Near-surface Defects Break Symmetry in Water Adsorption on CeO$_{2-x}$(111)

TL;DR

This study combines atomic force microscopy and first-principles calculations to reveal how subsurface defects in CeO$_{2-x}$(111) influence water adsorption, showing asymmetric water features near defects and identifying specific adsorption sites.

Contribution

It provides the first atomic-scale visualization of water adsorption asymmetry caused by subsurface defects on CeO$_{2-x}$(111) and links defect sites to water orientation and reactivity.

Findings

Water forms asymmetric boomerang-like features near defects.

Ce$^{3+}$ sites adjacent to vacancies are preferred adsorption sites.

Force signatures distinguish Ce$^{3+}$ from Ce$^{4+}$ centers.

Abstract

Water interactions with oxygen-deficient cerium dioxide (CeO$_2$) surfaces are central to hydrogen production and catalytic redox reactions, but the atomic-scale details of how defects influence adsorption and reactivity remain elusive. Here, we unveil how water adsorbs on partially reduced CeO$_{2-x}$(111) using atomic force microscopy (AFM) with chemically sensitive, oxygen-terminated probes, combined with first-principles calculations. Our AFM imaging reveals water molecules as sharp, asymmetric boomerang-like features radically departing from the symmetric triangular motifs previously attributed to molecular water. Strikingly, these features localize near subsurface defects. While the experiments are carried out at cryogenic temperature, water was dosed at room temperature, capturing configurations relevant to initial adsorption events in catalytic processes. Density functional theory identifies Ce$^{3+}$ sites adjacent to subsurface vacancies as the thermodynamically favored adsorption sites, where defect-induced symmetry breaking governs water orientation. Force spectroscopy and simulations further distinguish Ce$^{3+}$ from Ce$^{4+}$ centers through their unique interaction signatures. By resolving how subsurface defects control water adsorption at the atomic scale, this work demonstrates the power of chemically selective AFM for probing site-specific reactivity in oxide catalysts, laying the groundwork for direct investigations of complex systems such as single-atom catalysts, metal-support interfaces, and defect-engineered oxides.