Abstract
Scanning probe microscopy (SPM) techniques have had a great impact on research fields of surface science and nanotechnology during the last decades. They are used to investigate surfaces with scanning ranges between several 100 μm down to atomic resolution. Depending on experimental conditions, and the interaction forces between probe and sample, different SPM techniques allow mapping of different surface properties. In this work, scanning tunneling microscopy (STM) in air and under electrochemical conditions (EC-STM), atomic force microscopy (AFM) in air and scanning electrochemical potential microscopy (SECPM) under electrochemical conditions, were used to study different single crystalline surfaces in electrochemistry. Especially SECPM offers potentially new insights into the solid-liquid interface by providing the possibility to image the potential distribution of the surface, with a resolution that is comparable to STM. In electrocatalysis, nanostructured catalysts supported on different electrode materials often show behavior different from their bulk electrodes. This was experimentally and theoretically shown for several combinations and recently on Pt on Au(111) towards fuel cell relevant reactions. For these investigations single crystals often provide accurate and well defined reference and support systems. We will show heteroepitaxially grown Ru, Ir and Rh single crystalline surface films and bulk Au single crystals with different orientations under electrochemical conditions. Image studies from all three different SPM methods will be presented and compared to electrochemical data obtained by cyclic voltammetry in acidic media. The quality of the single crystalline supports will be verified by the SPM images and the cyclic voltammograms. Furthermore, an outlook will be presented on how such supports can be used in electrocatalytic studies.
Highlights
A comparison of the three techniques, scanning tunneling microscopy (STM), scanning electrochemical potential microscopy (SECPM) and atomic force microscopy (AFM), operating in air or under electrochemical conditions in constant current, constant potential and contact mode, is shown in order to evaluate the advantages and disadvantages of the different methods depending on the sample properties
The angle of the Au(111) terrace edge in the middle of the picture is 58.6 °; 120/60 ° step edges are characteristic of the Au(111) orientation. These results were directly compared with scanning electrochemical potential microscopy (SECPM)
While the electrochemical STM (EC-STM) image shown in Figure 2A monitors sharp edges of the single crystalline Ru(0001) structure, these edges appear fringed in the SECPM image (Figure 2B). These results indicate that the electronic properties of the step edges, i.e., conductivity mapped by STM and the potential distribution mapped by SECPM, may be different from the electronic properties of the smooth terraces
Summary
The cantilever oscillates preventing the tip from touching the surface and the AFM tip senses attractive forces (non-contact mode). This mode avoids that the tip or the surface of the sample are damaged which results in a decreased lateral resolution compared to contact mode. The hardware is similar to an EC-STM, the only modification consists of replacing the current pre-amplifier by a high input impedance potential difference amplifier Since both imaging techniques STM and SECPM are implemented in one head combined EC-STM/SECPM, studies of the same area of an electrode are possible, i.e., potential maps of the surface obtained at constant potential. As well as theoretical approaches, have to be done to clarify the feasibilities and limitations of this new technique including all above mentioned aspects
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