Partial Substitution of Cu Sites by Mg for the Improvement of CuWO₄ Photoanodes Performance

The photoelectrochemical properties of CuWO 4 (Mg x %) thin-films obtained by solution-based methods are investigated as a function of the material composition. The thin-films are prepared by spin-coating a single precursor solution onto FTO-coated glass substrates, followed by an annealing process at 550 °C. XRD, Raman, XPS, and electrochemical data studies indicate the formation of single-phase CuWO 4 (Mg x %), with Mg 2+ partially substituting Cu 2+ sites. Photoelectrochemical studies under monochromatic illumination show an 88.2% increase in photocurrent responses and a 2-fold increase in charge carriers bulk separation efficiency at 1.0 V vs RHE, upon replacing 2.5% of Cu by Mg. DFT calculations reveal that Mg incorporation rearranges electron density, shifting the position of magnesium toward an axial oxygen atom, increasing the covalent nature of the bond and decreasing the Cu–O bond length. It is proposed that a change in the localization of the electron density away from the sphere of influence of the oxygen atom, and toward the shared space of the covalent bond, leads to better carrier mobility and the generation of higher photocurrents.