Nanoscale

PAPER

Cite this: Nanoscale, 2015, 7, 12372

Received 22nd April 2015,Accepted 3rd June 2015

DOI: 10.1039/c5nr02604k

www.rsc.org/nanoscale

Re-shaping graphene hydrogels for effectivelyenhancing actuation responses†

Jiangli Xue,a Chuangang Hu,a Lingxiao Lv,a Liming Dai*b and Liangti Qu*a

The development of actuation-enabled materials is important for smart devices and systems. Among

them, graphene with outstanding electric, thermal, and mechanical properties holds great promise as a

new type of stimuli-responsive material. In this study, we developed a re-shaping strategy to construct

structure-controlled graphene hydrogels for highly enhanced actuation responses. Actuators based on

the re-shaped graphene hydrogel showed a much higher actuation response than that of the common

graphene counterparts. On the other hand, once composited with a conducting polymer (e.g., polypyr-

role), the re-shaped hybrid actuator exhibits excellent actuation behavior in response to electrochemical

potential variation. Even under stimulation at a voltage as low as 0.8 V, actuators based on the re-shaped

graphene-polypyrrole composite hydrogel exhibit a maximum strain response of up to 13.5%, which is the

highest value reported to date for graphene-based materials.

1. Introduction

Artificial actuators that can transform shapes or physical pro-perties in response to various stimuli have attracted a greatdeal of attention due to their wide applications, which rangefrom robots, sensors, smart switches and memory chips toprosthetic devices.1–6 It is very important to design materialswith rational structures and unique properties for the develop-ment of novel high-performance actuators. In this regard, gra-phene possesses many distinctive properties for applicationssuch as actuation-enabled materials. Upon fast electrical char-ging and discharging, graphene based materials can exhibitreversible contraction/expansion, which is attractive for actua-tion applications.7,8 Moreover, the physical characteristics ofassembled graphene materials can be easily tuned by control-ling their shapes, which effectively expand the scope of theirpotential applications.9–11 For example, we constructed a newtype of one-dimensional (1D) fibrillar actuator from graphene/polypyrrole fibers for use in multi-armed tweezers and micro-tweezers.12 We also fabricated region-asymmetrically patternedgraphene/graphene oxide (G/GO) fibers by the region-selectivelaser reduction of freshly spun GO fibers and demonstrated

their potential as an amazing walking robot.3,4 Once twisted,the graphene fibers (GFs) can behave as a reversible rotarymotor.13 Furthermore, the spring-like flexible GFs not onlyexhibit outstanding performance as a reversibly stretchableactuator but also allow the development of novel magnetostric-tion switches.14 Therefore, modulating the assembly of gra-phene materials in a shape controlled manner plays anessential role in the development of functional grapheneactuation systems.

Recent studies performed by us and other groups havedemonstrated that 1D fibers,3,4,13,14 2D films,8,15 and 3Dnetwork structures6,16 of graphene materials hold promise foractuator applications. In particular, Biener et al. fabricatedmacroscopic 3D nanographene (3D-NG) assembles thatshowed a linear strain amplitude of 2.2% at an applied poten-tial of ±1.0 V.16 Our group built a superior 3D graphene-poly-pyrrole hydrogel (G-PH) actuator with a maximum strain of2.5% under a square wave potential of ±0.8 V.6 More recently, aspongy graphene (sG) paper with loose porous structures,which was generated by electrically induced thermal reductionof GO paper, has been shown to provide a large strain variationof 2.4% under 0.1 Hz at a high voltage stimulation of 10 V.17

However, the aforementioned graphene-based electrical/electromechanical actuators still exhibit limited deformationefficiency. In this study, we develop a re-shaping strategy forthe fabrication of 3D graphene-based hydrogel actuators,which could be actuated under a relatively low voltage stimu-lation (±0.8 V) with a very large strain response of up to13.5%. To the best of our knowledge, it is the largest strainvalue that has been reported to date for graphene basedactuators.

†Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nr02604k

aKey Laboratory of Cluster Science, Ministry of Education of China, Beijing Key

Laboratory of Photoelectric/Electrophotonic Conversion Materials, School of

Chemistry, Beijing Institute of Technology, Beijing 100081, China.

E-mail: lqu@bit.edu.cn; Fax: +86 10 68918608; Tel: +86 10 68918608bCenter of Advanced Science and Engineering for Carbon (Case4Carbon),

Department of Macromolecular Science and Engineering, School of Engineering, Case

Western Reserve University, Cleveland, OH 44106, USA. E-mail: liming.dai@case.edu

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2. Experimental2.1 Synthesis of the samples

Preparation of re-shaped graphene hydrogel (RSGH). 3D gra-phene hydrogel (GH) was fabricated by the hydrothermal treat-ment of 2 mg mL−1 of homogeneous GO as reported in ourprevious study.6,18 Then, the GH was loaded into a syringewith a similar diameter. Some water was kept in the syringe tomaintain a wet condition. Thereafter, the GH was slowly com-pressed down along the syringe to a certain degree. To main-tain the stable structure, the final RSGH sample was left in thecontainer for 2–3 days to solidify the shapes before beingreleased.

Preparation of G-PH and re-shaped G-PH (RG-PH). In atypical procedure, 400 µL fresh pyrrole (Py) monomer wasadded to 8 ml aqueous dispersion of GO (1 mg mL−1) and thesolution was stirred uniformly, and then transferred into a10 mL Teflon-lined autoclave for hydrothermal treatment at180 °C for 12 h. Py was then electrochemically polymerizedinto PPy using 0.2 M NaClO4 as the electrolyte to produceG-PH.

The process for RG-PH preparation is similar to that ofRSGH as mentioned above.

2.2 Actuation investigation

All the electrochemical investigations were carried out in athree-electrode system using a CHI660D electrochemical work-station, where GH, RSGH, G-PH or RG-PH acted as the workingelectrode, and an Ag/AgCl (KCl, 0.1 M) and Pt wire were usedas the reference and counter electrodes, respectively. Thedetailed measurements and calculations of the strainresponses are described in our previous study.6

2.3 Characterization

The morphology of the samples was examined with a scanningelectron microscope (SEM, JSM-7001F). Transmission electronmicroscopy (TEM) images and energy dispersive spectroscopy(EDS) data were collected on a TecnaiG2 20ST (T20) at anacceleration voltage of 120 kV. Raman spectra were obtainedusing an RM 2000 Microscopic Confocal Raman Spectrometer(Renishaw PLC, England) with a laser at 514.5 nm. The X-raydiffraction (XRD) patterns were recorded with a Bruker D8-Advance X-ray powder diffractometer, and Cu Kα was used asthe radiation source (λ = 1.54 Å).

3. Results and discussion

Our strategy for the controllable fabrication of the re-shapedgraphene hydrogel (RSGH) is based on a simple physical com-pression process (Fig. 1). To start with, a 3D graphene hydrogel(GH) is fabricated by the hydrothermal treatment of 2 mgmL−1 homogeneous GO as reported previously.6,18,19 Toprepare the RSGH, a cylinder of GH (ca. 0.7 cm in diameter,1.0 cm in height) was loaded into a syringe with similar dia-meter (Fig. 1a and b). Some water was placed in the syringe to

maintain a wet condition, as shown in Fig. 1b. Thereafter, theGH was slowly compressed down to a certain degree. Becauseof the flexible nature of graphene sheets, shape deformationcould occur under the external force without cracks within theinner microstructures. To maintain the stable reshaped struc-ture, the final RSGH sample was left in the container for 2–3days for solidifying its shape before the release of the com-pression force. A sample with 30% compression in height iscalled 30% RSGH (short for 30%). Accordingly, 65% and 85%RSGHs (65% and 85%) were also prepared.

Fig. 2a schematically shows the structure evolution of GHduring the re-shaping process, as evidenced by scanning elec-tron microscope (SEM) imaging. Fig. 2b shows an inter-connected porous structure with a pore size of up to hundredsof micro-meters within the interwoven graphene layers(Fig. 2b) for a typical GH pillar with a height of about 1.0 cmand a diameter of about 0.7 cm (inset of Fig. 2b). As expected,the pores within the compressed GH samples becamegradually compact (Fig. 2c–e) and the corresponding GHheights gradually decreased, while their diameters remainedunchanged (insets of Fig. 2c–e).

The Raman spectrum of GH exhibits two prominent bandsat around 1350 and 1591 cm−1, which are attributable to theD and G bands of hydrothermally reduced graphene oxide(Fig. 3a). No shift in the D or G band was observed for all there-shaped samples, which indicates physical deformation onlyfor the re-shaping; this was also verified by the correspondingX-ray diffraction (XRD) patterns with similar diffraction peaksat 2θ = 24.2° (Fig. 3b). Moreover, Brunauer–Emmett–Teller(BET) measurements (Fig. 3c) revealed that the freeze-driedGH, 30%, 65% and 85% RSGHs have a similar specific surfacearea (SSA) of 275–300 m2 g−1, which indicates that grapheneinterlayer stacking, if any, was insignificant during com-pression even for the 85% RSGH.

Fig. 1 Schematic illustration of the fabrication process of RSGH. (a)Initial GH. (b) GH is loaded in the syringe with water. (c) The GH is com-pressed by pushing the piston. (d) The RSGH is released aftersolidification.

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The actuation response of the re-shaped GHs was investi-gated according to the method described in our previous study(ESI, Fig. S1†).6 Strain responsive tests were carried out in athree-electrode system using an Ag/AgCl electrode as the refer-ence electrode and a Pt wire as the counter electrode in anaqueous solution of 1 M NaClO4. A slice of Au sheet in closecontact with the GH or RSGHs pillar was used as the workingelectrode and support accompanied with a glass slide and anarrow strip of oxidized Si wafer as the mirror for laser reflec-tion. The strain change was recorded by a displacementsensor. A detailed description of the strain response calcu-lation can be found in the ESI of ref. 6.

Fig. 4a shows the strain responses for pristine GH (initial),30%, 65% and 85% RSGH under an applied square wavepotential of ±0.8 V with a cycle time of 50 s. The initial GHshowed a small strain of ca. 0.05%. However, 30% RSGHexhibited an enhanced strain of ca. 0.12%, which is similar tothat of our previously reported loaded GH.6 The 65% and 85%RSGHs showed strong strain responses of ca. 0.44% and 1.3%,respectively. The detailed actuation behaviors of the GH andRSGHs pillars are shown in Fig. 4b. All the response curves areuniform and consistent with each other at the applied voltageof ±0.8 V. The saturated strain for the 85% RSGH decreasesslightly in the first several cycles, which is probably due to itsown expansion upon release from the syringe. As the cyclenumber increases, the strain response tends to be stable(Fig. S2†). Under the applied potential of +0.8 V, the pillars of

GH or RSGHs are contracted and exhibit a decline in theirresponse curves, and vice versa. The observed actuationresponses could be attributed to non-Faradaic electrochemicaldoping and undoping.8

For the 85% RSGH actuator, the saturated strain was up to2.9% under ±0.8 V with a cycle time of 1000 s (Fig. S3a†),which is higher than 2.2% of 3D-NG assembles at the appliedpotential of ±1.0 V (ref. 16) and 2.4% of an sG paper under arelatively high potential of ±10 V.17 During the measured500 cycles, the high strain response remained almostunchanged (Fig. S3b†). For the reshaped samples from 30%RSGH to 85% RSGH, however, their strains increased graduallywith increasing applied voltage (Fig. S3c†) and cycle number(Fig. S3d†).

To further improve the actuation performance, we preparedG-PH by hydrothermal process and electro-polymerization (see

Fig. 2 (a) Schematic diagram of the structure evolution that GH under-goes during the re-shaping process. (b–e) Cross sectional SEM imagesof (b) initial GH, (c) 30%, (d) 65% and (e) 85% RSGH (Scale bars: 10 µm).Insets of (b–e) show the photos of each sample with a scale bar of0.5 cm.

Fig. 3 (a) Raman spectra, (b) XRD patterns, and (c) nitrogen adsorptionisotherms of the pristine GH, 35%, 65%, and 85% RSGHs.

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the Experimental section).6 Unlike GH actuators that are oper-ated directly by electrical charging and discharging processes,G-PH actuators are driven by faradaic doping and undopingprocesses,8 which could induce a larger actuation response.Fig. 5a shows a typical G-PH sample with a height and dia-meter of about 1.5 cm and 1.0 cm, respectively. The interior

microstructures of the G-PH skeleton were imaged using SEM,which shows an interconnected porous network (Fig. 5b).Energy dispersive X-ray spectroscopy (EDS) reveals that theG-PH has a predominant C peak with weak N and O peaks,which indicate the presence of an electrodeposited PPy layeron the graphene sheets (Fig. S4†). The observed Cl peak origi-nated from NaClO4, which was used as the electrolyte forelectro-polymerization. The TEM image shows that the poly-pyrrole (PPy) layer is uniformly coated over the graphenesheets. The high resolution TEM image given in the inset ofFig. 5c reveals a layered structure with graphene sheets sand-wiched between the electrodeposited PPy layers. Selected areaelectron diffraction (SAED) presents a ring pattern due toamorphous PPY coating on the graphene sheets (Fig. 5d).Raman spectra (Fig. 5e) exhibit the typical D and G bands ofgraphene and a series of peaks at ca. 930, 1060, 1243, 1336,1393 and 1586 cm−1 from the PPy coating, which confirm thesuccessful polymerization of pyrrole on the graphenesheets.6,19 The XRD pattern (Fig. 5f) shows a broad peak at2θ = 24.3°, which arises from the (002) plane of stackedgraphene sheets within the G-PH.20

The process to produce re-shaped G-PH (RG-PH), 30%RG-PH, 65% RG-PH and 85% RG-PH is similar to that for thepreparation of RSGHs, as mentioned above. Fig. 6a–c showsthe digital images of 30%, 65% and 85% RG-PH. It can beclearly seen that the G-PH could be re-shaped to have anyspecific height between those of the pristine RG-PH and 85%RG-PH. Even at 85% compression, the highly compact RG-PHstill maintained its structural integrity with an unchanged dia-meter and no obvious cracks, which is similar to the RSGH.The interior microstructures of the 30%, 65% and 85% RG-PHare revealed in Fig. 6d–f. As can be seen in Fig. 6d, the 30%RG-PH is full of elliptic holes, and the initial micropores(Fig. 5b) are squished into a more compact structure. A fluffy,but ordered, structure is observed for the 65% RG-PH (Fig. 6e),while a densely corrugated congregation appears for the 85%RG-PH (Fig. 6f).

Similar to the RSGHs, the variation trend of strain responseis gradually enhanced from the initial G-PH to the 85% RG-PH

Fig. 4 (a) Comparison of the strain response of the GH and 30%, 65%and 85% RSGHs under an applied square wave potential of 0.8 V with acycle time of 50 s. (b) Actuation behavior of the GH and 30%, 65% and85% RSGHs under an applied square wave potential of 0.8 V with a cycletime of 50 s.

Fig. 6 (a–c) Photographs of 30% RG-PH, 65% RG-PH and 85% RG-PH,respectively. The bars in (a–c) are 0.5 cm. (d–f ) SEM images of 30%RG-PH, 65% RG-PH and 85% RG-PH, respectively (Scale bars: 10 µm).

Fig. 5 (a) Photograph, (b) SEM image, and (c) TEM image of the G-PH.The inset in (c) is a magnified view of the G-PH sheet edge. (d) Thecorresponding EDS spectrum of the G-PH. (e) Raman spectra of theG-PH excited at 633 nm. (f ) XRD patterns of the G-PH.

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under the applied square wave potential of ±0.8 V with a cycletime of 50 s (Fig. 7a). It can be seen that the actuationresponse for the initial G-PH is relatively small due to its rela-tively poor electrical conductivity (Fig. S5†) or limited expan-sion space. 30% RG-PH has an actuation strain of up to 0.6%,which is similar to that we reported in previous work with aload of 20 g by weight,6 and the strain response of 65% RG-PHactuator is up to 1.0%. Impressively, the 85% RG-PH exhibitsan excellent response of ca. 5%, which is much higher thanthe other actuation strains mentioned above. Fig. 7b shows theuniform actuation behaviors of the G-PH and RG-PHs. When+0.8 V is applied, the G-PH or RG-PH actuators expand, whichis consistent with the uptrend of the response curves attribu-ted to the Faradaic electrochemical doping and undopingprocesses.

As can be seen in Fig. 8a, the 85% RG-PH actuator exhibitsregular actuation behavior under various square wave inputswith cycle times of 50 s. Under the same threshold, the actua-tion strain gradually increases with the stepwise increase ofvoltage, which indicates a strong dependence of the strain onthe applied potential. Fig. 8b shows the linear increase of thestrain response with an increase in the applied voltage. Even ata relatively low applied square wave potential of ±0.6 V, theRS-PH can reach a strain of 3.0%, which exceeds themaximum strain response generated by the 85% RSGH.

Fig. 9a shows the strain behavior of the 85% RG-PH actua-tor with different cycle times at a constant square wave poten-tial of ±0.8 V. A uniform strain response is evident for all thestates, which indicates that the actuator can promptly andreliably respond to external stimuli. It is interesting to notethat the strain also depends on the cycle time, which can besteadily enhanced by increasing the cyclic period at ±0.8 V(Fig. 9b). The saturated actuation strain reached a record of13.5% under ±0.8 V, which is considerably larger than that ofgraphene/MWCNT21 or graphene/Ag,22 approximately 17 timeslarger than that of the multi-walled carbon nanotubes-COOH/polymer (MWCNT-COOH) actuator under a similar rate butrelatively higher potential of ± 2.0 V,23 and about 6 times that

of the normal 3D G-PPy actuator,6 thus revealing the importanteffect of the corrugated structure, which is induced by re-shaping, on actuation performance.

For comparison, Table 1 24 lists performance datafor typical actuators based on graphene or othermaterials.6,16,17,21–23,25 Clearly, the 85% RG-PH actuator exhi-bits the highest strain.

The strain response of the 85% RG-PH actuator decreasesonly by ca. 10% over 500 measurement cycles, which indicatesa relatively stable actuation performance (Fig. S6†). It is alsofound that the changes of strain depend on the applied squarewave potentials with a linear relationship (Fig. S7a†). The cycletime is positively correlated to the strain response as well(Fig. S7b†). From G-PH to the 85% RG-PH, the actuationresponse has a stepwise increase under a constant potential ora constant cycle period.

As mentioned above, the re-shaping strategy is successfulfor achieving a high actuation response of a GH. The re-shaped compact structure indeed induced a shortenedpathway for the transmission of ions or charges and improvedelectrical conductivity26 while maintaining the 3D porous con-figuration. In other words, the corrugated structure has the

Fig. 7 Comparison of the strain response (a) and actuation behaviors(b) of the G-PH, 30% RG-PH, 65% RG-PH and 85% RG-PH under theapplied square wave potential of 0.8 V with a cycle time of 50 s.

Fig. 8 (a) The strain responses of the 85% RG-PH actuator underdifferent square wave potentials with a cycle time of 50 s. (b) The corres-ponding strain changes as a function of the applied voltages.

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ability for a larger expansion from the appropriate stimuli.According to the possible actuation mechanism, the samplevolume would be inflated when a voltage is applied for acertain time. The actuation process, which is induced bychanges in the inner corrugated structure, is schematicallyshown in Fig. 10a. As can be seen, the dense layers show tightbinding between graphene sheets. Under electrochemicalstimulation, the corrugated structure is prone to expand dueto the intercalation of charges or small mobile ions. Forexample, the G-PH actuator that is not subjected to the re-shaping treatment caused no obvious change in the mor-

phology compared to that of the initial G-PH even when thepositive potential was applied for a long time (Fig. S8a andb†). As seen in Fig. 10b and c, however, the morphologicalstructure of the initial RG-PH (Fig. 10b) is considerablydifferent from that of the same RG-PH (85%) under an appliedpotential (Fig. 10c). The compact layers of the RG-PH areswollen in a self-supportive manner. The obvious reconfigura-tion of the graphene sheets within the sample ensures thehigh strain response, as we have observed above. Contrarily,when a negative potential was applied, the 85% RS-PH actuatorshrunk, which led to a relatively small pore size (Fig. S9†).

The re-shaping strategy is indeed efficient for enhancingactuation responses (Fig. S10†). To further demonstrate thispoint, the G-PH with a height same as the 85% RG-PH samplewas prepared by cutting the initial G-PH, which showed a veryinsufficient response compared with the re-shaped counterpart(Fig. S11†).

4. Conclusions

In summary, we have developed a novel re-shaping strategy forthe fabrication of high strain-responsive actuators from gra-

Table 1 Summary of actuator performance with different electrodes from the literature and this article

Electrode Condition Strain (%) Ref.

3D G-PPy ±0.8 V, 0.00025 Hz 2.5 63D-NG ±1.0 V, 4 h cyclic time 2.2 16sG paper ±10 V, 0.1 Hz 2.4 17Graphene/MWCNT ±2 V, 1 Hz 0.0287 21Graphene/Ag ±2 V, 8.33 Hz 0.053 22MWCNT-COOH ±2 V, 0.05–0.005 Hz 0.78–0.8 23Shape memory alloy — 8 25GH ±0.8 V, 0.2 Hz 0.05 This work85% RSGH ±0.8 V, 0.2 Hz or 0.001 Hz 1.3–1.5 or 2.5–2.9G-PH ±0.8 V, 0.2 Hz 0.0985% RG-PH ±0.8 V, 0.2 Hz or 0.001 Hz 5.1–5.4 or 12.8–13.0

Fig. 10 (a) Schematic diagram of the actuation process for a RG-PHactuator under an applied voltage. SEM images of the microstructures ofthe 85% RG-PH at the initial state (b) or under an applied potential of+0.8 V for a certain time (c).

Fig. 9 (a) The strain responses of the 85% RG-PH actuator at an appliedsquare wave potential of 0.8 V with different cycle times. (b) The corres-ponding strain changes as a function of cyclic periods.

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phene-based hydrogels. This strategy is effective for actuatorsthat are based on not only GHs but also graphene composites.All the actuators studied in this work could be actuated undera low voltage (±0.8 V) stimulation and it is noticeable that themaximum actuation strain of the 85% RG-PH can reach arecord high of up to 13.5%, which is almost 17 times largerthan that of a MWCNT-COOH actuator, and is about 6 timesthat of normal 3D G-PPy actuators. The exceptionally largestrain amplitude at a low voltage and the availability of centi-meter sized 3D structured monolithic samples of GHs provideconsiderable room for the development of various macroscopicsmart materials and devices that can effectively respond toexternal stimuli.

Acknowledgements

We thank the financial support from NSFC (no. 21325415,21174019), National Basic Research Program of China(2011CB013000), Beijing Natural Science Foundation(2152028) and the 111 Project B07012.

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Paper Nanoscale

12378 | Nanoscale, 2015, 7, 12372–12378 This journal is © The Royal Society of Chemistry 2015

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