Open Access

Sani Y. Harouna-Mayer, Lars Klemeyer, Cecilia A. Zito, Johan Bielecki, Xuemei Cheng, Davide Derelli, Armando D. Estillore, Tjark L. R. Gröne, Lukas V. Haas, Romain Letrun, Chan Kim, Jayanath C. P. Koliyadu, Abhishek Mall, Parichita Mazumder, Diogo V. M. Melo, Adam R. Round, Amit K. Samanta, Abhisakh Sarma, Zhou Shen, Xiao Sun, Patrik Vagovic, Tamme Wollweber, Richard Bean, Jochen Küpper, Henry N. Chapman, Dorota Koziej, Kartik Ayyer

• Single-particle small-angle X-ray scattering (SP-SAXS) at X-ray free electron lasers (XFELs) enables quantitative analysis of morphological heterogeneity that is fundamentally inaccessible to ensemble-averaged in situ techniques. By recording diffraction snapshots from isolated particles, SP-SAXS resolves low-contrast, less abundant, or transient species within heterogeneous particle populations that would otherwise remain hidden to conventional X-ray techniques. We demonstrate this unique capability by investigating the solvothermal formation of CoO nanocrystal assemblies from a Co­(acac)3 precursor in benzyl alcohol. The single-particle data revealed amorphous, uniform-density Co­(acac)2 spheres as transient intermediates that directly crystallize into cavernous CoO nanocrystal assemblies, explaining why CoO forms as hierarchicalSingle-particle small-angle X-ray scattering (SP-SAXS) at X-ray free electron lasers (XFELs) enables quantitative analysis of morphological heterogeneity that is fundamentally inaccessible to ensemble-averaged in situ techniques. By recording diffraction snapshots from isolated particles, SP-SAXS resolves low-contrast, less abundant, or transient species within heterogeneous particle populations that would otherwise remain hidden to conventional X-ray techniques. We demonstrate this unique capability by investigating the solvothermal formation of CoO nanocrystal assemblies from a Co­(acac)3 precursor in benzyl alcohol. The single-particle data revealed amorphous, uniform-density Co­(acac)2 spheres as transient intermediates that directly crystallize into cavernous CoO nanocrystal assemblies, explaining why CoO forms as hierarchical aggregates rather than as isolated nanocrystals. These results establish SP-SAXS as a uniquely powerful framework for uncovering nonclassical nanoparticle formation pathways hidden in ensemble measurements.…