Visible Light Induced Hole Transport from Sensitizer to Co3O4 Water Oxidation Catalyst across Nanoscale Silica Barrier with Embedded Molecular Wires

Abstract

In an artificial photosynthetic system, separation of the catalytic sites for water oxidation from those of carbon dioxide reduction by a gas impermeable physical barrier is an important requirement for avoiding cross and back reactions. Here, an approach is explored that uses crystalline Co3O4 as an oxygen evolving catalyst and a nanometer-thin dense phase silica layer as the separation barrier. For controlled charge transport across the barrier, hole conducting molecular wires are embedded in the silica. Spherical Co3O4(4 nm)–SiO2(2 nm) core–shell nanoparticles with p-oligo(phenylenevinylene) wire molecules (three aryl units, PV3) cast into the silica were developed to establish proof of concept for charge transport across the embedded wire molecules. FT-Raman, FT-infrared, and UV–visible spectroscopy confirmed the integrity of the organic wires upon casting in silica. Transient optical absorption spectroscopy of a visible light sensitizer (ester derivatized [Ru(bpy)3]2+ complex) indicates efficient charge injection into Co3O4–SiO2 particles with embedded wire molecules in aqueous solution. An upper limit of a few microseconds is inferred for the residence time of the hole on the embedded PV3 molecule before transfer to Co3O4 takes place. The result was corroborated by light on/off experiments using rapid-scan FT-IR monitoring. These observations indicate that hole conducting organic wire molecules cast into a dense phase, nanometer thin silica layer offer fast, controlled charge transfer through a product-separating oxide barrier.