Direct assessment of the proton affinity of individual surface hydroxyls with non-contact atomic force microscopy

Abstract

The state of protonation/deprotonation of surfaces has far-ranging implications in all areas of chemistry: from acid-base catalysis${}^1$ and the electro- and photocatalytic splitting of water${}^2$, to the behavior of minerals${}^3$ and biochemistry${}^4$. The acidity of a molecule or a surface site is described by its proton affinity (PA) and pK${}_{\mathrm{a}}$ value (the negative logarithm of the equilibrium constant of the proton transfer reaction in solution). For solids, in contrast to molecules, the acidity of individual sites is difficult to assess. For mineral surfaces such as oxides they are estimated by semi-empirical concepts such as bond-order valence sums${}^5$, and also increasingly modeled with first-principles molecular dynamics simulations${}^{6,7}$. Currently such predictions cannot be tested - the experimental measures used for comparison are typically average quantities integrated over the whole surface or, in some cases, individual crystal facets${}^8$, such as the point of zero charge (pzc)${}^9$. Here we assess individual hydroxyls on In${}_2$O${}_3$(111), a model oxide with four different types of surface oxygen atoms, and probe the strength of their hydrogen bond with the tip of a non-contact atomic force microscope (AFM). The force curves are in quantitative agreement with density-functional theory (DFT) calculations. By relating the results to known proton affinities and pK${}_{\mathrm{a}}$ values of gas-phase molecules, we provide a direct measure of proton affinity distributions at the atomic scale.

References in corpus (4)

• Quantum ESPRESSO: a modular and open-source software project for quantum simulations of materials
• The mechanism of high-resolution STM/AFM imaging with functionalized tips
• Composition, structure, and stability of the rutile TiO_2(110) surface: oxygen depletion, hydroxylation, hydrogen migration and water adsorption
• Fast low-noise transimpedance amplifier for scanning tunneling microscopy and beyond

Cited by in corpus (6)

• Water Structures Reveal Local Hydrophobicity on the In2O3(111) Surface
• Standardization of chemically selective atomic force microscopy for metal-oxide surfaces
• Scanning tunneling microscopy using CO-terminated probes with tilted and straight geometries
• Reconstruction changes drive surface diffusion and determine the flatness of oxide surfaces
• Molecular Arrangements in the First Monolayer of Cu-Phthalocyanine on In${}_2$O${}_3$(111)
• Near-surface Defects Break Symmetry in Water Adsorption on CeO${}_{2-x}$(111)