Characterization of Metallic Iridium Nanoparticles Synthesized under Hydrothermal Conditions

This paper examines processes for the preparation of metallic iridium nanoparticles under hydrothermal conditions. The reduction of aqueous potassium hexachloroiridate(IV) solutions with sodium tetrahydridoborate in acidic and alkaline media at temperatures from 130 to 180°C has been shown to take 2–30 min and result in the formation of fine metallic iridium powder with a characteristic mosaic structure. The average size of the iridium(0) nanoparticles varies from 8 to 300 nm, depending on synthesis conditions, and the crystallite size is no greater than 10 nm. According to low-temperature nitrogen gas adsorption measurements, the specific surface area of the materials prepared in acid solutions ranges from 1 to 10 m2/g, and that of the materials prepared in alkaline solutions reaches 25 m2/g. X-ray photoelectron spectroscopy results demonstrate that the surface of the 8-nm-diameter iridium nanoparticles is covered with an oxide film. As shown by differential scanning calorimetry and thermogravimetry in an argon atmosphere, the fraction of iridium oxide compounds in the material with a specific surface area of 25 m2/g does not exceed 5 wt %.