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RESEARCH NEWSRESEARCH NEWS
Solid-state catalysis is one of the cornerstones of the modern chemical industry, yet knowledge of the underlying processes at play are often surprisingly limited, partly because the catalytic efficiency depends directly on the nanoscale morphology of the catalyst itself. Traditional assay techniques commonly fail to resolve details of this size, and report averages over a wide range of morphologies instead.The only way to establish a precise relation between particle structure and catalytic efficiency is to image the individual catalyst particles directly, and precisely such an approach has now been demonstrated by an Australian research group [Novo et al, Nat. Nanotechnology (2008) doi:10.1038/nnano.2008.246].At the heart of their study is the use of dark field microscopy. “To date most of this work has focused on understanding the optical properties of such small objects, yet we wanted to measure how fast redox reactions take place at individual catalytic particles,” explains Paul Mulvaney of the University of Melbourne. Through this technique the researchers were able to measure redox reactions of just a few hundred electrons per second, and showed that the growth of a single nanocrystal can be monitored in real-time.“We hope that this method will enable scientists to study catalysts in much more detail and to establish which types of crystal are the most efficient,” Paul continues. “In the longer term, we believe that these
measurements demonstrate that electrical modulation of the nanocrystal scattering provides a new method for developing plasmonic-based circuitry. Right now we are developing a method to inject electrons into the particle electrochemically, which is much more controlled than the chemical method, and will enable reversible and defined surface plasmon modulation. This will be the first step towards plasmonic switching.”
Peter Dedecker
The research and development of light
scattering in nanowire materials is of
large importance for the optimization
of the external efficiency of
nanowire-based photovoltaic devices
[Muskens et al. 10.1021/nl0808076],
by matching the wavelength of
the relevant light to the nanowire
diameter and the refraction index
of the filler medium, more light is
absorbed.
The optical properties of nanowire
layers relevant for solar cell
applications are governed by strong
multiple scattering of light. Nanowire
layers were synthesised on substrates
of InP, Si, and GaP, which are strongly
photonic and produce multiple
scattering. Using a supercontinuum
white-light source and visible and
near-infrared fiber spectrometers, both
specular and diffuse reflectance spectra
of the nanowire layers were measured
over the spectral region 0.7 to 2.2 eV
(1650-560 nm). Diffuse reflectance as
a function of energy was calculated for
the three nanowire materials.
The measurement of hemispherical
diffuse reflectance for different
thicknesses of nanowire in the Si
and GaP materials showed that light
scattering contributes significantly
as the wavelength becomes shorter
and more closely matches the
diameter of the nanowires, whilst
for InP nanowires diffuse reflectance
decreases sharply towards shorter
wavelengths. Successful suppression of
the hemispherical diffuse reflectance
of InP nanowires was demonstrated,
to below that of the corresponding
transparent effective medium.
The control and design of light
scattering in nanowire-based devices
is vital for increasing solar energy
collection performance, where
maximum spectral irradiance occurs in
the green spectral region.
Jon Hobden
Nanowires in the sunENERGY
Learning mechanical principles from ancient fish
Over 500 million years ago, in the Ordovician period, the
first dermal armour among vertebrates developed with the
rise of the now long-dead Ostracoderms. Today, researchers
from the Departments of Materials Science and Engineering
as well as Mechanical Engineering, both of the Massachusetts
Institute of Technology, are investigating more recent types of
fish body armour with regard to their mechanical properties
(Nature Materials, doi:10.1038/nmat2231). In particular,
they performed an experimental and computational study of
materials design principles in the armour of the ‘living fossil’
fish Polypterus senegalus , a member of the Polypteridae
family which appeared in the Cretaceous and has not changed
much since.
Benjamin Bruet and her group examined the features of both
the individual multi-layered, mineralized ganoid scales as well
as the array of interlocking scales as a whole. Evolution saw
to it that this body armour developed into a system both
rigid and flexible – an approach that Bruet and colleagues
try to learn from. “This could enable pathways to improved
bioinspired human body armour, and may provide clues to the
evolutionary origins of mineralized tissues,” says Bruet. They
found that many parameters, such as material variations, the
number and thickness of layers, the construction of junctions,
the prevailing geometries etc. significantly influence the
overall properties.
Each individual scale possesses a quad-layered structure, each
with its own quite unique set of properties, optimized for their
respective biological function.
In addition, the scientists found that the junctions between
material layers possess a gradual change in properties. These
junctions are capable of transferring load and redistribute
stress, thus relieving mechanical strain of the individual scales
and significantly enhancing the performance of the system as
a whole.
Researchers hope to gain insight into general principles
that may be applied in the design of improved engineered
biomimetic structural materials.
Michel Fleck
BIOMATERIALS
Dark field microscopy image of individual gold
nanocrystals of various shapes and sizes.
Single catalysts show their true colorsNANOSCOPIC MATERIALS
OCT-DEC 2008 | VOLUME 3 | NUMBER 5-68
NTv3n5-6p8_11.indd 8 11/11/2008 17:06:08