JP7799255B2 - Laminate and manufacturing method thereof - Google Patents

Description

Article 30, paragraph 2 of the Patent Act applies. September 2, 2020: Publication on the website of the lecture at the 2020 Autumn Meeting (167th) of the Japan Institute of Metals. September 15-18, 2020: Presentation at the 2020 Autumn Meeting (167th) of the Japan Institute of Metals.

The present invention relates to a laminate and a method for manufacturing the same.

Titanium dioxide is a material known as a photocatalyst. While titanium dioxide can absorb high-energy ultraviolet light, it has low light absorption efficiency in the visible light range. Therefore, there is a demand for photocatalysts that can absorb light in the visible light range.

Magneli phase oxides are known as photocatalysts that absorb light in the visible light region, such as the Magneli phase titanium oxide of Ti _n O _2n-1 .

Patent Documents 1 and 2 disclose methods for producing Ti _n O _2n-1 . Specifically, Patent Document 1 describes a method for producing a pellet-shaped sample by mixing TiO ₂ nanoparticles of approximately 10 nm to 30 nm with a hydrogen reducing agent such as CaH ₂ , LiH, NaH, MgH ₂ , LiAlH ₄ , or NaBH ₄ and pressurizing the mixture under an inert gas atmosphere, and then heating the pellet-shaped sample at a low temperature of approximately 350°C for several days for reduction while crushing the sample in the process, thereby producing a particulate Ti ₂ O ₃ or other composition. Patent Document 2 describes a method for producing a titanium oxide crystal having Ti _n O _2n-1 on a portion of the crystal surface of a titanium oxide represented by Li _α Ti _β O _γ by mixing and stirring a number of raw materials in a predetermined ratio and order and then heat-treating the mixture under predetermined temperature and pressure conditions.

JP 2012-214348 A International Publication No. 2016/159323

However, the manufacturing methods described in Patent Documents 1 and 2 require complex processing to form Magneli phase oxide, and there is no disclosure of a layered body made of Magneli phase oxide or a manufacturing method thereof. Furthermore, no laminated body is known that has a layered body made of Magneli phase oxide in the surface layer portion.

The present invention was made in consideration of the above circumstances, and aims to provide a method for producing a laminate that can easily produce a laminate having a metal oxide layer composed of Magneli phase oxide, and a laminate obtained by this production method.

After extensive research, the inventors discovered that when the surface of a metal substrate is oxidized in the atmosphere and then heat-treated in an environment controlled to reduce the oxygen partial pressure, metal ions diffuse outward, changing the composition and thickness of the metal oxide layer formed on the surface. This makes it possible to thin the metal oxide layer made of Magneli phase oxide and control the composition of the thin-layered Magneli phase oxide through a simple process. As a result, it is possible to form a laminate having a Magneli phase metal oxide layer on the surface, and it is also possible to easily form a Magneli phase metal oxide layer on a substrate with a complex shape.

In other words, the gist of the present invention is as follows:

(1) A laminate according to a first aspect of the present invention comprises a metal layer and a metal oxide layer formed on a surface portion of the metal layer and composed of Magneli-phase titanium oxide, the Magneli-phase titanium oxide being represented by a composition formula Ti _n O _2n-1 (n≧3).

(2) In the laminate according to the above aspect, the average thickness of the metal oxide layer composed of the Magneli phase titanium oxide may be 2 μm or more.

(3) In the laminate according to the above aspect, the metal oxide layer may have two regions with different compositions in the stacking direction, and the more distant the region from the metal layer, the lower the composition ratio of titanium element in the Magneli phase titanium oxide.

(4) Either of the two regions may be made of Ti ₄ O ₇ .

(5) In the laminate according to the above aspect, voids may be present between the metal layer and the metal oxide layer composed of the Magneli phase titanium oxide.

(6) A method for producing a laminate according to a second aspect of the present invention is a method for producing a laminate according to the first aspect, and includes a first step of preheating a titanium substrate in the atmosphere, and a second step of heat-treating the preheated titanium substrate under a low oxygen partial pressure.

(7) In the method for producing a laminate according to the above aspect, in the first step, the titanium substrate may be preheated at 650°C to 750°C, and in the second step, the preheated titanium substrate may be heat-treated at a temperature of 750°C to 950°C and an oxygen partial pressure of 1.0 x ^10-7 atm or less.

The present invention provides a laminate having a metal oxide layer composed of a Magneli phase oxide.

1 is a schematic cross-sectional view showing a cross section of a laminate according to an embodiment of the present invention. 1 shows the results of thin-film GI-XRD measurement of the sample of Example 1, which is a laminate according to the present embodiment. 1 is a TEM image showing a cross section of a sample of Example 1, which is a laminate according to the present embodiment. 1 is an example of an electron beam diffraction image of Ti ₅ O ₉ crystal grains. FIG. 2 is a cross-sectional view schematically showing a modified example of the laminate of FIG. 1 . FIG. 2 is a cross-sectional view schematically showing another modified example of the laminate of FIG. 1 . FIG. 2 is a cross-sectional view schematically showing another modified example of the laminate of FIG. 1 . 5A to 5C are diagrams illustrating an example of a second step in the method for manufacturing a laminate according to the present embodiment. 1 shows the results of thin film GI-XRD measurement of the laminate of Example 2. 1 shows the results of thin film GI-XRD measurement of the laminates of Examples 3 to 5. 1 shows the results of XRD measurement of the laminates of Comparative Examples 1 to 3. 1 shows the results of XRD measurement of the laminates of Comparative Examples 4 to 6. 10 shows the results of XRD measurement of the laminate of Comparative Example 7. 1 shows the results of thin film GI-XRD measurement of the laminates of Comparative Examples 8 and 9. 10 shows the results of thin film GI-XRD measurement of the laminate of Comparative Example 10. 10 shows the results of XRD measurement of the laminate of Comparative Example 11. 1 is a TEM image showing a cross section of a sample of Example 1. 1 is a TEM image showing a cross section of a sample of Example 2. 1 is a TEM image showing a cross section of a sample of Comparative Example 1. 1 is a TEM image showing a cross section of a sample of Comparative Example 2. 1A and 1B are an enlarged view of the vicinity of region _Rd of the sample of Example 1 and an electron beam diffraction image of region _Rd . 1 is an enlarged view of the vicinity of region _R1 of the sample of Comparative Example 1 and an electron beam diffraction image of region _R1 . 1 shows a HAADF-STEM image of region _Rb of the sample of Example 1 and an image obtained by Fourier transforming the image. 1A and 1B are diagrams showing the contrast intensity of XY lines in the HAADF-STEM image of the sample of Example 1 and the atomic arrangement of region _Ra . FIG. 1 is a diagram showing the reflectance of Example 1, Comparative Examples 1 and 2, and rutile-type white _TiO2 powder. FIG. 10 is a diagram showing the thermogravimetric curve of Comparative Example 12. FIG. 10 is a diagram showing an external appearance photograph and XRD measurement results of a sample in state c in Comparative Example 12.

An example of an embodiment of the present invention will be described in detail below with reference to the drawings. Note that the drawings used in the following description may show enlarged portions of the features of the present invention for the sake of clarity. For this reason, the dimensional proportions of each component may differ from the actual dimensions.

[Laminate]
1 is a schematic cross-sectional view showing a cross section of a laminate 100 according to this embodiment. The laminate 100 has a metal layer 10 and a metal oxide layer 20 made of Magneli-phase titanium oxide and provided on a surface layer portion of the metal layer 10.

(metal layer)
The metal layer 10 is a layer containing, for example, titanium element as a main component.

(Metal oxide layer)
The metal oxide layer 20 is provided above the metal layer 10, for example, on the surface of the metal layer 10. The metal oxide layer 20 in this embodiment is not limited to a configuration in which it is in direct contact with the metal layer 10, but also includes a configuration in which it is separated from the metal layer 10, for example, a configuration in which it is provided on the metal layer 10 via one or more other layers. Note that "above" does not necessarily coincide with the direction along the direction of gravity.

The metal oxide layer 20 is a layer made of Magneli phase titanium oxide. Magneli phase titanium oxide is a composition expressed by the composition formula Ti _n O _2n-1 (n≧3). Examples of such compositions include Ti O ₅ , Ti _{O 7} , Ti _{O 9} , _Ti O _{11 ,} and Ti _{O 13} .

Magneli phase titanium oxide is a rutile-type metal dioxide in which oxygen vacancies are periodically introduced. Magneli phase titanium oxide with n 3≦n≦9 has periodic planar defects on the (121) plane, while Magneli phase titanium oxide with n≧10 has periodic planar defects on the (132) plane.

The metal oxide layer 20 contains, for example, two or more compounds with different composition ratios. Note that all of the compounds contained in the metal oxide layer 20 are composed of Magneli phase titanium oxide.

Figure 1 shows an example in which the metal oxide layer 20 contains two or more compounds with different compositions. The metal oxide layer 20 has, for example, a first region 21, a second region 22, and a third region, in this order from the metal layer 10 side. The first region 21 is the region closest to the metal layer 10 and is in direct contact with the metal layer 10. The second region 22 is located farther from the metal layer 10 than the first region 21 and is disposed on the opposite side of the first region 21 from the metal layer 10. The third region 23 is located farther from the metal layer 10 than the second region 22 and is disposed on the opposite side of the second region 22 from the first region 21. In this embodiment, the third region 23 constitutes the outermost layer exposed on the surface.

Each of the first region 21, second region 22, and third region 23 is, for example, a region of a compound having the same composition, and is a region composed of crystal grains of each compound. The Magneli phase titanium oxides constituting the first region 21, second region 22, and third region 23 have, for example, different compositions. Furthermore, crystal grain boundaries exist between each of the first region 21, second region 22, and third region 23.

Of the three regions of metal oxide layer 20, the farther a region is from metal layer 10, the lower the titanium element composition ratio in the Magneli phase titanium oxide. That is, in the example shown in Fig. 1, the titanium element composition ratio in the Magneli phase titanium oxide contained in first region 21 is the highest, and the titanium element composition ratio in the Magneli phase titanium oxide contained in third region 23 is the lowest. An example of such a case is one in _which first region 21 is composed of the composition formula _Ti3O5 , second region ₂₂ is composed of the composition formula _Ti4O7 , and third region 23 is composed of the composition formula _{Ti5O9 .}

Each of the first region 21, the second region, and the third region 23 does not necessarily have to be made of a single compound having a single composition ratio, but may be a mixed crystal of two compounds having different composition ratios. Examples of such a mixed crystal include a case _where the third region 23 is made of a mixed crystal of _{TiO7 and TiO9} , and a case where the second region 22 is a (1-20 ₎ twin crystal of _TiO7 .

The fact that the metal oxide layer 20 is composed of Magneli phase titanium oxide can be confirmed by X-ray diffraction (XRD). FIG. 2 shows an example of the results of X-ray diffraction measurement of the metal oxide layer 20 of the laminate 100 according to this embodiment, showing the results of thin-film GI-XRD measurement. As shown in FIG. 2, in the XRD profile of the metal oxide layer 20, only peaks due to Magneli phase titanium oxide are detected, and peaks due to other titanium oxides such as Ti ₂ O are not detected. Note that the X-ray penetration depth in XRD is preferably adjusted according to the thickness of the metal oxide layer 20.

The composition of the Magneli phase titanium oxide that makes up the metal oxide layer 20 can be measured using a transmission electron microscope (TEM). To identify the composition of the Magneli phase titanium oxide, first, a TEM image showing a cross section of the laminate 100 is observed and the crystal grains are identified. Next, an electron diffraction image of the crystal grains is obtained. Based on this electron diffraction image, the composition of the Magneli phase titanium oxide that makes up each crystal grain can be identified.

3 is an example of a TEM image of the laminate 100 according to this embodiment. In this TEM image, it can be confirmed that the regions Ra, Rb, and Rc of the metal oxide layer ₂₀ are mixed crystals of _TiO , _TiO , _{TiO9 ,} and _{TiO7 ,} respectively _, using an electron beam analysis image described below.

Specifically, for example, the composition of a Magneli-phase titanium oxide represented by the composition formula Ti _n O _2n-1 (3≦n≦9) can be identified in an electron beam diffraction image by the number of atoms (spots) contained between the (000) spot through which the transmitted wave passes and the (121) _{R-TiO 2 corresponding to the rutile-type TiO 2} plane. The titanium atomic columns of a Magneli-phase titanium oxide represented by the composition formula Ti _n O _2n-1 (3≦n≦9) are arranged in an n-cycle. Therefore, if the number of spots located on the line segment connecting the (000) spot through which the transmitted wave passes and the (121) _{R-TiO 2} spot corresponding to the rutile-type AO ₂ plane is (N−1), the composition of the Magneli-phase titanium oxide constituting that region can be identified as _TiN O _2N-1 . For example, if the number of spots is three, the Magneli phase titanium oxide constituting the region can be identified as Ti ₄ O ₇ , and if the number of spots is four, it can be identified as Ti ₅ O _9. These spots equally divide the line segment connecting the (000) spot and the (121) _{R-TiO 2} spot, for example.

Figure 4 shows an example of an electron diffraction image for identifying the composition of Magneli-phase titanium oxide represented by the composition formula Ti _n O _2n-1 (3≦n≦9). In the electron diffraction image shown in Figure 4, there are four spots located on the line connecting the (000) spot through which the transmitted wave passes and the (121) _R-TiO 2 spot corresponding to the rutile-type TiO ₂ plane, and the line connecting these two spots is divided into five equal parts. Therefore, the crystal grains shown in the electron diffraction image in Figure 4 can be identified as Ti ₅ O ₉ .

Similarly, the composition of a Magneli-phase titanium oxide represented by the composition formula Ti _n O _2n-1 (10≦n) can be identified by the number of atoms (spots) between (000) and (132) in an electron diffraction image. Titanium atomic columns in a Magneli-phase titanium oxide represented by the composition formula Ti _n O _2n-1 (10≦n) are arranged in an n-cycle. Therefore, if the number of spots located on the line segment connecting the (000) spot and the (132) spot, through which the transmitted wave passes, is (N−1), the composition of the Magneli-phase titanium oxide constituting that region can be identified as Ti _N O _2N-1 . These spots are, for example, equally spaced apart by the line segment connecting the (000) spot and the (132) spot.

Furthermore, once the electron diffraction image can be indexed by the above method, even if (N-1) spots are not confirmed between the (000) spot and the (121) _R-TiO2 spot, or between the (000) spot and the (132) spot, it is possible to identify the composition of the crystal grains based on the relationship between the spacing and angles of each spot. Furthermore, even with the composition of these crystal grains, if the composition of these crystal grains is expressed by the composition formula Ti _n O _2n-1 , by performing orientation alignment, it is confirmed that (N-1) spots are located between the (000) spot and the (121) _R-TiO2 spot, or between the (000) spot and the (132) spot, depending on the composition of the crystal grains.

The Magneli phase titanium oxide constituting the metal oxide layer 20 preferably has crystal grains with a crystallite size of 100 nm to 1.0 μm, and more preferably is composed of crystal grains with a crystallite size of 100 nm to 1.0 μm.

For example, voids V are formed between the metal layer 10 and the metal oxide layer 20 (Figure 3). The size of the voids V in the stacking direction is, for example, on the order of tens to hundreds of nanometers, and they are distributed in the in-plane direction, for example, along the interface between the metal layer 10 and the metal oxide layer 20. For example, the voids V are approximately cylindrical voids several tens to hundreds of nanometers in size that are connected to each other in the in-plane direction of the interface within a range of several hundreds of nanometers from the interface between the metal layer 10 and the metal oxide layer 20 in the stacking direction toward the surface layer.

From the viewpoint of improving conductivity, the metal oxide layer 20 according to this embodiment preferably has an average thickness of 200 nm or more, more preferably 500 nm or more, and even more preferably 1 μm or more. The metal oxide layer 20 may also have an average thickness of 2 μm or less. The average thickness of the metal oxide layer 20 can be calculated by measuring the distance in the stacking direction from the surface of the outermost layer of the laminate to the voids in a TEM image at 10 points in the in-plane direction of the laminate and averaging the measurements.

From the viewpoint of improving conductivity, the third region 23 preferably has an average thickness of 200 nm or more, more preferably 500 nm or more, and even more preferably 1 μm or more. The third region 23 may have an average thickness of 2 μm or less. The average thickness of the third region 23 can be calculated by measuring the distance in the stacking direction from the boundary between the second region 22 and the third region 23 of the laminate to the boundary between the third region 23 and the void in the TEM image at 10 points in the in-plane direction of the laminate and averaging the measurements.

The metal oxide layer according to this embodiment has higher conductivity in the in-plane direction and the stacking direction than _{TiO2 and} a mixture of _TiO2 and Magneli phase titanium oxide. In addition, the metal oxide layer according to this embodiment has high light absorption efficiency in the visible light and near-infrared light regions, and is therefore expected to be used as a photocatalyst in the visible light and near-infrared light regions.

Note that while Figure 1 illustrates a configuration in which the first region 21, second region 22, and third region 23 are layered, the laminate according to this embodiment is not limited to this example. For example, while the surface of each layer is flat in Figure 1, it may also be curved or uneven. Furthermore, the thickness of each layer may not be uniform, and may instead include thin and thick portions.

Note that while Figure 1 shows a Magneli phase with three regions, the number of regions can be any number greater than or equal to one, and the metal oxide layer may consist of a single region with a specific composition ratio, or it may contain four or more regions.

Figures 5 to 7 are cross-sectional views schematically illustrating modified examples of the laminate 100 of Figure 1. In the laminates 101, 102, and 103 of Figures 5 to 7, components similar to those in the laminate 100 are designated by the same reference numerals, and descriptions thereof will be omitted.

5, the metal oxide layer 20 of the laminate 101 is composed of a first region 21 and a second region 22. The first region 21 is composed of, for example, a composition formula _Ti3O5 , and the second region ₂₂ is composed of, for example, a composition formula _Ti4O7 . The second region ₂₂ in the laminate 101 constitutes the outermost layer.
6, the metal oxide layer 20 of the laminate 102 is composed of a second region 22 and a third region 23. The second region 22 is composed of, for example, a composition formula _Ti4O7 , and the third region ₂₃ is composed of, for example, a composition formula _Ti5O9 . The third region 23 in the laminate ₁₀₂ constitutes the outermost layer.
7, the metal oxide layer 20 of the laminate 103 is composed of a second region 22. That is, in the laminate 103, the entire metal oxide layer 20 is composed of a single Magneli-phase titanium oxide consisting of a single compound having a specific composition ratio. The second region 22 in the laminate 103 constitutes the outermost layer.

As described above, the metal oxide layer of the laminate according to this embodiment may be composed of two regions having different compositions in the stacking direction, or may be composed of a single region. In this case, it is preferable that the region farther from the metal layer 10 has a lower titanium element composition ratio in the Magneli-phase titanium oxide. It is also preferable that one of the two regions is composed of _Ti4O7 , which has relatively excellent electrical properties, particularly electrical conductivity, among Magneli-phase titanium oxides (see FIGS. 5 and ₆ ). The Magneli-phase titanium oxide contained in each region may be the same as the Magneli-phase titanium oxide contained in the corresponding region in FIG. 1.

The thickness of each of the first region 21, second region 22, and third region 23 can be selected as desired. For example, in Figure 5, the thickness of the second region 22 is greater than the thickness of the first region 21, but the thickness of the second region 22 may be smaller than the thickness of the first region 21. Also, in Figure 6, the thickness of the second region 22 is greater than the thickness of the third region 23, but the thickness of the second region 22 may be smaller than the thickness of the third region 23.

Of the Magneli phase titanium oxides contained in each region, the Magneli phase titanium oxide represented by _the composition formula _Ti4O7 has the highest conductivity. Therefore, from the viewpoint of increasing the conductivity of the metal oxide layer 20, it is preferable that the thickest region among the first region 21, the second region 22, and the third region ₂₃ be composed of the composition formula _Ti4O7 . For example, it is preferable that the second region 22 be thickest among the first region 21, the second region 22, and the third region 23, and that the second region ₂₂ be composed of the Magneli phase titanium oxide represented by the composition formula _Ti4O7 .

[Method of manufacturing laminate]
The method for producing a laminate according to this embodiment is a method for producing a metal oxide layer according to the above embodiment, and includes a first step of preheating a titanium substrate, and a second step of heat-treating the preheated metal substrate under a low oxygen partial pressure.

(1st step)
In the first step, the metal substrate can be preheated in the atmosphere using a known heating furnace such as an electric furnace, etc. In the first step, the surface of the metal substrate can be converted into titanium dioxide.

Commercially available substrates may be used. The thickness of the substrate is, for example, 3 μm or more, and preferably 5 μm or more. By ensuring that the substrate is thick enough to provide a sufficient amount of titanium ions and titanium elements for outward diffusion, it is easier to control the formation of Magneli phase titanium oxide. The substrate may contain titanium as its main component, but it is preferable to use a substrate consisting only of titanium elements. The substrate may also contain a third element other than titanium. The substrate may be flat, or may have a complex shape including curved or bent portions.

The heat treatment temperature in the first step is, for example, 600°C to 800°C, preferably 650°C to 750°C. By controlling the heat treatment temperature in the first step within this range and performing the second step described below, it is possible to manufacture a laminate including a metal oxide layer according to the above embodiment. Titanium element undergoes a rapid oxidation reaction at approximately 800°C. Therefore, if the heat treatment temperature in the first step is excessively high, the oxidation reaction of titanium element on the surface may proceed excessively, and even when the second step described below is performed, a metal oxide layer composed of Magneli phase titanium oxide may not be obtained, or the surface may peel off.

The heat treatment time for the first step is, for example, 30 minutes to 5 hours, and preferably 30 minutes to 3 hours. Note that the heat treatment time here refers to the time the temperature is maintained after reaching the heat treatment temperature. The heat treatment time may be shortened if the heat treatment temperature is high, and may be lengthened if the heat treatment temperature is low. By controlling the heat treatment time within the above range, it is possible to prevent cracks and fractures from occurring.

(Second process)
In the second step, the preheated metal substrate is heat-treated under a low oxygen partial pressure. In the second step, the heat treatment can be performed using, for example, a known electric furnace. The same heating furnace as that used in the first step or a heating furnace different from that used in the first step can be used.

"Under a low oxygen partial pressure" means that the heat treatment is performed in a gas having an oxygen partial pressure sufficiently lower than that of air. The gas may be, for example, air or oxygen gas. The gas may be composed of oxygen alone, or may contain oxygen and an inert gas such as _N2 or Ar. However, the gas is preferably composed of oxygen alone or oxygen and an inert gas.

The oxygen partial pressure when the second step is carried out is, for example, 1.0×10 ⁻⁷ atm to 1.0×10 ⁻¹⁵ atm, and preferably 1.0×10 ⁻⁷ atm to 1.0×10 ⁻¹⁰ atm.

Oxygen partial pressure can be controlled by known methods. For example, this can be done by supplying air and an inert gas into the heating furnace and using an oxygen partial pressure controller to adjust the amount of air and/or inert gas supplied based on the measured oxygen partial pressure in the heating furnace.

When performing the second step on a preheated titanium substrate, the heat treatment temperature is, for example, 750°C to 950°C, preferably 780°C to 930°C, and more preferably 870°C to 930°C.

The heat treatment temperature during the second step is the maximum temperature during the second step, and it is preferable not to maintain the temperature at this heat treatment temperature. For example, during the second step, heating is performed at a predetermined temperature increase rate until the maximum temperature is reached, and after the maximum temperature is reached, preferably immediately after the maximum temperature is reached, heat is removed at a predetermined temperature decrease rate. The temperature increase rate during the second step is, for example, 10°C/min, and the temperature decrease rate is, for example, 3°C/min to 5°C/min.

In the heat treatment of the second step, after the maximum temperature is reached, the maximum temperature may be maintained for a certain period of time. The maximum temperature may be maintained for, for example, 15 hours or less, and preferably 10 hours or less.

Figure 8 is a diagram showing an example of the change in composition inside the sample during the second step. After the first step, the surface of the metal substrate becomes rutile dioxide. By performing the second step, titanium elements and titanium ions from the titanium substrate diffuse outward, forming Magneli phase titanium oxide from the inside of the sample. When the second step is completed, the metal oxide layer is composed of Magneli phase titanium oxide.

By controlling the oxygen partial pressure range and heat treatment temperature in the second step within the above ranges, it is possible to control the rate at which oxygen penetrates from the outside and the rate at which the outward-diffusing titanium element and its ions react with the oxygen taken up inside, thereby forming a metal oxide layer composed of Magneli phase titanium oxide.

By performing the second step, it is possible to control the rate at which titanium ions and titanium elements from the titanium substrate diffuse outward, the rate at which oxygen penetrates into the metal oxide layer from the outside, and the rate at which oxygen penetrated from the outside progresses inward, thereby forming a metal oxide layer composed of Magneli phase titanium oxide on the metal layer. Furthermore, because oxygen is being taken in from the outside and the outward diffusion of titanium elements from the metal layer continues during the second step, the thickness of the metal oxide layer after the second step is thicker than the thickness of the metal oxide layer immediately after the first step.

Furthermore, by adjusting the heat treatment temperature and time in the first step, and the oxygen partial pressure, heat treatment temperature and time in the second step, it is possible to adjust the thickness of the metal oxide layer composed of Magneli phase titanium oxide and the composition of Magneli phase titanium oxide in the formed metal oxide layer.

The laminate manufacturing method according to this embodiment allows for the safe and easy production of a laminate containing Magneli phase titanium oxide. The metal oxide manufacturing method according to this embodiment allows for the production of a laminate containing the desired Magneli phase titanium oxide by controlling heat treatment conditions such as temperature and oxygen partial pressure. For example, the thickness and chemical composition ratio of the metal oxide layer can be precisely controlled.

The laminate manufacturing method according to this embodiment can be carried out even when substrates with complex shapes are used, making it possible to produce flexible oxide semiconductors of any shape. The metal oxide layer may be peeled off from the laminate manufactured by the laminate manufacturing method according to this embodiment and used independently. When using the metal oxide layer independently, it can be peeled off from the metal substrate of the laminate, for example, by using tweezers to peel it off, or by using a concentrated acid solution such as aqua regia to dissolve only the metal substrate. It is also possible to form a laminate composed solely of the Magneli phase, or a layered material composed solely of the Magneli phase (Magneli phase sheet), by using a thin metal substrate of 3 μm or less as the starting material.

The laminate shown in Figures 5 to 7 is manufactured using a method similar to the method for manufacturing the laminate according to the above embodiment. By performing the second step under an appropriate oxygen partial pressure, a metal oxide layer consisting of two regions, or a metal oxide layer consisting of one region, can be formed.

The following describes examples of the present invention. The present invention is not limited to the following examples.

[Example 1]
First, a titanium substrate having a thickness of 5 μm was prepared. The titanium substrate used in Example 1 was composed of 99.9 mass % titanium and the remaining 0.1 mass % elements other than titanium.

Next, in the first step, an intermediate sample was prepared by placing the titanium substrate in an electric furnace and preheating it in an air atmosphere. The temperature increase rate during preheating of the titanium substrate was approximately 10°C/min, the heat treatment temperature was 700°C, and the heat treatment time was 3 hours. After the heat treatment, the substrate was cooled in the furnace (at a rate of approximately 5°C/min).

Next, in the second step, the intermediate sample was placed in an electric furnace and heat-treated in a gas having an oxygen partial pressure sufficiently lower than that of air. In addition to oxygen gas, argon gas was used as a process gas. The heat treatment was performed at a heat treatment temperature of 900°C, a heat treatment time of 0 minutes, and an oxygen partial pressure of 1.0 x ^10-10 atm. The oxygen partial pressure was controlled by an oxygen partial pressure controller. The temperature rise rate during the heat treatment was approximately 10°C/min. In the second step, after the temperature was raised to the maximum temperature of 900°C, the maximum temperature was not maintained, and the furnace was immediately cooled (cooled at a rate of approximately 5°C/min) to prepare the sample.

[Example 2]
A sample was prepared in the same manner as in Example 1, except that the oxygen partial pressure in the second step was changed to 1.0×10 ⁻⁷ atm.

[Example 3]
The samples were prepared in the same manner as in Example 1, except that the maximum temperature in the second step was changed to 800° C. and the maximum temperature was maintained for 10 hours. That is, the heating rate and furnace cooling rate during the heat treatment were the same as in Example 1.

[Example 4]
A sample was prepared in the same manner as in Example 1, except that the heat treatment temperature in the second step was changed to 800° C. and the maximum temperature was maintained for 5 hours.

[Example 5]
A sample was prepared in the same manner as in Example 1, except that the heat treatment temperature in the second step was changed to 800°C.

[Comparative Example 1]
A sample was prepared in the same manner as in Example 1, except that the second step was not carried out.

[Comparative Example 2]
A sample was prepared in the same manner as in Example 1, except that the oxygen partial pressure in the second step was changed to 1.0×10 ⁻⁵ atm.

[Comparative Example 3]
A sample was prepared in the same manner as in Example 1, except that the second step was carried out in an atmospheric pressure atmosphere.

[Comparative Example 4]
A sample was prepared in the same manner as in Example 1, except that the heat treatment temperature in the second step was changed to 1000° C. and the oxygen partial pressure was changed to 1.0×10 ⁻²⁰ atm.

[Comparative Example 5]
A sample was prepared in the same manner as in Example 1, except that the maximum temperature in the second step was changed to 1000°C.

[Comparative Example 6]
A sample was prepared in the same manner as in Example 1, except that the maximum temperature in the second step was changed to 1000° C. and the oxygen partial pressure was changed to 1.0×10 ⁻⁵ atm.

[Comparative Example 7]
A sample was prepared in the same manner as in Example 1, except that the oxygen partial pressure in the second step was changed to 1.0×10 ⁻⁵ atm and the maximum temperature was maintained for 5 hours.

[Comparative Example 8]
A sample was prepared in the same manner as in Example 1, except that the maximum temperature in the second step was changed to 800° C., the maximum temperature was maintained for 10 hours, and the oxygen partial pressure was changed to 1.0×10 ⁻⁵ atm.

[Comparative Example 9]
A sample was prepared in the same manner as in Example 1, except that the maximum temperature in the second step was changed to 800° C. and the oxygen partial pressure was changed to 1.0×10 ⁻⁵ atm.

[Comparative Example 10]
A sample was prepared in the same manner as in Example 1, except that the maximum temperature in the second step was changed to 900° C. and the oxygen partial pressure was changed to 1.0×10 ⁻⁵ atm.

[Comparative Example 11]
A sample was prepared in the same manner as in Example 1, except that the oxygen partial pressure in the second step was changed to 1.0×10 ⁻⁵ atm and the maximum temperature was maintained for 3 hours.

The sample preparation conditions after the first step in Examples 1 to 5 and Comparative Examples 1 to 11 are summarized in Table 1. In Table 1, conditions in which a heat treatment was performed after a step corresponding to the first step is indicated by a circle, and conditions in which a heat treatment was not performed after a step corresponding to the first step is indicated by an x.

(Sample Evaluation)
The samples prepared in Examples 1 to 5 and Comparative Examples 1 to 11 were measured and analyzed by the following methods.

(XRD measurement, thin film GI-XRD measurement)
First, an arbitrary cross section of the sample was cut out, and XRD and thin-film GI-XRD measurements were performed. XRD measurements were performed using a fully automated multipurpose X-ray diffractometer (Rigaku Corporation, model name: Utima IV) under the following measurement conditions: CuKα source, 40 kV tube voltage, 40 mA tube current, and 20°-80° incidence angle. Thin-film GI-XRD measurements were performed using a fully automated multipurpose X-ray diffractometer (Rigaku Corporation, model name: SmartLab) under the following measurement conditions: CuKα source, 45 kV tube voltage, 200 mA tube current, and 20°-60° incidence angle. Figure 9 shows the results of thin-film GI-XRD measurements performed on the sample of Example 2. Figure 10 shows the results of thin-film GI-XRD measurements performed on the samples of Examples 3 to 5. Figure 11 shows the results of XRD measurements performed on the samples of Comparative Examples 1 to 3. Fig. 12 shows the results of XRD measurement performed on the samples of Comparative Examples 4 to 6. Fig. 13 shows the results of XRD measurement performed on the sample of Comparative Example 7. Fig. 14 shows the results of thin-film GI-XRD measurement performed on the samples of Comparative Examples 8 and 9. Fig. 15 shows the results of thin-film GI-XRD measurement performed on the sample of Comparative Example 10. Fig. 16 shows the results of XRD measurement performed on the sample of Comparative Example 11. Note that "I" in Figs. 12 to 16 indicates that the first step is the same heat treatment step as the first step in Example 1.

2, 9, and 10, it was confirmed that the laminates of Examples 1 to 5 had a metal oxide layer made of Magneli-phase titanium oxide in the surface layer portion. Of these, it was confirmed that the main phase in the surface portion of Examples 1, 2, and ₄ was _Ti4O7 , which has the highest electrical conductivity among Magneli-phase titanium oxides.

On the other hand, it was confirmed that the laminates of Comparative Examples 1 to 11 did not have a metal oxide layer made of Magneli phase titanium oxide in the surface layer portion.

When only the first step was performed without the second step as in Comparative Example 1, it was confirmed that a metal oxide layer composed of rutile-type _TiO2 was formed on the surface layer.

When the oxygen partial pressure in the heat treatment of the second step was high, as in Comparative Example 2, Magneli phase titanium oxide was formed inside the laminate and near the interface with the metal layer, but rutile-type _TiO2 was formed in the surface layer, and a metal oxide layer made of Magneli phase titanium oxide was not formed.

When the heat treatment following the first step was also performed under atmospheric pressure, as in Comparative Example 3, Magneli phase titanium oxide was not formed.

When the heat treatment in the second step was carried out at a high temperature as in Comparative Examples 4 to 6, excessive oxygen was taken in, resulting in the formation of Ti ₂ O ₃ , TiO, etc. in the surface layer.

When the oxygen partial pressure was high in the second step as in Comparative Examples 7, 10, and 11, excessive oxygen was taken in, resulting in the formation of rutile-type TiO ₂ .

When the oxygen partial pressure in the second step was high, as in Comparative Examples 8 and 9, the surface was excessively oxidized even when the heat treatment temperature was low, and Magneli phase titanium oxide could not be obtained.

When the oxygen partial pressure was high and the heat treatment temperature in the second step was high, as in Comparative Examples 10 and 11, Magneli phase titanium oxide was formed, but rutile _TiO2 was also contained, and a metal oxide layer consisting of Magneli phase titanium oxide was not formed in the surface layer. In Comparative Examples 10 and 11, the heat treatment temperature was higher than in Comparative Examples 8 and 9, which promoted the outward diffusion of titanium elements and their ions in the metal layer, resulting in the formation of Magneli phase oxide. On the other hand, the high oxygen partial pressure caused excessive oxygen to be taken in from the outside, resulting in the surface layer consisting of rutile _TiO2 , and a laminate with a surface layer consisting of Magneli phase oxide was not obtained.

(Cross-section observation)
Next, the cross section of the sample of Example 1 was observed using a TEM. TEM images showing cross sections of the sample of Example 1 at different positions are shown in FIGS. 3 and 17. Observation of the TEM images in FIGS. 3 and 17 confirmed that the sample of Example 1 had a void V in a direction intersecting the stacking direction, had two layers sandwiching the void V, and that the surface layer of the two layers had multiple crystal grains ranging from several hundred nanometers to approximately 1.0 μm. Thin-film GI-XRD measurement confirmed that the surface layer of the sample was a metal oxide layer composed of Magneli-phase titanium oxide. Next, the distance in the stacking direction from the surface of the outermost layer of the metal oxide layer to the void was measured at 10 locations in the in-plane direction of the stack in the TEM image, and the average value was calculated to determine the average thickness of the metal oxide layer. The average thickness of the metal oxide layer was 3.2 μm.

As in Example 1, the cross section of the sample of Example 2 was observed using a TEM. Figure 18 shows a TEM image of the cross section of the sample of Example 2. From the TEM image in Figure 18, it was confirmed that the sample of Example 2 has a void V in a direction intersecting the stacking direction, that it has two layers sandwiching the void V, and that the surface layer of the two layers is composed of crystal grains ranging from several hundred nm to approximately 1.0 μm. Thin-film GI-XRD measurement confirmed that the layer on the surface side of the sample is a metal oxide layer composed of Magneli phase titanium oxide. The average thickness of the metal oxide layer was 2.9 μm.

As in Example 1, the cross section of the sample of Comparative Example 1 was observed by TEM. A TEM image showing the cross section of the sample of Comparative Example 1 is shown in FIG. 19. Observation of the TEM image in FIG. 19 confirmed that the sample of Comparative Example 1 has a void V in a direction intersecting the stacking direction, has two layers sandwiching the void V, and that the surface layer of the two layers is composed of crystal grains of several hundred nanometers. XRD measurement confirmed that the layer provided on the metal layer of the sample is composed of _TiO2 .

As in Example 1, the cross section of the sample of Comparative Example 2 was observed using a TEM. A TEM image showing the cross section of the sample of Comparative Example 2 is shown in FIG. 20. Observation of the TEM image in FIG. 20 confirmed that the sample of Comparative Example 2 has a void V in a direction intersecting the stacking direction, has two layers sandwiching the void V, and that the surface layer of the two layers is composed of crystal grains ranging from several hundred nm to approximately 1.0 μm. XRD measurement confirmed that the layer provided on the metal layer of the sample is composed of rutile-type _TiO2 and Magneli-phase titanium oxide.

(Electron Diffraction)
First, electron beam diffraction was performed on the sample of Example 1. Electron beam diffraction was performed on the crystal grains in regions R _a , R _b and R _c in FIG. 3 and region R _d in FIG.

In the electron diffraction image of region _Rc , the number of spots included between the (000) plane through which the transmitted wave passes and the (121) _{R-TiO2 plane} corresponding to the rutile-type TiO2 plane was three and four, respectively, dividing the line segment connecting the two points into _four and _five equal parts. Therefore, it was confirmed that the Magneli-phase titanium oxide located in region _Rc was a mixed crystal of _Ti4O7 and _Ti5O9 .
In the electron diffraction image of region _Rb , the number of spots between the (000) and (121) _R-TiO2 corresponding to the rutile TiO2 _plane was three, and the line segment connecting these two points was divided into four equal parts. Furthermore, the electron diffraction image was mirror-symmetric with respect to the line connecting the (000) and (120) spots. Therefore, it was confirmed that the Magneli-phase titanium oxide located in region _Rb was a ( _1-20 ) twin crystal of _Ti4O7 .
In the electron diffraction image of region R _a , the number of spots included between the (000) plane through which the transmitted wave passes and the (121) _R-TiO2 plane corresponding to the rutile-type TiO2 _plane was two, and the line segment connecting these two points was divided into three equal parts. Therefore, it was confirmed that the Magneli-phase titanium oxide located in region R _{a was Ti3O5} .

In Example 1, an enlarged view and an electron diffraction image of the vicinity of region _Rd in FIG. 17 are shown in FIG. 21. The electron diffraction image of region _Rd confirmed the presence of superlattice reflection between the (000) plane through which the transmitted wave passes and the ( ₁₂₁ ) _R-TiO2 plane, which corresponds to the rutile _TiO2 plane. Furthermore, the number of spots between the (000) plane and the (121) _{R-TiO2 plane} , which corresponds to the rutile TiO2 plane, was three, and the line segment connecting these two spots was divided into four equal parts. Therefore, it was confirmed that the Magneli-phase titanium oxide located in region Rd _{was Ti4O7} . Therefore, it was confirmed that planar defects existed periodically in the metal oxide layer.

Next, electron beam diffraction was performed on the sample of Example 2. Electron beam diffraction was performed on the crystal grains in regions R _e and R _f in FIG. 18. In the electron beam diffraction image of region R _e , the number of spots contained between (000) and (121) _R-TiO2 was five, and the line segment connecting these two spots was divided into six equal parts. Therefore, region R _e was confirmed to be Ti ₆ O _11. In the electron beam diffraction image of region R _f , the number of spots contained between (000) and (121) _R-TiO2 was three, and the line segment connecting these two spots was divided into four equal parts. Therefore, region R _f was confirmed to be Ti ₄ O ₇ .

Next, electron beam diffraction was performed on the sample of Comparative Example 1. Fig. 22 shows an enlarged view of the vicinity of region _R1 in Fig. ₁₉ and an electron beam diffraction image. From the electron beam diffraction image of region R1, it was confirmed that the crystal grains in region _R1 were composed of rutile-type _TiO2 .

Next, electron beam diffraction was performed on the sample of Comparative Example 2. Electron beam diffraction was performed on regions _R2 , _R3 , and _R4 in FIG. 20. From the electron beam diffraction image of region _R2 , it was confirmed that the crystal grains in region _R2 were composed of (101) _rutile twin crystals of _TiO2 . Furthermore, in the electron diffraction images of regions R3 _{and R4} , the number of spots included between the (000) plane through which the transmitted wave passes and the (121) _R-TiO2 plane corresponding to the rutile TiO2 _plane was three, and the line segment connecting these two spots was divided into four equal parts. Therefore, it was confirmed that regions R3 _{and R4} were ( _1-20 ) twin crystals of _Ti4O7 .
Furthermore, the electron beam diffraction results for Comparative Example 2 confirmed that Magneli phase titanium oxide is formed by outward diffusion of titanium element or ions when the second step is performed under a low oxygen partial pressure on a laminate structure having titanium dioxide on the surface, as shown in Figure 8. In a laminate in which _TiO2 remains as in Comparative Example 2, a laminate having a metal oxide layer composed of Magneli phase titanium oxide provided on the surface layer of the metal layer cannot be obtained.

(HAADF-STEM observation)
The atomic arrangement of the cross section of region R _B of the sample of Example 1 was confirmed using a high-angle annular dark-field scanning transmission microscope (HAADF-STEM) (instrument name: Cs-correced Titan3 microscope, manufactured by Fisher Scientific International Inc.). Figure 23 shows an image observed with HAADF-STEM (left image) and an enlarged image of the area within the frame on the left image, which was further subjected to Fourier filtering using Digital Micrograph software manufactured by Gatan. As a result of observation with HAADF-STEM, region R _B was identified as Ti ₄ O _7. Fourier filtering was performed at an acceleration voltage of 300 kV, an electron beam diameter of approximately 0.08 nm, and a convergence angle of 17.8 mrad.

Figure 24 shows the contrast intensity along the X-Y line of the HAADF-STEM image in Figure 23, and a schematic diagram of the atomic arrangement calculated based on the contrast intensity. The bright spots in the HAADF-STEM image were confirmed to be atomic columns of Ti atoms based on their contrast intensity (Z = 22). The contrast intensity of spots with lower contrast intensity than the Ti atomic columns was (Z = 8). Here, Z indicates the atomic number. In this crystal grain, the Ti atomic columns are arranged in a periodic pattern of four, which is consistent with the atomic arrangement of Ti ₄ O ₇ in previous research (Y. Le Page, M. Marezio, Journal Solid State Chemistry, Volume 53, (1984) pp. 13-21.). The low-brightness spots located around the Ti atomic columns in Figure 23 are presumed to be O atomic columns.

(Light absorption efficiency)
Next, the reflectance was measured to determine the light absorption efficiency of the samples of Example 1, Comparative Example 1, and Comparative Example 2, and the rutile white _TiO2 powder. The reflectance was measured in the range of 300 nm to 1500 nm using an ultraviolet-visible-near-infrared spectrophotometer (instrument name: UV-3600, manufactured by Shimadzu Corporation).

Figure 25 shows the reflectance (%) versus wavelength for the samples of Example 1, Comparative Example 1, and Comparative Example 2, and the rutile-type white _TiO2 powder. The sample of Example 1 exhibited low reflectance not only in the ultraviolet light region, but also in the visible light region and near-infrared light region. Specifically, the sample of Example 1 exhibited a reflectance of 25% or less in the range of approximately 400 nm to approximately 1800 nm. Furthermore, the sample of Example 1 exhibited a lower reflectance than Comparative Example 1 in the range of approximately 300 nm to approximately 1800 nm, and a lower reflectance than Comparative Example 2 in the range of approximately 350 nm to approximately 1250 nm. In particular, the sample of Example 1 exhibited a lower reflectance than either Comparative Example 1 or Comparative Example 2 in the visible light region, which is approximately 360 nm to approximately 830 nm. Therefore, it was found that the sample of Example 1 exhibited excellent light absorption efficiency in the visible light region and near-infrared light region.

(Resistance measurement)
Next, using a DC stabilized power supply (model number AD-8735D, manufactured by A&D Corporation), the four-terminal method was used to measure the resistance values in the plane direction and thickness direction of the constituent phases of the samples of Example 1, Comparative Example 1, and Comparative Example 2. When measuring the resistance value in the thickness direction, the metal oxide layer was not peeled off from the metal layer of the sample, and measurements were taken on the front and back of the sample.

As shown in Table 2, the resistance values in the plane direction and the thickness direction of the constituent phase (rutile-type _TiO2 ) of Comparative Example 1 were equal to or greater than the measurement limit (3 x ¹⁰⁴ Ω). The resistance value in the plane direction of the constituent phase (rutile-type TiO2 _{, Ti2O} , and _Ti4O7 ) of Comparative Example 2 was 0.56 _Ω , and the resistance value in the thickness direction was 1.93 Ω. On the other hand, the resistance value in the plane direction of the constituent phase (Magneli-phase titanium oxide ( _Ti5O9 and _Ti4O7 )) of Example 1 was 0.17 Ω, _and the resistance value in the thickness direction was 0.04 Ω. Compared with the resistance values of Comparative Examples 1 and 2, _it was found that both the resistance values in the plane direction and the thickness direction were lower, and in particular the resistance value in the thickness direction was significantly lower.

[Comparative Example 12]
A 1 cm x 1 cm x 5 μm titanium substrate was heated in air at a constant heating rate, and thermogravimetry (TG) was performed. The thermogravimetry was performed using a thermogravimetric differential calorimeter (NETCH, model STA2500) under the condition of a heating rate of 10°C/min. Figure 26 shows the thermogravimetric curve (TG curve) of the sample of Comparative Example 12. From the TG curve of the sample of Comparative Example 12, it was confirmed that the oxidation of the titanium substrate progressed as the temperature increased in the temperature range of about 600°C or higher, and that the gradient of weight change with temperature increased significantly at temperatures of 700°C, 800°C, and 900°C.

FIG. 27 shows photographs of the appearance of the sample in states a (room temperature), b (700°C), and c (1000°C) in FIG. 26, as well as the XRD measurement results for state c. As shown in the photographs, the surface of the sample in state c has fallen off. Furthermore, the XRD measurement results confirm that the surface of the sample in state c is rutile-type _TiO2 . From FIG. 27, it can be seen that the surface will fall off if the temperature in the first step is not controlled within a predetermined range.

The laminate having a metal oxide layer composed of Magneli phase titanium oxide according to the above embodiment can effectively utilize the visible light range and can accommodate complex shapes and arbitrary dimensions. Therefore, the laminate according to the above embodiment is expected to be used as a photocatalytic electrode or an optical sensor. The metal oxide layer of the laminate according to the above embodiment has high conductivity and may have high oxidation resistance, so it is expected to be used as a fuel cell.

10: Metal layer, 20: Metal oxide layer, 100, 101, 102, 103: Laminate, V: Void

Claims (7)

a metal layer; and a metal oxide layer formed on a surface portion of the metal layer and composed of Magneli phase titanium oxide,
The Magneli phase titanium oxide is represented by a composition formula Ti _n O _2n-1 (n≧3),
the metal oxide layer has two or three regions having different compositions in the stacking direction;
A laminate , wherein the value of n in the composition formula of one of the two or three regions is larger as the region is more distant from the metal layer . The laminate described in claim 1, wherein the average thickness of the metal oxide layer composed of Magneli phase titanium oxide is 2 μm or more. the metal oxide layer has two regions having different compositions in a stacking direction,
3. The laminate according to claim 1, wherein the region away from the metal layer has a larger value of n in the composition formula than the region on the metal layer side . 10. The stack of claim 1 _, wherein any of the two or three regions is composed of _Ti4O7 . The laminate described in any one of claims 1 to 4, having voids between the metal oxide layer composed of Magneli phase titanium oxide and the metal layer. A method for producing the laminate according to any one of claims 1 to 5,
A first step of preheating a titanium substrate in the atmosphere at a temperature lower than that of a second step to be performed later ;
a second step of heat-treating the preheated titanium substrate under a low oxygen partial pressure of 1.0×10 ⁻⁷ atm or less at a temperature higher than that of the first step . In the first step, the titanium substrate is preheated at 650°C to 750°C,
7. The method for producing a laminate according to claim 6, wherein in the second step, the preheated titanium substrate is heat-treated at a temperature of 750° C. to 950° C. and an oxygen partial pressure of 1.0×10 ⁻⁷ atm or less.