Methods for assessing the redox properties of the surface
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.
For larger particles the proportion of surface atoms versus atoms in the bulk will be too low to detect any change at the surface. However, if the particles are deposited on a conducting support, the use of the total electron yield detection mode could enhance the sensitivity to surface. This detection mode presents the strong advantage to be sensitive only of the 40-50 nm of the materials. Therefore in the case of larger nanoparticles it is still possible to obtain some information. In that case it is really important anyway to control and correct self-absorption effects and that the support is conductive.
In some specific cases Surface-XAS can by-pass such difficulties. Indeed in the case of flat material (for example multilayer components…) it is possible to use a grazing incident angle between the beam and the surface of the material leading to a strong enhance of the surface signal. In fact Surface-XAS is based on the theory of x-ray total reflection stating that at an incident angle below the critical angle the narrow collimated primary beam is totally reflected. The critical angle depends on the density of the materials but is generally lower than 2 or 3 mrad. In the case of nanoparticles, it implies that the nanoparticles are deposited on a flat support forming a quasi-perfect layer. Even if experimentally it is very difficult, this may be possible in some particular cases. The penetration of X-ray below the critical angle can reach 10 nm or even below which may enhance the signal from the surface of nanoparticles.