Michael W. Möller

Single Nanocrystals of Platinum Prepared by Partial Dissolution of Au-Pt Nanoalloys

Small metal nanoparticles that are also highly crystalline have the potential for showing enhanced catalytic activity. We describe the preparation of single nanocrystals of platinum that are 2 to 3 nanometers in diameter. These particles were generated and immobilized on spherical polyelectrolyte brushes consisting of a polystyrene core (diameter of ∼100 nanometers) onto which long chains of a cationic polyelectrolyte were affixed. In a first step, a nanoalloy of gold and platinum (a solid solution) was generated within the layer of cationic polyelectrolyte chains. In a second step, the gold was slowly and selectively dissolved by cyanide ions in the presence of oxygen. Cryogenic transmission electron microscopy, wide-angle x-ray scattering, and high-resolution transmission electron microscopy showed that the resulting platinum nanoparticles are faceted single crystals that remain embedded in the polyelectrolyte-chain layer. The composite systems of the core particles and the platinum single nanocrystals exhibit an excellent colloidal stability, as well as high catalytic activity in hydrogenation reactions in the aqueous phase.

Sphere-to-Rod Transition of Micelles formed by the Semicrystalline Polybutadiene-block-Poly(ethylene oxide) Block Copolymer in a Selective Solvent

We present a morphological study of the micellization of an asymmetric semicrystalline block copolymer, poly(butadiene)-block-poly(ethylene oxide), in the selective solvent n-heptane. The molecular weights of the poly(butadiene) (PB) and poly(ethylene oxide) (PEO) blocks are 26 and 3.5 kg · mol(-1) , respectively. In this solvent, micellization into a liquid PEO-core and a corona of PB-chains takes place at room temperature. Through a thermally controlled crystallization of the PEO core at -30 °C, spherical micelles with a crystalline PEO core and a PB corona are obtained. However, crystallization at much lower temperatures (-196 °C; liquid nitrogen) leads to the transition from spherical to rod-like micelles. With time these rod-like micelles aggregate and form long needles. Concomitantly, the degree of crystallinity of the PEO-cores of the rod-like micelles increases. The transition from a spherical to a rod-like morphology can be explained by a decrease of solvent power of the solvent n-heptane for the PB-corona chains: n-Heptane becomes a poor solvent at very low temperatures leading to a shrinking of the coronar chains. This favors the transition from spheres to a morphology with a smaller mean curvature, that is, to a cylindrical morphology.

Tailoring Shear-Stiff, Mica-like Nanoplatelets

This work introduces a novel facile method to produce shear-stiff, mica-like nanoplatelets by efficient exfoliation. The essence of this procedure is the nonreversible alteration of the interlamellar reactivity of a synthetic fluorohectorite by simple cation exchange. The possibility of switching from highly hydrated to collapsed interlayers permits a highly efficient exfoliation in the swollen state while providing shear-stiffness in the collapsed state. This method restricts cation exchange in the mica-like nanoplatelets to the outer surfaces, which represents a significant advantage for use in nanocomposites as compared to conventional organoclays which contain up to 40%/wt of organocations. It is expected that this new type of rigid, shear-stiff, clay-based nanoplatelets will be superior for reinforcement when used in composite materials like polymer layered silicate nanocomposites or artificial nacre.

UV‐Cured, Flexible, and Transparent Nanocomposite Coating with Remarkable Oxygen Barrier

A polymer-layered silicate nanocomposite coating is prepared by combining a novel synthetic lithium-hectorite and an UV-curable, cationic polyurethane. Oxygen transmission measurements clearly indicate the supremacy of the lithium-hectorite as compared to a standard montmorillonite. In addition, a very high degree of optical transparency of the nanocomposite coating is achieved, rendering this material highly interesting for flexible packaging and encapsulation applications.

Barrier Properties of Synthetic Clay with a Kilo‐Aspect Ratio

Intrinsic anisometry, the appearance in plate-like pseudocrystals (tactoids) with high aspect ratios, and their rich intercalation chemistry (cation-exchange, swelling) are some of the essential features of clays, which render them interesting as functional particles for a variety of applications. [ 1–3 ] Long before Nielsen’s [ 4 ] essay about the tortuous path theory appeared in the late 1960s, composite materials were prepared of inorganic platelets in a continuous macromolecular phase. [ 5 ] Aside from mechanically toughening of polymeric matrices via the incorporation of stiff platelets, [ 6–9 ] diffusion-barrier, [ 10–12 ] and fl ame retardant applications [ 13 ] are the main focus of current research. Since both, experiments and simulations have emphasized the key importance of high aspect ratios ( α ) of fi llers, in particular for gas permeability, [ 14 , 15 ] material scientists continue hunting for higher α exploring a variety of lamellar materials including graphene. [ 16 ]

How to maximize the aspect ratio of clay nanoplatelets

Melt-synthesis yielded lithium-fluorohectorites (Li-hect(x)) with variable layer charge (x = 0.4, 0.6, 0.8, 1.0). Counterintuitively, both tactoid diameter and intracrystalline reactivity increased concomitantly with increasing layer charge. This way hectorites with very large diameters were obtained (d(50%) = 48 μm) that nevertheless still spontaneously delaminate when immersed into water and nano-platelets with huge aspect ratios (>10 000) are formed. Melt-synthesis of Li-hect(x) has been performed in an open glassy carbon crucible allowing for easy scaling to batches of 500 g. These unprecedented huge aspect ratio fillers promise great potential for flame retardants and barrier applications.

Controlled Exfoliation of Layered Silicate Heterostructures into Bilayers and Their Conversion into Giant Janus Platelets

Ordered heterostructures of layered materials where interlayers with different reactivities strictly alternate in stacks offer predetermined slippage planes that provide a precise route for the preparation of bilayer materials. We use this route for the synthesis of a novel type of reinforced layered silicate bilayer that is 15 % stiffer than the corresponding monolayer. Furthermore, we will demonstrate that triggering cleavage of bilayers by osmotic swelling gives access to a generic toolbox for an asymmetrical modification of the two vis-à-vis standing basal planes of monolayers. Only two simple steps applying arbitrary commercial polycations are needed to obtain such Janus-type monolayers. The generic synthesis route will be applicable to many other layered compounds capable of osmotic swelling, rendering this approach interesting for a variety of materials and applications.