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O: Fachverband Oberflächenphysik

O 101: Poster Session VIII: Oxides and insulators: Adsorption and reaction of small molecules II

O 101.3: Poster

Donnerstag, 4. März 2021, 13:30–15:30, P

Direct assessment of the acidity of individual surface hydroxyls on In₂O₃(111) — •Margareta Wagner^1,2, Bernd Meyer³, Martin Setvin^{1, 4}, Michael Schmid¹, and Ulrike Diebold¹ — ¹TU Wien, Vienna, Austria — ²CEITEC BUT, Brno, Czech Republic — ³FAU Erlangen-Nürnberg, Erlangen, Germany — ⁴Charles University, Prague, Czech Republic

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 O_s(α-δ). Water dissociation leads to a pair of OH groups: the surface O_SH(β) and the water O_WH. 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.