In this study the performance enhancement effect of structural ordering for the oxygen reduction reaction (ORR) is systematically studied. Two samples of PtCu3 nanoparticles embedded on a graphitic carbon support are carefully prepared with identical initial composition, particle dispersion and size distribution, yet with different degrees of structural ordering. Thus we can eliminate all coinciding effects and unambiguously relate the improved activity of the ORR and more importantly the enhanced stability to the ordered nanostructure. Interestingly, the electrochemically induced morphological changes are common to both ordered and disordered samples. The observed effect could have a groundbreaking impact on the future directions in the rational design of active and stable platinum alloyed ORR catalysts.

The atomic structure and the chemistry of basal-plane inversion boundaries in Sb 2 O 3 -doped ZnO were investigated using quantitative transmission electron microscopy techniques. Electron microdiffraction and high-resolution transmission electron microscopy were used to determine the orientation of the polar c-axis on both sides of the inversion boundary and the translation state between the inverted ZnO domains. Quantitative energy-dispersive X-ray spectroscopy combined with high-resolution transmission electron microscopy allowed us to determine the exact amount and the arrangement of antimony in the boundary layer. Inversion boundaries are head-to-head oriented with a displacement vector of the oxygen sublattice of R IB ‫؍‬ 1 ⁄3[011 0] -0.102[0001]. The boundary plane consists of a highly ordered SbZn 2 monolayer in which the cations occupy the octahedral interstices of the structure. In the octahedral boundary layer, zinc and antimony atoms constitute a honeycomb superstructure with a threefold (3m) in-plane symmetry.

Editor: J. Fein Keywords:Amorphous iron sulfide Mackinawite Greigite Phase transformations XRD TEM Nanocrystalline iron sulfides form in diverse anoxic environments. The initial precipitate is commonly referred to as nanocrystalline mackinawite (FeS) or amorphous FeS. In order to better understand the structure of the initial precipitate and its conversion to mackinawite and greigite (Fe 3 S 4 ), we studied synthetic iron sulfide samples that were precipitated from hydrous solutions near room temperature. The transformation of precipitated FeS was followed in both aqueous and dry aging experiments using X-ray powder diffraction (XRD) and scanning and transmission electron microscopy (SEM/TEM) and selected-area electron diffraction (SAED). Under tightly controlled anoxic conditions the first precipitate was nanocrystalline mackinawite. In contrast, when anaerobic conditions during synthesis were not completely ensured, freshly precipitated iron sulfide was typically X-ray amorphous (FeS am ), and showed only one broad Bragg-peak at 2Θ = 16.5°(5.4 Å). A distribution of interatomic distances calculated from pair-distribution function analysis of SAED patterns of FeS am showed that only short-range (b 7 Å) order was present in the bulk of the material, with Fe mainly present in tetrahedral coordination. SEM and TEM images confirmed the poorly ordered structure and showed that FeS am formed aggregates of curved, amorphous sheets that contained 3-8 structurally ordered layers at their cores. Such layers are generally assumed to be structurally similar to the tetrahedral iron sulfide layers in mackinawite. However, both inter-and intralayer spacings measured in high-resolution TEM images (~5.3 to 6.3 and~3.0 to 3.1 Å, respectively) were significantly larger than the corresponding spacings in crystalline mackinawite (5.03 and 2.6 Å, respectively), suggesting that short-range structural order within the semi-ordered layers of FeS am was not mackinawite-like. In aqueous aging experiments at room temperature, FeS am transformed into a mixture of mackinawite and greigite in~2 months, and completely converted to platy greigite crystals after~10 months. These aqueous transformations were likely driven by excess sulfur in the reacting solutions. We also studied the conversions of nanocrystalline mackinawite. In order to accelerate phase transitions, the initial FeS precipitate was heated to 120°C, resulting in the formation of crystalline mackinawite within 2 h; at 150°C, the material converted directly to pyrrhotite. Finally, when stored in a dry state at room temperature, crystalline mackinawite converted to greigite in 3 months, much faster than in the equivalent experiments in the aqueous solution, probably as a result of a more oxidative environment. The distinction between FeS am and nanocrystalline mackinawite is significant, since conditions for the formation of both phases are present in natural settings. Our experiments in a well-sealed anaerobic chamber simulate iron sulfide formation under anoxic conditions, where...

Recent breakthrough of novel hierarchic materials, orchestrated through oriented attachment of crystal subunits, opened questions on what is the mechanism of their self-assembly. Using rutile-type TiO2, synthesized by hydrothermal reaction of Ti(IV)-butoxide in highly acidic aqueous medium, we uncovered the key processes controlling this nonclassical crystallization process. Formation of complex branched mesocrystals of rutile is accomplished by oriented assembly of precipitated fibers along the two low-energy planes, i.e. {110} and {101}, resulting in lateral attachment and twinning. Phase analysis of amorphous material enclosed in pockets between imperfectly assembled rutile fibers clearly shows harmonic ordering resembling that of the adjacent rutile structure. To our understanding this may be the first experimental evidence indicating the presence of electromagnetic force-fields that convey critical structural information through which oriented attachment of nanocrystals is made possible.

The effects of adding small quantities of SnO 2 to the basic ZnO-Bi 2 O 3 varistor composition were studied in terms of phase reactions, microstructural development, and the formation of inversion boundaries. Scanning and transmission electron microscopy studies showed that the inversion boundaries, triggered by the addition of SnO 2 , cause anisotropic grain growth in the early stages of sintering. ZnO grains that include inversion boundaries grow exaggeratedly, at the expense of normal grains, until they dominate the microstructure. Higher additions of SnO 2 lead to an increase in number of grains with inversion boundaries and to a more fine-grained microstructure. The increasing amount of secondary phases is also related to a higher level of SnO 2 addition; however, the influence of these phases on ZnO grain growth is subordinate to the role of inversion boundaries.