JP2012123990A - Color conversion element and color conversion method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To propose a color conversion element and a color conversion method thereof which allow color conversion by easy processes and offer high durability, and to provide a technique useful for a sensor, a display element or the like.SOLUTION: There are provided a color conversion element and a color conversion method. The color conversion element comprises a base material containing metal and a coloring layer containing an oxide contacting to at least part of the metal, in which an oxidation reduction potential of the metal is a more negative potential than a flat band potential of the oxide, and the color conversion element is used for chromism which causes coloration by contacting an ion medium to at least the metal to reduce the oxide and causes decolorization by exposing at least the oxide to oxidant to oxidize the oxide.

Description

  The present invention relates to a highly durable color conversion element capable of color conversion by an easy process and a color conversion method thereof.

  As a color conversion element, a photochromic element whose color is changed by light irradiation is known. For example, as disclosed in Patent Document 1, tungsten oxide is often used as a photochromic material, but the coloring process requires intense light irradiation to excite tungsten oxide. In general, a color conversion element using the principle of photochromism requires a strong light irradiation for coloring, and has a problem that a range of use is limited.

  On the other hand, an electrochromic element that changes color when an electric field is applied is also known as a color conversion element. For example, the color conversion element disclosed in Patent Document 2 is composed of an oxide coloring layer such as molybdenum oxide and a transparent conductive film as a counter electrode, and an electrolyte is filled between the coloring layer and the transparent conductive film. Yes. The color conversion element can be colored and decolored by applying an electric field to the color developing layer and the counter electrode. However, the color conversion element using the principle of electrochromism including the color conversion element disclosed in Patent Document 2 requires a power source for applying an electric field to the color conversion, and has a problem that the range of use is limited. was there. In addition, the color conversion element using the principle of electrochromism has a problem that care must also be taken in the electrolyte sealing technique in order to achieve long-term durability.

  Furthermore, in the photochromic element and the electromic element as described above, color conversion elements using organic molecules are disclosed in Patent Documents 3 and 4, for example. However, the color conversion element using these organic molecules has a problem that long-term thermal and chemical durability cannot be obtained.

Japanese Patent Laid-Open No. 9-230390 JP 2010-14917 JP-A-6-95289 JP-A-5-59354

F. D. Quarto, A. D. Paola, C. Sunseri, Electrochimica Acta, 26, 1177 (1981). Grunert et al. Journal of Catalysis, 107, 522-534 (1987).

  In view of the problems as described above, the problem to be solved by the present invention is to realize a highly durable color conversion element capable of performing color conversion in an easy process and a color conversion method thereof.

  Another object of the present invention is to realize a chromic element (color conversion element) or a color conversion method based on an oxidation-reduction reaction that does not require a power source (self-generating type).

  The color conversion element and color conversion method of the present invention employ the following configuration in order to solve the above-described problems.

  That is, the color conversion element of this invention is equipped with the base material containing a metal, and the colored layer containing the oxide which contacted the metal contained in the said base material at least partially. In the color conversion element of the present invention, the oxidation-reduction potential of the metal is more negative than the flat band potential of the oxide, and the oxide is reduced by bringing an ion medium into contact with at least the metal. It is characterized in that it is used for chromism, which is colored with oxidant and is decolored by exposing the oxidant to at least the oxide to oxidize the oxide.

  In the color conversion method of the present invention, coloring is performed by bringing the ion medium into contact with at least the metal of the color conversion element to reduce the oxide. Further, at least the oxide of the color conversion element is exposed to an oxidant to oxidize the oxide to remove the color.

  The oxide may be tungsten oxide. The tungsten oxide may be at least one of crystalline tungsten oxide and amorphous tungsten oxide.

  The metal may be aluminum. The oxide may be formed in a dot shape.

  In addition, the color conversion element of this invention may be equipped with the ion medium used for the reduction | restoration of the said oxidizing agent.

  According to the above configuration, in the color conversion element and the color conversion method according to the present invention, at least the metal is colored by contacting the ion medium, and at least the oxide is exposed to an oxidizing agent (atmosphere (oxygen) or the like). Therefore, color conversion is possible with an easy process. In addition, the color conversion element and the color conversion method according to the present invention do not require organic molecules, and thus have excellent long-term durability. Furthermore, according to the above configuration, at least the metal is colored by contacting with an ionic medium, and at least the oxide is decolored by exposure to an oxidizing agent (air (oxygen), etc.). An oxidation-reduction reaction can be caused, and color conversion can be realized.

  According to the present invention, it is possible to provide a highly durable color conversion element capable of performing color conversion in an easy process and a color conversion method thereof. Further, according to the present invention, it is possible to realize a chromic element (color conversion element) or a color conversion method based on an oxidation-reduction reaction that does not require a power source (self-generating type).

FIG. 3 is a diagram illustrating an overall image of a color conversion element of one embodiment of the present invention. FIG. 6 is a diagram illustrating an interface state between a colored layer and a substrate of a color conversion element of one embodiment of the present invention. The figure which shows the energy diagram of the color conversion element in the Example of this invention. The scanning electron micrograph of the color conversion element in the Example of this invention. The photograph before and after color conversion of the color conversion element (# 1 sample) in the Example of this invention. The photograph after the color conversion of the color conversion element (# 1 sample) in the Example of this invention and a scanning electron micrograph. The figure which shows the result of having measured the tungsten 4f orbit by the X-ray photoelectron spectroscopy of the color conversion element (# 1 sample) in the Example of this invention, and the photograph which shows a measurement point. The photograph of the color conversion characteristic of the color conversion element (# 2 sample) in the Example of this invention. The figure which shows the reflection spectrum before and behind color conversion of the color conversion element (# 2 sample) in the Example of this invention. The photograph before and after color conversion of the color conversion element (# 3 sample) in the Example of this invention. The figure which shows the change of the light absorbency in the various decoloring process of the color conversion element (# 2 sample) in the Example of this invention. The figure which shows the repeated coloring and decoloring characteristic of the color conversion element (# 2 sample) in the Example of this invention. The scanning electron micrograph of the surface after repeated characteristic evaluation of the color conversion element (# 2 sample) in the Example of this invention.

  Hereinafter, a color conversion element and a color conversion method according to an aspect of the present invention will be described as an embodiment (hereinafter referred to as “this embodiment”). However, the present invention is not limited to this embodiment.

  Hereinafter, the color conversion element of this embodiment is provided with the base material containing a metal, and the colored layer containing the oxide which contacted the metal contained in the said base material at least partially. In the color conversion element of the present embodiment, the oxidation-reduction potential of the metal is more negative than the flat band potential of the oxide, and at least the metal is brought into contact with the metal to reduce the oxide. It is characterized by being used for chromism that is colored by oxidization of the oxide by exposing the oxide to at least the oxide and oxidizing the oxide.

  Moreover, the color conversion method of this embodiment is colored by reducing the oxide by bringing the ion medium into contact with at least the metal of the color conversion element. Then, at least the oxide of the color conversion element is exposed to an oxidant to oxidize the oxide to remove the color.

  An example of the structure of the color conversion element and the color conversion method of this embodiment is shown in FIGS. The color conversion element of the present invention is not limited to these drawings. FIG. 1 shows an example of an overall image of a color conversion element according to this embodiment. FIG. 2 shows an example of an enlarged view of the interface between the colored layer and the substrate of the color conversion element according to this embodiment.

  Part of the oxide in the colored layer is in contact with part of the metal contained in the substrate. When the ion medium comes into contact with the metal, an electromotive force is generated due to a potential difference between the oxidation-reduction potential of the metal and the flat band potential of the oxide. By the electromotive force, the metal is oxidized, and the oxide is reduced and colored. The electromotive force for causing coloration is induced by the potential difference between the redox potential of the metal and the flat band potential of the oxide. That is, in order to cause coloring, that is, in order to reduce the oxide, it is necessary that the oxidation-reduction potential of the metal is more negative than the flat band potential of the oxide. In addition, when the oxide in a colored layer is a particulate form, it is not necessary for all the particles to contact a metal, and a part of oxide should just contact the metal. This is because the electromotive force is generated if even a part of the oxide is in contact with the metal.

  A color conversion mechanism when the metal according to this embodiment is aluminum and the oxide is tungsten oxide will be described with reference to the energy diagram of FIG. Note that the combination of metal and oxide in the color conversion element according to the present embodiment is not limited to the combination of aluminum and tungsten oxide.

The vertical axis in FIG. 3 represents the standard hydrogen electrode potential (NHE), with the upper side being a negative (base) potential and the lower side being a positive (noble) potential. The redox potential of aluminum is -1.662V, while the flat band potential of tungsten oxide is +0.2 to + 0.5V as disclosed in Non-Patent Document 1.
It is known to be in the range. Therefore, a potential difference (ΔE = 1.862 V to 2.162 V) is generated between aluminum and tungsten oxide. Aluminum is oxidized by the electromotive force due to this potential difference, and tungsten oxide is reduced.

  Although the oxidation number of tungsten present in tungsten oxide is stable at 6 valences in the atmosphere, it becomes pentavalent or tetravalent by the reduction reaction. Tungsten oxide before being reduced is yellow, but tungsten having an oxidation number of pentavalent or tetravalent is blue. Therefore, tungsten oxide changes from yellow to blue by the reduction reaction.

  Further, when the color conversion element containing tungsten oxide colored blue is exposed to an oxidizing agent such as oxygen, for example, in the atmosphere, tungsten oxide containing a pentavalent or tetravalent oxidation state is oxidized, and the oxidation number of tungsten. Will return to its original hexavalent state and decolorize. That is, the color of tungsten oxide returns from blue to yellow.

  Such coloring and decoloring of tungsten oxide is repeatedly manifested by repeating the above steps (reduction and oxidation).

  On the other hand, aluminum is oxidized and eluted into the ionic medium solution, or at least one of an oxide film, a hydroxide film, and an aluminate film is formed on the surface. When the color conversion element of this embodiment reaches the reduction saturation capacity of tungsten oxide, the oxidation reaction also stops, so the amount of aluminum eluted is very small and the colored layer is not peeled off. Can be expressed.

  The flat band potential of the oxide according to this embodiment is affected by its synthesis method, oxide crystallinity and microstructure, but as shown in Non-Patent Document 1, measurement of photocurrent and electrochemical It can be measured by simple measurement. Note that the oxidation-reduction potential of metal has been examined in detail conventionally.

  Note that tungsten oxide is preferably used as the oxide according to this embodiment. The tungsten oxide may be crystalline tungsten oxide or amorphous tungsten oxide. In the case of crystalline tungsten oxide, it changes to yellow before coloring and to blue after coloring, and in the case of amorphous tungsten oxide, it changes from transparent to white before coloring and to blue after coloring.

  As the oxide according to the present embodiment, any oxide may be used as long as its flat band potential is a positive potential than the oxidation-reduction potential of the metal contained in the substrate. The oxide that can be used as the colored layer in this embodiment depends on what is used as the metal contained in the substrate. For example, when aluminum is used as the metal contained in the base material, molybdenum oxide, manganese oxide, titanium oxide, vanadium oxide, nickel oxide, oxidation are used as oxides whose flat band potential is more negative than the oxidation-reduction potential of aluminum. At least one oxide selected from the group consisting of cobalt and niobium oxide can be used.

  These oxides may be crystalline oxides, amorphous oxides, or a mixture thereof. In order to perform multicolor conversion, the colored layer according to the present invention may include a combination of a plurality of oxides selected from the oxide group or tungsten oxide.

The base material according to the present embodiment includes a metal having a redox potential that is more negative than the flat band potential of the oxide. The substrate according to the present embodiment may be a metal substrate, or may be an inorganic polycrystal such as glass or ceramic coated with a metal substrate, a single crystal substrate, a plastic, a film, or the like. . The substrate need not be limited to a flat plate, and may be a substrate having a complicated shape such as glass, ceramics or metal, a porous foam, a honeycomb, a nonwoven fabric of glass or ceramics, glass fiber, glass or silica gel, etc. The bead-like substance may be used. The metal may have a structure kneaded into the substrate.

  The metal according to the present embodiment is preferably aluminum. Aluminum is light and inexpensive, and has a relatively negative (base) oxidation-reduction potential. Therefore, it can be suitably used for the color conversion element of the present invention.

  As the metal according to the present embodiment, any metal may be used as long as the oxidation-reduction potential is more negative than the flat band potential of the oxide. The metals that can be used depend on what is used as the oxide of the colored layer. For example, when the oxide is tungsten oxide, the metal according to the present embodiment is lithium, cesium, rubidium, potassium, barium, strontium, calcium, sodium, whose oxidation-reduction potential is more negative than the flat band potential of tungsten oxide. And at least one metal selected from the group consisting of magnesium, manganese, tantalum, zinc, chromium, iron, cadmium, cobalt, nickel, tin, lead, antimony, bismuth, and copper. The base material according to the present embodiment may include a plurality of metals selected from these groups of metals and aluminum.

  The colored layer according to this embodiment includes an oxide that causes a color change. The colored layer according to this embodiment may include a binder in order to increase the durability of the colored layer. Further, when the oxide is in the form of particles, it is not necessary that all the particles are in contact with the metal contained in the substrate, and color conversion characteristics can be obtained if some of the particles are in contact.

  The method for forming the colored layer on the substrate is preferably spin coating, flow coating, dip coating, spray coating, roll coating or the like. In addition to these wet synthesis methods, methods for forming a colored layer on a substrate include sputtering, CVD, plasma CVD, ion plating, MBE, and pulsed laser deposition (PLD). And so on.

  The color conversion element according to the present embodiment can be colored by bringing at least a metal contained in the base material into contact with the ion medium. This is because an oxide in contact with a metal can undergo a reduction reaction using electrons emitted by the metal being oxidized by an ionic medium. However, the ionic medium is preferably in contact with both the metal contained in the substrate and the oxide in the colored layer. For this reason, it is preferable that the said oxide is formed in the dot form on the base material containing the said metal. Moreover, when coating the colored layer containing an oxide on a base material uniformly, it is preferable that the film thickness is 1 micrometer or less.

When the ionic medium comes into contact with the metal and the oxide, the metal oxidation reaction and the oxide reduction reaction proceed simultaneously. In particular, the ionic medium is an aqueous solution in order to promote the metal oxidation reaction. It is preferable that the aqueous solution has a pH range of 0 to 5, or 9 to 14. By setting the pH of the ionic medium within this range, the oxidation reaction of the metal is promoted and the coloring speed is increased.

  When the metal is aluminum, the ionic medium is preferably an aqueous solution having a pH range of 0-4. When the pH of the ionic medium becomes alkaline, a hydroxide film is formed on the surface of the aluminum, and the color conversion characteristics deteriorate repeatedly. Therefore, when the metal is aluminum, the ionic medium is preferably acidic.

  The ionic medium is preferably an aqueous solution, but may not be an aqueous solution such as a gel electrolyte.

After the color conversion element according to the present embodiment is colored, it is decolored by being exposed to at least an oxidizing agent (in this embodiment, oxygen) to return to the original color. The exposure method to oxygen is, for example, leaving the device in the atmosphere. In addition, in order to increase the decolorization rate, the exposure method to oxygen may include a heat treatment that does not oxidize the metal contained in the substrate. When the metal is aluminum, the color conversion element according to the present embodiment is decolorized more quickly than when exposed to the atmosphere at room temperature by performing a heat treatment at 100 ° C. or higher in the atmosphere.

  In addition, as the oxidizing agent in the color conversion element according to the present embodiment, for example, at least one gaseous material selected from the group consisting of ozone, hydrogen peroxide, and halogen (fluorine, chlorine, bromine) may be used. good. The oxidant is selected from the group consisting of potassium nitrate, hypochlorous acid, chlorous acid, chloric acid, perchloric acid, iodine, permanganate, cerium ammonium nitrate, chromic acid, dichromic acid, and nitrobenzene. A solvent containing at least one substance may be used.

  When the use of the color conversion element of the present invention is started, the ion medium may be in contact with the metal surface in advance. Until the use of the color conversion element is started, the surface of the color conversion element of the present invention may be covered with a resin film or the like in order to prevent evaporation of the ion medium.

  In the color conversion element according to the present embodiment, the coloring and decolorization are repeated. The coloring process is a very easy process with contact with an ionic medium, and the decoloring process is exposure to the atmosphere, and it can be expected to provide useful technologies for sensors, display elements, and the like.

  Next, the present invention will be specifically described with reference to examples. However, the present invention is not limited to these examples.

1. Production of Color Conversion Element An aluminum substrate having a 1 cm square and a thickness of 1 mm was used as a base material, and an aqueous solution in which a commercially available crystallized tungsten oxide powder was dispersed was spin-coated on the aluminum substrate. Samples having a solid content concentration of tungsten oxide powder of 10 wt% and 1 wt% are designated as # 1 and # 2.

  Further, an amorphous tungsten oxide thin film was coated on a 1 cm square, 1 mm thick aluminum substrate by pulsed laser deposition (PLD method). This sample is designated as # 3 sample.

2. Structural analysis of color conversion element The structure of the color conversion element was observed using a scanning electron microscope (SEM: Hitachi High-Technologies Corporation, S-4800). FIG. 4 shows an SEM image of the sample. The SEM images of the # 1 and # 2 samples are images observed from an angle of 30 ° with respect to the surface of the aluminum substrate, and the SEM image of the # 3 sample is an image observed from the surface of the aluminum substrate. The film thicknesses of tungsten oxide in the samples # 1 and # 2 were 4 μm and 0.8 μm, respectively. The tungsten oxide in the # 3 sample was a porous structure grown in a columnar shape, and the metal base material (aluminum substrate) and the ionic medium could be contacted.

3. 3. Evaluation of color conversion characteristics 3-1. Color Conversion Characteristics of # 1 Sample FIG. 5 shows photographs before and after the # 1 sample was brought into contact with the ionic medium. The ionic medium used for the # 1 sample is an aqueous oxalate buffer solution with pH = 1.68. As a result, the tungsten oxide (colored layer) of the # 1 sample was spot-like and partially colored blue.

FIG. 6 shows the result of observing the colored portion with a scanning electron microscope (SEM: manufactured by Hitachi High-Technologies Corporation, S-4800). From FIG. 6, it became clear that there is a pinhole in the center colored blue. In other words, in the # 1 sample, the oxalic acid buffer aqueous solution was brought into contact with both the aluminum substrate and the tungsten oxide through this pinhole, so that the oxidation reaction of aluminum and the reduction reaction of tungsten oxide proceeded. It seems to have been colored. Therefore, it was found that the contact between the ion medium and the metal substrate is important for coloring the color conversion element according to the present invention.

Next, in order to investigate the oxidation number (valence) of tungsten oxide of the # 1 sample, X-ray photoelectron spectroscopy (XPS: Quantum 2000, manufactured by Physical Electronics) was used, and the 4f orbit of tungsten was analyzed by analysis with a beam size of 1 μm. It was measured. FIG. 7 shows the measurement results.

  In Non-Patent Document 2, the spectrum of tungsten obtained by the above measurement broadens its peak value and shifts to a lower energy side when tungsten having a valence (oxidation number) lower than hexavalence is generated. Has been shown to do. In this regard, as shown in FIG. 7, it was observed that the spectrum of tungsten before coloring (A in FIG. 7) was attributed to hexavalence. In addition, in the spectrum of the blue colored portion (C in FIG. 7), it was observed that the peak value was broadened and shifted to the low energy side, so the blue portion has a lower valence than hexavalent. It was found that tungsten was generated. From this, it became clear that coloring was induced by reduction of tungsten oxide.

3-2. Color Conversion Characteristics of # 2 Sample The color conversion characteristics of the # 2 sample were evaluated using three types of ion media. As an ionic medium, oxalic acid buffer aqueous solution (pH = 1.68), pure water (pH = 5.5) in which atmospheric oxides were dissolved, and boric acid buffer aqueous solution (pH = 9.18) were used. Further, as a decoloring step, the sample # 2 was left in the air at room temperature after coloring.

  FIG. 8 shows the color change of the # 2 sample. The # 2 sample was more uniformly colored over the entire surface than when the color test was performed on the # 1 sample under the same conditions (when an oxalic acid buffer aqueous solution was used as the ion medium). Thereby, it turned out that the one where the film thickness of tungsten oxide is thinner is preferable.

  In addition, the sample # 2 was decolored by being exposed to air at room temperature, and was again brought into contact with the ion medium. Thereby, as FIG. 8 showed, it turned out that coloring and decoloring express repeatedly.

In addition, in the sample # 2, when the boric acid buffer aqueous solution (pH = 9.18) is used as the ion medium, the coloring becomes non-uniform, and when the oxalic acid buffer aqueous solution (pH = 1.68) is used, the coloring becomes uniform. It was. This also revealed that the ionic medium should be acidic in order to uniformly color the # 2 sample (the metal contained in the base material is aluminum). When the ion medium was pure water, no significant color change occurred.

  Further, in order to quantitatively evaluate the color change in the process from coloring to decoloring, the diffuse reflectance was measured using a spectrophotometer (manufactured by JASCO Corporation, V-660). FIG. 9 shows the measurement results. As a result, an absorption profile due to the band gap transition of tungsten oxide was observed before coloring, but strong visible light absorption was exhibited by immersion in an ionic medium, and the reflectance was significantly reduced. After that, it was found that it was restored to the state before coloring again by exposure to the atmosphere.

3-3. Color Conversion Characteristics of # 3 Sample FIG. 10 shows photographs before and after contacting the # 3 sample with the ion medium. The ionic medium used to color the # 3 sample is an aqueous oxalate buffer solution with pH = 1.68. As a result, it was revealed that the # 3 sample, which is amorphous tungsten oxide, is highly colored.

4). Evaluation of Decoloring Process After coloring # 2 sample with an ion medium of oxalic acid buffer aqueous solution with pH = 1.68, the decoloring process is performed under various conditions, and the color change under the various conditions is measured with a spectrophotometer (Japan) V-660) manufactured by Spectroscopic Co., Ltd. and measured and evaluated. FIG. 11 shows changes in absorbance at a wavelength of 550 nm among absorbances calculated from diffuse reflectance.

  As a result, it was clarified that the sample colored blue in nitrogen does not return to the original, but decolorizes in the presence of oxygen. It was also found that heat treatment in the atmosphere promotes the oxidation reaction and improves the decolorization rate.

5. Evaluation of repetitive characteristics of color conversion Absorbance of # 2 sample when immersed in an aqueous oxalic acid buffer solution at pH = 1.68 for 10 minutes as a coloring process and repeated exposure 30 times in air at 120 ° C as a decolorizing process for 5 minutes The change was evaluated using a spectrophotometer (manufactured by JASCO Corporation, V-660). The results are shown in FIG. 12, and it has become clear that coloring and decoloring are repeatedly reproduced.

After evaluating the repeated characteristics, the surface microstructure was observed using a scanning electron microscope (SEM: manufactured by Hitachi High-Technologies Corporation, S-4800). The results are shown in FIG. 13, but there was no significant change in the surface structure and no peeling of the colored layer was observed.

  The color conversion element of the present invention can perform color conversion by a very easy process in which the coloring process is contact with an ionic medium and the decoloring process is exposure to oxygen. Moreover, since organic molecules are not used, repeated durability is also high. The color conversion element of the present invention can provide a technique useful for a sensor, a display element, a light shielding element, and the like.

Claims (10)

A substrate comprising a metal;
A colored layer comprising an oxide in contact with at least part of the metal contained in the substrate,
The redox potential of the metal is more negative than the flat band potential of the oxide,
It is used for chromism in which at least the metal is brought into contact with an ionic medium to reduce the oxide and colored, and at least the oxide is exposed to an oxidant and oxidized to decolorize the oxide. And a color conversion element.   The color conversion element according to claim 1, wherein the oxide is tungsten oxide.   The color conversion element according to claim 2, wherein the tungsten oxide is at least one of crystalline tungsten oxide and amorphous tungsten oxide.   The color conversion element according to claim 1, wherein the metal is aluminum.   The color conversion element according to claim 1, wherein the oxide is formed in a dot shape.   6. The color conversion element according to claim 1, wherein the ion medium is an aqueous solution, and the pH range is 0 to 5 or 9 to 14.   The color conversion element according to claim 1, wherein the oxidizing agent is oxygen. A substrate comprising a metal;
A colored layer comprising an oxide in contact at least in part with the metal contained in the substrate;
An ionic medium,
The redox potential of the metal is more negative than the flat band potential of the oxide,
It is used for chromism in which the ionic medium is brought into contact with at least the surface of the metal and colored by reducing the oxide, and at least the oxide is exposed to an oxidizing agent to decolorize the oxide. A color conversion element characterized by that. A base material containing a metal, and a colored layer containing an oxide provided on the base, wherein the metal and the oxide are in contact at least in part, and the oxidation-reduction potential of the metal is In the color conversion method of the color conversion element that is a negative potential than the flat band potential of the oxide,
A color conversion method comprising coloring at least the metal by bringing an ion medium into contact with the metal to reduce the oxide. A base material containing a metal, and a colored layer containing an oxide provided on the base, wherein the metal and the oxide are in contact at least in part, and the oxidation-reduction potential of the metal is In the color conversion method of the color conversion element that is a negative potential than the flat band potential of the oxide,
A color conversion method characterized in that at least the oxide is exposed to an oxidant to oxidize the oxide for decolorization.

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