Light-Driven Water Oxidation Using Polyelectrolyte Layer-by-Layer Chromophore-Catalyst Assemblies

Layer-by-Layer (LbL) polyelectrolyte self-assembly occurs by the alternate exposure of a substrate to solutions of oppositely charged polyelectrolytes or polyions. Here, we report the application of LbL to construct chromophore-catalyst assemblies consisting of a cationic polystyrene-based Ru polychromophore (PS-Ru) and a [Ru(tpy)(2-pyridyl-N-methylbenzimidazole) (OH₂)]²⁺ water oxidation catalyst (RuC), codeposited with poly(acrylic acid) (PAA) as an inert polyanion. These assemblies are deposited onto planar indium tin oxide (ITO, Sn:In₂O₃) substrates for electrochemical characterization and onto mesoporous substrates consisting of a SnO₂/TiO₂ core/shell structure atop fluorine doped tin oxide (FTO) for application to light-driven water oxidation in a dye-sensitized photoelectrosynthesis cell. Cyclic voltammetry and ultraviolet-visible absorption spectroscopy reveal that multilayer deposition progressively increases the film thickness on ITO glass substrates. Under an applied bias, photocurrent measurements of the (PAA/PS-Ru)₅/(PAA/RuC)₅ LbL films formed on FTO//SnO₂/TiO₂ mesoporous core-shell electrodes demonstrate a clear anodic photocurrent response. Prolonged photoelectrolysis experiments, with the use of a dual working electrode collector-generator cell, reveal production of O₂ from the illuminated photoanode with a Faradaic efficiency of 22%. This is the first report to demonstrate the use of polyelectrolyte LbL to construct chromophore-catalyst assemblies for water oxidation.