Bidentate Dicarboxylate Capping Groups and Photosensitizers Control the Size of IrO2 Nanoparticle Catalysts for Water Oxidation

Abstract

Dicarboxylic acid ligands (malonate, succinate, and butylmalonate) stabilize 2 nm diameter IrO₂ particles synthesized by hydrolysis of aqueous IrCl₆^2- solutions. Analogous monodentate (acetate) and tridentate (citrate) carboxylate ligands, as well as phosphonate and diphosphonate ligands, are less effective as stabilizers and lead to different degrees of nanoparticle aggregation, as evidenced by transmission electron microscopy. Succinate-stabilized 2 nm IrO₂ particles are good catalysts for water photo-oxidation in persulfate/sensitizer solutions. Ruthenium tris(2,2‘-bipyridyl) sensitizers containing malonate and succinate groups in the 4,4‘-positions are also good stabilizers of 2 nm diameter IrO₂ colloids. The excited-state emission of these bound succinate-terminated sensitizer molecules is efficiently quenched on a time scale of ∼30 ns, most likely by electron transfer to Ir(IV). In 1 M persulfate solutions in pH 5.8 Na₂SiF₆/NaHCO₃ buffer solutions, the excited-state of the bound sensitizer is quenched oxidatively on the time scale of ∼9 ns. Electron transfer from Ir(IV) to Ru(III) occurs with a first-order rate constant of 8 × 10² s^-1, and oxygen is evolved. The turnover number for oxygen evolution under these conditions was ∼150. The sensitizer−IrO₂ diad is thus a functional catalyst for photo-oxidation of water, and may be a useful building block for overall visible light water splitting systems.