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

Technical Summary: Near-surface Defects Break Symmetry in Water Adsorption on CeO2−x(111)

Problem Statement
Water interactions with oxygen-deficient cerium dioxide (CeO2) surfaces are critical for hydrogen production and catalytic redox reactions. However, the atomic-scale mechanisms by which defects influence water adsorption and reactivity remain elusive. Previous studies using Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM) on CeO2(111) surfaces, ranging from room temperature to 10 K, consistently imaged molecular water as wide, symmetric triangular features extending over three surface oxygen atoms. These features were attributed to a six-fold degenerate adsorption state where water molecules rapidly interconvert between molecular and hydroxyl pair configurations via low energy barriers. The challenge addressed in this work is to resolve the true atomic structure of water adsorption on partially reduced ceria and to determine how subsurface defects alter this symmetry and reactivity.

Methodology
The authors employed a combined experimental and theoretical approach:

• Experimental: Experiments were conducted using Non-Contact Atomic Force Microscopy (nc-AFM) at cryogenic temperatures (~4.8 K) with chemically sensitive, oxygen-terminated probes (modeled as rigid CO molecules). Water was dosed at room temperature (35°C) onto partially reduced CeO2−x(111) thin films grown on Cu(111), resulting in low coverage. The study utilized constant-height AFM imaging and force spectroscopy (frequency shift, $\Delta f$, curves) to probe atomic sites.
• Theoretical: First-principles calculations were performed using Density Functional Theory (DFT) within the VASP code. The study utilized the PBE exchange-correlation functional with DFT+U (Ueff = 4.5 eV) to treat Ce 4f electron localization and DFT-D3 for long-range dispersion corrections. Models included (3×3) and (4×4) supercells with subsurface oxygen vacancies (SSOV) to simulate the reduced surface.
• Simulation: To interpret the AFM data, the authors simulated constant-height images and force curves using the Full Density-Based Model (FDBM). This model combines short-range Pauli repulsion and electrostatic interactions (calculated via DFT with a rigid CO probe) with long-range van der Waals (vdW) interactions derived from experimental fits.

Key Results

1. Asymmetric Water Imaging: Contrary to the symmetric triangular motifs reported previously, the high-resolution AFM images reveal water molecules as sharp, asymmetric, boomerang-like features. These features connect the adsorption site to two neighboring oxygen atoms, breaking the surface's three-fold symmetry.
2. Role of Subsurface Defects: DFT calculations identify that water preferentially adsorbs on Ce3+ sites adjacent to subsurface oxygen vacancies (SSOV) rather than on Ce4+ sites of the pristine surface. The adsorption energy on Ce3+ (-0.79 eV) is more favorable than on Ce4+ (-0.70 eV). The presence of the SSOV induces lattice relaxations (oxygen atoms moving ~15 pm toward the bulk, Ce3+ moving ~7 pm toward vacuum) that break the local symmetry, stabilizing specific adsorption orientations.
3. Probe-Induced Dynamics: The observed "boomerang" shape is attributed to the probe-induced mobility of the water molecule. The low energy barriers between different adsorption configurations (molecular vs. hydroxyl pair) allow the water molecule to reorient or transition states during the AFM imaging process. Simulations show that the probe can induce transitions between molecular and hydroxyl pair forms, as well as rotations of the molecule, which manifest as the sharp, bond-like features in the images.
4. Spectroscopic Signatures of Ce3+: Force spectroscopy reveals distinct interaction signatures between Ce3+ and Ce4+ sites. Ce3+ sites exhibit a significantly deeper $\Delta f$ minimum (stronger attractive force) compared to Ce4+ sites. This is counter-intuitive given the extra electron screening but is explained by local structural relaxations that enhance the vertical electric field ($E_z$) and reduce Pauli repulsion. Experimental $\Delta f$ curves over cerium atoms near water molecules show this enhanced attraction, suggesting the presence of Ce3+ ions and nearby SSOVs.
5. Water Coverage and Defects: Water dosing at room temperature resulted in low coverage, which did not significantly increase with higher dosages. This suggests that water adsorption on CeO2(111) at room temperature is heavily dependent on the presence of defects (SSOVs and Ce3+), consistent with previous observations that fully oxidized surfaces do not adsorb water at room temperature.

Significance and Claims
The paper claims to resolve the long-standing ambiguity regarding the atomic structure of water on ceria surfaces. By combining chemically selective AFM with first-principles calculations, the authors demonstrate that:

• The previously observed symmetric triangular features were likely an artifact of lower resolution or different surface conditions, whereas the true ground state on reduced surfaces involves symmetry breaking driven by subsurface defects.
• Subsurface oxygen vacancies and the associated Ce3+ ions are the thermodynamically favored sites for water adsorption, dictating the orientation and reactivity of the adsorbate.
• Chemically selective AFM, particularly with oxygen-terminated probes, is a powerful tool for distinguishing between Ce3+ and Ce4+ centers and probing site-specific reactivity in oxide catalysts.
• The work lays the groundwork for investigating complex catalytic systems, such as single-atom catalysts and metal-support interfaces, where local defect structures and charge states critically influence chemical behavior.

The authors maintain a modest tone regarding the direct identification of subsurface vacancies, noting that while they cannot directly image the SSOV in the $\Delta f$ or dissipation signals, the spectroscopic signatures of the cerium atoms and the behavior of the water molecules provide strong indirect evidence for their presence and influence.