title: Rhodium(III) oxide
text: # 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.
  
  