Moreover, environmental conditions can cause surface defects, such as vacancies, which can further enhance the diffusion of oxygen into the bulk, leading to the initiation of metal oxide formation. iron atoms as a function of oxygen coverage. (c) The coverage-dependent oxygen adsorption energy on both Fe(100) and Fe(110) surfaces. Reprinted with permission from IOP sicence. (d) The schematic diagram of possible oxygen-induced initial iron oxidation pathways. Work function (Φ) is an important property of a surface that directly affects surface reactions such as the formation of crystallites, surface segregation, and chemical reactions. Work function is defined as the least energy required to remove an electron from the surface to a point in a vacuum. As seen from Table , the value of the work function increased with oxygen coverage because of growth in metal-surface dipole induced by the electronegative oxygen atoms. Such high electronegativity caused the transfer of electrons from the substrate to the oxygen adlayer. Moreover, since both sulphur and oxygen are group VI elements, Hugosson et al.