Polymers SECM studies

Although most amperometric SECM experiments involved ET reactions at the tip and/or substrate, interfacial IT processes can also be probed. Historically, the first IT reactions studied by SECM were ion-exchange processes at ionically and electronically conductive polymer films (48). The ions of interest were electrochemically active (e.g., Ec(CN)f or Br ) to enable amperometric detection at the tip. It was shown more recently that the tip process can be an IT reaction rather than an ET process if a micropipet electrode is used as an amperometric probe (49). In this section we consider two different types of IT reactions employed in SECM studies, i.e., facilitated IT and simple IT. 

From SECM studies of the electrochemical doping and undoping processes of poly-(didodecyl-terthiophene), the electron transfer between the polymer and a redox mediator in solution was investigated as a function of the doping state of the polymer [164], The electron transfer was found to occur at the polymer-electrolytic-solution interface and not inside the polymer film. The investigated polymer films are not permeable to redox species when placed in the neutral state and therefore behave as completely passivating films. 

SECM studies of electroosmosis in the persulfonated polymer Nation have been investigated using the single pore membrane shown in Figure 9.13. Cation transport numbers in Nation approach unity,and electroosmotic flow of water readily occurs in this material. The Nation-filled, single pore membrane is referred to hereafter as the mica/Nafion membrane. 

The SECM capacity for rapid screening of an array of catalyst spots makes it a valuable tool for studies of electrocatalysts. This technique was used to screen the arrays of bimetallic or trimetallic catalyst spots with different compositions on a GC support in search of inexpensive and efficient electrocatalytic materials for polymer electrolyte membrane fuel cells (PEMFC) [126]. Each spot contained some binary or ternary combination of Pd, Au, Ag, and Co deposited on a glassy carbon substrate. The electrocatalytic activity of these materials for the ORR in acidic media (0.5 M H2S04) was examined using SECM in a rapidimaging mode. The SECM tip was scanned in the x—y plane over the substrate surface while electrogenerating 02 from H20 at constant current. By scanning... 

A variety of studies have now been done that demonstrate that the SECM can carry out metal deposition, metal and semiconductor etching, polymer formation, and other surface modifications with high resolution. Such processes are discussed in Chapter 13. These SECM approaches have the advantage over analogous STM procedures in that the conditions of deposition or etching are usually known and well defined, based on electrochemical studies at larger electrodes. 

Most SECM measurements are carried out with the sample under a thick liquid layer, and thus the tip must be sheathed in an insulator to achieve high resolution. SECM measurements can also be carried out within a thin layer of water that forms on the surface of a sample in air. In this case very high resolution can be attained using tips without insulation (e.g., the usual W or Pt-Ir STM-type tips) because the tip area is defined by the small part of the tip that touches the liquid layer (34,35). Studies of mica surfaces, polymer films, and some biological samples as described below are possible by this technique. With this mode it is also possible to fabricate small metal structures in polymer films as demonstrated previously (36). High-resolution electrochemical deposition of silver nanostructures on mica surfaces in humid air was also achieved (35). For detailed discussion on SECM applications for fabrication, see Chapter 13. 

SECM can also be used to study the flux of species produced at a modified electrode surface, such as one with a film of polymer (Section 14.2.3). In one type of experiment, the tip is held at a potential where it can detect an electroactive ion released from the polymer film during a redox process (30-32). For example the SECM was used to detect the release of Br during the reduction of oxidized polypyrrole (PP) in the form, PP" Br . During a reductive cyclic voltammetric scan, Br was found to be released only in a later part of the scan, after an appreciable amount of cathodic charge had passed. This result suggested that during the early phase of the reduction the uptake of cations, rather than the release of anions, maintained charge balance in the film. 

SECM has been applied to the investigation of various technologically important materials and interfaces, for example, metallic corrosion [91-96], fuel cell electrocatalysts [97], semiconductor photocatalysts [12, 60-63, 98], conducting polymers [49, 50, 85, 86, 99-103], liquid-liquid and liquid-gas interfaces [29, 30, 68]. The SECM may be used to image the substrate topography and/or reactivity, or with the tip at a fixed location, to study the local kinetics of the interfacial reactions of interest. 

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