Photovoltage effects of sintered IrO₂ nanoparticle catalysts in water-splitting dye-sensitized photoelectrochemical cells

Water-splitting dye-sensitized photoelectrochemical cells (WS-DSPECs) utilize high surface area TiO₂ electrodes functionalized with light absorbing sensitizers and water oxidation catalysts. Because water splitting requires vectorial electron transfer from the catalyst to the sensitizer to the TiO₂ surface, attaching both sensitizer and catalyst to TiO ₂ in the correct sequence and stabilizing them under photoelectrochemical conditions has been a challenging problem. Rutile-phase IrO₂ nanoparticles can be deposited directly on the TiO₂ electrode by adsorbing citrate-capped amorphous IrO_x and then sintering at 450 °C. Electrodes functionalized with these nanocrystalline particles show higher activity than those made from ligand-capped amorphous IrO_x without sintering. In the WS-DSPEC, the Coulombic efficiency for oxygen evolution from the sintered nanoparticle photoelectrodes was near unity. The loading of colloidal IrO_x and IrO₂ particles onto the porous TiO₂ electrodes was quantified by neutron activation analysis. Photovoltage measurements suggest that at high catalyst loading the dominant charge recombination pathway is from photoinjected electrons to IrO₂.