Nio

Nickel(II) oxide, NiO, is an inorganic compound that forms a green solid and is commonly referred to simply as nickel oxide. It is the binary oxide of nickel and occurs in the rock-salt crystal structure, with Ni2+ and O2− in octahedral coordination.

In terms of physical properties, NiO has a wide bandgap on the order of 3.6 to 4.0 eV, making it a wide-bandgap p-type semiconductor. Its electrical conductivity is relatively low at room temperature and increases with temperature due to intrinsic charge carriers and nickel vacancies, which impart p-type behavior. Magnetically, NiO orders antiferromagnetically below a Néel temperature of about 523 K, with a type-II antiferromagnetic arrangement; some samples exhibit weak ferromagnetism due to spin canting.

NiO occurs naturally as the mineral bunsenite but is more commonly encountered in synthetic form. It is produced industrially by calcining nickel salts or hydroxides, or by the controlled oxidation of nickel precursors. The material is typically prepared as powders or as thin films for various applications.

Applications of NiO include use as a pigment, often referred to as nickel oxide green, in ceramics and glasses. It is also employed as a catalyst or catalyst support for oxidation reactions and as a functional material in electrochromic devices, gas sensors, and as a p-type component in thin-film electronics. In electrochemical contexts, NiO participates in redox couples involving Ni2+/Ni3+ (as NiOOH/Ni(OH)2) in alkaline media, enabling charge storage and related electrochemical processes.

Safety considerations for NiO align with those for nickel compounds; dust inhalation should be avoided, and appropriate handling precautions applied in industrial settings. In general, NiO is considered relatively stable, but nickel compounds are regulated due to potential health and environmental concerns.