Healing defects in hexagonal boron nitride(h-BN sheet)or doping it with oxygen can modify or restore its physical properties,which would increase its range of potential applications.Thus,it is very important to find an efficient method of healing or a BN sheet or doping it with oxygen.In this work,using density functional theory(DFT)calculations,we identified a mechanism for healing h-BN sheets with nitrogen vacancies(VN)or doping BN sheets with oxygen using NO2 molecules.The results indicate that such reactions involve three steps:(1)the chemisorption of NO2,(2)the incorporation of the N or O atom of NO2 into the defective h-BN sheet,and(3)the removal of the adsorbed O atom or NO molecule.We found that the proposed mechanism is theoretically possible and has the following advantages.First,the barrier is about 0.60 eV for the formation of the Odoped h-BN sheet.For the healing process,because the energy released during NO2 chemisorption(-4.94 eV)completely offsets the subsequent barrier(1.17 eV),a perfect h-BN sheet can easily be achieved by using NO2 and an h-BN sheet with VB defects as reactants.Second,no catalyst is needed,and thus there is no need for a purification step to remove the catalyst.Third,NO2,a toxic gas,can be used as a reactant and will then be reduced to O2 or NO.Fourth,NO2 shows high selectivity for vacancy defect sites.Our findings show that this is an effective theoretical method of synthesizing O-doped h-BN sheets or of healing defective h-BN sheets,which should prove useful in the design of h-BN sheet-based devices.