Metal complex photocatalysts are promising candidates in CO₂ photo-reduction systems in terms of its high efficiency and selectivity under visible light even in aqueous solution.[1] These systems, however, need the sacrificial reductive additives as electron donor due to the low oxidation power of photosensitizer unit. In this study, we developed novel “molecular” photocathodes consisting of Ru(II)-Re(I) metal complex photocatalyst (RuRe) and p-type semiconductor electrodes for visible-light-driven CO₂ reduction (Figure). The application of the molecular photocathodes for photoelectrochemical CO₂ reduction in aqueous solution was also examined.

RuRe consists of tris-diimine Ru(II) unit as a photosensitizer and tricarbonyl diimine Re(I) unit as a catalyst, and has methylphosphonate anchoring group to adsorb on the electrode surface. p-type NiO electrode was synthesized and hybridized with RuRe to obtain molecular photocathode (RuRe/NiO).[2] This RuRe/NiO photocathode displayed photocathodic responses under the visible light (λ_ex > 460 nm), which selectively photoexcites Ru(II) photosensitizer in RuRe, in a CO₂-purged aqueous electrolyte without the need for any sacrificial additives. It reflects that the photo-excited RuRe can receive electrons from an external circuit through the NiO electrode. The onset potential for cathodic photocurrent was approx. -0.1 V vs. Ag/AgCl in a CO₂-purged 50 mM NaHCO₃ aqueous solution. Photoelectrochemical CO₂ reduction using the RuRe/NiO photocathode was examined under the continuous irradiation at the potential of -0.7 V vs. Ag/AgCl, resulting that the catalytic amount of CO was observed. Its turnover number for CO formation, which was based on the amount of RuRe on the NiO, was 32 for 12 h irradiation. These results clearly suggest that the immobilized metal complex photocatalyst (RuRe) functions to drive photoreduction of CO₂ with using electrons supplied from the NiO electrode.

We next developed p-type CuGaO₂ electrode as an alternative of NiO for such molecular photocathode with RuRe.[3] The resulting RuRe/CuGaO₂ photocathode also shows photoelectrochemical activity for CO₂ reduction under visible light irradiation. The photocathodic responses was obtained from approx. +0.3 V vs. Ag/AgCl, which is equivalent to +0.9 V vs. RHE. This value of onset potential is around 0.4 V positive than that for RuRe/NiO, which means that the utilization of CuGaO₂ as an electrode material enlarged the region of working potential for CO₂ reduction to positive direction. This tendency agreed well to the flat band potentials of these semiconductor electrodes obtained from the electrochemical impedance spectroscopy. The turnover number of CO formation using the RuRe/CuGaO₂ photocathode reached to 125 for 15 h irradiation, while H₂ was also generated as byproduct. These results indicate the advantage of CuGaO₂ for usage of an electrode material for molecular photocathode from the aspect of efficient interfacial electron injection.

In addition, these photocathodes were investigated to combine with a CoO_x/TaON photoanode for the oxidation of water.[4] The constructed photoelectrochemical cells consisting of these molecular photocathodes and the CoO_x/TaON photoanode enabled the visible-light-driven catalytic reduction of CO₂ with water oxidation to obtain CO and O₂ as the products. These systems successfully demonstrated CO₂ reduction using water as an electron donor by means of combined photocatalytic abilities of both the molecular metal complex (RuRe) and the semiconductor material (TaON). To the best of our knowledge, these are the sole examples of visible-light-driven CO₂ reduction systems using water as the reductant, based on molecular photocatalysts with hybridizing semiconductor.

[1] Y. Yamazaki, H. Takeda and O. Ishitani, J. Photochem. Photobiol. C, 2015, 25, 106–137.; A. Nakada, K. Koike, K. Maeda and O. Ishitani, Green Chem., 2016, 18, 139–143.

[2] G. Sahara, R. Abe, M. Higashi, T. Morikawa, K. Maeda, Y. Ueda and O. Ishitani, Chem. Commun., 2015, 51, 10722–10725.; G. Sahara, H. Kumagai, K. Maeda, N. Kaeffer, V. Artero, M. Higashi, R. Abe and O. Ishitani, J. Am. Chem. Soc., 2016, 138, 14152–14158.

[3] H. Kumagai, G. Sahara, K. Maeda, M. Higashi, R. Abe and O. Ishitani, Chem. Sci., 2017, 8, 4242–4249.

[4] M. Higashi, K. Domen and R. Abe, J. Am. Chem. Soc., 2012, 134, 6968–6971.