Exceptional Tunability over Size and Density of Spontaneously Formed Nanoparticles via Nucleation Dynamics

The ex-solution process, in which metal nanoparticles are grown on a host oxide, can be used to synthesize nanocatalysts with excellent thermal and chemical durability via spontaneous heterogeneous nucleation. However, this technique lacks a means to control the particle size and density because the amounts of ex-solved metal elements vary with the reduction conditions. Here, we devise a strategy to achieve small particle sizes and high particle densities concurrently by controlling the temperature (T), oxygen partial pressure (pO₂) and ramping rate of the temperature. Quantitative analyses of Co particles ex-solved on Sr_0.98Ti_0.95Co_0.05O_3-δ thin films using ex situ SEM and in situ TEM reveal that the increasing T and decreasing the pO₂ lead to smaller particle sizes with higher density levels and vice versa, contrary to common ex-solution examples. We find that nucleation thermodynamics dictates such counterintuitive behaviors of particle characteristics, which are attributed to our specific ex-solution conditions in which particle interactions are minimized and all the Co atoms are ex-solved under highly reducible conditions. We also demonstrated the feasibility of our strategy via CO oxidation with typical powder-based catalysts, suggesting that this method can be extended to various chemical/electrochemical applications.