Analysis of silver by XPS is typically performed on the 3d region where the only likely overlaps will be the K 2s region and the Nb 3p region, though both of these may be easily deconvoluted due to large differences in doublet separation (or in the case of K 2s, no doublet).
Certain silver salts (e.g. AgCl) may undergo rapid reduction to Ag0 under the X-ray beam, although may still be differentiated by the auger parameter.(1)
Ag 3d has a large doublet separation of 6.0 eV, however small shifts in binding energy make it very difficult to accurately determine oxidation state.(3)
The O 1s spectra of Ag oxides may be complicated by the propensity of silver oxides to adsorb oxygen and thus, oxygen may be present in many forms (molecular, atomic, subsurface) which may differ from sample to sample.(4) Furthermore, the oxides are basic and may readily adsorb CO2 to form surface carbonates under the right conditions.(5)
Ag analysed by XPS is almost unique (with Cadmium) in that the binding energies of the oxides and other salts typically appear at binding energies lower than the metal. This was thought to be due to post-ionisation extra-atomic relaxation effects, where BEexp = BEisol – Δrel (BEexp = experimental binding energy, BEisol = theoretical binding energy of an isolated atom in this electronic configuration, Δrel = energy change due to solid state relaxation effects).(3) Gaarenstroom and Winograd(6) developed a scheme to separate the influences of initial-state effects (partial charge/ionic charge shift and lattice potential) from final-state effects (extra atomic relaxation energy) in order to determine why Ag (and Cd) behaved in such a manner. By assuming that the static relaxation energy (amount of energy an l-orbital reduces by when a hole is introduced to the k-subshell R(kl)) is independent of chemical matrix, dynamic extra-atomic relaxation energies could be calculated using equation 1:
Ek(jkl;X) = EB(j) – EB(k) – EB(l) + F(kl;X) + R(kl) + [ -ER(l) + ER(l*)] Equation 1
Where Ek(jkl;X) is the auger kinetic energy (can be calculated using atomic multiplet coupling theory), EB is the binding energy of a contributing orbital, F(kl;X) is the interaction energy between a k and an l hole in the X final state and ER is the dynamic relaxation term ([ -ER(l) + ER(l*)] accounts for 2 initial states – one with no hole and one with a k-hole).
By determining ER, Gaarenstroom and Winograd were able to determine that is was due to INITIAL state effects that the silver salt binding energies were lower than the metal, since the relaxation energies would imply a higher energy for the silver salts. Silver was calculated to have a low magnitude energy shift per unit charge (k), thought to be due to the specific configuration of d– and s-orbital screening (in both initial and final states). From this it was determined that the lattice potential and the low k combined with a high oxygen Coulomb repulsion (Up-p) was the driving force behind the negative EB shifts associated with silver salts.
Below are some typical binding energies associated with various silver salts.
|Species||Binding energy / eV||Charge Reference||Ref|
|AgO||367.5||C 1s (285 eV)||1|
|Ag2O||367.9||C 1s (285 eV)||1|
|AgF||368.0||C 1s (285 eV)||1|
|AgCl||368.3||C 1s (285 eV)||1|
|AgI||368.0||C 1s (285 eV)||1|
|Ag2CO3||368.0||C 1s (285 eV)||1|
|Ag2S||368.2||C 1s (285 eV)||1|
|Ag2SO4||368.0||C 1s (285 eV)||1|
|AgNO3||368.4||C 1s (285 eV)||1|
Because of the small shifts in binding energy for many Ag compounds, if it is possible to do so then the auger parameter may provide more information.
|Species||Auger parameter||Auger transition||Ref|
|Ag||726.0||3d5/2 – M4N4,5N4,5||1|
|AgO||724.2||3d5/2 – M4N4,5N4,5||1|
|Ag2O||724.3||3d5/2 – M4N4,5N4,5||1|
|AgF||722.8||3d5/2 – M4N4,5N4,5||1|
|AgCl||723.5||3d5/2 – M4N4,5N4,5||1|
|AgI||724.0||3d5/2 – M4N4,5N4,5||1|
|Ag2CO3||723.1||3d5/2 – M4N4,5N4,5||1|
|Ag2S||724.8||3d5/2 – M4N4,5N4,5||1|
|Ag2SO4||723.0||3d5/2 – M4N4,5N4,5||1|
|AgNO3||723.8||3d5/2 – M4N4,5N4,5||1|
Furthermore, Ferraria et al(3) reported distinct lineshapes for various Ag auger regions.
- Kaushik, V. K. (1991). “XPS core level spectra and Auger parameters for some silver compounds.” Journal of Electron Spectroscopy and Related Phenomena 56(3): 273-277. Read it online here.
- Data acquired by HarwellXPS.
- Ferraria, A. M., et al. (2012). “X-ray photoelectron spectroscopy: silver salts revisited.” Vacuum 86(12): 1988-1991. Read it online here.
- Hoflund, G. B., et al. (2000). “Surface characterization study of Ag, AgO, and Ag 2 O using x-ray photoelectron spectroscopy and electron energy-loss spectroscopy.” Physical Review B 62(16): 11126. Read it online here.
- Weaver, J. F. and G. B. Hoflund (1994). “Surface characterization study of the thermal decomposition of AgO.” The Journal of Physical Chemistry 98(34): 8519-8524. Read it online here.
- Gaarenstroom, S. and N. Winograd (1977). “Initial and final state effects in the ESCA spectra of cadmium and silver oxides.” The Journal of chemical physics 67(8): 3500-3506. Read it online here.