From the viewpoint of reducing carbon dioxide gas emission and cleaning energy, hydrogen energy systems have been attracting attention. When used as energy medium, hydrogen can generate electricity and heat using fuel cells, and hydrogen can generate heat and power by being combusted directly. In these cases, the final product is safe and innoxious water, creating a clean energy circulation cycle. Most of hydrogen as an energy medium is produced from petroleum or natural gas by cracking using a catalyst, although hydrogen as an energy medium exists naturally as well. It also is possible to produce hydrogen and oxygen by electrolyzing water, but this cannot be a fundamental solution because the electrolysis needs electric energy. It also is possible to use a system in which a solar cell converts light energy into electricity so as to perform electrolysis with this electric power. However, taking into consideration the manufacturing cost of the solar cell, energy consumption, and electricity storing technique, the hydrogen production method using such a system is not necessarily an effective method.
In contrast, hydrogen production using a photocatalyst is a system for producing hydrogen directly from water and sunlight, and can convert the sunlight energy effectively into hydrogen energy (see Patent Literatures 1, 2, and 3). However, even when an anatase titanium oxide, a typical photocatalyst, is used, the solar-to-hydrogen conversion efficiency is about 0.5%, which needs to be enhanced much further. A problem here is that the titanium oxide photocatalyst itself absorbs only ultraviolet rays with a wavelength of 400 nm or less in sunlight so as to be pumped. Therefore, a material that can be pumped by visible light is expected, that is, development of a visible-light-responsive photocatalytic material is expected. Meanwhile, cells, devices, and apparatuses for producing hydrogen efficiently also are being studied. They roughly can be categorized into powder type and electrode type.
The powder type is a system in which a powdered photocatalytic material is dispersed directly in an aqueous solution and particles of the photocatalytic material are irradiated with light so as to produce hydrogen and oxygen, and when certain amounts of the gases are produced, the oxygen and hydrogen are separated from each other. In contrast, the electrode type is a system in which an electrode obtained by applying a photocatalytic material to an ITO (Indium Tin Oxide) film or a conductive substrate to form a film thereon is used. In the electrode type, the electrode on which the photocatalyst film has been formed is connected, with a conducting wire, to a conductor, such as a platinum plate, that functions as a counter electrode, the electrode on which the photocatalyst film has been formed is irradiated with light so as to produce oxygen at this electrode, and electrons pumped at the same time when the oxygen is produced are guided to the counter electrode so as to produce hydrogen at the counter electrode. The powder type is simple in structure and convenient to use, but it is difficult to separate hydrogen and oxygen from each other, thereby lowering the efficiency. In the electrode type, since hydrogen and oxygen are produced at different electrodes, it is easy to separate hydrogen and oxygen from each other, but there is a restriction that it is necessary to form a photocatalytic material into an electrode.
Similarly to the above-mentioned systems in which hydrogen is used as an energy medium, solar cells, each being a device for converting sunlight directly into electricity, also are attracting a lot of attention from the viewpoint of cleaning energy. Usually, the solar cells have a mechanism in which a p-type semiconductor and an n-type semiconductor are bonded to each other, and the photoelectromotive force generated between the p-type semiconductor and the n-type semiconductor is extracted as electricity. In the solar cells, an Si crystalline body or amorphous Si is used as a main semiconductor material, and this is doped with a dopant to form the n-type semiconductor and the p-type semiconductor.
Currently, compounds having a photocatalytic property that allows water to be decomposed so as to produce hydrogen and oxygen are rare and few. The visible-light-pumped photocatalysts capable of decomposing water that have been reported so far are few, only Ta3N5, Ag3VO4 and so on. Thus, it has been expected to develop a visible-light-pumped photocatalytic material capable of absorbing visible light so as to be pumped and decomposing water into hydrogen and oxygen. When referred to by the type of material structure, the photocatalytic property has been found in perovskite oxides such as SrTiO3, BaTiO3, CaTiO3, and BaZrO3, and simple oxides such as WO3 (see Non-patent Literature 2, for example). However, there have been found very few oxides that are capable of decomposing water and can be used as a photocatalytic material that is pumped by visible light with a wavelength of 460 nm or more. Also, materials for which sunlight or a fluorescent lamp can be used, that is, visible-light-pumped materials, currently are very few among photocatalytic materials that decompose, or oxidize partially or reduce partially fats and oils and organic substances. As optically pumped semiconductors for solar cells, chalcogenides, such as CdS, CuInS, SiC, Te, Se, In, and Ga, have been used, but oxide semiconductors that are highly resistant to moisture and can absorb visible light so as to be pumped have not been found.