Carbon nanotube–cellulose composite aerogels for vapour sensing

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Abstract

Electrically conductive aerogels composed of carbon nanotubes (CNTs) and cellulose were used as vapour sensors for the first time. The vapour sensing behaviours of these aerogels were investigated by monitoring the electrical resistance change upon exposure to a series of volatile organic compound (VOC) vapours such as methanol, ethanol, toluene, and others. The effects of vapour types, CNT contents and vapour concentrations on relative electrical resistance change were analyzed. The results revealed that CNT–cellulose composite aerogels exhibit rapid response, high sensitivity and good reproducibility to both polar and nonpolar vapours. The dominant mechanism for these aerogels as vapour sensors is the resistance change from chemical molecule absorption by both CNT networks and cellulose matrix. This is mainly due to the unique three-dimensional (3D) porous structure of materials, which also broadens the scope of analytes to be detected. This work provides a novel concept for developing a new class of chemical sensors by combining the nature of CNTs with the 3D porous matrix.

Introduction

Gas sensors, including sensors for volatile organic compounds (VOCs), are attracting tremendous interest because of their widespread applications in industry, environmental monitoring, space exploration, gas-utility field operations and so on [1], [2]. Numerous sensing materials for gases have been reported previously, including metal oxide semiconductors, conjugated polymers, and other porous structured materials such as porous silicon [3], [4], [5]. Recent development of nanotechnology has created huge potential to build highly sensitive, low cost, portable sensors with low power consumption [1], [6]. In particular, carbon nanotubes (CNTs) are optimally suited to detect chemicals for their unique electronic properties and the large amount of carbon atoms present at their surface [6], [7]. The direct transduction of chemical information into an electrical signal associated to existing low power microelectronics and sensing technology makes them attractive materials [8]. However, CNT alone can only detect molecules having distinct electron donating or accepting ability, as for example, ammonia (NH3) as a donor or nitrogen dioxide (NO2) as an acceptor. For the detection of molecules that are only weakly adsorbed on CNT surface, the change in resistance is often too small [9], [10]. Electrically conductive polymer composites (CPCs) based on CNT were thus investigated widely for gas and vapour sensing, as they can combine the selectivity of both CNTs and polymer matrix. By different processes, such as mixing, coating and layer-by-layer spraying processes, a series of CNT–polymer composite has been developed and used for vapour sensing materials [11], [12], [13].

Since the most common gas sensing principle is the adsorption and desorption of gas molecules on sensing materials, it is quite understandable that by increasing the contact interfaces between the analytes and the sensing materials, the sensitivity can be significantly enhanced [1]. Aerogels are highly porous solids that hold gas (usually air) within the pores or three-dimensional (3D) networks of solid substances [14], [15]. On account of their unique features such as the extremely high surface-to-volume ratio and porous structure, aerogels are ideal for gas molecules adsorption and storage, and further for gas sensing. Although several kinds of CNT-based aerogels were already fabricated [16], [17], it is surprising that there are scarcely reports about such aerogels used for vapour sensing available until now. In our previous work, CNT–cellulose composite aerogels were fabricated based on a homogeneous dispersion of CNTs and cellulose in alkaline-urea aqueous solution [18]. It is a simple, efficient and environmental friendly process. The uniform dispersion of CNTs and favourable CNT–cellulose matrix interaction cause the excellent mechanical properties and thermal stability. The resultant composite aerogels had highly porous networks composed of nanometer-sized cellulose fibrils and carbon nanotubes. The composite aerogels have a nitrogen adsorption surface area of 140–160 m2/g, with a broad pore size distribution (such as macropores, mesopores and micropores which are open). Owing to these multiscale features, CNT–cellulose composite aerogels show good sensitivity to ambient pressure [18], which provides the potential for detection of chemical vapours.

In this work, the sensing properties of the CNT–cellulose composite aerogels will be monitored by exposing them to several VOC/dry air cycles. The electrical resistance changes at different test conditions (such as vapour concentrations, vapour types and CNT contents of aerogels) are analyzed. The sensitivity, reproducibility and selectivity of these materials as vapour sensors are comprehensively investigated.

Section snippets

Materials

Cellulose samples (cotton linters, DP 500) supplied by Hubei Chemical Fiber Group Ltd. (Xiangfan, China) were used. Commercially available MWCNTs (NC3150, purity +95%, Nanocyl S.A., Belgium) with an average diameter of 9.5 nm and an average length of 1.5 μm were used. Organic solvents, non-ionic surfactant Brij76 (polyoxyethylene (10) stearyl ether) and other reagents were of analytical grade and obtained from Sigma–Aldrich.

Preparation of CNT–cellulose composite aerogels

The cellulose/CNT composite aerogels were prepared according to Ref. [18]

Results and discussions

The surface and cross-sectional morphologies of CNT–cellulose composite aerogels with 3 wt% CNT loading are shown in Fig. 1b–d. The SEM micrographs indicate that the resultant composite aerogels had highly porous networks composed of nanometer-sized cellulose fibrils and carbon nanotubes. As reported earlier [18], the uniform dispersion of CNTs in cellulose matrix causes the excellent mechanical properties. The resistivity of the CNT–cellulose composite aerogels could be controlled by varying

Conclusions

In this work, electrically conductive CNT–cellulose composite aerogels as vapour sensors were investigated for the first time. These materials were demonstrated to be used as simple, reliable, highly sensitive, well reversible and reproducible sensors for VOCs analysis at room temperature. The possible dominant mechanism of resistance change of CNT–cellulose aerogels as vapour sensors is the adsorption of solvent molecules on the CNT surfaces which increases the distance between the individual

Acknowledgements

This work was supported by the German Research Foundation (DFG), project “Multifunctional cellulose based fibres, interphases and composites” (MA 2311/4-1)and “Multifunctional materials based on cellulose and graphene” (QI 94/1-1). We thank Mrs. J. Hiller for experimental assistance.

Haisong Qi received the PhD in Polymer Chemistry and Physics from Wuhan University in, China, 2008. He had worked in the Institute of Organic Chemistry and Macromolecular Chemistry in Friedrich-Schiller University of Jena, Germany, as a postdoctoral research fellow from 2008 to 2011. He is currently a scientific collaborator in the Department of Composite Materials in Leibniz Institute of Polymer Research Dresden, Germany. His research is focused on bio-based materials and functional materials.

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    Haisong Qi received the PhD in Polymer Chemistry and Physics from Wuhan University in, China, 2008. He had worked in the Institute of Organic Chemistry and Macromolecular Chemistry in Friedrich-Schiller University of Jena, Germany, as a postdoctoral research fellow from 2008 to 2011. He is currently a scientific collaborator in the Department of Composite Materials in Leibniz Institute of Polymer Research Dresden, Germany. His research is focused on bio-based materials and functional materials.

    Jianwen Liu received the Master degree in Mechanical Engineering from Technische Universität Dresden (TUD), Germany, in 2002; and PhD in Engineering Science from TUD in 2008. He is currently a scientific collaborator from 2002 in the Department of Composite Materials in Leibniz Institute of Polymer Research Dresden, Germany. His research is focused on composite materials and functional materials.

    Jürgen Pionteck received his PhD in physico-organic chemistry (Technische Universität Dresden, Germany) in 1988. He is currently scientific collaborator in the Department Functional Nanocomposites and Blends. His research is focused on synthesis and modification of polymers, reactive compatibilization of polymer blends, electrical conductive (nano-) composites, interfacial and surface properties, and thermodynamics of polymers and blends.

    Petra Pötschke received her diploma and PhD in mechanical engineering from the Technische Universität Dresden, Germany. Since 1988 she has been working at the Leibniz Institute of Polymer Research Dresden (IPF), Germany. In 1999/2000 she worked with a grant of the Max–Kade-Foundation at the University of Texas at Austin, USA on carbon nanotube filled composites and blends. She currently heads the Department Functional Nanocomposites and Blends. Her research is focused on the dispersion of nanofillers in polymer matrices, properties of nanocomposites and rheology of composites and blends.

    Edith Mäder received her PhD in Mechanical Engineering from Technische Universität Dresden (TUD) in 1974. After graduation she worked as Group Leader at Institute of Polymer Technology of Academy of Sciences in Dresden. She completed Habilitation (Dr.-Ing. habil.) at TUD in 2001, topic: “Interfaces, Interphases and Mechanical Properties of Composites”. From 2002 to 2013 she served as Head of Department “Composite Materials” at Leibniz Institute of Polymer Research Dresden and simultaneously at the Institute of Materials Science at TUD, where she is Honorary Professor in the area of Interphases in Composite Materials.

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