Nanoporous Gold: From Structure Evolution to Functional Properties in Catalysis and Electrochemistry.

Nanoporous gold (NPG) is characterized by a bicontinuous network of nanometer-sized metallic struts and interconnected pores formed spontaneously by oxidative dissolution of the less noble element from gold alloys. The resulting material exhibits decent catalytic activity for low-temperature, aerobic total as well as partial oxidation reactions, the oxidative coupling of methanol to methyl formate being the prototypical example. This review not only provides a critical discussion of ways to tune the morphology and composition of this material and its implication for catalysis and electrocatalysis, but will also exemplarily review the current mechanistic understanding of the partial oxidation of methanol using information from quantum chemical studies, model studies on single-crystal surfaces, gas phase catalysis, aerobic liquid phase oxidation, and electrocatalysis. In this respect, a particular focus will be on mechanistic aspects not well understood, yet. Apart from the mechanistic aspects of catalysis, best practice examples with respect to material preparation and characterization will be discussed. These can improve the reproducibility of the materials property such as the catalytic activity and selectivity as well as the scope of reactions being identified as the main challenges for a broader application of NPG in target-oriented organic synthesis.

Self-Detachment and Subsurface Densification of Dealloyed Nanoporous Thin Films.

Experiment shows thin films of dealloyed nanoporous gold (NPG) spontaneously detaching from massive gold base layers. NPG can also densify near its external surface. This is naturally reproduced by kinetic Monte Carlo (KMC) simulation of dealloying and coarsening and so appears generic for nanoscale network materials evolving by surface diffusion. Near the porous layer's external surface and near its interface with the base layer, gradients in the depth-profile of a laterally averaged mean surface curvature provide driving forces for diffusion and cause divergences of the net fluxes of matter, leading to accretion/densification or to erosion/disconnection. As a toy model, the morphology evolution of substrate-supported nanopillars by surface diffusion illustrates and confirms our considerations. Contrary to cylindrical nanowires, the ligaments in nanoporous materials exhibit pre-existing gradients in the mean curvature. The Plateau-Rayleigh long-wavelength stability criterion is then not applicable and the disconnection accelerated.

Verifying Larché-Cahn elasticity, a milestone of 20th-century thermodynamics.

Many materials phenomena are governed by the interaction between chemistry and mechanics. However, it was only in the second half of the 20th century that the theory of open system elasticity by Francis Larché and John W. Cahn concatenated the fields of solid mechanics and alloy chemistry. As the theory's central materials descriptors, the open system elastic parameters describe how solids deform under stress when solute can rearrange at equilibrium while the chemical potential is held constant. Here, we report experiments verifying the predictions for these parameters. We study the elasticity of nanoporous Pd-H and Pd-Au-H during load cycles imposed by a dynamic mechanical analyzer. Short diffusion paths afford fast equilibration of H in the local strain gradients that carry the macroscopic elastic deformation. The experiment is in excellent agreement with the theory, confirming a central prediction of one of the key contributions to 20th-century thermodynamics.

Electrocapillary Coupling at Metal Surfaces from First Principles: On the Impact of Excess Charge on Surface Stress and Relaxation.

We study the response of the surface stress to excess charge via ab initio simulation of metal surfaces in an external electric field. We focus on "simple" sp-bonded metals to gain insight into the mechanisms underlying electrocapillary coupling. Both the direct effect on the surface stress via charging of the bonds and the indirect effect resulting from the charge-induced relaxation are analyzed and discussed in relation to the trends of the coupling coefficients, which-owing to a Maxwell relation-are determined in terms of the response of the work function to strain. Al(111), Mg(0001), and Na(110) are investigated as prototypical sp-bonded metal surfaces with positive, vanishing, and negative coupling parameters, respectively. Mg(0001) and Al(111) exhibit an inward relaxation of the first atomic layer upon negative charging, whereas an outward relaxation occurs for Na(110). The indirect contribution of the relaxation to the coupling coefficient has the same sign as the total response and makes up about 30% of its magnitude for Al(111) and Na(110). Our study highlights that even the response behavior of the so-called simple metals is by no means readily captured within simple models.

Semiordered Hierarchical Metallic Network for Fast and Large Charge-Induced Strain.

Nanoporous metallic actuators for artificial muscle applications are distinguished by combining the low operating voltage, which is otherwise reserved for polymer-based actuators with interesting values of strain amplitude, strength, and stiffness that are comparable of those of piezoceramics. We report a nanoporous metal actuator with enhanced strain amplitude and accelerated switching. Our 3D macroscopic metallic muscle has semiordered and hierarchical nanoporous structure, in which µm-sized tubes align perpendicular with the sample surface, while nm-sized ligaments consist of the tube walls. This nanoarchitecture combines channels for fast ion transportation with large surface area for charge storage and strain generation. The result is a record reversible strain amplitude of 1.59% with a strain rate of 8.83 × 10-6 s-1 in the field of metallic based actuators. A passive hydroxide layer is self-grown on the metal surface, which not only contributes a supercapacitive layer, but also stabilizes the nanoporous structure against coarsening, which guarantees sustainable actuation beyond ten-thousand cycles.

Plastic Poisson's Ratio of Nanoporous Metals: A Macroscopic Signature of Tension-Compression Asymmetry at the Nanoscale.

The suggestion, based on atomistic simulation, of a surface-induced tension-compression asymmetry of the strength and flow stress of small metal bodies so far lacks experimental confirmation. Here, we present the missing experimental evidence. We study the transverse plastic flow of nanoporous gold under uniaxial compression. Performing mechanical tests in electrolyte affords control over the surface state. Specifically, the surface tension, γ, can be varied in situ during plastic flow. We find that decreasing γ leads to an increase of the effective macroscopic plastic Poisson ratio, νP. Finite element simulations of a network with surface tension confirm the notion that νP of nanoporous gold provides a signature for a local tension-compression asymmetry of the nanoscale struts that form the network. We show that γ promotes compression while impeding tensile elongation. Because the transverse strain is partly carried by the elongation of ligaments oriented normal to the load axis, the surface-induced tension-compression asymmetry acts to reduce νP. Our experiment confirms a decisive contribution of the surface tension to small-scale plasticity.

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.

Electrocapillary coupling at rough surfaces.

We investigate the impact of the surface roughness on the experimental value of the electrocapillary coupling coefficient, ς. This quantity relates the response of electrode potential, E, to tangential elastic strain, e, and also measures the variation of the surface stress, f, with the superficial charge density, q. We combine experiments measuring the apparent coupling coefficient ςeff for gold thin film electrodes in weakly adsorbing electrolyte with data for the surface roughness determined by atomic force microscopy and by the capacitance ratio method. We find that even moderate roughness has a strong impact on the value of ςeff. Analyzing the mechanics of corrugated surfaces affords a correction scheme yielding values of ς that are invariant with roughness and that agree with expectations for the true coupling coefficient on ideal, planar surfaces. The correction is simple and readily applied to experiments measuring ςeff from surface stress changes in cantilever bending studies or from the potential variation in dynamic electro-chemo-mechanical analysis.

Less Noble or More Noble: How Strain Affects the Binding of Oxygen on Gold.

Many heterogeneous catalysts exploit strained active layers to modulate reactivity and/or selectivity. It is therefore significant that density functional theory, as well as experimental approaches, find that tensile strain makes the gold surface more binding for oxygen, in other words, less noble. We show that this behavior does not apply when re-structuring of the gold surface is allowed to occur simultaneously with the adsorption of oxygen. In situ cantilever-bending studies show the surface stress to increase when oxygen species adsorb on a (111)-textured gold surface in aqueous H2 SO4 . This implies a positive sign of the electrocapillary coupling parameter and, hence, a trend for weaker oxygen binding in response to tensile strain. These conflicting findings indicate that different electrosorption processes, and specifically oxygen species adsorption on the bulk-terminated surface, exhibit fundamentally different coupling between the chemistry and the mechanics of the surface.

Electrocapillary coupling during electrosorption.

The electrocapillary coupling coefficient, ς, measures the response of the electrode potential, E, to tangential elastic strain at the surface of an electrode. Using dynamic electro-chemo-mechanical analysis, we study ς(E) simultaneously with cyclic voltammetry. The study covers extended potential intervals on Au, Pt, and Pd, including the electrosorption of oxygen species and of hydrogen. The magnitude and sign of ς vary during the scans, and quite generally the graphs of ς(E) emphasize details which are less obvious or missing in the cyclic voltammograms (CVs). Capacitive processes on the clean electrode surfaces exhibit ς < 0, whereas capacitive processes on oxygen-covered surfaces are characterized by ς < 0 on Au but ς > 0 on Pt and Pd. The findings of ς < 0 during the initial stages of oxygen species adsorption and ς > 0 for hydrogen electrosorption agree with the trend that tensile strain makes surfaces more binding for adsorbates. However, the large hysteresis of oxygen electrosorption on all electrodes raises the question: is the exchange current associated with that process sufficient for its measurement by potential response during small cyclic strain?

Coherent Phase Change in Interstitial Solutions: A Hierarchy of Instabilities.

Metal hydrides or lithium ion battery electrodes can take the form of interstitial solid solutions with a miscibility gap. This work discusses theory approaches for locating, in temperature-composition space, coherent phase transformations during the charging/discharging of such systems and for identifying the associated transformation mechanisms. The focus is on the simplest scenario, where instabilities derive from the thermodynamics of the bulk phase alone, considering strain energy as the foremost consequence of coherency and admitting for stress relaxation at free surfaces. The extension of the approach to include capillarity is demonstrated by an example. The analysis rests on constrained equilibrium phase diagrams that are informed by geometry- and dimensionality-specific mechanical boundary conditions and on elastic instabilities-again geometry-specific-as implied by the theory of open-system elasticity. It is demonstrated that some scenarios afford the analysis of chemical stability to be based entirely on a linear stability analysis of the mechanical equilibrium, which provides closed-form solutions in a straightforward manner. Attention is on the impact of the system geometry (infinitely extended or of finite size) and on the chemical (closed or open system) and mechanical (incoherent or coherent) boundary conditions. Transformation mechanism maps are suggested for documenting the findings. The maps reveal a hierarchy of instabilities, which depend strongly on each of the above characteristics. Specifically, realistic, finite-sized systems differ qualitatively from idealized systems of infinite extension. Among the transformation mechanisms exposed by the analysis are a uniform switchover to the other phase when the open system reaches its chemical spinodal, practical coherent nucleation, as well as chemo-elastically coupled spontaneous buckling modes, which may take the form of either, single-phase or dual-phase states.

Dynamic electro-chemo-mechanical analysis during cyclic voltammetry.

We report and validate a method for measuring the strain-response, ς, of the electrode potential of electrically conductive solids in a fluid electrolyte. Simultaneously with cyclic voltammetry, the electrode is subjected to cyclic elastic strain at frequencies of up to 100 Hz. We explore three independent strategies for separating the cyclic variation of potential or current from the voltammogram proper, and find that the results of all three are in quantitative agreement. By means of an example we explore dominantly capacitive processes at a gold electrode in H(2)SO(4) and HClO(4). The response parameter ς is not sensitive to the nature of the electrolyte. Yet, its value varies by more than a factor of two in the potential interval investigated. The potential of largest magnitude of ς agrees closely with the potential of zero charge.

Nanoporous Au-Pt alloys as large strain electrochemical actuators.

Nanoporous Au-Pt alloys with pore- and ligament size down to few nanometers were fabricated by dealloying Ag-Au-Pt. Owing to the small structure size and large specific surface area, the surface stress and its variation give rise to significant stress and strain in the bulk of these materials. In fact, dilatometry experiments find electrochemical actuation with large reversible strain amplitude. The linear strain reaches approximately 1.3% and strain energy density is up to 6.0 MJ/m(3). The associated stresses may approach the elastic limit of the alloy.

Scaling behavior of stiffness and strength of hierarchical network nanomaterials.

Structural hierarchy can enhance the mechanical behavior of materials and systems. This is exemplified by the fracture toughness of nacre or enamel in nature and by human-made architected microscale network structures. Nanoscale structuring promises further strengthening, yet macroscopic bodies built this way contain an immense number of struts, calling for scalable preparation schemes. In this work, we demonstrated macroscopic hierarchical network nanomaterials made by the self-organization processes of dealloying. Their hierarchical architecture affords enhanced strength and stiffness at a given solid fraction, and it enables reduced solid fractions by dealloying. Scaling laws for the mechanics and atomistic simulation support the observations. Because they expose the systematic benefits of hierarchical structuring in nanoscale network structures, our materials may serve as prototypes for future lightweight structural materials.

Electrocapillary maximum and potential of zero charge of carbon aerogel.

We present an electrochemical study of carbon aerogel (CA) in aqueous sodium fluoride solutions, focusing on the comparison of two quantities that are related to the potential of zero charge (pzc): the capacitance minimum and the 'electrocapillary maximum' of the surface forces. Capacitance minima are well resolved in our samples. Their potential emerges reproducibly as around 90 mV (vs. Ag/AgCl in KCl), similar to the value, 70 mV, of bulk glassy carbon which we use for comparison, and similar to previous reported pzc values for carbon materials. Significantly, no electrocapillary maximum is found in this potential range. This demonstrates that the pzc does not necessarily coincide with the potential of the maximum of surface stress. We also determined the area-specific capacitances, c(a) = 2.8 microF cm(-2), which agrees well with reports for the basal-plane of graphite single crystals. Our experiments yield large reversible strain amplitudes, up to 0.45%.

Datasets for the microstructure of nanoscale metal network structures and for its evolution during coarsening.

The datasets in this work are files containing atom position coordinates of volume elements approximating nanoporous gold made by dealloying and annealing. The material is represented in an as-prepared state and in various stages of coarsening, as described in Phys. Rev. Mater, 3 (2019) 076001. Realistic initial structures of different solid fractions have been constructed by the leveled-wave algorithm, approximating mixtures at the end of early-stage spinodal decomposition. The microstructural evolution during coarsening by surface diffusion was approximated by on-lattice kinetic Monte-Carlo simulation. The data sets refer to solid fractions from 0.22 to 0.50, providing for different initial connectivity of the bicontinuous structures. Coarsening at two temperatures, 900 K and 1800 K, explores two different degrees of surface energy anisotropy - more faceted at 900 K and more rough at 1800 K. Each structure takes the form of a face-centred cubic lattice with approximately 32 million sites. A site can be occupied by either void or atom. 3D periodic boundary conditions are satisfied. Tables list each structure's properties, and specifically the specific surface area, two different measures for the ligament size, the net topological genus as well as the scaled genus. The atom coordinate files may serve as the basis for geometry analysis and for atomistic as well as finite element simulation studies of nanoporous as well as spinodally decomposed materials. The data sets are accessible via the TORE repository at http://hdl.handle.net/11420/3253.

On the impact of capillarity for strength at the nanoscale.

The interior of nanoscale crystals experiences stress that compensates for the capillary forces and that can be large, in the order of 1 GPa. Various studies have speculated on whether and how this surface-induced stress affects the stability and plasticity of small crystals. Yet, experiments have so far failed to discriminate between the surface contribution and other, bulk-related size effects. To clarify the issue, here we study the variation of the flow stress of a nanomaterial while distinctly different variations of the two capillary parameters, surface tension, and surface stress, are imposed under control of an applied electric potential. Our theory qualifies the suggested impact of surface stress as not forceful and instead predicts a significant contribution of the surface energy, as measured by the surface tension. The predictions for the combined potential-dependence and size-dependence of the flow stress are quantitatively supported by the experiment. Previous suggestions, favoring the surface stress as the relevant capillary parameter, are not consistent with our experiment.

Electrocatalytic methanol oxidation with nanoporous gold: microstructure and selectivity.

The properties of Nanoporous Gold (NPG) obtained by the selective dissolution of Ag from an Au-Ag alloy can be tuned by the details of its fabrication, and specifically the residual Ag content is correlated to the ligament size of the material. We link this correlation to methanol electro-oxidation. Specifically, two different NPG types (obtained by potentiostatic dealloying) are compared with one obtained by free corrosion. They show remarkable differences in activity. Quantitative product analysis reveals that NPG shows nearly selective oxidation of CH3OH to HCOO- when NPG is used as an active electrode in contrast to planar Au. This trend can further be enhanced when applying finer nanoporous structures that are linked to a higher Ag content. X-ray photoelectron spectroscopy (XPS) reveals changes in the nature of residual Ag from which we conclude that Ag is not a passive component in the methanol oxidation process.

Structural relaxation in charged metal surfaces and cluster ions.

Space-charge layers can noticeably affect the properties of metal electrode surfaces, for instance by modifying the surface dielectric response or indirectly via the induced atomic relaxations. While there are efforts to exploit this concept for designing novel functional nanomaterials, the underlying microscopic processes are poorly understood. Here, we report on a density functional theory (DFT) study of atomic relaxation in Au cluster ions comprising up to 309 atoms. Suitable averages over atomic displacements respond to charging consistent with experimental observation on macroscopic Au single-crystal surfaces. Moreover, the overall DFT response is also consistent with predictions of a simple phenomenological model. Motivated by our observations, we propose a scenario in which the surface relaxation ("stretch") results from out-of-plane Hellman-Feynman forces exerted on the surface atoms by the excess charge, and where the in-plane surface stress represents essentially an elastic transverse contraction tendency of the surface layer in response to stretch.

Fast and Reversible Actuation of Metallic Muscles Composed of Nickel Nanowire-Forest.

Surface-charge-induced reversible and millimeter-scale deflection is found in a bilayered Ni cantilever upon cyclic potential triggering. The nanowire-forest structure, in which unidirectional primary nanowires are evenly separated by cross-linking subnanowires, ensures fast ion transport leading to a record-high strain response time ≈0.1 s. The actuation is sustainable beyond 800 cycles; the strain energy is compatible with human skeletal muscles.