Photocatalytic Water Splitting Using Oxynitride and Nitride Semiconductor Powders for Production of Solar Hydrogen

Abstract

57 Water splitting using a photocatalyst for direct solar hydrogen production from sunlight and water, has been regarded as an important approach to artificial photosynthesis.1,2 Solar hydrogen, which is the simplest solar fuel, can then be converted to various energy carriers such as organic hydrides, methanol, methane, and ammonia for transportation and storage. Thus, energy systems based on solar hydrogen can lead to a stable, secure, and ecologically-friendly society. Although solar fuel production using electricity from photovoltaic cells and concentrated solar thermal power is an existing technology, direct synthesis of solar fuels by artificial photosynthesis has more scalability in terms of system cost. To split water into hydrogen and oxygen, 1.23 V of energy is required, corresponding to a 2 electron reaction. An energy of 1.23 V is equivalent to a photon energy of 1000 nm, so that the visible light (400-800 nm in wavelength) that represents half of the solar energy spectrum can be thermodynamically used for water splitting. The challenge is to obtain an efficient photocatalyst that has an absorption edge in longer wavelength and the ability to split water. Highly efficient oxide photocatalysts, such as La-doped NaTaO3, and Zn-doped Ga2O3, have been reported with quantum efficiencies of over 50%.3-5 However, these oxides only absorb ultraviolet (UV) light with wavelengths shorter than 300 nm. In the solar spectrum at the earth’s surface, such short UV light is not present, so that these oxides cannot operate efficiently under solar irradiation. Most oxides have band gaps wider than the UV energy because of the deeper O 2p potential of the valence band. The valence band of nitrides is composed of the shallower N 2p potential, and thus their band gaps are generally narrower than those of oxides. For photocatalytic water splitting, photoexcited electrons in the conduction band reduce water to evolve hydrogen, and photogenerated holes in the valence band oxidize water to evolve oxygen. Therefore, the potential of the conduction band should be shallower than the reversible hydrogen potential and the potential of the valence band should be deeper than the reversible oxygen potential. In this article, the properties of oxynitride and nitride photocatalysts for water splitting are described. Surface modification of semiconductor photocatalyst particles with hydrogenor oxygen-evolving cocatalysts is an indispensable technique in the study of photocatalytic water splitting. In fact, hydrogen-evolution cocatalysts are required Photocatalytic Water Splitting Using Oxynitride and Nitride Semiconductor Powders for Production of Solar Hydrogen