Page 132 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

118 Principles and Methods

change phase and/or be resilient in such conditions. With regards to sen-
sitivity, the matrix or support of the studied element is of high importance.
But it seems difficult to detect elements at concentrations lower than
1–2 percent (w/w).
Application in the particular case of nanoparticles. For nanoparticles some
specificity exists when applying Mössbauer spectroscopy. When the
grain size of fine particles is smaller than a critical grain size, Dc, they
are composed of many single magnetic domains even though there is
no external magnetic field. Therefore, the magnetic fields will not be
stationed in a fixed direction like for a bulk material, but will rather
jump from one easy-magnetization direction to another; this is the
superparamagnetic phenomenon. The Dc varies from one mineral to
another. For instance, the Dc of a-Fe O is 20 nm. If the grain size of
3
2
iron oxide in composites is smaller than Dc, there appear superpara-
magnetic doublet lines in the Mössbauer spectra. Using this property,
Liu et al. [2005] combined XRD and Mössbauer spectroscopy on an
Fe O -Al O nanocomposite, and found that below 1373 K the aver-
3
3
2
2
age grain size is below 20 nm (i.e., it exhibited a superparamagnetic
behavior).
As Mössbauer spectroscopy probes the local structure of a target atom,
the crystallographic sites of doped nanoparticles can also be studied. For
example, Zhu et al. [2005] found that Fe can form superparamagnetic
a-Fe O phases in iron-doped TiO if the Fe concentration was higher
2
2
3
than 4 percent (w/w). This phenomena is also a consequence of the fact
that Fe does not substitute Ti in the TiO structure.
2
In the case of iron, Mössbauer spectroscopy is capable of differenti-
57
ating redox states and also atomic sites. The chemical shifts of Fe in
various solid systems are summarized in Figure 4.9.
Like XAS, Mössbauer spectroscopy is sensitive to the atomic arrange-
ment of the surface layer in a bulk solid. For metallic clusters of Au and
Pt, Mulder et al. [1996] showed that for nanoparticles composed of 55,

Fe/Mn Fe/Cr Metallic Fe FeAI 3 CeFe 2 Fe 2 O 3 FeCI 3 FeF 3 FeS FeI 2 FeCI 2 FeCI 2 .6H 2 O FeF 2

–0.2 –0.1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 (mm/s)
0
Metallic systems Fe 2+
Fe 3+
Figure 4.9 Chemical shifts of various iron species.