(Keynote) Photocatalytic and Photoelectrochemical Water Splitting and CO2 Reduction As Artificial Photosynthesis

Water splitting and CO2 fixation of uphill reactions can be regarded as artificial photosynthesis, because light energy is converted to chemical energy. In the present paper, I mention the trends of photocatalytic water splitting to generate solar hydrogen and then introduce our results of various metal oxide and sulfide photocatalysts, and photoelectrochemical cells for water splitting and CO2 reduction of uphill reactions, aiming at artificial photosynthesis.1) Rh and Sb-codoped SrTiO3 photocatalyst loaded with IrO2 is active for water splitting into H2 and O2 under visible light and simulated sunlight irradiations as a single particle type photocatalyst. This photocatalyst responds to 500 nm.2) SrTiO3:Rh of a H2-evolving photocatalyst and BiVO4 of an O2-evolving photocatalyst construct various type of Z-schematic photocatalyst systems with Fe3+/Fe2+, [Co(bpy)3]3+/2+, [Co(phen)3]3+/2+, and a conductive reduced graphene oxide (RGO) as an electron mediator and even without an electron mediator. It is noteworthy that a sheet photocatalyst consisting of SrTiO3:Rh,La and BiVO4 powders with a Au contacting layer shows a quite high activity.3) On the other hand, SnNb2O6 with 2.4 eV of the band gap that has previously been reported as a powdered photocatalyst been has arisen as a candidate of an alternative of a BiVO4 photoanode.4) Metal sulfide photocatalysts that are normally unstable for water splitting into H2 and O2 in the absence of an electron donor can be employed for Z-schematic photocatalyst systems for water splitting. Z-schematic photocatalyst systems combining metal sulfide photocatalysts as a H2-evolving photocatalyst with TiO2 (RGO/TiO2)5) and BiVO4+Co complex (an electron mediator) as an O2-evolving photocatalyst show activity for water splitting into H2 and O2.5) These photocatalyst materials can also be employed for photoelectrochemical system for solar water splitting.6,7) Ag/BaLa4Ti4O15 and Ag/NaTaO3:M (M=Ca, Sr, Ba, and La) photocatalysts with wide bandgaps show activities for CO2 reduction to form CO and HCOOH in an aqueous medium without any sacrificial reagents.8,9) O2 evolved with a stoichiometric amount indicating that water reacted as an electron donor indicating that an uphill reaction of CO2 reduction accompanied with water oxidation was achieved. CuGaS2-RGO/BiVO4 of a Z-scheme photocatalyst system is active for water splitting and CO2 reduction to CO under visible light irradiation without any sacrificial reagents. This is the first time to demonstrate CO2 reduction using water as an electron donor in a Z-schematic powdered photocatalyst system with visible light response.10) References 1) A. Kudo, Y. Miseki, Chem. Soc. Rev., 2009, 38, 253. 2) R. Asai, H. Nemoto, Q. Jia, K. Saito, A. Iwase, A. Kudo, Chem. Commun. 2014, 50, 2543. 3) Q. Wang, T. Hisatomi, Q. Jia, H. Tokudome, M. Zhong, C. Wang, Z. Pan, T. Takata, M. Nakabayashi, N. Shibata, Y. Li, I. Sharp, A. Kudo, T. Yamada, and K. Domen, Nature Mater, 2016, 15, 611. 4) R. Niishiro, Y. Takano, Q. Jia, M. Yamaguchi, A. Iwase, Y. Kuang, T. Minegishi, T. Yamada, K. Domen, and A. Kudo, Chem. Commun., 2017, 53, 629. 5) K. Iwashina, A. Iwase, Y. Hau Ng, R. Amal, A. Kudo, J. Am. Chem. Soc., 2015, 137, 604. 6) T. Kato, Y. Hakari, S. Ikeda, Q. Jia, A. Iwase, A. Kudo, J. Phys. Chem. Lett., 2015, 6, 1042. 7) Q. Jia, K. Iwashina, A. Kudo, Proc. Natl. Acad. Sci. USA, 2012, 109, 11564. 8) K. Iizuka, T. Wato, Y. Miseki, K. Saito, and A. Kudo, J. Am. Chem. Soc., 2011, 133, 20863. 9) H. Nakanishi, K. Iizuka, T. Takayama, A. Iwase, A. Kudo, ChemSusChem, 2017, 10, 112. 10) A. Iwase, S. Yoshino, T. Takayama, Y. H. Ng, R. Amal, A. Kudo, J. Am. Chem. Soc., 2016, 138, 10260.