Orbitals and Energies #
Note – these are listed in BINDING ENERGY
Sn 3d ≈ 485 eV
Sn 3s ≈ 884 eV
Sn 3p ≈ 715 eV
Sn 4s ≈ 137 eV
Sn 4p ≈ 89 eV
Sn 4d ≈ 24 eV
Sn 5p ≈ 1 eV
Common Overlaps for Sn 3d #
W 4p – Sc 2s – Po 4d – Rh 3p – Ir 4p – W 4p – Yb 4s – Ru 3p – Zn LMM (Al Ka X-rays)
Theory and Background #
The binding energies of different tin species, such as Sn(0), Sn(II), and Sn(IV), are often very close together, making it difficult to distinguish between them using X-ray photoelectron spectroscopy (XPS).(4-6) This is due to several factors:
Small Chemical Shifts: The chemical shift between different oxidation states of tin is small.(7) For example, the shift between Sn(0) and Sn(II, IV) in the 3d spectra of oxidized tin is about 1.7 eV. The difference between Sn(II) and Sn(IV) is even smaller, around 0.5-0.7 eV, which is difficult to resolve with standard XPS.(4,8) Some studies have reported even smaller shifts, with differences less than 0.2 eV between SnO (Sn(II)) and SnO2 (Sn(IV)).(6,9)
Counteracting Effects: The change in free-ion potential between Sn+ and Sn++ is cancelled by the change in the Madelung potential at tin sites between the two lattices, which leads to very small chemical shifts.(6)
Limitations of XPS: XPS has limitations in resolving power, which can make it difficult to distinguish between different tin oxidation states.(5) The Sn 3d core level spectra for SnO and SnO2 look very similar,(6) and the binding energies for Sn 3d5/2 are very close for SnO and SnO2.(9)
Surface Charging: Surface charges can cause shifts in the binding energy that do not originate from chemical bonds.(6) This effect is especially pronounced for semiconductors, where excitation during measurement with an X-ray source can induce photovoltages, resulting in surface band bending.(6)
Peak Overlap: The peaks for different oxidation states can overlap, further complicating analysis.(6,7)
Because of these small differences in binding energies, other techniques such as analysis of the valence band spectra,(2,3,7) Auger parameters,(3,5) or the separation between the Sn 4d core level and valence band peaks are often used to differentiate between tin species.(3)
Experimental Advice #
Reduction of Tin Oxides: Argon ion bombardment can reduce tin oxides to metallic tin. This is observed as a shift in the Sn 3d peaks to lower binding energies, characteristic of Sn(0).(10)
Preferential Sputtering: Ion bombardment can cause preferential sputtering of oxygen, leading to a transformation from SnO2 to SnO, and further to metallic tin. This effect is more pronounced in SnO than in SnO2, where metallic tin formation is more readily achieved through sputtering.(4)
Modified Auger Parameter: The modified Auger parameter (α’) can be used to analyze the chemical state of tin. This method relies on the observation of both photoelectrons (Sn 3d) and Auger electrons (Sn MNN) and is less sensitive to charging effects than binding energy analysis alone. As such it is advised to collect the Sn auger along with the 3d core line.(6)
Data Analysis Guidance #
Techniques such as correlation analysis, or auger analysis are very helpful in the understanding of Sn oxidation states.
In the case of tin oxides, the Sn 3d peak does not exhibit a discernible chemical shift between SnO and SnO2. However, the oxidation states can be determined by considering the metal to oxygen stoichiometry, and correlation analysis provides a new viewpoint on this method. By analysing the correlation between Sn and O, and incorporating a phase model, the average oxidation state of Sn can be determined at any point in a depth profile.
References #
- Axnanda, Stephanus, Wei-Ping Zhou, and Michael G. White. “CO oxidation on nanostructured SnO x/Pt (111) surfaces: unique properties of reduced SnO x.” Physical Chemistry Chemical Physics 14.29 (2012): 10207-10214. Read it online here.
- Stranick, Michael A., and Anthony Moskwa. “SnO by XPS.” Surface Science Spectra 2.1 (1993): 45-49. Read it online here.
- Stranick, Michael A., and Anthony Moskwa. “SnO2 by XPS.” Surface Science Spectra 2.1 (1993): 50-54. Read it online here.
- Themlin, Jean-Marc, et al. “Characterization of tin oxides by x-ray-photoemission spectroscopy.” Physical Review B 46.4 (1992): 2460. Read it online here.
- Kövér, L., et al. “Electronic structure of tin oxides: High‐resolution study of XPS and Auger spectra.” Surface and interface analysis 23.7‐8 (1995): 461-466. Read it online here.
- Wieczorek, Alexander, et al. “Resolving Oxidation States and X–site Composition of Sn Perovskites through Auger Parameter Analysis in XPS.” Advanced Materials Interfaces 10.7 (2023): 2201828. Read it online here.
- Sexton, B. A., A. E. Hughes, and K. Foger. “An X-ray photoelectron spectroscopy and reaction study of Pt Sn catalysts.” Journal of Catalysis 88.2 (1984): 466-477. Read it online here.
- Axnanda, Stephanus, Wei-Ping Zhou, and Michael G. White. “CO oxidation on nanostructured SnO x/Pt (111) surfaces: unique properties of reduced SnO x.” Physical Chemistry Chemical Physics 14.29 (2012): 10207-10214. Read it online here.
- Domashevskaya, E. P., et al. “XPS and XANES studies of SnO x nanolayers.” Journal of Structural Chemistry 49 (2008): 80-91. Read it online here.
- Ansell, R. O., et al. “Quantitative use of the angular variation technique in studies of tin by X-ray photoelectron spectroscopy.” Journal of Electron Spectroscopy and Related Phenomena 11.3 (1977): 301-313. Read it online here.
- Bhatt, Prajna, et al. “Correlation analysis in X-ray photoemission spectroscopy.” Applied Surface Science 672 (2024): 160808. Read it online here.
