Surface Chemical Structure of Isolated Aerosols by X-ray Photoelectron Spectroscopy
2016
X-ray photoelectron spectroscopy (XPS) is powerful tool to
investigate the surface chemical structure of any material.
However, when applied to nanoobjects, this technique faces
drawbacks due to interactions with a substrate, on which
nanoobjects have to be deposited, and sample charging
effects. We present a new experimental approach to XPS1
based on coupling soft x-ray synchrotron radiation with an in
vacuum beam of free nanoaerosols, focused by an
aerodynamic lens system (Fig 1a). Two examples of
experiments performed on the PLEIADES beamline at the
SOLEIL Synchrotron facility are presented to illustrate the
effectiveness of this approach.
In the first example, the structure of the Si/SiO2
interface is probed on isolated silicon nanocrystals previously
oxidized with ambient air1
(Fig. 1b) or by heat treatment
under air. Full characterization of the surface has been
achieved for different sizes of nanocrystals between 4 and 80
nm and with different oxidizing treatments. The technique
allows probing the presence of various oxidation states at the
interface and to deduce therefrom a thickness of the oxide
layer. For the smaller and more oxidized nanoparticles, a
relatively abrupt interface including Si = O double bonds is
highlighted.
In the second example, the adsorption of water on the
surface of TiO2 nanoparticles is investigated in the gas phase3
.
TiO2 free aerosols are exposed to a controlled pressure of
water vapor before being analyzed directly by XPS. The
technique allows here the observation of a predominantly
dissociative adsorption of water on the surface of TiO2 in its
very first stage, highlighting a largely covered surface by OH
groups (Fig. 1c).
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