High-Entropy Layered Double Hydroxides as Precursors for High-Entropy Oxides

Layered double hydroxides (LDHs) consist of positively charged metal cation layers with anion interlayers and water molecules, as following by formula, M_1-x²⁺M_x³⁺(OH)₂[A^n-]_x/nzH₂O. This material has advantages such as tunable metal composition layers, exchangeable interlayer anions, and memory effect, actively applying to the research fields of catalysts, batteries, drug delivery, and adsorption.<br/>Through the flexible design of tunable metal layers, LDHs can be expanded to a high entropy system. The reported high entropy system exhibits a lower adsorption activation in energy in comparison to conventional systems. This resulted from the various adsorption activation energy spectrums and the availability of multiple binding sites because of the mixing of multiple elements. Furthermore, high entropy LDHs can be easily transformed into high entropy layered double oxides (LDOs) through calcination. Nevertheless, there is a scarcity of research on high-entropy LDHs and high-entropy LDOs.<br/>To address this gap, 120 binary, and ternary LDHs were synthesized through co-precipitation, serving as a database for expanding to the quaternary and quinary systems with a machine-learning-based approach. Out of the 120 LDHs, 47 cases were successfully formed as LDH crystal structures, while 10 cases showed mixed phases with less than half consisting of the secondary phases, and the remainder failed to crystallize as LDHs. The thermal stability of synthesized 57 cases was conducted and the evaporation temperature of the parent interlayer anions varied depending on the combination of metal elements. Lastly, the order of iodate adsorption capacity within the ternary system was as follows: Mg-based, Mn-based, Co-based, Cu-based, and Ni-based. Furthermore, 70% of the ternary system showed an improvement in adsorption capacity compared to simple binary cases, underscoring the importance of expanding the high entropy LDH system. Moreover, the exploration of high entropy LDHs might be the role of precursors for expansion into the high entropy oxide research area.