(Invited) Photocatalytic and Photoelectrochemical Water Splitting Under Visible Light Irradiation

Photocatalytic water splitting into H2 and O2 using a semiconductor photocatalyst has received much attention recently due to the potential of this method for the clean production of H2 from water utilizing solar energy. Although a number of metal oxides have been reported to be active photocatalysts for the water-splitting reaction, most only function under UV light owing to the large band gap energy of the materials. Since almost half of all incident solar energy at the Earth’s surface falls in the visible region, the efficient utilization of visible light remains indispensable for realizing practical H2 production. However, it had been noted that it is intrinsically difficult to develop an oxide semiconductor photocatalyst that has both a sufficiently negative conduction band for H2 production and a sufficiently narrow band gap (i.e., <3.0 eV) for visible light absorption because of the highly positive valence band (at ca. +3.0 V vs. NHE) formed by the O 2p orbital. Although some non-oxide semiconductors, such as (oxy)sulfides, (oxy)nitrides, and organic dyes (including metal complexes) posses appropriate band levels for water splitting under visible light, they are generally unstable and readily become deactivated through photocorrosion or self-oxidation, rather than evolving O2. In order to split water under visible light, we have developed new type of water splitting system based on two-step photoexcitation between two different photocatalyst materials. Over a H2 evolution photocatalyst, the photoexcited electrons reduce water to H2 and holes oxidize a reductant (Red) to an oxidant (Ox). The Ox is reduced back to the Red by photoexcited electrons generated over an O2 evolution photocatalyst, where the holes oxidize water to O2. Based on the strategy, we have achieved overall water splitting using various visible light responsive photocatalysts, such as SrTiO3 doped with Cr, tantalum oxynitrides (TaON or BaTaO2N), and organic dyes, which work as a H2 evolution photocatalyst, combined with WO3 for O2 evolution in the presence of a shuttle redox mediator such as iodate/iodide (IO3 –/I–). The use of BaTaO2N or coumarin organic dye was demonstrating to be photoactive at wavelength up to ca. 700 nm, indicating the potential of a two-step water-splitting system for utilizing a broader band of visible spectrum. We also demonstrated that some oxy-halides such as Bi4NbO8Cl can stably oxidize water in the presence of iron Fe3+/Fe2+ redox under visible light, also affording Z-scheme water spliting under visible light. The introduction of two-step photoexcitation mechanism (Z-scheme) also enabled us to generate H2 separately from O2, which will be quite important in the practical application to avoid the explosion of the mixture, by employing simple porous glass filters that divide the two different photocatalysts but permit the transportation of redox couple. Some metal oxynitride materials possess appropriate band levels for water splitting as well as a narrow band gap that permits visible light absorption; are thus a promising candidate as photoanode materials for efficient water splitting under visible light with low applied bias. However, the introduction of N 2p orbitals in the valence band generates a new problem in stability that need to be solved. Most oxynitrides undergo self-oxidative deactivation some degree, in which photogenerated holes oxidize nitrogen anions to N2. Recently, we have developed a simple method for fabricating stable and efficient photoanodes of oxynitrides (TaON and BaTaO2N), in which highly dispersed nanoparticles of water-oxidizing catalyst (e.g., CoOx) loaded efficiently scavenge the photogenerated holes and effectively suppress self-oxidative deactivation of the surface. It was also demonstrated that co-loading of RhOx with CoOx significantly improved both the stability and density of photocurrent on the oxynitride photoanodes. These modifications enabled us to stably split water into H2 and O2 efficiently under visible light irradiation at a relatively low applied bias.