17 research outputs found
Dealloying of Platinum-Aluminum Thin Films Part II. Electrode Performance
Highly porous Pt/Al thin film electrodes on yttria stabilized zirconia
electrolytes were prepared by dealloying of co-sputtered Pt/Al films. The
oxygen reduction capability of the resulting electrodes was analyzed in a solid
oxide fuel cell setup at elevated temperatures. During initial heating to 523 K
exceptionally high performances compared to conventional Pt thin film
electrodes were measured. This results from the high internal surface area and
large three phase boundary length obtained by the dealloying process. Exposure
to elevated temperatures of 673 K or 873 K gave rise to degradation of the
electrode performance, which was primarily attributed to the oxidation of
remaining Al in the thin films.Comment: 5 pages, 4 figure
Dealloying of Platinum-Aluminum Thin Films Part I. Dynamics of Pattern Formation
Applying focused ion beam (FIB) nanotomography and Rutherford backscattering
spectroscopy (RBS) to dealloyed platinum-aluminum thin films an in-depth
analysis of the dominating physical mechanisms of porosity formation during the
dealloying process is performed. The dynamical porosity formation due to the
dissolution of the less noble aluminum in the alloy is treated as result of a
reaction-diffusion system. The RBS analysis yields that the porosity formation
is mainly caused by a linearly propagating diffusion front, i.e. the
liquid/solid interface, with a uniform speed of 42(3) nm/s when using a 4M
aqueous NaOH solution at room temperature. The experimentally observed front
evolution is captured by the normal diffusive
Fisher-Kolmogorov-Petrovskii-Piskounov (FKPP) equation and can be interpreted
as a branching random walk phenomenon. The etching front produces a gradual
porosity with an enhanced porosity in the surface-near regions of the thin film
due to prolonged exposure of the alloy to the alkaline solution.Comment: 4 pages, 5 figure
Adapting the concepts of nonaqueous sol–gel chemistry to metals: synthesis and formation mechanism of palladium and palladium–copper nanoparticles in benzyl alcohol
Benzyl alcohol is a versatile reaction medium for the synthesis of different types of nanoparticles. Its ability to act as an oxygen source gave access to metal oxide nanoparticles, while its reducing properties can be harnessed for the preparation of metals. Here we report the synthesis of Pd and PdCu nanoparticles in benzyl alcohol supplemented by a detailed mechanistic study for both systems. To elucidate the chemical formation mechanism of the Pd nanoparticles, we performed in situ attenuated total reflection ultraviolet–visible (ATR-UV–vis) and Fourier transform infrared spectroscopy (ATR-FTIR), providing information on the organic as well as on the inorganic side of the reaction. Potential gaseous products were analyzed by in situ gas chromatography (GC) and mass spectrometry (MS). We observed the formation of benzaldehyde, toluene, and dibenzyl ether as the three main organic products. The formation of the PdCu alloy nanoparticles was studied by ex situ powder X-ray diffraction (PXRD). A time-resolved study of the synthesis at 100 °C indicated that initially three types of particles formed, composed of an alloy with high Pd content, an alloy with high content of copper, and palladium particles, and only later in the reaction course they transformed into an alloy with a Pd-to-Cu ratio close to 1.ISSN:0928-0707ISSN:1573-484
Strategies to improve the electrical conductivity of nanoparticle-based antimony-doped tin oxide aerogels
We present different strategies to improve the electrical conductivity of antimony-doped tin oxide aerogels assembled from preformed nanosized building blocks. By adjusting the annealing atmosphere and temperature conditions, additional UV treatment to remove surface organics prior to annealing and by tuning the antimony content of the nanoparticles, different strategies are employed to influence the properties of the supercritically dried aerogels before and after gelation. In the framework of this study, also the formation of pure SnO2 particle-based aerogels could be achieved. Furthermore, we present an experimental setup for analyzing the electrical conductivity of porous and fragile aerogel monoliths based on a four-point probe. While the annealing atmosphere does not significantly affect the resistivity, UV treatment leads to a resistivity decrease in around 50 %. It is found that the resistivity of the samples can be tuned by altering the antimony content, offering very low-resistivity levels down to 4.5 Ω cm, while the surface areas remained high without significant crystal growth for the doped samples.ISSN:0928-0707ISSN:1573-484
Three-Dimensional Assembly of Yttrium Oxide Nanosheets into Luminescent Aerogel Monoliths with Outstanding Adsorption Properties
The preparation of macroscopic materials
from two-dimensional nanostructures
represents a great challenge. Restacking and random aggregation to
dense structures during processing prevents the preservation of the
two-dimensional morphology of the nanobuilding blocks in the final
body. Here we present a facile solution route to ultrathin, crystalline
Y<sub>2</sub>O<sub>3</sub> nanosheets, which can be assembled into
a 3D network by a simple centrifugation-induced gelation method. The
wet gels are converted into aerogel monoliths of macroscopic dimensions <i>via</i> supercritical drying. The as-prepared, fully crystalline
Y<sub>2</sub>O<sub>3</sub> aerogels show high surface areas of up
to 445 m<sup>2</sup>/g and a very low density of 0.15 g/cm<sup>3</sup>, which is only 3% of the bulk density of Y<sub>2</sub>O<sub>3</sub>. By doping and co-doping the Y<sub>2</sub>O<sub>3</sub> nanosheets
with Eu<sup>3+</sup> and Tb<sup>3+</sup>, we successfully fabricated
luminescent aerogel monoliths with tunable color emissions from red
to green under UV excitation. Moreover, the as-prepared gels and aerogels
exhibit excellent adsorption capacities for organic dyes in water
without losing their structural integrity. For methyl blue we measured
an unmatched adsorption capacity of 8080 mg/g. Finally, the deposition
of gold nanoparticles on the nanosheets gave access to Y<sub>2</sub>O<sub>3</sub>–Au nanocomposite aerogels, proving that this
approach may be used for the synthesis of catalytically active materials.
The broad range of properties including low density, high porosity,
and large surface area in combination with tunable photoluminescence
makes these Y<sub>2</sub>O<sub>3</sub> aerogels a truly multifunctional
material with potential applications in optoelectronics, wastewater
treatment, and catalysis
Conducting ITO Nanoparticle-Based Aerogels—Nonaqueous One-Pot Synthesis vs. Particle Assembly Routes
Indium tin oxide (ITO) aerogels offer a combination of high surface area, porosity and conductive properties and could therefore be a promising material for electrodes in the fields of batteries, solar cells and fuel cells, as well as for optoelectronic applications. In this study, ITO aerogels were synthesized via two different approaches, followed by critical point drying (CPD) with liquid CO2. During the nonaqueous one-pot sol–gel synthesis in benzylamine (BnNH2), the ITO nanoparticles arranged to form a gel, which could be directly processed into an aerogel via solvent exchange, followed by CPD. Alternatively, for the analogous nonaqueous sol–gel synthesis in benzyl alcohol (BnOH), ITO nanoparticles were obtained and assembled into macroscopic aerogels with centimeter dimensions by controlled destabilization of a concentrated dispersion and CPD. As-synthesized ITO aerogels showed low electrical conductivities, but an improvement of two to three orders of magnitude was achieved by annealing, resulting in an electrical resistivity of 64.5–1.6 kΩ·cm. Annealing in a N2 atmosphere led to an even lower resistivity of 0.2–0.6 kΩ·cm. Concurrently, the BET surface area decreased from 106.2 to 55.6 m2/g with increasing annealing temperature. In essence, both synthesis strategies resulted in aerogels with attractive properties, showing great potential for many applications in energy storage and for optoelectronic devices.ISSN:2310-286
Amorphous cobalt silicate nanobelts@carbon composites as a stable anode material for lithium ion batteries
During the past decade, tremendous attention has been given to the development of new electrode materials with high capacity to meet the requirements of electrode materials with high energy density in lithium ion batteries. Very recently, cobalt silicate has been proposed as a new type of high capacity anode material for lithium ion batteries. However, the bulky cobalt silicate demonstrates limited electrochemical performance. Nanostructure engineering and carbon coating represent two promising strategies to improve the electrochemical performance of electrode materials. Herein, we developed a template method for the synthesis of amorphous cobalt silicate nanobelts which can be coated with carbon through the deposition and thermal decomposition of phenol formaldehyde resin. Tested as an anode material, the amorphous cobalt silicate nanobelts@carbon composites exhibit a reversible high capacity of 745 mA h g−1 at a current density of 100 mA g−1, and a long life span of up to 1000 cycles with a stable capacity retention of 480 mA h g−1 at a current density of 500 mA g−1. The outstanding electrochemical performance of the composites indicates their high potential as an anode material for lithium ion batteries. The results here are expected to stimulate further research into transition metal silicate nanostructures for lithium ion battery applications.ISSN:2041-6520ISSN:2041-653
Colloidal Nanocrystal-Based BaTiO<sub>3</sub> Xerogels as Green Bodies: Effect of Drying and Sintering at Low Temperatures on Pore Structure and Microstructures
Although
aerogels prepared by the colloidal assembly of nanoparticles
are a rapidly emerging class of highly porous and low-density materials,
their ambient dried counterparts, namely xerogels, have hardly been
explored. Here we report the use of nanoparticle-based BaTiO<sub>3</sub> xerogels as green bodies, which provide a versatile route to ceramic
materials under the minimization of organic additives with a significant
reduction of the calcination temperature compared to that of conventional
powder sintering. The structural changes of the xerogels are investigated
during ambient drying by carefully analyzing the microstructure at
different drying stages. For this purpose, the shrinkage was arrested
by a supercritical drying step under full preservation of the intermediate
microstructure, giving unprecedented insight into the structural changes
during ambient drying of a nanoparticle-based gel. In a first step,
the large macropores shrink because of capillary forces, followed
by the collapse of residual mesopores until a dense xerogel is obtained.
The whole process is accompanied by a volume shrinkage of 97% and
a drop in surface area from 300 to 220 m<sup>2</sup> g<sup>–1</sup>. Finally, the xerogels are sintered, causing another shrinkage of
up to 8% with a slight increase in the average pore and crystal sizes.
At temperatures higher than 700 °C, an unexpected phase transition
to BaTi<sub>2</sub>O<sub>5</sub> is observed