Prof. Dr. Tobias Kraus

The chemical extraction of niobium and titanium carbonitride precipitates from microalloyed steels was studied. Steel samples and chemically synthesized reference nanoparticles were subjected to commonly used extraction protocols, and conditions were systematically varied. High acid concentrations led to particle etching with losses above 10%; long extraction times and small etchant volumes led to the formation of dense SiOx networks that engulfed the extracted particles. The addition of surfactants was found to reduce agglomeration and limit etching. We developed an optimized extraction protocol that can extract and retain particles with diameters below 10 nm with reduced etching and negligible network formation. The resulting particle dispersions are suitable both for efficient electron microscopy of large particle numbers in a single run and colloidal analysis of large numbers of particles in dispersion.

This work aims to elucidate the role of environmental humidity on the tribological behavior of steel surfaces lubricated with an ionic liquid comprised of a fluorinated phosphonium cation—tributyl-3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-octyl-phosphonium—and a dicyanamide anion (i.e. N(CN)2−). Ball-on-disk tribotests were carried out at room temperature and at various levels of relative humidity (RH). Water was found to be required to promote the formation of a tribofilm over the contact area. The reaction layer exhibited a patchy morphology, which resembles that observed formed with conventional antiwear additives such as ZnDTP. A surface-chemical analysis of the tribofilm indicated that the tribofilm is composed of fluorides, oxides, and phosphates, pointing to a stress-induced degradation of the ions and corrosion of the sliding counterparts, which is enabled by the presence of water at the sliding interface.

Gold nanorods (AuNRs) with conductive polymer shells are interesting colloidal building blocks for electronics. Hybrid particles with AuNR cores and poly(3,4-ethylenedioxythiophene) or polystyrene sulfonate (PEDOT:PSS) shells were prepared as stable aqueous dispersions. Film formation during the drying of such dispersions is known to affect the electric conductivity of the material. We observed the mechanisms of drying in thin, spray-coated films with grazing incidence small-angle X-ray scattering (GISAXS). A sparse, uniform monolayer formed because the anisotropic shape of the AuNR inhibited “coffee-ring” effects. We used generalized two-dimensional correlation (2DC) spectroscopy to analyze the GISAXS data and to decipher the microscopic structure formation of the film during drying. Four major scattering peaks were attributed to porous PEDOT, PSS, Au, and the substrate layer. Their time-dependent intensity indicated the sequence of film formation: AuNR with mobile shells arranged on the substrate first, PEDOT and then PSS dried sequentially around the gold core. We discuss the final phase-separation of PEDOT:PSS on the hybrid rods. ER

Abstract Pure and Nb-doped TiO2 photocatalysts with highly ordered alternating gyroid architecture and well-controllable mesopore size of 15 nm via co-assembly of a poly(isoprene)-block-poly(styrene)-block-poly(ethylene oxide) block copolymer are synthesized. A combined effort by electron microscopy, X-ray scattering, photoluminescence, X-ray photoelectron spectroscopy, Raman spectroscopy, and density functional theory simulations reveals that the addition of small amounts of Nb results in the substitution of Ti4+ with isolated Nb5+ species that introduces inter-bandgap states, while at high concentrations, Nb prefers to cluster forming shallow trap states within the conduction band minimum of TiO2. The gyroidal photocatalysts are remarkably active toward hydrogen evolution under UV and visible light due to the open 3D network, where large mesopores ensure efficient pore diffusion and high photon harvesting. The gyroids yield unprecedented high evolution rates beyond 1000 µmol h−1 (per 10 mg catalyst), outperforming even the benchmark P25-TiO2 more than fivefold. Under UV light, the Nb-doping reduces the activity due to the introduction of charge recombination centers, while the activity in the visible triple upon incorporation is owed to a more efficient absorption due to inter-bandgap states. This unique pore architecture may further offer hitherto undiscovered optical benefits to photocatalysis, related to chiral and metamaterial-like behavior, which will stimulate further studies focusing on novel light–matter interactions.

Abstract In searching for polymer‐based electrolytes with improved performance for lithium ion and lithium metal batteries, we studied block copolymer electrolytes with high amounts of bis(trifluoromethane)sulfonimide lithium obtained by macromolecular co‐assembly of a poly(isoprene)‐block‐poly(styrene)‐block‐poly(ethylene oxide) and the salt from tetrahydrofuran. Particularly, an ultra‐short poly(ethylene oxide) block of 2100 g mol−1 was applied, giving rise to 2D continuous lamellar microstructures. The macroscopic stability was ensured with major blocks from poly(isoprene) and poly(styrene), which separated the ionic conductive PEO/salt lamellae. Thermal annealing led to high ionic conductivities of 1.4 mS cm−1 at 20 °C with low activation energy and a superior lithium ion transference number of 0.7, accompanied by an improved mechanical stability (storage modulus of up to 107 Pa). With high Li:O ratios >1, we show a viable concept to achieve fast Li+ transport in block copolymers (BCP), decoupled from slow polymer relaxation.

Abstract Active nanocomposites are created with liquid inclusions that contain plasmonic gold nanoparticles inside a polymeric matrix. The alkylthiol-coated gold particles are designed to reversible agglomerate at certain temperatures, which changes the plasmonic coupling and thus optical properties. It is found that particles confined to the liquid inclusions inside the active composite retain this capability and cause macroscopic, temperature-dependent color change of the solid. The transition is fully reversible for at least 100 times and tunable in temperature via particle size and ligand. This method is suitable to “package” responsive dispersion in solid composites to exploit their dynamic properties in materials.

We demonstrate that rapid nanoparticle self-assembly is possible in organic solvents if the temperature is above the melting point of the particles' ligand shell. Flow experiments coupled to small-angle X-ray scattering reveal the agglomeration kinetics and agglomerate structures of alkylthiol-coated gold nanoparticles at different temperatures, interparticle potentials, and times. Our experiments allow to discriminate between the effects of long-range and short-range interactions on self-assembly: crystalline agglomerates formed for a wide range of potentials, but only at temperatures where the short-ranged mobility was sufficient. Rapid superlattice formation in less than 3 s was observed for strongly attractive potentials at high temperatures, implying an assembly rate that is sufficient for large-scale material synthesis. Strong attraction between the particles did not impede high-quality self-assembly when short-ranged mobility was provided by ligands above a specific temperature.

Metal grids with submicron line diameters are optically transparent, mechanically flexible, and suitable materials for transparent and flexible electronics. Printing such narrow lines with dilute metal nanoparticle inks is challenging because it requires percolation throughout the particle packing. Here, we print fully connected submicron lines of 3.2 nm diameter gold nanoparticles and vary the organic ligand shell to study the relation between colloidal interactions, ligand binding to the metal core, and conductivity of the printed lines. We find that particles with repulsive potentials aid the formation of continuous lines, but the required long ligand molecules impede conductivity and need to be removed after printing. Weakly bound alkylamines provided sufficient interparticle repulsion and were easy to remove with a soft plasma treatment after printing, so that grids with a transparencies above 90% and a conductivity of 150 Ω sq–1 could be printed.

Being able to predict and tune the colloidal stability of nanoparticles is essential for a wide range of applications, yet our ability to do so is currently poor due to a lack of understanding of how they interact with one another. Here, we show that the agglomeration of apolar particles is dominated by either the core or the ligand shell, depending on the particle size and materials. We do this by using Small-Angle X-ray Scattering and molecular dynamics simulations to characterize the interaction between hexadecanethiol passivated gold nanoparticles in decane solvent. For smaller particles, the agglomeration temperature and interparticle spacing are determined by ordering of the ligand shell into bundles of aligned ligands that attract one another and interdigitate. In contrast, the agglomeration of larger particles is driven by van der Waals attraction between the gold cores, which eventually becomes strong enough to compress the ligand shell. Our results provide a microscopic description of the forces that determine the colloidal stability of apolar nanoparticles and explain why classical colloid theory fails.

Inorganic nanoparticle cores are often coated with organic ligands to render them dispersible in apolar solvents. However, the effect of the ligand shell on the colloidal stability of the overall hybrid particle is not fully understood. In particular, it is not known how the length of an apolar alkyl ligand chain affects the stability of a nanoparticle dispersion against agglomeration. Here, small-angle X-ray scattering and molecular dynamics simulations have been used to study the interactions between gold nanoparticles and between cadmium selenide nanoparticles passivated by alkanethiol ligands with 12–18 carbons in the solvent decane. We find that increasing the ligand length increases colloidal stability in the core-dominated regime but decreases it in the ligand-dominated regime. This unexpected inversion is connected to the transition from ligand-dominated to core-dominated agglomeration when the core diameter increases at constant ligand length. Our results provide a microscopic picture of the forces that determine the colloidal stability of apolar nanoparticles and explain why classical colloid theory fails.