Dominik Kramer

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.

Approaching the theoretical strength in nanoporous Au

The mechanical properties of nanoporous Au have been investigated by uniaxial compression. Micron-sized columns were machined in the surface of nanoporous Au using a focused Ga+ beam and compressed with a flat punch in a nanoindenter. Using scaling laws for foams, the yield strength of the 15nm diameter ligaments is estimated to be 1.5GPa, close to the theoretical strength of Au. This value agrees well with extrapolations of the yield strength of submicron, fully dense gold columns and shows that in addition to foam density and structure, the absolute size of ligaments and cell walls can be used to tailor foam properties.The mechanical properties of nanoporous Au have been investigated by uniaxial compression. Micron-sized columns were machined in the surface of nanoporous Au using a focused Ga+ beam and compressed with a flat punch in a nanoindenter. Using scaling laws for foams, the yield strength of the 15nm diameter ligaments is estimated to be 1.5GPa, close to the theoretical strength of Au. This value agrees well with extrapolations of the yield strength of submicron, fully dense gold columns and shows that in addition to foam density and structure, the absolute size of ligaments and cell walls can be used to tailor foam properties.

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.

Response of the potential of a gold electrode to elastic strain

We describe the measurement of the response, sigma, of the potential of a metal electrode to elastic strain under open circuit conditions. The experimental response exhibits a frequency dependence due to Faraday loss currents, which become negligible beyond 30 Hz. For a (111)-textured gold film the experimental value at higher frequency, sigma = -1.83 V, compares well with the intrinsic value of Au(111) predicted by recent ab initio computation.

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.

Dependence of surface stress, surface energy and surface tension on potential and charge

A toy model and simple model functions are used to exemplify the relation between surface tension, surface energy and surface stress given by Shuttleworths equation. Variations of the surface tension of charged interfaces must obey Lippmanns equation. Variations Deltaf of the surface stress of electrodes would be either equal to or proportional to and smaller than those variations Deltagamma of the surface tension, if the potential of zero charge (pzc) did not depend on the surface strain epsilon. However, since the pzc E(0) is a function of strain epsilon, the basic dependence of the surface stress on the charge, f(q), is described by a sum of three terms: the first one is the surface stress of the uncharged surface. The second one varies linearly with the surface tension, gamma(q), as long as the amount of specific adsorption remains constant, and is quadratic in E and q for a potential-independent double layer capacitance. The third summand that contributes to f(q) is linear in q and is a direct consequence of the potential dependence E(0)(epsilon) of the pzc. This result should help to resolve the seeming discrepancy between previous work on the surface stress changes of electrodes: Most experimental and theoretical results supported either the view that the variations of surface stress are identical or similar to that of surface tension, i.e. have essentially a quadratic dependence on the electrode potential, or that the basic dependence is a linear function of the charge. The more comprehensive model description presented here allows an explanation of the different results and their seeming discrepancies without assuming experimental errors or fundamental thermodynamic problems. Furthermore, an estimate of the potential dependence of the surface modulus can be obtained.

Mechanical measurements on lithium phosphorous oxynitride coated silicon thin film electrodes for lithium-ion batteries during lithiation and delithiation

The development of large stresses during lithiation and delithiation drives mechanical and chemical degradation processes (cracking and electrolyte decomposition) in thin film silicon anodes that complicate the study of normal electrochemical and mechanical processes. To reduce these effects, lithium phosphorous oxynitride (LiPON) coatings were applied to silicon thin film electrodes. Applying a LiPON coating has two purposes. First, the coating acts as a stable artificial solid electrolyte interphase. Second, it limits mechanical degradation by retaining the electrodes planar morphology during cycling. The development of stress in LiPON-coated electrodes was monitored using substrate curvature measurements. LiPON-coated electrodes displayed highly reproducible cycle-to-cycle behavior, unlike uncoated electrodes which had poorer coulombic efficiency and exhibited a continual loss in stress magnitude with continued cycling due to film fracture. The improved mechanical stability of the coated silicon electrodes allowed for a better investigation of rate effects and variations of mechanical properties during electrochemical cycling.

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.