NO2 desorption from Carbon nanotubes filled with Ni nanoclusters

Gas sensing with carbon nanostructured materials has attracted a lot of interest and particularly single-walled carbon nanotubes (SWCNTs) have enormous potential for applications due to their unique structural and physical properties, such as large surface area to volume ratio and conductivity. However, the development of the next generation gas sensors stringent requirements, such as high sensitivity, high selectivity, fast response and optimal recovery, are imposed. SWCNTs have already been tested in those directions but not always the purity of the materials has been well controlled. This is the reason why many nanotube-based sensors have shown accuracy and durability problems that remain unsolved. One of the reasons is related to the use of nanotubes that do not meet the required purity and where defects play an important role as extremely reactive sites, responsible for the chemisorption of oxidizing gases that do not desorb. To have adsorption reversibility, ideally the system should have the least possible chemisorption events. An almost standard recovery procedure is heating the sensing object to clean it. However, this can have other implications in durability and alteration of the sensor´s properties.

In the present study executed at the SuperESCA beamline, we have used metallicity sorted SWCNTs filled with nickel(II) acetylacetonate in molecular form, and we have also subsequently transformed the filling into metal clusters, which remain encapsulated in the hollow core. With these materials we have been able to unfold two major challenges mentioned before: tuning the gas-tube interaction and achieving desorption of NO₂ at ambient temperature.
In order to control the sensitivity of the nanotubes to NO₂ at room temperature we made use of fast and high energy resolution photoemission in the core level (fast-XPS) and also in the valence band region. In situ photoemission experiments have revealed remarkable sensitivity and recovery at ambient temperature.

Regarding the selectivity criterion, the materials used in our experiments are an example of how to tailor specifically the selectivity towards NO₂ in a reusable sensor. We observed that in semiconducting nanotubes the chemical potential is pinned inside their energy gap shifted to the onset of the conduction band when filled with nanoclusters. This shows that cluster filling is a key to high sensitivity, opening the possibility for a very high desorption at ambient temperature.

In summary, this work represents an important step towards understanding the ability of SWCNTs to behave as highly gas sensitive objects capable of recovering at ambient temperature. Metallicity sorted SWCNT filled with metal nanoclusters have allowed us defining a pathway to achieve a reversible room temperature sensor for NO₂. We find that the electronic structure in the vicinity of the Fermi level, which is in turn strongly related to the electron transport properties, is reversibly influenced by the NO₂ adsorption mechanism. Inspired by this work, other reactive and poisonous gas species can be monitored by sensing targets with controlled and increased sensitivity and selectivity at room temperature.