The development of hierarchical inorganic assemblies for carbon dioxide photoreduction by water to fuel requires precise control of charge transport between light absorbers and catalysts, and across membranes that separate oxidation and reduction chemistries. Equally importantly, the challenge of matching the rate of photon absorption by chromophores with the speed of multi-electron catalysts for maximizing the productive use of absorbed light demands identification of kinetic bottlenecks of catalytic cycles in order to guide design improvements. We are developing Co3O4 core – silica shell nanotube assemblies in which the inner surface of the Co oxide tube serves as location for water oxidation catalysis while light absorbers and sites for CO2 reduction are situated on the outside of the silica shell. Charge transport from light absorber to Co oxide catalyst across the 2 nm proton conducting silica layer is controlled by the orbital energetics of embedded molecular wires (oligo-para(phenylene vinylene)). Transient optical absorption spectroscopy using spherical core-shell nanoparticles revealed ultrafast (200 ps) hole transfer from molecular light absorber to the Co3O4 catalyst across the silica membrane. Efficient light-driven charge flow across the silica membrane with embedded wires was quantified by photoelectrochemical measurements in short circuit configuration using planar constructs. The molecular structure and kinetics of transient surface intermediates (Co(IV) oxo and a superoxide) of water oxidation catalysis were elucidated by rapid-scan FT-infrared spectroscopy. Sites with different activities were found, with the fastest site exhibiting a high turnover frequency of 3/sec. Transient spectroscopic studies of the coupling of heterobinuclear light absorbers such as ZrOCo with silica embedded molecular wires, and with Cu cluster catalysts for CO2 reduction will be discussed.

Charge transport and reaction mechanisms of inorganic photocatalytic assemblies by transient infrared and optical spectroscopy

253rd National Meeting (2017)

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