<p>This work shows that a nano-coating of electrochromic polymer grown onto the ligaments of nanoporous gold causes reversible dimensional and color changes during electrochemical actuation. This combination of electromechanical and optical properties opens additional avenues for the applications of artificial muscles, i.e., a metallic muscle exhibits its progress during work by changing color that can be detected by optical means. (C) 2013 AIP Publishing LLC.</p>

De Hosson, J. T. M.

Detsi, E.

Onck, P. R.

This work shows that a nano-coating of electrochromic polymer grown onto the ligaments of nanoporous gold causes reversible dimensional and color changes during electrochemical actuation. This combination of electromechanical and optical properties opens additional avenues for the applications of artificial muscles, i.e., a metallic muscle exhibits its progress during work by changing color that can be detected by optical means. (C) 2013 AIP Publishing LLC

ARTS repository - University of Groningen

   University of GroningenElectrochromic artificial muscles based on nanoporous metal-polymer compositesDetsi, E.; Onck, P. R.; De Hosson, J. T. M.Published in:Applied Physics LettersDOI:10.1063/1.4827089IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2013Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Detsi, E., Onck, P. R., & De Hosson, J. T. M. (2013). Electrochromic artificial muscles based onnanoporous metal-polymer composites. Applied Physics Letters, 103(19), 193101-1-193101-3. [193101].https://doi.org/10.1063/1.4827089CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 07-06-2022Electrochromic artificial muscles based on nanoporous metal-polymercompositesE. Detsi, P. R. Onck, and J. T. M. De Hossona)Department of Applied Physics, Zernike institute for Advanced Materials, University of Groningen,Nijenborgh 4, 9747 AG Groningen, The Netherlands(Received 17 February 2013; accepted 20 August 2013; published online 4 November 2013)This work shows that a nano-coating of electrochromic polymer grown onto the ligaments ofnanoporous gold causes reversible dimensional and color changes during electrochemicalactuation. This combination of electromechanical and optical properties opens additional avenuesfor the applications of artificial muscles, i.e., a metallic muscle exhibits its progress during workby changing color that can be detected by optical means. VC 2013 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4827089]Electrochemical actuators base on nanoporous metalswith high surface-area-to-volume ratios, so-called metallicmuscles,1 represent a class of smart materials that undergoreversible dimensional changes upon changes in their inter-face electronic charge densities.2–6 Here, we show that thesemetallic muscles become smarter when a thin electrochromicpolymer coating is grown onto the ligaments of nanoporousmetals: In addition, to the reversible dimensional changestaking place in the nanoporous metal during electrochemicalactuation,2–6 the polymer coating undergoes reversiblechanges in color upon electro-oxidation/reduction.7,8 In thisletter, we use nanoporous gold (NPG) as metallic muscle andpolyaniline (PANI) as electrochromic conducting polymer todemonstrate the proof of principle.A 60mm-long metallic cantilever having a bilayerconfiguration was made.5,9 The active part consists of a thickNPG layer with thickness 20 lm and the support is madeof a relatively thin dense gold layer (7 lm). A schematicillustration of the bilayer strip cantilever is shown in Figure1(a) and the typical bicontinuous nanoporous morphology ofthe active layer is displayed by the scanning electron micro-graph of Figure 1(b). The average size of the ligaments andpores is 25 nm. A PANI coating with thickness between5 and 10 nm was grown onto these ligaments by electro-chemical oxidative polymerization of aniline from an aque-ous solution containing 0.5M of H2SO4 and 50mM ofaniline monomer.10 The scanning and transmission electronmicrographs of Figures 1(c) and 1(d), respectively, display atypical 5 nm thick uniform PANI coating grown onto theligaments of NPG.The first 10 cyclic voltammograms recorded during thegrowth of PANI onto the ligaments of NPG are shown inFigure 2. In the first cycle (red curve), the anodic depositioncurrent increases at 1.1V (versus Ag/AgCl referenceelectrode); this indicates that aniline initially oxidizes at arelatively high potential (1.1V). A similar behavior hasbeen reported during electrochemical deposition of anilineon diamond nanoparticles,11 and on noble metals includingbare platinum.12 The oxidation of aniline at a relatively highvoltage gives rise to degradation products, which are depos-ited onto the ligaments of NPG during subsequent cycles.This is illustrated by the two anodic peaks observed at0.5V and 0.7V during the second and third cycles (bluecurves).11 The standard two pairs of redox peaks that charac-terize the oxidation (oxi) and reduction (red) of aniline atappropriate redox potentials, i.e., oxi1/red1 at 0.3V/0.1Vand oxi2/red2 at 0.8V/0.6V only dominate from the fourthcycle (black curves). These two pairs of peaks are less pro-nounced compared to those reported during electrochemicaldeposition of aniline monomers onto bare noble metals elec-trode such as platinum.12 In fact, the positions and ampli-tudes of these two pairs of redox peaks are well-known to beaffected by the nature of the substrate onto which PANI isgrown. The gradual increase in the current intensities of thevoltammograms during subsequent cycles is commonlyattributed to the auto-catalytic property of PANI forpolymerization.In the following, we present the simultaneous reversiblechanges in dimension and in color in our NPG/PANI hybridmaterial. We concentrate on a qualitative description of thematerial response; it is referred to Refs. 2–6 for quantitativeFIG. 1. Microstructural characterization. (a) The as-synthesized bilayer stripcantilever is initially bent because during dealloying the thick NPG layershrinks,6,13 whereas the thin dense gold layer does not. (b) Scanning electronmicrograph showing the typical bicontinuous morphology of the thick NPGlayer. (c, d) Scanning and transmission electron micrographs showing a typi-cal 5 nm-thick PANI coating covering the ligaments of NPG.a)Author to whom correspondence should be addressed. Electronic mail:j.t.m.de.hosson@rug.nl. Tel.: þ31-50-363 4898. Fax: þ31-50-363 4881.0003-6951/2013/103(19)/193101/3/$30.00 VC 2013 AIP Publishing LLC103, 193101-1APPLIED PHYSICS LETTERS 103, 193101 (2013) This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:129.125.63.113 On: Thu, 14 Nov 2013 09:52:48studies on electrochemical actuation in nanoporous metals.After electrochemical polymerization, the NPG/PANI isactuated in the potential range between ÿ0.22V and 1.1V(versus Ag/AgCl reference electrode), using as electrolyte afresh aqueous solution containing 0.5M of H2SO4 (withoutaniline). Note that the as-synthesized bilayer strip cantileveris initially bent as schematized in Figure 1(a). This isbecause during the dealloying process the thick nanoporousgold layer contracts, i.e., the well-known volume contractionduring dealloying,6,13 whereas the dense gold support doesnot. Typically, the as-synthesized green NPG/PANI cantile-ver expands during the forward scan and at the meantime itbecomes dark blue. During reverse process the cantilever(initially dark blue) bends back to the initial shape and at themeantime it recovers its initial green color. This combinationof electromechanical and optical properties broadens thepotential of metallic muscles for many applications. Astraightforward application might involve metallic musclesthat can communicate while performing work, e.g., duringwork, the metallic muscle displays a certain color (in thiscase dark blue); when the work is done a different color isdisplayed (in this case green). Although electrochromicactuators can be made from pure polymer, the combinationof a metal and a polymer for electrochromic actuation pur-poses is more attractive, because metallic actuators are stron-ger and stiffer than polymer actuators.3The physical origin of the simultaneous changes indimension and in color of the cantilever is the following:The NPG/PANI hybrid material takes advantage of theactuation ability of nanoporous metals on the one hand,2–6and the electrochromic properties of PANI on the otherhand.7,8 Starting with electrochemical actuation, Figure 3(a)displays the cyclic voltammograms recorded during thefirst 10 actuation cycles. These voltammograms display apseudo-capacitive behavior associated with charge storageby the NPG/PANI composite.10 Chen and co-workers haverecently reported on a strong enhancement of the charge stor-age capacity of NPG/PANI hybrid materials in aqueouselectrolyte.10 This storage (release) of electronic charge bythe NPG/PANI composite satisfies the requirement for re-versible dimensional changes in conducting high surface-area-to-volume ratio nanostructured materials,2–6 whichexplains the expansion and bending of our cantilever.In addition to these reversible dimensional changes, thecantilever also undergoes reversible changes in color asschematized in Figure 3(b). As aforementioned, thesechanges in color are attributed to the electrochromic propertyof the PANI coating. Indeed, the color of PANI depends onits oxidation state and common colors include a dark bluecolor when PANI is fully oxidized (perningraniline), a greencolor when PANI is partially oxidized and doped (emeral-dine salt), and a transparent color when PANI is notoxidized, which corresponds to the fully reduced form ofPANI (leucoemeraldine).7,8,14Before actuation, the NPG/PANI cantilever may displaya green or a dark blue color depending on the doping state ofPANI. In the present case, the as-synthesized NPG/PANIcantilever is green, i.e., at the end of the electropolymeriza-tion procedure, the PANI coating is partially oxidized anddoped with H2SO4 (emeraldine salt).7,8,14 Due to the electro-chromic property of the PANI coating, the starting greencolor of our cantilever is susceptible to change to dark blueif PANI is further oxidized from its partial oxidation level toits full oxidation state. It is evident from Figure 3(a) that thetwo pairs of redox peaks oxi1/red1 and oxi2/red2 observedduring electrochemical polymerization from Figure 2 are stillpresent during electrochemical actuation, which indicatesthat PANI is being oxidized and reduced during actuation.These two redox activities in the PANI coating are responsi-ble for the reversible changes in color observed in the canti-lever during actuation: (A) During the forward anodic scanwhere the cantilever expands, green and dark blue colora-tions are observed (see illustration in Figure 3(b)). These twocolors are justified by the following: (i) when the cantileverstarts expanding, its initial green color is preserved at thefirst oxidation peak (i.e., oxi1 in Figure 3(a)). This is becausethat peak corresponds to the oxidation of residual transparentleucoemeraldine (fully reduced form of PANI) to the greenFIG. 3. Simultaneous reversible changesin dimension and in color: (a) Cyclicvoltammograms recorded during the first10 actuation cycles. (b) Simultaneouschanges in color and in dimension in thebilayer cantilever.FIG. 2. Electrochemical polymerization curves. First 10 cyclic voltammo-grams recorded during the growth of PANI onto the ligaments of nanopo-rous gold.193101-2 Detsi, Onck, and De Hosson Appl. Phys. Lett. 103, 193101 (2013) This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:129.125.63.113 On: Thu, 14 Nov 2013 09:52:48emeraldine (partially oxidized form).15,16 (ii) When the can-tilever continues to expand, its color changes from green todark blue. This is because at the second oxidation peak (i.e.,oxi2 in Figure 3(a)), the green emeraldine (partially oxidizedform of PANI) is further oxidized to dark blue perningrani-line (fully oxidized form of PANI).15,16 (B) During thereverse cathodic scan where the cantilever bends back to itsinitial shape, dark blue, and green colorations are observed(see illustration in Figure 3(b)). These two colors are justifiedas follows: (i) when the cantilever starts bending back to itsinitial shape, the starting dark blue color is preserved despitethe reduction activity present at the beginning of the reversesweep as illustrated by the second reduction peak (i.e., red2in Figure 3(a)). This dark blue color is still present becausethe peak red2 corresponds to the reduction of residual greenemeraldine (partially oxidized form of PANI) to the transpar-ent leucoemeraldine (fully reduced form of PANI).15,16Since the residual leucoemeraldine is colorless, the dark bluecolor of perningraniline (fully oxidized form of PANI) is stilldisplayed. (ii) As the cantilever continues to bend back, itscolor changes from dark blue to green. This is because at thefirst reduction peak (i.e., red1 in Figure 3(a)), the dark blueperningraniline (fully oxidized form of PANI) is partiallyreduced to the green emeraldine (partially oxidized form ofPANI).15,16 The redox activities at oxi1/red1 and oxi2/red2clearly justify the reversible changes in color observed dur-ing the relaxation and bending of the cantilever. These twophenomena (reversible dimensional and color changes) arewell-reproducible over many cycles because of the high sta-bility of PANI.In summary, we have demonstrated that a thin polymercoating can be exploited to add an additional functionality tonanoporous metals electrochemical actuators. By doing so, ametallic muscle becomes smarter because in addition to itsreversible dimensional changes, it also undergoes reversiblechanges in color. This combination of electromechanical andoptical properties opens other avenues for the applications ofartificial muscles, i.e., a metallic muscle exhibits its progressduring work by changing color that can be detected by opti-cal means.1R. H. Baughman, Science 300, 268 (2003).2J. Weissm€uller, R. N. Viswanath, D. Kramer, P. Zimmer, R. W€urschum,and H. Gleiter, Science 300, 312 (2003).3H.-J. Jin, X.-L. Wang, S. Parida, K. Wang, M. Seo, and J. Weissm€uller,Nano Lett. 10, 187 (2010).4J. Zhang, P. Liu, H. Ma, and Y. Ding, J. Phys. Chem. C 111, 10382(2007).5D. Kramer, R. N. Viswanath, and J. Weissm€uller, Nano Lett. 4, 793(2004).6E. Detsi, S. Punzhin, J. Rao, P. R. Onck, and J. T. M. De Hosson, ACSNano 6, 3734 (2012).7M. M. Gvozdenović, B. Z. Jugović, J. S. Stevanović, T. L. Trisović,and B. N. Grgur, in Electropolymerization, edited by E. Schab-Balcerzak(In Tech, 2011), Chap. IV, pp. 77–87.8L. Zhao, L. Zhao, Y. Xu, T. Qiu, L. Zhi, and G. Shi, Electrochem. Acta55, 491 (2009).9E. Detsi, Z. G. Chen, W. P. Vellinga, P. R. Onck, and J. T. M. De Hosson,Appl. Phys. Lett. 99, 083104 (2011).10X. Lang, L. Zhang, T. Fujita, Y. Ding, and M. Chen, J. Power Sources197, 325 (2012).11X. Y. Zhao, J. B. Zang, Y. H. Wang, L. Y. Bian, and J. K. Yu,Electrochem. Commun. 11, 1297 (2009).12A. S. Sarac, M. Ates, and B. Kilic, Int. J. Electrochem. Sci. 3, 777 (2008).13S. Parida, D. Kramer, C. A. Volkert, H. R€osner, J. Erlebacher, and J.Weissm€uller, Phys. Rev. Lett. 97, 035504 (2006).14J. Stejskal, P. Kratochvıl, and A. Jenkins, Polymer 37, 367 (1996).15C. V. Tuan, M. A. Tuan, N. V. Hieu, and T. Trung, Curr. Appl. Phys. 12,1011 (2012).16M. H. Pournaghi-Azar and B. Habibi, Electrochem. Acta 52, 4222 (2007).193101-3 Detsi, Onck, and De Hosson Appl. Phys. Lett. 103, 193101 (2013) This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:129.125.63.113 On: Thu, 14 Nov 2013 09:52:48

English

Electrochromic artificial muscles based on nanoporous metal-polymer composites

E. Detsi

P. R. Onck

J. T. M. De Hosson

Crossref

Applied Physics Letters

University of Groningen Digital Archive

   University of GroningenElectrochromic artificial muscles based on nanoporous metal-polymer compositesDetsi, E.; Onck, P. R.; De Hosson, J. T. M.Published in:Applied Physics LettersDOI:10.1063/1.4827089IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2013Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Detsi, E., Onck, P. R., & De Hosson, J. T. M. (2013). Electrochromic artificial muscles based onnanoporous metal-polymer composites. Applied Physics Letters, 103(19), 193101-1-193101-3. [193101].https://doi.org/10.1063/1.4827089CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 17-07-2023Electrochromic artificial muscles based on nanoporous metal-polymercompositesE. Detsi, P. R. Onck, and J. T. M. De Hossona)Department of Applied Physics, Zernike institute for Advanced Materials, University of Groningen,Nijenborgh 4, 9747 AG Groningen, The Netherlands(Received 17 February 2013; accepted 20 August 2013; published online 4 November 2013)This work shows that a nano-coating of electrochromic polymer grown onto the ligaments ofnanoporous gold causes reversible dimensional and color changes during electrochemicalactuation. This combination of electromechanical and optical properties opens additional avenuesfor the applications of artificial muscles, i.e., a metallic muscle exhibits its progress during workby changing color that can be detected by optical means. VC 2013 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4827089]Electrochemical actuators base on nanoporous metalswith high surface-area-to-volume ratios, so-called metallicmuscles,1 represent a class of smart materials that undergoreversible dimensional changes upon changes in their inter-face electronic charge densities.2–6 Here, we show that thesemetallic muscles become smarter when a thin electrochromicpolymer coating is grown onto the ligaments of nanoporousmetals: In addition, to the reversible dimensional changestaking place in the nanoporous metal during electrochemicalactuation,2–6 the polymer coating undergoes reversiblechanges in color upon electro-oxidation/reduction.7,8 In thisletter, we use nanoporous gold (NPG) as metallic muscle andpolyaniline (PANI) as electrochromic conducting polymer todemonstrate the proof of principle.A 60mm-long metallic cantilever having a bilayerconfiguration was made.5,9 The active part consists of a thickNPG layer with thickness 20 lm and the support is madeof a relatively thin dense gold layer (7 lm). A schematicillustration of the bilayer strip cantilever is shown in Figure1(a) and the typical bicontinuous nanoporous morphology ofthe active layer is displayed by the scanning electron micro-graph of Figure 1(b). The average size of the ligaments andpores is 25 nm. A PANI coating with thickness between5 and 10 nm was grown onto these ligaments by electro-chemical oxidative polymerization of aniline from an aque-ous solution containing 0.5M of H2SO4 and 50mM ofaniline monomer.10 The scanning and transmission electronmicrographs of Figures 1(c) and 1(d), respectively, display atypical 5 nm thick uniform PANI coating grown onto theligaments of NPG.The first 10 cyclic voltammograms recorded during thegrowth of PANI onto the ligaments of NPG are shown inFigure 2. In the first cycle (red curve), the anodic depositioncurrent increases at 1.1V (versus Ag/AgCl referenceelectrode); this indicates that aniline initially oxidizes at arelatively high potential (1.1V). A similar behavior hasbeen reported during electrochemical deposition of anilineon diamond nanoparticles,11 and on noble metals includingbare platinum.12 The oxidation of aniline at a relatively highvoltage gives rise to degradation products, which are depos-ited onto the ligaments of NPG during subsequent cycles.This is illustrated by the two anodic peaks observed at0.5V and 0.7V during the second and third cycles (bluecurves).11 The standard two pairs of redox peaks that charac-terize the oxidation (oxi) and reduction (red) of aniline atappropriate redox potentials, i.e., oxi1/red1 at 0.3V/0.1Vand oxi2/red2 at 0.8V/0.6V only dominate from the fourthcycle (black curves). These two pairs of peaks are less pro-nounced compared to those reported during electrochemicaldeposition of aniline monomers onto bare noble metals elec-trode such as platinum.12 In fact, the positions and ampli-tudes of these two pairs of redox peaks are well-known to beaffected by the nature of the substrate onto which PANI isgrown. The gradual increase in the current intensities of thevoltammograms during subsequent cycles is commonlyattributed to the auto-catalytic property of PANI forpolymerization.In the following, we present the simultaneous reversiblechanges in dimension and in color in our NPG/PANI hybridmaterial. We concentrate on a qualitative description of thematerial response; it is referred to Refs. 2–6 for quantitativeFIG. 1. Microstructural characterization. (a) The as-synthesized bilayer stripcantilever is initially bent because during dealloying the thick NPG layershrinks,6,13 whereas the thin dense gold layer does not. (b) Scanning electronmicrograph showing the typical bicontinuous morphology of the thick NPGlayer. (c, d) Scanning and transmission electron micrographs showing a typi-cal 5 nm-thick PANI coating covering the ligaments of NPG.a)Author to whom correspondence should be addressed. Electronic mail:j.t.m.de.hosson@rug.nl. Tel.: þ31-50-363 4898. Fax: þ31-50-363 4881.0003-6951/2013/103(19)/193101/3/$30.00 VC 2013 AIP Publishing LLC103, 193101-1APPLIED PHYSICS LETTERS 103, 193101 (2013) This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:129.125.63.113 On: Thu, 14 Nov 2013 09:52:48studies on electrochemical actuation in nanoporous metals.After electrochemical polymerization, the NPG/PANI isactuated in the potential range between ÿ0.22V and 1.1V(versus Ag/AgCl reference electrode), using as electrolyte afresh aqueous solution containing 0.5M of H2SO4 (withoutaniline). Note that the as-synthesized bilayer strip cantileveris initially bent as schematized in Figure 1(a). This isbecause during the dealloying process the thick nanoporousgold layer contracts, i.e., the well-known volume contractionduring dealloying,6,13 whereas the dense gold support doesnot. Typically, the as-synthesized green NPG/PANI cantile-ver expands during the forward scan and at the meantime itbecomes dark blue. During reverse process the cantilever(initially dark blue) bends back to the initial shape and at themeantime it recovers its initial green color. This combinationof electromechanical and optical properties broadens thepotential of metallic muscles for many applications. Astraightforward application might involve metallic musclesthat can communicate while performing work, e.g., duringwork, the metallic muscle displays a certain color (in thiscase dark blue); when the work is done a different color isdisplayed (in this case green). Although electrochromicactuators can be made from pure polymer, the combinationof a metal and a polymer for electrochromic actuation pur-poses is more attractive, because metallic actuators are stron-ger and stiffer than polymer actuators.3The physical origin of the simultaneous changes indimension and in color of the cantilever is the following:The NPG/PANI hybrid material takes advantage of theactuation ability of nanoporous metals on the one hand,2–6and the electrochromic properties of PANI on the otherhand.7,8 Starting with electrochemical actuation, Figure 3(a)displays the cyclic voltammograms recorded during thefirst 10 actuation cycles. These voltammograms display apseudo-capacitive behavior associated with charge storageby the NPG/PANI composite.10 Chen and co-workers haverecently reported on a strong enhancement of the charge stor-age capacity of NPG/PANI hybrid materials in aqueouselectrolyte.10 This storage (release) of electronic charge bythe NPG/PANI composite satisfies the requirement for re-versible dimensional changes in conducting high surface-area-to-volume ratio nanostructured materials,2–6 whichexplains the expansion and bending of our cantilever.In addition to these reversible dimensional changes, thecantilever also undergoes reversible changes in color asschematized in Figure 3(b). As aforementioned, thesechanges in color are attributed to the electrochromic propertyof the PANI coating. Indeed, the color of PANI depends onits oxidation state and common colors include a dark bluecolor when PANI is fully oxidized (perningraniline), a greencolor when PANI is partially oxidized and doped (emeral-dine salt), and a transparent color when PANI is notoxidized, which corresponds to the fully reduced form ofPANI (leucoemeraldine).7,8,14Before actuation, the NPG/PANI cantilever may displaya green or a dark blue color depending on the doping state ofPANI. In the present case, the as-synthesized NPG/PANIcantilever is green, i.e., at the end of the electropolymeriza-tion procedure, the PANI coating is partially oxidized anddoped with H2SO4 (emeraldine salt).7,8,14 Due to the electro-chromic property of the PANI coating, the starting greencolor of our cantilever is susceptible to change to dark blueif PANI is further oxidized from its partial oxidation level toits full oxidation state. It is evident from Figure 3(a) that thetwo pairs of redox peaks oxi1/red1 and oxi2/red2 observedduring electrochemical polymerization from Figure 2 are stillpresent during electrochemical actuation, which indicatesthat PANI is being oxidized and reduced during actuation.These two redox activities in the PANI coating are responsi-ble for the reversible changes in color observed in the canti-lever during actuation: (A) During the forward anodic scanwhere the cantilever expands, green and dark blue colora-tions are observed (see illustration in Figure 3(b)). These twocolors are justified by the following: (i) when the cantileverstarts expanding, its initial green color is preserved at thefirst oxidation peak (i.e., oxi1 in Figure 3(a)). This is becausethat peak corresponds to the oxidation of residual transparentleucoemeraldine (fully reduced form of PANI) to the greenFIG. 3. Simultaneous reversible changesin dimension and in color: (a) Cyclicvoltammograms recorded during the first10 actuation cycles. (b) Simultaneouschanges in color and in dimension in thebilayer cantilever.FIG. 2. Electrochemical polymerization curves. First 10 cyclic voltammo-grams recorded during the growth of PANI onto the ligaments of nanopo-rous gold.193101-2 Detsi, Onck, and De Hosson Appl. Phys. Lett. 103, 193101 (2013) This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:129.125.63.113 On: Thu, 14 Nov 2013 09:52:48emeraldine (partially oxidized form).15,16 (ii) When the can-tilever continues to expand, its color changes from green todark blue. This is because at the second oxidation peak (i.e.,oxi2 in Figure 3(a)), the green emeraldine (partially oxidizedform of PANI) is further oxidized to dark blue perningrani-line (fully oxidized form of PANI).15,16 (B) During thereverse cathodic scan where the cantilever bends back to itsinitial shape, dark blue, and green colorations are observed(see illustration in Figure 3(b)). These two colors are justifiedas follows: (i) when the cantilever starts bending back to itsinitial shape, the starting dark blue color is preserved despitethe reduction activity present at the beginning of the reversesweep as illustrated by the second reduction peak (i.e., red2in Figure 3(a)). This dark blue color is still present becausethe peak red2 corresponds to the reduction of residual greenemeraldine (partially oxidized form of PANI) to the transpar-ent leucoemeraldine (fully reduced form of PANI).15,16Since the residual leucoemeraldine is colorless, the dark bluecolor of perningraniline (fully oxidized form of PANI) is stilldisplayed. (ii) As the cantilever continues to bend back, itscolor changes from dark blue to green. This is because at thefirst reduction peak (i.e., red1 in Figure 3(a)), the dark blueperningraniline (fully oxidized form of PANI) is partiallyreduced to the green emeraldine (partially oxidized form ofPANI).15,16 The redox activities at oxi1/red1 and oxi2/red2clearly justify the reversible changes in color observed dur-ing the relaxation and bending of the cantilever. These twophenomena (reversible dimensional and color changes) arewell-reproducible over many cycles because of the high sta-bility of PANI.In summary, we have demonstrated that a thin polymercoating can be exploited to add an additional functionality tonanoporous metals electrochemical actuators. By doing so, ametallic muscle becomes smarter because in addition to itsreversible dimensional changes, it also undergoes reversiblechanges in color. This combination of electromechanical andoptical properties opens other avenues for the applications ofartificial muscles, i.e., a metallic muscle exhibits its progressduring work by changing color that can be detected by opti-cal means.1R. H. Baughman, Science 300, 268 (2003).2J. Weissm€uller, R. N. Viswanath, D. Kramer, P. Zimmer, R. W€urschum,and H. Gleiter, Science 300, 312 (2003).3H.-J. Jin, X.-L. Wang, S. Parida, K. Wang, M. Seo, and J. Weissm€uller,Nano Lett. 10, 187 (2010).4J. Zhang, P. Liu, H. Ma, and Y. Ding, J. Phys. Chem. C 111, 10382(2007).5D. Kramer, R. N. Viswanath, and J. Weissm€uller, Nano Lett. 4, 793(2004).6E. Detsi, S. Punzhin, J. Rao, P. R. Onck, and J. T. M. De Hosson, ACSNano 6, 3734 (2012).7M. M. Gvozdenović, B. Z. Jugović, J. S. Stevanović, T. L. Trisović,and B. N. Grgur, in Electropolymerization, edited by E. Schab-Balcerzak(In Tech, 2011), Chap. IV, pp. 77–87.8L. Zhao, L. Zhao, Y. Xu, T. Qiu, L. Zhi, and G. Shi, Electrochem. Acta55, 491 (2009).9E. Detsi, Z. G. Chen, W. P. Vellinga, P. R. Onck, and J. T. M. De Hosson,Appl. Phys. Lett. 99, 083104 (2011).10X. Lang, L. Zhang, T. Fujita, Y. Ding, and M. Chen, J. Power Sources197, 325 (2012).11X. Y. Zhao, J. B. Zang, Y. H. Wang, L. Y. Bian, and J. K. Yu,Electrochem. Commun. 11, 1297 (2009).12A. S. Sarac, M. Ates, and B. Kilic, Int. J. Electrochem. Sci. 3, 777 (2008).13S. Parida, D. Kramer, C. A. Volkert, H. R€osner, J. Erlebacher, and J.Weissm€uller, Phys. Rev. Lett. 97, 035504 (2006).14J. Stejskal, P. Kratochvıl, and A. Jenkins, Polymer 37, 367 (1996).15C. V. Tuan, M. A. Tuan, N. V. Hieu, and T. Trung, Curr. Appl. Phys. 12,1011 (2012).16M. H. Pournaghi-Azar and B. Habibi, Electrochem. Acta 52, 4222 (2007).193101-3 Detsi, Onck, and De Hosson Appl. Phys. Lett. 103, 193101 (2013) This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:129.125.63.113 On: Thu, 14 Nov 2013 09:52:48

University of Groningen

   University of GroningenElectrochromic artificial muscles based on nanoporous metal-polymer compositesDetsi, E.; Onck, P. R.; De Hosson, J. T. M.Published in:Applied Physics LettersDOI:10.1063/1.4827089IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2013Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Detsi, E., Onck, P. R., & De Hosson, J. T. M. (2013). Electrochromic artificial muscles based onnanoporous metal-polymer composites. Applied Physics Letters, 103(19), 193101-1-193101-3. [193101].https://doi.org/10.1063/1.4827089CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 12-11-2019Electrochromic artificial muscles based on nanoporous metal-polymercompositesE. Detsi, P. R. Onck, and J. T. M. De Hossona)Department of Applied Physics, Zernike institute for Advanced Materials, University of Groningen,Nijenborgh 4, 9747 AG Groningen, The Netherlands(Received 17 February 2013; accepted 20 August 2013; published online 4 November 2013)This work shows that a nano-coating of electrochromic polymer grown onto the ligaments ofnanoporous gold causes reversible dimensional and color changes during electrochemicalactuation. This combination of electromechanical and optical properties opens additional avenuesfor the applications of artificial muscles, i.e., a metallic muscle exhibits its progress during workby changing color that can be detected by optical means. VC 2013 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4827089]Electrochemical actuators base on nanoporous metalswith high surface-area-to-volume ratios, so-called metallicmuscles,1 represent a class of smart materials that undergoreversible dimensional changes upon changes in their inter-face electronic charge densities.2–6 Here, we show that thesemetallic muscles become smarter when a thin electrochromicpolymer coating is grown onto the ligaments of nanoporousmetals: In addition, to the reversible dimensional changestaking place in the nanoporous metal during electrochemicalactuation,2–6 the polymer coating undergoes reversiblechanges in color upon electro-oxidation/reduction.7,8 In thisletter, we use nanoporous gold (NPG) as metallic muscle andpolyaniline (PANI) as electrochromic conducting polymer todemonstrate the proof of principle.A 60mm-long metallic cantilever having a bilayerconfiguration was made.5,9 The active part consists of a thickNPG layer with thickness 20 lm and the support is madeof a relatively thin dense gold layer (7 lm). A schematicillustration of the bilayer strip cantilever is shown in Figure1(a) and the typical bicontinuous nanoporous morphology ofthe active layer is displayed by the scanning electron micro-graph of Figure 1(b). The average size of the ligaments andpores is 25 nm. A PANI coating with thickness between5 and 10 nm was grown onto these ligaments by electro-chemical oxidative polymerization of aniline from an aque-ous solution containing 0.5M of H2SO4 and 50mM ofaniline monomer.10 The scanning and transmission electronmicrographs of Figures 1(c) and 1(d), respectively, display atypical 5 nm thick uniform PANI coating grown onto theligaments of NPG.The first 10 cyclic voltammograms recorded during thegrowth of PANI onto the ligaments of NPG are shown inFigure 2. In the first cycle (red curve), the anodic depositioncurrent increases at 1.1V (versus Ag/AgCl referenceelectrode); this indicates that aniline initially oxidizes at arelatively high potential (1.1V). A similar behavior hasbeen reported during electrochemical deposition of anilineon diamond nanoparticles,11 and on noble metals includingbare platinum.12 The oxidation of aniline at a relatively highvoltage gives rise to degradation products, which are depos-ited onto the ligaments of NPG during subsequent cycles.This is illustrated by the two anodic peaks observed at0.5V and 0.7V during the second and third cycles (bluecurves).11 The standard two pairs of redox peaks that charac-terize the oxidation (oxi) and reduction (red) of aniline atappropriate redox potentials, i.e., oxi1/red1 at 0.3V/0.1Vand oxi2/red2 at 0.8V/0.6V only dominate from the fourthcycle (black curves). These two pairs of peaks are less pro-nounced compared to those reported during electrochemicaldeposition of aniline monomers onto bare noble metals elec-trode such as platinum.12 In fact, the positions and ampli-tudes of these two pairs of redox peaks are well-known to beaffected by the nature of the substrate onto which PANI isgrown. The gradual increase in the current intensities of thevoltammograms during subsequent cycles is commonlyattributed to the auto-catalytic property of PANI forpolymerization.In the following, we present the simultaneous reversiblechanges in dimension and in color in our NPG/PANI hybridmaterial. We concentrate on a qualitative description of thematerial response; it is referred to Refs. 2–6 for quantitativeFIG. 1. Microstructural characterization. (a) The as-synthesized bilayer stripcantilever is initially bent because during dealloying the thick NPG layershrinks,6,13 whereas the thin dense gold layer does not. (b) Scanning electronmicrograph showing the typical bicontinuous morphology of the thick NPGlayer. (c, d) Scanning and transmission electron micrographs showing a typi-cal 5 nm-thick PANI coating covering the ligaments of NPG.a)Author to whom correspondence should be addressed. Electronic mail:j.t.m.de.hosson@rug.nl. Tel.: þ31-50-363 4898. Fax: þ31-50-363 4881.0003-6951/2013/103(19)/193101/3/$30.00 VC 2013 AIP Publishing LLC103, 193101-1APPLIED PHYSICS LETTERS 103, 193101 (2013) This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:129.125.63.113 On: Thu, 14 Nov 2013 09:52:48studies on electrochemical actuation in nanoporous metals.After electrochemical polymerization, the NPG/PANI isactuated in the potential range between ÿ0.22V and 1.1V(versus Ag/AgCl reference electrode), using as electrolyte afresh aqueous solution containing 0.5M of H2SO4 (withoutaniline). Note that the as-synthesized bilayer strip cantileveris initially bent as schematized in Figure 1(a). This isbecause during the dealloying process the thick nanoporousgold layer contracts, i.e., the well-known volume contractionduring dealloying,6,13 whereas the dense gold support doesnot. Typically, the as-synthesized green NPG/PANI cantile-ver expands during the forward scan and at the meantime itbecomes dark blue. During reverse process the cantilever(initially dark blue) bends back to the initial shape and at themeantime it recovers its initial green color. This combinationof electromechanical and optical properties broadens thepotential of metallic muscles for many applications. Astraightforward application might involve metallic musclesthat can communicate while performing work, e.g., duringwork, the metallic muscle displays a certain color (in thiscase dark blue); when the work is done a different color isdisplayed (in this case green). Although electrochromicactuators can be made from pure polymer, the combinationof a metal and a polymer for electrochromic actuation pur-poses is more attractive, because metallic actuators are stron-ger and stiffer than polymer actuators.3The physical origin of the simultaneous changes indimension and in color of the cantilever is the following:The NPG/PANI hybrid material takes advantage of theactuation ability of nanoporous metals on the one hand,2–6and the electrochromic properties of PANI on the otherhand.7,8 Starting with electrochemical actuation, Figure 3(a)displays the cyclic voltammograms recorded during thefirst 10 actuation cycles. These voltammograms display apseudo-capacitive behavior associated with charge storageby the NPG/PANI composite.10 Chen and co-workers haverecently reported on a strong enhancement of the charge stor-age capacity of NPG/PANI hybrid materials in aqueouselectrolyte.10 This storage (release) of electronic charge bythe NPG/PANI composite satisfies the requirement for re-versible dimensional changes in conducting high surface-area-to-volume ratio nanostructured materials,2–6 whichexplains the expansion and bending of our cantilever.In addition to these reversible dimensional changes, thecantilever also undergoes reversible changes in color asschematized in Figure 3(b). As aforementioned, thesechanges in color are attributed to the electrochromic propertyof the PANI coating. Indeed, the color of PANI depends onits oxidation state and common colors include a dark bluecolor when PANI is fully oxidized (perningraniline), a greencolor when PANI is partially oxidized and doped (emeral-dine salt), and a transparent color when PANI is notoxidized, which corresponds to the fully reduced form ofPANI (leucoemeraldine).7,8,14Before actuation, the NPG/PANI cantilever may displaya green or a dark blue color depending on the doping state ofPANI. In the present case, the as-synthesized NPG/PANIcantilever is green, i.e., at the end of the electropolymeriza-tion procedure, the PANI coating is partially oxidized anddoped with H2SO4 (emeraldine salt).7,8,14 Due to the electro-chromic property of the PANI coating, the starting greencolor of our cantilever is susceptible to change to dark blueif PANI is further oxidized from its partial oxidation level toits full oxidation state. It is evident from Figure 3(a) that thetwo pairs of redox peaks oxi1/red1 and oxi2/red2 observedduring electrochemical polymerization from Figure 2 are stillpresent during electrochemical actuation, which indicatesthat PANI is being oxidized and reduced during actuation.These two redox activities in the PANI coating are responsi-ble for the reversible changes in color observed in the canti-lever during actuation: (A) During the forward anodic scanwhere the cantilever expands, green and dark blue colora-tions are observed (see illustration in Figure 3(b)). These twocolors are justified by the following: (i) when the cantileverstarts expanding, its initial green color is preserved at thefirst oxidation peak (i.e., oxi1 in Figure 3(a)). This is becausethat peak corresponds to the oxidation of residual transparentleucoemeraldine (fully reduced form of PANI) to the greenFIG. 3. Simultaneous reversible changesin dimension and in color: (a) Cyclicvoltammograms recorded during the first10 actuation cycles. (b) Simultaneouschanges in color and in dimension in thebilayer cantilever.FIG. 2. Electrochemical polymerization curves. First 10 cyclic voltammo-grams recorded during the growth of PANI onto the ligaments of nanopo-rous gold.193101-2 Detsi, Onck, and De Hosson Appl. Phys. Lett. 103, 193101 (2013) This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:129.125.63.113 On: Thu, 14 Nov 2013 09:52:48emeraldine (partially oxidized form).15,16 (ii) When the can-tilever continues to expand, its color changes from green todark blue. This is because at the second oxidation peak (i.e.,oxi2 in Figure 3(a)), the green emeraldine (partially oxidizedform of PANI) is further oxidized to dark blue perningrani-line (fully oxidized form of PANI).15,16 (B) During thereverse cathodic scan where the cantilever bends back to itsinitial shape, dark blue, and green colorations are observed(see illustration in Figure 3(b)). These two colors are justifiedas follows: (i) when the cantilever starts bending back to itsinitial shape, the starting dark blue color is preserved despitethe reduction activity present at the beginning of the reversesweep as illustrated by the second reduction peak (i.e., red2in Figure 3(a)). This dark blue color is still present becausethe peak red2 corresponds to the reduction of residual greenemeraldine (partially oxidized form of PANI) to the transpar-ent leucoemeraldine (fully reduced form of PANI).15,16Since the residual leucoemeraldine is colorless, the dark bluecolor of perningraniline (fully oxidized form of PANI) is stilldisplayed. (ii) As the cantilever continues to bend back, itscolor changes from dark blue to green. This is because at thefirst reduction peak (i.e., red1 in Figure 3(a)), the dark blueperningraniline (fully oxidized form of PANI) is partiallyreduced to the green emeraldine (partially oxidized form ofPANI).15,16 The redox activities at oxi1/red1 and oxi2/red2clearly justify the reversible changes in color observed dur-ing the relaxation and bending of the cantilever. These twophenomena (reversible dimensional and color changes) arewell-reproducible over many cycles because of the high sta-bility of PANI.In summary, we have demonstrated that a thin polymercoating can be exploited to add an additional functionality tonanoporous metals electrochemical actuators. By doing so, ametallic muscle becomes smarter because in addition to itsreversible dimensional changes, it also undergoes reversiblechanges in color. This combination of electromechanical andoptical properties opens other avenues for the applications ofartificial muscles, i.e., a metallic muscle exhibits its progressduring work by changing color that can be detected by opti-cal means.1R. H. Baughman, Science 300, 268 (2003).2J. Weissm€uller, R. N. Viswanath, D. Kramer, P. Zimmer, R. W€urschum,and H. Gleiter, Science 300, 312 (2003).3H.-J. Jin, X.-L. Wang, S. Parida, K. Wang, M. Seo, and J. Weissm€uller,Nano Lett. 10, 187 (2010).4J. Zhang, P. Liu, H. Ma, and Y. Ding, J. Phys. Chem. C 111, 10382(2007).5D. Kramer, R. N. Viswanath, and J. Weissm€uller, Nano Lett. 4, 793(2004).6E. Detsi, S. Punzhin, J. Rao, P. R. Onck, and J. T. M. De Hosson, ACSNano 6, 3734 (2012).7M. M. Gvozdenovic´, B. Z. Jugovic´, J. S. Stevanovic´, T. L. Trisovic´,and B. N. Grgur, in Electropolymerization, edited by E. Schab-Balcerzak(In Tech, 2011), Chap. IV, pp. 77–87.8L. Zhao, L. Zhao, Y. Xu, T. Qiu, L. Zhi, and G. Shi, Electrochem. Acta55, 491 (2009).9E. Detsi, Z. G. Chen, W. P. Vellinga, P. R. Onck, and J. T. M. De Hosson,Appl. Phys. Lett. 99, 083104 (2011).10X. Lang, L. Zhang, T. Fujita, Y. Ding, and M. Chen, J. Power Sources197, 325 (2012).11X. Y. Zhao, J. B. Zang, Y. H. Wang, L. Y. Bian, and J. K. Yu,Electrochem. Commun. 11, 1297 (2009).12A. S. Sarac, M. Ates, and B. Kilic, Int. J. Electrochem. Sci. 3, 777 (2008).13S. Parida, D. Kramer, C. A. Volkert, H. R€osner, J. Erlebacher, and J.Weissm€uller, Phys. Rev. Lett. 97, 035504 (2006).14J. Stejskal, P. Kratochvıl, and A. Jenkins, Polymer 37, 367 (1996).15C. V. Tuan, M. A. Tuan, N. V. Hieu, and T. Trung, Curr. Appl. Phys. 12,1011 (2012).16M. H. Pournaghi-Azar and B. Habibi, Electrochem. Acta 52, 4222 (2007).193101-3 Detsi, Onck, and De Hosson Appl. Phys. Lett. 103, 193101 (2013) This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:129.125.63.113 On: Thu, 14 Nov 2013 09:52:48

   University of GroningenElectrochromic artificial muscles based on nanoporous metal-polymer compositesDetsi, E.; Onck, P. R.; De Hosson, J. T. M.Published in:Applied Physics LettersDOI:10.1063/1.4827089IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2013Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Detsi, E., Onck, P. R., & De Hosson, J. T. M. (2013). Electrochromic artificial muscles based onnanoporous metal-polymer composites. Applied Physics Letters, 103(19), 193101-1-193101-3. [193101].https://doi.org/10.1063/1.4827089CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 26-04-2023Electrochromic artificial muscles based on nanoporous metal-polymercompositesE. Detsi, P. R. Onck, and J. T. M. De Hossona)Department of Applied Physics, Zernike institute for Advanced Materials, University of Groningen,Nijenborgh 4, 9747 AG Groningen, The Netherlands(Received 17 February 2013; accepted 20 August 2013; published online 4 November 2013)This work shows that a nano-coating of electrochromic polymer grown onto the ligaments ofnanoporous gold causes reversible dimensional and color changes during electrochemicalactuation. This combination of electromechanical and optical properties opens additional avenuesfor the applications of artificial muscles, i.e., a metallic muscle exhibits its progress during workby changing color that can be detected by optical means. VC 2013 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4827089]Electrochemical actuators base on nanoporous metalswith high surface-area-to-volume ratios, so-called metallicmuscles,1 represent a class of smart materials that undergoreversible dimensional changes upon changes in their inter-face electronic charge densities.2–6 Here, we show that thesemetallic muscles become smarter when a thin electrochromicpolymer coating is grown onto the ligaments of nanoporousmetals: In addition, to the reversible dimensional changestaking place in the nanoporous metal during electrochemicalactuation,2–6 the polymer coating undergoes reversiblechanges in color upon electro-oxidation/reduction.7,8 In thisletter, we use nanoporous gold (NPG) as metallic muscle andpolyaniline (PANI) as electrochromic conducting polymer todemonstrate the proof of principle.A 60mm-long metallic cantilever having a bilayerconfiguration was made.5,9 The active part consists of a thickNPG layer with thickness 20 lm and the support is madeof a relatively thin dense gold layer (7 lm). A schematicillustration of the bilayer strip cantilever is shown in Figure1(a) and the typical bicontinuous nanoporous morphology ofthe active layer is displayed by the scanning electron micro-graph of Figure 1(b). The average size of the ligaments andpores is 25 nm. A PANI coating with thickness between5 and 10 nm was grown onto these ligaments by electro-chemical oxidative polymerization of aniline from an aque-ous solution containing 0.5M of H2SO4 and 50mM ofaniline monomer.10 The scanning and transmission electronmicrographs of Figures 1(c) and 1(d), respectively, display atypical 5 nm thick uniform PANI coating grown onto theligaments of NPG.The first 10 cyclic voltammograms recorded during thegrowth of PANI onto the ligaments of NPG are shown inFigure 2. In the first cycle (red curve), the anodic depositioncurrent increases at 1.1V (versus Ag/AgCl referenceelectrode); this indicates that aniline initially oxidizes at arelatively high potential (1.1V). A similar behavior hasbeen reported during electrochemical deposition of anilineon diamond nanoparticles,11 and on noble metals includingbare platinum.12 The oxidation of aniline at a relatively highvoltage gives rise to degradation products, which are depos-ited onto the ligaments of NPG during subsequent cycles.This is illustrated by the two anodic peaks observed at0.5V and 0.7V during the second and third cycles (bluecurves).11 The standard two pairs of redox peaks that charac-terize the oxidation (oxi) and reduction (red) of aniline atappropriate redox potentials, i.e., oxi1/red1 at 0.3V/0.1Vand oxi2/red2 at 0.8V/0.6V only dominate from the fourthcycle (black curves). These two pairs of peaks are less pro-nounced compared to those reported during electrochemicaldeposition of aniline monomers onto bare noble metals elec-trode such as platinum.12 In fact, the positions and ampli-tudes of these two pairs of redox peaks are well-known to beaffected by the nature of the substrate onto which PANI isgrown. The gradual increase in the current intensities of thevoltammograms during subsequent cycles is commonlyattributed to the auto-catalytic property of PANI forpolymerization.In the following, we present the simultaneous reversiblechanges in dimension and in color in our NPG/PANI hybridmaterial. We concentrate on a qualitative description of thematerial response; it is referred to Refs. 2–6 for quantitativeFIG. 1. Microstructural characterization. (a) The as-synthesized bilayer stripcantilever is initially bent because during dealloying the thick NPG layershrinks,6,13 whereas the thin dense gold layer does not. (b) Scanning electronmicrograph showing the typical bicontinuous morphology of the thick NPGlayer. (c, d) Scanning and transmission electron micrographs showing a typi-cal 5 nm-thick PANI coating covering the ligaments of NPG.a)Author to whom correspondence should be addressed. Electronic mail:j.t.m.de.hosson@rug.nl. Tel.: þ31-50-363 4898. Fax: þ31-50-363 4881.0003-6951/2013/103(19)/193101/3/$30.00 VC 2013 AIP Publishing LLC103, 193101-1APPLIED PHYSICS LETTERS 103, 193101 (2013) This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:129.125.63.113 On: Thu, 14 Nov 2013 09:52:48studies on electrochemical actuation in nanoporous metals.After electrochemical polymerization, the NPG/PANI isactuated in the potential range between ÿ0.22V and 1.1V(versus Ag/AgCl reference electrode), using as electrolyte afresh aqueous solution containing 0.5M of H2SO4 (withoutaniline). Note that the as-synthesized bilayer strip cantileveris initially bent as schematized in Figure 1(a). This isbecause during the dealloying process the thick nanoporousgold layer contracts, i.e., the well-known volume contractionduring dealloying,6,13 whereas the dense gold support doesnot. Typically, the as-synthesized green NPG/PANI cantile-ver expands during the forward scan and at the meantime itbecomes dark blue. During reverse process the cantilever(initially dark blue) bends back to the initial shape and at themeantime it recovers its initial green color. This combinationof electromechanical and optical properties broadens thepotential of metallic muscles for many applications. Astraightforward application might involve metallic musclesthat can communicate while performing work, e.g., duringwork, the metallic muscle displays a certain color (in thiscase dark blue); when the work is done a different color isdisplayed (in this case green). Although electrochromicactuators can be made from pure polymer, the combinationof a metal and a polymer for electrochromic actuation pur-poses is more attractive, because metallic actuators are stron-ger and stiffer than polymer actuators.3The physical origin of the simultaneous changes indimension and in color of the cantilever is the following:The NPG/PANI hybrid material takes advantage of theactuation ability of nanoporous metals on the one hand,2–6and the electrochromic properties of PANI on the otherhand.7,8 Starting with electrochemical actuation, Figure 3(a)displays the cyclic voltammograms recorded during thefirst 10 actuation cycles. These voltammograms display apseudo-capacitive behavior associated with charge storageby the NPG/PANI composite.10 Chen and co-workers haverecently reported on a strong enhancement of the charge stor-age capacity of NPG/PANI hybrid materials in aqueouselectrolyte.10 This storage (release) of electronic charge bythe NPG/PANI composite satisfies the requirement for re-versible dimensional changes in conducting high surface-area-to-volume ratio nanostructured materials,2–6 whichexplains the expansion and bending of our cantilever.In addition to these reversible dimensional changes, thecantilever also undergoes reversible changes in color asschematized in Figure 3(b). As aforementioned, thesechanges in color are attributed to the electrochromic propertyof the PANI coating. Indeed, the color of PANI depends onits oxidation state and common colors include a dark bluecolor when PANI is fully oxidized (perningraniline), a greencolor when PANI is partially oxidized and doped (emeral-dine salt), and a transparent color when PANI is notoxidized, which corresponds to the fully reduced form ofPANI (leucoemeraldine).7,8,14Before actuation, the NPG/PANI cantilever may displaya green or a dark blue color depending on the doping state ofPANI. In the present case, the as-synthesized NPG/PANIcantilever is green, i.e., at the end of the electropolymeriza-tion procedure, the PANI coating is partially oxidized anddoped with H2SO4 (emeraldine salt).7,8,14 Due to the electro-chromic property of the PANI coating, the starting greencolor of our cantilever is susceptible to change to dark blueif PANI is further oxidized from its partial oxidation level toits full oxidation state. It is evident from Figure 3(a) that thetwo pairs of redox peaks oxi1/red1 and oxi2/red2 observedduring electrochemical polymerization from Figure 2 are stillpresent during electrochemical actuation, which indicatesthat PANI is being oxidized and reduced during actuation.These two redox activities in the PANI coating are responsi-ble for the reversible changes in color observed in the canti-lever during actuation: (A) During the forward anodic scanwhere the cantilever expands, green and dark blue colora-tions are observed (see illustration in Figure 3(b)). These twocolors are justified by the following: (i) when the cantileverstarts expanding, its initial green color is preserved at thefirst oxidation peak (i.e., oxi1 in Figure 3(a)). This is becausethat peak corresponds to the oxidation of residual transparentleucoemeraldine (fully reduced form of PANI) to the greenFIG. 3. Simultaneous reversible changesin dimension and in color: (a) Cyclicvoltammograms recorded during the first10 actuation cycles. (b) Simultaneouschanges in color and in dimension in thebilayer cantilever.FIG. 2. Electrochemical polymerization curves. First 10 cyclic voltammo-grams recorded during the growth of PANI onto the ligaments of nanopo-rous gold.193101-2 Detsi, Onck, and De Hosson Appl. Phys. Lett. 103, 193101 (2013) This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:129.125.63.113 On: Thu, 14 Nov 2013 09:52:48emeraldine (partially oxidized form).15,16 (ii) When the can-tilever continues to expand, its color changes from green todark blue. This is because at the second oxidation peak (i.e.,oxi2 in Figure 3(a)), the green emeraldine (partially oxidizedform of PANI) is further oxidized to dark blue perningrani-line (fully oxidized form of PANI).15,16 (B) During thereverse cathodic scan where the cantilever bends back to itsinitial shape, dark blue, and green colorations are observed(see illustration in Figure 3(b)). These two colors are justifiedas follows: (i) when the cantilever starts bending back to itsinitial shape, the starting dark blue color is preserved despitethe reduction activity present at the beginning of the reversesweep as illustrated by the second reduction peak (i.e., red2in Figure 3(a)). This dark blue color is still present becausethe peak red2 corresponds to the reduction of residual greenemeraldine (partially oxidized form of PANI) to the transpar-ent leucoemeraldine (fully reduced form of PANI).15,16Since the residual leucoemeraldine is colorless, the dark bluecolor of perningraniline (fully oxidized form of PANI) is stilldisplayed. (ii) As the cantilever continues to bend back, itscolor changes from dark blue to green. This is because at thefirst reduction peak (i.e., red1 in Figure 3(a)), the dark blueperningraniline (fully oxidized form of PANI) is partiallyreduced to the green emeraldine (partially oxidized form ofPANI).15,16 The redox activities at oxi1/red1 and oxi2/red2clearly justify the reversible changes in color observed dur-ing the relaxation and bending of the cantilever. These twophenomena (reversible dimensional and color changes) arewell-reproducible over many cycles because of the high sta-bility of PANI.In summary, we have demonstrated that a thin polymercoating can be exploited to add an additional functionality tonanoporous metals electrochemical actuators. By doing so, ametallic muscle becomes smarter because in addition to itsreversible dimensional changes, it also undergoes reversiblechanges in color. This combination of electromechanical andoptical properties opens other avenues for the applications ofartificial muscles, i.e., a metallic muscle exhibits its progressduring work by changing color that can be detected by opti-cal means.1R. H. Baughman, Science 300, 268 (2003).2J. Weissm€uller, R. N. Viswanath, D. Kramer, P. Zimmer, R. W€urschum,and H. Gleiter, Science 300, 312 (2003).3H.-J. Jin, X.-L. Wang, S. Parida, K. Wang, M. Seo, and J. Weissm€uller,Nano Lett. 10, 187 (2010).4J. Zhang, P. Liu, H. Ma, and Y. Ding, J. Phys. Chem. C 111, 10382(2007).5D. Kramer, R. N. Viswanath, and J. Weissm€uller, Nano Lett. 4, 793(2004).6E. Detsi, S. Punzhin, J. Rao, P. R. Onck, and J. T. M. De Hosson, ACSNano 6, 3734 (2012).7M. M. Gvozdenović, B. Z. Jugović, J. S. Stevanović, T. L. Trisović,and B. N. Grgur, in Electropolymerization, edited by E. Schab-Balcerzak(In Tech, 2011), Chap. IV, pp. 77–87.8L. Zhao, L. Zhao, Y. Xu, T. Qiu, L. Zhi, and G. Shi, Electrochem. Acta55, 491 (2009).9E. Detsi, Z. G. Chen, W. P. Vellinga, P. R. Onck, and J. T. M. De Hosson,Appl. Phys. Lett. 99, 083104 (2011).10X. Lang, L. Zhang, T. Fujita, Y. Ding, and M. Chen, J. Power Sources197, 325 (2012).11X. Y. Zhao, J. B. Zang, Y. H. Wang, L. Y. Bian, and J. K. Yu,Electrochem. Commun. 11, 1297 (2009).12A. S. Sarac, M. Ates, and B. Kilic, Int. J. Electrochem. Sci. 3, 777 (2008).13S. Parida, D. Kramer, C. A. Volkert, H. R€osner, J. Erlebacher, and J.Weissm€uller, Phys. Rev. Lett. 97, 035504 (2006).14J. Stejskal, P. Kratochvıl, and A. Jenkins, Polymer 37, 367 (1996).15C. V. Tuan, M. A. Tuan, N. V. Hieu, and T. Trung, Curr. Appl. Phys. 12,1011 (2012).16M. H. Pournaghi-Azar and B. Habibi, Electrochem. Acta 52, 4222 (2007).193101-3 Detsi, Onck, and De Hosson Appl. Phys. Lett. 103, 193101 (2013) This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:129.125.63.113 On: Thu, 14 Nov 2013 09:52:48

Electrochromic artificial muscles based on nanoporous metal-polymer composites

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