Stabilizing Small Molecules on Metal Oxide Surfaces Using Atomic Layer Deposition

Abstract

Device lifetimes and commercial viability of dye-sensitized solar cells (DSSCs) and dye-sensitized photoelectrosynthesis cells (DSPECs) are dependent on the stability of the surface bound molecular chromophores and catalysts. Maintaining the integrity of the solution-metal oxide interface is especially challenging in DSPECs for water oxidation where it is necessary to perform high numbers of turnovers, under irradiation in an aqueous environment. In this study, we describe the atomic layer deposition (ALD) of TiO₂ on nanocrystalline TiO₂ prefunctionalized with the dye molecule [Ru­(bpy)₂(4,4′-(PO₃H₂)­bpy)]²⁺ (RuP) as a strategy to stabilize surface bound molecules. The resulting films are over an order of magnitude more photostable than untreated films and the desorption rate constant exponentially decreases with increased thickness of ALD TiO₂ overlayers. However, the injection yield for TiO₂-RuP with ALD TiO₂ also decreases with increasing overlayer thickness. The combination of decreased injection yield and 95% quenched emission suggests that the ALD TiO₂ overlayer acts as a competitive electron acceptor from RuP*, effectively nonproductively quenching the excited state. The ALD TiO₂ also increases back electron transfer rates, relative to the untreated film, but is independent of overlayer thickness. The results for TiO₂-RuP with an ALD TiO₂ overlayer are compared with similar films having ALD Al₂O₃ overlayers