832 research outputs found
Monitoring Galvanic Replacement Through Three-Dimensional Morphological and Chemical Mapping
Galvanic replacement reactions on metal nanoparticles are often used for the
preparation of hollow nanostructures with tunable porosity and chemical
composition, leading to tailored optical and catalytic properties. However, the
precise interplay between the three-dimensional (3D) morphology and chemical
composition of nanostructures during Galvanic replacement is not always well
understood as the 3D chemical imaging of nanoscale materials is still
challenging. It is especially far from straightforward to obtain detailed
information from the inside of hollow nanostructures using electron microscopy
techniques such as SEM or TEM. We demonstrate here that a combination of
state-of-the-art EDX mapping with electron tomography results in the
unambiguous determination of both morphology transformation and elemental
composition of nanostructures in 3D, during Galvanic replacement of Ag
nanocubes. This work provides direct and unambiguous experimental evidence
leading to new insights in the understanding of the galvanic replacement
reaction. In addition, the powerful approach presented here can be applied to a
wide range of nanoscale transformation processes, which will undoubtedly guide
the development of novel nanostructures
Transmission Electron Microscopy on Interface Engineered Superconducting Thin Films
Transmission electron microscopy is used to evaluate different deposition techniques, which optimize the microstructure and physical properties of superconducting thin films. High-resolution electron microscopy proves that the use of an YBa2Cu2O buffer layer can avoid a variable interface configuration in YBa2Cu3O7 thin films grown on SrTiO3. The growth can also be controlled at an atomic level by using sub-unit cell layer epitaxy, which results in films with high quality and few structural defects. Epitaxial strain in Sr0 85La0 15CuO2 infinite layer thin films influences the critical temperature of these films, as well as the microstructure. Compressive stress is released by a modulated or a twinned microstructure, which eliminates superconductivity. On the other hand, also tensile strain seems to lower the critical temperature of the infinite layer
Barrier efficiency of sponge-like La2Zr2O7 buffer layers for YBCO-coated conductors
Solution derived La2Zr2O7 films have drawn much attention for potential
applications as thermal barriers or low-cost buffer layers for coated conductor
technology. Annealing and coating parameters strongly affect the microstructure
of La2Zr2O7, but different film processing methods can yield similar
microstructural features such as nanovoids and nanometer-sized La2Zr2O7 grains.
Nanoporosity is a typical feature found in such films and the implications for
the functionality of the films is investigated by a combination of scanning
transmission electron microscopy, electron energy-loss spectroscopy and
quantitative electron tomography. Chemical solution based La2Zr2O7 films
deposited on flexible Ni-5at.%W substrates with a {100} biaxial texture
were prepared for an in-depth characterization. A sponge-like structure
composed of nanometer sized voids is revealed by high-angle annular dark-field
scanning transmission electron microscopy in combination with electron
tomography. A three-dimensional quantification of nanovoids in the La2Zr2O7
film is obtained on a local scale. Mostly non-interconnected highly facetted
nanovoids compromise more than one-fifth of the investigated sample volume. The
diffusion barrier efficiency of a 170 nm thick La2Zr2O7 film is investigated by
STEM-EELS yielding a 1.8 \pm 0.2 nm oxide layer beyond which no significant
nickel diffusion can be detected and intermixing is observed. This is of
particular significance for the functionality of YBa2Cu3O7-{\delta} coated
conductor architectures based on solution derived La2Zr2O7 films as diffusion
barriers.Comment: Accepted for publication in Superconductor Science and Technolog
Porous nanostructured metal oxides synthesized through atomic layer deposition on a carbonaceous template followed by calcination
Porous metal oxides with nano-sized features attracted intensive interest in
recent decades due to their high surface area which is essential for many
applications, e.g. Li ion batteries, photocatalysts, fuel cells and
dye-sensitized solar cells. Various approaches were so far investigated to
synthesize porous nanostructured metal oxides, including self-assembly and
template-assisted synthesis. For the latter approach, forests of carbon
nanotubes are considered as particularly promising templates, with respect to
their one dimensional nature and the resulting high surface area. In this work,
we systematically investigate the formation of porous metal oxides (Al2O3,
TiO2, V2O5 and ZnO) with different morphologies using atomic layer deposition
on multi-walled carbon nanotubes followed by post deposition calcination. X-ray
diffraction, scanning electron microscopy accompanied with X-ray energy
dispersive spectroscopy and transmission electron microscopy were used for the
investigation of morphological and structural transitions at the micro- and
nano-scale during the calcination process. The crystallization temperature and
the surface coverage of the metal oxides and the oxidation temperature of the
carbon nanotubes were found to produce significant influence on the final
morphology
Formation and Thermal Stability of Gold-Silica Nanohybrids: Insight into the Mechanism and Morphology by Electron Tomography
Gold-silica hybrids are appealing in different fields of applications like
catalysis, sensors, drug delivery, and biotechnology. In most cases, the
morphology and distribution of the hetero-units play significant roles in their
functional behavior. Methods of synthesizing these hybrids, with variable
ordering of the hetero-units, are replete; however, a complete characterization
in three dimensions could not be achieved yet. A simple route to the synthesis
of Au-decorated SiO2 spheres is demonstrated and a study on the 3D ordering of
the hetero-units by scanning transmission electron microscopy (STEM) tomography
is presented at the final stage, intermediate stages of formation, and after
heating the hybrid. The final hybrid evolves from a soft self-assembled
structure of Au nano-particles. The hybrid shows good thermal stability up to
400 C, beyond which the Au particles start migrating inside the SiO2 matrix.
This study provides an insight in the formation mechanism and thermal stability
of the structures which are crucial factors for designing and applying such
hybrids in fields of catalysis and biotechnology. As the method is general, it
can be applied to make similar hybrids based on SiO2 by tuning the reaction
chemistry as needed
Atomic layer deposition-based tuning of the pore size in mesoporous thin films studied by in situ grazing incidence small angle x-ray scattering
Atomic layer deposition (ALD) enables the conformal coating of porous
materials, making the technique suitable for pore size tuning at the atomic
level, e.g., for applications in catalysis, gas separation and sensing. It is,
however, not straightforward to obtain information about the conformality of
ALD coatings deposited in pores with diameters in the low mesoporous regime (<
10 nm). In this work, it is demonstrated that in situ synchrotron based grazing
incidence small angle x-ray scattering (GISAXS) can provide valuable
information on the change in density and internal surface area during ALD of
TiO2 in a porous titania film with small mesopores (3-8 nm). The results are
shown to be in good agreement with in situ x-ray fluorescence data representing
the evolution of the amount of Ti atoms deposited in the porous film. Analysis
of both data sets indicates that the minimum pore diameter that can be achieved
by ALD is determined by the size of the Ti-precursor molecule
Atomic Layer Deposition-Based Synthesis of Photoactive TiO2 Nanoparticle Chains by Using Carbon Nanotubes as Sacrificial Templates
Highly ordered and self supported anatase TiO2 nanoparticle chains were
fabricated by calcining conformally TiO2 coated multi-walled carbon nanotubes
(MWCNTs). During annealing, the thin tubular TiO2 coating that was deposited
onto the MWCNTs by atomic layer deposition (ALD) was transformed into chains of
TiO2 nanoparticles (~12 nm diameter) with an ultrahigh surface area (137 cm2
per cm2 of substrate), while at the same time the carbon from the MWCNTs was
removed. Photocatalytic tests on the degradation of acetaldehyde proved that
these forests of TiO2 nanoparticle chains are highly photo active under UV
light because of their well crystallized anatase phase
Synthesis of a 3D network of Pt nanowires by atomic layer deposition on carbonaceous template
The formation of a 3D network composed of free standing and interconnected Pt
nanowires is achieved by a two-step method, consisting of conformal deposition
of Pt by atomic layer deposition (ALD) on a forest of carbon nanotubes and
subsequent removal of the carbonaceous template. Detailed characterization of
this novel 3D nanostructure was carried out by transmission electron microscopy
(TEM) and electrochemical impedance spectroscopy (EIS). These characterizations
showed that this pure 3D nanostructure of platinum is self-supported and offers
an enhancement of the electrochemically active surface area by a factor of 50
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