Page 128 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

114 Principles and Methods

XANES analysis is more sensitive than EXAFS to the site geometry and
the oxidation state of the target atom. Therefore, the combination of XANES
and EXAFS is very powerful for probing the speciation of elements.

Sample preparation. XAS has the ability to measure samples under var-
ious physical forms (solid, liquid, and gas) with little preparation, in con-
trast to other techniques such as TEM. In transmission mode, it is
crucial to prepare pellets (for solid samples) with no pinholes and con-
stant and appropriate thickness. It is also important that the size of the
particles in the sample not be much larger than one absorption length
(which is always the case for nanoparticles). Solutions produce the best
transmission samples. For nanoparticles in suspension it is essential to
prevent any settling during the measurement.
Limitations. The interpretation of EXAFS is limited in that it does not
provide any information about bond angles between atoms. Moreover,
it is not practically possible to distinguish between atoms that are in the
same line in the periodic table (Z ± 2). For example, it is not possible to
distinguish between O and N, whereas the differences between O and
S allow for their identification. This quite strong limitation is due to the
energy dependence of Zf skdZ and the phase shift function f skd . These
i
ij
two functions contain the information characteristic of the nature of the
scattering atoms. Unfortunately, the differences of Zf skdZ and f ij skd for
i
two atoms like O and N are not great enough to easily distinguish
between them (Figure 4.6).
In XANES analysis the theory is not as yet fully quantitative as is the
case for EXAFS and requires different physical considerations. Here, X-rays
from a synchrotron source may induce significant chemical changes
within the sample (e.g., oxydo-reduction processes), which has obvious
consequences for accurately characterizing the material of interest.

Application in the particular case of nanoparticles. XAS can be helpful in deter-
mining the structural evolution of nanoparticles as a function of the
nature of the ligands capping them. For example, Chemseddine et al.
[1997] demonstrated that the presence of acetate or thiolate modifies
the symmetry of CdS in the surface layer. Whereas in the bulk on the

Figure 4.6 Backscattering ampli-
tude factor Zf i skdZ for oxygen and
nitrogen.