Enhanced atomic oxygen adsorption on defective nickel surfaces: An ab initio study

In this work we have examined the influence of the presence of a monovacancy on the atomic oxygen adsorption process at nickel surfaces of orientation (111), (100), and (110). The presence of such a defect was neglected in earlier studies. And for the first time, we have studied oxygen segregation on a defective (111) surface. The results reveal a varying sensitivity of the oxygen adsorption energy to the state of the surfaces. When compared to the perfect surface, we have registered an energy gain of 0.22 eV in the process of oxygen adsorption on the (111) surface when a vacancy is present on it. However, the energetic gains for the other two surfaces, (100) and (110), are much less than that of the (111) surface: they are of the order of 0.1 eV. Comparing to the perfect surfaces, we have found that charge reconstruction in the neighborhood of the vacancy plays a major role in giving rise to the aforementioned energetic gains. Indeed, we find an increase in the charge density on the nickel atoms surrounding the vacancy, which leads to strengthening of the ionic Ni-O bond if the oxygen is adsorbed in its vicinity. As a means of studying the effect of the presence of the vacancy on the first stages of the growth of an oxide layer, we have looked at the segregation process of oxygen atoms at the three surfaces. Our results show that up to four oxygen atoms can aggregate favorably at the adsorption sites inside and in the vicinity of the monovacancy at the (111) surface. This number is reduced to two oxygen atoms at the (110) surface, and to only one oxygen atom at the (100) surface.