Georg Bendt

Pilot-scale synthesis of metal nanoparticles by high-speed pulsed laser ablation in liquids

The synthesis of catalysis-relevant nanoparticles such as platinum and gold is demonstrated with productivities of 4 g h(-1) for pulsed laser ablation in liquids (PLAL). The major drawback of low productivity of PLAL is overcome by utilizing a novel ultrafast high-repetition rate laser system combined with a polygon scanner that reaches scanning speeds up to 500 m s(-1). This high scanning speed is exploited to spatially bypass the laser-induced cavitation bubbles at MHz-repetition rates resulting in an increase of the applicable, ablation-effective, repetition rate for PLAL by two orders of magnitude. The particle size, morphology and oxidation state of fully automated synthesized colloids are analyzed while the ablation mechanisms are studied for different laser fluences, repetition rates, interpulse distances, ablation times, volumetric flow rates and focus positions. It is found that at high scanning speeds and high repetition rate PLAL the ablation process is stable in crystallite size and decoupled from shielding and liquid effects that conventionally occur during low-speed PLAL.

Off the Beaten Track-A Hitchhiker's Guide to Beryllium Chemistry.

This Minireview aims to give an introduction to beryllium chemistry for all less-experienced scientists in this field of research. Up to date information on the toxicity of beryllium and its compounds are reviewed and several basic and necessary guidelines for a safe and proper handling in modern chemical research laboratories are presented. Interesting phenomenological observations are described that are related directly to the uniqueness of this element, which are also put into historical context. Herein we combine the contributions and experiences of many scientist that work passionately in this field. We want to encourage fellow scientists to reconcile the long-standing reservations about beryllium and its compounds and motivate intense research on this spurned element. Who on earth should be able to deal with beryllium and its compounds if not chemists?

Deposition of topological insulator Sb2Te3 films by an MOCVD process

Layered Sb2Te3 films were grown by a MOCVD process on Al2O3(0001) substrates at 400 °C by use of i-Pr3Sb and Et2Te2 and characterized by SEM, AFM, XRD, EDX and Auger spectroscopy. The electrical sheet resistivity was measured in the range of 4 to 400 K, showing a monotonic increase with increasing temperature. The valence band structure probed by angle-resolved photoemission shows the detailed dispersions of the bulk valence band and the topological surface state of a quality no less than for optimized bulk single crystals. The surface state dispersion gives a Dirac point roughly 30 meV above the Fermi level leading to hole doping and the presence of bulk valence states at the Fermi energy.

Role of Composition and Size of Cobalt Ferrite Nanocrystals in the Oxygen Evolution Reaction

Sub‐10 nm CoFe2O4 nanoparticles with different sizes and various compositions obtained by (partial) substitution of Co with Ni cations have been synthesized by using a one‐pot method from organic solutions by the decomposition of metal acetylacetonates in the presence of oleylamine. The electrocatalytic activity of CoFe2O4 towards the oxygen evolution reaction (OER) is clearly enhanced with a smaller size (3.1 nm) of the CoFe2O4 nanoparticles (compared with 4.5 and 5.9 nm). In addition, the catalytic activity is improved by partial substitution of Co with Ni, which also leads to a higher degree of inversion of the spinel structure. Theoretical calculations attribute the positive catalytic effect of Ni owing to the lower binding energy differences between adsorbed O and OH compared with pure cobalt or nickel ferrites, resulting in higher OER activity. Co0.5Ni0.5Fe2O4 exhibited a low overpotential of approximately 340 mV at 10 mA cm−2, a smaller Tafel slope of 51 mV dec−1, and stability over 30 h. The unique tunability of these CoFe2O4 nanocrystals provides great potential for their application as an efficient and competitive anode material in the field of electrochemical water splitting as well as for systematic fundamental studies aiming at understanding the correlation of composition and structure with performance in electrocatalysis.

Synthesis of Bi2Te3 and (BixSb1−x)2Te3 nanoparticles using the novel IL [C4mim]3[Bi3I12]

The novel Bi-containing reactive ionic liquid [C4mim]3[Bi3I12], which was synthesized in quantitative yield by equimolar reaction of BiI3 and [C4mim]I, was used as a novel Bi-source for the ionothermal synthesis of Bi2Te3 nanoparticles by reaction with (Et3Si)2Te in the ionic liquid [C4mim]I. The solid state structure of [C4mim]3[Bi3I12] was determined by single crystal X-ray diffraction. In addition, the ionothermal synthesis of the single source precursor (Et2Sb)2Te and [C4mim]3[Bi3I12] yielded the ternary (BixSb1-x)2Te3 (x = 0.25, 0.5, 0.75) nanoparticles. The chemical composition and phase purity of the tetradymite-type materials were determined by EDX and XRD and the surface composition of the nanoparticles was further investigated by IR and XPS. In addition, the morphology of the nanoparticles was investigated by SEM and TEM.

Van der Waals epitaxial MOCVD-growth of (BixSb1−x)2Te3 (0 < x < 1) films

Epitaxial (BixSb1−x)2Te3 films with (0 < x < 1) were grown by the metal-organic chemical vapour deposition (MOCVD) process at 400 °C using the tailor-made precursors Et2Te2, i-Pr3Sb and Et3Bi. The films grown on Al2O3(0001) substrates show a very smooth surface morphology as shown by a scanning electron microscope (SEM), atomic force microscopy (AFM) and transmission electron microscopy (TEM), while those grown on Si(100) are rather polycrystalline. The chemical composition of the crystalline films (x-ray powder diffraction (XRD)) was investigated by energy-dispersive x-ray (EDX) and x-ray photoelectron spectroscopy (XPS), and the in-plane transport properties were measured, and a strong dependency from the bismuth content was found, which allows the tuning of the carrier concentration and mobility in a wide range.

Adjusting the catalytic properties of cobalt ferrite nanoparticles by pulsed laser fragmentation in water with defined energy dose

Highly active, structurally disordered CoFe2O4/CoO electrocatalysts are synthesized by pulsed laser fragmentation in liquid (PLFL) of a commercial CoFe2O4 powder dispersed in water. A partial transformation of the CoFe2O4 educt to CoO is observed and proposed to be a thermal decomposition process induced by the picosecond pulsed laser irradiation. The overpotential in the OER in aqueous alkaline media at 10 mA cm−2 is reduced by 23% compared to the educt down to 0.32 V with a Tafel slope of 71 mV dec−1. Importantly, the catalytic activity is systematically adjustable by the number of PLFL treatment cycles. The occurrence of thermal melting and decomposition during one PLFL cycle is verified by modelling the laser beam energy distribution within the irradiated colloid volume and comparing the by single particles absorbed part to threshold energies. Thermal decomposition leads to a massive reduction in particle size and crystal transformations towards crystalline CoO and amorphous CoFe2O4. Subsequently, thermal melting forms multi-phase spherical and network-like particles. Additionally, Fe-based layered double hydroxides at higher process cycle repetitions emerge as a byproduct. The results show that PLFL is a promising method that allows modification of the structural order in oxides and thus access to catalytically interesting materials.

Molecular Design for Tailoring a Single-Source Precursor for Bismuth Ferrite

Nearly phase-pure bismuth ferrite particles were formed by thermolysis of the single-source precursor [Cp(CO)2FeBi(OAc)2] (1) in octadecene at 245 °C, followed by subsequent calcination at 600 °C for 3 h. In contrast, the slightly modified compound [Cp(CO)2FeBi(O2C(t)Bu)2] (2) yielded only mixtures of different bismuth oxide phases, revealing the distinctive influence of molecular design in material synthesis. The chemical composition, morphology, and crystallinity of the resulting materials were investigated by X-ray diffraction, transmission electron microscopy, and energy-dispersive X-ray spectroscopy. In addition, the optical properties were investigated by Fourier transform infrared and UV-vis spectroscopies, showing a strong band gap absorption in the visible range at 590 nm (2.2 eV). The magnetic behavior was probed by vibrating-sample and superconducting quantum interference device magnetometry, as well as (57)Fe Mössbauer spectroscopy.

Low intrinsic c-axis thermal conductivity in PVD grown epitaxial Sb2Te3 films

Accurate determination and comprehensive understanding of the intrinsic c-axis thermal conductivity κc of thermoelectric layered Sb2Te3 is of high importance for the development of strategies to optimize the figure of merit in thin film devices via heterostructures and defect engineering. We present here high precision measurements of κc of epitaxial Sb2Te3 thin films on Al2O3 substrates grown by physical vapor deposition in the temperature range of 100 K to 300 K. The Kapitza resistances of the involved interfaces have been determined and subtracted from the film data, allowing access to the intrinsic thermal conductivity of single crystalline Sb2Te3. At room temperature, we obtain κc = 1.9 W/m K, being much smaller than the in-plane thermal conductivity of κab = 5 W/m K and even lower than the thermal conductivity of nano crystalline films of κnc ≈ 2.0–2.6 W/m K published by Park et al. [Nanoscale Res. Lett. 9, 96 (2014)]. High crystallinity and very low defect concentration of the films were confirmed ...