Band Gap Narrowing in a High-Entropy Spinel Oxide Semiconductor for Enhanced Oxygen Evolution Catalysis

Abstract

High-entropy oxides (HEOs), which contain five or more metal cations that are generally thought to be randomly mixed in a crystalline oxide lattice, can exhibit unique and enhanced properties, including improved catalytic performance, due to synergistic effects. Here, we show that band gap narrowing emerges in a high-entropy aluminate spinel oxide, (Fe0.2Co0.2Ni0.2Cu0.2Zn0.2)Al2O4 (A 5Al2O4). The 0.9 eV band gap of A 5Al2O4 is narrower than the band gaps of all parent spinel oxides. First-principles calculations for multicomponent AAl2O4 spinels indicate that the band gap narrowing arises from the broadening of the energy distribution of the 3d states due to variations in the electronegativities and crystal field splitting across the 3d transition-metal series. As a catalyst for the oxygen evolution reaction in an alkaline electrolyte, A 5Al2O4 reaches a current density of 10 mA/cm2 at an overpotential of 400 mV, outperforming all of the single-metal end members at an applied potential of 1.7 V vs RHE. Catalyst deactivation occurs after 5 h at 10 mA/cm2 and is attributed, based on elemental analysis and grazing-incidence X-ray diffraction, to the formation of a passivating layer that blocks the high-entropy oxide surface. This result helps to validate that the HEO is the active catalyst. The observation of band gap narrowing in A 5Al2O4 expands the scope of synergistic properties exhibited by high-entropy materials and offers insight into the question of how the electronic structure of multicomponent oxide materials can be engineered via a high-entropy approach to achieve enhanced catalytic properties.

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

“…The existence of a narrow band gap indicates that this material is a degenerate semiconductor (Figure a). The striking difference in band gaps between A 6 WO 4 and all of the parent compounds is consistent with the previously observed band structure reconstruction and narrowing in other transition metal HEOs, such as A 5 Al 2 O 4 , and rare earth HEOs, such as (Ce,Gd,La,Nd,Pr,Sm,Y)­O 2−δ , , as discussed above. In contrast to the tungstates, all of the molybdate parent compounds have a black color, as previously reported, with no observable band gaps in the visible or near-IR range (Figure b).…”

“…In addition, the large number of constituent metals can lead to a shift in orbital energies. One representative example is the recently reported HEO spinel (Fe,Co,Ni,Cu,Zn)­Al 2 O 4 , which exhibits a significant band gap narrowing relative to the parent phases . Such a band structure reconstruction can be attributed to the combined effect of the emergence of new t 2g and e g states near the Fermi level due to the electronegativity differences of the mixed transition metals and the crystal field splitting of the high entropy phase adopting the smallest energy difference of the parent phases …”

“…One representative example is the recently reported HEO spinel (Fe,Co,Ni,Cu,Zn)Al 2 O 4 , which exhibits a significant band gap narrowing relative to the parent phases. 10 structure reconstruction can be attributed to the combined effect of the emergence of new t 2g and e g states near the Fermi level due to the electronegativity differences of the mixed transition metals and the crystal field splitting of the high entropy phase adopting the smallest energy difference of the parent phases. 10 Because of their unique band structures and the synergy of mixed elements, HEOs can exhibit unique properties that are superior to those of their end members, including both bulk properties such as thermal conductivity, mechanical strength, and ionic conductivity, and surface properties such as catalytic activity.…”

“…10 structure reconstruction can be attributed to the combined effect of the emergence of new t 2g and e g states near the Fermi level due to the electronegativity differences of the mixed transition metals and the crystal field splitting of the high entropy phase adopting the smallest energy difference of the parent phases. 10 Because of their unique band structures and the synergy of mixed elements, HEOs can exhibit unique properties that are superior to those of their end members, including both bulk properties such as thermal conductivity, mechanical strength, and ionic conductivity, and surface properties such as catalytic activity. [1][2][3]5 The superior catalytic activities of high-entropy materials, in particular, indicate that the mixed metal sites that comprise them also persist at the surface and advantageously impact surface properties.…”

General Route to Colloidally Stable, Low-Dispersity Manganese-Based Ternary Spinel Oxide Nanocrystals

“…The synthesis of ternary spinel oxide nanocrystals is increasingly being pursued due to the diverse applications of these materials. − Manganese (Mn) ternary spinels have been targeted for a variety of energy applications such as supercapacitors, − lithium-ion batteries, − and as electrocatalysts for the oxygen reduction reaction (ORR) − and the oxygen evolution reaction (OER). − ,− Cobalt-manganese (Co-Mn) spinel oxides, in particular, have demonstrated outstanding performance as supercapacitors and electrocatalysts. ,,− ,,,, The normal spinel structure is AB 2 O 4 , where A is a 2+ cation with tetrahedral coordination and B is a 3+ cation with octahedral coordination; in inverse spinels, the 2+ cation occupies an octahedral site and half of the 3+ cations occupy the tetrahedral site; and in complex spinels, a mixture of normal and inverse spinel structures exists . Previous work on Co-Mn spinels suggests that it is a complex spinel structure .…”