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|Title:||ALS-CXRO Seminar Series|
|When:||08/27/2014 3:00 PM - 4:00 PM|
|Description:||ALS/CXRO Seminar Series|
Dr. Lionel Vayssieres
All-Oxide Quantum-confined Heteronanostructures & Devices
Series of materials and devices based on metal oxide (hetero)nanostructures consisting of surface controlled quantum dots and rods building-blocks have been fabricated utilizing low-cost and large scale aqueous chemical growth. The electronic structure and basic structural, optical, and photoelectrochemical properties of such novel visible light-active oxide semiconductors based on vertically oriented quantum rod-arrays have been thoroughly investigated. Doped and/or quantum dot sensitized bundle of iron oxide quantum rods which by intermediate band effects enable a full visible absorption profile while still being stable against photo-corrosion for efficient and low cost solar hydrogen generation by direct water splitting at neutral pH allowing therefore the use of the largest free natural resource on Earth, that is seawater, as unique electrolyte. The effect of quantum dot size on the chemical properties of materials and, in particular, on the surface chemistry of hydrated metal oxides has scarcely been reported. We have investigated the size effect of quantum dots on their aqueous surface chemistry. Indeed, the effect of size on the surface chemistry of metal oxides was demonstrated by the reversal of the surface acidity from acidic to neutral to basic by changing the size from 12 to 7.5 to 3.5 nm, respectively. Additional studies include the synthesis of large quantities of pure TiO2 anatase quantum dots without the use of surfactant. Thermodynamically and kinetically stable aqueous suspensions have been obtained at various concentration of Ti from which powders have been extracted by ultracentrifugation. In-depth analysis of the size dependent electronic structure over two orders of magnitude (2-200 nm) performed at synchrotron radiation reveals a direct effect of the nanoparticle size on the orbital character of TiO2 anatase quantum dots with important inference for enhanced electrical properties of large bandgap semiconductors. An experimental observation of spontaneous electron enrichment of metal d orbitals in a new transition metal oxide heteronanostructure with nanoscale dimensionality has also been recorded. Such a study has direct implications for the understanding of electron gradient formation at the interface of heteronanostructures. Finally, we also recently studied experimentally by XAS the interfacial electronic structure of various TCO related interfaces as well as the origin of losses in α-Fe2O3 electrodes and the cause of the widely reported order-of-magnitude photoanodic current increase upon short high temperature annealing as well as the contribution of orbital anisotropy on Hematite photoanodes and the size dependency of electrical conductivity of TiO2 quantum dots.