JP2024058626A - NiO film and NiO dispersion - Google Patents

Abstract

[Problem] To provide a NiO dispersion having high particle dispersibility, and to provide a NiO film having high light transmittance. [Solution] The NiO dispersion of the present invention contains NiO particles and a polyhydric alcohol. It is preferable that the NiO particles have a cumulative 50% diameter D50 in the volumetric particle size distribution measured by dynamic light scattering method of 5 nm or more and 50 nm or less. It is also preferable that the NiO dispersion contains a silane coupling agent and a titanium coupling agent. The NiO film contains NiO particles. The NiO particles have a cumulative 50% diameter Dn50 in the volumetric particle size distribution of 5 nm or more and 50 nm or less. The NiO film contains a compound having a polyhydric alcohol skeleton and containing at least one element selected from silicon, titanium, zirconium, and aluminum. [Selected Figure] None

Description

The present invention relates to a NiO film. The present invention also relates to a NiO dispersion liquid that is suitable for use in forming a NiO film.

Attempts are being made to apply NiO films formed from a dispersion containing NiO particles to, for example, thin-film transistor elements, solar cells, semiconductor secondary batteries, and electrochromic elements. Such NiO films are useful not only for their semiconductor and electrochromic properties, but also for their optical transparency.

Conventional techniques relating to a dispersion liquid containing NiO particles are known, for example, from Patent Documents 1 and 2.
Patent Document 1 describes a method in which a dispersion liquid consisting of nickel oxide particles obtained by heating nickel hydroxide in an air atmosphere and a dispersion medium consisting of methyl ethyl ketone, xylene, ethyl acetate and acrylic polyol is applied by a spin coater to form a coating film, and the coating film is dried to form an electrochromic layer.
Patent Document 2 describes an ink composition containing nickel oxide nanoparticles, a binder, a solvent, a dispersant, and a surfactant.

Japanese Patent Application Laid-Open No. 7-175417 JP 2020-97720 A

However, the NiO particle dispersions described in Patent Documents 1 and 2 do not have sufficient dispersibility of the NiO particles, and as a result, the NiO film formed from the dispersion does not have sufficiently high light transmittance.
Therefore, an object of the present invention is to provide a NiO dispersion liquid having high particle dispersibility, and a NiO film having high light transmittance.

As a result of intensive research by the inventors to solve the above problems, it was discovered that the dispersibility of NiO particles can be improved by using a specific compound as a dispersion medium in the NiO dispersion, or by using NiO particles with a specific particle size. It was also discovered that the light transmittance of the NiO film produced from the NiO dispersion can be increased.

The present invention was made based on the above findings and provides a NiO dispersion liquid containing NiO particles and a polyhydric alcohol.

The present invention also provides a NiO film containing NiO particles,
The NiO film has a cumulative 50% diameter _Dn50 of 5 nm or more and 50 nm or less in a volume particle size distribution of the NiO particles, the cumulative 50% diameter Dn50 being measured by observing the NiO film with a scanning electron microscope.

The present invention further provides a NiO film containing NiO particles,
The present invention provides a NiO film, which contains a compound having a polyhydric alcohol skeleton and containing at least one element selected from silicon, titanium, zirconium, and aluminum.

According to the present invention, a NiO dispersion liquid with high particle dispersibility is provided. In addition, according to the present invention, a NiO film with high optical transparency is provided.

The present invention will be described below based on preferred embodiments thereof. First, the NiO dispersion will be described.
The NiO dispersion liquid is prepared by dispersing NiO particles in a dispersion medium.
The NiO particles contained in the NiO dispersion are preferably composed of an oxide of divalent nickel, but it is permissible that some oxide of nickel other than divalent nickel is inevitably contained.
From the viewpoint of increasing the light transmittance of the NiO film produced using the NiO dispersion, it is preferable that the NiO particles are stably dispersed in the NiO dispersion, and from this viewpoint, it is preferable that the NiO particles are present in a monodisperse state as much as possible without agglomeration between particles.

In the present invention, from the viewpoint of increasing the dispersion stability of NiO particles in the NiO dispersion, it is preferable to use at least one of polyhydric alcohols, cyclic esters, terpineol, and dihydroterpineol as the dispersion medium. In particular, it is preferable to use polyhydric alcohols as the dispersion medium not only to increase the dispersion stability of NiO particles in the NiO dispersion, but also from the viewpoint of good compatibility with other components described later.

A polyhydric alcohol suitable for use as a dispersion medium is a compound in which a hydroxyl group (-OH) is bonded to two or more different carbon atoms in a hydrocarbon skeleton or an ether skeleton. The hydrocarbon skeleton and the ether skeleton may be a saturated or unsaturated aliphatic group. The hydrocarbon skeleton and the ether skeleton may be linear or cyclic. From the viewpoint of increasing the dispersion stability of NiO particles, the hydrocarbon skeleton and the ether skeleton are preferably linear.
The number of hydroxyl groups in the polyhydric alcohol is preferably 2 or 3, and more preferably 2, since this enhances the dispersion stability of the NiO particles in the NiO dispersion.

The total number of carbon atoms in the hydrocarbon skeleton is preferably 2 to 8, more preferably 2 to 6, and even more preferably 2 to 4, from the viewpoints of dispersion stability of the NiO particles and appropriate viscosity of the NiO dispersion.
The total number of carbon atoms in the ether skeleton is preferably 4 or more and 8 or less, and particularly preferably 4 or more and 6 or less, from the viewpoint of proper dispersion stability of NiO particles and viscosity of NiO dispersion liquid.

The polyhydric alcohol includes, for example, glycols.
Examples of glycols having a hydrocarbon skeleton as described above include ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,2-butanediol, 1,3-butanediol, 2,3-butanediol, 1,4-butanediol, pentamethylene glycol, and hexylene glycol.
Examples of glycols having an ether skeleton include diethylene glycol, triethylene glycol, tetraethylene glycol, 4-oxa-2,6-heptanediol, 2-(2-hydroxy-propoxy)-propan-1-ol, and 2-(2-hydroxy-1-methyl-ethoxy)-propan-1-ol.
These polyhydric alcohols can be used alone or in combination of two or more.

Among the various polyhydric alcohols mentioned above, it is preferable to use ethylene glycol, propylene glycol, diethylene glycol, and triethylene glycol from the viewpoint of further increasing the dispersion stability of NiO particles and from the viewpoint of more appropriate viscosity of NiO dispersion liquid. In particular, propylene glycol is particularly preferable from the viewpoint of increasing the light transmittance of the NiO film formed from the NiO dispersion liquid and of excellent film strength, since the crosslinked body formed by the reaction with the coupling agent described later has a highly three-dimensional structure due to its bent three-dimensional structure.

Examples of cyclic esters suitable for use as dispersion media include β-propiolactone, β-butyrolactone, γ-butyrolactone, β-valerolactone, γ-valerolactone, and δ-valerolactone.

The NiO dispersion may contain other dispersion media in addition to the above-mentioned dispersion media, such as organic solvents used to adjust the viscosity and surface tension of the NiO dispersion.
Examples of the organic solvent include aliphatic hydrocarbons, aromatic hydrocarbons, aliphatic monohydric alcohols, aromatic monohydric alcohols, aliphatic aldehydes, aromatic aldehydes, aliphatic ethers, aromatic ethers, aliphatic ketones, monohydric alcohols of aliphatic ketones, aromatic ketones, aliphatic carboxylic acids and their salts, aromatic carboxylic acids and their salts, amides of aliphatic carboxylic acids, amides of aromatic carboxylic acids, esters of aliphatic carboxylic acids, esters of aromatic carboxylic acids, etc. Among these, aliphatic monohydric alcohols are preferred due to their high compatibility with polyhydric alcohols. Among aliphatic monohydric alcohols, it is preferred to use saturated aliphatic monohydric alcohols, and it is particularly preferred to use saturated aliphatic monohydric alcohols having 1 to 4 carbon atoms. It is particularly preferred to use diacetone alcohol, methanol, or ethanol, because they have a low boiling point and can be easily removed in the baking process during the formation of the NiO film, and it is particularly preferred to use methanol.

When a dispersion medium other than a polyhydric alcohol is used, the dispersion medium is preferably contained in an amount of 60 parts by mass or less, and more preferably 50 parts by mass or less, per 100 parts by mass of all dispersion medium, from the viewpoint of obtaining favorable dispersion stability. Furthermore, the dispersion medium other than a polyhydric alcohol is preferably contained in an amount of 0.5 parts by mass or more, and more preferably 10 parts by mass or more, per 100 parts by mass of all dispersion medium, from the viewpoint of facilitating the formation of a NiO film.

As described above, the NiO dispersion contains polyhydric alcohol as a dispersion medium, which increases the dispersion stability of the NiO particles. As a result, the NiO dispersion exhibits high dispersion stability, with the settling velocity of the NiO particles being 0.1 mm/min or less. The slower the settling velocity, the more preferable it is, and for example, 0.01 mm/min or less, and even more preferably 0.001 mm/min or less. This settling velocity is measured at 25°C.

The NiO particles contained in the NiO dispersion preferably have a cumulative 50% diameter _D50 in the volumetric particle size distribution of the NiO particles measured by a dynamic light scattering method of 5 nm or more and 50 nm or less. From the viewpoint of the dispersion stability of the NiO particles in the dispersion, _D50 is more preferably 50 nm or less, and further more preferably 30 nm or less. On the other hand, there is no particular restriction on the lower limit of _D50 , and the smaller the better, but 5 nm is practical.

The concentration of NiO particles contained in the NiO dispersion is preferably 70% by mass or less, from the viewpoint of increasing the dispersion stability and adjusting the viscosity of the NiO dispersion to an appropriate range, and may be 50% by mass or less, or 30% by mass or less. In addition, the concentration of NiO particles is preferably 5% by mass or more, from the viewpoint of facilitating the formation of a NiO film from the NiO dispersion.

The NiO particles can be synthesized by a method known in the art, for example, by the following method.
First, an aqueous solution containing divalent nickel ions is mixed with a basic substance such as aqueous ammonia or an aqueous sodium hydroxide solution to neutralize the nickel ions, thereby forming a precipitate of nickel hydroxide in the liquid.
This nickel hydroxide precipitate is isolated by filtration, and is then washed and dried to obtain solid nickel hydroxide.
Solid nickel hydroxide is calcined in an air atmosphere to produce NiO. Before calcination, the solid nickel hydroxide may be subjected to a crushing step and a sieving step, if necessary, to adjust the particle size.
The produced NiO is pulverized and then sieved to obtain the desired NiO particles.

The NiO particles thus obtained are mixed with a polyhydric alcohol using a stirring mixer such as a bead mill, ball mill, or paint shaker to obtain the desired NiO dispersion.

In addition to the NiO particles and the dispersion medium, the NiO dispersion may contain other components in order to improve the properties of the NiO film formed from the dispersion. Examples of other components include coupling agents, surface tension modifiers, viscosity modifiers, and binder resins. In particular, if the NiO dispersion contains a coupling agent, the dispersion stability of the NiO particles is further improved, and not only the light transmittance of the NiO film is improved, but also the film strength of the NiO film formed by applying the dispersion to a substrate is improved. From this perspective, it is preferable to determine the amount of coupling agent contained in the NiO dispersion in relation to the amount of NiO particles contained in the NiO dispersion. Specifically, the content of the coupling agent per 100 parts by mass of NiO particles is preferably 1 part by mass or more, and more preferably 5 parts by mass or more. In addition, the content is preferably 50 parts by mass or less, and more preferably 40 parts by mass or less. In addition, when two or more types of coupling agents are used, the content refers to the total content of the two or more types of coupling agents.

It is preferable that the coupling agent contains various functional groups from the viewpoint of further increasing the adhesion between the NiO film and the substrate and further increasing the dispersion stability of the NiO particles. From this viewpoint, it is preferable that the coupling agent contains at least one selected from an epoxy group, a vinyl group, a styryl group, a methacryl group, an acrylic group, an amino group, an isocyanurate group, a ureido group, a mercapto group, and an isocyanate group.

In particular, when a coupling agent is contained in the NiO dispersion, two different types of coupling agents can be used as the coupling agent. In this case, by using one of the coupling agents containing an epoxy group, and further by containing another coupling agent of a different type in addition to the coupling agent, the dispersion stability of the NiO particles is further increased, and the NiO film formed has excellent light transmittance. In addition, the adhesion between the NiO film and the substrate is further increased, and the film strength of the NiO film is also excellent. The reason for this is that the epoxy group reacts with another coupling agent of a different type or with a hydroxyl group in a polyhydric alcohol to form a crosslinked body that has high light transmittance and high adhesion to the substrate.

Metal alkoxides can be used as coupling agents. Specifically, for example, silane coupling agents (silane alkoxides), titanium coupling agents (titanium alkoxides), aluminum coupling agents (aluminum alkoxides), and zirconium coupling agents (zirconium alkoxides) can be used. These coupling agents can be used alone or in combination of two or more. In particular, the use of a silane coupling agent in combination with a titanium coupling agent is preferable in terms of further increasing the dispersion stability of the NiO particles, thereby increasing the light transmittance of the NiO film, and increasing the adhesion between the NiO film and the substrate, resulting in excellent film strength of the NiO film.

Examples of silane coupling agents include vinyltrichlorosilane, vinyltrimethoxysilane, vinyltriethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-glycidoxypropyltriethoxysilane, p-styryltrimethoxysilane, 3-methacryloxypropylmethyldimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, 3-methacryloxypropyltriethoxysilane, 3-acryloxypropyltrimethoxysilane, N-2(aminoethyl)3-aminopropylmethyldimethoxysilane, N-2(aminoethyl)3-aminopropyltrimethoxysilane, N-2(aminoethyl)3-aminopropyltriethoxysilane, 3-aminopropyl propyltrimethoxysilane, 3-aminotriethoxysilane, 3-triethoxysilyl-N-(1,3-dimethyl-butylidene)propylamine, N-phenyl-3-aminopropyltrimethoxysilane, N-(vinylbenzyl)-2-aminoethyl-3-aminopropyltrimethoxysilane hydrochloride, 3-ureidopropyltriethoxysilane, 3-chloropropyltrimethoxysilane, 3-mercaptopropylmethyldimethoxysilane, 3-mercaptopropyltrimethoxysilane, bis(triethoxysilylpropyl)tetrasulfide, 3-isocyanatopropyltriethoxysilane, tetramethoxysilane, tetraethoxysilane, methyltrimethoxysilane, methyltriethoxysilane, dimethyltriethoxysilane, phenyltriethoxysilane, hexamethyldisilazane, hexyltrimethoxysilane, and decyltrimethoxysilane. Among these, those containing epoxy groups are preferred from the viewpoint of not only providing excellent light transmission when made into a NiO film, but also favorable film strength; specifically, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, and 3-glycidoxypropyltriethoxysilane are preferred.

Examples of titanium coupling agents include tetraisopropyl titanate, tetra-normal butyl titanate, butyl titanate dimer, tetra(2-ethylhexyl) titanate, tetramethyl titanate, titanium acetylacetonate, titanium tetraacetylacetonate, titanium ethylacetoacetate, titanium octanediolate, titanium lactate, titanium triethanolamine, and polyhydroxytitanium stearate.

Examples of aluminum coupling agents include aluminum isopropylate, monosec-butoxyaluminum diisopropylate, aluminum sec-butylate, aluminum ethylate, ethyl acetoacetate aluminum diisopropylate, aluminum tris(ethyl acetoacetate), alkyl acetoacetate aluminum diisopropylate, aluminum monoacetylacetonate bis(ethyl acetoacetate), aluminum tris(acetylacetonate), aluminum monoisopropoxy monooleoxyethyl acetoacetate, cyclic aluminum oxide isopropylate, cyclic aluminum oxide octylate, and cyclic aluminum oxide stearate.

Examples of zirconium coupling agents include zirconium normal propylate, zirconium normal butyrate, zirconium tetraacetylacetonate, zirconium monoacetylacetonate, zirconium bisacetylacetonate, zirconium monoethylacetoacetate, zirconium acetylacetonate bisethylacetoacetate, zirconium acetate, and zirconium monostearate.

Here, when a silane coupling agent and a titanium coupling agent are used in combination, the content of the titanium coupling agent relative to 100 parts by mass of the silane coupling agent is preferably 0.1 parts by mass or more, more preferably 0.5 parts by mass or more, and even more preferably 1.0 parts by mass or more. The content of the titanium coupling agent relative to 100 parts by mass of the silane coupling agent is preferably 5.0 parts by mass or less, more preferably 4.0 parts by mass or less, and even more preferably 3.0 parts by mass or less. By setting the content of the titanium coupling agent relative to the silane coupling agent in this way, not only is the dispersion stability of the NiO particles further improved, but the adhesion between the NiO film and the substrate is further improved, resulting in excellent film strength of the NiO film.

When a silane coupling agent and a titanium coupling agent are used in combination, the total content of the silane coupling agent and the titanium coupling agent relative to 100 parts by mass of polyhydric alcohol is preferably 0.005 parts by mass or more, more preferably 0.01 parts by mass or more, and particularly preferably 0.03 parts by mass or more. Furthermore, the total content of the silane coupling agent and the titanium coupling agent relative to 100 parts by mass of polyhydric alcohol is preferably 0.2 parts by mass or less, and more preferably 0.1 parts by mass or less. By setting the mass ratio in this way, not only is the dispersion stability of the NiO particles further increased, but the adhesion between the NiO film and the substrate is further increased, resulting in excellent film strength of the NiO film.

When using a combination of a silane coupling agent and a titanium coupling agent, the silane coupling agent preferably contains an epoxy group, because this not only further enhances the dispersion stability of NiO particles, but also further enhances the adhesion between NiO film and substrate, making the film strength of NiO film excellent.In this case, the titanium coupling agent preferably has an alkoxy group.
From a similar viewpoint, it is also preferable to use a silane coupling agent in combination with a titanium coupling agent containing an epoxy group.

When using a combination of a zirconium coupling agent and a titanium coupling agent instead of a combination of a silane coupling agent and a titanium coupling agent, the content of the titanium coupling agent relative to 100 parts by mass of the zirconium coupling agent is preferably 0.1 parts by mass or more. Also, the content of the titanium coupling agent relative to 100 parts by mass of the zirconium coupling agent is preferably 5.0 parts by mass or less. By setting the content of the titanium coupling agent relative to the zirconium coupling agent in this way, not only is the dispersion stability of the NiO particles further improved, but the light transmittance of the NiO film is also excellent.

When the NiO dispersion contains two or more coupling agents, the total content of the coupling agents relative to 100 parts by mass of the polyhydric alcohol is preferably 0.005 parts by mass or more, more preferably 0.01 parts by mass or more, and particularly preferably 0.03 parts by mass or more. The total content of the coupling agents relative to 100 parts by mass of the polyhydric alcohol is preferably 0.2 parts by mass or less, and more preferably 0.1 parts by mass or less. By setting the mass ratio in this way, not only is the dispersion stability of the NiO particles further increased, but the adhesion between the NiO film and the substrate is further increased, resulting in excellent film strength of the NiO film. As for two or more coupling agents, a combination of a silane coupling agent and a titanium coupling agent, a combination of a zirconium coupling agent and a titanium coupling agent, or a combination of a silane coupling agent, a titanium coupling agent and a zirconium coupling agent is preferred.

Next, we will explain the NiO film formed using the NiO dispersion liquid described above. The NiO film is formed, for example, by applying the NiO dispersion liquid to the surface of a substrate to form a coating film, and then baking the coating film in an air atmosphere. There are no particular limitations on the method for applying the NiO dispersion liquid, and known methods such as spin coating, inkjet printing, and application by a dispenser can be used.

The baking temperature is preferably 120°C or higher and 250°C or lower, from the viewpoint of sufficiently volatilizing the dispersion medium and reliably causing the reaction between the polyhydric alcohol and the coupling agent (if included). From this viewpoint, it is even more preferable that the baking temperature is 125°C or higher and 200°C or lower.

The substrate used to form the NiO film is preferably transparent to light. From this perspective, the substrate is preferably made of, for example, glass or plastic. However, other materials, such as metals and ceramics, may also be used as the substrate. In this specification, "light" refers to visible light. Note that a substrate is not essential for forming the NiO film.

The NiO particles present in the NiO film can be evaluated by the cumulative 50% diameter Dn ₅₀ in the volumetric particle size distribution of the NiO particles measured by observing the NiO film with a scanning electron microscope (hereinafter also referred to as "SEM").
_Dn50 is preferably 50 nm or less, more preferably 30 nm or less. There is no particular restriction on the lower limit of _Dn50 , and the smaller it is, the higher the light transmittance of the NiO film becomes, which is preferable, but _Dn50 of about 5 nm, particularly about 10 nm, is preferable from the viewpoint of suitably preventing aggregation of NiO particles in the NiO dispersion.
In the present invention, since the NiO dispersion contains a polyhydric alcohol, the degree of aggregation of the NiO particles is low. As a result, the degree of aggregation of the NiO particles in the NiO film also tends to be low. As a result, the NiO film exhibits high light transmittance.

The NiO dispersion liquid used for forming the NiO film contains polyhydric alcohol as described above. The NiO dispersion liquid preferably contains metal alkoxides such as silicon, titanium, aluminum and/or zirconium. Due to the use of the NiO dispersion liquid containing these components, the NiO film preferably contains a compound having a polyhydric alcohol skeleton and containing at least one element selected from silicon, titanium, zirconium and aluminum. In the NiO film, it is preferable that NiO particles are dispersed in this compound. By the NiO film having such a structure, not only the light transmittance of the NiO film is increased, but also the adhesion between the NiO film and the substrate is increased, and the film strength of the NiO film is also excellent.
From this viewpoint, it is more preferable that the compound has a polyhydric alcohol skeleton and contains at least two elements selected from silicon, titanium, zirconium and aluminum.More preferably, the compound has a polyhydric alcohol skeleton and contains silicon and titanium, or has a polyhydric alcohol skeleton and contains titanium and zirconium.In addition, in the NiO film, there may also be a compound that is crosslinked only by a coupling agent that contains at least two elements selected from silicon, titanium, zirconium and aluminum.
The above-mentioned polyhydric alcohol skeleton refers to a skeleton in which oxygen atoms are bonded to two or more different carbon atoms in a hydrocarbon skeleton or an ether skeleton, and an example of such a skeleton is a glycol skeleton.
Whether or not a compound having a polyhydric alcohol skeleton and containing at least one element selected from silicon, titanium, zirconium, and aluminum is present in the NiO film can be confirmed, for example, by the following method.
First, element mapping is performed on the cross section of the NiO film using a scanning electron microscope-energy dispersive X-ray analyzer (SEM-EDX) or the like to confirm the presence or absence of each element, and then gas chromatography-mass spectrometry (GC-MS) is used to confirm whether or not the film has a polyhydric alcohol skeleton.

In particular, when the compound is a crosslinked product of a silane coupling agent containing an epoxy group, a titanium coupling agent, and a polyhydric alcohol, the light transmittance of the NiO film is further increased, and the adhesion between the NiO film and the substrate is further increased, resulting in excellent film strength of the NiO film, which is preferable. In this case, it is preferable that the titanium coupling agent has an alkoxy group.

The thickness of the NiO film is preferably 50 nm or more. The thickness of the NiO film is more preferably 500 nm or less, and even more preferably 300 nm or less. By setting the thickness of the NiO film within this range, the light transmittance of the NiO film can be significantly increased. The light transmittance of the NiO film is preferably high, 85% or more, more preferably 88% or more, and even more preferably 90% or more, expressed as the transmittance of light with a wavelength of 450 nm.

The NiO film of the present invention is formed from NiO particles without impairing the properties of bulk NiO, and therefore the band gap of the NiO film is preferably 3.2 eV to 3.9 eV, more preferably 3.4 eV to 3.8 eV.
Similarly, the work function of the NiO film is preferably 5.2 eV or more and 5.7 eV or less, and more preferably 5.3 eV or more and 5.6 eV or less.

The NiO film of the present invention can be used in, for example, thin film transistor elements, solar cells, semiconductor secondary batteries, and electrochromic elements, taking advantage of the p-type semiconductor properties of NiO.

In relation to the above-mentioned embodiments, the present invention further discloses the following NiO film and NiO dispersion.
[1]
A NiO film containing NiO particles,
The NiO film has a cumulative 50% diameter Dn50 in a volume particle size distribution of the NiO particles, the cumulative 50% diameter _Dn50 being 5 nm or more and 50 nm or less, as measured by observing the NiO film with a scanning electron microscope.
[2]
A NiO film containing NiO particles,
The NiO film comprises a compound having a polyhydric alcohol skeleton and containing at least one element selected from the group consisting of silicon, titanium, zirconium, and aluminum.
[3]
The NiO film according to [2], wherein the compound contains at least two elements selected from silicon, titanium, zirconium, and aluminum.
[4]
The NiO film according to [2] or [3], wherein the compound is a crosslinked product of a silane coupling agent containing an epoxy group, a titanium coupling agent, and a polyhydric alcohol.
[5]
The NiO film according to any one of [1] to [4], having a transmittance of 85% or more for light having a wavelength of 450 nm.

[6]
The NiO film according to any one of [1] to [5], having a band gap of 3.2 eV or more and 3.9 eV or less.
[7]
The NiO film according to any one of [1] to [6], having a work function of 5.2 eV or more and 5.7 eV or less.
[8]
The NiO film according to any one of [1] to [7], having a thickness of 50 nm to 500 nm.
[9]
A NiO dispersion comprising NiO particles and a polyhydric alcohol.
[10]
The NiO dispersion according to [9], wherein the NiO particles have a cumulative 50% diameter _D50 in a volume particle size distribution measured by a dynamic light scattering method of 5 nm or more and 50 nm or less.
[11]
The NiO dispersion according to [9] or [10], wherein the settling rate of NiO particles is 0.1 mm/min or less.

[12]
The NiO dispersion according to any one of [9] to [11], further comprising a coupling agent.
[13]
the coupling agents are two different types of coupling agents,
The NiO dispersion according to [12], wherein one of the coupling agents is a coupling agent containing an epoxy group.
[14]
The coupling agent is a silane coupling agent and a titanium coupling agent,
The NiO dispersion liquid according to [12] or [13], wherein the content of the titanium coupling agent is 0.1 parts by mass or more and 5.0 parts by mass or less relative to 100 parts by mass of the silane coupling agent.

The present invention will be described in more detail below with reference to examples. However, the scope of the present invention is not limited to these examples. Unless otherwise specified, "%" means "% by mass."

Example 1
(1) Synthesis of NiO particles 174.61 g of nickel (II) nitrate hexahydrate was placed in a 2-liter beaker, and 74.64 g of 28% aqueous ammonia was added to generate nickel hydroxide in the liquid. The liquid was stirred while the aqueous ammonia was being added.
The obtained nickel hydroxide was washed with ultrapure water until the nitrate ion concentration in the washing solution became 150 ppm or less.
After washing, the nickel hydroxide dispersion was subjected to solvent substitution using IPA (denatured alcohol).
The nickel hydroxide dispersion after solvent replacement was dried in a thermostatic oven set at 70° C. to remove the solvent.
The solid nickel hydroxide thus obtained was pulverized using a grinder.
The crushed nickel hydroxide was sieved through a sieve with 75 μm openings, and the material that passed through the sieve was calcined in air at 250° C. for 2 hours and further calcined at 400° C. for 2 hours to obtain NiO.
The obtained NiO was sieved through a sieve with 45 μm openings, and the NiO particles that passed through the sieve were used in the next step.

(2) Preparation of Dispersion Liquid 4 g of NiO particles and 6 g of propylene glycol/methanol mixed solvent were added to a 50 ml resin container and stirred at 2000 rpm for 1 minute with a planetary centrifugal mixer (ARE-250 manufactured by Thinky Corporation) to obtain a NiO dispersion liquid. The propylene glycol/methanol mixed solvent had a composition of 70% propylene glycol and 30% methanol.

(3) First stage crushing treatment 80 g of zirconia beads (0.8 mmφ) was added to the NiO dispersion prepared in (2). Then, zirconia beads were further added, and the difference between the surface of the NiO dispersion after adding the zirconia beads and the top surface of the beads was adjusted to about several mm. Next, crushing treatment was performed for 1 hour using a paint shaker manufactured by Asada Iron Works Co., Ltd.
The crushed NiO dispersion was filtered under nitrogen gas using a stainless steel mesh with a mesh size of 20 μm, using a pressure filtration device (Advantec Toyo Co., Ltd.: Stirring Type Ultra Holder UHP-43K).
Next, the NiO dispersion remaining on the beads was washed away with a propylene glycol/methanol mixed solvent, and the NiO dispersion was collected.

(4) Second stage crushing treatment 127 g of zirconia beads (0.05 mm diameter) were added to the NiO dispersion prepared in (3), and the mixture was crushed for 3 hours using a paint shaker. 19.17 g of the NiO dispersion was used.
The crushed NiO dispersion was pressure filtered under nitrogen gas using a stainless steel mesh with a mesh size of 20 μm and a pressure filter. The NiO dispersion that had settled on the beads was then washed away using a propylene glycol/methanol mixed solvent, and the NiO dispersion was collected.

(5) First-stage removal of coarse particles The obtained NiO dispersion was passed through a membrane filter having a pore size of 0.8 μm to remove coarse particles contained in the NiO dispersion.

(6) Third-stage crushing treatment 143 g of zirconia beads (0.05 mm diameter) were added to the NiO dispersion prepared in (5), and crushing treatment was carried out for 1 hour using a paint shaker. 22.78 g of NiO dispersion was used.
The crushed NiO dispersion was filtered under nitrogen gas using a stainless steel mesh with a mesh size of 20 μm and a pressure filter. The NiO dispersion remaining on the beads was then washed away with methanol, and the NiO dispersion was collected.

(7) Second-stage coarse particle removal The obtained NiO dispersion was passed through a membrane filter having a pore size of 0.45 μm to remove coarse particles contained in the NiO dispersion, thereby obtaining a NiO dispersion from which the coarse particles had been removed (the NiO dispersion of Example 1).
The concentration of NiO particles in the NiO dispersion was 6.2%, the concentration of propylene glycol was 50.14%, and the concentration of methanol was 43.66%.

(8) Preparation of NiO film The NiO dispersion of Example 1 was applied onto a glass substrate having a thickness of 0.7 mm using a spin coater. The spin coating was performed at 4000 rpm for 10 seconds. The resulting coating film was baked in an incubator at 230° C. in air for 1 hour to obtain the NiO film of Example 1.

Example 2
To 1.2224 g of NiO dispersion with a NiO concentration of 12.93% obtained through the "(5) first-stage coarse particle removal" process of Example 1, 0.3493 g of methanol, 0.0296 g of a titanium coupling agent (Matsumoto Fine Chemical Co., Ltd.: X-1221 (containing 20% titanium compound)), and 0.0071 g of a silane coupling agent (Shin-Etsu Chemical Co., Ltd.: KBM403, an epoxy group-containing silane coupling agent (3-glycidoxypropyltrimethoxysilane)) were added to obtain a mixed solution. This mixed solution was stirred for 1 minute at 2000 rpm using a planetary centrifugal mixer, and then mixed for 10 minutes using a paint shaker to obtain a NiO dispersion.
0.0773 g of the titanium coupling agent was added to 1.0285 g of the NiO dispersion, and the mixture was stirred at 2000 rpm for 1 minute using a planetary centrifugal mixer, and then mixed for 10 minutes using a paint shaker to obtain a NiO dispersion (NiO dispersion of Example 2).
The concentration of NiO particles in the NiO dispersion is 8.68%, the concentration of propylene glycol is 40.92%, the concentration of methanol is 40.99%, the concentration of silane coupling agent is 0.48%, and the concentration of titanium coupling agent is 1.79%. Note that the "concentration of titanium coupling agent" referred to here is calculated as the titanium compound is contained in 100%, since the titanium compound in the titanium coupling agent used is contained in 20%.
The NiO dispersion of Example 2 was applied onto a glass substrate having a thickness of 0.7 mm using a spin coater. The spin coating was performed at 6000 rpm for 10 seconds. The resulting coating film was baked in an incubator at 230° C. in air for 1 hour to obtain a NiO film of Example 2.

Example 3
To 0.9994 g of NiO dispersion with a NiO concentration of 6.2% obtained through the "(7) second-stage coarse particle removal" process of Example 1, 0.0687 g of a titanium coupling agent (Matsumoto Fine Chemical Co., Ltd.: X-1221 (containing 20% titanium compound)) and 0.0058 g of a silane coupling agent (Shin-Etsu Chemical Co., Ltd.: KBM403, an epoxy group-containing silane coupling agent (3-glycidoxypropyltrimethoxysilane)) were added to obtain a mixed liquid. This mixed liquid was stirred at 2000 rpm for 1 minute with a planetary centrifugal mixer, and then mixed for 10 minutes with a paint shaker to obtain a NiO dispersion (NiO dispersion of Example 3).
The concentration of NiO particles in the NiO dispersion is 5.77%, the concentration of propylene glycol is 46.66%, the concentration of methanol is 40.63%, the concentration of silane coupling agent is 0.54%, and the concentration of titanium coupling agent is 1.28%. Note that the "concentration of titanium coupling agent" referred to here is calculated as the titanium compound is contained in 100%, since the titanium compound in the titanium coupling agent used is contained in 20%.
The NiO dispersion of Example 3 was applied onto a glass substrate using a spin coater. The spin coating was performed at 4000 rpm for 10 seconds. The resulting coating was baked in an incubator at 185° C. in air for 1 hour to obtain the NiO film of Example 3.

Example 4
A NiO dispersion and a NiO film of Example 4 were obtained in the same manner as in Example 3, except that no titanium coupling agent was used and the concentration of the silane coupling agent was adjusted to the following concentration.
In the NiO dispersion, the concentration of NiO particles was 6.16%, the concentration of propylene glycol was 49.84%, the concentration of methanol was 43.40%, and the concentration of the silane coupling agent was 0.59%.

Example 5
(1) Synthesis of NiO Particles 100.02 g of ultrapure water and 20.26 g of sodium hydroxide were placed in a 300-mL beaker and stirred to prepare an aqueous sodium hydroxide solution (A).
In a 3-liter beaker, 59.71 g of nickel (II) chloride hexahydrate and 2,501.22 g of ultrapure water were placed and stirred while heating with a hot stirrer set at 50° C. to prepare an aqueous nickel chloride solution (B).
118.20 g of the sodium hydroxide aqueous solution (A) was placed in a 5-liter stainless steel beaker and stirred while being heated to 50° C. with a heater. The nickel chloride aqueous solution (B) was added to the sodium hydroxide aqueous solution (A) over a period of 6 minutes with a tubing pump. After the addition was completed, the mixture was stirred for 60 minutes while maintaining the liquid temperature at 50° C. to prepare nickel hydroxide.
The obtained nickel hydroxide was washed with ultrapure water until the sodium ion concentration in the washing solution became 170 ppm or less.
After washing, the nickel hydroxide dispersion was subjected to solvent substitution using IPA (denatured alcohol).
The nickel hydroxide dispersion after solvent replacement was dried in a thermostatic oven set at 60° C. to remove the solvent.
The solid nickel hydroxide thus obtained was crushed using a grinder.
The crushed nickel hydroxide was sieved through a sieve with 45 μm openings, and the material that passed through the sieve was calcined in air at 250° C. for 6 hours to obtain NiO.
The obtained NiO was crushed in a grinder and then sieved through a sieve with 45 μm openings, and the NiO particles that passed through the sieve were used in the next step.

(2) Preparation of Dispersion Liquid 3.5 g of NiO particles and 14 g of propylene glycol were added to a 50 ml resin container, and the mixture was stirred at 2000 rpm for 1 minute using a planetary centrifugal mixer (ARE-250 manufactured by Thinky Corporation), and then degassed at 2200 rpm for 30 seconds to obtain a NiO dispersion liquid.

(3) First stage crushing treatment 110 g of zirconia beads (0.8 mmφ) was added to the NiO dispersion prepared in (2). Then, zirconia beads were further added, and the difference between the surface of the NiO dispersion after adding the zirconia beads and the top surface of the beads was adjusted to about several mm. Next, crushing treatment was performed for 8 hours using a paint shaker manufactured by Asada Iron Works Co., Ltd.
The crushed NiO dispersion was pressure-filtered under nitrogen gas using a stainless steel mesh with a mesh size of 20 μm using a pressure filter (Stirring Type Ultra Holder UHP-43K, manufactured by Advantec Toyo Co., Ltd.) The NiO dispersion that had settled on the beads was then washed away using propylene glycol, and the NiO dispersion was collected.

(4) First-stage coarse particle removal The obtained NiO dispersion was passed through a membrane filter having a pore size of 0.8 μm to remove coarse particles contained in the NiO dispersion.

(5) Second stage crushing treatment 20 g of the NiO dispersion prepared in (4) and 144 g of zirconia beads (0.1 mmφ) were added to a 50 ml resin container, and the mixture was crushed for 24 hours using a paint shaker.
The crushed NiO dispersion was pressure filtered under nitrogen gas using a stainless steel mesh with a mesh size of 20 μm and a pressure filter. The NiO dispersion that had settled on the beads was then washed away using propylene glycol, and the NiO dispersion was collected.

(6) Second-stage removal of coarse particles The obtained NiO dispersion was passed through a membrane filter having a pore size of 0.45 μm to remove coarse particles contained in the NiO dispersion.

(7) Third-stage crushing treatment: 16.5 g of the NiO dispersion prepared in (6) and 100 g of zirconia beads (0.05 mmφ) were added to a 50 ml resin container, and the mixture was subjected to a crushing treatment for 67 hours using a paint shaker.
The crushed NiO dispersion was pressure filtered under nitrogen gas using a stainless steel mesh with a mesh size of 20 μm and a pressure filter. The NiO dispersion remaining on the beads was then washed away using propylene glycol, and the NiO dispersion was collected.

(8) Third-stage coarse particle removal The obtained NiO dispersion was passed through a membrane filter having a pore size of 0.2 μm to remove coarse particles contained in the NiO dispersion, thereby obtaining a NiO dispersion from which the coarse particles had been removed.
The concentration of NiO particles in the NiO dispersion was 8.8%.

(9) Preparation of NiO dispersion To 1.95 g of the NiO dispersion obtained in (8), 2.48 g of diacetone alcohol was added, and the mixture was stirred at 2000 rpm for 1 minute using a planetary centrifugal mixer, and then degassed at 2200 rpm for 30 seconds to obtain a NiO dispersion of Example 5.

(10) Preparation of NiO film The NiO dispersion of Example 5 was applied onto a glass substrate having a thickness of 0.7 mm using a spin coater. The spin coating was performed at 4000 rpm for 10 seconds. The resulting coating film was baked in an incubator at 185° C. in air for 1 hour to obtain the NiO film of Example 5.

Example 6
To 1.0 g of the NiO dispersion obtained through the "(8) third-stage coarse particle removal" step of Example 5, 0.9155 g of diacetone alcohol, 0.0931 g of a titanium coupling agent (Matsumoto Fine Chemical Co., Ltd.: X-1221 (containing 20% titanium compound)), and 0.0044 g of a silane coupling agent (Shin-Etsu Chemical Co., Ltd.: KBM403, an epoxy group-containing silane coupling agent (3-glycidoxypropyltrimethoxysilane)) were added to obtain a mixed liquid. This mixed liquid was stirred for 1 minute at 2000 rpm using a planetary centrifugal mixer, and then degassed for 30 seconds at 2200 rpm to obtain the NiO dispersion of Example 6.

The NiO dispersion of Example 6 was applied onto a glass substrate having a thickness of 0.7 mm using a spin coater. The spin coating was performed at 6000 rpm for 10 seconds. The resulting coating was baked in an incubator at 185°C in air for 1 hour to obtain the NiO film of Example 6.

Example 7
To 1.0 g of the NiO dispersion obtained through the "(8) third-stage coarse particle removal" step of Example 5, 0.9132 g of diacetone alcohol, 0.0926 g of a titanium coupling agent (Matsumoto Fine Chemical Co., Ltd.: X-1221 (containing 20% titanium compound)), and 0.0044 g of a zirconium coupling agent (Matsumoto Fine Chemical Co., Ltd.: ZC-570) were added to obtain a mixed liquid. This mixed liquid was stirred at 2000 rpm for 1 minute using a planetary centrifugal mixer, and then degassed at 2200 rpm for 30 seconds to obtain the NiO dispersion of Example 7.

The NiO dispersion of Example 7 was applied onto a glass substrate having a thickness of 0.7 mm using a spin coater. The spin coating was performed at 6000 rpm for 10 seconds. The resulting coating was baked in an incubator at 185°C in air for 1 hour to obtain the NiO film of Example 7.

Example 8
To 1.0 g of the NiO dispersion obtained through the "(8) third-stage coarse particle removal" step of Example 5, 0.9135 g of diacetone alcohol, 0.0928 g of a titanium coupling agent (Matsumoto Fine Chemical Co., Ltd.: X-1221 (containing 20% titanium compound)), and 0.0045 g of a zirconium coupling agent (Matsumoto Fine Chemical Co., Ltd.: ZA-45) were added to obtain a mixed liquid. This mixed liquid was stirred at 2000 rpm for 1 minute using a planetary centrifugal mixer, and then degassed at 2200 rpm for 30 seconds to obtain the NiO dispersion of Example 8.

The NiO dispersion of Example 8 was applied onto a glass substrate having a thickness of 0.7 mm using a spin coater. Spin coating was performed at 6000 rpm for 10 seconds. The resulting coating was baked in an incubator at 185°C for 1 hour in air to obtain the NiO film of Example 8.

Comparative Example 1
0.15g of NiO particles and 9.85g of methyl ethyl ketone were added to a 50ml resin container and stirred with a planetary centrifugal mixer to obtain a NiO dispersion. 80g of zirconia beads (0.8mmφ) were added to the prepared NiO dispersion. Then, zirconia beads were further added, and the difference between the surface of the NiO dispersion after adding the zirconia beads and the top surface of the beads was adjusted to be about several mm. Next, a disintegration treatment was performed for 1 hour using a paint shaker.
The crushed NiO dispersion was pressure filtered under nitrogen gas using a stainless steel mesh with a mesh size of 20 μm and a pressure filter. The NiO dispersion that had settled on the beads was then washed away using methyl ethyl ketone, and the NiO dispersion (NiO dispersion of Comparative Example 1) was collected.
The concentration of NiO particles in the NiO dispersion was 1.5%, and the concentration of methyl ethyl ketone was 98.5%.

Comparative Example 2
A NiO dispersion of Comparative Example 1 was obtained in the same manner as in Comparative Example 1, except that the dispersion medium in the NiO dispersion of Comparative Example 1 was changed to 2-methoxy-1-methylethyl acetate.
The concentration of NiO particles in the NiO dispersion was 1.5%, and the concentration of 2-methoxy-1-methylethyl acetate was 98.5%.

〔evaluation〕
For the NiO dispersions obtained in the examples and comparative examples, the particle diameter _D50 was measured by the following method. Furthermore, the dispersion stability was evaluated by the following method.
The NiO films obtained in the examples were evaluated for thickness, _Dn50 of NiO particles, light transmittance, band gap, work function, and film strength by the following methods. In Comparative Examples 1 and 2, the dispersion stability of the NiO particles in the NiO dispersion was poor, so no NiO film was produced and the evaluation of the NiO film was omitted. This is indicated by "-" in Table 1.
The results are shown in Tables 1 and 2 below.

[Particle diameter _D50 of NiO particles]
The dispersions of the examples and comparative examples were diluted with a propylene glycol/methanol mixed solvent (mass ratio 50:50) so that the NiO concentration was 0.6%. The particle diameter D50 of the NiO particles in the diluted dispersions was measured using _a particle size measurement device (Malvern Panalytical: Zetasizer Nano ZS) based on the dynamic light scattering method.
In the comparative example, the NiO particles settled during measurement of the particle diameter _D50 , and therefore the measurement was not possible. In Table 1, this is indicated as "measurable."

[Dispersion stability of NiO dispersion]
The dispersion of the example was diluted with a propylene glycol/methanol mixed solvent (mass ratio 50:50) so that the NiO concentration was 0.6%, and the settling velocity of the NiO particles at 25° C. was measured using a solution stability evaluation device (Formal Action: Turviscan MA2000). The settling velocity was evaluated as follows. The evaluation results are shown in Tables 1 and 2.
A: Sedimentation rate is 0.001 mm/min or less.
B: Sedimentation rate is greater than 0.001 mm/min and not greater than 0.01 mm/min.
C: Sedimentation rate is more than 0.01 mm/min and 0.1 mm/min or less.
D: Sedimentation rate exceeds 0.1 mm/min.

[Thickness of NiO film]
The thickness of the NiO film of the example was measured using a step gauge (KLA-Tenchore's stylus profiler P-17).

[Dn ₅₀ of NiO particles in NiO film]
The following was carried out as a pretreatment for measuring the _Dn50 of NiO particles. That is, for Examples 1, 4, and 5, the glass substrate coated with the NiO film was fixed to an aluminum stub using carbon tape so that the NiO film surface was the upper surface. For Examples 2, 3, 6, 7, and 8, the NiO film was peeled off from the substrate and fixed to an aluminum stub using carbon tape. Next, using a sputtering device (Ion Coater IB-5 manufactured by Eiko Co., Ltd.), the NiO film surface was coated with Pt under the conditions of 3 mA x 2 min using a Pt target.
Next, the NiO film was photographed with a scanning electron microscope (SU7000 manufactured by Hitachi High-Technologies Corporation) at a magnification of 30,000 to 200,000 after automatically adjusting the contrast. From the electron microscope images, 200 or more particles that did not overlap were randomly selected, and the particle diameters (Heywood diameters) were measured, and the arithmetic average value thereof was taken as _Dn50 .

[Light transmittance of NiO film]
The light transmittance of the NiO film of the example was evaluated based on the transmittance of light with a wavelength of 450 nm using a spectrophotometer (U-4100 manufactured by Hitachi High-Tech Science Corporation).

[Band gap of NiO film]
The band gap of the NiO film of the example was measured using a spectrophotometer (U-4100 manufactured by Hitachi High-Tech Science Corporation).

[Work function of NiO film]
The work function of the NiO film of the example was measured using an atmospheric photoelectron yield spectrometer (AC-3, manufactured by Rika Keiki Co., Ltd.).

[Film strength of NiO film]
The film strength of the NiO film of the example was evaluated in accordance with JIS K 5600 using a scratch hardness tester (KT-VF2391, manufactured by Coatec Co., Ltd.).

As is clear from the results shown in Tables 1 and 2, the NiO dispersions obtained in the Examples have higher dispersion stability of NiO particles than the dispersions obtained in the Comparative Examples.
It is also found that the NiO films formed using the NiO dispersions obtained in each Example have high optical transparency, and furthermore, that the NiO films formed using the NiO dispersions obtained in each Example exhibit band gaps and work functions close to those of bulk NiO.
In particular, when a coupling agent is contained in the NiO dispersion, the NiO film formed using the dispersion has higher light transmittance and higher film strength.

Claims (14)

A NiO film containing NiO particles,
The NiO film has a cumulative 50% diameter Dn50 in a volume particle size distribution of the NiO particles, the cumulative 50% diameter _Dn50 being 5 nm or more and 50 nm or less, as measured by observing the NiO film with a scanning electron microscope. A NiO film containing NiO particles,
The NiO film comprises a compound having a polyhydric alcohol skeleton and containing at least one element selected from the group consisting of silicon, titanium, zirconium and aluminum. The NiO film according to claim 2, wherein the compound contains at least two elements selected from silicon, titanium, zirconium, and aluminum. The NiO film according to claim 2 or 3, wherein the compound is a crosslinked product of a silane coupling agent containing an epoxy group, a titanium coupling agent, and a polyhydric alcohol. The NiO film according to claim 1 or 2, which has a transmittance of 85% or more for light with a wavelength of 450 nm. The NiO film according to claim 1 or 2, having a band gap of 3.2 eV or more and 3.9 eV or less. The NiO film according to claim 1 or 2, having a work function of 5.2 eV or more and 5.7 eV or less. The NiO film according to claim 1 or 2, having a thickness of 50 nm or more and 500 nm or less. A NiO dispersion containing NiO particles and a polyhydric alcohol. The NiO dispersion according to claim 9 , wherein the NiO particles have a cumulative 50% diameter D ₅₀ in a volume particle size distribution measured by a dynamic light scattering method of 5 nm or more and 50 nm or less. The NiO dispersion according to claim 9 or 10, wherein the settling velocity of the NiO particles is 0.1 mm/min or less. The NiO dispersion according to claim 9 or 10, further comprising a coupling agent. the coupling agents are two different types of coupling agents,
The NiO dispersion according to claim 12 , wherein one of the coupling agents is a coupling agent containing an epoxy group. The coupling agent is a silane coupling agent and a titanium coupling agent,
The NiO dispersion according to claim 12, wherein the content of the titanium coupling agent is 0.1 parts by mass or more and 5.0 parts by mass or less relative to 100 parts by mass of the silane coupling agent.