Semiconductor particles for use in water splitting photocatalysts and method for synthesizing photocatalysts using the same

Abstract

To provide a photocatalyst capable of achieving high quantum efficiency as stably as possible in a water decomposition reaction by light and suitable for mass synthesis and practical use.SOLUTION: A photocatalyst formed by adding a co-catalyst to strontium titanate containing semiconductor particles and capable of causing a water decomposition reaction decomposing a water molecule into an oxygen molecule and a hydrogen molecule by light irradiation has scandium doped in the semiconductor particles. A method for synthesizing a semiconductor for the photocatalyst comprises the step of synthesizing the strontium titanate containing semiconductor particles having the doped scandium by mixing and calcining strontium titanate (SrTiO3) and scandium oxide (Sc2O3) in strontium chloride (SrCl2).SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to semiconductor particles used in a photocatalyst that induces a water-splitting reaction that splits water into hydrogen and oxygen, a photocatalyst prepared using the same, and a method for synthesizing the same. [Background technology]

[0002] Hydrogen gas is expected to be used as a clean next-generation fuel that does not produce carbon dioxide when burned. Hydrogen gas can be produced by a water decomposition reaction using light energy with a photocatalyst, so the development of a photocatalyst that can efficiently induce a water decomposition reaction using light is underway. As an example of such a highly efficient photocatalyst, Non-Patent Document 1 reports a photocatalyst with a quantum efficiency of approximately 1 in the water decomposition reaction and a method for synthesizing the same. According to Non-Patent Document 1, in short, a photocatalyst composed of oxides of Rh (rhodium), Cr (chromium), Co (cobalt) and the like attached to the crystal surface of a semiconductor particle (SrTiO3:Al) formed by doping aluminum (Al) on strontium titanate (SrTiO3) exhibits an (external) quantum efficiency of 96% in water splitting by irradiation with light of a wavelength of 350 to 360 nm ((external) quantum efficiency is given by dividing the number of reduced hydrogen atoms (= the number of hydrogen molecules generated × 2) by the number of irradiated photons. Hereinafter, in this specification, when "quantum efficiency" is mentioned, it means (external) quantum efficiency). The reason why such a high quantum efficiency was obtained is considered to be that in the structure of the photocatalyst, Rh / Cr2O3 and CoOOH are attached to different crystal surfaces of the SrTiO3:Al particle, respectively, and thus the transfer of charges generated by light toward the particle surface is achieved without the transfer of charges in the opposite direction. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] T. Takata, et al. Nature, volume 581, pages 411-414, 2020 Summary of the Invention [Problem to be solved by the invention]

[0004] In the above-mentioned Non-Patent Document 1, it is reported that the semiconductor particles of SrTiO3:Al that achieve a quantum efficiency of 96% are prepared by grinding and mixing powders of SrTiO3, Al2O3 and SrCl2 in an agate mortar, and firing the mixture (raw material mixture) in an alumina crucible at 1150°C for 10 hours while SrCl2 is dissolved. However, according to a reproduction experiment by the inventors of the present invention, in a water splitting reaction using a photocatalyst prepared from particles obtained by firing the above-mentioned raw material mixture at 1150°C for 10 hours as described in Non-Patent Document 1, the quantum efficiency only reached 8 to 28% (average value 16%, standard deviation 3.56% in 37 trials). In addition, the (external) quantum efficiency of the photocatalyst obtained by varying the calcination temperature and calcination time when calcining the SrTiO3:Al particles from the above raw material mixture was investigated. As shown in Table 1 below, it was confirmed that the average value of the (external) quantum efficiency for the calcination temperature and calcination time varies greatly, and the range of the calcination temperature and time conditions that give high efficiency is narrow. For example, referring to Table 1, it is confirmed that the quantum efficiency of the photocatalyst using the obtained semiconductor particles can change significantly just by changing the set temperature during calcination by about 20 degrees Celsius (when the calcination time is 10 hours, the calcination temperature is changed from 1150 degrees Celsius to 1175 degrees Celsius, and the quantum efficiency differs by about 30%.). In addition, the variation in quantum efficiency at each calcination temperature and calcination time was 3% to 10% in terms of standard deviation. This suggests that when preparing semiconductor particles with the composition of Non-Patent Document 1, whether or not a photocatalyst with high quantum efficiency can be obtained is easily affected by the temperature to which the raw material mixture is exposed during calcination. In fact, when SrTiO3:Al particles are fired from the above mixture, air is circulated in the firing furnace by a circulation fan so that the temperature distribution is uniform, but there is a possibility that almost no convection occurs in the crucible, resulting in uneven temperature, which may cause a decrease and variation in the quantum efficiency of the photocatalyst, etc. In any case, such a decrease or change or variation in the quantum efficiency of the photocatalyst with respect to the firing temperature and firing time during firing of the semiconductor particles is not desirable for mass synthesis and practical use of the photocatalyst. [Table 1]

[0005] Therefore, the inventors of the present invention conducted research and development of a photocatalyst suitable for mass synthesis and practical use, and found that by adding scandium to a raw material mixture containing strontium titanate and using semiconductor particles obtained by calcining the raw material mixture, a photocatalyst that stably achieves high quantum efficiency in a water splitting reaction by light can be prepared. Specifically, it was shown that when semiconductor particles obtained by doping strontium titanate with scandium are used, it is possible to obtain a photocatalyst that stably provides a quantum efficiency of more than 70% by adjusting the conditions. It was also shown that when strontium titanate is doped with scandium, a photocatalyst that provides a certain degree of high quantum efficiency can be obtained without doping with aluminum. This finding is utilized in the present invention.

[0006] Thus, the main object of the present invention is to provide a photocatalyst that achieves as stable a quantum efficiency as possible in a light-induced water splitting reaction, and that is suitable for mass synthesis and practical use.

[0007] Another object of the present invention is to provide semiconductor particles which provide the above-mentioned photocatalyst, a photocatalyst using the same, and a method for synthesizing the same. [Means for solving the problem]

[0008] According to one aspect of the present invention, the above object is achieved by semiconductor particles containing strontium titanate to which a co-catalyst has been added and which is used as a photocatalyst to induce a water splitting reaction in which water molecules are split into oxygen molecules and hydrogen molecules upon exposure to light, the semiconductor particles being strontium titanate doped with scandium.

[0009] In the above configuration, the "photocatalyst" is a substance that, when irradiated with light, can induce a water decomposition reaction and reduce water to generate oxygen molecules (oxygen gas) and hydrogen molecules (hydrogen gas). The "photocatalyst" is basically a substance formed by adding a co-catalyst to semiconductor particles containing strontium titanate, as in the photocatalyst described in Non-Patent Document 1 above, but in the present invention, the semiconductor particles containing strontium titanate are doped with scandium. The co-catalyst and the method of adding the co-catalyst to the semiconductor particles when using the semiconductor particles as a photocatalyst for a water decomposition reaction may be the same as those described in Non-Patent Document 1 or other conventional techniques. When the semiconductor particles containing strontium titanate are doped with scandium as in the present invention, the quantum efficiency of the photocatalyst can be more stably high, that is, the average value can be higher with less variation, as described in the embodiment section below, compared to the case where the semiconductor particles not doped with scandium are used as the photocatalyst.

[0010] The semiconductor particles of the photocatalyst in the present invention may be synthesized by a method including a step of synthesizing semiconductor particles containing strontium titanate doped with scandium by mixing strontium titanate (SrTiO3) and scandium oxide (Sc2O3) in strontium chloride (SrCl2) and baking the mixture. Therefore, according to another aspect of the present invention, the above object is achieved by a method for synthesizing the semiconductor particles of the photocatalyst, which is formed by adding a co-catalyst to semiconductor particles containing strontium titanate and which induces a water splitting reaction in which water molecules are split into oxygen molecules and hydrogen molecules by light irradiation, the method including a step of synthesizing the semiconductor particles.

[0011] In the above-mentioned method and the configuration of the semiconductor particles of the present invention, more specifically, in the synthesis of the semiconductor particles, scandium oxide may be mixed with strontium titanate in a molar ratio of 0.001 to 0.05, thereby obtaining a photocatalyst exhibiting a higher and less variable (smaller standard deviation) quantum efficiency than when the semiconductor particles are not doped with scandium.More preferably, in the synthesis of the semiconductor particles, scandium oxide may be mixed with strontium titanate in a molar ratio of 0.001 to 0.007, thereby obtaining a photocatalyst exhibiting an even higher quantum efficiency.

[0012] In addition, the above-mentioned semiconductor particle of the present invention may be further doped with aluminum, and in the synthesis of the semiconductor particle, strontium titanate (SrTiO3), scandium oxide (Sc2O3) and aluminum oxide (Al2O3) may be mixed in strontium chloride (SrCl2) and fired to dope strontium titanate with aluminum. More specifically, aluminum oxide may be mixed with strontium titanate at a molar ratio of 0.02. It has been found that doping with aluminum results in a photocatalyst exhibiting a higher quantum efficiency than that of scandium alone.

[0013] The semiconductor particles for the photocatalyst of the present invention are synthesized as particles in which strontium titanate is doped with scandium or aluminum by mixing strontium titanate and scandium oxide or aluminum oxide in strontium chloride and baking the mixture. In the baking, strontium chloride melts and becomes liquid, and in the liquid, scandium atoms or aluminum atoms enter the inside of strontium titanate and are doped, forming Sc-SrTiO3 or Al-Sc-SrTiO3 particles. Therefore, in the baking step, the baking temperature and time conditions may be appropriately set so that such Sc-SrTiO3 or Al-Sc-SrTiO3 particles are formed. Specifically, according to experiments, the baking conditions in the synthesis may be, but are not limited to, a baking temperature of 1150°C to 1200°C and a baking time of 10 to 30 hours.

[0014] When imparting photocatalytic activity to the semiconductor particles obtained according to the teachings of the present invention, as already mentioned, an appropriately selected co-catalyst may be added in the same manner as in the past. The addition of such a co-catalyst can be achieved by carrying out a step of adding the co-catalyst to the surface of the semiconductor particles dispersed in water by any method. Specifically, the co-catalyst to be added to the semiconductor particles can be, as in the case of Non-Patent Document 1, Rhodium-Chromium Oxide The co-catalysts may be Rh / Cr2O3 and cobalt oxide hydroxide (CoOOH), and these co-catalysts can be suitably added to the surface of the semiconductor particles dispersed in water by photoelectrodeposition. In the case of photoelectrodeposition, for example, the amounts of Rh, Cr, and Co may be about 0.1 wt%, 0.05 wt%, and 0.05 wt%, respectively, relative to the amount of the semiconductor particles, but are not limited thereto. Effect of the Invention

[0015] Thus, according to the present invention, a co-catalyst is added to semiconductor particles containing strontium titanate, and the semiconductor particles in the photocatalyst induce water splitting reaction in which water molecules are split into oxygen molecules and hydrogen molecules by light irradiation.By using strontium titanate doped with scandium, a photocatalyst with higher quantum efficiency and smaller variation in water splitting reaction can be synthesized with good reproducibility.The method and semiconductor particles of the present invention make the properties related to the quantum efficiency of the photocatalyst more stable, so the method and semiconductor particles of the present invention can be said to be more suitable for mass synthesis and practical use of photocatalysts for producing hydrogen gas.

[0016] Other objects and advantages of the present invention will become apparent from the following description of preferred embodiments of the invention. [Brief description of the drawings]

[0017] [Figure 1] FIG. 1 is a schematic diagram illustrating steps in a method for synthesizing semiconductor particles and a photocatalyst using the same according to the present invention. [Diagram 2] FIG. 2 is an electron microscope image of the photocatalyst synthesized by the method of the present invention. [Diagram 3] Figure 3 shows the quantum efficiency obtained by the photocatalyst synthesized according to the method of the present invention, when the scandium doped in the strontium titanate was changed. The bar graph shows the average value, and the error bar shows the standard deviation (three trials for each condition). The semiconductor particles were synthesized by mixing SrTiO3 and Sc2O3 or further Al2O3 in SrCl2 and firing at a firing temperature of 1200°C for 30 hours. [Explanation of symbols]

[0018] M…Agate mortar C...Alumina crucible H: Firing furnace V: Glass container W…aqueous solution P…Irradiation light BEST MODE FOR CARRYING OUT THEINVENTION

[0019] Synthesis method of semiconductor particles and photocatalyst for water splitting reaction The semiconductor particles for the water splitting photocatalyst according to this embodiment are synthesized by heating a raw material mixture of strontium titanate (SrTiO3) and scandium oxide (Sc2O3) or further aluminum oxide (Al2O3) mixed in strontium chloride (SrCl2) to a temperature at which SrCl2 melts and becomes liquid (however, a temperature at which SrTiO3, Sc2O3, and Al2O3 do not melt) to dope SrTiO3 with scandium atoms (Sc) or further aluminum atoms (Al) and make SrTiO3 a semiconductor (flux method).The photocatalyst is synthesized by adding a co-catalyst substance to the semiconductor particles obtained by the flux method, for example, by photoelectrodeposition (also called photoprecipitation method).

[0020] Referring to FIG. 1, in the method for synthesizing semiconductor particles, specifically, first, a large amount of SrCl2 powder is mixed with SrTiO3 powder, Sc2O3 powder, and / or Al2O3 powder (FIG. 1(A)). The powders may be mixed, for example, by grinding in an agate mortar (M) (for about 30 minutes). In such a mixture of powders (raw material mixture), the amount of SrCl2 may be, for example, about 10 times the amount of SrTiO3 in terms of molar ratio. The amount of Al2O3 powder, if used, may be the same as in Non-Patent Document 1, and may be, for example, about 0.02 in terms of molar ratio to the amount of SrTiO3. According to experiments by the inventors of the present invention, the amount of Sc2O3 powder may be 0.001 to 0.05 in terms of molar ratio to the amount of SrTiO3, and it has been found that a photocatalyst with high quantum efficiency can be obtained when the molar ratio of Sc2O3 powder is preferably 0.001 to 0.007, and more preferably when the molar ratio of Sc2O3 powder is 0.001 to 0.005, or 0.003 to 0.005.

[0021] Thereafter, the raw material mixture of the powders is transferred to a crucible for firing, for example, an alumina crucible (C), and fired in a firing furnace (H) (FIG. 1(B)). In this step, the firing temperature may be a temperature (874°C or higher) at which SrCl2 melts and becomes liquid, and a temperature (2072°C or lower) at which SrTiO3, Sc2O3, and Al2O3 do not melt, as described above, and may be, for example, about 1000 to 1400°C, and preferably 1150 to 1200°C according to the inventors of the present invention. The firing time during which the raw material mixture is exposed to the above temperature is a time sufficient for SrTiO3 to be doped with Sc or further Al and for SrTiO3 to become a semiconductor, and according to experiments by the inventors of the present invention, the firing time may be 10 to 30 hours, and preferably about 30 hours.

[0022] After the above-mentioned firing step, when the fired product is cooled to room temperature, water (distilled water may be used) is added to the crucible (C), and the fired product in the crucible is dispersed as particles in the water while being stirred by applying ultrasonic waves using an ultrasonic agitator or the like, and the fired product may be recovered by suction filtration or the like. Such particulate fired product is the semiconductor particles (Sc-SrTiO3 or Al-Sc-SrTiO3) for the photocatalyst according to this embodiment. The recovered semiconductor particles may then be washed with water. This washing may be carried out until the pH of the washing water becomes 7 and the washing water does not contain chlorine. Thus, the semiconductor particles may be dried after washing.

[0023] In order to make the semiconductor particles function as photocatalysts, a co-catalyst is added to the crystal surface of the semiconductor particles. It is believed that such a co-catalyst prevents the charges (electrons and holes) generated by the irradiation of light in the semiconductor particles from moving back to the inside of the semiconductor particles after the charges move to the surface of the semiconductor particles in the water splitting reaction by light. In this embodiment, the co-catalyst may be added to the semiconductor particles by any method, and typically, as already mentioned, the co-catalyst may be added to the semiconductor particles by precipitating the co-catalyst on the crystal surface of the semiconductor particles dispersed in water by the photoelectrodeposition method, as in the case of Non-Patent Document 1. Specifically, first, the semiconductor particles are dispersed in water in a transparent container such as a glass container (FIG. 1(C)). Note that ultrasonic waves may be applied to the water (semiconductor particle dispersion) in which the semiconductor particles are dispersed so that the semiconductor particles are uniformly dispersed. Thereafter, salts serving as the raw material for the promoter are added, and light P is irradiated to perform a process for precipitating a metal oxide that serves as the promoter on the surface of the semiconductor particles (FIG. 1(D)).

[0024] More specifically, as a co-catalyst to be added to the surface of the semiconductor particles, similarly to the case of Non-Patent Document 1, Rhodium-Chromium OxideWhen Rh / Cr2O3 and cobalt oxide hydroxide (CoOOH) are used, the process may be carried out as follows. That is, first, an aqueous solution of rhodium chloride (RhCl3) is added to the semiconductor particle dispersion so that rhodium (Rh) is 0.1 wt% relative to the amount of semiconductor particles, and the semiconductor particle dispersion is irradiated with light from a xenon lamp (300 W, 20 mA) at atmospheric pressure for 10 minutes. Next, an aqueous solution of potassium chromate (K2CrO4) is added to the semiconductor particle dispersion so that chromium (Cr) is 0.05 wt% relative to the amount of semiconductor particles, and the semiconductor particle dispersion is irradiated with light from a xenon lamp (300 W, 20 mA) at atmospheric pressure for 5 minutes. Then, an aqueous solution of cobalt nitrate (Co(NO3)2) is added to the semiconductor particle dispersion so that the cobalt (Co) is 0.05 wt% relative to the amount of semiconductor particles, and light from a xenon lamp (300 W, 20 mA) is irradiated for 5 minutes under atmospheric pressure. Then, as shown in FIG. 1(D), Rh-Cr oxide and Co hydroxide oxide are attached to the surface of the semiconductor particles, and the prepared composite material functions as a photocatalyst that causes a water splitting reaction by light. FIG. 2 is an electron microscope image of the prepared photocatalyst, and as shown in the figure, the photocatalyst has a particulate form. In the step of adding a cocatalyst to the surface of the semiconductor particles by the photoelectrodeposition method, the concentration of salts for the cocatalyst added to the semiconductor particle dispersion may be appropriately adjusted. According to an experimental example described later, it has been found that, for example, when the concentration of salts for the cocatalyst is four times the above, the quantum efficiency is significantly reduced, so it is preferable that the concentration of the salts is adjusted so as not to be excessive. The co-catalyst may be added to the surface of the semiconductor particles by an impregnation method (adding a salt to a dispersion liquid and applying heat) in addition to the photoelectrodeposition method.

[0025] As described above, in the photocatalyst prepared by using the semiconductor particles formed by doping scandium into strontium titanate according to this embodiment, the quantum efficiency [hydrogen molecules x 2 / number of irradiated photons] in the water splitting reaction by light is at least 50% on average, preferably more than 75%, with a standard deviation of about 1% (0.4 to 4.4%), as will be explained later. Therefore, according to the method for synthesizing the semiconductor particles and photocatalyst according to this embodiment, compared to the conventional case where scandium is not used (external quantum efficiency is 16 to 67% on average, and the standard deviation can reach 12% (3.2 to 12.2%)), it is possible to provide semiconductor particles and photocatalysts that have less variation, that is, that provide a stable and higher quantum efficiency.

[0026] The reason why photocatalysts using semiconductor particles made of strontium titanate doped with scandium exhibit stable and higher quantum efficiency is currently under investigation, and it has been confirmed that the shape of the semiconductor particles doped with scandium is a more rounded polyhedron (see Figure 2). In a rounded polyhedral structure, the corners of the cubic crystals are cut off, exposing many of the facets of the strontium titanate, resulting in surfaces that are prone to accumulating electrons and surfaces that are prone to accumulating holes. This may be because charge separation is further promoted and the water splitting reaction occurs more efficiently.

[0027] Experimental Example According to the teaching of the present embodiment, the semiconductor particles of strontium titanate doped with scandium and the photocatalyst using the same were synthesized, and the quantum efficiency of the photocatalyst was measured to verify the effectiveness of the present embodiment. It should be understood that the following experimental examples are merely illustrative of the effectiveness of the present embodiment and do not limit the scope of the present invention.

[0028] The synthesis of semiconductor particles was carried out according to the above-mentioned process. Specifically, first, SrCl2 powder, SrTiO3 powder, and Sc2O3 powder or further Al2O3 powder were ground and mixed in an agate mortar for 30 minutes. The molar ratio of SrCl2, SrTiO3, and Al2O3 (only when used) was 10:1:0.02. The molar ratio of Sc2O3 was between 0.001 and 0.05 (0.1 to 5 mol%) with respect to SrTiO3. The powder mixture was transferred to an alumina crucible and then fired in a firing furnace with various firing temperatures and firing times. In the firing process, the temperature was raised from room temperature to the firing temperature in 2 hours, and after the firing time had elapsed, the mixture was allowed to cool to room temperature for 6 hours. After cooling, distilled water was added to the crucible containing the fired product, and ultrasonic waves were applied using an ultrasonic agitator to stir the mixture. The fired product in the crucible (including those adhering to the inner wall of the crucible) was dispersed in water in a particulate form, and collected by suction filtration. The collected particulate fired product was then washed with distilled water. In the washing, the pH of the water after washing was confirmed using pH test paper, and the presence or absence of chlorine in the water after washing was confirmed by adding 0.1M silver nitrate to the water after washing to confirm whether silver chloride was generated or not. Since the pH of the water after washing was 7 and chlorine was no longer detectable, washing was performed. The particulate fired product after washing, i.e., the semiconductor particles, was then dried at 70°C.

[0029] In preparing the photocatalyst using the above semiconductor particles, 100 mg of semiconductor particle powder was dispersed in 100 ml of distilled water in a heat-resistant glass container (400 ml). Then, first, an aqueous solution of rhodium chloride (RhCl3) was added to the semiconductor particle dispersion so that rhodium (Rh) was 0.1 wt% relative to the amount of semiconductor particles, and the semiconductor particle dispersion was irradiated with light from a xenon lamp (300 W, 20 mA) at atmospheric pressure for 10 minutes, then an aqueous solution of potassium chromate (K2CrO4) was added to the semiconductor particle dispersion so that chromium (Cr) was 0.05 wt% relative to the amount of semiconductor particles, and the semiconductor particle dispersion was irradiated with light from a xenon lamp (300 W, 20 mA) at atmospheric pressure for 5 minutes in the same manner as above, and finally, an aqueous solution of cobalt nitrate (Co(NO3)2) was added to the semiconductor particle dispersion so that cobalt (Co) was 0.05 wt% relative to the amount of semiconductor particles, and the semiconductor particle dispersion was irradiated with light from a xenon lamp (300 W, 20 mA) at atmospheric pressure for 5 minutes in the same manner as above. The irradiation with light from the xenon lamp was performed with a quartz plate lid on the glass container. Thus, the semiconductor particle dispersion liquid after the treatment was used as it was as a solution in which the photocatalyst was dispersed (photocatalyst dispersion liquid) for measuring the quantum efficiency.

[0030] In the measurement of the quantum efficiency of the photocatalyst, first, the glass container containing the photocatalyst dispersion liquid was degassed with a vacuum pump, and then filled with argon gas to replace the air in the glass container with argon gas. Then, the glass container was connected to a gas chromatograph via a glass pipe, and the photocatalyst dispersion liquid in the glass container was irradiated with light from a xenon lamp (300 W, 20 mA) through a 365 nm bandpass filter to induce a water splitting reaction to generate hydrogen gas. In the detection of the amount of hydrogen gas generated, the hydrogen gas generated in the glass pipe was stored during the two-hour light irradiation, and the stored gas was introduced into the gas chromatograph to detect the amount of hydrogen gas (measurements were performed three times at 30-minute intervals). 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 ​​the detection data portion corresponding to hydrogen gas was prepared in advance using a standard gas with a known number of moles of hydrogen gas, and the number of moles generated was determined from the area of ​​the detection data portion of the hydrogen gas introduced into the gas chromatograph from the glass pipe using the calibration curve. Meanwhile, for the number of photons irradiated to the photocatalyst dispersion in the glass container, the wattage P (amount of energy per unit time) of the total light irradiated to the photocatalyst dispersion in the glass container used for the measurement was measured with a photodiode sensor, and the number of photons I incident on the photocatalyst dispersion per unit time was calculated using the following formula. I( / s)=P(W)×λ(m) / [h(J·s)×c(m / s)] Here, λ is the wavelength of the irradiated light, h is the Planck constant, and c is the speed of light. The quantum efficiency was calculated by the following formula. Quantum efficiency (%) = n ( / s) × NA × 2 / I × 100 Here, n is the number of moles of hydrogen gas generated per unit time, and NA is Avogadro's number.

[0031] As a result, first, when a photocatalyst was prepared using semiconductor particles synthesized by varying the firing temperature and firing time during firing of the powder mixture, the quantum efficiency measured for the photocatalyst under each firing temperature and firing time condition was as shown in Table 2 below. The powder mixture used was one in which scandium oxide was added at a molar ratio of 0.005 (0.5 mol%) to strontium titanate in the presence of aluminum oxide at a molar ratio of 0.02 (2 mol%) to strontium titanate. [Table 2] With reference to the results of Table 2 above, when scandium oxide was added to the powder mixture and the mixture prepared was fired as described above, the average quantum efficiency was higher and the standard deviation was smaller under all conditions than when scandium was not added (see Table 1). In particular, under the firing temperature and firing time conditions of Non-Patent Document 1, the average quantum efficiency increased from 16% to 63%. Furthermore, when the firing temperature was 1185°C or higher and scandium was added, a photocatalyst was prepared that stably exhibited a quantum efficiency of more than 70% to 75%. The above results suggest that when semiconductor particles are synthesized by doping strontium titanate with scandium, the quantum efficiency of the photocatalyst using the semiconductor particles can be increased and its variation can be reduced.

[0032] Next, the amount of scandium oxide added to the powder mixture during semiconductor particle synthesis was varied to confirm the dependence of the quantum efficiency of the photocatalyst on the amount of scandium, as shown in the graph in Figure 3. The firing temperature and firing time during semiconductor particle synthesis were 1200°C and 30 hours (optimum conditions in the results in Table 2). The semiconductor particles were synthesized in the powder mixture with strontium titanate in the presence of 0.02 (2 mol%) aluminum oxide and in the absence of aluminum oxide.

[0033] 3, in the photocatalysts using semiconductor particles synthesized by adding scandium oxide at a molar ratio of 0.001 to 0.05 (0.1 to 5 mol%) to strontium titanate, the quantum efficiency was higher than that in the case where only aluminum was doped in the semiconductor particles (when scandium oxide was 0 mol%), and exceeded 50%. In particular, when scandium oxide was 0.1 to 0.7 mol%, the quantum efficiency was approximately 70% or more, and when scandium oxide was 0.5 mol%, the quantum efficiency exceeded 75% in all cases, with a standard deviation below 1%. Furthermore, in the case of the photocatalysts using semiconductor particles synthesized in the absence of aluminum oxide (without Al doping), the quantum efficiency was higher than that in the case where only aluminum was doped. These results show that when semiconductor particles made of strontium titanate doped with scandium are used as a photocatalyst, a higher quantum efficiency is achieved than when semiconductor particles doped with aluminum only are used. In particular, when scandium oxide is at 0.1 to 0.7 mol% in the presence of aluminum oxide, a quantum efficiency of around 70% or more can be stably achieved (with a smaller standard deviation).

[0034] In Table 2, the semiconductor particles used in the photocatalyst that gave the maximum quantum efficiency (78% ± 0.81%) (calcination temperature 1200°C, calcination time 30 hours, 0.02 aluminum oxide and 0.005 scandium oxide added in molar ratios to strontium titanate) were used, and when the salt concentrations when the promoters were added were increased to four times the above (Rh: 0.1 → 0.4 wt%; Cr: 0.05 → 0.2 wt%; Co: 0.05 → 0.2 wt%), the quantum efficiency of the resulting photocatalyst decreased to 35% ± 2.94%. This suggests that the salt concentrations when the promoters are added should be around 0.1 wt% Rh, around 0.05 wt% Cr, and around 0.05 wt% Co relative to the amount of semiconductor particles.

[0035] The above description has been given in relation to the embodiment of the present invention, but it will be apparent to those skilled in the art that many modifications and changes can be easily made thereto, and that the present invention is not limited to the embodiment exemplified above, but can be applied to various devices without departing from the concept of the present invention.

Claims

1. A method for synthesizing semiconductor particles to be used as a photocatalyst that induces a water splitting reaction in which water molecules are split into oxygen molecules and hydrogen molecules by irradiation with light, comprising the steps of: The semiconductor particles are synthesized by doping strontium titanate with scandium and aluminum, and mixing scandium oxide in a molar ratio of 0.001 to 0.007 with respect to the strontium titanate (SrTiO 3 ).

2. The method of claim 1, wherein the semiconductor particles are synthesized by mixing aluminum oxide in a molar ratio of 0.02 to strontium titanate.

3. The method of claim 1, comprising the step of synthesizing the semiconductor particles by mixing the strontium titanate (SrTiO 3 ), the scandium oxide (Sc 2 O 3 ) and aluminum oxide (Al 2 O 3 ) in strontium chloride (SrCl 2 ) and firing the mixture.

4. The method according to claim 3, wherein in the synthesis process of the semiconductor particles, the firing temperature is 1150°C to 1200°C and the firing time is 10 to 30 hours.

5. A method for synthesizing a photocatalyst that induces a water splitting reaction in which water molecules are split into oxygen molecules and hydrogen molecules by light irradiation, using semiconductor particles synthesized by the method of claim 1, comprising the steps of: Adding a promoter to the surfaces of the semiconductor particles dispersed in water The method includes:

6. 6. The method of claim 5, wherein the co-catalyst is rhodium-chromium oxide (Rh / Cr ２ O ３ ) and cobalt oxide hydroxide (CoOOH).

7. 6. The method of claim 5, wherein in the step of applying the co-catalyst, the co-catalyst is applied to the surface of the semiconductor particles dispersed in water by photoelectrodeposition.

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