In Situ Study of Spinel Ferrite Nanocrystal Growth Using Liquid Cell Transmission Electron Microscopy

We report transition metal oxide nanocrystal formation in a liquid cell using transmission electron microscopy (TEM). The growth of M–Fe–oxide (M = Ni, Mn, Co, or Zn) nanoparticles from a growth solution of metal acetylacetonates dissolved in oleylamine, oleic acid, and benzyl ether was studied. Nickel iron oxide nanocrystals with spinel structure were obtained under electron beam irradiation of the Ni–Fe growth solution, whereas iron oxide nanocrystals were achieved with Mn remaining in the Mn–Fe growth solution. Similarly, we achieved cobalt iron oxide nanocrystals in the Co–Fe precursor solution, while iron oxide nanoparticles were obtained in the Zn–Fe solution. By tracking nanoparticle size evolution as a function of time along the Ni–Fe–oxide nanoparticle growth trajectories, we found the growth kinetics follow a Lifshitz–Slyozov–Wagner (LSW) model suggesting surface reaction-limited growth. Ex situ characterization shows elemental distribution and structural and valence state of the different nanoparticles. The trend of nanoparticle growth in a liquid cell shares many similarities with those in “one-pot” flask synthesis by thermal heating. We compare reduction potentials (E_r) of the metal ions and thermal decomposition temperatures (T_d) of the precursors and correlate them with nanoparticle growth in a liquid cell under TEM. We found a tendency to form mixed metal ion oxide nanoparticles instead of single metal ion (iron) oxides when the two precursors have similar values of T_d and metal ion reduction potential. The higher T_d and smaller E_r values of Mn and Zn precursors than those of Fe precursor, as well as Ni and Co precursors, may have resulted in the single metal ion (iron) oxide formation in M–Fe (M = Mn and Zn) precursor systems. This study sheds light on nanoparticle growth mechanisms by liquid cell TEM. In situ study of oxide nanocrystal growth using liquid cell TEM provides the opportunity to explore solution chemistry during nanocrystal growth beyond the nanoparticle growth that occurs in a TEM cell.