Compressible, Elastic, and Pressure-Sensitive Carbon Aerogels

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Compressible, elastic, and pressure-sensitive carbon aerogel derived from 2D titanium carbide nanosheets and bacterial cellulose for wearable sensors Zehong Chen, Yijie Hu, Hao Zhuo, Linxiang Liu, Shuangshuang Jing, Linxin Zhong, Xinwen Peng, and Run-cang Sun Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b00259 • Publication Date (Web): 16 Apr 2019 Downloaded from http://pubs.acs.org on April 17, 2019

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Chemistry of Materials

Compressible,

elastic,

and

pressure-sensitive

carbon

aerogel

derived

from

2D titanium carbide nanosheets and bacterial cellulose for wearable sensors

Zehong Chen a,‖, Yijie Hu a,‖, Hao Zhuo a, Linxiang Liu a, Shuangshuang Jing a, Linxin Zhong a,*,

a

Xinwen Peng a,*, and Run-cang Sun b

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

Guangzhou, 510641, China. b

Centre for Lignocellulose Science and Engineering, College of Light Industry and Chemical

Engineering, Dalian Polytechnic University, Dalian 116034, China. *

Corresponding

authors.

E-mail:

[email protected]

[email protected] (X. Peng)

‖ The

two authors contributed equally to this paper.

1 ACS Paragon Plus Environment

(L.

Zhong)

and

Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Compressible and elastic carbon aerogels (CECAs) hold great promise for applications in wearable electronics and electronic skins. MXenes, as new two-dimensional materials with extraordinary properties, are promising materials for piezoresistive sensors. However, the lack of sufficient interaction among MXene nanosheets makes them difficult to fabricate CECAs. Herein, a lightweight CECA is fabricated by using bacterial cellulose fiber (BC) as a nano binder to connect MXene (Ti3C2) nanosheets into continuous and wave-shaped lamellas. The lamellas are highly flexible and elastic, and the oriented alignment of these lamellas results in a CECA with super compressibility and elasticity. Its ultrahigh structural stability can withstand an extremely high strain of 99% for more than 100 cycles and long-term compression at 50% strain for at least 100 000 cycles. Furthermore, it has a high sensitivity that demonstrates not only an ultrahigh linearity but also a broad working pressure range (010 kPa). Especially, the CECA has a high linear sensitivity in almost the whole workable strain range (0-95%). In addition, it has very low detection limits for tiny stain and pressure. These features enable the CECA-based sensor as a flexible wearable device to monitor both subtle and large biosignals of human body.


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Introduction CECAs possess chemical and thermal stability, excellent conductivity, shape memory, and can transfer external pressure or strain into current signal, thus attracting great interest in the applications in wearable pressure- or strain-sensing electronics and electronic skins.1-4 In the past five years, a series of CECAs are fabricated from carbon nanotube (CNT),5-7 graphene or graphene oxide (GO),8-13 and their composites14-16 via solvent thermal9,10 or hydrothermal treatment,11,15 freeze-casting,8,12,13 and chemical vapor deposition (CVD).7,14,16 For example, a compressible and elastic CNT foam was fabricated by CVD method and could be compressed for 1000 cycles with high stress retention.7 Qiu et al.8 described an ultralight cellular monolith from partially reduced graphene oxide (rGO) by freeze casting, and the monolith could rapidly recover from 80% compression. Gao et al.15 fabricated a compressible and elastic CNT/GO carbon aerogel by polydopamine crosslinking and reducing. The aerogel could maintain 90% height after 100 compression cycles at 80% strain. However, owing to the brittleness feature of carbon, the present CECA-based sensing materials generally suffer from low compressibility, elasticity, and poor fatigue resistance. Furthermore, from an application point of view, wide-range linear sensitivity is very important and highly demanded for pressure sensors to capture accurate signals within broad pressure range. Although Gao et al.12 and Zhuo et al.17 reported CECAs with high mechanical performance and/or ultra-low detection limits for strain and pressure, respectively, high linear sensitivity has not been achieved. Therefore, the major problems of the present CECAs for piezoresistive sensors are (1) low linear sensitivity and (2) narrow sensing pressure range. Fabricating CECAs simultaneously possessing super mechanical performance and high linear sensitivity within a broad pressure or strain range remains a great challenge currently. MXenes are new groups of two-dimensional (2D) materials with chemical formula of Mn+1XnTX, where M is an early transition metal such as Ti, V, Nb, X represents carbon and/or nitrogen, and Tx stands for various surface terminations. MXenes exhibit large specific 3 ACS Paragon Plus Environment


surface area, high electrical conductivity, favorable mechanical strength, and thus hold promising applications in separation,18 energy storage,19-20 sensor,21 electrochemical catalysis,22,23 and electromagnetic interference shielding.24 Some stretchable materials, such as MXene/CNT composite25 and MXene multilayers,26 have been obtained from MXene nanosheets for strain or bending angle sensing. However, the weak interaction among MXene sheets makes it difficult to assemble them into compressible and elastic macrostructure for piezoresistive sensors. To solve this issue, Yue et al.27 used polymer sponge as a flexible substrate to fabricate a compressible composite via dipping-coating process. However, the thermal and chemical instability of polymer restricts its application in some areas. By wrapping MXene inside the GO aerogel, Ma et al.28 prepared a compressible MXene/rGO aerogel with a high sensitivity and fast response time. Nevertheless, high compressibility, excellent fatigue resistance, wide-range linear sensitivity, and low detect limits are not achieved for the present CECA. Herein, a lightweight CECA with both exceptionally high mechanical and sensing performances is fabricated by using BC as a nano binder to join MXene (Ti3C2) nanosheets into a lamellar macrostructure. BC has high aspect ratio and flexibility, and can easily entangle with each other to connect MXene nanosheets together. The sufficient connect strength among Ti3C2 nanosheets gives rise to continuous, oriented, and wave-shaped lamellas, which present high flexibility and elasticity, as well as fatigue resistance. Moreover, the CECA is a highly sensitive material that can not only accurately record the output signals at extremely wide-range pressure and strain, but also capture tiny pressure changes. These advantages allow the CECA have potential applications in flexible wearable devices for detecting biosignals. Results and discussion Fabrication and characterization of carbon aerogel. As shown in Figure 1a, C-MX/BCx (C, MX, BC, and x represent carbonization, Ti3C2, bacterial cellulose, and the mass ratio of 4 ACS Paragon Plus Environment

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BC to Ti3C2, respectively) aerogel is fabricated by the mixing of Ti3C2 and BC, directional freezing, freeze-drying, and carbonization (Experimental Section). For comparison, pure Ti3C2 aerogel (A-MX), carbonized Ti3C2 aerogel (C-MX) at 700 °C, pure bacterial cellulose aerogel (A-BC), and carbonized bacterial cellulose aerogel (C-BC) at 700 °C are synthesized by the same method. Ti3C2, prepared by selectively etching out the “Al” layer from Ti3AlC2 using HCl/LiF mixing solution, is full of abundant hydroxyl and F element.29 After exfoliation, the colloid solution of Ti3C2 nanosheets reveals a Tyndall scattering effect, indicating good dispersion and nanometer size (Figure S1a). The exfoliated Ti3C2 nanosheets are very thin, as shown in SEM and AFM images (Figure 1b and 1c). As indicated from AFM image (Figure 1c), Ti3C2 nanosheets have a thickness of about 1.3 nm, which is very close to the theoretical thickness of Ti3C2 single layer (~1 nm).30-32 Considering that the insufficient interaction among Ti3C2 nanosheets will lead to weak macrostructure, two key points are proposed to fabricate MXene-based carbon aerogel: (1) connecting Ti3C2 nanosheets into large layers that can efficiently transfer stress and thus improve the mechanical strength of aerogel, and (2) forming a continuous and lamellar structure to guarantee satisfactory compression and sensing performances. AFM images reveal that BC possesses a diameter of ~50 nm and a length of more than 5 μm, as shown in Figure 1d and Figure S2. It can link Ti3C2 nanosheets together to form continuous layers, and thus producing a lamellar structure after directional freezing-casting and freeze-drying (Figure 1a).



Figure 1. Fabrication of C-MX/BC-x carbon aerogel and the morphologies of Ti3C2 nanosheets, aerogels and carbon aerogels. (a) Illustration of fabricating C-MX/BC-x carbon aerogel. (b) SEM image of Ti3C2 sheets on anodic aluminum oxide (AAO). AFM images of (c) Ti3C2 nanosheets and (d) BC. SEM images of (e) pure Ti3C2 aerogel (A-MX), (f) carbonized Ti3C2 aerogel (C-MX), (g) pure BC aerogel (A-BC), (h) BC-derived carbon aerogel (C-BC), (i) 6 ACS Paragon Plus Environment

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Ti3C2/BC aerogel (A-MX/BC-2), and (j) carbonized Ti3C2/BC aerogel (C-MX/BC-2), respectively. Ti3C2/BC aerogel (A-MX/BC-x) was prepared by a facile directional freezing method. As shown in Figure S3a and S3b, pure Ti3C2 aerogel (A-MX) has a ragged surface and its carbon aerogel also shows rough surface and tends to dust. In a sharp contrast, pure BC aerogel (ABC), Ti3C2/BC aerogel (A-MX/BC-2 as an example) and their carbon aerogels possess a smooth surface without dusting (Figure S3c-S3f). The pure Ti3C2 aerogel (A-MX) and carbonized Ti3C2 aerogel (C-MX) are composed of incontinuous, disordered, and small fragments (Figure 1e and 1f); while pure BC aerogel (A-BC) and carbon aerogel (C-BC) exhibit a disordered structure with large and highly kinky layers (Figure 1g and 1h). AMX/BC aerogel (A-MX/BC-2 as an example), however, consists of parallel and continuous lamellas (Figure 1i), and this structure can be well reserved after annealing (Figure 1j). The lack of connecting strength among Ti3C2 nanosheets is expected to gives rise to an incontinuous and disordered structure of Ti3C2 aerogel and its carbon aerogel. However, the high aspect ratio and high flexibility of BC result in the easy entanglement among them, and thus lead to a continuous but disordered microstructure. Therefore, BC facilitates continuous assembly of Ti3C2 nanosheets, while Ti3C2 nanosheets can induce the parallel alignment of BC layers owing to their 2D sheet feature. The combination of Ti3C2 and BC gives rise to a continuous and oriented lamellar architecture. During the directional freeze-casting, BC and MXene nanosheets are expelled by ice crystals (Figure S4), and the entanglement of BC forms a interconnected network (Figure S5a and S5b) to link MXene nanosheets together, thus resulting in continuous layers (Figure 1a). The AFM image of A-MX/BC-2 reveals that MXene nanosheets are connected by embedding in the BC network (Figure S5c). After carbonization, BC nanofibers in C-MX/BC-2 are still recognizable (Figure S5d), while the embedded MXene nanosheets are not as clear as those in C-MX/BC-2, indicating that structure change occurs to MXene. Therefore, after carbonization, BC and MXene still 7 ACS Paragon Plus Environment


connect with each other except for the carbonization of BC and MXene. Moreover, Elemental mapping of C-MX/BC-2 suggests a humongous distribution of C, Ti, and O, indicating the good dispersion of carbonized Ti3C2 nanosheets (Figure S6). It has been widely reported that strong hydrogen bonds form between atoms H and O or F, which has higher electro negativity than hydrogen (H).33 Many previous works reported that strong hydrogen bonds form among BC nanofibers in BC-based hydrogel,34 aerogel,35 nanofibers36 or film37. Such hydrogen bonds can stabilize the structure and ensure the strength of these materials. It is also reported that MXene nanosheets can form hydrogen bonds with other macromolecules with hydroxyl groups, such as polyvinyl alcohol38 and cellulose39. Therefore, hydrogen bonds can form between MXene and BC owing to their abundant H, O, or F elements. Moreover, the FTIR spectra of A-BC and A-MX/BC-2 were recorded to investigate the possible hydrogen bonds between BC and MXene or among BC nanofibers. As shown in Figure S7, the peak at 3344 cm-1 of A-BC spectrum is attributed to stretch vibrations of hydroxyl groups in cellulose.40,41 As for the spectrum of A-MX/BC-2, the narrowed peak at 3344 cm-1 and lower intensity imply that the hydroxyl groups in BC are disturbed, indicating the formation of hydrogen bonds between MXene and BC or among BC nanofibers.37,40 These hydrogen bonds favor the formation of ordered wavy lamellas and ensure the structural stability of aerogel and carbon aerogel. By contrast, RC-MX/BC-2 fabricated by random freeze-casting reveals a disordered structure, indicating that the directional freeze-casting plays an important role in forming the ordered lamellar structure (Figure S8). As illustrated in Figure S4, the ice crystals mainly grow from left to right because of the temperature gradient during freeze-casting. The directionally growing ice expels MXene/BC suspension, and thus results in ordered lamellar structure. It is also observed that A-MX/BC-2 has relatively flat lamellas with smooth surface (Figure 1i, and Figure S9a-S9c), while C-MX/BC-2 contains wave-shaped lamellas (Figure 1j and Figure S9d-S9f), which may result from the volume shrinkage of BC at annealing. During carbonization, the volume decrease of BC is severer than that of MXene (Figure S3) because 8 ACS Paragon Plus Environment

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of the higher weight loss of BC (Figure S10). Therefore, the flat lamellas are crumpled into wave-shaped architecture. A-MX/BC-2 carbonized at different temperatures (300, 500, and 700 °C) also suggest that the flat lamellas become wave-shaped gradually with the increase of temperature (Figure S11). According to the above analysis, the formation of the wave-shaped lamellas structure can be ascribed to the connecting effect of BC, directional freeze-casting strategy, and the carbonization process. XRD patterns and Raman spectra of the as-prepared samples are shown in Figure S12. The XRD spectrum of pure Ti3C2 aerogel (A-MX) reveals a strong peak at 2θ=5.9°, assigned to the (002) plane of Ti3C2.29 After pyrolysis, the peak of Ti3C2 disappears and the characteristic peaks of TiO2 are clearly observed in the patterns of C-MX and C-MX/BC-2 (rutile and anatase are marked with green and orange asterisks, respectively), confirming the formation of TiO2.42 The broad and low-density peak at 2θ=24.3° corresponding to the (002) graphitic plane indicates the partial graphitization structure in C-MX, C-BC, and C-MX/BC-2, suggesting that both Ti3C2 and BC are pyrolyzed to low-degree graphitic carbon (Figure S12a). As shown in Figure S12b, the Raman spectrum of A-MX reveals a sharp peak at 154 cm-1, corresponding to the Eg(1) of anatase vibrational modes.43 And after carbonization, two additional broad peaks at 428 and 610 cm-1, representing B1g and Eg(3) of anatase, respectively, are observed for C-MX.43 All carbon aerogels display two peaks at 1340 and 1598 cm-1 assigned to the disordered (D) and graphitic (G) bands, respectively, again indicates the partial graphitization structure in these samples.



Figure 2. Compressibility and elastic mechanisms of the as-prepared carbon aerogels. Stress-strain curves of (a) C-MX, (b) C-BC, and (c) C-MX/BC-2. Elastic mechanisms of (d) C-MX, (e) C-BC, and (f) C-MX/BC-2. Super compressibility, elasticity, and fatigue resistance of carbon aerogel. Figure S13 and Figure 2 demonstrate the compressibility and elasticity of the as-prepared aerogels and their carbon aerogels. Before carbonization, A-BC, A-MX, and A-MX/BC aerogels (Figure S13a-S13c) display a significant loss of height upon compression, demonstrating remarkable plastic deformation. After carbonization, C-MX remains no elasticity and collapses for only one compression cycle (Figure 2a and Figure S13d), owing to the weak interconnection among carbonized MXenes. Although C-BC can withstand 500 compression cycles, remarkable structural destruction is observed (Figure 2b and Figure S13e), with a stress retention of only 58.3% at 50% strain. However, C-MX/BC-x (C-MX/BC-2 as an example) can maintain its original shape without significant structural deformation (Figure 2c and Figure S13f), with extremely high height retention (98.4%) and stress retention (94.5%), suggesting remarkably improved compressibility and elasticity. Additionally, C-MX/BC-2 10 ACS Paragon Plus Environment

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can also be bended in a large angle and then recover its original shape (Figure S13g). By contrast, RC-MX/BC-2 prepared via random freeze-casting reveals a distinct collapse with a stress retention of only 68.1% (100 cycles at 50% strain, Figure S14), indicating a poor mechanical performance. These results demonstrate that C-MX/BC-2 has a stable structure, which can be ascribed to the continuous wave-shaped layers. The compressible and elastic mechanisms of C-MX, C-BC, and C-MX/BC-2 are displayed in Figure 2d-2f, respectively. CMX is composed of incontinuous fragments that tend to collapse when pressure is applied, because the stress cannot transfer among these incontinuous fragments (Figure 2d). For C-BC, the carbon layers are highly crimp and disordered, and the stress transfer is not efficient and stress concentration easily occurs, producing cracks during repeated compression (Figure 2e). On the contrary, C-MX/BC-2, with the connect of BC, has a lamellar structure with continuous and parallel wave-shaped carbon layers that can efficiently transfer stress and undergo a relatively small geometric deformation at high compression strain, without yielding structural plastic deformation or damage (Figure 2f). Therefore, the lamellar structure ensures high compressibility and elasticity. Furthermore, to investigate the effect of cellulose raw materials on the mechanical performance of carbon aerogel, C-MX/MC-2, C-MX/MFC-2, C-MX/CNC-2 and CMX/CNF-2 were synthesized by replacing the BC with methylcellulose (MC), microfiber cellulose (MFC), cellulose nanocrystals (CNC), and cellulose nanofiber (CNF), respectively. As shown in Figure S15, the stress retentions at 50% strain of C-MX/MC-2, C-MX/MFC-2, C-MX/CNC-2, and C-MX/CNF-2 are 63.2% (5 cycles), 63.0% (100 cycles), 80.9% (500 cycles), and 85.5% (500 cycles), respectively, which are lower than that of C-MX/BC-2 (94.5%, 500 cycles). MFC has micro size (10-30 μm in diameter, Figure S16a), which has lower surface specific area to connect MXene as compared with, BC, CNC, and CNF. BC is much longer (>5 μm) than CNC (~500 nm) and CNF (~1 μm) (Figure S16b-S16d), resulting



in easier entanglement among BC fibers to connect MXene nanosheets together. Therefore, BC is preferred to prepare aerogel with high compressibility.

Figure 3. SEM images and compressibility of C-MX/BC-x with different Ti3C2 contents. SEM images of cross sections from (a) C-MX/BC-10, (b) C-MX/BC-5, (c) C-MX/BC-2, and (d) C-MX/BC-1. Stress-strain curves of (e) C-MX/BC-10, (f) C-MX/BC-5, (g) C-MX/BC-2, and (h) C-MX/BC-1. Since Ti3C2 nanosheets play an important role in inducing oriented wave-shaped lamellar structure, the characteristic morphologies and mechanical performances of C-MX/BC-x 12 ACS Paragon Plus Environment

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prepared with different Ti3C2 contents (mass ratio of BC to Ti3C2 = 10:1, 5:1, 2:1, and 1:1) are investigated, as shown in Figure S17 and Figure 3. All A-MX/BC-x aerogels exhibit regular shapes (Figure S17a-S17d); annealing leads to significant volume shrinkage (Figure S17eS17h). C-MX/BC-1 with the highest Ti3C2 content remains the largest volume (Figure S17d and S17h). C-MX/BC-10 with the lowest Ti3C2 content exhibits the least ordered structure with randomly aligned carbon layers (Figure 3a), which is similar to the morphology of C-BC (Figure 1h). With the increase of Ti3C2 content, carbon aerogels (C-MX/BC-5 and C-MX/BC2) display highly ordered wave-shaped lamellas (Figure 3b and 3c). When Ti3C2 content is further increased to 50% (C-MX/BC-1), the lamellar structure becomes compact, and is composed of relatively flat carbon layers (Figure 3d). These results demonstrate that Ti3C2 nanosheets facilitate the formation of highly-oriented lamellas, while BC favors the wave shape. The combination of Ti3C2 and BC with desirable ratio can effectively control the lamellar structure of carbon aerogel, and thus is expected to tailor its mechanical performances. The density of carbon aerogel significantly increases from 13.9 to 34.9 mg cm3 as

the ratio of BC to Ti3C2 decreases from 10:1 to 1:1 (Table S1).

The mechanical performances of C-MX/BC-x are shown in Figure 3e-3h, Table S2 and S3. After 500 compression cycles at 50% strain, C-MX/BC-10 with the least Ti3C2 content has stress and height retentions of 75.3% and 88.6%, respectively (Figure 3e), while C-MX/BC-5 displays higher stress and height retentions of 88.2% and 95.5%, respectively (Figure 3f). The unstable structure and relatively poor elastic strength of C-MX/BC-10 can be mainly attributed to the inefficient stress transfer because of its disordered structure. Impressively, CMX/BC-2 can almost maintain its original height and exhibit a stress retention of high up to 94.5% after 500 cycles (Figure 3g). However, a remarkable deformation occurs to C-MX/BC1 when Ti3C2 content increases to 50%, with a height retention of only 79.4% after 500 cycles (Figure 3h). The results suggest that large amount of Ti3C2 tends to produce a fragile structure, owing to the weak link among the carbonized nanosheets. Therefore, the continuous and 13 ACS Paragon Plus Environment


wave-shaped lamellas with sufficient linking strength among Ti3C2 nanosheets are highly flexible and elastic. Their ordered alignments facilitate the efficient stress transfer along the whole lamellar structure.

Figure 4. Super compressibility and fatigue resistance of C-MX/BC-2. (a) Stress-strain curves at different compression strains. (b) Stress-strain curves at a high strain of 95% for 300 cycles. (c) Stress-strain curves at an extreme strain of 99% for 100 cycles. (d) Stress-strain curves at 50% strain for 100 000 cycles. (e) Comparison of stress retentions of C-MX/BC-2 with those of other materials reported in literature. (f) Stress-strain curves before and after being compressed continuously for 15 days at 50% strain. SEM images of C-MX/BC-2 after (g and h) 5 000 and (i and j) 100 000 cycles.

The compressibility and fatigue resistance of C-MX/BC-2 are further investigated since it has a better mechanical performance than those of other samples (Figure 4). The stress-strain curves of C-MX/BC-2 at 10%-90% strain are crescent and steepen as the increase of strain (Figure 4a). C-MX/BC-2 can undergo a very high strain of 95% with a stress of 10.3 kPa 14 ACS Paragon Plus Environment

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(Figure 4b), and can almost recover its original height (97.1%) with similar strain-stress curves after 100 cycles. An excellent reversible compression can be maintained after 300 cycles at 95% strain. Even under an extreme strain of 99% for 100 cycles, C-MX/BC-2 remains a height retention of 90.6% (Figure 4c and Movie S1), indicating a superior compressibility and elasticity. As shown in Figure 4d, C-MX/BC-2 displays an outstanding fatigue resistance with a stress retention of 90.9% after 5 000 cycles at 50% strain. It can undergo a long-term compression for high up to 100 000 cycles, with 93.3% height retention and 73.6% stress retention (Figure 4d). As shown in Table S4 and Figure 4e, the outstanding mechanical performances of C-MX/BC-2 are superior to those of other compressible carbon materials, including graphene aerogels,17,44-46 GO/CNT composite aerogels,47 carbonaceous nanofibrous aerogels,48 and so on49-52. As shown in Figure 4g and 4h, no damage occurs to the wave-shaped lamellas after 5 000 compression cycles, and even after 100 000 cycles, no significant fracture can be observed for the continuous and ordered lamellar structure (Figure 4i and 4j), further indicating superior elasticity and flexibility of the wave-shaped lamellas. When being compressed continuously for 15 days at 50% strain, C-MX/BC-2 can almost recover its original height without deformation (Figure 4f). The stress-strain curve after compression for 15 days is similar to that of the original one, with a stress retention of 94.3%.



Figure 5. Strain- and pressure-current responses and sensitivity of C-MX/BC-2. (a) Current response to different compression strains. (b) Current response to pressure ranging 16 ACS Paragon Plus Environment

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from 20 Pa to 1500 Pa. (c) Current stability for 1000 cycles at 50% strain. (d) The first and last five current cycles at 50% strain obtained from figure (c). (e) R/R0 at 50% strain for 1000 cycles. (f) The linear sensitivity at a wide pressure range of 0-10 kPa. (g) Comparison of sensitivity of C-MX/BC-2 with those of other sensing materials. (h) Current response to water drops. (i) Gauge factor at strain of less than 6.8%. (j) Current responses to different bending angles. The electrical performances of carbon aerogel. To investigate the sensing performances of C-MX/BC-2, a series of electrical response behaviors are carried out, as shown in Figure 5. The conductivities of the as-prepared samples are shown in Table S5. A-MX has a high conductivity, while C-MX shows a relatively low conductivity, suggesting that the change in the structure of MXene during annealing reduces the conductivity. A-BC and A-MX/BC-2 show extremely low conductivities, because of the dielectric BC. However, after annealing, conductivities of C-BC and C-MX/BC-2 significantly increase, which is due to the carbonization of BC. The carbonization of BC results in a high conductivity of C-BC because of the formation of conductive carbon. However, the conductivity of C-MX/BC-2 is lower than that of C-BC, which can be attributed to their difference in morphology and the structural change of MXene. During annealing, the structural change of MXene (formation of TiO2, Figure S12) reduces the conductivity of MXene53-55, and thus results in a relatively low conductivity for C-MX/BC. Moreover, the conductivity of the carbon aerogel highly depends on the contact area among the conductive carbon lamellas. A more compact structure can result in higher contact area among lamellas, and therefore a higher conductivity. C-BC exhibits a disordered structure with relatively high contact area among lamellas (Figure 1h), which contributes to its high conductivity. C-MX/BC-2 (Figure 1j), however, reveals an ordered and wave-shaped structure with large space among carbon lamellas, which may give rise to a high resistance. Figure 5a displays the current responses of C-MX/BC-2 to different 17 ACS Paragon Plus Environment


compression strains. The current rapidly rises and declines at compression and release, respectively, suggesting that the real-time current is highly sensitive to compression strain. It is observed that the current intensity increases as strain increasing from 10% to 90%, indicating that C-MX/BC-2 can distinguish different levels of compression strains with a wide working range. Since the conductivity of the carbon aerogel highly depends on the contact area, when applying a pressure, the space among lamellas reduces and the contact area among lamellas increases (Figure S18), resulting in a corresponding decrease in contact resistance. Figure 5b shows the current change upon various pressures. The current intensity proportionally increases with pressure, demonstrating an excellent pressure-current response behavior. To evaluate the current response stability, we conducted 1000 cycles of compression (50% strain) on C-MX/BC-2. As shown in Figure 5c and 5d, the current shows no significant change and maintains nearly identical amplitude after 1000 cycles, revealing a stable current responding performance of the lamellar structure. The increase in current within the initial 300 cycles may result from the slight reduce in the height of C-MX/BC-2 (98.4% height retention within the initial 500 compression cycles, Table S3). The height decrease can give rise to a minor structural change (a more compact structure), and thus increase in current. The sight decrease in current after 300 cycles may result from the fracture of some link points among the carbon lamellas during cyclic compression. On the whole, this fracture is not noticeable in terms of the excellent fatigue resistance and structural stability of the aerogel (Figure 4). Figure 5e shows the normalized electrical resistance (R/R0) of C-MX/BC-2. It is found that the R/R0 linearly drops at the strain of less than 15%, suggesting a rapid increase in contact area among carbon lamellas. Furthermore, R/R0 curve maintains the similar shape even after 1000 cycles, indicating a very stable R/R0, which well agrees with the result in Figure 5c. Therefore, C-MX/BC-2 has a highly stable conductivity during cycling compression, which can be due to the flexible and elastic carbon layers.


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A high linear relationship between ΔI/I0 and pressure in a wide pressure range is extremely important for pressure sensor to capture accurate information from output signal. However, up to now, maintaining a high linear relationship at wide pressure range is still a great challenge for the present carbon-based pressure sensors.28,56-59 Sensitivity (S) can be defined as S = δ(ΔI/I0)/δP

(1)

where ΔI is the relative change in current, I0 is the initial current without loading, and δP is the change of the applied pressure. As shown in Figure 5f, C-MX/BC-2 has a high linear sensitivity of 12.5 kPa-1 at a wide pressure ranging from 0 to 10 kPa, with a very high linearity (R2= 0.999), indicating not only an ultrahigh linear relationship between the loading pressure and electrical signal, but also a wide working pressure range. Furthermore, a pressure of 10 kPa relates to ~95% strain (Figure 4b), which means that the carbon aerogel can obtain accurate output signal from an extremely wide compression strain of 0-95%. The high and wide-range linear sensitivity not only surpasses to those of the present carbon aerogels,28,56,57,60 but also higher than those of flexible substrate-loaded carbon,58,59,61 and carbon films62-64 (Figure 5g). Although some carbon-based sensors can work at wide-range pressure (0-10 kPa), their sensitivities are extremely low, for example, 0.03 kPa-1 (3-10 kPa) for a polyurethane sponge61 and 0.92 kPa-1 (2-10 kPa) for a graphene film63. The high sensitivity and excellent linearity within a broad pressure range can be attributed to: (1) the highly compressible lamellar structure ensures a wide-range pressure or strain, and (2) the flexible and stable wave-shaped carbon lamellas is supposed to produce a proportional increase in contact area with applied pressure or strain. Therefore, it is the unique architecture that results in a linear sensitivity within large pressure or strain range. The detection limit for pressure, was measured by dropping tiny water droplet on the sensor (the inset in Figure 5h), and each water drop equates to 1.0 Pa. As shown in Figure 5h, the current increases with adding water drops, and the detection limit is low to 1.0 Pa, allowing C-MX/BC-2 a sensitive material to detect tiny pressure. 19 ACS Paragon Plus Environment


Gauge factor (GF), as an essential parameter of sensitivity for strain, can be defined as GF= (ΔR/R0)/ε

(2)

where ΔR is the relative change in resistance, R0 is the initial resistance without applied strain, and ε is the applied stain. As shown in Figure 5i, C-MX/BC-2 has a good linear relationship between ΔR/R0 and strain (R2= 0.996, strain < 6.8 %) with a high GF of 6.14, and it can detect a tiny strain of as low as 0.5%. Therefore, C-MX/BC-2 is a highly sensitive material that can not only accurately record the output signals at wide-range pressure or strain, but also capture the tiny change in pressure and strain. In addition, as shown in Figure 5j, the sensor can be bended in large angle, and the real-time current intensity increases proportionally with the bending angle. These results demonstrate that C-MX/BC-2 is a multifunctional material for deformation detect, making it an ideal alternative for sensor applications.

Figure 6. Applications of C-MX/BC-2 for biosignal detection. (a) Assembly of a sensor. (b) Current signals from speaking “carbon” and “super”. (c) Current signals from face expressions. (d) Detection of jugular venous pulse of humans. (e) Detection of human arm 20 ACS Paragon Plus Environment

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pulse. Currents signals from (f) finger, (g) elbow, and (h) wrist bending. (i) Responsive and recovery time.

Applications of carbon aerogel for wearable devices. Considering the outstanding mechanical and strain- or pressure-current responding performances, C-MX/BC-2 was used as a wearable device by sandwiching a carbon aerogel between two pieces of PET substrates adhered with Ni electrodes, as shown in Figure 6a. When attaching the sensor to human throat, it can record the current changes from speaking different words, such as “carbon” and “super” (Figure 6b). The sensor can also detect the expressions of human face, such as puffing and smiling, as shown in Figure 6c. Furthermore, the sensor can monitor the jugular venous pulse of humans (Figure 6d). When the sensor is attached to a human arm (Figure 6e), the pulse fluctuant signal is regular and obvious with an interval of about 0.88 s. Two characteristic peaks representing “percussion” (P) and “diastolic” (D)28 (the inset of Figure 6e) can be detected clearly, indicating its high sensitivity. In addition, the sensor can be also used to detect human joint movements, such as the bending of finger, elbow and wrist, as shown in Figure 6f-6h. With the increasing angle caused by bending finger, the current increases (Figure 6f). The current rises rapidly when elbow bending (Figure 6g). Repeatable signals can be obtained from wrist bending (Figure 6h). The sensor shows rapid response (167 ms) and recovery (121 ms) abilities (Figure 6i), allowing it a sensitive sensor to meet the practical application. As shown in Figure S19 and Movie S2, the sensor can be used as a piezoelectric regulator to continuously control the brightness of LEDs by applying different pressures. Therefore, the superior mechanical performance, ultrahigh and wide-range linear sensitivity, as well as low detect limit, make the carbon aerogel have promising applications in wearable electronic devices.

Conclusion 21 ACS Paragon Plus Environment


A CECA is successfully fabricated from Ti3C2 nanosheets by using BC as a binder. Ti3C2 nanosheets and BC play important roles in the formation of order microstructure and continuous texture, respectively, and thus realizing structural engineering. As compared with the present CECAs, our carbon aerogel exhibits the following advantages because of the untra-stable lamellar structure containing continuous and oriented wave-shaped carbon layers: (1) ultrahigh compressibility and superelasticity; (2) high sensitivity and linearity; (3) wide linear range; and (4) low detection limits for pressure and strain. These superiorities make the carbon aerogel a promising candidate for piezoresistive sensors and wearable devices. Our innovative approach will be very helpful to design outstanding CECAs from various 2D nano blocks.

Experimental Section Materials. Ti3AlC2 power was purchased from Beijing Kaifatetao Technology Co., Ltd. HCl and LiF were purchased from Aladdin. Bacterial cellulose (with TEMPO treatment) suspension (BC, 0.5 wt%) was supplied kindly by Guilin Qihong Technology Co., Ltd, China. Fabrication of exfoliated MXene nanosheets. MXene (Ti3C2) was obtained by selectively etching the Al from Ti3AlC2 using HCl and LiF. Typically, 1.0 g Ti3AlC2 was added into 20 mL HCl (6 M) containing 1.0 g LiF. And after magnetic stir at 35 °C for 24 h, the resultant product was washed by deionized water and centrifuged at 4000 rpm for several times until the pH of the supernatant >6. The above clay-like Ti3C2 flakes were then dispersed in deionized water and ultra-sonicated for 3 h to obtain exfoliated Ti3C2 nanosheets. And the unexfoliated MXene was removed by centrifugation at 5000 rpm for 15 min. Then the exfoliated Ti3C2 suspension was freeze-dried in a lyophilizer (-58 °C, 0.22 mbar). Fabrication of aerogels. For the fabrication of aerogel, 50 mg exfoliated Ti3C2 nanosheets was added into 20 g BC suspension (0.5 wt%, 100 mg BC), and the mixture was stirred for 10 min and then ultra-sonicated for 60 min to make sure the homogeneous mixing of BC and 22 ACS Paragon Plus Environment

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Ti3C2. Then, a certain amount of the above mixture (13 mL) was placed in a plastic box (38 mm * 28 mm * 18 mm) that was tied to an open lidless steel box. With liquid nitrogen inside the lidless steel box, ice nucleus grew along the horizontal direction induced by the temperature gradient. After completely freeze-casting, the frozen box was freeze-dried in a lyophilizer (-58 °C, 0.22 mbar) for 60 h to obtain A-MX/BC-2 aerogel (A, MX, BC, and 2 represents aerogel, Ti3C2, bacterial cellulose, and the mass ratio of BC to Ti3C2, respectively). A-MX/BC-10, A-MX/BC-5, and A-MX/BC-1 with different mass ratios of BC to MXene (10:1, 5:1, and 1:1, respectively) were prepared by the same method. For comparison, RMX/BC-2 prepared via random freeze-casting was also obtained by immersing a plastic box containing Ti3C2/BC suspension (BC to Ti3C2=2:1) in liquid nitrogen, and then freeze-drying. To investigate the effect of cellulose morphology on the mechanical strength of carbon aerogel, C-MX/MC-2, C-MX/MFC-2, C-MX/CNC-2, and C-MX/CNF-2 were synthesized by replacing BC with methylcellulose (MC), microfiber cellulose (MFC), cellulose nanocrystal (CNC), and cellulose nanofiber (CNF), respectively. Pure Ti3C2 and pure BC aerogels, named as A-MX and A-BC, were also fabricated from Ti3C2 and BC suspension by the same method. Fabrication of carbon aerogels. The carbon aerogels were obtained by annealing the aerogels in a tube furnace under flowing Ar. A-MX/BC-x (x=10, 5, 2, and 1), RA-MX/BC-2, A-MX/MC-2, A-MX/MFC-2, A-MX/CNC-2, A-MX/CNF-2, A-MX, and A-BC aerogels were heated to 700 °C at a heating rate of 3 °C min-1 and kept at 700 °C for 2 h to obtain carbon aerogels, named as C-MX/BC-x (x=10, 5, 2, and 1), RC-MX/BC-2, C-MX/MC-2, CMX/MFC-2, C-MX/CNC-2, C-MX/CNF-2, C-MX, and C-BC, respectively. Characterizations and Measurements. The morphologies of all samples were observed by scanning electron microscopy (SEM) combined with EDS mapping (Merlin, Zeiss) and transmission electron microscopy (TEM, JEM-2100F) at an acceleration voltage of 200 kV. The atomic force microscopy (AFM) images were conducted by a Bruker Multi Mode 8 scanning probemicroscope (SPM, VEECO) in tapping mode. X-ray diffraction (XRD) 23 ACS Paragon Plus Environment


patterns were conducted by a Bruker D8 diffractometer using Cu-Kɑ radiation as an X-ray source. Raman spectra were recorded on a Raman spectrometer (LabRAM ARAMIS-Horiba Jobin Yvon) operating with 532 nm excitation. Infrared (IR) data were recorded on a Fourier transform IR spectrometer (VERTEX 70, Bruker Corp., Germany). Thermal gravity analysis (TGA) was recorded on a simultaneous thermal analyzer (Pyris Diamond TG/DSC-200, US) from room temperature to 700 °C with a heating rate of 3 °C min-1 in a nitrogen atmosphere. Compression, elasticity, and cycling tests were performed on a compressive instrument (Instron 5565). The electrical current was measured by electrochemical workstation (CHI 660E) or 2400 digital source-Meter. The resistance was recorded by a multimeter (VC 890D). Assembly and sensing test. C-MX/BC-x-based sensor was fabricated by embedding a carbon aerogel in two pieces of poly (ethylene terephthalate) (PET) substrates adhered with Ni sheets. The strain and loading pressure were conducted by Instron 5565. The real-time current and resistance were recorded by electrochemical workstation (CHI 660E, applying a voltage of 1 V) and multimeter (VC 890D), respectively. The current-stress response in low pressure region was recorded by dropping tiny water droplet on the surface of the assembled sensor. The biosignal detections were recorded by a 2400 digital source-Meter.

Acknowledgements This work is supported by Guangdong Natural Science Funds for Distinguished Young Scholar (2017A030306029 and 2016A030306027), Guangdong Special Support Program (2017TQ04Z837), Natural Science Foundation of Guangdong Province (2016A030313487), and Fundamental Research Funds for the Central Universities. Supporting Information Digital photographs of the Tyndall effect of Ti3C2 and BC suspensions; AFM image of BC; Digital photographs of A-MX, C-MX, A-BC, C-BC, A-MX/BC-2, and C-MX/BC-2; Illustration of the directional freeze-drying and annealing strategy; AFM images of A-BC, A24 ACS Paragon Plus Environment

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MX/BC-2 and C-MX/BC-2; FTIR spectra of A-BC and A-MX/BC-2; SEM image of CMX/BC-2, and EDS elemental mappings of C, O, and Ti; Digital photograph and SEM image of RC-MX/BC-2; Microstructural change of the composite Ti3C2/BC aerogel before and after annealing; Thermal gravity analysis (TGA) curves for A-BC, A-MX and A-MX/BC-2; Microstructural change of the composite Ti3C2/BC aerogel (A-MX/BC-2 as an example) with annealing; XRD and Raman patterns of the as-prepared samples; Digital photographs of the compressibility of A-MX, A-BC, A-MX/BC-2, C-MX, C-BC, and C-MX/BC-2, and the excellent bendability of C-MX/BC-2; Stress-strain curves of RC-MX/BC-2; Stress-strain curves of carbon aerogels from other celluloses; SEM image of MFC and AFM images of CNC, CNF and BC; Digital photographs of A-MX/BC-10, A-MX/BC-5, A-MX/BC-2, AMX/BC-1, C-MX/BC-10, C-MX/BC-5, C-MX/BC-2, and C-MX/BC-1; Sensing mechanism of C-MX/BC-2 and the corresponding SEM images of C-MX/BC-2 at different strains; Digital photographs of the brightness of a series of LEDs controlled by loading different pressure on C-MX/BC-2. A movie of the compression of C-MX/BC-2 at 99% strain; A movie of the brightness of a series of LEDs controlled by loading different pressure on C-MX/BC-2. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Conflict of Interest The authors declare no conflict of interest.

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The Table of Contents


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Compressible, Elastic, and Pressure-Sensitive Carbon Aerogels

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