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Methods for Structural and Chemical Characterization of Nanomaterials 111

spectrum and sometimes in the ultraviolet zone) is slightly lowered or
raised by inelastic interactions with the vibrational modes. It is a pow-
erful tool for investigating the structural and morphological properties
of solids at a local level [see, for example, Ferraro and Nakamoto, 1994].
In a simple approach, Raman spectra can fingerprint the nature or type
of crystal phases. Raman can in some cases assess the crystal, or amor-
phous nature, of minerals. These properties of Raman spectroscopy,
however, are not particular to nanoparticles.
Sample preparation and limitations. One very interesting point concerning
Raman spectroscopy is that the sample can be solid or in solution. A lim-
itation of Raman spectroscopy, however, is that the sensitivity of the
technique is dependent upon the material being characterized.
Application in the particular case of nanoparticles. Raman spectroscopy is a
powerful tool to identify the nature of nanoparticles. Like XRD, the posi-
tion of Raman peaks can be considered as a fingerprint for different
minerals. More than XRD, overlapping of peaks can lead to difficulties in
identifying the constituent minerals in a complex matrix. In some cases
like TiO 2 , for which the Raman peaks are particularly intense, it is pos-
sible to distinguish between the different polymorphs. Taking for exam-
ple anatase and rutile, both of which are polymorphs of TiO 2 , the Raman
spectroscopy may be used to differentiate between them. For anatase, six
1 1 1
different peaks are recorded at 144 cm (Eg), 197 cm (Eg), 397 cm
1 1
(B1g), 518 cm (A1g and B1g, unresolved), and 640 cm (Eg). On the
1
other hand, for rutile, three different peaks are detected at 144 cm
1 1
(B1g), 448 cm (Eg), and 613 cm (A1g) (a fourth very weak band cor-
1
responding to the B2g mode also exists at 827 cm ) [Robert et al., 2003].
In the particular case of nanoparticles, the signal is strongly affected by
particle size as well as shape. This point will be detailed further later in
this chapter.
Element-specific techniques
X-ray absorption spectroscopy (XAS)
Operating principles. X-ray absorption spectroscopy (XAS) is one of the
most powerful techniques for probing the local atomic structure in a vast
array of materials. XAS is a short-range order method that can be used
regardless of the sample’s physical state (crystalline, amorphous, in solu-
tion, or in a gas phase). Another important property of XAS is that it is
an element-specific technique, which is in contrast to other spectroscopic
methods such as Raman spectroscopy. The operating principle of this
method requires that the incident X-ray beam energy be scanned from
below to above, the binding energy of the core shell electrons of the target
atom. By doing so, one observes an abrupt increase in the absorption coef-
ficient corresponding to the characteristic absorption edge of the selected