The Effect of Water on the 2-Propanol Oxidation Activity of Co-Substituted LaFe₁₋Co_xO₃ Perovskites

Perovskites are interesting oxidation catalysts due to their chemical flexibility enabling the tuning of several properties. In this work, we synthesized LaFe1−xCoxO3 catalysts by co-precipitation and thermal decomposition, characterized them thoroughly and studied their 2-propanol oxidation activity under dry and wet conditions to bridge the knowledge gap between gas and liquid phase reactions. Transient tests showed a highly active, unstable low-temperature (LT) reaction channel in conversion profiles and a stable, less-active high-temperature (HT) channel. Cobalt incorporation had a positive effect on the activity. The effect of water was negative on the LT channel, whereas the HT channel activity was boosted for x > 0.15. The boost may originate from a slower deactivation rate of the Co3+ sites under wet conditions and a higher amount of hydroxide species on the surface comparing wet to dry feeds. Water addition resulted in a slower deactivation for Co-rich catalysts and higher activity in the HT channel state

Spatial resolution and radiation damage in quantitative high-resolution STEM-EEL spectroscopy in oxides

The chemical analysis on the atomic scale in a scanning transmission electron microscope bears a number of challenges. These are an unambiguous assignment of a spectroscopic signal to a sample location and sufficient signal above noise for quantification. Modern aberration-corrected optics provide intense electron probes allowing for the highest spatial resolution and beam current density possible. On the other hand, non-destructive analysis requires low irradiation doses, so that there is a limit to the achievable signal-to-noise ratio. Here, we employ the StripeSTEM method that sacrifices the resolution in one spatial dimension in return for decreased radiation damage to the sample. Using this technique, radiation damage effects and achievable quantification accuracy are examined on the example of bulk SrTiO3 and a one unit cell thick layer of LaAlO3 in SrTiO3. The results show that valency artefacts are expected for conventional recording conditions where the electron dose is concentrated to a few atomic columns. Likewise a high accuracy for measuring the oxygen defect chemistry without radiation damage requires spreading out the irradiation dose. (C) 2011 Elsevier Ltd. All rights reserved

StripeSTEM, a technique for the isochronous acquisition of high angle annular dark-field images and monolayer resolved electron energy loss spectra

A technique capable of producing monolayer resolved electron energy loss (EEL) spectroscopy data along one direction in crystal structures is introduced. Unambiguous assignment of EEL spectra to atomic planes is possible via the execution of high angle annular dark-field (HAADF) imaging and EEL spectrum acquisition in parallel. The recording of instrumental instabilities in the HAADF image during the measurement enables a proper quantification by virtue of post-acquisition correction. Compared to the conventional line profile technique a dose reduction by several orders of magnitude can be achieved. The technique is applied to bulk SrTiO(3) and ZnO:In(2)O(3) in order to explore its capabilities and limits. Monolayer resolution was achieved for the Ti-L(23) and In-M(45) core-losses. Multislice calculations were carried out for the purpose of assessing the residual delocalisation of the inelastic signal. Fundamental limits to the resolution are imposed by dynamical dispersion of the electron wave in the crystal combined with the extension of the inelastic potential. In the present case, owing to the requirement of a high beam current, the geometrical probe size cannot be neglected when compared to the width of an inelastic scattering potential

Inorganic WS2 nanotubes revealed atom by atom using ultra-high-resolution transmission electron microcopy

The characterization of nanostructures to the atomic dimensions becomes more important, as devices based on a single particle are being produced. In particular, inorganic nanotubes were shown to host interesting properties making them excellent candidates for various devices. The WS2 nanotubes outperform the bulk in their mechanical properties offering numerous applications especially as part of high strength nanocomposites. In contrast, their electrical properties are less remarkable. The structure-function relationship can be investigated by aberration-corrected high-resolution transmission electron microscopy (HRTEM), which enables the insight into their atomic structure as well as performing spectroscopic measurements down to the atomic scale. In the present work, the deciphering of atomic structure and the chiral angle of the different shells in a multiwall WS2 nanotube is demonstrated. In certain cases, the helicity of the structure can also be deduced. Finally, first electron energy loss spectra (EELS) of a single tube are presented, acquired by a new acquisition technique that allows for high spatial resolution (denoted StripeSTEM). The measured band gap values correspond with the values found in literature for thin films, obtained by spectroscopic techniques, and are higher than the values resulting from STM measurements

Intermixing and charge neutrality at DyScO3/SrTiO3 interfaces

Recently, interfaces between complex insulating oxides have attracted much attention due to their broad spectrum of electronic properties. Joining two materials of different polarity call provide highly conducting layers. The polar discontinuity delivers the driving force for a charge accumulation in the interfacial region which has been demonstrated for lanthanum-based perovskite interfaces with SrTiO3. Here it is shown that the polar discontinuity can be accommodated by variations in composition of cation lattice planes at the polar oxide interface between DyScO3 and SrTiO3, Where DyScO3 holds the same polarity as the lanthanum-based perovskites. An intermixing extending over two monolayers at the interfaces for both the Dy-Sr sublattice and the Sc-Ti sublattice is quantified. As a result, charge neutrality is established by electrical compensation between neighbouring atomic planes. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Self-assembly of biaxial discorectangular lead carbonate nanosheets into stacked ribbons studied by SAXS and HAADF-STEM tomographic tilt series

The self-assembling behaviour of 2.6 nm thin PbCO3 nanoplatelets with discorectangular shape and uniform width and thickness occurring after their formation in nonionic water-in-oil microemulsions has been investigated using synchrotron small angle X-ray scattering (SAXS) and (scanning) transmission electron microscopy ((S)TEM). The presence of attractive depletion forces originating from the ubiquitous microemulsion droplets triggers a new type of superstructure at low particle concentration. Instead of the universally observed formation of face-to-face assembled lamellar mesostructures, the nanosheets self-organise into extended ribbon structures, whereby each on top lying sheet is displaced by a constant shift in the length and width directions leading to a so far unprecedented staggered zigzag-type stack assembly with restricted height. This type of stacking gives rise to a complex interference pattern in the isotropic small angle scattering of the stacked ribbon assemblies (SRAs) in reverse micellar solution. Different to the, for lamellar-structured nanosheets typical, diffraction peaks at multiples of the wave vector corresponding to one particular repeat distance, the scattering peaks measured in this study are asymmetric, displaying a shoulder on their low wave vector side. The asymmetric shape of the observed face-to-face correlation peaks indicates that the SRAs do not extend in one direction only. Their scattering behaviour is analysed by expanding the Kratky–Porod structure factor for stacking plates into three dimensions. High-angle annular dark-field (HAADF)-STEM tilt series have complementary been acquired to retrieve three-dimensional structural information on the SRAs in the dry state and to confirm the model used for the refinement of the SAXS data