JP2015059049A - Semiconductor material, photo-electrode material, photocatalyst material, and method of producing semiconductor material - Google Patents

Abstract

A material with improved photocurrent response is obtained.
The present invention relates to a semiconductor material in which a transition metal element M is doped in a crystal having a composition represented by AB ₂ O ₄ . The doping of M is performed by substituting a part of the A or B site with M, doping M between lattices in the AB ₂ O ₄ crystal, or doping the crystal grain boundary of AB ₂ O ₄ with M. Done either in one or a combination of these. The crystal system is a calcium ferrite type or a spinel structure.
[Selection] Figure 1

Description

The present invention relates to a material having a crystal having an AB ₂ O ₄ composition and a method for producing the material.

  In photocatalysts, solar cells, and the like, there is a demand to use a p-type semiconductor having a conduction band lower end potential as close to the base as possible.

CaFe ₂ O ₄ is known as a semiconductor exhibiting p-type characteristics composed of inexpensive elements with abundant resources, but because of its low electrical conductivity, photoresponsiveness and photocatalytic activity as a photocatalyst or battery Is low.

As a means for improving electric conductivity, doping with other elements can be considered. Non-Patent Document 1 discloses improvement of p-type characteristics using a CaFe ₂ O ₄ photoelectrode doped with Na and Mg. This photoelectrode is produced by pyrolyzing a nitride containing Ca, Fe, and Mg at 1000 ° C. in an oxygen atmosphere. As a result of the photoelectrochemical measurement in the aqueous solution, the photocurrent increased from the potential position (negative potential) lower than 0 V (vs RHE (reversible hydrogen electrode)) compared to undoped CaFe ₂ O ₄ . However, in this photoresponse characteristic, a positive current flows at a potential position (positive potential) nobler than 0 V, and a negative current flows at a negative potential, and the behavior is not p-type or n-type semiconductor. In other words, it is a characteristic in which a considerable amount of leakage current is superimposed, and the characteristic as a semiconductor is considered to be very small.

Matsumoto, Y .; Sugiyama, K .; Sato, E. I., “IMPROVEMENT OF CaFe2O4PHOTOCATHODE BY DOPING WITH Na AND Mg” Journal of Solid State Chemistry 1988, 74, 117

  As described above, the optical response characteristic of the material of Non-Patent Document 1 is not sufficient, and it cannot be said that the characteristic is particularly a p-type semiconductor. Furthermore, in order to synthesize the material, a high temperature treatment of 1200 ° C. is necessary, and it is often difficult to synthesize the material in combination with other materials.

The semiconductor material according to the present invention is a semiconductor material in which a transition metal element M is doped in a crystal having a composition represented by AB ₂ O ₄ , and the doping of M includes a part of the A or B site. Substitution with M, doping M between lattices in the AB ₂ O ₄ crystal, or doping M into the grain boundary of AB ₂ O ₄ , or a combination thereof, and the crystal system Is a calcium ferrite type or a spinel structure.

  In one embodiment, the semiconductor material includes at least one of Ca, Sr, Cu, Mg, Ba, and Zn as A and at least one of Fe and Bi as B.

  Further, the semiconductor material according to an embodiment includes at least one of Ag, Cu, and Au as the transition metal element M.

  In the semiconductor material according to one embodiment, the M / B ratio x of the surface composition observed by X-ray photoelectron spectroscopy is 0 <x <2.6.

  In addition, the semiconductor material according to one embodiment has a calcium ferrite type crystal system.

  Moreover, the semiconductor material which concerns on one Embodiment has increased the light absorbency of visible light compared with the undoped thing by doping M.

  In addition, the photoelectrode material according to an embodiment is a photoelectrode material using the semiconductor material described above, and is irradiated with light in a solution under an external potential of 0.0 V with respect to an Ag / AgCl electrode standard. It produces a larger photocathode current than CaFe 2 O 4 not doped with M.

  The photocatalytic material according to one embodiment is a photocatalytic material using the semiconductor material described above, and causes a reduction reaction of water or oxygen when irradiated with visible light.

A method for manufacturing a semiconductor material according to an embodiment is the above-described method for manufacturing a semiconductor material, wherein the target of AB ₂ O _{4 and} the target of M or its oxide are simultaneously formed by RF magnetron sputtering. By performing sputtering by applying electric power, simultaneous film formation and heat treatment in the range of 600 ° C. to 700 ° C. to crystallize.

  A method for manufacturing a semiconductor material according to an embodiment is the above-described method for manufacturing a semiconductor material, in which any one of oxides, carbonates, lead nitrates, and acetates of A, B, and M is mixed in water. It is formed by calcination after removing moisture by evaporation under reduced pressure.

According to the present invention, it is possible to obtain a material of AB ₂ O ₄ having sufficient photoresponse characteristics as a p-type semiconductor. Further, the material can be obtained by a relatively low temperature treatment of 600 ° C. to 700 ° C.

Oxygen reduction current CaFe ₂ O ₄ by introduction of Ag (Current mA) - is a potential diagram showing changes in (Potential V) characteristics (600 ° C. heat treatment). Oxygen reduction current CaFe ₂ O ₄ by introduction of Ag (Current mA) - is a potential diagram showing changes in (Potential V) characteristics (650 ° C. heat treatment). A diagram (600 ° C. heat treatment) showing a change in X-ray diffraction pattern of CaFe ₂ O ₄ by introduction of Ag. A diagram (650 ° C. heat treatment) showing a change in X-ray diffraction pattern of CaFe ₂ O ₄ by introduction of Ag. Is a graph showing changes in Ag CaFe ₂ O ₄ optical absorption spectra and monochromatic light-current conversion efficiency with and without the introduction of (IPCE).

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that the present invention is not limited to the embodiments described herein.

"Basic embodiment"
The semiconductor material according to the embodiment of the present invention has a crystal system having a composition represented by AB ₂ O ₄ . And the transition metal element M is doped in the crystal | crystallization which has this composition. Here, the transition metal element M is doped by replacing a part of the site of A with M, doping M between lattices in the AB ₂ O ₄ crystal, or crystal grain boundaries of AB ₂ O _4. Or any combination thereof. Therefore, the transition metal element M is doped while maintaining the basic AB ₂ O ₄ crystal system. Note that the crystal system that fluctuates the composition represented by AB ₂ O ₄ is a calcium ferrite type or a spinel structure.

Highly efficient optical electrode, in building a photocatalyst and a solar cell device, for example, CO ₂ conduction band potential to a possible base side than the reduction potential and the hydrogen generation potential is located, supply electrons efficiently from the surface to the outside Development of p-type semiconductor materials that can be developed is desired.

For example, CaFe ₂ O ₄ , one of calcium ferrite materials having a composition of AB ₂ O ₄ , is a p-type semiconductor whose conduction band bottom potential is located on the relatively base side, but its electrical conductivity is extremely low, The electrons cannot be efficiently supplied to the outside. On the other hand, the M-doped AB ₂ O _{4 of} this embodiment shows a higher electrical conductivity than the undoped one. In addition, an increase in absorbance in the visible light region is observed due to the doping of the transition metal element M.

  Therefore, if the semiconductor material of this embodiment is applied as a photoelectrode or a photocatalyst, the target substance can be efficiently reduced. Moreover, if it uses as a p-type layer of a solar cell, there is a possibility that a device having a large release voltage and showing high photoelectric conversion efficiency can be constructed. Furthermore, because of the effect of the transition metal element M doping, high photoresponsiveness can be seen in a heat treatment at a relatively low temperature compared to undoped at about 600 ° C., so that it can be synthesized at a lower cost.

Here, although the reason why the photocurrent response of the semiconductor material having the composition of AB ₂ O ₄ is improved by doping with the transition metal M has not been clarified, it has been reported that metal doping of the oxide semiconductor has been reported so far. The following is inferred from the case where the improvement of the p-type characteristics is realized.

B in AB ₂ O ₄ exists mainly as trivalent ions B ³⁺ , but it is considered that B ⁴⁺ is generated when M ²⁺ replaces A ²⁺ or B ³⁺ by M ⁺ . B ⁴⁺ forms an acceptor level at a lower potential than the top of the valence band, and generates holes in the valence band. For this reason, it is interpreted that the conductivity and the absorbance in the visible region are improved. Further, it is considered that Ca ²⁺ (99 pm) or Fe ³⁺ (64 pm) and Ag ⁺ (126 pm) having greatly different ionic radii may be replaced to improve crystallinity and expose a new crystal phase. . As a result of these, it is presumed that a semiconductor material exhibiting high photocathode responsiveness was obtained. Therefore, a configuration in which M replaces A or B is considered suitable.

In addition, in CaFe ₂ O ₄ , the conduction band is formed by Fe 3d and 4s orbitals, and it is assumed that the mobility may be improved by involving the molecular orbitals of Ag.

  The element A preferably includes at least one of Ca, Sr, Cu, Mg, Ba, and Zn, and the element B preferably includes at least one of Fe and Bi. is there.

  Moreover, it is preferable that at least one of Ag, Cu, and Au is included as the transition metal element M.

  Further, the doping amount of the transition metal element M is such that the M / B ratio x of the surface composition observed by X-ray photoelectron spectroscopy is 0 <x <2.6, preferably 0 <x <1.7, more preferably Has been confirmed to be 0.05 <x <0.4.

Such a photocathode larger than CaFe ₂ O ₄ not doped with M when it is irradiated with light in a solution under an external potential of 0.0 V with respect to an Ag / AgCl electrode, based on an Ag / AgCl electrode. Those that generate current are suitable as photoelectrode materials.

  In addition, a semiconductor material in which the absorbance of visible light is increased by doping with M compared to an undoped material is suitable as a photocatalytic material, and causes a reduction reaction of water and oxygen when irradiated with visible light. What is to be done is suitable.

<Production method>
Examples of the method for manufacturing the semiconductor material as described above include the following methods.

Using an RF magnetron sputtering apparatus, sputtering is performed by simultaneously applying electric power using an AB ₂ O ₄ target or an oxide thereof as a target. As a result, the radiation from both targets is deposited, so that AB ₂ O ₄ doped with M can be formed. Moreover, it heat-processes in the range of 600 to 700 degreeC. Thereby, AB ₂ O ₄ is crystallized, and a semiconductor doped with Ag in an M-doped CaFe ₂ O ₄ crystal is obtained.

  Also, by mixing any of oxides of A, B, and M, or carbonates, lead nitrates, and acetates in water, evaporating and removing moisture by reducing the pressure, and then firing to form, The semiconductor material of this embodiment can be manufactured.

“Embodiment 1”
In the first embodiment, CaFe ₂ O ₄ is doped with Ag. Thus, (i) a portion of the Ca or Fe site of CaFe ₂ O ₄ is replaced with Ag, (ii) Ag is doped between the lattices in the CaFe ₂ O ₄ crystal, (iii) CaFe ₂ O Ag-doped CaFe ₂ O ₄ is obtained by either or a combination of Ag-doped grain boundaries of _four crystals.

<Production method>
In this example, RF magnetron sputtering is used. Sputtering is performed in Ar plasma using CaFe ₂ O ₄ and Ag as targets, and an Ag-doped CaFe ₂ O ₄ film is deposited on the substrate. Thereafter, heat treatment is performed to crystallize CaFe ₂ O ₄ . Thereby, a semiconductor doped with Ag in a CaFe ₂ O ₄ crystal is obtained.

Here, in this example, the use as a photoelectrode is considered, and the semiconductor is deposited on the transparent electrode by using ATO (Sb-doped SnO ₂ ) deposited on the transparent glass. Moreover, heat processing shall be 600 to 700 degreeC. This makes it possible to use a glass substrate on which ATO is deposited. The heat treatment is aimed at crystallization and is performed in an oxygen gas atmosphere.

The conditions of this manufacturing method are summarized as follows.
Target: CaFe ₂ O ₄ and Ag
・ Sputtering gas: Ar, 50 sccm
-Input power: CaFe ₂ O ₄ : 500 W, Ag: 0, 20, 35, 40, 45, 50 W
Substrate: a transparent glass on the transparent electrode ATO (Sb-doped SnO ₂₎ the 100nm deposited shall Film thickness: about 200nm
-Post heat treatment temperature: 600, 650, 700, 2 hours at any one of 750 ° C-Post heat treatment atmosphere: under oxygen stream (in oxygen atmosphere)

In Example 1-4, Ag was doped by sputtering, but a wet synthesis method described later can also be adopted for Ag doping.
<Example 1-4>
○ Example 1
Thin film electrode synthesized by applying power of 500 W to the CaFe ₂ O ₄ target and 35 W to the Ag target and synthesized by heat treatment at 600 ° C. for 2 hours ○ Example 2
Thin film electrode synthesized by applying power of 500 W to a CaFe ₂ O ₄ target and 20 W to an Ag target and synthesized by heat treatment at 650 ° C. for 2 hours. Example 3
Thin film electrode synthesized by applying power of 500 W to the CaFe ₂ O ₄ target and 35 W to the Ag target and synthesized by heat treatment at 650 ° C. for 2 hours ○ Example 4
Thin film electrode synthesized by applying power of 500 W to the CaFe ₂ O ₄ target and 50 W to the Ag target and synthesized by heat treatment at 650 ° C. for 2 hours <Comparative Example 1-2>
○ Comparative Example 1
A thin film electrode formed by applying 500 W power to a CaFe ₂ O ₄ target and synthesized by heat treatment at 600 ° C. for 2 hours in an oxygen stream. Comparative Example 2
Thin film electrode formed by applying 500 W power to CaFe ₂ O ₄ target and heat-treated at 650 ° C. for 2 hours under oxygen stream

“Embodiment 2”
In the second embodiment, CaFe ₂ O ₄ is doped with Cu. Thus, (i) a portion of the Ca or Fe site of CaFe ₂ O ₄ is replaced with Cu, (ii) Cu is doped between the lattices in the CaFe ₂ O ₄ crystal, (iii) CaFe ₂ O Cu-doped CaFe ₂ O ₄ is obtained by either or a combination of Cu-doped grain boundaries of _four crystals.

<Example 5-8>
Next, examples of the present invention with different synthesis methods and forms are shown.

Example 5
An aqueous solution in which calcium acetate monohydrate and iron nitrate nonahydrate were dissolved in distilled water was prepared, and an aqueous solution in which copper nitrate trihydrate was dissolved in distilled water was prepared. These were weighed and mixed so that the molar ratio of Ca and Fe was 1: 2, and the molar ratio of Cu and Ca was 0.5%. This mixed solution was mixed for 15 minutes and then dried at 120 ° C. Firing was performed at 450 ° C. for 2 hours in the atmosphere, and then sintering was performed at 1000 ° C. for 6 hours. A CaFe ₂ O ₄ film containing Cu was formed by dispersing the powder in ethanol and applying a thin coating on platinum night at 1,200 ° C. for 2 hours.

Example 6
An aqueous solution in which calcium acetate monohydrate and iron nitrate nonahydrate were dissolved in distilled water was prepared, and an aqueous solution in which copper nitrate trihydrate was dissolved in distilled water was prepared. These were weighed and mixed so that the molar ratio of Ca and Fe was 1: 2, and the molar ratio of Cu and Ca was 1.5%. This mixed solution was mixed for 15 minutes and then dried at 120 ° C. Firing was performed at 450 ° C. for 2 hours in the atmosphere, and then sintering was performed at 1000 ° C. for 6 hours. A CaFe ₂ O ₄ film containing Cu was formed by dispersing the powder in ethanol and applying a thin coating on platinum night at 1,200 ° C. for 2 hours.

Example 7
An aqueous solution in which calcium nitrate tetrahydrate and bismuth nitrate pentahydrate were dissolved in distilled water was prepared, and further an aqueous solution in which copper nitrate trihydrate was dissolved in distilled water was prepared. These were weighed and mixed so that the molar ratio of Ca and Bi was 1: 2, and the molar ratio of Cu and Ca was 0.5%. A mixed solution obtained by adding ethylenediaminetetraacetic acid and citric acid to this was mixed for 30 minutes, dried at 120 ° C., and calcined at 350 ° C. for 2 hours in the air. A CaBi ₂ O ₄ film containing Cu was formed by dispersing the powder in ethanol and applying a thin coating on platinum overnight at 800 ° C. for 6 hours.

Example 8
An aqueous solution in which calcium nitrate tetrahydrate and bismuth nitrate pentahydrate were dissolved in distilled water was prepared, and further an aqueous solution in which copper nitrate trihydrate was dissolved in distilled water was prepared. These were weighed and mixed so that the molar ratio of Ca and Bi was 1: 2, and the molar ratio of Cu and Ca was 1.5%. A mixed solution obtained by adding ethylenediaminetetraacetic acid and citric acid to this was mixed for 30 minutes, dried at 120 ° C., and calcined at 350 ° C. for 2 hours in the air. A CaBi ₂ O ₄ film containing Cu was formed by dispersing the powder in ethanol and applying a thin coating on platinum overnight at 800 ° C. for 6 hours.

<Comparative Example 3-4>
○ Comparative Example 3
An aqueous solution in which calcium acetate monohydrate and iron nitrate nonahydrate were each dissolved in distilled water was prepared, and these were weighed and mixed so that the molar ratio of Ca to Fe was 1: 2. This mixed solution was mixed for 15 minutes and then dried at 120 ° C. Firing was performed at 450 ° C. for 2 hours in the atmosphere, and then sintering was performed at 1000 ° C. for 6 hours. A CaFe ₂ O ₄ film was formed by dispersing this powder in ethanol and applying a thin coating on platinum overnight at 1200 ° C. for 2 hours.

○ Comparative Example 4
An aqueous solution in which calcium nitrate tetrahydrate and bismuth nitrate pentahydrate were dissolved in distilled water was prepared, and these were weighed and mixed so that the molar ratio of Ca and Bi was 1: 2. A mixed solution obtained by adding ethylenediaminetetraacetic acid and citric acid to this was mixed for 30 minutes, dried at 120 ° C., and calcined at 350 ° C. for 2 hours in the air. A CaBi ₂ O ₄ film was formed by dispersing the powder in ethanol and applying a thin coating on platinum overnight at 800 ° C. for 6 hours.

<Test results>
For the photoresponsive cyclic voltammetry measurement of the thin film electrode samples obtained in Example 1-8 and Comparative Example 1-4, refer to 0.2 M K ₂ SO ₄ aqueous solution saturated with oxygen using a potentiostat. This was performed while changing the bias potential with respect to the electrode. Ag / AgCl was used for the reference electrode and Pt was used for the counter electrode. A 500 W xenon lamp (manufactured by Asahi Spectroscope) was used as the irradiation light source. Light irradiation was performed while adjusting the light in the entire wavelength region of the xenon lamp to 45 mW / cm ² and continuously turning on and off with a chopper.

FIG. 1 and FIG. 2 show changes in oxygen reduction current characteristics of CaFe ₂ O ₄ (photocurrent profile in an oxygen atmosphere) depending on the amount of Ag introduced. The horizontal axis represents the potential (Potential / unit V) with respect to the Ag / AgCl reference electrode, and the vertical axis represents the current (Current / unit mA). In FIG. 1, the electrode was heat-treated at 600 ° C. in an oxygen atmosphere after film formation, and FIG. 2 was heat-treated at 650 ° C. in an oxygen atmosphere for 2 hours after film formation.

In FIG. 1, Example 1 (solid line) is a photocurrent profile in an oxygen atmosphere for CaFe ₂ O ₄ that is 35 W-powered, doped with Ag, and heat-treated at 600 ° C., according to light irradiation. A photocathode response corresponding to electron donation from the electrode to oxygen was observed as the current changed. On the other hand, Comparative Example 1 (broken line) is a photocurrent profile of an undoped CaFe ₂ O ₄ electrode in an oxygen atmosphere, and even when the potential is swept to the base side while repeating the light ON / OFF, the optical response is not observed. I couldn't. From this, it can be seen that the introduction of Ag makes it possible to develop the photoresponsiveness even under the heat treatment conditions of 600 ° C. in which only CaFe ₂ O ₄ does not show the photoresponsiveness.

FIG. 2 shows Ag (20 W (Example 2), 35 W (Example 3), 50 W (Example 4)) doped CaFe ₂ O ₄ electrode and undoped (Comparative Example 2) CaFe ₂ O heat-treated at 650 ° C. _The photocurrent profile in the oxygen atmosphere of ₄ electrodes is shown. When heat-treated at 650 ° C., a photocathode response was observed even with an undoped CaFe ₂ O ₄ electrode (Comparative Example 2). The electrode produced by co-sputtering with Ag20W (Example 2) is almost the same as Comparative Example 2, but slightly higher and lower in response to on / off compared to Comparative Example 2, and a slight increase in photocurrent was observed. It was.

  In the electrode (Example 3) in which the amount of Ag was increased by introducing the input power to Ag as 35 W, the photocurrent increased significantly. On the other hand, in the electrode (Example 4) manufactured by supplying 50 W of electric power to Ag, the photoresponse was lowered and the contribution of dark current was observed. In Example 3, which showed the highest photoresponse, the photocurrent value at an applied voltage of 0 V was 245 μA, corresponding to 15 times that in the case of no doping (Comparative Example 2).

3 and 4 show changes in the X-ray diffraction pattern of CaFe ₂ O ₄ due to the difference in the amount of Ag introduced. The thin film was heat-treated at 600 ° C. (FIG. 3) or 650 ° C. (FIG. 4) in an oxygen atmosphere for 2 hours after film formation.

FIG. 3 shows an X-ray diffraction result (diffraction pattern) of a film that was processed at 600 ° C. for 2 hours under an oxygen stream after film formation. The thin film of CaFe ₂ O ₄ without Ag introduced in Comparative Example 1 (lower thin line in the figure) did not show diffraction lines, whereas the thin film of CaFe ₂ O ₄ with Ag introduced in Example 1 Showed a strong diffraction line (upper dark solid line in the figure). The diffraction lines at 2θ = 38, 44, 64 ° were assigned to Ag, and all other diffraction lines were assigned to CaFe ₂ O ₄ . Thus, in the comparative example in which Ag is not introduced, there is no diffraction line attributed to CaFe ₂ O ₄ , crystallization is not performed, and when Ag is introduced, the diffraction line attributed to CaFe ₂ O ₄ is It can be seen that crystallization is occurring. That is, it can be seen that Ag promotes crystallization of CaFe ₂ O ₄ . By promoting this crystallization, it can be interpreted that in Example 1, a photoresponse was developed even at a low temperature heat treatment of 600 ° C.

FIG. 4 shows an X-ray diffraction pattern when the heat treatment temperature is 650 ° C. When the heat treatment temperature was 650 ° C., the thin film into which Ag was not introduced also showed diffraction lines. Focusing on the change with respect to the amount of introduced Ag, the introduction of Ag reduces the diffraction line (2θ = 19 °) of the (200) plane of CaFe ₂ O ₄ and the diffraction line of the sum of the (320) and (040) planes ( An increase of 2θ = 34 ° was observed. From this, it can be seen that Ag causes a change in the crystallinity of CaFe ₂ O ₄ . In the sample into which Ag is introduced at 50 W, the peak attributed to Ag is mainly, suggesting that Ag is present in excess.

The visible ultraviolet absorption spectrum of CaFe ₂ O ₄ (Comparative Example 2: gray) heat-treated at 650 ° C. in FIG. 5 and CaFe ₂ O ₄ (Example 3: black) formed by applying 35 W power to Ag and The current conversion efficiency (IPCE) of monochromatic light of a wavelength is shown. IPCE is a plot of current conversion efficiency (IPCE) of incident light when monochromatic light of each wavelength is irradiated while applying a voltage of 0 V vs Ag / AgCl.

  From the UV-visible absorption spectrum, it can be seen that by introducing Ag, the absorption band in the visible region spreads to the longer wavelength side, and light having a wider wavelength can be used. Furthermore, when comparing IPCE of each wavelength, the current conversion efficiency of any wavelength was significantly increased by introducing Ag. From this, it can be seen that the cause of the improvement in the photoresponsiveness due to the introduction of Ag in this material is not only the extension of the absorption wavelength.

  In addition, when the ultraviolet-visible absorption spectrum was measured about the thin film obtained in Example 1-4 and Comparative Example 1-2, all showed absorption in the wavelength range of 600 nm or less. In particular, in the thin film doped with Ag, an absorption band was also observed in a longer wavelength region.

  In addition, a significant decrease in resistivity was confirmed by introducing Ag. When the resistivity of each electrode was measured by the four-probe method, in Comparative Example 2 in which Ag was not introduced, the resistivity could not be measured greatly, but when the input power to Ag was 20 and 35 W, The resistivity dropped significantly, and the resistivity became almost constant when power was applied at 35 W or more. It is suggested that the decrease in resistivity due to the introduction of Ag may contribute to an increase in photoresponsiveness.

From the above facts, the introduction of Ag into CaFe ₂ O ₄ causes a change in crystallinity, a longer absorption wavelength, and a decrease in resistivity. As a result, a significant improvement in photoresponsiveness is realized. It is thought.

Table 1 shows the photocurrent values at 0.0 VvsAg / AgCl of each photoelectrode. For CaFe ₂ O ₄ system, as compared with CaFe ₂ O ₄ of Comparative Example 3, it was improved in both of the irradiation of the entire light irradiation and the visible light only. Further, the photocurrent values of the Cu-doped CaFe ₂ O ₄ of Examples 5 and 6 are also improved in both the all-light irradiation and the visible light-only irradiation.

As for the CaBi ₂ O ₄ system, as compared to CaBi ₂ O ₄ of Comparative Example 4, the photocurrent value of Cu doped CaBi ₂ O ₄ of Example 7 and 8 are improved.

Thus, it was confirmed that the crystal system doped with transition metal elements Ag and Cu can increase the photocurrent in CaFe ₂ O ₄ which is a calcium ferrite type semiconductor material and CaBi ₂ O _{4 having} a spinel structure. . This is because a transition metal element such as Ag or Cu is doped into the AB ₂ O ₄ crystal, a part of the A or B site is replaced with a transition metal element such as Ag or Cu, and the AB ₂ O ₄ crystal. Either by doping a transition metal element between the lattices in the medium or by doping a transition metal element in the grain boundary of AB ₂ O ₄ , or a combination thereof, whereby the crystallinity of AB ₂ O ₄ Changes and the photoresponsiveness is improved.

As the crystal system, a calcium ferrite type or a spinel structure is adopted, and by introducing a transition metal element therein, the crystallinity of AB ₂ O ₄ can be changed. In particular, as a technique for doping a transition metal element, a technique of performing sputtering of the transition metal element simultaneously with the formation of AB ₂ O ₄ by sputtering, or an aqueous salt solution containing necessary components is mixed and then dried. Although a technique can be adopted, a process for crystallizing AB ₂ O ₄ is essential at this time. That is, the semiconductor material according to this embodiment is a crystal of AB ₂ O ₄ doped with a transition metal element, and it is important to crystallize by heat treatment or the like.

Claims (10)

A semiconductor material doped with a transition metal element M in a crystal having a composition represented by AB ₂ O ₄ ,
The doping of M is performed by substituting a part of the A or B site with M, doping M between lattices in the AB ₂ O ₄ crystal, or doping the crystal grain boundary of AB ₂ O ₄ with M. Done in any one or a combination of these
A semiconductor material whose crystal system is calcium ferrite type or spinel structure. The semiconductor material according to claim 1,
A semiconductor material containing at least one of Ca, Sr, Cu, Mg, Ba, Zn as A and at least one of Fe, Bi as B. A semiconductor material according to claim 1 or 2,
A semiconductor material containing at least one of Ag, Cu, and Au as the transition metal element M. The semiconductor material according to any one of claims 1 to 3,
A semiconductor material characterized in that the M / B ratio x of the surface composition observed by X-ray photoelectron spectroscopy is 0 <x <2.6. The semiconductor material according to claim 4,
A semiconductor material whose crystal system is a calcium ferrite type structure. A semiconductor material according to any one of claims 1 to 5,
A semiconductor material in which the absorbance of visible light is increased as a result of doping with M, compared to the undoped material. A photoelectrode material using the semiconductor material according to any one of claims 1 to 5,
A photoelectrode material that generates a photocathode current greater than that of CaFe ₂ O ₄ not doped with M when irradiated with light in a solution under an external potential of 0.0 V with respect to an Ag / AgCl electrode. A photocatalytic material using the semiconductor material according to any one of claims 1 to 5,
A photocatalytic material that causes a reduction reaction of water and oxygen when irradiated with visible light. It is a manufacturing method of the semiconductor material according to any one of claims 1 to 5,
By RF magnetron sputtering, the AB ₂ O ₄ target and the M or its oxide as a target are simultaneously sputtered by applying power, and at the same time, in the range of 600 ° C. to 700 ° C. A method for producing a semiconductor material, which is crystallized by heat treatment. It is a manufacturing method of the semiconductor material according to any one of claims 1 to 5,
A method for producing a semiconductor material, comprising forming any of oxides or carbonates of A, B and M, carbonate, lead nitrate, and acetate in water, evaporating and removing the water by depressurization, followed by firing.