Arc Synthesis, Crystal Structure, and Photoelectrochemistry of Copper(I) Tungstate

A little-studied p-type ternary oxide semiconductor, copper(I) tungstate (Cu₂WO₄), was assessed by a combined theoretical/experimental approach. A detailed computational study was performed to solve the long-standing debate on the space group of Cu₂WO₄, which was determined to be triclinic P1. Cu₂WO₄ was synthesized by a time-efficient, arc-melting method, and the crystalline reddish particulate product showed broad-band absorption in the UV-visible spectral region, thermal stability up to ∼260 °C, and cathodic photoelectrochemical activity. Controlled thermal oxidation of copper from the Cu(I) to Cu(II) oxidation state showed that the crystal lattice could accommodate Cu²⁺ cations up to ∼260 °C, beyond which the compound was converted to CuO and CuWO₄. This process was monitored by powder X-ray diffraction and X-ray photoelectron spectroscopy. The electronic band structure of Cu₂WO₄ was contrasted with that of the Cu(II) counterpart, CuWO₄ using spin-polarized density functional theory (DFT). Finally, the compound Cu₂WO₄ was determined to have a high-lying (negative potential) conduction band edge underlining its promise for driving energetic photoredox reactions.