Making Hollow Iron Oxide Nanocapsules

Making Hollow Iron Oxide Nanocapsules

Author: Liam Critchley

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.


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