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We present a systematic study, using atomic force and high resolution electron microscopy, of the morphology and crystal surface structure of hematite nanoparticles (HNP) synthesized from hydrolysis. We demonstrate that the HNP shape is unique, associated with facets belonging to the {104} family.
Nanometrical-sized particles are today largely used for various purposes because of the huge surface area (interesting for catalysis and adsorption phenomena) and because of their specific properties dependent on their size and shape (for instance, grain boundary effects on mechanical properties, quantum size effects on electrical and optical properties, relaxation and surface phenomena on magnetic properties). These effects give rise to interesting technological applications which need however dimensionally and morphologically well-defined particles, the more often free from aggregation and of various mean sizes. In specific conditions, nanometrical and non-aggregated HNPs of well-defined morphology and forming stable sols are obtained with different shapes. In fact, the particle shape is the more often determined from electron and/or atomic force microcopies, and/or X-ray diffraction experiments, giving rise to ambiguities on the effective shape of particles. Transmission electron microscopy shows projections of the particles along the optical axis of the microscope. To unambiguously determine particle's shapes, a brief observation of a few micrographs is certainly not enough. On the other hand, previous studies performed by AFM on nanoparticles have underlined the difficulties to accurately determine the shape and size of the particles, especially if they are aggregated.
We show how Tapping Mode atomic force microscopy in association with high resolution transmission electron microscopy can be effectively and coherently used. Sample deposition was improved to obtain well-isolated HNP for AFM imaging. By pushing the limits of equipment resolution and achieving imaging of these nanoparticles as never before, the identification of crystal faces of the HNP by AFM was possible. This is a significant step in our current efforts to apply atomic force microscopy on nanoparticles research. |