Substrate generation-tip collection

Fig. 37.3. Schematics of the GC mode, (a) Substrate-generation/tip-collection mode and (b) tip-generation/substrate-collection mode.

Historically, the first SECM-type experiments were carried out to measure concentration profiles in the diffusion layer generated by a macroscopic substrate [3, 26]. This type of measurement represents substrate generation/tip collection (SG/TC) mode. When the tip is moved through the thick diffusion layer produced by the substrate, the changes in iT reflect local variations of concentrations of redox species (Fig. 3b). Ideally, the tip should not perturb the diffusion layer at the substrate. This is easier to achieve with a potentiometric tip, which is a passive sensor and does not change concentration profiles of electroactive species. 

Recently, Colley et al. [104] studied the distribution of electrochemical activity in microarray electrodes. The array contained 50-pm diameter boron-doped regions spaced 250 pm apart in an intrinsic diamond disk. Reaction rate imaging was done in the substrate generation/tip collection mode. The electroactive boron doped regions were biased at a suitable potential to reduce the mediator, [Ru(NH3)6]3+, and the product of the reduction reaction was collected at the tip. Two-dimensional scans over different regions (Fig. 21) revealed wide variations in local electroactivity. 

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. 

The substrate generation/tip collection (SG/TC) mode with an ampero-metric tip was historically the first SECM-type measurement performed (32). The aim of such experiments was to probe the diffusion layer generated by the large substrate electrode with a much smaller amperometric sensor. A simple approximate theory (32a,b) using the well-known c(z, t) function for a potentiostatic transient at a planar electrode (33) was developed to predict the evolution of the concentration profile following the substrate potential perturbation. A more complicated theory was based on the concept of the impulse response function (32c). While these theories have been successful in calculating concentration profiles, the prediction of the time-de-pendent tip current response is not straightforward because it is a complex function of the concentration distribution. Moreover, these theories do not account for distortions caused by interference of the tip and substrate diffusion layers and feedback effects. 

The latest contribution to the theory of the EC processes in SECM was the modeling of the substrate generation/tip collection (SG/TC) situation by Martin and Unwin (40). Both the tip and substrate chronoamperometric responses to the potential step applied to the substrate were calculated. From the tip current transient one can extract the value of the first-order homogeneous rate constant and (if necessary) determine the tip/substrate distance. However, according to the authors, this technique is unlikely to match the TG/SC mode with its high collection efficiency under steady-state conditions. 

There are three methods of SECM that can be used to characterize and measure homogeneous reactions of the type identified above the feedback mode, the tip generation/substrate collection (TG/SC) mode, and the substrate generation/tip collection (SG/TC) mode. For illustrative purposes, these modes are compared schematically in Figure 1 for a case where a species B produced by reduction of a species A at a generator electrode undergoes a first-order chemical reaction in solution (characterized by a rate constant, k,). 

It is also possible to carry out substrate generation/tip collection (SG/TC) experiments, where the tip probes the products of a reaction at the substrate. However this approach to studies of homogeneous kinetics is less straightforward, since the larger substrate electrode does not attain a steady-state condition and the collection efficiency for this case, ijU, is much less than unity, even in the absence of a homogeneous kinetic complication. This mode has been used, however, for looking at concentration profiles above a substrate (27). 

Recently, the substrate-generation/tip-collection mode of SECM was exploited to confirm the release of oligomers by PEDOT films electrogenerated from aqueous solutions... 

A number of SECM modes of operation are possible [28,48]. The tip can be used to detect a species generated at the substrate surface (substrate generation/tip collection mode or SG/TC mode), a method that is particularly useful in corrosion studies. Alternatively, the tip generation/substrate collection (TG/SC) mode is usefiil for kinetic measurements. Additional information may be obtained by... 

Sanchez-Sanchez CM, Rodriguez-Lopez J, Bard AJ (2008) Scaiming electrochemical microscopy. 60. Quantitative calibration of the SECM substrate generation/tip collection mode and its use for the study of the oxygen reduction mechanism. Anal Chem 80(9) 3254-3260... 

Martin RD, Unwin PR (1998) Theory and experiment for the substrate generation tip collection mode of the scanning electrochemical microscope application as an approach for measuring the diffusion coefficient ratio of a redox couple. Anal Chem 70 (2) 276-284. doi 10.102l/ac97068Ip... 

To study the kinetics of electron transfer, SECM can be operated in two modes a feedback mode (53, 69) and a substrate generation-tip collection mode (53). In the feedback mode, a mediator redox couple is required to probe the interface. The tip is poised to a potential well above Eredox- The steady-state current observed due to hemispherical diffusion of O toward the tip is expressed as (Figure 9.29a)... 

In addition to the amperometric feedback mode described above, other amperometric operation modes are also possible. For example, in the substrate generation/tip collection (SG/TC) mode, ip is used to monitor the flux of electroactive species from the substrate and vice versa for the tip generation/substrate collection (TG/SC) mode. These operation modes will be described in Section 12.3.1.3 and are useful in studies of homogeneous reactions that occur in the tip-substrate gap (see Section 12.4.2) and also in the evaluation of catalytic activities of different materials for useful reactions, e.g., oxygen reduction and hydrogen oxidation (see Section 12.4.3). In addition to the amperometric methods, other techniques, e.g., potentiometric method is also applicable for SECM and will be discussed in Section 12.3.2. We will also update the techniques suitable for the preparation of SECM amperometric tips in Section 12.3.1.1 and potentiometric probes in Section 12.3.2.2. 

Figure 12.26 SECM feedback (A) and substrate generation/tip collection (B) mode used in the corrosion study.

The diffusion coefficient ratio of a redox couple can also be measured with SECM using an approach developed by Unwin (13,14). With this technique, the ratio of the steady-state collector current for the substrate generation/tip collection (SG/TC) mode to the steady-state collector current measured in the feedback mode (at the same tip-substrate separation) gives directly the ratio of diffusion coefficients for the redox couple. The advantage of this approach is that no knowledge of the tip-substrate separation, the electrode sizes, or the mediator concentration is required. 

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