Breaking the Heat Barrier—Colloidal Synthesis of Ultra-Small, Monodisperse High Entropy Oxide Nanocrystals

Despite recent interest in high entropy materials, there have been no reports of high entropy oxide (HEO) nanocrystals synthesized using colloidal methods. Conventional synthesis methods focus on high temperatures (TΔS) to overcome enthalpy and form a multi-metal phase, often resulting in large, polydisperse nanocrystals. In contrast, colloidal synthesis – which involves precursor decomposition within an organic ligand matrix at low temperatures and ambient pressure – produces the highest-quality nanocrystals with minimal size dispersity and stable solutions without agglomeration.<br/><br/>In this work, we challenge the conventional belief that high temperatures are necessary to thermodynamically drive HEO formation. Instead, we focus on the kinetics of precursor to monomer conversion, hypothesizing that similar decomposition rates among precursors can achieve HEO phases at lower temperatures, improving size and size dispersion control. We successfully leverage the low solubility product (K_sp) of metal oxides to synthesize HEO nanocrystals at low temperatures, producing the smallest (&lt;4 nm) and most monodisperse (&lt;15% size dispersity) HEOs to date. Our findings demonstrate a promising approach for producing spinel HEO nanocrystals with excellent size and dispersity control. Additionally, we show that this method enables the tunability in the number of cations, incorporating fewer cations into the lattice (from 1 to 6) than the high-entropy 5-cation rule suggests. This finding indicates that multicomponent phase formation in spinel oxides is not necessarily entropically driven.<br/><br/>We apply these HEO nanocrystals as electrocatalysts, showing promising activity towards the oxygen evolution reaction (OER) in alkaline media, with an overpotential of 345 mV at 10 mA/cm², only 75 mV higher than the precious metal RuO₂ catalyst at the same current density.<br/><br/>Reference: J.L. Rowell, M. Kang, D. Yoon, K.Z. Jiang, Y. Jia, H.D. Abruña, D.A. Muller, R.D. Robinson, <i>JACS Communication</i> (accepted) 2024