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. 2023 Mar 27;123(8):4972–5019. doi: 10.1021/acs.chemrev.2c00766

Figure 3.

Figure 3

(a) Sequential STM images of a CoTPP monolayer on Au(111) in 0.1 M HClO4 saturated with oxygen at different potentials. Reproduced with permission from ref (426). Copyright 2016 John Wiley & Sons, Ltd. (b) Scheme explaining the concept. When the local environment between the STM tip and the sample changes (in this case, when the tip is over a terrace (I) versus a step (II)), the tunneling barrier also changes in a way that is driven by the changes in approaching and departing reactants and products. In this scenario, increased tunneling-current noise is likely to appear when the tip is over a step edge, which is more active than the terrace sites. If the STM is operated in constant-current mode, then the noise is revealed in the measured z-position (the height of the STM tip over the sample). (III) STM line scans (constant-current mode) obtained over a Pt(111) surface in 0.1 M HClO4 when the potential of the sample is either sufficiently negative or too positive to initiate the HER (which is therefore either “ON” or “OFF”). (IV) Typical STM line scan over the Pt(111) surface in 0.1 M HClO4 under HER conditions. (c) Histograms of the tunneling-current noise over a polycrystalline Au surface (I) and corresponding histograms obtained over a polycrystalline Pt surface (II), results of repetitive ORR “ON” and “OFF” STM noise experiments at the same terrace of a Pt(111) sample (III), STM line scans at the surface of a Pt(111) electrode under ORR conditions, where both terraces and steps can be visualized (IV). Reproduced with permission from ref (427). Copyright 2017 Springer Nature. (d) n-EC-STM measurement of a Pt(111) step and adjacent terraces. With decreasing potential, the overall noise level increases. For data evaluation, the terrace data (on the left) are used for calibration for the relation between noise level and activity on this sample (I), histograms after separating the data for terrace (black) and step (red). At every applied sample potential, the step shows a higher noise level relative to the terrace, as evident from the broader and less intense distribution. All potentials given are referred to the Pt quasi-reference electrode (II). Reproduced with permission from ref (428). Copyright 2021 John Wiley & Sons, Ltd.