Superelastic carbon aerogel with ultrahigh and wide-range linear

Oct 31, 2018 - However, fabricating carbon materials with super elasticity, fatigue resistance, high and wide-range linear sensitivity for pressure or...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 40641−40650

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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,† Linxin Zhong,*,† Runcang Sun,*,‡ and Xinwen Peng*,† †

State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510641, China Centre for Lignocellulose Science and Engineering and Liaoning Key Laboratory Pulp and Paper Engineering, Dalian Polytechnic University, Dalian 116034, China

ACS Appl. Mater. Interfaces 2018.10:40641-40650. Downloaded from pubs.acs.org by UNIV OF GOTHENBURG on 11/30/18. For personal use only.

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S Supporting Information *

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, and 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 as a nanoreinforcement 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 50 000 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 a small angle change. These superiorities render its applications in various wearable devices. KEYWORDS: carbon aerogel, chitosan, cellulose nanocrystal, compressible, sensitivity

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ment,11 freeze-casting,12 and CVD.13 A graphene elastomer with superelasticity could be prepared via the freeze-casting method.12 The final product could recover from 98% compression, and its intersheet 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 increased the Young’s modulus and energy storage modulus of the material. To further improve the mechanical performances of compressible carbon materials, a lamellar structure was developed by directional freeze-casting.17,18 For example, a well-designed carbon monolith with a multiarch 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

INTRODUCTION Highly sensitive and compressible carbon materials have drawn substantial attention in the applications of health monitoring, sensors, wearable devices, and artificial intelligence because of their various advantages such as mechanical flexibility, high conductivity, chemical stability, and lightweight properties.1,2 Using flexible polymers as elastic subtracts, carbon/polymer composites are fabricated for pressure sensing, such as aligned carbon nanotubes (CNTs)/graphene−polydimethylsiloxane (PDMS),3 single-walled CNT/PDMS,4 and polyurethane/ carbon sponges,5 which show good mechanical properties. However, their sensitivities are limited because soft substrates have no contribution to conductivity but high density. The CNT is an attractive one-dimensional nanocarbon to form compressible all-carbon materials because of its high aspect ratio, mechanical robustness, and good conductivity.6−8 For instance, a lightweight and compressible CNT foam could be fabricated using the chemical vapor deposition (CVD) method.6 The jointwelded 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 twodimensional candidates for fabricating compressible carbon materials through solvent thermal9,10 or hydrothermal treat© 2018 American Chemical Society

Received: September 5, 2018 Accepted: October 31, 2018 Published: October 31, 2018 40641

DOI: 10.1021/acsami.8b15439 ACS Appl. Mater. Interfaces 2018, 10, 40641−40650

Research Article

ACS Applied Materials & Interfaces

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.

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) the cellulose nanocrystal (CNC) acts as nanoreinforcement or support, and (ii) a lamellar structure is created and carefully engineered by controlling FeCl3 concentration. Because of 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.

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 the high-pressure region, which is another challenge for current carbon materials. In addition, the high cost (requiring a 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 CNTs, 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

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MATERIALS AND METHODS

Materials. Chitosan (deacetylation ≥85%, viscosity = 200 cps), cellulose powder (size ≈ 90 μm) and analytical grade FeCl3·6H2O were purchased from Aladdin, China. 40642

DOI: 10.1021/acsami.8b15439 ACS Appl. Mater. Interfaces 2018, 10, 40641−40650

Research Article

ACS Applied Materials & Interfaces Fabrication of CNC Suspension. The 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 two 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 mechanical 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 the 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 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 FeCl 3 concentrations (0.0008, 0.0024, 0.004, and 0.0056 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, 1.2, 2.0, and 2.8 wt %, respectively. Other processing conditions are the same with that of the Fe−CS/CNC−C mentioned above. Characterizations. Micromorphologies of aerogels and carbon aerogels were observed on a 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 a 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 SourceMeter. The applied voltage was 1.0 V for all of the current and resistance measurements. Assembly and Testing of the 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 microloads 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 SourceMeter.

solvent, without CNC), as shown in Figure S1a (Supporting Information). Carbonization at 800 °C produces a carbon aerogel Fe−CS−C with remarkable volume shrinkage, as shown Figure S1b. The CNC, derived from renewable cellulose, is a highly crystalline and rod-like nanoparticle with a high 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 the CNC (high up to 56.7 mg/cm3), indicating that the CNC acts as a nanosupport 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,h. 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. Intensive absorption at 1600 cm−1 (G band) demonstrates the existence of the ordered graphitic structure, whereas absorption at 1340 cm−1 (D band) is attributed to the defects or disorders of carbon. The intensity ratios of the G band to D band (IG/ID) of these carbon aerogels show no significant difference, ranging from 1.04 to 1.07, suggesting that the CNC and chitosan are converted into lowdegree 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 the 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, FeNx, and FeCx.23,24 The SEM images of aerogels before carbonization (Figure S4a,b, 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 the ice crystal extrude chitosan chains to form a sheet-like structure, and an ordered lamellar structure is produced after carbonization (Figure 1b). As shown in Figure S4c,d, the Fe−CS/CNC also shows a sheet-connected architecture with oriented channels, and Fe−CS/CNC−C exhibits an aligned lamellar structure (Figure 1c). Fe−CNC (Figure S4e,f) and HAC−CS/CNC (Figure S4g,h) consist of irregularly aligned sheets, and the structures are well reserved after carbonization (Figure 1d,e). Because 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 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

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RESULTS AND DISCUSSION As shown in Figure 1a, the compressible and elastic carbon aerogel is prepared by mixing chitosan and the CNC in FeCl3 aqueous solution, directional freeze-casting, and carbonization. The chitosan chain is composed of β-(1−4)-acetylamino-2deoxy-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. Afterward, the chitosan solution is directionally froze by liquid nitrogen and then freeze-dried to obtain aerogel Fe−CS (FeCl3 solution as a 40643

DOI: 10.1021/acsami.8b15439 ACS Appl. Mater. Interfaces 2018, 10, 40641−40650

Research Article

ACS Applied Materials & Interfaces

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, and Fe7−CS/CNC−C, respectively.

stable structure and a higher mechanical compression strength because of 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,f), suggesting high mechanical anisotropy because of the directional arrange of wave-like carbon layers. Because FeCl3 solution plays a very important role in forming the 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 same conditions, as shown in Figures 2 and S7 (Supporting Information). It is found that the aerogel Fe1−CS/CNC with the lowest FeCl3 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 integrated and stronger (Figure S7b− d), and the alignment of its carbon sheets becomes ordered and

from these carbon aerogels. For the carbon sheet from Fe−CS− C without CNC (Figure S5a,b), 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,d, 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,f). Figure S6 (Supporting Information) and Figure 1f−i demonstrate the mechanical compressibility 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 a 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); however, Fe−CNC−C and HAC−CS/ CNC−C suffer from significant permanent plastic deformation, indicating poor elasticity (Figure 1h,i), 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 40644

DOI: 10.1021/acsami.8b15439 ACS Appl. Mater. Interfaces 2018, 10, 40641−40650

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ACS Applied Materials & Interfaces

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 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 50 000 cycles at 50% strain. (f) Static compression test at 50 and 70% strains for 60 days. (g) Rebounding a zirconia ball.

continuous (Figure 2b−d). 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 the 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−h, Tables S2 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), whereas Fe3−CS/CNC−C (Figure 2f) can retain 89% of its original height after 1000 40645

DOI: 10.1021/acsami.8b15439 ACS Appl. Mater. Interfaces 2018, 10, 40641−40650

Research Article

ACS Applied Materials & Interfaces

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. (c) Normalized resistance R/R0 for high up to 30 000 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 GF. (i) Real-time current response to different angles.

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 increases. 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 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. 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 region where stress rises sharply, reflecting the densification of the 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 S1 (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 50 000 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 50 000 loading−unloading cycles (Figure S9, 40646

DOI: 10.1021/acsami.8b15439 ACS Appl. Mater. Interfaces 2018, 10, 40641−40650

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ACS Applied Materials & Interfaces

Figure 5. Applications of Fe5−CS/CNC−C for body motion recognition and biosignal detection. (a) Assembly of the 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 the speaking “sensor” and “chitosan”. (f) Detecting the human blood pressure signal.

the contact area due to the large space among layers. As shown in Figure 4c, R/R0 retains a similar shape even after 10 000 cycles at 50% strain, and then slightly increases after 30 000 compression cycles. The ultrastable 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 pressure sensing materials, such as the carbon aerogel,2 flexible substrate-loaded carbon,1,38 and carbon film40 cannot maintain their linear sensitivity at such a broad pressure region. Thus, lots of motions cannot be well defined. The sensitivity (S) is defined as

Supporting Information). As shown in Table S4 (Supporting Information), except lamellar rGO aerogel,17 the outstanding fatigue resistance of Fe 5−CS/CNC−C surpasses other compressible carbon materials, including the 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 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 S2 (Supporting Information). These results highlight superior mechanical performances of the as-prepared carbon aerogel because of (1) the orderly aligned wave-shaped carbon layers, (2) the nanoreinforcement or support of the CNC for the carbon sheets, and (3) large compression space among carbon sheets. 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 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

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−f shows the sensitive response of current to pressure or strain. It is 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 the electrical signal output and pressure applied. The ultrahigh and wide-range linear sensitivity is not only superior to the present all-carbonbased materials 2,41 and flexible substrate-loaded carbons1,38,42−45 but also carbon films,40,46 as shown in Figure 4e. Although there is a graphene aerogel that 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 carbon-based 40647

DOI: 10.1021/acsami.8b15439 ACS Appl. Mater. Interfaces 2018, 10, 40641−40650

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sensing materials1,2,38,40,41,43−46 and many other kinds of sensing materials, such as the 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). The gauge factor (GF) is another important parameter of sensitivity, which depends on the electrical resistance of a sensor when strain is applied.49 The GF can be defined as GF = (ΔR /R 0)/ε

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b15439. 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 50 000 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 concentrations at 50% strain; fatigue resistance of different sorts of compressible aerogels (PDF) Compression test of Fe5−CS/CNC−C at 90% strain (AVI) Rebound of a zirconia ball by using Fe5−CS/CNC−C (AVI) Fe5−CS/CNC−C as a conductor to change the brightness of LED via finger bending (AVI)

(2)

where ΔR is the change of resistance with applied strain, 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 strain change with a 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 PET substrates adhering with nickel electrodes, it can be bended because of the excellent mechanical properties (Figure S12, Supporting Information), and its conductivity regularly increases with the 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. 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,c). More intuitively, we connect the sensor into a light-emitting diode (LED) circuit, and the brightness of LED lights can be continuously controlled via finger bending, as shown in Figure S13 and Movie S3 (Supporting Information). Furthermore, it can distinctly detect the tiny movements of face, such as smiling and puffing (Figure 5d). Figure 5e demonstrates that the resistance signal varies when we speak different words (e.g., “sensor” and “chitosan”). In addition, the assembled sensor can detect the human pulse (Figure 5f). These results suggest promising applications of the carbon aerogel in various wearable devices.

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Research Article

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (L.Z.). *E-mail: [email protected] (R.S.). *E-mail: [email protected] (X.P.). ORCID

Chuanfu Liu: 0000-0002-3151-7956 Linxin Zhong: 0000-0002-0639-4333 Runcang Sun: 0000-0003-2721-6357 Xinwen Peng: 0000-0002-4575-256X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the 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.

CONCLUSIONS

A compressible carbon aerogel with both ultrahigh mechanical performances and superior sensitivity is obtained from chitosan and the 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 low-pressure regime (