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

Abstract

W ater 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 NaTaO 3 , and Zn-doped Ga 2 O 3 , have been reported with quantum efficiencies of over 50%. [3][4][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 Hydrogenby Jun Kubota and Kazunari Domen in most cases. A simple illustration of photocatalytic water splitting is shown in Fig. 1. Photoexcited electrons in the conduction band need to be smoothly transferred to the hydrogen-evolution cocatalyst particles without release of energy. The first requirement for the cocatalyst is a smaller barrier for migration of electrons from the...

Hydrogen Peroxide Production on a Carbon Nitride–Boron Nitride‐Reduced Graphene Oxide Hybrid Photocatalyst under Visible Light

“…The spatial charge separation and selective two‐electron O 2 reduction on the catalyst successfully produces H 2 O 2 with 0.20 % SCC efficiency. This is similar to the highest values obtained by overall water splitting on powder catalysts such as (Ga 1− x Zn x )(N 1− x O x ) and Ru‐SrTiO 3 :Rh/BiVO 4 . The photocatalytic H 2 O 2 synthesis therefore has potential to be a new artificial photosynthesis for solar fuel production.…”

“…The SCC efficiency on g‐C 3 N 4 /PDI‐BN 0.2 ‐rGO 0.05 is 0.27 %, which is higher than that of g‐C 3 N 4 /PDI (0.10 %), g‐C 3 N 4 /PDI‐BN 0.2 (0.19 %), and g‐C 3 N 4 /PDI‐rGO 0.05 (0.20 %). So far, two kinds of powdered photocatalysts, such as (Ga 1‐ x Zn x )(N 1‐ x O x ) and Ru‐SrTiO 3 :Rh/BiVO 4 , promote overall water splitting with high SCC efficiency (ca. 0.2 %), although both systems need noble metals.…”

“…Artificial photosynthesis is one very important subject to convert solar energy into chemical energy for sustainable production of clean solar fuels . Overall water splitting by powdered photocatalysts have been studied extensively . They successfully produce H 2 from earth‐abundant water, but suffer from several unavoidable problems: 1) expensive metal components are necessary for the catalysts; 2) the storage and transportation of H 2 are basically difficult; and 3) the reaction needs inert gas atmosphere or low pressure condition.…”

Nanocatalysts for Solar Water Splitting and a Perspective on Hydrogen Economy

“…Band bending is caused by the creation of an electronic structure at the interface which can lead to selective trapping of electrons and holes. Depositing metal particles on the surface of a photocatalyst forms either a barrier, the so called Schottky barrier, or an interface called Ohmic contact, depending on the work function of the co‐catalyst and the Fermi level of the semiconductor . For example, noble metal (e.g., Pt) nanoparticles form a Schottky barrier with most semiconductors and extract electrons from the semiconductor to generate H 2 .…”

Tantalum nitride for photocatalytic water splitting: concept and applications

“…However, these types of materials are effective only for half of the reaction (water oxidation). For overall water splitting, there is a limited choice of photocatalysts, such as Ta 3 N 5 , TaON, or some material from the family of oxynitride perovskites [16][17][18][19][20][21][22][23][24]. Achieving one step overall water splitting with one single photocatalyst presents a practical way in term of engineering design of the reactions.…”