Adsorption sensitivity of Pd-doped SWCNTs to small gas molecules

The adsorption of three types of gas molecules (SO2, CH3OH, and CH4) on to Pd-doped (5,5) single-walled carbon nanotubes (Pd-SWCNTs) has been investigated within density functional theory. The Pd-SWCNTs can be utilized as good sensors for the three types of small molecules due to strong binding energy and large electron charge transfer between the Pd-SWCNTs and these small molecules. The adsorption features of the three types of molecules are all different from one another. The adsorption of SO2 and CH3OH on to the Pd-SWCNTs is attributed to chemical adsorption and physical adsorption, respectively, while the interaction between CH4 and the Pd-SWCNT is electrostatic interaction and a bound complex of (Pd-SWCNT)–(CH4)+ forms due to charge transfer between them. Density of states and molecular orbital of the Pd-SWCNT and M–Pd-SWCNT (M = SO2, CH3OH, and CH4) were analyzed to interpret the changes in electronic structure, conductivity, and binding energy upon adsorption.