Cölfen, Helmut
0000-0002-1148-0308 Berufsbeschreibung
Nachname
Vorname
Name
Suchergebnisse Publikationen
Non-stoichiometric hydrated magnesium-doped calcium carbonate precipitation in ethanol
The effect of Mg2+ on the precipitation pathway of CaCO3 in absolute ethanol has been studied to investigate the role of ion solvation in the crystallization process. Our data reveal that high concentrations of Mg2+ promote the precipitation of an amorphous transient phase together with non-stoichometric hydrated phases of calcium carbonate.
Symbiosis of Silica Biomorphs and Magnetite Mesocrystals
Silica biomorphs are extraordinary inorganic superstructures formed via autocatalytic co‐precipitation and bottom‐up self‐assembly of alkaline‐earth carbonates and silica. However, they show no inherent functionality except for their striking textural motifs and curved morphologies. This work presents strategies to magnetize silica biomorphs, thus creating thermally stable ceramic microswimmers with unique elaborate shapes. This is achieved by growing super paramagnetic magnetite mesocrystals on and around the complex curved surfaces of biomorphs, while keeping their morphology and maintaining mesocrystal integrity. Selective mesocrystal formation on certain parts of the substrates is induced by chemical modification of the biomorph surface, increasing the loading of magnetite on the silica–carbonate structures and, in suitable cases, rendering them able to respond to external magnetic fields and move as microswimmer entities. In this way, the complex ultrastructure of silica biomorphs is successfully used as a template for functional ceramics. Furthermore, selective dissolution of the carbonate core from the biomorphs leads to hollow magnetic structures that could be filled with actives, thus serving as microcarriers with considerable loading capacity.
Putting a new Spin on it : Gradient Centrifugation for Analytical and Preparative Applications
Gradient centrifugation is an important technique in chemistry, biology, materials science and engineering. It has big potential beyond the well-known centrifugation for separation of molecules and particles. Various possibilities for special analysis and separation of particles, but also preparative applications like the production of gradient materials and controlled polymerizations exist. In all examples, a gradient of physical and/or chemical properties is generated by centrifugation and used for the further application. In this concept article, we present selected examples of gradient centrifugation to show important developments in the field and discuss their applications, potential, and limitations. We conclude by analyzing future trends of gradient centrifugation that are relevant for academic and industrial usage.
New Horizons of Non-Classical Crystallization
Non-Classical Crystallization (NCC) summarizes a number of crystallization pathways, which differ from the classical lay-er-by-layer growth of crystals involving atomic/molecular building units. Common to NCC is that the building units are larger and include nanoparticles, clusters or liquid droplets, providing multiple handles for their control at each elementary step. Therefore, many different pathways towards the final single crystals are possible and can be influenced by appropriate experimental parameters or additives at each step of crystal formation and growth. As a consequence, NCC allows for a plethora of crystallization strategies towards complex crystalline (hybrid)materials. In this perspective, we summarize the current state of the art with a focus on the new horizons for NCC with respect to mechanistic understanding, high-performance materials and new applications. This gives a glimpse on what will be possible in the future using these crystallization approaches: Examples are new electrode and storage materials, (photo)catalysts, building materials, porous or crystalline materials with complex shape, structural hierarchy and anisotropic single crystals.
High-Resolution Analysis of Small Silver Clusters by Analytical Ultracentrifugation
Although silver particles are used in various applications and a countless amount of synthesis routes exists, their formation mechanism is still poorly understood. Especially the first species formed directly after nucleation challenge analysis methods with their small size and transient nature. Analytical ultracentrifugation (AUC) has already proven to provide high size resolution and therefore enables the characterization of early nucleation species. Herein, we present an experiment of multiwavelength (MWL)-AUC of silver clusters, which revealed seven different cluster species. They consist of less than 10 atoms and therefore represent the first species formed after nucleation. Using MWL-AUC, UV/vis spectra could be allocated to each of them, which is shown for the first time. These findings establish MWL-AUC as a high-resolution tool to investigate a nucleation mechanism for silver and other metal nanoparticles.
Bio‐Inspired Synthesis of Hematite Mesocrystals by Using Xonotlite Nanowires as Growth Modifiers and Their Improved Oxygen Evolution Activity
Bio-inspired synthesis of functional materials with highly ordered structure and tunable properties is of particular interest, but efficient approaches that allow the access of these materials are still limited. We describe here a novel method for the preparation of hematite particles by using xonotlite nanowires (XNWs) as growth modifiers. The concentration of the XNWs has a profound effect on the final morphology of the products, while the concentration of the iron(III) ions can significantly control the size of the hematite particles. Moreover, the underlying mechanism of such XNWs modified bioinspired mineralization process has been proposed. Remarkably, the obtained hematite particles exhibit good oxygen evolution reaction (OER) catalytic performance. They can afford a current density of 10 mA cm-2 with an overpotential of 370 mV, a small Tafel slope of 65 mV decade-1 and show good stability in alkaline electrolyte. Such strategy preparing functional materials by using nanowires as the growth modifiers can be applied for constructing various materials with hierarchical structures and excellent performance in the future.
Binary colloidal nanoparticles with a large size ratio in analytical ultracentrifugation
Sedimenting colloidal particles may feel surprisingly strong buoyancy in a mixture with other particles of a considerably larger size. In this paper we investigated the buoyancy of colloidal particles in a concentrated binary suspension in situ in a centrifugal field. By dispersing two different fluorescence labeled silica nanoparticles with a large size ratio (90 nm and 30 nm, size ratio = 3) in a refractive index matching solvent, we used a multi-wavelength analytical ultracentrifuge to in-situ measure the concentration gradients of both particles. The concentration of 90 nm silica nanoparticles was used to calculate the effective solvent density for the 30 nm silica nanoparticles. Then the Boltzmann equation for a sedimentation equilibrium concentration gradient with locally varying effective solvent density was used to fit the concentration gradient of 30 nm silica nanoparticles, which describes the experimental result well. This finding proves the validity of effective buoyancy in colloidal mixtures and provides a good model to study sedimenting polydisperse colloids.
Liquid Metastable Precursors of Ibuprofen as Aqueous Nucleation Intermediates
The nucleation mechanism of crystals of small organic molecules, postulated based on computer simulations, still lacks experimental evidence. In this study we designed an experimental approach to monitor the early stages of the crystallization of ibuprofen as pharmaceutically eminent molecule and a model system for small organic molecules. We found that ibuprofen undergoes liquid-liquid phase separation prior to nucleation. The binodal and spinodal limits of the corresponding liquid-liquid miscibility gap were localized by combining potentiometric titration with 1 H NMR spectroscopy and additional analyses. We confirmed the liquid character of this initially formed phase by applying PFG-STE self-diffusion experiments ( 1 H NMR) and found an increase in viscosity sustaining the kinetic stability of the dense liquid intermediate. Intermolecular distances of ibuprofen within the dense liquid phase were found to be similar to those in the crystal forms, according to 2D 1 H- 1 H NOESY measurements. Hence, this dense liquid phase is identified as a precursor phase within a nucleation pathway of ibuprofen, in which densification is followed by structural order generation. Fundamentally, this discovery bears the opportunity and promise to enrich poorly soluble pharmaceuticals beyond classical solubility limitations in aqueous environments.
Functionalized Multiwalled CNTs in Classical and Nonclassical CaCO3 Crystallization
Multiwalled carbon nanotubes (MWCNTs) are interesting high-tech nanomaterials. MWCNTs oxidized and functionalized with itaconic acid and monomethylitaconate were demonstrated to be efficient additives for controlling nucleation of calcium carbonate (CaCO3) via gas diffusion (GD) in classical as well as nonclassical crystallization, yielding aragonite and truncated calcite. For the first time, all amorphous calcium carbonate (ACC) proto-structures, such as proto calcite-ACC, proto vaterite-ACC and proto aragonite-ACC, were synthesized via prenucleation cluster (PNC) intermediates and stabilized at room temperature. The MWCNTs also showed concentration-dependent nucleation promotion and inhibition similar to biomolecules in nature. Incorporation of fluorescein-5-thiosemicarbazide (5-FTSC) dye-labeled MWCNTs into the CaCO3 lattice resulted in fluorescent hybrid nanosized CaCO3. We demonstrate that functionalized MWCNTs offer a good alternative for controlled selective crystallization and for understanding an inorganic mineralization process.
Controlled Preparation of Nanoparticle Gradient Materials by Diffusion
This article describes a new way to analyze data from the interpersonal circumplex (IPC) for interpersonal behavior. Instead of analyzing Agency and Communion separately or analyzing the IPC’s octants, we propose using a circular regression model that allows us to investigate effects on a blend of Agency and Communion. The proposed circular model is called a projected normal (PN) model. We illustrate the use of a PN mixed-effects model on three repeated measures data sets with circumplex measurements from interpersonal and educational psychology. This model allows us to detect different types of patterns in the data and provides a more valid analysis of circumplex data. In addition to being able to investigate the effect on the location (mean) of scores on the IPC, we can also investigate effects on the spread (variance) of scores on the IPC. We also introduce new tools that help interpret the fixed and random effects of PN models.Nanoparticle gradient materials combine a concentration gradient of nanoparticles with a macroscopic matrix. This way, specific properties of nanoscale matter can be transferred to bulk materials. These materials have great potential for applications in optics, electronics, and sensors. However, it is challenging to monitor the formation of such gradient materials and prepare them in a controlled manner. In this study, we present a novel universal approach for the preparation of this material class using diffusion in an analytical ultracentrifuge. The nanoparticles diffuse into a molten thermoreversible polymer gel and the process is observed in real-time by measuring the particle concentrations along the length of the material to establish a systematic understanding of the gradient generation process. We extract the apparent diffusion coefficients using Fick’s second law of diffusion and simulate the diffusion behavior of the particles. When the desired concentration gradient is achieved the polymer solution is cooled down to fix the concentration gradient in the formed gel phase and obtain a nanoparticle gradient material with the desired property gradient. Gradients of semiconductor nanoparticles with different sizes, fluorescent silica particles, and spherical superparamagnetic iron oxide nanoparticles are presented. This method can be used to produce tailored nanoparticle gradient materials with a broad range of physical properties in a simple and predictable way.