A microscope for imaging surface chemistry

When a chemical reaction involves a solid surface—a catalyst in an industrial reactor, for example, or a slowly corroding steel pipe—the results are messy and hard to understand. The problem is that every atom on a surface is a little bit different: In contrast to laboratory surface-science experiments, which typically use pristine crystalline facets with identically situated atoms, real surfaces are full of crags and defects. Each atom connects slightly differently to its neighbors and thus has slightly different chemical properties.

Those chemical distinctions are extremely difficult to assess directly. Standard experimental measurements can quantify only the average reaction output for the whole surface; they can’t track individual reactant molecules to see where on the surface they reacted.

Now the Technical University of Vienna’s Ulrike Diebold , Margareta Wagner, and their colleagues have adapted an atomic force microscope (AFM) to measure a surface chemical property—affinity for hydrogen ions, otherwise known as acidity—with atomic resolution.

For their proof-of-principle experiment, they used a crystalline surface. But the material they chose, indium oxide, has a sufficiently complex structure that its oxygen atoms (shown in red in the figure) can sit at the surface in four distinct chemical environments. When the researchers added hydrogen (white) to the mix by introducing water molecules that dissociate on the surface, the O atoms from the water (yellow) occupy a fifth type of chemically distinct site.

Diebold and her colleagues used an AFM tip coated in hydroxylated In₂O₃—the same material as the surface—with a single OH group dangling from the end. As the O atom on the tip approaches a surface-bound H atom, the two experience an attractive force (represented by the dotted line in the figure). The attraction sets up a tug-of-war for the H atom between the tip and the surface. The tip always loses: The terminal O atom already has one H atom, so the attraction it feels to a second H is weaker than the chemical bond holding the H to the surface. In fact, the more strongly the H clings to the surface O atom, the more weakly it’s attracted to the tip.

With their AFM, the researchers measured the force of attraction between the tip and H atoms on all five distinct O sites, and they quantitatively related their results to what theory predicts for the strength of the surface O–H bonds. By comparing their measurements with quantum calculations on molecules of known acidity, they derived a value for the acidity of each surface site: a measure of how readily an O atom will give up its H in an acid–base reaction, or how likely a bare O site is to draw in a new H from the surroundings.

Although the hydroxylated In₂O₃ surface contains only five types of O sites, the researchers imagine using the same method to look at irregular surfaces with countless chemically distinct sites. By imaging the H affinity atom by atom, they’ll get a much clearer view of how a reaction on the surface is likely to proceed. (M. Wagner et al., Nature 592, 722, 2021 .)