Page 122 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

108 Principles and Methods

Characterization of atomic structure
The characterization of the atomic arrangement of nanoparticles can
be performed to determine both the long-range (i.e., the crystal param-
eters) and short-range order of a material’s atomic structure. Of
particular interest is the arrangement of atoms at the material’s sur-
face. The various characterization techniques that are available for
these purposes may or may not be element specific. Therefore, care
must be taken to select an appropriate characterization method. We will
therefore discuss first the nonspecific techniques followed by the
element-specific ones.

Nonspecific techniques
X-ray diffraction
Operating principles. X-ray scattering was first used to study the long-range
order of the atomic arrangement in crystals in the early 20th century
(Wyckoff, 1964). More recently, it has been applied to characterize nano-
materials. The pattern, position, intensity, and shape of the peaks in X-
ray diffraction (XRD) are all influenced by the atomic structure—that
is, atomic interatomic distances and crystal cell parameters. The com-
parison of the diffraction pattern of unknown samples with the diffrac-
tion pattern of reference compounds from the database is necessary to
identify the nature of minerals present in mixtures. XRD is a common
technique used in mineralogy, and details to determine or refine atomic
position can be found elsewhere.
Furthermore, information regarding the size of nanoparticles may
also be derived from the XRD pattern, as particle size strongly affects
the peak width. Particle size may be calculated based on the Sherrer
formula, which states:

S 5 l/w cos u (1)
where S is the particle size, the wavelength of the beam, the
diffraction angle and w is the width of the peak at half-maximum.

Sample preparation. X-ray diffraction instruments may have a number
of different configurations resulting in different types of sample hold-
ers. Powder diffraction requires a fine and homogeneous powder.
Diffraction setup for “single” crystal analysis is not really adapted to
study small objects since the size of the beam is much larger than indi-
vidual nanoparticles. For nanoparticles, the sample needs to be dry and
deposited either on a sample holder or in capillary tubes.
Application in the particular case of nanoparticles. For nanoparticles, as for
other types of materials, it is essential to determine both the crys-
tallinity and polymorph type (i.e., two or more minerals having the