SECM

Scanning electrochemical microscopy seeks to overcome the lack of sensitivity and selectivity of the probe tip in STM and AFM to the substrate identity and chemical composition. It does this by using both tip and substrate as independent working electrodes in an electrochemical cell, which therefore also includes auxiliary and reference electrodes. The tip is a metal microelectrode with only the tip active (usually a metal wire in a glass sheath). At large distances from the substrate, in an electrolyte solution containing an electroactive species the mass-transport-limited current is therefore 

There are two types of application of this microscopic technique  

The resolution of the technique is limited by the size of the microelectrode tip, at present 200 nm. In the future, reduction of tip size to tens of nanometres by use of novel microelectrode fabrication procedures should increase the applicability of SECM. 

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SECM Scanning electrochemical microscopy [40] An STM serves as microelectrode to reduce electroactive species Electrochemical reactions on surfaces... 

The teclmique of scaiming electrochemical microscopy (SECM) [62] uses the same apparatus as in electrochemical STM, but instead of measuring tunnelling currents, the reaction O + ue —> R (where O and R... 

Figure Bl.19.12. Basic principles of SECM. (a) With ultramicroelectrode (UME) far from substrate, diflfiision leads to a steady-state current, ij, (b) UME near an insulating substrate. Flindered diflhision leads to < ij, 3D. (c) UME near a conductive substrate. Positive feedback leads to go. (Taken from [62],...

SECM is a scaiming-probe teclmiqiie introduced by Bard et aJ in 1989 [49, and M ] based on previous studies by the same group on in situ STM [ ] and simultaneous work by Engstrom et aJ [53 and M], who were the first to show that an amperometric microelectrode could be used as a local probe to map the concentration profile of a larger active electrode. SECM may be envisaged as a chemical microscope based on faradic current changes as a microelectrode is moved across a surface of a sample. It has proved iisefiil for... 

With SECM, almost any kind of electrochemical measurement may be carried out, whether voltaimnetric or potentiometric, and the addition of spatial resolution greatly increases the possibilities for the characterization of interfaces and kinetic measurements [, and 59]. It may be employed as an electrochemical tool... 

The high rate of mass transfer in SECM enables the study of fast reactions under steady-state conditions and allows the mechanism and physical localization of the interfacial reaction to be probed. It combines the usefid... 

FIGURE 2-17 Principles of SECM. (a) Tip far from the substrate surface diffusion of O leads to steady-state current. (b) Tip near a conductive substrate positive feedback of O. (c) Tip near the insulating substrate hindered diffusion of O. c = concentration a = radius of tip. (Reproduced with permission from reference 55.)... 

FIGURE 2-18 SECM image of a gold minigrid surface. (Reproduced with permission from reference 57.)... 

Explain clearly how SECM images the microdistribution of the electrochemical activity of composite electrode materials. 

Propose a SECM experiment for mapping the distribution of an oxidase enzyme within a carbon composite surface. (Note that the enzyme generates hydrogen peroxide in the presence of its substrate and oxygen.)... 

A typical configuration of a SECM system is shown in Fig. 36.6. In this case the solution contains oxidized (Ox) species (denoted mediators) that are reduced on the active part of the microelectrode yielding the reduced (Red) species. The figure also shows a possible reaction of the Red species with the electrode, with the reaction rate If is very large, the approach of the tip to the surface will result in an increase in the reduction reaction (current) on the tip because the regeneration of Ox on the tip will be more efficient in a smaller gap. On tfie otfier fiand, if k is close to zero, the only effect of the tip approach to the surface wifi be the depletion of the Ox species in the gap upon reduction, whose diffusion from the bulk of the solution is now hindered by the probe. These two mechanisms, which result in the positive and negative feedback operation modes, can be used to map the reaction rate k, on the surface. 

A different operation mode can be empfoyed in SECM if some species are only generated via a reaction on the substrate and they are electrochemicaUy active for... 

SECM has also been used to deposit a gold microstructure on n-Si(lll) and indinm oxide. In these experiments, Au is dissolved anodically from the ultramicro electrode under constant potential according to the reaction... 

The previous analysis indicates that although the voltammetiic behavior suggests that the aqueous phase behaves as a metal electrode dipped into the organic phase, the interfacial concentration of the aqueous redox couple does exhibit a dependence on the Galvani potential difference. In this sense, it is not necessary to invoke potential perturbations due to interfacial ion pairing in order to account for deviations from the Butler Volmer behavior [63]. This phenomenon has also been discarded in recent studies of the same system based on SECM [46]. In this work, the authors observed a potential independent ket for the reaction sequence. 

This result was taken as an experimental eonfirmation of the model developed by Sehmiekler [7]. However, it appeared somehow eontradictory with other results obtained with SECM. It was also suggested that eoneentration polarization phenomena occurring at the aqueous side are negligible as the whole potential drop is presumably developed in the benzene phase. This assumption can be qualitatively verified by evaluating a simplified expression for the potential distribution based on a back-to-back diffuse double layer [40,113],... 

A consistent picture for dynamics of heterogeneous ET has been emerging in the last 5 years with the development of new experimental approaches. Techniques such as AC impedance, modulated and time-resolved spectroscopy, SECM, and photoelectrochemical methods have extended our knowledge of charge-transfer kinetics to a wide range of time scales. This can be exemplified by comparing impedance analysis, which is limited to k of... 

We begin by reviewing the principles of SECM methods, and present an overview of the instrumentation needed for experimental studies. A major factor in the success of SECM, in quantitative applications, has been the parallel development of theoretical models for mass transport. A detailed treatment of the theory for the most common SECM modes that have been used to study liquid interfaces is therefore given, along with key results from these models. A comprehensive assessment of the applications of SECM is provided and the prospects for the future developments of the methodology are highlighted. 

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