Hello,
I am having a debate with some colleagues about how we believe a non-stoichiometric oxide would be observed in XPS. Let's say we have an oxide in the form of AB2-x (A = transition metal, B = oxygen), in which x = 0 - 0.5. The sesquioxide AB1.5 has the oxidation state of A(III), and the dioxide AB2 would be the A(IV) state. Would an intermediate phase in a graded stoichiometry (e.g. AB1.75) appear as a superposition of A(III) & A(IV) chemical states, or would it appear as its own intermediate doublet pair? This would mean it has a binding energy for each main line that falls between the A(III) & A(IV) binding energy values, not necessarily being attributed to either of them.
Would the lack of oxygens atoms in the coordination lead to an intermediate mixed oxidation state with A(III) & A(IV) states both present in an amorphous oxide?
My thinking is that even for an intermediate non-stoichiometric oxide, you would still be able to construct a fit from the A(III) and A(IV) chemical species. The contrary thinking is that each non-stoichiometric oxide would be shifted in binding energy position between the A(III) and A(IV) main lines.
Please let me know if I can clarify this question.
Thank you.
I imagine it depends whether 2x A(3.5), or A(IV)+A(III) has lower energy in that particular structure. In the latter case, you would have two types of A sites, so I imagine the structure would get a little distorted. I'm not sure how to answer this without someone doing DFT, but possibly others are more knowledgeable.
The other point is you assume that the off-stoichiometry is compensated entirely by adjustments to the A oxidation state. However, I imagine in theory you could also have charged vacancies/interstitials in the structure, you could form a 2-phase material (2x AB1.75 --> AB1.5 + AB2), your O oxidation state could deviate from 2- (unusual but observed in several Li ion battery cathodes on charging)... there are many possibilities. In each case, I imagine XPS will give you a clue what you made.
Keith White
I draw your attention to the fact that the oxidation state is a conditional value and is not equal to the true charge of an atom in a molecule. The peaks in XPC show not the energy of a chemical bond, but the binding energy of an electron, for example, in the 2S state with the atom nucleus in a molecule. All electron states will be observed in your two different oxides and in another supposedly mixed oxide. By the states of the electrons, you yourself need to draw a conclusion by comparing the XPS of three oxides and using other methods.
Both answers before give you important information about the chemical background of your oxides and the XPS. Maybe it is still possible to observe characteristic features in the spectrum. Depending on metal and literature, plasmon loss and shake-off satellites as well as Auger parameter might have been already related to your oxide or can be observed and compared to the stoichiometric oxide.
Dear Dr. Keith White ,
I suggest you to have a look at the following, interesting papers:
-Non-stoichiometric oxide and metal interfaces and reactions
R.A. Bennett, J.S. Mulley, M. Basham,·M. Nolan,·S.D. Elliott,·P.A. Mulheran
Appl Phys A, 96: 543–548 (2009)
Available on RG at: https://www.researchgate.net/publication/226332421_Non-stoichiometric_oxide_and_metal_interfaces_and_reactions
-Non-equilibrium Oxidation States of Zirconium during Early Stages of Metal Oxidation
Wen Ma, F. William Herbert, Sanjaya D. Senanayake and Bilge Yildiz
Papers: BNL-111675-2015-JA
Available at: https://www.bnl.gov/isd/documents/92582.pdf
-Study of SiO2/Si Interface by Surface Techniques
Rodica Ghita, Constantin Logofatu, Catalin-Constantin Negrila, Florica Ungureanu, Costel Cotirlan, Adrian-Stefan Manea, Mihail-Florin Lazarescu and Corneliu Ghica
Chapter of book: Crystalline Silicon - Properties and Uses (2011)
Available at: https://www.intechopen.com/books/crystalline-silicon-properties-and-uses/study-of-sio2-si-interface-by-surface-techniques
-New insight on early oxidation stages of austenitic stainless steelfromin situXPS analysis on single-crystalline Fe-18Cr-13Ni
Li Ma, Frederic Wiame, Vincent Maurice, Philippe Marcus
Corrosion Science, Elsevier, 140, pp.205-216 (2018)
Available at: https://hal.archives-ouvertes.fr/hal-02349690/document
-Direct observation of pure pentavalent uranium in U2O5thin films by high resolution photoemission spectroscopy
T. Gouder, R. Eloirdi & R. Caciuffo
ScientificREPORTS, 8:8306 (2018)
Available at: https://www.nature.com/articles/s41598-018-26594-z.pdf?origin=ppub
Best regards, Pierluigi Traverso.
That's a great discussion especially for a person who is interested in oxides XPS.
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