Visible light-responsive photocatalyst

Abstract

To provide a visible light-responsive photocatalyst having superior photocatalytic effect.SOLUTION: The present invention relates to a visible light-responsive photocatalyst comprising gallium oxyhydroxide, and also relates to a visible light-responsive photocatalyst comprising gallium oxyhydroxide or gallium oxide, and zinc, zinc oxide, titanium, or titanium oxide.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a visible light responsive photocatalyst. [Background technology]

[0002] Photocatalysts absorb light energy and generate electrons (e - ) and holes (h + ) pair, but this hole (h + ) converts the moisture on the catalyst surface into hydroxyl radicals, which have strong oxidizing power, and these hydroxyl radicals decompose organic matter such as pollutants, dirt, and bacteria, thereby exerting their function. The effect is proportional to the amount of light energy absorbed and is related to the band gap energy of the photocatalyst. The lower the band gap energy Eg, the greater the light energy that can be absorbed, and is related to the upper limit of the wavelength λ that can be absorbed by the following formula: Eg = Planck's constant × speed of light / λ = 1240 / λ

[0003] Titanium oxide (TiO2), which is widely used as a photocatalyst, has a bandgap energy of 3.2 eV and can utilize ultraviolet light with a wavelength of 388 nm or less, but it can only utilize just under 3% of the total energy of sunlight. Patent Document 1 discloses an efficient method for producing monoclinic bismuth vanadate (BiVO4), which has an even lower bandgap energy of 2.4 eV and can utilize the energy of visible light and ultraviolet light with a wavelength of 517 nm or less, but it can only utilize just under 19% of the total energy of sunlight.

[0004] Other known visible light responsive photocatalysts include graphite carbon nitride g-C3N4 (Non-Patent Document 1) and a calcined product of melamine C3H6N6 and BiVO4 (Patent Document 2). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2020-111476 [Patent Document 2] Patent Publication No. 2021-137789 [Non-patent literature]

[0006] [Non-Patent Document 1] "GS Alliance synthesizes and commercializes graphitic carbon nitride (g-C3N4), a visible-light-responsive photocatalyst that can also be used in artificial photosynthesis," published October 4, 2018, https: / / www.atpress.ne.jp / news / 167523 Summary of the Invention [Problem to be solved by the invention]

[0007] The visible light responsive photocatalysts disclosed in Patent Documents 1 and 2 and Non-Patent Document 1 do not have sufficient photocatalytic effects. An object of the present invention is to obtain a visible light responsive photocatalyst that has excellent photocatalytic effects. [Means for solving the problem]

[0008] The present inventors have discovered that gallium oxyhydroxide exhibits a higher photocatalytic effect than gallium oxide, which is known to have a photocatalytic effect, and have completed the present invention. Furthermore, the present inventors have discovered that adding titanium, titanium oxide, zinc, or zinc oxide to gallium oxyhydroxide or gallium oxide exhibits a higher visible light-responsive photocatalytic effect than a photocatalyst using gallium oxide alone, and have completed the present invention.

[0009] That is, the present invention (1) is a visible light responsive photocatalyst containing gallium oxyhydroxide.

[0010] The present invention (2) is a visible light responsive photocatalyst containing gallium oxyhydroxide or gallium oxide, and titanium, titanium oxide, zinc, or zinc oxide.

[0011] The present invention (3) is the visible light responsive photocatalyst according to the present invention (2), which has an average particle size of 0.01 to 15 μm.

[0012] The present invention (4) is the visible light responsive photocatalyst according to the present invention (2) or (3), wherein the average long side is 0.04 to 15 μm and the average short side is 0.01 to 10 μm.

[0013] The present invention (5) provides a method for dispersing gallium in water by ultrasonic irradiation; A step of removing water from the obtained dispersion and drying it The method for producing the visible light responsive photocatalyst according to any one of the present inventions (1) to (4) includes the steps of:

[0014] The present invention (6) is the method for producing a visible light responsive photocatalyst according to the present invention (5), wherein the frequency of the ultrasonic waves to be irradiated is 10 to 100 kHz.

[0015] The present invention (7) is a method for producing a visible-light responsive photocatalyst according to the present invention (5) or (6), wherein the ultrasonic irradiation time is 1 to 5 hours.

[0016] The present invention (8) is a method for producing a visible light responsive photocatalyst according to any one of the present inventions (5) to (7), further comprising a step of heat-treating the particles obtained by drying at a temperature of 400°C or less.

[0017] The present invention (9) is a method for producing a visible light responsive photocatalyst according to any one of the present inventions (5) to (8), wherein zinc, zinc oxide, titanium or titanium oxide is dispersed together with gallium in the dispersion step. [Effects of the Invention]

[0018] The visible light-responsive photocatalyst of the present invention exhibits a higher visible light catalytic effect than a photocatalyst made only of gallium oxide, which is known to have a photocatalytic effect. Furthermore, the production method of the present invention can utilize existing equipment as is, achieving low cost photocatalysts, and is expected to have effects such as water purification by decomposing organic environmental pollutants under visible light, hydrogen generation by water decomposition, sterilization, and deodorization. [Brief explanation of the drawings]

[0019] [Figure 1] 1 shows XRD charts of particles produced in Examples 1 to 3 and Comparative Examples 1 to 5. [Figure 2] 1 shows the photocatalytic effects of the visible light responsive photocatalysts prepared in Examples 1 to 3 and Comparative Examples 1 to 5. [Figure 3] 1 shows XRD charts of the visible light responsive photocatalysts prepared in Examples 1, 4, 6, and 7. [Figure 4] The photocatalytic effects of no photocatalyst, Ga2O3 (commercially available), and the visible light responsive photocatalyst prepared in Example 4 are shown. [Figure 5] 1 shows the photocatalytic effects of the visible light responsive photocatalysts prepared without a photocatalyst and in Examples 1 and 5 to 7. [Figure 6] 1 shows the photocatalytic effects of the visible light responsive photocatalysts prepared in Examples 7 to 11 and without the photocatalyst. [Figure 7] 1 shows a scanning electron microscope photograph of the gallium oxyhydroxide particles produced in Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0020] <<Visible light responsive photocatalyst>> The first visible light responsive photocatalyst of the present invention is characterized by containing gallium oxyhydroxide. Gallium oxyhydroxide has a higher photocatalytic effect than gallium oxide, which is known to have a photocatalytic effect. Note that as long as it contains gallium oxyhydroxide, it may contain other impurities such as zinc and titanium.

[0021] Gallium oxyhydroxide (GaOOH) can be synthesized by irradiating liquid gallium metal with ultrasound in water. Details will be explained later.

[0022] The second visible light responsive photocatalyst of the present invention is characterized by containing gallium oxyhydroxide or gallium oxide, and zinc, zinc oxide, titanium or titanium oxide.

[0023] Gallium oxide (GaO) can be synthesized by oxidizing Ga metal, for example, by heat treatment in the atmosphere, and depending on the heat treatment temperature, α-, β-, γ-, δ-, and ε-types are produced. In the present invention, any of the types can be used, but α- and β-types are preferred, and α-type is more preferred.

[0024] In the visible light responsive photocatalyst, when zinc or zinc oxide is contained, the content of gallium oxyhydroxide or gallium oxide is preferably 0.001 to 50 mass%, more preferably 0.01 to 30 mass%, while when titanium or titanium oxide is contained, the content of titanium is preferably 0.001 to 10 mass%, more preferably 0.05 to 5 mass%.

[0025] The average particle size of the visible light responsive photocatalyst is preferably 0.01 to 15 μm, more preferably 0.1 to 1.5 μm. Furthermore, the visible light responsive photocatalyst is preferably a particle having a square pillar shape. The average long side of the square pillar is preferably 0.04 to 15 μm, more preferably 40 to 1000 nm, and the average short side is preferably 0.010 to 10 μm, more preferably 10 to 800 nm.

[0026] <<Method for producing visible light responsive photocatalyst>> The method for producing a visible light responsive photocatalyst of the present invention is characterized by comprising the steps of dispersing gallium in water by ultrasonic irradiation, and removing water from the resulting dispersion and drying it. By irradiating ultrasonic waves in water, gallium can be converted into gallium hydroxide.

[0027] It is preferable to use purified water or ultrapure water as the water. The amount of water added is preferably 0.1 to 100 ml, more preferably 1 to 50 ml, per 1 g of gallium. If a large amount of water is used during ultrasonic irradiation, the particle size of the gallium hydroxide particles tends to increase. Liquid gallium metal is used as the gallium. Since the melting point of gallium is 25°C, if the reaction system is below 25°C, it is preferably heated to 30°C or higher, more preferably 40°C or higher.

[0028] The frequency of the ultrasonic waves to be irradiated is preferably 10 to 100 kHz, more preferably 20 to 50 kHz, and the duration of the ultrasonic irradiation is preferably 1 to 50 hours, more preferably 2 to 5 hours.

[0029] In the dispersion step, it is preferable to disperse zinc, zinc oxide, titanium, or titanium oxide together with gallium in order to improve the photocatalytic effect. Zinc and zinc oxide can be uniformly mixed with gallium, so they can be dispersed together with gallium as they are. On the other hand, it is not easy to uniformly mix titanium and titanium oxide with gallium, so it is preferable to previously calcinate gallium, titanium, or titanium oxide to obtain a powder, disperse it in water, and then irradiate it with ultrasound. The calcination temperature when previously calcining gallium, titanium, or titanium oxide is preferably 300 to 1500°C, and more preferably 600 to 1200°C. The calcination time is preferably 2 to 200 minutes, and more preferably 10 to 100 minutes.

[0030] It is preferable that the method further includes a step of heat-treating the particles obtained by drying at a temperature of 400° C. or less. The heat-treatment temperature is more preferably 100° C. or less. Heat-treatment at a temperature exceeding 400° C. tends to produce γ-, δ-, and ε-types, resulting in a decrease in photocatalytic activity.

[0031] The photocatalytic activity of a visible light-responsive photocatalyst can be evaluated by a decolorization test using, for example, a methylene blue solution or a Congo red solution. The ease of decomposition varies depending on whether the dye used is cationic or anionic, but the visible light-responsive photocatalyst of the present invention tends to decompose Congo red more easily than methylene blue.

[0032] In the case of methylene blue, a 0.5 mM methylene blue solution is mixed with a visible light responsive photocatalyst and then irradiated with visible light for 1 hour. The visible light responsive photocatalyst is then removed by centrifugation, and the light transmittance of the resulting solution at a wavelength of 662 nm is measured. The recovery rate calculated using the following formula is preferably 80% or more, more preferably 85% or more, and even more preferably 90% or more. Recovery rate [%] = (solution transmittance / water transmittance) x 100

[0033] In the case of Congo Red, a 0.1 mM Congo Red solution is mixed with a visible light-responsive photocatalyst and then irradiated with visible light for 1 hour. The visible light-responsive photocatalyst is then removed by centrifugation, and the light transmittance of the resulting solution at a wavelength of 497 nm is measured. The recovery rate calculated using the following formula is preferably 80% or more, more preferably 85% or more, and even more preferably 90% or more. Recovery rate [%] = (solution transmittance / water transmittance) x 100

[0034] The visible light responsive photocatalyst of the present invention can be suitably used for deodorization, sterilization, and antifouling purposes. It can also be applied to water systems to be purified using visible light, and is particularly suitable for decomposing endocrine disruptors such as nonylphenol, bisphenol A, and natural estrogens present in the water systems to be purified. [Example]

[0035] Examples of the present invention will be described below, but the present invention is not limited to the following examples.

[0036] Example 1 1 g of Ga and 10 ml of purified or ultrapure water were placed in a screw cap vial and irradiated with 42 kHz ultrasound at 50°C for 2 hours to disperse the Ga into particles. The dispersion was transferred from the screw cap vial to a petri dish and then evaporated in a drying oven at 120°C for 2 hours to produce gallium oxyhydroxide (GaOOH) particles. Figure 7 shows a scanning electron microscope image of the resulting gallium oxyhydroxide particles. The average particle diameter of the visible light-responsive photocatalyst was 1 μm. The average long side was 315 nm and the average short side was 71 nm.

[0037] Examples 2 to 3, Comparative Examples 1 to 5 The gallium oxyhydroxide produced in Example 1 was heat-treated (annealed) for 4 hours at 250°C (Example 2), 350°C (Example 3), 450°C (Comparative Example 1), 550°C (Comparative Example 2), 650°C (Comparative Example 3), 750°C (Comparative Example 4), and 850°C (Comparative Example 5) to produce the photocatalysts of Examples 2-3 and Comparative Examples 1-5.

[0038] <Crystalline> XRD measurements were performed on the obtained particles. The results are shown in Figure 1. Particles without heat treatment showed a GaOOH peak, particles heat-treated at 400°C and 500°C showed an α-Ga2O3 peak, and particles heat-treated at 600°C or higher showed a β-Ga2O3 peak. This confirmed that as the firing temperature increased, the crystal structure changed from GaOOH to α-Ga2O3 and then to β-Ga2O3.

[0039] <Photocatalytic effect> The photocatalytic effect of the particles prepared in Examples 2 and 3 and Comparative Examples 1 to 5 was measured (however, new catalysts prepared by heat treatment at a temperature 50°C lower than the sample used for XRD measurement were used). Specifically, 3 mL of a 0.1 mM Congo Red (CR) solution and 0.1 g of the prepared photocatalyst were placed in a plastic container and irradiated with visible light for 10 minutes, followed by centrifugation using a centrifuge tube (13,200 rpm, 30 minutes). The supernatant was transferred to a quartz cell, and the transmittance was measured using a spectrophotometer. The results are shown in Figure 2. An increase in transmittance was confirmed for the photocatalyst that was not heat-treated (Example 1) and the photocatalyst that was heat-treated at 400°C or less.

[0040] Example 4 Ga2O3 (commercially available, a mixture of α- and β-types) and ZnO were mixed in a molar ratio of 1:2 (Ga2O3: 0.18744 g, ZnO: 0.16278 g) and fired in a tubular furnace at 1000°C for 12 hours while flowing Ar (20 sccm) to produce the photocatalyst of Example 4 (a mixture of Ga2O3 and ZnO). The average particle diameter of the visible light-responsive photocatalyst was 1 μm. The average long side was 1000 nm and the average short side was 400 nm.

[0041] Example 5 The photocatalyst of Example 5 (a mixture of GaOOH and ZnO) was prepared in the same manner as in Example 4, except that the GaOOH prepared in Example 1 was used instead of Ga2O3. The average particle diameter of the visible light-responsive photocatalyst was 1.0 μm. The average long side was 800 nm, and the average short side was 400 nm.

[0042] Example 6 Purified water or ultrapure water and 1 g of liquid Ga metal were placed in a 13.5 cc screw cap bottle, and 0.01 g (1 wt%) of ZnO powder was added to the pure water. Then, 42 kHz ultrasound was applied at 50°C for 2 hours. The suspended solution was transferred to a petri dish, and the pure water was evaporated in a drying oven to produce the photocatalyst of Example 6 (a mixture of GaOOH and ZnO). The liquid Ga was converted to GaOOH by ultrasound irradiation. The average particle diameter of the visible light-responsive photocatalyst was 1 μm. The average long side was 800 nm, and the average short side was 400 nm.

[0043] Example 7 0.01 g (1 wt%) of Ti wire was added to 1 g of liquid Ga metal and calcined in an electric furnace (in air) at 1000 °C for 60 minutes. The resulting calcined product was placed in a 13.5 cc screw cap vial together with purified or ultrapure water and irradiated with 42 kHz ultrasound at 50 °C for 2 hours. The suspended solution in the screw cap vial after ultrasonic treatment was transferred to a petri dish and evaporated in a drying oven at 120 °C for 2 hours to obtain the photocatalyst of Example 4 (a mixture of GaOOH and Ti or TiO). The liquid Ga was converted to liquid metal Ga with at least partial β-GaO oxide present on the surface and titanium mixed inside by calcination, and then converted to GaOOH particles containing titanium or titanium oxide by subsequent ultrasound irradiation. The average particle diameter of the visible light-responsive photocatalyst was 0.5 μm. The average long side was 800 nm and the average short side was 400 nm. However, some were over 10 μm, and some were less than 100 nm in length.

[0044] <Crystallinity (XRD)> Figure 3 shows the XRD measurement results for the photocatalysts prepared in Examples 1, 4, 6, and 7, and Ga2O3 (commercially available, a mixture of α- and β-types). In Examples 6 and 7, GaOOH peaks were observed at 23°, 36°, and 55°, indicating the presence of GaOOH in the photocatalyst. On the other hand, in Example 4, Ga2O3 peaks were observed, indicating the presence of Ga2O3 in the photocatalyst.

[0045] In the photocatalyst of Example 4, which used ZnO, the peak at 23° disappeared. Furthermore, in the photocatalysts of Examples 4 and 6, ZnO peaks were observed at 31° and 36°, indicating the presence of ZnO in the photocatalyst. In Example 7, Ti peaks were observed at 62° and 38°, indicating the presence of Ti in the photocatalyst.

[0046] <Photocatalytic effect> 3 mL of 0.1 mM Congo Red (CR) solution and 0.1 g of the photocatalysts prepared in Preparation Example 1 and Examples 4 to 7 were placed in a plastic container and irradiated with visible light for 10 minutes. After that, the container was centrifuged (13,200 rpm, 30 minutes) using a centrifuge tube. The supernatant was transferred to a quartz cell, and the transmittance was measured using a spectrophotometer. The results are shown in Figures 2 and 3.

[0047] From Figure 4, it can be seen that the photocatalyst of Example 4, which contains ZnO, exhibits a higher photocatalytic effect than the commercially available Ga2O3. Note that the figure shows the results for Congo Red, which is difficult to decompose, but it is estimated that the difference between the two would be greater if methylene blue, which is easily decomposed, were used.

[0048] 5 shows that the photocatalytic effect on the resistant Congo Red was large even when GaOOH was used alone in Example 1. The photocatalysts of Examples 5 and 6, which contained ZnO, and the photocatalyst of Example 7, which contained Ti and / or TiO, were found to have an even greater photocatalytic effect on the resistant Congo Red.

[0049] Examples 8 to 11 The same experiment as in Example 7 was conducted, except that 0.01 g (1 wt%) of Ti wire was replaced with 0.0001 g (0.01 wt%), 0.001 g (0.1 wt%), 0.005 g (0.5 wt%), and 0.02 g (2.0 wt%) of Ti wire. The photocatalysts (mixtures of GaOOH and Ti or TiO) of Examples 8 to 11 were obtained. The average particle diameters of the visible light-responsive photocatalysts were 1 μm (Example 8), 1 μm (Example 9), 1 μm (Example 10), and 1 μm (Example 11), respectively. The average long and short sides were 800 nm, 400 nm (Example 8), 800 nm, 400 nm (Example 9), 800 nm, 400 nm (Example 10), 800 nm, and 400 nm (Example 11), respectively.

[0050] The photocatalytic effect was measured in the same manner as in Example 7, and the results are shown in Figure 6. Even when the Ti content was 0.01 wt%, a high photocatalytic effect was obtained. [Industrial Applicability]

[0051] The present invention provides a visible-light-responsive photocatalyst with excellent photocatalytic activity, which can purify environmental pollutants, for example, by using visible light to decompose endocrine disruptors such as nonylphenol, bisphenol A, and natural estrogens contained in water systems to be purified.

Claims

1. A visible light-responsive photocatalyst containing gallium oxyhydroxide.

2. A visible light responsive photocatalyst comprising gallium oxyhydroxide or gallium oxide and zinc, zinc oxide, titanium or titanium oxide.

3. 3. The visible light responsive photocatalyst according to claim 2, wherein the average particle size is 0.01 to 15 μm.

4. 4. The visible light responsive photocatalyst according to claim 2, wherein the average long side is 0.04 to 15 μm and the average short side is 0.01 to 10 μm.

5. Dispersing gallium in water by ultrasonic irradiation; and A step of removing water from the obtained dispersion and drying it 3. A method for producing the visible light responsive photocatalyst according to claim 1 or 2, comprising:

6. 6. The method for producing a visible light responsive photocatalyst according to claim 5, wherein the frequency of the ultrasonic waves to be irradiated is 10 to 100 kHz.

7. 6. The method for producing a visible light responsive photocatalyst according to claim 5, wherein the ultrasonic irradiation time is 1 to 5 hours.

8. The method for producing a visible light responsive photocatalyst according to claim 5, further comprising a step of heat treating the particles obtained by drying at a temperature of 400°C or less.

9. 6. The method for producing a visible light responsive photocatalyst according to claim 5, wherein zinc, zinc oxide, titanium or titanium oxide is dispersed together with gallium in the dispersing step.

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