Ultrathin Cerium Oxide Islands on Cu(111): Work Function and Reaction with Oxygen and Hydrogen at RT

Abstract Ultrathin oxide films can significantly change the work function (WF) of metal surfaces, with large consequences for the adsorption of nanoparticles and molecules on the films. In contrast to other oxides, little is known about the impact of ultrathin cerium oxide (ceria) films on the metal WF, particularly with regard to the reducibility of ceria. Here, it is shown that oxidized ceria islands in the submonolayer range reduce the WF of Cu(111) by at least −0.25 eV, while reduced ceria islands strongly decrease the metal WF by more than −1 eV, as measured by Kelvin probe force microscopy (KPFM) under ultrahigh-vacuum (UHV) conditions. A possible model suggests that the significant WF modifications upon reduction are due to an electron transfer from ceria’s oxygen vacancies into the Cu support, as most of the interfacial Ce ions are already in the 3+ state after the thin-film preparation. As observed by KPFM, reduced ceria islands become immediately oxidized at room temperature (RT) when exposed to a few Langmuir of O2 due to the filling of oxygen vacancies with O. In contrast, H2 at RT generally does not affect ceria, nor does it affect the WF.