X-ray Absorption Spectroscopy (XAS): In the case of metallic
nanoparticles (silver, gold…) but also metallic oxides (TiO2,
CeO2, Fe2O3, Fe3O4…) or quantum dots, the redox state of the
metal can be determined by XAS. The XANES part (X-ray Absorption
Near Edge Structure) of the spectra is the more sensitive to the
redox state and the EXAFS part (Extended X-ray Absorption Fine
Structure) give information on the local atomic environment of
the targeted atom. Moreover XAS spectra recorded at the L-edge
are more sensitive to the redox state than spectra recorded at
the K-edge. There are also some drawbacks. First of all,
studying the L-edge in the case of metals from the first
transition series implies to use low X-ray energy and therefore
to put samples in vacuum cells. The second limitation is that
the particles are entirely irradiated (bulk + surface).
Therefore XAS will be only sensitive to the surface redox
properties of nanoparticles with a large surface/volume ratio
i.e. with a diameter smaller than 20-30 nm