2020, Trepka, Bastian

2018-10-25, Trepka, Bastian, Stiegeler, Julian, Wimmer, Ilona, Fonin, Mikhail, Polarz, Sebastian

EuO is unique, because it belongs to the few solids combining semiconducting properties (E_gap = 1.1 eV) with native ferromagnetism. For future applications of EuO, e.g. as spin-filters or for sensors, one has to learn how defined nanostructures can be prepared. Unlike other ceramic oxides, there are no established soft-chemistry routes (e.g. sol-gel) towards EuO nanomaterials e.g. porous materials. This is due to the labile nature of the oxidation state Eu(+ii). We present a particle-based method leading to a EuO aerogel. Instead of making the target material directly, we use nanoparticles of an organic-inorganic hybrid phase (Eu₂O₃-benzoate) and assemble those into an aerogel, followed by the transformation into phase-pure EuO. It is shown that organic aldehydes act as capping agents for controlling the morphogenesis of the hybrid particles. Depending on the steric demand of the aldehyde, one obtains plate-like particles or nanorods with increasing aspect ratio. The particles form a gel, when the aspect ratio is increased to >20. After supercritical drying, one receives a nanorod-based aerogel. Treatment of the latter with Eu-vapor leads to reduction of the Eu₂O₃ domains to EuO while retaining the aerogel structure. Proof of ferromagnetism in the resulting EuO aerogel was delivered by SQUID measurements.

2019-12-17, Donner, Adrian, Trepka, Bastian, Theiss, Sebastian, Immler, Fabian, Traber, Johanna, Polarz, Sebastian

Next-generation surfactants provide extended functionality apart from their amphiphilic properties. We present two novel metallosurfactants characterized by a N-heterocyclic carbene (NHC) head bearing Cu(I) and Fe(II). An innovative approach for their application in emulsion polymerizations under ATRP conditions was developed. Thereby the complexes fulfilled the role of emulsifiers, active catalysts and stabilization agents at once. Polymerization of methyl methacrylate (MMA) yielded stable PMMA colloids in water with the catalyst located at the surface of the colloids. The termination of PMMA with a bromine moiety enabled the subsequent co-polymerization with styrene via macroinitiation and PMMA-PS core-shell particles were obtained. Gel permeation chromatography (GPC) and selective gradient NMR experiments revealed a covalent linkage between the PMMA core and the PS shell.

2018-01, Trepka, Bastian, Erler, Philipp, Selzer, Severin, Kollek, Tom, Boldt, Klaus, Fonin, Mikhail, Nowak, Ulrich, Wolf, Daniel, Lubk, Axel, Polarz, Sebastian

Semiconductors with native ferromagnetism barely exist and defined nanostructures are almost unknown. This lack impedes the exploration of a new class of materials characterized by a direct combination of effects on the electronic system caused by quantum confinement effects with magnetism. A good example is EuO for which currently no reliable routes for nanoparticle synthesis can be established. Bottom-up approaches applicable to other oxides fail because of the labile oxidation state +II. Instead of targeting a direct synthesis, the two steps—“structure control” and “chemical transformation”—are separated. The generation of a transitional, hybrid nanophase is followed by its conversion into EuO under full conservation of all morphological features. Hierarchical EuO materials are now accessible in the shape of oriented nanodisks stacked to tubular particles. Magnetically, the coupling of either vortex or onion states has been found. An unexpected temperature dependence is governed by thermally activated transitions between these states.

2019-05-01, Sutter, Sebastian, Trepka, Bastian, Siroky, Stephan, Hagedorn, Kay, Theiss, Sebastian, Baum, Peter, Polarz, Sebastian

The maximization of activity is a general aim in catalysis research. The possibility for light-triggered enhancement of a catalytic process, even if the process is not photochemical in nature, represents an intriguing concept. Here, we present a novel system for the exploration of the latter idea. A surfactant with a catalytically active head group, a protonated polyoxometalate (POM) cluster, is attached to the surface of a gold nanoparticle (Au NP) using thiol coupling chemistry. The distance of the catalytically active center to the gold surface could be adjusted precisely using surfactants containing hydrocarbon chains (C_n) of different lengths ( n = 4-10). Radiation with VIS-light has no effect on the catalytic activity of micellar aggregates of the surfactant. The situation changes, as soon as the surfactants have been attached to the Au NPs. The catalytic activity could almost be doubled. It was proven that the effect is caused by coupling the surface plasmon resonance of the Au NPs with the properties of the POM head group. The improvement of activity could only be observed if the excitation wavelength matches the absorption band of the used Au NPs. Furthermore, the shorter the distance between the POM group and the surface of the NP, the stronger is the effect. This phenomenon was explained by lowering the activation energy of the transition state relevant to the catalytic process by the strong electric fields in the vicinity of the surfaces of plasmonic nanoparticles. Because the catalytic enhancement is wavelength-selective, one can imagine the creation of complex systems in the future, a system of differently sized NPs, each responsible for a different catalytic step and activated by light of different colors.

2016, Liu, Yi, Claes, Nathalie, Trepka, Bastian, Bals, Sara, Lang, Peter R.

In this article we report on the synthesis and characterization of a system of colloidal spheres suspended in an aqueous solvent which can be refractive index-matched, thus allowing for investigations of the particle near-wall dynamics by evanescent wave dynamic light scattering at concentrations up to the isotropic to ordered transition and beyond. The particles are synthesized by copolymerization of a fluorinated acrylic ester monomer with a polyethylene-glycol (PEG) oligomer by surfactant free emulsion polymerization. Static and dynamic light scattering experiments in combination with cryo transmission electron microscopy reveal that the particles have a core shell structure with a significant enrichment of the PEG chains on the particles surface. In index-matching DMSO/water suspensions the particles arrange in an ordered phase at volume fraction above 7%, if no additional electrolyte is present. The near-wall dynamics at low volume fraction are quantitatively described by the combination of electrostatic repulsion and hydrodynamic interaction between the particles and the wall. At volume fractions close to the isotropic to ordered transition, the near-wall dynamics are more complex and qualitatively reminiscent of the behaviour which was observed in hard sphere suspensions at high concentrations.