SEMICONDUCTOR PARTICLES USED IN WATER-SPLITTING PHOTOCATALYST, PHOTOCATALYST FORMED OF SEMICONDUCTOR PARTICLES, AND SYNTHESIS METHOD THEREOF

Abstract

Semiconductor particles are used as a photocatalyst for inducing a water-splitting reaction where water molecules decompose into oxygen molecules and hydrogen molecules by addition of a co-catalyst and light irradiation, the semiconductor particles including strontium titanate doped with scandium. A synthesis method of a semiconductor for the photocatalyst includes a synthesis step of synthesizing the semiconductor particles including strontium titanate doped with scandium by mixing strontium chloride (SrCl.sub.2), strontium titanate (SrTiO.sub.3), and scandium oxide (Sc.sub.2O.sub.3) and firing the mixture.

Claims

1. Semiconductor particles that are used as a photocatalyst for inducing a water-splitting reaction where water molecules decompose into oxygen molecules and hydrogen molecules by light irradiation with a co-catalyst, the semiconductor particles comprising strontium titanate doped with scandium.

2. The semiconductor particles according to claim 1, wherein the semiconductor particles are synthesized by mixing strontium chloride (SrCl.sub.2), strontium titanate (SrTiO.sub.3), and scandium oxide (Sc.sub.2O.sub.3) to obtain a mixture and firing the mixture.

3. The semiconductor particles according to claim 2, wherein the semiconductor particles are synthesized by mixing the scandium oxide at a molar ratio of 0.001 to 0.05 with respect to strontium titanate.

4. The semiconductor particles according to claim 3, wherein the semiconductor particles are synthesized by mixing the scandium oxide at a molar ratio of 0.001 to 0.007 with respect to strontium titanate.

5. The semiconductor particles according to claim 1, characterized in that the semiconductor particles are further doped with aluminum.

6. The semiconductor particles according to claim 5, wherein the semiconductor particles are synthesized by mixing strontium chloride (SrCl.sub.2), strontium titanate (SrTiO.sub.3), scandium oxide (Sc.sub.2O.sub.3) to obtain a mixture, and aluminum oxide (Al.sub.2O.sub.3) and firing the mixture.

7. The semiconductor particles according to claim 6, wherein the semiconductor particles are synthesized by mixing aluminum oxide at a molar ratio of 0.02 with respect to strontium titanate.

8. The semiconductor particles according to claim 2, wherein a firing temperature is 1150° C. to 1200° C. and a firing time is 10 hours to 30 hours when the semiconductor particles are synthesized.

9. A photocatalyst comprising: the semiconductor particles according to claim 1; and rhodium chromium oxide (Rh/Cr.sub.2O.sub.3) and cobalt hydroxide oxide (CoOOH) added as a co-catalyst to the semiconductor particles.

10. The photocatalyst according to claim 9, wherein the co-catalyst is added by photodeposition to surfaces of the semiconductor particles dispersed in water.

11. A synthesis method of semiconductor particles including strontium titanate in a photocatalyst for inducing a water-splitting reaction where water molecules decompose into oxygen molecules and hydrogen molecules by light irradiation, the photocatalyst being formed by adding a co-catalyst to the semiconductor particles including strontium titanate, and the synthesis method comprising a synthesis step of synthesizing the semiconductor particles including strontium titanate doped with scandium by mixing strontium chloride (SrCl.sub.2), strontium titanate (SrTiO.sub.3), and scandium oxide (Sc.sub.2O.sub.3) to obtain a mixture and firing the mixture.

12. The synthesis method according to claim 11, wherein in the synthesis step, the scandium oxide is mixed at a molar ratio of 0.001 to 0.05 with respect to strontium titanate.

13. The synthesis method according to claim 12, wherein in the synthesis step, the scandium oxide is mixed at a molar ratio of 0.001 to 0.007 with respect to strontium titanate.

14. The synthesis method according to claim 11, wherein in the synthesis step, the semiconductor particles are further doped with aluminum by further mixing aluminum oxide (Al.sub.2O.sub.3) and firing the mixture.

15. The synthesis method according to claim 14, wherein in the synthesis step, aluminum oxide is mixed at a molar ratio of 0.02 with respect to strontium titanate.

16. The synthesis method according to claim 11, wherein in the synthesis step, a firing temperature is 1150° C. to 1200° C. and a firing time is 10 hours to 30 hours.

17. A synthesis method of synthesizing a photocatalyst using semiconductor particles synthesized using the synthesis method according to claim 11, the photocatalyst inducing a water-splitting reaction where water molecules decompose into oxygen molecules and hydrogen molecules by light irradiation, and the synthesis method of synthesizing the photocatalyst comprising an addition step of adding the co-catalyst to surfaces of the semiconductor particles dispersed in water.

18. The synthesis method according to claim 17, wherein the co-catalyst is rhodium chromium oxide (Rh/Cr.sub.2O.sub.3) and cobalt hydroxide oxide (CoOOH).

19. The synthesis method according to claim 18, wherein in the addition step, the co-catalyst is added by photodeposition to the surfaces of the semiconductor particles dispersed in water.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Features, advantages, and technical and industrial significance of exemplary embodiments of the disclosure will be described below with reference to the accompanying drawings, in which like signs denote like elements, and wherein:

[0018] FIG. 1 is a schematic diagram showing a step of a synthesis method of semiconductor particles according to the disclosure and a photocatalyst formed of the semiconductor particles;

[0019] FIG. 2 is an electron microscope image of the photocatalyst synthesized using the method according to the disclosure; and

[0020] FIG. 3 shows a quantum efficiency that is obtained by the photocatalyst synthesized using the method according to the disclosure, the quantum efficiency being a value when scandium doped with strontium titanate is changed. The bar graph represents an average value, and the error bar represents a standard deviation (the number of trials for each condition was three). The semiconductor particles were synthesized by mixing SrTiO.sub.3 and Sc.sub.2O.sub.3 or further mixing Al.sub.2O.sub.3 in SrCl.sub.2 and firing the mixture at a firing temperature of 1200° C. for 30 hours.

DETAILED DESCRIPTION OF EMBODIMENTS

[0021] Synthesis Method of Semiconductor Particles for Water-Splitting Reaction Photocatalyst and Photocatalyst

[0022] Semiconductor particles for a water-splitting reaction photocatalyst according to an embodiment are synthesized by mixing strontium chloride (SrCl.sub.2), strontium titanate (SrTiO.sub.3), and scandium oxide (Sc.sub.2O.sub.3) or further mixing aluminum oxide (Al.sub.2O.sub.3) to obtain a raw material mixture, heating the raw material mixture to a temperature at which SrCl.sub.2 melts to become a liquid (temperature at which SrTiO.sub.3, Sc.sub.2O.sub.3, and Al.sub.2O.sub.3 do not melt), and doping SrTiO.sub.3 with scandium atoms (Sc) or further with aluminum atoms (Al) to form a semiconductor of SrTiO.sub.3 (flux method). A photocatalyst is synthesized by adding a material as a co-catalyst, for example, by photodeposition to the semiconductor particles obtained using the flux method.

[0023] Specifically, in the synthesis method of the semiconductor particles, first, SrTiO.sub.3 powder and Sc.sub.2O.sub.3 powder or further Al.sub.2O.sub.3 powder are mixed with a large amount of SrCl.sub.2 powder with reference to FIG. 1. (step A of FIG. 1). The powders are mixed, for example, by grinding the powders in an agate mortar M (about 30 minutes). In the mixture (raw material mixture) of the powders, the amount of SrCl.sub.2 may be about 10 times with respect to the amount of SrTiO.sub.3, for example, by molar ratio. When the powders are used, the amount of the Al.sub.2O.sub.3 powder may be the same as described in T. Takata et al., Nature, Volume 581, 411-414 (2020) and may be, for example, about 0.02 by molar ratio with respect to the amount of SrTiO.sub.3. According to an experiment by the present inventors, the amount of the Sc.sub.2O.sub.3 powder may be 0.001 to 0.05 by molar ratio with respect to the amount of SrTiO.sub.3 and found that, when the molar ratio of the Sc.sub.2O.sub.3 powder is 0.001 to 0.007, 0.001 to 0.005, or 0.003 to 0.005, a photocatalyst with a high quantum efficiency can be obtained.

[0024] Next, the raw material mixture of the powders is transferred to a crucible for firing, for example, an alumina crucible C and is fired in a firing furnace H (step B in FIG. 1). In this step, the firing temperature is a temperature (874° C. or higher) at which SrCl.sub.2 melts to become a liquid as described above, may be a temperature (2072° C. or lower) at which SrTiO.sub.3, Sc.sub.2O.sub.3, and Al.sub.2O.sub.3 do not melt, may be, for example, about 1000° C. to 1400° C., and may be 1150° C. to 1200° C. In addition, the firing time for which the raw material mixture is exposed to the temperature is a period of time that is sufficient for doping SrTiO.sub.3 with Sc or further with Al to form a semiconductor of SrTiO.sub.3. According to the experiment by the present inventors, the firing time may be 10 hours to 30 hours or about 30 hours.

[0025] When the fired material is cooled to normal temperature after the firing step, water (or distilled water) is added to the crucible C, the fired material in the crucible is dispersed in water as particles while ultrasonic waves are applied to the fired material using an ultrasonic stirrer or the like to stir the fired material, and the fired material is further collected by filtration under reduced pressure or the like. The particulate fired material is semiconductor particles (Sc—SrTiO.sub.3 or Al—Sc—SrTiO.sub.3) for the photocatalyst according to the embodiment. The collected semiconductor particles may be cleaned with water. The semiconductor particles may be cleaned until the pH of cleaning water is 7 and the cleaning water does not include chlorine. Thus, the semiconductor particles may be dried after being cleaned.

[0026] In order to function the semiconductor particles as a photocatalyst, the co-catalyst is added to crystal facets of the semiconductor particles. It is presumed that the co-catalyst prevents charge (electrons and holes) produced by the light irradiation in the semiconductor particles and transferred to surfaces of the semiconductor particles in a photoinduced water-splitting reaction from being transferred backward again to the inside of the semiconductor particles. In the embodiment, the co-catalyst may be added to the semiconductor particles using any method. Typically, as described above, by depositing the co-catalyst on the crystal facets of the semiconductor particles dispersed in water by photodeposition as in T. Takata et al., Nature, Volume 581, 411-414 (2020), the co-catalyst may be added to the semiconductor particles. Specifically, first, the semiconductor particles are dispersed in water in a transparent container V such as a glass container (step C in FIG. 1). Ultrasonic waves are applied to water W (semiconductor particle dispersion liquid) in which the semiconductor particles are dispersed such that the semiconductor particles are uniformly dispersed. Next, a treatment of adding a salt as a raw material of the co-catalyst and depositing a metal oxide as the co-catalyst on surfaces of the semiconductor particles by irradiation of light P is executed (step D in FIG. 1).

[0027] More specifically, when rhodium chromium oxide (Rh/Cr.sub.2O.sub.3) and cobalt hydroxide oxide (CoOOH) are used as the co-catalyst to be added to the surfaces of the semiconductor particles as in T. Takata et al., Nature, Volume 581, 411-414 (2020), the following step may be executed. That is, first, a rhodium chloride (RhCl.sub.3) aqueous solution is added to the semiconductor particle dispersion liquid such that the amount of rhodium (Rh) is 0.1 wt % with respect to the amount of the semiconductor particles, and the semiconductor particle dispersion liquid is irradiated with light from a xenon lamp (300 W, 20 mA) at the atmospheric pressure for 10 minutes. Next, a potassium chromate (K.sub.2CrO.sub.4) aqueous solution is added to the semiconductor particle dispersion liquid such that the amount of chromium (Cr) is 0.05 wt % with respect to the amount of the semiconductor particles, and the semiconductor particle dispersion liquid is irradiated with light from a xenon lamp (300 W, 20 mA) at the atmospheric pressure for 5 minutes. Next, a cobalt nitrate (Co(NO.sub.3).sub.2) aqueous solution is added to the semiconductor particle dispersion liquid such that the amount of cobalt (Co) is 0.05 wt % with respect to the amount of the semiconductor particles, and the semiconductor particle dispersion liquid is irradiated with light from a xenon lamp (300 W, 20 mA) at the atmospheric pressure for 5 minutes. As a result, as schematically illustrated in step D of FIG. 1, the Rh—Cr oxide and the Co hydroxide oxide are deposited on the surfaces of the semiconductor particles, and thus the prepared composite functions as a photocatalyst for inducing a photoinduced water-splitting reaction. FIG. 2 is an electron microscope image of the prepared photocatalyst, and the photocatalyst has a form of particles as illustrated in the drawing. In the addition step of adding the co-catalyst to surfaces of the semiconductor particles by photodeposition, the concentration of the salt for the co-catalyst to be added to the semiconductor particle dispersion liquid may be appropriately adjusted. It was found that, as in Experimental Examples described below, for example, when the concentration of the salt for the co-catalyst is four times that described above, the quantum efficiency significantly decreases. In some embodiments, the salt concentration is adjusted not to be excessive. In addition, the addition of the co-catalyst to the surfaces of the semiconductor particles may be executed using an impregnation method (method of adding the salt to the dispersion liquid and applying heat) in addition to photodeposition.

[0028] In the photocatalyst that is prepared using the semiconductor particles formed by doping strontium titanate with scandium as described above in the embodiment, as described below, the average value of the quantum efficiency [hydrogen molecules×2/number of emitted photons] in the photoinduced water-splitting reaction is at least 50%, more than 75%, and the standard deviation is about 1% (0.4% to 4.4%). Accordingly, in the synthesis method of the semiconductor particles and the photocatalyst according to the embodiment, as compared to a technique in the related art where scandium is not used (the average value of the external quantum efficiency is 16% to 67%, the standard deviation is 12% (3.2% to 12.2%)), semiconductor particles and a photocatalyst that have a smaller variation and stably achieve a higher quantum efficiency can be provided.

[0029] The reason why the photocatalyst that is formed of the semiconductor particles obtained by doping strontium titanate with scandium stably exhibits a high quantum efficiency is presumed to be as follows. According to a current investigation, it was verified that the shape of the semiconductor particles doped with scandium is a polyhedron having a larger roundness (refer to FIG. 2). Therefore, in the rounded polyhedral structure, corners of the cubic are cut, such that most of facets of strontium titanate are exposed, and thus a face where electrons are likely to accumulate and a face where holes are likely to accumulate are formed. As a result, charge separation is further accelerated, and the water-splitting reaction is caused to efficiently occur.

Experimental Examples

[0030] Semiconductor particles formed by doping strontium titanate with scandium and a photocatalyst formed of the semiconductor particles were synthesized according to the instructions of the embodiment. The quantum efficiency of the photocatalyst was measured, and the effectiveness of the embodiment was verified. It should be understood that the following experimental examples are examples for verifying the effectiveness of the embodiment and do not limit the range of the disclosure.

[0031] The synthesis of the semiconductor particles was performed in the following steps. Specifically, first, SrCl.sub.2 powder, SrTiO.sub.3 powder, and Sc.sub.2O.sub.3 powder or further Al.sub.2O.sub.3 powder were crushed and mixed in an agate mortar for 30 minutes. A molar ratio between SrCl.sub.2, SrTiO.sub.3, and Al.sub.2O.sub.3 (only when used) was 10:1:0.02. The molar ratio of Sc.sub.2O.sub.3 was 0.001 to 0.05 (0.1 mol % to 5 mol %) with respect to SrTiO.sub.3. The mixture of the powders was transferred to an alumina crucible and was fired in a firing furnace while setting the firing temperature and the firing time to various values. In the firing step, the mixture was heated from room temperature to the firing temperature for 2 hours, and was allowed to cool to room temperature for 6 hours after the elapse of the firing time. After allowing the mixture to cool, distilled water was added to the crucible containing the fired material, ultrasonic waves were applied thereto using an ultrasonic stirrer to stir the fired material, and the fired material in the crucible (those deposited to the inner wall of the crucible were dispersed) was dispersed in water in the form of particles and collected by filtration under reduced pressure. Next, the collected particulate fired material was cleaned with distilled water. When the pH of water after cleaning was checked using pH test paper whether or not chlorine was present in the water after cleaning was checked based on whether or not silver chloride was produced after adding 0.1 M silver nitrate to the water after cleaning, the pH of the water after cleaning was 7, and chlorine was not detected. Therefore, the fired material was cleaned. The cleaned particulate fired material, that is, the semiconductor particles were dried at 70° C.

[0032] In the preparation of the photocatalyst using the semiconductor particles, 100 mg of the semiconductor particle powder was dispersed in 100 ml of distilled water in a heat-resistant glass container (400 ml). First, a rhodium chloride (RhCl.sub.3) aqueous solution was added to the semiconductor particle dispersion liquid such that the amount of rhodium (Rh) was 0.1 wt % with respect to the amount of the semiconductor particles, and the semiconductor particle dispersion liquid was irradiated with light from a xenon lamp (300 W, 20 mA) at the atmospheric pressure for 10 minutes. Next, a potassium chromate (K.sub.2CrO.sub.4) aqueous solution was added to the semiconductor particle dispersion liquid such that the amount of chromium (Cr) was 0.05 wt % with respect to the amount of the semiconductor particles, and the semiconductor particle dispersion liquid was irradiated with light from a xenon lamp (300 W, 20 mA) at the atmospheric pressure for 5 minutes using the same method as described above. Finally, a cobalt nitrate (Co(NO.sub.3).sub.2) aqueous solution was added to the semiconductor particle dispersion liquid such that the amount of cobalt (Co) was 0.05 wt % with respect to the amount of the semiconductor particles, and the semiconductor particle dispersion liquid was irradiated with light from a xenon lamp (300 W, 20 mA) at the atmospheric pressure for 5 minutes using the same method as described above. The irradiation of light using the xenon lamp was performed after covering the glass container with a lid of a quartz plate. Thus, the treated semiconductor particle dispersion liquid was used as it is as a solution (photocatalyst dispersion liquid) in which the photocatalyst was dispersed for the measurement of the quantum efficiency.

[0033] In the measurement of the quantum efficiency of the photocatalyst, first, the glass container containing the photocatalyst dispersion liquid was deaerated using a vacuum pump and was filled with argon gas. As a result, air in the glass container was replaced with argon gas. Next, the glass container was connected to a gas chromatograph through a glass pipe, the photocatalyst dispersion liquid in the glass container was irradiated with the light from the xenon lamp (300 W, 20 mA) through a 365 nm band pass filter, such that the water-splitting reaction was induced to produce hydrogen gas. In the detection of the amount of hydrogen gas produced, produced hydrogen gas accumulated in the glass pipe while the light irradiation was executed for 2 hours, the accumulated hydrogen gas was introduced into the gas chromatograph, and the amount of hydrogen gas was detected (the measurement was performed three times on a 30-minute basis). In the detection of the amount of hydrogen gas in the chromatograph, a calibration curve between the number of moles of hydrogen and the area of a detection data part corresponding to hydrogen gas was generated using standard gas where the number of moles of hydrogen gas was known. Using this calibration curve, the number of moles produced was determined from the area of the detection data part of the hydrogen gas introduced from the glass pipe into the gas chromatograph. On the other hand, regarding the number of photons emitted to the photocatalyst dispersion liquid in the glass container, the total wattage P (the amount of energy per unit time) of light emitted to the photocatalyst dispersion liquid in the glass container used for the measurement was measured using a photodiode sensor, and the number of photons I incident on the photocatalyst dispersion liquid per unit time was calculated from the following expression.

I(/s)=P(W)×λ(m)/[h(J.Math.s)×c(m/s)]

[0034] Here, λ represents the wavelength of emitted light, h represents the Planck's constant, and c represents the light speed. The quantum efficiency was calculated from the following expression.

Quantum Efficiency (%)=n(/s)×NA×2/I×100

[0035] Here, n represents the number of moles of hydrogen gas produced per unit time, and NA represents the Avogadro's number.

[0036] In the results, when the photocatalyst was prepared using the semiconductor particles synthesized while changing the firing temperature and the firing time to various values during the synthesis of the powder mixture, the quantum efficiency measured in the photocatalyst under each of the conditions of the firing temperatures and the firing times was as shown in Table 2 below. As the powder mixture, a mixture to which scandium oxide was added at a molar ratio of 0.005 (0.5 mol %) with respect to strontium titanate in the presence of aluminum oxide at a molar ratio of 0.02 (2 mol %) with respect to strontium titanate was used.

TABLE-US-00002 TABLE 2 Quantum Efficiency (Average Value ± Standard Deviation) when Firing Temperature and Time during Synthesis of Semiconductor Particles are changed Firing Temperature (° C.) 1150 1185 1200 Firing 10 63% ± 4.4% 71% ± 1.8% 76% ± 0.4% Time 15 72% ± 2.1% 75% ± 2.3% 75% ± 1.4% (Hours) 30 68% ± 3.4% 76% ± 0.6% 78% ± 0.8% The number of trials for each condition was three

[0037] Referring to the results of Table 2, when the mixture that was prepared by adding scandium oxide to the powder mixture was fired as described above, the average value of the quantum efficiency was higher and the standard deviation was smaller under all of the conditions as compared to the results where scandium was not added (refer to Table 1). In particular, under the conditions of the firing temperature and the firing time described in T. Takata et al., Nature, Volume 581, 411-414 (2020), the average value of the quantum efficiency increased from 16% to 63%. In addition, when the scandium was added at a firing temperature of 1185° C. or higher, a photocatalyst that stably achieved a quantum efficiency of more than 70% to 75% was prepared. The result shows that, when the semiconductor particles are synthesized by doping strontium titanate with scandium, the quantum efficiency of the photocatalyst formed of the semiconductor particles increases and a variation in quantum efficiency can be reduced.

[0038] Next, when the dependence of the quantum efficiency of the photocatalyst on the amount of scandium was checked by changing the amount of scandium oxide to be added to the powder mixture during the synthesis of the semiconductor particles to various values, the dependence was as shown in the graph of FIG. 3. The firing temperature and the firing time during the synthesis of the semiconductor particles were 1200° C. and 30 hours (the optimal conditions among the results of Table 2). The semiconductor particles were synthesized in the presence of 0.02 (2 mol %) of aluminum oxide with respect to the strontium titanate in the powder mixture, and the semiconductor particles were synthesized in the absence of aluminum oxide.

[0039] Referring to FIG. 3, in the photocatalyst formed of the semiconductor particles that were synthesized by adding scandium oxide at a molar ratio of 0.001 to 0.05 (0.1 mol % to 5 mol %) with respect to strontium titanate, the quantum efficiency was high at more than 50% in all of the cases as compared to the results where the semiconductor particles were doped with only aluminum (the amount of scandium oxide was 0 mol %). In particular, when the amount of scandium oxide was 0.1 mol % to 0.7 mol %, the quantum efficiency was about 70% or more. In all of the examples where the amount of scandium oxide was 0.5 mol %, the quantum efficiency was more than 75% and the standard deviation was less than 1%. Further, in the photocatalyst (not doped with Al) formed of the semiconductor particles that were synthesized in the absence of aluminum oxide, the quantum efficiency was higher than that in the photocatalyst formed of the semiconductor particles doped with only aluminum. The results show that: when the semiconductor particles obtained by doping strontium titanate with scandium were used for the photocatalyst, a higher quantum efficiency can be achieved than the semiconductor particles doped with only aluminum; and in particular, when the amount of scandium oxide is 0.1 mol % to 0.7 mol % in the presence of aluminum oxide, a quantum efficiency of about 70% or more can be stably achieved (with a smaller standard deviation).

[0040] In the semiconductor particles (firing temperature: 1200° C., firing time: 30 hours, 0.02 of aluminum oxide and 0.005 of scandium oxide were added by molar ratio with respect to strontium titanate) used for the photocatalyst that achieved the maximum quantum efficiency (78%±0.81%) in Table 2, when the salt concentrations during the addition of the co-catalyst were increased to four times those described above (Rh: 0.1 wt % 0.4 wt %; Cr: 0.05 wt % 0.2 wt %; Co: 0.05 wt % 0.2 wt %), the quantum efficiency of the obtained photocatalyst decreased to 35%±2.94%. This result shows that, regarding the salt concentrations during the addition of the co-catalyst, that Rh is about 0.1 wt %, Cr is about 0.05 wt %, and Co is about 0.05 wt % with respect to the amount of the semiconductor particles.

[0041] The above description relates to the embodiment of the disclosure, but various modifications and changes can be easily made by those skilled in the art. The disclosure is not limited to only the above exemplary embodiment, and it is obvious that the disclosure is applicable to various devices within a range not departing from the concepts of the disclosure.