Unterpotentiale

Unterpotentiale is a term used in electrochemistry to describe the deposition of a metal adatom onto a substrate at potentials more positive than the equilibrium potential for bulk deposition. This underpotential deposition occurs because strong interactions between the adatoms and the substrate stabilize a submonolayer or monolayer film, making it energetically favorable before the potential reaches the value where bulk metal would nucleate and grow.

The mechanism rests on the adatom–substrate binding energy being sufficiently large to lower the chemical potential of the adatom when it resides on the surface. As a result, deposition proceeds at potentials where, for the same metal in solution, bulk deposition would not yet occur. In voltammetric measurements, Unterpotentiale are typically manifested as distinct, sharp waves or steps in the vicinity of the bulk deposition signals, corresponding to the formation of discrete surface coverages. The exact potentials and coverages depend on the substrate crystallography, the metal pair, and the electrolyte composition.

Commonly studied systems include copper on gold, bismuth on copper, and lead on gold, among others. Cu UPD on Au is one of the most thoroughly investigated examples and serves as a benchmark for understanding interfacial energetics and monolayer formation. Observations of Unterpotentiale contribute to insights about surface structure, adsorbate ordering, and the factors governing ultrathin-film growth.

Applications of Unterpotentiale research include the controlled fabrication of ultrathin films for catalysis and sensor technologies, templating of nanostructures, and fundamental studies of electrode–surface interactions. The concept remains a cornerstone in surface electrochemistry and electrochemical nanotechnology.

See also: underpotential deposition, overpotential deposition, electrodeposition, surface science.