Structural color-exhibiting material and its manufacturing method

Abstract

To provide a structure color expression material with an excellent environmental responsivity to surrounding vapor, and a method for manufacturing the material.SOLUTION: The structure color expression material has a laminate structure D made of a composite body of several different types of particles, at least one of the composite particles of the composite body being a tabular particle 2, other composite particles 3 of the composite body being among tabular particles 2, and a gap G being formed in the tabular particles 2.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a structural color developing material having environmental responsiveness and a method for producing the structural color developing material. [Background technology]

[0002] Structural color is a coloring phenomenon caused by the interference of visible light due to highly regular geometric structures, and is known to be closely related to the coloring phenomena of natural products and living organisms, such as the coloring of gemstones such as opals and insect wings.

[0003] In recent years, research into the application of such coloring mechanisms in structures formed by the accumulation of synthesized organic polymer particles or inorganic particles has been attracting attention. In particular, the structural color of colloidal crystals formed spontaneously by silica colloidal particles is known as a representative example.

[0004] Structural colors exhibited by closely packed monodispersed spherical particles with perfectly uniform particle diameters have been widely studied.

[0005] In order to artificially produce such structural colors, it is necessary to strictly control both the particle size and the aggregate structure of the monodispersed spherical particles that form the basis for the structural color. Therefore, compared to when dyes are used, many strictly controlled processes are required to obtain a structural color-producing material, and there are also problems that need to be improved in terms of cost, etc.

[0006] Therefore, the above-mentioned problems have been solved by forming a laminate structure including a plate-like structure and exhibiting structural color due to so-called Bragg reflection. For example, Patent Document 1 discloses a technology relating to a structural color exhibiting material using such a laminate structure including a plate-like structure.

[0007] The technology described in Patent Document 1 forms a regular laminate structure using plate-like structures in which gluconic acid is bonded to the surface of layered titanic acid, and then incorporates a hydrogen-bonding compound such as glycerin between the plate-like structures to form an integrated structure in which the plate-like structures and the liquid or solid phases of the hydrogen-bonding compound are alternately laminated, thereby obtaining a structural color-producing material that exhibits structural color through Bragg reflection.The structural color-producing material described in Patent Document 1 can control the stacking spacing of the plate-like structures by changing the ratio of the hydrogen-bonding compound to the plate-like structures, and the wavelength of the structural color that is produced can be controlled by changing the stacking spacing of the plate-like structures. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Patent No. 6654319 (pages 4-5, Figure 3) Summary of the Invention [Problem to be solved by the invention]

[0009] In Patent Document 1, for example, a structural color-producing material having an alternating laminate structure of a plate-like structure and a liquid phase of a hydrogen-bonding compound is disclosed. When gas or other components dissolve into the liquid phase from the outside, the refractive index of the liquid phase changes, and the light diffraction conditions change, thereby changing the wavelength of the structural color. This structural color change makes the structural color-producing material potentially applicable as an environmentally responsive sensor for various gases, including water vapor. However, because the dissolution of various gases from the outside into the liquid phase is diffusion-limited, the response speed of the structural color change is very slow, and there are problems with the responsiveness for use as a sensor.

[0010] The present invention has been made in view of these problems, and an object of the present invention is to provide a structural color-exhibiting material that has excellent environmental response to surrounding vapors, and a method for producing the same. [Means for solving the problem]

[0011] The structural color-exhibiting material of the present invention that solves the above problems comprises: It has a laminated structure made of a composite of multiple types of particles, At least one of the constituent particles in the composite is a plate-like particle, The composite is characterized in that other constituent particles of the composite are present between the plate-like particles, and voids are formed between the plate-like particles. According to this feature, in the laminate structure of particle composites, vapor enters the minute voids formed between the plate-like particles by the other constituent particles, causing capillary condensation, and liquid or solid matter is adsorbed onto the plate-like particles, turning the voids into a liquid or solid phase. This changes the refractive index in the voids of the laminate structure, changing the light diffraction conditions and thereby changing the wavelength of the structural color. Furthermore, because the capillary condensation of vapor in the minute voids formed between the plate-like particles occurs in an extremely short time, the structural color-producing material has excellent environmental response to surrounding vapor.

[0012] The other constituent particles are characterized in that they are spacer particles that determine the stacking intervals of the plate-like particles. According to this feature, by controlling the particle size and number of other constituent particles, which are spacer particles present between the plate-like particles, it is possible to control the stacking spacing of the plate-like particles and the resulting structural color development characteristics.

[0013] The stacking interval of the plate-like particles is characterized by being 1000 nm or less. According to this feature, capillary condensation of vapor is likely to occur in the gaps formed between the plate-like particles, which makes it easier to change the wavelength at which the structural color is expressed.

[0014] The plate-like particles are characterized by containing cerium oxide as a main component. This feature allows the particles to easily assume the shape of plate-like particles and have a high refractive index, which allows them to easily form a laminate structure and exhibit structural colors. Here, the term "main component" refers to the substance that accounts for the largest proportion of the components constituting the plate-like particles per mass, and for example, the component constituting the plate-like particles may contain 50 mass% or more of cerium oxide.

[0015] The other constituent particles are characterized in that they contain a sugar derivative as a main component. According to this feature, the sugar derivative particles adhere to the surface of the plate-like particles to easily form a complex. Here, the main component is the substance that is the most abundant component per mass among the components that make up the other constituent particles, and for example, the sugar derivative may be contained in an amount of 50 mass% or more as a component that makes up the other constituent particles.

[0016] The sugar derivative is characterized by being a sugar alcohol. According to this feature, fine crystal particles of sugar alcohol adhere to the surface of the plate-like particles to form a complex, and thus the stacking intervals of the plate-like particles can be precisely controlled.

[0017] The sugar alcohol is characterized by being sorbitol. According to this feature, the stacked state of the laminate structure of the complex of the plate-like particles and the sorbitol particles is easily aligned, so that light scattering is reduced and structural color is easily exhibited.

[0018] The method for producing the structural color-exhibiting material of the present invention comprises the steps of: synthesizing a platelet particle sol by peptization of the platelet particle gel; mixing a sugar derivative with the plate-like particle sol; The method is characterized by comprising a step of evaporating the solvent from the mixed solution of the plate-like particle sol and the sugar derivative. According to this feature, by evaporating the solvent from a mixed solution of a plate-like particle sol synthesized by peptization and a sugar derivative, spontaneous precipitation of a composite of plate-like particles and sugar derivative particles and accumulation of a layered structure occur, making it possible to obtain a structural color-expressing material through a simple process.

[0019] It is characterized by having a deflocculation temperature of 55°C to 115°C. According to this feature, light scattering caused by an increase in the particle size of the plate-like particles is suppressed, and structural colors are easily exhibited. [Effects of the Invention]

[0020] As described above, the present invention can provide a structural color-exhibiting material that is excellent in environmental response to surrounding vapors, and a method for producing the same. [Brief explanation of the drawings]

[0021] [Figure 1] 1 is a diagram showing an image of a laminate structure of a structural color developing material according to an embodiment of the present invention. [Figure 2] FIG. 1 is a diagram showing an image of a state in which the stacking intervals of plate-like particles are controlled by spacer particles in the stacked structure of the structural color generating material according to an embodiment. [Figure 3] 1 is a diagram showing an image of a state in which voids in a laminate structure of a structural color developing material according to an embodiment are in a liquid phase or a solid phase. FIG. [Figure 4] 1 is a diagram showing an image of a laminate structure formed from a structural color developing material according to an embodiment. FIG. [Figure 5] FIG. 2 shows XRD patterns of dry powders of cerium oxide particle-dispersed sols synthesized at different peptization temperatures in Example 1. [Figure 6] FIG. 2 is a photograph showing the state of a cerium oxide particle-dispersed sol synthesized by changing the peptization temperature in Example 1. [Figure 7] FIG. 2 is a diagram showing the transmittance spectra of cerium oxide particle-dispersed sols synthesized by changing the peptization temperature in Example 1. [Figure 8] FIG. 2 is a graph showing the particle size distribution of cerium oxide particles in cerium oxide particle dispersion sols synthesized by changing the peptization temperature in Example 1. [Figure 9] FIG. 2 is a photograph showing the change in structural color of a cerium oxide particle-sorbitol particle composite thin film in a dry state and in a hygroscopic state, which was prepared using a cerium oxide particle dispersion sol synthesized at different peptization temperatures in Example 1. [Figure 10] FIG. 2 is a photograph showing the reversibility of the structural color change of a cerium oxide particle-sorbitol particle composite thin film in a dry state and in a hygroscopic state in Example 1. [Figure 11](a) is an SEM image of the dried powder of the cerium oxide particle-dispersed sol synthesized at a peptization temperature of 95°C in Example 1, and (b) is an SEM image of the dried state of a cerium oxide particle-sorbitol particle composite thin film produced using the cerium oxide particle-dispersed sol synthesized at a peptization temperature of 95°C. [Figure 12] FIG. 1 shows UV-Vis transmittance spectra in a dry state and a hygroscopic state of a cerium oxide particle-sorbitol particle composite thin film produced using a cerium oxide particle dispersion sol synthesized in Example 1 at a peptization temperature of 55° C. to 95° C. [Figure 13] FIG. 1 shows UV-Vis transmittance spectra in a dry state and a hygroscopic state of a cerium oxide particle-sorbitol particle composite thin film produced using a cerium oxide particle dispersion sol synthesized in Example 1 at a peptization temperature of 115° C. to 155° C. [Figure 14] FIG. 1 shows UV-Vis transmittance spectra of a cerium oxide particle-sorbitol particle composite thin film and a cerium oxide particle-mannitol particle composite thin film in a dry state and a hygroscopic state, both of which were prepared using the cerium oxide particle dispersion sol synthesized at a peptization temperature of 95°C in Example 2, and an image of the integrated state of the laminate structure. DETAILED DESCRIPTION OF THE INVENTION

[0022] Hereinafter, embodiments of the present invention will be described in detail with reference to Figures 1 to 14. However, the present invention can be embodied in many different forms and is not limited to the following embodiments and examples.

[0023] As shown in Figure 1, the structural color developing material according to the present invention (hereinafter referred to as "the material") has a structural color domain D as a laminate structure made up of a composite of multiple types of particles. More specifically, the material is composed of a composite of plate-like particles 2 and spacer particles 3 as other constituent particles, and minute voids G are formed between the plate-like particles 2 due to the presence of the spacer particles 3 in a columnar shape. In other words, the spacer particles 3 are scattered so as to contact only part of the surface of the plate-like particles 2, and the spacer particles 3 are not in contact with the other parts of the surface of the plate-like particles 2, forming the voids G.

[0024] Furthermore, in this material, the plate-like particles 2 accumulate to form a plate-like structure. This allows multiple plate-like structures to spontaneously stack in one direction to form a layer. By aligning the stacking spacing of the plate-like particles 2 in this plate-like structure with the spacer particles 3 that are combined with the plate-like particles 2, structural color can be expressed as structural color domain D through so-called Bragg reflection. Furthermore, in this material, as shown in Figure 2, by changing the amount of spacer particles 3 that are combined with the plate-like particles 2, it is possible to control the stacking spacing of the plate-like particles 2 in the plate-like structure, i.e., the size of the gap G, and therefore it is possible to change the wavelength of the structural color expressed by the structural color domain D.

[0025] The stacking interval of the plate-like particles 2 in this material is not limited as long as the capillary condensation phenomenon of vapor occurs in the gaps G formed between the plate-like particles 2. For example, to enable the appearance of structural colors in the visible region, the stacking interval of the plate-like particles 2 is preferably 0.1 nm or more and 1000 nm or less, and more preferably 1 nm or more and 500 nm or less.

[0026] The thickness of the plate-like structures in this material is not limited as long as it can maintain a sufficient stacking distance between the plate-like particles 2 to allow the appearance of structural color. For example, to allow the appearance of structural color in the visible region, the thickness of the plate-like structures is preferably 0.001 μm or more and 1 μm or less.

[0027] In addition, in this material, the component of the plate-like particles 2 is not limited as long as it can form plate-like particles, but is preferably, for example, cerium oxide, zinc oxide, titanate compounds, phosphates (Zr, Ti, etc.), or hydroxyapatite, with cerium oxide being particularly preferred. By using cerium oxide as the component of the plate-like particles 2, due to its high refractive index and its tendency to easily take the form of plate-like particles with strong two-dimensional anisotropy, it is easy to form a layered structure in which many plate-like structures are stacked, and structural color is easily expressed.

[0028] In addition, while conventional structural color-producing materials (see Patent Document 1) require the entire surface of the plate-like structure to be modified with sugar acid, chemical modification of the surface of the plate-like particles 2 is not essential in this material. Therefore, it is possible to use various components capable of forming plate-like particles, such as cerium oxide.

[0029] In this material, the components of the spacer particles 3 are not particularly limited as long as they can be attached to the surfaces of the plate-like particles 2, but a water-soluble sugar derivative is preferred. The sugar derivative is not particularly limited and is selected from sugar alcohols and glucogenic amino acids, with sugar alcohols being particularly preferred. By using a sugar alcohol as a component of the spacer particles 3, a complex can be formed in which fine crystal particles of the sugar alcohol are attached to the surfaces of the plate-like particles 2, and the spacer particles 3 can precisely control the stacking spacing of the plate-like particles 2. The form of attachment is not particularly limited, but a chemical bond, particularly a coordinate bond, is preferred.

[0030] As the sugar alcohol, sorbitol, mannitol, pentaerythritol, xylitol, etc. can be preferably used, and sorbitol, which has high solubility, is particularly preferred.

[0031] Furthermore, the content of the sugar derivative in this material is not particularly limited as long as the stacking spacing of the plate-like particles 2 can be appropriately controlled, but it is preferably 0.05% by mass to 20% by mass, and more preferably 0.1% by mass to 10% by mass, relative to the total solid content.

[0032] Furthermore, the mass ratio of the plate-like particles to the sugar derivative in this material is not particularly limited as long as a laminate structure is formed by a complex of the plate-like particles and the sugar derivative, but is preferably 0.01-20:10-100, and more preferably 0.1-10:20-100.

[0033] As shown in Figure 3, minute voids G formed between plate-like particles 2 in this material become liquid or solid due to the infiltration of surrounding vapor and the occurrence of capillary condensation. At this time, the stacking spacing of the plate-like particles 2 determined by the spacer particles 3 is maintained and remains almost unchanged even when the voids become liquid or solid. "Almost unchanged stacking spacing of the plate-like particles 2" means that the spacing remains unchanged within an increase range of about 50%. In this embodiment, the vapor components that are subject to capillary condensation include not only substances that condense into liquids as gases, but also substances that sublimate into solids as gases.

[0034] For example, the refractive index of the minute gaps G formed between the plate-like particles 2 in this material changes significantly between the refractive index (n=1.00) when air is present (hereinafter referred to as the "dry state") and the refractive index (n=1.33) when surrounding water vapor enters, causing capillary condensation and liquid water being present.

[0035] Furthermore, the capillary condensation phenomenon in the tiny gaps G formed between the plate-like particles 2 in this material occurs in a very short time, allowing the wavelength of the structural color to be changed in rapid response to changes in the surrounding humidity, etc.

[0036] In addition, the material is not limited to water vapor (water), and the vapor components are not limited as long as they become liquid or solid due to capillary condensation in the tiny gaps G formed between the plate-like particles 2.

[0037] As described above, this material reacts to various vapors in the environment, and the wavelength of the structural color it emits changes rapidly depending on the vapor pressure, making it a structural color-expressing material with excellent environmental responsiveness to surrounding vapors.

[0038] Next, a method for producing the structural color-exhibiting material according to the present invention (hereinafter referred to as "the method for producing the material") will be described.

[0039] The method for producing this material includes the steps of synthesizing a plate-like particle sol by peptizing a plate-like particle gel, mixing a sugar derivative with the plate-like particle sol, and evaporating the solvent from the mixed solution of the plate-like particle sol and the sugar derivative.

[0040] Specifically, a plate-like particle gel is first left to stand in water at low temperature to synthesize a plate-like particle sol in which plate-like particles having two-dimensional anisotropy are dispersed by peptization. Peptization allows for both uniform dispersion of the plate-like particles and control of the morphology and particle size of the plate-like particles. The solvent used for peptization is not limited to water, and is not particularly limited as long as it can stably disperse the plate-like particles and dissolve the sugar derivative.

[0041] Next, a sugar derivative is mixed with the plate-like particle sol to prepare a mixed solution, and the solvent is evaporated from the mixed solution, thereby increasing the concentration of the sugar derivative while maintaining a uniform dispersion of the plate-like particles 2 in the mixed solution, as shown in FIG. 4, and resulting in the precipitation of a composite of the plate-like particles 2 and the sugar derivative particles 30. The composite precipitation refers to the precipitation of the plate-like particles 2 and the sugar derivative particles 30 in a complex state in the mixed solution. Furthermore, by evaporating the solvent from the mixed solution and drying it, a regular layered structure (structural color domain D) can be formed by the complex of the plate-like particles 2 and the sugar derivative particles 30. Furthermore, at this time, the layered structure spontaneously aggregates.

[0042] As described above, the manufacturing method for structural color-expressing materials involves evaporating the solvent from a mixed solution of a plate-like particle sol synthesized by peptization and a sugar derivative, which causes spontaneous precipitation of a composite of plate-like particles and sugar derivative particles and accumulation of a layered structure, thereby allowing structural color-expressing materials to be obtained in a simple process. [Example]

[0043] Here, the structural color developing material of Example 1 according to the above embodiment was actually produced, and its effects were confirmed. Specific explanations are given below.

[0044] (Synthesis of cerium oxide particle dispersion sol) First, 50 ml of a 0.1 M aqueous solution of cerium nitrate and 50 ml of a 0.2 M aqueous ammonia were mixed, stirred, and allowed to stand to prepare an aqueous solution containing a cerium hydroxide precipitate.

[0045] Thereafter, the aqueous solution containing the cerium hydroxide precipitate was centrifuged, and the solvent was removed to obtain a cerium hydroxide precipitate (gel).

[0046] This cerium hydroxide precipitate was then dispersed in 100 ml of water, and the mixture was left to stand for 24 hours while being heated at atmospheric pressure at 55°C, 75°C, and 95°C, respectively, to deflocculate the precipitate, yielding a cerium oxide particle-dispersed sol in which cerium oxide particles were dispersed as plate-like particles. Similarly, cerium hydroxide precipitate was dispersed in 100 ml of water, and the mixture was left to stand for 24 hours while being heated at 95°C, and then further heated in an autoclave for 24 hours at 115°C, 135°C, and 155°C, respectively, to deflocculate the precipitate, yielding a cerium oxide particle-dispersed sol.

[0047] The crystallinity of the particles in the cerium oxide particle dispersion sol obtained by peptization was investigated. The XRD patterns of the powders obtained by drying the cerium oxide particle dispersion sols processed at each peptization temperature at 75°C for 12 hours are shown in Figure 5.

[0048] As shown in Figure 5, in the cerium oxide particle dispersion sols processed at each peptization temperature, all peaks were attributed to cerium oxide diffraction peaks, confirming the production of cerium oxide. It was also confirmed that the half-width of the peaks did not change significantly even when the temperature conditions were changed.

[0049] Next, the state of the cerium oxide particle-dispersed sol obtained by peptization was examined. Photographs of the cerium oxide particle-dispersed sols treated at each peptization temperature are shown (see Figure 6).

[0050] As shown in Figure 6, no precipitates were formed in any of the cerium oxide particle-dispersed sols processed at each peptization temperature, and a milky white sol was obtained that maintained a stable dispersion. In particular, the cerium oxide particle-dispersed sol heated in an autoclave exhibited a dispersion state with stronger light scattering than the cerium oxide particle-dispersed sol heated at atmospheric pressure. This is presumably due to an increase in the size of the cerium oxide particles dispersed in the sol.

[0051] Next, we investigated in detail the effect of the deflocculation temperature on the generation of cerium oxide particles. The transmittance spectra of cerium oxide particle-dispersed sols processed at various deflocculation temperatures are shown in Figure 7.

[0052] As shown in Figure 7, the transmittance of the cerium oxide particle-dispersed sols processed at each deflocculation temperature was 0% at wavelengths below 370 nm. This indicates absorption due to interband electronic transitions. Furthermore, at deflocculation temperatures of 55°C, 75°C, and 95°C, transmittance was 60% or higher in the visible light region above 500 nm. This indicates the production of stable dispersion sols with minimal light scattering. Furthermore, transmittance tended to decrease with increasing deflocculation temperature. This indicates a change in light scattering due to changes in the particle size of the cerium oxide particles. Furthermore, at deflocculation temperatures above 115°C, transmittance in the visible light region above 500 nm significantly decreased to below 40%. This is presumably due to increased particle size of the cerium oxide particles.

[0053] Next, the particle size distribution of the cerium oxide particles in the cerium oxide particle dispersion sols treated at each peptization temperature was measured (see FIG. 8).

[0054] From the scattering intensity distribution shown in Figure 8, it was confirmed that particles with a diameter of approximately 40 nm were generated in the cerium oxide particle dispersion sol at a peptization temperature of 75°C. It was also confirmed that as the peptization temperature increased to 55°C, 95°C, 115°C, and 135°C, the generation of particles with diameters of approximately 100 nm to 400 nm increased stepwise.

[0055] From the above results, it was confirmed that the particle size of the cerium oxide particles can be controlled by the temperature when the cerium hydroxide precipitate is peptized with water.

[0056] (Preparation of cerium oxide particle-sorbitol particle composite thin film) Next, cerium oxide particle-sorbitol particle composite thin films with a laminated structure of cerium oxide particles were prepared from the cerium oxide particle dispersion sols processed at each peptization temperature, and the appearance and control of their structural color were investigated.

[0057] First, a glass substrate was immersed in a mixed solution of 20 g of cerium oxide particle dispersion sol treated at a deflocculation temperature of 55°C to 155°C and 0.0001 mol of sorbitol, and then dried at 55°C for 24 hours.

[0058] Another glass substrate was placed on top of the dry cerium oxide particle-sorbitol particle composite thin film formed on the surface of the glass substrate to block the influence of external moisture, etc. In this state, the structural color of the dry cerium oxide particle-sorbitol particle composite thin film, described below, was evaluated and the UV-Vis transmittance spectrum was measured.

[0059] A beaker filled with water was prepared, and the glass substrate placed on top was removed. The exposed cerium oxide particle-sorbitol particle composite thin film was placed in the beaker so that the liquid water did not come into contact with the beaker. The beaker was then sealed with plastic wrap and left to stand at room temperature for 4 hours, allowing the cerium oxide particle-sorbitol particle composite thin film to absorb moisture in an atmosphere of saturated water vapor pressure. After the moisture absorption treatment, another glass substrate was placed on top of the humidified cerium oxide particle-sorbitol particle composite thin film, and the structural color of the humidified cerium oxide particle-sorbitol particle composite thin film and the UV-Vis transmittance spectrum were measured, as described below.

[0060] The thickness of the cerium oxide particle-sorbitol particle composite thin film did not change between the dry and humid states.

[0061] Next, to confirm the change in the structural color of the cerium oxide particle-sorbitol particle composite thin film, we present photographs of the cerium oxide particle-sorbitol particle composite thin film prepared using the cerium oxide particle dispersion sols processed at various peptization temperatures in both the dry and humid states (see Figure 9).

[0062] As shown in Figure 9, all cerium oxide particle-sorbitol particle composite thin films prepared using cerium oxide particle-dispersed sols treated at peptization temperatures between 55°C and 155°C exhibited a yellow structural color in the dry state, confirming that there was no difference in the structural color depending on the peptization temperature. Furthermore, cerium oxide particle-sorbitol particle composite thin films treated with moisture absorption at room temperature exhibited the following structural colors: green at a peptization temperature of 55°C, red at a peptization temperature of 75°C, blue at a peptization temperature of 95°C, and light blue at a peptization temperature of 115°C. Furthermore, cerium oxide particle-sorbitol particle composite thin films prepared using cerium oxide particle-dispersed sols treated at peptization temperatures of 135°C and 155°C became opaque in the hygroscopic state, demonstrating no structural color.

[0063] Thus, it was confirmed that the wavelength of the structural color displayed by a cerium oxide particle-sorbitol particle composite thin film prepared using a cerium oxide particle-dispersed sol processed at a peptization temperature of 55° C. to 115° C., preferably 55° C. to 95° C., changes upon contact with water vapor. It was also confirmed that the structural color displayed upon contact with water vapor may be related to the peptization temperature during synthesis of the cerium oxide particle-dispersed sol, i.e., the particle size of the cerium oxide particles.

[0064] Furthermore, as shown in Figure 10, it was confirmed that the structural color of the cerium oxide particle-sorbitol particle composite thin film changes reversibly through repeated drying and moisture absorption. This indicates that vapor that enters the voids formed between the plate-like cerium oxide particles undergoes capillary condensation to become a liquid or solid phase, and then the liquid evaporates into gas, or the solid sublimes into gas, returning the film to a dry state with air remaining in the voids.

[0065] Here, we show SEM images of powders obtained by drying and grinding cerium oxide particle-dispersed sols treated at a deflocculation temperature of 95°C, and of a dried cerium oxide particle-sorbitol particle composite thin film prepared using the cerium oxide particle-dispersed sol treated at a deflocculation temperature of 95°C (see Figure 11).

[0066] As shown in Figure 11(a), the dried powder of the cerium oxide particle dispersion sol shows aggregation of plate-like cerium oxide particles with a particle size of 100 nm or less, and as shown in Figure 11(b), the cerium oxide particle-sorbitol particle composite thin film shows voids formed between the plate-like cerium oxide particles by the sorbitol particles used as spacers.

[0067] Next, the UV-Vis transmittance spectra of cerium oxide particle-sorbitol particle composite thin films prepared using cerium oxide particle dispersion sols treated at each peptization temperature are shown in the dry and humid states (see Figures 12 to 14).

[0068] As shown in Figure 12, the cerium oxide particle-sorbitol particle composite thin film prepared using a cerium oxide particle-dispersed sol processed at peptization temperatures between 55 and 95°C exhibited high transmittance of over 80% at wavelengths above 500 nm in the dry state, and only absorption due to interband electronic transitions at wavelengths below 400 nm was observed. In contrast, the cerium oxide particle-sorbitol particle composite thin film prepared using a cerium oxide particle-dispersed sol processed at a peptization temperature of 55°C exhibited a minimum transmittance, or reflection, peak at a wavelength of 500 nm in the humidified state. This indicates that this cerium oxide particle-sorbitol particle composite thin film diffracts and reflects light with a wavelength near 500 nm.

[0069] Furthermore, it was confirmed that a cerium oxide particle-sorbitol particle composite thin film prepared using a cerium oxide particle dispersion sol processed at a peptization temperature of 75°C exhibits a minimum peak in transmittance at a wavelength of around 660 nm in a hygroscopic state.Furthermore, it was confirmed that a cerium oxide particle-sorbitol particle composite thin film prepared using a cerium oxide particle dispersion sol processed at a peptization temperature of 95°C exhibits a minimum peak in transmittance at a wavelength of 430 nm and at twice that wavelength, 860 nm, in a hygroscopic state.

[0070] The transmittance spectrum of the moisture-treated cerium oxide particle-sorbitol particle composite thin film clearly shows peaks due to diffraction reflection, suggesting that a thin film structure consisting of regularly-assembled laminate structures (structural color domain D) of cerium oxide particle-sorbitol particle composites is formed. It is speculated that water vapor penetrates the gaps between the cerium oxide particle plates, causing capillary condensation and forming a liquid phase, which produces structural color due to diffraction. In particular, the cerium oxide particle-sorbitol particle composite thin film fabricated using a cerium oxide particle dispersion sol processed at a peptization temperature of 95°C exhibits a peak at 860 nm for the first-order diffraction light (nλ=2d sinθ(θ=90°)) at wavelengths of 860 nm and 430 nm for the second-order diffraction light (n=2). The presence of second-order diffraction light also suggests that structural color is produced by reflection from the multilayer laminate structure.

[0071] Furthermore, as shown in Figure 13, it was confirmed that the cerium oxide particle-sorbitol particle composite thin film prepared using a cerium oxide particle-dispersed sol processed at a peptization temperature of 115°C exhibited a minimum transmittance peak at a wavelength of around 760 nm in a hygroscopic state. Furthermore, it was confirmed that the cerium oxide particle-sorbitol particle composite thin film prepared using a cerium oxide particle-dispersed sol processed at peptization temperatures of 135°C and 155°C did not exhibit a clear minimum transmittance peak. This is presumably because the increase in cerium oxide particle size with increasing peptization temperature changed the state of composite formation between the cerium oxide particles and sorbitol particles, resulting in a decrease in the regularity of the laminate structure (loss of interference).

[0072] In addition, for cerium oxide particles with large particle sizes, increasing the amount of sorbitol particles combined can prevent uneven distribution of sorbitol particles precipitated on the surface of the plate-like particles of cerium oxide particles, thereby making the stacking spacing of the plate-like particles uniform.

[0073] From the above results, it was confirmed that in a cerium oxide particle-sorbitol particle composite system prepared using a cerium oxide particle dispersion sol processed at a deflocculation temperature of 55°C to 115°C, preferably 55°C to 95°C, highly regular laminate structures are spontaneously formed, and these laminate structures are regularly aligned and integrated, thereby favorably expressing structural color. [Example]

[0074] Next, the structural color developing material of Example 2 according to the above embodiment was actually produced, and its effects were confirmed.

[0075] (Preparation of cerium oxide particle-mannitol particle composite thin film) A cerium oxide particle-mannitol particle composite thin film was prepared using a cerium oxide particle-dispersed sol that had been treated at a peptization temperature of 95°C in the same manner as in Example 1, except that mannitol was used instead of sorbitol.

[0076] Mannitol, like sorbitol, is a sugar alcohol and an isomer of sorbitol. Although mannitol has the same hydrophilicity as sorbitol, its solubility is less than one-tenth that of sorbitol.

[0077] As shown in Figure 14, the transmittance of the cerium oxide particle-mannitol particle composite thin film prepared using the cerium oxide particle dispersion sol processed at a peptization temperature of 95°C was 40% or less at wavelengths of 500 nm or longer, demonstrating a significant decrease in transmittance compared to the thin film prepared using sorbitol. This indicates that the orientation of the cerium oxide particles in the cerium oxide particle-mannitol particle composite thin film is becoming increasingly disordered.

[0078] In contrast, the cerium oxide particle-sorbitol particle composite thin film and the cerium oxide particle-mannitol particle composite thin film, both of which were prepared using a cerium oxide particle dispersion sol processed at a deflocculation temperature of 95°C, showed minimal diffraction peaks at wavelengths of around 430 nm to 470 nm in a humidified state, confirming that a regular laminate structure exhibiting structural color was formed in both thin films.

[0079] From the above results, it can be inferred that the cerium oxide particle-mannitol particle composite thin film maintains the laminate structure (structural color domain D) that exhibits structural color, but the accumulation state of the laminate structure becomes disordered and irregularly aligned (see Figure 14) compared to the cerium oxide particle-sorbitol particle composite thin film, resulting in a decrease in transmittance.

[0080] The difference in the state of accumulation of the laminate structure is presumably related to the timing of the precipitation of sugar alcohol particles and the subsequent aggregation of cerium oxide particles, and is presumably due to the progress of the accumulation of the laminate structure as the composite precipitation of cerium oxide particles and sugar alcohol particles influence each other. Furthermore, the regularly aligned state of accumulation of the laminate structure of the cerium oxide particle-sorbitol particle composite thin film is presumably due to the close timing of the precipitation of sorbitol particles and the subsequent aggregation of cerium oxide particles.

[0081] Furthermore, the precipitation of cerium oxide particles and sugar alcohol particles will either result in composite precipitation or separate precipitation, in which cerium oxide particle aggregation and sugar alcohol particle precipitation occur separately, depending on the solubility of the sugar alcohol particles (spacer particles) and the trigger for nucleation in the solution. For example, if the solubility of the sugar alcohol particles is low, the sugar alcohol particles will precipitate individually at an earlier stage than the cerium oxide particle aggregation, resulting in separate precipitation. However, if the solubility of the sugar alcohol particles is high, the sugar alcohol particles will precipitate close to the cerium oxide particle aggregation, resulting in composite precipitation, as described above. It is also believed that the precipitation pattern of cerium oxide particles and sugar alcohol particles can be controlled by changing the drying rate of the solution or by drying under controlled precipitation conditions using a physical technique such as ultrasound irradiation to induce nucleation of the sugar alcohol particles.

[0082] As described above, these embodiments and examples can provide a structural color developing material that is excellent in environmental response to surrounding vapors, and a method for producing a structural color developing material.

[0083] Furthermore, the structural color-exhibiting material of the present invention can control the particle size and number of the sugar derivative particles used as spacer particles by changing the concentration of the sugar derivative mixed with the plate-like particle sol, the solvent evaporation rate in the mixed solution of the plate-like particle sol and the sugar derivative, etc. This allows the stacking spacing of the plate-like particles and the resulting structural color-exhibiting characteristics to be controlled.

[0084] Furthermore, the structural color-producing material of the present invention can impart selectivity to vapor molecules entering the gaps between the plate-like particles by controlling the chemical properties of the spacer particles and the surface properties of the plate-like particles. One method for controlling the surface properties of the plate-like particles is, for example, chemically modifying the surface of the plate-like particles with a surface coupling agent, thereby controlling the frequency of nucleation when spacer particles precipitate on the surface of the plate-like particles, thereby controlling the size of the spacer particles and the deposition density of the spacer particles on the surface. It is also believed that hydrophobizing the surface of the plate-like particles with a silanol-based coupling agent or a titanate-based coupling agent can reduce the deposition density of the spacer particles, increase the particle size of each spacer particle, and increase the stacking spacing between the plate-like particles. Conversely, it is also believed that increasing the number of hydroxyl groups on the surface of the plate-like particles to make them hydrophilic can increase the deposition density of the spacer particles, reduce the particle size of each spacer particle, and decrease the stacking spacing between the plate-like particles.

[0085] Furthermore, the structural color-exhibiting material of the present invention can be used as a highly sensitive humidity sensor that is portable and easy to use, since the color change occurs reversibly.

[0086] Furthermore, since the structural color developing material of the present invention has a structure that does not contain a liquid phase in a dry state, it can be made into a form that is easy to handle, such as a film. [Industrial Applicability]

[0087] The present invention has industrial applicability as a structural color-producing material and a method for producing a structural color-producing material that reacts with water vapor or various vapors in the environment and rapidly changes its color wavelength depending on the vapor pressure. The present invention has a wide range of applications, including ultraviolet and infrared blocking materials, moisture-proof material production, and cosmetics, as well as the use of the structural color-producing material in combination with other materials as pigments.

Claims

1. It has a laminated structure made of a composite of multiple types of particles, At least one of the constituent particles in the composite is a plate-like particle, A structural color-exhibiting material characterized in that other constituent particles in the composite are attached to the surfaces of the plate-like particles by coordinate bonds and scattered in a columnar shape between the plate-like particles, and voids are formed in places where the other constituent particles are not present.

2. 2. The structural color generating material according to claim 1, wherein the other constituent particles are spacer particles that determine the stacking intervals of the plate-like particles.

3. 3. The structural color generating material according to claim 1, wherein the stacking interval of the plate-like particles is 1000 nm or less.

4. 4. The structural color generating material according to claim 1, wherein the plate-like particles contain cerium oxide as a main component.

5. 5. The structural color generating material according to claim 1, wherein the other constituent particles are mainly composed of a sugar derivative.

6. 6. The structural color generating material according to claim 5, wherein the sugar derivative is a sugar alcohol.

7. 7. The structural color generating material according to claim 6, wherein the sugar alcohol is sorbitol.

8. synthesizing a plate-like particle sol in which plate-like particles are dispersed by peptizing the plate-like particle gel at a peptizing temperature of 55°C to 115°C; mixing a sugar derivative with the synthesized plate-like particle sol; a step of evaporating a solvent from the mixed solution of the plate-like particle sol and the sugar derivative, A method for producing a structural color-expressing material, characterized in that the sugar derivative is attached to the surface of the plate-like particles by coordinate bonds and scattered in a columnar shape between the plate-like particles, and voids are formed in areas where the sugar derivative is not present.

9. 9. The method for producing a structural color-exhibiting material according to claim 8, wherein the sugar derivative is sorbitol.

Images