Page 160 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

146 Principles and Methods

Evolution of the oxidation state
at the surface
For metallic and some oxide nanoparticles, the oxidation state of ele-
ments composing the objects might evolve at the surface. Many of the
techniques previously described in this chapter may be used to probe the
evolution of the elemental oxidation state. Some techniques, however,
like X-ray photoelectron spectroscopy (XPS), can be used to specifically
probe the surface or near-surface region.

X-ray photoelectron spectroscopy (XPS)
Operating principle. XPS is based on the measurement of the energies of
photoelectrons emitted as a result of the interaction between an incident
X-ray beam with matter. As it measures the energies emitted from pho-
toelectrons, XPS is classified as an electron spectroscopy technique. The
main difference between electron and X-ray spectroscopies is the depth
to which the surface is characterized. For example, electrons travel
through a extremely short distance in a solid before losing their energy,
while X-rays penetrate deeper into the solid matrix. Therefore, XPS is a
surface or near-surface sensitive technique, as it is sensitive to a depth
of between 1 to 5 nms. On the other hand, X-rays are used to character-
ize the bulk structure. However, for nanoparticles smaller than 20 nm,
XPS can also be considered as a bulk sensitive technique!

Sample preparation. XPS is classified as a surface sensitive technique, but
only for relatively large surfaces. For analysis of nanoparticles with XPS,
the best preparation method is to deposit them on a clean and flat surface.
More than for TEM, XPS requires an ultra-high vacuum chamber. It is
therefore important to dry the system before analyzing it using XPS.
When examining interactions between nanoparticles and living cells,
sample preparation can be quite complicated as it is imperative to avoid
any chemical modifications during drying (e.g., oxidation). Samples with
large surface areas, or with volatile components, should ideally be dried
and placed in a vacuum chamber prior to insertion into the XPS equipment.

Application to nanoparticles. The ejected electrons correspond to core pho-
toemission. The core level peaks for a given atom can exhibit different
binding energies due to symmetry or oxidation state effects. In particu-
lar, core-level photoemission can be very sensitive to changes in the oxi-
dation state of an element. As an example, for TiO 2 optical irradiation can
lead to the formation of charge carriers by optical absorption across the
band gap. These charge carriers can directly participate in redox processes
on the TiO 2 surface. XPS can distinguish the different Ti oxidation
states—that is, the Ti2p 3/2 photoemission varies from 455.3 eV for TiO
2+ 3+ 3+ 4+
(Ti ) to 456.7 eV for Ti 2 O 3 (Ti ), 457.6 eV for Ti 3 O 5 (2xTi , Ti ), and