Detection of CO₂ using CNT-based sensors: Role of Fe catalyst on sensitivity and selectivity

The adsorption of CO₂ on surfaces of graphene and carbon nanotubes (CNTs), decorated with Fe atoms, are investigated using the self-consistent-charge density-functional tight-binding (SCC-DFTB) method, neglecting the heat effects. Fe ad-atoms are more stable when they are dispersed on hollow sites. They introduce a large density of states at the Fermi level (N_F); where keeping such density low would help in gas sensing. Furthermore, the Fe ad-atom can weaken the C[dbnd]O double bonds of the chemisorbed CO₂ molecule, paving the way for oxygen atoms to drain more charges from Fe. Consequently, chemisorption of CO₂ molecules reduces both N_F and the conductance while it enhances the sensitivity with the increasing gas dose. Conducting armchair CNTs (ac-CNTs) have higher sensitivity than graphene and semiconducting zigzag CNTs (zz-CNTs). Comparative study of sensitivity of ac-CNT-Fe composite towards various gases (e.g., O₂, N₂, H₂, H₂O, CO and CO₂) has shown high sensitivity and selectivity towards CO, CO₂ and H₂O gases.