R. N. Viswanath

Surface-chemistry-driven actuation in nanoporous gold

Although actuation in biological systems is exclusively powered by chemical energy, this concept has not been realized in man-made actuator technologies, as these rely on generating heat or electricity first. Here, we demonstrate that surface-chemistry-driven actuation can be realized in high-surface-area materials such as nanoporous gold. For example, we achieve reversible strain amplitudes of the order of a few tenths of a per cent by alternating exposure of nanoporous Au to ozone and carbon monoxide. The effect can be explained by adsorbate-induced changes of the surface stress, and can be used to convert chemical energy directly into a mechanical response, thus opening the door to surface-chemistry-driven actuator and sensor technologies.

Surface stress-charge response of a (111)-textured gold electrode under conditions of weak ion adsorption.

We report a cantilever bending investigation into the variation of surface stress, f, with surface charge density, q, for (111)-textured thin films of gold in aqueous NaF and HClO 4. The graphs of f(q) are highly linear, and the surface stress-charge coefficients, d f/d q, are -1.95 V for 7 mM NaF and -2.0 V for 10 mM HClO 4 near the potential of zero charge. These values exceed some previously published experimental data by a factor of 2, but they agree with recent ab initio calculations of the surface stress-charge response of gold in vacuum.

Tuneable magnetic susceptibility of nanocrystalline palladium

A charge-induced reversible variation of the paramagnetic susceptibility χ of Pd is reported. In situ charging was performed by means of a nanocrystallite-electrolyte composite. The charge-induced variation of χ is analyzed taking into account the modification of the charge carrier density by surface charging and the effect of charge-induced pressure on the nanocrystallites. The present studies may open up the way to switchable ferromagnetism.

Charge induced Variation of the Magnetization in Nanoporous Ni-Pd

Changes in the electronic structure in superficial space-charge regions may substantially affect the properties of metals near their surface. In materials with a nanoscale porosity and with a high surface to volume ratio, changes in the properties of even a thin surface layer may have a noticeable effect on the properties of the entire material. In nanoporous metals immersed in an electrolyte, the space charge can be induced as a part of the electric double layer at the metal-electrolyte interface. Here we present first experiments on the effect of surface charging in a nanoporous metal on the magnetism. We report reversible changes in the magnetic moment in Ni-Pd alloys. As possible origins of the dependency of the magnetization on the surface charge density we discuss band filling and, alternatively, magnetostriction. X-ray diffraction and dilatometry reveal a considerable strain amplitude, about of 6×10 -4 , resulting from surface charging; this corresponds to a surface-induced pressure in the crystal lattice, in the order of 0.3 GPa.

Charge-influenced structural properties of electrically connected platinum nanoparticles

We present results of a study motivated by the recent suggestion that the properties of nanocrystalline materials with a large surface-to-volume ratio can be tuned by inducing spacecharge regions at interfaces by means of an applied voltage. As an example, we investigate the reversible variation of the lattice constant of platinum nanoparticles immersed in an aqueous 1M KOH electrolyte as a function of applied potential. It is found that a reversible volumetric strain of up to 1.2 % can be induced, corresponding to pressures of up to 3.2 GPa. We present the experimental set-ups for in-situ X-ray diffraction with an electrochemical cell. The variation of the space charge at the metal-electrolyte interface results in a variation of the surface stress f as a function of the applied potential, which is not an electrocapillary effect.

A neutron-scattering investigation of the magnetic structure and magnetic excitations in nanocrystalline Tb

The magnetic structure and magnetic excitations in nanocrystalline Tb have been investigated by neutron diffraction and neutron spectroscopy. This is a report on the long-range magnetic order and the magnetic excitations in a nanocrystalline elemental rare earth. Refinement of the neutron-diffraction data reveals an “average” magnetic structure of each crystallite which contains a significant out-of-plane component to the magnetic moment as well as a suppression of the high-temperature antiferromagnetic phase observed for coarse-grained Tb. The inelastic-neutron-scattering measurements reveal the presence of a magnetic excitation of approximately 10 meV at 2.5 K. The excitation energy decreases with increasing temperature. The origins of this excitation are discussed with particular reference to the magnetic modes at the zone center observed for single-crystal Tb.

Reversible Strain in Porous Metals Charged in Electrolytes

Nanoporous metal samples with millimetre size were prepared either by compacting nanocrystalline powders or by dealloying, the dissolution of the less noble metal(s) of an alloy. The samples were immersed in an electrolyte, and their length was measured as a function of the applied potential in-situ in a dilatometer. The results obtained for nanocrystalline platinum, nanoporous gold and for gold platinum alloys show that the length varies in dependence of the surface charge. The strain amplitude of nanocrystalline platinum was 0.15%, and even larger strains have been measured using an Au-Pt alloy. This strain is comparable to commercial piezoceramics, but it is achieved using smaller voltages. The strain measured for nanoporous gold prepared by dealloying was smaller than that mainly due to the larger structure size (20 nm structure size compared to 6 nm Pt crystallite size), but in the case of gold, it was possible to prepare stable composite structures of a metal foil and of the nanoporous gold. If such a bimetallic foil is charged, it is found to bend. Due to the mechanical amplification of the contraction or expansion of the nanoporous part of the foil, it was possible to observe the effect of electric charges on the surface stress of metals directly with the naked eye for the first time. These results demonstrate that nanoporous metals might be useful for actuator applications and for the study of surface strain effects. Furthermore, they are the first realization of a general concept that suggests that most of the properties of conducting nanomaterials can be tuned by controlling the surface charge.