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 Y₂O₃ 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 Y₂O₃ aerogels show high surface areas of up to 445 m²/g and a very low density of 0.15 g/cm³, which is only 3% of the bulk density of Y₂O₃. By doping and co-doping the Y₂O₃ nanosheets with Eu³⁺ and Tb³⁺, 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 Y₂O₃–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 Y₂O₃ 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 BaTiO₃ 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 BaTiO₃ 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 m² g^–1. 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 BaTi₂O₅ is observed