Naturally Dried Graphene-Based Nanocomposite Aerogels with

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Naturally Dried Graphene-Based Nanocomposite Aerogels with Exceptional Elasticity and High Electrical Conductivity Yaqian Zhang, Li Zhang, Gongzheng Zhang, and Huanjun Li* School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing, 100081 P.R. China

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S Supporting Information *

ABSTRACT: Materials combining high porosity, mechanical durability, and multifunctionality have drawn significant research interest because of their potential in engineering applications. Herein, the porous air-dried nanocomposite aerogels containing reduced graphene oxide (RGO) and chitosan (CS) are fabricated by self-assembling an aqueous dispersion of graphene oxide and chitosan with the addition of hydroiodic acid (HI) followed by recasting the hybrid hydrogel with an ice-template method. The strong cross-linked composite aerogels obtained have reversible compressibility, exceptional elasticity, and high electrical conductivity, which are derived from the restacking inhibition and steric hindrance of the polymer chains. What’s more, the successive soaking−drying experiments indicate that the as-prepared graphene-based aerogels exhibit excellent environmental stability and reuseability. The regenerated electrical conductivity remains almost the same and more than 90% of its maximum compressive stress at a strain of up to 92% is retained after five cycles. This makes them ideal candidates for potential applications in areas of supercapacitors and energy storage. KEYWORDS: air-dried, nanocomposite aerogel, elasticity, electrical conductivity, environmental stability

1. INTRODUCTION Graphene, a new carbon material with one-atom thickness and two-dimensional single layer, has emerged as a crucial nanomaterial in various fields including supercapacitors, actuators, thermal insulators, energy storage, and electronic devices owing to its prominent intrinsic properties.1−7 Selfassembling graphene nanosheets is a simple and facile method for the fabrication of novel materials, such as one-dimensional nanostructures, and two-dimensional graphene films.8−10 It is widely assumed that the development of three-dimensional (3D) structures of graphene will further expand its significance in practical exploitation. Recently, 3D graphene aerogels/ foams/sponges are successfully fabricated according to a wide variety of processing techniques, such as in situ self-assembly, chemical cross-linking, electrochemical synthesis, and chemical vapor deposition.11−16 The as-prepared graphene aerogels consisting of 3D interconnected micropores and mesopores are ultralightweight and exhibit superelasticity, high electrical conductivity, and exceptional potential in many practical applications.17−20 Of particular note, in many cases, the approaches mentioned above need specific drying techniques (freeze-drying or supercritical CO2 drying) to remove the liquid from the hydrogels and then obtain the final aerogels with minimal distortion of volume and structure. However, the application of specialized instrumentation results in high energy consumption, low production, and difficult preparation of the graphene © XXXX American Chemical Society

aerogels. Consequently, the ambient drying technique has arisen as a more competitive option for graphene aerogels fabrication because it saves energy, is extremely convenient, and has non-special device requirements. Nevertheless, the resultant aerogels normally suffer from inevitable volume shrinkage and structure cracking due to the capillary pressure caused by solvent evaporation under ambient temperature and pressure conditions, which eventually results in the collapse of the 3D architectures, making them inappropriate for the fabrication of other functional materials or nanocomposites.21−24 Therefore, the preparation of high-performance graphene aerogels based on the naturally dried technique is still challenging. Even though many graphene aerogels with excellent properties have been developed, the fabrication of graphenebased nanocomposite aerogels with exceptional elasticity and satisfactory electrical conductivity are rarely reported.25,26 The poor dispersion of graphene in polymeric matrix and the weak interaction between the two components would be the factors limiting the application of graphene-based aerogels. Chitosan (CS), the partially deacetylated derivative of chitin, is of vital importance in the fabrication of functional materials owing to its low cost and environmental benefits.27−30 In addition to having biocompatible and biodegradable characteristics, the Received: March 22, 2018 Accepted: June 4, 2018 Published: June 4, 2018 A

DOI: 10.1021/acsami.8b04689 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. Preparation of the RGO aerogel (a) and RGSA (b). 2.2. Characterization. The Fourier transform infrared (FTIR) spectra of the samples were recorded on a Nicolet 6700 instrument (Thermal Scientific, USA) from 400 to 4000 cm−1. The thermogravimetric analysis (TGA) curves were carried out from 40 to 800 °C using a TA Instruments Q50 thermogravimetric analyzer (TGA) with a heating rate of 10 °C/min in a highly purified N2 atmosphere. Raman spectra were measured on a confocal laser micro-Raman spectrometer (Invia/RENISHAW) equipped with a charge-coupled device (CCD) camera (1024 × 256 pixels). X-ray photoelectron spectroscopy (XPS) was conducted on a PHI QUANTERA-II SXM instrument at 25 W under a vacuum lower than 10−6 Pa. The detailed information for the SEM and XRD measurements refers to that found in ref 29. The mechanical properties of RGSA were performed by Shimadzu electronic universal testing machine ASG-J with compression at ambient conditions. The compressive stress−strain curves were measured at a strain rate of 10 mm/min. All samples were cut into cylinders with diameters of 10 mm and gauge lengths of 20 mm. For each composition, at least five samples were put into test in all mechanical measurements to acquire reliable values. The electrical conductivities of the graphene-based aerogels were measured at room temperature with a conventional four-probe technique.

abundant amino and hydroxyl groups endow CS with the potential for cross-linking reactions with graphene and graphene oxide (GO).31,32 Therefore, CS is one of the most appropriate natural materials for the fabrication of graphenebased nanocomposites. Tunability of the physical properties of graphene-based aerogels is less reported, so it is of great interest to explore the preparations of ultralight graphene-based aerogels with controlled properties. In this work, we report the fabrication of an air-dried reduced graphene oxide (RGO)-chitosan hybrid aerogel (RGSA) with superelasticity and high electrical conductivity through hydrothermal assembly of the aqueous suspension of GO and CS followed by the freezing−thawing technique. The resulting composite aerogels are robust enough to resist the reduced solvent evaporation capillary pressure and show no noticeable volume shrinkage during the convenient air-drying process. Furthermore, the as-formed hybrid materials exhibit remarkable properties including large reversible compressibility, high cycling electrical conductivity, and excellent environmental stability and reusability, demonstrating the potential for applications of sensors, supercapacitors, micro-electromechanical systems, and environmental applications.

3. RESULTS AND DISCUSSION Two types of macroscopic graphene-based aerogels were synthesized from GO or GO/CS suspension by a hydrothermal method followed by freezing−thawing, as illustrated in Figure 1. The 3D architectures of a chemically converted RGO wet gel were developed via the self-assembly of an aqueous GO dispersion with the addition of a chemical reducing agent. After additional freezing in an ordinary refrigerator and dried under ambient temperature and pressure conditions, the RGO dry aerogel was finally obtained (Figure 1a). The aggregation and formation of the RGO hydrogel is driven by the hydrophobic and π−π stacking interaction of the conjugated structures during the chemical reduction process triggered by HI. In contrast, the RGSA was assembled in a different category owing to the unique cross-linking interactions between RGO and CS. Combined with the oxygen-containing groups of GO, the amino and hydroxyl functional groups of CS which are involved in the self-assembly of RGO hydrogel endow the RGO/CS with better dispersion abilities than original RGO in an aqueous solution. Consequently, the RGSH shows less volume shrinkage and exhibits lower density and higher porosity than the RGO wet gel (Figure 1b). Interestingly, it was found that the shape of the RGSH is reactor-dependent. Various shapes of graphene-based hydrogels, such as cylindrical, tubular, and conical, could be successfully fabricated depending on the relative shapes of the

2. EXPERIMENTAL SECTION 2.1. Preparation. HI (hydroiodic acid, 55%−57%) was purchased from Beijing Chemical Reagents Co., Ltd. (Beijing). All the other materials used (chitosan, natural graphite powder, and inorganic reagents) were the same as described in the literature.29 Graphene oxide was synthesized from natural graphite powder according to a modified Hummer’s method as reported in our previous papers.10,29 Chitosan was dissolved in an aqueous solution of 1 wt % acetic acid to form a 2 wt % solution. Then the desired amount of CS solution was added into the suspension of GO (60 mL, 2 mg/mL), followed by vigorous stirring for several minutes using a vortex mixer. The resulting homogeneous mixture was heated at 95 °C for 8 h with the addition of HI (2 mL) as a reducing agent under atmospheric pressure to get a RGO/CS hydrogel (RGSH). The wet hybrid hydrogel was immersed in 1 wt % NaOH solution for 3 h to neutralize the residual acetic acid, followed by washing with ultrapure water to eliminate the needless alkaline in the sample. Then the obtained hydrogel was completely frozen at −20 °C in an ordinary refrigerator for 6 h. With drying of the RGSH under ambient temperature and pressure conditions, the final RGSA was obtained. The contents of CS in the homogeneous mixtures were varied from 0 to 20 wt %, and the pure RGO aerogel was prepared for comparison through the same procedure. Four types of samples with 5%, 10%, 15%, and 20% by weight (wt %) CS of the composite aerogels were named as RGSA-5, RGSA-10, RGSA-15, and RGSA-20, respectively. B


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pressure is related to the solvent surface tension (γ), contact angle (θ), and pore radius (r) according to the Laplace formula:

reactor utilized (Figure S1, Supporting Information). The results demonstrated that the shrinkage of the as-prepared hydrogel during the hydrothemal assembly process is isotropic, and it provides a chance to prepare shape-controlled graphenebased hydrogels as desired. What’s more, it could be clearly seen from Figure 1 that the shape and volume of the asprepared RGSA and RGO aerogel remained almost the same as the original hybrid and RGO hydrogel after the air-drying. FTIR experiments were carried out to verify the chemical structure and interaction of the RGSA, as shown in Figure 3a. As expected, the hybrid aerogel exhibited distinct IR features related to the CS while these peaks did not exist in the RGO aerogel. CS was identified by its three characteristic absorbance bands centered at 3018, 1708, and 1530 cm−1 in the spectrum of RGSA, which correspond to the −OH bending vibration, CO stretching vibration of −NHCO−, and the N−H bending of −NH2, respectively. Compared to pure CS, the cross-linking reactions of RGO sheets with CS are the major cause of the red shifts of the corresponding absorption peaks. In the spectrum of RGO aerogel, most of the oxygencontaining functional groups of GO were successfully eliminated by HI, suggesting a high degree of reduction. The results were further confirmed by X-ray diffraction (XRD) analysis (Figure S2). A new wide diffraction peak at 23.5°, corresponding to an interlayer spacing of 0.38 nm, provides strong evidence for the strengthened restoration of sp2 conjugation regions and π−π stacking interactions of graphene sheets in graphene-based aerogels. Apparently, self-assembly of the aqueous suspension in the presence of a reducing agent occurred, which resulted in the formation of a 3D interconnected structure of the hydrogel. The XPS spectra were conducted to verify the successful reduction and cross-linking of the RGSA (Figure S3). For GO, the dominant peaks at around 285.1, 286.8, and 289.0 eV were ascribed to CC/C−C in aromatic rings, C−O (epoxy and alkoxy), and carboxyl, respectively. The C 1s spectra of the RGO aerogel exhibited a substantial decrease in the intensities of the oxygen-related peaks, demonstrating that most of the oxygen functional groups were removed by HI. A new peak at 285.9 eV corresponding to C−O−C bonds appeared in the spectrum of RGSA, which indicates the formation of ether bridges in the process of cross-linking reaction. In addition, a full range survey scan of the RGSA provided the composition of elemental C at 81.96% and that of elemental O at 11.86%. The C/O atomic ratio of the RGSA was about 6.91 for elemental analysis, briefly lower than that of the RGO aerogel (8.42) because of the existence of cross-linked CS. Raman spectroscopy was performed to illustrate the binding nature of CS with the RGO aerogel. As shown in Figure S4, the RGO aerogel exhibits a typical reduced graphene oxide spectrum featuring the D-band at around 1352 cm−1 and the G-band at around 1587 cm−1, showing a peak intensity ratio for the D- and G-bands (ID/IG) of 1.47. Furthermore, the phenomenon of peak downshifts of two bands to 1350 and 1584 cm−1 is interpreted as a result of charge transfer between the RGO aerogel and CS. Combined with the FT-IR and XPS analyses, it is reasonable to conclude that the CS was successfully incorporated into the interconnecting networks of the RGO aerogel through self-assembly. As illustrated in the Introduction, the graphene aerogels usually suffer from severe volume shrinkage or even not being stiff enough to resist the large solvent evaporation capillary pressure (P), leading to an unstable framework. The capillary

P = ( −2γ cos(θ ))/r

(1)

The equation clearly revealed that the capillary pressure could be effectively reduced by enlarging the pore radius when the contact angle and surface tension remain constant. In addition, enhancing the framework stiffness will be another path to minimize the volume shrinkage and structure cracking of the aerogels, which is beneficial for realizing ambient drying. Recent studies have revealed that the prefreezing technique was triumphantly applied to the aqueous suspension of nanoparticles to produce 3D porous architectures, which are templated by forming an ice crystal in the hydrogels.33,34 And slower cooling rates of freezing will lead to larger pore dimensions.35 Thus, to increase the pore diameters of the aerogels, the as-formed wet hydrogels were prefrozen before the air-drying process. Figure 2 shows the typical cross-

Figure 2. Cross-sectional SEM images of the RGO aerogel (a) and RGSA-10 (b). (c) SEM images of the RGSA with different contents of CS. (d) Bulk density of the RGSA as a function of CS content.

sectional SEM images of RGSA and RGO aerogels. The 3D macropores of the two types of aerogels were continuously interconnected and uniformly distributed as depicted in Figure 2a,b and Figure S6. CS acted as an inflating agent that expanded the pore volume, which in turn reduced the density of the RGSA. The pore size exhibited a monotonic increase from ∼35 to 95 μm when CS content increased from 0 to 15 wt %, and decreased to ∼65 μm for RGSA-20 (Figure S7). As shown in Figure 2d, the bulk density of the hybrid aerogels drastically decreased with the addition of CS, and then gradually reduced to the minimum value for RGSA-15. When the initial CS content was increased to 20 wt %, the density of the RGSA-20 picked up slightly. This phenomenon was visualized by the corresponding SEM images of the RGSA. As can be seen in Figure 2c, the hybrid aerogel showed a typical porous structure with a relatively larger average pore size compared to that of the neat RGO aerogel. Moreover, the pore walls consisting of the RGO/CS sheets thinned and stretched with the addition of CS. At the minimum of the curve in Figure 2d, the hybrid aerogel C


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Figure 3. (a) FTIR spectrum of CS, RGO aerogel, and RGSA-10. (b) Compressive stress−strain curves of RGSA with different loadings of CS. Inset: the corresponding compression modulus of each cycle. (c) Stress−strain curves of RGO aerogel and RGSA-10 at 92% strain. Inset: experimental snapshots of one compression cycle of RGSA-10. (d) Stress−strain curves of RGSA-10 with different cycles at 92% strain. Inset: the variation of height as a function of cycle numbers. The stress−strain curves of RGO aerogel (e) and RGSA-10 (f) at 70% strain for the first 5 cycles. Inset: the corresponding Young’s modulus for the first 5 cycles.

with 15 wt % CS exhibited the lowest density of 10.8 mg cm−3 and the largest pore diameter of 95 μm. However, when the initial CS content was higher than 15 wt %, the size of the pores decreased and the nanosheets layers of the pore walls increased because of the stacking of excess CS, resulting in a higher bulk density. N2 sorption measurements were conducted to study the pore sizes distribution of the RGO aerogel and RGSA. As shown in Figure S8, the surface areas of the RGO aerogel and RGSA-10, which were calculated by the Brunauer−Emmett−Teller equation using N2 adsorption−desorption isotherms, were estimated to be 351.65 and 304.76 m2 g−1, respectively. For the RGO aerogel, a typical type-IV isotherm characteristic with an adsorption hysteresis shown in Figure S8 indicated that there are plenty of mesopores existing in the aerogel. However, the RGSA-10 showed a type-II adsorption−desorption isotherm, corresponding to typical macroporous characteristics, which is incompatible with their micrometer-sized pore dimensions (Figure S7). In addition, the different isotherm behaviors of the RGO aerogel and RGSA are consistent with the divergences in pore diameter established by Barret−Joyner−Halenda (BJH) pore distribution (Figure S9). In the RGO aerogel, most of the pore volumes are presented as mesopores with pore diameters

of ∼30 nm. For the RGSA, macropores are prevalent, with pore diameters in the range of ∼20−200 nm. It was also noticed that the RGSA shows a relatively large pore volume and high porosity. Especially, the RGSA-15 has a large pore volume of 2.65 cm3 g−1 and high porosity of 93.48% as compared to those of RGO aerogel (pore volume of 0.71 cm3 g−1 and porosity of 86.52%). These results can be explained by the presence of extended CS chains during the formation of the RGSA aerogel. CS acted as an inflating agent expands the pore size, which in turn improves the porosity of the hybrid aerogel. The enlarged pore radium of the RGSA could be interpreted by the restricted restacking and steric hindrance. The crosslinking reactions of CS and RGO make RGO sheets more dispersible and easy to exfoliate in an aqueous solution. The restacking of the exfoliated RGO nanosheets was restrained even after the formation of the 3D porous architecture, which in turn thinned out the pore walls composed of the hybrid nanosheets, giving rise to a large pore diameter and interconnecting network. Besides, the steric hindrance effect of CS also contributes to the enlarged pore dimensions. A series of SEM images demonstrated that the pore walls of RGSA thinned and stretched while the pore sizes were enhanced with the addition of CS. D


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Figure 4. (a) Strain-dependent electrical conductivity of the RGO aerogel and RGSA-10. (b) Variation of Rt/R0 of the RGSA-10 with 90% compressive strain in one cycle. (c) Variation of Rt/R0 of the RGSA-10 with 10 compression and release cycles at strains of 90%. (d) Electrical conductivity of the RGSA as a function of CS content.

compressed for 100 cycles at a strain of 92%, the hybrid aerogel could fully recover to its original volume with the maximum stress decreased by only 9.2%, suggesting that the as-formed RSGA shows repeatable and reversible superelasticity (Figure 3d). The inset plot of Figure 3d shows the cyclic compression experiments of RGSA-10 with strains up to 70% and 92%, respectively. The aerogel height remains almost the same compared to its original value, indicating that the as-formed hybrid aerogels exhibit perfect reversible and repeatable compression capability. Compared to the graphene aerogel fabricated via freeze-drying or supercritical-drying techniques,19,35,38−40 the resulting aerogel prepared through the airdrying technique exhibit even better mechanical properties, such as larger maximum strain (92%) and higher ultimate stress (137.6 KPa), which means that the naturally drying technique is not only inexpensive and easy to scale up but also has guaranteed quality for graphene-based aerogel production. The results of cyclic compression testing for five cycles at strain of up to 70% are shown in Figure 3e,f. The dissipation of mechanical energy during the loading−unloading process results in hysteresis loops. As can be seen, the cyclic compression profiles of RGSA-10 (Figure 3f) and RGO aerogel (Figure 3e) exhibit the same tendency of variability. The first compression cycle is different from the subsequent ones, showing a higher Young’s modulus and maximum ultimate compressive stress. The hysteresis loop shrinks for the second cycle with only 3.2% and 6.9% reduction of the ultimate stress for RGSA-10 and RGO aerogel compared to the first one, respectively, and remains almost unchanged after the second cycle. Importantly, Young’s modulus and compressive stress of the hybrid aerogel under 70% compression strain are 18.7 KPa and 27.3 KPa, respectively, which makes the aerogel framework capable of resisting the solvent evaporation capillary pressure to form a stable 3D architecture with no observable volume shrinkage.

In addition to enlarging the pore sizes, the chemical crosslinking of CS is also a significant contributing factor to the mechanical durability of the RGSA since covalent interaction offers an effective path for load transfer. The mechanical properties of the RGSA were obviously improved with the addition of CS owing to the strong interactions between the two components. The compressive tests were carried out along the axial direction to investigate the effect of chemical crosslinking interactions on the physical properties of the RGSA. As shown in Figure 3b, the compression modulus increased almost linearly with an increase of CS contents from 0% to 15%, while decreasing with 20% loading of CS. RGSA-15 exhibited relatively high modules of 22.3 KPa and compressive stress of 30.4 KPa, which were 1.9 and 4.6 times greater than those of RGO aerogel, respectively. This result indicated that some addition of CS could enhance the framework stiffness of the RGSA, which is beneficial for less volume shrinkage. The mechanical properties of the RGSA-10 and RGO aerogel were tested under ambient conditions, and the successive cyclic compressive stress−strain curves are shown in Figure 3. The aerogel can be compressed into a pellet (>90% compression) under pressure and recover to its original shape and volume once the applied pressure is removed. The superelastic behavior of the resulting RGO aerogel and RGSA are significantly superior to other graphene-based aerogel,22,36,37 and comparable to the naturally dried graphene aerogel reported by Zhang and co-workers.25 In the loading process, the aerogel gradually contracted and then were conformally densified. During the unloading process, the sample timely recovered along with the removing external force, and the morphologies of the aerogel could be completely restored after the pressure was fully released. With the addition of CS, the compressive stress of the RSGA-10 at 92% strain is about 137.6 KPa, which represents a more than threefold improvement in performance relative to RGO aerogel of the same strain (Figure 3c). More specifically, even after being E


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Figure 5. (a) Compressive stress−strain curves of RGSA-10 with different recycling times. (b) The height variation of RGSA-10 as a function of cycle numbers. Inset: a series of pictures showing the aerogel height with repeated soaking and drying.

Recently, there has been increasing demand for the development of conductive aerogels by various strategies. In addition to the repeatable and reversible compressive deformability, the as-prepared RGSA also exhibits straindependent electrical conductivity due to the combination of compressibility and conductivity. The conductivity of the graphene-based aerogel is very sensitive to the strain. As shown in Figure 4a, the electrical conductivity of the RGO aerogel and RGSA increased nearly linearly to 257.7 and 278.2 S/m, respectively, by increasing the strain to 90%. The macropores inside the aerogels make the compressed samples capable of dissipating the external energy by shutting themselves off without disrupting the framework. During the compression process, many new contact points are formed, which is conducive to the electrical transport, resulting in an improvement of the electrical conductivity. The sensitivity of the hybrid aerogel is higher than RGO aerogel. Considering the reversible compressive ability, we carried out some experiments to illustrate the stability of the strain-dependent electrical conductivity of RGSA. As shown in Figure 4b, the normalized electrical resistance (Rt/R0) decreased by 94.5% when the aerogel was compressed to 90%, and if gradually released, the external stress Rt/R0 of the sample recovered very well. Because of the excellent compressibility, removing load is accompanied by the disappearance of the newly built contacts and then the electrical resistance returns to its original value. Furthermore, the response stability of Rt/R0 of the hybrid aerogel to compressive strain was investigated by alternate supply of compress and release stimulations time after time. It can be seen from Figure 4c that the response is quite stable over 10 compression and release cycles, giving the conclusion that the hybrid aerogel is a promising candidate for developing pressuresensitive materials. We further investigated the effect of CS concentrations on the strain-dependent electrical conductivity of the RGSA. The results are shown in Figure 4d. As expected, RGSA-15 is more sensitive to the compressive strain and reaches the maximum electrical conductivity of 294.8 S/m at the strain of 90%, which is 3.8 times higher than the original value. The RGSA-20 has decreased mechanical properties and reduced electrical conductivity due to the aggregation of excess CS. The regeneration and stability of the hybrid aerogels is also an important factor for assessing their potential for practical applications. The as-formed RGSA was immersed in ultrapure water followed by drying under ambient temperature and pressure conditions to obtain a regenerated hybrid aerogel, and the soaking−drying cycles were successively conducted five times to explore the environmental stability of the RGSA.

Figure 5a shows the effect of recycling time on the compressive stress of the dried RGSA at a strain of 92%. As is clear, the ultimate stress of regenerated sample was still very high, and more than 90% of its maximum compressive strength was retained after five cycles, suggesting good stability and recyclability of the RGSA. Figure 5b shows the cyclic soaking−drying testing of the hybrid aerogel. The height of the dry sample was measured as 92.9% of the corresponding wet one after five cycles. And the inset pictures displayed the variation of aerogel height with repeated soaking and drying. Moreover, the electrical conductivity of the as-prepared RGSA after five cycles remained almost the same compared to its original value (Figure S10), indicating that the hybrid aerogel shows perfect environmental stability and reusability, which makes the RGSA an ideal candidate for practical applications as compressible electrode materials.

4. CONCLUSION In conclusion, we have developed a natural drying strategy to prepare 3D architectures of graphene-based aerogels via a twostep process. The hydrothermal reduction process gives rise to a self-assembled composite hydrogel with little stacking and the reducing agent eliminates most of the functional groups and constructs a highly porous framework; the followed freezing− thawing technique results in a porous aerogel with no noticeable volume shrinkage during the air-drying process. The obtained aerogels exhibit superelasticity, high electrical conductivity, and excellent environmental stability. The selfassembly and cross-linking chemistries of CS and RGO contribute to the high porosity and low density yet exceptional mechanical properties including compressive strength and resilience. Compared to the conventional freezing techniques, this strategy developed here is highly efficient, inexpensive, and easy to scale up for commercial production. Thus, our study provides much inspiration for the design and fabrication of elastic conducting polymer aerogels for a variety of practical applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b04689. Details of the characterization of RGO aerogel and RGSA (PDF) F


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AUTHOR INFORMATION

Corresponding Author

*Phone/fax: 86-10-68918530. E-mail: [email protected]. ORCID

Yaqian Zhang: 0000-0002-5861-1352 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (21736001, 21174017), the Beijing Municipal Natural Science Foundation of China (2102040), and the Cultivation Project for Technology Innovation Program of BIT (2011CX01032).

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REFERENCES

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