Electrochemical scanning tunneling microscopy (EC-STM) can map the electrode surface with a resolution down to the nanometer or even atomic scale while at the same time allowing potential control. In particular, active sites can be identified, and local activity can be assigned by evaluating noise features in the STM signal.^[1] The underlying principle is that in the course of a reaction, continuous changes in the composition and structure of the tunneling medium lead to continuous changes in the tunneling barrier. In Figures 1a and b, the idea is sketched with the active sites in rose color. Under reaction conditions (“on”), the active sites are reflected as spikes in the signal compared to less active sites.

An example of such a measurement on an IrO_x surface under oxygen evolution reaction (OER) conditions is shown in Figure 1c. An increased noise level can be seen when the reaction is switched “on” compared to “off”. In Figure 1d, height profiles in the scan direction across the step edge marked in Figure 1c are given. When the OER is “on” (blue), multiple spikes can be seen in the STM signal compared to “off” (brown).^[2] This signal can be further evaluated to extract the local reactivity, e.g. of step edges and terrace sites. Besides IrO_x for the OER, platinum and its alloys are of interest as a catalyst for the oxygen reduction reaction in this presentation.

References

[1] R. W. Haid, R. M. Kluge, Y. Liang, A.S. Bandarenka, Small Methods 4 (2021) 2000710.

[2] R. M. Kluge, R. W. Haid, A. S. Bandarenka, J. Catal 396 (2021) 14-22.

Figure 1 | a,b) Principle of the n-EC-STM technique to identify electrocatalytic active sites. c,d) n-EC-STM measurement results on amorphous IrO_x in 0.1 M HClO₄. Reprinted from reference [2]. © 2021 Elsevier Inc.