Analyzing Oxygen Vacancy Using X-Ray Photoelectron Spectroscopy

Metal oxides are often used in ceramics, and they are commonly found in the Earth’s crust (1). Apart from ceramics, metal oxides are used in other industries such as petrochemicals, medicine, and environmental analysis. They are also widely used in catalysis, superconductivity, magnetism, and ferroelectricity (2).

The functionality and design of metal oxides is heavily influenced by accurate analysis of oxygen vacancies. Oxygen vacancies help give the metal oxides its important physiochemical properties (2). Oxygen vacancies in metal oxides are pivotal to their electronic and structural properties, but their accurate measurement has long been a subject of debate (3).

Colorful 3D Illustration of the Reduction of Metal Oxides at Microscopic Level | Image Credit: © artefacti - stock.adobe.com

In a recent study, Jiayue Wang of Stanford University, in collaboration with researchers from the Peter Grünberg Institute and Lawrence Berkeley National Laboratory, examined this issue at length. In particular, the research team explored several new strategies to enhance the reliability of oxygen vacancy quantification using X-ray photoelectron spectroscopy (XPS) (3).

Also known as electron spectroscopy for chemical analysis, XPS is a technique designed to measure and analyze the surface chemistry of a given material (4). It is often used for characterizing metal oxides, yet a common pitfall has persisted: the misinterpretation of the 531–532 eV feature in the O 1s spectra as a direct indicator of oxygen vacancies (3). The researchers sought to explore ways how this pitfall could be resolved. To do so, the research team investigated three alternate approaches that could be used to assess oxygen vacancies accurately and efficiently.

First, the research team suggested to use XPS to monitor changes in the valence state of cations (3). Oxygen vacancies often cause cations to lower their oxidation states, so by using XPS, researchers should be able to infer the presence and concentration of oxygen vacancies (3).

Second, the research team suggested that evaluating the surface oxygen-to-cation stoichiometry through normalized oxygen spectral intensity could also be a solution (3). As oxygen is released and vacancies form, the relative intensity of the oxygen spectrum diminishes (3).

And finally, the researchers discussed how there could also be value in tracking shifts in the binding energy. Oxygen vacancies lead to electron doping, which increases the electron chemical potential in oxides (3). Tracking this could be a reliable pathway for oxygen vacancy analysis.

After describing these three proposed methods in detail, the research team discussed when each method is best suited to be used. Ultimately, the researchers’ conclusion was that the best method to use out of the proposed solutions depends on two important things: the specific material being used and understanding the physicochemical and spectroscopic behavior of a given material (3).

These alternative XPS techniques hold significant implications for the future of materials science. Improved quantification of oxygen vacancies will enable deeper insights into the structural and electronic dynamics of metal oxides, paving the way for innovations in energy storage, catalysis, and electronic devices (3).

Oxygen vacancies are important in making metal oxides have enhanced conductivity (5). This study advances work in this space by explaining how XPS can be used to analyze oxygen vacancies. By debunking traditional assumptions and introducing a few alternative methods that could improve oxygen vacancies, Wang and colleagues provide a robust framework for more precise investigations of metal oxides (3).

This study, published in the Journal of the European Ceramic Society, shows how these advancements highlighted in the paper can help propel the material technology industry forward.

References

1. Yu, X.; Marks, T. J.; Facchetti, A. Metal Oxides for Optoelectronic Applications. Nature Mater. 2016, 15, 383–396. DOI: 10.1038/nmat4599
2. Wang, Z.; Lin, R.; Huo, Y.; Li, H.; Wang, L. Formation, Detection, and Function of Oxygen Vacancy in Metal Oxides for Solar Energy Conversion. Adv. Funct. Mater. 2022, 32 (7), 2109503. DOI: 10.1002/adfm.202109503
3. Wang, J.; Mueller, D. N.; Crumlin, E. J. Recommended Strategies for Quantifying Oxygen Vacancies with X-ray Photoelectron Spectroscopy. J. Eur. Cer. Soc. 2024, 44 (15), 116709. DOI: 10.1016/j.jeurceramsoc.2024.116709
4. Thermo Fisher Scientific, X-Ray Photoelectron Spectroscopy. Thermo Fisher. Available at: https://www.thermofisher.com/us/en/home/materials-science/xps-technology.html#:~:text=Surface%20modification%20can%20be%20used,surface%20elements%20(except%20hydrogen).(accessed 2024-11-25).
5. Gunkel, F.; Christensen, D. V.; Chen, Y. Z.; Pryds, N. Oxygen Vacancies: The (In)visible Friend of Oxide Electronics. Appl. Phys. Lett. 2020, 116, 120505. DOI: 10.1063/1.5143309