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

Abstract

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, AWO4 and B2Mo3O8, where A and B are 3d transition metals. The HEOs A6WO4 (A = Mn, Fe, Co, Ni, Cu, and Zn) and B2 5Mo3O8 (B = Mn, Fe, Co, Ni, and Zn), as well as all accessible single-metal AWO4 and B2Mo3O8 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. A6WO4 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. B2 5Mo3O8 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 A6WO4 and B2 5Mo3O8 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, A6WO4 and B2 5Mo3O8 also exhibited stable performance without significant dissolution during 10 h stability experiments at a constant current.

Preparation and Properties of a High-Entropy Wolframite-Type Antiferromagnet, (Mn0.2Co0.2Ni0.2Cu0.2Cd0.2)WO4

“…(ii) We avoided Fe 2+ , which is prone to oxidation, as is evident from Figure S1, where the point of the six-component material shows an abnormally reduced unit cell volume (as well as one of points corresponding to FeWO 4 ); indeed, X-ray photoelectron spectroscopy (XPS) data showed the presence of Fe 3+ and Cu + in the six-component material; on the other hand, the two Fe-free five-component materials (that from ref and ours) have unit cell volumes in good agreement with the general trend (points 1 and 3 in Figure S1).…”

“…Because no magnetic studies of wolframites containing more than two divalent cations were known from literature, we started the preparation and study of a five-component wolframite-type magnet, (Mn 1/5 Co 1/5 Ni 1/5 Cu 1/5 Cd 1/5 )­WO 4 (TWO 4 for short), being sure that this will be a pioneering work [a recent publication on (Mn 1/5 Co 1/5 Ni 1/5 Cu 1/5 Zn 1/5 )­WO 4 did not report magnetic properties]. However, in May 2023, a paper appeared on (Mn 1/6 Fe 1/6 Co 1/6 Ni 1/6 Cu 1/6 Zn 1/6 )­WO 4 with T N of 30 K . Actually, our work brings sufficiently new information and may be a useful addition to the previous publications:…”

Structural and Electronic Factors behind the Electrochemical Stability of 3D-Metal Tungstates under Oxygen Evolution Reaction Conditions

“…As anticipated, the core-level Ni 2p spectrum, recorded with a post-CA sample, exhibited a single envelope and spin–orbit components corresponding to the binding energy of Ni 2+ (Figure b). However, in a very recent study by Schaak et al, CoWO 4 is claimed to be more stable than NiWO 4 during a 10 h chronopotentiometry (CP) study, where degradation followed by dissolution of NiWO 4 was noticed after 6 h of CP study due to the voltage delivered beyond 1.85 V (vs RHE) …”

High Entropy Oxides: Mapping the Landscape from Fundamentals to Future Vistas

“…Magnetization data reveals that (f) A 6 WO 4 exhibits AFM behavior with a TN of 30 K, while (g) B 2 5 Mo 3 O 8 displays paramagnetic properties. Panels (f–g) are adapted with permission from ref . Copyright 2023 American Chemical Society.…”