Flexible Circuits and Soft Actuators by Printing Assembly of Graphene

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Flexible Circuits and Soft Actuators by Printing Assembly of Graphene Wenbo Li, Fengyu Li, Huizeng Li, Meng Su, Meng Gao, Yanan Li, Dan Su, Xingye Zhang, and Yanlin Song ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04235 • Publication Date (Web): 28 Apr 2016 Downloaded from http://pubs.acs.org on May 3, 2016

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ACS Applied Materials & Interfaces

Flexible Circuits and Soft Actuators by Printing Assembly of Graphene Wenbo Li, 1, 2 Fengyu Li,*, 1 Huizeng Li, 1, 2 Meng Su, 1, 2 Meng Gao, 1, 2 Yanan Li, 1, 2 Dan Su, 1, Xingye Zhang, 1 and Yanlin Song*,1 1

Key Laboratory of Green Printing, Institute of Chemistry, Chinese Academy of Sciences

ICCAS), Beijing Engineering Research Center of Nanomaterials for Green Printing Technology, Beijing National Laboratory for Molecular Sciences (BNLMS), Beijing 100190, P. R. China 2

University of Chinese Academy of Sciences, Beijing, 100049, P. R. China

*Corresponding authors: [email protected], [email protected] KEYWORDS printed electronics, graphene patterns, induced assembly, flexible circuits, soft actuators

ABSTRACT

An effective way to improve the electrical conductivity of printed graphene patterns was demonstrated by realizing the assembly of giant graphene oxide sheets during the printing process. The synergetic effect of printing-induced orientation and evaporation-induced interfacial assembly facilitated the formation of laminar-structured patterns. The resulting patterns after

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chemical reduction showed excellent electrical conductivity in printed graphene electronics. Due to the high conductivity, mechanical flexibility and the advantage in pattern designing, printed graphene electrodes were applied in electrical-driven soft actuator, which can realize controllable deformation with low driving voltage. Such achievements will be of great significance for the development of graphene-based flexible and printed electronics.

INTRODUCTION

The combination of printing technique and nanotechnology promotes the development of printed electronics,1,2 which provides a robust way for nanomaterials assembling into desirable electrical devices through facial, efficient and custom printing process.3,4 Especially, it is exploring the emerging fields of large-area, flexible and wearable electronics.5-7 As the intrinsic flexibility, chemical stability, high conductivity and low cost,8 graphene and its derivatives are well suited as building blocks for printed electronics. Recently, great progress has been made in graphene-based electronics through high-throughput printing strategies such as inkjet printing, screen printing, gravure printing and direct ink writing, etc.9-25 By these printing approaches, well developed functional inks were the key points to achieve high electrical performance of the devices. Pristine graphene has been utilized in the printing ink, while it has tough difficulty to be dispersed in solvents so that it sustains repeated printing passes and reduced efficiency.9 By alternative means, pristine graphene was modified or composited with polymer so as to be welldispersed for the solution processing method and meet the requirement of printability,10,13,15 while it turns out the loss of electrical property and requires additional extensive annealing procedure to remove the organics. In contrast, graphene oxide (GO) could form a stable dispersion due to the abundant functional groups.26-28 Therefore, a water-based GO ink is


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available, which has convenience in various printing systems. After printing, an in-situ reduction process is required to remedy the oxidation-induced defects on graphene sheets. However, the electrical conductivity of the patterns by this method is far from satisfactory.18, 20 It is still a challenge to regain the electrical properties of pristine graphene from GO inks. As the two-dimensional instinct, GO has advantages in forming the laminar-structured films/papers.29 Moreover, greater aspect ratio of building blocks and their better alignment can give rise to a higher electrical conductivity of the films due to the closer stacking forms and lower inter-sheet contact resistance,30 which has been proved in macroscopic assembly of graphene. These versatile strategies have promising prospects in printing of graphene to tremendously enhance the properties. However, the difficulty lies in matching the assembly of graphene with the requirement for printing. In this work, we presented an effective way to promote the electrical conductivity of printed graphene patterns by realizing the assembly of giant graphene oxide (GGO) sheets during the printing process. Direct ink writing offers the shear force to induce the orientation of GGO sheets, and subsequently the upward force of the evaporating flow facilitates the formation of distinct laminar structure. Followed by chemical reduction, enhanced conductivity of up to 4.51 (± 0.18) ×104 S/m was achieved, which is among the highest records in printed graphene electronics. By virtue of the remarkable electrical conductivity and inherent mechanical flexibility, large-area and uniform patterns were printed for the application in flexible circuits. With the advantage in pattern designing, printed graphene electrodes were applied in electrical-driven soft actuator, which can realize controllable deformation with low driving voltage. RESULTS AND DISCUSSION


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We synthesized GGO sheets according to modified Hummers’ method,31, 32 as described in the Supporting Information. The representative thickness is characterized as 0.8 nm by the atomic force microscope (AFM) image (Figure 1a), indicating the monolayered attribute of the dispersion. The lateral size of the GGO sheets distributes from 4 µm to 38 µm and the average size is 15.6 µm, as calculated by the scanning electron microscope (SEM) images (Figure 1b and Figure S1). GGO sheets as building blocks exhibit outstanding behavior in vacuum filtrated graphene films and wet-spun graphene fibers,33, 34 while they are seldom adopted in printing process. In typical approaches such as inkjet printing, the diameter of printing nozzle (Ф) regularly defaults to be tens of micrometers, thus the increased GO sizes beyond the safe range (below 1/20 of Ф) would restrict the printing fluency and cause clogging.35 Generally, it is a tradeoff between GO sizes and printing strategies. Direct ink writing offers an attractive way to break the routine and meet the printability with the demanding GO sizes. The printing nozzles have series options of diameters ranging from sub-micrometer to millimeter scale to satisfy with the inks. More importantly, the extrusion based procedure plays a crucial role in directing the orientation36-38 of GGO sheets to pass through the nozzle during printing. With such high aspect ratio (width/thichness ≈ 2.0×104), GGO sheets were induced alignment along the flowing direction under the shear force and extensional flow field. The alignment of GGO during printing process is illustrated in Figure 1c. Inks for direct writing are viscosity depended to achieve the printing requirement.39-43 The assynthesized GGO dispersion was concentrated by distillation to form the paste-like inks with high contents. We prepared a broad range of GGO inks with concentration between 10 and 30 mg/mL. Figure 1e shows the viscosity as a function of shear rate for GGO inks of varying concentrations. The GGO inks showed typical shear-thinning behavior of non-Newtonian fluid,


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which is necessary for the extrusion through micronozzles under ambient conditions. Figure 1f indicates the elastic modulus (G') as a function of shear stress for GGO inks of varying concentrations. In the linear viscoelastic region, G' rises 1~2 orders of magnitude as the GGO content increases from 10 to 30 mg/mL. Figure S2 presents the loss modulus (G'') as a function of shear stress for GGO inks of varying concentrations. The plateau value of G' is higher than that of G'' for each ink, which demonstrates that the GGO inks behave like solid state materials. Moreover, the broad range of rheological behavior is convenient for tailoring the printed features (Figure S3). Note that with the utilization of GGO, the viscosity values and elastic modulus of our inks are higher than that of the previous reported GO ink with same concentration.40, 41 It means that equivalent rheological properties could be obtained with lower GGO contents. The pattern width can be controlled by adjusting the concentrations of the inks and the printing parameters (Figure S4). The optimized ink with concentration of 20 mg/mL was screened for the printing (Figure 1d), which possessed superior rheological properties to match the printability.


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Figure 1. Direct writing with GGO inks. (a) AFM image of GGO deposited on mica. (b) SEM image of GGO deposited on silicon. (c) Schematic diagram illustrating alignment of GGO during printing process. (d) Optical image of the printing process and the modified ink with concentration of 20 mg/mL in a 3 cc barrel (insert). (e, f) Viscosity and shear elastic modulus as a function of shear rate for GGO inks of varying concentrations (10, 15, 20, 25, 30 mg/mL, respectively). Direct ink writing is well known as a patterning technique to build up structures with functional materials.2,18,39-41 We demonstrate this patterning approach by printing patterns onto a glass substrate using the GGO ink, which is deposited through a 200 µm nozzle. The printed patterns are consistent and uniform in a large area (Figure 2a), indicating the excellent stability of the GGO ink in printing process. Figure 2b shows the printed features with resolution of 200 µm at optimized parameters (pressure = 5 psi, speed = 10 mm/s), which is in complete agreement with the diameter of nozzle used for printing. By decreasing the center-to-center spacing below 200 µm, the adjacent ink filaments joined together, and the film-like GGO structure could be obtained after liquid drying (Figure S5). We analyzed the structure of GGO film by XRD analysis (Figure 2c). The diffraction peak at 9.8° corresponds to characteristic 001 peak of GGO. The d-spacing of 9.01 Å indicates the densely stacking of GGO sheets. Besides, a weak diffraction peak at 19.8° is close to the 002 peak located in 26.4° from natural graphite.44 It demonstates that the GGO sheets in printed films have large conjugated plane and similar stacking structure with graphite. SEM images in Figure 2d-f present the microscopic morphologies of the printed GGO patterns. From an overall perspective, it shows continuous lines with smooth edges and flat surface morphology (Figure 2d). From the constructed specification, the printed patterns show wrinkles with irregular distribution on the surface area


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(Figure 2e), which are caused by constriction and overlapping of the GGO sheets during the drying process. The laminar structure in cross section further proves the alignment and densely stacking of GGO sheets (Figure 2f). SEM images from cross session of the patterns with varying printing parameters are shown in Figure S6, which indicate the layered structures of GGO sheets. GO has advantages in forming the layered stacking structure by various strategies in macroscopic assembly of graphene.29, 45-47 The extrusion based printing procedure is similar to the spinning approach of GO fibers and films. The concentrated inks resemble the spinning dopes of GO liquid crystals (LCs).30, 34, 38 The shear force occurs when the fluid passing through the nozzle, by which the GO LCs progressively tuned into alignment. While in order to match the printability, the concentrations of our inks are much higher than the spinning dopes of GO LCs, which process differences on rheological properties. Moreover, the distinct rheological behaviors of the inks would affect the self-assembly of the GGO sheets, as well as their transformation into ordered solid materials. In this case, patterned GGO film was formed under the synergistic effect of printing-induced orientation and evaporation-induced interfacial assembly. After the ink filament was deposited on the substrate, GGO sheets pre-aligned along the printing direction. The concentrated ink possessed sufficient high storage elastic modulus to maintain the printed filamentary shape (Figure 1f). As the solvent evaporation, the viscosity of the ink got higher and further resulted in a pinning state at the gas-liquid-solid three-phase contact line. The ink filament went through longitudinal contraction (Figure S7). GGO sheets were driven by the upward force of the evaporating flow to be assembled at the liquid/air interface.48 Meanwhile, the wet film was compressed in the vertical direction as evaporation occurred. At this stage, the compressing force plays a key role to force the GGO sheets into alignment. The sheets gradually evolved to a more


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consistent arrangement (Figure S8). Due to π-π conjugation effect and hydrogen-bonding interaction,29,44 the close packed GGO lines with ordered film-like structure were achieved. The assembly process is illustrated in Figure 2g.

Figure 2. The morphology and structure of printed patterns. (a, b) Optical images of large-area GGO arrays printed on glass substrate. (c) XRD pattern of printed GGO film. SEM images of the parallel GGO lines (d), surface morphology (e) and cross morphology (f) of the printed patterns. (g) Schematic diagram illustrating evolution of GGO sheets assembly in drying process. Efficient reduction is necessary for the printed GGO to achieve high conductivity. Herein, GGO is reduced by chemical reduction of hydroiodic acid (HI) to prepare reduced graphene oxide (RGO).49, 50 As shown in Figure 3a, it turns from brown color to metallic luster of the printed patterns before and after HI treatment. XRD pattern of RGO displays a wide peak at


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24.2°, with a d-spacing of 3.67 Å (Figure 3b). The interlamellar spacing of RGO is much more narrow than that of GGO, and quite close to that of natural graphite. This is due to partial removal of the oxygen-containing groups in the reduction process. It is confirmed by the XPS spectra that the epoxy/hydroxyl (C–O), carbonyl (C=O) and carboxyl (O–C=O) are greatly diminished while carbon-carbon bond (C-C) has an accordingly enhancement (Figure 3c). In the Raman spectra (Figure 3d), it shows obvious band shifts from GGO to RGO. The GGO films have plenty of sp3 carbon due to the functional groups. After chemical treatment, small graphitic domains formed on the basal plane. The structure of GGO films altered with a high quantity of defects, corresponding to the increase in D band. Thus the increase of ID/IG reveals the partial restoration of sp2 carbon in RGO films. The increase of I2D/IS3 ratio further indicates the restoration of conjugation structure after reduction.44,

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We investigated the electrical

conductivity of GGO and RGO using a four-probe method by Keithley 4200 Semiconductor Characterization System (Figure 3e and Figure S9). The conductivity value of GGO patterns was measured to be 5.22 (± 0.26)×10-3 S/m. In comparison, the RGO circuits exhibited an electrical conductivity of 4.51 (± 0.18) ×104 S/m, which is with similar value of high-quality graphene made by vacuum filtrated and wet-spun method,30, 33 and among the highest records in reported printed graphene electronics (Figure 3f). The electrical conductivity is averaged from ten samples with the detailed data and calculated values in Table S1. The high conductivity is attributed to the introduction of GGO as the printing ink, as well as the ordered structure by printing assembly. It can be expected to further enhance the conductivity by optimizing the size and distribution of GO for the inks,51,

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or by additional post-processsing method such as

thermal annealing33, 51 and rolling compression14, 22 for the graphene patterns.


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Figure 3. Structural characterization of the printed patterns. High conductivity is achieved after reduction. (a) Digital images of the printed patterns before and after chemical reduction by HI. (b) XRD pattern of RGO lines. C1s XPS spectra (c) and Raman spectra (d) of GGO and RGO patterns. (e) Current-voltage tests of GGO and RGO patterns. (f) Comparison in electrical conductivities of graphene by different printing techniques.


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Our printing strategy of GGO also demonstrates to be versatile on both rigid and flexible substrates (Figure S10). Towards the application in flexible electronics, we prepared RGO circuits on polydimethylsiloxane (PDMS) and performed bending test to examine the electrical stability (Figure 4a). A custom-built micropositioner was used to carry out the experiment. The reported values were averaged from 5 specimens. Under compression, the relative resistance (R/R0) had slightly changes with the bending radius decreasing from 30 mm to 2 mm. The relative resistance maintained changes within 5% after 2000 bending times at the radius of 5 mm (Figure 4b). The appearance of the RGO circuit has almost no breakages after the bending test, as shown in the inserts of Figure 4b. Some wrinkles appear in the circuit as a result of the mechanical deformation (Figure S11), while they have little impact on the electrical resistance. In most areas, the RGO circuits keep well contact with the substrates due to the high adhesive force (Figure S12). These results indicate the mechanical flexibility of the printed RGO circuits. To further demonstrate the reliable flexibility of the circuits, we fabricated a series of parallel RGO electrodes with a 3 mm center-to-center spacing on PDMS substrate (Figure 4c). After then, LED chips were mounted to the electrodes connected by silver paste. The LED chips were powered by a 9 V battery, and displayed bright illumination at bending and twisting states.


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Figure 4. The flexible RGO circuits for the bending test. (a) The relative resistance change of RGO circuit as a function of bending radius under compression. The inserts illustrate the experimental setup. (b) The relative resistance change of RGO circuit as a function of bending times at a bending radius of 5 mm. Optical images of the RGO circuit before and after the bending test are shown in the inserts. (c) Large area flexible RGO circuits are prepared and integrated with LED chips, displaying bright illumination when twisted by hands and wrapped on a roller pen. Taking advantages of the high conductivity and stable flexibility of the printed RGO patterns, we produced soft actuator53, 54 with bilayer structure of RGO/PDMS (Figure 5a). With given operating voltages, the device could achieve deformations in large-strain. The principle is based


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on asymmetrical constriction/swelling of RGO/PDMS under electricity-induced Joule effect.55-58 RGO serves as active electrodes to respond the electric signal and generate heat. PDMS elastomer serves as the passive substrate and deforms when voltage applied. Graphene has intrinsic negative coefficient of thermal expansion (~-10-6 K-1), which is significant different from that of PDMS (3.1×10-4 K-1).56 When the bilayer structure is electrically heated, the fast temperature rise will cause asymmetrical thermal expansion of the two materials. In specific, there would be a slight constriction of RGO electrodes and a relative obvious swelling of PDMS. As a result, the bilayer would bend towards the RGO side along the length direction. The device was prepared with settled printing parameters and processes, thus the ratio of the active/passive layer thickness (dRGO/dPDMS) was relatively constant. The thicknesses of the RGO and PDMS layers were 0.81 ± 0.12 µm and 60 ± 5 µm, respectively (Figure S13). U-shaped actuators with the same width of 3 mm and lengths varying 10 mm, 15 mm, 20 mm were fabricated for characterization (Figure S14). Their electrical resistances were measured to be 214 ± 18 Ω, 327 ± 20 Ω, 450 ± 23Ω, respectively. The U-shaped actuators were held vertically to character the actuating performance, and the resulting bending displacements were monitored with digital camera. Figure 5b presents the maximum bending angle (θmax) of the actuators as a function of driving voltage. For each sample, the θmax gets larger when the input voltage increases. Lower voltage is required to attain a certain deformation for the actuator with shorter pattern length that has smaller electrical resistance as well. These results can be explained as follows. The electrothermal actuating system is equivalent to pure resistor element circuit, thus the Joule's laws can be described here as Q = (U2/R) t. The quantity of heat (Q) is in direct proportion to the driving voltage (U) for the same device, and U is in direct proportion to the electrical resistance (R) at constant Q. It also demonstrated that the high conductivity of RGO


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electrodes by our printing strategy is beneficial to the actuation with low driving voltage. Figure 5c shows the reversible shape change of the actuator with 10 mm length under operating voltage of 13.5 V. It spent 5 s to reach the maximum bending angle at 300°, and took another 9 s to recovery its initial position. The reversible shape change of the actuator in cyclic is shown in Figure S15.

Figure 5. Fabrication of soft actuator with printed RGO electrodes. (a) Schematic diagram of the RGO/PDMS bilayer actuator under electrical stimulation. (b) Maximum bending angle of the actuators as a function of driving voltage. (c) Digital images of actuator at various bending angles, which demonstrate the reversible shape change of the actuator. (d) Hand-shaped actuator with its original shape and various gestures by independent control of the fingers.


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With the advantage in pattern designing, a complex hand-shaped actuator was fabricated (Figure 5d). The fingers were individually controlled by five independent circuits, and various gestures can be performed by tuning the electric signals. By tailoring the ink composition and printing parameters, we could easily control the line width and thickness to make arbitrary shapes. Actuators with more complex geometries and deformations could also be realized by this versatile and efficient method. The printed active devices are expected to be applied in soft actuators, bionic robots, artificial muscles, smart sensors, etc. CONCLUSIONS We have demonstrated a convenient and efficient approach for patterning of highly conductive graphene through direct-write printing assembly of GGO. The synergetic effect of printinginduced orientation and evaporation-induced interfacial assembly facilitated the formation of laminar-structured GGO patterns. The resulting RGO after chemical reduction showed remarkable electrical conductivity in printed graphene electronics. The RGO circuits withstand large-stain folding and maintain steady performance during repeated bending tests. Large-area and highly conductive electrodes were fabricated and used for flexible circuits. An electro-driven soft actuator with bilayer structure of RGO/PDMS has been achieved with low driving voltage. Multifunctional devices with complex geometry and controllable deformation can be prepared by the convenient printing method, which will be of great significance for the development of graphene-based flexible and printed electronics. METHODS Materials, synthesis of GGO, and instruments sections are described in the Supporting Information.


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Ink preparation. The as-synthesized GGO dispersion was concentrated by distillation with a 60 °C water bath. The solid loading of the concentrated paste was test by thermogravimetric analysis (TGA). Then the pastes were diluted by deionized water with gentle stirring to yield the inks with concentrations of 10, 15, 20, 25, 30 mg/mL. Rheological properties of the ink are characterized using an MCRxx2 Rheometer (Anton Paar) with a 25 mm flat plate geometry at 25 °C. Direct ink writing of GGO. The patterns were printed using a multi-axis dispensing system (2400, EFD). The GGO inks (with contents from 10 to 30 mg/mL) were housed in a syringe (3cc barrel, EFD Inc.) attached by a luer-lok to a micronozzle (200 µm inner diameter). An airpowered fluid dispenser (Ultimus I, EFD, Inc.) was used to provide appropriate pressure to extrude the ink through the nozzle. The printed patterns were dried in ambient atmosphere for 12 h and then vacuum dried at 60 °C for 3 h. Chemical reduction of GGO patterns. The printed GGO patterns were placed in an vacuum desiccator with a drop of HI (40%) aqueous solution. Then the desiccator was heated to 100 °C and kept for 3 h. After the sample cooled to room temperature, the patterns were washed by water and ethanol in sequence and vacuum dried at 60 °C for 3h. Fabrication of the actuators. PDMS (Dow Corning Sylgard 184, 10:1 ratio of precursor and curing agent) was spin-coated on PET films for 60 s at speed of 1000 rpm and then cured at 80 °C for 3h. GGO patterns were printed and converted to RGO by HI reduction. The unnecessary part of PDMS was cut out along the outline of the patterns by surgical blade. The RGO/PDMS bilayer was then peeled off from the PET substrate carefully. The electrodes were connected to a power supply by copper tapes.


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ASSOCIATED CONTENT Supporting Information. The size distribution of the GGO sheets, loss modulus of the inks, tailored printed features, optical images of GGO film, cross-session SEM images of the patterns with varying printing parameters, characterization of the evolution of ink filament assembly in drying process, patterns of GGO printed on various substrates, method and detailed data by the four-probe test, enlarged view of the circuits after bending cycles, adhesive forces of printed RGO on various substrates, cross section of the RGO/PDMS bilayer, optical image and I-V test of U-shaped actuators, reversible shape change of the actuator. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Fengyu Li, E-mail: [email protected] *Yanlin Song, E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources The authors thank the financial support by the 973 Program (No. 2013CB933004), the National Nature Science Foundation (Grant Nos. 51203166, 51473172, 51473173, 21203209, 21301180 and 21303218), the 863 Program (No. 2013AA030802) and the “Strategic Priority Research Program” of Chinese Academy of Sciences (Grant No. XDA09020000).


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors thank the financial support by the 973 Program (No. 2013CB933004), the National Nature Science Foundation (Grant Nos. 51203166, 51473172, 51473173, 21203209, 21301180 and 21303218), the 863 Program (No. 2013AA030802) and the “Strategic Priority Research Program” of Chinese Academy of Sciences (Grant No. XDA09020000). REFERENCES (1) Ahn, J. H.; Kim, H. S.; Lee, K. J.; Jeon, S.; Kang, S. J.; Sun, Y.; Nuzzo, R. G.; Rogers, J. A., Heterogeneous Three-Dimensional Electronics by Use of Printed Semiconductor Nanomaterials. Science 2006, 314, 1754-1757. (2) Ahn, B. Y.; Duoss, E. B.; Motala, M. J.; Guo, X. Y.; Park, S. I.; Xiong, Y. J.; Yoon, J.; Nuzzo, R. G.; Rogers, J. A.; Lewis, J. A., Omnidirectional Printing of Flexible, Stretchable, and Spanning Silver Microelectrodes. Science 2009, 323, 1590-1593. (3) Zhang, Z.; Zhang, X.; Xin, Z.; Deng, M.; Wen, Y.; Song, Y., Controlled Inkjetting of a Conductive Pattern of Silver Nanoparticles Based on the Coffee-Ring Effect. Adv. Mater. 2013, 25, 6714-6718. (4) Carlson, A.; Bowen, A. M.; Huang, Y.; Nuzzo, R. G.; Rogers, J. A., Transfer Printing Techniques for Materials Assembly and Micro/Nanodevice Fabrication. Adv. Mater. 2012, 24, 5284-5318. (5) Javey, A.; Nam, S.; Friedman, R. S.; Yan, H.; Lieber, C. M., Layer-by-Layer Assembly of Nanowires for Three-Dimensional, Multifunctional Electronics. Nano Lett. 2007, 7, 773777.


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Table of Contents Graphic


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Flexible Circuits and Soft Actuators by Printing Assembly of Graphene

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