Underpotential deposition (UPD), in

electrochemistry Electrochemistry is the branch of physical chemistry concerned with the relationship between Electric potential, electrical potential difference and identifiable chemical change. These reactions involve Electron, electrons moving via an electronic ...

, is a phenomenon of electrodeposition of a

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

(typically reduction of a

metal A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, electricity and thermal conductivity, heat relatively well. These properties are all associated wit ...

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 convent ...

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 Equilibrium may refer to: Film and television * ''Equilibrium'' (film), a 2002 science fiction film * '' The Story of Three Loves'', also known as ''Equilibrium'', a 1953 romantic anthology film * "Equilibrium" (''seaQuest 2032'') * ''Equilibr ...

( 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 Substrate may refer to: Physical layers *Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached ** Substrate (aquatic environment), the earthy material that exi ...

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 a gas). In electrochemical cells, electrodes are essential parts that can consist of a varie ...

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

crystal lattice In crystallography, crystal structure is a description of ordered arrangement of atoms, ions, or molecules in a crystal, crystalline material. Ordered structures occur from intrinsic nature of constituent particles to form symmetric patterns that ...

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 entities, commonly atoms or molecules. Monolayers can also be made out of cells. ''Self-assembled monolayers'' form spontaneously on surfaces. Monolayers of layered crystals like graphene and molyb ...

of M (sometimes up to two monolayers). The electrodeposition of a metal on a substrate of the same metal occurs at an

equilibrium potential In a biological membrane, the reversal potential is the membrane potential at which the direction of ionic current reverses. At the reversal potential, there is no net flow of ions from one side of the membrane to the other. For channels that are pe ...

, 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 boundary, grain bound ...

s than on

polycrystalline A crystallite is a small or even microscopic crystal which forms, for example, during the cooling of many materials. Crystallites are also referred to as grains. Bacillite is a type of crystallite. It is rodlike with parallel longulites. S ...

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