Superhydrophobic Ultraflexible Triple-Network Graphene

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Superhydrophobic Ultraflexible Triple-Network Graphene/ Polyorganosiloxane Aerogels for High-Performance Multifunctional Temperature/Strain/Pressure Sensing Array Guoqing Zu, Kazuyoshi Kanamori, Kazuki Nakanishi, and Jia Huang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b02437 • Publication Date (Web): 26 Jul 2019 Downloaded from pubs.acs.org on July 27, 2019

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Chemistry of Materials

Superhydrophobic Ultraflexible Triple-Network Graphene/Polyorganosiloxane Aerogels for High-Performance Multifunctional Temperature/Strain/Pressure Sensing Array

Guoqing Zu, *,a Kazuyoshi Kanamori, *,b Kazuki Nakanishi, c Jia Huang a

aSchool

of Materials Science and Engineering, Tongji University, Shanghai 201804, P. R.

China bDepartment

of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa,

Sakyo-ku, Kyoto 606-8502, Japan cInstitute

of Materials and Systems for Sustainability, Nagoya University, Furo-cho, Chikusa-

ku, Nagoya 464-8601, Japan

*Corresponding authors： E-mail: [email protected] (G. Zu) [email protected] (K. Kanamori)

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ABSTRACT : Recently, many efforts have been made to develop various smart sensors. However, achieving flexible multifunctional sensors combining excellent sensing of temperature, strain, and pressure with a single material is still challenging. Here, we report unprecedented

superhydrophobic

(rGO)/polyorganosiloxane

ultraflexible

aerogels

and

reduced

graphene

high-performance

oxide

multifunctional

temperature/strain/pressure sensors based on these aerogels. GO nanosheets are first crosslinked and reduced with (3-aminopropyl)triethoxysilane (APTES) to obtain APTESmodified

rGO

aerogels,

which

polyvinylmethyldimethoxysilane

are

then

further

covalently

crosslinked

with

polymers and vinylmethyldimethoxysilane via co-

polycondensation to afford rGO/polyorganosiloxane aerogels. The resulting aerogels exhibit a coralline-like triple-network nanostructure consisting of rGO nanosheets, polyvinylpoly(methylsiloxane), and poly(vinylmethylsiloxane) that are crosslinked with each other. The aerogels combine superhydrophobicity, high compressibility, high bendability, superelasticity, excellent machinability, and temperature-, strain-, and pressure-sensitive conductivity, which is

a

combination

not

observed

with

traditional

materials.

In

addition,

an

rGO/polyorganosiloxane aerogel-based flexible multifunctional sensing array combining sensing of temperature (20−100 ℃), strain (in the wide range of 0.1−80 %), and pressure (in the wide range of 10 Pa−110 kPa) with high sensitivity and high durability against compression, bending, and humidity has been demonstrated for the first time.

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INTRODUCTION New materials for smart sensing are increasingly needed with the rapid development of artificial intelligence. Graphene, graphene oxide (GO), reduced graphene oxide (rGO), and their derivatives have drawn a lot of interest for their unique properties, such as excellent electrical conductivity and good mechanical properties, and their potential applications in sensing of temperature, pressure, humidity, chemicals, etc.1-6 Recently, three-dimensional (3-D) rGO aerogels and graphene aerogels have been widely studied for their attractive properties such as low bulk density, high porosity, and good electrical conductivity. They are recognized as promising candidates as pressure/strain sensors,7-9 adsorbents/absorbents,

10,11

catalysts,

12-14

and energy storage materials.15-17 Nonetheless, the low hydrophobicity, low elasticity, and poor bendability of traditional graphene and rGO aerogels significantly restrict their practical applications in smart sensing and flexible devices. It is reported that rGO aerogels with an anisotropic porous structure or a periodic microlattice structure show improved elasticity against compression compared to that of rGO aerogels with a random structure.8,18-22 Hybridizing graphene with carbon nanotube may afford graphene/carbon nanotube hybrid aerogels with high elasticity because of their synergistic effect.23-25 Crosslinking of rGO aerogels with a crosslinker such as glutaraldehyde, ethylenediamine, and borate is an effective method to improve the elasticity of rGO aerogels.2628

It is noteworthy that the rGO aerogels that are crosslinked with methyltriethoxysilane,

perfluorodecyltrimethoxysilane, and perfluorodecyltrichlorosilane exhibit not only improved mechanical properties but also enhanced hydrophobicity by introducing hydrophobic 3



groups.10,29,30 In addition, incorporating flexible organic polymers such as polyimide, polyacrylamide, polyvinyl alcohol, and resorcinol-formaldehyde can effectively improve the compressibility and elasticity of rGO-based aerogels.11,27,31,32 The rGO/polyimide nanocomposite aerogels also show good bending flexibility because of the flexible polyimide in rGO aerogels.32 However, to the best of our knowledge, there is no report on the preparation of rGO-based aerogels that combine superhydrophobicity, high bendability, and superelasticity. There are some reports on elastic rGO-based aerogels with pressure- or strain-sensitive conductivity, such as graphene/polyimide aerogels,32 graphene aerogels with sacrificial skeleton of melamine foam,7 rGO/MXene aerogels,9 and graphene/carbon nanotube aerogels.22,25 Some of these aerogels show good pressure or strain sensing performances and are used for the detection of the movement and configuration of robots and human motions such as wrist bending, finger bending, wrist or neck blood pulse.7,9,25 Nonetheless, there is no report on the preparation of rGO aerogel-based multifunctional sensing arrays that combine temperature, strain, and pressure sensing. Herein, we report highly flexible multifunctional rGO/polyorganosiloxane nanocomposite aerogels obtained by a novel crosslinking strategy. This strategy is based on the following three steps. First, GO nanosheets are crosslinked and reduced with (3-aminopropyl)triethoxysilane (APTES) to obtain APTES-modified rGO aerogels. Second, vinylmethyldimethoxysilane (VMDMS) is radically polymerized in the presence of a radical initiator to afford chainlike polyvinylmethyldimethoxysilane (PVMDMS) polymers. Third, the APTES-modified rGO aerogels are further covalently crosslinked with PVMDMS polymers and VMDMS via copolycondensation to afford rGO/polyorganosiloxane aerogels. The resulting nanocomposite 4


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aerogels exhibit a triple-network structure consisting of rGO nanosheets, polyvinylpoly(methylsiloxane) (PVPMS, (CH2CH(Si(CH3)O2/2))n), and poly(vinylmethylsiloxane) (PVMS, (CH2=CHSi(CH3)O2/2)n) that are crosslinked with each other. They combine low bulk density, superhydrophobicity, high compressibility, high bendability, superelasticity, excellent machinability, and temperature-, strain-, and pressure-sensitive conductivity. Moreover, we demonstrate that the rGO/polyorganosiloxane aerogel-based flexible sensing array shows multifunctionality combining excellent temperature, strain, and pressure sensing. To the best of our knowledge, this is the first time to achieve high-performance temperature, strain, and pressure sensing in a single aerogel.

RESULTS AND DISCUSSION The reaction of GO nanosheets with APTES, VMDMS, and PVMDMS during the synthesis of rGO/polyorganosiloxane aerogel is schematically presented in Figure 1a. In the first step, nanosheets of GO are supposed to be crosslinked and reduced simultaneously with APTES to afford 3-D APTES-modified rGO aerogels consisting of inter-crosslinked rGO nanosheets and APTES-derived siloxanes on the surface of the nanosheets. C−N coupling may occur by the reaction of epoxy groups on the GO nanosheets with −NH2 groups of APTES. 26,28 One indirect evidence of the crosslinks between GO nanosheets and APTES is obtained from the gelation time of the gels. After adding APTES in the GO suspension, the gelation time of the APTESmodified gel (3 h), while no gelation occurs for the APTES solution without GO suspension. In the second step, radical polymerization of VMDMS in the presence of a radical initiator (di-tert-butyl peroxide, DTBP)

5



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is supposed to form chainlike PVMDMS polymers with controlled degree of polymerization and conversion.33 Under the DTBP concentration of 1 mol %, the measured degree of polymerization and conversion of PVMDMS are 40.5 and 95 %, respectively.

33

In the third

step, co-polycondensation of PVMDMS and VMDMS would occur in the presence of a strong base catalyst (tetramethylammonium hydroxide, TMAOH), affording a flexible doubly crosslinked polyorganosiloxane network structure consisting of PVPMS and PVMS (Figure S1). At the same time, rGO aerogels are supposed to be further covalently crosslinked with PVMDMS and VMDMS via co-polycondensation of PVMDMS (or VMDMS) and APTESderived siloxanes on the surface of rGO nanosheets. Finally, the rGO/polyorganosiloxane aerogels with a triple-network structure consisting of inter-crosslinked rGO nanosheets, PVPMS, and PVMS could be obtained. The synthesis route, schematic of structure variations during preparation, and the resulting aerogels are presented in Figure 1. The preparation of the triple-network rGO/polyorganosiloxane aerogels involves several steps including APTES modification, radical polymerization of VMDMS, and copolycondensation of PVMDMS and VMDMS. In spite of this, the preparation is carried out at relatively low temperature (≤300 ℃) without using highly toxic reductant, and the gel is dried via simple freeze drying instead of supercritical drying, which will contribute to their applications.

6


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Figure 1. Synthesis route of rGO/polyorganosiloxane aerogels. (a) Synthesis route and schematics of structure variations and reaction of GO nanosheets with APTES, VMDMS, and PVMDMS during preparation. (b) Photograph of A4G. (c,d) Excellent machinability shown by shaping with a knife.

The sample names “A1”, “A2”, “A3”, and “A4” indicate the polyorganosiloxane aerogels, while “ATG” indicates the APTES-modified rGO aerogel. The sample names “A2G” and “A4G” indicate the rGO/polyorganosiloxane aerogels. In this strategy, an appropriate amount of unpolymerized VMDMS is added in addition to the radically polymerized PVMDMS polymers 7



during co-polycondensation, which is supposed to enhance the hydrophobicity and flexibility of the resulting aerogels. The chemical structure of the obtained polyorganosiloxane aerogels is investigated by FTIR spectra (Figure S2). The broad band located at ~1080 cm−1 is ascribed to the asymmetric stretching of Si−O−Si bonds, while the band located at 779 cm−1 is assigned to the asymmetric stretching of Si−C bonds.33 The bands at 1406 cm−1 and 1262 cm−1 are attributed to the deformation of C−H bonds, while the bands at 2962 cm−1, 2920 cm−1, and 2850 cm−1 are ascribed to the stretching of C−H bonds.34,35 Besides, the bands at 3056 cm−1 and 1600 cm−1 correspond to the stretching of =C−H bonds and C=C bonds in the vinyls, respectively.36 These bonds result from the PVPMS and PVMS in the polyorganosiloxane aerogels A1, A2, and A3. From the FTIR spectra we can also see that intensity of the bands corresponding to vinyl groups (3056 cm−1 and 1600 cm−1) increases with an increasing inclusion of unpolymerized VMDMS during preparation. In the case of rGO/polyorganosiloxane aerogels, all the bands that are mentioned above in the FTIR spectra of the polyorganosiloxane aerogel A2 are also observed in that of the rGO/polyorganosiloxane aerogels A2G and A4G (Figure 2a). In the FTIR spectra of the APTES-modified rGO aerogels (ATG), the bands at ~1580 cm−1 and ~1150 cm−1 are ascribed to stretching of C=C bonds of unoxidized graphitic domains (a characteristic peak of rGO) and the asymmetric stretching of Si−O−Si bonds, respectively.32 By comparing the FTIR spectra of A2, ATG, and A2G, it shows that polyorganosiloxanes consisting of PVPMS and PVMS are successfully incorporated in the resulting rGO/polyorganosiloxane aerogels. In addition, A2, A2G, and A4G exhibit only a small amount of –OH groups (Figure 2a). Besides, the X-ray diffraction (XRD) spectra of A2G and A4G exhibit a broad peak at ~23° (Figure 2b), 8


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corresponding to rGO in the rGO/polyorganosiloxane aerogels. 7,17

Figure 2. Chemical and porous structures and hydrophobicity. (a) FTIR and (b) XRD spectra of typical aerogels. (c) Nitrogen adsorption-desorption isotherms and (d) pore size distributions of typical aerogels. SEM images of the (e) polyorganosiloxane aerogel A2, (f) APTES-modified rGO aerogel ATG, and (g) rGO/polyorganosiloxane aerogel A2G. TEM images of (h) A2, (i) ATG, and (j) A2G. (k) Water contact angles of typical aerogels.

The resulting polyorganosiloxane aerogels exhibit a tunable, highly porous network structure,

9



which is confirmed by the nitrogen adsorption measurements (Figure 2c,d and Figure S3) and SEM images (Figure 2e, Figure S4 and Figure S5a). The porous structure becomes more inhomogeneous and the sizes of pores and particles become larger with a larger amount of unpolymerized VMDMS during preparation. As presented in SEM images (Figure S4a), A1 shows abundant pores with a pore size in the range of 50−700 nm, which can also be confirmed in the nitrogen adsorption isotherm and the pore size distribution (Figure S3). By contrast, as shown in SEM images (Figure 2e and Figure S4b), the pore sizes of A2 and A3 are mainly in the range of 100 nm−3 μm and 2−20 μm, respectively. Meanwhile, the sizes of particles increase from 50−150 nm for A1 to 100−300 nm for A2 and then 1.5−3 μm for A3. The reason for this structure variation may be explained as follows. The PVMDMS polymers resulting from the radical polymerization have abundant hydrolyzable alkoxy groups and give relatively high crosslinking density, which suppresses the macroscopic phase separation between the hydrophobic condensates and polar solvent during hydrolytic polycondensation. On the contrary, the unpolymerized VMDMS tends to form cyclic and chainlike species during polycondensation process, promoting macroscopic phase separation, which results in an inhomogeneous structure with larger pores and particles. Accordingly, the SSA of A1 is as high as 540 m2 g−1, while that of A2 is 72 m2 g−1 (Table 1). Because of the low concentration of the precursor, the polyorganosiloxane aerogels A1−A4 exhibit low bulk density in the range of 29−43 mg cm−3.

Table 1. Starting compositions and physical properties of typical aerogels based on polyorganosiloxane and rGO/polyorganosiloxane. 10


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sample

VMDMS a)

DTBP

VMDMS b) BzOH/Si

(1st step)

/mol%

(2nd step) /mol mol−1 /mol mol−1 /mol mol−1 /mg cm−3 /m2 g−1

/mL

H2O/Si

TMAOH/Si

ρc)

SBETd)

/mL

A1

1.0

1.0

0.5

18.3

8.0

0.083

43

540

A2

1.0

1.0

1.0

18.4

8.0

0.083

40

72

A3

1.0

1.0

2.0

14.2

8.0

0.070

35

A4

1.0

1.0

1.0

25.3

8.0

0.083

29

61

A2G

1.0

1.0

1.0

18.4

8.0

0.083

36

204

A4G

1.0

1.0

1.0

25.3

8.0

0.083

24

185

a)

The amount of VMDMS used for the preparation of PVMDMS polymers via radical

polymerization.

b)

polycondensation.

The amount of the unpolymerized VMDMS added during coc)

Bulk density.

d)

Brunauer-Emmett-Teller (BET) SSA obtained from

nitrogen adsorption measurement.

The measured bulk density and SSA of ATG are 2.7 mg cm−3 and 355 m2 g−1, respectively. The SSAs of the resulting rGO/polyorganosiloxane aerogels A2G and A4G are 204 and 185 m2 g−1, respectively (Table 1). The lower SSAs of A2G and A4G compared to that of ATG are because the incorporated polyorganosilxoanes of A2 (72 m2 g−1) or A4 (61 m2 g−1) show lower SSAs. As shown in the pore size distributions (Figure 2d), A2G and A4G have less small-sized mesopores compared to those of ATG. It is found that the bulk density of A2G (36 mg cm−3) and A4G (24 mg cm−3) is slightly lower than that of A2 and A4, respectively. This results from the different drying methods and the ultralow bulk density of ATG. The shrinkage of A2 and A4 that are obtained via ambient pressure drying is relatively larger than that of A2G and A4G that are obtained via freeze drying. Meanwhile, the ultralow bulk density of ATG has little 11



effect on the resulting bulk density of A2G and A4G. These two factors lead to the lower bulk density of A2G and A4G. The reason why different drying methods are applied for polyorganosiloxane aerogels and the triple-network aerogels is as follows. Polyorganosiloxane aerogels can be obtained via ambient pressure drying. However, after the incorporation of rGO, the resulting triple-network rGO/polyorganosiloxane aerogels via ambient pressure drying show larger shrinkage compared with pristine polyorganosiloxane aerogels via ambient pressure drying. In order to obtain low-density triple-network aerogels with smaller shrinkage, freeze drying is used. The APTES-modified aerogel ATG exhibits a 3-D highly porous network structure that consists of inter-crosslinked rGO nanosheets (Figure 2f) and APTES-derived siloxane nanoparticles (with a particle size mainly in the range of 20−50 nm) that are distributed on the surface of rGO nanosheets (Figure 2i). After incorporating polyorganosiloxanes in rGO networks, the resulting rGO/polyorganosiloxane aerogels (A2G and A4G) combine the intercrosslinked rGO nanosheets and inter-crosslinked VMDMS-derived polyorganosiloxane nanoparticles (well distributed on the surface of rGO nanosheets) with similar sizes as those of the corresponding polyorganosiloxane aerogels (A2 and A4), showing a coralline-like structure (Figure 2g and Figure S5).This unique structure results from the crosslinks of rGO nanosheets with PVMDMS polymers and VMDMS. From high-resolution TEM images (Figure 2j and Figure S6) we can see that the polyorganosiloxanes in A2G and A4G contain small nanoparticles with a particle size in the range of 30−70 nm, which are similar to those of A2 (Figure 2h). Because of their highly porous structures with abundant hydrophobic groups (methyl groups, 12


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vinyl groups, and hydrocarbon chains) and a small amount of hydrophilic –OH groups as shown in the FTIR spectra, the polyorganosiloxane aerogels A2 and A4 exhibit superhydrophobicity with a water contact angle as high as ~167° (Figure 2k). After incorporating superhydrophobic polyorganosiloxanes in rGO networks, the resulting rGO/polyorganosiloxane aerogels also exhibit superhydrophobicity with the same water contact angle (167°) as that of A2 and A4, the value of which is higher than that of APTES-modified rGO aerogel ATG (141°) (Figure 2k). Besides, A2 is thermally stable up to ~230 ℃, above which oxidation of hydrocarbon polymers, vinyl groups, and methyl groups occurs (Figure S7). The thermal stability of A2G is higher than that of A2, which is probably because the incorporated rGO has a relatively higher thermal stability. Benefiting from their unique doubly crosslinked polyorganosiloxane network structures consisting of flexible PVPMS and PVMS, the polyorganosiloxane aerogels exhibit high flexibility in both compression and bending. As shown in Figure 3a,b, A1, A2, A3, and A4 are compressed with 80 % strain without fracture and then spring back to their original sizes after the force is removed, indicating high compressibility and elasticity. Meanwhile, as presented in Figure S8, A2 is bent by hand with a large bending deformation without fracture and then recovers its original shape after bending, indicating high bendability and elasticity. Besides, As shown in Figure 3a,b, the compressive stress of A1 is higher than that of A2 and A4 with the same compressive strain in the range of 0−80 %. The higher compressive stress may result in lower sensitivity for pressure sensing. A3 exhibits lower compressive stress, but the processibility becomes lower. For these reasons, A2 and A4 are used for the preparation of rGO/polyorganosiloxane aeorgels. 13



Figure 3. Mechanical flexibility and elasticity. (a-c) Stress-strain curves of uniaxial compression-decompression tests on typical aerogels. The insets are Photographs of a uniaxial compression-decompression test on A2. (d) Stress-strain curves of A2G with different compressive strains. (e) Compression cycle performance of A2G. (f) Comparison of water contact angle, recoverable compressive strain, and bendability of the present aerogels and other reported elastic rGO-based aerogels. (g) Photographs of a uniaxial compression-decompression

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test on A2G. (h) A hand bending test on A2G. (i,j) Schematics of structure variations of rGO/polyorganosiloxane aerogels during 90 % compression-decompression and bending.

The APTES-modified rGO aerogel ATG springs back to 63 % from 80 % compression (Figure S9), showing lower elasticity compared to that of the polyorganosiloxane aerogels. It is noteworthy that the rGO/polyorganosiloxane aerogels A2G and A4G exhibit high compressibility, high bendability, and high elasticity. A2G and A4G are compressed with 80 % strain without fracture and then spring back after the force is removed (Figure 3c). In addition, as shown in stress-strain curves (Figure 3b,c and Figure S9), the compressive strength of rGO/polyorganosiloxane aerogel A2G is much higher than that of pristine polyorganosiloxane aerogel A2 and pristine APTES-modified aerogel ATG, which may result from the crosslinks of rGO nanosheets with PVMDMS polymers and VMDMS. A2G also perfectly recovers from 90 % compression (Figure 3d,g). In addition, A2G recovers nearly its original size after compression-decompression with 80 % strain for 500 cycles, indicating an excellent compression cycle performance (Figure 3e). The recoverable compressive strain (90 %) together with the water contact angle (167°) of A2G and A4G are among the highest in the reported rGO-based aerogels so far (Figure 3f). The recoverable compressive strain of A2G and A4G is higher than that of MXene/rGO aerogel (60 %),9 polyimide/graphene aerogel (50 %),32 and graphene-coated carbon nanotube aerogel (80 %).39 The recoverable compressive strain and water contact angle of the triple-network aerogels (A2G and A4G) are also higher than those of doubly crosslinked aerogels based on PVPMS (80 %, 131–154°)

33,40

and

polyvinylpolydimethylsiloxane (PVPDMS) (80 %, 142–157°).41 The A2G also exhibits high 15



bendability that is comparable to that of A2 (Figure 3h), which is rarely reported for rGO-based aerogels. Their unique triple-network structures consisting of inter-crosslinked rGO nanosheets, PVPMS, and PVMS probably contribute to the high flexibility of rGO/polyorganosiloxane aerogels. The structure variations of rGO/polyorganosiloxane aerogels during 90 % compression-decompression and bending are schematically presented in Figure 3i,j. Besides, various desired shapes of rGO/polyorganosiloxane aerogels can be obtained simply by cutting with a knife without fracture, indicating excellent machinability (Figure 1c,d). The resulting rGO/polyorganosiloxane aerogels are electrically conductive because of the inter-crosslinked conductive rGO nanosheets in aerogels. It should be noted that the electrical resistance of A2G decreases linearly with increasing temperature in the range of 25–100 ℃ (Figure 4a). It decreases 50.2 % when the temperature increases from 25 ℃ to 100 ℃. The conductance of rGO versus temperature could be described by the following equation:1 G = Gh·exp(–H/T1/3)+Gt

(1)

where G is conductance, H is a hopping parameter, Gh·exp(–H/T1/3) represents the contribution of the charge hopping, Gt represents the contribution of the quantum tunneling. High temperature probably promotes thermally activated charge hopping through the rGO nanosheet junction, increasing the electrical conductance of rGO, which decreases the resistance of the rGO/polyorganosiloxane aerogels (Figure 5).1,2,5 The normalized electrical resistance of A2G remains nearly unchanged after compression-decompression with 50 % strain for 1000 cycles, indicating excellent durability against compression (Figure 4b). In addition, the normalized electrical resistance remains nearly unchanged after repeated bending to a 1 cm radius and releasing for 100 cycles, indicating excellent durability against bending (Figure 4c). 16


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Furthermore, probably because of its superhydrophobicity, A2G exhibits good durability against humidity with relative humidity in the wide range of 20–90 % (Figure 4d). These results indicate that the rGO/polyorganosiloxane aerogels are suitable as flexible temperature sensors.

Figure 4. Temperature, strain, and pressure sensing of A2G. (a) Normalized electrical resistance versus temperature. (b) Normalized electrical resistance versus temperature after compressiondecompression with 50 % strain for 10, 100, and 1000 cycles. (c) Normalized electrical

17



resistance after repeated bending to a 1 cm radius and releasing for 100 cycles. (d) Normalized electrical resistance versus relative humidity. (e,f) Normalized electrical resistance versus compressive strain. (g) Durability test on A2G over 10000 cycles with 50 % compressive strain. (h) Normalized electrical resistance versus compressive strain under different temperatures. (i,j) Normalized electrical resistance versus pressure. (k) Responses to diverse pressures and the corresponding compressive strains (in parentheses).

Figure 5. Schematic of mechanism of temperature, strain, and pressure sensing of A2G-based sensor with two kinds of structures. (a) The sandwich structure. (b) The structure with two electrodes on the same side, which is used for sensing array in this work.

The rGO/polyorganosiloxane aerogels exhibit not only temperature-sensitive conductivity but also strain- and pressure-sensitive conductivity. The normalized electrical resistance of A2G decreases significantly with increasing compressive strain in the wide range of 0–80 %

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(Figure 4e,f). It decreases 1.5 %, 7.3 %, 37.5 %, 67 %, and 84 % at 0.1 %, 1 %, 10 %, 50 %, and 80 % compressive strain, respectively, and exhibits good recovery during the decompression process. After compression-decompression with 50 % strain for 1000 and 10000 cycles, the electrical resistance-compressive strain curve remains nearly unchanged, indicating an excellent cycle performance (Figure 4e,g). More contacts among rGO nanosheets are produced upon compression, which increases the conduction paths and thus decreases the electrical resistance (Figure 5). The resistance recovers upon decompression because of the recovery of the superelastic network of A2G. With increasing temperature, the electrical resistance-compressive strain curve of A2G shifts to the lower electrical resistance (Figure 4h). However, the shape of the curve remains nearly unchanged. Similar to the electrical resistance variations with compressive strain, the normalized electrical resistance of A2G decreases significantly with increasing stress (pressure) in the wide range of 0–110 kPa (Figure 4i,j). It decreases 1.5 %, 7 %, 46 %, 69 %, and 83.5 % at 10 Pa, 0.1 kPa, 1 kPa, 10 kPa, and 100 kPa, respectively, and recovers well during decompression process. It also shows fast responses to diverse pressures and the corresponding compressive strains (Figure 4k). The mechanism of temperature, strain, and pressure sensing of A2G is schematically presented in Figure 5. As we can see, the resulting rGO/polyorganosiloxane aerogel A2G exhibits high sensitivity and high durability in sensing of strain, pressure, and temperature, which allow it to be promising in the practical applications of temperature/strain/pressure sensors and multifunctional wearable electronics. Temperature, strain, and pressure can be recognized using the A2G-based sensors by the following method. We may use one A2G-based sensor without pressing or loading to recognize 19



the temperature first. In this case, temperature can be determined by the resistance changes of this sensor. As shown in the stress-strain curves of A2G (Figure 3c), pressure (stress) is accompanied by strain for the sensors based on A2G. In other words, when the sensor is pressed, strain occurs at the same time. After the temperature is determined, pressure and strain can be determined at the same time by the resistance changes of another A2G-based sensor combined with the effect of the obtained temperature on the resistance changes. In order to demonstrate the potential applications of the rGO/polyorganosiloxane aerogels in multifunctional wearable electronics, a flexible 4 × 5 rGO/polyorganosiloxane aerogel-based sensing array has been prepared. Au circuits are thermally evaporated onto a transparent and flexible polypropylene substrate through a shadow mask (Figure 6a). The circuits are attached by 20 cuboid samples of A2G with the same size using adhesives, followed by fixing a blue insulation layer with the same size on top of each aerogel with adhesives, affording a flexible 4 × 5 temperature/strain/pressure sensing array (Figure 6b,c,d).

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Figure 6. Multifunctional temperature/strain/pressure sensing array based on A2G. (a) a 4 × 5 sensor pixel array on a flexible and transparent substrate. (b,c) The flexible 4 × 5 rGO/polyorganosiloxane aerogel-based sensing array. (d) Schematic of the sensing array structure. (e) The sensing array attached on the outside surface of a transparent glass cup with hot water and the corresponding signal output. (f) The sensing array attached on the outside surface of an inclined transparent glass cup with hot water and the corresponding signal output. (g) A bracelet on the flat sensing array and the signal output. (h) A bracelet on the curved sensing array and the signal output. (i) A crawling insect on the flat sensing array and the signal 21



output.

The sensing array is attached on the outside surface of a transparent glass cup with hot water to measure the temperature distribution of the cup (Figure 6e,f). The two-dimensional normalized electrical resistance distribution agrees with the level of the hot water in the glass cup. In addition, a bracelet is put on the flat or curved sensing array to determine the strain (or pressure) distribution generated by the loading (Figure 6g,h). The signal output is consistent with the shape and position of the bracelet. Furthermore, a crawling insect on the flat sensing array can be clearly monitored (Figure 6i).

CONCLUSION In summary, highly flexible multifunctional triple-network rGO/polyorganosiloxane aerogels have been synthesized via a novel crosslinking strategy. This strategy is based on crosslinking and reducing of GO nanosheets with APTES to obtain APTES-modified rGO aerogels, followed by further covalent crosslinking of the rGO aerogels with VMDMS and PVMDMS polymers. The resulting rGO/polyorganosiloxane aerogels exhibit a triple-network nanostructure consisting of rGO nanosheets, PVPMS, and PVMS that are crosslinked with each other. Incorporating polyorganosiloxanes in rGO aerogels significantly enhances the hydrophobicity, elasticity, and flexibility of the resulting nanocomposite aerogels. A combination of superhydrophobicity (water contact angle up to 167°), high compressibility, high bendability, superelasticity (reversible 90 % compressive strain), excellent machinability, and temperature-, strain-, and pressure-sensitive conductivity has been achieved in a single 22


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rGO/polyorganosiloxane aerogel. In addition, we demonstrate that the rGO/polyorganosiloxane aerogel-based flexible sensing array exhibits multifunctionality combining temperature, strain, and pressure sensing with high sensitivity and high durability against compression, bending, and humidity. This study is expected to provide new concepts to synthesize flexible multifunctional porous materials and design multifunctional wearable electronics based on these materials.

EXPERIMENTAL SECTION Materials. Graphite flake (325 mesh), vinylmethyldimethoxysilane (VMDMS), and tetramethylammonium hydroxide (TMAOH, 25 wt % in water) were obtained from SigmaAldrich, Co. (USA). (3-Aminopropyl)triethoxysilane (APTES) and di-tert-butyl peroxide (DTBP) were purchased from Tokyo Chemical Industry Co., Ltd. (Japan). Tert-butyl alcohol, sodium nitrate (NaNO3), sulfuric acid (H2SO4, 98 %), potassium permanganate (KMnO4), hydrogen peroxide (H2O2, 35 %), hydrochloric acid (HCl, 36 %), sodium hydroxide (NaOH), benzyl alcohol (BzOH), n-hexane, and isopropyl alcohol (IPA) were obtained from Kishida Chemical Co., Ltd. (Japan). Distilled water was purchased from Hayashi Pure Chemical Ind., Ltd. (Japan). All the chemical reagents were used as received. Preparation of GO Suspension. GO suspension was prepared via a modified hummers’ method.42 1.0 g of graphite flake, 0.5 g of NaNO3, and 23.0 mL of H2SO4 were mixed in an icebath with stirring. 3.0 g of KMnO4 was added to the suspension slowly to prevent the temperature from exceeding 20 ℃. The suspension was then subjected to heat treatment at 35 ℃ for 30 min, after which it was diluted by slowly adding 46 mL of distilled water. After stirring 23



for 15 min, the suspension was further diluted by adding 70 mL of distilled water. 12 mL of 6 wt % H2O2 solution was then added to the suspension. A yellow-brown filter cake was obtained after filtering of the suspension. The filter cake was washed with 120 mL of 6 wt % HCl solution to remove the metal ions. The obtained suspension was then washed with distilled water and centrifuged three times. The concentration of the GO suspension was adjusted to be 10 mg mL−1 and subjected to ultrasonic agitation for 20 min before use. Preparation of APTES-Modified rGO Aerogel. 0.75 mg of NaOH and 0.06 mL of H2O were first mixed and added into 1.2 mL of GO suspension (10 mg mL−1) in an ice-bath with stirring. 0.012 mL of APTES and 6.79 mL of H2O were then mixed and added into the above suspension with stirring. After stirring for 3 min, the mixture was then transferred into a container and sealed. The container was heated at 95 ℃, where the gel formed within 30 min. The gel was aged at 95 ℃ for 3 d and washed with tert-butyl alcohol at 50 ℃ three times (each 8 h). After being frozen in a freezer, the gel was freeze-dried at 40 Pa and −50 ℃ by a freeze dryer (FDU-12AS, Tokyo Rikakikai Co., Ltd., Japan) to afford an aerogel. Finally, the aerogel was put into a tubular furnace and subjected to heat treatment at 300 ℃ under pure nitrogen flow (400 mL min−1) for 10 h to obtain an APTES-modified rGO aerogel (ATG). Preparation of Polyorganosiloxane Aerogels. An appropriate amount of VMDMS and DTBP (1 mol %) were placed in a hydrothermal reactor. After flushing with argon, the reactor was sealed and heated at 120 ℃ for 48 h to afford a transparent and viscous liquid that mainly contained PVMDMS polymers. BzOH, VMDMS, H2O, and TMAOH with a specific molar ratio were added into the above liquid in the listed order under stirring. Details of the starting compositions were listed in Table 1. After stirring for 3 min, the sol was then sealed and placed 24


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in a forced convection oven at 95 ℃, where the gel formed within 1 h. The gel was then aged at 95 ℃ for 4 d and washed with IPA at 60 ℃ three times (each 8 h) to remove the residual chemicals. After that, the gel was washed with n-hexane at 50 ℃ three times (each 8 h), followed by ambient pressure drying at 80 ℃ for 1 h to afford a polyorganosiloxane aerogel. Preparation of rGO/Polyorganosiloxane Aerogels. An appropriate amount of VMDMS and DTBP (1 mol %) were placed in a hydrothermal reactor. After flushing with argon, the reactor was sealed and heated at 120 ℃ for 48 h to afford a transparent and viscous liquid. BzOH, VMDMS, H2O, and TMAOH with a specific molar ratio were added into the above liquid in the listed order under stirring. After stirring for 3 min, the APTES-crosslinked rGO aerogel was placed in the above sol, allowing the sol to be absorbed into the porous matrix of rGO aerogel quickly. The obtained rGO composite gel was then sealed and placed in a forced convection oven at 95 ℃ and aged at this temperature for 4 d. After washing with tert-butyl alcohol at 50 ℃ three times (each 8 h), the gel was frozen in a freezer and freeze-dried at 40 Pa and −50 ℃ by a freeze dryer to afford an rGO/polyorganosiloxane aerogel. Preparation of Multifunctional Temperature/Strain/Pressure Sensing Array. A flexible and transparent polypropylene sheet was used for the substrate. After the substrate was cleaned with distilled water and ethanol several times, flexible Au circuits were thermally evaporated onto the substrate through a shadow mask to form a 4 × 5 sensor pixel array. The circuits were attached by 20 cuboid rGO/polyorganosiloxane aerogels with the same size using adhesives. A blue insulation layer with the same size was fixed on top of each aerogel with adhesives, affording a flexible 4 × 5 temperature/strain/pressure sensing array based on rGO/polyorganosiloxane aerogels. 25



Characterizations. The chemical structures of typical aerogels were analyzed by a Fourier transform infrared (FTIR) spectroscope (IRAffinity-1, Shimadzu Corp., Japan) and a powder X-ray diffractomer (XRD, RINT Ultima III, Rigaku Corp., Japan) using Cu Kα (λ = 0.154 nm) as an incident beam. The morphology was observed using a transmission electron microscope (TEM, JEM-2100F, JEOL, Japan) and a scanning electron microscope (SEM, JSM-6060S, JEOL, Japan). Water contact angles were determined by a Drop Master (DM-561Hi, Kyowa Interface Science Co., Ltd., Japan) with water droplet volume of 3 μL. The porous structures of typical aerogels were investigated by nitrogen adsorption-desorption isotherms, SSAs, and pore size distributions, which were measured using a nitrogen adsorption analyzer (BELSORP-mini, BEL Japan, Inc., Japan). The SSAs were derived from the adsorption branch of the isotherms by the Brunauer−Emmett−Teller (BET) method. The pore size distributions were obtained from the adsorption branch of the isotherms by the Barrett−Joyner−Halenda (BJH) calculation. The thermal stability was investigated using a thermogravimetric−differential thermal analyzer (TG-DTA, TG 8120, Rigaku Corp, Japan) at a heating rate of 5 ℃ min−1 in air. The uniaxial compression-decompression tests on polyorganosiloxane aerogels, rGO aerogels, and rGO/polyorganosiloxane aerogels were carried out with a material tester (EZGraph, Shimadzu Corp., Japan). The cross-head speed during tests was 20 mm min−1. For the compression-decompression tests, cylindrical aerogels with diameter × height of (8−16) × (6−16) mm were used. Electrical resistance versus temperature, relative humidity, and compression/bending cycle number was measured with a digital multimeter (15B, Fluke, USA) by a two-probe method. 26


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Electrical resistance versus compressive strain and pressure was measured by a material tester (EZGraph, Shimadzu Corp., Japan) combined with the digital multimeter. The signal output of the 4 × 5 rGO/polyorganosiloxane aerogel-based flexible sensing array generated by different temperatures and loadings was measured by the digital multimeter.

Supporting Information. Schematic of synthesis of polyorganosiloxane aerogel, FIIR spectra, nitrogen adsorption measurement, SEM images, TEM images, TG curves, a hand bending test, and a uniaxial compression-decompression test of typical aerogels.

Acknowledgements This work was financially supported by National Key Research and Development Program of China (2017YFA0204600), KAKENHI (17K06015) from the Japan Society for the Promotion of Science (JSPS), and Incubation Program of Kyoto University.

Author Contributions G. Z. designed and fabricated materials. G. Z. and J. H characterized the materials. G. Z. wrote manuscript. K. K. and K. N. supervised this project.

Conflict of Interest The authors declare no conflict of interest.

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Superhydrophobic Ultraflexible Triple-Network Graphene

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