Martirez, John Mark Pinat
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PublicationSurface Reconstructions of Perovskite-Type Oxides: Their Exotic Structures and Modified Chemistry(2015-01-01) Martirez, John Mark PinatThe ability to manipulate the atomic and electronic structures and stoichiometry of surfaces is of utmost importance in optimizing heterogeneous catalysts. A critical requirement in this endeavor is a deep thermodynamic and kinetic understanding of surface reconstruction behavior, under various thermal and chemical constraints. We explore the reconstruction behaviors (structure and chemistry) of Ti-based perovskite type oxides: BaTiO3, PbTiO3 and SrTiO3. The former two exhibit ferroelectricity. We find that these oxides undergo surface reconstruction transformations that generally result in enrichment of their catalytically active component: Ti. These reconstructions show rich bonding and structural motifs that affect the active sites' reactivity and accessibility. In addition to the thermodynamic understanding of the surface reconstructions, we introduce the kinetic tunability of the surface reconstruction. We demonstrate this from a particular surface phase coexistence observed in BaTiO3, namely the c(2x2) and c(4x4), where the diffusion behavior of the TiO units that compose both surfaces strongly dictate their degree of agglomeration. This work emphasizes that employing kinetics in addition to thermodynamics is sometimes needed in explaining surface phase transformations. We also explore the unique chemistry that these reconstructions enable, specifically the reduced Ti-rich reconstruction of BaTiO3 and the oxidized TiO2-rich double-layer reconstructions of SrTiO3. In the former, a promising route for hydrogen production is found upon reaction with water. In the latter, low thermodynamic barriers for the oxidation of water to O2 has been found on the reconstructed surface relative to the typical native termination and the surface of rutile-phase TiO2. Finally, we show that in ferroelectrics, BaTiO3 and PbTiO3, these surface transformations can be tuned with the help of an electric field. An applied electric field changes the material's polarization, which then alters the surface electronic properties, and thereby also affects their sensitivity towards stoichiometric changes.