Hierarchical supercrystalline nanocomposites through the self-assembly of organically-modified ceramic nanoparticles

Abstract

Biomaterials often display outstanding combinations of mechanical properties thanks to their hierarchical structuring, which occurs through a dynamically and biologically controlled growth and self-assembly of their main constituents, typically mineral and protein. However, it is still challenging to obtain this ordered multiscale structural organization in synthetic 3D-nanocomposite materials. Herein, we report a new bottom-up approach for the synthesis of macroscale hierarchical nanocomposite materials in a single step. By controlling the content of organic phase during the self-assembly of monodisperse organically-modified nanoparticles (iron oxide with oleyl phosphate), either purely supercrystalline or hierarchically structured supercrystalline nanocomposite materials are obtained. Beyond a critical concentration of organic phase, a hierarchical material is consistently formed. In such a hierarchical material, individual organically-modified ceramic nanoparticles (Level 0) self-assemble into supercrystals in face-centered cubic superlattices (Level 1), which in turn form granules of up to hundreds of micrometers (Level 2). These micrometric granules are the constituents of the final mm-sized material. This approach demonstrates that the local concentration of organic phase and nano-building blocks during self-assembly controls the final material’s microstructure, and thus enables the fine-tuning of inorganic-organic nanocomposites’ mechanical behavior, paving the way towards the design of novel high-performance structural materials.

Iron oxide-based nanostructured ceramics with tailored magnetic and mechanical properties: development of mechanically robust, bulk superparamagnetic materials

“…56 Sintering can start as soon as the oleic acid preventing direct contact between the ceramic nanoparticles is removed. According to TGA data, most of the organic material is eliminated in the 150-400 C temperature range, with the maximum loss starting at 300 C. 21 Such a wide temperature window for the removal of oleic acid is usually associated with the presence of ligands that are differently bound to the nanoparticles' surface, 57 to the ligands' decomposition or desorption at the surface, 58 and to their high connement in-between the nanoparticles themselves. Another important factor to keep in mind for the sintering behavior of the nanocomposites is the narrow and monomodal size distribution of the nanoparticles, which hampers grain growth aer the onset of densication.…”

Mapping the Mechanical Properties of Hierarchical Supercrystalline Ceramic-Organic Nanocomposites

“…The nanocomposites’ starting building blocks are iron oxide nanoparticles (Fe 3 O 4 -NPs) with a diameter of 18.6 ± 0.1 nm (as determined via small-angle x-ray scattering, SAXS) [ 30 ], surface-functionalized with oleyl phosphate (OPh, 21 wt%) and suspended in toluene. After a self-assembly step via solvent evaporation, such a concentration of organic phase leads to a hierarchical structure composed of supercrystalline grains in an organic-rich matrix, and thus the material will be called “hierarchical” in the following.…”

“…After a self-assembly step via solvent evaporation, such a concentration of organic phase leads to a hierarchical structure composed of supercrystalline grains in an organic-rich matrix, and thus the material will be called “hierarchical” in the following. To obtain a non-hierarchical SC material, the organic content needs to be reduced to 8 wt%—the quantity needed to obtain a single ligand monolayer on the NPs’ surface [ 30 ]. In the following, this material will be called “homogeneous SC” (NP diameter 18.4 ± 0.1 nm, assessed via SAXS) [ 29 ].…”

“…By controlling the concentration of the organic phase in the starting colloidal suspension, it has become possible to induce the formation of hierarchical composites made of supercrystalline bricks, already during the self-assembly step. The resulting material is a bulk, mm-size nanocomposite, made of supercrystalline quasi-spherical grains surrounded by an organic-rich matrix [ 30 ]. Even though the elongation of the grains and the distribution of the organic matrix are not yet optimized to perfectly fit the criteria that make nacre so fracture-tough, this kind of material represents an important step forward towards architected nanocomposites that are both hierarchically structured and ultimately consisting of ultra-strong nano-building blocks.…”

“…Even though the elongation of the grains and the distribution of the organic matrix are not yet optimized to perfectly fit the criteria that make nacre so fracture-tough, this kind of material represents an important step forward towards architected nanocomposites that are both hierarchically structured and ultimately consisting of ultra-strong nano-building blocks. A preliminary analysis of these new nanocomposites’ mechanical properties has shown the expected bimodal distribution of stiffness and hardness [ 30 ]. There are however several questions still to be answered in order to discern the effects of the organic ligand in the hierarchical structure and overall material’s toughening.…”