Nickeloxide

Nickel oxide, NiO, is an inorganic compound composed of nickel in the +2 oxidation state and oxide ions. It is a solid that ranges in color from pale green to black, depending on particle size and impurities. NiO adopts the rock-salt structure, with nickel ions in octahedral coordination to oxide ions. It has a relatively wide band gap and behaves as a p-type semiconductor when nonstoichiometry introduces nickel vacancies.

In terms of properties, NiO is typically an insulator with a fundamental band gap around 3.6 eV and an optical gap near 4.0 eV. Its electrical conductivity is enhanced in nonstoichiometric samples due to nickel vacancies, which create holes. Magnetically, NiO is antiferromagnetic below its Néel temperature of about 523 K; above this temperature it becomes paramagnetic. The material is chemically stable under many conditions but can be reduced to metallic nickel at high temperatures in reducing environments.

Preparation methods include calcination of nickel hydroxide or nickel carbonate to NiO, roasting nickel metal in air, or decomposition of nickel salts such as nickel nitrate. Sol-gel, hydrothermal, and other modern synthesis routes also yield nanostructured NiO with controlled morphology.

Applications for NiO are diverse. It has long been used as a ceramic pigment, providing green hues in glazes and enamels. NiO serves as a precursor and component in nickel-based catalysts and has been studied as a p-type semiconductor for electrochromic devices and other thin-film electronics. It is also investigated for gas sensing and energy storage applications, often in nanostructured forms.

Safety considerations include the general hazards of nickel compounds, which can be toxic and potentially carcinogenic with inhalation exposure; appropriate handling and protective equipment are advised.