Reducing structural defects and oxygen-containing functional groups

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Functional Inorganic Materials and Devices

Reducing structural defects and oxygen-containing functional groups in GO- hybridized CNTs aerogels: Simultaneously improve the electrical and mechanical properties to enhance pressure sensitivity Xianzhang Wu, Xiaohong Liu, Jinqing Wang, Jingxia Huang, and Shengrong Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12578 • Publication Date (Web): 10 Oct 2018 Downloaded from http://pubs.acs.org on October 11, 2018

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ACS Applied Materials & Interfaces

Reducing Structural Defects and Oxygen-Containing Functional Groups in GOHybridized CNTs Aerogels: Simultaneously Improve the Electrical and Mechanical Properties to Enhance Pressure Sensitivity Xianzhang Wua,b, Xiaohong Liua, Jinqing Wanga,*, Jingxia Huanga,b, and Shengrong Yanga,*

a

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, P. R. China

b

University of Chinese Academy of Sciences, Beijing, 100080, P. R. China

* Corresponding authors, [email protected] (J. Q. Wang);[email protected] (S. R. Yang) Fax: 0086-931-4968019 Tel: 0086-931-4968076 Keywords: aerogels, graphene oxide liquid crystals, carbon nanotube, induced self-assembly

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Abstract Three-dimensional graphene oxide-carbon nanotube (GO-CNT, abbreviated as GCNT) aerogels can find wide applications in various fields. Especially, low-density GCNT aerogels featuring both high conductivity and superelasticity are essential requirements for the construction of highly sensitive pressure sensor. However, simultaneous improvement on the electrical and mechanical properties of low-density GCNT is still a great challenge owing to their disordered microstructure, severe structural defects and massive oxygen-containing functional groups. Here, a structurally ordered and less defective GCNT aerogel featuring both high conductivity and superelasticity has been fabricated through alkali induced self-assembly of GO liquid crystals (GO LCs) and CNTs. Our methodology relies on the double roles of KOH solution as dispersant for CNTs and an inducer for the self-assembly of GO LCs nanosheets. The less-defective CNTs acting as reinforcement material contributes to the robust structure networks, leading to the significantly improved conductivity (2.4 S m-1) and elasticity (14.3 kPa) of GCNT. Benefiting from these outstanding properties of GCNT aerogels, the assembled pressure sensor exhibits an ultra-high sensitivity of 1.22 kPa-1, rapid response time of 28 ms and excellent cycling stability, which enables it as a high-performance sensing platform to monitor various human motions in real time.

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1. Introduction Three-dimensional (3D) graphene oxide-hybridized carbon nanotubes (GO-CNTs) composites represent characteristics of high conductivity and decent mechanical properties, which allow wide applications such as energy storage, thermal insulation, oil– water separation and flexible sensors.1-9 Unfortunately, since most reported GO-CNTs are based on self-assembly technology, they present low electrical conductivity due to their massive oxygen-containing functional groups on GO sheets. Meanwhile, most GO-CNTs also have suffered from poor mechanical properties because of their extremely disordered microstructures. Therefore, to design and fabricate low-density GO-CNTs featuring both high elasticity and electrical conductivity remain a challenging task. Currently, several strategies have been developed to improve these properties, including chemical reduction, chemical vapor deposition (CVD) and microwave assisted method.10-13 For instance, as presented by Gao et al.,14 an ultralight 3D material based on carbon nanotubes (CNTs) and giant graphene was fabricated by freeze-drying and subsequent chemical reduction, which exhibited extremely low density and excellent electrical conductivity. However, the reinforcement effect of CNTs has been greatly limited owing to severely structural defects and oxygen-containing functional groups introduced by the mixture of concentrated H2SO4 and HNO3 during purification processes. Direct growth of CNTs on the GO surface by the CVD and microwave assisted process is a very attractive technique, in which the obtained composite has less structural defects and exhibits high 3 / 30



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conductivity. Nevertheless, it typically yields short and tedious operation. Furthermore, the GO acting as a matrix for embedding GNTs is responsible for effective reinforcement in the GO-CNTs. However, the structure of GO is uncontrollable. Therefore, the resulted GO-CNTs generally exhibit unsatisfactory mechanical and conductive performances. The recent strategy is focused on assembling GO sheets to produce a long-range ordered structure by the ice crystal template technique.15-19 However, the density of GO-CNTs has not been effectively reduced, greatly restricting their practical applications, especially in flexible sensors. Additionally, the electrical conductivity and mechanical properties of GO-CNTs are difficult to be optimized simultaneously because these properties show strong correlation with its density;

20,21

thusly, these materials are often difficult to possess excellent electrical conductivity and high mechanical strength because of their sparse conducting channel and cluttered structure.22,23 Evidently, fabrication strategies that enable simultaneous decent mechanical and conductivity properties for GO-CNTs are rare. GO liquid crystals (GO LCs) provided the additional benefit of being they can achieve phase-transformation from ordered fluids into ordered solids and are widely used.24-27 For example, Yao, et al. prepare long-range ordered microstructures with low density by induced self-assembly technique.14 However, these materials still hold unsatisfactory mechanical and conductivity properties because of the concentration limitation of GO LCs suspension (below 7 mg ml-1). To overcome the above-mentioned drawbacks, a new fabrication technique of GO-CNTs, combination of high conductivity, 4 / 30


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decent mechanical properties and ultralow density, is urgently needed to be developed Herein, we highlight a liquid crystals self-assembly strategy to fabricate GCNT aerogels featuring extremely low density, superior elasticity and excellent electrical conductivity. Our approach is based on the double effects of alkali (KOH): 1) Inducing the assembly of GO LCs into ultralight 3D hierarchical structures with tunable density and performance. 2) Effective dispersing CNTs without introducing excessive structural defects and oxygen-containing functional groups on its surfaces, which is usually unavoidable in traditional acid treatment processes. As a result, the less-defective CNTs acting as reinforcement material contributes to the robust structure networks, leading to the significantly improved conductivity and elasticity of GCNT. Furthermore, derived from the synergistic effects between electric CNTs and hierarchical graphene cell walls, the prepared GCNT realize all the desired properties, including high electrical conductivity (2.4 S m-1), superior elasticity (14.3 kPa), and extremely low density (0.84 mg cm-3). This work provides a feasible approach for constructing multifunctional 3D graphene aerogels, which are potentially viable for artificial skins and flexible sensing devices. 2. Results and discussion The highly ordered 3D GCNT is built up by integrating GO LCs and CNTs through self-assembly process, followed by freezedrying and calcining under nitrogen atmosphere (Figure 1a). GO LCs gel, where GO LCs nanosheets could be stably dispersed in water with single-layer structure (Figure S1, Supporting Information), was mixed with the 5 / 30



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KOH solution containing CNTs (Figure S2, Supporting Information), and then the mixture was self-assembled into hydrogels by hydrothermal treatment. After that, the hydrogels were freeze dried and subsequently calcinated to obtain the ultralight GCNTs (Figure 1c, Figure S3, Supporting Information). GO LCs is chosen for the fabrication because it is known to be a highly ordered microstructure, therefore, making this a basis scaffold for the assembly of ordered structure. Most remarkably, the KOH solution plays double roles for the preparation of GCNT: first, KOH can effectively stabilize CNTs without aggregation. CNTs can be dispersed by KOH solution, apparently due to the highly negative potential of approximately –19.29 mV on the CNTs surface that efficiently blocks the CNTs aggregation through electrostatic repulsion (Figure S4, Supporting

Information);

secondly,

KOH

can

induce

the

formation

of

hierarchical-cellular GCNT, whose mechanisation originates from three aspects: 1) GO LCs sheets are partially deoxygenated by KOH to restore their conjugated structures; 2) KOH can enhance the electrostatic repulsion between GO LCs sheets to increase the fluidity of GO LCs gel; 3) KOH can extend the rigid domains of GO LCs sheets, enabling them to self-assemble into a highly ordered microstructure.29 During hydrothermal process, well arranged GO LCs sheets and CNTs could be assembled into an ordered hierarchical structure through hydrogen bonds and π-π stacking interactions between the delocalised electrons in both the conjugated regions of GO sheets and the CNTs.30,31

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Figure 1. (a) Schematic showing the synthetic steps. 1. GO LCs gel is fabricated using a modified Hummers method. 2. CNTs are dispersed in KOH solution by strong sonication to form the CNTs suspensions. 3. GCNT hydrogel is prepared via self-assembly technique. 4. GCNT aerogel is fabricated by freeze drying and subsequent calcining. (b) POM microscopic image of GO LCs + CNTs + KOH suspension; Scale bar = 400 μm. (c) GCNT aerogel with ultralight weight (ρ = 0.84 mg cm-3) standing on dandelion. SEM images of (d-e) GCNT at different magnifications and (f) GO/CNT aerogels; Scale bars = 500 μm for d and f while 100 μm for e. 7 / 30



In order to understand the mechanism of KOH-induced effect, the arrangements of GO LCs sheets in GO LCs + CNTs suspension with different dispersants and CNT concentrations are characterized by POM observations. As shown in Figure S5a (Supporting Information), the birefringence domains are observed in GO LCs suspension. Upon adding the KOH solution containing CNTs, a well-organized longitudinal texture was obtained (Figure 1b). In contrast, the GO LCs + CNTs aqueous solution showed much more disordered phase, owing to the absence of KOH (Figure S5b). Moreover, UV-vis spectra show that the absorption peak of the GO LCs + KOH suspension is blue shifted from 230 nm to 210 nm (Figure S6a, Supporting Information), confirming the partial deoxygenation and recovery of the conjugated structures of GO LCs sheets in KOH solution. The KOH assisted strategy can overcome the formation of excessive defects to enhance conductivity, which is quite difficult for traditional acid treatment processes.12, 15 Raman characterization is used to confirm the reduction of GCNT’ structure defects. As shown in Figure 2a, the intensity ratio of ID/IG is 1.09 for GCNT, lower than that value of 1.25 for the acid purified GO/CNT, indicating that GCNT has fewer defects than GO/CNT on the surface of GO LCs sheets and CNTs. Figure 2b shows the XPS spectra, where the O1s peak intensity has significantly increased for GO/CNT and the C/O atomic ratio has decreased to 1.2 compared to 2.3 for the GCNT. The C1s spectra are presented in Figure 2c, comparing the spectra of GO/CNT, the peak intensities associating with 8 / 30


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oxygen-containing components of GCNT are significantly decreased, and C-C become dominant in GCNT. The FTIR spectra of GCNT also exhibit much weaker peaks of oxygen functional groups (OH, C=O and C-O) than the acid purified GO/CNT (Figure S6b, Supporting Information). These abovementioned results imply that GCNT contains fewer defects and oxygen functional groups on the surface of CNTs. Understandably, the electrical conductivity of GCNT can be greatly improved because it contains fewer defects and oxygen-containing groups. SEM images (Figure 1d, e) reveal that GCNT displays a honeycomb-like hierarchical structure that is quite different from the disordered microstructure of acid purified GO/CNT (Figure lf). This finding confirms that the inducing effect of the KOH can directly prompt the morphological evolution of GCNT with aunique honeycomb-like hierarchical structures.31 Intriguingly, the microstructures of GCNT are influenced by CNT dose. With a higher CNT dose of 20 wt%, a core–shell-like hierarchical structure of GCNT is obtained (Figure S7b, Supporting Information). When the CNT dose is further increased to 30 wt%, the disordered microstructure of GCNT is observed (Figure S7d, Supporting Information), due to the high concentration of CNTs hindering the efficient assembly of GO LCs sheets into an ordered structure (Figure S8a-c, Supporting Information). GCNT is subsequently characterized by TEM to investigate the dispersion of CNTs in the cellular walls and struts of GCNT. The uniformly dispersed CNTs in GCNT are clearly observed from low-magnification TEM images (Figure 2e, Figure S9a, 9 / 30



Supporting Information). The CNTs have a uniform diameter in the range of 2 to 8 nm (Figure S9a, Supporting Information) and lengths of tens to hundreds of micrometers. High-resolution TEM (HRTEM) image further shows the intimate contact between CNTs and GO layer, uniform lattice fringes with cross connection over the entire image region (Figure 2f, Figure S9b, Supporting Information). Evidently, the uniformly distributed CNTs enhance the mechanical properties of GCNT and provide a large number of electronic channels to improve electrical conductivity of GCNT synchronously. This simple strategy for CNT dispersion under mild conditions in KOH solution has not been attempted previously.

Figure 2. Structural and chemical composition characterizations of samples. (a) Raman and (b) XPS survey spectra of GCNT and GO/CNT aerogels. XPS C1s spectra of (c) GCNT and GO/CNT. TEM images of d) the typical CNTs and (e) GCNT. (f) Magnified TEM image of the GCNT; Scale bars = 50 nm for d and e while 5 nm for f.

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The bulk electrical conductivity of GCNT is plotted as a function of CNT content in Figure 3b and further compared with conventional graphene aerogels (Figure 3c). Unlike the GO/CNT prepared by acid purification process, the GCNT displays a quite high electrical conductivity at similar CNT content. The significant improvement in conductivity for the GCNT is attributed to the following facts: first, the interconnected 3D GCNT networks provide an effective channel for rapid electron transport; second, the less defective and oxygen-containing CNTs greatly promote the charge percolation. As expected, the electrical conductivity of GCNT increas with the increase of the CNTs content. A high conductivity of ∼2.4 S m−1 is achieved with ∼15 wt% of CNT content (Figure. 3a, b), which is four times of magnitude higher than the reported conductivities for GO/CNT at 50 wt% (∼0.6 S m−1)14, graphene aerogels (1.0 S m−1 ) materials

33-37

32

and other 3D

(Figure. 3c). It should be pointed out that 15 wt % is the highest CNTs

content that can form an ordered microstructure, and further increase of CNTs content is very difficult because the high dose of CNTs hinders the assembly of GO LC sheets into an ordered 3D structure (Figure S7, Supporting Information). Therefore, the percolation threshold for the GCNT in this study is estimated to be 15 wt%. Significantly, the light-emitting diode (LED) strip can be illumined when connected with a GCNT aerogel, and its brightness fluctuates when the pressure is imposed and released (Movie S1, Supporting Information), indicating the good conductivity of GCNT aerogel.

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Figure 3. Electrical characteristics of the GCNT-based sensor. (a) Photograph of the resistance measurement processes for GCNT with an electrical conductivity of 2.4 S m−1 (84.4 Ω in GCNT with diameter of 13.8 mm and height of 21.7 mm). (b) The relationship between electrical conductivity and CNTs content of GCNT. (c) The electrical conductivities of selected GCNT at relative low densities and several carbon-based 3D materials with low density reported in literatures. (d) I–V curves of GCNT under different pressures. (e) Multiple-cycle tests of repeated compress and release with different pressures. The minimum detectable pressure is as low as 10 Pa. (f) Durability test of GCNT under repeated compress and release pressure of 1 kPa.

The synergistic effect between hierarchical architecture and CNT ribs brings ultra-high compressibility, and the GCNT aerogel can tolerate an extremely large strain of 90% under stress up to 23.4 kPa without mechanical fracture (Movie S2, Supporting 12 / 30


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Information). Figure 4a shows the compressive stress–strain (σ-ε) curves for the GCNT at maximum strains (ε) of 35, 65, and 90 %, respectively. The loading process exhibits two distinct regions, including the approximately linear elastic region at ε6% marked by the steep increases of stress owing to continuously decreasing pore volume. Interestingly, the slope of the loading curve increases very fast with strain corresponding to significant densification with a large degree of recovery during unloading, demonstrating the super-elasticity of the honeycomb-like GCNT aerogel. Comparatively, the disordered GO/CNT suffers from buckling and structural collapse under large strain compression, which ultimately results in the stress reduction (Figure S10b, Supporting Information). The ultimate stress (ε=90%) of the GCNT is approximately 23.4 kPa much higher than that of the disordered GO/CNT (18.2 kPa) (Figure 4a and c). The results of multi-cycle compression testing of the GCNT with strain up to 80% are presented in Figure 4b. During the first compression cycle, the Young’s modulus of GCNT with the density of 1.48 mg cm-3 is 14.3 kPa, great higher than those of CS-GO scaffold (9.4 kPa) 36 and other graphene aerogels (Table S1, Supporting Information). However, the GO/CNT suffers from folding, buckling or structural collapse after the compression test of only 100 cycles (see the insert in Figure 4d), indicating that the disordered GO/CNT is very brittle. In contrast, the structure of GCNT does not collapse after 3000 cycle compressions and remained its original shape (Figure 4b). The maximum strain of multi-cycle compression is 80%, which is much 13 / 30



higher than previous maximum strain of ≈50% (Table S1, Supporting Information). Additionally, no significant decreases in ultimate stress and Young’s modulus are observed after 100 cyclic compressions at a strain of 80% (Figure 4e). The superelasticity of our GCNT can be assigned to the synergistic effect between hierarchical architecture and CNT ribs. The unique hierarchical cellular structure with closer to closed-cell of our GCNT has enabled the superelasticity;35,39 meanwhile, the CNTs are responsible for effective reinforcement in the hierarchical cellular structure, which strengthens the cell walls and struts to prevent structural damage or collapse at compression (a model is shown in Figure 4f). To further understand the superelasticity of GCNT, the structure revolution during the compression process is observed by the optical microscope, as shown in Figure 4g-i. In the loading process, the cellular walls and struts of GCNT shrink perpendicular to the direction of compressive force. Once the loading is released, the cell morphologies can be totally restored without structural fatigue due to the high elastic restoring forces of the cell walls and struts. Moreover, large cells generally tend to deform into several small cells under external pressure and can immediately recover to their original shape. Therefore, the synergistic effect between hierarchical architecture and CNT rib endow GCNT with superelasticity. The intrinsic high electrical conductivity of GCNT aerogel, together with the superelasticity and extremely low density, endow it with high pressure sensitivity. As shown in Figure 3d, GCNT displays a good linear I-V response under different pressures, 14 / 30


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and the slope of I-V curves with improving the pressures from 0 to 1500 Pa, demonstrating the GCNT has good sensitivity of resistive responses. Figure 3e shows the resistive responses of GCNT aerogel to repeated compression cycles. This pressure sensor presents a stable and continuous response at detect pressure as low as 10 Pa. The cycling stability of the GCNT pressure sensor is further tested under a pressure of 1 kPa and presented in Figure 3f. It clearly shows that the high signal-to-noise ratios are well maintained and the relative resistance (ΔR/R0) exhibits negligible changes after 3000 compression cycles, indicating a high reliability and durability of GCNT aerogel as pressure sensors. The pressure sensitivity of GCNT can be defined as S =δ(ΔR/R0)/δP, where ΔR is the relative resistance change, R0 denotes the resistance without applied pressure, and P denotes the applied pressure.40-43 As shown in Figure 5a, the pressure sensitivity curve displays two obvious linear regimes as the applied pressure is improved: in the pressure range of < 1 kPa, the S is as high as 1.22 kPa-1, while a reduced S of 0.39 kPa-1 can be obtained in the pressure range of >1kPa. The decrease of S value is attributable to the increased elastic resistance with improving the pressure. In comparison, the S value of 1.22 kPa-1 for GCNT aerogel is much higher than those of gold nanowires (1.14 kPa-1)

44

and RGO-PU-HT-P sponges (0.26 kPa-1) 45. Furthermore,

the sensor exhibits an ultrafast response time of 28 ms, which is superior the recently reported values (47-110 ms) of other strain sensors.46-48

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Figure 4. Mechanical performances of the GCNT aerogel with a density of 1.48 mg cm-3. The stress-strain curves of (a) GCNT and (c) GO/CNT at set strains of 35%, 65%, and 90%, respectively. Stress-strain curves of multicycle compressions at 80% strain upon (b) GCNT for 3000 cycles and (d) GO/CNT for 100 cycles; Insert photographs show the corresponding samples. (e) The ultimate stress and Young’s modulus for 100 compression cycles. (f) The schematic model of a single GCNTs cell wall. (g-i) Optical microscope images of GCNT during the compression process.

Due to its outstanding performances, our GCNT-based pressure sensor (Figure S11, Supporting Information) can detect directly physiological signs including arterial 16 / 30


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pulse, swallowing action and finger folding, respectively. As shown in the inset of Figure 5c, a photograph of a pressure sensor conformally attached on a human wrist to detect the pulse is presented. The real-time response recording the wrist arterial pulse presented in Figure 5c shows waveforms with a periodicity of 63 beats min−1. The detected signal shows the distinct separated fluctuated Q, R and S curves, which are caused by the systolic and diastolic blood pressure, the ventricular pressure, and the reflected pressure. A characteristic carotid artery pressure curve is also obtained with three clearly distinguishable peaks (Figure 5d), demonstrating the ultra-sensitivity of the flexible strain sensor. Figure 5e shows the real-time response of saliva swallowing action. The relative resistance of ∆R/R0 measured by the saliva swallowing is roughly four times higher than that by carotid artery due to high contact pressure. Furthermore, index finger activities with different folding angles are also continuously monitored by attaching this flexible pressure sensor onto opisthenar (Figure 5f). The folding degree of the index finger is reflected by the different relative resistance of ∆R/R0, with a larger ∆R/R0 value corresponding to a higher folding angle. The ∆R/R0 value from the index finger folding of 90o (28.9%) is higher than that of 15o (3.5%) due to high contact pressure, thus indicating that the pressure sensor has a broad sensing range and is suitable for detecting large-scale human motions. These results suggest that this pressure sensor can be used to detect tiny human motions, and applied as a wearable device to detect additional physical signals for human’s health in real-time and human-machine interfaces.

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Figure 5. Sensing performances of the GCNT-based sensor. (a) Normalized relative resistance as a function of compressible strain and the corresponding linear fittings. (b) Instant response of the flexible strain sensor, which exhibits response time of 28 ms. (c) Measurement of the wrist artery pulse signals of a heartbeat under normal frequency (67 beats min-1); Insert photographs show the blood pressure monitoring using the strain sensor with an active area of 10 mm × 10 mm. Detections of (d) carotid artery pulses and (e) saliva swallowing in real time. (f) Detection of finger folding in real time. Inset photographs show finger folding with different angle in the test. 3. Conclusions In summary, we have developed a synergistic assembly strategy to fabricate hierarchical cellular-structured GCNT aerogel with high electrical conductivity, extremely low density and superelasticity by using KOH solution as the CNTs dispersant. The unique hierarchical microstructure of GCNT aerogel combining the advantages of 18 / 30


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both graphene cell walls and CNT ribs, endows GCNT not only with structure-derived superelasticity (14.3 kPa) but also with high electrical conductivity (2.4 S m-1). Benefiting from the excellent properties of GCNT aerogel, the assembled pressure sensor exhibits an ultra-high sensitivity of 1.22 kPa-1, rapid response time of 28 ms and excellent cycling stability, which enable it to be employed as a sensing platform for monitoring various human motions in real time. Moreover, we envision that such exceptional aerogel will make the assembled sensors promising for human healthcare, artificial skins and human–machine interface applications. 4. Experimental section Preparation of GCNT aerogels: GO LCs gel was synthesized from natural graphite powder (40 μm) using a modified Hummers method according to the literature.26 Meanwhile, CNTs were dispersed in KOH solution (0.32 mol/L) to form the CNTs suspensions. In a typical fabrication procedure for GCNT (CNTs 15 wt%) with a density of 1.48 mg cm-3, 15mL GO LCs gel (3.0 mg mL-1) was mixed with 15mL CNTs suspension (0.5 mg mL-1) by vigorous stirring, then a centrifuge was used to further mix the suspensions at 5000 rpm for 30 min. The prepared mixture was placed in a 50 mL Teflon vessel and maintained at 180 °C for 12 h to obtain the GCNT hydrogel. Due to the hydrogen bonds and π-π stacking interactions between the delocalised electrons in both the conjugated regions of GO sheets and the CNTs, the well-arranged GO LCs sheets and CNTs could directionally overlapping joint and then be assembled into an ordered hierarchical structure under the induction of KOH. Subsequently the as-formed hydrogel 19 / 30



was subjected to dialysis in solvent composed of deionized water and ethanol (volume ratio of 100:1) for 12 h and then freeze dried for 12 h. Finally, the obtained GCNT aerogel was further reduced by annealing at 850 °C for 20 min under flowing nitrogen. Other GCNT aerogels with densities ranging from 0.85 to 12.06 mg cm-3 were also fabricated by tuning the concentration of the GO LCs gel. Fabrication of GO/CNT aerogels: CNTs were purified in a mixture of H2SO4/HNO3 (3:1) at 100 °C for 1 h, and then filtrated and washed with deionized water. 15m L CNTs aqueous dispersion (0.5 mg mL-1) was mixed with 15m L GO LCs gel (3.0 mg mL-1) by vigorous stirring for 2 h. The mixture was placed in a 50 mL Teflon vessel and maintained at 180 °C for 12 h to obtain a GO/CNT hydrogel and then freeze dried for 12 h. Finally, the obtained GO/CNT monolith was further reduced by annealing at 850 °C for 20 min under flowing nitrogen to get the GO/CNT aerogels. Sensor preparation: Copper pieces were pasted onto the upper surface of the obtained GCNT aerogels as sensing component and connected with copper wire to form electrodes. Polydimethylsiloxane (PDMS) elastomer and cross-linker were mixed and then the bubbles were removed under vacuum. The GCNT with electrode was encapsulated by the PDMS mixture and annealed at 70 oC for 2 h. Characterizations. The morphology and microstructure of the fabricated samples were characterized by scanning electron microscope (SEM, JSM-5601LV) and transmission electron microscope (TEM, TF20). X-ray photoelectron spectrum (XPS) was recorded on an ESCALab MKII X-ray photoelectron spectrometer using monochromatic Al-Kα 20 / 30


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irradiation as exciting source. Polarized optical microscope (POM) was taken on a LEICA DM2500P. Raman analysis was taken using a LabRAM HRUV spectrometer. The surface functional groups of the fabricated samples were studied using a TENSON 27 Fourier transform infrared spectrometer (FTIR). Atomic force microscope (AFM) images of GO LC sheets were taken in the tapping mode, performed on a BRUKER-Veeco. Zeta potential measurements were carried out using a DB-525 zeta potential analyzer. The compression performances were tested on AGS-X at a 2 mm/min for loading and unloading tests. The electrical conductivity and electrical response were recorded by the source meter Model 2450. Supporting Information AFM image of GO LCs nanosheets; photographs of CNTs + H2O suspension and GO LCs + CNTs + KOH suspension; weight measurement processes for the GCNT; Photograph of the zeta potential measurement; POM and optical images of GO LCs suspensions and GO LCs + CNTs + KOH suspension; FTIR and UV-vis spectra of GO LCs; SEM images of the GCNT aerogels with different CNTs contents; TEM image of the typical GCNT; photographs of the compression performance for GCNTs and GO/CNT.

Acknowledgements We gratefully acknowledge the funding support from the National Natural Science Foundation of China (Grant Nos. 51675514 and 51575507). 21 / 30



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Conflict of Interest The authors declare no conflict of interest. References: (1) Sun, G.; Zhang, X.; Lin, R.; Yang, J.; Zhang, H.; Chen, P. Hybrid Fibers Made of Molybdenum Disulfide, Reduced Graphene Oxide, and Multi-walled Carbon Nanotubes for Solid-State, Flexible, Asymmetric Supercapacitors. Angew. Chem. Int. Ed. Engl. 2015, 54 , 4651-4656. (2) Shin, M. K.; Lee, B.; Kim, S. H.; Lee, J. A.; Spinks, G. M.; Gambhir, S.; Wallace, G. G.; Kozlov, M. E.; Baughman, R. H.; Kim, S. J. Synergistic Toughening of Composite Fibres by Self-Alignment of Reduced Graphene Gxide and Carbon Nanotubes. Nat. Commun. 2012, 3, 650. (3) Im, H.; Kim, J. Thermal Conductivity of A Graphene Oxide-Carbon Nanotube Hybrid/Epoxy Composite.Carbon 2012,50, 5429-5440. (4) Han, P.; Yue, Y.; Liu, Z.; Xu, W.; Zhang, L.; Xu, H.; Dong, S.; Cui, G. Graphene Oxide

Nanosheets/Multi-Walled

Carbon

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As

an

Excellent

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Flexible and Highly Sensitive Strain Sensors Fabricated by Pencil Drawn for Wearable Monitor. Adv. Funct. Mater. 2015, 25, 2395-2401.

TOC Reducing Structural Defects and Oxygen-Containing Functional Groups in GOHybridized CNTs Aerogels: Simultaneously Improve the Electrical and Mechanical Properties to Enhance Pressure Sensitivity

A structurally ordered and less defective graphene oxide-hybridized carbon nanotube (GCNT) aerogel, featuring both high conductivity (2.4 S m-1) and superelasticity (14.3 kPa), is fabricated through alkali induced self-assembly technique. The GCNT-based pressure sensor exhibits an ultra-high sensitivity (1.22 kPa-1), and can be used to monitor 29 / 30



various human motions in real time.

30 / 30


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Figure 1. (a) Schematic showing the synthetic steps. 1. GO LCs gel is fabricated using a modified Hummers method. 2. CNTs are dispersed in KOH solution by strong sonication to form the CNTs suspensions. 3. GCNT hydrogel is prepared via self-assembly technique. 4. GCNT aerogel is fabricated by freeze drying and subsequent calcining. (b) POM microscopic image of GO LCs + CNTs + KOH suspension; Scale bar = 400 μm. (c) GCNT aerogel with ultralight weight (ρ = 0.84 mg cm-3) standing on dandelion. SEM images of (de) GCNT at different magnifications and (f) GO/CNT aerogels; Scale bars = 500 μm for d and f while 100 μm for e. 167x169mm (150 x 150 DPI)



Figure 2. Structural and chemical composition characterizations of samples. (a) Raman and (b) XPS survey spectra of GCNT and GO/CNT aerogels. XPS C1s spectra of (c) GCNT and GO/CNT. TEM images of d) the typical CNTs and (e) GCNT. (f) Magnified TEM image of the GCNT; Scale bars = 50 nm for d and e while 5 nm for f. 169x84mm (299 x 299 DPI)


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Figure 3. Electrical characteristics of the GCNT-based sensor. (a) Photograph of the resistance measurement processes for GCNT with an electrical conductivity of 2.4 S m−1 (84.4 Ω in GCNT with diameter of 13.8 mm and height of 21.7 mm). (b) The relationship between electrical conductivity and CNTs content of GCNT. (c) The electrical conductivities of selected GCNT at relative low densities and several carbon-based 3D materials with low density reported in literatures. (d) I–V curves of GCNT under different pressures. (e) Multiple-cycle tests of repeated compress and release with different pressures. The minimum detectable pressure is as low as 10 Pa. (f) Durability test of GCNT under repeated compress and release pressure of 1 kPa.



Figure 4. Mechanical performances of the GCNT aerogel with a density of 1.48 mg cm-3. The stress-strain curves of (a) GCNT and (c) GO/CNT at set strains of 35%, 65%, and 90%, respectively. Stress-strain curves of multicycle compressions at 80% strain upon (b) GCNT for 3000 cycles and (d) GO/CNT for 100 cycles; Insert photographs show the corresponding samples. (e) The ultimate stress and Young’s modulus for 100 compression cycles. (f) The schematic model of a single GCNTs cell wall. (g-i) Optical microscope images of GCNT during the compression process.


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Figure 5. Sensing performances of the GCNT-based sensor. (a) Normalized relative resistance as a function of compressible strain and the corresponding linear fittings. (b) Instant response of the flexible strain sensor, which exhibits response time of 28 ms. (c) Measurement of the wrist artery pulse signals of a heartbeat under normal frequency (67 beats min-1); Insert photographs show the blood pressure monitoring using the strain sensor with an active area of 10 mm × 10 mm. Detections of (d) carotid artery pulses and (e) saliva swallowing in real time. (f) Detection of finger folding in real time. Inset photographs show finger folding with different angle in the test.



The average lateral size of the GO LC sheets is measured as 3.5 ± 1.8 µm with a thickness of about 0.8 nm, indicating it can be stably dispersed in water with single-layer structure.


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Figure S2. (a) The photographs of CNTs dispersed in deionized water (left) and 0.32 M KOH solution (right).



Figure S3. The weight measurement processes for the GCNT aerogel with a density of 0.84 mg cm-3 (6.76 mg in 8.05 cm-3).


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Figure S4. Photograph of the zeta potential measurement for GO LCs +CNTs + KOH suspensions for GCNT with a value of -19.29 eV.



Figure S5. POM images of (a) GO LCs suspensions and (b) the GO LCs + CNTs aqueous solution. Scale bar: 400 μm.


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Figure S6. (a) UV-vis spectra of GO LCs, GO LCs + CNTs and GO LCs + KOH suspensions. (b) FT-IR spectra of GCNT and GO/CNT aerogel. 188x75mm (300 x 300 DPI)



Figure S7. SEM images of the GCNT aerogels with different CNTs contents of (a) 15 wt%, (b) 20 wt%, (c) 25 wt%, and (d) 30 wt%.


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Figure S8. POM images of GO LCs suspensions with different CNTs contents of (a) 20 wt%, (b) 25 wt%, and (d) 30 wt%. Scale bar: 400 μm.



Figure S9. (a) TEM image of the typical GCNT. (b) Magnified TEM image of the GCNT.


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Figure S10. The photographs of the compression performance for (a) GCNTs and (b) GO/CNT at 80% strain for 100 cycles 219x127mm (299 x 299 DPI)



Figure S11. Photograph of the assembled strain sensor based on GCNT aerogel. 706x702mm (72 x 72 DPI)


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Reducing structural defects and oxygen-containing functional groups

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