Photocatalytic water splitting and CO₂ reduction are promising reactions to solve resources, energy, and environment issues, and achievement of carbon neutral. These reactions are regarded as artificial photosynthesis, because light energy is converted to storable chemical energy. In the present paper, various metal oxide and sulfide photocatalysts for water splitting and CO₂ reduction using water as an electron donor are introduced and discussed.

(1) Photocatalytic water splitting for green H₂ production

Rh_0.5Cr_1.5O₃/AgTaO₃ (BG 3.4 eV) of a valence-band-controlled metal oxide photocatalyst gave 40% of an apparent quantum yield at 340 nm and 0.13% of a solar to hydrogen conversion efficiency. However, this photocatalyst responds to only UV light because of the wide band gap. It is a challenging topic to develop photocatalysts responding to light with long wavelength. Rh and Sb-codoped SrTiO₃ photocatalyst loaded with IrO₂ was 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 520 nm of visible light. Moreover, we recently developed Ir-doped SrTiO₃ that showed the activity for water splitting up to 600 nm of visible light. On the other hand, SrTiO₃:Rh and several metal sulfides of a H₂-evolving photocatalyst and BiVO₄ of an O₂-evolving photocatalyst constructed 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. Various types of Z-scheme systems for water splitting under visible light irradiation were also successfully developed by employing Rh- and Ir-doped metal oxide powdered materials with relatively narrow energy gaps (EG) to utilize wide range of visible light.

(2) Photocatalytic CO₂ reduction using water as an electron donor

Ag cocatalyst-loaded ALa₄Ti₄O₁₅ (A = Ca, Sr, and Ba) and tantalates photocatalysts such as NaTaO₃:Ba with 3.79–4.1 eV of band gaps showed activities for CO₂ reduction to form CO and HCOOH in an aqueous medium without any sacrificial reagents. CO is the main reduction product rather than H₂ even in an aqueous medium. Especially, the Ag/NaTaO₃:Ba photocatalyst gave ca. 90% of the selectivity for the CO₂ reduction. When Rh-Ru cocatalyst is used instead of Ag, CH₄ was formed with ca. 10% of the selectivity. Thus, an uphill reaction of CO₂ reduction accompanied with water oxidation was achieved using the Ag- and Rh-Ru-loaded metal oxide photocatalysts with wide bandgaps. We also constructed a Z-scheme system consisting of (CuGa)_0.5ZnS₂ of a CO₂-reducing photocatalyst with BiVO₄ of an O₂-evolving photocatalyst and RGO of an electron mediator for CO₂ reduction using water as an electron donor under visible light irradiation. The Z-scheme system gave H₂, O₂, and CO simultaneously under visible light irradiation. When the Z scheme system was modified with Co(dmbpy), the selectivity for CO formation reached higher than 90% accompanied with O₂ evolution under visible light irradiation even in an aqueous solution.