Cobalt(II) tungstate is an inorganic compound with the chemical formula CoWO4. It is a greenish-blue to blue-black crystalline solid and belongs to the wolframite family of transition-metal tungstates.
Preparation Cobalt(II) tungstate can be prepared by precipitation from aqueous solutions of cobalt(II) sulfate and sodium tungstate:
CoSO4 + Na2WO4 → CoWO4↓ + Na2SO4
Nanocrystalline CoWO4 can also be prepared hydrothermally. Hydrothermal syntheses without templates or surfactants have produced nanocrystals with sizes on the order of tens of nanometres.
Structure and properties Cobalt(II) tungstate crystallizes in the monoclinic crystal system with the wolframite-type structure, in space group P2/c (No. 13). Its structure contains distorted CoO6 and WO6 octahedra linked through shared edges and corners to form a three-dimensional framework.
CoWO4 is paramagnetic at room temperature and becomes antiferromagnetic at low temperature. Nanocrystalline CoWO4 has a reported Néel temperature of about 40 K, somewhat lower than bulk material because of reduced exchange coupling in nanoscale particles.
It is also a semiconductor. Reported optical band gaps are typically in the vicinity of 2.7–2.9 eV, and CoWO4 has been described as showing p-type electrical conductivity.
CoWO4 is nearly insoluble in water but dissolves in hot concentrated acids.
Occurrence A material with the composition CoWO4 has been found in burning coal-mine waste and is known as krasnoselskite. Because it is of anthropogenic origin, it is not recognized by the International Mineralogical Association as a valid mineral species. It is classified structurally with the wolframite group.
Applications Cobalt(II) tungstate has been investigated as a photocatalyst and electrocatalyst. Recent research has examined CoWO4-based materials for oxygen evolution, oxygen reduction, nitrate reduction, photocatalytic hydrogen evolution, carbon dioxide reduction and degradation of organic pollutants.
It has also been used as a ceramic pigment and has been investigated as a catalyst and functional oxide material.
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