Fluoride Substitution: Quantifying Surface Hydroxyls of Metal Oxides with Fluoride Ions

Abstract

Surface hydroxyls (OH) are crucial for heterogeneous catalysis in water. However, they are commonly characterized at solid–gas interfaces (e.g., FTIR, XPS, TGA), which may not represent the surface in aqueous environments. Here, the surface OH of five catalytically relevant oxides (Al₂O₃, ZrO₂, TiO₂, Fe₂O₃, Co₃O₄) are quantified by substituting them with F⁻ ions at pH 3–10, where the surface fluoride (F) density is evaluated by XPS using the geometry factor for spherical particles. These results show that the surface F density peaks at around pH 4 across all oxides, but decreases at more basic pH due to increased OH⁻ competition. Generally, oxides more abundant in surface OH can also accommodate more surface F, establishing F⁻ ions as effective probes. While terminal F are likely the preferential substitution product, bridging F also appear to form at lower pH levels. Furthermore, fluoride substitution is applied to a series of Co₃O₄ gradually enriched with defects using pulsed laser defect engineering in liquid (PUDEL). This approach reveals a linear correlation between laser processing and surface OH density, which aligns with a previously observed improvement in OER activity, and is supported by additional DFT calculations here. This work will stimulate further studies adopting fluoride substitution to better understand the relationship between surface chemistry and catalytic processes in aqueous environments.