Organic-inorganic hybrid photocatalyst for carbon dioxide reduction

Efficient hybrid photocatalysts for carbon dioxide reduction were developed from dye-sensitized TiO₂ nanoparticles and their catalytic performance was optimized by ternary organic/inorganic components. Thus, the hybrid system consists of (E)-2-cyano-3-(5'-(5''-(p-(diphenylamino)phenyl)thiophen-2''-yl)thiophen-2'-yl)-acrylic acid as a sensitizer and fac-[Re(4,4'-bis(diethoxyphosphorylmethyl)-2,2'-bipyridine)(CO)₃Cl] as a reduction catalyst (ReP), both of which have been fixed onto TiO₂ semiconductors (s-TiO₂, h-TiO₂, d-TiO₂). Mott-Schottky analysis on flat-band potential (E_fb) of TiO₂ mesoporous films has verified that E_fb can be finely modulated by volume variation of water (0 to 20 vol%). The increase of added water resulted in substantial positive shifts of E_fb from -1.93 V at 0 vol% H₂O, to -1.74 V (3 vol% H₂O), to -1.56 V (10 vol% H₂O), and to -1.47 V (20 vol% H₂O). As a result, with addition of 3-10 vol% water in the photocatalytic reaction, conversion efficiency of CO₂ to CO increased significantly reaching a TON value of ∼350 for 30 h. Catalytic activity enhancement is mainly attributed to (1) the optimum alignment of E_fb by 3-10 vol% water with respect to the of the dye and E_red of ReP for smooth electron transfer from photo-excited dye to RePvia the TiO₂ semiconductor and (2) the water-induced acceleration of chemical processes on the fixed ReP. In addition, the energy level was further tuned by variation of the dye and ReP amounts. We also found that the intrinsic properties of TiO₂ sources (morphology, size, agglomeration) exert a great influence on the overall photocatalytic activity of this hybrid system. Implications of the present observations and reaction mechanisms are discussed in detail.