Page 130 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

116 Principles and Methods

EXAFS can be a powerful tool for characterizing nanomaterials when
used along with XRD. Indeed, the decrease in the size of crystalline
particles leads to an increase in the FWHM of the XRD peaks. But a
decrease in particle crystallinity without any change in size leads to the
same effect. For very small particles (<3 to 4 nm) the XRD spectra are
quite noisy, and accurate information concerning the minerals is diffi-
cult to extract. Using EXAFS in combination with XRD can help in
solving this limitation. The Debye-Waller factor determined by EXAFS
modeling is related to the disorder of the particles. For example Choi
et al. [2005] observed an increase of the static disorder as TiO parti-
2
cle size decreased. Moreover, a volume contraction as particle size
decreases has been highlighted by a decrease of the Ti-Ti interatomic
distances.

Mössbauer spectroscopy
Operating principles. While XAS spectroscopy is based on the measure-
ment of electronic transitions, the Mössbauer effect involves the inter-
action of radiation (i.e., the resonant absorption) with the nuclei of the
atoms of a solid. Here, -rays are used to probe the nuclear energy levels
related to the local electron configuration and the electric and magnetic
fields of the solid. To date, Mössbauer spectroscopy has been mainly used
to study Fe nanoparticles, but Au and Pt materials can also be studied
by following nuclear transitions [see, for example, Mulder et al., 1996].
Like XAS, the Mössbauer spectroscopy is element specific. Mössbauer
spectra consist of plotting the transmission of rays as a function of their
source velocity. A Mössbauer spectrometer consists of a vibrating mech-
anism that imparts a Doppler shift to the source energy and then to a
source. In the absence of any magnetic field, the Mössbauer spectrum
consists of one or two absorption maxima between I1/2 and I3/2 nuclear
levels (Figure 4.8). The difference between the ground and excited state
levels is called the chemical or isomer shift, , which is described accord-
ing to the following relationship:

4p 2 2 dR 2 2
d 5 Ze R a b[ Zcs0dZ 2 Zcs0dZ SOURCE ] (3)
5 R ABS
where Z is the nuclear charge, dR is the difference between the radii of
the ground and excited states, R is the mean radius of the ground and
2 2
excited states, and Zcs0dZ ABS and Zcs0dZ SOURCE are the electron density
of the absorbant and source, respectively.
When a magnetic field exists it will influence the resonant nuclei by
splitting the nuclear spin of the ground and excited states into various
new states. This phenomenon leads to multiple transitions, where the
position and absorption intensity are related to hyperfine interactions
between the resonant nuclei and the electrons surrounding them. The