Inspired by nature one of the most promising and challenging approaches is to synthesize nanocomposites, which consist of a combination of soft organic and hard ceramic materials.1-3 Iron oxide such as magnetite nanoparticles proved to be very suitable for the synthesis of nanocomposite, because nature makes use of iron oxide in chiton radular teeth. Indeed, chiton teeth are one of the hardest and toughest materials known in nature.4
Furthermore, iron oxide nanoparticles are being extensively studied with regard to their size and shape-controlled synthesis. Preparation methods for nanocomposites demands for a superior quality of the NPs. In particular, properties like size, monodispersity, crystal structure and shape are of paramount importance. As the most prominent examples we synthesize monodisperse magnetite nanoparticles as spheres, rods, cubes and octopods.
We present the successful manufacturing of a nanocomposite consisting of oleic acid coated spherical iron oxide nanoparticle with exceptional isotropic mechanical properties.5 We developed an easy concept to link iron oxide nanoparticles in a well-ordered superstructure by oleic acid molecules during a thermal process. The synthesis process is divided into four stages: sedimentation, drying, pressing and heat treatment.
Nanoindentation in the nanocomposite shows an increase of the elastic modulus and hardness with increasing annealing temperature. Despite this compaction and heat treatment, HRTEM and SAXS investigations of milled material clearly show that each particle is still isolated from adjacent particles by an organic layer. XPS measurements of the material provide further evidence for thermally induced chemical changes in the iron oxide/oleic acid system and in addition we exclude a significant fraction of oleic acid graphitisation up to 350 °C.
The exceptional mechanical properties - bending modulus of 114GPa, hardness of up to 4GPa and strength of up to 630MPa - are dominated by the covalent backbone of the linked oleic acid molecules. To our knowledge these are the highest combined values of elastic modulus, strength and nanohardness ever reported for a synthetic bioinspired organic/inorganic nanocomposite.
1. Fratzl, P. & Weinkamer, R. Nature’s hierarchical materials. Prog. Mater. Sci. 52, 1263–1334 (2007).
2. Sellinger, A. et al. Continuous self-assembly of organic-inorganic nanocomposite coatings that mimic nacre. Nature 394, 256–260 (1998).
3. Tang, Z., Kotov, N. a, Magonov, S. & Ozturk, B. Nanostructured artificial nacre. Nat. Mater. 2, 413–418 (2003).
4. Weaver, J. C. et al. Analysis of an ultra hard magnetic biomineral in chiton radular teeth. Mater. Today 13, 42–52 (2010).
5. Dreyer, A. et al. Organically linked iron oxide nanoparticle supercrystals with exceptional isotropic mechanical properties. Nat. Mater. 15, 522–528 (2016).