The acid content of individual hydroxyl groups on a metal oxide surface was determined for the first time using non-contact atomic force microscopy (AFM) and a hydroxyl-functionalized tip.1 With the new setup, researchers in Austria, Germany and the Czech Republic were able to directly assess the proton affinity of individual sites by examining the strength of their hydrogen bonds with the tip. “The indium oxide surface we examined has four different oxygen atoms with different reactivity, so that we can investigate the proton affinity of four oxygen atoms in one experiment,” says Margareta Wagner from TU Wien. She believes that this type of experiment could one day be used to adjust the chemical reactivity of solids, atom by atom.
Leo Gross, a scientist at the IBM Research Laboratory in Zurich, Switzerland who was not involved in the study, says proton affinity – a measure of the acidity of a system and a key variable in many chemical reactions – is typically determined by characterizing processes, in which hydrogen is exchanged between reactants. “This usually happens in large ensembles of molecules, so it is difficult to find out where on a surface the reaction of a single species took place,” he says. Wagner adds that surfaces are heterogeneous and that the way individual atoms are bound to their neighbors affects their reactivity.
With a high-resolution atomic force microscope with a modern qPlus sensor, the researchers have now been able to solve these problems. They first exposed single crystals of indium oxide to small amounts of water vapor to adsorb OH groups and protons on the surface, and then performed AFM experiments in an ultra-high vacuum chamber at low temperature. Eventually, they used theoretical models to quantify proton affinities based on known values.
“It is a mechanical testing process at the atomic level,” explains Franz Giessibl from the University of Regensburg, who developed the qPlus sensor a few years ago.2 “Basically, you grab a proton at a certain point with your left hand and an OH group with your right hand and see how much they attract.”
Wagner explains that the AFM tip is placed over a proton on the surface and the force is measured when the tip approaches that proton. “The OH group on the tip and the oxygen atom on the surface compete for the proton,” she says. If the tip can interact strongly with the proton, it means that the bond between the proton and the surface oxygen atom is weak and the proton affinity of the latter is low. This also works the other way round: A weak interaction between the tip and the proton implies a high proton affinity of the oxygen atom on the surface. ‘With the new method, the team was able to measure the acid content at certain points on an oxide surface and compare the experimental values with theoretical calculations.
Pavel Jelínek from the Czech Academy of Sciences notes that this is a clever way to study the chemical properties of individual atoms on surfaces. “With this method, you can map the reactivity of the surface at the atomic level,” he says. “This is of indispensable value to any scientist interested in chemical reactions on surfaces.” The approach works for other oxides as well. The researchers say it can be used as a general instrument for measuring proton affinities, atom by atom.