# Copper(I) oxide

Copper(I) oxide or cuprous oxide is the inorganic compound with the formula Cu$_2$O. It is one of the principal oxides of copper, the other being copper(II) oxide or cupric oxide (CuO).The compound can appear either yellow or red, depending on the size of the particles. Cuprous oxide is found as the mineral cuprite. It is a component of some antifouling paints, but also has other applications including some that exploit its property as a semiconductor.

## Preparation

Copper(I) oxide may be produced by several methods. Most straightforwardly, it arises via the oxidation of copper metal:
    4 Cu + O$_2$ → 2 Cu$_2$O
Additives such as water and acids affect the rate as well as the further oxidation to copper(II) oxides. It is also produced commercially by reduction of copper(II) solutions with sulfur dioxide.
Alternatively, it may be prepared via the reduction of copper(II) acetate with hydrazine:
    4 Cu(O$_2$CCH$_3$)$_2$ + N$_2$H$_4$ + 2 H$_2$O→ 2 Cu$_2$O + 8 CH$_3$CO$_2$H + N$_2$
Aqueous cuprous chloride solutions react with base to give the same material. In all cases, the color of the cuprous oxide is highly sensitive to the procedural details. Cu$_2$O degrades to copper(II) oxide in moist air.
Formation of copper(I) oxide is the basis of the Fehling's test and Benedict's test for reducing sugars. These sugars reduce an alkaline solution of a copper(II) salt, giving a bright red precipitate of Cu$_2$O.
It 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.

## Properties

Like all copper(I) compounds, cuprous oxide is diamagnetic. It does not readily hydrate to cuprous hydroxide.
Copper(I) oxide dissolves in concentrated ammonia solution to form the colourless complex [Cu(NH$_3$)$_2$]$^+$, which is easily oxidized in air to the blue [Cu(NH$_3$)$_4$(H$_2$O)$_2$]$^{2+}$.
Cuprous oxide is attacked by acids. Hydrochloric acid gives the chloride complex CuCl$^−$ $_2$. Sulfuric acid and nitric acid produce copper(II) sulfate and copper(II) nitrate, respectively.

## Structure

In terms of their coordination spheres, copper centres are 2-coordinated and the oxides are tetrahedral. The structure thus resembles in some sense the main polymorphs of SiO$_2$, but cuprous oxide's lattices interpenetrate. Cu$_2$O crystallizes in a cubic structure with a lattice constant a$_l$ = 4.2696 Å. The copper atoms arrange in a fcc sublattice, the oxygen atoms in a bcc sublattice. One sublattice is shifted by a quarter of the body diagonal. The space group is Pn3m, which includes the point group with full octahedral symmetry.

## Applications

The dominant use of cuprous oxide is as a component of antifouling paints.
Cuprous oxide is also commonly used as a pigment and a fungicide.

### Semiconductor and related uses

Rectifier diodes based on this material have been used industrially as early as 1924, long before silicon became the standard. Copper(I) oxide is also responsible for the pink color in a positive Benedict's test. In the history of semiconductor physics, Cu$_2$O is one of the most studied materials. Many Semiconductor applications have been demonstrated first in this material:
* Semiconductor diodes
* Phonoritons ("a coherent superposition of exciton, photon, and phonon")
The lowest excitons in Cu$_2$O are extremely long lived; absorption lineshapes have been demonstrated with neV linewidths, which is the narrowest bulk exciton resonance ever observed. The associated quadrupole polaritons have low group velocity approaching the speed of sound. Thus, light moves almost as slowly as sound in this medium, which results in high polariton densities. Another unusual feature of the ground state excitons is that all primary scattering mechanisms are known quantitatively. Cu$_2$O 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 Cu$_2$O that the Kramers–Kronig relations do not apply to polaritons.
In December 2021, Toshiba disclosed a transparent cuprous oxide (Cu$_2$O) thin-film solar cell. The cell achieved an 8.4% energy conversion efficiency, the highest efficiency ever reported for any cell of this type as of 2021. The cells could be used for high-altitude platform station applications and electric vehicles.

## Similar compounds

An example of natural copper(I,II) oxide is the mineral paramelaconite, Cu$_4$O$_3$ or Cu$^I$ $_2$Cu$^{II}$ $_2$O$_3$.