The modified tip of an atomic force microscope can probe individual atoms on a surface. Image: TU Wien.
The modified tip of an atomic force microscope can probe individual atoms on a surface. Image: TU Wien.

The degree of acidity or alkalinity of a substance is crucial for its chemical behavior. The decisive factor is the so-called proton affinity, which indicates how easily an entity accepts or releases a single proton. But while it is easy to measure this factor for molecules, it is much more difficult for surfaces, because atoms on surfaces have very different proton affinities depending on where they sit.

Now, for the first time, researchers at the Vienna University of Technology (TU Wien) in Austria have succeeded in making this important physical quantity experimentally accessible. Using a specially modified atomic force microscope, they were able to study the proton affinity of individual atoms on a surface. The researchers report this novel microscopy technique, which should prove particularly useful for analyzing catalysts at an atomic scale, in a paper in Nature.

"All previous measurements of surface acidity had one severe drawback," says Ulrike Diebold from the Institute of Applied Physics at TU Wien. "Although the surface atoms behave chemically differently, one could only ever measure the average value."

This meant researchers couldn't determine which atoms contributed to chemical reactions, and to what extent, which made it impossible to adjust surfaces at the atomic scale to favor certain chemical reactions. But that is exactly what is needed when looking to develop more effective catalysts for hydrogen production, for example.

"We analyzed surfaces made of indium oxide," says Margareta Wagner, who carried out the measurements in Diebold's lab. "They are particularly interesting because there are five different types of OH [hydroxide] groups with different properties on the surface."

By using a special trick, the researchers were able to study these OH groups individually. This trick involved placing a single OH group at the tip of an atomic force microscope, which was then precisely positioned over one specific atom on the surface. A force acts between the OH group on the tip and the OH group directly below it on the indium oxide surface, and this force depends on the distance between the two OH groups.

"We vary the distance between the tip and the surface and measure how this changes the force," explains Wagner. "This gives us a characteristic force curve for each OH group on the surface of a material." The shape of this force curve provides information about how well the respective oxygen atoms on the indium oxide surface hold their protons – or how easily they will release them.

Obtaining an actual value for the proton affinity required further theoretical work, which was carried out by Bernd Meyer at the Friedrich-Alexander-University Erlangen-Nürnberg in Germany. Using elaborate computer simulations, the force curve of the atomic force microscope could be translated in a simple and precise way into values for proton affinity.

"This is quite crucial for the further development of catalysts," says Meyer. "We know that atoms of the same type behave quite differently depending on their atomic neighbors and the way they are incorporated into the surface."

For example, it can make a big difference whether the surface is perfectly smooth or has atomic-scale steps. Atoms with a smaller number of neighbors sit at the edges of such steps, and they can potentially significantly improve or worsen chemical reactions.

"With our functionalized scanning force microscope tip, we can now precisely investigate such questions for the first time," says Diebold. "This means that we no longer have to rely on trial and error, but can precisely understand and improve chemical properties of surfaces."

This story is adapted from material from TU Wien, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.