Electrostatic stability of insulating surfaces: Theory and applications

We analyze the electrostatic stability of insulating surfaces in the framework of the bulk modern theory of polarization. We show that heuristic arguments based on a fully ionic limit find formal justification at the microscopic level, even in solids where the bonding has a mixed ionic/covalent character. Based on these arguments, we propose simple criteria to construct arbitrary non-polar terminations of a given bulk crystal. We illustrate our ideas by performing model calculations of several LaAlO3 and SrTiO3 surfaces. We find, in the case of ideal LaAlO3 surfaces, that cleavage along a higher-index (n10) direction is energetically favorable compared to the polar (100) orientation. In the presence of external adsorbates or defects the picture can change dramatically, as we demonstrate in the case of H2O/LaAlO3(100).Comment: 18 pages, 10 figure

Evolution of the Surface Structures on SrTiO3_3(110) Tuned by Ti or Sr Concentration

The surface structure of the SrTiO$_3$(110) polar surface is studied by scanning tunneling microscopy and X-ray photoelectron spectroscopy. Monophased reconstructions in (5$\times$1), (4$\times$1), (2$\times$8), and (6$\times$8) are obtained, respectively, and the evolution between these phases can be tuned reversibly by adjusting the Ar$^{+}$ sputtering dose or the amount of Sr/Ti evaporation. Upon annealing, the surface reaches the thermodynamic equilibrium that is determined by the surface metal concentration. The different electronic structures and absorption behaviors of the surface with different reconstructions are investigated.Comment: 10 pages, 14 figure

Epitaxial Stabilization of Face Selective Catalysts

Abstract Selective, active, and robust catalysts are necessary for the efficient utilization of new feedstocks. Faceselective catalysts can precisely modify catalytic properties, but are often unstable under reaction conditions, changing shape and losing selectivity. Herein we report a method for synthesizing stable heterogeneous catalysts in which the morphology and selectivity can be tuned precisely and predictably. Using nanocrystal supports, we epitaxially stabilize specific active phase morphologies. This changes the distribution of active sites of different coordination, which have correspondingly different catalytic properties. Specifically, we utilize the different interfacial free-energies between perovskite titanate nanocube supports with different crystal lattice dimensions and a platinum active phase. By substituting different sized cations into the support, we change the lattice mismatch between the support and the active phase, thereby changing the interfacial free-energy, and stabilizing the active phase in different morphologies in a predictable manner. We correlate these changes in active phase atomic coordination with changes in catalytic performance (activity and selectivity), using the hydrogenation of acrolein as a test reaction. The method is general and can be applied to many nanocrystal supports and active phase combinations. Keywords Epitaxy Á Perovskite Á Platinum Á Heterogeneous catalysis Á Hydrogenation Á Acrolein Controlling the morphology of catalytic metal nanoparticles has incredible potential for improving selectivity and yield. This is because catalytic properties often depend upon the coordination of active site atoms We have recently observed that oriented oxide nanocrystal supports can epitaxially stabilize a specific orientation and morphology of the active phas

Victim contact work and the probation service A study of service delivery and impact

SIGLEAvailable from British Library Document Supply Centre-DSC:m00/21819 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Transition from reconstruction toward thin film on the (110) surface of strontium titanate.

The surfaces of metal oxides often are reconstructed with a geometry and composition that is considerably different from a simple termination of the bulk. Such structures can also be viewed as ultrathin films, epitaxed on a substrate. Here, the reconstructions of the SrTiO3 (110) surface are studied combining scanning tunneling microscopy, transmission electron diffraction, and Xray absorption spectroscopy, and analyzed with density functional theory calculations. While SrTiO3 (110) invariably terminates with an overlayer of titania, with increasing density its structure switches from n×1 and 2×n. At the same time the coordination of the Ti atoms changes from a network of corner-sharing tetrahedra to a double layer of edge-shared octahedra with bridging units of octahedrally coordinated strontium. This transition from the n×1 to 2×n reconstructions is a transition from a pseudomorphically stabilized tetrahedral network towards an octahedral titania thin film with stress-relief from octahedral strontia units at the surface