Contact potentials, Fermi level equilibration and surface charging
This paper focuses on contact electrification from thermodynamic equilibration of the electron electrochemical potentials of two conductors upon contact. The contact potential difference generated in bimetallic macro- and nanosystems, the Fermi level after the contact, and the amount and location of the charge transferred from one metal to the other are discussed. Spheres in contact, Janus particles and core-shell particles are three geometries considered. Additionally, the force between the two spheres in contact with each other is calculated and found to be attractive. A simple electrostatic model for calculation of charge distribution and potential profiles both in vacuum and in aqueous electrolyte solution is described. Immersion of these bimetallic systems into an electrolyte solution leads to a formation of an electric double layer at the metal-electrolyte interface. This Fermi level equilibration and the associated charge transfer can at least partly explain experimentally observed different electrocatalytic, catalytic and optical properties of multimetallic nanosystems in comparison to systems composed of pure metals. For example, the shifts on the surface plasmon resonance peaks in bimetallic core-shell particles seem to result at least partly from contact charging.