Page 124 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

110 Principles and Methods

Ceria before thermal treatment
Ceria after thermal treatment

PDR(r)

3 4 5 6 7 8
r(Å)
Figure 4.3 PDF of ceria before and after thermal treatment
(adapted from Mamontov and Egami, 2000).

used for studying the nonperiodic structure of matter in noncrystalline
materials (glasses) [Warren, 1990]. The local structure determined by the
pair distribution function is the probability of finding an atom at a dis-
tance r from a reference z atom. The PDF transforms the signal obtained
in the reciprocal space (wave vector space Q 5 4p sin u/l with the scat-
u
l
tering angle and wavelength of the incident beam) to the real space
(interatomic distance space). The spatial resolution is directly linked to
1
the Q range scanned (greater than 20 Å ). Therefore, to obtain a high
interatomic size resolution it is necessary to measure data using high
X-ray energy (> 100 KeV), that is to say, low wavelength (<0.12 Å). The
high X-ray energy is a strong limitation of this technique, since it requires
a synchrotron radiation source. The PDF analysis of ceria nanoparticles
[Mamontov and Egami, 2000] illustrates the interest of such a tool.
Using neutron diffraction, the authors have shown that the nanoscale
ceria had Fenkel-type oxygen defects. The defects disappeared after a ther-
mal treatment as shown in the post-thermal treatment PDF curve, which
exhibits a higher intensity compared to the ceria before thermal treat-
ment, for example, a higher number of interatomic distances (Figure 4.3).

Raman spectroscopy
Operating principles. Raman spectra result from the scattering of elec-
tromagnetic radiation by the molecules in solid bulk materials. The
energy of the incident light beam (usually in the visible region of the