Optical and electron microscopy studies of Schiller layer formation and structure

Iridescent Schiller layers were prepared by centrifugation of β-FeOOH sols with an initial particle concentration of 1014 particles/mL, reducing the Schiller layer formation time from over 2 months to 3 weeks. The formation and structure of the Schiller layers were investigated using optical and transmission electron microscopy. Microscopy studies revealed the self-assembly to proceed by the formation of two-dimensional particle arrays followed by the stacking of these arrays to form the final iridescent state. Varying the pH showed that Schiller layer formation occurs only in the pH range 1.4–2.0, indicating that electrostatic interactions play a pivotal role in the self-assembly. Decreasing the particle concentration of the sols was found to inhibit the assembly. DLVO theory and order–disorder phase transition models were found to be insufficient to accurately model the experimental behavior. Several approaches were investigated in an attempt to make ferrimagnetic arrays from the Schiller layers. The most promising was via electron beam irradiation, which transforms the β-FeOOH into γ-Fe2O3 without altering the shape of the nanorods.

TEM micrograph showing self-assembled array of nanorods.Figure optionsDownload as PowerPoint slide