Rhodium(III) oxide
# Rhodium(III) oxide

Rhodium(III) oxide (or Rhodium sesquioxide) is the inorganic compound with the formula $Rh_2O_3.$ It is a gray solid that is insoluble in ordinary solvents.

## Structure

$Rh_2O_3$ has been found in two major forms. The hexagonal form adopts the corundum structure. It transforms into an orthorhombic structure when heated above 750 °C.

## Production

Rhodium oxide can be produced via several routes:

* Treating $RhCl_3$ with oxygen at high temperatures.
* Rh metal powder is fused with potassium hydrogen sulfate. Adding sodium hydroxide results in hydrated rhodium oxide, which upon heating converts to $Rh_2O_3.$
* Rhodium oxide thin films can be produced by exposing Rh layer to oxygen plasma.
* Nanoparticles can be produced by the hydrothermal synthesis.

## Physical properties

Rhodium oxide films behave as a fast two-color electrochromic system: Reversible yellow ↔ dark green or yellow ↔ brown-purple color changes are obtained in KOH solutions by applying voltage ~1 V.

Rhodium oxide films are transparent and conductive, like indium tin oxide (ITO) - the common transparent electrode, but $Rh_2O_3$ has 0.2 eV lower work function than ITO. Consequently, deposition of rhodium oxide on ITO improves the carrier injection from ITO thereby improving the electrical properties of organic light-emitting diodes.

## Catalytic properties

Rhodium oxides are catalysts for hydroformylation of alkenes, $N_2O$ production from NO, and the hydrogenation of CO.

