Semi-ordered hierarchical metallic network for fast and large charge

Letter pubs.acs.org/NanoLett

Semiordered Hierarchical Metallic Network for Fast and Large Charge-Induced Strain Chuan Cheng,*,† Lukas Lührs,† Tobias Krekeler,‡ Martin Ritter,‡ and Jörg Weissmüller†,§ †

Institute of Materials Physics and Technology and ‡Electron Microscopy Unit, Hamburg University of Technology, 21073 Hamburg, Germany § Institute of Materials Research, Helmholtz-Zentrum Geesthacht, 21502 Geesthacht, Germany S Supporting Information *

ABSTRACT: Nanoporous metallic actuators for artificial muscle applications are distinguished by combining the low operating voltage, which is otherwise reserved for polymerbased 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. KEYWORDS: Artificial muscle, nanoporous metal, electrochemical actuation, hierarchical nanoporous structure, charge-induced strain

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Nanoporous nickel is a candidate for low-cost electrochemical actuation.11,12,17,18 Dealloyed nanoporous nickel with ligament size of 10−20 nm can generate strain amplitudes in the order of 1% in alkaline solution; however, charging times of 800−8000 s were reported, which require improvement.19 Template-deposited nickel nanowire-forests actuate much faster, in the order of 0.1 s; however, their strain amplitude was only 0.01%, a result of the large ligament size (∼100−200 nm).13 The strain amplitudes of electrochemical actuators decrease sharply with increasing of the charging rate,12 especially for ligament size down to 10 nm. In other words, the increased strain amplitude reached by the large surface area is counteracted with the low strain response due to the slow diffusion. Thus, to reach large strain amplitude under fast charging rates, well designed nanoarchitectures, which can facilitate ions transportation, is a critical step. For instance, the tortuosity, which depends on the connectivity and meandering of the pores, equals 1 for straight channels parallel to the flow direction, while it equals 3 for isotropically distributed pores of dealloyed nanoporous gold.20 Ordered nanoporous structures may be made by lithography or templating, yet this limits the sample size. Thus, the challenge is to integrate very small ligaments (e.g., ∼10 nm) into well-organized nanoporous

anoporous and high surface area conductive materials based on carbons1−4 or metals5 exhibit reversible macroscopic volume change when functionalized by contact with electrolyte and subjected to cyclic electrical charging/ discharging processes. They take advantage of the large surfacearea-to-volume-ratio of nanoporous structures to accumulate charge-induced local straining between surface atoms into macroscopic dimension change of the whole sample through reversible electrochemical processes at the electrode/electrolyte interface.6 In contrast to piezoelectric actuators, which can require operating voltages in the order of kV, these nanomaterials work with ∼1 V while generating similar or even larger strain amplitudes. This makes them promising candidates for artificial muscle applications as in microrobotics7 and electrochemo-mechanical systems.8 Compared with the polymerbased low-voltage actuators, metallic actuators are distinguished by their higher strength and stiffness.9,10 Metallic actuators may be further improved by (i) using lowcost nanoporous metals instead of commonly used noble metals;11,12 (ii) designing nanoarchitectures with multiple length scales to accelerate ion transport kinetics,16 which leads to fast strain response;13,14 (iii) decorating surface oxides or slow diffusive elements to stabilize metallic nanoporous structure against coarsening for sustainable actuation;15,16 or (iv) coating by a thick (several molecular spacing) pseudocapacitive layer to enhance the strain amplitude.16 However, so far, one actuating material that can integrate all of the above properties has not been reported. © XXXX American Chemical Society

Received: April 11, 2017 Revised: July 8, 2017 Published: July 24, 2017 A

DOI: 10.1021/acs.nanolett.7b01526 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 1. Characterization of semiordered hierarchical nanoporous nickel with dual-scaled structures. (a) Optical image of a hierarchical nanoporous Ni dealloyed in 0.2 M (NH4)2SO4 at 55 °C for ∼20 h. The inset shows side-view of the sample. (b) SEM top-view of μm-sized tubes with axis normal to the surface of panel a. (c) Enlarged SEM image inside the tube wall of panel b. (d) Tube structure with axis normal to the surface of another nanoporous Ni sample, dealloyed under the sample condition as panel a except a longer time of ∼29 h. The inset shows enlarged tube mouth. (e) TEM image of a tube wall. (f) Enlarged TEM image of nanoporous structure inside the tube wall. Inset shows selected area diffraction pattern within the tube wall. (g−i) TEM-EDX mapping of several nanoligaments within tube wall. More SEM images of the porous structures in various samples were provided in Figures S2 and S5−S7.

lattice parameter is 3.698 Å, consistent with literature data for fcc structured γMn−Ni at this atomic ratio.27 Typical microscopic images of the nanoporous Ni after dealloying are shown in Figure 1. A semiordered pore structure with dual length scales, in which the μm-sized tubes are unidirectional oriented vertical to the sample surface (ordered) and the nm-sized ligaments in the tube walls have a bicontinuous structure (disordered), can be observed. Monolithic nanoporous samples with macroscopic size in 3D have been obtained, see the sample size of 2.2 mm × 1.0 mm × 0.8 mm in Figure 1a. The μm-sized tubes were formed with their axes perpendicular with the surface of the sample (Figures 1b and S2). At the tube mouth, the outer tube diameter is 1.05 ± 0.68 μm and the tube wall thickness is 206 ± 18 nm, where the error bars denote the standard deviation of the values measured from scanning electron microscopy (SEM) images. The tube walls are composed of nm-sized ligaments with a diameter of 10 ± 2.7 nm (Figure 1c). Transmission electron microscopy (TEM) image of a piece of tube wall is shown in Figure 1e, and the corresponding magnified image is shown in Figure 1f. These TEM images confirm the tubes walls are composed of nanoporous structures, which is consistent with the nanoporous structure observed at the tube walls by SEM shown Figure 1c. The diameter of nanoligaments is 100-fold lesser than the outer tube diameter. The inset of Figure 1f shows the selected area diffraction pattern (area ∼800 nm in diameter) of a tube wall. It shows a single, fcc structured crystal. X-ray diffraction confirms the asdealloyed nanoporous sample was fcc structured Ni, with no diffraction peaks from master alloy or metal oxides (later in Figure 2g, black curve). Figure 1g shows a TEM image of several individual ligaments; the corresponding energydispersive X-ray spectroscopy (EDX) map in Figures 1h and 1i show uniform distribution of both Ni and Mn elements. The atomic ratio of Ni/Mn was 96.5:3.5 (Figure S3). Both the Ni crystalline structure and low residual Mn content indicate that the master alloy has been fully dealloyed into nanoporous Ni.

architectures through a high yield process to achieve advanced actuation performances with large strain amplitude and fast strain response. Here, we report a semiordered and hierarchical nanoporous nickel actuator with mm size in three dimensions, synthesized by one-step chemical dealloying of a single-phase alloy. The μm-sized tubes are uniaxially oriented toward the outer sample surface, while the tube walls are composed of nm-sized ligaments, with diameter ∼100 times smaller than that of the tubes. Compared with other hierarchical but random porous structures,19,21,22 the straight and large tube channels are along with the ion migration direction, thus having less hindrance for ions and enhancing the charging/discharging rates,20 while the nanosized ligaments on the tube walls provide sufficient surface area for charge storage and large strain generation. Results and Discussion. Preparation. Our synthesis protocol is described below in the Methods. The one-step chemical dealloying protocol was used, which worked with a single-phase Ni30Mn70 master alloy. This is different from conventional methods, which obtained random hierarchical structures. For instance, hierarchical nanoporous metals were dealloyed either from multiphase alloys through successively etching of different active phases19,21 or from single-phase alloys through multisteps of dealloying/annealing/dealloying processes.14,23,24 Note that hierarchical nanoporous metals with dual microscopic length scale can also be dealloyed from coldrolled alloy precursors, in which the larger length scale inherited from the microband textures of the precursor made by coldrolling.25,26 Differently, here, no cold-rolling was performed to obtain the 3D bulk hierarchical nanoporous metals, in which the channels with larger length scale are along with the ion flow direction. Ni30Mn70 alloy was first casted by induction melting, heat treated to obtain a single-phase, and then cut into 3D millimeter sizes directly from ingot for dealloying (Methods). Microstructure. For the master alloy, X-ray diffraction confirms only one group of peaks corresponding to a facecentered cubic (fcc) crystalline structure (Figure S1). The B


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Figure 2. Electrochemical actuation of semiordered and hierarchical nanoporous Ni actuators. (a) Illustration of in situ setup for measurement of charge-induced strain of the actuators. (b) Cyclic voltammetry (CV) at 1 mV s−1 in the range of [−1.1, 0.7] V versus Hg/HgO for five cycles in 1 M NaOH electrolyte. (c) Strain (right) and potential (left) against time corresponding to panel b. (d) Specific charge against strain relationship corresponding to panel b. (e) Strain against time during CVs in the same potential range as panel b but with scan rates increasing from 5 to 1000 mV s−1. Fifteen cycles were performed at each scan rate. (f) Strain against cycle number (bottom) and time (top) for 10 000 cycles at 100 mV s−1 within the same potential region as panel b. Inset shows a snapshot of 20 cycles. (g) X-ray diffraction of as-dealloyed nanoporous Ni (top) and nanoporous Ni after electrochemical actuation (bottom).

(60 s, which corresponds to H adsorption and H2 evolution, respectively. Formation Mechanism of the Hierarchical Nanoporous Structure. The presence of multiple length scales in our nanoporous structure is unusual and remarkable. One-step dealloying of single-phase solid solutions generally leads to ligaments and pores with only a single, well-defined length scale. Nanoporous Au,37 Pt,29 Au−Pt alloy,16 and Ag38 exemplify this. We emphasize the uniform composition (see the EDX mapping of Figure S9) and large grain size (100 μm according to the electron backscatter diffraction (EBSD) in Figure S10) of the master alloys in our study. The μm-sized tubes (e.g., Figure 1b,d) must therefore form during dealloying and are not related to structure in the master alloy. As a conceivable origin of the tubes, we advertise the large volume shrinkage during dealloying. The lattice parameter of the master alloy, 3.698 Å, exceeds that of the dealloyed nanoporous Ni, 3.524 Å, by 4.94%. Dealloying thus entails a volume reduction by 15.6%, which suggests large stresses at the dealloying front. Crack formation may relieve the shrinkage-induced stress. In fact, ordered crack patterns are ubiquitous in shrinkage scenarios, for instance in basalt columns formed during cooling and mud cracks formed during drying.39−41 Shrinkage cracks tend to propagate vertically to the sample surface, maximizing the stress release or minimizing the strain energy by a characteristic crack spacing and symmetry. These parameters have been found to depend sensitively on stress gradients, which result from humidity gradients during drying or from temperature gradients during cooling.42 For instance, ordered crack patterns, with evenly distributed cracks in the crosssection, can be formed under constant evaporation rate during drying of slurries (e.g., Figure 1e of ref 40); the crack spacing E


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Nano Letters from the EDX measurements (Figures S11c,d and S12c,d), which implies the samples have already been fully dealloyed. From Figures S11a,b and S12a,b, the columns are uniformly nanoporous at this stage. Extended dealloying for 20 h under the same conditions generates the structures of Figure 4f and g. The corrosion has here progressed further and has converted the originally homogeneously nanoporous columns into hollow tubes, which nanoporous structure left on the tube walls. By means of explanation, we present the following tentative picture. The dealloying solution is known to dissolve Ni after Mn is removed.43 The internal tensile stress within each column, which is induced also by volume shrinkage during dealloying, may not be large enough to form subcracks. However, the internal stress may be concentrated in the center of each columns; the extra strain energy would make Ni effectively less noble, facilitating dissolution in the center. Thus, the formation of hierarchical nanoporous nickel involves two successive steps. In the first step, Mn is almost completely removed and the master alloy dealloyed into a porous Ni network at the scale of ∼10 nm. The volume deficit associated with the lattice contraction when going from Mn−Ni to Ni is largely accommodated by an ordered crack pattern, perpendicular to the sample surface. The cracks define parallel ordered columns; their size depends on the dealloying rate. In the second step, the columns are further corroded to form hollow tubes with nanoporous walls. Stress concentration in the center of the columns is a conceivable explanation for the preferential corrosion of the nanoporous Ni there. In summary, semiordered and hierarchical nanoporous nickel with dual-scaled porous structures was obtained by one step dealloying of a single-phase alloy. The well-organized nanoporous structure facilitates fast ion transfer kinetics through large sized tubes with their mouths oriented toward the surface of the sample and the channel parallel with ion diffusion direction, while at the same time, it maintains a large surface area by nanoligaments on the tube walls. By integrating the unique nanoporous structure and a self-grown surpercapacitive hydroxide layer on the surface of nickel, reversible and sustainable actuations with charge-induced strain of 1.59% were obtained, the largest value obtained under the same charging conditions compared with the state-of-the-art electrochemical actuators. The unique hierarchical structure is formed due to the volume shrinkage induced ordered cracks during dealloying. Because of the low cost and high performance, the research may pave a solid foundation toward commercialization of metallic based electrochemical actuators. Methods. Synthesis of Nanoporous Nickel. Ni30Mn70 alloy ingot was casted by an induction furnace (EkoHeat, AMBRELL) in Ar atmosphere by using Ni and Mn metals (>99.99% pure, ChemPUR). The ingot was cut into disks (∼16 mm in diameter and ∼2.5 mm in thickness) by a mechanical saw and annealed at 900 °C for 24 h under vacuum and then quenched in water. The disks were mechanically polished and cut into cuboids by diamond wire saw. Bulk hierarchical nanoporous Ni was synthesized by one-step chemical dealloying in 0.2 M (NH4)2SO4, under constant temperatures of 55 °C with dealloying time from 20−38 h. For dealloying of metal foils, the master alloy was first cool rolled into ∼170 μm thick foil and then annealed under vacuumed at 800 °C for ∼3 h. After that one side was coated with a layer of nail polish for electric isolation, and the other side was electrochemical deposited with a solid Ni layer, under a constant current density of −10 mA cm−2, within an electrolyte of 0.15 M NiSO4

+ 0.6 M HBO3 at room temperature. After that, the nail polish was washed away by acetone. Materials Characterization. The crystalline structures of the master alloy and nanoporous Ni were analyzed by X-ray diffraction in a Bruker D8 powder diffractometer (Cu Kα). For the XRD test, the nanoporous Ni was first compressed into powder so that the inner part of the dealloyed sample can also be characterized. The morphology of nanoporous structures were observed by SEM carried out in a Zeiss Supra 55 VP FEGSEM combined with EDX, and also by TEM carried out in a FEI Talos F200X. The grain size and orientation of the master alloy was determined by Zeiss Supra 55 VP with an Oxford HKL Nordlys EBSD system. The specific surface area of the nanoporous Ni was measured by BET method through nitrogen absorption/desorption processes at 77 K using a Quantachrome Autosorb surface analyzer. Macroscopic photos of bulk nanoprous Ni were captured by optical microscopy (Leica M205C, Germany). Measurement of Actuation. The linear actuation strain of cuboid-shaped bulk nanoporous Ni was measured in situ in an electrochemical cell, combined with a computer controlled dilatometer (Linseis, L75 vertical dilatometer) and a potentiostat (Autolab, PGSTAT302N). A glass push rod, which was connected with the displacement sensor of the dilatometer, had a constant compressive pressure (∼0.2 MPa) on the top surface of the sample to maintain a close contact between the push rod and the sample. The electrochemical cell was filled with 1 M NaOH electrolyte with the nanporous Ni as working electrode, a piece of carbon clothes as counter electrode, and a commercial Hg/HgO reference electrode (Sensortechnik Meinsberg, Germany).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.7b01526. X-ray diffraction, EDX, and EBSD mapping of the master alloy; additional SEM images, EDX, BET characterization, and actuation measurements of dealloyed nanoporous nickel (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. uk. ORCID

Chuan Cheng: 0000-0003-1271-2315 Jörg Weissmüller: 0000-0002-8958-4414 Present Address

C.C., Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, United Kingdom. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C.C. thanks the Humboldt Research Fellowship from Alexander von Humboldt Foundation, Germany. We acknowledge the financial support by DFG via SFB “M3”, subproject B2. F


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Semi-ordered hierarchical metallic network for fast and large charge

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