Exploring partially reduced CeO   2 (111) surface at the atomic scale using scanning probe microscopy

Cerium dioxide (CeO ${ }_2$ ) is extensively studied due to its exceptional redox properties, which are closely related to oxygen vacancy formation and the associated charging of cerium atoms from Ce ${ }^{4+}$ to Ce ${ }^{3+}$ . These charged species play an important role in promoting active sites in CeO ${ }_2$ -based catalysts. The existence of Ce ${ }^{3+}$ atoms is typically characterized by means of surface spectroscopic techniques, because the direct atomic-scale observation and discrimination of Ce ${ }^{3+}$ ions from Ce ${ }^{4+}$ atoms remains challenging. Here, we use simultaneous scanning tunneling microscopy (STM) and atomic force microscopy (AFM) complemented by force spectroscopy to characterize candidates to Ce ${ }^{3+}$ atoms on partially reduced CeO ${ }_2$ (111) samples. While STM images reveal electronic modulations of the atomic contrast in the form of an inhomogeneous shading, AFM clearly differentiates these electronic features from the true topographic atomic structure. The chemical reactivity of these candidates to Ce ${ }^{3+}$ atoms is quantified against the Ce ${ }^{4+}$ counterparts by means of force spectroscopy using carbon monoxide functionalized probes. This study demonstrates that the combination of STM with AFM and force spectroscopy bears great potential to provide robust atomic-level insights into the chemistry of defects at ceria surfaces.