An object of the present invention is to provide a photocatalyst for water splitting, which can form a water splitting device that is excellent in durability and responsiveness to visible light and excellent in the amount of generated gas, and a water splitting device having the photocatalyst for water splitting. A photocatalyst for water splitting according to the embodiment of the present invention is a photocatalyst for water splitting, which is used for an electrode that generates gas by irradiation with light in a state of being immersed in water, and includes a compound represented by a formula, (Ln).sub.2CuO.sub.4. In the formula, Ln represents a lanthanoid, and a part of Ln's may be substituted with an element of Groups II to IV of the periodic table.

Described herein are systems incorporating a Casimir cavity, such as an optical Casimir cavity or a plasmon Casimir cavity. The Casimir cavity modifies the zero-point energy density therein as compared to outside of the Casimir cavity. The Casimir cavities are paired in the disclosed systems with product generating devices and the difference in zero-point energy densities is used to directly drive the generation of products, such as chemical reaction products or emitted light.

An object of the present invention is to provide a photocatalytic electrode for water splitting and a water splitting device excellent in the onset potential. The water splitting device of the present invention is a water splitting device which generates gases from a photocatalytic electrode for hydrogen generation and a photocatalytic electrode for oxygen generation by irradiating the photocatalytic electrode for hydrogen generation and the photocatalytic electrode for oxygen generation with light, and includes a bath to be filled with an electrolytic aqueous solution and the photocatalytic electrode for hydrogen generation and the photocatalytic electrode for oxygen generation each disposed in the bath. The photocatalytic electrode for hydrogen generation has a p-type semiconductor layer, an n-type semiconductor layer provided on the p-type semiconductor layer, and a co-catalyst provided on the n-type semiconductor layer. The p-type semiconductor layer is a semiconductor layer containing a CIGS compound semiconductor containing Cu, In, Ga, and Se, and a molar ratio of Ga to a total molar amount of Ga and In in the CIGS compound semiconductor is 0.4 to 0.8.

A CaTiO.sub.3TiO.sub.2 composite electrode and method of making is described. The composite electrode comprises a substrate with an average 2-12 m thick layer of CaTiO.sub.3TiO.sub.2 composite particles having average diameters of 0.2-2.2 m. The method of making the composite electrode involves contacting the substrate with an aerosol comprising a solvent, a calcium complex, and a titanium complex. The CaTiO.sub.3TiO.sub.2 composite electrode is capable of being used in a photoelectrochemical cell for water splitting.

A photoelectrochemical device includes a substrate, a first titanium nitride (TiN) layer coated on the substrate, and a first nitrogen-doped titanium dioxide (NTiO.sub.2) layer coated on the first TiN layer. The photoelectrochemical device has enhanced photoelectric conversion efficiency and can be made by a simple, effective method, thereby shortening the manufacturing time and lowering the manufacturing cost thereof.

A photoelectrochemical device includes a substrate, a first titanium nitride (TiN) layer coated on the substrate, and a first nitrogen-doped titanium dioxide (NTiO.sub.2) layer coated on the first TiN layer. The photoelectrochemical device has enhanced photoelectric conversion efficiency and can be made by a simple, effective method, thereby shortening the manufacturing time and lowering the manufacturing cost thereof.

A photoelectrochemical cell includes a cathode with a front and back cathode surface, an anode with front and back anode surfaces, a conductive connector between the cathode and the anode, and an optical waveguide configured to direct sunlight to the back surfaces of the cathode and anode. The cathode is adapted for photoelectric generation of electrons at the back cathode surface and electrolytic generation of hydrogen at the front cathode surface. Similarly, the anode is adapted for photoelectric generation of electrons at the back anode surface and electrolytic generation of oxygen at the front anode surface. The photoelectrochemical cell may also include a waveguide optical concentrator coupled to the waveguide.

Photoelectrochemical cells for the oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid and/or 2,5-diformylfuran are provided. Also provided are methods of using the cells to carry out the electrochemical and photoelectrochemical oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid and/or 2,5-diformylfuran.

An object of the invention is to provide a water splitting device having a low electrolysis voltage and excellent gas separation performance. The water splitting device of the invention is a water splitting device that generates gases from the positive electrode and the negative electrode, the water splitting device including: a bath to be filled with an electrolytic aqueous solution; the positive electrode and the negative electrode disposed in the bath; and a polymer membrane that is ion-permeable and is disposed between the positive electrode and the negative electrode in order to separate the electrolytic aqueous solution filling the bath into the positive electrode side and the negative electrode side, wherein the positive electrode and the negative electrode are installed at a predetermined distance from the polymer membrane, and the moisture content of the polymer membrane is 40% or more.

Electrochemical and photoelectrochemical cells for the oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid and/or 2,5-diformylfuran are provided. Also provided are methods of using the cells to carry out the electrochemical and photoelectrochemical oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid and/or 2,5-diformylfuran.