JP7767733B2 - Material manufacturing method and photocatalyst - Google Patents

Description

The invention was published in the Japanese Journal of Applied Physics, Volume 60, Number 1, 010903 (2021) on December 17, 2020, subject to Article 30, Paragraph 2 of the Patent Act. The invention was published in the final examination of the doctoral program in the Department of Materials Systems Science, Graduate School of Nanobioscience, Yokohama City University on March 24, 2021.

This disclosure relates to a material manufacturing method and a photocatalyst.

In recent years, nanostructures such as metal nanorods and metal oxide nanorods have been found to possess properties that dramatically improve upon those of conventional materials, or even possess properties not found in conventional materials. For this reason, nanostructures are expected to be used as next-generation functional materials, including electromagnetic wave absorbing materials, battery electrode materials, catalyst materials, semiconductor materials, electron emitter materials, optical materials, and strength-reinforcing materials.

As a method for producing such nanostructures, for example, Patent Document 1 describes a method for producing a catalyst in which a support and an organometallic complex are dispersed in a liquid, and then the organometallic complex is thermally decomposed to precipitate metal particles on the surface of the support, thereby supporting metal particles with a particle size of 1 nm or less on the support surface.

JP 2008-149280 A

As mentioned above, nanostructures are expected to be next-generation functional materials, and there is a demand for the development of new materials that include nanostructures.

The purpose of this disclosure is to provide a material production method that can produce new materials, and a photocatalyst.

In order to solve the above problem, a material manufacturing method according to one embodiment of the present disclosure is a plasma CVD apparatus having a chamber, a sputtering target provided in the chamber and composed of a predetermined element, and a plasma discharge unit that discharges a plasma flow into the chamber , wherein the plasma constituting the plasma flow discharged by the plasma discharge unit collides with the sputtering target, the method comprising the steps of: supplying a reactive gas containing at least a carbon compound gas into the chamber; discharging a plasma flow into the chamber; and doping the carbon nanowalls with the predetermined element while forming carbon nanowalls on a substrate provided in the chamber; and, after performing the doping step, stopping the supply of the reactive gas into the chamber while maintaining the discharge of the plasma flow , wherein the predetermined element is an element having a lower sputtering rate than iron .

Furthermore, after performing the stopping step, the method may further include a step of reducing carbon from the carbon nanowalls doped with the specified element.

It will become possible to manufacture new materials.

FIG. 1 is a flowchart illustrating the process flow of a material manufacturing method according to an embodiment. FIG. 2 is a diagram illustrating a specific configuration of a plasma CVD apparatus. FIG. 3 is a diagram illustrating carbon nanowalls. FIG. 4 is a diagram illustrating the material produced in the forming process. FIG. 5 is a diagram illustrating the material produced by the carbon reduction process. FIG. 6 is a diagram illustrating the material produced in the second modified example. FIG. 7 is a diagram showing an SEM image of the first example. FIG. 8 shows the results of analysis by XPS. FIG. 9 shows the results of photoelectrochemical measurement using Example A. FIG. 10 is a diagram showing an SEM image of the second example.

One embodiment of the present disclosure will be described in detail below with reference to the accompanying drawings. Dimensions, materials, and other specific values shown in this embodiment are merely examples to facilitate understanding and, unless otherwise specified, do not limit the present disclosure. Note that in this specification and drawings, elements that have substantially the same function and configuration are designated by the same reference numerals to avoid redundant explanation, and elements not directly related to this disclosure are not shown.

[Material manufacturing method]
1 is a flowchart illustrating the process flow of a material production method according to this embodiment. As shown in FIG. 1, the material production method according to this embodiment includes a formation step S110, a sputtering step S120, and a carbon reduction step S130. Details of the formation step S110, the sputtering step S120, and the carbon reduction step S130 will be described below.

[Formation step S110]
The forming step S110 is a step of forming carbon nanowalls on a substrate using a plasma CVD (Chemical Vapor Deposition) apparatus 100 and doping the carbon nanowalls with a predetermined element (hereinafter referred to as a "dope element").

Figure 2 is a diagram illustrating the specific configuration of the plasma CVD apparatus 100. As shown in Figure 2, the plasma CVD apparatus 100 includes a chamber 110, a substrate holder 120, a target holder 130, and a plasma gun 140.

The chamber 110 is formed with a gas supply port 112. A reactive gas and an inert gas are supplied into the chamber 110 through the gas supply port 112. The reactive gas contains at least a carbon compound gas (e.g., methane ( _CH4 )). The reactive gas may contain hydrogen ( _H2 ) in addition to the carbon compound gas. The inert gas is, for example, argon (Ar).

The flow rate of the carbon compound gas supplied into the chamber 110 is, for example, 5 sccm (standard cc/min) or more and 20 sccm or less (e.g., 10 sccm). The flow rate of the hydrogen supplied into the chamber 110 is, for example, 0 sccm or more and 40 sccm or less (e.g., 10 sccm). The flow rate of the inert gas supplied into the chamber 110 is, for example, 60 sccm or more and 90 sccm or less (e.g., 80 sccm).

A vacuum pump (not shown) is also connected to chamber 110. The vacuum pump maintains the pressure inside chamber 110 at a predetermined pressure (e.g., 3.0E-3 Torr or higher and 8.0E-3 Torr or lower).

The chamber 110 is also equipped with a heater (not shown). The heater maintains the temperature inside the chamber 110 at 450°C or higher and 750°C or lower, preferably 500°C or higher and 650°C or lower. This allows the plasma CVD apparatus 100 to efficiently form carbon nanowalls.

The substrate holder 120 is provided within the chamber 110. The substrate holder 120 holds the substrate S. The substrate S contains elements that easily form carbides, such as silicon (Si), titanium (Ti), tantalum (Ta), zirconium (Zr), and niobium (Nb).

The target holder 130 is provided within the chamber 110. The target holder 130 holds the sputtering target T. The target holder 130 also continuously (always) applies a voltage (DC) at a predetermined frequency to the sputtering target T.

In this embodiment, the sputtering target T is composed of a metal element with a lower sputtering rate than iron. Under argon plasma with an irradiation energy of 600 eV (hereinafter referred to as "sputtering conditions"), the sputtering rate of iron is 1.26. The sputtering target T may be composed of, for example, aluminum (Al, sputtering rate 1.24 under the above sputtering conditions), iridium (Ir, sputtering rate 1.17 under the above sputtering conditions), osmium (Os, sputtering rate 0.95 under the above sputtering conditions), molybdenum (Mo, sputtering rate 0.93 under the above sputtering conditions), rhenium (Re, sputtering rate 0.91 under the above sputtering conditions), hafnium (Hf, sputtering rate 0.83 under the above sputtering conditions), or beryllium (Be, sputtering rate 0.94 under the above sputtering conditions). It is composed of one metal element (doping element) selected from the group consisting of 0.80), zirconium (Zr, sputtering rate 0.75 under the above sputtering conditions), vanadium (V, sputtering rate 0.70 under the above sputtering conditions), niobium (Nb, sputtering rate 0.65 under the above sputtering conditions), tungsten (W, sputtering rate 0.62 under the above sputtering conditions), tantalum (Ta, sputtering rate 0.62 under the above sputtering conditions), and titanium (Ti, sputtering rate 0.58 under the above sputtering conditions), or an alloy of multiple metal elements.

The plasma gun 140 (plasma emission unit) emits a plasma flow PF into the chamber 110. The plasma gun 140 can utilize various existing technologies (e.g., JP 2008-056546 A), so a detailed description will be omitted here. When the plasma gun 140 emits the plasma flow PF, the plasma that constitutes the plasma flow PF decomposes the reactive gas and collides with the sputtering target T. As a result, carbon nanowalls are formed on the substrate S, and the atoms that constitute the sputtering target T are ejected (sputtered) from the sputtering target T and doped into the carbon nanowalls.

Below, we will explain three processing patterns that can be performed as part of the formation process S110.

[First processing pattern of forming step S110]
In the first processing pattern, first, the substrate S is held by the substrate holder 120, and a reactive gas is supplied into the chamber 110 through the gas supply port 112. Then, the plasma gun 140 is driven simultaneously with the application of a voltage to the sputtering target T by the target holder 130. In other words, the doping of the doping element into the carbon nanowalls is carried out simultaneously with the start of the formation of the carbon nanowalls on the substrate S. As a result, the carbon nanowalls doped with the doping element are formed on the substrate S.

Figure 3 is a diagram illustrating a carbon nanowall. Figure 3A is a first diagram illustrating a carbon nanowall. Figure 3B is a second diagram illustrating a carbon nanowall. Figure 3C is a third diagram illustrating a carbon nanowall. Figure 3D is a fourth diagram illustrating a carbon nanowall. Note that in Figures 3B and 3D of this embodiment, the perpendicularly intersecting X-axis, Y-axis, and Z-axis are defined as shown.

Carbon nanowalls are graphene sheets (carbon atoms (C) are indicated by white circles in Figure 3A) as shown in Figure 3A, which stand upright (grown vertically) on the surface of a substrate S as shown in Figure 3B. When carbon nanowalls are formed on a substrate S, a graphite layer or amorphous carbon layer is first formed on the surface of the substrate S, as shown in Figures 3C and 3D. The graphite layer or amorphous carbon layer is formed on the surface of the substrate S along the in-plane direction of the surface of the substrate S. For example, the graphite layer or amorphous carbon layer is formed on the surface of the substrate S parallel to the in-plane direction of the surface of the substrate S.

The carbon nanowalls are formed so that they extend (stretch) in a direction perpendicular to the surface of the substrate S (the Y-axis direction in Figures 3B and 3D) via a graphite layer or amorphous carbon layer. The thickness of the carbon nanowalls in the direction parallel to the substrate S (the X-axis direction in Figure 3B) is 1 nm or more and 100 nm or less. The distance between the carbon nanowalls in the X-axis direction in Figure 3B is 1 nm or more and 10 μm or less.

Carbon nanowalls have a self-organizing function. Therefore, simply by generating plasma in a reactive gas atmosphere in the plasma CVD apparatus 100, the carbon nanowalls grow so that they extend in a direction perpendicular to the surface of the substrate S (the Y-axis direction in Figures 3B and 3D), forming nanometer-sized voids (gaps) between them. Here, nanometer size refers to a size between 1 nm and 10 μm.

In the first processing pattern, the carbon nanowalls are doped with a doping element at the same time as the formation of carbon nanowalls on the substrate S begins.

Figure 4 is a diagram illustrating the material 200 produced in the formation process S110. Figure 4A is a diagram illustrating the material 200 produced in the first processing pattern formation process S110. Figure 4B is a diagram illustrating the material 200 produced in the second processing pattern formation process S110. Figure 4C is a diagram illustrating the material 200 produced in the third processing pattern formation process S110. In addition, in Figures 4A, 4B, and 4C of this embodiment, the perpendicular X-axis, Y-axis, and Z-axis are defined as shown.

As shown in FIG. 4A, the material 200 (hereinafter referred to as material 200A) produced by the first processing pattern includes a substrate S, an underlayer 210, and a CNW layer 220. The underlayer 210 is a layer formed along the in-plane direction of the surface of the substrate S. The underlayer 210 is composed of graphite doped with a doping element (or amorphous carbon doped with a doping element). The CNW layer 220 is composed of a plurality of doping element-containing carbon nanowalls 222. The doping element-containing carbon nanowalls 222 are carbon nanowalls doped with a doping element. The doping element-containing carbon nanowalls 222 extend from the underlayer 210 in a direction perpendicular to the surface (in-plane direction) of the substrate S (the Y-axis direction in FIG. 4A). Furthermore, in the CNW layer 220, the doped-element-containing carbon nanowalls 222 extend from the underlayer 210, maintaining nanometer-sized gaps 222a between adjacent doped-element-containing carbon nanowalls 222.

By performing the formation process S110 with the first processing pattern, it is possible to dope not only the CNW layer 220 but also the underlayer 210 with a doping element. This causes the elements that make up the substrate S to bond with the doping element, strengthening the bond between the underlayer 210 (CNW layer 220) and the substrate S. For example, if a silicon substrate is used as the substrate S and titanium is used as the doping element, the result will be Ti-Si, which will strengthen the bond between the CNW layer 220 (underlayer 210) and the substrate S.

In this way, a material 200A is produced, comprising an underlayer 210 (thin film) and a plurality of doping-element-containing carbon nanowalls 222 standing upright from the underlayer 210, each having a thickness of 1 nm or more and 10 nm or less and spaced apart from each other by a distance of 50 nm or more and 1 μm or less.

The execution time of the formation process S110, i.e., the time for supplying the reactive gas, the time for driving the plasma gun 140, and the time for applying voltage to the sputtering target T, is from 3 minutes to 24 hours, and preferably from 10 minutes to 360 minutes.

If the execution time of the formation process S110 is less than 3 minutes, the doping element-containing carbon nanowalls 222 will not be formed. Furthermore, if the execution time of the formation process S110 is more than 24 hours, it will take an unnecessary amount of time.

Therefore, by setting the execution time of the formation step S110 in the plasma CVD apparatus 100 to between 3 minutes and 24 hours, it becomes possible to efficiently form doping element-containing carbon nanowalls 222.

[Second processing pattern of forming step S110]
First, the substrate S is held by the substrate holder 120. Then, in the second processing pattern, a reactive gas is supplied into the chamber 110 through the gas supply port 112, and the plasma gun 140 is driven without applying a voltage to the sputtering target T. After a first predetermined time has elapsed since the start of driving the plasma gun 140, the application of a voltage to the sputtering target T is started to perform sputtering. That is, after the start of formation of carbon nanowalls on the substrate S, the carbon nanowalls are doped with a doping element after the first predetermined time has elapsed. Here, the first predetermined time is the time required for a graphite layer or an amorphous carbon layer to be formed on the substrate S, i.e., the time required for the carbon nanowalls to be formed. Then, carbon nanowalls doped with the doping element are formed on the substrate S, specifically, on the graphite layer (or amorphous carbon layer).

Specifically, as shown in FIG. 4B, material 200 (hereinafter referred to as material 200B) manufactured using the second processing pattern includes a substrate S, an underlayer 230, and a CNW layer 220. The underlayer 230 is a layer formed along the in-plane direction of the surface of the substrate S. Unlike material 200A, the underlayer 230 is composed of graphite (or amorphous carbon) that does not contain any doping elements.

By performing the formation process S110 with the second processing pattern, it is possible to produce material 200B in which only the carbon nanowalls are doped with the doping element.

[Third Processing Pattern of Forming Step S110]
First, the substrate S is held by the substrate holder 120. Then, a reactive gas is supplied into the chamber 110 through the gas supply port 112, and the plasma gun 140 is driven without applying a voltage to the sputtering target T. In the third processing pattern, after a second predetermined time has elapsed since the plasma gun 140 was started to be driven, a voltage is applied to the sputtering target T to perform sputtering. That is, after the formation of carbon nanowalls on the substrate S is started, the doping of the doping element into the carbon nanowalls is performed after the second predetermined time has elapsed. Here, the second predetermined time is a predetermined time after the formation of carbon nanowalls on the substrate S is started. Then, carbon nanowalls doped with the doping element are formed on the carbon nanowalls.

Specifically, as shown in FIG. 4C, material 200 (hereinafter referred to as material 200C) manufactured using the third processing pattern includes a substrate S, an underlayer 230, and a CNW layer 240. Unlike the CNW layer 220 of materials 200A and 200B, the CNW layer 240 is partially (tip) composed of carbon nanowalls 242 doped with a doping element. Specifically, as shown in FIG. 4C, the carbon nanowalls 242 are not doped with a doping element from the base end (the end on the underlayer 230 side) to a predetermined position, but are doped with a doping element from the predetermined position to the tip. The CNW layer 240 includes a first layer 244 and a second layer 246. The first layer 244 is formed on the underlayer 230. The first layer 244 is composed of carbon nanowalls 244a that do not contain a doping element. The second layer 246 is formed on the first layer 244. The second layer 246 is continuous with the first layer 244. The second layer 246 is composed of doped element-containing carbon nanowalls 246a. The doped element-containing carbon nanowalls 246a are carbon nanowalls doped with a doped element.

Note that, similar to material 200B, the carbon nanowalls 242 extend from the underlayer 230 in a direction perpendicular to the surface (in-plane direction) of the substrate S (Y-axis direction in Figure 4C). Furthermore, in the CNW layer 240, the carbon nanowalls 242 extend from the underlayer 230 while maintaining nanometer-sized gaps 242a.

By performing the formation process S110 with the third processing pattern, it is possible to produce material 200C in which only a portion of the carbon nanowalls are doped with the doping element.

[Sputtering step S120]
The sputtering step S120 is a step of stopping the supply of the reactive gas into the chamber 110 after the formation step S110 is performed, while maintaining the emission of the plasma flow (driving the plasma gun 140) and the application of a voltage to the sputtering target T. Note that in the sputtering step S120, a pulse voltage is applied to the sputtering target T.

By performing the sputtering step S120, the portion of the material 200 containing the doping element can be further doped with the doping element. For example, in the material 200A produced by the first processing pattern, the base layer 210 and the doping-element-containing carbon nanowalls 222 are further doped with the doping element. In the material 200B produced by the second processing pattern, the doping-element-containing carbon nanowalls 222 are further doped with the doping element, while the base layer 230 is hardly doped with the doping element. In the material 200C produced by the third processing pattern, the second layer 246 (the doping-element-containing carbon nanowalls 246a) is further doped with the doping element, while the base layer 230 and the first layer 244 (the carbon nanowalls 244a) are hardly doped with the doping element.

In other words, carbon nanowalls, graphite, or amorphous carbon that has been doped with a doping element in the formation process S110 is further doped with the doping element by performing the sputtering process S120. On the other hand, carbon nanowalls, graphite, or amorphous carbon that has not been doped with a doping element in the formation process S110 will not be doped with the doping element even if the sputtering process S120 is performed.

Note that the execution time of the sputtering step S120, i.e., the operating time of the plasma gun 140 and the application time of voltage to the sputtering target T, is from 4 minutes to 200 minutes, preferably from 5 minutes to 60 minutes, when the doping element is titanium.

If the sputtering step S120 is performed for less than 4 minutes, further doping of the carbon nanowalls, graphite, or amorphous carbon doped with the doping element will be insufficient. In this case, performing the carbon reduction step S130 described below will result in the formation of metal oxide nanowalls.

On the other hand, if the sputtering step S120 is performed for more than 200 minutes, the gaps 222a between the doping-element-containing carbon nanowalls 222 or the gaps 242a between the doping-element-containing carbon nanowalls 246a will be filled with the doping element. In this case, when the carbon reduction step S130 is performed, metal oxide nanowalls that are spaced apart from each other will no longer be formed.

Therefore, by setting the execution time of the sputtering process S120 in the plasma CVD apparatus 100 to between 4 minutes and 200 minutes, it becomes possible to produce metal oxide nanowalls that are spaced apart when the carbon reduction process S130 is performed.

In addition, when the doping element is an element other than titanium, the execution time of the sputtering process S120 is equal to or longer than the time Tmin (minutes) calculated from the following formula (1) and equal to or shorter than the time Tmax (minutes) calculated from the following formula (2).
Tmin={4×(SRFe-SRX)}/(SRFe-SRTi)...Formula (1)
Tmax={200×(SRFe-SRX)}/(SRFe-SRTi)...Formula (2)
In the above formulas (1) and (2), SRFe is the sputtering rate of iron (i.e., the sputtering rate of 1.26 under the above sputtering conditions). SRTi is the sputtering rate of titanium (i.e., the sputtering rate of 0.58 under the above sputtering conditions). SRX is the sputtering rate of a doping element other than titanium (the sputtering rate under the above sputtering conditions). For example, if the doping element is aluminum, SRX is 1.24.

Furthermore, the temperature inside the chamber 110 during the sputtering step S120 is maintained at 25°C or higher and 750°C or lower, preferably 500°C or higher and 650°C or lower. This allows the plasma CVD apparatus 100 to efficiently further dope the carbon nanowalls, graphite, or amorphous carbon that has already been doped with the doping element.

Note that the pressure inside the chamber 110 during the sputtering process S120 is substantially the same as that during the formation process S110. Furthermore, during the sputtering process S120, no reactive gas is supplied to the interior of the chamber 110, but an inert gas is. The flow rate of argon supplied into the chamber 110 is substantially the same as that during the formation process S110.

[Carbon reduction step S130]
The carbon reduction step S130 is a step of reducing carbon from carbon nanowalls, graphite, or amorphous carbon doped with a doping element. In this embodiment, the carbon reduction step S130 is a step of heating the material 200 in a predetermined atmosphere. Here, the predetermined atmosphere is an atmosphere containing at least oxygen (oxidizing atmosphere), such as air.

In addition, the atmospheric pressure during the carbon reduction step S130 is equal to or higher than atmospheric pressure. This improves the efficiency of carbon desorption (reduction efficiency) from the material 200.

Furthermore, the ambient temperature in the carbon reduction step S130 (hereinafter referred to as "ambient temperature") is 400°C or higher and 1400°C or lower, and preferably 400°C or higher and 750°C or lower. If the ambient temperature is below 400°C, carbon cannot be removed from the material 200. On the other hand, if the ambient temperature is above 1400°C, the structures (CNW layer 220, CNW layer 240) will no longer be able to maintain their nanostructure.

The execution time of the carbon reduction step S130 is from 10 minutes to 24 hours, preferably from 6 minutes to 1200°C. If the execution time of the carbon reduction step S130 is less than 10 minutes, carbon cannot be sufficiently removed, and the doping element cannot be sufficiently oxidized. On the other hand, if the execution time of the carbon reduction step S130 is more than 24 hours, there is almost no increase in the carbon removal rate or the doping element oxidation rate compared to when the execution time is 24 hours. For example, when the doping element is titanium, there is almost no difference in IPCE (photoelectric conversion efficiency) between when the execution time of the carbon reduction step S130 is from 10 minutes to 24 hours and when the execution time is more than 24 hours.

By performing the carbon reduction process S130, carbon can be removed from material 200 (the carbon in material 200 can be reduced).

Figure 5 is a diagram illustrating the material 300 produced in the carbon reduction process S130. In Figure 5 of this embodiment, the perpendicularly intersecting X-axis, Y-axis, and Z-axis are defined as shown.

When the carbon reduction process S130 is performed on the material 200A produced in the first processing pattern formation process S110, a material 300 (hereinafter referred to as material 300A) including a substrate S, an underlayer 310, and a nanostructure layer 320 is produced, as shown in Figure 5.

The underlayer 310 is a layer (thin film) in which carbon has been removed from the underlayer 210. Therefore, the underlayer 310 is composed of an oxide (metal oxide) of the doped element, and is a layer formed along the in-plane direction of the surface of the substrate S.

The nanostructure layer 320 is a layer in which carbon has been removed from the CNW layer 220. Therefore, the nanostructure layer 320 is a layer composed of structures 322 in which oxides (metal oxides) of doping elements are arranged in the shape of nanowalls. In other words, the structures 322 are metal oxides in the shape of carbon nanowalls. Specifically, the nanostructure layer 320 includes multiple structures 322 extending (standing) in a direction perpendicular to the surface of the substrate S (the Y-axis direction in FIG. 5). The thickness of the structures 322 in a direction parallel to the substrate S (the X-axis direction in FIG. 5) is 1 nm to 1 μm (e.g., 1 nm to 10 nm), and the distance between the structures 322 (gaps 322a) in the X-axis direction in FIG. 5 is 50 nm to 1 μm.

Furthermore, as described above, material 200A is formed by bonding the elements that make up substrate S with the doped elements of underlayer 210. Therefore, even when carbon reduction step S130 is performed, the bond between underlayer 310 and substrate S is maintained. In other words, material 300A is formed by bonding underlayer 310 (nanostructure layer 320) and substrate S.

In this way, material 300A is produced, comprising an underlayer 310 (thin film) and a plurality of structures 322 extending upright from underlayer 310, each having a thickness of 1 nm or more and 10 nm or less and spaced apart from each other by a distance of 50 nm or more and 1 μm or less.

Furthermore, in the carbon reduction process S130, the carbon reduction rate can be adjusted by controlling the atmospheric temperature, atmospheric pressure, and heating time. In other words, by controlling the atmospheric temperature, atmospheric pressure, and heating time in the carbon reduction process S130, it is possible to remove all of the carbon from the material 200, or it is also possible to remove a portion (a predetermined amount) of the carbon from the material 200.

The material 300A produced in this manner can be used as a photocatalyst. That is, by performing the formation step S110, the sputtering step S120, and the carbon reduction step S130, a photocatalyst (material 300A) having nanowall-shaped structures 322 containing at least a metal oxide is produced.

As described above, the material production method according to this embodiment can produce novel materials, such as nanowall-shaped metal oxides (metal oxide nanowalls).

The material production method according to this embodiment also performs a sputtering step S120 in addition to the formation step S110. Metal elements have catalytic properties. Therefore, when doping with a metal element having a lower sputtering rate than iron, performing the formation step S110 and carbon reduction step S130 without performing the sputtering step S120 will not produce a nanowall-shaped metal oxide. Therefore, when doping with a metal element having a lower sputtering rate than iron, the sputtering step S120 is performed in addition to the formation step S110. This makes it possible to produce a nanowall-shaped metal oxide even when using a metal element having a lower sputtering rate than iron.

[First Modification]
In the above embodiment, the carbon reduction step S130 is described as heating the material 200 in an oxidizing atmosphere. However, the carbon reduction step S130 may be any step as long as it can reduce carbon from carbon nanowalls, graphite, or amorphous carbon doped with a doping element.

For example, the carbon reduction process S130 may be a process of heating the material 200 in a reducing atmosphere. In this case, a material can be produced that includes a base layer composed of a doping element (metal) and formed along the in-plane direction of the surface of the substrate S, and a nanostructure layer composed of a structure in which the doping element (metal) is configured in a nanowall shape. Also, a structure in which the doping element (metal) is configured in a nanowall shape (nanowall-shaped metal) can be produced.

The nanowall-shaped metal structures produced in this way can be used in sensors, imaging devices, optical devices, and other devices that utilize the surface plasmon effect.

[Second Modification]
In the above embodiment and the first modified example, the carbon reduction step S130 is described, in which the material 200 is heated in a predetermined atmosphere. However, the carbon reduction step S130 may be any step as long as it can reduce carbon from carbon nanowalls, graphite, or amorphous carbon doped with a doping element.

For example, the carbon reduction step S130 may be a step of exposing the material 200 to a predetermined plasma atmosphere. Here, the predetermined plasma atmosphere is, for example, hydrogen (H ₂ ) plasma, nitrogen (N ₂ ) plasma, or oxygen plasma.

The pressure of the plasma atmosphere is between 10 Pa and 100 Pa (similar to the degree of vacuum during ECR plasma processing and RF plasma processing).

Furthermore, the temperature of the plasma atmosphere (hereinafter referred to as the "plasma atmosphere temperature") is from room temperature (e.g., 25°C) to 800°C, preferably from 500°C to 700°C, and more preferably 600°C. If the plasma atmosphere temperature is below room temperature, carbon cannot be removed from material 200. If the plasma atmosphere temperature exceeds 800°C, the structure (CNW layer 220, CNW layer 240) will no longer be able to maintain its nanostructure.

By exposing material 200 to a plasma atmosphere, carbon can be removed from material 200. When exposing material 200 to a plasma atmosphere, it is advisable to perform oxygen plasma treatment from the opposite side of substrate S, i.e., from the tip side of CNW layer 220 and the tip side of second layer 246. This allows oxidation treatment of carbon from the tip side of CNW layer 220 and the tip side of second layer 246.

Figure 6 is a diagram illustrating a material 300 manufactured according to the second modified example. Figure 6A is a first diagram illustrating a material 300 manufactured according to the second modified example. Figure 6B is a second diagram illustrating a material 300 manufactured according to the second modified example. In Figures 6A and 6B of this embodiment, the perpendicularly intersecting X-axis, Y-axis, and Z-axis are defined as shown.

When the material 200B produced in the second processing pattern formation step S110 is exposed to a plasma atmosphere, a material 300 (hereinafter referred to as material 300B) including a substrate S, an underlayer 230, and a nanostructure layer 320 is produced, as shown in FIG. 6A.

Furthermore, when the material 200C produced in the third processing pattern formation step S110 is exposed to a plasma atmosphere, a material 300 (hereinafter referred to as material 300C) including a substrate S, an underlayer 230, and a nanostructure layer 340 is produced, as shown in Figure 6B.

The nanostructure layer 340 is a layer in which carbon has been removed from the CNW layer 240. The nanostructure layer 340 includes multiple structures 342 that extend (stand) perpendicular to the surface of the substrate S (Y-axis direction in Figure 6B). The thickness of the structures 342 in the direction parallel to the substrate S (X-axis direction in Figure 6B) is 1 nm or more and 10 nm or less, and the distance between the structures 342 (gaps 342a) in the X-axis direction in Figure 6B is 50 nm or more and 1 μm or less.

Specifically, the nanostructure layer 340 is a layer composed of nanowall-shaped structures 342, one part (tip end) of which is composed of a doped element (metal) and the other part (base end) of which is composed of carbon nanowalls 244a. In other words, the nanostructure layer 340 is composed of a first layer 244 and a second layer 346. The second layer 346 includes nanowall-shaped structures 346a composed of a doped element.

The material 300C manufactured in this manner can be used as a device. Specifically, when light is irradiated onto the second layer 346, the structures 346a are excited and electrons are emitted. The emitted electrons then flow to the base layer 230 via the first layer 244 (carbon nanowalls 244a). Therefore, the material 300C can be used as a device that detects light.

Material 300C can also be used as a photoanode in devices that generate hydrogen through water splitting (e.g., Ji-Wook Jang et al., Enabling unassisted solar water splitting by iron oxide and silicon, Nature Communications 6 (2015)).

In addition, the carbon reduction rate can be adjusted by controlling the plasma atmosphere temperature, plasma atmosphere pressure, and plasma treatment time. In other words, by controlling the plasma atmosphere temperature, plasma atmosphere pressure, and plasma treatment time, it is possible to remove all of the carbon from material 200, or it is also possible to remove a portion (a predetermined amount) of the carbon from material 200.

[First Example]
A silicon substrate was used as the substrate S, titanium was used as the doping element, and the material was manufactured using a plasma CVD apparatus 100 .

In Example A, the execution time of the formation process S110 was 360 minutes, the execution time of the sputtering process S120 was 30 minutes, and the execution time of the carbon reduction process S130 (in an air atmosphere) was 1 hour. Furthermore, in Example A, the temperature inside the chamber 110 during the formation process S110 and sputtering process S120 was 600°C, and the ambient temperature during the carbon reduction process S130 (in an air atmosphere) was 500°C.

In the comparative example, the formation process S110 was performed for 360 minutes, the sputtering process S120 was not performed, and the carbon reduction process S130 (in an air atmosphere) was performed for 1 hour. In addition, in the comparative example, the temperature inside the chamber 110 during the formation process S110 was set to 600°C, and the ambient temperature during the carbon reduction process S130 (in an air atmosphere) was set to 500°C.

The material was then observed with a scanning electron microscope (SEM) after the sputtering step S120 in Example A but before the carbon reduction step S130, after the carbon reduction step S130 in Example A, after the formation step S110 in the comparative example but before the carbon reduction step S130, and after the carbon reduction step S130 in the comparative example.

Figure 7 shows SEM images of Example 1. Figure 7A shows an SEM image of Example A after the sputtering step S120 and before the carbon reduction step S130. Figure 7B shows an SEM image of Example A after the carbon reduction step S130. Figure 7C shows an SEM image of Example A after the formation step S110 and before the carbon reduction step S130. Figure 7D shows an SEM image of Example A after the carbon reduction step S130.

As shown in Fig. 7A, titanium-doped carbon nanowalls were confirmed in Example A after the sputtering step S120 and before the carbon reduction step S130. Also, as shown in Fig. 7B, nanowall-shaped titanium oxide (TiO ₂ ) was confirmed in Example A after the carbon reduction step S130.

On the other hand, as shown in Figure 7C, in the comparative example, titanium-doped carbon nanowalls were confirmed after the formation process S110 and before the carbon reduction process S130. However, as shown in Figure 7D, in the comparative example, no nanowall-shaped titanium oxide was confirmed after the carbon reduction process S130.

Therefore, it was confirmed that by performing the sputtering process S120 in addition to the formation process S110, the titanium is reinforced and nanowall-shaped titanium oxide can be produced after the carbon reduction process S130 is performed. In other words, it was confirmed that performing only the formation process S110 without performing the sputtering process S120 would not allow nanowall-shaped titanium oxide to be produced after the carbon reduction process S130 is performed.

In addition, the material after carbon reduction step S130 in Example A was analyzed using XPS (X-ray photoelectron spectroscopy) and Raman spectroscopy.

Figure 8 shows the results of XPS analysis. In Figure 8, the horizontal axis represents the sputtering depth [nm] in the XPS analysis. In Figure 8, the vertical axis represents the atomic content [AT% (atomic %)]. In Figure 8, squares represent oxygen (O). In Figure 8, triangles represent titanium (Ti). In Figure 8, circles represent carbon (C).

As shown in Figure 8, the material produced by Example A was confirmed to contain titanium and oxygen throughout the entire depth of the structure, with almost no carbon.

In addition, crystal structure evaluation using Raman spectroscopy confirmed that titanium oxide with an anatase structure was produced in Example A.

Photoelectrochemical measurements were also performed using Example A. Specifically, experiments were performed using a three-electrode electrochemical measurement cell that utilized an FTO (fluorine-doped tin) film with Example A attached as the counter electrode. A platinum wire was used as the working electrode of the three-electrode electrochemical measurement cell. A 0.1 M aqueous sodium sulfate solution (pH 6.4) was used as the electrolyte. The potential was scanned at 20 mV/s.

Then, ultraviolet (UV) light with a wavelength of 254 nm and a light intensity of 0.83 mW/cm ₂ was turned on and off to measure the current flowing between the working electrode and the counter electrode.

Fig. 9 shows the results of photoelectrochemical measurement using Example A. In Fig. 9, the horizontal axis represents the voltage value [V] relative to the reference electrode, and the vertical axis represents the current density [µA/cm ₂ ].

9, when no ultraviolet light was irradiated, the current density was 0 μA/cm ₂ , meaning that no current flowed. On the other hand, when ultraviolet light was irradiated, the current density exceeded 0 μA/cm ₂ , meaning that current flowed.

These results confirm that the nanowall-shaped titanium oxide produced in Example A functions as a photocatalyst.

[Second Example]
A silicon substrate was used as the substrate S, titanium was used as the doping element, and the material was manufactured using a plasma CVD apparatus 100 .

In Example B, the formation process S110 was performed for 10 minutes, the sputtering process S120 for 5 minutes, and the carbon reduction process S130 (in an air atmosphere) for 60 minutes. Furthermore, in Example B, the temperature inside the chamber 110 during the formation process S110 and sputtering process S120 was set to 650°C, and the ambient temperature during the carbon reduction process S130 (in an air atmosphere) was set to 500°C.

In Example C, the formation process S110 was performed for 60 minutes, the sputtering process S120 was performed for 60 minutes, and the carbon reduction process S130 (in an air atmosphere) was performed for 60 minutes. Furthermore, in Example C, the temperature inside the chamber 110 during the formation process S110 and sputtering process S120 was set to 500°C, and the ambient temperature during the carbon reduction process S130 (in an air atmosphere) was set to 500°C.

In Example D, the formation process S110 was performed for 360 minutes, the sputtering process S120 for 30 minutes, and the carbon reduction process S130 (in an air atmosphere) for 60 minutes. Furthermore, in Example D, the temperature inside the chamber 110 during the formation process S110 and sputtering process S120 was set to 600°C, and the ambient temperature during the carbon reduction process S130 (in an air atmosphere) was set to 600°C.

In the formation step S110 of Examples B to D, the flow rate of methane supplied into the chamber 110 was 10 sccm, the flow rate of hydrogen was 10 sccm, and the flow rate of argon was 80 sccm.

The materials produced in Examples B to D were then observed using a scanning electron microscope (SEM).

Figure 10 shows an SEM image of Example 2. Figure 10A shows an SEM image of Example B. Figure 10B shows an SEM image of Example C. Figure 10C shows an SEM image of Example D.

As shown in FIGS. 10A to 10C, it was confirmed that nanowall-shaped titanium oxide (TiO ₂ ) could be produced in all of Examples B to D.

[Third Example]
A silicon substrate was used as the substrate S, titanium was used as the doping element, and the material was manufactured using the plasma CVD apparatus 100. In the third example, the execution time of the formation step S110 was set to 30 minutes, the execution time of the sputtering step S120 was set to 360 minutes, and the execution time of the carbon reduction step S130 (in the air atmosphere) was set to 1 hour or 10 hours.

In addition, in the third example, the crystalline structure of titanium oxide depending on the ambient temperature during the carbon reduction step S130 (in air) was analyzed using Raman spectroscopy.

It was confirmed that when the ambient temperature of the carbon reduction step S130 is between 400°C and 450°C and the execution time is one hour, the structure contains titanium oxide and graphite with an anatase structure. In other words, it was confirmed that carbon nanowalls containing titanium oxide can be produced by performing the carbon reduction step S130 (in air) at an ambient temperature of between 400°C and 450°C.

It was also confirmed that when the ambient temperature of the carbon reduction step S130 was between 600°C and 750°C and the execution time was 10 hours, the structure contained titanium oxide with both anatase and rutile structures. In other words, it was confirmed that by performing the carbon reduction step S130 (in air) at an ambient temperature of between 600°C and 750°C, it was possible to produce carbon-free, nanowall-shaped titanium oxide. It was also confirmed that the higher the ambient temperature, the less titanium oxide with anatase structure there was and the more titanium oxide with rutile structure there was.

Although the embodiments have been described above with reference to the accompanying drawings, it goes without saying that the present disclosure is not limited to such embodiments. It is clear that a person skilled in the art could conceive of various modifications or alterations within the scope of the claims, and it is understood that these also naturally fall within the technical scope.

For example, in the above-described embodiment, the doping element is a metal element with a sputtering rate lower than that of iron. However, the doping element may also be a non-metal element (including a metalloid element) with a sputtering rate lower than that of iron. For example, the doping element may be either or both of silicon (Si) and germanium (Ge). In this case, a nanowall-shaped structure composed of a non-metal element is produced.

The doping element may also be an element (metal element or non-metal element) that has a sputtering rate equal to or higher than that of iron.

The doping element may also be two or more types of metal elements. In this case, a nanowall-shaped structure composed of an alloy containing two or more types of metal elements is produced. The doping element may also be a metal element and a non-metal element. In this case, a nanowall-shaped structure containing a metal element and a non-metal element is produced.

Furthermore, in the above embodiment, an example was described in which the structures 322 stand upright vertically from the surface of the substrate S. However, the structures 322 may stand upright from the substrate S, and there is no limitation on the angle.

In the above embodiment, a configuration in which carbon is completely removed in the carbon reduction process S130 has been described as an example. However, a desired amount of carbon may be left behind in the carbon reduction process S130. Note that if the carbon is completely removed from materials 200B and 200C in the carbon reduction process S130, only nanowall-shaped structures 322 are produced. Furthermore, if the carbon reduction process S130 involves leaving a certain amount of carbon in materials 200B and 200C, materials with less carbon than materials 200B and 200C are produced.

Furthermore, in the above embodiment, an example was given in which the carbon reduction process S130 was performed. However, the carbon reduction process S130 is not a required component. By performing the formation process S110 and the sputtering process S120, new materials 200A, 200B, and 200C can be manufactured.

Furthermore, in the above embodiment, an example was given of manufacturing a structure 322 composed of a nanowall-shaped metal oxide. However, the nanowall-shaped structure may contain an additive in addition to the metal oxide. The additive is any one or more of all elements except hydrogen (H), helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). When the nanowall-shaped structure contains an additive in addition to the metal oxide, the photocatalytic function can be improved.

In the above embodiment, the target holding unit 130 applies a constant voltage during the formation process S110 and a pulse voltage during the sputtering process S120. However, the target holding unit 130 may apply a constant voltage during both the formation process S110 and the sputtering process S120, may apply a pulse voltage during both the formation process S110 and the sputtering process S120, or may apply a pulse voltage during the formation process S110 and the sputtering process S120, or may apply a pulse voltage during the formation process S110 and a constant voltage during the sputtering process S120.

This disclosure can contribute, for example, to Sustainable Development Goal (SDG) Goal 6, "Ensure access to water and sanitation for all," and Goal 7, "Ensure access to affordable, reliable, sustainable and modern energy."

T: Sputtering target S110: Forming step S120: Sputtering step S130: Carbon reduction step 100: Plasma CVD apparatus 110: Chamber 140: Plasma gun (plasma emission part)
300A Material (Photocatalyst)
322 Structure

Claims (2)

In a plasma CVD apparatus having a chamber, a sputtering target provided in the chamber and composed of a predetermined element, and a plasma discharger for discharging a plasma flow into the chamber, wherein the plasma constituting the plasma flow discharged by the plasma discharger collides with the sputtering target, the apparatus comprises: supplying a reactive gas containing at least a carbon compound gas into the chamber; discharging the plasma flow into the chamber; and doping the carbon nanowalls with the predetermined element while forming carbon nanowalls on a substrate provided in the chamber;
After performing the doping step, stopping the supply of the reaction gas into the chamber while maintaining the emission of the plasma flow;
Including,
A material manufacturing method, wherein the predetermined element is an element having a lower sputtering rate than iron. The material production method according to claim 1, further comprising, after performing the stopping step, a step of reducing carbon from the carbon nanowalls doped with the predetermined element.