Sascha Vongehr

Ag Dendrite-Based Au/Ag Bimetallic Nanostructures with Strongly Enhanced Catalytic Activity

Dendritic Ag/Au bimetallic nanostructures have been synthesized via a galvanic replacement reaction (GRR) of Ag dendrites in a chlorauric acid (HAuCl4) solution. After short periods of time, one obtains structures with protruding flakes; these will mature into very porous structures with little Ag left over. The morphological, compositional, and crystal structural changes involved with reaction time t were analyzed by using scanning and transmission electron microscopy (SEM and TEM, respectively), energy-dispersive X-ray spectrometry (EDX), and X-ray diffraction. High-resolution TEM combined with EDX and selected area electron diffraction confirmed the replacement of Ag with Au. A proposed formation mechanism of the original Ag dendrites developing pores while growing Au flakes cover this underlying structure at longer reaction times is confirmed by exploiting surface-enhanced Raman scattering (SERS). Catalytic reduction of 4-nitrophenol (4-NP) by sodium borohydride (NaBH4) is strongly enhanced, implying promising applications in catalysis.

A Review on Diverse Silver Nanostructures

This article reviews recent advances in the utilization of various water based synthesis routes towards the shape-controlled synthesis of silver nanoparticles and microstructures in a diverse range of shapes and sizes from several nanometers to micrometers. A variety of very simple one-pot methods, at times employing commercial microwave ovens, inexpensive low power ultrasound cleaners, or two-electrode electro-chemistry, can be surprisingly effective in the controlled synthesis of a wide range of nanostructured products, if only parameters are carefully chosen. Many approaches which are adopted include synthesis of Ag nanostructures with various shapes in solution, doping of Ag nanoparticles on unmodified silica and on/inside carbon spheres, kinetically controlled growth of Ag micro-particles with novel nanostructures on flat substrates, and galvanic replacement towards bimetallic Ag-Au dendrites and carbon composites. Characterizations of shape, composition and microstructure are carried out via scanning and transmission electron microscopy, various spectroscopy methods, N 2 absorption measurements and suchlike. The involved growth mechanisms are investigated in order to discover new means towards better control. Size, location and shape control, including micro- and nanostructure features, allows tuning the products properties towards desired applications. We focus on the optical properties and catalytic activities, but also the stability of compounds can be an issue of interest.

Highly catalytic spherical carbon nanocomposites allowing tunable activity via controllable Au–Pd doping

We report the synthesis of highly catalytic spherical carbon composite particles with Au-Pd bimetallic nanoparticle doping using a microwave-assisted technique that allows control over the location of the nanoparticles (NPs), putting them into stable interior, but still near-surface locations (within a 100 nm thick shell). First, composite particles with Pd NPs inside of nanoporous carbon spheres (CSs) were synthesized. Subsequent immersion of the composite particles in HAuCl(4) solutions containing PVP led to an addition of Au near the Pd. Au-Pd/CS composites with Au:Pd atomic ratios varying from 0.4 to 4.6 were prepared. The growth of Au and its location relative to the carbons surface and the Pd are discussed. The catalytic activity towards the reduction of 4-nitrophenol is tunable via the Au:Pd atomic ratio. Optimizing the composition increases the activity a hundredfold over that of the corresponding monometallic Pd/CS. The catalytic activity arises from the synergy between different contributing mechanisms, here especially the interaction between the carbon matrix and metals, metal-metal interfaces, and the hydrogen absorption capabilities of Pd.

A high energy density asymmetric all-solid-state supercapacitor based on cobalt carbonate hydroxide nanowire covered N-doped graphene and porous graphene electrodes

In order to achieve high energy densities, an asymmetric all-solid-state supercapacitor is developed by synthesizing a novel composite of cobalt carbonate hydroxide (CCH) nanowire covered N-doped graphene (NG) as positive and porous NG as negative electrodes. The CCH–NG composite is obtained from a one-step hydrothermal method, where optimization of the CCH content triples the specific capacitance of porous NG, reaching 1690 F g−1 at 1.0 A g−1. The optimal composite exhibits a remarkable cycling stability retaining 94.2% of the initial capacitance after 10 000 cycles, and good rate capability (still 1358 F g−1 at 10 A g−1). The assembled asymmetric supercapacitor based on the optimal composite has a high discharge areal capacitance of 153.5 mF cm−2 (at 1.0 mA cm−2), can cycle reversibly in the high-voltage region of 0–1.9 V, and thus provide superior energy and power densities (0.77 W h m−2 and 25.3 W m−2).

The Missing Memristor has Not been Found

In 1971, not only the theoretical and by definition already existing ‘ideal memristor’ concept was introduced, but a real memristor device was suggested on grounds of the already known real inductors. The latter is a scientifically significant hypothesis grounded in fundamental symmetries of basic physics, here electro-magnetism. 2008 claimed the discovery of the “missing memristor.” Controversy arose: The devices were not new, and the hypothesized device needs magnetism but has no material memory, while the available devices constitute analogue memory that would work in a world without magnetism. Nevertheless, even the originator of the prediction accepted the discovery. Defenders of the 2008 claim emphasize that the devices are not merely ‘memristive systems,’ which is however a distinction defined in 1976, not 1971. We clarify widely confused concepts and maintain that the originally hypothesized real memristor device is missing and likely impossible. The argument is illustrated also by finding an ideal mechanical memristor element and purely mechanical memristive systems, and hypothesizing a real mechanical memristor device that requires inert mass just like the 1971 implied device requires magnetic induction.

Work functions, ionization potentials, and in between: Scaling relations based on the image-charge model

We reexamine a model in which the ionization energy of a metal particle is associated with the work done by the image-charge force in moving the electron from infinity to a small cutoff distance just outside the surface. We show that this model can be compactly, and productively, employed to study the size dependence of electron removal energies over the range encompassing bulk surfaces, finite clusters, and individual atoms. It accounts in a straightforward manner for the empirically known correlation between the atomic ionization potential (IP) and the metal work function (WF),

High-Performance Flexible Solid-State Carbon Cloth Supercapacitors Based on Highly Processible N-Graphene Doped Polyacrylic Acid/Polyaniline Composites.

\mathrm{I}\mathrm{P}/\mathrm{W}\mathrm{F}\ensuremath{\sim}2.

Unusual pickup statistics of high-spin alkali agglomerates on helium nanodroplets

We formulate simple expressions for the model parameters, requiring only a single property (the atomic polarizability or the nearest-neighbor distance) as input. Without any additional adjustable parameters, the model yields both the IP and WF within

Growing ultracold sodium clusters by using helium nanodroplets

\ensuremath{\sim}10%

3D nitrogen-doped graphene/Co(OH)2-nanoplate composites for high-performance electrochemical pseudocapacitors

for all metallic elements, simulates the concentration dependence of the WF of regular binary bulk alloys, and matches the size evolution of the IP of finite metal clusters for a large fraction of the experimental data. The parametrization takes advantage of a remarkably constant numerical correlation between the nearest-neighbor distance in a crystal, the cube root of the atomic polarizability, and the image-force cutoff length. The paper also includes an analytical derivation of the relation of the outer radius of a cluster of close-packed spheres to its geometric structure.