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Sensors and Actuators B 248 (2017) 657–664

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

jo ur nal home page: www.elsev ier .com/ locate /snb

lectrochemical actuation of nickel hydroxide/oxyhydroxide atub-volt voltages

in-Wa Kwan, Nga-Yu Hau, Shien-Ping Feng, Alfonso Hing-Wan Ngan ∗

epartment of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, China

r t i c l e i n f o

rticle history:eceived 13 February 2017ccepted 3 April 2017vailable online 4 April 2017

eywords:

a b s t r a c t

The development of materials capable of actuating at low triggering voltages is crucial for applicationssuch as compact prime movers for soft, micro robots. In this paper, the electrochemical actuation ofNi(OH)2/NiOOH in alkaline environment at low voltages is demonstrated and studied for the first time.Specifically, sub-micron layers of Ni(OH)2/NiOOH deposited on Ni films a few microns thick by anodicelectrodeposition were found to undergo reversible contraction by ∼0.1% in an alkaline solution under

lectrochemical actuatorsow voltagesnodic electrodepositionickel hydroxide/oxyhydroxides

a low voltage of only ∼0.4 V, causing a 5-mm long bi-layer actuator to bend by ∼3.5 mm at its end,giving a large device strain of ∼70%. The actuation mechanism is found to be due to a redox reaction inthe Ni(OH)2/NiOOH couple. By masked electrodeposition, successful fabrication of actuators of variousshapes and actuation performance was also demonstrated. The required electrolyte can be packaged withthe actuating material to achieve a stand-alone actuating device.

rtificial muscles

oft robotics

. Introduction

The last two decades have seen a surge in interest on devel-ping small, compact and self-contained actuating materials forpplications as artificial muscles, to replace bulky pneumatic orydraulic actuators, motors or engines, for robotics [1]. Conven-ional piezoelectric ceramics actuate with large rate response of10%/s and good strains of ∼0.1% which is already close to the elas-ic limit of the material, but they do so only under high triggeringoltages typically in the kV range [2–4]. Dielectric elastomers alsohow similar rate response and even much larger actuating strains,ut again, they typically require kV voltages to actuate [5–9]. Theilo-volt requirements of these materials make them unsuitableor compact designs, as in micro-robots. Of other known types ofctuating materials, a few can actuate at low voltages in the 1–10 Vange. Conducting polymers can actuate from less than a volt to

few volts giving good strains of a few%, but their rate responses slow due to the reliance on ion diffusion within the polymero cause actuation [10–13]. Carbon nanotubes can also actuate at

few volts, with both good strain and rate responses [14–17].anoporous noble metals such as Au and Pt are known to actuate

n electrolytic environments under low voltages by changes in sur-ace stress associated with the electrical double layer (EDL) at thelectrolyte-metal interface [18–22]. On the other hand, nanoporous

∗ Corresponding author.E-mail address: hwngan@hku.hk (A.H.-W. Ngan).

ttp://dx.doi.org/10.1016/j.snb.2017.04.009925-4005/© 2017 Elsevier B.V. All rights reserved.

© 2017 Elsevier B.V. All rights reserved.

Ni, as a non-noble metal much cheaper than noble counterparts, hasalso been found to exhibit electrochemical actuation [23–26]. Cyclicvoltammetry and electrical impedance spectroscopy indicated thata redox couple on the Ni surfaces is responsible for the electrochem-ical actuation [24–26], and Ni(OH)2/NiOOH was suggested as sucha possible redox couple [24–26], although no concrete proof hasbeen obtained so far.

The redox couple Ni(OH)2 ↔ NiOOH is a well-known materialsystem for use as electrodes in nickel–metal hydride recharge-able batteries, in addition to other usages including catalysts andsensors [27]. While the redox reaction in Ni hydride rechargeablebatteries during charging and discharging can cause mechanicalswelling and shrinkage in the electrode which will limit the lifeof the battery [28–31], in the present work, we explore a promis-ing use of such electrochemomechanical effects as electrochemicalactuators. Specifically, the unit cell volume of Ni(OH)2 can decreaseby more than 10% when oxidized into NiOOH [32], and reversibleredox zipping between the two phases Ni(OH)2 and NiOOH isknown to happen within low voltage ranges of less than one volt[33,34]. Such remarkable properties should make Ni(OH)2/NiOOH apromising material system for electrochemical actuation under lowtriggering voltages. In this work, we successfully demonstrate thatNi(OH)2/NiOOH directly fabricated by anodic electrodeposition onNi films can undergo highly reversible and stable electrochemicalactuation under low voltages of only ∼0.4 V in an alkaline medium,

with the intrinsic actuation strain higher than 0.1%. Through properdevice engineering, we also demonstrate that high device strainsapproaching unity can be produced from the intrinsic actuation

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train of such a material system. Furthermore, by applying maskedlectrodeposition in the fabrication, Ni(OH)2/NiOOH actuators ofarious shapes can be built. These show that Ni(OH)2/NiOOH is aromising and versatile material system for developing artificialuscles for soft or micro robots.

. Experimental

.1. Fabrication of the actuators

Ni(OH)2/NiOOH was deposited on thin films of Ni metal the fab-ication of which is first described here. The Ni films were platedn fluorine doped tin oxide (FTO) glass by a Ni Plating Kit (Caswell)sing a CHI 660E (CH Instruments) electrochemical workstation.o make actuators of a cantilever shape, a mask of 50 �m-thickhemical-resist sticker (Max Bepop, CM-200E) with rectangularuts of 1 mm × 15 mm was first adhered to the FTO. The plating bathas a 3-electrode electrolytic cell using a Pt mesh counter elec-

rode. Voltages of −1.2 V vs SCE for various durations ranging from.75 min to 12.5 min were applied to obtain different Ni thicknessNi. After rinsing by DI water, rectangular Ni films with a smoothnd shiny surface were obtained.

For the electrodeposition of Ni(OH)2/NiOOH, lacquer was firstainted on the Ni films (still supported by FTO) so that only

mm × 5 mm areas would be plated with Ni(OH)2/NiOOH. Theame 3-electrode plating bath and electrochemical workstationere used, with the solution changed into 0.13 M NiSO4, 0.1 MaOAc and 0.13 M of Na2SO4. The plating anodic current densitysed was mainly 0.4 mA/cm2 for 30 min under vigorous stirring.

uniform black or dark brown layer, which was subsequentlyharacterized to be Ni(OH)2/NiOOH, was observed. Other currentensities (from 0.04 to 1 mA/cm2) and plating durations (from 5o 50 min) were also studied. After rinsing and detaching the lac-uer and mask by tweezers, the rectangular bilayers comprisingi(OH)2/NiOOH on top of Ni films were carefully peeled off from

he FTO. The uncoated region of Ni was attached to Cu tape (3MTM)o form the cantilever-actuators as shown in the inset of Fig. 1a. Forach plating parameter, 5 samples were made and each was testedor 10 cycles under cyclic voltammetry, giving 50 measurementsor the end-deflection data. The data points in Fig. 3 are obtainedrom the mean of the end-deflection normalized with the length ofhe actuator, with the error bars indicating the standard deviations.

.2. Actuation tests

The actuation tests were carried out in a 3-electrode cellFig. 1a). In each test, a cantilever-actuator was held vertically ashown. Voltage was applied to the actuator by an electrochemicalorkstation (LK2006A, Lanlike) under 1 M NaOH against SCE refer-

nce electrode. The end-deflection of the cantilever-actuators wasideo-recorded by a compact digital camera (DSC-H70, Sony Co.).or some actuators with smaller end deflections, an optical micro-cope (Olympus Co.) was used. A CCD camera (DXC-107P, Sonyo.) was connected to the microscope via a microscope attachmentWV-9005, Matsushita Comm. Industrial Co. Ltd). The capturedideos were processed by a computer freeware Kinovea to obtainhe numerical data for the end-deflection measurement. Fig. 1b,chow the schematic diagram and photographs of an actuator under-oing reduction and oxidization during the test.

.3. Characterization of the actuators

The thickness of the Ni substrate was measured by a DektakXT®

tylus profiler (Bruker). Microstructural characterization was car-ied out by scanning electron microscopy (SEM), transmissionlectron microscopy (TEM), X-ray photoelectron spectroscopy

ators B 248 (2017) 657–664

(XPS) and X-ray diffraction (XRD). A Leo 1530 FEG SEM and a HitachiS4800 FEG SEM were used to image the surface morphologies andthickness of the Ni(OH)2/NiOOH. TEM imaging and selected areaelectron diffraction (SAED) were performed by a JEOL 2010 TEM.The TEM samples were prepared by oxidizing or reducing the elec-trodeposited Ni(OH)2/NiOOH in 1 M NaOH for 30 min at 0.4/0 V vsSCE. Then, the Ni(OH)2/NiOOH layer was scratched off from the Nisubstrate and adhered onto a formvar film.

XPS was performed in a Kratos Axis Ultra Spectrometer usinga monochromatic Al K� source. Glazing incidence XRD was per-formed in a Rigaku Smartlab diffractometer with an incidence angleof 0.5◦ using a monochromatic Cu K� source (� = 1.5406 Å). In addi-tion to the oxidized/reduced Ni(OH)2/NiOOH samples describedabove, a bare Ni sample prepared by the first electrodeposition stepwas also characterized for comparison purposes.

3. Results

3.1. Actuation behavior

Bilayered cantilevers comprising a thin layer of theNi(OH)2/NiOOH actuating material backed by a passive Ni layerwere fabricated as described in Experimental section. On applyinga positive potential to the cantilever actuators in 1 M NaOH solutionas shown in Fig. 1a, they exhibit large bending towards the side ofthe Ni(OH)2/NiOOH layer, corresponding to contractive actuationof the latter (Fig. 1b,c). The actuation is reversible during potentialcycling, as shown in the example in Supplementary Movie 1.Accompanying the actuation, the color of the Ni(OH)2/NiOOHlayer also turned slightly darker upon oxidation (increasing)and paler upon reduction (decreasing potential), as shown inSupplementary Movie 2–the color change here is a good indicationof an electrochemical reaction. The end-deflection per unit lengthof the cantilever-actuator (D/L), used here as a measure of thedevice strain of the cantilever-actuator, was measured under cyclicpotential scanning from 0 to 0.425 V vs SCE at a rate of 25 mV/s. Theresults are plotted against the applied voltage for different Ni filmthickness tNi in Fig. 2a. The actuation was faster for smaller tNi, withthe highest D/L reaching ∼0.7 for the smallest tNi = 1.3 �m studied.The hysteresis in the response is likely caused by time-dependentfactors including kinetic, mass-transfer and ohmic resistances, asquasi-reversibility is observed from the cyclic voltammetry, wherethe ratio of peak currents for oxidation to reduction is smaller than1 and the difference in the peak potentials is larger than 59 mV[35]. Also, redox reaction of Ni(OH)2/NiOOH was shown to exhibittime-independent hysteresis, which is thought to be caused by theion intercalation or phase separation during the reaction [36].

The cyclic voltammetry (CV) in Fig. 2b indicates clearly a redoxreaction with an oxidation peak at ∼0.37 V and a reduction peakat ∼0.25 V, which are broadly similar to other CV reported for theredox couple Ni(OH)2/NiOOH in the literature [34,37]. The chargetransfer to the actuator from the workstation was calculated byintegrating current over time, and as shown in Fig. 2c, the rise andfall of the charge transferred, which corresponds to the progressionof the oxidation reaction (i.e. amount of oxidant formed) accordingto the Faradaic law, coincide with the rise and fall of D/L, indicat-ing that the actuation was mainly caused by the redox reaction,as a result of the volume decrease on oxidation. Fig. 2d shows theD/L and the voltage applied over 10 typical cycles for the 1.3 �mtNi actuator. Fig. S2 in the Supplementary Information shows the

last 36 cycles of a 100-cycle test under 0.2–0.36 V vs SCE voltagecycling at 25 mV/s, for a 1.3 �m tNi cantilever-actuator which hadbeen tested for 400 cycles in prior. The D/L is very steady, indicatingthe high stability of the system.

K.-W. Kwan et al. / Sensors and Actuators B 248 (2017) 657–664 659

Fig. 1. Actuators and actuation test cell. a. Schematic showing the actuation testing system. The working electrode (WE), counter electrode (CE) and reference electrode (RE)of the electrochemical workstation are connected to the cantilever actuator, platinum mesh and standard calomel electrode respectively. The end-deflection of the actuatori t digitb r. c. Phd

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s video-recorded by a CCD camera through a microscope or directly by a compacackground grid of 1 mm grid spacing. b. Schematic diagram of cantilever-actuatouring an actuation test (Scale bar: 5 mm).

Fig. 3a shows the measured device strain D/L in 1 M NaOH ofantilever-actuators with different Ni thicknesses tNi, all with theiri(OH)2/NiOOH layers deposited at 0.4 mA/cm2 for 30 min. It cane seen that the device strain decreases as the Ni layer becomeshicker, which is not surprising since a thicker Ni layer wouldmpose higher rigidity constraint opposing the actuation of thei(OH)2/NiOOH layer. When the actuating layer is thin compared

o the Ni supporting layer, the device strain is given by the Hsuehodel as

D

L≈ 3εox

(EaENi

)Lta

t2Ni

(1)

here �ox is the oxidation actuation strain of the Ni(OH)2/NiOOHayer, Ea,Ni is the Young modulus of the deposited Ni(OH)2/NiOOHr Ni layer and ta,Ni is their thickness (see Supplementary Informa-ion for details). The inset in Fig. 3a shows that the measured devicetrain D/L obeys an inverse relation with tNi2 according to Eq. (1),nd the �ox obtained from the slope is 0.16 ± 0.01%.

Fig. 3b,c show the effects of the plating time and current densityn the thickness of the Ni(OH)2/NiOOH layer and the device strain/L. It can be seen that the Ni(OH)2/NiOOH thickness increasesith plating time. The same also applies to the increasing plating

urrent density except at the highest value of 1 mA/cm2, at whichignificant bubble formation occurred on the plating surface. Theesults show that the device strain D/L generally increases with thei(OH)2/NiOOH thickness, in accordance with Eq. (1). The device

train D/L exhibits large scatter at the plating time of 50 min, athich holes formed on the samples, possibly due to the partial

issolution of Ni by the anodic current during the prolonged elec-rodeposition. Therefore, there is a limiting plating time (40 min)nd plating current density (0.6 mA/cm2) for the electrodepositionf Ni(OH)2/NiOOH.

With the present bilayered cantilever-actuators behaving as aype of prime mover, a relevant property to consider would be theiroad-displacement characteristics. For a skin-effect actuator of aantilever shape, the load is more conveniently represented by an

al camera. Inset shows photograph of a real, typical cantilever-actuator against aotographs captured from a cantilever-actuator when being reduced and oxidized

applied moment M, as shown in Fig. S3 in the Supplementary Infor-mation. To determine the moment vs deflection characteristics ofthe present cantilever-actuator, one way would be to use a testerto apply different values of moment load M to the actuator andsee its response. However, a bending-moment tester is not easy tobuild, and so the moment vs deflection characteristics are consid-ered theoretically in Supplementary Information. A linear relationis found for the bending-moment load vs D/L, giving a parabolicrelation between the external work density and D/L. Therefore, themaximum work density is exerted at halved the maximum devicestrain.

3.2. Self-contained actuating assembly

The bilayered cantilever-actuators described so far requires analkaline electrolyte environment to operate and this may pose alimitation on application. However, the required liquid phase elec-trolyte can be packaged into an assembly of the actuating materialto achieve a stand-alone device that can actuate in dry ambientconditions. One possible design is shown in Fig. 3d. Here, a thin liq-uid film of the 1 M NaOH electrolyte was sandwiched between twobilayered cantilever-actuators of the type shown in Fig. 1b,c; theliquid film was held in place simply by surface-tension effects. Thetwo cantilever-cantilevers were connected to the electrodes of theelectrochemical workstation as shown in Fig. 3d. In this design, theNi(OH)2/NiOOH actuating layers of the two cantilever-actuators areexposed to the electrolyte film and facing each other, so that as ana.c. signal is applied by the electrochemical workstation, the twocantilever-actuators would take turn to actuate on half cycles of theapplied voltage, resulting in oscillatory bending of the device. Snap-shots of the actual operation of the device assembly in air under avoltage scan rate of 1 V/s from −2 V to 2 V are shown in Fig. 3d

together with the corresponding cyclic voltammogram (CV), andSupplementary Movie 3 shows the entire process. Bending of thedevice can be seen to occur in synchrony with the oxidation peaksin the CV under both directions. The device can actually actuate

660 K.-W. Kwan et al. / Sensors and Actuators B 248 (2017) 657–664

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ig. 2. Cyclic actuation test results. a. Device strain D/L of the cantilever-actuatoroltammetry of tNi = 1.3 �m. c. D/L of, and charge transferred across, the cantileverested in CV of 0–0.425 V vs SCE and 25 mV/s, under 1 M NaOH.

ather fast under larger voltage scan rates – Supplementary Movies, 5 and 6 show the operation of the device under 10 V/s, 50/s and00 V/s, respectively, where large actuation amplitudes were stillchieved.

.3. Microstructure of the actuating layer

Fig. 4a,b show that the electrodeposited Ni(OH)2/NiOOH wasn the form of uniform solid layers, and this is due to the vigor-us stirring of the electrolyte during the deposition process. This

s important for the actuation, since Eq. (1) indicates that for theame intrinsic actuation strain �ox, a larger device strain D/L can bebtained by a larger Ea of the actuation layer. Previous attemptso electrodeposit Ni(OH)2/NiOOH using similar electrolyte andoltage-current conditions, but without stirring of the electrolyte,roduced nanoporous structures of the material [38], and this isonfirmed in the present work as discussed in the Supplementarynformation. A nanoporous structure would have a lower Young’s

odulus than a solid layer of the same material, and according toq. (1), the actuation performance of the device will be hampered.

Fig. 4c shows dark-field (DF) TEM images of the reduced and

xidized actuation material taken using portions of the lower-rder diffraction rings in the diffraction patterns as indicated inig. 4d. The larger bright spots indicated by red arrows are there-ore nano-crystals, and it can be seen that the oxidized sample

ferent Ni thickness tNi against the voltage applied. b. The corresponding of cyclictor of tNi = 1.3 �m. d, D/L and the applied voltage against time. The actuation was

contained larger crystals than the reduced sample. The correspond-ing selected area electron diffraction (SAED) patterns are shown inFig. 4d. Broad diffraction rings are seen in the SAED implying lim-ited crystallinity. Comparing the oxidized and reduced states, theradii of the diffraction rings are about the same, although the small-est ring observed in the oxidized sample is not found in the reducedsample. Also, their relative intensities appear to be different, whichagrees with the difference in crystallinity as observed in the DFimages. The inter-planar spacing measured from the ring radii can-not be matched with a single structure among the known phasesof Ni(OH)2, NiOOH and NiO, as shown in Table 1.

The XPS spectrum in Fig. 5a confirms that the first electrode-posited layer was Ni, and the characteristic peaks of Ni 2p spectrumof Ni(OH)2 at 860.9 and 855.6 eV [39] are found in the elec-trodeposited Ni(OH)2/NiOOH layer both after being oxidized andreduced. These peaks are slightly shifted from those of Ni (858.4 and852.6 eV). The O 1s peak at 531.2 eV corresponds to OH− (Fig. 5b),and the oxidized sample has an extra hump at around 529 eV, whichmay correspond to �-NiOOH or NiO [40].

From the XRD results in Fig. 5c, sharp peaks for Ni (111), (200)and (220) planes are observed at 2� = 44.7◦, 52.0◦ and 76.7◦, again

showing that the first electrodeposited layer was Ni. The curvesfor both oxidized and reduced Ni(OH)2/NiOOH show extra peakscompared to that of Ni (Fig. 5d shows enlarged views of the lastpeaks). The 2� of the peak position for the oxidized sample are

K.-W. Kwan et al. / Sensors and Actuators B 248 (2017) 657–664 661

Fig. 3. Device performance under different fabrication conditions. a. Measured peak device strain D/L of cantilever-actuators in 1 M NaOH vs their Ni thickness tNi , with theNi(OH)2/NiOOH layers deposited at 0.4 mA/cm2 for 30 min in all cases. b,c. Effects of plating time (b) and current density (c) on the thickness of Ni(OH)2/NiOOH ta and devicestrain D/L, with tNi = 1.9 �m. The plating current density for b was 0.4 mA/cm2, and the plating time for c was 30 min. (Number of data measurement n = 50 for D/L, n = 10 for ta .Error bars: standard deviation) d. Self-contained actuating assembly – (left) schematic showing construction and (right) snapshots (from Supplementary Movie 3) showingactuation in air and the corresponding cyclic voltammogram under cyclic voltage scan rate of 1 V/s from −2 V to +2 V.

662 K.-W. Kwan et al. / Sensors and Actuators B 248 (2017) 657–664

Fig. 4. Electron microscopy. a–b. SEM images of the electrodeposited Ni(OH)2/NiOOH: cross section (a) and top view (b) (scale bar: 50 nm). c. TEM dark-field (DF) images ofthe reduced and oxidized Ni(OH)2/NiOOH taken using the portions of the diffraction patterns encircled in d (common scale bar: 100 nm). Nano-crystals shown up as brightspots are indicated by red arrows. d. Selected area electron diffraction patterns of the reduced and oxidized Ni(OH)2/NiOOH (common scale bar: 5 nm−1). The dashed circlesindicate the positions of the objective aperture when the DF images in c were taken. The bright nano-crystals in c therefore have crystal structures corresponding to thediffraction rings inside the circles (indicated by arrows). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of thisarticle.)

Table 1The matching of inter-planar spacing measured by SAED and XRD. NiO (JCPDS cards No. 04-0835), �/�-Ni(OH)2 [27], �/�-NiOOH (JCPDS cards No. 06-0075) [47].

Inter-planar spacing measured by SAED (Å) Inter-planar spacing measured by XRD (Å)

Oxidized Reduced Oxidized Reduced

4.93 / 5.37 /2.50 / (111) �-Ni(OH)2 2.34 (102) �-NiOOH 2.56 (111) �-Ni(OH)2

2.10 2.09 (200) NiO(105) �-NiOOH

1.48 1.47 (220) NiO 1.39 (110) �-NiOOH 1.52 (301) �-Ni(OH)2

1s

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(301) �-Ni(OH)2

(111) �-Ni(OH)2

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8.0◦, 38.4◦ and 67.2◦, and 16.5◦, 35.0◦ and 60.9◦ for the reducedample. The corresponding inter-planar spacing is given in Table 1.

From the cyclic actuation tests, the CV in Fig. 2b and theharge against voltage in Fig. 2c show that the actuation wasaused by a redox reaction. The peak potentials match those ofhe Ni(OH)2 ↔ NiOOH redox reaction [34]. However, as is shownn the inset in Fig. 2b, a small hump appeared on the oxidationeak, suggesting two phases in the actuating layer undergoing thexidation reaction. The main peak is likely associated with the oxi-ation of �-Ni(OH)2, while the hump on slightly higher potentialuggests the oxidation of �-Ni(OH)2. In Fig. S4 of the Supplemen-

ary Information, the structure electrodeposited by an unstirredlating bath was shown to be nanoporous, as is similar to Chang38]. The CV of this structure under 1 M NaOH clearly shows twoeaks, which are possibly the redox peaks for the � ↔ � and � ↔ �

(110) �-NiOOH

phase of Ni(OH)2 ↔ NiOOH [41]. This suggests that the redox reac-tions involved �/�-Ni(OH)2 and �/�-NiOOH in the Ni(OH)2/NiOOHlayer. (It is noted that at the same plating current density andtime, the actuation of the nanoporous structure was smaller thanthe solid counterpart, showing that the structure density plays animportant role in the actuation as discussed earlier.) Also, the colorchange observed in the actuation tests agrees with the known elec-trochromic properties of Ni(OH)2/NiOOH [42]. The color changewas less significant in the solid structure; however, an almostreversible change in color can be observed in the nanoporous struc-ture.

Given that in Table 1, the inter-planar spacing measured fromSAED and XRD cannot be matched with a single structure of Nioxide, hydroxides or oxyhydroxides from the literature, the depo-sition may be a mixture of NiO, �/�-Ni(OH)2 and �/�-NiOOH.

K.-W. Kwan et al. / Sensors and Actuators B 248 (2017) 657–664 663

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ig. 5. X-ray Characterization. a–c. Ni 2p (a) and O 1s (b) XPS spectra and grazing

he last XRD peaks for oxidized and reduced Ni(OH)2/NiOOH.

he SAED resembles that of a turbostratic structure with short-r intermediate-range order [43]. The first peak for the XRD ofhe reduced sample could be �- or �-Ni(OH)2; the (100) inter-lanar spacing of the former is 4.62 Å and the (001) of the latter

s 4.61 Å [27]. The higher measured spacing might be caused byhe high degree of hydration that leads to a stacking fault disorder44,45]. Furthermore, from the weak and broad peaks in the XRDFig. 5c,d), the structure of the Ni(OH)2/NiOOH is poorly crystallized46], which agrees with the SAED results. Therefore, the presenti(OH)2/NiOOH layer may contain turbostratic �-Ni(OH)2 [34] andisordered hydroxides/oxyhydroxides. From the 2�-position of theeaks in the XRD results, the inter-planar spacing of the oxidizedample is 8% smaller than that of the reduced sample. The per-entage change is comparable with the known reduction of 10%nd 12% respectively in the lattice constants a and c for the oxida-ion �-Ni(OH)2 → �-NiOOH. For �-Ni(OH)2 → �-NiOOH; the latticeonstant a decreases by 10% while c increases by 4% [32,47]. Theontraction of electrodeposited �-Ni(OH)2 → �-NiOOH had beeneasured by atomic force microscopy [48,49], which agrees in the

rder of magnitude with the present results. Thus, all in all, it isonvincing enough to conclude that the actuating mechanism ishe redox reaction: Ni(OH)2 ↔ NiOOH.

. Discussion

The intrinsic actuating strain of ∼0.16% of the presenti(OH)2/NiOOH is comparable to some of the nanoporous metals,

nce XRD (c) for Ni and oxidized and reduced Ni(OH)2/NiOOH. d. Enlarged views of

but is lower than the best nanoporous metals [22] or electroactivepolymers [1]. The main advantage of the present material, how-ever, is the low voltage of ∼0.4 V required to trigger the actuation.The intrinsic actuation strain in fact does not matter, as it is alsoshown here that an intrinsic strain of ∼0.16% can be engineered togive a large device strain of ∼70%, by a bilayered design of the can-tilever actuators. As indicated by Eq. (1), such a large device strainis accomplished by a large L/tNi ratio of ∼3800 in the present study,in addition to using vigorous electrolyte stirring to achieve a solidactuation layer with a higher Ea as discussed above. A further advan-tage of the present actuating material is that it can be fabricatedeasily into different ratios of thickness and various shapes. As thethickness of Ni(OH)2/NiOOH is linearly related to the plating timeand plating current density (Fig. 3), actuators with a desired devicestrain or even actuation shape can be easily fabricated. As shown inthe Supplementary Movie 7–9: branched-, circular- and star-shapeactuators have been successfully fabricated and demonstrated. Fur-thermore, it has been demonstrated that, through packaging theliquid electrolyte into an assembly, self-contained devices that canactuate in dry environments can be achieved. These show the ver-satile potential of the present material system for use as artificialmuscles in micro or soft robotics. However, as an electrochemi-cal actuating material, a limitation of the present Ni(OH)2/NiOOH

material system is that it requires an alkaline electrolyte environ-ment to operate. Future work can focus on packaging the materialwith a closed material system that provides the electrolytic envi-ronment in a self-contained way.

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[during electrochemical oxidation-reduction monitored by in-situatomic-Force microscopy, J. Electroanal. Chem. 385 (2) (1995) 273–277.

[49] Y. Hu, D.A. Scherson, Potential-induced plastic deformations of nickel hydrouselectrodes in alkaline electrolytes: an in situ atomic force microscopy study, J.

64 K.-W. Kwan et al. / Sensors an

. Conclusions

We have demonstrated the electrochemical actuation of elec-rodeposited Ni(OH)2/NiOOH with an intrinsic actuating strain�ox) of ∼0.16%, and developed cantilever-actuators made ofi(OH)2/NiOOH on Ni which can actuate with a device strain (D/L)s large as 70%, at just ∼0.4 V in NaOH. The actuation mechanisms the redox reaction Ni(OH)2 ↔ NiOOH involving volume change.he Ni(OH)2/NiOOH layer was found to comprise multiple phases of

ow-crystallinity Ni(OH)2, NiOOH, and possibly NiO. The thicknessatios of the Ni(OH)2/NiOOH and Ni layers can be easily varied forontrol of the device strain, load-bearing capability and work den-ity, and masked electrodeposition of Ni(OH)2/NiOOH also allowsctuators of various 2-D shapes to be made. The alkaline electrolyteequired for the electrochemical actuation can be packaged in anssembly of the actuating material to achieve a device that canctuate in air in a self-contained way.

cknowledgements

The work was funded by a grant from the Research Grants Coun-il (Project No. 17206114) of the Hong Kong Special Administrativeegion, as well as the Kingboard Endowed Professorship in Mate-ials Engineering. We thank F.Y.F. Chan for help with TEM samplereparation, D.Y.K. Mak and R.K.M. Ho for help with TEM operationnd N.K.C. Ho for help with XPS operation.

ppendix A. Supplementary data

Supplementary Information and Supplementary Movie 1–9ssociated with this article can be found, in the online version, atttp://dx.doi.org/10.1016/j.snb.2017.04.009.

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