Method for manufacturing photoelectrode and photoelectrode

Abstract

The present invention is a method for manufacturing a photoelectrode, including: preparing a transparent conductive substrate comprising a transparent conductive layer on a substrate; synthesizing a sol from titanium alkoxide and alkyl ammonium hydroxide; applying the sol onto the transparent conductive layer; and forming a titanium oxide layer by sintering the sol on the transparent conductive layer at a temperature below 400°C, wherein the molar ratio R ([NR4+] / [Ti+]) of alkyl ammonium ions (NR4 +) in the alkyl ammonium hydroxide to titanium ions (Ti+) in the titanium alkoxide is in the range 0.125-0.5.

Description

Method for fabricating photoelectrodes, and photoelectrodes

[0001] This invention relates to a method for fabricating a photoelectrode, and to a photoelectrode itself.

[0002] To combat global warming, reducing greenhouse gas emissions is essential, leading to the development of clean energy sources such as solar power and fuel cells that use hydrogen as their primary energy source. Photoelectrodes containing metal oxide semiconductors are being considered for use as photoelectrodes in photoelectric conversion elements used in solar cells, and as photoelectrodes for converting water into hydrogen fuel using sunlight.

[0003] Japanese Patent Publication No. 2018-110182

[0004] Patent Document 1 discloses a photoelectrode fabricated by sintering a titanium oxide layer on a transparent conductive layer on a transparent substrate at 500°C. However, the resistance value of this transparent conductive layer sometimes increased. On the other hand, forming the titanium oxide layer by sintering at a lower temperature may suppress the increase in resistance value of the transparent conductive layer. However, the titanium oxide layer formed by such sintering may not become crystalline, which can lead to a decrease in photoelectric effect performance.

[0005] The object of the present invention is to provide a photoelectrode in which the titanium oxide layer is crystallized and the increase in the resistance value of the transparent conductive layer is suppressed.

[0006] Embodiments of the present invention provide the following method for producing a liquid photoelectrode and a photoelectrode.

[0007] (1) According to one embodiment, the method includes: preparing a transparent conductive substrate having a transparent conductive layer on a substrate; synthesizing a sol from titanium alkoxide and alkylammonium hydroxide; coating the sol onto the transparent conductive layer; and sintering the sol on the transparent conductive layer at a temperature of less than 400°C to form a titanium oxide layer, wherein the titanium ions (Ti) in the titanium alkoxide ＋ ) and the alkylammonium ion (NR ４ ＋ ) molar ratio R([NR ４＋] / [Ti ＋ A method for fabricating a photoelectrode is provided, wherein the coefficient of

[0008] (2) According to another embodiment, a method for producing the photoelectrode described in (1) is provided, wherein the transparent conductive layer contains indium tin oxide.

[0009] (3) According to one embodiment, a photoelectrode is provided comprising: a substrate; a transparent conductive layer on the substrate; a titanium oxide layer on the transparent conductive layer; and a mixed layer between the transparent conductive layer and the titanium oxide layer, the mixed layer comprising titanium and at least one metal in the transparent conductive layer, wherein the titanium oxide layer includes a crystalline structure, and the thickness of the mixed layer is 30 nm or less.

[0010] (4) According to another embodiment, a photoelectrode is provided comprising: a substrate; a transparent conductive layer on the substrate; a titanium oxide layer on the transparent conductive layer; and a mixed layer between the transparent conductive layer and the titanium oxide layer, the mixed layer comprising titanium and at least one metal in the transparent conductive layer, wherein the titanium oxide layer has a crystalline structure, and the ratio of the thickness of the mixed layer to the total thickness of the transparent conductive layer and the mixed layer is 30% or less.

[0011] (5) According to another embodiment, a photoelectrode is provided comprising: a substrate; a transparent conductive layer on the substrate; a titanium oxide layer on the transparent conductive layer; and a mixed layer between the transparent conductive layer and the titanium oxide layer containing titanium and at least one metal in the transparent conductive layer, wherein the titanium oxide layer has a crystalline structure, and by X-ray photoelectron spectroscopy while argon etching from the titanium oxide layer toward the transparent conductive layer, the content of titanium, oxygen, and the metal in the transparent conductive layer at each position in the thickness direction is measured, and the argon etching time when the content ratio (at%) of the at least one metal in the transparent conductive layer to the total content of titanium, oxygen, and metal exceeds 1 at%, and the argon etching time when the content ratio (at%) of titanium to the total content of titanium, oxygen, and metal becomes less than 1 at%, and the time difference between the argon etching times is 0.75 minutes or less.

[0012] (6) According to another embodiment, the photoelectrode described in any of (3) to (5) is provided, wherein the titanium oxide layer has a critical load value of 30 mN or more in a microscratch test in accordance with JIS R 3255.

[0013] (7) According to another embodiment, a photoelectrode according to any one of (3) to (6) is provided, wherein the transparent conductive layer comprises indium tin oxide and the at least one metal in the transparent conductive layer is indium.

[0014] According to the present invention, it is possible to provide a photoelectrode in which the titanium oxide layer is crystallized and the increase in the resistance value of the transparent conductive layer is suppressed.

[0015] The attached drawings are included in the specification and constitute part thereof, illustrating embodiments of the present invention and are used to explain the principles of the present invention together with the description thereof. A schematic cross-sectional view of a photoelectrode according to this embodiment.

[0016] The embodiments will be described in detail below with reference to the attached drawings. Note that the following embodiments do not limit the invention as defined in the claims, and not all combinations of features described in the embodiments are essential to the invention. Two or more features from the multiple features described in the embodiments may be combined arbitrarily. Furthermore, identical or similar configurations will be given the same reference numeral, and redundant descriptions will be omitted.

[0017] To facilitate understanding of the method for fabricating the photoelectrode according to this embodiment, the configuration of the fabricated photoelectrode will be described first.

[0018] <Photoelectrode according to this embodiment> The photoelectrode according to this embodiment comprises a substrate, a transparent conductive layer on the substrate, a titanium oxide layer on the transparent conductive layer, and a mixed layer between the transparent conductive layer and the titanium oxide layer, the mixed layer containing titanium and at least one metal in the transparent conductive layer. The titanium oxide layer has a crystalline structure, and the photoelectrode can have excellent photoelectric effect performance or photocatalytic performance. The mixed layer is defined by X-ray photoelectron spectroscopy as described later, and the mixed layer is absent or almost absent. In other words, the thickness of the mixed layer is 30 nm or less. The absence or almost absence of the mixed layer can suppress the increase in the resistance value of the transparent conductive layer.

[0019] Figure 1 is a schematic cross-sectional view of a photoelectrode according to this embodiment. The photoelectrode 1000 comprises a substrate 100, a transparent conductive layer 200 on the substrate 100, and a titanium oxide layer 300 on the transparent conductive layer 200. Furthermore, the photoelectrode 1000 has a mixed layer 400 between the transparent conductive layer 200 and the titanium oxide layer 300, which contains titanium and at least one metal in the transparent conductive layer. The stacking direction of each layer is sometimes referred to as the thickness direction X.

[0020] (Substrate) The substrate 100 is not particularly limited as long as it can support the transparent conductive layer 200 and the titanium oxide layer 300, and may be made of inorganic or organic materials. If high rigidity is required, inorganic materials may be used, and such inorganic materials include ceramics, glass, metals, etc. Examples of glass include borosilicate glass, soda-lime glass, white glass, and quartz glass. If flexibility is required, organic materials may be used, and such organic materials include resins, such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate (PC), and polyethersulfone (PES). Furthermore, as will be described later, since the titanium oxide layer 300 of the photoelectrode according to this embodiment is formed by sintering at a temperature of less than 400°C, such organic materials can be used.

[0021] Furthermore, the substrate 100 may be made of a transparent material or an opaque material. When light is incident on the titanium oxide layer 300 via the substrate 100 and the transparent conductive layer 200, the substrate 100 may be made of a transparent material. Note that a transparent material is a material with high transmittance of visible light. The substrate 100 is in the form of a plate, and its shape is not particularly limited; it can be a cube, a rectangular prism, a disc, etc. The thickness of the substrate 100 can be appropriately selected according to the size of the photoelectrode 1000, for example, it can be in the range of 50 μm to 40 mm.

[0022] (Transparent conductive layer) The transparent conductive layer 200 is not particularly limited as long as it supports the titanium oxide layer 300 and is transparent and conductive. Examples include indium tin oxide (ITO), tin oxide, and fluorinated tin oxide. The transparent conductive layer 200 may be a single layer or a laminate of multiple different layers. Note that a transparent material is a material with high transmittance of visible light. The surface resistance of the transparent conductive layer 200 is not particularly limited, but it should be less than 30 Ω / □, may be 20 Ω / □ or less, or may be 10 Ω / □ or less. This can reduce voltage loss in the photoelectrode. The surface resistance of the transparent conductive layer 200 should be as low as possible, but 5 Ω / □ or more is practical. The thickness of the transparent conductive layer 200 can be appropriately selected according to the size of the photoelectrode 1000, for example, it can be in the range of 0.01 μm to 5 mm.

[0023] (Titanium Oxide Layer) The titanium oxide layer 300 is not particularly limited as long as it can act as part of a photoelectric conversion element or convert water into hydrogen, etc., through the action of light. Also, the titanium oxide layer 300 is titanium dioxide (TiO ２The titanium oxide layer 300 can be a layer of the photoelectrode 1000. The titanium oxide layer 300 has a crystalline structure, and the crystalline structure may be polycrystalline or monocrystalline. This improves the photoelectric conversion performance or photocatalytic performance of the titanium oxide layer 300. The crystalline structure of titanium oxide can be anatase type, brookite type, or rutile type, but the crystalline structure of titanium oxide can be anatase type. The titanium oxide layer 300 may also be a titanium oxide layer doped with at least one of metals such as lithium, magnesium, vanadium, and niobium. The thickness of the titanium oxide layer 300 can be appropriately selected according to the size of the photoelectrode 1000, and can be in the range of 0.01 μm to 5 mm, for example.

[0024] (Adhesion) The titanium oxide layer 300 may have excellent adhesion to the transparent conductive layer 200. The adhesion of the titanium oxide layer 300 can be evaluated by performing a micro-scratch test in accordance with JIS R 3255 (Test method for adhesion of thin films on a glass substrate). The adhesion of the titanium oxide layer 300 may have a critical load value of 30 mN or more in the micro-scratch test, or it may be 40 mN or more, or it may be 50 mN or more. This will give the photoelectrode excellent durability.

[0025] (Mixed layer) The mixed layer 400 is located between the transparent conductive layer 200 and the titanium oxide layer 300 and contains titanium and at least one metal in the transparent conductive layer. In other words, the mixed layer 400 is a layer in which at least some of the metal contained in the transparent conductive layer 200 and the titanium oxide layer 300 are present in the region where the transparent conductive layer 200 and the titanium oxide layer 300 are in contact. Furthermore, the at least some of the metal contained in the transparent conductive layer 200 in the mixed layer 400 can be the metal with the highest content among the metals in the transparent conductive layer 200.

[0026] The mixed layer 400 can be defined using X-ray photoelectron spectroscopy (XPS). First, from the titanium oxide layer 300 toward the transparent conductive layer 200, argon etching using argon ion plasma or the like and measurement by XPS are alternately performed to measure titanium, oxygen, and metals in the transparent conductive layer (for example, indium, tin, etc.) at each position in the thickness direction X. The measurement interval in the thickness direction by XPS may be set according to the etching rate, the measurement device, and the like. Next, the argon etching time when the content ratio of one metal (for example, indium) of the transparent conductive layer to the total content of titanium, oxygen, and metals in the transparent conductive layer first exceeds 1 at%, and the argon etching time when the content ratio of titanium to the total content of titanium, oxygen, and metals in the transparent conductive layer first becomes less than 1 at% are obtained, and the region existing between these argon etching times is defined as the mixed layer 400.

[0027] In the mixed layer 400 according to the present embodiment, the time difference between the argon etching time when the content ratio of one metal (for example, indium) of the transparent conductive layer first exceeds 1 at% and the argon etching time when the content ratio of titanium first becomes less than 1 at% is 0.75 minutes or less, may be 0.6 minutes or less, may be 0.5 minutes or less, may be 0.4 minutes or less, may be 0.3 minutes or less, may be 0.2 minutes or less, may be 0.1 minutes or less, or may be 0.05 minutes or less. If the mixed layer 400 has a region or thickness corresponding to this time difference, an increase in the resistance value of the transparent conductive layer can be suppressed. On the other hand, the lower limit of the time difference is not particularly limited, and the time difference does not need to exist. In other words, the mixed layer 400 does not need to exist.

[0028] Furthermore, the relationship between argon etching time and thickness can be obtained by performing argon etching on a transparent conductive layer having a known thickness and measuring the time taken therefor. Since the mixed layer 400 is mainly composed of a substance that constitutes a transparent conductive layer such as indium tin oxide, the relationship between argon etching time and thickness for a transparent conductive layer can be regarded as equivalent to the relationship between argon etching time and thickness for the mixed layer 400. From this, the thickness t1 of the mixed layer 400 can be obtained from the relationship between the argon etching time and thickness of the transparent conductive layer and the time difference of the argon etching time of the mixed layer 400 as described above.

[0029] For example, under the XPS measurement conditions described later, it has been confirmed that measuring a depth direction of 40 nm in indium tin oxide requires one minute, so an argon etching time of 0.1 minutes can be converted to a thickness of 4 nm. Accordingly, the thickness t1 of the mixed layer 400 is 30 nm or less, may be 25 nm or less, may be 20 nm or less, or may be 16 nm or less. This reduces disturbances in the composition and crystal structure of the transparent conductive layer, and can suppress an increase in the resistance value of the transparent conductive layer. On the other hand, the lower limit of the thickness t1 of the mixed layer 400 is not particularly limited, and the mixed layer 400 does not need to be present.

[0030] <Photoelectrode According to Another Embodiment> A photoelectrode according to another embodiment includes a substrate, a transparent conductive layer on the substrate, a titanium oxide layer on the transparent conductive layer, and a mixed layer containing titanium and at least one metal in the transparent conductive layer between the transparent conductive layer and the titanium oxide layer. The titanium oxide layer has a crystal structure, and the photoelectrode can have excellent photoelectric effect performance or photocatalytic performance. Further, the ratio of the thickness of the mixed layer to the total thickness of the transparent conductive layer and the mixed layer is 30% or less. As described above, the thickness of the mixed layer is smaller than that of the transparent conductive layer, and an increase in the resistance value of the transparent conductive layer can be suppressed.

[0031] Furthermore, the ratio of the thickness t1 of the mixed layer to the total thickness t2 of the transparent conductive layer 200 and the mixed layer 400 is 30% or less, may be 20% or less, or may be 16% or less. This reduces disturbances in the composition and crystal structure of the transparent conductive layer, and can suppress an increase in the resistance value of the transparent conductive layer. On the other hand, the lower limit of the ratio of the thickness t1 of the mixed layer to the total thickness t2 of the transparent conductive layer 200 and the mixed layer 400 is not particularly limited, and may be 0%.

[0032] The application of the photoelectrode according to the present embodiment is not particularly limited, and the photoelectrode can be a photoelectrode for a photoelectric conversion element used in solar cells or the like, or a photoelectrode for converting water into hydrogen fuel using sunlight.

[0033] <Method for producing the photoelectrode according to the present embodiment> The method for producing the photoelectrode according to the present embodiment includes: preparing a transparent conductive substrate having a transparent conductive layer on a substrate; synthesizing a sol from titanium alkoxide and alkylammonium hydroxide; applying the sol onto the transparent conductive layer; and sintering the sol on the transparent conductive layer at a temperature lower than 400°C to form a titanium oxide layer. In addition, titanium ions (Ti ＋ ) and alkylammonium ions (NR ４ ＋ ) have a molar ratio R ([NR ４ ＋ / [Ti ＋ ) in the range of 0.125 to 0.5. This allows the sol to crystallize and form a crystalline titanium oxide layer 300. Furthermore, since a mixed layer containing titanium and the metal in the transparent conductive layer is formed with a thickness of 30 nm or less, or is substantially not formed, an increase in the resistance value of the transparent conductive layer can be suppressed.

[0034] (Preparation of Transparent Conductive Substrate) A transparent conductive substrate may be prepared by preparing a substrate and forming a transparent conductive layer on it. The substrate can be the same as the one described in <Photoelectrode according to this embodiment>. The method for forming the transparent conductive layer is not particularly limited, and the transparent conductive layer may be formed using film formation techniques such as sputtering, CVD, and coating. Furthermore, the transparent conductive layer is formed using the raw materials necessary to obtain the transparent conductive layer described in <Photoelectrode according to this embodiment> in these film formation techniques. In another embodiment, a commercially available transparent conductive substrate with a transparent conductive layer already formed on it may be prepared.

[0035] (Synthesis of Sol) The synthesis of the sol is the sol-gel method, and it is synthesized from titanium alkoxide and alkylammonium hydroxide. In one embodiment, a predetermined molar ratio R([NR)) is added to an aqueous solution of alkylammonium hydroxide of a predetermined concentration. ４ ＋ ] / [Ti ＋ By adding titanium alkoxide and stirring so that the mixture becomes ], and allowing it to stand at a predetermined temperature for a predetermined time, the titanium alkoxide can be hydrolyzed to synthesize a titanium dioxide sol. Water may be added to the mixture of titanium alkoxide and alkylammonium hydroxide for concentration adjustment, etc. Examples of water include deionized water, distilled water, and pure water.

[0036] Titanium alkoxides contain titanium ions (Ti ＋ ) and alkylammonium ions (NR) in alkylammonium hydroxide ４ ＋ ) molar ratio R([NR ４ ＋ ] / [Ti ＋ The sol is added to the aqueous solution of alkylammonium hydroxide so that the molar ratio R is in the range of 0.125 to 0.5. By setting the molar ratio R to this range, the sol coated on the transparent conductive layer described later can be sintered and crystallized at a temperature of less than 400°C, and the adhesion of the titanium oxide layer to the transparent conductive layer is further improved.

[0037] Molar ratio R([NR ４＋ ] / [Ti ＋ The molar ratio R may be 0.125 or more and 0.5 or less, 0.15 or more, 0.175 or more, 0.2 or more, 0.45 or less, 0.3 or less, or 0.25 or less. By setting the molar ratio R to 0.125 or more, the sol can be converted into a gel at an appropriate rate. Furthermore, by setting the molar ratio R to 0.5 or less, the sol coated on the transparent conductive layer described later can be sintered and crystallized at a temperature of less than 400°C, reducing the remaining amorphous titanium oxide and improving the photoelectric conversion performance or photocatalytic performance of the titanium oxide layer 300.

[0038] Titanium alkoxides are not particularly limited and may be used individually or in combination of two or more types. Examples of alkoxy groups include methoxy, ethoxy, n-propoxy, i-propoxy, n-butoxy, i-butoxy, sec-butoxy, tert-butoxy, and 2-ethylhexoxy groups. Examples of titanium alkoxides include tetramethoxytitanium, tetrapropoxytitanium, tetraisopropoxytitanium, tetrabutoxytitanium, and tetrapentoxytitanium.

[0039] Alkylammonium hydroxide can be a quaternary ammonium salt represented by the following formula (1). NR ４ OH (1)

[0040] The alkylammonium hydroxide is not particularly limited and can be a water-soluble quaternary ammonium salt, and may be used alone or in combination of two or more types. The alkyl group R in formula (1) can be a methyl group, an ethyl group, an n-propyl group, an i-propyl group, an n-butyl group, an i-butyl group, etc. Examples of alkylammonium hydroxides include tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide, tetraisopropylammonium hydroxide, tetrabutylammonium hydroxide, and tetraisobutylammonium hydroxide. By using such alkylammonium hydroxides, sufficient crystallization can be achieved by sintering at temperatures below 400°C.

[0041] The concentration of the aqueous solution of alkylammonium hydroxide is not particularly limited and may be 10% by mass or more, 20% by mass or more, 40% by mass or more, 60% by mass or less, or 50% by mass or less.

[0042] Furthermore, the temperature and time of hydrolysis are not particularly limited; the temperature can be 100°C or lower, or it can be hydrothermally treated at 100°C or higher, and the time can be from 1 to 10 hours.

[0043] (Application of Sol) The sol is applied onto a transparent conductive layer. The application method is not particularly limited and includes spin coating, dip coating, bar coating, spray coating, blade coating, slit die coating, gravure coating, reverse coating, screen printing, print transfer, and inkjet.

[0044] (Sintering of Sol) The sol coated on the transparent conductive layer is sintered at a temperature of less than 400°C. The coated sol is dried as desired and then fired at a temperature of less than 400°C. The sintering temperature is not particularly limited as long as it is less than 400°C, and can be 370°C or lower, 350°C or lower, 300°C or lower, 150°C or higher, or 200°C or higher. By setting the sintering temperature to less than 400°C, the formation of a mixed layer can be suppressed, and the increase in the resistance value of the transparent electrode can be suppressed. Furthermore, in the method for manufacturing the photoelectrode according to this embodiment, even if the sol is sintered at less than 400°C, a crystalline titanium oxide layer 300 is formed. By setting the sintering temperature to 150°C or higher, the crystallinity of the titanium oxide layer is improved, and the photoelectric conversion performance or photocatalytic performance is improved.

[0045] The sintering time is not particularly limited and can be 0.5 hours or more, 1 hour or more, 2 hours or more, or 7 hours or less, 3 hours or less, or 2 hours or less. By setting the sintering time to 0.5 hours or more, the crystallinity of the titanium oxide layer is improved, and the photoelectric conversion performance or photocatalytic performance is improved. By setting the sintering time to 7 hours or less, the conductive performance of the transparent conductive layer can be maintained.

[0046] The present invention will be described in more detail below with reference to examples. However, the present invention is not limited in any way to the following examples.

[0047] (Example 1) Titanium isopropoxide (TIP) is added to a 40% by mass aqueous solution of tetrabutylammonium hydroxide (TBA) with titanium ions (Ti ＋ ) and tetrabutylammonium ions (NR ４ ＋ The mixture was then added so that the molar ratio R was 0.5. After that, titanium ions (Ti ＋Purified water was added and the mixture was stirred until the concentration of the solution was 0.1 mol%. The mixture was then left to stand and heated at 80°C for 7 days. The resulting titanium dioxide sol dispersion was spin-coated to a thickness of 1 μm onto a transparent conductive substrate having a 100 nm thick layer of indium tin oxide (ITO) on a glass substrate, and sintered at 300°C for 2 hours to produce a titanium dioxide photoelectrode. The obtained titanium dioxide photoelectrode was subjected to XRD measurement under the conditions described later, and its crystallization was confirmed by detecting the peak of anatase-type titanium dioxide.

[0048] (Example 2) Molar ratio R([NR ４＋ ] / [Ti ＋ A titanium oxide photoelectrode was prepared in the same manner as in Example 1, except that titanium isopropoxide (TIP) was added so that the ratio was 0.125. The obtained titanium oxide photoelectrode was subjected to XRD measurement under the conditions described later, and it was confirmed that it had crystallized by detecting the peak of anatase-type titanium oxide.

[0049] (Example 3) A titanium oxide photoelectrode was prepared in the same manner as in Example 1, except that tetramethylammonium hydroxide (TMAOH) was used instead of tetrabutylammonium hydroxide (TBA). The obtained titanium oxide photoelectrode was subjected to XRD measurement under the conditions described below, and it was confirmed that it had crystallized by detecting the peak of anatase-type titanium oxide.

[0050] (Example 4) A titanium oxide photoelectrode was prepared in the same manner as in Example 1, except that titanium butoxide (TBO) was used instead of titanium isopropoxide (TIP). The obtained titanium oxide photoelectrode was subjected to XRD measurement under the conditions described below, and it was confirmed that it was crystallized by detecting the peak of anatase-type titanium oxide.

[0051] (Comparative Example 1) Molar ratio R([NR ４＋ ] / [Ti ＋A titanium oxide photoelectrode was prepared in the same manner as in Example 1, except that titanium isopropoxide (TIP) was added so that the ratio was 0.75. The obtained titanium oxide photoelectrode was subjected to XRD measurement under the conditions described later, and it was confirmed that no anatase-type titanium oxide peak was detected, indicating that it was not crystallized.

[0052] (Comparative Example 2) A titanium oxide photoelectrode was prepared in the same manner as in Example 1, except that the titanium oxide sol on a transparent conductive substrate was sintered at 400°C for 2 hours. The obtained titanium oxide photoelectrode was subjected to XRD measurement under the conditions described later, and it was confirmed that it had crystallized by detecting the peak of anatase-type titanium oxide.

[0053] (Comparative Example 3) A titanium oxide photoelectrode was prepared in the same manner as in Example 1, except that an aqueous ammonia (NH3) solution was used instead of tetrabutylammonium hydroxide (TBA). The obtained titanium oxide photoelectrode was subjected to XRD measurement under the conditions described later, and it was confirmed that no anatase-type titanium oxide peak was detected, indicating that it was not crystallized.

[0054] (Surface Sheet Resistance) The surface sheet resistance (Ω / □) of the transparent conductive layer (ITO) in the portion of the titanium oxide layer not formed in the titanium oxide photoelectrode prepared in Example 1 was measured using a device manufactured by Mitsubishi Chemical Analytec Co., Ltd. (product name "Loresta-GP MCP-T600"). The surface sheet resistance (Ω / □) of the transparent conductive layer (ITO) in the portion of the titanium oxide layer not formed in the titanium oxide photoelectrodes prepared in Examples 2 to 4 and Comparative Examples 1 to 3 was measured in the same manner as in Example 1. The evaluation criteria are as follows: ○: Surface sheet resistance less than 30 Ω / □ ×: Surface sheet resistance 30 Ω / □ or more

[0055] (Evaluation of the mixed layer) The presence or absence of a mixed layer in the titanium oxide photoelectrode fabricated in Example 1 was determined using X-ray photoelectron spectroscopy (XPS). The equipment and analysis conditions used for XPS are shown below. (X-ray photoelectron spectroscopy (XPS) analyzer) ・ULVAC-PHIE, product name: "Quantum 2000" ・X-ray source: AlKα (1486.6 eV) ・X-ray beam diameter: 100 μm (Ion gun conditions) ・Beam voltage: 1 kV ・Ion current: 25 mA (Analysis conditions) ・Power value: 25 W ・Voltage: 15 kV ・Removal angle: 45 degrees ・Vacuum level: 5.0 × 10⁻⁶ －８ Pa

[0056] In the titanium oxide photoelectrode fabricated in Example 1, argon etching using argon ion plasma and XPS measurements were alternately performed from the titanium oxide layer toward the ITO layer to measure the content of titanium, oxygen, indium, and tin at each position in the thickness direction X. Next, the time difference in argon etching time was determined from the argon etching time when the ratio of indium content to the total content of titanium, oxygen, indium, and tin ([In / (Ti+O+In+Sn)]×100) first exceeded 1 at%, and the argon etching time when the ratio of titanium content to the total content of titanium, oxygen, indium, and tin ([Ti / (Ti+O+In+Sn)]×100) first fell below 1 at%. The mixed layers in the titanium oxide photoelectrodes fabricated in Examples 2 to 4 and Comparative Examples 1 to 3 were evaluated in the same manner as in Example 1. The evaluation criteria are shown below. ◎: Time difference is 0.4 minutes or less, i.e., the thickness of the mixed layer is 16 nm or less. ○: Time difference is greater than 0.4 minutes and 0.75 minutes or less, i.e., the thickness of the mixed layer is greater than 16 nm and 30 nm or less. ×: Time difference is greater than 0.75 minutes, i.e., the thickness of the mixed layer is greater than 30 nm.

[0057] (Crystallization) The titanium dioxide sol prepared in Example 1 was dried to form a powder. The exothermic peak of this powder was measured using a differential scanning calorimetry device. The titanium dioxide sol was calcined at the peak top temperature of the obtained exothermic peak, and XRD measurements were performed to confirm that titanium dioxide had crystallized. The peak top temperature was defined as the crystallization temperature. In the same manner as in Example 1, the crystallization of titanium dioxide was confirmed in the titanium dioxide sols prepared in Examples 2 to 4 and Comparative Examples 1 to 3, and the crystallization temperatures were determined. The evaluation criteria are shown below. ◎: Crystallization temperature is 300°C or below only. 〇: Crystallization temperature is below 400°C only (excluding 300°C or below only). ×: Crystallization temperature is 400°C or above. XRD measurements were performed using a fully automated horizontal X-ray diffractometer (Rigaku Corporation, SMART LAB) under the following conditions. • CuKα1 line: 0.15406 nm • Scanning range: 20°–90° • X-ray output setting: 45 kV–200 mA • Step size: 0.020° • Scan speed: 0.5° min －１ -4°min －１

[0058] (Adhesion) The titanium oxide layer in the titanium oxide photoelectrode prepared in Example 1 was subjected to a micro-scratch test in accordance with JIS R 3255 (Test method for adhesion of thin films on a glass substrate) using a micro-scratch tester manufactured by Resca Co., Ltd., and its adhesion was evaluated. The adhesion of the titanium oxide layers prepared in Examples 2 to 4 and Comparative Examples 1 to 3 was also evaluated in the same manner as in Example 1. The evaluation criteria and conditions for the adhesion test are shown below. Evaluation Criteria ○: Critical load value of 30 mN or more ×: Critical load value of less than 30 mN Test Conditions - Indenter slitus diameter: 25 um - Indenter spring constant: 100 g / m ２ • Scratching speed: 20 μm / sec • Load application speed: 0.83 mN / sec • Excitation amplitude: 100 μm • Excitation frequency: 45 Hz

[0059] Table 1 shows the raw materials and sintering temperatures for Examples 1 to 4 and Comparative Examples 1 to 3, along with the results for surface sheet resistance, mixed layer evaluation, crystallization temperature, and adhesion.

[0060]

[0061] The titanium oxide photoelectrodes of Examples 1 to 4 showed excellent surface sheet resistance, mixed layer evaluation, crystallization temperature, and adhesion results. Therefore, the titanium oxide layers of Examples 1 to 4 had a crystalline structure, and the photoelectrodes of Examples 1 to 4 had virtually no mixed layer; thus, the photoelectrodes of Examples 1 to 4 possessed excellent photoelectric conversion or photocatalytic performance.

[0062] The sols in Comparative Examples 1 and 3 had high crystallization temperatures, and the titanium oxide layer did not have a crystalline structure. The photoelectrode in Comparative Example 2 had a thick mixed layer.

[0063] Although embodiments of the invention have been described above, the invention is not limited to the embodiments described above, and various modifications and changes are possible within the scope of the gist of the invention.

[0064] This application claims priority based on Japanese Patent Application No. 2025-039499, filed on 12 March 2025, and all of its contents are incorporated herein by reference.

Claims

1. The process includes: preparing a transparent conductive substrate having a transparent conductive layer on a substrate; synthesizing a sol from titanium alkoxide and alkylammonium hydroxide; coating the sol onto the transparent conductive layer; and sintering the sol on the transparent conductive layer at a temperature of less than 400°C to form a titanium oxide layer, wherein the titanium ions (Ti) in the titanium alkoxide... ＋ ) and the alkylammonium ion (NR ４ ＋ ) molar ratio R([NR ４＋ ] / [Ti ＋ A method for fabricating a photoelectrode, wherein the coefficient of the photoelectrode is in the range of 0.125 to 0.

5.

2. The method for producing a photoelectrode according to claim 1, wherein the transparent conductive layer contains indium tin oxide.

3. A photoelectrode comprising: a substrate; a transparent conductive layer on the substrate; a titanium oxide layer on the transparent conductive layer; and a mixed layer between the transparent conductive layer and the titanium oxide layer, the mixed layer comprising titanium and at least one metal in the transparent conductive layer, wherein the titanium oxide layer has a crystalline structure and the thickness of the mixed layer is 30 nm or less.

4. A photoelectrode comprising: a substrate; a transparent conductive layer on the substrate; a titanium oxide layer on the transparent conductive layer; and a mixed layer between the transparent conductive layer and the titanium oxide layer, the mixed layer comprising titanium and at least one metal in the transparent conductive layer, wherein the titanium oxide layer has a crystalline structure, and the ratio of the thickness of the mixed layer to the total thickness of the transparent conductive layer and the mixed layer is 30% or less.

5. The photoelectrode according to claim 3 or 4, wherein the titanium oxide layer has a critical load value of 30 mN or more in a micro-scratch test in accordance with JIS R 3255.

6. The photoelectrode according to claim 3 or 4, wherein the transparent conductive layer contains indium tin oxide, and the at least one metal in the transparent conductive layer is indium.

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