4 research outputs found
The innovative capacity of voluntary organisations I Exploring the innovations and the innovators
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Mobility and Poisoning of Mass-Selected Platinum Nanoclusters during the Oxygen Reduction Reaction
A major
challenge in electrocatalysis is to understand the effect
of electrochemical processes on the physicochemical properties of
nanoparticle or nanocluster (NC) ensembles, especially for complex
processes, such as the oxygen reduction reaction (ORR) considered
herein. We describe an approach whereby electrocatalysis at a small
number of well-defined mass-selected Pt NCs (Pt<sub>923±37</sub>, diameter, <i>d</i> ≈ 3 nm), deposited from a cluster
beam source on carbon-coated transmission electron microscopy (TEM)
grids, can be measured by a scanning electrochemical cell microscopy
(SECCM) setup, in tandem with a range of complementary microscopy
and spectroscopy techniques. The SECCM setup delivers high mass transport
rates and allows the effects of transient reactive intermediates to
be elucidated for different Pt surface coverages (NC spacing). A major
observation is that the ORR activity decreases during successive electrochemical
(voltammetric) measurements. This is shown to be due to poisoning
of the Pt NCs by carbon-/oxygen-containing moieties that are produced
by the reaction of reactive oxygen intermediates (RIs), generated
by the ORR, with the carbon support. The effect is most prominent
when the Pt surface coverage on the carbon support is low (<6%).
Furthermore, the NC deposition impact energy drastically affects the
resulting Pt NC stability during electrochemistry. For lower impact
energy, Pt NCs migrate as a consequence of the ORR and are rearranged
into characteristic groups on the support. This previously unseen
effect is caused by an uneven flux distribution around individual
NCs within the ensemble and has important consequences for understanding
the stability and activity of NC and nanoparticle arrays
Recrystallization of Highly-Mismatched Be<sub><i>x</i></sub>Zn<sub>1–<i>x</i></sub>O Alloys: Formation of a Degenerate Interface
We investigate the effect of thermally
induced phase transformations on a metastable oxide alloy film, a
multiphase Be<sub><i>x</i></sub>Zn<sub>1–<i>x</i></sub>O (BZO), grown on Al<sub>2</sub>O<sub>3</sub>(0001)
substrate for annealing temperatures in the range of 600–950
°C. A pronounced structural transition is shown together with
strain relaxation and atomic redistribution in the annealed films.
Increasing annealing temperature initiates out-diffusion and segregation
of Be and subsequent nucleation of nanoparticles at the surface, corresponding
to a monotonic decrease in the lattice phonon energies and band gap
energy of the films. Infrared reflectance simulations identify a highly
conductive ZnO interface layer (thicknesses in the range of ≈10–29
nm for annealing temperatures ≥800 °C). The highly degenerate
interface layers with temperature-independent carrier concentration
and mobility significantly influence the electronic and optical properties
of the BZO films. A parallel conduction model is employed to determine
the carrier concentration and conductivity of the bulk and interface
regions. The density-of-states-averaged effective mass of the conduction
electrons for the interfaces is calculated to be in the range of 0.31<i>m</i><sub>0</sub> and 0.67<i>m</i><sub>0</sub>. A
conductivity as high as 1.4 × 10<sup>3</sup> S·cm<sup>–1</sup> is attained, corresponding to the carrier concentration <i>n</i><sub>Int</sub> = 2.16 × 10<sup>20</sup> cm<sup>–3</sup> at the interface layers, and comparable to the highest conductivities
achieved in highly doped ZnO. The origin of such a nanoscale degenerate
interface layer is attributed to the counter-diffusion of Be and Zn,
rendering a high accumulation of Zn interstitials and a giant reduction
of charge-compensating defects. These observations provide a broad
understanding of the thermodynamics and phase transformations in Be<sub><i>x</i></sub>Zn<sub>1–<i>x</i></sub>O
alloys for the application of highly conductive and transparent oxide-based
devices and fabrication of their alloy nanostructures
Universality of hair as a nucleant: exploring the effects of surface chemistry and topography
The ability to control crystal nucleation through the simple addition of a nucleating agent (nucleant) is desirable for a huge range of applications. However, effective nucleating agents are known for only a small number of systems, and many questions remain about the mechanisms by which they operate. Here, we explore the features that make an effective nucleant and demonstrate that the biological material hair─which naturally possesses a chemically and topographically complex surface structure─has excellent potential as an effective nucleating agent. Crystallization of poorly soluble compounds in the presence of hairs from a range of mammals shows that nucleation preferentially occurs at the cuticle step edges, while a novel microdroplet-based methodology was used to quantify the nucleating activities of different hairs. This showed that the activities of the hairs can be tuned over a wide range using chemical treatments. Analysis of the hair structure and composition using atomic force microscopy, scanning ion conductance microscopy, and X-ray photoelectron spectroscopy demonstrates that surface chemistry, surface topography, and surface charge all act in combination to create effective nucleation sites. This work therefore contributes to our understanding of heterogeneous nucleating agents and shows that surface topography as well as surface chemistry can be used in the design or selection of universal nucleating agents.</p