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

Abstract

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 “VO” and zinc vacancy “VZn” are put under scope. The results show that both pristine and defected graphene with VO 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 VZn 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 VZn while the molecule H2S is reducing to VZn 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 VZn and we concluded that VZn persists to yield selective adsorption to only CO2 and H2S molecules.