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M. Wagner, B. Meyer, M. Setvin, M. Schmid, U. Diebold:
"Direct assessment of the acidity of individual surface hydroxyls on In₂O₃(111)";
Poster: DPG-Frühjahrstagung (DPG Spring Meeting) of the Surface Science Division, SurfaceScience21, virtual; 04.03.2021.

Non-contact atomic force microscopy is a versatile tool to investigate properties of individual atoms or molecules. Here we report a novel approach to determine the acidity of individual surface hydroxyls, which is directly linked to the proton affinity (PA) of the involved O atoms. The PA - the tendency to gain or lose a proton - is crucial e.g. in acid-base catalysis and the electro- and photocatalytic splitting of water.

The testcase of this study is the stoichiometric In2O3(111) surface, which has four inequivalent surface O atoms Os(α-δ). Water dissociation leads to a pair of OH groups: the surface OSH(β) and the water OWH. The remaining surface O atoms O(α, γ, δ) can be protonated via manipulation with the tip. We probe the strength of their H bond with a functionalized tip of a nc-AFM via F(z)-spectroscopy and find quantitative agreement with density-functional theory (DFT) calculations. By relating the results to known PAs of gas-phase molecules, we can calibrate our data and determine the PA of different surface sites of In2O3(111) with atomic precision. Measurements on hydroxylated TiO2 and zirconia extend our method to other oxides. The trends of the site-specific PA values agree well with the expectations based on area-averaging techniques.