Stimuli-deformable graphene materials: from nanosheet to macroscopic assembly

Stimuli-deformable graphene materials: from nanosheet to macroscopic assembly

Materials Today  Volume 00, Number 00  November 2015 RESEARCH: Review RESEARCH Stimuli-deformable graphene materials: from nanosheet to macroscop...

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Materials Today  Volume 00, Number 00  November 2015

RESEARCH: Review

RESEARCH

Stimuli-deformable graphene materials: from nanosheet to macroscopic assembly Fei Zhao, Yang Zhao, Nan Chen and Liangti Qu* Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, Key Laboratory of Cluster Science, Ministry of Education, School of Chemistry, Beijing Institute of Technology, Beijing 100081, PR China

Stimulus-induced deformation (SID) of graphene-based materials has triggered rapidly increasing research interest due to the spontaneous response to external stimulations, which enables precise configurational regulation of single graphene nanosheets (GNSs) through control over the environmental conditions. While the micro-strain of GNS is barely visible, the deformation of graphenebased macroscopic assemblies (GMAs) is remarkable, thereby presenting significant potential for future application in smart devices. This review presents the current progress of SID of graphene in the manner of nanosheets and macroscopic assemblies in both the experimental and theoretical fronts, and summarizes recent advancements of SID of graphene for applications in smart systems. Introduction Considerable interest has been paid to Graphene [1] due to its potential in a variety of research areas, such as structural materials [2–7], electronics [8–12], and energy storage [13–20]. Graphene has a carrier mobility of up to 200,000 cm2 V 1 s 1, a thermal conductivity of 3000–5000 W m 1 K 1 at room temperature [1,21], a high surface area of 2630 m 2 g 1 [22], good optical transparency of 97.3% [23] and excellent mechanical strength with a Young’s modulus of 1.0 TPa [24] by its two-dimensional (2D) single-atom layer conjugate structure. However, the chemical activity and dispersibility of single layered graphene nanosheets (GNSs) are limited by their unreactable and tightly stacked honeycomb conjugate structure, hindering further applications of graphene. In this context, the functionalization of graphene is a promising strategy for the large scale preparation of graphene derivatives, which presents unexpected properties and partially maintains the performance of pristine graphene at the expense of producing structural defects. Various functionalization methods, including chemical modification, physical mixture and self-assembly have been developed in the past few years to promote intensive study of graphene derivatives [25–29]. As a consequence, a series of novel graphene-based smart materials, which provide advanced functions beyond the intrinsic properties of graphene, were presented [30–40]. *Corresponding author. Qu, L. ([email protected])

Smart materials refer to those lifeless materials that assimilate animated functions such as sensing, deformation, coloring and self-healing to adaptively react to changes in the environment in a constructive and mostly repeatable way. Among them, the stimulus-induced deformation (SID) enabled materials that shrink, swell, bend or swing in response to external stimuli have attracted enormous attention owing to their potential for use in intelligent robots, biomedical devices and electro-mechanical systems [41,42]. Currently, deformable materials can be constructed from polymer and/or nano-materials which are able to spontaneously vary their conformation with environmental changes such as pH, temperature, electrical, light and other stimuli [43–53]. The microcosmic change of morphology induced by conformation variation was amplified in their macroscopic assemblies, hence causing remarkable deformations. In view of the excellent electrical, thermal and mechanical properties mentioned above, graphene-based environmentally responsive materials have emerged as promising candidates for next generation smart deformable materials with special functions (e.g. conductivity or chemical stability) in comparison to other stimuli deformable materials based on polymers or metals, and have become a fastgrowing research field. Benefiting from the mass production of the building blocks based on GNSs, the fabrication of graphene-based macroscopic assemblies (GMAs) with functional micro- and/or nano-structures

1369-7021/ß 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). http://dx.doi.org/10.1016/ j.mattod.2015.10.010

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hydrogen bonding and electrostatic forces, which have been used for the assembly of GO to generate a microcapsule capable of releasing the drug molecule [62]. It suggests that the oxygencontaining groups provide an adjustable surface state for the regulation of inner- and inter-sheet interactions. Moreover, as molecular-weight dependent weak interactions, Van der Waals forces become significant in the GNSs, and hence the surface adhesion effect allows the GNSs to conform to other surfaces, which is useful for manipulating their deformation [63]. The interplay mentioned above is affected by the environmental conditions of GNSs, leading to a variety of deformation behavior. Additionally, the deformation induced inner-sheet stress could act as a restoring force, tending to bring the GNSs back to initial planar configuration. Therefore, GNSs will maintain a folded conformation when the attractive force is stronger than the restoring force, and return back to planar configuration once the force balance reversed. FIGURE 1

Schematic illustration of strategies toward stimuli-deformable graphene materials from nanosheet to macroscopic assembly.

became possible. Characteristic of the building blocks and assembling method, GMAs exhibit enhanced chemical, electrical and mechanical properties together with many new functions such as ionic adsorption, electrochemical catalysis and stimuli response [54–57]. Therefore, the development of functionalized architectures derived from GNS-based building blocks is indeed of considerable interest, and research toward GMA enabled smart deformable materials has been a central focus for materials scientists in recent years. To date, a number of synthetic methods have been presented to prepare GMA with novel functions, which have previously been summarized by comprehensive reviews [58–61]. However, a targeted overview on GMA based smart deformable materials is still absent. Thereby, in this review, we will summarize recent advancements on the fabrication of graphene-based SID systems contributed by us and other groups, mainly focusing on the environmentally responsive graphene nano-materials and their SID enabled assemblies as shown in Fig. 1. Since the GNSs are able to act as an actuating component and passive platform, the actuation-mode-dependent SID behaviors of GMAs are also discussed.

SID mechanism of GNSs The SID of GNSs, including planar expansion and spatial warping through a fold event of graphene, depends on the stretch and rotation of carbon–carbon bonds, which are predominated by the surface chemical state. For instance, on a defect-free surface of GNS, the remarkable p–p interactions between the polycyclic aromatic hydrocarbon structures enabled graphite-derived or graphene lattice-derived aggregations. While there are many possible routes toward the regulation of the surface chemical state of GNSs, the current widely accepted strategy is the modified Hummers’ method [25], which can oxidize graphite to graphite oxide nanosheet, where the graphene oxide (GO) are obtained by the subsequent solvent-exfoliation of the graphite oxide. The oxygencontaining groups are bonded on the GNS, introducing structural defects and surface charging. Meanwhile, the presence of the oxygen-containing groups will facilitate the interactions of

Deformation by stretching chemical bonds Chemical bonds stretching in GNSs Theoretical studies of expansion behavior in GNSs induced by charge injection and ionic liquid immersion have revealed that the electrostatic double layer (EDL) could lead to a strain of more than 1% and its contribution to the overall strain is always higher than that of quantum-mechanical strain (0.2%) from charge injection of 0.08 e per C atom with electric potentials of 1 V (Fig. 2a) [64]. Meanwhile, these results also show that maximizing the electrolyte-accessible surface areas will enhance the EDL effect, and hence enlarge the deformation of GNSs. Despite the lack of direct experimental observation of the expansion behavior for a single GNS, the stretching effect of graphene chemical bonds has been determined to be in part due to the stabilization of thermal fluctuations across the sheet when freely suspended on a micro-fabricated scaffold in air or vacuum [65]. The atomic scale observation revealed that the single layered GNS were not perfectly flat but exhibited microscopic deformations with a local strain of up to 1% induced by carbon-carbon bond stretching, which was as large as theoretical predictions and was sustainable without plastic deformation and the generation of defects. Furthermore, this inner-sheet strain was also presented in those solution-dispersed GNSs [66]. When the average sizes of GNSs ranged hundreds of nanometers, the partial stretch of chemical bonds resulted in small wrinkles on the surface of planar structure. For the smaller GNSs, a slight bending behavior has also been demonstrated.

Chemical bond stretching in GO The simulation study of GO demonstrated that a highly ordered compound of GO is able to mimic the behavior of mammalian skeletal muscle to provide large expansive strain of 10% and contractive strain of 4.8% upon electron injection of 0.15 e per C-atom within 1 ns (Fig. 2b) [67]. More impressively, beyond a reversible strain of 6.3%, the huge 28% irreversible strain is shown to be the result of a change in the atomic structure of GO from a metastable clamped state to a stable unzipped configuration (Fig. 2c) [68]. Significantly, this strain generation mechanism makes it possible to hold a constant strain of 23.8% upon removal of the input power, enabling this material for long-term, lowpower switching applications.

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FIGURE 2

(a) Schematic illustration of charge injection induced strains of GNSs. Reproduced with permission from Ref. [64] Copyright American Chemical Society (2012). (b) Unit cell (denoted by dotted lines) of C4-asym-unzip GO. The inset table lists the a and b lattice parameters for C4O-asym-unzip GO and pristine graphene, as well as the associated ab initio k-point grids. C and O atoms are represented by small-silver and large-purple spheres, respectively. Reproduced with permission from Ref. [65] Copyright AIP Publishing LLC (2013). (c) Schematic illustration of hole injection induced strains of GNSs. Reproduced with permission from Ref. [66] Copyright American Chemical Society (2012).

Deformation by carbon skeleton bending Carbon skeleton bending in GNSs Recent theoretical investigations have demonstrated a series of out-of-plane deformation of GNSs dominated by carbon skeleton bending, which can be controlled by various external stimuli, such as temperature change [69,70], edge-stress [71,72], and surface tension [73]. For instance, Iijima and coworkers demonstrated that the edges of adjacent GNSs can be closed by a thermal treatment, and hence equivalently achieve the folded GNSs [69]. Xu et al. [70] further reported that the conformation of GNSs can be controlled by a cooperation of thermal effect and other experimental conditions such as surface modifications, substrate and size of GNS. Shenoy et al. [71] showed that edge stresses introduced intrinsic ripples in GNSs even in the absence of thermal effect. Compressive edge stresses of the sheets result in various warping shapes with a certain amplitude and penetration depth proportional to the strength. This effect can lead to the twisting, scrolling and shrinking conformation for rectangular GNSs. Moreover, the results from both finite deformation beam theory and molecular dynamic simulations indicate that the flattened GNSs are metastable [72], and hence water nanodroplets can activate and guide the deformation of planar graphene nanostructures [73]. Once the nanodroplets initiate conformational changes and overcome deformation barriers associated with the nanostructures, the rapid bending, folding, sliding, rolling or zipping of

the planar sheets was achieved. Since GNSs are flexible in 2D plane, a stretch deformation can be geometrically achieved by curl (Fig. 3a) and fold process with a self-overlap of GNSs (Fig. 3b). If the graphene is hydrogenated on one side, it can completely scroll up and remains stable at room temperature [74] with a certain radius depending the water content [75]. The experimental results agree very well with those theoretical investigations. As described by Xie et al. [76], the surface tension exerted on the GNS during drying can lead to the scrolling of the planar layer. Therein, the substrate, edge condition and shape strongly influenced the ability of GNSs to deform. Additionally, since the initial energy barrier associated with the deformation was overcome by capillarity of solvent evaporation, the regulation of the drying process also altered the scrolling behavior of GNSs. Furthermore, GNSs can be dispersed with the aid of a range of surfactants at low concentration in solvent systems, where zeta potential of the GNSs is largely controlled by ionic surfactants, hence the shape of GNSs can be controlled by the use of surfactants with a certain concentration. Zhang and coworkers demonstrated that surfactants can accurately control the deformation of GNSs by tuning the electrostatic potential barrier and steric potential barrier of graphene [77]. However, depending on the type of surfactant, the residual surfactants are not easily removed from carbon nanomaterials, and may lead to a decrease in electrical performance [78]. Furthermore, defamation of GNSs can also be 3

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by electrostatic interaction in acidic environments. In line with the self-overlap strategy for folding GO, the flat GO layer can be converted into the crumpled paper-ball-like morphology, presenting a new possibility for scalable control of their deformation [83].

SID behavior of GMA

RESEARCH: Review FIGURE 3

(a) Roll and (b) fold of a GNS, which is activated and guided by a nanodroplet of water. Reproduced with permission from Ref. [73] Copyright American Chemical Society (2009). Model sheets of (c) GO and (d) SLG with the geometry calculated with the PM6/MOZYME method: unfolded bands and maximum folded structures corresponding to local minima on the potential energy surface. Reproduced with permission from Ref. [81] Copyright American Chemical Society (2012).

achieved by the exertion of an electrostatic field of appropriate intensity. Along with the mechanism for edge stress induced conformational changes of graphene, electric fields introduce charged edges and lift GNSs from the substrate surface. The combination of these two factors is sufficient to cause the scrolling or folding of GNSs [79].

Carbon skeleton bending in GO On the other hand, different from the pure graphene system, GO nanosheets have remarkable dispersibility and chemical reactivity, where the presence of oxygen groups on the graphene surface introduces a charging effect, making GO more attractive in terms of processability [80]. Therefore, the deformation of GO can be realized in efficient ways including salt induced assembly [81], pHdriven conformational change [82] and the surface charge effect [83]. The conformation of GO is directly inferred from functionalized regions that render inter-sheet interactions. For example, repulsion between GO caused by surface charges can be overcome by adjusting the concentration of salt, and thus the GO tends to aggregate, which is useful for deformation control of GO [81]. Additionally, this surface charge induced interaction can also be controlled by pH. The high pH value of the chemical environment results in deprotonation of the hydrophilic oxygen-containing groups in GO, establishing negative surface charge. It is reported that the zeta potential of GO is below 30 mV when the pH is less than 3 and can reach 45 mV at the pH approaching 10 [82]. The GO sheet has sufficient charge to repulse the other GO sheets and remains isolated when the zeta potential is more negative than 30 mV. Additionally, at low pH, GO is observed to appear folded as shown in Fig. 3c, presenting tighter folded configuration than GNS (Fig. 3d). Thereby, the self-overlap of the GO sheet was guided

GMAs have been attracting significant attention as they not only possess the intrinsic properties of 2D graphene but also provide advanced functions with improved performance in various applications. Benefiting from the remarkable interactions mentioned above, GMAs can be obtained through different procedures, including the direct growth of graphene on metal foams [7], selfassembly of GO during reduction [84], solvent exchange and evaporation induced self-assembly [85], freeze-drying of GO solution [86], hydrothermal treatment [87–89], and so on. Considering that GMAs are constructed by incomputable GNSs, the tiny deformations of each single sheet as mentioned above will be amplified in different degrees and hence obvious deformations can be achieved. Moreover, beyond the intra-sheet interaction, various inter-sheets bridging effects are presented by those assembled structures. Therefore, the regulation of the interplay between architecturally connected sites can be of use to control the deformation of GMAs. In this section, the SID behavior of GMAs are generally classified as SID achieved by a graphene active component and SID supported by a graphene flexible skeleton.

Graphene as an active component of SID Graphene-based SID The excellent electrical, thermal and optical properties of graphene enable a series of graphene-based stimuli-sensitive deformable structures. For instance, utilizing the electrical properties of graphene, Chen and coworkers demonstrated an electrically actuated deformation of free-standing flexible graphene-based papers [90]. An elongation strain of 0.064% was observed at an applied potential of 1 V versus saturated calomel electrode in 1 M NaCl solution, and the strain could be improved to 0.1% by the addition of magnetic Fe3O4 nanoparticles to lessen the sheet stacking of GNSs. In a different way, we have realized an all-graphene based deformable structure by asymmetrically modifying the opposite surfaces of a monolithic graphene film using hexane plasma (HexP) and oxygen plasma (O-P), respectively (Fig. 4a). Therein, the O-P treated graphene strip displayed a reversible length change of up to 0.2% in response to the applied voltages, which was about 20–50 times larger than that of its Hex-P treated counterpart (Fig. 4b) [91]. As a result, the GMA presented a saturated curvature of ca. 0.6 cm 1 despite the rigidity of the graphene strip and its displacement decreased only by ca. 10% over 90 measurement cycles. It was also found that O-P treatment of a freestanding graphene film made by direct filtration of aqueous graphene colloidal suspensions could notably improve the deformation of the graphene film to a strain of ca. 0.85% at 1.2 V [92], which is four times higher than that for carbon nanotube (CNT) based deformable structures, hence demonstrating that tiny deformations of each single sheet was amplified in GMAs to achieve the macroscopical deformations [93]. The deformation of the O-P treated GMA was adequately load-tolerant, only suffering a slight reduction at 6.1 MPa (Fig. 4c,d). In order to further increase the deformation, a polypyrrole (PPy)–graphene bilayer GMAs was fabricated (Fig. 4e) [94].

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FIGURE 4

(a) Illustration of asymmetric plasma treatments of the GMA with hexane and oxygen, and the corresponding surface wettability. Water contact angles for the Hex-P (left inset) and O-P (right inset) treated surfaces are ca. 908 and ca. 158, respectively. (b) The length changes of the GMA treated with Hex-P and O-P under applied square wave potential of 1.2 V, L and DL are defined as the length and length change of the GMA. Reproduced with permission from Ref. [89] Copyright American Chemical Society (2010). (c) The apparatus used for characterizing the load-tolerant, highly strainresponsive GMA, and (d) the image of a final fabricated device. Reproduced with permission from Ref. [91] Copyright The Royal Society of Chemistry (2011). (e) Schematic illustration of the SID mechanism, where the charges in each electrode are completely balanced by ions from the electrolyte. Reproduced with permission from Ref. [92] Copyright The Royal Society of Chemistry (2012).

The combination of graphene and PPy film was devised to harvest their synergetic function for remarkable deformation, and the PPy–graphene bilayer film demonstrated reversible warping with a large bending ability of about 1208, superior to that of the pure graphene films and overcoming the relatively poor mechanical strength of PPy films. As another improved all-graphene deformable structure, Hu et al. [95] presented a low voltage induced deformation of foldable corrugated structure of GMA, as shown in Fig. 5a,b. A macroscopic length contraction with a strain of 2.4% is presented for the GMA under applied voltage of 10 V within 0.5 s (Fig. 5c,d), which is probably due to the interlayer gas expansion and the collective effect of the GNSs with intrinsic negative coefficient of thermal expansion (CTE). The negative CTE of this GMA along the length direction is estimated to be

FIGURE 5

Cross sectional SEM images of (a) a GO paper and (b) spumous GMA paper. Schematic setup for measuring the thickness displacement of the spumous GMA paper. (c) Displacement of the spumous GMA paper along the thickness direction under 0.1 Hz square wave voltage. (d) Schematic diagram of the corrugated structure-causing deformation process under the applied voltage. Reproduced with permission from Ref. [93] Copyright The Royal Society of Chemistry (2014). (e) Schematic diagram of the proposed polymer-improved spumous GMA paper. Reproduced with permission from Ref. [94] Copyright The Royal Society of Chemistry (2014).

approximately 10 4/8C while the positive CTE along thickness direction is about 10 3/8C. Later, they selected polydimethylsiloxane (PDMS) to improve the deformable GMA (Fig. 5e) and realized an ultra-large bending curvature of about 1.2 cm 1 at 10 V for 3 s with a high displacement-to-length ratio of ca. 0.79 [96], which is much larger than that of the other electromechanical deformable GMA [97–99]. On the other hand, beyond the electrothermal effect, direct heating is also an efficient way to fabricate deformable structures based on the CTE of graphene. Theoretically, graphene has demonstrated distinctive large negative CTE (contraction upon heating) values ranging from 7 ppm K 1 for single-layer GNS to 1 81.2 ppm K for graphene nano-crystal [100,101]. Conley et al. [102] studied the curvature of bimorph graphene–SiNx and graphene–Au cantilevers and compared the strain, CTE, and the adhesion force acting on GNS attached to SiNx and Au substrates (Fig. 6a). They found that all the cantilevers were made to be considerably bent toward the graphene side at room temperature, and the degree 5

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(a) Schematic illustration of temperature dependence of curvature for graphene bimetallic-like cantilevers. (b) Temperature dependence of the cantilever during the initial heating (red)/cooling (blue) cycle immediately after fabrication. (c) The temperature-dependence of the profile for the same device in subsequent heating/cooling cycles. Reproduced with permission from Ref. [100] Copyright American Chemical Society (2011).

of bending became temperature dependent following an initial annealing step (Fig. 6b,c). Meanwhile, some irreversible deformation during the initial heating/cooling process turned reversible in successive temperature cycling.

SID in GO-based assemblies Zhu et al. [103] presented a deformable GO paper that experienced reversible contraction/expansion upon heating/cooling between 30 and 808C, which is similar to the negative thermal expansion of graphene. They carefully investigated this large equivalent negative thermal expansion of GO up to 130.14 ppm K 1 and found that it in fact originated from thermo-hydration effects that were highly dependent on the relative humidity and the state of water in GO. Along with this mechanism, Mu et al. [104] prepared GMA with a gradient reduced graphene oxide/graphene oxide (rGO/ GO) structure, which provided reversible, fast (0.3 s), powerful (7.5  105 N kg 1 force output) and controllable mechanical deformation and recovery.

Graphene as flexible skeleton of SID Apart from playing an active role in deformable GMAs, graphene has been used as a mechanical support in the active component/ graphene composite systems for deformation purposes. Ruoff and coworkers showed a deformable GMA prepared by sequential filtration of multi-walled carbon nanotubes (MWCNTs) and then GO platelets to form a bilayer of MWCNT–GO paper [105]. The water content of 17% in the GO layer was confirmed by thermal gravimetric analysis, while the MWCNT layer was free of water content, indicating an asymmetric swell of GO and MWCNT layers depending on the water. As a result, the bilayer paper initially rolled up with the MWCNT side facing outward at a low RH of 12% at room temperature, gradually unrolled to an almost flat state around a RH of 55% to 60%, started to curl in the opposite

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direction of GO side when the RH exceeded 60%, and eventually fully curled at a RH of 85%. Lu et al. [106] demonstrated that ionic liquid could be inserted to separate the layers of paralleled GNSs (Fig. 7a), producing a GMA with ionic liquid of 66.7 wt%, and presenting variation in thickness as large as 98% under a 2 V electrical stimulation. However, the unstable parallel structure (due to the lack of binder and large volume deformation) led to the breakdown of the electrode under high voltage, long duration time or repetitive cycles. To address this issue of instability, an improvement was carried out by introducing MWCNTs to reduce the restacking of rGO, and the rGO–MWCNT system exhibited an excellent long-term stability of up to a million cycles [107]. The GO presented excellent hydrophilia so that water molecules can act as inserts in a GO-water system (Fig. 7b), while the desiccation of GO will lead to a shrinking of GO papers (Fig. 7c). Consequently, the deformation was deemed to originate from the difference between the amounts of inserted water in both layers under different RH values. Recently, the region-asymmetric deformable fibriform GMA was fabricated by our group in virtue of the laser positioning reduction of the freshly spun GO fibers [108]. The asymmetric graphene fiber was conveniently fabricated by scanning a laser beam along the preformed GO fiber, where the scanned area was converted to rGO and the unexposed region remained unchanged. As expected, a rapid bending to the rGO side occurred once the GMA was exposed to moisture with a RH of 80%, while the fiber recovered its initial state as the environmental humidity regressed to the ambient condition of 25%. This process was fully reversible with an average motion rate of ca. 88 s 1. Moreover, a regionspecific asymmetric GMA was also designed for use as complex deformation-predefined systems. By rational design and localization of the laser-induced rGO regions along the GO fiber, more sophisticated shape changes can be achieved. Furthermore, we also reconstruct the intrinsic configuration of GNSs within the GMA and achieved a new type of moisture-driven rotational motor by rationally designing the rotary processing of the freshly spun GO fiber hydrogel [109]. As schematically illuminated in Fig. 8a, the deformable GMA can be conveniently fabricated by simply rotating the freshly spun GO hydrogel fiber along the axis. The twisting process has largely remolded the intrinsic structure of GF. As shown in Fig. 8b, the initial axis oriented arrangement of GO sheets becomes a helical configuration after the twisting process and the compact GNSs on the surface of the GO fiber conform to the rotary direction (Fig. 8c). The formed helical geometry of obtained GMA would enable the reversible torsional rotation under the alternation of humidity with stable speed of up to 5000 rpm (Fig. 8d,e). Along with this asymmetric moisture inserting strategy, more and more SID systems based on GMA have been presented recently (Fig. 8f,g) [110–113].

State-of-the-art applications The SID process has enabled lifeless GMAs to work as living organisms, thereby these GMAs were able to realize some complex functions of living organisms through specific design. For instance, a voltaic-responsive smart claw was fabricated by GMA/ polymer bilayer [96]. As shown in Fig. 9a,b, a ‘‘tri-finger’’ mechanical gripper could quickly grab and put down objects with an applied on and off voltage. Considering that the weight of these

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FIGURE 7

(a) Illustration of ion insertion induced graphite expansion (Reproduced with permission from Ref. [104] Copyright Royal Society of Chemistry 2012.) and (b) water adsorption/desorption process in GO paper (Reproduced with permission from Ref. [101] Copyright American Chemical Society 2011). (c) Structural changes in response to water absorption and desorption are shown in the AFM images. The resulting deformation is clearly visible. The blue regions in the sketches indicate the rGO face of the GMA, while the brown regions indicate the GO face. Reproduced with permission from Ref. [105] Copyright Nature Publishing Group (2011).

three ‘‘fingers’’ is about 36 mg and the weight of the grabbed small objects (plastic foams) was 40 mg, this gripper can grab objects which are a little heavier than the gripper itself. On the other hand, the SID effect was applied to transform various chemical, optical and thermal energies to mechanical energy, which was considered as an ideal energy source for electrical power. Cheng et al. [109] developed a new type of humidity triggered electric generator

(Fig. 9c,d), which will produce power using mechanical work induced by the variation of ambient moisture. To demonstrate this idea, a twisted GO fiber was attached with a magnet bar at one end, which was located in the center of several copper coils. When the whole device was exposed to the varied humidity, the fast rotation of twisted GO fiber could drive the magnet, behaving in the same way, thus inducing the generation of electricity in the 7

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(a) Scheme of the twisted GMA fabrication. (b and c) SEM images of directly as-prepared GMA and twisted GMA, respectively. (d) Schematic rotation of a twisted GMA with a paddle at the low (left) and high (right) humidity. When the moisture increases (right), the GMA can drive the paddle rotating fast; then the paddle can reverse to the initial state when the moisture decreases (left). (e) The durability test of GMA undergoing repeated RH changes, showing forward (the environment humidity changed from RH = 20% to 85%) and backward (RH = 85% to 20%) rotation speed versus cycle numbers. Reproduced with permission from Ref. [107] Copyright Wiley-VCH (2014). (f ) Dependence of the curvature of the GO/rGO bilayer GMA upon RH. (g) Schematic illustration of SID properties and mechanism of the GO/rGO bilayer GMA. Reproduced with permission from Ref. [111] Copyright Wiley-VCH (2014).

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FIGURE 9

(a) A photograph of ‘‘tri-finger’’ mechanical grippe (b) A larger object is grabbed by the ‘‘tri-finger’’ gripper with applied voltage of 10 V. Reproduced with permission from Ref. [94] Copyright The Royal Society of Chemistry (2014). (c) The scheme of the designed alternating current generator. (d) The short-circuit current of the generator tested under humidity changes between 20% and 85%. Reproduced with permission from Ref. [107] Copyright Wiley-VCH (2014). (e) A crawler paper robot fabricated by using GO/RGO bilayer paper as smart legs. ‘‘On’’ and ‘‘off’’ means turning on and turning off the humidity, respectively. Reproduced with permission from Ref. [111] Copyright Wiley-VCH (2014).

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copper coils. Mu et al. [104] presented an electrostatic spinning strategy to prepare poly(vinylidene fluoride) (PVDF) attached GMA, where the SID of the GMA stretches the PVDF element, and generates an open circuit voltage of up to 4 V. Furthermore, as a preliminary exploration of future smart systems, the SID effect of GMAs was used in self-driven robots. The prototype device of a moisture-responsive crawler robot was fabricated with GMA ‘‘legs’’ and a paper ‘‘body’’ as shown in Fig. 9e [113]. When a moisturerich environment was applied to the robot, the two GMA ‘‘legs’’ gathered up, leading to significant bending performance toward the right. Once the moisture was replaced by dry air, the GMA legs became unbent, pushing the whole robot toward the right. These results indicate that the SID enabled GMAs holds great promise for the development of smart materials and devices.

Summary and outlook SID enabled GMA can be constructed by mass produced GNSs. Upon the formation of asymmetric structures, the GMAs exhibit bend, curl and swing behaviors, while the homogeneous structures normally correspond to strain changes. The deformation ability of GMAs is controlled by a configuration change of each GNS, inducing an enlargement of the interlayer spacing and/or a synergistic effect of them. Since various shapes can be easily obtained by structural design, these SID enabled GMAs have been used to fabricate smart devices and their performance is comparable to those of counterparts based on polymers with high cost. The SID effect of GMA is also promising for applications in sensing, drug delivery and energy transformers. Nevertheless, several challenges still remain in this field. Firstly, as described above, all the techniques developed for fabricating GMAs have to face the problem of scaling up, thus a low-cost and productive method for construct GMAs with critically controllable micro-/nano-structures is still required. Secondly, the mechanical strength of GMAs without assistance of polymers is unsatisfactory. It is of great theoretical and practical interest to develop an allgraphene based SID system with both flexibility and toughness for important applications (e.g. structural materials and reinforcement). Thirdly, theoretical simulations have predicted the great potential of the SID effect for practical applications. However, these applications have rarely been realized experimentally. Undoubtedly, more effort from both academia and industry will speed up the development of this emerging area, and the wide applications of graphene-based smart deformable systems will be realized.

Acknowledgements We thank the financial support from the 973 Program of China (2011CB013000) and NSFC (21325415, 21174019, 21301018), Beijing Natural Science Foundation (2152028) and 111 Project 807012. References [1] [2] [3] [4] [5] [6] [7]

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