Graphene Thin Films by Noncovalent-Interaction-Driven Assembly of Graphene Monolayers for Flexible Supercapacitors

Article

Graphene Thin Films by NoncovalentInteraction-Driven Assembly of Graphene Monolayers for Flexible Supercapacitors Guo-Fei Wang, Haili Qin, Xiang Gao, ..., Heng-An Wu, Huai-Ping Cong, Shu-Hong Yu [email protected] (H.-P.C.) [email protected] (S.-H.Y.)

HIGHLIGHTS A freestanding, transparent, and ultrathin GO film was fabricated The strongest noncovalent interaction between melamine and GO was quantified The flexible GO film presented high mechanical and electromechanical performances Ultrathin reduced GO film showed potential as an all-solid-state flexible supercapacitor

Yu and colleagues have fabricated freestanding, highly transparent, ultrathin graphene films under the synergy of the unique wrinkled structure of GO nanosheets and strong noncovalent interaction between GO and melamine. The films exhibited excellent tailored behavior, mechanical properties, electromechanical stability, and flexible supercapacitor performance. The design and synthetic strategy in this work could promote the realization of nextgeneration mobile electronic devices.

Wang et al., Chem 4, 1–15 April 12, 2018 ª 2018 Elsevier Inc. https://doi.org/10.1016/j.chempr.2018.01.008

Please cite this article in press as: Wang et al., Graphene Thin Films by Noncovalent-Interaction-Driven Assembly of Graphene Monolayers for Flexible Supercapacitors, Chem (2018), https://doi.org/10.1016/j.chempr.2018.01.008

Article

Graphene Thin Films by NoncovalentInteraction-Driven Assembly of Graphene Monolayers for Flexible Supercapacitors Guo-Fei Wang,1,5 Haili Qin,1,5 Xiang Gao,3 Yi Cao,3 Wei Wang,3 Feng-Chao Wang,4 Heng-An Wu,4 Huai-Ping Cong,1,* and Shu-Hong Yu2,6,*

SUMMARY

The Bigger Picture

Macroscopic assembly can create advanced materials with hierarchical structure and translate the properties of individual building blocks into ensembles for specific applications. Here, we demonstrate the fabrication of freestanding graphene oxide (GO) films composed of only two Langmuir-Blodgett-induced monolayers under the synergy of wrinkled GO nanosheets and strong noncovalent interaction between GO and melamine. We found that the strongest noncovalent interaction ever measured (1 nN) was a synergistic effect of a charge-transfer interaction and hydrogen bonding. The as-obtained film delivered a large optical transmittance of 84.6% at 550 nm and a high mechanical strength of 45 MPa at a notable elongation of 3.5%. The reduced GO film, with a resistance of 420 U sq 1, exhibited excellent electromechanical stability for 10,000 cycles at a bend radius of 1.5 mm. With the merits of a unique structure and outstanding properties, such an ultrathin film demonstrates its potential in the application of all-solid-state flexible supercapacitors.

All-solid-state supercapacitors are considered a state-of-the-art power supply for miniaturized electronic devices. Flexible, ultrathin graphene films, which have the excellent properties of individual graphene nanosheets and are lightweight because of their small thickness, are attractive when used as flexible power sources. However, it is still a great challenge to construct such graphene assemblies because of the lack of an efficient assembly method and strong interfacial interaction. In this work, we demonstrate the fabrication of freestanding, transparent, and ultrathin GO films with high mechanical and electromechanical performances based on strong noncovalent interactions between melamine and GO through a modified Langmuir-Blodgett strategy. Benefitting from ultrathin thickness, unique structure and mechanical and electrical properties, the reduced GO films obtained are highly competitive as all-solid-state flexible supercapacitors.

INTRODUCTION Macroscopic-scale assemblies are emerging as new material systems by the translation of the excellent properties of individual building blocks into advanced ensembles.1–4 For safe and efficient electronic systems with light weight and flexible properties, all-solid-state supercapacitors have been considered a state-of-the-art power supply for miniaturized electronic devices that can efficiently avoid leakage of harmful electrolytes.5,6 From a practical point of view, graphene film represents an easily processable macroscopic architecture with periodic alignments and attractive properties.7 To date, flexible graphene films with controlled thickness in the range of hundreds of nanometers to micrometers have been fabricated by various solution-phase manipulations for flexible power sources.8–11 For a filmassembled all-solid-state supercapacitor, the thickness of the film arising from a thick stack hinders the permeation of electrolyte and disturbs the establishment of electrical double layers (EDLs), resulting in capacitance decay. Therefore, developing a novel technology for construction of graphene films with few layers as a flexible electrode is of significance and necessity. Up until now, thin graphene films obtained through chemical vapor deposition (CVD)-grown12–14 or solution-phaseassembled15–17 methods need the support of various substrates, such as polyethylene terephthalate and glass, because of the lack of enough interlayer interaction. The complicated transfer procedure and wafer-dependent limitation reduce the processability of these films and further prevent their use. In addition, the thin films obtained should have excellent mechanical performance to endure large

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mechanical deformations and extended cycling operation when used as a flexible device. With these opposites in mind, we learn from nature and find that the high performance of natural nacre is attributed to its hierarchical micro- and nanostructure and strong organic-inorganic interfacial interactions.18,19 Intense efforts have been devoted to creating nacre-like graphene films with a ‘‘brick and mortar’’ microstructure and enhanced interlayer interactions.20–22 The organic mortar as a tough and viscoelastic glue is essential for the mechanical performance of the composite materials. As stress accumulates, the interactions between organic molecules and graphene layers can break and reform consequently to dissipate energy, conferring the high strength and stiffness of the graphene materials. Hitherto, the organic molecules used in graphene assemblies were either selected serendipitously or in a trialand-error fashion. Going a step further in the rational design of microstructures and interface interactions, transparent, nonwafer, and flexible graphene film is available. One challenge is to find suitable organic molecules that can establish strong interfacial interactions with neighboring graphene layers and allow us to understand the molecular origin of the high mechanical performance. The other crucial aspect is to develop an effective assembly strategy for achieving optimized micro- and nanoscaled arrangements in the film for large extensibility. Here, we introduce the modified Langmuir-Blodgett (LB) approach for the fabrication of a freestanding, transparent, and ultrathin GO film with a high GO content by sequential deposition of GO monolayers and melamine molecules. We produced a 22-nm-thick flexible GO film, composed of only two LB-induced GO monolayers sandwiched with melamine, under the synergy of the unique winkled structure of GO nanosheets and strong adhesive interaction between GO and melamine. The unique swollen property of the fiber network in the cellulose membrane contributes to the remarkable wrinkled structure of GO film, which interlocks nanosheets of the same layer and increases the contacts between two layers. Through a systematic screening of 11 amine-containing molecules, we discovered that melamine shows the strongest noncovalent interaction with GO by exhibiting a rupture force of more than 1 nN. Importantly, this rupture force is within the force range of typical covalent bonding. Such strong binding originates from the synergistic effects of charge-transfer interactions between GO and the triazine moiety of melamine, together with binding of the amino groups of melamine to GO. The GO film obtained exhibited transparency as high as 84.6% at 550 nm, excellent fold-resistant flexibility, notable extensibility, and good chemical stability. The reduced GO (RGO) film, with a resistance of 420 U sq 1, presented high electromechanical stability with less than 1% resistance variation after 10,000 cycles at a small bending radius of 1.5 mm. Evaluated as a supercapacitor electrode, ultrathin RGO films exhibited a high capacitance performance of 197.3 F/cm3 at a current density of 500 mA/cm3. Furthermore, the thin film suggested its usefulness as an all-solid-state flexible supercapacitor by presenting a large specific capacitance, a stable electromechanical capability under large bending deformation, and good cycling stability. The high-performance graphene films reported in this study could find broad applications as next-generation mobile electronic systems.

1Anhui

Province Key Laboratory of Advanced Catalytic Materials and Reaction Engineering, School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei 230009, P.R. China

2Division

of Nanomaterials and Chemistry, Hefei National Research Center for Physical Sciences at Microscale, CAS Centre for Excellence in Nanoscience, Collaborative Innovation Center of Suzhou Nano Science and Technology, Department of Chemistry, University of Science and Technology of China, Hefei 230026, P.R. China

3Department

of Physics, Nanjing University, Nanjing 210093, P.R. China

4Department

of Modern Mechanics, University of Science and Technology of China, Hefei 230027, P.R. China

5These

RESULTS Assembly of Freestanding GO Film The typical fabrication procedures for freestanding GO film are schematically illustrated in Figure 1. In the first stage, the GO monolayer was assembled on the

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6Lead

authors contributed equally

Contact

*Correspondence: [email protected] (H.-P.C.), [email protected] (S.-H.Y.) https://doi.org/10.1016/j.chempr.2018.01.008

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Figure 1. Schematic Illustrations for the Fabrication of Freestanding GO Film Step 1: a GO monolayer was deposited on the cellulose membrane by the LB technique. Step 2: the GO monolayer was wrinkled as a result of the shrinkage of the cellulose membrane during the drying process. Step 3: melamine molecules were adsorbed on the surface of the GO monolayer. Step 4: the GO film with the desired thickness was prepared by repetition of the above three steps. Note that the last layer of the assembled film was deposited with GO nanosheets. Step 5: the freestanding GO film was formed by dissolution of the cellulose membrane in acetone. After the floating GO film was captured with a metal grip from acetone and removed from the grip, the flexible GO film was fabricated. See also Figures S1–S5 and Movie S1.

cellulose membrane by the LB technique. The GO nanosheets used here were micrometers in size and 1.5 nm thick and were prepared according to the modified Hummers method23 (Figure S1A). With the neutral charge of GO nanosheets in 1-butanol, as determined by zeta-potential measurements (Figure S2), 1-butanol was selected as the ideal spreading solvent for GO in the LB deposition process.24 The surface pressure-area isotherm was monitored to reveal the change in coverage density of the GO monolayer the during compression process by a tensiometer (Figure S3). After the surface pressure was increased from 10 to 25 mN/m, as observed from atomic force microscopy (AFM) imaging, the separately dispersed GO sheets touched each other, forming a close-packed monolayer and tiling the entire surface (Figure S1A and S1B). With further compression to 40 mN/m, the GO nanosheets overlapped and were wrinkled (Figure S1C). On the basis of these analyses, a pressure of 25 mN/m was fixed for the LB deposition to give a balance between the prevention of overstacked GO monolayers and full interaction of neighboring GO sheets. Then, the dried membrane loaded with GO sheets was immersed into 0.1 wt % of melamine solution, which produced a periodic layer of GO and melamine by virtue of the strong interactions between melamine and the GO nanosheets. GO films with the desired thickness, designated by the symbol GOn, were prepared by sequential repetition of these steps. Here, n represents the number of GO monolayers. Note that the last layer of the film was deposited with GO nanosheets. As revealed from the identical decrease in surface pressure when one GO layer in region b of the isotherm was repeatedly collected (Figure S3), a constant amount of GO

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Figure 2. Physical Properties of Freestanding, Transparent GO2 Film (A–D) Photographs showing the freestanding GO 2 film of various shapes, including a square (A), triangle (B), pentagon (C), and circle (D), demonstrating its good processability. (E) Photograph of the transparent GO 2 film floating in the air. (F and G) Photographs of the GO2 film still retaining its structural integrity after being immersed in 1 M HCl (F) or NaOH (G) solutions for 24 hr and then lifted with tweezers. (H) AFM image of freestanding GO 2 film and the corresponding height profile. See also Movie S2.

nanosheets was collected, which enable the uniformity of the assembled film. In the second stage (Figure S4), a piece of free, integrated GO film floating on the liquid surface was obtained after complete dissolution of the cellulose membrane in acetone, as revealed from the smooth surface on scanning electron microscopy (SEM) and AFM imaging (Figure S5). Then, a metal grip, carefully breaking through the surface tension of the liquid, was applied to capture the film from the acetone. After the metal framework was removed with tweezers, a piece of freestanding, well-shaped GO film was obtained (Movie S1). Figures 2A–2D show highly transparent, freestanding GO2 films of various geometric shapes, including a triangle, square, pentagon, and circle (Movie S2), indicative of their good processability. As shown in Figure 2E, the density of the film is ultralight, and it can float in air. Its thickness was determined to be 22 nm from the AFM height profile (Figure 2H). Abundant wrinkles are clearly observed on the corresponding AFM image. As shown in Figures 2F and 2G, the assembled films endured lifting with tweezers and retained their integrated film morphology after being immersed

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Figure 3. Compositional Characterization of Freestanding GO6 Film (A–D) TEM image of GO6 film (A) and elemental mappings of C (B), O (C), and N (D). (E) Cross-sectional SEM image of GO 6 film. (F) EDX spectrum of GO 6 film. See also Figures S6–S11.

in HCl and NaOH for 24 hr, indicating their outstanding corrosion-resistant performance as a result of the chemical resistance of the GO nanosheets and wrinkled structures, as well as strong adhesion by the melamine molecules between neighboring GO layers. For accuracy, we used GO6 film to characterize its composition. The low contrast in the transmission electron microscopy (TEM) image in Figure 3A and the well-defined cross-sectional SEM image in Figure 3E indicate the thickness of GO6 film at nanometer scale. The corresponding elemental mappings present uniform distribution of C, N, and O in the film (Figures 3B–3D and S6). As determined from the dominant 80.5 wt % of C and 1.3 wt % of N in the energy-dispersive X-ray (EDX) spectrum (Figure 3F), such film is composed of 98.1 wt % of GO nanosheets and 1.9 wt % of melamine. No typical diffraction peak of GO at 9 –10 was detected in the X-ray diffraction (XRD) pattern of the GO6 film (Figure S7), implying that the GO nanosheets are in a nonaggregated state in each layer under the spacing effect of winkles seen on the AFM images (Figure 2H). The Role of Cellulose Membrane in the Formation of Freestanding GO Film Cellulose membrane was found to play an important role in the formation of the thin freestanding GO film. An SEM image of the membrane used shows an interconnected network with a pore size centered at 425 nm (Figure S8). As illustrated in Figure S9, when the membrane was wetted during collection of the GO nanosheets in water, the cellulose network was loosely crosslinked because of the intrinsically swollen characteristic of such a polymer substrate. An SEM image of the freeze-dried membrane loaded with a fresh LB-assembled GO layer demonstrates that the pore size of the network significantly increased to 950 nm on average (Figure S10A). After the membrane was dried naturally, the GO sheets interlocked with each other obviously after the network of the attached cellulose substrate had shrunk. Therefore, an integrated, wrinkled GO lamella covered the surface of the membrane (Figure S10B). However, because of the restriction of interlocked GO nanosheets, the membrane (with a pore size of 650 nm) could not recover to its original state. The height of

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the dried GO monolayer after dissolution of the cellulose membrane approached 12 nm according to AFM imaging, confirming the wrinkled structure of the first GO layer (Figure S11A). This unique microstructure maximizes the interactions between GO nanosheets in each lamella. Because of the abundant wrinkles in the first layer, the height of the GO2 film reached a reasonable value of 22 nm on AFM rather than deposition of multilayer film without wrinkles from flat GO nanosheets, which is consistent with the XRD results (Figure S7). Quantification of Interactions between GO and Melamine The melamine molecules between the two GO layers make a big contribution to the formation of this kind of unique film, as shown by the collapsed state of the GO film without melamine when the membrane was dissolved in acetone and couldn’t be collected by the metal grip (Figure S12). To obtain molecular insights into the interactions between melamine and GO in the film, we used AFM-based single-molecule force spectroscopy, a method for directly measuring the break strength of bonding motifs with pico-Newton resolution,25–28 to quantify the binding force between melamine and GO at the single-molecule level (Figure 4). Furthermore, to screen for the most suitable adhesion molecule, we measured the binding strength between GO and ten other molecules, including fatty amines, aromatic amines, and amino-group triazines (Figure S13). We brought a Si3N4 cantilever tip flanked with melamine through a polyethylene glycol (PEG) linker into contact with the GO sheets to measure the rupture forces (Figure 4A). Figure 4B shows a set of three representative force-extension curves for approach (gray) and retraction (red) of the melamine-functionalized tip from a GO surface. Control experiments using an unmodified cantilever or surface revealed the rupture events in force-extension curves from the break of the melamine-GO interactions (Figure S14). The fitted force-extension curves from the extended worm-like chain model for polymer elasticity confirmed single-molecule rupture events (Figure S15).29 According to Figure 4B, the rupture force of melamine-GO was more than 1 nN, stronger than that of any noncovalent bonds measured so far by single-molecule AFM and comparable with that of covalent bonds (1–2 nN).26,30 Moreover, such interaction is reversible, i.e., can reform after breakage, indicative of its noncovalent nature. In contrast, according to representative force-extension curves, the rupture forces of other molecules with GO are much smaller, ranging from 100 to 600 pN (Figures 4C and S16). To further understand the contributions of different parts of melamine to the binding strength, we measured the rupture forces of ethylenediamine (EDA), 2-amino-1,3,5triazine (ATA), and melamine at different loading rates. EDA and ATA served as reference molecules because one of their amino groups links with the PEG linker in AFM experiments, and they contain only a free amino group and a free triazine for GO binding, respectively. Figure 4D shows the rupture-force distribution of these three molecules at a pulling speed of 1,000 nm s 1. As revealed from the force values of 213.8 pN for EDA, 309.2 pN for ATA, and 1,027 pN for melamine, the binding strength of melamine is more than the combined contribution of both amine and triazine and the synergistic effects between different binding parts. Figure 4E summarizes the binding forces at different loading rates. On the basis of the calculations using the Bell-Evans model,25,31 we found that Dx for melamine, EDA, and ATA was 0.09 G 0.04, 0.4 G 0.1, and 0.07 G 0.01 nm, respectively. The Dx for EDA was consistent with that for hydrogen-bonding interactions.32,33 The Dx values for melamine and ATA were shorter than that of typical noncovalent bonds34,35 but were similar to that for a charge-transfer complex.36 Typically, shorter Dx leads to higher rupture forces,37 which explains the extremely high mechanical strength of melamine. The free-energy barriers for the rupture of melamine, EDA, and ATA were

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Figure 4. Single-Molecule AFM Experiments Directly Measure the Binding Strength between Different Organic Molecules and GO Surfaces (A) Schematic of the single-molecule experiments. Melamine was linked to the AFM cantilever tip through a PEG linker by one of the amino groups. Shorter mPEG was used to minimize nonspecific tip-surface interactions. (B) Three representative force-extension curves for the rupture of melamine-GO interactions. The peak value measured the force at which melamine detached from the GO surface. Because we used a soft cantilever (spring constant of 50 pN/nm) for the force measurement, the cantilever was slow to return to the resting flat conformation after each rupture event. Therefore, the force curves show a descending slope after each force peak, reflecting the gradual conformational change of the cantilever. (C) Histogram of the rupture forces for 11 different adhesion molecules with GO surfaces, including fatty amines (ethylenediamine [EDA], diethylenetriamine [DETA], triethylenetetramine [TETA], and tetraethylenepentamine [TEPA]), aromatic amines (aniline and three phenylenediamine derivatives: o-PDA, m-PDA, and p-PDA), and amino-group triazines (2-amino-1,3,5-triazine [ATA], 2,4-diamino-1,3,5-triazine [DATA], and melamine). Error bars show the SD with a sample size of 3. (D) The rupture force distributions for melamine, EDA, and ATA with GO surfaces at a pulling speed of 1,000 nm s 1 . The insets show the corresponding molecular structures (gray, carbon; blue, nitrogen; white, hydrogen).

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Figure 4. Continued (E) The linear-log plots of rupture forces versus loading rates for melamine, EDA, and ATA. The lines correspond to the fits to the Bell-Evans model. Error bars show the SD with a sample size of 3. See also Figures S12–S18.

83.2 G 10.2, 76.3 G 5.0, and 37.2 G 4.3 kJ mol 1, respectively. Interestingly, the binding energy of melamine to GO seemed to be mainly contributed by the amino groups, whereas the triazine ring defined a short Dx with a specific flat/flat orientation throughout. Such cooperative behavior makes it the strongest noncovalent interaction determined to date. To understand the chemistry of melamine-GO interactions, we carried out Fourier-transform infrared (FTIR) spectroscopy on GO, melamine, GO/melamine film, and RGO/melamine film (Figure S17). In the spectrum of GO/melamine film, the characteristic bands of NH2 stretching vibration at 3,000–3,500 cm 1 of melamine disappeared, and the N–H deformation vibration mode was red shifted from 1,653 to 1,628 cm 1, indicating the formation of hydrogen bonds between NH2 and GO. Moreover, the vibration bands of triazine at 1,551 and 1,194 cm 138 were significantly damped and red shifted to 1,510 and 1,175 cm 1, respectively. This suggests that the vibration of the triazine ring was constrained as a result of strong interactions with GO.39 With the chemical reduction of HI, in the FTIR spectrum of RGO/melamine film, the characteristic bands of C=O at 1,733 cm 1, C–OH at 1,370 cm 1, and C–O–C at 1,230 cm 1 disappeared as a result of the removal of oxygen-containing functional groups on the GO nanosheets. However, the red-shifted bands around 1,513 and 1,174 cm 1 belonging to the triazine vibrations were still present, implying that the specific flat/flat-orientated interaction between RGO and melamine was maintained. According to our density functional theory (DFT) calculations, nitrogen atoms in the triazine ring of melamine are more electronegative and can act as good chargetransfer acceptors through hydrogen bonds, etc. Such electron-transfer and hydrogen-bonding formations then result in strong interactions between the triazine ring and GO (Figure S18). These results allow us to understand the molecular origin of the strong noncovalent interactions between melamine and GO. The charge transfer from GO to the triazine ring of melamine leads to strong interaction strength as well as a short rupture distance between them, which is key for the cooperativity of the charge-transfer interactions between the triazine ring of melamine and GO and of hydrogen-bonding interactions between the amine groups and the GO surface. Therefore, melamine serves as a sticky ‘‘mortar’’ to tightly glue neighboring GO nanosheets together, giving rise to such unique GO films. Transparency and Mechanical Properties of GO and RGO Films Because of the precisely arranged structure and strong interfacial interaction, as shown in Figure 5A, freestanding GO2 film delivered a high transmittance of 84.6% at 550 nm and 90.7% at 800 nm. Moreover, the optical transparency of the assembled GOn films was closely related to the number of GO layers (Figure S19). The transmittance was reduced by 3.4% when the film was assembled with one more GO lamella, consistent with 2.3% of absorption of visible light for the CVDgrown graphene monolayer.40,41 Even with ten GO layers, the transmittance in the whole visible light region was still more than 55%, indicating good arrangement of GO nanosheets in the film. After being chemically reduced by HI, the resulting RGO2 film still showed a high transparency of 79.2% at 550 nm.

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Figure 5. Optical, Mechanical, and Electromechanical Properties of GO and RGO Films (A) Transmittance spectra of GO2 film and RGO 2 film. The inset shows the RGO 2 film. (B) Typical tensile stress-strain curves of GO 3 film and the corresponding RGO 3 film. (C) Photographs of RGO 3 film under a continuous bending deformation. (D) Resistance variation of RGO 3 film at a bending radius up to 1.5 mm during the first bending cycle. (E) Variation of the electrical resistance of RGO 3 film during ten bending cycles. For each cycle, the film was gradually bent to a radius of 1.5 mm and then straightened to its original state. (F) Variation of the resistance of RGO 3 film as a function of a long-term bending cycle at a bending radius of 1.5 mm. The inset shows the resistance changes after 10, 100, 1,000, and 10,000 bending cycles. See also Figures S19–S23.

Although the assembled transparent films were only nanometers thick, they exhibited excellent mechanical performance. To quantify the mechanical strength of the films, we applied the tensile stress-strain measurement in Figure 5B to a GO3 film with a thickness of 32 nm (Figure S11B). The tensile strength and Young’s modulus of the GO3 film were calculated to be 45 and 690 MPa, respectively. Notably, its ultimate strain was up to 3.5%, several times higher than the highest reported value of 0.3%–1.2% for GO films (Figure S20).22,42,43 The GO3 film could resist folding twice with overlap without breaking and recovered its original state after being unfolded (Figure S21), demonstrating the fine toughness and flexibility of the as-formed film. Different from the smoothly stacked structure in GO film, the abundant wrinkles in the layered GO nanosheets presented here make a great contribution to such good extensibility, consistent with the wrinkled graphene sheets in graphene fibers.44,45 In addition, the self-adaptable abilities between melamine molecules as well as organic molecules and GO sheets are a benefit when GO-melamine motifs are elongated under tension. After chemical reduction of HI, the tensile stress of the corresponding film improved to 60 MPa at a maximum strain of 2.9%, which represents one of the best values for the strain of RGO films reported in previous work (Figure S22). The RGO film obtained could endure continuous bending deformation to a large degree, as shown in Figure 5C. Such a high strength is ascribed to the increased interactions between graphene sheets followed by the removal of oxygenated groups and the resulting decreased interlayer

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distance, as well as the tightly interlocking structures from wrinkling of the graphene sheets (Figure S23). Electromechanical Stability of RGO Film The combination of excellent transparency, electronic conductivity, and mechanical robustness discussed above makes this freestanding film a promising candidate for use as a flexible transparent conductor. We investigated the electromechanical stability of RGO3 film with an electrical conductivity of 420 U sq 1 by recording its resistance variation under bending deformations with a high-precision mechanical system. As shown in Figures 5D and 5E, for the first ten cycles, the electrical resistance slowly increased along with a decrease in the bending radius, giving a slight change of 0.45% up to a bending radius of 1.5 mm. After the bending force was released, the film was straightened, and the resistance completely recovered to its original value. Note that the curve of resistance variation as a function of the bending radius since the 1,000th cycle was flat, indicating that the film resistance was stable during the bending process. Even after 10,000 bending-releasing cycles, the irreversible resistance of the film was less than 0.8% at a small bending radius of 1.5 mm (Figure 5F and inset), much lower than the reported 2.7% increase in a graphene-foam-based flexible conductor at a bending radius of 2.5 mm.46 Ultrathin RGO Films Assembled for All-Solid-State Flexible Supercapacitors To study the electrochemical capacitive performance of the as-obtained ultrathin RGO films, we performed electrochemical evaluations by using a two-electrode system. For comparison, RGO films of different thicknesses were also assessed. Figure 6A shows the cyclic voltammetry (CV) curves of the RGO films as supercapacitor electrodes 30, 70, and 210 nm thick at a scan rate of 50 mV/s in the voltage region of 0–0.8 V. The nearly rectangular shapes in the CV curves indicate a typical EDL capacitor. The 30-nm-thick RGO film showed the highest current response, revealing its great potential as a capacitive material. We also performed galvanostatic charge-discharge measurements at a current density of 500 mA/cm3 in a voltage window of 0–0.8 V to disclose the performance of RGO films as supercapacitor electrodes. As shown in Figure 6B, the ultrathin 30 nm RGO film delivered an impressive volumetric capacitance of 197.3 F/cm3, which was much larger than that at 70 nm (134.1 F/cm3) and 210 nm (50.9 F/cm3), in accordance with the above CV results. Given that high volumetric capacitance was significant for realizing a compact supercapacitor device for real applications,47 this ultrathin graphene film was assessed as a highly competitive candidate among the carbon-based films reported so far (Table S1). For systematic research on capacitance performance, RGO films of different thicknesses were measured as electrodes at various current densities (Figure 6C). The capacitance decreased continuously with increasing current density from 100 to 5,000 mA/cm3 as a result of less efficient diffusion and penetration of ions into the electrode, as well as electrode overpotential at high current density.48,49 The 30-nm-thick electrode at a current density as high as 5,000 mA/cm3 still maintained a high capacitance of 94 F/cm3. However, the 70- and 210-nm-thick films reached only 46.3 and 30.4 F/cm3, respectively. In addition, when the scan rates were elevated, the 30-nm-thick film demonstrated linearly increased responses of current density in both the charge and discharge processes (Figures 6E and S24), indicating its ideal capacitive behavior and good rate capability.50 Moreover, 94.7% of the initial capacitance was retained after 7,500 cycles for the 30-nm-thick RGO film, indicating its outstanding cycling stability (Figure 6F). Even after 1 month of storage, the electrode still showed good stability, as revealed by acceptable changes in the CV curves and charge-discharge curves in comparison with the initially assembled electrode (Figures S25A and S25B). To

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Figure 6. Electrochemical Performance of RGO Film Electrodes Using a Two-Electrode Cell (A) CV curves at a scan rate of 50 mV/s. (B) Galvanostatic charge-discharge curves at a current density of 500 mA/cm 3 . (C) Volumetric capacitances at different current densities. (D) Electrochemical impedance spectra for RGO films with different thicknesses as supercapacitor electrodes. (E) Variations of current densities of 30-nm-thick RGO film electrodes during charge and discharge with different scan rates at a constant voltage of 0.4 V. (F) Cycling stability of 30-nm-thick RGO film electrodes at a current density of 2,000 mA/cm 3 .

understand the effect of the structure and thickness on the capacitance performance, we carried out electrochemical impedance spectroscopy of three electrodes (Figure 6D). The 30-nm-thick film electrode delivered the steepest slop in the lowfrequency region, as an index of the smallest electrolyte diffusion resistance, indicating that fast transport channels of electrolyte ions arise from its lowest thickness and abundant wrinkled structure in the film.51 In our case, ion transport from bulk electrolyte to the current collector throughout the film played a crucial role in the capacitor performance. Apparently, the distance and resistance of ion transport were improved in the electrodes with increased film thickness, which was unfavorable for the specific capacitance especially at high current density.52–54 Moreover, the thicker film needed a large number of ions to migrate from the bulk electrolyte; otherwise, the ionic resistance increased significantly by depletion of the ionic concentration. This deterioration was more pronounced for the thickest graphene film electrodes because graphene nanosheets with a high aspect ratio were prone to align perpendicularly to the direction of the ion field. Therefore, the capacitor performance of the film was improved by a decreased thickness. From the above results and the reported work, the superiority of such an ultrathin graphene film as a supercapacitor has been fully testified (Figure S26). In view of the striking capacitance performance of the 30-nm RGO films, we integrated two pieces of film with a H2SO4-PVA gel electrolyte to assemble an all-solid-state supercapacitor. To investigate the flexibility and mechanical stability of the as-obtained device, we studied the electrochemical performance under bending at angles from 0 to 180 (Figure 7A). Compared with the CV curve obtained

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Figure 7. Electrochemical Performance of an All-Solid-State Flexible Supercapacitor Assembled from 30-nm-Thick RGO Films (A) CV curves at a scan rate of 50 mV/s at different bending degrees. The inset shows the all-solidstate flexible supercapacitor assembled from 30-nm-thick RGO films. (B) CV curves at different scan rates. (C) Galvanostatic charge-discharge curves at different current densities. (D) Cycling stability at a current density of 2,000 mA/cm 3 . See also Figures S24–S26 and Table S1.

without bending, curves obtained after bending showed no significant change in their shape or area, even at 180 , revealing the device’s excellent mechanical stability. CV curves at different scan rates ranging from 50 to 2,000 mV/s are shown in Figure 7B. The well-defined rectangular shape even at a high scan rate and rapid current response within the potential ranges illustrates fast and efficient diffusion of ions in the electrode. Its outstanding capacitance performance was also demonstrated by charge-discharge curves at different current densities (Figure 7C). For instance, the flexible all-solid-state supercapacitor assembled by the 30-nm RGO film delivered a volumetric capacitance of 124.5 F/cm3 at a current density of 1,000 mA/cm3, and 91.4% and 81.8% of the initial value were maintained after 7,500 cycles and 1 month of storage, respectively, demonstrating its excellent stability (Figures 7D, S25C, and S25D).

DISCUSSION In summary, flexible, transparent, and ultrathin GO films with a controllable nanoscale thickness and unique wrinkled microstructure have been fabricated by means of an elaborate LB technique involving the swollen property of cellulose membrane and strong interactions between melamine and GO nanosheets. We discovered that melamine containing three amino groups and a triazine ring shows remarkably strong and reversible binding to GO with a break force as high as 1 nN, representing the strongest noncovalent interaction reported so far. Freestanding GO films with a tailored shape, a thickness as low as 22 nm, a high transmittance of 84.6% at 550 nm, and excellent mechanical performance with extensibility of 3.5% were prepared. The resulting transparent and ultrathin RGO films exhibited outstanding electromechanical stability with a slight resistance variation of 0.75% after 10,000 cycles at a

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small bending radius of 1.5 mm. In use as a supercapacitor, the ultrathin 30-nm RGO film displayed a large specific capacitance of 197.3 F/cm3 at a current density of 500 mA/cm3 and remarkable cycling stability. Furthermore, the ultrathin RGO film demonstrated its potential in the assembly of an all-solid-state flexible supercapacitor by delivering a high specific capacitance of 124.5 F/cm3 at a current density of 1,000 mA/cm3, excellent electromechanical stability under large bending deformation, and stable cycling capability with 91.4% retention of the initial capacitance after 7,500 cycles. The strategy presented in this work could provide a general method for the assembly of a broad class of low-dimensional nanoscale building blocks into flexible-film-like materials with new functions and could promote developments in the field of next-generation flexible electronic systems.

EXPERIMENTAL PROCEDURES Preparation of Freestanding GO Films In a typical procedure, GO was first prepared through acid oxidation of graphite flakes according to a modified Hummers method. Cellulose membrane was used as the substrate for deposition of GO film. The LB trough (Nima Technology, KN2006) was filled with Millipore Milli-Q water as subphase at room temperature. 5 mL of GO dispersion in 1-butanol (0.2 mg mL 1) was spread onto the water surface drop by drop with a glass syringe. After being stabilized for 30 min, the GO monolayer was compressed by barrier movement at 20 mm min 1; the surface pressure was monitored with a tensiometer and a Wilhelm plate. At a surface pressure of 25 mN/m, the cellulose membrane was dipped vertically into the trough and lifted at a speed of 1 mm min 1. In this way, the first GO monolayer was transferred and deposited onto the membrane. After drying, the membrane obtained was immersed into 0.1 wt % melamine solution for 2 min and then rinsed in deionized water. The GO film with the desired thickness was fabricated by repetition of the above procedures. The last layer was always a GO layer in our experiments. Then, the cellulose membrane carrying the GO film was soaked in acetone for 6 hr. To dissolve the membrane completely, fresh acetone was changed twice. A metal grip was then used to capture the floating GO film from the acetone. After the metal framework was removed with tweezers, a piece of freestanding, well-shaped GO film was fabricated. RGO films were prepared by chemical reduction of GO films with hydroiodic acid (40%) at 70 C overnight.

SUPPLEMENTAL INFORMATION Supplemental Information includes Supplemental Experimental Procedures, 26 figures, 1 table, and 2 movies and can be found with this article online at https://doi. org/10.1016/j.chempr.2018.01.008.

ACKNOWLEDGMENTS We acknowledged funding support from the National Natural Science Foundation of China (21571046, 51732011, 21761132008, 21431006, and 21503063), the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (21521001), the Chinese Academy of Sciences (CAS) Key Research Program of Frontier Sciences (QYZDJ-SSW-SLH036), the National Basic Research Program of China (2014CB931800), the Users with Excellence and Scientific Research Grant from the CAS Hefei Science Center (2015HSC-UE007), the Program for New Century Excellent Talents in University (2013JYXR0654), the Fundamental Research Funds for the Central Universities (JZ2016HGPA0735 and JZ2017HGTB0197), and the Anhui Provincial Natural Science Foundation (1708085MB30). This work was partially

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carried out at the University of Science and Technology of China Center for Micro and Nanoscale Research and Fabrication.

AUTHOR CONTRIBUTIONS H.-P.C. and S.-H.Y. supervised the project, conceived the idea, designed the experiments, and wrote the paper. G.-F.W. and H.Q. planned and performed the experiments, collected and analyzed the data, and wrote the paper. X.G., Y.C., and W.W. performed the single-molecule AFM experiments, analyzed the data, and wrote the paper. F.-C.W. and H.-A.W. carried out the DFT calculations and discussed the data. All authors discussed the results and commented on the manuscript.

DECLARATION OF INTERESTS The authors declare no competing interests. Received: September 11, 2017 Revised: December 14, 2017 Accepted: January 10, 2018 Published: March 8, 2018

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