It is known that certain metal complexes based on rhenium or ruthenium catalyze CO2 reduction into CO or HCOOH photocatalytically with high selectivity and quantum yields. However, the oxidation ability of metal complex photocatalysts is generally low, requiring a strong electron donor for operation. On the other hand, semiconductors have high activity and stability for oxidation reactions, although the selectivity to CO2 reduction is low. Here we developed a new system for CO2 reduction using a visible-light-responsive semiconductor and a binuclear metal complex having a redox photosensitizer and a catalytic unit. In this reaction scheme, both the semiconductor and the photosensitizer unit of the metal complex undergo photoexcitation. The conduction band electrons in the semiconductor move to the excited state of the sensitizer unit, producing a one-electron reduced species. After subsequent intermolecular electron transfer from the one-electron-reduced species to the catalyst unit, CO2 is reduced into HCOOH (or CO), while holes left in the valence band of the semiconductor participate in the oxidation reactions. Because of the electron transfer pathway, this is called “Z-scheme” CO2 reduction system. We found that TaON, CaTaO2N, Y2Ta2O5N2, and C3N4 became active component for the Z-scheme system driven by visible light (> 400 nm) in combination with a suitable binuclear metal complex.
As the component of water oxidation half cycle, we developed a new powdered photocatalyst consisting of Co(OH)2 and TiO2. TiO2 is known to be an active photocatalyst, but only works under UV irradiation. By contrast, the Co(OH)2/TiO2 hybrid photocatalyst is capable of absorbing visible light with wavelengths of up to 850 nm and oxidizing water into oxygen gas, even though it consists of only earth-abundant elements only. To our knowledge, this system provides the first demonstration of a photocatalytic material capable of water oxidation upon excitation by visible light up to such a long wavelength.