On the wrong assignment of the XPS O1s signal at 531–532 eV attributed to oxygen vacancies in photo- and electro-catalysts for water splitting and other materials applications

•A large number of articles are wrongly attributing an XPS O1s signal at about 531–532 eV to oxygen vacancies.•The signal attributed to oxygen vacancies is that of adventitious hydroxyls of water.•A binary or mixed metal oxide containing surface oxygen vacancies will be oxidized instantaneously in ambient conditions.•Examples of surface science studies on model surfaces are given and discussed. In recent years there has been a large number of articles wrongly attributing, an XPS O1s signal at about 531–532 eV to oxygen vacancies. These studies are based on ex situ measurements of powder materials. By definition, photoelectron spectroscopy relies on measuring the kinetic energy of an electron removed from the core or valence levels of an atom in a compound. Therefore, a photoelectron signal originating from a missing oxygen atom is not possible. The signal attributed to oxygen vacancies is simply that of adventitious hydroxyls of water that are inevitably adsorbed on oxides in ambient conditions. XPS O1s of reduced and non-reduced single crystals and thin films have been studied in details for decades in surface science, some of which are given here. In addition, a binary or mixed metal oxide material or catalyst containing surface oxygen vacancies will be oxidized instantaneously in ambient conditions due to the strong adsorption energy of water (typically about 1 eV) and its high sticking probability (typically equal 1) on defected oxides) and therefore ex-situ measurements of surface oxygen vacancies are not possible.