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Published April 28, 2021 | Version v1
Journal article Open

Direct assessment of the acidity of individual surface hydroxyls

  • 1. Central European Institute of Technology (CEITEC), Brno University of Technology, Brno, Czech Republic
  • 2. Interdisciplinary Center for Molecular Materials, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany.
  • 3. Department of Surface and Plasma Science, Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic.
  • 4. Institute of Applied Physics, TU Wien, Vienna, Austria.

Description

The state of deprotonation/protonation of surfaces has far-ranging implications in
chemistry, from acid–base catalysis1 and the electrocatalytic and photocatalytic
splitting of water2, to the behaviour of minerals3 and biochemistry4. An entity’s acidity
is described by its proton affinity and its acid dissociation constant pKa (the negative
logarithm of the equilibrium constant of the proton transfer reaction in solution). The
acidity of individual sites is difficult to assess for solids, compared with molecules. For
mineral surfaces, the acidity is estimated by semi-empirical concepts, such as
bond-order valence sums5, and increasingly modelled with first-principles molecular
dynamics simulations6,7. At present, such predictions cannot be tested—experimental
measures, such as the point of zero charge8, integrate over the whole surface or, in
some cases, individual crystal facets9. Here we assess the acidity of individual hydroxyl
groups on In2O3(111)—a model oxide with four different types of surface oxygen atom.
We probe the strength of their hydrogen bonds with the tip of a non-contact atomic
force microscope and find quantitative agreement with density functional theory
calculations. By relating the results to known proton affinities of gas-phase molecules,
we determine the proton affinity of the different surface sites of In2O3 with atomic
precision. Measurements on hydroxylated titanium dioxide and zirconium oxide
extend our method to other oxides.

Notes

This work was supported by the Austrian Science Fund (FWF), project V 773-N (Elise-Richter-Stelle, M.W.) and Z 250-N27 (Wittgenstein Prize, U.D.), as well as the German Research Foundation (DFG), Research Unit FOR 1878 (funCOS, B.M.). M.W. and U.D. also acknowledge funding under the Horizon 2020 Research and Innovation Programme under the grant agreement number 810626. M. Setvin acknowledges the support of GAUK Primus/20/SCI/009. Computational resources were provided by LRZ Garching (project pn98fa)and RRZ Erlangen.

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