Size effect on oxygen vacancy formation and gaseous adsorption in ZnO nanocrystallites for gas sensors: a first principle calculation study

Abstract

The first principle calculation based on density function theory was used to investigate the crystallite size effect of ZnO semiconductor, the size of which was controlled from 0.325 to 1.625 nm. The formation energy of oxygen vacancy increased from 3.855 to 6.256 eV, showing incremental difficulties in the formation of defects. The densities of states of ZnO super cells with complete and defective (110) surface were calculated, concluding positive dependences of carrier mobility and conductivity on crystallite size. The oxygen species of O− and O2− were considered as adsorbates on ZnO super cells with defective (110) surface. In case of crystallite size below 1.3 nm, the adsorption energy of O− increased with crystallite size, while the one of O2− kept constant, inferring that the oxygen adsorption was dominated by O−. A competitive adsorption between both species was found when crystallite size was over 1.3 nm. The adsorption of O− was inhibited by the incremental adsorption energy of O2−, which was the dominated species on ZnO crystallite surface.