ADSORPTION OF H2 AND O2 GASES ON ZnO WURTZOID NANOCRYSTALS: A DFT STUDY

In the present work, we apply wurtzoids nanocrystals with density functional theory to explain the sensitivity of ZnO nanostructures towards hydrogen and oxygen molecules. Present results of ZnO nanocrystals’ sensing to H2 and O2 molecules show a reduction in the energy gap and hence electrical resistivity of ZnO nanocrystals upon attachments of these molecules in agreement with experiment. The results also show that higher temperatures increase the sensitivity of ZnO wurtzoids towards H2 and O2 molecules with the maximum sensitivity approximately at 390∘C and 417∘C for H2 and O2 molecules, respectively, after which it begins to decline according to calculated Gibbs free energy. These temperatures are comparable with experimentally reported operating temperatures of 325∘C and 350∘C for the two gases, respectively. The main reaction mechanism is the dissociation of H2 or O2 molecules on ZnO nanocrystal surface in which hydrogen and oxygen atoms are attached to neighboring Zn and O surface atoms. The removal of these molecules from the surface is also performed by the formation of H2 and O2 molecules prior to their removal from the ZnO nanocrystal surface. Electronic charge transfers to the adsorbed atoms and molecules confirm and illustrate the mechanism mentioned above.