Finding a cheap and effective method to split water is crucial if hydrogen is to fulfill its potential as a clean energy source on a global scale, and metal oxides are one of the catalyst types being investigated for this purpose. However, the atomic-scale insights into the behavior of water molecules on metal oxide surfaces needed to further this research have proved difficult to obtain.

Using an imaging technique called scanning tunneling microscopy (STM), Yousoo Kim and Maki Kawai at RIKEN and co-workers in Japan1 have now been able to ‘watch’ the dissociation of a single water molecule on an ultrathin magnesium oxide (MgO) film.

In STM, a metal tip set at a certain voltage is scanned across a surface. The voltage causes electrons to ‘tunnel’ between the tip and the surface, resulting in a current that varies depending on the features of the surface. From this scanned map, an image of what is happening on the surface can be constructed.

Fig. 1: The splitting of single water molecules (blue) on an ultrathin MgO film (gray) placed on a silver surface (white) can be observed by the response of a scanning tunneling microscope tip (top).

The insulating properties of MgO, however, make it difficult to observe the water dissociation reaction using this technique. “Such a study demands that the conductivity of the substrate be sufficient for electron tunneling,” Shin explains. To achieve this condition, the researchers placed the MgO film on a conducting silver surface (Fig. 1).

The team saw that, as expected, water splitting only occurs at defects on the MgO surface. “The most significant aspect of the work, however, is that we were able to selectively excite not only the vibrational state but also the electronic state of the molecule to achieve the chemical reaction at the single-molecule level,” Shin says. ”The vibrationally induced dissociation of a single water molecule was previously impossible to achieve on metal surfaces,” he adds.

As well as being catalysts in their own right, metal oxides can be used to tune the catalytic properties of metal nanoparticles supported on their surface. “The chemical reactions occurring in the presence of metal nanoparticles supported by MgO films is one of the promising systems that can be investigate on the basis of our findings,” says Shin. “In this system, the reaction barrier can be reduced by the catalytic effects of the metal nanoparticles, while the lifetime of an electron in the molecule can be enhanced by the insulating MgO film.”