Strategy of Constructing Light-Weight and Highly Compressible

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Functional Nanostructured Materials (including low-D carbon)

The strategy of constructing light-weight and highly compressible graphene-based aerogels with ordered unique configuration for wearable piezoresistive sensors Xiaowei He, Qiongzhen Liu, Weibing Zhong, Jiahui Chen, Dengming Sun, Haiqing Jiang, Ke Liu, Wenwen Wang, Yuedan Wang, Zhentan Lu, Mufang Li, Xue Liu, Xiaojun Wang, Gang Sun, and Dong Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 06 May 2019 Downloaded from http://pubs.acs.org on May 6, 2019

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ACS Applied Materials & Interfaces

The Strategy of Constructing Light-Weight and Highly Compressible GrapheneBased Aerogels with Ordered Unique Configuration for Wearable Piezoresistive Sensors Xiaowei He1, Qiongzhen Liu1,

2*,

Weibing Zhong3, Jiahui Chen1, Dengming Sun1,

Haiqing Jiang1, Ke Liu1, Wenwen Wang1, Yuedan Wang1, Zhentan Lu1, Mufang Li1, Xue Liu1, Xiaojun Wang1, Gang Sun2, Dong Wang1, 3* 1Hubei

Key Laboratory of Advanced Textile Materials & Application, Wuhan, 430200,

China 2Division

of Textiles and Clothing, University of California, Davis, California

95616, United States 3College

of Chemistry, Chemical Engineering and Biotechnology, Donghua University,

Shanghai, 201620, China *Corresponding

authors：

1. Qiongzhen Liu, [email protected] 2. Dong Wang, [email protected] Abstract: 3D graphene aerogels (GAs) have attracted huge attentions from researchers due to their great potentials in vast applications. The hydrothermal reaction combined with freeze-drying using GO (Graphene oxide) as a precursor has proven to be an effective method for obtaining a relatively well-structured pure GAs. However, the insufficient mechanical strength and low compressibility of the materials still limit their practical applications. Here, we report the microstructure-induced strong mechanical anisotropy of these monolithic GAs in transverse direction (TD) and longitudinal

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direction (LD), which has never been considered to be related to structural vulnerability. To overcome this anisotropy and enhance the structure, we hereby introduce our selfmade poly (vinyl alcohol)-co-polyethylene (PVA-co-PE) nanofibers and low molecular weight PVA as structural enhancers into the original 3D network to form a novel nanofibers-graphene composite aerogel. Intriguingly, a unique configuration is formed in the GA, in which the highly aligned stacked rGO sheets serve as framework (cellular walls), and the nanofibers act as crosslinking columns anchored between the walls to support the structure along TD, while the micro/nanosized PVA lamellae as binders. The resulting aerogel (referred to as GNPA) has excellent compressive resilience along TD and exhibits ultra-high Gauge Factor (14387%) at a very subtle strain (0.23%) in piezoresistive properties. The GNPA-TD has also been assembled into a verity of wearable sensors and demonstrates the great potential for wireless human pressure sensing. In short, this study offers an extremely simple and effective method for developing graphene aerogels with a strong mechanical structure and paves the way for the application of 3D graphene in wearable sensors. Keywords: Graphene aerogel; Anisotropy; poly (vinyl alcohol)-co-polyethylene nanofibers;

Mechanical

properties;


Wearable

sensors

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1. Introduction 3D graphene aerogels (GAs) have attracted great attentions due to the realization of macroscopic assembly of the 2D graphene nanosheets and their potentials in a wide variety of applications, such as energy storage/conversion, adsorption, insulation, catalysis, damping materials, sensing and so on1-4. The ideal GAs are expected to have 3D porous network and can possess excellent electrical, thermal and mechanical properties of the 2D graphene sheets5-6. Presently, pure GAs are mainly prepared by chemical reduction, hydrothermal reduction7 or ice templating8 method using graphene oxide (GO) as a precursor followed with various drying process and post-treatments. In these processes, many of the oxygen-containing functionalities in the GO have been partially reduced, and the hydrogen bonds between the reduced graphene oxide (rGO) sheets are severely weakened when further subjected to ice sublimation upon drying 9. Therefore, the 3D porous network is mainly formed by the overlapping of the rGO sheets. This makes the structure collapse easily when undergoing deformation, due to the strong π-π stacking and Van der Waals' force between the graphene sheets10. Obviously, the weak mechanical strength of the pure GAs greatly limits their practical applications and thus how to enhance the structure strength is an urgent issue to be solved. Compared with other drying methods (i.e. supercritical CO2 fluid drying), freezedrying is considered to be the optimal method of forming high-quality aerogels with various designed shapes. Naturally, this technique is commonly used to prepare the GAs. It is worth noting the ice-templating effect caused by the growth of ice crystals is



unavoidable during freeze-drying. Thus, this will facilitate the formation of monolithlike aerogels having highly ordered cellular pores aligned along the ice growth direction8. In addition, hydrothermal chemistry is a simple, efficient and green method. In recent years, it has been found that in the hydrothermal process, the GO sheets can be partially reduced to rGO sheets, while the rGO sheets can be simultaneously selfassembled into a 3D hydrogel by sheets overlapping7. Subsequently, the graphene (rGO) aerogels can be formed by slowly replacement of liquid phase in the rGO hydrogels with gas through the freeze-drying process. Therefore, the combination of hydrothermal reduction and freeze-drying using GO as a precursor is considered as an effective method for preparing the GAs. It is also worth mentioning that ethylenediamine (EDA) or other amines are commonly used as reducing agents and crosslinkers to enhance the network of GA10. As a result, the GAs tend to have a highly ordered 3D network while maintaining good shape integrity externally by this method. However, the insufficient mechanical strength, lack of enough elasticity and the sacrifice of electrical conductivity are the main drawbacks. In view of the above problems, considerable studies have been carried out by introducing various cross-linking agents or structural strengthening factors in the preparation of the GAs, especially during hydrothermal reactions. Zhong et al.11 developed a composite CNT/GA aerogel via freeze-drying of the suspension of unzipped and partially exfoliated multi-walled carbon nanotubes. The microstructure showed that the carbon nanotubes (CNTs) were just like veins to support the graphene sheets (laminas), exhibiting good electrical conductivity and stable structural integrity


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upon cyclic compression. Similarly, Wan et al.

12

designed a highly compressive

graphene-carbon nanotube aerogel by embedding CNTs into the network during a hydrothermal process. Recently, Xu et al. 13 achieved a significantly enhanced structure of GA which could be dried at room temperature by employing borate as cross-linkers during hydrothermal reduction. Ma et al.

3

utilized MXene (Ti3C2T2) nanosheets to

support the network of graphene aerogel and formed a superelastic structure. Zhou et al.14 enhances the interaction among rGO layers via addition of cellulose nanocrystalline and low-molecular-weight carbon precursors, thus realizing a flexible and superstable structure for rGO‐based carbon aerogels. Moreover, various metal ions (Fe3+, Zn2+, Nd3+, etc.) are also employed as enhancers for the graphene-based aerogels as reviewed by Gong15. These studies suggest that structural enhancers can act as ideal spacers to effectively prevent restacking of graphene sheets, while simultaneously interacting with GO or rGO sheets via coordination bonds, hydrogen bonds, van der Waals forces, dipole interactions, electrostatic interactions or chemical bonds. Of course, these works are effective and enlightening, and contribute greatly to improve structure strength of the GAs. However, these efforts have not fundamentally elucidated how to overcome structural vulnerability and how to provide both improved mechanical and electrical properties for GAs. In this work, we have taken a deep insight into the strong structural anisotropy of the monolithic pure GAs via a freeze-drying formation method. As we examined, the mechanical strength of these porous monoliths in the transverse direction (TD) are much weaker than that in the longitudinal direction (LD). When observing the microstructure, larger rGO sheets always tend to align in the



LD to form cellular walls, while smaller sized rGO sheets are spliced between the walls along TD. Obviously, the aerogels along TD lack sufficient cross-linking reinforcement or mechanical support. Moreover, this strong mechanical anisotropy of pure GAs often exacerbates their structural fragility, especially when subjected to pressure-induced shear stress. Therefore, we need to develop a strategy to improve the mechanical strength of the GAs in both TD and LD directions while maintaining excellent electrical conductivity. Based on the above understanding, PVA-co-PE polymeric nanofibers and low-molecular PVA crosslinkers were introduced into the network as structural enhancer to form novel graphene-nanofiber composite aerogels. As expected, a unique cellular porous network was formed during the hydrothermal process, and the mechanical strength of the novel GA in the transverse direction (TD) was significantly enhanced. More interestingly, the highly aligned rGO sheets serve the cellular walls, nanofibers act like columns, and PVA lamellae acts as binders. Especially along the TD, the nanofibers function like elastic springs that support the cellular walls, avoiding structure collapse in case of stress or shear. Delightingly, this configuration also imparts excellent compressive resilience and superior cyclic performance to the graphenenanofibers composite aerogel (designated as GNPA). It is worthy noting that this microstructure provides a very large resistance variation due to the geometric effects and high compressibility. Taking into account the above advantages, we further

evaluated the piezoresistive performance of the GNPA and finally proved the great potential of the aerogel for wearable pressure sensors, demonstrating its ultra-high sensitivity to subtle strain or pressure.


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2. Materials and Methods 2.1. Materials Fine graphite powders (D50 < 400 nm) were purchased from Makclin Biochemical Co., Ltd. The Poly (vinyl alcohol-co-ethylene) (PVA-co-PE) nanofibers (Mn=78495 g mol-1) were massively produced and supplied by our lab16-19. Low molecular weight PVA (1799) was purchased from Aladdin. Chemicals including concentrated sulfuric acid, potassium permanganate, phosphorus pentoxide, potassium persulfate, acetone, hexane, toluene and hydrochloric acid were obtained from Sinopharm Group Co., Ltd (China). Deionized water was made in our lab. All the chemicals were used as received without further purification. 2.2. Preparation Of The Elastic Graphene/Nanofiber Composite Aerogel (GNPA) The schematic illustration of the preparation process of the GNPA and all other aerogels used for comparative studies is shown in Figure 1. In brief, the GNPA was readily prepared by a combination of hydrothermal reduction and freezing-drying methods. In preparation, fine crystalline graphite powders were used as raw material for obtaining graphene oxide (GO) according to a modified Hummers method20-21. Frist, a PVA-co-PE nanofibers suspension (milky white) and a small molecular PVA solution (5 mL, 2 wt. %, transparent) were added to the aqueous GO solution (5 mg/mL) to form a homogeneous mixed GO solution. The Ethylenediamine (EDA) was then added to the GO solution before the subsequent hydrothermal reaction process. During hydrothermal process (120 oC for 14h), the rGO, nanofibers and PVA crosslinkers formed a hydrogel with a 3D network by self-assembly. After natural cooling to the



room temperature, the hydrogel was dialyzed in an ethanol-water solution (w/w=1: 10) for 24 h to remove residues and prevent possible cracks during freeze-drying. The hydrogel was then freeze-dried to obtain the highly porous aerogel with ordered cellular pores. Afterwards, a post-reduction by gaseous phase of hydrazine hydrate (N2H4) was employed to obtain the final GNPA aerogel. As schemed, the GNPA eventually forms a unique configuration, that is, nanofiber bundles are connected between the rGO cellular walls, while the PVA crosslinkers consolidate the anchoring between the rGO sheets and the nanofiber bundles. This configuration further proves beneficial to the mechanical properties and piezoresistive performance of the GNPA. Also noted in the Figure 1, the pure graphene-based aerogels (GAs), graphene-PVA composite aerogels (GPAs), graphene-nanofibers composite aerogels (GNAs) were prepared by the same method as control samples. 2.3. Assembly and Test of The GNPA Based Pressure Sensors The GNPAs were firstly carefully cutted into smaller pieces with various designed dimensions. Then, make an aluminum foil strip and one end of a copper wire firmly adhere to the adhesive side of an insulating tape to form one piece. The other piece was prepared in the same way. After that, sandwiched the two pieces with the smaller GNPA aerogel while leaving two ends of copper wires for measurement. As such, the change in electrical resistance under varying deformation can be measured by an Autolab system (PGSTAT302, Switzerland). The external pressure was applied by a motorized test stands for tension and compression testing (Mark10, ESM301). For the assembly of wireless wearable sensors, a commercial Bluetooth module (BIAZE D13) was fixed


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to a badge for signal transmission. The badge was connected to a headband or a wristband with an embedded thin GNPA aerogel and fixed by interconnecting sewed copper wires. The signals thus could be recorded by a mobile using our wireless sensors. For a better understanding, the assembly details of the headband wireless pressure sensor were illustrated in Figure S1 in the Supporting Information. 2.4. Characterizations The as-prepared GO sheets were characterized by a high-resolution transmission electron microscopy (HRTEM, FEI TF20) and an atomic force microscope (NT-MDT, Prima). The porous structure of all the rGO aerogels were observed by a scanning electron microscope (JEOL, IT-300). A Fourier-transform infrared spectroscopy (FTIR) (TENSOR-27) in the attenuated total reflection (ATR) mode at a range of 4000400 cm-1 was used for all the test sample. A Raman spectroscopy (LabRAM HR800, JY) at an excitation wavelength of 514 nm was also employed for chemical characterization. Besides, an X-ray diffractometer (Bruker D8 ADVANCE XRD) and an X-ray photoelectron spectroscopy (XPS) (Escalab 250Xi, Thermo Fisher Scientific) were also employed. The apparent density of the aerogels was calculated as mass divided by the geometric volume (Va). The porosity of all the aerogels is defined as

Porosity  (Va  Vs ) / Va 100% , wherein Vs is the skeleton volume or true volume, which is measured by a true density tester (AccuPyc II 1340, micromeritics) using He gas replacement method. The compressive stress-strain measurements of all aerogels were carried out using an Instron Model 5965 equipped with two testing plates and a load cell of 1000 N. The



cylindrical aerogel sample for compression test has a dimension about Φ10 mm×8 mm. A pre-load 0.2 N was applied in advance to make sure close contact with two plates for test samples. The strain rate was controlled to be 5 mm min-1 and the cyclic loading was carried out at a displacement of 30 mm min-1. The Young’s modulus was evaluated from stress-strain curves in the elastic portion (ε

Strategy of Constructing Light-Weight and Highly Compressible

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