Switchable changes in the conductance of single-walled carbon nanotube networks on exposure to water vapour

Abstract

We have discovered that wrapping single-walled carbon nanotubes (SWCNTs) with ionic surfactants induces a switch in the conductance-humidity behaviour of SWCNT networks. Residual cationic vs.anionic surfactant induces a respective increase or decrease in the measured conductance across the SWCNT networks when exposed to water vapour. The magnitude of this effect was found to be dependent on the thickness of the deposited SWCNT films. Previously, chemical sensors, field effect transistors (FETs) and transparent conductive films (TCFs) have been fabricated from aqueous dispersions of surfactant functionalised SWCNTs. The results reported here confirm that the electrical properties of such components, based on randomly orientated SWCNT networks, can be significantly altered by the presence of surfactant in the SWCNT layer. A mechanism for the observed behaviour is proposed based on electrical measurements, Raman and UV-Vis-NIR spectroscopy. Additionally, the potential for manipulating the sensitivity of the surfactant functionalised SWCNTs to water vapour for atmospheric humidity sensing was evaluated. The study also presents a simple method to establish the effectiveness of surfactant removal techniques, and highlights the importance of characterising the electrical properties of SWCNT-based devices in both dry and humid operating environments for practical applications.

Humidity Sensing Behavior of Endohedral Li-Doped and Undoped SWCNT/SDBS Composite Films

“…In this situation, most of the SWCNT surface would be covered with water molecules, and further water addition would elicit a small or no change in resistance. Our results differ from the results recently reported by Evans et al [26], who used SWCNTs in combination with anionic and cationic surfactants. It is also important to mention, that the SWCNT fabrication process and the film fabrication parameters (e.g., film thickness, type of SWCNTs, substrate type of surfactant) in ref.…”

“…It is also important to mention, that the SWCNT fabrication process and the film fabrication parameters (e.g., film thickness, type of SWCNTs, substrate type of surfactant) in ref. [26] differed from the parameters of the present work. Evans et al [26] reported a drop in conductance for semiconducting SWCNTs in the presence of water when they used anionic surfactants (e.g., sodium deoxycholate (DOC), sodium cholate (SC)) due to electrostatic interactions inducing an increase in the p-type nature of semiconducting SWCNTs.…”

“…[26] differed from the parameters of the present work. Evans et al [26] reported a drop in conductance for semiconducting SWCNTs in the presence of water when they used anionic surfactants (e.g., sodium deoxycholate (DOC), sodium cholate (SC)) due to electrostatic interactions inducing an increase in the p-type nature of semiconducting SWCNTs. In the present work electrostatic interactions do not dominate the conduction behavior.…”

“…We assumed that strong interaction between water molecules and SWCNTs/SDBS induced the resistance change in our samples. In the presence of sufficient water, SDBS molecules adsorbed on the nanotube surface could be rearranged in such a way that the hydrophilic ends were oriented away from the nanotubes [26,42]. This rearrangement would enable water molecules to interact with the SWCNTs and allow electrons to hop from the water molecules to the SWCNTs.…”

“…In fact, concerning the area of sensors, SWCNTs are particularly attractive because they provide ballistic electron transport and have excellent ability to exchange charge with several chemical species. The way surfactants impact the electrical properties of SWCNTs has recently been studied [26]. In the presence of cationic surfactants, conductance increases upon exposure to water; as for anionic surfactants, the opposite effect arises.…”