Nanoporous‐Gold‐Polypyrrole Hybrid Materials for Millimeter‐Sized Free Standing Actuators

Abstract

An important materials parameter of actuators is their strain-charge coupling coefficient, ψ = δε/δq V , where ε denotes the linear, uniaxial strain. For PPy, the magnitude and sign of ψ depend on the synthesis conditions, the species of transferred ions, the solvent and the mechanical boundary conditions. [4,5,11,21-23] Besides the magnitude of the straincharge coupling, the response time is an important characteristic of actuators. For PPy, the underlying charge transport kinetics include both, ionic transport and the formation of electronically conducting zones along the polymer chains. [24,25] If the potential interval is restricted so that the PPy is kept in its oxidized state with high electronic conductivity, the ion exchange is rate controlling. Similar to classical interdiffusion, the time constant for equilibration after a change in E increases with the square of the diffusion distance. [24-26] Acceptable response times thus require small actuator dimensions in at least one direction. Therefore, many studies investigate PPy as a thin film (typically not exceeding 10 µm) on an electrically conductive substrate. Examples for such actuators are bilayered bending cantilevers [27-29] which may exhibit roughened interfaces for enhanced adhesion, [30] multilayered axial strain actuators [31] or coated fibers spun to yarns. [10] Nanometer-thin PPy films allow for particularly fast reaction times. [15,32] For PPy films with a thickness of about 10 µm, the actuation response time increases to minutes. [33,34] Free-standing PPy actuators with a thickness of ≈100 µm and a cross-sectional gradient in microstructure have been investigated in the context of wireless actuation. [35,36] They exhibit similar response times. For even thicker geometries, the response will be prohibitively slow. PPy actuators with spatial dimensions of mm or above can be realized with PPy-coated nanoskeleton structures, such as multi-walled carbon nanotube networks [37] or electrospun microribbons. [38] In these architectures, a liquid electrolyte in the pore space provides a fast pathway for long-range ion transport. Metal-PPy-electrolyte hybrid materials obtained by the imbibition of PPy-coated nanoporous metals [39] represent a particularly interesting case, because their skeleton structure is load-bearing under compression. This enables actuation against an external compressive load by free-standing macroscopic bodies without a supporting device structure. Furthermore, the mechanical behavior of nanoporous metals has been intensively studied. [40-45] This provides the opportunity for a This work studies the actuation of hybrid materials made from nanoporous gold, polypyrrole, and aqueous electrolyte. The deposition protocol affords a conformal polypyrrole coating on the entire internal interface of millimetersized nanoporous metal specimens made by dealloying. The hybrid material emerges when the remaining pore space is filled with perchloric acid. The metal serves as load-bearing and electronically conductive substrate, the polypyrrole...

In Situ Temperature Dependent Optical Characterization and Modeling of Dealloyed Thin‐Film Nanoporous Gold Absorbers

“…L cL σ ( ) = (10) in which "c" is an empirical fit constant. Now, the increase in ligament diameter can account for an increase in surface roughness which allows for formulating a theoretical value of RSR as a function of roughness and ligament diameter using Equations ( 7), ( 8) and ( 10…”

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area, [1] high catalytic activity, [2][3][4][5][6] chemical inertness and size-dependent properties such as plasmonic resonance, [7,8] superior light absorptivity and biocompatibility, and a large selection of surface functionalization chemistries to construct optical biosensors, [9] electromechanical actuators, [10] light-activated nanotherapeutics [11] and thermo-nano-plasmonics. [12] While many of its recent uses are in room-temperature applications, there is an emerging interest in using it at elevated temperatures (i) as selective absorbers in thermophotovoltaics, [13] photothermal therapies [14] and laser micropatterning, [15] and (ii) as catalysts in CO oxidation for hydrogen gas purification.

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Robust Metallic Actuators Based on Nanoporous Gold Rapidly Dealloyed from Gold–Nickel Precursors

“…[ 5a ] Although our ligament size is much larger (120 nm, Figure 2), the strain amplitudes at the same scan rate are two times more, i.e., 5.0 × 10 −4 (in 1 m H 2 SO 4 ) and 5.2 × 10 −4 (in 1 m NaOH). For a np‐Au dealloyed from Au 25 Ag 75 and annealed to the same ligament size of 120 nm, the strain amplitude was 4.7 × 10 −5 at 10 mV s −1 , [ 41 ] i.e., only one‐tenth of our samples. The large strain amplitude here should be attributed to the high relative density (36%, Figure 2) than previously reported np‐Au (25% in ref.…”

“…[ 5a ] and 27% in ref. [ 41 ] ), which leads to good network connectivity so that more effective ligaments contribute to the macroscopic volume change, rather than dangling ligaments [ 42 ] that have free‐ends and do not carry loads or contribute to the structure stiffness (see later for details).…”

Electrochemical Behavior of Nanoporous Gold/Polypyrrole Supercapacitor under Deformation

“…NPG is a bi-continuous network constructed by ligaments and pores, both of which can be tuned from several to hundreds of nm, giving rise to large specific surface area, providing numerous sites for the deposition of foreign active materials [16,[22][23][24][25]. In addition, the porous structure can evidently eliminate structure damage caused by the volume change during electrochemical processes [26] while persevering efficient electrolyte transport.…”