Metal-Oxide Interfaces: Toward Design via Control of Defect Density
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The nature and strength of the bond between a metal and an oxide varies, often sensitively, with the constituent materials and their preparation, e.g., surface morphology, metal coverage, gas phase atmosphere, etc. A number of different adhesion mechanisms have been reported, including weak van der Waals dispersion forces between induced surface dipoles, electrostatic polarization binding, and strong ionic and covalent bonding (the latter being more directional). Several reviews on the general science of metal–oxide interfaces have been recently published. Recommended background literature includes the excellent reviews on the surface science of metal oxides by Henrich and Cox, and on metal–oxide interfaces by Finnis and by Campbell.
Single metal atoms tend to form relatively strong chemical bonds to most oxide surfaces. This is mainly the result of a severe under-coordination of the metal atom (compared to its favored bulk state), which is therefore reactive with the surface. These ionic or covalent metal–oxide bonds are correspondingly quite short, normally around 1–2 Å. Typical bond strengths range from 1 to several eV, and are normally referred to as adsorption or desorption energies—defined as the energy required to remove a metal atom to the vacuum level. The metal–oxide bonds tend to further strengthen at various oxide surface defects, e.g., anion/cation vacancies, which can trap and immobilize diffusing metal atoms (as discussed in detail below).