J. Phys. Chem. A 2004, 108, 9115-9119
9115
Kinetics of Electron Transfer and Oxygen Evolution in the Reaction of [Ru(bpy)3]3+ with Colloidal Iridium Oxide Natalie D. Morris, Masahiro Suzuki,† and Thomas E. Mallouk* Department of Chemistry, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802 ReceiVed: May 9, 2004; In Final Form: August 13, 2004
The kinetics of electron transfer and oxygen evolution at citrate-stabilized IrOx‚nH2O colloids were studied by time-resolved UV-visible spectrosopy and by steady-state photolysis of [Ru(bpy)3]2+ (bpy ) 2,2′-bipyridyl) and persulfate in a hexafluorosilicate/bicarbonate buffer. Time-resolved studies of the reaction of [Ru(bpy)3]3+ with these colloids show an initial fast electron transfer, corresponding to oxidation of Ir(III) to Ir(IV). Further oxidation of surface Ir atoms occurs concomitantly with oxygen evolution with a second-order rate constant of 1.3 × 106 M-1 s-1. Both the time-resolved reduction of [Ru(bpy)3]3+ by IrOx‚nH2O and the photocatalytic oxygen evolution under non-light-limited photolysis conditions have a H/D kinetic isotope effect (KIE) of 1.0. This contrasts with significantly higher KIE values for oxygen evolution from molecular cis,cis-[(bpy)2Ru(OH2)]2O]4+ and [(terpy)(H2O)MnIII(O)2(OH2)terpy)]3+ water oxidation catalysts. This is consistent with the conclusion that, under the conditions of most photocatalytic experiments (∼10-4 M [Ru(bpy)3]2+ concentration), electron transfer from the colloid to the oxidized sensitizer rather than formation of a surface-bound hydroperoxy species is the rate-determining step in photocatalytic oxygen evolution.
Introduction The photochemical cleavage of water to generate hydrogen and oxygen has been studied since the early 1970s1 and today remains one of the major challenges in the field of chemistry.2 Overall water splitting has been demonstrated in series and tandem photoelectrochemical cells. However, these cells are either inefficient (because they employ polycrystalline photoelectrodes)3 or very expensive (because they involve the use of complex single-crystal heterostructures).4 Particles of wide-bandgap oxide semiconductors such as SrTiO3 and KTaO3 modified with Ni/NiO are inexpensive and perform the water splitting reaction reasonably well (quantum efficiency >20%), but they can do so only by using ultraviolet (UV) light.5-10 Although cation and anion doping of oxide semiconductors has recently been shown to give photoactivity in the visible region,11-14 the quantum yields of overall water splitting remain quite low (
