Researchers Successfully Switch Oxygen Molecules On and Off

Oxygen atoms are extremely reactive, yet the world is surrounded by this aggressive element and it does not spontaneously burn. How come? The reason is that normal O₂ molecules, are not specifically reactive.

At the Vienna University of Technology, researchers have been able to selectively switch individual O₂ molecules deposited on a titanium oxide surface between a non-reactive to a reactive state using a special force microscope. This method was viewed in high-resolution images for the first time.

Activation: Heat, or Electrons

There are several ways to switch a stable, non-reactive O₂ molecule into a reactive state. You can increase the temperature - that happens when you burn things. Alternatively, you can add an additional electron to the molecules, this also makes them chemically active.

Martin Setvin, Institute for Applied Physics at the Vienna University of Technology

Setvin is a member of the research group of Prof. Ulrike Diebold.

This method of activating oxygen molecules by incorporating electrons is ubiquitous - all living organisms employ this trick, and current fuel cells also function in this way. At the TU Wien, Setvin and colleagues are now able to trigger individual O₂ molecules at will with a force microscope, and discover how the process takes place at the atomic scale.

In the experiments, O₂ molecules were analyzed on the surface of a titanium oxide crystal at very low temperatures. Titanium oxide is an interesting material used in numerous areas - from the self-cleaning, dirt-repellent mirrors to coating for artificial hip joints. It is also a photocatalyst, which means that it can trigger chemical reactions when irradiated with light.

Seeing and Feeling Atoms

The success of the oxygen experiments was mainly attributed to a sophisticated atomic-force microscope, purchased by Prof. Diebold using proceeds of her 2014 Wittgensteinpreis Award.

A tiny needle is vibrated and moved across the surface. When the atoms at the very end of the tip come close to the surface, the tip feels a force and the oscillation changes. From this tiny change, one can create an image showing where the atoms are. Essentially, the reactive oxygen molecules that have an extra electron exert a stronger force on the tip than the unreactive ones, and thus we can distinguish them.

Ulrike Diebold, Professor, Institute for Applied Physics at the Vienna University of Technology

Interestingly, it is also possible for an extra electron to be injected to an individual oxygen molecule with the same tip, and then view the transition from the inactive to the active state. The same method also takes place when the titanium oxide’s surface is irradiated with light – electrons are let loose inside the material, and can rise to the surface to trigger one of the oxygen molecules.

Whether we add an electron using the microscope or by irradiating the titanium oxide - the end  result is the same. Our method gives us a whole new level of control over this process, and opens up new possibilities for investigating the inner workings of photocatalysts.

Ulrike Diebold, Professor, Institute for Applied Physics at the Vienna University of Technology