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⁾

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