Critical examination of the use of x-ray photoelectron spectroscopy (XPS) O 1s to characterize oxygen vacancies in catalytic materials and beyond

In 2021, Idriss published a manuscript outlining that an assignment commonly utilized in catalyst research is not feasible. Despite this, the idea that you can measure oxygen vacancies (OV) with ex situ x-ray photoelectron spectroscopy (XPS) persists. A review of 427 manuscripts citing Idriss’s work found that for the citations related to catalysis (55%), those with an interest in OV were more likely to avoid using XPS to identify OV. Regardless, many catalysis researchers continue to rely on erroneous application of XPS data. Of the citations not related to catalysis research (45%), these were more likely to utilize the more obscure approaches. Surprisingly, two thirds of the citations showed no apparent interest in the assignment, an observation shared between both groups. We believe that many of these citations have failed to understand the original publication and/or are using it as a general citation for O 1s assignments. When the data were viewed as a function of publication year, three observations were made: (1) The number of citations using XPS for OV is increasing, (2) those utilizing it as a direct measure are decreasing, and (3) one alternative method is gaining popularity. Herein, we provide evidence that most approaches are generally not applicable, or if they are, can only be successfully performed in-operando using highly specialized equipment with carefully planned methodologies; less than 3% of the citations reviewed met these criteria. It is envisaged that this publication will assist in improving the quality of XPS data interpretation in relation to XP O 1s spectroscopy.