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

Magnetic Force Microscopy Study of Multiscale Ion-Implanted Platinum in Silica Glass, Recorded by an Ultrafast Two-Wave Mixing Configuration

This study explores magnetization exhibited by nanoscale platinum-based structures embedded in pure silica plates. A superposition of laser pulses in the samples produced periodic linear arrangements of micro-sized structures. The samples were integrated by PtO2 microstructures (PtOΣs) with dispersed Pt oxide nanoparticles in their surroundings. The characterization of the materials was performed by high transmission electron microscopy studies. Furthermore, topographical and magnetic effects on the sample surfaces were analyzed by atomic force microscopy and magnetic force microscopy, respectively. The magnetic measurements indicated an enhancement in the gradient phase shift and in the gradient force related to the magnetic PtOΣs. The possibility of tuning the magnetic characteristics of the samples through contact with a Nd2Fe14B magnet was demonstrated. This process corresponds to an innovative method for obtaining magnetic PtOΣs induced by laser pulses. Moreover, an increase in the compactness of the silica with platinum-based structures was confirmed by an evaluation of the effective elastic modulus with reference to pure silica. The multimodal magnetic structures studied in this work seem to be candidates for developing high-density magnetic storage media.

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 PdH_β to PdH_α 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 PdH_α and PdH_β 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.

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.