Crystallization within Intermediate Amorphous Phases Determines the Polycrystallinity of Nanoparticles from Coprecipitation.

Intense research on nanocrystals synthesized in solution is motivated by their original physical properties, which are determined by their sizes and shapes on various scales. However, morphology control on the nanoscale is limited by our understanding of crystallization, which is challenged by the now well-established prevalence of noncrystalline intermediates. In particular, the impact of such intermediates on the final sizes and crystal quality remains unclear because the characterization of their evolution on the nanometer and millisecond scales with nonperturbative analyses has remained a challenge. Here we use in situ X-ray scattering to show that the nucleation and growth of YVO4:Eu nanocrystals is spatially restrained within amorphous, nanometer-scaled intermediates. The reactivity and size of these amorphous intermediates determine (i) the mono versus polycrystalline character of final crystals and (ii) the size of final crystals. This implies that designing amorphous intermediates themselves that form in <6 ms is one of the keys to controlled bottom-up syntheses of optimized nanoparticles.

Self-Confined Nucleation of Iron Oxide Nanoparticles in a Nanostructured Amorphous Precursor.

Crystallization from solution is commonly described by classical nucleation theory, although this ignores that crystals often form via disordered nanostructures. As an alternative, the classical theory remains widely used in a "multistep" variant, where the intermediate nanostructures merely introduce additional thermodynamic parameters. However, this variant still requires validation by experiments addressing indeed proper time and spatial scales (millisecond, nanometer). Here, we used in situ X-ray scattering to determine the mechanism of magnetite crystallization and, in particular, how nucleation propagates at the nanometer scale within amorphous precursors. We find that the self-confinement by an amorphous precursor slows down crystal growth by 2 orders of magnitude once the crystal size reaches the amorphous particle size (∼3 nm). Thus, not only the thermodynamic properties of transient amorphous nanostructures but also their spatial distribution determine crystal nucleation.

Toward a Chemical Control of Colloidal YVO4 Nanoparticles Microstructure.

Rare-earth-doped oxides are a class of compounds that have been largely studied in the context of the development of luminescent nanocrystals for various applications including fluorescent labels for bioimaging, MRI contrast agents, luminescent nanocomposite coatings, etc. Elaboration of colloidal suspensions is usually achieved through coprecipitation. Particles exhibit emission properties that are similar to the bulk counterparts, although altered by crystalline defects or surface quenching species. Focusing on YVO4:Eu, one of the first reported systems, the aim of this work is to revisit the elaboration of nanoparticles obtained through a simple aqueous coprecipitation route. The objective is more precisely to get a better understanding of the parameters affecting the particles' internal microstructure, a feature that is poorly controlled and characterized. We show that the hydroxyl concentration in the precursor solution has a drastic effect on the particles' microstructure. Moreover, discrepancies in the reported particle structure are shown to possibly arise from the carbonation of the strongly basic orthovanadate precursor. For this study, SAXS/WAXS is shown to be a powerful tool to characterize the multiscale structure of the particles. It could be shown that playing on the precursor composition, it may be varied between almost monocrystalline nanocrystals to particles exhibiting a hierarchical microstructure well described by a surface fractal model. This work provides a new methodology for the characterization of nanoparticles microstructure and opens new directions for its optimization in view of applications.

What makes zeolitic imidazolate frameworks hydrophobic or hydrophilic? The impact of geometry and functionalization on water adsorption.

We demonstrate, by means of Grand Canonical Monte Carlo simulation on different members of the ZIF family, how topology, geometry, and linker functionalization drastically affect the water adsorption properties of these materials, tweaking the ZIF materials from hydrophobic to hydrophilic. We show that adequate functionalization of the linkers allows one to tune the host-guest interactions, even featuring dual amphiphilic materials whose pore space features both hydrophobic and hydrophilic regions. Starting from an initially hydrophobic material (ZIF-8), various degrees of hydrophilicity could be obtained, with a gradual evolution from a type V adsorption isotherm in the liquid phase to a type I isotherm in the gas phase. This behavior is similar to what was described earlier in families of hydrophobic all-silica zeolites, with hydrophilic "defects" of various strength, such as silanol nests or the presence of extra-framework cations.

The delicate balance of phase speciation in bimetallic nickel cobalt nanoparticles.

Bimetallic nickel-cobalt nanoparticles are highly sought for their potential as catalytic and magnetic nanoparticles. These are typically prepared in organic solvents in the presence of strong stabilizing ligands such as tri-n-octylphosphine (TOP). Due to the variety of cobalt crystallographic phases and to the strong interaction of the ligands with the metallic surfaces, forming fcc nanoparticles rather than a phase mixture is a challenging endeavor. Here, using a two-step synthesis strategy that aims at a core-shell nickel-cobalt morphology, we demonstrated that many parameters have to be adjusted: concentration of the metal precursors, stoichiometry of TOP, and heating program from room temperature to 180 °C. We found optimized conditions to form size-controlled fcc NiCo nanoparticles from preformed Ni nanoparticles, and the phase attribution was confirmed with a combination of X-Ray diffraction on powder and X-Ray absorption spectroscopy at the Co K edge. We then investigated the early stages of Co nucleation on the nickel using a lower stoichiometry of Co, down to 0.05 equiv. vs. Ni. Using X-ray photoelectron spectroscopy and scanning transmission electron microscopy coupled to energy-dispersive X-Ray spectroscopy and electron energy loss spectroscopy, we showed that cobalt reacts first on the nickel nanoparticles but easily forms cobalt-rich larger aggregates in the further steps of the reaction.