Invited Paper EH-TuM4
Visible Light Responsive Photocatalysts toward Solar Hydrogen Production
Tuesday, December 13, 2016, 9:00 am, Room Lehua
Water splitting into H2 and O2 using semiconductor photocatalysts has received much attention recently due to the potential for the clean production of H2 from water utilizing solar energy [1]. Because 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. We have developed a new type of photocatalytic water splitting system, mimicking the mechanism of photosynthesis in green plants [2-6]. In this system, the water splitting reaction is broken up into two stages: one for H2 evolution and the other for O2 evolution; these are combined by using a shuttle redox couple (Red/Ox) in the solution. 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. This system reduces the energy required to drive each photocatalysis process, allowing visible light to be utilized more efficiently than in conventional system. We have achieved water splitting using various visible light responsive photocatalysts, such as SrTiO3 doped with Cr [3], tantalum oxynitrides (TaON or BaTaO2N) [4], and organic dyes [5], which work as a H2 evolution photocatalyst, combined with tungsten oxide (WO3) for O2 evolution in the presence of a shuttle redox mediator such as iodate/iodide. Some oxyhalides such as Bi4NbO8Cl have also been proven as stable and efficient O2-evolving photocatalysts under visible light, enabling a Z-scheme overall water splitting with iron redox [6]. We have also demonstrated that the porous photoanodes of tantalum oxynitrides can stably oxidize water with significantly high quantum efficiency, after loading of IrO2 or CoOx nanoparticles as a cocatalyst; enabling water splitting under visible light with a relatively small applied bias [7].
References
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[2] Bull. Chem. Soc. Jpn. 84 (2011) 1000.
[3] J. Phys. Chem. B 109 (2005) 16052.
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[5] J. Am. Chem. Soc. 135 (2013) 16872.
[6] J. Am. Chem. Soc. 38 (2016) 2082.
[7] a) J. Am. Chem. Soc., 132 (2010) 11828; b) Energy Environ. Sci. 4 (2011) 4138; c) J. Am. Chem. Soc. 134 (2012) 6968; d) J. Am. Chem. Soc. 135 (2013) 10238.