2. Physical approach to a surface

What we've just described for metals, ceramics and, more generally, ionic compounds, also applies to oxides. Every metal (except gold) is covered in contact with air by a so-called natural oxide (or hydroxide) film. This film is very thin, ranging in thickness from a few units to ten nanometers, depending on the nature of the substrate. The result is that, depending on the orientation of the surface, the electrostatic characteristics of the first planes vary.

The electronic structure of atoms is disturbed in the vicinity of a surface, due to bond breakage (dangling bonds) and structural distortions. Even for a clean, ideal surface, with no structural or compositional defects (non-stoichiometry), the shape of the energy bands is disturbed relative to that of the volume, so that electron partitioning between anions and cations is altered and the band gap reduced....