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Towards new anisotropic materials with tunable magnetic properties

The bottom-up approach is a very powerful method for designing nanomaterials. When anisotropic building-blocks are assembled together, an important goal is to find easy methods to elaborate nanocomposite materials with a truly macroscopic anisotropy, with the same orientation for all the building-blocks. Silica mesoporous materials with an ordered porosity, consisting in micron-long nanochannels are a good example of such anisotropic nanomaterials. To fully benefit of an assembly of parallel nanochannels, it is necessary to obtain macroscopic alignment of all the channels over a few mm3 or even cm3. Indeed, only such a large oriented sample is able to exhibit anisotropic properties at the macroscopic scale, providing for example anisotropic diffusion coefficients, a crucial fact for controlling the transport of any active species loaded in the porosity. More generally, a truly macroscopic orientation will provide that any physical response such as thermal, mechanical, optical or magnetic will be anisotropic, an important fact for engineering applications. Mesoporous silica/iron oxide nanocomposites are attracting much attention because the pores can be loaded with a range of active species, like drugs or contaminants, and at the same time they can be manipulated using magnetic forces. They are of interest for many fields such as drug delivery, biological imaging, wastewater treatment or catalysis.

Rod-like mesoporous magnetic particles containing iron oxide nanocrystals. (a) Scheme of a particle showing the iron oxide nanocrystals inside the nanochannels (diameter 7 nm) (b) Electron diffraction showing the preferential crystallographic orientation of the nanocrystals inside the nanochannels (c) Alignment under magnetic field of the particles revealed by X-ray scattering (d) chains of magnetized particles observed in optical microscopy.

Here, we succeeded to assemble anisotropic mesoporous silica (SiO2) particles, made magnetic by embedding iron oxide (Fe2O3) nanocrystals inside their porosity. These micron size colloidal mesoporous particles have a highly anisotropic rod-like shape (aspect ratio 10) and can be easily oriented using a magnetic field ( 200 mT) when dispersed in a solvent. 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 Fe2O3 iron oxide have been stabilized, the superparamagnetic \gamma-phase and the rarest multi-ferroic \epsilon-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 Fe2O3/silica particles. Moreover, in the case of the \epsilon-phase, nanocomposites with original anisotropic magnetic properties are in view.
Because the iron oxide loading is only of 8% of the porous volume, these objects are good candidates for applications in the fields of remediation and magnetically guided delivery of drugs. Indeed, the remaining available porous volume can be used to load active species for different purposes. They are also appealing for studying the complex magnetic properties of the \epsilon-phase, which is attracting a lot of interest due to its unique multi-ferroic properties. The achievement of macroscopic alignment is here important because it confers strongly anisotropic magnetic properties to the nanocomposites. For example, magnetic materials with tiny magnetic domains - down to a few nanometers- could be designed for magnetic data storage applications.
In addition, when dispersed in a solvent, these particles are promising new magneto-rheological fluids, where varying the amount of iron atoms could give adjustable properties. Last, but not least, starting from the colloidal suspensions, one could polymerize the solvent under magnetic field in order to ‘freeze’ the orientation of the particles and get a completely solid nanocomposite. In conclusion, these Fe2O3/silica particles are very promising building blocks to form strongly anisotropic nanocomposites, with a very homogeneous orientation over macroscopic scales.

Reference :

Alignment under Magnetic Field of Mixed Fe2O3/SiO2 Colloidal Mesoporous Particles induced by Shape Anisotropy
J. G. Li, G. Fornasieri, A. Bleuzen, M. Gich, A. Gloter, F. Bouquet & M. Impéror-Clerc
Small , DOI : 10.1002/smll.201602272 (2016).

Contact :

Marianne Impéror
Jheng Guang Li

Collaborations :

Equipe d’Anne Bleuzen (ICMMO, Orsay)
Equipe de Marti Gich (ICMAB, Barcelona, Spain)