Superelastic Carbon Aerogel with Ultrahigh and Wide-Range Linear

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Functional Inorganic Materials and Devices

Superelastic carbon aerogel with ultrahigh and wide-range linear sensitivity Yijie Hu, Hao Zhuo, Zehong Chen, Kunze Wu, Qingsong Luo, Qingzhong Liu, Shuangshuang Jing, Chuanfu Liu, Run-Cang Sun, Linxin Zhong, and Xinwen Peng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b15439 • Publication Date (Web): 31 Oct 2018 Downloaded from http://pubs.acs.org on November 1, 2018

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ACS Applied Materials & Interfaces

Superelastic Carbon Aerogel with Ultrahigh and Wide-range Linear Sensitivity

Yijie Hu1, Hao Zhuo1, Zehong Chen1, Kunze Wu1, Qingsong Luo1, Qingzhong Liu1, Shuangshuang Jing1, Chuanfu Liu1, Linxin Zhong1, *, Runcang Sun2, *, and Xinwen Peng1, *

1

State Key Laboratory of Pulp and Paper Engineering, South China University of Technology,

Guangzhou, 510641, China. 2

Centre for Lignocellulose Science and Engineering and Liaoning Key Laboratory Pulp and Paper

Engineering, Dalian Polytechnic University, Dalian 116034, China. * Corresponding authors. E-mail: [email protected] (L. Zhong), [email protected] (R. Sun), and [email protected] (X. Peng)

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Abstract Compressible and elastic carbon materials offer many advantages and have promising applications in various electronic devices. However, fabricating carbon materials with super elasticity, fatigue resistance, high and wide-range linear sensitivity for pressure or strain remains a great challenge. Herein, a facile and sustainable route is developed to fabricate a carbon aerogel with not only superior mechanical performances but also exceptionally high and wide-range linear sensitivity, by using chitosan as a renewable carbon source and cellulose nanocrystal (CNC) as a nano reinforcement or support. The as-prepared carbon aerogel with wave-shaped layers shows high compressibility, super elasticity, stable strain-current response, and excellent fatigue resistance (94% height retention after 50000 cycles). More importantly, it demonstrates both an ultrahigh sensitivity of 103.5 kPa-1 and a very wide linear range of 0-18 kPa. In addition, the carbon aerogel has a very low detection limit (1.0 Pa for pressure and 0.05% for strain). The carbon aerogel also can be bended to detect small angle change. These superiorities render its applications in various wearable devices.

Keywords: carbon aerogel; chitosan; cellulose nanocrystal; compressible; sensitivity


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Introduction Highly sensitive and compressible carbon materials have drawn substantial attention in the applications of health monitoring, sensors, wearable devices, and artificial intelligence due to their various advantages such as mechanical flexibility, high conductivity, chemical stability, and lightweight.1,2 Using flexible polymers as elastic subtracts, carbon/polymer composites are fabricated for pressure sensing, such as aligned carbon nanotubes/graphene-PDMS,3 SWNT/PDMS,4 and polyurethane/carbon sponges,5 which show good mechanical properties. However, their sensitivities are limited because soft substrates have no contribution to the conductivity but a high density. Carbon nanotube (CNT) is an attractive one-dimensional (1D) nanocarbon to form compressible all-carbon materials due to its high aspect ratio, mechanical robustness, and good conductivity.6-8 For instance, a lightweight and compressible CNT foam could be fabricated using chemical vapor deposition (CVD) method.6 The joint-welded foam showed structural robustness and could be compressed for 1000 cycles with high stress retention. Graphene and graphene oxide (GO) are also considered as excellent two-dimensional (2D) candidates for fabricating compressible carbon materials through solvent thermal9,10 or hydrothermal treatment,11 freeze-casting,12 and CVD.13 A graphene elastomer with super-elasticity could be prepared via freeze-casting method. 12 The final product could recover from 98% compression, and its inter-sheet interactions could be adjusted by changing annealing conditions. Compressible graphene/CNT foams could also be fabricated by coating graphene onto CNTs and exhibited excellent fatigue resistance.14-16 Kim et al. 15 transferred single-walled CNTs into a superelastic material by coating them with graphene, which exhibited no change in mechanical properties after 2000 compressive cycles at 60% strain. The coating also 3 ACS Paragon Plus Environment


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increased the Young's modulus and energy storage modulus of the material. To further improve the mechanical performances of compressible carbon materials, lamellar structure was developed by directional freeze-casting.17,18 For example, a well-designed carbon monolith with multi-arch microstructure possessed unexpected mechanical performances, and could maintain 98% height after 2.5×105 cycles at 50% strain.17 However, high sensitivity remains a great challenge for the present compressible carbon materials. Furthermore, from an application point of view, wide-range linear sensitivity is highly desired for pressure or stain sensors to discriminate a subtle change in high-pressure region, which is another challenge for current carbon materials. In addition, the high cost (requiring metal catalyst and expensive equipment), low production rate, and complicated synthesis process (e.g. post treatment),19 as well as their high dependence on petroleum resources, hinder the applications of CNT, graphene, and GO in such materials. Chitosan20 and cellulose21 are the most abundant, renewable, and low-cost carbon sources in nature, and thus attractive alternatives to petroleum resources for preparing low-cost and sustainable carbon materials. Herein, we propose a facile and sustainable route to fabricate carbon aerogel with both exceptionally high mechanical performances and wide-range linear sensitivity by using chitosan as a sustainable carbon precursor. To achieve such outstanding performances, two key considerations are proposed: (i) CNC acts as a nano reinforcement or support, and (ii) a lamellar structure is created and carefully engineered by controlling FeCl3 concentration. Due to its unique properties, the carbon aerogel can detect a very low pressure or a deformation change, and have promising applications in flexible wearable devices for detecting biosignals.

Materials and methods 4 ACS Paragon Plus Environment

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Materials. Chitosan (deacetylation≥85%, viscosity=200 cps), cellulose powder (size≈90 μm) and analytical grade FeCl3·6H2O were purchased from Aladdin, China. Fabrication of CNC suspension. CNC was prepared by acid hydrolysis. Typically, 15 g cellulose was added into 150 mL 64 wt% H2SO4 solution, and the mixture was stirred at 55 °C for 60 min. Then the reaction was stopped by adding 1 L cold deionized water, and the suspension was centrifuged for 10 min (5000 rpm) for 2 times to separate the CNC. The obtained CNC was dialyzed against deionized water to obtain CNC suspension with 0.5 wt% concentration. Fabrication of Fe-CS, Fe-CNC, Fe-CS/CNC, and HAC-CS/CNC aerogels. In a typical procedure for the fabrication of Fe-CS/CNC, 0.05 g FeCl3·6H2O was dissolved in 50 mL 0.5 wt% CNC suspension by mechanically stirring. Then, 0.25 g chitosan powder was added to the suspension and stirred for 1 h. The obtained suspension was transferred to a plastic rectangular box which was tied to an open lidless steel box filled with liquid nitrogen to complete freeze casting. The frozen sample was then freeze-dried to obtain Fe-CS/CNC aerogel. Fe-CS without CNC and Fe-CNC without chitosan were fabricated by dissolving 1% chitosan or homogenizing 1% CNC in 0.004 mol/L FeCl3 solution, respectively. For comparison, aerogel HAC-CS/CNC was prepared by replacing FeCl3 solution with 2 wt% acetic acid solution. Fabrication of Fe-CS-C, Fe-CNC-C, Fe-CS/CNC-C, and HAC-CS/CNC-C carbon aerogels. Fe-CS, Fe-CNC, Fe-CS/CNC, and HAC-CS/CNC were annealed in a tube furnace from about 30 °C to 800 °C with a heating rate of 3 °C/min under an argon atmosphere, and held at 800 °C for 2 h to obtain carbon aerogels Fe-CS-C, Fe-CNC-C, Fe-CS/CNC-C, and HAC-CS/CNC-C, respectively. Fabrication of Fex-CS/CNC-C with different FeCl3 concentrations. The carbon aerogels prepared with different FeCl3 concentrations (0.0008 mol/L, 0.0024 mol/L, 0.004 mol/L, and 0.0056 5 ACS Paragon Plus Environment


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mol/L) are named as Fe1-CS/CNC-C, Fe3-CS/CNC-C, Fe5-CS/CNC-C, and Fe7-CS/CNC-C, respectively. The iron contents in these aerogels (based on the total weight of chitosan and CNC) are 0.4 wt%, 1.2 wt%, 2.0 wt%, and 2.8 wt%, respectively. Other processing conditions are the same with that of Fe-CS/CNC-C mentioned above. Characterizations. Micromorphologies of aerogels and carbon aerogels were observed on transmission electron microscope (TEM) (JEM-2100F) and scanning electron microscope (SEM) (Merlin, Zeiss). Raman and X-ray diffraction (XRD) patterns were recorded on a Raman spectrometer (LabRAM ARAMIS-Horiba Jobin Yvon) operating with 532 nm laser and a Bruker D8 diffractometer, respectively. Mechanical compression and cycle were carried out on Instron 5565 equipped with a 100 N load cell, and samples were placed between two compression stages. The resistance of samples was recorded on a multimeter (VC 890D), and the current was recorded on an electrochemical workstation (CHI660E) and a 2400 digital source-Meter. The applied voltage was 1.0 V for all of the current and resistance measurements. Assembly and testing of compressible sensor. The carbon aerogel-based sensor was assembled by fitting the carbon aerogel into two Ni electrodes adhered to thick poly(ethylene terephthalate) (PET) substrates. The tiny strain and micro loads were provided by Instron 5565. The compression-induced resistance changes were recorded on a multimeter (VC 890D). The bending angle was conducted by a protractor. The real-time current signals were recorded on a 2400 digital source-Meter.

Results and Discussion As shown in Figure 1a, the compressible and elastic carbon aerogel is prepared by mixing chitosan 6 ACS Paragon Plus Environment

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and CNC in FeCl3 aqueous solution, directional freeze-casting and carbonization. Chitosan chain is composed of β-(1-4)-acetylamino-2-deoxy-β-D-glucose. The abundant amino groups can be cationized in acid solutions such as FeCl3 and acetic acid solution, resulting in mutual repulsion of the protonated amino groups and therefore the dissolution of chitosan. Afterwards, the chitosan solution is directionally froze by liquid nitrogen and then freeze-dried to obtain aerogel Fe-CS (FeCl3 solution as a solvent, without CNC), as shown Figure S1a (Supporting Information). Carbonization at 800 °C produces a carbon aerogel Fe-CS-C with remarkable volume shrinkage, as shown Figure S1b.



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Figure 1. Fabrication, microstructures, compression behaviors and elasticity mechanism of the as-prepared carbon aerogels. a) Schematic illustration of fabricating aerogel and corresponding carbon aerogel. b-e) SEM images and f-i) stress-strain plots of Fe-CS-C, Fe-CS/CNC-C, Fe-CNC-C and HAC-CS/CNC-C, respectively. j) Schematic illustration of the super compression and elasticity mechanism of Fe-CS/CNC-C. CNC, derived from renewable cellulose, is a highly crystalline and rod-like nanoparticle with high 8 ACS Paragon Plus Environment

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aspect ratio, as shown in Figure S2 (Supporting Information). It has been widely used as a reinforcement for nanocomposites because of its high strength and low density.22 It is found that the addition of CNC produces aerogel Fe-CS/CNC (Figure S1c) and carbon aerogel Fe-CS/CNC-C with a higher volume retention (Figure S1d). Table S1 (Supporting Information) shows that Fe-CS/CNC-C has a low density of 17.5 mg/cm3, which is much lower than that of Fe-CS-C without CNC (high up to 56.7 mg/cm3), indicating that CNC acts as a nano support to produce a lightweight carbon aerogel. Pure CNC aerogel Fe-CNC without chitosan is also prepared (Figure S1e), and its carbon aerogel Fe-CNC-C shows the lowest volume shrinkage (Figure S1f) and the lowest density (9.2 mg/cm3, Table S1), further confirming its support effect. When acetic acid solution is used as a solvent, aerogel HAC-CS/CNC and carbon aerogel HAC-CS/CNC-C were prepared for comparison, as shown in Figure S1g and S1h. There is no significant difference in density between Fe-CS/CNC-C and HAC-CS/CNC-C (Table S1). Figure S3a (Supporting Information) shows the Raman spectra of the four samples. An intensive absorption at 1600 cm-1 (G band) demonstrates the existence of ordered graphitic structure, while absorption at 1340 cm-1 (D band) is attributed to the defects or disorders of carbon. The intensity ratios of G band to D band (IG/ID) of these carbon aerogels show no significant difference, ranging from 1.04 to 1.07, suggesting that CNC and chitosan are converted into low-degree graphitic carbon during annealing. The characteristic peaks from XRD patterns are shown in Figure S3b. All of the samples show a broad and low-intensity peak at 2θ=24° attributed to the graphite (002) plane, confirming a low-degree graphitization. Additionally, Fe-CS-C, Fe-CNC-C, and Fe-CS/CNC-C have characteristic peaks of iron element with various forms: 2θ = 24.4° and 30.0° (Fe2O3), 35.2° and 64.9° (Fe3O4), 36.1° (FeNx), 41.9°, 43.1°, and 44.6° (FeCx), indicating the presence of Fe2O3, Fe3O4, 9 ACS Paragon Plus Environment


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FeNx, and FeCx.23,24 The SEM images of aerogels before carbonization (Figure S4a and S4b, Supporting Information) show that Fe-CS is composed of large-size sheets with ordered channels. During directional freezing by liquid nitrogen, the formation and growth of ice crystal extrude chitosan chains to form sheet-like structure, and an ordered lamellar structure is produced after carbonization (Figure 1b). As shown in Figure S4c and S4d, Fe-CS/CNC also shows sheet-connected architecture with oriented channels, and Fe-CS/CNC-C exhibits an aligned lamellar structure (Figure 1c). Fe-CNC (Figure S4e and S4f) and HAC-CS/CNC (Figure S4g and S4h) consist of irregularly-aligned sheets, and the structures are well reserved after carbonization (Figure 1d and 1e). Since the complexation or coordination between chitosan and Fe3+ occurs mainly through the amino group and –OH in chitosan,25 an interaction among chitosan macromolecules will happen, which is supposed to produce continuous chitosan layers after freeze-drying. However, this interaction is absent in the acetic acid solution, resulting in less continuous and small layers. Furthermore, these results also suggest that chitosan favor the formation of continuous and ordered layers. Figure S5 (Supporting Information) reveals the lamelliform carbon sheets from these carbon aerogels. For the carbon sheet from Fe-CS-C without CNC (Figure S5a and S5b), it has many particles with regular lattice fringe, relating to various iron particles. Except for the lattice fringe of iron particles, there are lots of rods on the carbon sheet of Fe-CS/CNC-C, as indicated by the arrows in Figure S5c and S5d, which can be attributed to the carbonized CNCs (referring to the TEM images of pure CNC in Figure S2). These well-dispersed carbonized CNCs are expected to enhance the mechanical performances of carbon aerogel. These carbonized CNCs also can be found in the carbon sheet of HAC-CS/CNC-C (Figure S5e and S5f). Figure S6 (Supporting Information) and Figure 1f-1i demonstrate the mechanical compressibility 10 ACS Paragon Plus Environment

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of the as-prepared samples before and after carbonization, respectively. All of the aerogels before carbonization significantly lose their heights upon compression at 60% strain (Figure S6), demonstrating very poor elasticity, which can be ascribed to the weak strength among CNCs and/or the softness and high hydrophilicity of chitosan. After carbonization, Fe-CS-C exhibits tiny height loss after 10 compression cycles, as indicated in Figure 1f. Fe-CS/CNC-C can recover its original height after 10 cycles at 60% strain (Figure 1g); while Fe-CNC-C and HAC-CS/CNC-C suffer from significant permanent plastic deformation, indicating poor elasticity (Figure 1h and 1i), which can be due to the disorder architecture. Furthermore, the stress retention of Fe-CS/CNC-C (85%) after 10 cycles is much higher than those of Fe-CS-C (73%), Fe-CNC-C (55%) and HAC-CS/CNC-C (75%), demonstrating that Fe-CS/CNC-C has a more stable structure and a higher mechanical compression strength due to the stacking wave-like layers and CNC reinforcement or support. After carbonization, both chitosan and CNC are converted to carbon that is brittle yet somewhat elastic at relatively small deformation. When the compression stress is applied perpendicularly to the wave-like layers, the arch-shaped architecture can undergo a large geometric deformation without collapse because of its small material strain,17 and thus recover its original shape immediately upon compression force releasing (Figure 1j). When applying stress parallel to the wave-like layers (front and side direction), structural destruction and significant stress loss occur to Fe-CS/CNC-C (Figure S6e and S6f), suggesting high mechanical anisotropy due to the directional arrange of wave-like carbon layers.



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Figure 2. SEM images and compressibilities of Fe-CS/CNC-C with different FeCl3 concentrations. a-d) SEM images and e-h) stress-strain plots of Fe1-CS/CNC-C, Fe3-CS/CNC-C, Fe5-CS/CNC-C, Fe7-CS/CNC-C, respectively.

Since FeCl3 solution plays a very important role in forming aligned lamellar structure, the impact of FeCl3 concentration on the microstructure and mechanical performances of Fe-CS/CNC-C is investigated. Samples (Fe1-CS/CNC-C, Fe3-CS/CNC-C, Fe5-CS/CNC-C, and Fe7-CS/CNC-C) with different FeCl3 concentrations were prepared under the same condition, as shown in Figure 2 and Figure S7 (Supporting Information). It is found that the aerogel Fe1-CS/CNC with the lowest FeCl3 12 ACS Paragon Plus Environment

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concentration (0.0008 mol/L) demonstrates a remarkable volume shrink during freeze-casting (Figure S7). Its carbon aerogel Fe1-CS/CNC-C exhibits a disordered structure (Figure 2a). With the increase of FeCl3 concentration, the aerogel becomes more integrity and stronger (Figure S7b-S7d), and the alignment of its carbon sheets becomes ordered and continuous. (Figure 2b-2d). Especially, with the highest FeCl3 concentration of 0.0056 mol/L (Fe7-CS/CNC-C), the carbon sheets are much larger (Figure 2d). It is speculated that a higher FeCl3 concentration will give rise to a stronger interaction between chitosan and Fe3+, which results in more continuous or larger wave-like layers. Figure S8 indicates that all of the carbon aerogels have no significant difference in both IG/ID ratio (ranging from 1.05 to 1.08) and XRD pattern (except the intensities of various iron forms caused by different FeCl3 concentrations). As shown in Figure 2e-2h, Table S2 (Supporting Information) and S3 (Supporting Information), increasing FeCl3 concentration also leads to a higher height retention and compression strength. Specifically, Fe1-CS/CNC-C has the lowest height retention (66.7% after 10 cycles) at 50% strain (Figure 2e), while Fe3-CS/CNC-C (Figure 2f) can retain 89% of its original height after 1000 loading-unloading cycles. Impressively, Fe5-CS/CNC-C (Figure 2g) and Fe7-CS/CNC-C (Figure 2h) can almost retain their original heights after 1000 cycles, which indicates that the wave-like layers are highly compressible and elastic. However, cracks tend to occur to Fe7-CS/CNC-C during cyclic compression (as indicated by the white dashed curves in Figure 2h), suggesting less flexibility of the large carbon sheets. The stress at 50% strain also remarkably increases from 1.6 kPa (Fe3-CS/CNC-C) to 10.5 kPa (Fe7-CS/CNC-C) as FeCl3 concentration increasing. Furthermore, the stress retentions of Fe3-CS/CNC-C and Fe5-CS/CNC-C after 1000 compression cycles are high up to 84.0% and 83.0%, which are better than that of Fe7-CS/CNC-C (78.2%), as shown in Table S3. Thus, increasing FeCl3 13 ACS Paragon Plus Environment


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concentration leads to more ordered and continuous carbon sheets, which contributes to the excellent mechanical strength. However, too much FeCl3 leads to larger carbon sheets with less flexibility.

Figure 3. Super compressibility, elasticity and fatigue resistance of Fe5-CS/CNC-C. a) Stress-strain plots at different compression strains. b) Height retentions after 10 cycles at different 14 ACS Paragon Plus Environment

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compression strains. c) Stress-strain plots at 90% strain for 10 cycles. d) Stress-strain plots at 70% strain for 300 cycles. e) Stress-strain plots for 50000 cycles at 50% strain. f) Static compression test at 50% and 70% strains for 60 days. g) Rebounding a zirconia ball.

As shown in Figure 3a, unlike the stress-strain curves of conventional open-cell foams, which show three characteristic deformation regions (e.g., an initial linear elastic region relating to elastic deformation, a plateau stage corresponding to elastic buckling, and a stress sharply rising region reflecting the densification of cell wall),1, 26 the curve of Fe5-CS/CNC-C shows a crescent shape that heightens dramatically with increasing strain. It can undergo an extremely high strain of 95% with a maximum stress of 280 kPa. Figure 3b demonstrates that the carbon aerogel exhibits no height loss at 10-50% strain after 10 compression cycles and only 2% height loss at a high strain of 70%, without yielding significant plastic deformation. Even at very high stains of 90% and 95%, Fe5-CS/CNC-C can recover 88% and 82% of its original height, respectively. The corresponding strain-stress curves retain the similar shape after 10 cycles, as shown in Figure 3c and Movie 1 (Supporting Information). At 70% strain, the carbon aerogel maintains 90% of its original height after 300 compression cycles (Figure 3d), demonstrating high compressibility and elasticity. It can be compressed for high up to 50000 cycles at 50% strain, as shown in Figure 3e. Specifically, the carbon aerogel can completely recover its original height within 100 cycles and retain 94% height after 50000 loading-unloading cycles (Figure S9, Supporting Information). As shown in Table S4 (Supporting Information), except lamellar rGO aerogel,17 the outstanding fatigue resistance of Fe5-CS/CNC-C surpasses other compressible carbon materials, including CNT sponge,6 graphene aerogel,18,

27-34

CNT/graphene

composite aerogel,14,16 and so on.35-37 No significant height loss occurs to Fe5-CS/CNC-C when the 15 ACS Paragon Plus Environment


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sample is continuously compressed at 50% strain for 60 days (Figure 3f). And it can undergo the shock of a zirconia ball and easily rebound it without structural damage, revealing excellent elasticity and high mechanical strength, as shown in Figure 3g and Movie 2 (Supporting Information). These results highlight superior mechanical performances of the as-prepared carbon aerogel due to (1) the orderly aligned wave-shaped carbon layers, (2) the nano reinforcement or support of CNC for the carbon sheets, and (3) large compression space among carbon sheets.

Figure 4. Strain- and pressure-current or resistance responses and sensing abilities of Fe5-CS/CNC-C. a) Real-time current at different compression strains. b) Stable strain-current response for 1000 cycles at 30% and 50% strains, respectively. c) The normalized resistance R/R0 for 16 ACS Paragon Plus Environment

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high up to 30000 cycles at 50% strain. d) Ultrahigh and wide-range linear sensitivity. e) Comparison of sensitivity among different carbon materials. f) Ultrahigh sensitivity for detecting tiny load. g) Comparison of detection limits among different carbon materials. h) Tiny strain-resistance curve and gauge factor. i) Real-time current response to different angles.

Figure 4a shows the real-time current responses of Fe5-CS/CNC-C at different strains. The current versus voltage curve without strain shows that the I0 is about 0.8 mA at 1.0 V (Figure S10, Supporting Information). Figure 4a shows that the current remarkably increases upon compression and immediately decreases at release, showing a very fast response time. The conductivity of compressible materials relates to the contact resistance of conductive blocks. As shown in Figure 4a (the inset), during the compression process, the contact area among the carbon layers increases, leading to the decrease of resistance. Figure 4b shows the current response stability of the carbon aerogel within 1000 cycles at 30% and 50% strains, respectively. The current slightly decreases in the initial 250 cycles and then becomes stable in the following cycles. Figure S11 (Supporting Information) shows the normalized resistances (R/R0) of Fe5-CS/CNC-C at different strains. All of the curves are identical, again demonstrating a super stable structure. At a strain of less than 10%, R/R0 linearly decreases, suggesting a rapid increase in contact area due to the large space among layers. As shown in Figure 4c, R/R0 retains a similar shape even after 10000 cycles at 50% strain, and then slightly increases after 30000 compression cycles. The ultra stable R/R0 can be attributed to the superior mechanical performances of Fe5-CS/CNC-C. The linear relationship of ΔI/I0 and pressure is very helpful to obtain accurate information from the sensor output.38 A gentle touch usually relates to 1-10 kPa.39 However, most of present carbon-based 17 ACS Paragon Plus Environment


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pressure sensing materials, such as carbon aerogel,2 flexible substrate-loaded carbon,1, 38 and carbon film,40 cannot maintain their linear sensitivity at such broad pressure region. Thus, lots of motions cannot be well defined. The sensitivity (S) is defined as S = δ(ΔI/I0)/δP

(1)

where ΔI is the change in current with applied pressure, I0 is the initial current without applied pressure, and δP is the change of the applied pressure. Figure 4d-4f shows the sensitive response of current to pressure or strain. It’s notable that the sensitivity of Fe5-CS/CNC-C is high up to 27.2 kPa-1, and its linear range is exceptionally wide (0-18 kPa, Figure 4d), demonstrating a steady and sensitive relationship between electrical signal output and pressure applied. The ultrahigh and wide-range linear sensitivity is not only superior to the present all-carbon-based materials2, 41 and flexible substrate-loaded carbons,1, 38, 42-45 but also carbon films,40, 46 as shown in Figure 4e. Although there is a graphene aerogel has a larger linear range of 0-65 kPa, its sensitivity is very low (only 0.18 kPa-1), far from meeting actual application requirements.31 Furthermore, at tiny pressure (≤ 10 Pa), the sensitivity of Fe5-CS/CNC-C is high up to 103.5 kPa-1 (Figure 4f), which is the highest value reported. It is also found that the detection limit for pressure is low to 1.0 Pa, which is superior to most of carbon-based sensing materials1,2,38,40,41,43-46 and many other kinds of sensing materials, such as PPy/PDMS film47 and copper nanowire-based aerogel,48 as indicated in Figure 4g. This advantage allows the carbon aerogel to obtain accurate pressure data in tiny pressure detection, such as the pulse monitoring. The ultrahigh and wide-range linear sensitivity makes the carbon aerogel perceive not only subtle pressure (10 kPa). Gauge factor (GF) is another important parameter of sensitivity, which depends on the electrical 18 ACS Paragon Plus Environment

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resistance of a sensor when strain is applied.49 GF can be defined as GF= (ΔR/R0)/ε

(2)

where ΔR is the change of resistance with applied stain, R0 is the initial resistance without strain, and ε is the applied stain. Figure 4h shows the linear relationship between ΔR/R0 and strain, and the GF is high up to 21.7, which makes the carbon aerogel an ideal strain sensor to detect a tiny stain change with significant output signal. The assembled device can also detect a small compressing height change as low as 5 μm (equal to 0.05% change in stain, Figure 4h). In addition, when fitting the carbon aerogel into two poly(ethylene terephthalate) (PET) substrates adhering with nickel electrodes, it can be bended due to the excellent mechanical properties (Figure S12, Supporting Information), and its conductivity regularly increases with bending angle, as shown in Figure 4i. Upon bending, the carbon aerogel is squeezed to produce compression, and thus decreases contact resistance, indicating its potential application in angle detection. This unique performance, together with fast response, super elasticity, and durability, allows Fe5-CS/CNC-C to have practical applications in strain and pressure sensors.



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Figure 5. Applications of Fe5-CS/CNC-C for body motion recognition and biosignal detect. a) The assembly of pressure sensor. b) Current signals from elbow bending. c) Current signals from finger bending (the insets show the relative bending degree). d) Current signals from face expression. e) Current signals from speaking “sensor” and “chitosan”. f) Detecting human blood pressure signal.

Figure 5 shows the applications of the highly-sensitive carbon aerogel in detecting body motion and biosignals. Figure 5a shows the assembly of Fe5-CS/CNC-C into a simple sensor device. The sensor can detect human joint actions, such as elbow and finger bendings (Figure 5b and 5c). More intuitively, we connect the sensor into a LED circuit, and the brightness of LED lights can be continuously controlled via finger bending, as shown in Figure S13 and Movie 3 (Supporting Information). Furthermore, it can distinctly detect the tiny movements of face, such as smiling and puffing (Figure 5d). Figure 5e demonstrates that resistance signal varies when we speak different words (for example, “sensor” and “chitosan”). In addition, the assembled sensor can detect human pulse (Figure 5f). These results suggest the promising applications of the carbon aerogel in various wearable devices.

Conclusions A compressible carbon aerogel with both ultrahigh mechanical performances and superior sensitivity is obtained from chitosan and CNC by carefully engineering the lamellar structure. The as-prepared carbon aerogel exhibits high compressibility, super elasticity, fatigue resistance, and stable strain-current response. More importantly, a wide-range linear sensitivity can be obtained from 0 to 18 kPa, with a high value of 27.2 kPa-1; the sensitivity can even achieve 103.5 kPa-1 in a 20 ACS Paragon Plus Environment

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low-pressure regime (< 10 Pa). In addition, the carbon aerogel shows a very low detection limit for both pressure and strain. These superiorities allow the carbon aerogel to have promising applications in pressure/strain sensors and wearable devices.

Acknowledgements This work is supported by National Natural Science Foundation of China (21506068 and 31430092), Guangdong Natural Science Funds for Distinguished Young Scholar (2017A030306029 and 2016A030306027), Natural Science Foundation of Guangdong Province (2016A030313487), State Key Laboratory of Pulp and Paper Engineering (2017TS05), and Fundamental Research Funds for the Central Universities.

Supporting Information Pictures of as-prepared aerogels and carbon aerogels; TEM images of CNC; SEM images of Fe-CS, Fe-CS/CNC, Fe-CNC, and HAC-CS/CNC; TEM images of Fe-CS-C, Fe-CS/CNC-C, and HAC-CS/CNC-C; Raman and XRD patterns of Fe-CS-C, Fe-CNC-C, Fe-CS/CNC-C, and HAC-CS/CNC-C; Stress-strain plots of Fe-CS, Fe-CS/CNC, Fe-CNC, and HAC-CS/CNC; Pictures of Fe-CS/CNC with different FeCl3 concentrations; Raman and XRD patterns of Fe1-CS/CNC-C, Fe3-CS/CNC-C, Fe5-CS/CNC-C, and Fe7-CS/CNC-C; Height and stress retentions of Fe5-CS/CNC-C for 50000 cycles at 50% strain; The normalized resistances of Fe5-CS/CNC-C at different strains; Bending Fe5-CS/CNC-C with different angles; LED responses at different bending degrees; Densities of the as-prepared carbon aerogels; Height retentions of carbon aerogels with different FeCl3 concentrations at 50% strain; Stress retentions of carbon aerogels with different FeCl3 21 ACS Paragon Plus Environment


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concentrations at 50% strain; Fatigue resistance of different sorts of compressible aerogels. A video showing the compression test of Fe5-CS/CNC-C at 90% strain. A video showing the rebound of a zirconia ball by using Fe5-CS/CNC-C. A video showing Fe5-CS/CNC-C as a conductor to change the brightness of LED via finger bending.

Conflict of Interest The authors declare no conflict of interest.

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Superelastic Carbon Aerogel with Ultrahigh and Wide-Range Linear

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