Sensors and Actuators B 115 (2006) 488–493

Electrochemical actuator based on monolithic polypyrrole–TiO2

nanoparticle composite film

Ximin He, Gaoquan Shi ∗Department of Chemistry and Key Lab of Organic Optical Electronics and Molecular Engineering of Education Commission,

Tsinghua University, Beijing 100084, PR China

Received 16 June 2005; received in revised form 8 October 2005; accepted 12 October 2005Available online 16 November 2005

Abstract

A polypyrrole (PPy) composite film with a pure PPy zone and a PPy–TiO2 nanoparticle composite zone in its cross-sectional direction wassynthesized by direct oxidation of pyrrole successively in an electrolyte with or without TiO2 nanoparticles. The volume content of TiO2 in thecomposite film was tested to be lower than 0.9%. The actuator fabricated from this monolithic PPy film bends uniformly in two directions with amaximum bending angle larger than 90◦ and has a high response rate (∼115◦ s−1 at 1.0 V versus SCE) and a long lifetime (>20,000 cycles) in a1t©

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M LiClO4 aqueous solution. The high performance of this actuator resulted from the non-delamination and intrinsic asymmetric structure andhe high conductivity of PPy film.

2005 Elsevier B.V. All rights reserved.

eywords: Polypyrroles; Nanocomposites; Electrochemistry

. Introduction

Electrochemical soft actuators made of conducting polymersCP) have been studied extensively in the last decades. Dur-ng the redox processes, electrochemomechanical deformationECMD) of CP films is significantly affected by the film mor-hology [1]. Since a typical CP film has a symmetric structure,t exhibits uniform volume changes during the redox processnd cannot do bending movement as an actuator. Thus, in gen-ral, a flexible and electrochemically inert layer, which is notubjected to volume changes under electrochemical stimuli iseeded to be glued to the free-standing CP film. Therefore, mostf the CP actuators have been constructed based on either bilayer2,3] or multilayer [4–6] configuration. However, the inevitableelamination in the multilayer actuators shortens their lifetime,ince the physical adhesion between the inert layer and the CPayer(s) cannot sustain repeated volume alteration at their inter-aces [7,14]. Moreover, the inert layer contributes additionaleight of the actuator and the energy consumed by the defor-ation at the interfaces decreases the actuator efficiency.

In comparison to conventional multilayered actuators, mono-lithic actuators have advantages of higher efficiency and betterstructural stability. Wang and co-workers prepared monolithicpolyaniline (Pani) electrochemical actuators with microvoidsdistributed across the cross-section [8]. Although the actua-tors had a very long lifetime (>325,000 cycles), they couldonly be operated at a high driving potential (±2 V) in acidicmedium (1 mol l−1 HCl) and could not do uniform bending.The main problem is the low conductivity of the Pani film(∼0.1 S cm−1). Then, they fabricated another actuator based ona monolithic asymmetric polyaniline-graphite composite film,consisting of graphite-rich and -lacking layers [9]. The highconductivity resulted from the incorporated graphite in thePani film, and the asymmetric distribution of graphite acrossthe cross-section allows for lower operation potential, largerbending angles and also large life cycles (120,000). However,the film was prepared through a multi-step procedure, anda high content of graphite powder (∼70%) in the compos-ite film reduced the efficiency and mechanical strength of theactuator.

A PPy film grown in a slender pipe of polytetrafluoroethylene

∗ Corresponding author. Tel.: +86 10 6277 3743; fax: +86 10 6277 1149.E-mail address: gshi@tsinghua.edu.cn (G. Shi).

has a considerable vertical anisotropy [10] and can performbending movements. Recently, we also electrochemically syn-thesized an anisotropic PPy film from a mixed solvent of acetoni-

925-4005/$ – see front matter © 2005 Elsevier B.V. All rights reserved.

oi:10.1016/j.snb.2005.10.011

X. He, G. Shi / Sensors and Actuators B 115 (2006) 488–493 489

Fig. 1. Scheme of the preparation procedure of anisotropic PPy–TiO2 composite films.

trile and dichloromethane containing a high concentration of ananion surfactant of sodium 1,4-bis(2-ethylhexyl)sulfosuccinate(AOT) [11]. The film has compact and porous zones in its ver-tical direction and the actuator fabricated from this film can douniform bending in a neutral medium and has high performance.However, the preparation of this film needs organic solvents anda large amount of expensive AOT. Moreover, the porous struc-ture may decrease the mechanical strength of the actuator. In thispaper, we desire to report that high performance actuators canalso be fabricated from an anisotropic PPy-oxide nanoparticlecomposite film formed by direct co-deposition of PPy and thenanoparticles from an aqueous medium. Here, choosing TiO2nanoparticles as the example is nothing special, mainly becauseit can be commercially obtained. It was found that the introduc-tion of a little amount of TiO2 nanoparticles (<0.9%, by volume)can improve greatly the performance of the single-layer PPyactuator.

2. Experimental

2.1. Materials

Pyrrole (99%, Hongyu Chem. Co., Beijing, China) was usedafter distillation under reduced pressure. Dodecylbenzene sul-fonate acid (DBSA, >99%, Acros), anhydrous lilhium perchlo-rnu

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pMRe6rs

The procedure of preparation of the anisotropic PPy–TiO2composite films is schematically illustrated in Fig. 1. First,a pure PPy layer was deposited on the working electrode bydirect oxidation of 0.1 mol l−1 pyrrole in an aqueous solution of0.1 mol l−1 DBSA galvanostatically at 1.6 mA cm−2(Fig. 1a) for700 s (112 mC cm−2). Secondly, 0.2, 0.4 or 0.8 g TiO2 nanoparti-cle powder was added into the electrolyte and dispersed by ultra-sonication for 15 min (the content of TiO2 in the electrolyte wascalculated to be 0.0025, 0.005 or 0.01 g ml−1), then a PPy–TiO2layer was successively deposited on the PPy layer under mag-netic stirring for another 700 s (112 mC cm−2) (Fig. 1b). Allsolutions were de-aerated by bubbling dry nitrogen, and main-tained a light nitrogen overpressure during the experiments.Finally, the composite film was peeled off from the surface ofthe working electrode, washed repeatedly with de-ionized waterand dried under vacuum at room temperature for characteriza-tions. A pure PPy film was also synthesized under the sameconditions for comparing with the PPy–TiO2 composite film.The thickness of the films was measured to be approximately15 �m by the use of a thickness monitor (CH-1-ST, Shanghai,China).

2.3. Characterizations

The scanning electron micrographs (SEM) were taken out byuUw(ibCpvfiotfse

ate (>99%, Jinke Inst. Fine Chem., Tianjin, China) and TiO2anoparticles with diameters of 20–30 nm (Deggusa, USA) weresed as received.

.2. Electrochemical syntheses of anisotropic PPy–TiO2

lms

Electrochemical syntheses were performed at room tem-erature in a one-compartment cell by the use of aodel 273 potentiostat–galvanostat (EG&G Princeton Appliedesearch) under computer control. The working and counterlectrodes were both a stainless steel sheet (AISI 304,.0 cm × 2.0 cm) and placed 2.0 cm apart. All potentials wereeferred to an Ag/AgCl wire that was immersed directly in theolution.

sing a scanning electron microscope (SEM; Sirion-200, FEI,SA). The elemental compositions of the composite film surfaceere determined by using an energy dispersive X-ray detector

EDX; Serion-200 132-10, EDAX, USA). The thermal stabil-ty and the weight content of TiO2 in the film were studiedy the use of a thermal gravimetric analyzer (TGA 2050, TAo.). The dc conductivity was measured by conventional four-robe technique. The electrochemical analyses including cyclicoltammetry and the bending movements of the single-layer PPylms (size = 15 mm × 2 mm × 0.015 mm) were carried out in ane-compartment cell by the use of a 1 mol l−1 aqueous solu-ion of LiClO4 as the electrolyte. The working electrode was areestanding PPy film, the counter electrode was a stainless steelheet and all the potentials were referred to a saturated calomellectrode (SCE).

490 X. He, G. Shi / Sensors and Actuators B 115 (2006) 488–493

3. Results and discussion

3.1. Morphology and conductivity of PPy–TiO2 film

The appearances of the two surfaces of a 15 �m thickPPy–TiO2 composite film are different: the TiO2-rich side is dullblack; while the TiO2-free side is black and shiny. However, thetwo surfaces of a 15 �m thick pure PPy film have nearly thesame appearance. The SEM images of a pure and a PPy–TiO2composite film are shown in Fig. 2. As can be seen from thisFig. 2a, for a 15 �m pure PPy film, the surface in contact withthe electrolyte is compact and smooth. However, for a PPy–TiO2film, the surface in contact with the electrolyte, e.g. the TiO2-richside, is rough and irregular (Fig. 2b), while its TiO2-free side isflat and compact (Fig. 2c). The composite film does not show anapparent interface between its TiO2-rich and TiO2-free layers,and the nanoparticles distribute asymmetrically across its cross-section as shown in Fig. 2d. The SEM micrographs suggest thatthe incorporation of TiO2 nanoparticles affects the morphologyof the film significantly and makes the TiO2-rich side look to berelative loosely piled up with PPy and TiO2 nanoparticles.

The chemical composition of the TiO2-rich surface of thecomposite film was examined by EDX analysis (Fig. 3). Theresult indicates the existence of elements Ti (from TiO2), C,N, O (from pyrrole) and S (from DBS). The results of TGAexaminations indicated that the film prepared from the mediac1obfit

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Fig. 3. EDX spectrum of the TiO2-rich side of the PPy–TiO2 film.

3.2. Electrochemistry and actuations of the PPy–TiO2 film

The cyclic voltammograms (CVs) of an as-grown PPy–TiO2film (size = 15 mm × 2 mm × 0.015 mm) containing 0.40%(v/v) TiO2 and coated on the working electrode surface or thesame film in freestanding state were carried out in an aqueoussolution of 1.0 mol l−1 LiClO4 at 30 mV s−1, and the results areshown in Fig. 4. As can be seen from the figure, each CV cyclehas a couple of broad redox waves. Furthermore, the films canbe cycled repeatedly between the conducting (oxidized) andinsulating (neutral) states without significant decomposition.The CV of a composite film coated on a metallic electrodeshows a reduction wave at approximately −0.58 V and anoxidation wave at approximately −0.10 V (Fig. 4). Thus, thepotential difference between the oxidation and reduction currentpeaks (�Ep) is calculated to be 0.48 V. On the other hand, theCV of a freestanding PPy–TiO2 film shows a broad reductionwave centered at approximately −0.80 V and an oxidation wave

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ontaining 0.005 or 0.01 g ml−1 TiO2 had a TiO2 content of.21 or 2.57% (w/w), respectively. Thus, the volume contentsf TiO2 nanoparticles in the composite films were calculated toe 0.40 and 0.87% (v/v), respectively. In contrast, the pure PPylm showed no weight residue as the sample was heated in air

o over 650 ◦C.The conductivity of the PPy–TiO2 film was measured to be

pproximately 16.0 S cm−1. This value is much higher than thatf Pani [12]. Furthermore, these films are flexible enough to beolded or shaped into various structures for the fabrication ofctuators.

ig. 2. SEM images of (a) a PPy film without TiO2 and (b–d) a PPy–TiO2 filmhe surface in contact with the electrolyte (TiO2 rich side), (c) the surface in con

trochemically prepared at 1.6 mA cm−2 with a thickness of 15 �m: (a) and (b)ith the electrode (TiO2 free side) and (d) the cross-section.

X. He, G. Shi / Sensors and Actuators B 115 (2006) 488–493 491

Fig. 4. Cyclic voltammograms of (a) a PPy–TiO2 film grown at 1.6 mA cm−2 on a stainless steel electrode and (b) a free-standing PPy–TiO2 film (solid line) anda pure PPy film (dashed line). All films are in size of 15 mm × 2 mm × 0.015 mm and are studied in a 1.0 mol l−1 aqueous solution of LiClO4 at a scan rate of30 mV s−1.

Fig. 5. Photographs of bending motion of a PPy–TiO2 composite film actuator, which is held as TiO2-rich side in left, at various redox potential of (a) −1.0 V, (b)−0.8 V, (c) −0.6 V, (d) 0.2 V and (e) 0.5 V (vs. SCE).

at approximately 0.30 V (Fig. 4b, solid line). The �Ep (∼1.1 V)is larger than that of the film coated on a metallic electrode(∼0.48 V). In contrast, the CV of a free-standing pure PPy filmhas a broad oxidation wave at 0.08 V and a broad reduction waveat −0.62 V, and has a �Ep of only ∼0.7 V (Fig. 4b, dashed line).This is mainly because the conductivity of the composite film islower than that of the pure PPy film and also much lower than thatof the metallic electrode. Similar phenomena were observed bycarrying out the cyclic voltammetry at different scan rates. Theresults demonstrated that the reversibility of the electrochem-ical processes increased, while the electrochemical behaviordifference between the pure PPy and the PPy–TiO2 compositefilms decreased with the decreasing the potential scan rate.

The bending movements of the pure PPy and composite filmswith different TiO2 contents were studied electrochemically in a

1.0 mol l−1 LiClO4 aqueous solution. It was found that the purePPy film could bend only to one side, while the asymmetricPPy–TiO2 film could bend to both sides. This is mainly becausethe PPy–TiO2 film has a much better anisotropic structure thanthat of a pure PPy film.

As demonstrated in Fig. 5, a composite film containing 0.4%(v/v) TiO2 of 10 mm × 2 mm × 0.015 mm can uniformly bendfrom the position of 0◦ (vertical) to +60◦ (Fig. 5d) and +90◦(Fig. 5e) as it was oxidized at 0.2 and 0.5 V (versus SCE), respec-tively, or to −60◦ (c) and −90◦ (Fig. 5b) when reduced at −0.6and −0.8 V (versus SCE), respectively. Furthermore, this filmwas curled at −1.0 V, indicating its maximum bending angle ismuch larger than −90◦.

Fig. 6 plots the bending movement rates of a pure PPy and aPPy–TiO2 film at different driving potentials. The PPy–TiO2

F TiO2

ig. 6. Plots of movement rates of a free-standing (a) pure PPy film and (b) PPy– film at different driving potentials in a 1.0 mol l−1 aqueous solution of LiClO4.

492 X. He, G. Shi / Sensors and Actuators B 115 (2006) 488–493

film has a bending rate of ∼115◦ s−1 at a driving potentialof 1.0 V (versus SCE), which is about four times that of thepure PPy film (∼31◦ s−1) at the same potential. The move-ment rate of the PPy–TiO2 film increases quadratically withan increase of the applied driving potential in the range of−1.0 to +1.0 V (versus SCE), while the bending motion of thePPy film increases linearly. Moreover, the movement rate of thePPy–TiO2 film (Fig. 6b) is much higher than those of a single-layer AOT (sodium 1,4-bis(2-ethylhexyl)sulfosuccinate) dopedPPy film (∼60◦ s−1 at 1.0 V)[11], the single-layer polyani-line film (∼20◦ s−1 at 1.0 V) [8,12] and PPy bi-ionic actuators(∼30◦ s−1 at 1.0 V) [13].

In lifetime test, the PPy film was subjected to multiple poten-tial steps of reduction at −0.8 V and oxidation at 0.8 V at 0.5 Hz(bending angle >45◦). After 30,000 cycles, the film still canbend to 90◦ at a potential of 0.8 V. However, its movement ratedecreased with increasing the cycling number. After 22,000 or30,000 cycles, the bending movement rate of the film decreasedfrom 100 to about 45◦ s−1 or 5◦ s−1, respectively. This resultindicated that the monolithic PPy–TiO2 film maintained the abil-ity of bending, particularly the maximum bending angle was 90◦after the multiple potential steps. Accordingly, the lifetime ofthe actuator based on this monolithic PPy–TiO2 film is muchhigher than those of the PPy/Au actuators (∼2000 cycles) [14].The high performance of this actuator is mainly because the PPyfilm has a high conductivity and can do uniform bending at al

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4. Conclusion

An actuator based on a monolithic PPy–TiO2 nanoparti-cle composite film with a pure PPy layer and a PPy–TiO2composite layer has been successfully prepared by using anelectrochemical co-deposition technique. The incorporationof a little amount of TiO2 nanoparticles changed the mor-phology and properties of the PPy film greatly and con-structed a vertical anisotropic configuration. The actuator fab-ricated from this monolithic PPy–TiO2 composite film has alow driving potential, large bending angle and fast responserate, mainly due to the high conductivity and good mechan-ical properties of the film. Furthermore, there is no physi-cal interface between the pure and composite layers of thefilm. This overcomes the strong friction generated between thestretching and supporting layer of the multilayer actuators andextends the lifetime. The simple and cheap method developedhere can be used to synthesize various anisotropic conduct-ing polymer-nanoparticle composite films and correspondingactuators.

Acknowledgement

This work was supported by National Science Foundation ofChina (Nos. 50533030, 50225311).

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ow driving potential and without delamination problems.The cyclic voltammograms of the film after 30,000 cycles

ending movements under multiple potential steps of reductiont −0.8 V and oxidation at 0.8 V at 0.5 Hz (bending angle >45◦)howed a weak and broad oxidation wave at 0.90 V and a reduc-ion wave at −0.80 V. Their current densities are much lowerhan those of the original film. This is mainly because the filmxperienced an enormous number of repeated redox processesaused the decomposition or over-oxidation of the PPy chains8].

.3. Mechanism of actuation

The as-grown PPy film doped with DBS anions is in anxidized state, and therefore, it is a cation-driven (or cationwelling) film. When the film was reduced, the large DBS anionsimmobile counter ions) could not be ejected from the film. Forhe first several cycles, in order to maintain the overall chargeeutrality, the electrolyte cations (Li+ ions) and water moleculesrom the electrolyte were incorporated into the film, and the filmwelled [15,16]. After that, the ClO4

− anions in the electrolyteay also participate in the ionic motion. Thus, the electrochemi-

al process was dominated by cation driving, while being slightlyffected by the incorporation of ClO4

−. As the monolithic asym-etric PPy–TiO2 film has a PPy density gradient along the film

ross-section, it bends to the TiO2-rich side, as observed in thectuation test. On the other hand, when the film is oxidized,maller cations were released from the TiO2-free zone than thoserom the TiO2-rich zone, which causes the PPy-free zone toontract more than the TiO2-rich zone, so the film bends to theiO2-free side direction.

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Biographies

Ximin He received her bachelor from Shanghai Jiaotong University in 2003.She then entered the graduate school of Tsinghua University as a mastercandidate (2004–2007).

Gaoquan Shi received his BS (1985) and PhD (1992) degrees from NanjingUniversity. He joined Nanjing University as an assistant professor in 1992,and then was promoted to an associate professor in 1993 and a professorin 1995. He moved to Department of Chemistry, Tsinghua University as aprofessor (2000 to date). His interests focus on syntheses and applicationsof conducting polymers.