The creation of tailored inorganic nanoparticles not only relies on the crystalline structure, but also the size, shape, and morphology of the particles. Tailoring nanoparticles with well-defined properties has been a synthetic challenge. One approach for this is topotactic conversion, i.e., structural changes to a crystalline solid which may include loss or gain of material and, in this case, can convert nanoparticles to a different structure and composition.
Beth S. Guiton, University of Kentucky, Lexington, USA, and colleagues have created hollow nanostructures by decomposing β-FeOOH nanorods (NRs) into various hollow iron oxide nanocapsules. The NRs were synthesized via a hydrothermal method. When annealing the β-FeOOH NRs in air at 500–700 °C, α-Fe2O3 capsules were formed. In a high-vacuum environment, depending on the annealing temperature, phase transformations from β-FeOOH to β-Fe2O3, γ-Fe2O3, Fe3O4, and FeO were observed during the hollowing process.
The researchers were able to track the morphological progress and the nanocapsule formation using in-situ transmission electron microscopy (TEM)—from the initial morphology through a series of metastable intermediate phases to a final thermodynamically stable phase. The method of formation of the nanocapsules has been termed “shell-induced Ostwald ripening”. Even though all routes produced hollowed-out nanocapsules, the specific route of formation for the final morphology was found to be highly dependent on the crystal structure of the starting material.
- Shell-Induced Ostwald Ripening: Simultaneous Structure, Composition, and Morphology Transformations during the Creation of Hollow Iron Oxide Nanocapsules,
Lei Yu, Ruixin Han, Xiahan Sang, Jue Liu, Melonie P. Thomas, Bethany M. Hudak, Amita Patel, Katharine Page, Beth S. Guiton,
ACS Nano 2018.