Photophysical and photocatalytic properties of Li₂M(WO₄)₂ (M = Co and Ni)

Li₂M(WO₄)₂ (M = Co and Ni) were synthesized by a conventional solid-state reaction method and characterized by powder x-ray diffraction, Brunauer-Emmet-Teller (BET) measurement, ultraviolet-visible (UV-vis) diffuse reflectance spectra, Raman spectroscopy, and photocatalytic evaluation measurements. Photocatalytic water splitting results showed that Li₂M(WO₄)₂ (M = Co and Ni) exhibited abilities for H₂ evolution with Pt cocatalyst from an aqueous methanol solution and for O₂ evolution from an aqueous AgNO₃ solution under UV light irradiation. Theoretical calculation, absorbance analysis, and photocatalytic H₂ evolution experiment revealed that the position of W 5d level shifted to the negative side with respect to the reduced potential of H⁺/H₂. The photocatalytic H₂ evolution over Li₂M(WO₄)₂ is discussed from the view of crystal and electronic structure point.