Cobalt Oxide-Silica Core-Shell Nanotubes for Photodriven CO2 Reduction by H2O
Final Report Abstract
By introducing a new synthetic approach that consists of surface attachment of a tripodal anchor followed by linking to a wire molecule, the accuracy of loading level and ease of tuning of the surface concentration of wires in ultrathin silica separation membrane was enabled, which is important for establishing a tight electronic communication between heterobinuclear units and water-oxidation catalyst, i.e. Co oxide, through molecular wires. Infrared, Raman and optical spectroscopy prove the structural integrity and reveal that surface attachment of the PV3 molecules and casting into amorphous silica does not result in structural changes compared to the free wire molecules. At the same time, spectral signature of the interaction of PV3 with the silica environment was uncovered by FT-Raman and optical spectroscopy. Hole transfer was observed from transient CoIII formed by TiIVOCoII → TiIIIOCoIII charge transfer excitation of the chromophore to PV3 molecule within 8 ns (and likely ultrafast), which can outcompete undesired charge transfer processes. The finding that hole transfer from anchored light absorber to embedded molecular wire is observed without molecularly defined linkage, relaxes the need for developing a very difficult synthesis that couples the two components covalently to each other. This is important for practical reasons and described in my review article more in detail. Combined with previously observed electron transfer from concurrently generated transient ZrIII to Cu oxide catalyst for CO2 reduction, the result demonstrates for the first time transfer of the transient hole on the Co donor center to membrane-embedded wire molecule, which is known to inject into a Co oxide catalyst on the ultrafast time scale. With the results reported here, all interfacial charge transfer processes of a heterobinuclear light absorber coupled to nanoparticle catalysts for CO2 reduction and H2O oxidation across a nanoscale silica separation membrane have individually been demonstrated and open up the exploration of complete CO2 + H2O photosynthetic cycles using the Co oxide-silica/PV3 core-shell architecture. Heterobinuclear units grafted onto silica were discovered by the Frei group about a decade ago and are a unique class of chromophores, which stand out due to their robustness, overall broad absorption profile in the visible regime, molecularly defined electronic states (no delocalization of electrons and holes), and their catalytic activity. Only the long-year expertise made it possible to study these units in detail. To get a deeper understanding, model systems consisting of pyridine bound to ZrIVOCoII or TiIVOCoII binuclear units on silica nanoparticles were synthesized and characterized. Transient absorption spectroscopy revealed that pyridine is selectively and reversibly reduced on the sub-10 ns timescale only by transient Zr(III) from the MMCT state, and is extremely long-lived (> 200 ms), which again is important for outcompeting undesired processes. This represents the first time-resolved study of electron transfer from a binuclear unit to an organic acceptor. The discovery guides the design of electronic coupling of heterobinuclear light absorbers to alternative electron accepting (as opposed to hole accepting) organic wire molecules embedded in ultrathin silica separation membrane for developing 2-photon systems. The Co oxide-silica/PV3(a)-TiIVOCoII core shell construct hereby is expanded by an overlayer of electron accepting PV3(b) (from TiIII) embedded in another ultrathin silica coupled to surface grafted ZrIVOFeIII.
