Principles of SECM

3 Basic principles of scanning electrochemical microscopy (SECM) (A) far from the substrate, diffusion leads to a steady-state current, iT (B) near a conductive substrate, feedback diffusion leads to iT iT 0O (C) near an insulating substrate, hindered diffusion leads to iT iT 0O. (Reprinted with permission from A. J. Bard, G. Denuault, C. Lee, D. Mandler, and D. O. Wipf, Acc. Chem. Res. 23, 357 (1990). Copyright 1990 American Chemical Society.) 

As discussed above, there are two modes of this type. In the TG/SC mode, the tip is held at a potential where an electrode reaction occurs and the substrate is held at a different potential where a product of the tip reaction will react and thus be collected. In most cases the substrate is considerably larger than the tip, so that the collection efficiency, given by is/h (where s is the substrate current), is essentially 1 (100%) for a stable tip-generated species, R. If R reacts on transit from tip to substrate, is/iT becomes smaller, and its change with separation, d, allows determination of the rate constant of the homogeneous reaction (Chapter 7). 

The alternative mode is the substrate generation-tip collection (SG/TC) mode. In this case the tip probes the reactions that are occurring on a substrate. For example, a scan in the z direction can produce the concentration profile, while a scan over the surface can identify hot spots, where reactions occur at a higher rate. 

4 Diffusion-controlled steady-state tip current as a function of tip-substrate separation. (A) Substrate is a conductor (B) substrate is an insulator. (From Ref. 2.) 

A related method involves the use of the tip reaction to perturb a reaction at a surface an example of this approach is SECM-induced desorption (SECMID) (22). For example, the adsorption/desorption kinetics of protons on a hydrous metal oxide surface can be studied in an unbuffered solution by bringing the tip near the surface and reducing proton (to hydrogen) at the tip. This causes a local change in pH that results in proton desorption from the surface. The tip current can be used to study the kinetics of proton desorption and diffusion on the surface (Chapter 12). 

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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],...

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.)... 

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. 

The apparatus and basic principles of SECM [28] were described in Section 5.6. 

This volume is devoted to a complete and up-to-date treatment of scanning electrochemical microscopy (SECM). In this introductory chapter, we cover the historical background of the technique, the basic principles of SECM, and an overview of some of its applications (covered in more depth in later chapters). A number of reviews of this field have also been published G-6). 

FIG. 12 Principles of SECM-induced dissolution using copper sulfate pentahydrate as an example substrate. Note that the diagram is not to scale as the tip-crystal separation is typically <0.01 r.. 

Figure 16.4.2 Principles of SECM, showing (a) hemispherical diffusion to the disk-shaped tip far from substrate, (b) blocking of diffusion by insulating substrate, (c) positive feedback at a conductive substrate.

Fig. 7.11. Principle of SECM with the UME far away from a surface (top), approaching an insulating surface (middle) and approaching a conductive surface kept at a suitable electrode potential (bottom)...

To fully understand the beneflts of coupling SECM with AFM, one of course first needs to understand the operating principle of SECM and be aware of its limitations for exploring nanosystems. The reader is referred to Chapter 18 and to the literature for a detailed presentation of these topics that we only briefly recall in the following. 

The principles of SECM surface modification have also been integrated into microdevices [166,167]. Surfaces that prevent protein adsorption based on heparin have been activated with electrochemically... 

Recent methodological advances have greatly increased the capacity of SECM to characterize interfaces at the nanoscale and to obtain molecular-level chemical information. This thoroughly updated edition retains original chapters describing the principles of SECM measurements, instrumentation, preparation of SECM probes, imaging methodologies, and theory and offers ... 

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