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Abstract
We reported here a facile, green, and simple method to fabricate moisture-responsive graphene actuators by moderate flash reduction of graphene oxides (GO) films. Due to the limited light transparency and thermal relaxation, the oxygen containing groups (OCGs) on the GO sheets could be selectively removed from the radiation side, forming a photoreduction gradient along the lateral direction of the GO film. In this manner, we obtained a reduced GO (RGO)/GO bilayer film. The RGO/GO film can bend to the RGO side when exposed to moisture because RGO and GO layers show different absorption capabilities of water molecules. Taking advantage of this controllable deformation, we fabricated moisture-responsive actuators including a crawler and claw robots.
© 2017 Optical Society of America
1. Introduction
Actuators that have special response to external stimulations have attracted enormous research interests, revealing great potential in a broad range of applications including adaptive optics, tissue engineering and biomimetic actuations [1–10]. Generally, stimuli-responsive actuators can be fabricated by precise assembly of multilayer structures using different materials [11–14]. Since there exist an obvious stress mismatch between different material layers, the multilayer structured film would occur predictable deformations such as bending, twisting, and coiling motions under external stimulations (e.g., light [15], moisture [16], temperature [17], or solvents [18–20]). As typical examples, Gracies et al. developed a series of micro-grippers using multi-layer material structures, which could be actuated by thermal stimulation [21], magnetic field [22], and laser [23]. To date, despite such a kind of actuators have been successfully fabricated using different materials with the help of diverse processing technologies, continued efforts in the development of stimuli-responsive actuators using novel material systems are still highly desired with the aim of further extending new applications.
Recently, graphene and its related materials (e.g., graphene oxides, GO) have drawn much attentions, because this new member of carbon materials family exhibits outstanding physical/chemical properties such as extraordinary mechanical strength [24–26], flexibility [27–29], good stability [30–32], high electron mobility [33–36], excellent biocompatibility [37–39], and others [39–43]). Consequently, graphene and related materials have been recognized as superior candidates for actuators design [44]. Various light-active, electro-active and thermal response actuators based on graphene have been successfully developed [45–49]. As a successful example, Qu et al. fabricated fiber-type actuators by laser local reduction of GO fibers [50]. In their work, selective modification of GO fiber using a focused laser makes the GO fiber very sensitive to environmental moisture, controllable deformation such as bending, folding and S-shape deformation has been successfully realized. In our previous work, we prepared the RGO/GO bilayers paper using unilateral sunlight or UV irradiated [51, 52]. The self-controlled photoreduction process directly leads to the formation of a moisture responsive RGO/GO bilayer structure. However, despite of these successes, open problems with respect to the precise control over the photoreduction gradient, facilitation of fabrication process, and improvement of the fabrication efficiency of the stimuli-responded RGO/GO actuators are still challenging.
Herein, we demonstrated a very facile approach to fabricate RGO/GO moisture-responsive actuators through moderate flash reduction of GO paper. The flash irradiation induced photothermal reduction can effectively remove most of the oxygen containing groups (OCGs) on the surface. Due to the limited light transmittance and thermal relaxation, a photoreduction gradient formed along the lateral direction, instead of total reduction of the GO film. Taking advantage of the anisotropic bilayer structure, the resultant RGO/GO film could be actuated by moisture. We further demonstrated moisture-responsive graphene actuators including a crawler and a claw as typical examples.
2. Experiment section
2.1 Preparation of GO paper
GO was synthesized from natural graphite powder using the Hummer’s method. The GO paper was prepared by vacuum filtration of the GO aqueous solution through a membrane filter (0.22 µm in pore size), followed by air drying at room temperature. Finally, the GO paper was peeled off for further use.
2.2 Sample characterization
SEM images were obtained using a JEOL JSM-7500 field-emission scanning electron microscopy (FE-SEM). Powder X-ray diffraction (XRD) patterns were collected on a Rigaku D/MAX 2550 diffractometer with Cu Kα radiation (λ = 1.5418 Å). Raman spectroscopy were recorded on a Jobin–Yvon T64000 Raman spectrometer equipped with a liquid nitrogen-cooled argon ion laser at 514.5 nm (Spectra-Physics Stabilite2017) as the excitation source; the laser power used was about 10 mW at the samples with an average spot size of 1 µm in diameter. X-ray photoelectron spectroscopy (XPS) was recorded using an ESCALAB 250 spectrometer. We obtained the interlayer surface of the RGO-GO bilayer film through a mechanical exfoliation method using a general tape. The controlled humidity environments were achieved using saturated aqueous solutions of MgCl2, K2CO3, NaBr, NaCl, KCl, and K2SO4 in a closed glass vessel, which yielded ≈33%, 44%, 57%, 75%, and 86% relative humidity (RH), respectively. All of the measurements were conducted in air at room temperature.
3. Results and discussion
Figure 1 shows the schematic illustration of the design principle and fabrication procedure of moisture-responsive RGO/GO bilayer film. The GO film was prepared by vacuum filtration of GO aqueous and dried in air. Subsequently, moderate flash treatment was employed to reduce the GO film from one side. The experiment was carried out in ambient condition. Moderate flash reduction of the GO films can be done with a single flash from the Xenon lamp equipped on a common digital camera. Typical flash energies applied to the samples were about 2 J/cm2. We tune the irradiation intensity by changing the distance between camera flash and GO samples. It is worthy point out that relative long distance between the flash lamp and the GO simple is critical to realize moderate flash reduction. For a moderate flash reduction, the distance is about 20-30 mm. Close-up flash treatment of GO with a distance less than 20 mm would lead to thorough reduction of GO, even burning of the GO film; whereas further increase of the irradiation distance would not trigger the photoreduction at all. Generally, the wavelength of a flash covers a broad spectral range from visible light to IR (from ~300 nm to 1200 nm), in which the IR irradiation contributes mainly to the photoreduction. In the case of moderate flash treatment, due to the limited light transmission and thermal relaxation, the flash light cannot totally reduce the GO paper into RGO. In this way, anisotropic photoreduction occurs, OCGs gradient forms along the lateral direction of the GO paper (RGO/GO bilayer film). Since the GO sheets can adsorb water molecules by forming hydrogen bond, whereas the RGO sheets would interact with water through much weaker Van der Waals force, the GO layer can absorb more water molecules than RGO. The asymmetric expansion in the GO layer would lead to bending deformation towards the RGO side when the RGO/GO bilayer film was exposed in moisture. Despite our previous works have proved that sunlight and UV irradiation are also workable for controllable GO reduction, the flash method reported here is more cost-effective. Large-area RGO/GO bilayer film would form within a second.
Fig. 1 The schematic illustration of the design principle and fabrication procedure of a moisture responsive RGO/GO bilayer film.
After moderate flash treatment, an obvious color change could be observed. We cut the GO film into an asymmetric star shape using a knife. The yellow-brown GO film turns black, which indicates the removal of OCGs, as shown in Fig. 2(a). However, when we observe the back side of the RGO film, we found that it still takes on yellow-brown, Fig. 2(b), which suggest the incomplete photoreduction. To get further insight into this photoreduction gradient, we further investigate the section structure of the graphene film by scanning electron microscopy (SEM). Figure 2(c) displays the section morphology of the flash treated GO film. After the flash photoreduction, the stacked structure, in the upside section, is extensively exfoliated because most of the OCGs have been drastically removed in the form of gas (e.g., CO2, CO, H2O, etc), which leads to the formation of an expanded structure. However, the back side remains well stacked layered structure. In this regard, a photoreduction gradient formed naturally, without any additional control. The formation of this RGO/GO bilayer structure could be attributed to the incomplete photoreduction. Since the incident light has been preferentially absorbed by the frond layer of GO, the photothermal reduction of GO occurs at the front surface first. Due to the limited light transmittance and thermal relaxation, the thick GO film cannot be fully reduced when the flash intensity is not high enough. Therefore, a RGO/GO bilayer film formed instead of a RGO film. It is worthy point out that special attention should be paid to the flash intensity or the distance between GO film and the light source. High flash intensity, very short exposure distance or multi-exposure would lead to a fully reduced sample or even burn the entire RGO film. We also found that the thickness GO influences the bilayer structure. A GO film with a thickness larger than 5μm is suitable for such controllable photoredcution. Thin GO film would be totally reduced into RGO upon flash reduction.
Fig. 2 Photograph of a star-shape RGO/GO bilayer film (a) RGO side; (b) GO side. (c) SEM images of the RGO/GO film.
Besides, we also used X-ray diffraction (XRD) to study the structural transformation of GO. The XRD pattern of the GO sheets in Fig. 3(a) shows a typical diffraction peaks at 2θ = 11.55°, which corresponds to a d-spacing of 0.76 nm. The diffraction pattern indicate an ordered layered structure of GO. However, after flash treatment, the diffraction peak disappeared, which suggests that the RGO layers become disorder after the removal of OCGs. Raman spectra are also employed to investigate the structural changes before and after flash treatment, as shown in Fig. 3(b). Typically, GO and RGO film exhibits two characteristic peaks at 1350 cm−1 (D band) and 1585 cm−1 (G band), respectively. Remarkably, the ID/IG slight decreased from 0.91 with respect to GO to 1.12 in the case of RGO, which indicates that more edge defects is formed due to the fragmentation of GO sheets, in good agreement with the SEM images. After flash reduction, the graphene sheets have been broken into small pieces, which bring additional defects obviously.
To quantitative measure the elemental composition change of the GO surfaces before and after flash treatment, we characterized the front RGO surface, the inter layer, and the GO side using X-ray photoelectron spectroscopy (XPS). The inter layer is prepared by a micromechanical cleavage method, which is between the GO and RGO layers. As shown in Fig. 4(a), the C/O ratios of the RGO side, the inter layer, and the GO side is about 11.69, 2.65 and 2.22, respectively. The gradually decreased C/O ratio confirms the reduction gradient along the lateral direction of the GO paper. Besides, as shown in Fig. 4(b), the C1s spectra of three samples are divided into three peaks for C-C (284.6 eV, non-oxygenated ring carbon), C-O (286.7 eV, hydroxyls and epoxies), and C = O (288.2 eV, carbonyls), respectively. The main difference among them is the different C-O peaks contents. From the front RGO surface to the GO back side, the contents of C-O peaks increased from 12.5% (RGO) to 37.6% (GO). This result demonstrates that OCGs have been preferentially removed at the surface exposed to flash irradiation.
Fig. 4 (a) Survey XPS spectra and (b) C1s XPS of the reverse side (GO), the inter layer, and the front side (RGO).
The unique RGO/GO bilayer structure makes the resultant graphene film very sensitive to environmental moisture. Since the GO sheets absorb water molecules via hydrogen-bonding interaction, whereas the RGO sheets absorb water molecules via weaker Van der Waals forces, water molecules would be preferentially adsorbed by GO layer. As a result, the GO layer would absorb more water molecules than the RGO layer. The selective absorption of water in the GO layer would induce an obvious expansion of GO layer, which directly leads to the force mismatch between GO sheets and RGO layers. Therefore, the RGO/GO paper will bend towards the RGO side in moisture. In our previous work, we have investigated the interlayer spacing changes under different humidity by XRD [52], in which the sample under high humidity showed relative larger interlayer spacing. This result confirms the moisture response mechanism of the RGO/GO bilayer structure. We further investigate the dependence of the bending angles of the RGO/GO bilayer film and GO film on RH, as shown in Fig. 5(a). Generally, the bending angles increase with the RH and the max bending angle is ~85° when the humidity is 86% RH. On the contrary, the pristine GO paper hardly bends in moisture. The bending and unbending deformations are reversible. For the RGO/GO bilayer film, the response and recovery times are ~16 s and ~17 s, respectively. Figure 5(b) shows the highly repeatable cycling behavior upon 4 cycles.
Fig. 5 (a) Dependence of the bending angles of the RGO/GO film and GO film on RH. (b) Responsive and recovery properties of the RGO/GO film and GO film.
The moisture-responsive properties of the RGO/GO paper enables to design and fabricate smart actuators. In this work, we demonstrate a crawler and a claw robot as typical examples. Figure 6 shows the moisture-driven crawler robot. We just cut the RGO/GO bilayer film into an insect shape with four legs. In this four-leg design, the crawler is free-standing, additional supporter is not necessary. When we switch the environmental humidity, the crawler robot would move forward, as shown in Fig. 6. After switching for several times, the crawler robot moved 3.5 mm in 12 s. In addition to the crawler, we also assembled 8 ribbons (5 mm*1 mm) into an eight-claw shape using dual adhesive tapes, as shown in Fig. 7. The claws would deform from open to closed state within 12 s. When we the moisture condition is replaced by dry gas, it would take additional 56 s for the claws to recover its open state. The smart claws could be used for capturing and release of target objects. It does not need external energy supply systems. The changes of environmental humidity would mediate a controllable manipulation of the smart claws. In the regard, the smart RGO/GO paper may find broad application in the development of smart devices.
4. Conclusions
In conclusion, a moisture-responsive RGO/GO bilayer film has been successfully fabricated by moderate flash reduction of GO paper from one side. Moderate flash treatment was accomplished by exposing the GO paper to the Xenon lamp at relative long distance. Due to the limited light absorption and thermal relaxation, OCGs of the GO paper cannot be totally removed, a photoreduction gradient was observed along the lateral direction of the GO paper, forming a RGO/GO bilayer film. Because the GO and RGO layers show different absorption capabilities of water molecules, the RGO/GO bilayer film was very sensitive to moisture, reversible bending deformation from 0 to 85° could be observed when the RH was switched between 33% and 86%. Making full use of this moisture responsive property, typical humidity-driven actuators including the crawler and the claw robot have been demonstrated. We deem that this facile, green and simple method provide an effective avenue to prepare smart RGO/GO bilayer film hold great promise for the development of the graphene based intelligent devices.
Funding
National Natural Science Foundation of China (#61522503, #61376123, #61435005, #61377048 and # 21603083).
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