Metal-iron oxide composite thin film and hydrogen generation device using the same

Abstract

To provide a composite thin film having a structure in which nanoparticles of an element of which work function is relatively large, such as Pt, are dispersed in a matrix of α-Fe2O3.SOLUTION: A metal-iron oxide composite thin film includes 0.01-10 at.% Pt and has a structure in which a nanoparticle X1 comprising the metal and of which a particle size is 2-15 nm is dispersed in a matrix X2 comprising a hematite expressed by a general formula of Fe2O3-δ (however, 0≤δ≤0.32).SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to composite thin films of metal and iron oxide. [Background technology]

[0002] A thin film was formed by sputtering in an Ar atmosphere using a TiO2 disk and a Pt wire with a diameter of 0.5 mm as targets. The thin film was then heat-treated at 600°C in air, resulting in a Pt 4+ It has been reported that Pt, which was dissolved in TiO2 in the form of Pt, becomes Pt after heat treatment and undergoes phase separation from TiO2 (see, for example, Non-Patent Document 1). [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] T. sasaki, N. Koshizak, S. Terauchi, H. Umebara, Y. Matsumoto, M. Koinuma, PREPARATIONOFPt / TiO2NANOCOMPOSITEFILMSUSINGCO-SPUTTERINGMETHOD, NanostructuredMaterials, 8 (1997) 1077. DISCLOSURE OF THEINVENTION [Problem to be solved by the invention]

[0004] Incidentally, α-Fe2O3 (hematite) absorbs light in the ultraviolet to visible light range to generate electron-hole pairs, and Pt has a large work function and tends to accumulate the generated electrons. Therefore, if a thin film in which Pt and α-Fe2O3 are phase-separated can be created, it will be possible to cause a reduction reaction by Pt and an oxidation reaction by α-Fe2O3 on the surface of the thin film, and it is expected that the range of application of this function will be expanded.

[0005] Therefore, an object of the present invention is to provide a composite thin film having a structure in which nanoparticles of an element with a relatively large work function, such as Pt, are dispersed in an α-Fe2O3 matrix. [Means for solving the problem]

[0006] In the sputtering method, the greater the difference in heat of formation between the oxide of the element used in the nanoparticles and the oxide used in the matrix, the more thermodynamically stable the element is and the more likely it is to be nanoscaled. For example, if the element is Pt, the heat of formation ΔH of its oxide PtO2 is 0 is +40.3 kcal / mol. In contrast, the heat of formation of TiO2, ΔH 0 is -225.8 kcal / mol, and the heat of formation of α-Fe2O3 (hematite), ΔH 0 is -196.3 kcal / mol. Therefore, when α-Fe2O3 is used as the matrix, the thermal stability of Pt is lower than when TiO2 is used as the matrix, and it is predicted that phase separation between Pt and hematite is difficult.

[0007] However, according to the research of the present inventors, it has been found that, contrary to this prediction, it is possible to realize a composite thin film in which a metal having a relatively large work function, such as Pt, and hematite are phase-separated.

[0008] The metal-iron oxide composite thin film of the present invention based on this finding is as follows: Pt nanoparticles containing 0.01-10 at.% Pt and with a particle size of 2-15 nm are formed according to the general formula Fe2O 3-δ (where 0≦δ≦0.32) is dispersed in a matrix of hematite.

[0009] The hydrogen generation device of the present invention uses the metal-iron oxide composite thin film to generate hydrogen gas by electrolyzing water through an oxidation reaction by the Pt nanoparticles and a reduction reaction by the matrix. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram illustrating the configuration of a metal-iron oxide composite thin film according to one embodiment of the present invention. [Diagram 2] FIG. 1 is an explanatory diagram showing the composition analysis results of a number of Pt-doped α-Fe2O3 thin film samples. [Diagram 3] FIG. 1 is an explanatory diagram of the light wavelength dependence of the light transmittance of each of several samples (Pt-doped α-Fe2O3 thin films) with different heat treatment times. [Figure 4] FIG. 1 shows XRD patterns of metal-iron oxide composite thin films (Pt content: 6.3 at.%) with different heat treatment times. [Diagram 5] A partially enlarged view of the XRD pattern in Figure 4. [Figure 6] FIG. 2 is an explanatory diagram regarding magnetization of metal-iron oxide composite thin films (Pt content: 6.3 at.%) with different heat treatment times. [Figure 7] FIG. 1 shows Raman spectra of a number of samples having different Pt contents. [Figure 8] FIG. 1 shows XRD spectra of a number of samples having different Pt contents. [Figure 9] FIG. 1 is an explanatory diagram of the particle size of Pt nanoparticles in each of a number of samples having different Pt contents. [Figure 10] FIG. 2 is an explanatory diagram showing the light wavelength dependence of the light transmittance of each of a number of samples having different Pt contents. [Figure 11] FIG. 1 is a configuration explanatory diagram of a hydrogen generation device according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] (composition) As an embodiment of the present invention, as shown in FIG. 1, a metal-iron oxide composite thin film contains 0.01 to 10 at. % Pt, and Pt nanoparticles X1 made of the metal and having a particle size of 2 nm to 15 nm are represented by the general formula FeO 3-δ(wherein 0≦δ≦0.32) is dispersed in a matrix X2 made of hematite. The Pt content is preferably 0.5 to 9.5 at.%, more preferably 2.0 to 8.0 at.%, and even more preferably 3.5 to 6.5 at.%. δ representing the amount of oxygen vacancy in hematite is preferably 0 to 0.22, more preferably 0 to 0.12, and even more preferably 0 to 0.06.

[0012] (Manufacturing method) A composite target consisting of a hematite (α-Fe2O3) target with Pt chips placed on it is placed in a thin film production apparatus (e.g., a sputtering apparatus). The number and / or size of the Pt chips are adjusted to adjust the atomic percentage of Pt in the composite thin film. A substrate, such as a Corning #7059 glass substrate, is then sputter-etched for an appropriate period of time, after which an AC voltage is applied to the target to deposit a Pt-doped α-Fe2O3 thin film (as-deposited thin film) in an atmosphere such as Ar gas.

[0013] Figure 2 shows the composition analysis results for several Pt-doped α-Fe2O3 thin film samples. From Figure 2, it can be seen that the composition of the Pt-doped α-Fe2O3 thin film is close to the phase separation line (the dashed line on the left side of Figure 2). In addition, the oxygen concentration of the Pt-doped α-Fe2O3 thin film is low, suggesting the presence of oxygen vacancies in the thin film.

[0014] Next, the Pt-added α-Fe2O3 thin film was heat-treated in air at a predetermined heat treatment temperature, for example, in the temperature range of 673 to 973 K, for a predetermined heat treatment period, for example, in the range of 10 minutes to 500 days. As a result, phase separation of Pt and α-Fe2O3 progressed, and Pt nanoparticles X1 containing 0.01 to 10 at.% Pt and having a particle size of 2 nm to 15 nm were formed according to the general formula Fe2O 3-δA metal-iron oxide composite thin film having a structure in which iron oxide is dispersed in a matrix X2 made of hematite represented by δ (where 0≦δ≦0.32) is produced (see FIG. 1). The metal-iron oxide composite thin film is taken out of the chamber after the inside of the chamber is purged with an appropriate gas such as nitrogen.

[0015] Figure 3 shows the measurement results of the optical wavelength dependence of the optical transmittance of each of several samples obtained by heat-treating a Pt-doped α-Fe2O3 thin film (AsDepo thin film) with a Pt content adjusted to 6.3% in air at 673K, with different heat treatment times (0hr, 82hr, 226hr, 23days, 53days, 113days). It can be seen from Figure 3 that the optical transmittance of the sample increases as the heat treatment time increases.

[0016] Figure 4 shows the XRD patterns of the metal-iron oxide composite thin films (Pt content 6.3 at.%) with different heat treatment times. Figure 5 shows an enlarged view of a part of the XRD pattern. Specifically, the XRD patterns were measured using X-ray diffraction (XRD) with CuKα radiation (Rigaku, RAD-X, Japan). Figures 4 and 5 show that as the heat treatment time increases, the (116) peak and (018) peak derived from α-Fe2O3 gradually become higher, the (111) peak derived from Pt gradually becomes higher, and the (422) and (511) peaks derived from Fe3O4 disappear.

[0017] From the results shown in FIG. 3, FIG. 4 and FIG. 5, it is understood that the oxygen deficiency of α-Fe2O3 (hematite) constituting the matrix X2 is improved as the heat treatment time is increased.

[0018] Figure 6 shows the magnetization (magnetization at 10 kOe) of metal-iron oxide composite thin films (Pt content 6.3 at.%) with different heat treatment times. Specifically, the magnetization of the thin films was measured at ambient temperature using a vibrating sample magnetometer (Tamakawa, TM-VSM-2430, Japan). It can be seen from Figure 6 that the magnetization decreases with the heat treatment time. The inset in Figure 6 shows the magnetization curves in the as-formed state and after heat treatment for 113 days in air. In the as-formed state, hysteresis suggesting ferromagnetism is clearly observed, whereas it disappears after heat treatment for 113 days. Hematite is a parasitic ferromagnet at room temperature, and the magnetization is negligibly small. This shows that as the heat treatment time increases, the oxygen deficiency of hematite is compensated and the original composition ratio is obtained.

[0019] Figure 7 shows the Raman spectra of several samples obtained by heat-treating several as-deposited thin films with different Pt contents (0 at.%, 1.6 at.%, 3.8 at.%, 4.6 at.%, 7.0 at.%, 9.4 at.%) in air at 673 K for 113 days. It can be seen from Figure 7 that all the peaks are derived from α-Fe2O3 (hematite).

[0020] Figure 8 shows the XRD patterns of several samples obtained by heat-treating several as-deposited thin films with different Pt contents (0 at.%, 3.8 at.%, 6.3 at.%, 9.4 at.%) in air at 673 K for 113 days. It can be seen from Figure 8 that the higher the Pt content, the higher the Pt-derived (111) peak in the sample becomes.

[0021] FIG. 9 shows the particle size of Pt (nanoparticles X1) in each of a number of samples obtained by heat-treating a number of as-deposited thin films with different Pt doping amounts (3.8 at.%, 6.3 at.%, 7.0 at.%, 9.4 at.%) in air at 673 K for 113 days. FIG. 9 also shows the particle size of Pt in a number of as-deposited thin films with different Pt doping amounts (6.3 at.%, 7.0 at.%, 9.4 at.%). The particle size t of Pt nanoparticles X1 is calculated by the Scherrer coefficient of 0.9, the wavelength λ of X-rays, the diffraction line width B of the (111) peak derived from Pt, and the diffraction angle θ B It was calculated according to Scherrer's formula (1) using

[0022] t=0.9λ / Bcosθ B ...(1).

[0023] It can be seen from FIG. 9 that the particle size of the Pt nanoparticles X1 in the sample increases as the amount of Pt added increases.

[0024] FIG. 10 shows the measurement results of the optical wavelength dependence of the optical transmittance of each of a number of samples obtained by heat-treating a number of as-deposited thin films with different Pt doping amounts (0 at.%, 1.6 at.%, 3.8 at.%, 6.3 at.%, 9.4 at.%) in air at 673 K for 113 days. It can be seen from FIG. 10 that with an increase in Pt concentration, the wavelength at which the optical transmittance becomes almost zero (optical absorption edge) shifts to the longer wavelength side and the steepness becomes gentler. As shown in FIG. 9, Pt exists in the thin film at the nanoscale, but does not exhibit optical absorption (surface plasmon resonance) associated with nanoscaling like Au and Cu, so the shift in the optical absorption edge and the change in steepness shown in FIG. 10 are due to optical scattering by Pt nanoparticles X1.

[0025] The hydrogen generation apparatus according to one embodiment of the present invention shown in Fig. 11 includes a substrate 1 having a metal-iron oxide composite thin film 2 according to one embodiment of the present invention formed on its surface, and a light irradiation device 4 that irradiates the metal-iron oxide composite thin film 2 with light. The substrate 1 may form a wall of a chamber and / or a flow path that is in contact with water. A window or an opening may be formed in the chamber and / or the flow path, and the light from the light irradiation device 4 may be irradiated through the window onto the metal-iron oxide composite thin film 2 that constitutes the inner surface of the wall of the chamber and / or the flow path. In an environment in which the metal-iron oxide composite thin film 2 is irradiated through a window, the light irradiation device 4 may be omitted.

[0026] In the hydrogen generator having the above configuration, the Pt nanoparticles X1 and the matrix X2 made of α-Fe2O3 are phase-separated in the metal-iron oxide composite thin film 2, so that a reduction reaction by the metal and an oxidation reaction by α-Fe2O3 can be caused on the surface of the metal-iron oxide composite thin film 2, and hydrogen gas can be generated by electrolysis of water. [Explanation of symbols]

[0027] 1‥Base material 2. Metal-iron oxide composite thin film 4‥Light irradiation device X1: Pt nanoparticles X2...The Matrix.

Claims

1. Pt nanoparticles containing 0.01 to 10 at. % Pt and having a particle size of 2 nm to 15 nm are represented by the general formula Fe 2 O 3-δ (where 0≦δ≦0.32) Metal-iron oxide composite thin film.

2. 2. A hydrogen generating device that uses the metal-iron oxide composite thin film according to claim 1 to generate hydrogen gas by electrolyzing water through an oxidation reaction caused by the Pt nanoparticles and a reduction reaction caused by the matrix.

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