Trepka, Bastian
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Controlling the density of hydrothermally grown rutile TiO2 nanorods on anatase TiO2 films
Due to their interface properties, compounds of anatase membranes and hydrothermally grown rutile TiO2 nanorods are valuable materials for (opto-) electronic applications. So far, dense nanorod arrays are typically grown on seeds such as polycrystalline rutile TiO2, fluorine-doped tin oxide (FTO) or seed particles in dispersion and the anatase modification is added subsequently. Nanorods grown on existing anatase films usually suffer from poor adhesion. In this study, we demonstrate the fabrication of anatase films that act directly as seed layers for the hydrothermal growth. The presented compounds offer a strong adhesion between the two TiO2 modifications and the substrate which resists even extensive sonication. So far, the density of nanorods is controlled with the HCl concentration, which affects also their size and shape. We control the density of nanorods with the average grain size of the anatase film without affecting their size and shape. This offers new scientific insights and applications of specific anatase/rutile compounds. The grain size of the anatase films is adjusted with the post-annealing temperature after film deposition. To satisfy the requirements of different applications, we provide suitable anatase seed layers with different deposition techniques such as sputter deposition, spray pyrolysis, and atmospheric spatial atomic layer deposition (SALD).
Influence of substrates and rutile seed layers on the assembly of hydrothermally grown rutile TiO2 nanorod arrays
Rutile TiO2 nanorod arrays (NRAs) are applicable in various prospective technologies. Hydrothermal methods present a simple technique to fabricate such NRAs. In this report, we present the fabrication of seed layers for the hydrothermal growth of rutile TiO2 nanorods via sputter deposition, electron-beam evaporation, and sol-gel method and study the influence of each on the growth behavior. To satisfy the requirements of numerous applications, p-type silicon, platinum, levitating carbon membranes, a template made of polystyrene spheres, and commercial fluorine tin oxide (FTO) were employed as substrates. We document the structural properties of the TiO2 seed layers and describe the relationship between the characteristics of the seed crystals, the growth evolution, and the appearance of as-grown nanorods. Various growth stages of rutile TiO2 nanorods are compared depending on whether they are grown on polycrystalline TiO2 or FTO seed layers. In both cases, a homogenous TiO2 bottom layer is formed at the seed layer/substrate interface, which is essential for electronic applications such as hybrid solar cells. Detached NRAs illustrate the effect of rutile FTO and TiO2 on the porosity of this bottom layer. Further details about the formation process of this layer are obtained from the growth on confined seed layers fabricated by electron-beam lithography.
Sunlight-Triggered Nanoparticle Synergy: Teamwork of Reactive Oxygen Species and Nitric Oxide Released from Mesoporous Organosilica with Advanced Antibacterial Activity
Colonization of surfaces by microorganisms is an urging problem. In combination with the increasing antibiotic resistance of pathogenic bacteria, severe infections are reported more frequently in medical settings. Therefore, there is a large demand to explore innovative surface coatings that provide intrinsic and highly effective antibacterial activity. Materials containing silver nanoparticles have been developed in the past for this purpose, but this solution has come into criticism due to various disadvantages like notable toxicity against higher organisms, the high price, and low abundance of silver. Here, we introduce a new, sunlight-mediated organosilica nanoparticle (NP) system based on silver-free antibacterial activity. The simultaneous release of nitric oxide (NO) in combination with singlet oxygen and superoxide radicals (O2•-) as reactive oxygen species (ROS) leads to the emergence of highly reactive peroxynitrite molecules with significantly enhanced biocidal activity. This special cooperative effect can only be realized, if the ROS-producing moieties and the functional entities releasing NO are spatially separated from each other. In one type of particle, Rose Bengal as an efficient singlet oxygen (1O2) producer was covalently bound to SH functionalities applying thiol-ene click chemistry. "Charging" the second type of particles with NO was realized by quantitatively transferring the thiol groups into S-nitrosothiol functionalities. We probed the oxidation power of ROS-NP alone and in combination with NO-NP using sunlight as a trigger. The high antibacterial efficiency of dual-action nanoparticles was demonstrated using disinfection assays with the pathogenic bacterium Pseudomonas aeruginosa.
NCH-Metallosurfactants as Active Polymerization Catalysts
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.
Light-Triggered Boost of Activity of Catalytic Bola-Type Surfactants by a Plasmonic Metal-Support Interaction Effect
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 (Cn) 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.
Nanomorphology Effects in Semiconductors with Native Ferromagnetism : Hierarchical Europium (II) Oxide Tubes Prepared via a Topotactic Nanostructure Transition
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.
Sacrificial Templating : A Route to Europium-II Oxide (EuO) Particles with Arbitrary Shape Prepared Indirectly by Hostile Takeover
Shape–property correlations of nanocrystals have recently moved into focus in materials science research. Magnetic properties, for instance, depend strongly on shape. Because crystal morphology is determined by a certain set of lattice planes (hkl) representing the surfaces, the achievable shapes are set by crystallographic symmetry. For instance, for a cubic crystal (90° angles) system it is very hard to realize hexagonal crystal shapes (120° angles). Breaking this paradigm is, thus, highly challenging. Here, we present a synthesis concept suitable for the synthesis of atypical Europium(II) oxide (EuO) particles. EuO is interesting as it is one of few materials known that belong to the class of intrinsic ferromagnetic semiconductors. It is shown that ZnO nanoparticles (rod-like, hexagonal platelets, spherical, and dumbbells) act as a sacrificial template and can be converted to the corresponding EuO particles by preservation of structural features, when treated with Eu-vapor. The transformation proceeds via a Kirkendall mechanism. This new reaction pathway enhances the accessibility of EuO particles tremendously and enables systematic studies on the magnetic shape property relationships of this highly functional metal oxide.
EurOgels : A ferromagnetic semiconductor with a porous structure prepared via the assembly of hybrid nanorods
EuO is unique, because it belongs to the few solids combining semiconducting properties (Egap = 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.
A combined 3D and 2D light scattering study on aqueous colloidal model systems with tunable interactions
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.