Organic–inorganic hybrid photocatalyst for carbon dioxide reduction

Abstract

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 (<italic>E</italic>)-2-cyano-3-(5′-(5′′-(<italic>p</italic>-(diphenylamino)phenyl)thiophen-2′′-yl)thiophen-2′-yl)-acrylic acid as a sensitizer and <italic>fac</italic>-[Re(4,4′-bis(diethoxyphosphorylmethyl)-2,2′-bipyridine)(CO)₃Cl] as a reduction catalyst (<bold>ReP</bold>), both of which have been fixed onto TiO₂ semiconductors (s-TiO₂, h-TiO₂, d-TiO₂). Mott–Schottky analysis on flat-band potential (<italic>E</italic>_fb) of TiO₂ mesoporous films has verified that <italic>E</italic>_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 <italic>E</italic>_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 <italic>E</italic>_fb by 3–10 vol% water with respect to the <graphic xmlns:xlink="http://www.w3.org/1999/xlink" id="ugt1" xlink:href="http://pubs.rsc.org/FD/2017/c6fd00222f/c6fd00222f-t1..gif"/> of the dye and <italic>E</italic>_red of <bold>ReP</bold> for smooth electron transfer from photo-excited dye to <bold>ReP</bold><italic>via</italic> the TiO₂ semiconductor and (2) the water-induced acceleration of chemical processes on the fixed <bold>ReP</bold>. In addition, the energy level was further tuned by variation of the dye and <bold>ReP</bold> 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.