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.
Supercrystalline
nanocomposite materials with micromechanical properties
approaching those of nacre or similar structural biomaterials can
be produced by self-assembly of organically modified nanoparticles
and further strengthened by cross-linking. The strengthening of these
nanocomposites is controlled via thermal treatment, which promotes
the formation of covalent bonds between interdigitated ligands on
the nanoparticle surface.
In this work, it is shown how the extent of the mechanical properties
enhancement can be controlled by the solvent used during the self-assembly
step. We find that the resulting mechanical properties correlate with
the Hansen solubility parameters of the solvents and ligands used
for the supercrystal assembly: the hardness and elastic modulus decrease
as the Hansen solubility parameter of the solvent approaches the Hansen
solubility parameter of the ligands that stabilize the nanoparticles.
Moreover, it is shown that self-assembled supercrystals that are subsequently
uniaxially pressed can deform up to 6 %. The extent of this deformation
is also closely related to the solvent used during the self-assembly
step. These results indicate that the conformation and arrangement
of the organic ligands on the nanoparticle surface not only control
the self-assembly itself but also influence the mechanical properties
of the resulting supercrystalline material. The Hansen solubility
parameters may therefore serve as a tool to predict what solvents
and ligands should be used to obtain supercrystalline materials with
good mechanical properties.
To integrate the exceptional properties of nanoscale building blocks into macroscopic devices, manufacturing methods that achieve control of nanoparticle (NP) assembly up to macroscale dimensions have to be developed. [1][2][3][4][5][6][7][8] In this regard, structural hierarchy emerges as a powerful strategy for creating functional materials, involving a precise control of composition and shape across several length scales. [1,[9][10][11][12][13] NPs can be assembled into larger structures by exploiting and further controlling their inherent intermolecular and surface forces. [3,14] Thus, by taking advantage of the short-range forces, colloidal self-assembly appears as a successful bottom-up approach for the development of new materials and their further integration into novel devices. Moreover, by specifically designing the nanobuilding blocks, the macroscopic behavior of the resulting engineered materials can be
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