Synthesis and Magnetic, Optical, and Electrocatalytic Properties of High-Entropy Mixed-Metal Tungsten and Molybdenum Oxides.

High-entropy oxides (HEOs) are of interest for their unique physical and chemical properties. Significant lattice distortions, strain, and tolerance for high-vacancy concentrations set HEOs apart from single-metal or mixed-metal oxides. Herein, we synthesized and characterized the structures and compositions, along with the optical, magnetic, and electrocatalytic properties, of two families of high-entropy mixed-metal tungsten and molybdenum oxides, AWO 4 and B 2 Mo 3 O 8 , where A and B are 3d transition metals. The HEOs A 6 WO 4 (A = Mn, Fe, Co, Ni, Cu, and Zn) and B 2 5 Mo 3 O 8 (B = Mn, Fe, Co, Ni, and Zn), as well as all accessible single-metal AWO 4 and B 2 Mo 3 O 8 parent compounds, were synthesized using high-temperature solid-state methods. X-ray photoelectron spectroscopy analysis of the surfaces revealed that the HEOs largely had the metal oxidation states expected from the bulk chemical formulas, but in some cases they were different than in the parent compounds. A 6 WO 4 exhibited antiferromagnetic (AFM) ordering with a Néel temperature of 30 K, which is less than the average of its AFM parent compounds, and had a narrow band gap of 0.24 eV, which is much lower than all of its parent compounds. B 2 5 Mo 3 O 8 was paramagnetic, despite the existence of AFM and ferromagnetic ordering in several of its parent compounds and had no observable band gap, which is analogous to its parent compounds. Both A 6 WO 4 and B 2 5 Mo 3 O 8 exhibited superior catalytic activity relative to the parent compounds for the oxygen evolution reaction, the oxidation half reaction of overall water splitting, under alkaline conditions, based on the overpotential required to reach the benchmark surface area normalized current density. Consistent with literature predictions of OER durability for ternary tungsten and molybdenum oxides, A 6 WO 4 and B 2 5 Mo 3 O 8 also exhibited stable performance without significant dissolution during 10 h stability experiments at a constant current.