Flexible Gas Sensing Fibre Based on Bismuthous Sulfide Nanobelts-Sensitized Graphene Nanoplatelets (GNPs) Nanocomposites
Authors: Zhao Yunong , Li Hua-Yao , Liu Jingyao , Hu Zhixiang , Li Long , Zhai Bohui , Su Huyin , Tian Zhilai , Liu Huan 
Published: May 2020

Introduction
Gas sensors with high flexibility and favorable extensibility have increasingly attractive prospects. Conventional rigid ceramics or semiconductor-based gas sensors have restricted flexibility and ductility, limiting their use in flexible devices and wearable electronics. Flexible gas sensors possess broad application prospects in the fields of wearable electronics, artificial intelligence, medical health, etc. Two-dimensional (2D) layered nanomaterials have attracted attention due to unique properties, which possess tunable band gaps and excellent mobility inherently. For example, graphene is usually utilized as gas sensing material due to high specific surface area and high carrier mobility [1]. However, the properties of pristine graphene nanoplatelets (GNPs) are influenced by significant adsorption of oxygen in an air environment, leading to instability and carrier mobility recession, limiting the practical application of GNPs gas sensors in atmosphere. The combination of quantum dots or nanobelts and 2D materials could build sensitized nanocomposites, which improves sensing performance of gas sensors.
Flexible and ultrafast-response nitrogen dioxide (NO2) gas sensors based on cotton fibre that applies the mechanism of coupling and hydrogen bonding interaction were prepared by facile layered dip-coating methods. The gas sensors were fabricated utilizing cotton fibre as flexible substrate and bismuth sulfide (Bi2S3) nanobelts-sensitized GNPs nanocomposites as sensing layer. The flexible gas sensing fibre demonstrated ultrafast and recoverable response (67 s/45 s for 500 ppb gas), good reproducibility and mechanical robustness. The favorable sensing performance and high flexibility of fibre gas sensors will open potential prospects in intelligent sensing system and wearable electronics.

Experimental

Preparation of coupling reagent modified cotton fibre: The cleansed fibre substrate was first modified by 3-aminopropyl) triethoxysilane (APTES: coupling reagent) via dipping in APTES ethanol solution, and then rinsed with deionized water and dried in drying oven (100 ℃, 15 min). The organic functional group in coupling agent can be combined with the fiber substrate, meanwhile the silanoxy group in coupling agent is reactive with inorganic substances. When coupling agent is positioned between fiber substrate and gas sensing layer, bonding structure of cotton fibre-coupling agent-sensing layer can be formed.
Synthesis of Bi2S3 nanobelts-sensitized graphene nanoplatelets nanocomposites: Solvothermal method was used for Bi2S3 nanobelts synthesis reported in literatures [2]. And GNPs/Sodium laurylsulfonate (SDL) ethanol solution (solution A) and Bi2S3/polyethylene glycol (PEG) ethanol solution (solution B) were prepared for next synthesis. Finally, solution A were injected into solution B under magnetic stirring for six hours to prepare Bi2S3/GNPs nanocomposites.
Flexible gas sensor fabrication: The modified fibre was dipped in Bi2S3/GNPs solution for 30 min through dip-coating method, then treated with Pb(NO3)2 for surface ligand exchange to remove insulating organic ligands capping on Bi2S3 surface, lastly annealed in drying oven (60 ℃, 30 min).

Results and Conclusions

The stereoscopic illustration of electronic fibre gas sensor is shown in figure 1 (a). After two steps dip-coating processes, modified layer and outer Bi2S3/GNPs nanocomposites layer were successively coated on the surface of cotton fibre. Figure 1 (b) shows cross-section structure of fiber-based gas sensors, illustrating bonding structure of fibre-coupling agent-sensing layer. As for Bi2S3/GNPs nanocomposites layer, the intermolecular hydrogen bond [3] between sulfonic groups (-SO3H) of SDL and hydroxyl groups (-OH) of PEG could contribute to the interfacial adhesion between substrate and sensing layer (figure 1 (c)).
For one thing, cotton fiber has advantages of large specific surface area and rich skeletal structure, which helps the combination of fiber skeleton and sensing materials. For another, Bi2S3-sensitized GNPs nanocomposites constructed an excellent receptor-transducer gas-sensing layer. Considering the dual-mode characteristics of fast carrier transfer on graphene and strong adsorption of gas molecules in Bi2S3 nanobelts, fiber-based gas sensors demonstrated ultrafast and recoverable response (67 s/45 s for 500 ppb gas), good reproducibility and mechanical robustness (figure 1(f-g)). The device showed excellent reversibility and negligible baseline drift following cycling responses (figure 1(g)). Compared with recently reported fibre-based gas sensors (Figure 1(h)), fibre-based gas sensors showed significantly enhanced response and recovery time. This work indicates that the flexible gas sensing fibre could be used to establish wearable electronics and real-time atmosphere information monitoring system.

References

[1] H. Qiu, et., Electrical characterization of back-gated bi-layer MoS2 field-effect transistors and the effect of ambient on their performances, Applied Physics Letters. 100 (2012) 123104. doi: 10.1063/1.3696045.
[2] G.H. Chen, et., Fabrication of ultrathin Bi2S3 nanosheets for high performance, flexible, visible-NIR photodetectors, Small. 11 (2015) 2848-2855. doi: 10.1002/smll.201403508.
[3] T.J. Wang, et., Effects of poly (ethylene glycol) on electrical conductivity of poly (3, 4-ethylenedioxythiophene)-poly (styrenesulfonic acid) film, Applied Surface Science. 250 (2005) 188-194. doi: 10.1016/j.apsusc.2004.12.051.
[4] Y.J. Yun, et., Ultrasensitive and highly selective graphene-based single yarn for use in wearable gas sensor, Scientific Reports. 5 (2015) 10904. doi: 10.1038/srep10904.
[5] Y.J. Yun, et., Highly stretchable, mechanically stable, and weavable reduced graphene oxide yarn with high NO2 sensitivity for wearable gas sensors, RSC Advances. 8 (2018) 7615-7621. doi: 10.1039/C7RA12760J.
[6] R. You, et., Fabrication of flexible room-temperature NO2 sensors by direct laser writing of In2O3 and graphene oxide composites, Sensors and Actuators B: Chemical. 277 (2018) 114-120. doi: 10.1016/j.snb.2018.07.179.
[7] Y.J. Yun, et., E-textile gas sensors composed of molybdenum disulfide and reduced graphene oxide for high response and reliability, Sensors and Actuators B: Chemical. 248 (2016) 829-835. doi: 10.1016/j.snb.2016.12.028.

Figure 1