Layered oxysulfide, visible light-responsive photocatalyst, and water splitting method

Abstract

This invention provides a novel material that possesses interlayer hydration ability, ion exchange ability, visible light absorption ability, and water resolution ability. [Solution] A layered oxysulfide is provided, which is composed of alternating layers of oxysulfide and interlayer ions, possessing interlayer hydration ability that allows water molecules to be inserted between layers, and interlayer ion exchange ability that allows interlayer ions to be exchanged for heterogeneous ions, and which also functions as a photocatalyst by absorbing visible light. Furthermore, a visible light-responsive photocatalyst containing the layered oxysulfide and a method for producing hydrogen and / or oxygen using the layered oxysulfide are also provided.

Description

[Technical Field]

[0001] This disclosure relates to layered oxysulfides, visible light-responsive photocatalysts, and water splitting methods. [Background technology]

[0002] Two-dimensional materials (layered compounds) possess unique structural properties such as the insertion of guest molecules into the interlayers and the exchange of interlayer ions. These properties have been utilized in various fields, including catalysts, batteries, adsorbents, and solid electrolytes. Among these, layered oxide semiconductors have long attracted attention as photocatalytic materials for hydrogen production through water splitting using sunlight. Layered oxide semiconductors are generally composed of anionic nanosheets in oxide layers and interlayer ions (mostly alkali metal cations), and these extremely thin nanosheets themselves function as semiconductors.

[0003] Among these layered oxides, there are compounds that possess the property of allowing water molecules to be inserted between layers (interlayer hydration ability), and such layered oxides achieve efficient water splitting under ultraviolet light irradiation by utilizing the interlayer space as a reaction field (Non-Patent Literature 1). Furthermore, interlayer ion exchange is also possible in many layered oxides, and the development of novel photocatalysts utilizing this ion exchange ability (Non-Patent Literature 2) and the exfoliation of layers by inserting bulky ions between layers (nanosheet formation) have also been reported, and improvements in water splitting efficiency have been achieved through these methods as well (Non-Patent Literature 3).

[0004] While the layered oxide semiconductors described above are very attractive as photocatalytic materials, most of them have the disadvantage of having a wide band gap and being able to utilize only ultraviolet light. The number of photons in the ultraviolet region (200-400 nm) accounts for only a few percent of the solar spectrum, and even if the photocatalyst could utilize all of these photons for the water splitting reaction, the solar energy conversion efficiency would only be about 2%. On the other hand, if the usable range is expanded to the visible light region, the efficiency improves dramatically to about 16% up to 600 nm and about 32% up to 800 nm. Therefore, it can be said that the use of visible light is essential for improving water splitting efficiency.

[0005] However, since the upper end of the valence band of oxide semiconductors is composed of oxygen's 2p orbital, it is fixed at a position about 3V deeper than the oxidation-reduction potential of water. This presents a dilemma: if the upper end of the valence band is capable of reducing water, the band gap of the oxide semiconductor inevitably becomes larger than 3eV, making visible light absorption impossible.

[0006] Recently, complex anionic compound (e.g., oxynitrides, oxysulfides) semiconductors have a narrower band gap than oxide semiconductors and are being actively studied as visible light-responsive photocatalytic materials (Non-patent documents 4, 5). These contain anions with lower electronegativity than oxygen anions (e.g., nitrogen anions and sulfur anions), which raises the upper end of the valence band compared to oxides and results in a narrower band gap. However, most complex anionic compound photocatalysts are not layered compounds, or if they are layered compounds, they lack interlayer hydration and ion exchange capabilities. Therefore, they have not been able to utilize the attractive properties of layered compounds as reported for the aforementioned layered oxide photocatalysts.

[0007] In recent years, only the layered oxynitride K2LaTa2O6N has been reported to possess interlayer hydration and ion exchange capabilities, and to generate hydrogen from water when irradiated with visible light in the presence of a sacrificial reducing agent (Non-Patent Literature 6), but it did not possess the ability to generate oxygen from water. [Prior art documents] [Non-patent literature]

[0008] [Non-Patent Document 1] J. Catal. 1989, 120, 337. [Non-Patent Document 2] J. Ceram. Soc. Jpn. 2007, 115, 511. [Non-Patent Document 3] Chem. Mater. 2002, 14, 4390. [Non-Patent Document 4] Nat Mater 2019, 18, 827. [Non-Patent Document 5] J. Am. Chem. Soc. 2023, 145, 3839. [Non-Patent Document 6] J. Mater. Chem. A 2023, 11, 9485. [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] As described above, layered oxides possessing interlayer hydration and ion exchange capabilities are an attractive group of materials for water splitting photocatalysts, but they have the problem of not having visible light absorption capabilities. On the other hand, while complex anion compound photocatalysts have visible light absorption capabilities, most of them are either not layered compounds, or even if they are layered compounds, they lack interlayer hydration and ion exchange capabilities. Furthermore, very recently, a layered oxynitride material has been discovered that possesses interlayer hydration and ion exchange capabilities and can produce hydrogen from water under visible light irradiation, but this is limited to only one example, and it does not have the ability to produce oxygen from water. In other words, there are no layered compound photocatalysts that have the ability to decompose water into hydrogen and oxygen under visible light, as well as interlayer hydration and ion exchange capabilities.

[0010] This disclosure aims to provide a novel material that possesses intercalation hydration ability, ion exchange ability, visible light absorption ability, and water resolution ability (both water reduction ability and water oxidation ability) in order to improve upon the problems of the prior art, and further aims to provide various similar materials. [Means for solving the problem]

[0011] The Disclosing Parties have diligently conducted research to achieve the above objectives, and as a result have developed a layered oxysulfide capable of achieving these objectives, thus completing this disclosure.

[0012] That is, the present disclosure relates to the following layered oxysulfide which is composed of an alternating stack of an oxysulfide layer and an interlayer ion, has interlayer hydration ability, ion exchange ability, and visible light absorption ability, and functions as a photocatalyst for water splitting under visible light irradiation.

[0013] <Aspect 1> It is composed of an alternating stack of an oxysulfide layer and an interlayer ion, has an interlayer hydration ability that allows water molecules to be inserted between layers and an interlayer ion exchange ability that allows interlayer ions to be exchanged with different ions, and absorbs visible light and functions as a photocatalyst, layered oxysulfide. <Aspect 2> The layered oxysulfide according to Aspect 1, which is a Ruddlesden-Popper type compound having the following formula (1) or a Dion-Jacobson type compound having the following formula (1): M , , 4-x , , , , x , , , M’ z M” n O [[ID=​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​4-x S x The layered oxysulfide according to any one of Aspects 1 to 4, having (Ln is a lanthanoid element, 0 < x < 4). <Aspect 6> A visible light-responsive photocatalyst comprising the layered oxysulfide according to any one of Aspects 1 to 5. <Aspect 7> The layered oxysulfide, and Rhodium particles and / or iridium oxide particles supported on the layered oxysulfide The photocatalyst according to Aspect 6, comprising. <Aspect 8> A method for producing hydrogen and / or oxygen, comprising irradiating visible light to a dispersion in which the layered oxysulfide according to any one of Aspects 1 to 5 is dispersed in water to decompose the water. [Effect of the Invention]

[0014] The present disclosure newly synthesized a layered oxysulfide composed of an alternating stack of an oxysulfide layer and interlayer ions, having both interlayer hydration ability, ion exchange ability, and visible light absorption ability, and demonstrated that it decomposes water under visible light irradiation and functions as a photocatalyst for water splitting. Starting from this discovery, the possibility of newly synthesizing various layered oxysulfides having similar characteristics and systematically controlling their physical properties can be expected. In addition, like the layered compounds that have been used in various fields so far, the layered oxysulfide can be expected to be applied to uses such as catalysts, batteries, adsorbents, and solid electrolytes. [Brief Description of the Drawings]

[0015] [Figure 1] It is a figure which shows the X-ray diffraction pattern of each layered oxide and each layered oxysulfide synthesized in Comparative Example 1 and Example 1. [Figure 2] It is a figure which shows the synchrotron X-ray diffraction pattern and crystal structure of the layered oxysulfides NaNdTiO4−xSx and NaSmTiO4−xSx synthesized in Example 1. [Figure 3]This figure shows the X-ray diffraction patterns of the samples obtained after stirring the layered oxysulfides NaNdTiO4-xSx and NaSmTiO4-xSx synthesized in Example 1 in pure water or hydrochloric acid. [Figure 4] This figure shows the light absorption characteristics of each layered oxide and each layered oxysulfide synthesized in Comparative Example 1 and Example 1. [Figure 5] This figure shows the results of DFT calculations to determine the density of states of the layered oxide NaLaTiO4 and layered oxysulfide NaLaTiO3S or NaLaTiO2S2 synthesized in Comparative Example 1 and Example 1. [Figure 6] This figure shows the time course of hydrogen or oxygen gas production from aqueous solutions of sodium sulfide and sodium sulfite or silver nitrate under visible light irradiation for each layered oxide and each layered oxysulfide synthesized in Comparative Example 1 and Example 1. [Modes for carrying out the invention]

[0016] Layered oxysulfides The layered oxysulfide of this disclosure is composed of alternating layers of oxysulfide and interlayer ions, possesses interlayer hydration ability and interlayer ion exchange ability, and functions as a photocatalyst by absorbing visible light.

[0017] The layered oxysulfides of this disclosure may be Ruddlesden-Popper type compounds or Dion-Jacobson type compounds, particularly Ruddlesden-Popper type compounds having the following formula (1), or Dion-Jacobson type compounds having the following formula (1): M y M' z M" n O 3n+1-x S x …(1) [During the ceremony, M and M' are, independently, hydrogen, alkali metals, alkaline earth metals, or rare earth metals. M'' is a transition metal, If it is a Ruddlesden-Popper type compound, then y+z=n+1, 1≦n, 0 <x<3n+1であり、かつ If it is a Dion-Jacobson type compound, then y=1, z=n-1, 2≦n, 0 <x<3n+1である。〕。

[0018] M, M', and M'' are not limited, but may be, for example, the following: M=H, Na, K, Rb, Cs, M'=Y,Ln (lanthanide elements), Ca, Sr, Ba, M” = Ti, Nb, Ta.

[0019] The following are specific examples of equation (1).

[0020] A concrete example of the Ruddlesden-Popper type (y+z=n+1, 1≦n, 0 <x<3n+1): H y La z Ti n O 3n+1-x S x …(1-1a) H y Pr z Ti n O 3n+1-x S x …(1-2a) H y Nd z Ti n O 3n+1-x S x …(1-3a) H y Sm z Ti n O 3n+1-x S x …(1-4a) H y EU z Ti n O 3n+1-x S x …(1-5a) H y Gd z Ti n O 3n+1-x S x …(1-6a) H y Y z Ti n O 3n+1-x S x…(1-7a) Now y La z Of n SHE 3n+1-x S x …(1-8a) Now y Pr z Of n SHE 3n+1-x S x …(1-9a) Now y Yes z Of n SHE 3n+1-x S x …(1-10a) Now y Sm z Of n SHE 3n+1-x S x …(1-11a) Now y Eu z Of n SHE 3n+1-x S x …(1-12a) Now y Good z Of n SHE 3n+1-x S x …(1-13a) Now y Yes z Of n SHE 3n+1-x S x …(1-14a) K y La z Of n SHE 3n+1-x S x …(1-15a) H y La z Of n SHE 3n+1-x S x …(1-16a) Now y La z Of n SHE 3n+1-x S x …(1-17a) K y La z Of n SHE3n+1-x S x …(1-18a)

[0021] Further specific examples of the Ruddlesden-Popper type (for formulas (1-1) to (1-1), 0 < x < 4, and for formulas (1-16b) to (1-18b), 0 < x < 10): HLaTiO 4-x S x …(1-1b) HPrTiO 4-x S x …(1-2b) HNdTiO 4-x S x …(1-3b) HSmTiO 4-x S x …(1-4b) HEuTiO 4-x S x …(1-5b) HGdTiO 4-x S x …(1-6b) HYTiO 4-x S x …(1-7b) NaLaTiO 4-x S x …(1-8b) NaPrTiO 4-x S x …(1-9b) NaNdTiO 4-x S x …(1-10b) NaSmTiO 4-x S x …(1-11b) NaEuTiO 4-x S x …(1-12b) NaGdTiO 4-x S x …(1-13b) NaYTiO 4-x S x …(1-14b) KLaTiO 4-x S x …(1-15b) H2La2Ti3O 10-x Sx …(1 - 16b) Na2La2Ti3O 10-x S x …(1 - 17b) K2La2Ti3O 10-x S x …(1 - 18b)

[0022] Specific examples of the Dion - Jacobson type (y = 1, z = n - 1, 2 ≤ n, 0 < x < 3n + 1): H y La z Nb n O 3n+1-x S x …(1 - 19a) Na y La z Nb n O 3n+1-x S x …(1 - 20a) K y La z Nb n O 3n+1-x S x …(1 - 21a) Rb y La z Nb n O 3n+1-x S x …(1 - 22a) Cs y La z Nb n O 3n+1-x S x …(1 - 23a) H y La z Ta n O 3n+1-x S x …(1 - 24a) Na y La z Ta n O 3n+1-x S x …(1 - 25a) K y La z Ta n O 3n+1-x S x …(1 - 26a) Rb y Laz Dad n O 3n+1-x S x ...(1-27a) Cs y Day z Dad n O 3n+1-x S x …(1-28a) A y Approx z No n O 3n+1-x S x ...(1-29a) So y Approx z No n O 3n+1-x S x …(1-30a) K y Approx z No n O 3n+1-x S x …(1-31a) Rs y Approx z No n O 3n+1-x S x …(1-32a) Cs y Approx z No n O 3n+1-x S x …(1-33a) A y Sr z No n O 3n+1-x S x …(1-34a) K y Sr z No n O 3n+1-x S x …(1-35a) A y Ba z No n O 3n+1-x S x …(1-36a) K y Ba z No n O 3n+1-x S x …(1-37a) H y Ca z Ta n O 3n+1-x S x …(1 - 38a) K y Ca z Ta n O 3n+1-x S x …(1 - 39a) H y Sr z Ta n O 3n+1-x S x …(1 - 40a) K y Sr z Ta n O 3n+1-x S x …(1 - 41a) H y Ba z Ta n O 3n+1-x S x …(1 - 42a) K y Ba z Ta n O 3n+1-x S x …(1 - 43a)

[0023] Further specific examples of the Dion - Jacobson type (for formulas (1 - 19b) to (1 - 28b), 0 < x < 7, and for formulas (1 - 29b) to (1 - 43b), 0 < x < 10): HLaNb2O 7-x S x …(1 - 19b) NaLaNb2O 7-x S x …(1 - 20b) KLaNb2O 7-x S x …(1 - 21b) RbLaNb2O 7-x S x …(1 - 22b) CsLaNb2O 7-x S x …(1 - 23b) HLaTa2O 7-x Sx …(1-24b) NaLaTa2O 7-x S x …(1-25b) KLaTa2O 7-x S x …(1-26b) RbLaTa2O 7-x S x …(1-27b) CsLaTa2O 7-x S x …(1-28b) HCa2Nb3O 10-x S x …(1-29b) NaCa2Nb3O 10-x S x …(1-30b) KCa2Nb3O 10-x S x …(1-31b) RbCa2Nb3O 10-x S x …(1-32b) CsCa2Nb3O 10-x S x …(1-33b) HSr2Nb3O 10-x S x …(1-34b) KSr2Nb3O 10-x S x …(1-35b) HBa2Nb3O 10-x S x …(1-36b) KBa2Nb3O 10-x S x …(1-37b) HCa2Ta3O 10-x S x …(1-38b) KCa2Ta3O 10-x S x …(1-39b) HSr2Ta3O 10-x S x …(1-40b) KSr2Ta3O 10-x S x …(1-41b) HBa2Ta3O10-x S x …(1-42b) KBa2Ta3O 10-x S x …(1-43b)

[0024] Among these, the following are preferred (0 < x < 4, preferably 1 ≤ x ≤ 2): NaLaTiO 4-x S x …(1-8b) NaPrTiO 4-x S x …(1-9b) NaNdTiO 4-x S x …(1-10b) NaSmTiO 4-x Sx …(1-11b) NaEuTiO 4-x S x …(1-12b) NaGdTiO 4-x S x …(1-13b) HNdTiO 4-x S x …(1-3b) HSmTiO 4-x S x …(1-4b).

[0025] The synthesis method of each of the layered oxysulfides represented by the formula (1) is not limited. For example, each of the layered oxysulfides represented by the formula (1) can be synthesized by using particles of Ruddlesden-Popper type layered oxides (such as NaNdTiO4, NaSmTiO4, etc.) or a mixture of sodium carbonate (Na2CO3), titanium oxide (TiO2), and at least one of lanthanoid oxides (such as Nd2O3, Sm2O3, etc.) in an arbitrary composition as a raw material and heat-treating them under a hydrogen sulfide stream. [[ID=​​​​​ The shape of each layered oxysulfide shown in formula (1) is not limited, but when used as a photocatalyst in the water splitting method of this disclosure, it is preferably in the form of particles (powder). The average particle diameter is preferably 0.7 to 10 μm, and more preferably 3 to 5 μm. Here, this average particle diameter can be determined as the average value of the area circle equivalent diameter when 100 or more particles are observed with a scanning electron microscope (SEM).

[0028] Each layered oxysulfide shown in formula (1) has interlayer hydration ability, making it possible to obtain hydrates in which water is inserted between the layers. For example, by suspending each layered oxysulfide in pure water or hydrochloric acid and stirring for a long time, it is possible to obtain hydrates in which water is inserted between the layers.

[0029] Each layered oxysulfide represented by formula (1) possesses interlayer ion exchange capacity, making it possible to obtain compounds in which their interlayer ions are exchanged for other types of ions. For example, by suspending each layered oxysulfide in pure water or hydrochloric acid and stirring for a long time, it is possible to obtain proton oxysulfides in which some or most of the interlayer ions (e.g., Na cations) are exchanged for protons.

[0030] Each layered oxysulfide shown in formula (1) possesses visible light absorption ability, thereby absorbing visible light and functioning as a photocatalyst. In particular, each layered oxysulfide shown in formula (1) has water decomposition capabilities (oxygen production and hydrogen production), and therefore can be used individually as a photocatalyst to decompose water upon visible light irradiation. Specifically, the upper end of the valence band of each layered oxysulfide is mainly composed of sulfur, which has a lower electronegativity than oxygen, so its upper end level is shifted significantly to the negative side compared to ordinary metal oxides, and as a result it has visible light absorption ability. Furthermore, the positions of the upper end of the valence band and the lower end of the conduction band of each layered oxysulfide are appropriate for the reduction and oxidation of water, respectively, giving it water decomposition capabilities (both water reduction and oxidation capabilities).

[0031] Visible light-responsive photocatalyst The visible light-responsive photocatalyst of this disclosure comprises the layered oxysulfide of this disclosure.

[0032] As described above, the layered oxysulfides of this disclosure absorb visible light and function as photocatalysts, and in particular absorb visible light and function as photocatalysts, to decompose water and thereby produce oxygen and / or hydrogen. Therefore, a photocatalyst of this disclosure, including such a layered oxysulfide, can decompose water and generate hydrogen and / or oxygen upon irradiation with visible light.

[0033] The visible light-responsive photocatalyst of this disclosure may include the layered oxysulfide of this disclosure, and rhodium particles and / or iridium oxide particles supported on the layered oxysulfide. In this case, the rhodium (Rh) particles can function as hydrogen generation sites, and the iridium oxide (IrO2) particles can function as oxygen generation sites.

[0034] 《Water splitting method》 A method of producing hydrogen and / or oxygen according to the present disclosure includes irradiating a dispersion in which the layered oxysulfide of the present disclosure is dispersed in water with visible light to decompose the water.

[0035] As described above, the layered oxysulfides of this disclosure absorb visible light and function as photocatalysts, and in particular absorb visible light and function as photocatalysts, can decompose water and thereby produce oxygen and / or hydrogen. Therefore, according to the water splitting method of this disclosure, in which a dispersion of the layered oxysulfides of this disclosure is dispersed in water is irradiated with visible light, water can be split and hydrogen and / or oxygen can be produced. In the method of this disclosure, sunlight can be used as the visible light.

[0036] When irradiating with visible light, for example, a xenon lamp equipped with a light cutoff filter can be used to irradiate only with visible light. In addition, sunlight can be used as the visible light in the method of this disclosure. In the method of this disclosure, the content of the layered oxysulfide of this disclosure in water is not limited, but in the step of using the layered oxysulfide of this disclosure as a photocatalyst, 0.01 to 0.5 g per 180 mL of water is preferred, and about 0.05 g is more preferred. [Examples]

[0037] The present disclosure will be specifically described below with reference to examples and comparative examples. However, the present disclosure is not limited to the examples.

[0038] The details of the physical property evaluation (crystal structure analysis, elemental composition identification, shape observation, light absorption characteristic evaluation, Mott-Schottky plot measurement, and quantum chemical (DFT) calculation) and photocatalytic activity evaluation of the samples (layered sulfides) synthesized in the examples and comparative examples are as follows.

[0039] <Crystal structure analysis> The crystal structure of each sample was identified using an X-ray diffractometer (Rigaku, MiniFlexII). CuKα rays generated at 30kV and 15mA were monochromatized using a carbon monochromator, and measurements were taken in the range of 2θ = 3 to 70 degrees. The crystal structure analysis of each sample was performed using a synchrotron X-ray diffractometer (SPring8, BL02B2), with monochromatic light of λ = 0.4139219 Å, and measurements were taken in the range of 2θ = 1.92 to 50.00 degrees. For refinement of the crystal structure, commercially available analysis software (Jana2006) was used, and the operation was repeated until the R factor (an indicator of the reliability of the analysis) was reduced.

[0040] <Elemental composition> The elemental composition of each sample was evaluated using an energy-dispersive X-ray spectrometer (Oxford, XMAX). Characteristic X-rays were detected with an acceleration voltage of 10 kV.

[0041] <Shape Observation> The powder form of each sample was observed using a scanning electron microscope (NVision 40, Carl Zeiss-SIINT) by scattering the sample powder onto carbon tape attached to a brass sample stage.

[0042] <Light absorption properties> The light absorption characteristics of each sample were evaluated using a UV-Vis spectrophotometer (Shimadzu, UV-2600i).

[0043] Specifically, measurements were performed using an integrating sphere with a standard reflector (BaSO4) as a reference, and the sample powder was measured under the following conditions: scanning range: 200-1400 nm, scanning speed: medium, sampling interval: 1.0 nm.

[0044] <Mott-Schottky plot measurement> A silver-silver chloride (Ag / AgCl) electrode was used as the reference electrode, a Pt electrode as the counter electrode, and a transparent conductive substrate (FTO) coated with each sample particle by squeegee was attached as the working electrode. Measurements were performed in a 0.1 mol / L sodium phosphate (Na2HPO4+Na3PO4) aqueous solution adjusted to pH 12 using sodium hydroxide, and the flat band potential was estimated from the x-intercept of the plot.

[0045] <Quantum chemistry (DFT) calculation> DFT calculations were performed using commercially available software (CASTEP) to determine the density of states (DOS) for each sample.

[0046] <Evaluation of photocatalytic activity - hydrogen generation capacity> The hydrogen generation capacity of each sample was evaluated by photocatalytic reactions using sodium sulfide and sodium sulfite as electron donors.

[0047] Specifically, 0.05 g of each sample particle was added to a Pyrex® reaction vessel along with aqueous solutions of sodium sulfide and sodium sulfite (10 mM, 180 mL). Then, an aqueous solution of rhodium chloride (RhCl3) was added to the photocatalyst particles to an amount equivalent to 1.0 wt% Rh, and the particles were dispersed by ultrasonic irradiation in an ultrasonic cleaner for approximately 30 seconds. The rhodium chloride was added to support rhodium (Rh) nanoparticles on the sample, which function as hydrogen generation sites on the photocatalyst.

[0048] The reaction vessel was connected to the flow system, and air contained in it was removed by flowing Ar gas through the flow system and into the reaction solution at a flow rate of 20 mL / min. For light irradiation, a commercially available 300W xenon lamp (Cermax, irradiation wavelength range 300-800 nm) was used, and a cutoff filter (HOYA, L-42) was attached to cut off wavelengths below 400 nm in order to irradiate only visible light.

[0049] The generated gas was analyzed using a gas chromatograph (GL Sciences, GC3210D, TCD detector, MS-5A column, Ar carrier) connected to the gas flow system.

[0050] <Evaluation of photocatalytic activity - oxygen generation capacity> The oxygen-producing capacity of each sample is determined by monovalent silver ions (Ag + This was evaluated by an oxygenation reaction using ) as an electron acceptor.

[0051] Specifically, 180 mL of silver nitrate (AgNO3) aqueous solution and 0.05 g of each sample particle were added to a Pyrex® reaction vessel, and the particles were dispersed by ultrasonic irradiation in an ultrasonic cleaner for approximately 30 seconds.

[0052] The reaction vessel was connected to the flow system, and air was removed from both the flow system and the reaction solution by flowing Ar gas at a rate of 20 mL / min. As with the hydrogen production capacity test, only visible light was used for irradiation, and the resulting gas was analyzed.

[0053] Prior to the reaction, iridium oxide (IrO2) colloidal particles were supported on the samples as oxygen generation sites using the following procedure. Specifically, 0.75 g of each sample particle and an IrO2 colloidal solution equivalent to 1.0 wt% of IrO2 per sample particle were added to a vial, and the particles were dispersed by ultrasonic irradiation using an ultrasonic cleaner. Then, the vial was covered with aluminum foil and stirred for 40 minutes while shielding from light, to adsorb and support the IrO2 colloidal particles onto each sample particle.

[0054] Comparative Example 1: Synthesis of layered oxide and evaluation of its physical properties <Synthesis of layered oxides> Layered oxide (NaMTiO4(M=La,Pr,Nd,Sm,Eu,Gd)) was synthesized using the following procedure.

[0055] Specifically, sodium carbonate (Na2CO3), metal oxide (M2O3), and titanium oxide (TiO2) were added in an elemental ratio of Na:M:Ti = 1.4:1:1 so that 1 g of layered oxide could be synthesized. These were then placed in an agate mortar and kneaded thoroughly with a pestle. After that, the mixture was calcined at 900°C or 950°C for 30 minutes to synthesize the layered oxide powder.

[0056] <Evaluation of physical properties of layered oxides> X-ray diffraction results of the obtained powder samples revealed peaks attributed to NaMTiO4 (see Figure 1). Elemental analysis of NaNdTiO4 and NaSmTiO4 powder samples using energy-dispersive X-ray spectroscopy also confirmed their theoretical compositions. The absorption edges of these samples, with the exception of NaEuTiO4, were all below 400 nm; for example, the band gap of NaLaTiO4 was estimated to be 4.04 eV (see Figure 3).

[0057] The semiconductor properties of each sample, estimated from these results and Mott-Schottky plot measurements, were investigated (see Figure 3 and Table 1). From the density of states (DOS) of NaLaTiO4 obtained by DFT calculation, it was confirmed that the upper end of the valence band is occupied by oxygen 2p orbitals, while the lower end of the conduction band is occupied by titanium 3d orbitals (see Figure 5). These property evaluation results were consistent with those reported in previous studies.

[0058] <Evaluation of photocatalytic activity of layered oxides> We investigated hydrogen production from sodium sulfide and sodium sulfite aqueous solutions using NaNdTiO4 and NaSmTiO4, but no hydrogen production was observed in either case (see Figure 6). We also investigated oxygen production from silver nitrate aqueous solution, but no oxygen production was observed in either case. Thus, it was confirmed that layered oxides do not function as photocatalysts for water splitting under visible light irradiation.

[0059] Example 1: Synthesis of layered oxysulfide and evaluation of its physical properties <Synthesis of layered oxysulfides> Layered oxysulfide (NaMTiO) is produced by following the procedure below. 4―x S x (M=La,Pr,Nd,Sm,Eu,Gd)) was synthesized.

[0060] Specifically, the layered oxide (NaMTiO4) powders obtained in Comparative Example 1 (or a mixed powder of sodium carbonate (Na2CO3), metal oxide (M2O3), and titanium oxide (TiO2)) were placed on an alumina board and set in a Pyrex® reaction tube (approximately 40 cm in length and 4 cm in inner diameter). The mixed powder was prepared by adding the raw materials in an elemental ratio of Na:M:Ti = 1.4:1:1 so that 1 g of layered oxysulfide could be synthesized, and then mixing them thoroughly in an agate mortar using a pestle. While hydrogen sulfide (H2S) diluted to 5 vol% with argon (99.99%) flowed through the reaction tube at a flow rate of approximately 100 mL / min, each layered oxide was heated at 950°C for 2 hours, and then allowed to cool naturally while replacing the inside of the reaction tube with argon. This yielded each layered oxysulfide (see Figure 1).

[0061] <Evaluation of the physical properties of layered oxysulfides> The X-ray diffraction patterns of each layered oxysulfide powder showed peaks shifted to lower angles than the diffraction peaks of the raw material layered oxide. This result indicates that the layered oxysulfide NaMTiO4, in which the oxygen atoms of the layered oxide NaMTiO4 are partially replaced by sulfur atoms with larger ionic radii due to hydrogen sulfide treatment, is a result of the treatment of NaMTiO4. 4―x S x This suggested that it had been synthesized.

[0062] Furthermore, using the Jana2006 analysis software, we analyzed a sample that was relatively close to single-phase: NaNdTiO 4―x S x and NaSmTiO 4―x S x The crystal structures of these materials were determined by analyzing their synchrotron X-ray diffraction patterns (see Figure 2).

[0063] Furthermore, elemental analysis using an energy-dispersive X-ray spectrometer confirmed that the sample had the composition described above. The X-ray diffraction patterns of these obtained crystal structures were as described above for NaMTiO 4―x S xThe X-ray diffraction patterns were also confirmed to match those of (M=La,Pr,Eu,Gd). Particle shape observation using a scanning electron microscope confirmed that all samples consisted of particles with a size ranging from several hundred nanometers to several micrometers.

[0064] To evaluate the intercalation hydration capacity of each layered acid sulfide, NaNdTiO 4―x S x and NaSmTiO 4―x S x When the layers were stirred in pure water (60°C) for 72 hours, the diffraction peaks shifted to lower angles (see Figure 3). This indicates that the interlayer distance of each layered oxysulfide increased due to the insertion of water molecules between the layers, and the lattice constant derived from the stacking direction increased. Furthermore, it was confirmed that vacuum drying of the hydrated layered oxysulfides caused water molecules to detach from the interlayers, shifting the diffraction peaks to higher angles. However, during stirring in pure water, some ion exchange between sodium ions in the layered oxysulfides and protons in the water occurred, causing the diffraction peaks to shift to higher angles than those of each layered oxysulfide. This was also confirmed by elemental analysis using an energy-dispersive X-ray spectrometer. From the above, it was confirmed that the layered oxysulfides possess interlayer hydration ability.

[0065] To evaluate the interlayer ion exchange capacity of each layered acid sulfide, NaNdTiO 4―x S x and NaSmTiO 4―x S x When the mixture was stirred in hydrochloric acid for 24 hours, the diffraction peaks shifted to lower angles (see Figure 3). This result, as described above, indicates intercalation hydration ability.

[0066] When each of these hydrated layered oxysulfides was vacuum-dried, water molecules were removed as described above, and the diffraction peak shifted to a higher angle. The diffraction peak obtained at this time was shifted to a higher angle than the diffraction peak obtained by stirring in pure water and then vacuum-drying. This indicates that the ion exchange between sodium ions in the layered oxysulfides and protons in hydrochloric acid proceeded to the greatest extent. Furthermore, elemental analysis using an energy-dispersive X-ray spectrometer also confirmed that the decrease in sodium after stirring was greater when stirred in hydrochloric acid than when stirred in pure water. These results confirm that the layered oxysulfides possess interlayer ion exchange capacity.

[0067] Table 1 shows the semiconductor properties of each sample estimated from light absorption characteristics evaluated using a UV-Vis spectrophotometer (Figure 4) and Mott-Schottky plot measurements. Unlike conventional layered oxides (NaNdTiO4, NaSmTiO4), it can be seen that the layered oxysulfides have a band gap (<3.0 eV) that can absorb visible light, as well as a lower conduction band level (<-0.71 V vs. NHE, at pH=12) and an upper valence band level (>0.52 V vs. NHE, at pH=12) that can reduce and oxidize water.

[0068] [Table 1]

[0069] DFT calculations for NaLaTiO4, NaLaTiO3S (x=1), and NaLaTiO2S2 (x=2) all show that the lower end of the conduction band is occupied by titanium 3d orbitals (see Figure 5). On the other hand, the upper end of the valence band was occupied by oxygen 2p orbitals in NaLaTiO4, and by sulfur 3p orbitals in NaLaTiO3S and NaLaTiO2S2. From these findings, it became clear that, compared to conventional layered oxides, layered oxysulfides have a significantly negative upper valence band level because the upper end of the valence band is formed by sulfur, which has a lower electronegativity than oxygen. As a result, they have a sufficiently negative lower conduction band level that allows for visible light absorption and water reduction.

[0070] <Evaluation of photocatalytic activity of layered oxysulfides> Each layered oxysulfide NaMTiO 4-x S x Using (M=La,Pr,Nd,Sm,Eu,Gd), hydrogen production from aqueous solutions of sodium sulfide and sodium sulfite was investigated, and in both cases, hydrogen production was confirmed under visible light irradiation (see Figure 6). Furthermore, oxygen production from aqueous solution of silver nitrate was also investigated, and the result was NaNdTiO 4-x S x Oxygen production was confirmed under visible light irradiation.

[0071] Furthermore, X-ray diffraction, elemental analysis, and absorption spectroscopy results after the photocatalytic reaction revealed that each layered oxysulfide functions stably as a photocatalyst. In addition, X-ray diffraction after the photocatalytic reaction confirmed a shift of the diffraction peaks indicating the interlayer hydration ability of each layered oxysulfide to lower angles.

Claims

1. It is composed of alternating layers of oxysulfide and interlayer ions. It possesses interlayer hydration ability that allows water molecules to be inserted between layers, and interlayer ion exchange ability that allows interlayer ions to be exchanged for heterogeneous ions, and It absorbs visible light and functions as a photocatalyst. Layered sulfur oxides.

2. The layered oxysulfide according to claim 1, which is a Ruddlesden-Popper type compound having the following formula (1), or a Dion-Jacobson type compound having the following formula (1): M ｙ M’ ｚ M” ｎ O ３ｎ＋１－ｘ S ｘ …(1) [During the ceremony, M and M' are, independently, hydrogen, alkali metals, alkaline earth metals, or rare earth metals. M'' is a transition metal, If it is a Ruddlesden-Popper type compound, then y + z = n + 1, 1 ≤ n, 0 < x < 3n + 1, and If it is a Dion-Jacobson type compound, then y = 1, z = n-1, 2 ≤ n, and 0 < x < 3n + 1.

3. The layered oxysulfide according to claim 1, which is a Ruddlesden-Popper type compound.

4. It is a Ruddlesden-Popper type compound, and Formula MM'M"O ４－ｘ S ｘ (0 < x < 4) The layered sulfur oxide according to claim 1.

5. Formula NaLnTiO ４－ｘ S ｘ The layered oxysulfide according to claim 1, having ４－ｘ S ｘ (Ln is a lanthanoid element and 0 < x < 4).

6. A visible light-responsive photocatalyst comprising a layered oxysulfide according to any one of claims 1 to 5.

7. The aforementioned layered oxysulfide, and Rhodium particles and / or iridium oxide particles supported on the aforementioned layered oxysulfide The photocatalyst according to claim 6, including the above.

8. A method for producing hydrogen and / or oxygen, comprising irradiating a dispersion in which a layered oxysulfide according to any one of claims 1 to 5 is dispersed in water with visible light to decompose water.

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