Cobalt Oxide Core - Silica Shell Units for Artificial Photosynthesis

Invited Speaker, Heinz Frei, presentation 022
Publication date: 31st March 2013

The long term goal of our research is the direct conversion of carbon dioxide and water with visible light to a liquid fuel in a nanoscale assembly. Focusing on robust inorganic molecular light absorbers and metal oxide nanocatalysts, geometries are explored that afford the coupling of the components across a proton transmitting nanoscale silica layer under separation of the water oxidation catalysis from all other photosynthetic processes. Using recently developed heterobinuclear charge-transfer units anchored on silica as visible light chromophores, Co₃O₄ particles as multi-electron catalysts for water oxidation,^[1,2] and core-shell geometry for separating the O₂ evolution from light absorber and reduction chemistry, we are developing an assembly for closing the photosynthetic cycle on the nanoscale.

            Starting out with spherical Co₃O₄(4 nm)/SiO₂(2 nm) core/shell particles, we have developed the materials chemistry for embedding molecular wires of type oligo(paraphenylenevinylene) (OPPV) into the silica shell for controlled hole transport from a visible light sensitizer on the outside to the Co₃O₄ catalyst core on the inside. Transient optical absorption spectroscopy revealed efficient hole injection into the embedded OPPV (3 aryl units) molecules followed by fast (microsecond or less) transfer to the Co oxide particle.^[3,4] The result opens up an approach for using nanoscale silica layers with embedded organic wire molecules for separating the water oxidation catalyst from light absorber and reductive chemistry by an impermeable barrier. Co oxide - silica core - shell nanotubes afford the proper geometry for realizing water oxidation catalysis in a separate space (i.e. inside Co₃O₄ tube) from light absorber (outside SiO₂ shell). We have explored synthetic methods for preparing Co₃O₄ nanotubes and dense silica shells around them with embedded OPPV wires of precise thickness and high uniformity. The core/shell nanotube design has the proper geometry for closing the photosynthetic cycle under product separation.

            Monitoring of visible light sensitized water oxidation at Co₃O₄ nanoparticles in aqueous solution by rapid-scan ATR-FT-IR spectroscopy allowed for the first time the detection of surface reaction intermediates of water oxidation on a metal oxide catalyst in a time resolved fashion. Two intermediates of the 4-electron oxidation process were identified.^[5,6] In parallel, we have developed binuclear ZrOCo(II) units covalently anchored on the silica surface for visible light induced reduction of CO₂ to CO and formate.

[1] F. Jiao and H. Frei, Angew. Chem. Int. Ed. 48, 1841 (2009). [2] F. Jiao and H. Frei, Energy Environ. Sci. 3, 1018 (2010). [3] H.S. Soo, A. Agiral, A. Bachmeier, and H. Frei, J. Am. Chem. Soc. 134, 17104 (2012). [4] A. Agiral, H.S. Soo, and H. Frei, submitted. [5] N. Sivasankar, W.W. Weare, and H. Frei, J. Am. Chem. Soc. 133, 12976-12979 (2011). [6] M. Zhang and H. Frei, submitted.

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