Cryoaerogels and Cryohydrogels as Efficient Electrocatalysts

Additive-free cryoaerogel coatings from noble metal nanoparticles are prepared and electrochemically investigated. By using liquid nitrogen or isopentane as cooling medium, two different superstructures are created for each type of noble metal nanoparticle. These materials (made from the same amount of particles) have superior morphological and catalytic properties as compared to simply immobilized, densely packed nanoparticles. The morphology of all materials is investigated with scanning electron microscopy (SEM). Electrochemically active surface areas (ECSAs) are calculated from cyclic voltammetry measurements. The catalytic activity is studied for the ethanol oxidation reaction (EOR). Both are found to be increased for superstructured materials prepared by cryoaerogelation. Furthermore, cryoaerogels with cellular to dendritic structure that arise from freezing with isopentane show the best catalytic performance and highest ECSA. Moreover, as a new class of materials, cryohydrogels are created for the first time by thawing flash-frozen nanoparticle solutions. Structure and morphology of these materials match with the corresponding types of cryoaerogels and are confirmed via SEM. Even the catalytic activity in EOR is in accordance with the results from cryoaerogel coatings. As a proof of concept, this approach offers a novel platform towards the easier and faster production of cryogelated materials for wet-chemical applications

Magnetic aerogels from FePt and CoPt3 directly from organic solution

Here the synthesis of magnetic aerogels from iron platinum and cobalt platinum nanoparticles is presented. The use of hydrazine monohydrate as destabilizing agent triggers the gelation directly from organic solution, and therefore a phase transfer to aqueous media prior to the gelation is not necessary. The aerogels were characterized through Transmission Electron Microscopy, Scanning Electron Microscopy, Powder X-Ray Diffraction Analysis and Argon Physisorption measurements to prove the formation of a porous network and define their compositions. Additionally, magnetization measurements in terms of hysteresis cycles at 5 K and 300 K (M-H-curves) as well as zero field cooled-field cooled measurements (ZFC-FC measurements) of the dried colloids and the respective xero- and aerogels were performed, in order to analyze the influence of the gelation process and the network structure on the magnetic properties

Fabrication of two-dimensional Au@FePt core-shell nanoparticle arrays by photochemical metal deposition

In this report, we experimentally demonstrate that single platinum nanoparticles exhibit the necessary catalytic activity for the optically induced reduction of H[AuCl4] complexes to elemental gold. This finding is exploited for the parallel Au encapsulation of FePt nanoparticles arranged in a self-assembled two-dimensional array. Magnetic force microscopy reveals that the thin gold layer formed on the FePt particles leads to a strongly increased long-term stability of their magnetization under ambient conditions