Photocatalyst

Abstract

To provide a highly stable photocatalyst that can maintain high activity for a long period of time. [Solution] A photocatalyst having a perovskite crystal, wherein it has a stepped structure composed of terraces and steps on its surface, and in the observed image of the surface, the occupancy rate of the projected area of ​​the stepped structure in the total projected area is 20% or more. Furthermore, it is preferable that the terraces are composed of {100} faces, and the steps are composed of {100} faces or {110} faces.

Description

[Technical Field]

[0001] This invention relates to a photocatalyst. [Background technology]

[0002] Non-patent document 1 discloses a particulate photocatalyst using aluminum-doped strontium titanate that can split water with an external quantum efficiency of up to 96% using light with a wavelength of 350-360 nm. It also discloses that by selectively photoelectrodepositing a co-catalyst onto the photocatalyst, the hydrogen and oxygen generation reactions can be separately promoted on different crystal surfaces of the photocatalyst. [Prior art documents] [Patent Documents]

[0003] [Non-Patent Document 1] Takata, Tsuyoshi, et al., "Photocatalytic water splitting with a quantum efficiency of almost unity", Nature 581.7809, 2020, p.411-414 [Overview of the project] [Problems that the invention aims to solve]

[0004] Conventional photocatalysts with perovskite crystals exhibit high quantum efficiency immediately after fabrication, but a challenge is that their catalytic activity decreases with prolonged light irradiation. Therefore, there is a need for highly stable photocatalysts that can maintain high activity over long periods. [Means for solving the problem]

[0005] The photocatalyst according to an application example of the present invention is A photocatalyst having a perovskite crystal, It has a stepped structure consisting of terraces and steps on the surface, In the observed image of the surface, the occupancy rate of the projected area of ​​the stepped structure in relation to the total projected area is 20% or more. [Brief explanation of the drawing]

[0006] [Figure 1] This is a schematic diagram showing a photocatalyst according to an embodiment. [Figure 2] This is a schematic plan view showing the terraces and steps that constitute the stepped structure when the main surface shown in Figure 1 is viewed from above. [Figure 3] This is a cross-sectional view along line AA in Figure 2. [Figure 4] Figure 2 is a cross-sectional view along line BB. [Figure 5] This is a partially enlarged view of Figure 3, a schematic diagram illustrating the function of a stepped structure. [Figure 6] Figure (A) shows an electron microscope image of the photocatalyst according to the embodiment, and Figure (B) shows a marker superimposed to identify the step-like structure in the image. [Figure 7] Figure (A') shows an electron microscope image of the photocatalyst related to the comparative example, and Figure (B') shows a marker superimposed to identify the step-like structure in this image. [Figure 8] This is a cross-sectional view showing a support for the photocatalyst according to the embodiment, and a co-catalyst supported on the surface of the support. [Figure 9] Table 1 shows the manufacturing conditions for the photocatalysts in each example and comparative example, as well as the evaluation results of the manufactured photocatalysts. [Figure 10] The following are images of the photocatalyst obtained in Example 1: an observation image of the support before the co-catalyst was supported and its schematic diagram; an observation image of the Pt support after a Pt support test was performed on the support shown in the observation image and its schematic diagram; and an observation image of the CoOy support after a CoOy support test was performed on the support shown in the observation image and its schematic diagram. [Figure 11]Regarding the photocatalyst obtained in the comparative example, the observation image of the carrier before loading the cocatalyst and its schematic diagram, the observation image of the Pt-loaded carrier and its schematic diagram after performing the Pt loading test on the carrier shown in the observation image, and the observation image of the CoOy-loaded carrier and its schematic diagram after performing the CoOy loading test on the carrier shown in the observation image. [Figure 12] It is a graph comparing the transition of the gas generation rate over 11 days for the photocatalyst obtained in Example 1 and the photocatalyst obtained in the comparative example.

Embodiments for Carrying Out the Invention

[0007] Hereinafter, the photocatalyst according to the present invention will be described in detail based on the embodiments shown in the accompanying drawings.

[0008] 1. Photocatalyst 1.1. Outline of the Photocatalyst First, the outline of the photocatalyst according to the embodiment will be described.

[0009] FIG. 1 is a schematic diagram showing a photocatalyst 1 according to the embodiment. The photocatalyst 1 shown in FIG. 1 is in a particulate form and has a crystal body of a perovskite-type crystal. Note that the photocatalyst 1 shown in FIG. 1 is usually used in the form of a powder in which a plurality of particles are aggregated, but only one of them is illustrated in FIG. 1. Note that the outer shape of the photocatalyst 1 shown in FIG. 1 shows one of the typical shapes. Therefore, the outer shape of the photocatalyst 1 is not limited to the shape shown in FIG. 1.

[0010] The photocatalyst 1 is used as a catalyst for the water decomposition reaction under light irradiation. For this reason, for example, by applying sunlight or the like to the photocatalyst 1 in contact with water, hydrogen as a renewable energy can be produced at a low cost.

[0011] The outer shape of the photocatalyst 1 shown in FIG. 1 is particulate, but it is not limited thereto. For example, it may be fibrous, needle-like, scaly, substrate-like,块状, etc. On the other hand, the particulate photocatalyst 1 has appropriate fluidity and can secure a large specific surface area. Thereby, a photocatalyst 1 that is easy to handle and has high catalytic efficiency can be realized.

[0012] Examples of perovskite crystal structures include cubic, tetragonal, orthorhombic, and monoclinic crystals. Of these, cubic perovskite crystals are preferably used in photocatalyst 1. Cubic perovskite crystals are composed of composite oxides represented by the general formulas ABO3 or ABO2N, for example. A is at least one element selected from the group consisting of Sr (strontium), Na (sodium), K (potassium), and Ba (barium). B is at least one element selected from the group consisting of Ti (titanium) and Ta (tantalum).

[0013] The photocatalyst 1 shown in Figure 1 has a stepped structure 2 on its surface. The stepped structure 2, also called a terrace-step structure, is a structure that includes a terrace, which is a flat surface, and steps, which intersect the terrace and form a difference in height.

[0014] As a result of the inventors' studies, it was found that the presence of such a stepped structure 2 on the surface in a predetermined area ratio allows for the creation of a highly active catalytic reaction field for the water splitting reaction in the photocatalyst 1, and also enhances the stability of the catalyst. In other words, the photocatalyst 1 having the stepped structure 2 on its surface in a predetermined area ratio does not experience a decrease in catalytic activity even with prolonged light irradiation, and can maintain high activity over a long period of time. The configuration of the photocatalyst 1 according to the embodiment will be described in detail below.

[0015] 1.2.Composition The composition of photocatalyst 1 consists of materials that make up a perovskite-type crystal.

[0016] Specific examples of composite oxides include strontium titanate (SrTiO3), sodium tantalate (NaTaO3), and potassium tantalate (KTaO3). Of these, strontium titanate (SrTiO3) is preferably used. Strontium titanate has high quantum efficiency in water splitting reactions and is preferably used as the parent catalyst material for photocatalyst 1.

[0017] Furthermore, metal elements may be added to the composite oxide. This allows for control of the electronic state of the composite oxide, adjusting the wavelength of light to which the photocatalyst 1 responds, or improving quantum efficiency. Examples of metal elements that can be doped into the composite oxide include Al (aluminum), Na (sodium), Mg (magnesium), Ga (gallium), In (indium), La (lanthanum), Rh (rhodium), Ir (iridium), Cr (chromium), Ru (ruthenium), etc., and one or more of these may be used.

[0018] The amount of metal ions added is not particularly limited, but is preferably 0.05 atomic% to 10 atomic% of the total composite oxide, more preferably 0.1 atomic% to 5 atomic%, and even more preferably 0.3 atomic% to 3 atomic%. This further enhances the catalytic activity of photocatalyst 1.

[0019] 1.3. Crystal structure The surface of photocatalyst 1 shown in Figure 1 is composed of an aggregate of several flat surfaces. In this specification, "flat surface" is not limited to strictly flat surfaces, but may include some irregularities or distortions. Typically, crystal faces of perovskite-type crystals are exposed on such flat surfaces.

[0020] Figure 1 shows an example of the Miller indices of crystal planes exposed on the surface of photocatalyst 1. In the example shown in Figure 1, the flat planes included on the surface of photocatalyst 1 are either {100} planes, {110} planes, or {111} planes.

[0021] When the surface of photocatalyst 1 is observed with an electron microscope, the flat surface with the largest projected area in the obtained observation image is defined as the "main surface P". The electron microscope used is a scanning electron microscope (SEM), a transmission electron microscope (TEM), or a scanning transmission electron microscope (STEM).

[0022] In the example shown in Figure 1, multiple {100} faces are exposed on the surface of the photocatalyst 1, one of which is the main face P. The main face of the photocatalyst may also be a {110} face. In other words, it is preferable that the main face of the photocatalyst be a {100} face or a {110} face. These crystal faces are highly active faces with high catalytic activity. Therefore, photocatalysts with these crystal faces as their main faces will have high catalytic activity.

[0023] The perovskite crystal is the parent catalyst of photocatalyst 1. Inside the perovskite crystal, electrons and holes excited by light move in opposite directions. Due to this phenomenon, sites are formed on the surface of photocatalyst 1 that donate electrons to water to produce hydrogen, and sites that donate holes to water to produce oxygen. The perovskite crystal also functions as a support for the co-catalyst described later.

[0024] 1.4.Step structure The stepped structure 2 is identified when the photocatalyst 1 is observed with an electron microscope, as terraces and steps with a difference in brightness appear as elongated, band-like structures in the observed image. In Figure 1, the area occupied by the stepped structure 2 in the observed image is indicated by dots.

[0025] Figure 2 is a schematic plan view showing the terrace 21 and step 22 that constitute the stepped structure 2 when the main surface P shown in Figure 1 is viewed from above.

[0026] As shown in Figure 2, the stepped structure 2 consists of a terrace 21, which is a surface parallel to the main surface P (flat surface), and a step 22, which is a surface intersecting the terrace 21. The terrace 21 and the step 22 extend in an elongated shape in approximately the same direction.

[0027] The arrangement of the stepped structure 2 on the surface of the photocatalyst 1 is not particularly limited, but as shown in Figure 1, the stepped structure 2 is distributed so as to surround at least one flat surface on the surface of the photocatalyst 1. This allows the stepped structure 2, which serves as a highly active catalytic reaction field, to be distributed evenly and uniformly on the surface of the photocatalyst 1. As a result, a photocatalyst 1 with high catalytic efficiency can be realized.

[0028] Furthermore, it is preferable that the stepped structure 2 is distributed so as to surround the {100} plane or the {110} plane. The {100} plane and the {110} plane are the active planes. Therefore, by distributing the stepped structure 2 so as to surround the active plane, a photocatalyst 1 with particularly high catalytic efficiency can be realized. In Figures 1 and 2, as an example, the stepped structure 2 is distributed so as to surround the {100} plane.

[0029] Figure 3 is a cross-sectional view along line AA in Figure 2. Line AA in Figure 2 is a straight line drawn connecting a flat plane (principal plane P), which is a {100} plane, with another {100} plane adjacent to the flat plane via other crystal planes.

[0030] As shown in Figure 3, when the photocatalyst 1 is cut in a plane containing the AA line, a periodic structure in which terraces 21 and steps 22 are arranged alternately appears on the cut surface. In this case, the surface that drops in from the edge of the main surface P shown in Figure 3 is defined as "step 22". In the periodic structure shown in Figure 3, the terraces 21 and steps 22 are arranged alternately in the direction away from the main surface P, starting from step 22. Both the terraces 21 and steps 22 shown in Figure 3 are {100} planes.

[0031] Figure 4 is a cross-sectional view of the line BB in Figure 2. The line BB in Figure 2 is a straight line drawn connecting the {100} plane, which is a flat plane (principal plane P), and the {110} plane, which is adjacent to the flat plane via other crystal planes.

[0032] As shown in Figure 4, when the photocatalyst 1 is cut in a plane containing the BB line, a periodic structure appears on the cut surface in which terraces 21 and steps 22 are arranged alternately. In this case, the plane that drops in from the edge of the main plane P shown in Figure 4 is defined as "step 22". In the periodic structure shown in Figure 4, the terraces 21 and steps 22 are arranged alternately in the direction away from the main plane P, starting from step 22. The terraces 21 shown in Figure 4 are {100} planes, and the steps 22 shown in Figure 4 are {110} planes.

[0033] As described above, the stepped structure 2 is a unique structure in which crystal planes with the same Miller indices and crystal planes with different Miller indices are adjacent to each other within a narrow range. Furthermore, as previously mentioned, the stepped structure 2 functions as a highly active catalytic reaction field for the water splitting reaction.

[0034] Figure 5 is a partially enlarged view of Figure 3, and is a schematic diagram illustrating the function of the stepped structure 2. As shown in Figure 5, the stepped structure 2 includes a ridge portion 23. The ridge portion 23 is the part near the edge located at the boundary between the terrace 21 and the step 22. This ridge portion 23 forms a site distinct from the terrace 21 and the step 22.

[0035] Specifically, within a perovskite crystal, electrons excited by light e - The electrons e selectively move to terrace 21 or step 22 over a short distance (near the excitation site). As a result, terrace 21 and step 22 move to the electrons e - It becomes a separation site and a hydrogen generation site that generates hydrogen from water. On the other hand, holes excited by light h + It selectively moves to the ridge portion 23 over a short distance. As a result, the ridge portion 23 has holes h + It becomes a separation site and an oxygen generation site that produces oxygen from water.

[0036] In this way, in the stepped structure 2, electron e - The region that moves and separates, and the hole h +The region that moves and separates, and the region that comes close to a narrow range. As a result, the excited electron e - and hole h + When moving in the photocatalyst 1, each moving distance can be shortened. As a result, the electron e - and hole h + The probability of recombination is reduced, and a decrease in photocatalytic activity can be suppressed.

[0037] In addition, as a result of the study by the inventor of the present application, it has been found that it is important that such a stepped structure 2 exists on the surface at a predetermined area ratio. Specifically, in the photocatalyst 1 according to the present embodiment, in the observation image of the surface, the occupancy ratio of the projected area of the stepped structure 2 in the entire projected area is 20% or more. That is, when the projected area of the entire particles of the photocatalyst 1 shown in the observation image is 100%, the ratio of the projected area of the stepped structure 2 is 20% or more. By providing the stepped structure 2 at such a ratio, in the photocatalyst 1, the electron e - and hole h + The suppression of recombination becomes remarkable. As a result, a highly stable photocatalyst 1 capable of maintaining high activity for a long time can be realized.

[0038] In addition, since the terrace 21 and the step 22 are surfaces facing concave spaces, they are less likely to receive external forces. Therefore, for example, by supporting a cocatalyst on such a surface, the dropout of the cocatalyst due to external forces can be suppressed. Therefore, by having the stepped structure 2 at a predetermined area ratio, high activity can be maintained for a long time.

[0039] FIG. 6 is a diagram (A) of an observation image of the photocatalyst 1 according to the embodiment by an electron microscope, and a diagram (B) showing a marker identifying the stepped structure 2 superimposed on this observation image.

[0040] In the observation image (A) shown in Figure 6, two photocatalyst 1 particles are surrounded by a dashed line as an example. For these two particles, the area enclosed by this dashed line corresponds to the "total projected area." Also in this observation image (A), a step-like structure 2 distributed over a wide area can be seen. In Figure 6 (B), the area occupied by this step-like structure 2 is filled in with a white marker. The area of ​​this white marker corresponds to the "projected area of ​​the step-like structure 2." To calculate the "occupancy rate of the projected area of ​​the step-like structure 2," for five or more photocatalysts 1 shown in one observation image (A), the ratio of the projected area of ​​the step-like structure 2 to the total projected area is calculated, and the average value is calculated. This average value is taken as the "occupancy rate of the projected area of ​​the step-like structure 2."

[0041] Furthermore, the occupancy rate of the stepped structure 2 is 20% or more, preferably 25% or more, and more preferably 30% or more. If the occupancy rate of the stepped structure 2 falls below the lower limit, electrons e - and hole h + Because the effect of suppressing recombination is limited, the stability of photocatalyst 1 decreases.

[0042] On the other hand, while there is no upper limit to the occupancy rate of the stepped structure 2, considering the increased difficulty of manufacturing, it is preferably 90% or less, more preferably 80% or less, and even more preferably 70% or less.

[0043] The observed image (A) should be an image with an imaging range that simultaneously captures five or more photocatalysts 1. The five or more photocatalysts 1 may be randomly selected, but preferably they are selected in order of largest projected area. It is also acceptable to select photocatalysts 1 that overlap, or those that are partially cut off at the edge of the observed image (A).

[0044] The length of one side of the imaging range of the observed image (A) is preferably 500 nm or longer, and more preferably 800 nm to 2000 nm.

[0045] Figure 7 shows an electron microscope image (A') of the photocatalyst 1' related to the comparative example, and a figure (B') overlaying a marker that identifies the step-like structure 2' in the same image.

[0046] In the observation image (A') shown in Figure 7, two photocatalyst particles 1' are surrounded by a dashed line as an example. In Figure 7(B'), the area occupied by the stepped structure 2' is filled in with a white marker. The method for calculating the "occupancy rate of the projected area of ​​the stepped structure 2'" in the comparative example photocatalyst 1' is the same as in the case of photocatalyst 1 according to the embodiment.

[0047] In the photocatalyst 1' shown in Figure 7, the occupancy rate of the projected area of ​​the step-like structure 2' is small, less than 20%. In this case, in the photocatalyst 1', electron e - and hole h + The suppression of recombination is insufficient, making it impossible to maintain high activity for a long period of time.

[0048] The width W shown in Figure 5 is the width of the terrace 21. The width W of the terrace 21 is not particularly limited, but is preferably 3 nm to 25 nm, more preferably 4 nm to 20 nm, and even more preferably 5 nm to 15 nm. If the width W of the terrace 21 is within the above range, the repeating period between the terrace 21 and step 22 and the ridge portion 23 can be optimized. This makes it possible to realize a photocatalyst 1 that has particularly high catalytic activity and has a higher density of hydrogen generation sites and oxygen generation sites.

[0049] Furthermore, if the width W falls below the lower limit, the difficulty of forming the terraces 21 may increase, or the effective area of ​​the hydrogen generation site may decrease. On the other hand, if the width W exceeds the upper limit, the difficulty of forming the terraces 21 may increase, or it may become difficult to maintain catalytic activity over a long period of time.

[0050] The height H shown in Figure 5 is the height of step 22. The height H of step 22 is not particularly limited, but is preferably 3 nm to 25 nm. If the height H of step 22 is within the above range, the repeating period between terrace 21 and step 22 and the ridge portion 23 can be optimized. This makes it possible to realize a photocatalyst 1 that has particularly high catalytic activity and has a higher density of hydrogen generation sites and oxygen generation sites.

[0051] Furthermore, if the height H falls below the lower limit, the difficulty of forming step 22 may increase, or the effective area of ​​the hydrogen generation site may decrease. On the other hand, if the height H exceeds the upper limit, the difficulty of forming step 22 may increase, or it may become difficult to maintain catalytic activity over a long period of time.

[0052] Methods for measuring the width W of the terrace 21 and the height H of the step 22 include, for example, measurement methods using observation images obtained from a transmission electron microscope (TEM) or scanning transmission electron microscope (STEM). In this case, the photocatalyst 1 may be sliced ​​to prepare thin sections, and observation images may be obtained from these thin sections.

[0053] The stepped structure 2 is preferably a structure in which terraces 21 and steps 22 are repeated alternately multiple times. In such a structure, hydrogen generation sites and oxygen generation sites that are densely close to each other can be distributed more widely. This makes it possible to realize a photocatalyst 1 that is highly stable and has high catalytic efficiency.

[0054] The number of repetitions of terraces 21 and steps 22 in the independent stepped structure 2 is not particularly limited, but is preferably 5 or more, and more preferably 10 or more. This yields a photocatalyst 1 with particularly high stability and catalytic efficiency. Furthermore, there is no upper limit to the number of repetitions, but considering the increased difficulty of manufacturing, it is preferably 50 or less. The independent stepped structure 2 refers to, for example, one of those arranged to surround the {100} plane in Figure 1 and separated from other stepped structures 2 via flat surfaces such as the {110} plane and the {111} plane.

[0055] The dihedral angle θ shown in Figure 5 is the angle of the dihedral angle formed by the terrace 21 and the step 22 toward the space. The dihedral angle θ is preferably 90° or more, and more preferably 90° to 100°. With this configuration, it is possible to suppress an increase in the difficulty of forming the stepped structure 2. In addition, when water comes into contact with the stepped structure 2, the water contact efficiency can be increased. Furthermore, it is possible to suppress the detachment of the co-catalyst supported on the terrace 21 and the step 22. If the dihedral angle θ falls outside the above range, it may become more difficult to manufacture the stepped structure 2, the water contact efficiency may decrease and the efficiency of the water splitting reaction may decrease, and the supported co-catalyst may become more prone to detachment. The method for measuring the dihedral angle θ is the same as the method for measuring the width W and height H.

[0056] 1.5. External shape of the photocatalyst As mentioned above, the external shape of the photocatalyst 1 is preferably particulate. The average particle size of the particulate photocatalyst 1 is preferably 50 nm to 30,000 nm, more preferably 80 nm to 1,000 nm, and even more preferably 100 nm to 500 nm. If the average particle size is within the above range, the photocatalyst 1 can be handled as fine particles (fine powder). In other words, because it is a fine powder, it has a high degree of freedom in shape and a wide range of arrangement options, so a photocatalyst 1 can be obtained that can easily realize water splitting devices of various structures.

[0057] The average particle size of photocatalyst 1 is measured as follows: First, the particulate photocatalyst 1 is observed using an electron microscope to obtain an image. Next, one photocatalyst 1 that does not overlap with other photocatalysts 1 in the observed image is selected. Then, the diameter of the circumscribed circle of the projected image of the selected photocatalyst 1 is defined as the particle diameter. In this way, the particle diameters are determined for 10 or more photocatalysts 1, and the average value is defined as the "average particle diameter".

[0058] The average aspect ratio (short axis diameter / long axis diameter) of the particulate photocatalyst 1 is not particularly limited, but is preferably 0.5 or more and 1.0 or less, and more preferably 0.6 or more and 1.0 or less. If the average aspect ratio is within the above range, a photocatalyst 1 that is relatively difficult to manufacture and has excellent handling properties can be realized.

[0059] The average aspect ratio of photocatalyst 1 is measured as follows:

[0060] First, the particulate photocatalyst 1 is observed using an electron microscope to obtain an image. Next, one photocatalyst 1 that does not overlap with other photocatalysts 1 in the observed image is selected. Then, the projection image of the selected photocatalyst 1 is enclosed by two parallel lines. The distance between the two lines when it is widest is defined as the major axis diameter, and the distance when it is narrowest is defined as the minor axis diameter. In this way, the aspect ratio is determined for 10 or more photocatalysts 1, and the average value is defined as the "average aspect ratio".

[0061] 1.6. Co-catalyst The photocatalyst 1 may have a support (major catalyst) composed of a perovskite-type crystal, as well as a co-catalyst supported on the surface of the support.

[0062] Figure 8 is a cross-sectional view showing the carrier 10 of the photocatalyst 1 according to the embodiment, and the co-catalysts 31 and 32 supported on the surface of the carrier 10.

[0063] The co-catalyst 31 is a hydrogen generation co-catalyst, supported on the hydrogen generation site, and has functions such as increasing hydrogen generation efficiency and extending the lifespan of the hydrogen generation site. As shown in Figure 8, the co-catalyst 31 is mainly supported on the terraces 21 and steps 22 of the support 10. Since the terraces 21 and steps 22 are separated from each other via the ridge portions 23, the size of the supported co-catalyst 31 is controlled to be minute and uniform. The size of the co-catalyst 31 is thought to affect the electronic structure of the co-catalyst 31 and thus influence the function of the co-catalyst 31. As mentioned above, the width W of the terraces 21 and the height H of the steps 22 are controlled to a range of a few nanometers to a range of tens of nanometers, so the size of the co-catalyst 31 is optimized to be about the same or smaller. This makes it possible to further increase hydrogen generation efficiency and sufficiently extend the lifespan of the hydrogen generation site. Although not shown in Figure 8, the co-catalyst 31 may also be supported on the flat surface of the support 10.

[0064] Examples of materials that make up the co-catalyst 31 include materials containing at least one element selected from the group consisting of Rh (rhodium), Pt (platinum), Ru (ruthenium), Ni (nickel), and Au (gold). Furthermore, the materials that make up the co-catalyst 31 may be metals containing these metal elements, metal oxides, or metal hydroxides. In addition, they may be composite oxides or mixed oxides of these metal elements with other metal elements. Examples of composite oxides include rhodium-chromium composite oxide (RhCrOx). Examples of mixed oxides include rhodium-chromium mixed oxide (RhCrOx).

[0065] The co-catalyst 32 is an oxygen-generating co-catalyst, supported on the oxygen-generating site, and has functions such as increasing oxygen generation efficiency and extending the lifespan of the oxygen-generating site. As shown in Figure 8, the co-catalyst 32 is mainly supported on the ridge portions 23 of the support 10. Since the ridge portions 23 are separated from other ridge portions 23 via terraces 21 and steps 22, the size of the supported co-catalyst 32 is controlled to be minute and uniform. The size of the co-catalyst 32 is thought to affect the electronic structure of the co-catalyst 32 and thus influence the function of the co-catalyst 32. As mentioned above, the width W of the terraces 21 and the height H of the steps 22 are controlled to be around a few nanometers to several tens of nanometers, so the width of the ridge portions 23 is controlled to be even narrower. Therefore, the size of the co-catalyst 32 is optimized to be about the same as or less than the width of the ridge portions 23. This makes it possible to further increase oxygen generation efficiency and sufficiently extend the lifespan of the oxygen-generating site.

[0066] Examples of materials that make up the co-catalyst 32 include materials containing at least one selected from the group consisting of Ir (iridium), Co (cobalt), Ru (ruthenium), and Fe (iron). Furthermore, the materials that make up the co-catalyst 32 may be metal oxides or metal hydroxides. Examples of metal oxides include cobalt oxide (CoOy).

[0067] Furthermore, if the stepped structure 2 is distributed to surround the flat surface, the co-catalysts 31 and 32 will also be distributed to surround the flat surface. This improves the efficiency of charge transfer within the photocatalyst 1, further enhancing the catalytic activity of the photocatalyst 1 and allowing the high activity to be maintained for a longer period of time.

[0068] The average particle size of the co-catalysts 31 and 32 is preferably 0.1 nm to 50 nm, and more preferably 0.5 nm to 20 nm. The average particle size of the co-catalysts 31 and 32 can be determined by averaging the particle sizes of 10 or more co-catalysts 31 and 32, which are determined by transmission electron microscopy (TEM) or scanning transmission electron microscopy (STEM).

[0069] The amount of co-catalysts 31 and 32 supported is not particularly limited, but when the mass of the carrier 10 is 1, it is preferably 0.0001 or more and 0.1 or less, and more preferably 0.001 or more and 0.05 or less.

[0070] By setting the average particle size and supported amount of co-catalysts 31 and 32 within the above range, it is possible to suppress light absorption by the co-catalysts 31 and 32 (preventing sufficient light from reaching the support 10) while effectively dispersing and supporting the co-catalysts 31 and 32 on the surface of the support 10. Furthermore, it is possible to form more hydrogen generation sites and oxygen generation sites.

[0071] Furthermore, the auxiliary catalysts 31 and 32 may be provided as needed, and either one or both may be omitted.

[0072] 2. Method for producing photocatalysts Next, we will describe an example of a method for producing photocatalyst 1. Below, we will explain the method for producing the carrier and the method for supporting the co-catalyst on the produced carrier.

[0073] 2.1. Method for manufacturing a carrier The carrier 10 shown in Figure 8 is a perovskite-type crystal. The carrier 10 is manufactured, for example, by the flux method. The flux method is manufactured through four steps: (1) formation of a melt or solution, (2) melting or dissolving of the crystalline raw material into the melt or solution, (3) precipitation and grain growth of the crystal in the melt or solution, and (4) separation of the precipitated crystal from the flux or solution that formed the melt. In the following description, a method using a melt will be described as an example.

[0074] First, the flux and the crystalline raw material are mixed. For example, in the synthesis of Al-doped SrTiO3, commercially available SrTiO3 and Al2O3 nanoparticles are placed in an automated agate mortar and pestle and mixed and ground. This yields a mixture of flux and crystalline raw material. Preferably, the flux used is a substance that does not react with the crystalline raw material and is easily separated from the crystalline raw material and the support 10. Examples of such fluxes include halides, carbonates, sulfates, and low-melting-point oxides. In addition, increasing the mixing time in the agate mortar tends to increase the proportion of the stepped structure formed in the crystal.

[0075] Next, the resulting mixture is heated to form a melt. Then, the resulting melt is heated to react with the crystalline raw material. This causes crystals to precipitate in the melt and promotes grain growth of the precipitated crystals. The heating temperature can be, for example, 800°C to 1600°C. The holding time at this heating temperature can be, for example, 30 minutes or more. The atmosphere during heating can be, for example, an oxidizing atmosphere such as air or oxygen.

[0076] Next, the crystals are separated and recovered by solid-liquid separation. For example, if the flux is water-soluble, the flux can be removed and the crystals separated by washing with deionized water or pure water. This yields the carrier 10.

[0077] 2.2. Method for supporting the co-catalyst As shown in Figure 8, the carrier 10 has a stepped structure 2. The method for supporting the co-catalysts 31 and 32 on this stepped structure 2 will be described below.

[0078] Methods for supporting the co-catalysts 31 and 32 include, for example, photoelectrodeposition, impregnation, and adsorption / calcination. Of these, photoelectrodeposition is preferred. In photoelectrodeposition, the co-catalysts 31 and 32 can be selectively supported at the desired position. The following explanation will use photoelectrodeposition as an example.

[0079] First, the carrier 10 is suspended in distilled water to prepare a suspension. Next, a first precursor solution of the co-catalyst 31 is added to the suspension. If the constituent material of the co-catalyst 31 is, for example, a rhodium-chromium composite oxide, then an aqueous solution of RhCl3 can be used as the first precursor solution. Next, the suspension to which the first precursor solution has been added is irradiated with light. The wavelength of the light to be irradiated is not particularly limited, but a wavelength to which the carrier 10 responds is preferred, and a wavelength of 300 nm or more and 500 nm or less is more preferred. The irradiation time is adjusted as appropriate depending on the illuminance of the light, but for example, it is 3 minutes or more and 10 hours or less.

[0080] Next, after stopping the irradiation of light, the second precursor solution and the third precursor solution are added to the suspension. If the constituent material of the co-catalyst 31 is, for example, a rhodium-chromium composite oxide, the second precursor solution can be an aqueous K2CrO4 solution. If the constituent material of the co-catalyst 32 is, for example, cobalt oxide, the third precursor solution can be an aqueous Co(NO3)2 solution. Next, the suspension to which the second and third precursor solutions have been added is irradiated with light. The wavelength of the irradiated light is not particularly limited, but the wavelength to which the support 10 responds is preferred, and 300 nm to 500 nm is more preferred. The irradiation time is adjusted as appropriate depending on the illuminance of the light, but for example, it is 3 minutes to 10 hours.

[0081] Through the irradiation of light as described above, rhodium-chromium composite oxide is photoelectrodeposited onto the terraces 21 and steps 22 of the carrier 10, thereby supporting the co-catalyst 31. In addition, cobalt oxide is photoelectrodeposited onto the ridge portions 23 of the carrier 10, thereby supporting the co-catalyst 32. In this way, photocatalyst 1 is obtained.

[0082] 3. Hydrogen production equipment Photocatalyst 1 is used, for example, in a hydrogen production apparatus. The hydrogen production apparatus comprises a container having a transparent light incident surface for irradiating light onto the photocatalyst 1, a water supply device for supplying water to the container, a hydrogen separation unit for separating the generated hydrogen, and a hydrogen storage unit for storing the generated hydrogen. Sunlight is preferably used as the light irradiated onto the photocatalyst 1.

[0083] 4. Effects achieved by the above embodiment As described above, the photocatalyst 1 according to the embodiment is a photocatalyst having a perovskite-type crystal and has a stepped structure 2 composed of terraces 21 and steps 22 present on its surface. Furthermore, in the surface observation image, the occupancy rate of the projected area of ​​the stepped structure 2 in the total projected area is 20% or more.

[0084] With this configuration, a highly stable photocatalyst 1 that can maintain high activity for a long period of time can be obtained.

[0085] In the photocatalyst 1 according to the above embodiment, it is preferable that the stepped structure 2 has a structure in which terraces 21 and steps 22 are repeated alternately multiple times.

[0086] This configuration allows for a wider distribution of hydrogen and oxygen generation sites that are densely and closely located relative to each other. This makes it possible to realize a photocatalyst 1 that is highly stable and highly catalytically efficient.

[0087] In the photocatalyst 1 according to the above embodiment, the perovskite crystal is preferably cubic. Furthermore, it is preferable that the surface of the photocatalyst 1 has a flat surface composed of {100} planes.

[0088] With this configuration, since the {100} plane is the active surface, the stepped structure 2 is distributed around this active surface, thereby realizing a photocatalyst 1 with particularly high catalytic efficiency.

[0089] In the photocatalyst 1 according to the above embodiment, the terrace 21 may be composed of {100} surfaces. In this case, step 22 is preferably composed of {100} surfaces or {110} surfaces.

[0090] With this configuration, the stepped structure 2 becomes a unique structure in which crystal planes with the same Miller indices and crystal planes with different Miller indices are adjacent to each other in a narrow area. For this reason, the stepped structure 2 functions as a highly active catalytic reaction field for the water splitting reaction.

[0091] In the photocatalyst 1 according to the above embodiment, it is preferable that the dihedral angle θ between the terrace 21 and the step 22 is 90° or more.

[0092] This configuration makes it possible to suppress the difficulty of forming the stepped structure 2. Furthermore, it allows for higher water contact efficiency when water comes into contact with the stepped structure 2. Additionally, it suppresses the shedding of the co-catalysts supported on the terraces 21 and steps 22.

[0093] In the photocatalyst 1 according to the above embodiment, the stepped structure 2 may be distributed so as to surround the flat surface. With this configuration, a photocatalyst 1 with particularly high catalytic efficiency can be realized.

[0094] In the photocatalyst 1 according to the above embodiment, the width W of the terrace 21 is preferably 3 nm or more and 25 nm or less.

[0095] This configuration allows for the optimization of the repeating cycle between the terrace 21 and step 22 and the ridge portion 23. This makes it possible to realize a photocatalyst 1 with particularly high catalytic activity and a higher density of hydrogen generation sites and oxygen generation sites.

[0096] The photocatalyst 1 according to the above embodiment is preferably in the form of parts with an average particle diameter of 50 nm or more and 30,000 nm or less.

[0097] With this configuration, the photocatalyst 1 can be handled as fine particles (fine powder). In other words, because it is a fine powder, it has a high degree of freedom in shape and a wide range of arrangement options, making it possible to obtain a photocatalyst 1 that can easily realize water splitting devices of various structures.

[0098] The photocatalyst 1 according to the above embodiment may have co-catalysts 31 and 32 that come into contact with the stepped structure 2.

[0099] This configuration allows for higher hydrogen and oxygen production efficiency, as well as significantly extending the lifespan of the hydrogen and oxygen production sites.

[0100] In the above embodiment, the photocatalyst 1 preferably contains strontium titanate as a perovskite-type crystal. In this case, the co-catalyst may contain rhodium-chromium mixed oxide, rhodium-chromium composite oxide, or cobalt oxide.

[0101] This configuration is preferable as the parent catalyst material for photocatalyst 1 because it increases the quantum efficiency of the water splitting reaction of strontium titanate. Furthermore, rhodium-chromium mixed oxide and rhodium-chromium composite oxide function as hydrogen production co-catalysts, and cobalt oxide functions as oxygen production co-catalysts. Therefore, by including these substances as co-catalysts, it is possible to increase the hydrogen production efficiency and oxygen production efficiency, and extend the lifespan of the hydrogen production sites and oxygen production sites.

[0102] The photocatalyst 1 according to the above embodiment is preferably used in a water splitting reaction. With this configuration, hydrogen can be produced cheaply as a renewable energy source simply by exposing it to sunlight or other light sources.

[0103] Although the photocatalyst according to the present invention has been described above based on the illustrated embodiments, the present invention is not limited thereto.

[0104] For example, the photocatalyst according to the present invention may have any components added to the above embodiment. [Examples]

[0105] Next, specific embodiments of the present invention will be described. 5. Manufacturing of photocatalysts 5.1. Examples 1-3 First, aluminum-doped strontium titanate (Al:SrTiO3) particles were prepared using the flux method. Analysis by X-ray diffraction revealed that the obtained particles possessed a cubic perovskite crystal structure.

[0106] Next, the obtained particles were used as a carrier, and the co-catalyst was loaded onto the carrier by photoelectrodeposition. This yielded the photocatalysts of Examples 1 to 3. In Examples 1 to 3, the occupancy rate of the stepped structure was varied by changing the manufacturing conditions of the carrier.

[0107] 5.2. Comparative Examples The photocatalyst was prepared in the same manner as in Example 1, except that the manufacturing conditions for the support were changed.

[0108] The manufacturing conditions for the photocatalysts are shown in Table 1 (Figure 9). Figure 9 is Table 1, which shows the manufacturing conditions for the photocatalysts of each example and comparative example, and the evaluation results of the manufactured photocatalysts.

[0109] The average particle size of the photocatalysts prepared in each example and comparative example was 200-500 nm. The average particle size of the co-catalyst was 0.2 to 10 nm.

[0110] On the surface of each photocatalyst, the {100} plane, {110} plane, and {111} plane were observed as flat surfaces. Furthermore, it was observed that the step-like structure was distributed surrounding the {100} plane.

[0111] The width of the terrace and the height of the steps were 1 to 15 nm, respectively, and the dihedral angle between the terrace and the steps was 90°. The number of repetitions of the terrace and steps ranged from 5 to 20.

[0112] 6. Evaluation of the support (matrix catalyst) For each example and comparative example, a support was prepared for the photocatalyst obtained before the co-catalyst was attached. Tests were then conducted on this support to evaluate the presence or absence of electron separation sites and hole separation sites. The test methods and evaluation methods for the test results are described below.

[0113] 6.1. Test to evaluate the presence or absence of electron separation sites (Pt-supported test) 0.1 g of each support, 30 mL of distilled water, and an aqueous solution of hexachloroplatinum(IV) containing 0.0001 g of Pt were placed in a 50 mL screw-top bottle and mixed to obtain a mixture. Next, the mixture was irradiated with light (ultraviolet light) at a wavelength of 365 nm for 10 minutes while stirring. After that, the support was removed from the mixture and dried to obtain a Pt support.

[0114] 6.2. Test to evaluate the presence or absence of hole separation sites (CoOy loading test) Specifically, 0.1 g of each support, 30 mL of distilled water, and an aqueous solution of Co(NO3)2 containing 0.00005 g of Co were placed in a 50 mL screw-top bottle and mixed to obtain a mixture. Next, the mixture was irradiated with light (ultraviolet light) at a wavelength of 365 nm for 10 minutes while stirring. After that, the support was removed from the mixture and dried to obtain a CoOy support.

[0115] 6.3. Evaluation of carriers, Pt carriers, and CoOy carriers Figure 10 shows, for the photocatalyst obtained in Example 1, an observation image 91 of the carrier 10 before the co-catalyst was supported and its schematic diagram 91A, an observation image 92 of the Pt support after a Pt support test was performed on the carrier 10 shown in observation image 91 and its schematic diagram 92A, and an observation image 93 of the CoOy support after a CoOy support test was performed on the carrier 10 shown in observation image 91 and its schematic diagram 93A.

[0116] In the observation image 91 of Figure 10, it was observed that the step-like structure 2 was present on the photocatalyst carrier 10 obtained in Example 1. Schematic diagram 91A shows that the step-like structure 2 is composed of terraces 21, steps 22, and ridge portions 23.

[0117] In the observation image 92 of Figure 10, Pt particles were observed to have precipitated on terraces 21 and step 22 contained within the step-like structure 2. Schematic diagram 92A schematically shows the precipitated Pt particles. These Pt particles are thought to have precipitated due to the reduction of the precursor. Therefore, the observation image 92 suggests the presence of electron separation sites on terraces 21 and step 22.

[0118] In the observation image 93 of Figure 10, CoOy particles were observed to have precipitated on the ridge portion 23 of the step-like structure 2. Schematic Figure 93A schematically shows the precipitated CoOy particles. These CoOy particles are thought to have precipitated due to oxidation of the precursor. Therefore, the observation image 93 suggests the presence of hole separation sites on the ridge portion 23.

[0119] Figure 11 shows, for the photocatalyst obtained in the comparative example, an observation image 94 of the carrier 10' before the co-catalyst was supported and its schematic diagram 94A, an observation image 95 of the Pt-supported body after a Pt-supporting test was performed on the carrier 10' shown in observation image 94 and its schematic diagram 95A, and an observation image 96 of the CoOy-supported body after a CoOy-supporting test was performed on the carrier 10' shown in observation image 94 and its schematic diagram 96A.

[0120] In the observation image 94 of Figure 11, it was observed that the support 10' of the photocatalyst obtained in the comparative example had many flat surfaces, while there were almost no step-like structures. Schematic Figure 94A schematically shows the absence of step-like structures.

[0121] In the observation image 95 in Figure 11, the deposition of Pt particles on a flat surface was observed. Schematic Figure 95A schematically shows the deposited Pt particles.

[0122] In the observation image 96 of Figure 11, it was observed that CoOy particles were deposited on a flat surface separate from the flat surface where Pt particles were deposited. Schematic Figure 96A schematically shows the deposited CoOy particles.

[0123] 7. Evaluation of the catalytic activity of photocatalysts The catalytic activity of the photocatalysts obtained in each example and comparative example was evaluated for water splitting reactions using the following method.

[0124] First, 0.1 g of photocatalyst was suspended in 50 mL of distilled water to prepare a suspension. Next, the obtained suspension was placed in a glass container and sealed. Then, the suspension was stirred with a stirrer while irradiating the sealed glass container with light of a wavelength of 365 nm from a 200 W high-pressure mercury lamp. Next, the gas generated from the suspension was collected by water displacement through a tube connected to the glass container. The gas generation rate was then measured daily while monitoring the light irradiation for 11 days.

[0125] Next, the gas generation rate measured on day 1 was set to 100, and the ratio of the gas generation rate measured on day 11 was calculated. The calculation results are shown in Table 1 (Figure 9).

[0126] As shown in Table 1, the gas generation rate on day 11 of the photocatalysts obtained in each example was almost the same as the gas generation rate on day 1. In other words, high activity could be maintained for a long period of time. In contrast, the gas generation rate on day 11 of the photocatalysts obtained in the comparative example was found to be significantly lower than the gas generation rate on day 1.

[0127] Figure 12 is a graph comparing the gas generation rate over 11 days for the photocatalyst obtained in Example 1 and the photocatalyst obtained in the comparative example. The horizontal axis of Figure 12 represents time (days), and the vertical axis represents the value obtained by converting the volume of gas generated in 24 hours to the volume of gas generated in 1 hour (gas generation rate [mL / h]).

[0128] As shown in Figure 12, the gas generation rate of the photocatalyst obtained in Example 1 remained almost constant for 11 days. In contrast, the gas generation rate of the photocatalyst obtained in the comparative example decreased over time.

[0129] The results above confirm that the present invention makes it possible to realize a highly stable photocatalyst that can maintain high activity for a long period of time. [Explanation of Symbols]

[0130] 1...Photocatalyst, 1'...Photocatalyst, 2...Stepped structure, 2'...Stepped structure, 10...Carrier, 10'...Carrier, 21...Terrace, 22...Step, 23...Ridge section, 31...Co-catalyst, 32...Co-catalyst, 91...Observed image, 91A...Schematic diagram, 92...Observed image, 92A...Schematic diagram, 93...Observed image, 93A...Schematic diagram, 94...Observed image, 94A...Schematic diagram, 95...Observed image, 95A...Schematic diagram, 96...Observed image, 96A...Schematic diagram, H...Height, P...Main surface, W...Width, e - ...electron, h + ...hole, θ...dihedral angle

Claims

1. A photocatalyst having a perovskite crystal, It has a stepped structure consisting of terraces and steps on the surface, The photocatalyst is characterized in that, in the observed image of the surface, the occupancy rate of the projected area of ​​the stepped structure in the total projected area is 20% or more.

2. The photocatalyst according to claim 1, wherein the stepped structure is a structure in which the terrace and the steps are repeated alternately multiple times.

3. The aforementioned perovskite-type crystal is cubic, The photocatalyst according to claim 1 or 2, wherein the surface is a flat surface composed of a {100} plane.

4. The terrace is composed of {100} surfaces. The photocatalyst according to claim 3, wherein the step is comprised of a {100} plane or a {110} plane.

5. The photocatalyst according to claim 4, wherein the dihedral angle between the terrace and the step is 90° or more.

6. The photocatalyst according to claim 3, wherein the stepped structure is distributed so as to surround the flat surface.

7. The photocatalyst according to claim 1 or 2, wherein the width of the terrace is 3 nm or more and 25 nm or less.

8. The photocatalyst according to claim 1 or 2, wherein the photocatalyst is particulate with an average particle diameter of 50 nm or more and 30,000 nm or less.

9. The photocatalyst according to claim 1 or 2, having a co-catalyst that contacts the stepped structure.

10. The perovskite-type crystal contains strontium titanate, The photocatalyst according to claim 9, wherein the co-catalyst comprises a rhodium-chromium mixed oxide, a rhodium-chromium composite oxide, or cobalt oxide.

11. A photocatalyst according to claim 1 or 2, used in a water splitting reaction.

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