It is indispensable to develop science and technology to produce green hydrogen and utilize carbon dioxide for achievement of carbon neutrality, and solution of resources, energy, and environmental issues. Artificial photosynthesis using a semiconductor photocatalyst is one of the candidates for it. Representative artificial photosynthesis is green hydrogen production and CO₂ reduction using water as an electron donor. Achievement of practical green hydrogen production using a low-cost powdered photocatalyst system will bring significant academic and social impacts. However, photocatalyst materials had been so limited and the activities had been very low until the middle of 1990s. In such background, Prof. Kudo has developed various photocatalysts for water splitting based on his original strategies for design of a photocatalyst material. He has also demonstrated photocatalytic CO₂ reduction with a high selectivity using water as an electron donor by finding of original cocatalysts for the CO₂ reduction. His major achievements are listed below.

1. Development of metal oxide photocatalyst showing high quantum yield
It was found that NiO/NaTaO₃:La showed 57% of a high quantum yield for water splitting into H₂ and O₂ in a stoichiometric amount. When the powdered photocatalyst loaded on a glass substrate was irradiated with UV light in just water, bubbling of H₂ and O₂ evolved was visually observed. It had been believed in 1990s that highly efficient water splitting would have been impossible using a powdered photocatalyst. In contrast to the negative common sense, finding of this photocatalyst has proven that highly efficient water splitting is actually possible using only powder of a photocatalyst and H₂O; it is a very simple system.

2．Development of visible light-driven metal oxide and sulfide photocatalysts by original design strategies
Development of visible light-driven photocatalyst has been a key issue to utilize solar light in artificial photosynthesis. He has developed various metal oxide and sulfide photocatalysts based on transition metal doping, valence band control, and solid solution formation by original band engineering and crystal engineering for inorganic materials. Various metal oxide photocatalysts have been developed by codoping of Rh, Ir, Ru, Cr, and Ni with Ta⁵⁺ and Sb⁵⁺ into TiO₂ and SrTiO₃ hosts of a wide band gap photocatalyst. Transition metal doping had been regarded as no good strategy for the design of visible light driven photocatalyst in 1980s, because doped cations worked as a recombination center between photogenerated electrons and holes. In contrast to the common sense, he has proven that manifestation of visible light response is possible if suitable host photocatalysts and dopants are chosen. Bi(III), Sn(II), Ag(I), and Cu(I) are effective components of visible light driven photocatalysts in a strategy of valence band control. Bi6s, Sn5s, Ag4d, and Cu3d orbitals contribute to the formation of the valence band maxima resulting in narrowing the band gaps. BiVO₄ with 2.4 eV of a band gap is the representative photocatalyst. Highly crystalline BiVO₄ with active facets for redox reactions of water can easily be synthesized in an aqueous medium under ambient condition. The BiVO₄ has widely been studied for photocatalytic and photoelectrochemical water oxidation in the research field of artificial photosynthesis such as producing a solar fuel.

3．Development of single particulate and Z-schematic photocatalyst systems working under visible light irradiation for water splitting
SrTiO₃:Rh,Sb, SrTiO₃:Ir,Sb,Al, SrTiO₃:Ru,Sb,Al, and SrTiO₃:Cr,Al of a single particulate metal oxide photocatalyst have been developed for efficient water splitting under visible light irradiation by optimizing a doping amount, a synthetic process, and a cocatalyst for original photocatalysts developed by doping. Among them, SrTiO₃:Ir,Sb,Al and SrTiO₃:Ru,Sb,Al photocatalysts respond to 600 nm in visible light. Various types of Z-scheme photocatalysts were developed using SrTiO₃:Rh, BiVO₄, and electron several mediators such as Fe^3+/2+, Co(bpy)₃^3+/2+, reduced graphene oxide (RGO), and poly-3,4-ethylenedioxythiophene (PEDOT). The Z-scheme photocatalyst without an electron mediator was also constructed. These results have widely been applied for various Z-scheme photocatalyst systems for water splitting.

4．Development of photocatalysts for CO₂ reduction using water as an electron donor
It is indispensable to use water as an electron donor for photocatalytic CO₂ reduction as an artificial photosynthesis. It was found that Ag was an effective cocatalyst for the photocatalytic CO₂ reduction to form CO with a high selectivity. When the Ag cocatalyst was loaded on Sr or Ba-doped NaTaO₃ and BaLa₄Ti₄O₁₅ photocatalysts, CO formed with >90% of the selectivity even in an aqueous suspension system. Moreover, Rh-Ru and Pd-Ni cocatalysts loaded on Sr-doped NaTaO₃ photocatalyst gave not CO but CH₄, C₂H₆, and C₃H₈ of hydrocarbons. These achievements are the first examples for the highly selective CO₂ reduction accompanied with water oxidation to O₂ in an aqueous medium. When (CuGa)_0.5ZnS₂ of a CO₂-reducing photocatalyst was used with BiVO₄ of an O₂-evolving photocatalyst and RGO of a solid electron mediator to construct a Z-scheme photocatalyst, photocatalytic CO₂ reduction to form CO proceeded accompanied with O₂ evolution under visible light irradiation in a simple aqueous suspension system.

As described above, Prof. Kudo has established a photocatalyst library (data base) by his original strategies for design of new photocatalyst materials. Various kinds of single particulate and Z-schematic photocatalysts were developed for water splitting to form green hydrogen, and CO₂ reduction to form CO and hydrocarbons using water as an electron donor. These achievements have impacted in photochemistry, catalytic chemistry, electrochemistry, and material science of various fields in chemistry. He also contributes to enlightenment of chemistry by demonstration of water splitting using these unique photocatalysts in schools and for citizens. Therefore, his achievements are recognized as worthy of the Chemical Society of Japan Award.