Our goal is to complete the photosynthetic cycle of CO2 reduction by H2O under membrane separation of the half reactions on the nanoscale in order to minimize side reactions and charge transport efficiency losses, and enable the design of scalable systems. Co oxide nanotubes surrounded by ultrathin silica shells with embedded molecular wires (para-oligo(phenylene vinylene)) for tight control of electron transport are being developed as water oxidation catalyst-membrane assemblies driven by all-inorganic heterobinuclear light absorbers (e.g. ZrOCo). On the reductive side, a photodeposition method using the ZrOCo charge transfer chromophore allowed the spatially directed assembly of a cuprous oxide nanocluster for CO2 reduction.

Femtosecond transient absorption spectroscopy of photo-induced hole transfer to Co oxide catalyst water oxidation catalyst across the silica-embedded wires allowed the direct observation of charge arrival on the wire molecule, which takes place in less than a ps. Charge separation was indicated by the emerging wire radical cation. Subsequent forward transfer of the positive charge to the Co oxide particle occurred in 250 ps, exceeding known hole transfer rates from anchored molecular light absorbers to metal oxide catalyst particles for water oxidation by several orders of magnitude. Arrival of holes on Co3O4 was indicated by bleach at 485 nm. The finding indicates that molecular light absorbers coupled to metal oxide catalysts by silica-embedded oligo(phenylene vinylene) offers an approach for integrated artificial photosystems featuring efficient hole transfer while enabling product separation on the nanoscale.

The nanometer-thin dense phase silica layers with embedded organic wires covalently anchored on the surface of Co oxide water oxidation catalyst were shown by visible light sensitized electrochemical measurements to tightly control transport of charges across the proton conducting, O2 impermeable membrane by the orbital energetics of the wire molecules. Visible light sensitization using Ru(bpy)3 resulted in short circuit current (27 e-s-1wire-1), consistent with favorable alignment of the [Ru(bpy)3]3+ potential with respect to the HOMO energy of the wire. By contrast, visible light-generated reduced Sn porphyrin did not induce current because the potential is situated in the HOMO-LUMO gap of the wire. The finding demonstrates rectifying property of the light absorber-wire assembly.

Mechanisms of water H2O and CO2 reduction on metal or metal oxide nanoparticle catalysts are elucidated by time-resolved ATR FT-IR spectroscopy under photocatalytic conditions by identifying transient surface intermediates. Recent new insights from the spectroscopic studies will be discussed.