Single-Particle X-ray Scattering Reveals a High Local Supersaturation of Precursors as the Origin of CoO Assembly Formation

TL;DR

This study uses single-particle X-ray scattering to uncover that high local supersaturation of precursors drives the formation of hierarchical CoO nanocrystal assemblies, revealing transient amorphous intermediates and the assembly pathway.

Contribution

The paper demonstrates the application of SP-SAXS to resolve transient intermediates and heterogeneity in nanoparticle formation, providing new insights into CoO assembly mechanisms.

Findings

Identification of amorphous Co(acac)$_2$ spheres as intermediates

Direct crystallization into cavernous CoO assemblies

High local supersaturation as the assembly origin

Abstract

Single-particle small-angle X-ray scattering (SP-SAXS) enables quantitative morphological analysis by recording diffraction snapshots from isolated particles using X-ray free-electron laser (XFEL) pulses. Unlike conventional X-ray techniques, which average over the entire illuminated sample volume, SP-SAXS resolves low-contrast, less abundant, or transient species within heterogeneous particle populations that would otherwise remain hidden. Here, we apply SP-SAXS to investigate the solvothermal formation of CoO nanocrystal assemblies from a Co(acac)$_3$ precursor in benzyl alcohol. The single-particle data reveal amorphous, uniform-density Co(acac)$_2$ spheres as transient intermediates that directly crystallize into cavernous CoO nanocrystal assemblies, which explains why CoO forms as hierarchical aggregates rather than as isolated nanocrystals. These results demonstrate that SP-SAXS provides a powerful framework for disentangling morphological heterogeneity in nanoparticle formation processes.