Abstract
In surface science and model catalysis, cerium oxide (ceria) is mostly grown as an ultra-thin film on a metal substrate in the ultra-high vacuum to understand fundamental mechanisms involved in diverse surface chemistry processes. However, such ultra-thin films do not have the contribution of a bulk ceria underneath, which is currently discussed to have a high impact on in particular surface redox processes. Here, we present a fully oxidized ceria thick film (180 nm) with a perfectly stoichiometric CeO2(111) surface exhibiting exceptionally large, atomically flat terraces. The film is well-suited for ceria model studies as well as a perfect substitute for CeO2 bulk material.
Highlights
Cerium oxide is a most important material in heterogeneous catalysis[1] due to its high oxygen storage capacity (OSC) based on the oxidation and reduction of cerium ions[2]
We found that annealing the ceria film in ultra-high vacuum (UHV) at temperatures above 1050 K yields the high-temperature surface morphology characterized by large atomically rough terraces[18] and a strongsurface reduction[17]
For the first two annealing steps performed at 1080 K (Fig. 1(a)), remainders of pyramids dominating the low-temperature surface morphology are found as for films annealed in UHV18
Summary
Cerium oxide (ceria) is a most important material in heterogeneous catalysis[1] due to its high oxygen storage capacity (OSC) based on the oxidation and reduction of cerium ions[2]. The CeO2(111)[6,7,8] and its reduced variant Ce2O3(111)[9,10] have been studied, and both reduction as well as re-oxidation have been demonstrated[10,11,12,13,14] Such studies are not suitable for understanding surface processes on thick ceria material as oxygen diffusion from the bulk may strongly influence the surface reaction state[2]. We show that a straightforward three step procedure in ultra-high vacuum (UHV) following MBE growth of the film yields a fully oxidized CeO2(111) surface with exceptionally wide, clean and atomically flat terraces. This is substantiated by UHV noncontact atomic force microscopy (NC-AFM) and photoelectron spectroscopy (XPS) measurements. Our work demonstrates that even ceria surfaces that are exposed to the ambient air and transferred into the UHV can be cleaned and oxidized by this three step procedure
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