Copper(II) tungstate is an inorganic compound with the chemical formula CuWO4. It is a yellow-brown crystalline solid that is practically insoluble in water. CuWO4 is an n-type semiconductor and has been investigated as a photocatalyst and as a photoanode material for photoelectrochemical water oxidation.
Structure and properties Anhydrous copper(II) tungstate crystallizes in the triclinic crystal system, in the space group P. Its structure is a distorted derivative of the wolframite structure.
The unit-cell parameters have been reported as approximately a = 4.703 Å, b = 5.839 Å and c = 4.878 Å, with α = 91.8°, β = 92.5°, γ = 82.8° and Z = 2. Both Cu(II) and W(VI) are six-coordinate. The CuO6 octahedra are strongly elongated as a result of the Jahn–Teller effect associated with Cu2+, whereas the WO6 octahedra are less distorted.
CuWO4 is structurally related to monoclinic wolframites such as MnWO4, but the stronger Jahn–Teller distortion of Cu(II) lowers its symmetry to triclinic.
At high pressure, CuWO4 undergoes a structural phase transition at about 9 GPa from its triclinic structure to a monoclinic P2/c wolframite-type structure. A second high-pressure transformation occurs above approximately 22 GPa.
A dihydrate, CuWO4·2H2O, is also known. It forms a monoclinic hydrated structure and can dehydrate on heating to form anhydrous triclinic CuWO4.
CuWO4 is a semiconductor with an indirect optical band gap of about 2.2–2.3 eV. Thin films of CuWO4 behave as n-type semiconductors, and their comparatively narrow band gap allows absorption of part of the visible spectrum.
Preparation Copper(II) tungstate can be prepared by precipitation from aqueous solutions of copper(II) nitrate and sodium tungstate:
Cu(NO3)2 + Na2WO4 → CuWO4↓ + 2 NaNO3
The material precipitated from aqueous solution may initially contain CuWO4·2H2O; heating converts the hydrated material into anhydrous CuWO4.
Anhydrous CuWO4 can also be prepared by a high-temperature solid-state reaction between copper(II) oxide and tungsten trioxide:
CuO + WO3 → CuWO4
Solid-state syntheses are typically carried out at several hundred degrees Celsius; modern preparations commonly use temperatures around 750–800 °C.
Applications Copper(II) tungstate has attracted interest as a visible-light-active semiconductor. Polycrystalline CuWO4 thin films have been studied as photoanodes for the photoelectrochemical oxidation of water because of their n-type conductivity and indirect band gap of approximately 2.25 eV.
CuWO4 has also been investigated as a photocatalyst for degradation of organic contaminants in water. Its absorption in the visible region and comparatively good stability in aqueous media make it useful in experimental photocatalytic systems.
Other investigated applications include humidity and gas sensors, electrochemical electrodes and energy-storage materials.
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