DFT insights into the electronic properties and adsorption of NO2 on metal-doped carbon nanotubes for gas sensing applications

Abstract

The gas sensing process involves complicated and precise adsorption mechanisms, and the selection of the right material is mainly based on “guess-and-check” procedures, which necessitates the search for a more systematic approach to discover optimum materials with required specifications. Herein, we report the use of ab initio first-principles methods to expedite the process of finding the material with the highest potential compared to in-lab trial and error. We focused on the NO₂ adsorption mechanism of Cu-, Pt- and Ti-doped single-walled carbon nanotubes (SWCNTs). Modelling the isolated NO₂ molecule indicated the formation of a band gap between the 3π* HOMO and 5σ* LUMO levels, which were found to be the electronic states primarily involved in the bonding process. Metal doping decreased the system stability and altered the SWCNT structure. NO₂ exposure caused the transfer of electrons to the gas molecules, thereby enhancing the p-type conductivity of the M-SWCNT structure. The highest charge transfer to the NO₂ molecule was observed for Ti-SWCNTs (0.456 eV), while the lowest was found for Cu-SWCNTs (0.351 eV). Ti-SWCNTs exhibited the highest stability and sensitivity as a potential NO₂ gas sensing material out of the 3 investigated metal dopants.