JP2012140255A - 2d bronze type tungsten oxide nanosheet, method for production thereof, and photocatalyst and photochromic device using the nanosheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a tungsten oxide nanosheet having a 2D bronze structure that two or more of the 6-membered ring structures comprising a metal-oxygen octahedron are regularly arranged in a sheet cross-sectional direction, and also to provide a method for production thereof and a device using the same.SOLUTION: The layered poly(tungstic acid) comprising a host layer having the 2D bronze structure is converted into a hydrogen ion exchanger having a solid acid property while maintaining its host structure, and a single layer peeling phenomenon is induced by introducing a bulky cation between the layers. Thereby, the tungsten oxide nanosheet is obtained, which has the 2D bronze structure that two or more of the 6-membered ring structures comprising a metal-oxygen octahedron are regularly arranged in a sheet cross-sectional direction.

Description

  The present invention relates to a tungsten oxide nanosheet having a two-dimensional (2D) bronze structure, a method for producing the same, and a photocatalyst and a photochromic device using the same, and more particularly, a six-membered ring structure composed of a metal-oxygen octahedron has a sheet 2D bronze-type tungsten oxide nanosheets regularly arranged in the cross-sectional direction and having a one-dimensional tunnel in the sheet plane direction, its production method comprising a precursor acid treatment step and a release agent mixing step, and a photocatalyst using them The present invention relates to a photochromic element.

  Conventionally, a two-dimensional sheet material that has been exfoliated to the nano level has been obtained by exfoliating clay minerals and laminar compounds having various compositions and structures such as sulfides, oxides, hydroxides, and graphite. Many are synthesized. These nanosheets have both a two-dimensional bulk size and a one-dimensional nanosize, and can be expected to have both good bulk handling properties and unique physical properties due to the nanosize effect. In recent years, graphene obtained from the exfoliation of graphite has been applied to a wide range of fields, from transparent electrodes to electrochemical capacitors and secondary battery materials. Layered compound exfoliation technology is of great interest from both basic and application viewpoints. It has been. In many cases, nanosheets obtained from single-layer exfoliation of layered compounds are electrostatically charged, so by using self-assembled alternating adsorption method, Langmuir-Blodgett method, reaggregation method, electrophoresis method, etc., from the thin film It has excellent material moldability and designability that can build materials in various forms from powder to nano-scale, and has the potential to be applied to a wide range of material fields.

  Among the nanosheet group, oxide-based nanosheets are rich in composition and structure, and are particularly valuable for industrial use due to the expression of functionality and the ease of synthesis and handling. For example, in the case of lepidochrosite-type nanosheets, high dielectric properties can be maintained even at the level of several nanometers (for example, Patent Document 1), and a giant magneto-optical effect that cannot be seen in the bulk depending on the type of dopant. As a very thin functional material, it has attracted particular attention from the device field. Recently, it has been shown that the creation of perovskite-type nanosheets with a thicker skeleton than lepidoccrosite can achieve higher dielectric constant retention (for example, Patent Document 2). In addition to composition, nanosheet search focusing on structure Is an important key.

Named after a well-known mineral / crystal structure, a lipidocrosite type (for example, Patent Document 3), an α-NaFeO ₂ type (for example, Patent Document 4), and a perovskite type (for example, Patent Document 5 and non-patent documents) Patent Document 1) A 2D pyrochlore nanosheet (for example, Patent Document 6) has already been established with a method of synthesizing an alkali layered oxide as a starting material. However, as with perovskite, single-layer nanosheets with a two-dimensional bronze structure known as a versatile structure such as ferroelectric and superconductivity are hardly known except for polytungstic acid as the alkali layered compound itself. Was not synthesized for.

Tungsten oxides have a wide range of industrial applications as functional materials such as photocatalysts, electrochromic elements, photochromic elements, and desulfurization / denitration catalysts. It is known that Rb ₄ W ₁₁ O ₃₅ exists as an alkali layered polytungstic acid having a 2D bronze type host layer (for example, Non-Patent Document 2). This host layer is formed of a porous skeleton in which a plurality of six-membered ring structures composed of metal-oxygen octahedrons are regularly arranged in the cross-sectional direction of the host layer. Therefore, if the 2D bronze type host layer is thicker than 1 nm and can be peeled off as a single layer, the nanosize of several nanometers, which is different from the nanosheet group that handles 1 nm or less of the lipidocrosite type or α-NaFeO ₂ type host layer, The creation of nanosheets with unique functionality that takes advantage of the features of both 2D and 2D bronze structures is expected.

  However, the crystal structure of alkali layered polytungstic acid is often very complex with many large unit cells, so it is difficult to analyze even if diffraction data is acquired. However, polytungstic acid still has many problems such as being easily discolored and difficult to synthesize a single phase or a single crystal. For this reason, there are few examples of soft chemical research such as identifying and extracting single-phase polytungstic acid and deriving new substances from their ion exchange reactions.

  Therefore, it is desirable that a single layer peeling phenomenon is induced by soft chemical treatment of an alkali layered polytungstic acid composed of a host layer having a 2D bronze structure, and a tungsten oxide nanosheet having an unprecedented 2D bronze structure is obtained. Further, further utilization of such 2D bronze type tungsten oxide nanosheets is desired.

WO2007 / 094244 WO2008 / 078652 JP 2001-270022 A JP 2003-201221 A JP 2009-292680 A JP 2009-1441A

Chem. Mater. , 1990, 2, 279 Inorg. Mater. 1998, 34, 845 Chem. Mater. , 2002, 14, 3524

  In the present invention, a plurality of six-membered ring structures composed of metal-oxygen octahedrons arranged in a sheet cross-sectional direction are regularly arranged in a different structure and composition from the flaky material obtained by single-layer peeling of a conventional layered compound. This is to provide a 2D bronze-type tungsten oxide nanosheet having a one-dimensional tunnel-like structure, a method for producing the same, a photocatalyst using the 2D bronze-type tungsten oxide nanosheet, and a photochromic element.

Therefore, as a result of intensive studies, the present inventors have focused on layered tungsten oxide in which a host layer having a 2D bronze structure is stacked in order to solve the above-described problem, and break the basic crystal structure from this oxide. It was found that a single layer could be peeled without any problems. That is, a host layer having a one-dimensional tunnel represented by the composition formula Rb ₄ W ₁₁ O ₃₅ is laminated, and an alkali layered crystal (see FIG. 1) is used as a starting material. Oxidation with a 2D bronze structure by inducing a hydrogen ion exchanger with solid acidity without causing the 2D bronze structure of the host layer to break down and then acting on it with a bulky cation-containing solution It has been found that the nanosheets can be separated into a single layer up to a thickness of 1 nm to 4 nm or less, which is a single layer level. The present invention has been made on the basis of these findings and successes, and provides a 2D bronze-type nanosheet, a production method thereof, a photocatalyst using the same, and a photochromic device.

That is, the present invention comprises the constituent elements described in the following (1) to (8), and has succeeded in providing a nanosheet having a unique property having a 2D bronze structure. .
(1) A nanosheet having a 2D bronze structure having a one-dimensional tunnel-like structure in which a plurality of six-membered ring structures composed of metal-oxygen octahedrons are regularly arranged in the sheet cross-sectional direction. A 2D bronze-type tungsten oxide nanosheet represented by 35.
(3) The nanosheet according to (2), wherein the crystallographic thickness is 1 nm to 4 nm and the lateral size is in the range of submicron to several millimeters.
(4) The nanosheet according to item (2), which is obtained by exfoliating a single layer of a layered tungsten oxide having a structure in which a host layer having a 2D bronze structure is laminated.
(5) The nanosheet according to (2), wherein the metal-oxygen octahedral six-membered ring structure (bronze structure hole) includes two to three layers on the sheet side surface.
(6) A proton or oxonium ion is introduced between layers while maintaining the host layer structure of a layered tungsten oxide having a structure in which a host layer having a 2D bronze structure is laminated, and then a bulky cation is introduced between the layers. A method for producing a nanosheet having a 2D bronze structure, characterized in that a solution in which a two-dimensional sheet-like substance separated by a single layer is dispersed is obtained.
(7) A photocatalyst using the 2D bronze-type tungsten oxide nanosheet according to any one of (2) to (5) above.
(8) A photochromic element using the 2D bronze-type tungsten oxide nanosheet according to any one of (2) to (5).

  The tungsten oxide nanosheet according to the present invention has a 2D bronze structure. Such a 2D bronze nanosheet may have a nanosize in the sheet cross-sectional direction and a bulk size in the sheet plane direction. Such flaky nanomaterials of size and properties are not only easier to handle than other nanomaterials, but also excellent in film forming properties and coating properties, and can be used as thin film materials and coating materials. In addition, the 2D bronze structure can function as an open channel by ion exchange of alkali ions, and thus is useful as a sensor or a catalyst using adsorption of various molecules and gases.

  The 2D bronze nanosheet according to the present invention is composed of tungsten oxide having versatile industrial utility value such as photocatalyst, electrochromic element, photochromic element, desulfurization / denitration catalyst. The 2D bronze nanosheet after the single-layer peeling has the same properties and includes a very useful usage mode, and is expected to contribute greatly to industrial development. That is, in recent years, window glass anti-fogging / self-cleaning technology using photocatalysis has attracted attention as an energy-saving technology. In order to express these functionalities not only outdoors but also indoors, further enhancement of sensitivity and utilization of visible light are indispensable, and various single material systems or composite material systems are energetically searched for. The 2D bronze-type tungsten oxide nanosheet of the present invention has a band gap narrower than the band gap of many existing semiconductor oxide nanosheets because of the maximum thickness of 4 nm derived from the 2D bronze structure and the metal-oxygen octahedral bonding structure. Application development such as use of the photocatalytic effect and photochromic properties in an ultraviolet / visible light environment can be considered. Furthermore, 2D bronze-type tungsten oxide nanosheets can be handled as polyanions, and because of their excellent material moldability by the layer-by-layer method described above, a completely new photocatalyst / photochromic thin film material can be produced. Expected and its significance is extremely large.

The cross-sectional schematic diagram of the 2D bronze type tungsten oxide nanosheet by this invention. The figure which shows the ultraviolet-visible light absorption spectrum of the colloidal solution of each density | concentration of Example 2, and the inset which plotted the light absorbency and density | concentration of 255 nm. The figure which shows the TEM image which adsorb | sucked to the Cu grid the 2D bronze-type tungsten oxide nanosheet which peeled (a) single layer of Example 2, and (b) the figure which shows the electron diffraction pattern. (A) AFM image of thin film formed on Si substrate in Example 2 and (b) A diagram showing its cross-sectional profile. _{(A) Rb 4 W 11 O} 35 and it in 16 provisions of nitric acid (b) 1, (c) 3, shows a powder X-ray diffraction pattern of a sample treated (d) 5 times. The figure which shows the recovery rate of the tungsten oxide weight when the 2D bronze-type tungsten oxide nanosheet colloid solution of Example 2 is centrifuged at each rotation speed. The figure which shows the ultraviolet-visible light absorption spectrum when the 2D bronze-type tungsten oxide nanosheet of Example 3 and an organic cation are laminated | stacked 10 times alternately, and the inset which plotted the light absorbency of 255 nm for every lamination | stacking frequency. (A) as-grown thin film in which 2D bronze-type tungsten oxide nanosheets and organic cations of Example 3 were alternately laminated 10 times, (b) UV irradiation for 1 hour, (c) UV light irradiation for 4 hours, (d) UV light irradiation for 24 hours The figure which shows the X-ray diffraction pattern after. The figure which shows the ultraviolet-visible light absorption spectrum before and behind ultraviolet irradiation of the 2D bronze-type tungsten oxide nanosheet colloidal solution of Example 4. FIG.

  Embodiments of the present invention will be described below with reference to the drawings. Like elements are given like reference numerals to avoid duplicate description.

In the present invention, an alkali layered polytungstic acid composed of a host layer having a 2D bronze structure (see FIG. 1) is used as a starting material. As such a substance, an alkali layered oxide represented by a composition formula Rb ₄ W ₁₁ O ₃₅ can be given. In addition to the alkali layered oxide, any layered compound composed of a host layer having a 2D bronze structure suitable for the intended purpose of the present invention can be used. In this example, a layered oxide having the above composition is used and treated with an acidic aqueous solution such as nitric acid to convert it into a hydrogen ion-containing substance Rb _4-x H _x W ₁₁ O ₃₅ (0 ≦ x ≦ 4) hydrate, Next, by mixing and shaking in an aqueous solution containing a bulky guest represented by a quaternary ammonium ion that functions as a bulky guest, the hydrated layered polytungstic acid is peeled to a single-layer thickness, and a 2D bronze structure is formed. A flaky nanosheet dispersion solution is obtained without breaking.

The alkali layered polytungstic acid Rb ₄ W ₁₁ O _{35 according to the} present invention is a sheet in which a six-membered ring structure composed of metal-oxygen octahedron similar to a tungsten bronze structure is regularly arranged in three layers in the sheet cross-sectional direction. It is formed from a host layer having a 2D bronze structure in which a one-dimensional channel is formed in the cross-sectional direction (see FIG. 1: orthorhombic a = 1.4653 (3) nm, b = 2.568 (3) nm, c = 0.7684 (3) nm). By applying an acid treatment to the layered compound and then adding a release accelerator, a colloidal solution in which nano-level flaky nanomaterials dispersed in the individual host layers are dispersed is obtained.

  In order to advance the peeling reaction, it is necessary to extract part or all of alkali ions by acid treatment and introduce hydrogen ions or oxonium ions between layers to give solid acidity. For the acid treatment, hydrochloric acid, sulfuric acid, nitric acid or the like can be used.

The amount of x in the composition formula Rb _4-x H _x W ₁₁ O ₃₅ hydrate (0 ≦ x ≦ 4), which is a product after ion exchange, can be changed depending on the type, concentration, and number of treatments of the acid. However, in order for the exfoliation reaction to proceed well, it is necessary to ion-exchange x = 1 or more corresponding to the amount of Rb ions between layers in the unit cell. At this time, it is difficult to completely extract rubidium, and in order to make rubidium less than 2 in the above composition formula, it is necessary to repeatedly use high-concentration hydrochloric acid, nitric acid, sulfuric acid, etc., which is economically undesirable. Finally, considering the cost and the number of days, acid treatment is performed until nitric acid having a concentration of 10 N or more is (solution) / (solid) = 100 cm ³ g ^{−1 and} the solid-liquid ratio is about 2 in the composition formula. It is desirable.

By mixing and shaking a hydrogen ion exchanger Rb _4-x H _x W ₁₁ O ₃₅ hydrate (0 ≦ x ≦ 4) in which rubidium is exchanged with hydrogen ions and a peeling accelerator in an aqueous solution, When the exfoliation reaction of the layers proceeds, a solution in which nanosheets having a 2D bronze structure thinned to nanoscale are dispersed can be obtained. At this time, by adjusting the molar ratio of the hydrogen ions in the hydrogen ion exchanger to the peeling accelerator, it is possible to obtain a 2D bronze-type tungsten oxide nanosheet having a single layer peeled as a main component.

As the peeling accelerator used in the present invention, tetrabutylammonium ion, tetrapropylammonium ion, tetraethylammonium ion, tetramethylammonium ion, n-propylamine, n-ethylamine, and ethanolamine can be used. Ions (hereinafter, TBA ⁺ ) are particularly effective.

Hydrogen ion exchanger Rb _4-x H _x W ₁₁ O ₃₅ hydrate (0 ≦ x ≦ 4) with respect to hydrogen ions, the ratio of the release accelerator TBA ⁺ is in the range of 0.5-2. To give a yellow dispersion. When the molar ratio is less than 0.5, peeling may not proceed sufficiently. If the molar ratio exceeds 2, the host 2D bronze structure may collapse. More preferably, when the molar ratio = 1, the 2D bronze-type tungsten oxide nanosheet can be obtained with the highest yield. Unreacted substances generated under these conditions can be removed by further centrifuging the dispersion. The obtained yellow supernatant has light absorption characteristics based on the band gap as shown in FIG. From the inset plotting the shoulder (255 nm) of this absorption peak, it can be seen that the colloidal solution is dispersed in a state where the layered compound is separated to the host monolayer because it follows the Lambert-Beer rule. The rise of the absorption peak becomes a straight line when plotted by the power of the absorbed light energy to the 0.5th power, indicating that it is based on an indirect transition with a band gap of about 3.3 eV.

The colloidal solution obtained by adding a peeling accelerator to the hydrogen ion exchanger Rb _4-x H _x W ₁₁ O ₃₅ hydrate (0 ≦ x ≦ 4) was dropped on the TEM observation grid and observed after drying. Then, a sheet-like substance having a uniform contrast and a sharp edge as shown in FIG. 3A can be observed over most of the grid. This uniform and thin contrast is attributed to the fact that the sheet-like material found in the colloid is a nanosheet that has been subdivided to the nano level by monolayer delamination of the precursor. The observed electron diffraction pattern of the nanosheet shown in FIG. 3B could be indexed with a two-dimensional rectangular lattice similar to the host layer of the precursor layered polytungstic acid bulk (a = 1) .4 nm, c = 0.78 nm). This result is the basis that the nanosheets in the colloid form a two-dimensional skeleton having the same 2D bronze structure as the host layer of the starting material shown in FIG.

The sheet plane direction of the nanosheet having a 2D bronze structure subdivided to the nano level obtained from the single layer peeling of the original layered compound depends on the size of the mother crystal used for peeling. If a size of the order of several millimeters is used as the starting material, the resulting 2D bronze-type tungsten oxide nanosheet is also on the order of several millimeters at the maximum. When mixed with the release accelerator and shaken, the yield is improved, but the size in the sheet plane direction tends to be reduced. When synthesis is performed with priority given to yield using a micron-sized mother crystal, it generally has a lateral size of submicron to several microns. Actually, from a tungsten oxide nanosheet colloid solution obtained after shaking at 180 rpm, a nanosheet monolayer was adsorbed on a Si substrate by a self-organized adsorption method using a cationic polymer, and an atomic force microscope (AFM: see FIG. 4). ), A laterally sized nanosheet of about 0.1 to 1 micron can be observed. The crystallographic thickness of these 2D bronze-type tungsten oxide nanosheets is about 2.5 nm when the van der Waals radius of oxygen is considered for one host layer, and shows a good agreement with the height of about 3 nm estimated from AFM observation images. ing. This result shows that the flaky tungsten oxide adsorbed on the Si substrate forms a two-dimensional skeleton having the same 2D bronze structure as the host layer of the starting material Rb ₄ W ₁₁ O ₃₅ shown in FIG. It becomes the side evidence to show.

The 2D bronze-type tungsten oxide nanosheets obtained as the above dispersion solution can re-aggregate the nanosheets by controlling the pH of the liquid phase and the electrolyte concentration, or by heating or freeze-drying. Fine particles having a specific surface area can be produced. At this time, by coexisting alkali ions, organic molecules, and complexes, it can be reconstructed and designed into a layered compound sandwiching various cationic species. In addition, since the 2D bronze-type tungsten oxide nanosheet has an anionic property, it utilizes an electrostatic self-assembly reaction with an organic polymer such as a polycation or an inorganic polymer obtained by peeling off a composite hydroxide. Accordingly, it is possible to induce the composite material or to form alternately adsorbed films on various substrates (for example, Si, SiO ₂ , ITO, Al, Ni, etc.). For example, when alternately adsorbing films with organic cations on a SiO ₂ substrate, the light absorption spectrum increases in proportion to the number of times 2D bronze-type tungsten oxide nanosheets are adsorbed as shown in FIG. It is possible to easily form an ultrathin film having characteristics or film thickness in a large area. In addition, because it is possible to form a film by using Langmuir-Blodgett method or electrophoresis method, construct materials that meet the industrial needs of various film forming technologies such as precise film thickness control, roll to roll, and large area. can do.

  The molar ratio of tungsten (W) to oxygen (O) in the 2D bronze-type tungsten oxide nanosheet of the present invention satisfies 11:35 except for substances that are adsorbed or inserted into or removed from the one-dimensional tunnel. If the molar ratio of W and O deviates from the above relationship, the 2D bronze structure in FIG. 1 may not be maintained, and nanosheets may not be obtained.

  In addition, the 2D bronze-type tungsten oxide nanosheet of the present invention can retain the metal-oxygen octahedral six-membered ring structure of the host layer with high crystallinity. The diameter of the hole of the six-membered ring structure is 0.3 nm considering the ionic radius of oxygen. Since this size is similar to that of general zeolite, it functions as an adsorbent / absorbent for water and specific molecules by removing alkali ions in the hole. In particular, unlike zeolite, the 2D bronze-type tungsten oxide skeleton expresses optical functionality, so that unique functional material expression combining adsorption ability and optical functionality is expected.

  The 2D bronze-type tungsten oxide nanosheet according to the present invention has a narrow band gap of about 3.3 eV, which is narrower than the tungsten oxide nanoparticles (5 nm or less) and the 2D pyrochlore-type tungsten oxide nanosheet conventionally known from the thickness and structure thereof. It is possible to decompose and remove organic substances by photocatalytic reaction. Therefore, the present invention can be applied to other application ranges such as water splitting, photoinduced hydrophilicity, and solar cells using a photocatalytic reaction of visible light / ultraviolet light response.

  Furthermore, the 2D bronze-type tungsten oxide nanosheet according to the present invention generates a photochromic reaction in response to visible light / ultraviolet light as in the case of hydrated tungsten trioxide. In a tungsten oxide photochromic material, an absorption region is created by the adsorption of cation species such as protons. Therefore, increasing the surface area on which many cation species can be adsorbed is an important issue. The 2D bronze-type tungsten oxide nanosheet according to the present invention has a two-dimensional sheet shape with a large surface area, and alkali ions in the host layer are replaced with protons to create a large space. It may be a photochromic material that achieves efficiency. Specifically, the 2D bronze-type tungsten oxide nanosheet according to the present invention may be applied to sunglasses, smart glass, automobile window glass, and the like. In addition, when combined with a nanomaterial having semiconducting properties such as titanium oxide, it is expected to function as a nanoscale energy storage photocatalyst.

  Hereinafter, the present invention will be specifically described based on examples. However, these examples are for the purpose of disclosure as an aid for easily understanding the present invention, and are not intended to limit the present invention.

Example 1
WO ₃ and Rb ₂ CO ₃ were mixed at a ratio of 11: 2, pre-fired at 750 ° C. for 1 hour, pulverized and mixed again, and then fired at 850 ° C. for 12 hours. From the powder X-ray diffraction pattern of the fired product (FIG. 5a), it was confirmed that Rb ₄ W ₁₁ O ₃₅ was obtained in a single phase (orthorhombic crystal: a = 1.4653 (3) nm, b = 2. 568 (3) nm, c = 0.7684 (3) nm).
The powder X-ray diffraction pattern of the substance obtained by reacting the synthesized yellow powder Rb ₄ W ₁₁ O ₃₅ with 16 N nitric acid at a solid-liquid ratio of (solution) / (solid) = 100 cm ³ g ⁻¹ is shown in FIG. Shown in Although the nitric acid treatment is performed once, the diffraction peak at d = 2.59 nm (2θ = 3.4 °) due to the reflection between the layers of the original layered compound Rb ₄ W ₁₁ O ₃₅ is slightly shifted to a low angle. It was the same pattern, and it was found that the basic crystal structure was maintained without dissolving even when contacted with high-concentration nitric acid. From the ICP emission analysis and atomic absorption analysis of the obtained powder sample, the ratio of Rb: W was determined, and it was found that Rb ions were extracted from 4:11 to 3.3: 11. It is suggested that is progressing.

  Next, when the number of acid treatments was increased to 3, 5 while taking one day of reaction time, the peak due to reflection between layers was further shifted to a lower angle (see FIGS. 5c and d). Finally, a diffraction peak at d = 2.64 nm (2θ = 3.3 °) was detected by performing the treatment five times. The Rb: W ratio was 3: 0: 11 and 2.4: 11 when the treatment was performed 3 or 5 times, respectively, indicating that the Rb ions can be further extracted with the number of treatments. This extraction amount is larger than the Rb ions between the layers, and it is considered that some Rb ions in the host layer are also exchanged with protons or oxonium ions, and that the 2D bronze type host layer itself exhibits an ion exchange reaction. Suggests. Further, when the X-ray diffraction pattern of the sample treated five times was indexed with the original orthorhombic lattice, a = 1.464 (1) nm, b = 2.578 (2) nm, c = 0.664. (2) It was found that the unit cell can be assigned by nm. At this time, the peaks at 2θ = 23.3, 24.3 °, 33.9 °, 36.7 °, 49.8 ° indicate the two-dimensional periodic structure of the 2D bronze type host layer (02, 40, 42, 60, 80), and it was found that even when the nitric acid treatment was carried out five times or more, the 2D bronze structure of the host layer was not destroyed, and the ion exchange reaction in the interlayer / host layer proceeded. This indicates that more Rb ions can be extracted by increasing the number of acid treatments, but it is necessary to reduce to at least Rb: W = 3: 11 in order to react with the peeling accelerator. If the efficiency is taken into consideration, it is desirable to carry out at least 3 times with a nitric acid having a concentration of 16N.

As described above, as an optimum ion exchange condition, a powder sample of Rb ₄ W ₁₁ O ₃₅ and 16 N nitric acid at a ratio of (solution) / (solid) = 100 cm ³ g ⁻¹ were mixed and stirred. This was exchanged with fresh nitric acid every day and repeated five times, and after ion exchange treatment, the solid residue was recovered by washing with water and air drying. Chemical The solid analysis to hydrogen ion substitution type layered tungstate powder: It was found that _{(Rb 2.4 H 1.6 W 11 O} 35 · nH 2 O n is in a dry state dependent) is.

(Example 2)
Dry body when calculated as n = 0 from the hydrogen-type layered tungstic acid powder obtained in Example 1 (Rb _2.4 H _1.6 W ₁₁ O ₃₅ · nH ₂ O: n depends on the dry state) A sample was taken so that the weight of the solution became 0.8 g, and added to 200 cm ³ of tetrabutylammonium hydroxide aqueous solution (TBAOH) and shaken (180 rpm) for about 10 days while being shielded from light at room temperature. At this time, when the concentration of TBAOH was changed to a molar ratio TBA ⁺ / H ⁺ = 0.1 to 10 with the ion-exchangeable protons in the solid, in the range of TBA ⁺ / H ⁺ = 0.5 to 2, After standing, the yellow dispersion and the settled unpeeled component were visually confirmed. In particular, when TBA ⁺ / H ⁺ = 1, a dispersion liquid having a small amount of precipitated components and seeming darkest was obtained. Although not shown, when the molar ratio was less than 0.5 and greater than 2, the supernatant after standing was a substantially transparent solution, and it was visually confirmed that peeling did not proceed.

Therefore, the sediment component was removed by centrifuging the dispersion obtained under the condition of TBA ⁺ / H ⁺ = 1 for 30 minutes at 300 to 2000 rpm. Each of the separated supernatants was dried and then heated in air to 500 ° C. and weighed to determine the yield. As a result, it was found that about 40 to 60% could be recovered (see FIG. 6). At this time, the fact that the tungsten oxide recovery rate decreases as the rotational speed of the centrifugal separation increases can be considered that the tungsten oxide fine particles having a large sheet size preferentially settled. That is, there is a possibility that the particle size can be controlled to some extent by controlling the rotation speed of the centrifugal separation.

The dispersion obtained under the condition of TBA ⁺ / H ⁺ = 1 was centrifuged at 2000 rpm from the viewpoint of obtaining a material having a uniform size as much as possible. The obtained yellow supernatant was diluted with ultrapure water (specific resistance value: 18 MΩcm) to have different concentrations of 11.4, 5.7, 1.14 and 0.57 (× 10 ⁻⁶ moldm ⁻³ ), respectively. Dilute to Then, when the ultraviolet-visible light absorption spectrum was measured about the diluted solution of each density | concentration, the strong absorption peak which starts from 380 nm vicinity was observed (refer FIG. 2). A plot of absorbance and concentration at a shoulder of 255 nm was prepared, and as shown in the inset of FIG. 7, it follows Lambert-Beer's law, so that a tungsten oxide nanosheet generated by monolayer delamination was obtained as a highly dispersed colloidal solution. It was shown that. When the molar extinction coefficient of Rb _2.4 H _1.6 W ₁₁ O ₃₅ ² -dispersed particles was determined from the slope of the straight line approximating the plot of the inset, it was a large value of 9 × 10 ^{4 at} 255 nm. It turns out that it absorbs light.

  In addition, it was found that the ultraviolet-visible light absorption spectrum becomes a straight line (not shown) when the absorption photon energy is raised to the 0.5th power, and has a band gap based on an indirect transition of about 3.3 eV. . This band gap is narrower than that of many existing semiconductor oxide nanosheets (for example, 2D pyrochlore-type tungsten oxide nanosheets are 3.6 eV, and lipidocrosite-type titanium oxide nanosheets are 3.8 eV). As a photocatalyst material covering up to, it is advantageous for application development such as decomposition / removal of organic components, water decomposition, photo-induced hydrophilicity, and solar cells.

Next, a colloidal solution obtained under the conditions of TBA ⁺ / H ⁺ = 1, 2000 rpm for 30 minutes was dropped on a Cu grid, and TEM observation was performed. In this colloidal solution, a sheet-like substance having a uniform contrast and a sharp edge can be observed as shown in the TEM image of FIG. This uniform and thin contrast is attributed to the fact that the sheet-like material found in the colloidal solution is a nanosheet that has been subdivided to the nano level by monolayer peeling of the precursor. The observed electron diffraction pattern of the nanosheet shown in FIG. 3B shows the host layer (a = 1.464 (1) nm, c = 0.664 (2)) of the precursor layered polytungstic acid bulk material. Indexing was possible with the same two-dimensional rectangular lattice as in n) (a = about 1.4 nm, c = about 0.78 nm). This result is the basis on which the nanosheet dispersed in the colloid forms a two-dimensional skeleton having the same 2D bronze structure as the host layer of the starting material shown in FIG.

Next, the Si substrate whose surface was cleaned was immersed in 100 cm ³ of an aqueous solution of polydiallyldimethylammonium chloride (PDDA: 2.5 gdm ⁻³ , 10 minutes) to coat the surface of the Si substrate with a polycation. The colloidal solution obtained under the conditions of TBA ⁺ / H ⁺ = 1, 2000 rpm for 30 minutes was diluted to 0.4 gdm ⁻³ with ultrapure water (specific resistance value: 18 MΩcm), and the Si substrate coated with polycation was immersed for 10 minutes. did. At this time, the 2D bronze-structured tungsten oxide nanosheet exfoliated with a single layer had a negative charge and could be preferentially adsorbed on the substrate by a layer-by-layer self-assembly reaction with a polycation.

The result of AFM observation of the obtained thin film sample is shown in FIG. In FIG. 4A, a large number of sheet-like substances having sharp edges with a lateral size of about 100 nm to 1 μm could be observed. This is in good agreement with the sheet-like material observed by TEM. From the cross-sectional profile of the sheet-like material shown in FIG. 4B, it was found that the sheet thickness was about 3 nm, indicating that it was a flaky material having a large two-dimensional anisotropy, that is, a nanosheet. This value corresponds to one layer of the 2D bronze type host layer of the original layered compound having a crystallographic thickness of 2.5 nm including the van der Waals radius of oxygen, and is in the form of a sheet adsorbed on the Si substrate. This is evidence that the material forms a two-dimensional skeleton having the same 2D bronze structure as the host layer of the starting material Rb ₄ W ₁₁ O ₃₅ shown in FIG. Moreover, when the coverage was calculated | required from the histogram (not shown) of height, it turned out that it is a 2D bronze-type tungsten oxide nanosheet monolayer film of about 50% (overlap part is less than 10%).

As described above, by mixing and shaking with bulky ammonium ions according to the present invention, layered polytungstic acid Rb _4-x H _x W ₁₁ O ₃₅ hydrate (0 ≦ x ≦ 4) having a 2D bronze structure host layer is obtained. A single-layer exfoliation phenomenon was induced, and it was shown that it can be taken out as an extremely thin sheet-like material “2D bronze-type tungsten oxide nanosheet”.

(Example 3)
PDDA aqueous solution (2.5 gdm ⁻³ , 10 minute immersion) and nanosheet colloidal solution (0.4 gdm ⁻³ ) used for the synthesis of the single layer film coated with about 2% of the 2D bronze type tungsten oxide nanosheet of Example 2 Were used to alternately stack on the SiO ₂ substrate. First, the substrate was immersed in an aqueous PDDA solution for 10 minutes to adsorb polycations, washed with water, and then immersed in a colloidal solution of tungsten oxide nanosheets to synthesize a single layer film. The process of immersing again in the PDDA aqueous solution for 10 minutes and then immersing in the tungsten oxide nanosheet colloidal solution was further repeated nine times. At this time, the result of monitoring by ultraviolet-visible light absorption spectrum measurement every time the 2D bronze-type tungsten oxide nanosheet is adsorbed is shown in FIG.

  Also in the 2D bronze type tungsten oxide nanosheet adsorbed on the substrate 10 times in total, a spectrum having the same shape as the colloidal ultraviolet-visible light absorption spectrum of FIG. 2 was obtained. From the plot of the absorbance at the shoulder portion (255 nm) of the absorption peak, which increases with decreasing wavelength, and the number of laminations, it was revealed that the absorbance increased in proportion to the number of laminations. This indicates that layer-by-layer alternate adsorption occurs due to electrostatic self-assembly reaction between the 2D bronze-type tungsten oxide nanosheet and the organic cation. In addition, the X-ray diffraction pattern (see FIG. 8a) of the nanosheet thin film adsorbed on the substrate a total of 10 times shows a diffraction peak of d = 3.4 nm corresponding to the distance between the stacked upper and lower nanosheets and its higher-order reflection. Was observed, indicating a multilayer film having an organic / inorganic hybrid superlattice in which 2D bronze-type tungsten oxide nanosheets are accumulated.

The organic / inorganic hybrid multilayer film in which the 2D bronze type tungsten oxide nanosheets were accumulated was irradiated with light (546 nm emission line intensity 5.8 mWcm ⁻² nm ⁻¹ ) with a HgXe lamp. When X-ray diffraction measurement was performed after light irradiation for 1 hour (see FIG. 8b), a peak at d = 3.4 nm (2θ = 2.6 °) corresponding to the distance between the original nanosheets disappeared, and d = 3 A new peak of .1 nm (2θ = 2.9 °) was observed. Furthermore, as shown in FIGS. 8b and 8c, it was found that the interlayer distance finally decreased to d = 2.8 nm (2θ = 3.1 °) by light irradiation for 4 hours or more. This decrease of about 0.6 nm can be attributed to the fact that PDDA was decomposed to ammonium ions. Actually, in the semiconductor titanium oxide nanosheet / PDDA multilayer film, an interlayer reduction of about 0.5 nm has been found by ultraviolet irradiation (Non-patent Document 3).

  As described above, the 2D bronze-type tungsten oxide nanosheet of the present invention is an ultrathin film having an arbitrary film thickness from a single layer film to a multilayer film by a self-organization reaction with a polycation utilizing the negative charge, and is large at room temperature. Area film formation is possible. In addition to the good material formability of such nanosheets, it has the feature of having a thick crystal structure as a 2D bronze structure and a relatively narrow band gap compared to existing nanosheet systems, and is intended for the decomposition and removal of organic substances. Development of a visible / ultraviolet light responsive photocatalytic coating material can be expected. Furthermore, it can be widely applied to water splitting, photoinduced hydrophilicity, solar cells and the like related to the photocatalytic reaction.

Example 4
The colloidal solution obtained in Example 2 (TBA ⁺ / H ⁺ = 1, 2000 rpm for 30 minutes) was diluted to 0.4 gdm ⁻³ using ultrapure water (specific resistance value: 18 MΩcm). Next, the diluted colloidal solution was put into a cell with a sealing film having a 1 cm optical path length, and deoxygenated by introducing nitrogen gas into the solution. With this as the initial state, ultraviolet-visible light absorption spectrum measurement was performed. Next, light irradiation was performed for 10 minutes (546 nm emission line intensity 5.8 mWcm ⁻² nm ⁻¹ ) using an HgXe lamp, and ultraviolet-visible light absorption spectrum measurement was performed again. The state of the cell before and after the light irradiation and the ultraviolet-visible light absorption spectrum are shown in FIG.

  The color of the colloidal solution changed from yellow to blue by light irradiation. From the ultraviolet-visible light absorption spectrum, it was found that the absorbance significantly increased in the visible / infrared light region of 600 to 1200 nm with light irradiation. Therefore, when the difference between the UV-visible light absorption spectra before and after the light irradiation was taken (FIG. 9 inset), the absorbance increased as the wavelength increased, and the maximum in the infrared region of 1200 nm, which is the maximum value in the measurement. It turns out that it shows a change. In the characteristics evaluation in this water dispersion system, since the change in the infrared absorption spectrum of water due to a slight temperature change is large, correct measurement at a wavelength longer than 1200 nm is difficult, but even at wavelengths longer than 1200 nm from the shape of the differential spectrum. It can be expected to absorb enough.

  As described above, the 2D bronze-type tungsten oxide nanosheet of the present invention is a photochromic and electrochromic material like other hydrated tungsten oxides. However, it is completely different from conventional 2D pyrochlore-type nanosheets and nanoparticles in terms of a crystal structure that is important in considering photochromism. This difference is considered to be reflected in the photochromic characteristics. From the above, the 2D bronze structure nanosheet exhibits a characteristic of absorbing infrared light stronger than visible light. This light absorption characteristic like a window through which visible light passes is advantageous for application development to ultraviolet and infrared cut coating materials required in automobiles, housing industries, and the like.

In each of the above examples, an example in which Rb ₄ W ₁₁ O ₃₅ is used as a mother crystal has been shown. However, if the layered compound has a 2D bronze structure similar to Rb ₄ W ₁₁ O ₃₅ , Rb ₄ W It is believed that compounds other than ₁₁ O ₃₅ could be used and there is no reason to exclude this. For example, A ₄ W ₁₁ O ₃₅ (A = Li, Na, K, Cs) is conceivable as an alkali layered polytungstic acid having the same composition ratio, but none of these has reported a synthesis method. Among the materials that can be used as starting materials, Rb ₄ W ₁₁ O ₃₅ is most preferred at this stage.

  The present invention provides a single-layer exfoliation of a layered compound in which a 2D bronze type host layer forming a one-dimensional tunnel in which a six-membered ring structure composed of metal-oxygen octahedrons is regularly arranged in the sheet cross-sectional direction is accumulated. In this way, we have succeeded in obtaining nanosheets with a 2D bronze structure that are rich in film formability and coating properties, and are used in various fields such as catalysts, various thin film materials and coating materials by taking advantage of their unique morphology and crystal structure. It is expected that In particular, a six-membered ring structure composed of a metal-oxygen octahedron can be used as an ion exchange site as described above, which is a great advantage for sensors and catalysis utilizing adsorption of hydrogen, water, molecules, etc. It is expected. Furthermore, if ion conductivity is imparted by introducing alkali ions into the 2D bronze structure, it is expected to exhibit excellent characteristics as a coating material for solid electrolytes and electrode materials such as lithium ion secondary batteries.

Tungsten itself is recognized as an extremely important metal in the industry because it is used in large quantities in filaments and cutting tools, etc., but it is a material with a low crustal abundance and also has a large bias in the production area. There is. In Japan, tungsten is almost entirely imported, so in the development of materials that use tungsten, it will be increasingly recycled in the future. Yes. In particular, tungsten oxide is or dimming mirror for motor vehicles, NO _x cracking catalyst in the flue gas, such as factories and power stations, it is widespread also widely present as a desulfurization and denitration catalyst used in the petrochemical and petroleum refining, Industrial use value is very high. The 2D bronze nanosheet of the present invention is also a tungsten oxide-based nanomaterial, and is useful in that it has a shape and properties different from those of existing materials.

  In recent years, anti-fogging / self-cleaning technology for window glass using photocatalytic action has become widespread as an energy-saving technology. In order to effectively express these functions not only outdoors but also indoors, it is indispensable to further increase the sensitivity and use visible light. Addition of promoters such as dissimilar metals to nitrogen-doped titanium oxide and tungsten oxide Various systems such as composite materials have been energetically searched. The 2D bronze-type tungsten oxide nanosheets of the present invention are narrower than the band gap of many existing semiconductor oxide nanosheets because of the maximum thickness of 4 nm derived from the 2D bronze structure and the bonded structure of metal-oxygen octahedron. Despite the system of tungsten alone, the photocatalytic effect is exhibited by ultraviolet / visible light irradiation. In other words, the nanosheet can be applied in various fields that utilize the photocatalytic effect, from organic matter decomposition to water decomposition, light-induced hydrophilicity, solar cells, etc. It is expected that the spread of the photocatalytic material will be promoted by achieving the above.

  Furthermore, like other tungsten oxides, the 2D bronze-type tungsten oxide nanosheets obtained by the present invention are also photochromic and electrochromic materials. Compared to bulk materials, nanosheets have the advantage of shape that they are thin and have a large area, and by using the internal adsorption sites unique to the 2D bronze structure, strong color development is expected in a very small amount. Because of the above-mentioned self-organization reaction method, etc., it is rich in film forming and coating properties, and the film thickness can be easily controlled. Therefore, highly efficient, low-cost photochromic and electrochromic materials have been developed through the environmentally low route, sunglasses, and smart. It is expected that a completely new light control thin film using the same component such as glass will be produced.

That is, a tungsten oxide nanosheet having a 2D bronze structure is provided by the present invention, and is expected to be actively used and utilized in material design that requires the same component including the above-described uses. Significance is enormous.

Claims (10)

A nanosheet obtained by exfoliating a precursor comprising a host layer having a two-dimensional (2D) bronze structure. 2. The nanosheet according to claim 1, wherein the 2D bronze structure is a one-dimensional tunnel-like structure in which a plurality of six-membered ring structures composed of metal-oxygen octahedrons are regularly arranged in the sheet cross-sectional direction of the nanosheet. Characteristic nanosheet.   A tungsten oxide nanosheet having a 2D bronze structure.   The tungsten oxide nanosheet according to claim 3, wherein a molar ratio of W and O in the skeleton composed of the metal-oxygen octahedron satisfies 11:35.   4. The tungsten oxide nanosheet according to claim 3, wherein the 2D bronze structure is a one-dimensional tunnel-like structure in which a plurality of six-membered ring structures composed of metal-oxygen octahedrons are regularly arranged in the sheet cross-sectional direction of the tungsten oxide nanosheet. A tungsten oxide nanosheet, characterized in that   The tungsten oxide nanosheet according to claim 3, wherein the tungsten oxide nanosheet has a sheet cross-sectional thickness of 1 nm to 4 nm or less. 4. The tungsten oxide nanosheet according to claim 3, wherein the tungsten oxide nanosheet is formed by exfoliating a single layer of a layered tungsten oxide represented by a general formula Rb ₄ W ₁₁ O _{35. 5} .   A method for producing a tungsten oxide nanosheet having a 2D bronze structure, wherein an acid treatment step of acid-treating a precursor, a mixing step of mixing the acid-treated layered tungsten oxide and a cationic peeling accelerator A method for producing a tungsten oxide nanosheet having a 2D bronze structure.   A photocatalyst using the tungsten oxide nanosheet according to any one of claims 3 to 7. A photochromic element using the tungsten oxide nanosheet according to any one of claims 3 to 7.