Alignment under Magnetic Field of Mixed Fe₂O₃/SiO₂Colloidal Mesoporous Particles Induced by Shape Anisotropy

Abstract

When using the bottom-up approach with anisotropic building-blocks, an important goal is to find simple methods to elaborate nanocomposite materials with a truly macroscopic anisotropy. Here, micrometer size colloidal mesoporous particles with a highly anisotropic rod-like shape (aspect ratio ≈ 10) have been fabricated from silica (SiO ) and iron oxide (Fe O ). When dispersed in a solvent, these particles can be easily oriented using a magnetic field (≈200 mT). A macroscopic orientation of the particles is achieved, with their long axis parallel to the field, due to the shape anisotropy of the magnetic component of the particles. The iron oxide nanocrystals are confined inside the porosity and they form columns in the nanochannels. Two different polymorphs of Fe O iron oxide have been stabilized, the superparamagnetic γ-phase and the rarest multiferroic ε-phase. The phase transformation between these two polymorphs occurs around 900 °C. Because growth occurs under confinement, a preferred crystallographic orientation of iron oxide is obtained, and structural relationships between the two polymorphs are revealed. These findings open completely new possibilities for the design of macroscopically oriented mesoporous nanocomposites, using such strongly anisotropic Fe O /silica particles. Moreover, in the case of the ε-phase, nanocomposites with original anisotropic magnetic properties are in view.

Epsilon‐Fe2O3 Nanocrystals inside Mesoporous Silicas with Tailored Morphologies of Rod, Platelet and Donut

“…In a previous study, we already designed promising γ‐Fe 2 O 3 and ϵ‐Fe 2 O 3 /mesoporous silica particles with a long rod morphology, and we have shown that they undergo macroscopic alignment when submitted to an external magnetic field . Here, we extend this study to the preparation of ϵ‐Fe 2 O 3 /mesoporous silica nanocomposites with two other shapes for the mesoporous silica hard template, namely platelet and donut.…”

“…For fabrication of ϵ‐Fe 2 O 3 nanocrystals inside the mesochannels by solvent‐free impregnation, the same procedure was followed for the three samples, with a thermal treatment under air at 1000 °C, as detailed in the experimental section and in Table . Figure shows the evolution of the SAXS patterns for the three kinds of samples after one and two impregnations.…”

“…These magnetic nanocomposites have great potential because of the outstanding physical properties of the ϵ‐Fe 2 O 3 iron oxide polymorph and because of the tailored shape of the particles. Anisotropy of the rod and platelet particles is very promising for alignment purposes using magnetic field . For instance, one can foresee that their alignment could be driven by a magnetic force in a polymerizing solution to prepare magnetically oriented materials at low cost …”

“…Preparation of the Fe 2 O 3 /silica particles . The solvent‐free impregnation strategy is based on our previous publication . First, 83 mg of the Fe 2 O 3 precursor Iron(III) Nitrate Nonahydrate, Fe(NO 3 ) 3 ⋅ 9H 2 O, Aldrich) and 50 mg of the mesoporous silica powder (i. e. the mass ratio of precursor to mesoporous silica needed to fill 100% of the pore volume) were placed into two separated containers (Fe 2 O 3 in the mortar and silica in the crucible) in an oven at 80 °C for 30 min.…”

Macroscopic Magnetic Anisotropy Induced by the Combined Control of Size, Shape and Organization of NiFe Prussian Blue Analog Nanoparticles in an Ordered Mesoporous Silica Monolith

“…In that case the ZFC maximum is close to the value expected for diluted nanocrystals. Magnetic shape anisotropy arising from dipolar interaction within single-chains of nanoparticles was already evidenced in ordered mesoporous silica powders containingc hains of metallic Co nanoparticles [23] or iron oxide particles [24] and in iron oxide nanoparticles assembled in chains that were prepared ChemNanoMat 2017, 3,833 -840 www.chemnanomat.org by soft templating based on bacteria [25] and organic molecules, [26] or inducedb yamagnetic field. [27] To the besto fo ur knowledge,t his is the first example of enhanced collective properties induced by the directed assembly of coordination nanocrystals into 1D-chains.…”