Chemistry Reference
In-Depth Information
nanoparticles can be achieved with thicknesses ranged from a few tens of nanometres
up to a few micrometers.
The main structural characteristics have to be studied by means of diffraction
techniques (X-ray, neutrons, EXAFS,…) and transmission electron microscopy to
check their crystalline state and the homogeneity in size and aggregation from well
established methodology. In this way, because of its local probe sensitivity,
57
Fe
Mössbauer spectrometry contributes in a better characterization of the local atomic
order, i.e. chemical homogeneity intra- and internanoparticles and in the differ-
entiation of surface and core Fe species. In addition, this spectroscopic tool is
highly sensitive to the presence of superparamagnetic fluctuations in the case of
magnetic nanoparticles, particularly to ferromagnetic, antiferromagnetic and fer-
rimagnetic species.
Indeed, the intrinsic magnetic properties of nanoparticles are strongly depen-
dent on the size while the extrinsic ones are correlated to the dipolar interactions.
By comparing to bulk materials, nanoparticles possess a very high ''surface to
volume'' ratio and their size scale favour thus quantum mechanical effects. Con-
sequently, finite size effects and the large atomic ratio surface/volume originate
unusual surface magnetic effects, which are obviously more and more important
when the size decreases. The symmetry breaking of the lattice favours thus the
reduction of the increase of the magnetic moments, the occurrence of broken
exchange bonds leading to the surface anisotropy, which does compete with core-
surface interactions and dipole-dipole interactions.
The transmission conventional Mössbauer spectrometry gives rise to hyperfine
structures which result from the set of Fe probes located in the sample composed
of an assembly of magnetic nanoparticles. The spectra consist of broadened and
overlapped lines emerging usually of both magnetic sextets and quadrupolar
doublets, the proportions of which are temperature dependent: their interpretation
of the spectrum consists in describing by means of a minimum of magnetic and
quadrupolar components characterized by physical hyperfine parameters corre-
lated to the chemical nature of the nanoparticles contained in the sample. Con-
sequently, it becomes easier to investigate monodisperse, chemically
homogeneous nanoparticles which are well distributed into a matrix or homoge-
neously aggregated and the Mössbauer study of assemblies of nanoparticles should
require first accurate preliminary characterization of structural and microstructural
properties. In such a case, the modelling of the hyperfine structure observed by
means of
57
Fe Mössbauer spectrometry must distinguish surface from bulk Fe
species from magnetic and/or structural aspects and bring relevant information to
model intrinsic and extrinsic magnetic properties. On the contrary, a non homo-
geneous assembly of nanoparticles will lead to Mössbauer spectra the description
of which cannot be well achieved with physical meaning because of the super-
imposition of different complex hyperfine structures. Consequently, it remains
quite difficult to model accurately the thermal transformations of Fe-bearing
materials at the nanoscale because of the non homogeneous process originating
distribution of sizes.
