+ ) : SOS = 14:1 (weight ratio) (3) Colloidal dispersion Dispersed in NMF to produce oxide nanosheets. 3+ and Gd 3+ The surfactant crystals containing Ce were obtained (ICP emission spectroscopy). 3+ and Sm 3+We obtained surfactant crystals incorporating Sm-doped ceria (ICP emission spectroscopy). Figure 8 shows the surfactant-ceria composite (Sm-doped) (1) synthesized by the present invention (TEM image, SAED pattern). The surfactant-ceria composite (Sm-doped) confirmed the synthesis of single-crystalline ceria nanosheets. Figure 9 shows the surfactant-ceria composite (Sm-doped) (2) synthesized by the present invention (TEM image, HR-TEM image). The surfactant-ceria composite (Sm-doped) showed a porous structure, confirming the retention of single crystallinity. Figure 10 shows the surfactant-ceria composite (Sm-doped) (3) synthesized by the present invention (TEM image, EDS spectrum). Comparing the peak intensities at 5.29 keV and 5.65 keV for the surfactant-ceria composite (Sm-doped), we confirmed the intensity ratio of 6:4. The synthesis of Sm-doped ceria nanosheets was confirmed. Figure 11 shows an example of a free-standing film of oxide nanosheets of ceria (CeO2), one of the oxide nanosheets of the present invention. Ceria (CeO2) oxide nanosheets exhibit excellent oxide ion conductivity and are useful as oxide ion conductors when constructed as free-standing films (in the stacking direction). Free-standing nanosheet laminate films were constructed from colloids containing ceria (CeO2) oxide nanosheets by suction filtration. Figure 12 shows the ionic conductivity of single-crystalline ceria nanosheets of the present invention. Nanosheets exhibit ionic conductivity three to four orders of magnitude higher than bulk nanosheets. The oxide ion conductivity of ceria, an oxide, dramatically improves upon nanosheet formation. Figure 13 shows the application of nanosheet synthesis using surfactant crystals of the present invention as a template. We successfully synthesized surfactant crystals (octadecyl sulfate and octadecyl sulfonate) using divalent and trivalent metal ions. Figure 14 shows the application of nanosheet synthesis using solid surfactants of the present invention as a template. By applying the oxide nanosheet manufacturing method of the present invention and using a solid surfactant as a template, we were able to synthesize nanosheets of a variety of non-layered oxides and hydroxides, such as yttrium hydroxide (Y(OH)3), gallium oxyhydroxide (GaOOH), and gallium(III) oxide (γGa2O3).By applying the method for producing oxide nanosheets of the present invention, it was possible to synthesize nanosheets of various non-layered oxides and hydroxides containing elements such as Fe, Ga, Bi, Al, Zn, In, Y, Sn, Sc, and Gd.

[0022] The present invention will be described in detail below.

[0023] In this specification, the term "containing" is a concept that encompasses all of "comprise," "consist essentially of," and "consist only of."

[0024] In this specification, when a numerical range is expressed as "A to B," it means "not less than A and not more than B."

[0025] [1] Oxide nanosheet synthesis technology (Figure 1) Nanosheets are a trend in the creation of two-dimensional materials. Nanosheets are expected to have unique properties due to their two-dimensional structure, and are next-generation materials that will exhibit new functions through precision assembly, such as two-dimensional monolayer assembly and superlattice formation. Nanosheet colloids (colloidal nanosheets) are important for the development of nanosheet applications, such as catalysts, electrodes, and dielectric materials, through precision assembly.

[0026] Conventionally, nanosheets have been synthesized by exfoliation from layered materials or by bottom-up synthesis of non-layered inorganic compounds. Conventionally, non-layered single-crystalline nanosheets have been synthesized on a substrate. Conventionally, non-layered single-crystalline nanosheets have not been obtained as independently dispersed colloids in a solvent.

[0027] The present invention encompasses oxide nanosheets of single-crystalline non-layered compounds and their synthesis methods. Ceria-based non-layered compounds are expected to be used in fuel cells, exhaust gas catalysts, and reforming catalysts, and are required to have high reactivity, long-term stability, high strength, reliability, and reduced manufacturing costs. According to the present invention, colloidal oxide nanosheets of single-crystalline non-layered compounds can be easily synthesized by using a solid anionic surfactant as a template and synthesizing the oxide nanosheet between its layers.

[0028] In the method for producing oxide nanosheets of the present invention, preferably, (pretreatment step) a metal salt (raw material for oxide (e.g., cerium (III)), such as Ce2(SO4)3·nH2O or Ce(NO3)3·6H2O) is mixed with an anionic surfactant (e.g., SOS) to synthesize surfactant-cerium composite crystals.

[0029] In the method for producing oxide nanosheets of the present invention, next, (1) surfactant-cerium complex crystals and chaotropic anions (NO3 - etc.) and monovalent cations (NH4 + The anionic surfactant-oxide nanosheet composite is produced by mixing the anionic surfactant with a salt (NH4NO3, etc.) and carrying out hydrothermal synthesis.

[0030] In the method for producing an oxide nanosheet of the present invention, next, (2) the obtained anionic surfactant-oxide nanosheet composite is exfoliated to produce an oxide nanosheet.

[0031] The oxide of the oxide nanosheet obtained through the above process is a compound without a layered structure. The nanosheet of the oxide nanosheet is a single crystal. The oxide nanosheet exists individually as a colloidal dispersion in the solvent.

[0032] Conventionally, nanosheets have been synthesized using soft aggregates such as micelles (surfactant liquid crystal phases) as templates, but it has not been possible to synthesize single-crystalline nanosheets with a thickness of 1 nm.

[0033] The present invention enables the synthesis of oxide nanosheets between layers of a solid anionic surfactant (such as SOS) as a template. The oxide nanosheets of the present invention are extremely high-quality colloidal oxide nanosheets.

[0034] The method for producing oxide nanosheets of the present invention enables the formation of free-standing ultrathin films with a thickness of approximately 1 nm by solution processing. The present invention utilizes a solid anionic surfactant as a stronger template, and synthesizes nanosheets between the surfactant layers, thereby enabling the synthesis of colloidal single-crystal oxide nanosheets.

[0035] In the present invention, oxide nanosheets can be synthesized by hydrothermal synthesis (hydrothermal treatment) under conditions in which a chaotropic salt (a salt composed of a chaotropic anion and a monovalent cation) is present (preferably at a high concentration), as conditions for precipitating a single-crystalline material between the surfactant layers.

[0036] The method for producing oxide nanosheets of the present invention is highly reproducible, has a very wide process window, and is useful for industrial processes.

[0037] FIG. 1 shows the process for producing an oxide nanosheet according to the present invention, in which an anionic surfactant (L β This figure shows an example of a method for synthesizing a non-layered colloidal single-crystalline oxide nanosheet (single-crystalline ceria) using a single-crystalline oxide nanosheet (a single-crystalline ceria phase). The sheet-like material in Figure 1 is the single-crystalline nanosheet of the present invention.

[0038] The anionic surfactant (SOS, etc.) forms a solid phase (L β The present invention provides a solid phase (L β By utilizing the nano-layer structure, it is possible to synthesize non-layered colloidal single-crystalline oxide nanosheets (such as single-crystalline ceria).

[0039] First, an anionic surfactant (such as SOS)-metal ion (such as cerium) complex is formed.

[0040] Next, the metal ions (such as cerium) undergo hydrolysis and polycondensation, forming layers of several atoms (nanosheets with a thickness of approximately 1 nm or less) between the layers of the anionic surfactant, thereby synthesizing a surfactant-nanosheet composite.

[0041] Next, the resulting surfactant-nanosheet composite (surfactant-nanosheet composite) is placed in an amide solvent (such as NMF) and subjected to ultrasonic exfoliation, yielding a colloidal solution of non-layered colloidal single-crystalline oxide nanosheets (single-crystalline ceria).

[0042] The present invention makes it possible to prepare a colloidal solution in which non-layered colloidal single-crystalline oxide nanosheets exist as individual, independent dispersions.

[0043] Furthermore, nanosheets can be assembled to form free-standing films several tens of micrometers thick, which can be used as oxide-ion conductors.

[0044] The crystal structure and single crystallinity of the oxide nanosheet of the present invention can be confirmed by image observation using a TEM. The oxide nanosheet produced by the method for producing an oxide nanosheet of the present invention can be confirmed by combining image observation using a transmission electron microscope (TEM), electron diffraction, and thickness measurement using an atomic force microscope (AFM).

[0045] (Pretreatment step) A step of mixing a metal salt and an anionic surfactant to synthesize surfactant-metal ion complex crystals In the method for producing oxide nanosheets of the present invention, preferably, first, a metal salt (raw material for oxide (cerium (III), etc.), Ce2(SO4)3.nH2O, Ce(NO3)3.6H2O, etc.) and an anionic surfactant (SOS, etc.) are mixed to synthesize a surfactant-metal ion complex.

[0046] In the method for producing oxide nanosheets of the present invention, it is possible to directly synthesize a surfactant-metal ion complex by mixing a metal salt, a surfactant, and a chaotropic anion in the next synthesis step without going through this pretreatment step.

[0047] Step (1): A process for producing an anionic surfactant-oxide nanosheet complex by mixing a surfactant-metal ion complex crystal and a salt consisting of a chaotropic anion and a monovalent cation in water containing a trace amount of an amide solvent (e.g., NMF). The process for producing oxide nanosheets (non-layered colloidal single-crystalline oxide nanosheets) involves the following steps: (1) First, in water containing a trace amount of an amide solvent (e.g., NMF), a surfactant-metal ion complex crystal and a salt consisting of a chaotropic anion (NO3 - etc.) and monovalent cations (NH4 +The process includes a step of mixing an anionic surfactant with a salt (NH4NO3, etc.) to produce an anionic surfactant-oxide nanosheet (CeO2, etc.) composite.

[0048] In step (1), a composite of a layered anionic surfactant and an oxide (anionic surfactant-oxide nanosheet composite) is prepared.

[0049] The metal ions are hydrolyzed and polycondensed in water containing a small amount of an amide solvent (such as NMF) to form oxide nanosheets.

[0050] The solid phase of anionic surfactants (L β The anionic surfactant-oxide nanosheet composite (phase) is the part that serves as a template for the anionic surfactant-oxide nanosheet composite. The hydrophilic part of the anionic surfactant is located on the oxide nanosheet side of the anionic surfactant-oxide nanosheet composite. The hydrophobic part of the anionic surfactant is located inside the template for the anionic surfactant-oxide nanosheet composite.

[0051] The method for producing oxide nanosheets of the present invention is extremely simple and has a wide process window (the range of processing conditions allowed for achieving the purpose of the process). In the method for producing oxide nanosheets of the present invention, the surfactant used as a template can be reused, and hydrothermal synthesis is a low-temperature synthesis, so it is superior to conventional techniques in terms of process cost and process time.

[0052] (1-1) Amide-based solvents: Oxide nanosheets can be formed by using a trace amount of an amide-based solvent, such as N,N-dimethylformamide (DMF), formamide, N-methylformamide (NMF), or formaldehyde, preferably NMF.

[0053] The amide solvent may be used alone or in the form of a mixture (blend) of two or more kinds.

[0054] (1-2) Metal Salts Metal salts are salts that constitute metal ions that are the raw materials for surfactant-metal ion complex crystals.

[0055] The metal constituting the metal salt is preferably a divalent and / or trivalent metal, since it can exist stably in water.

[0056] The metal constituting the metal salt is more preferably a divalent, trivalent, and / or tetravalent metal, and more preferably magnesium (Mg), calcium (Ca), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), aluminum (Al), gallium (Ga), indium (In), cadmium (Cd), rhodium (Rh), yttrium (Y), lanthanum (La), cerium (Ce), or gadolinium (Gd) is used.

[0057] The metal ions constituting the metal salt are preferably metals that become simple oxides when they are converted into oxide nanosheets.

[0058] The metal ions constituting the metal salt constitute an oxide that can easily enter between the layers of the anionic surfactant when forming a complex of anionic surfactant and oxide nanosheets.

[0059] The metals constituting the metal salt may be used singly or in the form of a mixture (blend) of two or more metals.

[0060] The metal salts preferably used include various nitrates, sulfates, hydrochlorides, etc., such as cerium (III) nitrate hexahydrate (Ce(NO3)3.6H2O) and cerium (III) sulfate n-hydrate (Ce2(SO4)3.nH2O).

[0061] The metal salt may be used alone or in the form of a mixture (blend) of two or more kinds.

[0062] (1-3) Anionic surfactants (anionic surfactants) Surfactants Anionic surfactants form a solid phase (L βThe anionic surfactant preferably used is sodium dodecyl sulfate (NaDS, SDS), sodium octyl sulfate, sodium decyl sulfate, sodium hexadecyl sulfate, sodium stearyl sulfate, sodium octadecyl sulfate (SOS), sodium octadecyl sulfonate, or the like, and more preferably SOS.

[0063] The surfactant may be used alone or in the form of a mixture (blend) of two or more of these surfactants.

[0064] (1-4) Salts consisting of chaotropic anions and monovalent cations In the Hofmeister series, the thiocyanate ion (SCN) has a low ability to structure water (low Hofmeister order). - ), iodide ion (I - ), nitrate ions (NO3 - Chaotropic anions such as guanidinium and guanidium increase the disorder of water and can extend the structure of anionic surfactants through hydrophobic interactions. Chaotropic molecules are the molecules that dissolve in the anionic surfactant (solid phase (L β It is possible to control the swelling behavior of the polymer phase and shrink the space between the layers.

[0065] By using a salt consisting of a chaotropic anion and a monovalent cation, the anionic surfactant can be stably present in water containing a trace amount of an amide solvent, and the solid phase (L β phase) can be formed.

[0066] The chaotropic anion is preferably bromide (Br - ), nitrate ions (NO3 - ), perchlorate ion (ClO4 - ), iodide ion (I - ), thiocyanate ion (SCN - ) etc. are used.

[0067] The monovalent cation is preferably an ammonium ion (NH + ), potassium ions (K+ ), cesium ions (Cs + ), sodium ions (Na + ), guanidinium ion (((NH2)2C=NH2) + ) etc. are used.

[0068] Salts consisting of chaotropic anions and monovalent cations are preferably ammonium nitrate (NH4NO3), ammonium iodide (NH4I), ammonium bromide (NH4Br), ammonium thiocyanate (NH4SCN), sodium nitrate (NaNO3), potassium nitrate (KNO3), cesium nitrate (CsNO3), potassium thiocyanate (KSCN), cesium thiocyanate (CsSCN), guanidinium thiocyanate ((NH2)2C=NH2)SCN), cesium iodide (CsI), etc.

[0069] The amount of salt consisting of a chaotropic anion and a monovalent cation used is adjusted to a concentration (weight % (wt%)) in water containing a trace amount of an amide solvent of preferably 10 wt% to 20 wt%, more preferably 12 wt% to 18 wt%, and even more preferably 13 wt% to 15 wt% (particularly preferably about 14 wt%), and the reaction is allowed to proceed.

[0070] (1-5) Hydrothermal Synthesis In step (1), a metal salt, an anionic surfactant, and a salt consisting of a chaotropic anion and a monovalent cation are mixed in water preferably containing a trace amount of an amide solvent, and an anionic surfactant-oxide nanosheet composite is produced by hydrothermal synthesis.

[0071] The hydrothermal synthesis is preferably carried out by placing the reaction solution in a pressure-resistant vessel (a sealed heat-resistant vessel such as an autoclave) under self-generated pressure or gas pressure, with stirring or in a static state.

[0072] The reaction temperature for hydrothermal synthesis is preferably about 80°C to 180°C, more preferably about 90°C to 150°C, and even more preferably about 120°C, in order to increase the reaction rate.

[0073] The reaction time for the hydrothermal synthesis is preferably about 1 to 7 days, more preferably about 1 to 5 days, and even more preferably about 2 days.

[0074] The hydrothermal synthesis is preferably carried out using a sealed heat-resistant and pressure-resistant vessel such as an autoclave under pressure at about 120° C. for about 2 days.

[0075] Preferred embodiment of reaction solution (weight ratio) In step (1), Ce(NO3)3.6H2O (metal salt), SOS (anionic surfactant) (forming a mixture of metal salt and anionic surfactant), more preferably surfactant-metal ion complex crystals, are mixed with NH4NO3 (a salt composed of a chaotropic anion and a monovalent cation) in water preferably containing a trace amount of NMF (amide solvent), and the mixture is pressurized in a sealed heat-resistant and pressure-resistant container such as an autoclave at about 120°C for about 2 days to produce an anionic surfactant-oxide nanosheet complex by hydrothermal synthesis.

[0076] The reaction solution is a mixture of a metal salt (such as Ce(NO3)3·6H2O) and an anionic surfactant (such as SOS), preferably surfactant-metal ion complex crystals, a salt composed of a chaotropic anion and a monovalent cation (such as NH4NO3), water, and an amide solvent (such as NMF) adjusted in the following preferred weight ratio (surfactant-metal ion complex crystals: salt composed of a chaotropic anion and a monovalent cation: water: amide solvent), and by proceeding with the reaction, an anionic surfactant-oxide nanosheet complex can be successfully synthesized.

[0077]

[0078] Step (2): Following step (1), the method for producing oxide nanosheets (non-layered colloidal single-crystalline oxide nanosheets) next involves exfoliating the anionic surfactant-oxide nanosheet composite obtained in step (1). By exfoliating the anionic surfactant-oxide nanosheet composite, a colloidal solution (dispersion) of oxide nanosheets can be obtained.

[0079] The oxide of the oxide nanosheet obtained through step (2) is a compound that does not have a layered structure.

[0080] The oxide nanosheet obtained through step (2) is a single crystal.

[0081] The oxide nanosheets obtained through step (2) exist as a colloidal dispersion in the solvent.

[0082] In step (2), the anionic surfactant-oxide nanosheet composite is subjected to ultrasonic treatment, preferably in an amide solvent, following step (1), to exfoliate the anionic surfactant-oxide nanosheet composite.

[0083] The solvent used in step (2) may be the same as the solvent used in step (1).

[0084] The use of an amide solvent weakens the bond between the anionic surfactant and the metal ion, allowing for the successful formation of oxide nanosheets. The amide solvent used following step (1) is preferably N,N-dimethylformamide (DMF), formamide, N-methylformamide (NMF), formaldehyde, or the like, more preferably N-methylformamide (NMF).

[0085] The amide solvent may be used alone or in the form of a mixture (blend) of two or more of these solvents.

[0086] (2-1) Ultrasonic Treatment In step (2), the anionic surfactant-oxide nanosheet composite (amide solvent) obtained in step (1) is preferably subjected to ultrasonic treatment for approximately 30 minutes. Furthermore, preferably, the colloidal dispersion after peeling is recovered by centrifugation, and an amide solvent is added again before ultrasonic treatment. The method for producing oxide nanosheets of the present invention can produce oxide nanosheets with a thickness of approximately 1 nm or less by ultrasonic peeling in an amide solvent (e.g., NMF).

[0087] (2-2) Colloidal Solution of Oxide Nanosheets The method for producing oxide nanosheets is preferably to obtain a colloidal solution of oxide nanosheets.

[0088] The method for producing oxide nanosheets can prepare a colloidal solution (dispersion) of oxide nanosheets either by removing the anionic surfactant from the anionic surfactant-oxide nanosheet composite or by peeling off the oxide nanosheets.

[0089] A dispersion liquid containing exfoliated layers (oxide nanosheets) each having a thickness of several oxide atoms can be obtained. The resulting dispersion liquid contains oxide nanosheets, and these oxide nanosheets can exist as individual, independent dispersions.

[0090] The thickness of this oxide nanosheet is approximately 1 nm or less (the thickness of a few atoms of the oxide), and preferably about 1 nm, which is a thickness that can be called a nanosheet.

[0091] The resulting colloidal solution contains an anionic surfactant, but the oxide nanosheets are selectively aggregated by centrifugation, and the supernatant solvent is then removed to remove only the surfactant.

[0092] After removing the surfactant, the oxide nanosheets can be redispersed in various solvents.

[0093] The oxide nanosheet of the present invention is useful as a solid electrolyte and catalyst for fuel cells.

[0094] When oxide nanosheets are used as solid electrolytes, the manufacturing process can produce oxide nanosheets as free-standing membranes. Nanosheet membranes are materials that can be used as non-layered nanosheets.

[0095] When oxide nanosheets are used as catalysts, the production process involves drying colloids containing oxide nanosheets, filling the dried powder of oxide nanosheets into a column or the like, and passing a gas through it.

[0096] [2] Oxide Nanosheet The oxide of the oxide nanosheet of the present invention (non-layered colloidal single-crystalline oxide nanosheet) is a compound with a crystalline structure that does not have a layered structure.

[0097] The oxide nanosheet of the present invention is a single crystal.

[0098] The oxide nanosheets of the present invention exist individually as a colloidal dispersion in a solvent.

[0099] By using the colloid of the oxide nanosheet of the present invention (non-layered colloidal single-crystalline oxide nanosheet), the specific surface area of ​​the nanosheet is high, so it is possible to create precisely assembled nanosheet films, aggregates suitable for use in catalysts, etc. By accumulating nanosheets, it is possible to construct devices, etc.

[0100] Freeze-drying the colloidal solution of oxide nanosheets of the present invention makes it possible to produce powders with a high specific surface area, and suction-filtering the colloidal solution of oxide nanosheets makes it possible to produce a free-standing film in which the nanosheets are aligned and assembled.

[0101] Among the oxide nanosheets of the present invention, ceria (CeO2) oxide nanosheets are particularly useful as fuel cell materials, as they are single-crystalline nanosheets with unique surfaces and exhibit excellent oxide-ion conductivity. Furthermore, ceria (CeO2) oxide nanosheets exhibit excellent surface proton conductivity and exhibit excellent catalytic activity due to surface exposure, making them useful as catalyst supports. Furthermore, ceria (CeO2) oxide nanosheets also exhibit excellent oxide-ion conductivity, making them useful as oxide-ion conductors when constructed into freestanding membranes.

[0102] The oxide nanosheet (atomic membrane) of the present invention can be applied to technical fields such as catalyst carriers and fuel cells, depending on the type of oxide.

[0103] Ceria nanomaterials are useful as catalyst supports and electrolytes for fuel cells. The conductivity and mobility of oxide ions contribute to their usefulness as catalyst supports and electrolytes for fuel cells. The ceria (CeO2) oxide nanosheets (single-crystalline ceria nanosheets) of the present invention exhibit ionic conductivity that is more than 1,000 times higher than that of ceria nanomaterials synthesized using conventional techniques, demonstrating their superiority.

[0104] (1) Oxides that make up oxide nanosheets The oxides that make up oxide nanosheets are compounds with a crystalline structure that does not have a layered structure (non-layered compounds).

[0105] The oxides are preferably oxides of divalent, trivalent and / or tetravalent metals.

[0106] The metal constituting the oxide is preferably a divalent, trivalent, and / or tetravalent metal, and more preferably magnesium (Mg), calcium (Ca), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), aluminum (Al), gallium (Ga), indium (In), cadmium (Cd), rhodium (Rh), yttrium (Y), lanthanum (La), cerium (Ce), gadolinium (Gd), etc.

[0107] The oxide is preferably a simple oxide (a compound of a single chemical composition in which one type of metal element and oxygen are chemically bonded).

[0108] The oxide is preferably a simple oxide, more preferably magnesium oxide (MgO), calcium oxide (CaO), manganese(II) oxide (MnO), manganese(II,III) oxide (Mn3O4), manganese(III) oxide (Mn2O3), iron(II) oxide (FeO), iron(III) oxide (Fe2O3), cobalt(II) oxide (CoO), cobalt(III) oxide (Co2O3), nickel(II) oxide (NiO), nickel(III) oxide (Ni2O3), copper oxide ( These include zinc oxide (ZnO), aluminum oxide (III) (Al2O3), gallium oxide (Ga2O3), indium oxide (In2O3), cadmium oxide (CdO), rhodium oxide (Rh2O3), yttrium oxide (Y2O3), lanthanum oxide (La2O3), cerium oxide (III) (Ce2O3), cerium oxide (IV) (CeO2), and gadolinium oxide (Gd2O3).

[0109] The oxide is preferably an oxide that can be synthesized from metal ions that can enter between the layers of the anionic surfactant when forming an anionic surfactant-oxide nanosheet complex.

[0110] (2) Oxide nanosheets (non-layered colloidal single-crystalline oxide nanosheets) Oxide nanosheets are single crystals (the direction of the crystal axis does not change regardless of the position on the nanosheet).

[0111] The oxide nanosheets exist as a colloidal dispersion in the solvent.

[0112] Oxide nanosheets exist as individual, independent dispersions (free-standing). The term "free-standing" refers to the fact that oxide nanosheets can be handled independently in a solvent without being supported by a substrate or other support (no support is required).

[0113] (3) Thickness and Size of Oxide Nanosheet The oxide nanosheet is made of a single crystal of oxide and preferably has a thickness of approximately 1 nm or less (a layer equivalent to a few atoms). The nanosheet of the present invention does not usually have a particle-like form.

[0114] The oxide nanosheet is made of a single crystal of oxide, and preferably has a lateral size of 100 nm to 5 μm.

[0115] The oxide nanosheet is composed of a single crystal of an oxide, and preferably has a theoretical specific surface area of ​​100 m 2 / g~500m 2 / g, and more preferably 200m 2 / g (150m 2 / g~450m 2 / g). For example, the specific surface area is calculated by taking the thickness of cerium (IV) oxide (CeO2) as 1 nm and its density as 7.22 g / cm 3 It can be theoretically calculated as:

[0116] (4) Colloidal Solution Containing Oxide Nanosheets The present invention encompasses colloidal solutions containing the oxide nanosheets of the present invention.

[0117] The oxide nanosheets exist individually as a colloidal dispersion in the solvent.

[0118] Oxide nanosheets are stably dispersed in solvents, and by controlling their assembly, it is possible to produce two-dimensionally assembled films and powders with high specific surface areas.

[0119] The oxide nanosheet of the present invention has excellent ionic conductivity of oxide ions. Compared to existing products, the oxide nanosheet has a unique single-crystal surface and good ionic conductivity of oxide ions, making it useful as a catalyst for solid oxide fuel cells (SOFCs).

[0120] Ceria (CeO2) oxide nanosheets exhibit high ionic conductivity due to the mobility of oxygen vacancies. The mobility of oxide ions in ceria (CeO2) leads to improved catalytic activity.

[0121] The oxide nanosheet of the present invention (non-layered colloidal single-crystalline oxide nanosheet) is useful as a seed material for applications such as catalyst supports and fuel cell catalysts because of its high specific surface area.

[0122] Hereinafter, the embodiments of the present invention will be described more specifically based on examples.

[0123] However, the present invention is not limited to the scope of the examples.

[0124] (1) Synthesis of oxide nanosheets (Figure 1) (Synthesis of non-layered colloidal single-crystalline oxide nanosheets) Step (1) As a pretreatment, surfactant-metal ion complex crystals were synthesized by mixing cerium (III) nitrate hexahydrate (Ce(NO3)3·6H2O) or cerium (III) sulfate n-hydrate (Ce2(SO4)3·nH2O) (metal salt, oxide (cerium (III)) raw material)) in the presence of sodium octadecyl sulfate (SOS) (anionic surfactant).

[0125] Using an autoclave (a sealed, heat-resistant, pressure-resistant container), surfactant-metal ion complex crystals (surfactant crystals) and ammonium nitrate (NH4NO3) (a salt consisting of a chaotropic anion and a monovalent cation) were mixed in water containing a trace amount of N-methylformamide (NMF) (an amide solvent), and hydrothermal synthesis (120°C, 2 days) was carried out to produce an SOS-cerium oxide (CeO2) nanosheet complex.

[0126] The amount of ammonium nitrate used was adjusted so that the concentration (weight % (wt%)) in the amide solvent was 1 wt% to 20 wt% (preferably 14 wt%), and the reaction was carried out.

[0127] The reaction solution was prepared by mixing surfactant crystals, NH4NO3, water, and NMF in a weight ratio of 0.08:0.5:3.5:0.1, and the reaction was carried out.

[0128] The hydrothermal synthesis was carried out at 120°C for 2 days.

[0129] Step (2) Next, the SOS-cerium oxide (CeO2) nanosheet composite obtained in step (1) was dispersed in NMF and subjected to ultrasonic treatment for about 30 minutes to obtain a dispersion liquid in which a layer having a thickness equivalent to several atoms of cerium oxide (CeO2) had peeled off.

[0130] This dispersion is a colloidal solution containing individual cerium oxide (CeO2) nanosheets (free-standing bodies) (colloidal dispersion).

[0131] By centrifugal separation, unpeeled material was removed, and cerium oxide (CeO2) nanosheets with a thickness of approximately 1 nm or less (approximately 0.95 nm) were produced.

[0132] The thickness of the cerium oxide (CeO2) nanosheet is 4+ ) and represents the thickness of the nanosheet.

[0133] Consideration of conditions for synthesizing oxide nanosheets Surfactant-cerium ion complex crystal: 0.08 g Water: 3.5 mL NH4NO3: 0.5 g (consider the type of salt; a concentration of about 14 wt% (a very large amount of salt) is preferable) NMF: 0.1 mL (consider the type of solvent)

[0134]

[0135] By using a salt consisting of a chaotropic anion and a monovalent cation, SOS was stably present in NMF, enabling successful synthesis of cerium oxide (CeO2) nanosheets.

[0136]

[0137] By using an amide-based organic solvent, the interaction between cerium and the surfactant was weakened, allowing for the successful synthesis of cerium oxide (CeO2) nanosheets.

[0138] (2) Evaluation of cerium oxide (CeO2) nanosheets (ceria nanosheets) (Figs. 2 to 6) Figures 2 and 3 show low-acceleration-voltage atomic image observations (SEM images) of the synthesized ceria nanosheets. The cerium oxide (CeO2) nanosheets were formed as ceria nanosheets with a fluorite structure and an exposed 111 plane (Fig. 2). The contrast matched the thickness of the ceria (0.95 nm). 3+ ) The nanosheets formed were ceria nanosheets containing four cerium layers (Figure 3).

[0139] Figure 4 shows the stability evaluation (AFM image) of the synthesized ceria nanosheets. Cerium oxide (CeO2) nanosheets maintain their single crystallinity even after sintering, confirming their thermal stability. They maintain their single crystallinity even when sintered at 700°C.

[0140] Figures 5 and 6 show the temperature-variable environmental control (Raman spectroscopy) of the synthesized ceria nanosheets. The formation of cerium oxide (CeO2) nanosheets was confirmed with exposed 111 planes, which have many surface defects (Figure 5). At medium and low temperatures, the cerium oxide (CeO2) nanosheets exhibited Ce 3+The maintenance of surface defects due to the generation of ions was confirmed (Fig. 6).

[0141] (3) Introduction of multiple metal species into surfactant crystals in cerium oxide (CeO2) nanosheets (ceria nanosheets) (Figures 7 to 10) Figure 7 shows the introduction of multiple metal species into surfactant crystals in synthesized ceria nanosheets (non-layered colloidal single-crystalline oxide nanosheets).

[0142] (1) Precursors: A combination of Ce(NO3)3·6H2O and Gd(NO3)3·6H2O (gadolinium(III) nitrate hexahydrate) A combination of Ce(NO3)3·6H2O and Sm(NO3)3·6H2O (samarium nitrate hexahydrate)

[0143] (2) Synthesis of intermediates In N-methylformamide (NMF), ammonium nitrate (NH4NO3) was added, and in the presence of sodium octadecylsulfonate (SOS), (i) Ce(NO3)3·6H2O and Ga(NO3)3·6H2O were combined, or (ii) Ce(NO3)3·6H2O and Sm(NO3)3·6H2O were combined, and the intermediates (anionic surfactant-oxide nanosheet composites) were prepared by hydrothermal synthesis. (Ce 3+ , and Gd 3+ , or Sm 3+ ) : SOS = 14:1 (molar ratio)

[0144] (3) Colloidal dispersion: Oxide nanosheets were prepared by dispersing in NMF.

[0145] Ce 3+ and Gd 3+ The surfactant crystals containing the compound (I) were obtained (ICP emission spectroscopy analysis).

[0146] Ce 3+ and Sm 3+ The surfactant crystals containing the compound (I) were obtained (ICP emission spectroscopy analysis).

[0147] Figures 8 to 10 show the surfactant-ceria composite (Sm-doped) that was synthesized. The surfactant-ceria composite (Sm-doped) confirmed the synthesis of single-crystalline ceria nanosheets (Figure 8, TEM image, SAED pattern). The surfactant-ceria composite (Sm-doped) showed a porous structure, but maintained its single crystallinity (Figure 9, TEM image, HR-TEM image). A comparison of the peak intensities at 5.29 keV and 5.65 keV for the surfactant-ceria composite (Sm-doped) confirmed an intensity ratio of 6:4. The synthesis of Sm-doped ceria nanosheets was confirmed (Figure 10, TEM image, EDS spectrum).

[0148] (4) Construction of a free-standing film (Figure 11) Figure 11 shows an example of the construction of a free-standing film of ceria (CeO2) oxide nanosheets, one of the oxide nanosheets of the present invention. Among the oxide nanosheets of the present invention, ceria (CeO2) oxide nanosheets in particular exhibit good oxide ion conductivity, and when constructed as a free-standing film, are useful as oxide ion conductors. A free-standing nanosheet laminate film was constructed from a colloid containing ceria (CeO2) oxide nanosheets by suction filtration.

[0149] (5) Ionic conductivity of single-crystalline ceria nanosheets (Figure 12) Figure 12 shows the ionic conductivity of the single-crystalline ceria nanosheets of the present invention. Compared to bulk ceria nanosheets, the ionic conductivity of ceria nanosheets is improved by three to four orders of magnitude. The oxide ion conductivity of ceria is dramatically improved by forming it into nanosheets.

[0150] (6) Application of nanosheet synthesis using solid surfactants as templates (Figs. 13 and 14) Figure 13 shows the application of nanosheet synthesis using surfactant crystals of the present invention as templates. We have successfully synthesized surfactant crystals (octadecyl sulfate and octadecyl sulfonic acid) using divalent and trivalent metal ions.

[0151] Figure 14 shows the application of nanosheet synthesis using the solid surfactant template of the present invention. By applying the oxide nanosheet production method of the present invention and synthesizing using the solid surfactant template, we were able to synthesize nanosheets of various non-layered oxides and hydroxides, such as yttrium hydroxide (Y(OH)), gallium oxyhydroxide (GaOOH), and gallium(III) oxide (γG2O3).

[0152] By applying the method for producing oxide nanosheets of the present invention, we have also been able to synthesize nanosheets of a variety of non-layered oxides and hydroxides containing various elements such as Fe, Ga, Bi, Al, Zn, In, Y, Sn, Sc, and Gd.

[0153] (7) Industrial Applicability The present invention makes it possible to synthesize oxide nanosheets (non-layered colloidal single-crystalline oxide nanosheets) between layers of a solid anionic surfactant (such as SOS) as a template.

[0154] The method for producing oxide nanosheets of the present invention enables the formation of ultrathin films of several nanometers in thickness by solution processing. The present invention utilizes a solid anionic surfactant as a stronger template, and synthesizes nanosheets between the surfactant layers, thereby enabling the synthesis of colloidal single-crystal oxide nanosheets.

[0155] The present invention enables the synthesis of oxide nanosheets by hydrothermal synthesis in the presence of a salt consisting of a chaotropic anion and a monovalent cation, which is a condition for precipitating a single-crystalline material between the surfactant layers.

[0156] The method for producing oxide nanosheets of the present invention is highly reproducible, has a very wide process window, and is useful for industrial processes.

[0157] The oxide nanosheet of the present invention is a colloidal oxide nanosheet of extremely high quality. By using the colloidal oxide nanosheet of the present invention, it is possible to create precisely assembled nanosheets suitable for use in membranes, catalysts, etc., due to the high specific surface area of ​​the nanosheets.

[0158] Among the oxide nanosheets of the present invention, ceria (CeO2) oxide nanosheets are particularly useful as fuel cell materials because they are single-crystalline nanosheets with unique surfaces and exhibit excellent oxide ion conductivity. Ceria (CeO2) oxide nanosheets also exhibit excellent surface proton conductivity and exhibit excellent catalytic activity due to the exposed surface, making them useful as catalyst supports.

[0159] Among the oxide nanosheets of the present invention, oxide nanosheets of gallium oxyhydroxide (GaOOH) and gallium(III) oxide (γG2O3) are particularly useful for manufacturing power devices (power semiconductors, etc.) that exhibit a Baliga figure of merit that exceeds that of diamond.

[0160] The oxide nanosheet (atomic membrane) of the present invention can be applied to technical fields such as catalyst carriers and fuel cells, depending on the type of oxide.

Claims

1. A colloidal solution containing oxide nanosheets, The oxide constituting the oxide nanosheet is a compound having a crystalline structure that does not have a layered structure, The oxide nanosheet is a non-layered colloidal single-crystalline oxide nanosheet, A colloidal solution in which the oxide nanosheets exist individually as a colloidal dispersion in a solvent.

2. The colloidal solution according to claim 1 , wherein the oxide of the oxide nanosheet is a simple oxide.

3. The colloidal solution according to claim 1 , wherein the oxide of the oxide nanosheet is an oxide of a divalent, trivalent, and / or tetravalent metal.

4. The colloidal solution according to claim 1, wherein the oxide of the oxide nanosheet is an oxide that can be synthesized from metal ions that can enter between the layers of the anionic surfactant when forming an anionic surfactant-oxide nanosheet complex.

5. The colloidal solution of claim 1 , wherein the oxide nanosheets are free-standing.

6. The colloidal solution according to claim 1 , wherein the oxide nanosheet has a thickness of approximately 1 nm or less.

7. The colloidal solution according to claim 1, wherein the oxide nanosheets have a lateral size of 100 nm to 5 μm.

8. The oxide nanosheet has a theoretical specific surface area of ​​100 m ２ / g~500m ２ 2. The colloidal solution of claim 1, wherein the colloidal solution has a viscosity of 1000 saturates.

9. (delete)

10. A method for producing an oxide nanosheet, comprising: (1) A process for producing an anionic surfactant-oxide nanosheet composite by mixing anionic surfactant-metal ion complex crystals and a salt consisting of a chaotropic anion and a monovalent or divalent cation that does not react with water to be hydrolyzed in water containing a trace amount of an amide solvent; and (2) a step of removing the surfactant from the anionic surfactant-oxide nanosheet composite obtained in step (1); A method for producing an oxide nanosheet, comprising:

11. The method for producing an oxide nanosheet according to claim 10, wherein the step (1) is a step of mixing anionic surfactant-metal ion complex crystals, a chaotropic anion, and a salt consisting of a monovalent or divalent cation that does not react with water to hydrolyze in water containing a trace amount of an amide solvent, and producing an anionic surfactant-oxide nanosheet complex by hydrothermal synthesis.

12. The method for producing an oxide nanosheet according to claim 10, wherein the method for producing an oxide nanosheet comprises obtaining a colloidal solution of the oxide nanosheet.

13. The method for producing an oxide nanosheet according to claim 12, wherein the oxide nanosheet is a non-layered colloidal single-crystalline oxide nanosheet.

14. The oxide constituting the oxide nanosheet is cerium (IV) oxide (CeO ２ 2. The colloidal solution according to claim 1, wherein