Scanning probe microscope electrochemical techniques

SECM (Scanning electrochemical microscopy) is a technique to characterize the local electrochemical nature of various materials by scanning a probe microelectrode [1,2]. The spatial resolution of SECM is inferior to the conventional scanning probe microscopes such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM) as the fabrication of the probe, microelectrode, with nanometer sizes is quite difficult and the faradaic current of the microprobe is very small (often picoamps or less). However, SECM has unique characteristics that cannot be expected for STM and AFM SECM can image localized chemical reactions and it also can induce localized chemical reactions in a controlled manner. 

In our days the SECM is recognized as the member of numerous scanning probe microscopic techniques. Similarly to the other scanning probe microscopic methods, it employs a microsized measuring probe, three-dimensional positioning devices, computerized data collection, and evaluation. Special feature is, however, that in SECM electrochemical microprobes are used. 

The electrochemical methods discussed in the previous chapters can provide a wealth of information about the electrode-electrolyte interface and the processes that occur there. However these methods are typically macroscopic ones, that is, they are based on measurements over large areas compared to the size of a molecule or the unit cell of a crystal surface. To provide structural information about the electrode, surface microscopic methods are needed. In Chapters 16 and 17, we consider a number of other techniques that supplement the purely electrochemical methods. This chapter is devoted to scanning probe methods, and the next describes spectroscopic and other methods. 

Alternatively, a higher rate of mass transport in steady-state measurements with a larger UME can be obtained by using it as a tip in the scanning electrochemical microscope (SECM). The SECM has typically been employed for probing interfacial ET reactions [29]. Recently, micropipettes have been used as SECM probes (see Section IV.B below) [8b,30]. Although the possibility of probing simple and assisted IT at ITIES by this technique was demonstrated, no actual kinetic measurements have yet been reported. 

Scanning electrochemical microscopy (SECM the same abbreviation is also used for the device, i.e., the microscope) is often compared (and sometimes confused) with scanning tunneling microscopy (STM), which was pioneered by Binning and Rohrer in the early 1980s [1]. While both techniques make use of a mobile conductive microprobe, their principles and capabilities are totally different. The most widely used SECM probes are micrometer-sized ampero-metric ultramicroelectrodes (UMEs), which were introduced by Wightman and co-workers 1980 [2]. They are suitable for quantitative electrochemical experiments, and the well-developed theory is available for data analysis. Several groups employed small and mobile electrochemical probes to make measurements within the diffusion layer [3], to examine and modify electrode surfaces [4, 5], However, the SECM technique, as we know it, only became possible after the introduction of the feedback concept [6, 7],... 

An alternative electrochemical approach to the measurement of fast interfacial kinetics exploits the use of the scanning electrochemical microscope (SECM). A schematic of this device is shown in Fig. 14 the principle of the method rests on the perturbation of the intrinsic diffusive flux to the microelectrode, described by Eq. (34) above. A number of reviews of the technique exist [109,110]. In the case of the L-L interface, the microelectrode probe is moved toward the interface once the probe-interface separation falls within the diffusion layer, a perturbation of the current-distance response is seen, which can be used to determine the rate of interfacial processes, generally by numerical solution of the mass-transport equations with appropriate interfacial boundary conditions. The method has been... 

Other than for electrodes immersed in bulk electrolyte, on electrodes covered by ultrathin layers the electrode potential may differ significantly across the electrode surface. Hence, localised measurements are of interest, being performed by scanning the tip across the sample. This was first applied for organic coated metals where the coating was electrochemically delaminating, driven by corrosion [12-14, 29], Even on the submicron scale the Kelvin probe technique can be applied for such studies, and then based on a modified atomic force microscope, see [34, 35]. Recent developments are the combination of Kelvin probe and SECM [36] and the use of Kelvin probe for hydrogen detection [37]. 

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