Abstract
Carbon nanotubes are a natural choice as gas sensor components given their high surface to volume ratio, electronic properties, and capability to mediate chemical reactions. However, a realistic assessment of the interaction of the tube wall and the adsorption processes during gas phase reactions has always been elusive. Making use of ultraclean single-walled carbon nanotubes, we have followed the adsorption kinetics of NO2 and found a physisorption mechanism. Additionally, the adsorption reaction directly depends on the metallic character of the samples. Franck–Condon satellites, hitherto undetected in nanotube–NOx systems, were resolved in the N 1s X-ray absorption signal, revealing a weak chemisorption, which is intrinsically related to NO dimer molecules. This has allowed us to identify that an additional signal observed in the higher binding energy region of the core level C 1s photoemission signal is due to the C=O species of ketene groups formed as reaction byproducts . This has been supported by density functional theory calculations. These results pave the way toward the optimization of nanotube-based sensors with tailored sensitivity and selectivity to different species at room temperature.
Highlights
Among the extraordinary physical and chemical properties that carbon nanotubes (CNTs) have, their large surface area to volume ratio is appealing for applications where the adsorption of other molecules plays an important role.1À8 Several studies are available, which use both multiwalled (MW) and singlewalled (SW) CNTs aiming at understanding the mechanisms that affect their electronic properties upon exposure to different molecular species.9À14 The literature in this field is very rich, but to achieve a gas sensing device that overcomes the problems related to accuracy and long-term stability without compromising sensitivity and selectivity15À19 requires further investigation
We have investigated the physicochemical effects in the interaction of nitrogen oxides and the outer wall of SWCNTs, which are sorted according to metallic character
Using ultrapure separated SWCNTs has allowed us to identify that the chemical reactions are different and take place at different rates from what has been reported for unsorted material
Summary
Among the extraordinary physical and chemical properties that carbon nanotubes (CNTs) have, their large surface area to volume ratio is appealing for applications where the adsorption of other molecules plays an important role.1À8 Several studies are available, which use both multiwalled (MW) and singlewalled (SW) CNTs aiming at understanding the mechanisms that affect their electronic properties upon exposure to different molecular species.9À14 The literature in this field is very rich, but to achieve a gas sensing device that overcomes the problems related to accuracy and long-term stability without compromising sensitivity and selectivity15À19 requires further investigation. Whether this adsorption is covalent chemisorption or charge transfer mediated, and if it changes for metallic vs semiconducting SWCNTs
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