Speaker
Description
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 In$_2$O$_3$(111) surface, which has four inequivalent surface O atoms O$_\mathrm{S}$($\alpha$-$\delta$). Water dissociation leads to a pair of OH groups: the surface O$_\mathrm{S}$H($\beta$) and the water O$_\mathrm{W}$H. The remaining surface O atoms O($\alpha$, $\gamma$, $\delta$) 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 In$_2$O$_3$(111) with atomic precision. Measurements on hydroxylated TiO$_2$ 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.
