Relevance of defects in ZnO nanotubes for selective adsorption of H₂S and CO₂ gas molecules: Ab-initio investigation

The adsorption of CO₂ and H₂S gas molecules on pristine and defected ZnO nanotubes (ZnO-NTs) is theoretically investigated using the density-functional theory (DFT). The interactions of each molecule with various sites on the pristine and defected ZnO-NT, with oxygen vacancy “V_O” and zinc vacancy “V_Zn” are put under scope. The results show that both pristine and defected graphene with V_O cannot capture these two molecules and yield physisorption processes. Amazingly, the zinc-vacancy can strongly bind to these two molecules and yields chemisorption processes. Although the capture of CO₂ on V_Zn is not associated with dissociation, it causes a large effect on the electronic structure in turning the characteristics into metallic as well as it induces a magnetic moment into the defect site. On the other hand, the chemisorption of H₂S is associated with a molecular dissociation (H₂S → H + HS), in which H-atom binds to the dangling bonds of oxygen from within the defect vicinity while HS molecule exhibits a physisorption. The molecule of CO₂ is oxidizing to V_Zn while the molecule H₂S is reducing to V_Zn with enormous charge transfer occurring between molecule and defect due to the large binding energy. For sake of selectivity, many other molecules were tested for adsorption on V_Zn and we concluded that V_Zn persists to yield selective adsorption to only CO₂ and H₂S molecules.