Martin Panthöfer

Influence of a Nano Phase Segregation on the ThermoelectricProperties of the p-Type Doped Stannite CompoundCu_(2+x)Zn_(1−x)GeSe_4

Engineering nanostructure in bulk thermoelectric materials has recently been established as an effective approach to scatter phonons, reducing the phonon mean free path, without simultaneously decreasing the electron mean free path for an improvement of the performance of thermoelectric materials. Herein the synthesis, phase stability, and thermoelectric properties of the solid solutions Cu(2+x)Zn(1-x)GeSe(4) (x = 0-0.1) are reported. The substitution of Zn(2+) with Cu(+) introduces holes as charge carriers in the system and results in an enhancement of the thermoelectric efficiency. Nano-sized impurities formed via phase segregation at higher dopant contents have been identified and are located at the grain boundaries of the material. The impurities lead to enhanced phonon scattering, a significant reduction in lattice thermal conductivity, and therefore an increase in the thermoelectric figure of merit in these materials. This study also reveals the existence of an insulator-to-metal transition at 450 K.

Multifunctional Two-Photon Active Silica-Coated Au@MnO Janus Particles for Selective Dual Functionalization and Imaging

Monodisperse multifunctional and nontoxic Au@MnO Janus particles with different sizes and morphologies were prepared by a seed-mediated nucleation and growth technique with precise control over domain sizes, surface functionalization, and dye labeling. The metal oxide domain could be coated selectively with a thin silica layer, leaving the metal domain untouched. In particular, size and morphology of the individual (metal and metal oxide) domains could be controlled by adjustment of the synthetic parameters. The SiO2 coating of the oxide domain allows biomolecule conjugation (e.g., antibodies, proteins) in a single step for converting the photoluminescent and superparamagnetic Janus nanoparticles into multifunctional efficient vehicles for theranostics. The Au@MnO@SiO2 Janus particles were characterized using high-resolution transmission electron microscopy (HR-)TEM, powder X-ray diffraction (PXRD), optical (UV-vis) spectroscopy, confocal laser fluorescence scanning microscopy (CLSM), and dynamic light scattering (DLS). The functionalized nanoparticles were stable in buffer solution or serum, showing no indication of aggregation. Biocompatibility and potential biomedical applications of the Au@MnO@SiO2 Janus particles were assayed by a cell viability analysis by coincubating the Au@MnO@SiO2 Janus particles with Caki 1 and HeLa cells. Time-resolved fluorescence spectroscopy in combination with CLSM revealed the silica-coated Au@MnO@SiO2 Janus particles to be highly two-photon active; no indication for an electronic interaction between the dye molecules incorporated in the silica shell surrounding the MnO domains and the attached Au domains was found; fluorescence quenching was observed when dye molecules were bound directly to the Au domains.

Extraordinary Performance of Carbon-Coated Anatase TiO2 as Sodium-Ion Anode.

The synthesis of in situ polymer‐functionalized anatase TiO2 particles using an anchoring block copolymer with hydroxamate as coordinating species is reported, which yields nanoparticles (≈11 nm) in multigram scale. Thermal annealing converts the polymer brushes into a uniform and homogeneous carbon coating as proven by high resolution transmission electron microscopy and Raman spectroscopy. The strong impact of particle size as well as carbon coating on the electrochemical performance of anatase TiO2 is demonstrated. Downsizing the particles leads to higher reversible uptake/release of sodium cations per formula unit TiO2 (e.g., 0.72 eq. Na+ (11 nm) vs only 0.56 eq. Na+ (40 nm)) while the carbon coating improves rate performance. The combination of small particle size and homogeneous carbon coating allows for the excellent electrochemical performance of anatase TiO2 at high (134 mAh g−1 at 10 C (3.35 A g−1)) and low (≈227 mAh g−1 at 0.1 C) current rates, high cycling stability (full capacity retention between 2nd and 300th cycle at 1 C) and improved coulombic efficiency (≈99.8%).

Bismuth‐Catalyzed Growth of SnS2 Nanotubes and Their Stability

Along with carbon nanotubes, non-carbon nanostructureshave attracted much attention over the past few years. Owingto their unusual geometry and promising physical properties,the study of inorganic fullerene nanostructures has becomeone of the key topics in nanoscale research since the firstreport on WS

Controlled synthesis of linear and branched Au@ZnO hybrid nanocrystals and their photocatalytic properties

Colloidal Au@ZnO hybrid nanocrystals with linear and branched shape were synthesized. The number of ZnO domains on the Au seeds can be controlled by the solvent mixture. Imidazole-functionalized Au@ZnO hybrid nanocrystals were soluble in water and exhibited a greatly enhanced photocatalytic activity compared to ZnO nanocrystals. The pristine heterodimeric NPs were characterized using transmission electron microscopy (TEM), X-ray diffraction (XRD), and UV-Vis spectroscopy.

Thermoelectric transport in Cu7PSe6 with high copper ionic mobility.

Building on the good thermoelectric performances of binary superionic compounds like Cu2Se, Ag2Se and Cu2S, a better and more detailed understanding of phonon-liquid electron-crystal (PLEC) thermoelectric materials is desirable. In this work we present the thermoelectric transport properties of the compound Cu7PSe6 as the first representative of the class of argyrodite-type ion conducting thermoelectrics. With a huge variety of possible compositions and high ionic conductivity even at room temperature, the argyrodites represent a very good model system to study structure-property relationships for PLEC thermoelectric materials. We particularly highlight the extraordinary low thermal conductivity of Cu7PSe6 below the glass limit, which can be associated with the molten copper sublattice leading to a softening of phonon modes.

Synthesis, Characterization, and Hierarchical Organization of Tungsten Oxide Nanorods: Spreading Driven by Marangoni Flow

Tungsten oxide nanorods were synthesized by a soft chemistry approach using tungsten alkoxide and trioctyl amine and oleic acid as the surfactants. The optical properties of the nanorods were studied. The nanorods were found to be soluble in a wide range of solvents like chloroform, cyclohexane, and so on. Upon solvent evaporation, the nanorods formed hierarchically organized solid state structures. Depending on the solvent used, the nanorods organized in different mesostructures. Moreover, the organization of the nanorods from mixtures of polar and nonpolar solvents was studied. Here, the Marangoni effect resulting from differences in the surface tensions of the two solvents was found to play a role in the organization of the nanorods. Furthermore, dip coating of the nanorod solutions on a mica substrate resulted in the formation of a uniform thin film of the nanorods, which may be useful for a variety of applications such as in electrochromic devices and in organic light emitting devices (OLEDs) using tungsten oxide as a buffer layer.

Structure analysis of titanate nanorods by automated electron diffraction tomography.

A hitherto unknown phase of sodium titanate, NaTi(3)O(6)(OH)·2H(2)O, was identified as the intermediate species in the synthesis of TiO(2) nanorods. This new phase, prepared as nanorods, was investigated by electron diffraction, X-ray powder diffraction, thermogravimetric analysis and high-resolution transmission electron microscopy. The structure was determined ab initio using electron diffraction data collected by the recently developed automated diffraction tomography technique. NaTi(3)O(6)(OH)·2H(2)O crystallizes in the monoclinic space group C2/m. Corrugated layers of corner- and edge-sharing distorted TiO(6) octahedra are intercalated with Na(+) and water of crystallization. The nanorods are typically affected by pervasive defects, such as mutual layer shifts, that produce diffraction streaks along c*. In addition, edge dislocations were observed in HRTEM images.

Interaction of Alkaline Metal Cations with Oxidic Surfaces: Effect on the Morphology of SnO2 Nanoparticles

Reaction pathways to SnO(2) nanomaterials through the hydrolysis of hydrated tin tetrachloride precursors were investigated. The products were prepared solvothermally starting from hydrated tin tetrachloride and various (e.g., alkali) hydroxides. The influence of the precursor base on the final morphology of the nanomaterials was studied. X-ray powder diffraction (XRD) data indicated the formation of rutile-type SnO(2). Transmission electron microscopy (TEM) studies revealed different morphologies that were formed with different precursor base cations. Data from molecular dynamics (MD) simulations provide theoretical evidence that the adsorption of the cations of the precursor base to the faces of the growing SnO(2) nanocrystals is crucial for the morphology of the nanostructures.

Thermally highly stable amorphous zinc phosphate intermediates during the formation of zinc phosphate hydrate.

The mechanisms by which amorphous intermediates transform into crystalline materials are still poorly understood. Here we attempt to illuminate the formation of an amorphous precursor by investigating the crystallization process of zinc phosphate hydrate. This work shows that amorphous zinc phosphate (AZP) nanoparticles precipitate from aqueous solutions prior to the crystalline hopeite phase at low concentrations and in the absence of additives at room temperature. AZP nanoparticles are thermally stable against crystallization even at 400 °C (resulting in a high temperature AZP), but they crystallize rapidly in the presence of water if the reaction is not interrupted. X-ray powder diffraction with high-energy synchrotron radiation, scanning and transmission electron microscopy, selected area electron diffraction, and small-angle X-ray scattering showed the particle size (≈20 nm) and confirmed the noncrystallinity of the nanoparticle intermediates. Energy dispersive X-ray, infrared, and Raman spectroscopy, inductively coupled plasma mass spectrometry, and optical emission spectrometry as well as thermal analysis were used for further compositional characterization of the as synthesized nanomaterial. (1)H solid-state NMR allowed the quantification of the hydrogen content, while an analysis of (31)P{(1)H} C rotational echo double resonance spectra permitted a dynamic and structural analysis of the crystallization pathway to hopeite.