The discovery of quasicrystals--crystalline structures that show order while lacking periodicity--forced a paradigm shift in crystallography. Initially limited to intermetallic systems, the observation of quasicrystalline structures has recently expanded to include 'soft' quasicrystals in the fields of colloidal and supermolecular chemistry. Here we report an aperiodic oxide that grows as a two-dimensional quasicrystal on a periodic single-element substrate. On a Pt(111) substrate with 3-fold symmetry, the perovskite barium titanate BaTiO3 forms a high-temperature interface-driven structure with 12-fold symmetry. The building blocks of this dodecagonal structure assemble with the theoretically predicted Stampfli-Gähler tiling having a fundamental length-scale of 0.69 nm. This example of interface-driven formation of ultrathin quasicrystals from a typical periodic perovskite oxide potentially extends the quasicrystal concept to a broader range of materials. In addition, it demonstrates that frustration at the interface between two periodic materials can drive a thin film into an aperiodic quasicrystalline phase, as proposed previously. Such structures might also find use as ultrathin buffer layers for the accommodation of large lattice mismatches in conventional epitaxy.
Butane activation has been studied using three types of sulfated zirconia materials, single crystalline epitaxial films, nanocrystalline films, and powders. A surface phase diagram of zirconia in interaction with SO(3) and water was established by DFT calculations, which was verified by LEED investigations on single-crystalline films and by IR spectroscopy on powders. At high sulfate surface densities a pyrosulfate species is the prevailing structure in the dehydrated state; if such species are absent, the materials are inactive. Theory and experiment show that the pyrosulfate can react with butane to give butene, H(2)O and SO(2), hence butane can be activated via oxidative dehydrogenation. This reaction occurred on all investigated materials; however, isomerization could only be proven for powders. Transient and equilibrium adsorption measurements in a wide pressure and temperature range (isobars measured via UPS on nanocrystalline films, microcalorimetry and temporal analysis of products measurements on powders) show weak and reversible interaction of butane with a majority of sites but reactive interaction with <5 micromol g(-1) sites. Consistently, the catalysts could be poisoned by adding sodium to the surface in a ratio S/Na = 35. Future research will have to clarify what distinguishes these few sites.
Nickel(II) hydroxide is a well-known material for the oxygen evolution reaction (OER) in alkaline media, particularly when iron is incorporated into its lattice. Moderate heat treatment of nickel(II) hydroxide (≤700 °C) leads to the formation of nickel(II) oxide (nano)particles, which exhibit reduced OER activity the higher the heat treatment temperature was. In this work, we report that heat treatment of nickel(II) hydroxide in air at even higher temperatures (60 min at 900 °C) results in an oxide material with high OER activity superior to that of the nickel(II) hydroxide. Similarly, the stability of the nickel(II) oxide under electrochemical conditions is increased compared to nickel(II) hydroxide. Electrochemical in situ Raman measurements show the formation of surface nickel oxy-hydroxides (NiOOH) at positive potentials and are significantly affected by the initial heat treatment. From XPS, Raman, and XRD results, it is concluded that a Ni 3+ -enriched phase, possibly a higher-valent mixed nickel oxide, is present at the surface of the nickel(II) oxide sample treated at 900 °C resulting in an increased OER activity compared to NiOOH. This basic understanding of high-temperature-treated nickel oxide may contribute to resolving the present stability issues of OER electrocatalysts and may help to leverage alkaline electrolysis as important key technology for a renewable energy supply.
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