Margitta Uhlemann

The anodic stripping voltammetry of nanoparticles: electrochemical evidence for the surface agglomeration of silver nanoparticles

Analytical expressions for the anodic stripping voltammetry of metallic nanoparticles from an electrode are provided. First, for reversible electron transfer, two limits are studied: that of diffusionally independent nanoparticles and the regime where the diffusion layers originating from each particle overlap strongly. Second, an analytical expression for the voltammetric response under conditions of irreversible electron transfer kinetics is also derived. These equations demonstrate how the peak potential for the stripping process is expected to occur at values negative of the formal potential for the redox process in which the surface immobilised nanoparticles are oxidised to the corresponding metal cation in the solution phase. This work is further developed by considering the surface energies of the nanoparticles and its effect on the formal potential for the oxidation. The change in the formal potential is modelled in accordance with the equations provided by Plieth [J. Phys. Chem., 1982, 86, 3166-3170]. The new analytical expressions are used to investigate the stripping of silver nanoparticles from a glassy carbon electrode. The relative invariance of the stripping peak potential at low surface coverages of silver is shown to be directly related to the surface agglomeration of the nanoparticles.

Electrochemical hydrogenation of Mg65Cu25Y10 metallic glass

Rapidly quenched ribbons of a Mg65Y10Cu25 metallic glass were electrochemically charged up to a maximum hydrogen content of about 3.7 wt.%. The hydrogen content was determined by hot extraction. The microstructure of different hydrogen-charged samples was investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The thermal behaviour was studied by differential scanning calorimetry (DSC) and thermal desorption analysis (TDA). Samples heated to selected temperatures were characterised by XRD. With increasing hydrogen content a change from a single-phase amorphous to a very fine nanocrystalline microstructure was observed, which is a consequence of hydride-forming reactions at room temperature. This strongly affects the thermal behaviour. With increasing fraction of nanocrystalline phases in the hydrogenated samples, grain growth processes are more pronounced than crystallisation of the residual amorphous phase for temperatures up to 623 K. Correspondingly, the fraction of nanocrystalline products of hydride-forming reactions, i.e. YH3, MgH2 and Cu2Mg, increases. Significant hydrogen desorption occurs at temperatures above 623 K and is mainly related to the reverse of those hydriding reactions.

The influence of Co and Ga additions on the corrosion behavior of nanocrystalline NdFeB magnets

Abstract Isotropic nanocrystalline Nd14Fe80B6 and Nd12Dy2Fe73.2Co6.6Ga0.6B5.6 magnets with different grain sizes in the range of 60–600 nm have been produced from melt-spun materials by hot pressing at 700 °C and subsequent annealing at 800 °C for 0.5–6 h. The microstructure has been characterized using XRD, SEM, energy dispersive X-ray analysis, and Kerr microscopy. The corrosion behavior of NdFeB magnets has been examined on 0.1 M H2SO4 by in situ inductively coupled plasma solution analysis, gravimetric and electrochemical techniques. The corrosion hydrogen absorption/desorption behavior has been investigated by thermal desorption analysis and hot extraction methods. Partial substitution of Fe with Co and Ga leads to an improvement in corrosion resistance and reduces the affinity and binding energy for hydrogen in these materials. Coarsening of the microstructure results in a better corrosion performance of these materials. The corrosion behavior of the magnets in relation to phase composition, phase distribution and grain size is discussed in terms of dissolution, hydrogenation and mechanical degradation.

Get More Out of Your Data: A New Approach to Agglomeration and Aggregation Studies Using Nanoparticle Impact Experiments

Anodic particle coloumetry is used to size silver nanoparticles impacting a carbon microelectrode in a potassium chloride/citrate solution. Besides their size, their agglomeration state in solution is also investigated solely by electrochemical means and subsequent data analysis. Validation of this new approach to nanoparticle agglomeration studies is performed by comparison with the results of a commercially available nanoparticle tracking analysis system, which shows excellent agreement. Moreover, it is demonstrated that the electrochemical technique has the advantage of directly yielding the number of atoms per impacting nanoparticle irrespective of its shape. This is not true for the optical nanoparticle tracking system, which requires a correction for the nonspherical shape of agglomerated nanoparticles to derive reasonable information on the agglomeration state.

Hydrogenation and its effect on the crystallisation behaviour of Zr55Cu30Al10Ni5 metallic glass

Abstract Zr 55 Cu 30 Al 10 Ni 5 metallic glass exhibits high thermal stability, and as it contains early and late transition metal elements (ETM/LTM), it is of interest to study its hydrogenation properties. Charging melt-spun ribbons electrochemically to different hydrogen-to-metal (H/M) ratios and following the effusion of hydrogen by thermal desorption analysis (TDA) reveals hydrogen desorption from high interstitial-site energy levels at temperatures below 623 K. Zirconium hydrides are formed above 623 K. At higher temperatures partial desorption of hydrogen occurs. Simultaneously, transformation to different hydride phases takes place in the order tetragonal e-Zr-hydride, cubic δ-Zr-hydride and a mixture of (α+β)-Zr-hydride. Thermal stability investigations by differential scanning calorimetry (DSC) point out the exothermic peaks of formation/transformation to different Zr-hydride phases. The formation of zirconium hydride causes depletion in the number of free Zr atoms leading, in turn, to different crystalline phases upon crystallisation. X-ray diffraction (XRD) reveals the formation of different crystalline phases for different H/M ratios. For a H/M-ratio of 0.37 a hexagonal AlZr 2 phase forms at 753 K, whereas for high hydrogen contents of 0.7 2 Zr forms already at 713 K. In contrast, the uncharged ribbons crystallise at 771 K by formation of a mixture of metastable fcc NiZr 2 -type phase, orthorhombic NiZr and tetragonal CuZr 2 phases.

Electrochemical Deposition of Co under the Influence of High Magnetic Fields

The effect of uniform, vertically oriented high magnetic fields up to 13 T on the electrodeposition of Co has been investigated in dependence on the cell and electrode geometry as well as the orientation and strength of the magnetic flux density by means of cyclic voltammetry, chronoamperometric measurements, and atomic force microscopy investigations. In the majority of cases, the limiting current density i l i m increases with increasing magnetic flux densities independent of the cell geometry and orientation. The current efficiency of Co increases with increasing magnetic flux densities only in magnetic fields aligned parallel to the electrodes due to the magnetohydrodynamic (MHD) effect. The morphology of the deposits exhibits randomly oriented round-shaped grains. The electrochemical behavior of horizontal electrodes with magnetic fields oriented perpendicular to the surface is strongly dependent on the electrode geometry. The current efficiency of the Co deposition on flat electrodes increases for low magnetic flux densities and keeps constant for high magnetic fields. In contrast, for wall electrodes the current efficiency decreases strongly even for low magnetic fields. These results are caused by overlapping effects of two types of convection, macro-MHD- and micro-magneto convection due to gradients of the concentration and the magnetic susceptibility. This leads to a modified morphology.

Effect of hydrogen on Zr65Cu17.5Al7.5Ni10 metallic glass

Abstract Glassy Zr65Cu17.5Al7.5Ni10 alloy samples with 0.13 and 0.46 at.% oxygen were prepared by single-roller melt-spinning. Subsequently, the ribbons were electrolytically charged to different hydrogen-to-metal ratios 0.035≤H/M≤1.6. The effect of hydrogen on the thermal stability and on the crystallisation behaviour of the alloy samples was studied by means of differential scanning calorimetry (DSC), X-ray diffraction (XRD) and transmission electron microscopy (TEM). With increasing hydrogen content, a reduction of the undercooled liquid region and a significant change in the crystallisation process was observed. Hydrogen was found to suppress the oxygen-triggered formation of the metastable quasicrystalline phase, whereas the metastable f.c.c.-phase formation seems to be less affected. Furthermore, hydrogen-induced copper segregation was detected after long-term charging to high H/M ratios at room temperature and already for samples with low hydrogen contents upon heating. At very high H/M ratios, the thermal behaviour of the investigated Zr65Cu17.5Al7.5Ni10 alloy samples is determined by preferential zirconium hydride formation and successive decomposition causing the formation of new stable intermetallic phases with lower zirconium content.

In situ analysis of three-dimensional electrolyte convection evolving during the electrodeposition of copper in magnetic gradient fields.

A novel three-dimensional particle tracking velocimetry technique was used to examine the flow during electrodeposition of Cu. For the first time electrode-normal, circumferential, and radial velocities were spatially resolved during deposition in superimposed low and high magnetic gradient fields. In this way the complex interaction of magnetic field gradient force and Lorentz force induced convective effects could be measured and analyzed. Magnetic field gradient force induced electrolyte flow was detected only in high gradient magnetic fields, and it was found to be directed toward regions of gradient maxima. Since this electrode-normal flow causes enhanced transport of Cu(2+) ions from the bulk electrolyte to those regions of the working electrode where maxima of magnetic gradients are present, a structured deposit is formed during diffusion-limited electrodeposition. Lorentz force driven convection was observed during deposition in the low and the high magnetic gradient experiments. The overall fluid motion and the convection near the working electrode were determined experimentally and discussed with regard to the acting magnetic forces and numerical simulations.

Dynamics of Single Hydrogen Bubbles at a Platinum Microelectrode.

Bubble dynamics, including the formation, growth, and detachment, of single H2 bubbles was studied at a platinum microelectrode during the electrolysis of 1 M H2SO4 electrolyte. The bubbles were visualized through a microscope by a high-speed camera. Electrochemical measurements were conducted in parallel to measure the transient current. The periodic current oscillations, resulting from the periodic formation and detachment of single bubbles, allow the bubble lifetime and size to be predicted from the transient current. A comparison of the bubble volume calculated from the current and from the recorded bubble image shows a gas evolution efficiency increasing continuously with the growth of the bubble until it reaches 100%. Two different substrates, glass and epoxy, were used to embed the Pt wire. While nearly no difference was found with respect to the growth law for the bubble radius, the contact angle differs strongly for the two types of cell. Data provided for the contact point evolution further complete the image of single hydrogen bubble growth. Finally, the velocity field driven by the detached bubble was measured by means of PIV, and the effects of the convection on the subsequent bubble were evaluated.

Magnetic vortex observation in FeCo nanowires by quantitative magnetic force microscopy

An approach is presented that allows quantifying the three dimensional magnetization pattern of a magnetic nanoobject from measured two dimensional Magnetic Force Microscopy (MFM) data. This is based on a MFM deconvolution approach, which quantitatively determines the effective surface charges, on a micromagnetic calculation of the total magnetic charges at and below the sample surface, and on a projection of the lower lying charges onto the sample surface for a comparison of the such obtained effective surface charges with the experimentally determined ones. Thus, by making use of the depth sensitivity of MFM and by applying a quantitative contrast analysis, we are able to reconstruct the inhomogeneous magnetization state at the end of individual cylindrical Fe52Co48 nanowires arranged in a triangular array. As a result, we prove the existence of a magnetic vortex state at their ends.