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Item type:Publication, Access status: without files , Surface-driven actuation: Sign reversal under load and surface load-memory effect(2019-06-24); ; ; ; Motivated by suggestions that hybrid nanomaterials from nanoporous metal and aqueous electrolyte can be used as actuators, we study the impact of an external load on the actuation behavior of nanoporous gold impregnated with aqueous electrolyte. At no load, we observe the well-documented trend for a more positive electrode potential prompting elongation of the nanoporous body. For purely capacitive electrode processes we confirm that the elastic response to external load is simply superimposed on the potential-induced elongation, so that the strain per electric charge is invariant with the load. The observations so far are consistent with the expectation for surface-stress driven actuation in a linear elastic materials system. Surprisingly, however, actuation in the regime of oxygen electrosorption responds strongly to loading: as the load is increased, the strain per charge gradually drops to zero and even inverts its direction. In other words, the actuator moves backward when asked to do work against a substantial external load. Furthermore, we demonstrate that the length change in response to lifting the oxygen adsorbate layer depends on the load that was present at the instant of oxysorption. This "load memory effect" has analogies to shape-memory behavior in massive alloys. Yet, contrary to shape-memory alloys, the microscopic origin is here a surface phase transition. We argue that the observation is a signature of the reorientation of local surface domains with anisotropic surface stress, and that the required atomic transport process acts only while mobile adatoms are supplied during the deposition or lifting of the oxygen adsorbate layer.Publicationtype: Journal Article151 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Mechanical response of nanoporous metals: A story of size, surface stress, and severed struts(2018-01-01); ; Nanoporous metals made by dealloying are macroscopic network architectures that can contain ∼1015 nanoscale struts or ligaments per sample. Their mechanical performance is critical to their applications as functional or lightweight high-strength materials. Testing nanoporous metals at the macroscopic scale offers opportunities for unraveling the properties of nanoscale solids in general. The central questions in this area include whether the macroscopic strength and elastic modulus of nanoporous metals can be correlated with the properties of nanoscale ligaments by the classical Gibson-Ashby equations, whether the dealloying-made network structure differs from the conventional foam metals, how network connectivity influences mechanical response, and how ligament size and surface properties affect the elastic and plastic response of nanoscale solids and that of nanoporous metals, particularly the tension-compression asymmetry in strength. This article reviews the fundamental observations of the mechanical response of nanoporous metals with a focus on gold and the emerging understanding of the aforementioned issues.Publicationtype: Journal Article145 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Porous gold with a nested-network architecture and ultrafine structure© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. A preparation strategy is developed for monolithic samples of nanoporous gold with a hierarchical structure comprising two nested networks of solid "ligaments" on distinctly different structural length scales. The electrochemical dealloying protocol achieves a large retention of less noble element in a first corrosion step, thereby allowing an extra corrosion step which forms a separate structural hierarchy level. The beneficial impact of adding Pt to the Ag-Au master alloys that are more conventionally used in dealloying approaches to nanoporous gold is demonstrated. At ≈6 nm, the lower hierarchy level ligament size emerges extremely small. Furthermore, Pt favors the retention of Ag during the first dealloying step even when the master alloy has a high Au content. The high Au content reduces the corrosion-induced shrinkage, mitigating crack formation during preparation and favoring the formation of high-quality macroscopic (mm-sized) samples. The corrosion effectively carves out the nanoscale hierarchical ligament structure from the parent crystals tens of micrometers in size. This is revealed by X-ray as well as electron backscatter diffraction, which shows that the porous crystallites inherit the highly ordered, macroscopic crystal lattice structure of the master alloy.Publicationtype: Journal ArticleScopus© Citations 65 309 347 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Electrical stiffness modulation : confirming the impact of surface excess elasticity on the mechanics of nanomaterials(2014-06-13); ; ; Local variations in the stiffness at surfaces may affect the elastic response of nanostructures, yet experiments disagree on the magnitude and even sign of the surface excess elastic constants. The present study reports the variation in the effective macroscopic stiffness of bulk samples of nanoporous gold when the surface state is modulated under potential control in an electrochemical environment. Using in situ experiments in a dynamic mechanical analyzer to measure the storage and loss moduli, we show that adsorption of ≤1 atomic monolayer of oxygen species as well as a capacitively controlled excess of electrons at the surface stiffen the material, while oxygen desorption/electron depletion enhance the compliance. Relative changes in the effective stiffness of up to 8% imply the variation of a surface excess elastic constant of the order of 60 N m , much larger than the absolute value of that constant deduced from previous atomistic simulation studies of clean surfaces. Since the electrode potential affects exclusively the surface, our observations provide conclusive evidence for the impact of local stiffness variation at surfaces on the effective elastic response of nanostructures.Publicationtype: Journal Article59 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Catalytic activity of nanostructured Au : scale effects versus bimetallic/bifunctional effects in low-temperature CO oxidation on nanoporous Au(Beilstein-Institut zur Förderung der Chemischen Wissenschaften, 2013-02-19); ; ; ; The catalytic properties of nanostructured Au and their physical origin were investigated by using the low-temperature CO oxidation as a test reaction. In order to distinguish between structural effects (structure-activity correlations) and bimetallic/bifunctional effects, unsupported nanoporous gold (NPG) samples prepared from different Au alloys (AuAg, AuCu) by selective leaching of a less noble metal (Ag, Cu) were employed, whose structure (surface area, ligament size) as well as their residual amount of the second metal were systematically varied by applying different potentials for dealloying. The structural and chemical properties before and after 1000 min reaction were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The catalytic behavior was evaluated by kinetic measurements in a conventional microreactor and by dynamic measurements in a temporal analysis of products (TAP) reactor. The data reveal a clear influence of the surface contents of residual Ag and Cu species on both O2 activation and catalytic activity, while correlations between activity and structural parameters such as surface area or ligament/crystallite size are less evident. Consequences for the mechanistic understanding and the role of the nanostructure in these NPG catalysts are discussed.Publicationtype: Journal ArticleScopus© Citations 40 311 262 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , A material with electrically tunable strength and flow stress(American Association for the Advancement of Science (Assoc.), 2011-06-03); The selection of a structural material requires a compromise between strength and ductility. The material properties will then be set by the choice of alloy composition and microstructure during synthesis and processing, although the requirements may change during service life. Materials design strategies that allow for a recoverable tuning of the mechanical properties would thus be desirable, either in response to external control signals or in the form of a spontaneous adaptation, for instance in self-healing. We have designed a material that has a hybrid nanostructure consisting of a strong metal backbone that is interpenetrated by an electrolyte as the second component. By polarizing the internal interface via an applied electric potential, we accomplish fast and repeatable tuning of yield strength, flow stress, and ductility. The concept allows the user to select, for instance, a soft and ductile state for processing and a high-strength state for service as a structural material.Publicationtype: Journal Article10