Co3O4

Co3O4, or cobalt(II,III) oxide, is a mixed-valence oxide of cobalt with the formula Co3O4. It adopts a spinel-type structure in which Co2+ ions occupy tetrahedral sites and Co3+ ions occupy octahedral sites, giving the empirical formulation often described as CoO·Co2O3. The material is a black, insoluble solid that is thermally stable in air up to several hundred degrees Celsius.

Synthesis and forms: It can be prepared by calcining cobalt(II) salts, such as cobalt nitrate, cobalt carbonate, or cobalt oxalate, in air. Other routes include precipitation from cobalt salts followed by annealing, hydrothermal synthesis, and combustion or templating methods. Nanostructured forms (nanosheets, nanorods, porous nanoparticles) are common to increase surface area for catalysis and electrochemistry.

Properties: Co3O4 is a p-type semiconductor with a relatively narrow band gap (reported around 1.5–1.7 eV, depending on preparation). It is widely studied for catalytic applications, including oxidation of carbon monoxide and volatile organic compounds, and as a black solid electrode material in electrochemical devices.

Applications: In energy storage, Co3O4 serves as an active material in lithium-ion battery anodes—via a conversion reaction that forms metallic cobalt and Li2O—and in supercapacitors. It is also used in electrochromic coatings and gas-sensing devices, where its redox properties enable reversible changes in optical or electrical response.

Safety and handling: Co3O4 poses typical hazards associated with fine metal oxides, including respiratory irritation if inhaled. Appropriate engineering controls, protective equipment, and waste disposal procedures should be followed as for cobalt compounds.