Highly Robust Hybrid Photocatalyst for Carbon Dioxide Reduction: Tuning and Optimization of Catalytic Activities of Dye/TiO₂/Re(I) Organic–Inorganic Ternary Systems

Herein we report a detailed investigation of a highly robust hybrid system (sensitizer/TiO₂/catalyst) for the visible-light reduction of CO₂ to CO; the system comprises 5′-(4-[bis­(4-methoxy­methyl­phenyl)­amino]­phenyl-2,2′-dithiophen-5-yl)­cyano­acrylic acid as the sensitizer and (4,4′-bis­(methyl­phosphonic acid)-2,2′-bipyridine)­Re^I(CO)₃Cl as the catalyst, both of which have been anchored on three different types of TiO₂ particles (s-TiO₂, h-TiO₂, d-TiO₂). It was found that remarkable enhancements in the CO₂ conversion activity of the hybrid photocatalytic system can be achieved by addition of water or such other additives as Li⁺, Na⁺, and TEOA. The photocatalytic CO₂ reduction efficiency was enhanced by approximately 300% upon addition of 3% (v/v) H₂O, giving a turnover number of ≥570 for 30 h. A series of Mott–Schottky (MS) analyses on nanoparticle TiO₂ films demonstrated that the flat-band potential (<i>V</i>_fb) of TiO₂ in dry DMF is substantially negative but positively shifts to considerable degrees in the presence of water or Li⁺, indicating that the enhancement effects of the additives on the catalytic activity should mainly arise from optimal alignment of the TiO₂ <i>V</i>_fb with respect to the excited-state oxidation potential of the sensitizer and the reduction potential of the catalyst in our ternary system. The present results confirm that the TiO₂ semiconductor in our heterogeneous hybrid system is an essential component that can effectively work as an electron reservoir and as an electron transporting mediator to play essential roles in the persistent photocatalysis activity of the hybrid system in the selective reduction of CO₂ to CO