Freezing-induced interfacial growth of polypyrrole layers on hierarchical carbon aerogels for robust ultrasensitive pressure sensors
Graphical Abstract
Introduction
Wearable devices have been widely used in the field of human health, for which different types of wearable devices have been studied such as bionic skin, piezoresistive sensors, gas sensors, etc (Iqbal et al., 2021, Li et al., 2022). Wearable devices or electronic skins could benefit greatly from mechanically fixable carbon aerogels that can detect body movements and transform them into electrical impulses (Costa et al., 2019, Ha et al., 2018, Nag et al., 2017, Wen et al., 2020). Carbon aerogels have significant advantages in terms of stability and sensitivity compared to conductive hydrogels and membranes (Jian et al., 2017, Lee and Park, 2020, Wang et al., 2019). The “Top to down” and “Bottom to up” strategies are two widely used paths for constructing carbon aerogels. The “Top to down” strategy involves directly annealing and carbonizing three-dimensional porous carbon precursors such as wood to form carbon aerogels. However, the mechanical properties of the obtained carbon aerogels highly depended on the wood species (Chen et al., 2020b, Zhao et al., 2021). The "Bottom to Up" process, on the other hand, includes creating carbon aerogels out of nanomaterials, which has several appealing characteristics including a diverse variety of raw material sources and a programmable structure (Long et al., 2020, Zhou and Xu, 2020, Zhuo et al., 2018). Thus, the “Bottom to up” strategy was widely studied and lots of nanomaterials such as carbon nanotubes (Doshi et al., 2019, Li et al., 2019a), graphene (Bae et al., 2013, Fan et al., 2021, Nag et al., 2018), and graphene oxide (Ma et al., 2018) have been developed to construct carbon aerogels. The ease of assembly, toughness, and excellent electrical conductivity also make them outstanding in terms of sensing materials (Peng et al., 2018). However, their practical applicability has been severely limited due to their poor mechanical qualities, high cost, difficult preparation procedure, and hazardous reactants involved in the reaction process. As a result, there is a pressing need to produce green and sustainable functional carbon aerogels that meet both environmental and high-performance sensing requirements.
Biomass materials are proven to be ideal carbon precursors, which are carbohydrates with high carbon content (Asim et al., 2019, Bhat et al., 2020, Huang et al., 2019a, Sam et al., 2020). To date, several biomass-derived carbon aerogels have been successfully developed from cotton (Li et al., 2015), wood (Chen et al., 2020a), bacterial cellulose (BC) (Chen et al., 2021b), cellulose nanofibers (CNF) (Chen et al., 2021a), water chestnuts (Zhang et al., 2020), and wood pineapples (Lee et al., 2020). Among them, CNF with the high ratio aspect is easy to form a 3D interconnect network to obtain high-strength aerogels, which have been extensively used to prepare carbon aerogels. However, the inevitable shrinkage that occurs during carbonization, and low elasticity, especially the narrow pressure range highly limit their further application. As a result, using the "Bottom to Up" technique to create carbon aerogels with mechanical flexibility and excellent conductivity remains problematic. Polypyrrole is a highly practical conductive polymer with the potential to modify conductivity, sensitivity, and applicability (Gunasekara et al., 2022, Tai et al., 2022). It can be blended or coated onto different materials by a variety of methods which is now widely used as a performance-enhancing material for piezoresistive sensors (Luo et al., 2017, Qin et al., 2022).
In this study, carbon aerogels were prepared from sustainable CNF using a "Bottom to up" approach. CNF aerogels (CAs) were made using a freeze-casting process, and the CNF-based carbon aerogels (CCAs) were achieved by carbonizing them in an N2 environment. Moreover, the PPy was equally polymerized on the nanoscale skeleton CCAs to obtain PPy-CCAs composites (CCAs-P) by freezing-induced in situ polymerization, which significantly improved the mechanical flexibility and sensing capabilities. As a result, the compressible elastic CCAs-P demonstrated excellent sensitivity to both minor and large deformations. This work provides a new technique to fabricate mechanically fixable carbon-based aerogels composites for flexible wearable devices.
Section snippets
Materials
Cellulose nanofiber (CNF) suspension was obtained from Tianjin Woodelf biotechnology Co. Ltd. Ferric chloride hexahydrate (FeCl3·6H2O), sodium 5-sulfosalicylate (NaSSA), and cyclohexane were obtained from Aladdin biochemical Polytron Technologies Inc. Pyrrole monomer was provided by Shanghai Macklin Biochemical Co. Ltd. All materials were used without further purification.
Preparation of cellulose nanofiber carbon aerogels (CCAs)
First, a mold with a diameter of 28 mm and a height of 32 mm was filled with 15 g (2 wt%) cellulose nanofiber suspension.
Results and discussion
The preparation process of 3D layered porous CCAs-P using a freezing-induced in situ polymerization strategy is shown in Scheme 1. Firstly, the CNF suspension was freeze-dried to obtain aerogels with a layered porous structure and then carbonized at 1000 °C under an N2 environment to prepare CNF-based carbon aerogels (CCAs). After that, the PPy was interfacially grown on the surface of the CCAs through freezing-induced. It is worth noting that the cyclohexane tended to crystallize at 5 °C (Lv
Conclusions
The "Bottom to up" technique was used to create a mechanically flexible biomass-derived carbon aerogel composite. The synthesized CCAs-P had a three-dimensional porous structure, and the PPy nanoparticles were uniformly covered on the surface of the CCAs framework. The CCAs-P exhibited outstanding conductivity of 30 mS m−1, strong mechanical compression stress of 800 kPa (at 80% strain), and good elastic recovery capabilities. Furthermore, the CCAs-P showed exceptional sensing capabilities and
CRediT authorship contribution statement
Ziheng Li: Conceptualization, Investigation, Formal analysis, Data curation, Writing – original draft. Lumin Chen: Investigation, Data curation, Formal analysis. Mengya Mu: Investigation, Formal analysis. Houyong Yu: Writing – review & editing, Supervision. Yingzhan Li: Conceptualization, Writing – review & editing. Xiang Chen: Resources.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This work was supported by Outstanding Youth Project of Zhejiang Provincial Natural Science Foundation [LR22E030002], Zhejiang Provincial Natural Science Key Foundation of China [LZ20E030003], and the Young Elite Scientists Sponsorship Program by CAST [2018QNRC001].
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