Copper(I) oxide

Copper(I) oxide or cuprous oxide (Cu2O) is an oxide of copper. It is insoluble in water and organic solvents. Copper(I) oxide dissolves in concentrated ammonia solution to form the colourless complex [ Cu(NH3)2 ] +, which easily oxidizes in air to the blue [ Cu(NH3)4(H2O)2 ] 2+. It dissolves in hydrochloric acid to form HCuCl2 (a complex of CuCl), while dilute sulfuric acid and nitric acid produce copper(II) sulfate and copper(II) nitrate, respectively.

Copper(I) oxide is found as the mineral cuprite in some red-colored rocks. When it is exposed to oxygen, copper will naturally oxidize to copper(I) oxide, but this takes extensive periods of time. Artificial formation is usually accomplished at high temperature or at high oxygen pressure. With further heating, copper(I) oxide will form copper(II) oxide.

Formation of copper(I) oxide is the basis of the Fehling's test and Benedict's test for reducing sugars which reduce an alkaline solution of a copper(II) salt and give a precipitate of Cu2O.

Cuprous oxide forms on silver-plated copper parts exposed to moisture when the silver layer is porous or damaged; this kind of corrosion is known as red plague.

General applications
Cuprous oxide is commonly used as a pigment, a fungicide and an antifouling agent for marine paints.

Applications as semiconductor
Copper(I) oxide was the first substance known to behave as a semiconductor. Rectifier diodes based on this material were used industrially as early as 1924, long before silicon became the standard.

Copper(I) oxide shows four well understood series of excitons with resonance widths in the range of neV. The associated polaritons are also well understood; their group velocity turns out to be very low, almost down to the speed of sound. That means light moves almost as slow as sound in this medium. This results in high polariton densities, and effects like Bose-Einstein condensation, the dynamical Stark effect and phonoritons have been demonstrated.

Another extraordinary feature of the ground state excitons is that all primary scattering mechanisms are known quantitatively. Cu2O was the first substance where an entirely parameter-free model of absorption linewidth broadening by temperature could be established, allowing the corresponding absorption coefficient to be deduced. It can be shown using Cu2O that the Kramers-Krönig relations do not apply to polaritons.