Growth Kinetics of Cobalt Carbonate Nanoparticles Revealed by Liquid-Phase Scanning Transmission Electron Microscopy

Abstract

Amorphous precursor phases have been observed in many crystallization processes in aqueous solution, yet their growth kinetics are not fully understood because of their often transient and unstable nature. In this work, we try to close this knowledge gap by employing liquid-phase scanning transmission electron microscopy (LPSTEM) to study the growth kinetics using amorphous cobalt carbonate (CoCO3) as a model system. Using simultaneous acquisition of bright-field and annular dark-field LPSTEM images, we observe that the volume of the amorphous CoCO3 nanoparticles grows linearly with time. By quantifying and extrapolating electron beam effects to zero dose, the growth rate at native solution conditions is obtained. From the observed linear volume growth and the observation of 10 nm-sized nanoparticles by cryotransmission electron microscopy (cryoTEM), we conclude that growth of amorphous CoCO3 nanoparticles proceeds through assembly of the 10 nm primary particles. Our observations not only provide unique insights into the nanoscale growth kinetics of an amorphous phase in aqueous solution that may open the way to improved synthesis protocols of the Co catalyst for Fischer-Tropsch synthesis.