Diffusion-reaction model of positron annihilation for complex defect pattern.

The increasing structural complexity in modern material science is often associated with grain sizes in theµm- and the sub-µm-regime. Therefore, when positron annihilation is applied for studying free-volume type defects in such materials, positron trapping at grain boundaries (GBs) cannot be neglected, even when other defect types are in the primary focus. For this purpose, the available diffusion-reaction model for positron trapping and annihilation at GBs is extended to competitive trapping at two different types of intragranular defects. Closed-form expressions for the mean positron lifetime and the relative intensities of the defect-specific positron lifetime components are given. The model is presented for cylindrical-shaped crystallites, but is valid in the general sense for spherical symmetry as well with appropriate replacements. The model yields the basis for properly determining defect concentrations, even for the inconvenient but common case that one intragranular defect type exhibits a lifetime component similar to that in GBs. It turns out, that positron trapping at GBs matters even forµm-sized crystallites and should not be neglected for precise studies of intragranular defects.

Porosity evolution and oxide formation in bulk nanoporous copper dealloyed from a copper-manganese alloy studied by in situ resistometry.

The synthesis of bulk nanoporous copper (npCu) from a copper-manganese alloy by electrochemical dealloying and free corrosion as well as the electrochemical behaviour of the dealloyed structures is investigated by in situ resistometry. In comparison to the well-established nanoporous gold (npAu) system, npCu shows strongly suppressed reordering processes in the porous structure (behind the etch front), which can be attributed to pronounced manganese oxide formation. Characteristic variations with the electrolyte concentration and potential applied for dealloying could be observed. Cyclic voltammetry was used to clarify the electrochemical behaviour of npCu. Oxide formation is further investigated by SEM and EDX revealing a hybrid composite of copper and manganese oxide on the surface of a metallic copper skeleton. Platelet-like structures embedded in the porous structure are identified which are rich in manganese oxide after prolonged dealloying. As an outlook, this unique heterogeneous structure with a large surface area and the inherent properties of manganese and copper oxides may offer application potential for the development of electrodes for energy storage and catalysis.

Positronium chemistry of a Fe2+/3+ solution under electrochemical control.

The positronium chemistry of a Fe2+/3+ solution is studied under full electrochemical control. For this novel approach to positronium electrochemistry, a suitable cell setup is used, which allows simultaneously both electrochemical measurements and positron annihilation spectroscopy. For the Fe2+/3+ redox couple, positronium serves as an ideally suited atomic probe owing to the rather different positronium chemistry of Fe2+ (spin conversion) and Fe3+ (total positronium inhibition and oxidation). This enabled the precise in situ monitoring of oxidation and reduction by means of positron lifetime upon slow cycling voltammetry or galvanostatic charging. The variation of the mean positron lifetime with the Fe2+/3+ concentration ratio could be quantitatively described by a reaction rate model for positronium formation and annihilation. An asymmetric behavior of the variation of the mean positron lifetime with applied potential, as compared to the simultaneously recorded symmetric current-potential curve, could be explained by the stronger influence of Fe3+ on the characteristics of positronium formation and annihilation. The highly reversible galvanostatic charging behavior monitored by positron lifetime underlines the attractive application potentials of positronium electrochemistry for in situ studies of iron-based redox-flow battery electrolytes.

In Situ Study of Nanoporosity Evolution during Dealloying AgAu and CoPd by Grazing-Incidence Small-Angle X-ray Scattering.

Electrochemical dealloying has become a standard technique to produce nanoporous network structures of various noble metals, exploiting the selective dissolution of one component from an alloy. While achieving nanoporosity during dealloying has been intensively studied for the prime example of nanoporous Au from a AgAu alloy, dealloying from other noble-metal alloys has been rarely investigated in the scientific literature. Here, we study the evolution of nanoporosity in the electrochemical dealloying process for both CoPd and AgAu alloys using a combination of in situ grazing-incidence small-angle X-ray scattering (GISAXS), kinetic Monte Carlo (KMC) simulations, and scanning transmission electron microscopy (STEM). When comparing dealloying kinetics, we find a more rapid progression of the dealloying front for CoPd and also a considerably slower coarsening of the nanoporous structure for Pd in relation to Au. We argue that our findings are natural consequences of the effectively higher dealloying potential and the higher interatomic binding energy for the CoPd alloy. Our results corroborate the understanding of electrochemical dealloying on the basis of two rate equations for dissolution and surface diffusion and suggest the general applicability of this dealloying mechanism to binary alloys. The present study contributes to the future tailoring of structural size in nanoporous metals for improved chemical surface activity.

Electrochemical switching of positronium triplet quenching.

Switching of positronium triplet quenching could successfully be demonstrated by electrochemical means in an aqueous K3[Fe(CN)6] electrolyte. For this purpose a suitable cell was designed to combine positron annihilation with electrochemical measurements. Highly reversible substantial variations of the mean positron lifetime τm could be observed upon electrochemical switching between the oxidation states Fe(CN)63- and Fe(CN)64-, arising from oxidation of positronium by Fe(CN)63-. Dynamic in situ measurements in dependence of potential exhibit a hysteresis like behavior of τm which perfectly correlates with the shift between the reduction and oxidation peaks simultaneously monitored by cyclic voltametry.

Nanoporous gold electrodes modified with self-assembled monolayers for electrochemical control of the surface charge.

The electrochemical behaviour of nanoporous gold modified with self-assembled monolayers is investigated with regard to its point of zero charge (pzc) and proton transfer reaction. Due to their high surface-to-volume ratio and conductivity, nanoporous electrodes represent promising materials for numerous applications, including the immobilization of biomolecules in biotechnology and biosensing. Therefore, the fundamental understanding and controllability of the surface state of the electrode is essential. To achieve a precise surface charge control, nanoporous gold (npAu) is modified with self-assembled monolayers (SAMs) of different lengths (3-mercaptopropionic acid (MPA) and 16-mercaptohexadecanoic acid (MHDA)). Cyclic voltammetry and impedance spectroscopy are used to determine the pzc. The most distinct pzc, and thus the most precise charge control, is found for the long-chain MHDA. Subsequently, the proton transfer reaction was investigated as a function of pH and scan rate. The observed protonation/deprotonation reaction was qualitatively well in line with the literature for planar gold electrodes, albeit the fraction of electrochemical controllable SAMs increased by a factor of 10 compared to planar electrodes indicating attractive application potential.

Influence of high-pressure torsion deformation on the corrosion behaviour of a bioresorbable Mg-based alloy studied by positron annihilation.

The effect of high-pressure torsion (HPT) on the corrosion behavior of extruded ZX00 (Mg-0.45wt%Zn-0.45wt%Ca) in phosphate buffered saline solution is investigated. MgCaZn alloys are promising candidates for the use as bioresorbable implant materials and, therefore, are in the focus of current research. To improve their strength, severe plastic deformation, e.g. via the technique of HPT, can be used. Positron lifetime spectroscopy (PLS) is applied as sensitive tool for studying open-volume defects which evolve during HPT processing and subsequent corrosion. The studies were complemented by electrochemical impedance spectroscopy (EIS). In the uncorroded state, grain boundaries are the major type of positron trap as quantitatively analysed by means of diffusion-reaction models for positron trapping and annihilation in fine-grained alloys. Upon corrosion, positronium formation and annihilation indicate larger open-volume structures, such as pores and cracks, in the emerging corrosion product and oxide layers. Both PLS and EIS clearly show that HPT-deformation strongly reduces the resistance against corrosion. Evidence is found for corrosion-induced open-volume defects, presumably related to hydrogen, in deeper parts of the material below the corrosion layer.

Adsorption and desorption of self-assembled L-cysteine monolayers on nanoporous gold monitored by in situ resistometry.

Surface modifications of nanoporous metals have become a highly attractive research field as they exhibit great potential for various applications, especially in biotechnology. Using self-assembled monolayers is one of the most promising approaches to modify a gold surface. However, only few techniques are capable of characterizing the formation of these monolayers on porous substrates. Here, we present a method to in situ monitor the adsorption and desorption of self-assembled monolayers on nanoporous gold by resistometry, using cysteine as example. During the adsorption an overall relative change in resistance of 18% is detected, which occurs in three distinct stages. First, the cysteine molecules are adsorbed on the outer surface. In the second stage, they are adsorbed on the internal surfaces and in the last stage the reordering accompanied by additional adsorption takes place. The successful binding of cysteine on the Au surface was confirmed by cyclic voltammetry, which showed a significant decrease of the double-layer capacitance. Also, the electrochemically controlled desorption of cysteine was monitored by concomitant in situ resistometry. From the desorption peak related to the (111) surface of the structure, which is associated with a resistance change of 4.8%, an initial surface coverage of 0.48 monolayers of cysteine could be estimated.

Magneto-Ionic Switching of Superparamagnetism.

Electrochemical reactions represent a promising approach to control magnetization via electric fields. Favorable reaction kinetics have made nanoporous materials particularly interesting for magnetic tuning experiments. A fully reversible ON and OFF switching of magnetism in nanoporous Pd(Co) at room temperature is demonstrated, triggered by electrochemical hydrogen sorption. Comprehensive magnetic characterization in combination with high-resolution scanning transmission electron microscopy reveals the presence of Co-rich, nanometer-sized clusters in the nanoporous Pd matrix with distinct superparamagnetic behavior. The strong magneto-ionic effect arises from coupling of the magnetic clusters via a Ruderman-Kittel-Kasuya-Yoshida-type interaction in the Pd matrix which is strengthened upon hydrogen sorption. This approach offers a new pathway for the voltage control of magnetism, for application in spintronic or microelectromagnetic devices.

Redox processes in sodium vanadium phosphate cathodes - insights from operando magnetometry.

Operando magnetic susceptibility measurements of sodium ion cathode materials during repetitive electrochemical cycling enable a continuous and bulk sensitive monitoring of the transition metal oxidation states. Such measurements on NaxV2(PO4)3 identified vanadium to be the only ion undergoing oxidation/reduction processes upon battery operation. For the initial battery charging-discharging cycle as well as for the first cycle after prolonged room temperature storage, however, peculiarities within the magnetic susceptibility measurements indicate parasitic side reactions, likely on the cathode surface.

Hydrogen-induced plasticity in nanoporous palladium.

The mechanical strain response of nanoporous palladium (npPd) upon electrochemical hydrogenation using an in situ dilatometric technique is investigated. NpPd with an average ligament diameter of approximately 20 nm is produced via electrochemical dealloying. A hydrogen-induced phase transition from PdHß to PdHα is found to enable internal-stress plasticity (or transformation-mismatch plasticity) in nanoporous palladium, which leads to exceptionally high strains without fracture as a result of external forces. The high surface stress in the nanoporous structure in combination with the internal-stress plasticity mechanism leads to a peculiar strain response upon hydrogen sorption and desorption. Critical potentials for the formation of PdHα and PdHß in npPd are determined. The theoretical concepts to assess the plastic strain response of nanoporous samples are elucidated, taking into account characteristics of structure and deformation mechanism.

Enhanced Charging-Induced Resistance Variations of Nanoporous Gold by Dealloying in Neutral Silver Nitrate Solution.

Nanoporous gold (np-Au), produced by dealloying in silver nitrate solution exhibits extraordinary high surface-to-volume ratios of more than 20 m2/g which represents an excellent prerequisite for property tuning by surface charging. Upon electrochemical charging in aqueous KOH solution, the electrical resistance is observed to vary reversibly by up to 88%. The charge coefficient, thus the sensitivity of the resistance toward the imposed charge per mol, is however significantly smaller compared to conventionally prepared np-Au, etched in nitric acid solution. While the strong resistance variation observed in the present work can directly be related to the high charge transfer due to extraordinary fine porosity, the charge coefficients can be understood with regards to the matrix resistance of the respective materials, which is strongly influenced by dealloying residuals.

Dealloying progress during nanoporous structure evolution analyzed by in situ resistometry.

The progress of dealloying, an electrochemical synthesis method capable of producing nanoporous structures with bulk outer dimensions, is studied by in situ resistometry. The resistance increases by three orders of magnitude while nanoporous gold or platinum is formed. Simultaneous monitoring of charge flow and electrical resistance increase proves to be an ideal combination for analyzing the etching progress, which in accordance with recent studies can be demonstrated to occur in two steps referred to as 'primary (or bulk) dealloying' and 'secondary (or ligament) dealloying'. A model is developed, which describes the resistance increase during etching as governed by the reduction of the master alloy backbone in favor of the nanoporous structure. This new approach allows an evaluation of the etching front propagation (primary dealloying) as well as the status of the already porous structure (secondary dealloying).

In situ characterization of hydrogen absorption in nanoporous palladium produced by dealloying.

Palladium is a frequently used model system for hydrogen storage. During the past few decades, particular interest was placed on the superior H-absorption properties of nanostructured Pd systems. In the present study nanoporous palladium (np-Pd) is produced by electrochemical dealloying, an electrochemical etching process that removes the less noble component from a master alloy. The volume and electrical resistance of np-Pd are investigated in situ upon electrochemical hydrogen loading and unloading. These properties clearly vary upon hydrogen ad- and absorption. During cyclic voltammetry in the hydrogen regime the electrical resistance changes reversibly by almost 10% upon absorbing approximately 5% H/Pd (atomic ratio). By suitable loading procedures, hydrogen concentrations up to almost 60% H/Pd were obtained, along with a sample thickness increase of about 5%. The observed reversible actuation clearly exceeds the values found in the literature, which is most likely due to the unique structure of np-Pd with an extraordinarily high surface-to-volume ratio.

A high-stability non-contact dilatometer for low-amplitude temperature-modulated measurements.

Temperature modulated thermophysical measurements can deliver valuable insights into the phase transformation behavior of many different materials. While especially for non-metallic systems at low temperatures numerous powerful methods exist, no high-temperature device suitable for modulated measurements of bulk metallic alloy samples is available for routine use. In this work a dilatometer for temperature modulated isothermal and non-isothermal measurements in the temperature range from room temperature to 1300 K is presented. The length measuring system is based on a two-beam Michelson laser interferometer with an incremental resolution of 20 pm. The non-contact measurement principle allows for resolving sinusoidal length change signals with amplitudes in the sub-500 nm range and physically decouples the length measuring system from the temperature modulation and heating control. To demonstrate the low-amplitude capabilities, results for the thermal expansion of nickel for two different modulation frequencies are presented. These results prove that the novel method can be used to routinely resolve length-change signals of metallic samples with temperature amplitudes well below 1 K. This high resolution in combination with the non-contact measurement principle significantly extends the application range of modulated dilatometry towards high-stability phase transformation measurements on complex alloys.

Electrochemically Tunable Resistance of Nanoporous Platinum Produced by Dealloying.

The extremely high surface-to-volume ratio of nanoporous platinum (np-Pt) produced by dealloying was applied for tuning electrical resistance by surface charging. In the as-dealloyed state, a characteristic sign-inversion of the charging-induced resistance variation occurs, which can be associated with the electronic structure of PtO. After electrochemical reduction, the relative resistance variations of np-Pt of up to 58% could be generated by electrochemically induced adsorption and desorption, which was 1 order of magnitude larger compared with that of cluster-assembled nanocrystalline Pt. Although the maximum resistance variation was also higher than that of dealloyed nanoporous gold (np-Au), the resistance variation related to the imposed charge was reduced owing to the higher bulk resistance of Pt compared with that of Au. The sign-inversion behavior of the resistance could be recovered by re-oxidation.

Electrochemical cell for in situ electrodeposition of magnetic thin films in a superconducting quantum interference device magnetometer.

An electrochemical cell is designed and applied for in situ electrodeposition of magnetic thin films in a commercial SQUID magnetometer system. The cell is constructed in such a way that any parasitic contribution of the cell and of the substrate for electrodeposition to the magnetic moment of the deposited film is reduced to a minimum. A remanent minor contribution is readily taken into account by a proper analysis of the detected signal. Thus, a precise determination of the absolute magnetic moment of the electrodeposited magnetic film during its growth and dissolution is achieved. The feasibility of the cell design is demonstrated by performing Co electrodeposition using cyclic voltammetry. For an average Co film thickness of (35.6 ± 3.0) atomic layers, a magnetic moment per Co atom of (1.75 ± 0.11) µ(B) was estimated, in good agreement with the literature bulk value.

Grain boundary excess volume and defect annealing of copper after high-pressure torsion.

The release of excess volume upon recrystallization of ultrafine-grained Cu deformed by high-pressure torsion (HPT) was studied by means of the direct technique of high-precision difference dilatometry in combination with differential scanning calorimetry (DSC) and scanning electron microscopy. From the length change associated with the removal of grain boundaries in the wake of crystallite growth, a structural key quantity of grain boundaries, the grain boundary excess volume or expansion [Formula: see text] m was directly determined. The value is quite similar to that measured by dilatometry for grain boundaries in HPT-deformed Ni. Activation energies for crystallite growth of [Formula: see text] and [Formula: see text] are derived by Kissinger analysis from dilatometry and DSC data, respectively. In contrast to Ni, substantial length change proceeds in Cu at elevated temperatures beyond the regime of dominant crystallite growth. In the light of recent findings from tracer diffusion and permeation experiments, this is associated with the shrinkage of nanovoids at high temperatures.

In situ monitoring magnetism and resistance of nanophase platinum upon electrochemical oxidation.

Controlled tuning of material properties by external stimuli represents one of the major topics of current research in the field of functional materials. Electrochemically induced property tuning has recently emerged as a promising pathway in this direction making use of nanophase materials with a high fraction of electrode-electrolyte interfaces. The present letter reports on electrochemical property tuning of porous nanocrystalline Pt. Deeper insight into the underlying processes could be gained by means of a direct comparison of the charge-induced response of two different properties, namely electrical resistance and magnetic moment. For this purpose, four-point resistance measurements and SQUID magnetometry were performed under identical in situ electrochemical control focussing on the regime of electrooxidation. Fully reversible variations of the electrical resistance and the magnetic moment of 6% and 1% were observed upon the formation or dissolution of a subatomic chemisorbed oxygen surface layer, respectively. The increase of the resistance, which is directly correlated to the amount of deposited oxygen, is considered to be primarily caused by charge-carrier scattering processes at the metal-electrolyte interfaces. In comparison, the decrease of the magnetic moment upon positive charging appears to be governed by the electric field at the nanocrystallite-electrolyte interfaces due to spin-orbit coupling.

SQUID magnetometry combined with in situ cyclic voltammetry: A case study of tunable magnetism of [Formula: see text]-Fe2O3 nanoparticles.

SQUID magnetometry combined with in situ cyclic voltammetry by means of a three-electrode chemical cell opens up novel potentials for studying correlations between electrochemical processes and magnetic behaviour. The combination of these methods shows that the charge-induced variation of the magnetic moment of nanocrystalline maghemite ([Formula: see text]-Fe2O3) of about 4% strongly depends on the voltage regime of charging. Upon positive charging, the charge-induced variation of the magnetic moment is suppressed due to adsorption layers. The pronounced charge-sensitivity of the magnetic moment in the regime of negative charging may either be associated with a redox reaction or with charge-induced variations of the magnetic anisotropy or magnetoelastic coupling.