- Nickel(II) oxide (NiO) is a binary inorganic compound composed of nickel in the +2 oxidation state and oxygen. It typically appears as a fine, green to black crystalline powder, depending on the method of preparation and particle size.
- NiO adopts a rock salt (NaCl-type) cubic crystal structure, where Ni²⁺ and O²⁻ ions are arranged in a face-centered cubic lattice. This structure imparts high thermal and chemical stability, making NiO suitable for various industrial, electronic, and catalytic applications.
- Nickel(II) oxide occurs naturally in the rare mineral bunsenite, but it is primarily synthesized through industrial processes. The most common method involves the thermal decomposition (calcination) of nickel(II) carbonate (NiCO₃), nickel(II) hydroxide (Ni(OH)₂), or nickel nitrate (Ni(NO₃)₂) at elevated temperatures (typically 400–600 °C), producing NiO as a stable, anhydrous solid. The color and reactivity of NiO can vary with particle morphology, impurities, and synthesis conditions, with blacker forms often exhibiting higher surface area and reactivity due to partial non-stoichiometry or defects.
- NiO is extensively used in ceramics, glass coloration, and catalysis. It serves as a precursor for producing metallic nickel through hydrogen or carbon reduction and is a component of ferrites used in magnetic and electronic applications. In rechargeable batteries, especially lithium-ion batteries, NiO is employed as an anode material due to its high theoretical capacity and favorable electrochemical properties. It is also utilized in catalysts for hydrogenation reactions, electrochromic devices, and gas sensors, owing to its semiconducting behavior and ability to undergo redox transformations.
- From an electronic standpoint, NiO is considered a p-type semiconductor with a wide band gap (typically around 3.6–4.0 eV), and its conductivity is influenced by intrinsic defects such as nickel vacancies or interstitial oxygen. This makes it suitable for applications in transparent conducting films, solar cells, and thin-film transistors. Its electronic properties are of particular interest in spintronic and photonic device research.
- Despite its utility, nickel(II) oxide poses significant health hazards. It is classified as a carcinogenic substance (Group 1, IARC) due to its ability to release Ni²⁺ ions, which can cause DNA damage upon chronic exposure. Inhalation of NiO dust can lead to respiratory issues, and prolonged contact may result in skin sensitization. Accordingly, strict occupational safety measures are required when handling NiO in industrial or laboratory environments.
