One Single Graphene Oxide Film for Responsive Actuation - ACS

Sep 16, 2016 - Graphene, because of its superior electrical/thermal conductivity, high surface area, excellent mechanical flexibility, and stability, ...
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One Single Graphene Oxide Film for Responsive Actuation Huhu Cheng,†,‡ Fei Zhao,† Jiangli Xue,† Gaoquan Shi,‡ Lan Jiang,§ and Liangti Qu*,† †

Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, Key Laboratory of Cluster Science, Ministry of Education of China, School of Chemistry, Beijing Institute of Technology, Beijing 100081, People’s Republic of China. ‡ Department of Chemistry, Tsinghua University, Beijing 100084, People’s Republic of China § Laser Micro-/Nano-Fabrication Laboratory, School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, People’s Republic of China S Supporting Information *

ABSTRACT: Graphene, because of its superior electrical/ thermal conductivity, high surface area, excellent mechanical flexibility, and stability, is currently receiving significant attention and benefit to fabricate actuator devices. Here, a sole graphene oxide (GO) film responsive actuator with an integrated self-detecting sensor has been developed. The film exhibits an asymmetric surface structure on its two sides, creating a promising actuation ability triggered by multistimuli, such as moisture, thermals, and infrared light. Meanwhile, the built-in laser-writing reduced graphene oxide (rGO) sensor in the film can detect its own deformation in real time. Smart perceptual fingers in addition to rectangular-shaped and even four-legged walking robots have been developed based on the responsive GO film. KEYWORDS: graphene, actuator, self-sensing, robots

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combined with another component to form asymmetric bilayer structures. Our previous work demonstrated that graphene/ graphene oxide (GO) films and fibers exhibit advanced bending movement and can be used as responsive fingers and even robots.32−34 A high-speed rotation motor has also been achieved by the spirally ordered assembly of GO sheets within fibers.35 However, the fabrication process for those special structures needs extensive regulation, and the final devices are generally in the absence of multifunctional integration. Herein, a sole GO film responsive actuator with an integrated self-detecting sensor has been developed. The GO film was prepared by directly casting the GO suspension on the flat plate. Evaporation-induced GO sheets assemble into asymmetric microstructures on the surface of GO film, leading to the reversible bending deformation in response to multistimuli, including light, temperature, and moisture. The in situ laserwriting circuits along the GO film form the built-in integrated sensor, and the actuation behavior of the GO film is detected in real time. More impressively, the GO film has adequate versatility to be tailored and assembled into smart perceptual

mart materials have attracted much attention because of their intelligent response to external stimulus such as living things. Examples include electronic skins,1−4 artificial hydrophobic/hydrophilic surfaces,5−9 electrochromic smart windows,10−14 and shape-changeable actuators.15−22 In recent years, smart actuators that can modulate their shapes are especially conspicuous because they can transfer external energy to mechanic movements directly in response to single or multiple thermal, magnetic, electric, moist, and optical stimuli. Various types of actuators based on metal alloy, polymer, carbon nanomaterials, or their composite materials have been available.15−22 However, the development of an actuation system by an extremely simple method is highly desirable, and the integration of more functions, such as sensing capacity to directly detect the external stimulus in situ during the actuation process, still remains one of the key challenges.23,24 Graphene, a two-dimensional material containing one layer of carbon atoms, possesses unique mechanical, electrical, thermal, and optical properties as well as high specific surface area and good stabilities,25−30 which make it an outstanding candidate for fabricating smart actuation systems.31−46 For example, graphene films can take elongation or contraction movements under electrochemical stimulus.31 For more sophisticated shape-changeable movements, graphene is always © 2016 American Chemical Society

Received: July 18, 2016 Accepted: September 16, 2016 Published: September 16, 2016 9529

DOI: 10.1021/acsnano.6b04769 ACS Nano 2016, 10, 9529−9535

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Figure 1. Fabrication scheme of the GO film responsive actuator. (a) Aqueous GO suspension was coated on a smooth substrate. (b) Process of solvent evaporation from GO film. (c) GO film with a rough top surface after solvent evaporation. (d) Obtained GO film with asymmetric surface structure after peeling off from the smooth substrate. (e) Film actuator with laser-writing reduced graphene oxide (rGO) circuit as the built-in sensor in the film.

Figure 2. (a) Photograph of the obtained GO film. (b,c) Photographs of the smooth side and rough side of the film, respectively. (d,e) SEM and (f,g) AFM images (30 × 30 μm) of the smooth side and rough side of the film, respectively. (h) In situ AFM images (30 × 30 μm) of the rough surface of the film under different relative humidity. Scale bars: (a) 1 cm; (b,c) 5 mm; (d,e) 5 μm.

asymmetrical surface characteristic of the as-fabricated GO film. The smoothness of the GO film can be identified by the optical microscopy and scanning electron microscopy (SEM) images (Supporting Information, Figure S1). Compared with the rough surface, the smooth surface is much brighter (Supporting Information, Figure S1d). SEM images (Figure 2d,e) show that the rough surface has apparent wrinkles, while the smooth side is relatively flat. Atomic force microscopy (AFM) verifies that the roughness is 33 nm for the smooth surface with a maximum peak height of about 244 nm (Figure 2f). In contrast, the opposite side has a roughness of 212 nm, and the maximum peak height is 1.57 μm (Figure 2g), which is about 7 times that of the smooth side. The adsorption and desorption process of water molecules within GO layers rich with oxygen-related functional groups caused the fast and reversible expansion/contraction of GO films.33,40,46 In situ AFM results indeed demonstrate that

fingers in addition to rectangular-shaped and even four-legged walking robots.

RESULTS AND DISCUSSION The specific GO film can be simply fabricated by casting the aqueous GO suspensions (5 mg mL−1) on a smooth substrate (e.g., glass plate) (Figure 1a), followed by being naturally dried in air (Figure 1b). The as-formed film has a very flat surface on the side that is in contact with the substrate, while the other side is relatively rough due to the solvent evaporation-induced wrinkles on the top surface (Figure 1c,d). Meanwhile, the sensing devices are integrated into the GO film by direct laser writing on the flat surface (Figure 1e) and can detect its own deformation under a multistimulus response process. Figure 2a shows a 60 cm2 GO film prepared by this method. The tweezers are clearly reflected on the smooth surface (Figure 2b), while no obvious reflection is observed on the rough surface of the GO film (Figure 2c), indicating the 9530

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Figure 3. (a) Schematic of the rGO circuit as the built-in sensor’s fabrication by direct laser writing. (b) Photo of the rGO circuit in the GO film. (c) SEM image of the rGO region in b. (d) Photo of the film (about 2.5 cm long and 0.5 cm wide) deformation under different relative humidity. (e) Curvature of the film (θ, red line), relative electrical resistance changes of the built-in rGO sensor (ΔR/R0, blue line), and the RH change (black line) curves. (f) Linear relationship between ΔR/R0 changes of the built-in rGO sensor and the curvature (θ) of the film. Scale bars: (b) 5 mm; (c) 0.5 mm.

ratio for rGO is about 9:1, which is much higher than that for GO (2:1) as determined by the corresponding energydispersive X-ray spectra (EDS) (Supporting Information, Figure S7), implying the effective reduction of GO into the conductive rGO (∼200 Ω cm−1). X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) further confirm the laser-writing production of rGO (Supporting Information, Figures S8 and S9). As expected, the film (about 9 μm thick, 2.5 cm long, and 5 mm wide) can bend to the smooth side (Figure 3d, left) when the RH increased to 85% in the environment. The film bending curvature (θ, Figure S10) increases while the film becomes shorter and thinner (Figure S11). It can recover to the initial state (Figure 3d, middle) and then bend to the other side when the RH changes to 0% (Figure 3d, right). This process is fully reversible, and the response time of the actuator is less than 1 s when exposing the sample to moisture. On the other hand, the control experiment illustrated that the GO film will always bend to the relatively smooth side of the film (Supporting Information, Figure S12), which further demonstrated that the asymmetric responsiveness of the GO film depended on the roughness difference on the surface. Meanwhile, the bending states of the film in Figure 3d can be easily detected in real time by the built-in rGO sensor in the film (Supporting Information, Figure S13). As shown in Figure 3e and Supporting Information, Figure S14, the film bending curvature (θ), the relative electrical resistance changes (ΔR/R0) of the built-in rGO sensor, and the surrounding RH changes

humidity changes cause the large reversible roughness evolution on the surface of GO film (Figure 2h). The wrinkles on the rough surface are in a relaxation state at high relative humidity (RH = 60%) due to the intake of water molecules, which shrink into many more small folds when the RH decreases to 5% because water loss induced the contraction of GO sheets. The process is reversible, as indicated in Figure 2h. For the smooth surface of the GO film, however, there are no significant changes observed due to its uniformity (Supporting Information, Figure S2). Meanwhile, the GO film has a thickness change of about 1.5%. The hydrophilicity is almost the same on the two sides (Supporting Information, Figure S3), illustrating the asymmetric responsiveness attributed to the asymmetric morphology of the GO film. Despite the asymmetric surface structure, the GO film has a uniform inner layer stacking of GO sheets (Supporting Information, Figure S4), indicating the film could have good mechanical property and could benefit the applications in the actuation device. The tensile test results show that the film is strong (Supporting Information, Figure S5a) and is flexible enough to be bent easily (Supporting Information, Figure S5b). After laser writing the reduced graphene oxide (rGO) circuit on the film (Figure 4a), we observed no significant strength change (Supporting Information, Figure S5a). Figure 3b,c shows the optical and SEM images of the rGO circuit embedded in GO film, which acts as the sensing unit. The laser irradiation induced an effective reduction of GO as reported previously (Supporting Information, Figure S6).47,48 The C/O atomic 9531

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Figure 4. Actuation performance of film triggered by temperature and IR light. Photographs of (a) temperature and (b) IR-induced reversible bending of the film. (c) Three finger responsive device. The electrical feedback signal of the deformation for each “finger” was reflected in real time under IR irradiation. The fingers and the corresponding electrical feedback signals are marked as 1, 2, and 3, respectively.

are synchronized. The film bends to the smooth side, and θ is about 75°, while the ΔR/R0 of the built-in rGO sensor (the absolute resistance value is about 1.6 × 104 Ω) increases about 10% of the initial value along with the RH increase (85%). Meanwhile, the small applied voltage (0.1 V) on the sample and detected current (∼6.2 μA) in the testing process has no significant effect on the actuation performance (Supporting Information, Figure S15). The electrical resistance changes of the rGO sensor are directly associated with the factors of the film bending states and surrounding humidity changes (Supporting Information, Figures S16−S19), resulting in a good linear relationship between the bending curvature (θ) and relative electrical resistance changes (ΔR/R0) (Figure 3f). In other words, the responsive deformation of the film can be directly detected by the electrical resistance changes of the built-in rGO sensor. For example, the electrical resistance decreases in Figure 3f mean that the film has bent to the other side in Figure 3d. The above results indicate that the film actuator developed here exhibits excellent actuation ability, and the actuation behavior can be detected in real time. Not limited to humidity, the film actuator can also work in response to thermal stimulus when the RH in the environment is greater than 10% because the temperature change can affect water adsorption and desorption in the GO film.40,41,46 As shown in Figure 4, it exhibits fast and reversible thermal responsive actuation. Figure 4a shows that the film bends to the rough side when approaching a heat panel (∼100 °C) because of the thermally induced water desorption in the GO sheets, which is just like the behavior at low RH. The relative electrical resistance decreases in the built-in rGO sensor, and the bending curvature increases with the increasing temperature (Supporting Information, Figure S20). Infrared (IR) light is a remote thermal source, and it will also cause reversible deformation of the film accordingly. Figure 4b and Figure S21 demonstrate the good synchronization relationship between relative electrical resistance changes of the built-in sensor and the film actuation states. When the film is exposed to IR light, water desorption causes the film to bend to the rough side, and simultaneously, the electrical resistance of built-in rGO sensor does not accurately represent the bending state. In the high-temperature-

triggered actuation process, the maximum temperature detected on the sample is about 78.6 °C when the preset heating panel of 100 °C is close to the sample (Supporting Information, Figure S22), and the IR-induced temperature on the film is about 38.6 °C (Supporting Information, Figure S23). This is much lower than the decomposition temperature49,50 (∼200 °C in Supporting Information, Figure S24) of GO. The time of the sample exposed to IR irradiation or high temperature is less than 1 s every time. Therefore, the effect of the high temperature and IR light on the sample in our experiment is negligible. After 500 cycle tests (Supporting Information, Figure S25), the bending curvature is still greater than 95% of the initial value, indicating favorable actuation cycling ability of the film. The promising multistimulus actuation with self-deformation detection of the film actuator makes it possible to design and fabricate graphene-based smart devices. A three finger responsive device (Supporting Information, Movie S1) composed of GO strips with a width of 5 mm has been designed in Figure 4c1. The motion of each finger can be controlled by IR irradiation, and the corresponding deformation is detected by the built-in rGO sensor in real time (Supporting Information, Figure S26). For example, when IR light irradiates on the NO.1 “finger”, it can comply with the IR light irradiation to generate the bending behavior, accompanying the detected electrical signal, while the other two fingers stay upright (Figure 4c2). The electrical signal variation of NO.1 and NO.3 fingers indicates that the bending movements occurred on the NO.1 and NO.3 fingers (Figure 4c3). Information on all three fingers at the bending state is accessible, as shown in Figure 4c4. The feedback information by the built-in sensor can easily reveal the bending behavior of each finger. In addition to the “smart fingers”, various types of microwalker devices can be fabricated. As shown in Figure 5a, a “rectangle” walker can crawl on a ratchet substrate (Supporting Information, Figure S27) just like other walkers reported by different groups including us.43−45 Once the IR irradiation cycle is in the on and off state, the walker begins to move through the repetitive bending and straight deformation. 9532

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Supporting Information, Figure S30c) until the legs support the device again. With alternation of the surrounding stimulus, the device can move forward on the smooth glass substrate step by step (Supporting Information, Figure S31). Figure 5e and Supporting Information Movie S3 show that the device moves about 1 cm after four cycles despite the absence of systematical device optimization.

CONCLUSIONS A single GO film responsive actuator with an integrated selfdetecting sensor has been developed by directly casting the GO suspension onto a flat plate and laser writing built-in circuits. The actuator exhibits excellent responsive deformation in response to moist, thermal, and IR stimulus due to the asymmetrical surface structure, and the actuating information can be detected in real time by the in situ formed rGO sensor. The GO film has adequate versatility to be tailored and assembled into smart perceptual fingers in addition to rectangular-shaped and even four-legged walking robots. This study not only offers a strategy for producing monolayer actuators for complex deformation by simply controlling the surface morphology but also supplies a brand platform for the development of the multifunctional actuators and smart walkers with self-deformation sensing ability. METHODS Preparation of GO Film with a Built-in Sensor. A graphene oxide suspension (5 mg mL−1) was prepared by a modified Hummers’ method as reported in our previous papers.51,52 The GO suspension was first coated on a commercial transparent glass. After being dried in air, the GO film was peeled off from the glass carefully. The rGO circuits were fabricated by exposing the film under a laser beam with a laser power of about 2 mW. Characterization. The morphology of the samples was examined by SEM (JSM-7001F) and TEM (JEM-2010). EDS of the samples were taken on a JSM-7001F SEM unit. XRD patterns were obtained using a Netherlands 1,710 diffractometer with a Cu Kα irradiation source (λ = 1.54 Å). AFM images were taken on an Innova atomic force microscope workstation. XPS data were obtained with an ESCALab220i-XL electron spectrometer from VG Scientific using 300 W Al Kα radiation. The base pressure was about 3 × 10−9 mbar. The binding energies were referenced to the C 1s line at 284.8 eV from adventitious carbon. Mechanical property test was conducted with an AGS-X material testing system (SHIMADZU). The strain rate for 1 cm gauge length is 0.2 mm/min with a preload of 0.5 N. The thermal images were collected by a thermal imager (Fluke TiX660). Actuation and Electrical Measurements. The humidity actuation performance was measured in an enclosed container, and relativity humidity was controlled by the flow of dry nitrogen or nitrogen-containing moisture. Thermal actuation was measured using a temperature-programmed heating plate. The IR light was from a commercial infrared source (100 W), and the distance was 20 cm from the sample to the source. All of the actuation movements were recorded by a digital camera. The data of curvature was obtained from the analysis of the camera record. The electrical signals were recorded using a CHI 660D electrochemical workstation.

Figure 5. (a) Photograph of a “rectangle” walker on a ratchet paper. (b) Series of photographs of the “rectangle” walker crawling on the ratchet paper in response to on/off states of the IR. (c) Detected current changes along with the “rectangle” walker’s steps in b. (d) Deformation of three feet “crane” under temperature change. Left: initial state at room temperature. Right: lifting state at 80 °C. (e) Series of photographs of a four-legged walker moving forward on a smooth glass. Scale bars: (a,d) 1 cm.

With the alternation of the IR switching on/off, the device walks forward step by step. The moving rate is about 2 mm per step, and it walks a total of 6 mm after three cycles, as shown in Figure 5b and Supporting Information, Movie S2. Meanwhile, the device’s walk step can be detected by the built-in rGO sensor, as shown in Figure 5c. When the walker takes a step forward, a peak of the electric signal is detected. Three peaks in Figure 5c correspond to three steps in Figure 5b very well. Additionally, a temperature-triggered three feet “crane” (Figure 5d and Supporting Information, Figure S28) and a bipedal “reptile” (Supporting Information, Figure S29) have also been fabricated. When the temperature increases to around 80 °C, the crane can bend its legs and stand up and even lift a paperboard (Supporting Information, Figure S28) with the same weight as the crane. After the temperature goes back to normal, it recovers to the flat state. The reptile in Supporting Information, Figure S29, can crawl forward easily with the alternation of temperature change. Apart from the devices mentioned above, a predesigned fourlegged walker can move on a smooth glass substrate. The device in Figure 5e contains four GO legs and a horizontal body with two support bars of different lengths, as schematically shown in Supporting Information, Figure S30. When the RH increases, all of the legs bend and drive the whole device forward until the bars support the device (Figure 5e and Supporting Information, Figure S30b). Then the legs recover to the initial state when exposed to IR light (Figure 5e and

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b04769. Additional characterizations of the film including photos, AFM, SEM, EDS, XRD, and XPS results (Figures S1− S8); illustration of the actuation testing process and performance (Figures S9−S18); and schematic diagram 9533

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and photos of actuation devices (Figures S19−S22) (PDF) Movie S1: three finger responsive device (AVI) Movie S2: a “rectangle” walker (AVI) Movie S3: four-legged walking robots (AVI)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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DOI: 10.1021/acsnano.6b04769 ACS Nano 2016, 10, 9529−9535

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DOI: 10.1021/acsnano.6b04769 ACS Nano 2016, 10, 9529−9535