59 research outputs found
Nanoscale assembly processes revealed in the nacroprismatic transition zone of Pinna nobilis mollusc shells
Intricate biomineralization processes in molluscs engineer hierarchical
structures with meso-, nano-, and atomic architectures that give the final
composite material exceptional mechanical strength and optical iridescence on
the macroscale. This multiscale biological assembly inspires new synthetic
routes to complex materials. Our investigation of the prism-nacre interface
reveals nanoscale details governing the onset of nacre formation using
high-resolution scanning transmission electron microscopy. A wedge polishing
technique provides unprecedented, large-area specimens required to span the
entire interface. Within this region, we find a transition from nanofibrillar
aggregation to irregular early-nacre layers, to well-ordered mature nacre
suggesting the assembly process is driven by aggregation of nanoparticles
(~50-80 nm) within an organic matrix that arrange in fiber-like polycrystalline
configurations. The particle number increases successively and, when critical
packing is reached, they merge into early-nacre platelets. These results give
new insights into nacre formation and particle-accretion mechanisms that may be
common to many calcareous biominerals.Comment: 5 Figure
Strain Relaxation in Core-Shell Pt-Co Catalyst Nanoparticles
Surface strain plays a key role in enhancing the activity of Pt-alloy
nanoparticle oxygen reduction catalysts. However, the details of strain effects
in real fuel cell catalysts are not well-understood, in part due to a lack of
strain characterization techniques that are suitable for complex supported
nanoparticle catalysts. This work investigates these effects using strain
mapping with nanobeam electron diffraction and a continuum elastic model of
strain in simple core-shell particles. We find that surface strain is relaxed
both by lattice defects at the core-shell interface and by relaxation across
particle shells caused by Poisson expansion in the spherical geometry. The
continuum elastic model finds that in the absence of lattice dislocations,
geometric relaxation results in a surface strain that scales with the average
composition of the particle, regardless of the shell thickness. We investigate
the impact of these strain effects on catalytic activity for a series of Pt-Co
catalysts treated to vary their shell thickness and core-shell lattice
mismatch. For catalysts with the thinnest shells, the activity is consistent
with an Arrhenius dependence on the surface strain expected for coherent strain
in dislocation-free particles, while catalysts with thicker shells showed
greater activity losses indicating strain relaxation caused by dislocations as
well.Comment: 23 pages,7 figures, includes appendi
Epitaxy of hexagonal ABO quantum materials
Hexagonal O oxides (, = cation) are a rich materials class for
realizing novel quantum phenomena. Their hexagonal symmetry, oxygen trigonal
bipyramid coordination and quasi-two dimensional layering give rise to
properties distinct from those of the cubic O perovskites. As bulk
materials, most of the focus in this materials class has been on the rare earth
manganites, MnO ( = rare earth); these materials display coupled
ferroelectricity and antiferromagnetic order. In this review, we focus on the
thin film manifestations of the hexagonal O oxides. We cover the
stability of the hexagonal oxides and substrates which can be used to template
the hexagonal structure. We show how the thin film geometry not only allows for
further tuning of the bulk-stable manganites but also the realization of
metastable hexagonal oxides such as the FeO that combine
ferroelectricity with weak ferromagnetic order. The thin film geometry is a
promising platform to stabilize additional metastable hexagonal oxides to
search for predicted high-temperature superconductivity and topological phases
in this materials class.Comment: The following article has been accepted by Applied Physics Review
Real-time imaging of activation and degradation of carbon supported octahedral Pt–Ni alloy fuel cell catalysts at the nanoscale using in situ electrochemical liquid cell STEM
Octahedrally shaped Pt–Ni alloy nanoparticles on carbon supports have demonstrated unprecedented electrocatalytic activity for the oxygen reduction reaction (ORR), sparking interest as catalysts for low-temperature fuel cell cathodes. However, deterioration of the octahedral shape that gives the catalyst its superior activity currently prohibits the use of shaped catalysts in fuel cell devices, while the structural dynamics of the overall catalyst degradation are largely unknown. We investigate the time-resolved degradation pathways of such a Pt–Ni alloy catalyst supported on carbon during cycling and startup/shutdown conditions using an in situ STEM electrochemical liquid cell, which allows us to track changes happening over seconds. Thereby we can precisely correlate the applied electrochemical potential with the microstructural response of the catalyst. We observe changes of the nanocatalysts’ structure, monitor particle motion and coalescence at potentials that corrode carbon, and investigate the dissolution and redeposition processes of the nanocatalyst under working conditions. Carbon support motion, particle motion, and particle coalescence were observed as the main microstructural responses to potential cycling and holds in regimes where carbon corrosion happens. Catalyst motion happened more severely during high potential holds and sudden potential changes than during cyclic potential sweeps, despite carbon corrosion happening during both, as suggested by ex situ DEMS results. During an extremely high potential excursion, the shaped nanoparticles became mobile on the carbon support and agglomerated facet-to-facet within 10 seconds. These experiments suggest that startup/shutdown potential treatments may cause catalyst coarsening on a much shorter time scale than full collapse of the carbon support. Additionally, the varying degrees of attachment of particles on the carbon support indicates that there is a distribution of interaction strengths, which in the future should be optimized for shaped particles. We further track the dissolution of Ni nanoparticles and determine the dissolution rate as a function of time for an individual nanoparticle – which occurs over the course of a few potential cycles for each particle. This study provides new visual understanding of the fundamental structural dynamics of nanocatalysts during fuel cell operation and highlights the need for better catalyst-support anchoring and morphology for allowing these highly active shaped catalysts to become useful in PEM fuel cell applications.TU Berlin, Open-Access-Mittel - 201
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