Underpotential deposition (UPD), in

electrochemistry Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an out ...

, is a phenomenon of electrodeposition of a

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...

(typically reduction of a

metal A metal (from ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, e ...

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

to a solid metal) at a

potential Potential generally refers to a currently unrealized ability. The term is used in a wide variety of fields, from physics to the social sciences to indicate things that are in a state where they are able to change in ways ranging from the simple r ...

less negative than the equilibrium ( Nernst) potential for the reduction of this metal. The equilibrium potential for the reduction of a metal in this context is the potential at which it will deposit onto itself. Underpotential deposition can then be understood to be when a metal can deposit onto another material more easily than it can deposit onto itself.

Interpretation

The occurrence of underpotential deposition is often interpreted as a result of a strong interaction between the electrodepositing metal M with the substrate S (of which the

electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials ...

is built). The M-S interaction needs to be energetically favoured to the M-M interaction in the

crystal lattice In geometry and crystallography, a Bravais lattice, named after , is an infinite array of discrete points generated by a set of discrete translation operations described in three dimensional space by : \mathbf = n_1 \mathbf_1 + n_2 \mathbf_2 + n ...

of the pure metal M. This mechanism is deduced from the observation that UPD typically occurs only up to a

monolayer A monolayer is a single, closely packed layer of atoms, molecules, or cells. In some cases it is referred to as a self-assembled monolayer. Monolayers of layered crystals like graphene and molybdenum disulfide are generally called 2D materials. C ...

of M (sometimes up to two monolayers). The electrodeposition of a metal on a substrate of the same metal occurs at an equilibrium potential, thus defining the reference point for the underpotential deposition. Underpotential deposition is much sharper on

monocrystal In materials science, a single crystal (or single-crystal solid or monocrystalline solid) is a material in which the crystal lattice of the entire sample is continuous and unbroken to the edges of the sample, with no grain boundaries.RIWD. "Re ...

s than on polycrystalline materials.J.O"M.Bockris, A.K.N.Reddy and M.Gamboa-Aldeco, "Modern Electrochemistry", 2nd edition, Volume 2A, Kluwer Academic/Plenum Publishers, 2000.

References

Electrochemistry Electrochemical potentials {{electrochem-stub