Electroactive layers behavior

Cyclic voltammetry (CV) has found widespread application in investigating and characterizing modified electrode processes. " Characterizing modifying layers under conditions of thin layer behavior has received particular attention. In the absence of diffusional limitations and under conditions of complete oxida-tion/reduction of electroactive centers, thin layer/surface-type behavior prevails. The ideal model for voltammetric behavior under such conditions was considered, and the following features are characteristic ... 

The observed complexity of the Se(IV) electrochemistry due to adsorption layers, formation of surface compounds, coupled chemical reactions, lack of electroactivity of reduction products, and other interrelated factors has been discussed extensively. Zuman and Somer [31] have provided a thorough literature-based review with almost 170 references on the complex polarographic and voltammetric behavior of Se(-i-IV) (selenous acid), including the acid-base properties, salt and complex formation, chemical reduction and reaction with organic and inorganic... 

Case II Reversible or Ouasi-Reversible Redox Species. If the tip-sample bias is sufficient to cause the electrolysis of solution species to occur, i.e., AEt > AEp, ev, the proximity of the STM tip to the substrate surface (d < 10 A) implies that the behavior of an insulated STM tip-substrate system may mimic that of a two-electrode thin-layer cell (TLC)(63). At the small interelectrode distances required for tunneling, a steady-state concentration gradient with respect to the oxidized (Ox) and and reduced (Red) electroactive species should be established between the tip and the substrate, and the resulting steady-state current will augment that present as a result of the convection of electroactive species from the bulk solution. In many cases, this steady state current is predicted to overwhelm the convective currents, so this situation is of concern when STM imaging under electrochemical conditions (64). 

The lure of new physical phenomena and new patterns of chemical reactivity has driven a tremendous surge in the study of nanoscale materials. This activity spans many areas of chemistry. In the specific field of electrochemistry, much of the activity has focused on several areas (a) electrocatalysis with nanoparticles (NPs) of metals supported on various substrates, for example, fuel-cell catalysts comprising Pt or Ag NPs supported on carbon [1,2], (b) the fundamental electrochemical behavior of NPs of noble metals, for example, quantized double-layer charging of thiol-capped Au NPs [3-5], (c) the electrochemical and photoelectrochemical behavior of semiconductor NPs [4, 6-8], and (d) biosensor applications of nanoparticles [9, 10]. These topics have received much attention, and relatively recent reviews of these areas are cited. Considerably less has been reported on the fundamental electrochemical behavior of electroactive NPs that do not fall within these categories. In particular, work is only beginning in the area of the electrochemistry of discrete, electroactive NPs. That is the topic of this review, which discusses the synthesis, interfacial immobilization and electrochemical behavior of electroactive NPs. The review is not intended to be an exhaustive treatment of the area, but rather to give a flavor of the types of systems that have been examined and the types of phenomena that can influence the electrochemical behavior of electroactive NPs. 

Examples of electroactive NP materials discussed in the review include Ti02, Mn02, iron oxides, other metal oxides, hydroxides and oxyhydroxides and Prussian Blue. We use the term electroactive N Ps to refer to the faradaic electroactivity in such materials and to distinguish them from NPs comprised of metals (such as Au, Ag, Pt, Co, etc.) or semiconductors (such as CdS, CdSe, etc.). This distinction is based on the ability of many electroactive NPs to undergo faradaic oxidation or reduction of all of the metal (redox) centers in the NP. This is in contrast to the behavior of many metal and semiconductor NPs for which oxidation or reduction is fundamentally an interfacial, double-layer process. This deflnition is somewhat arbitrary, since the smallest metal and semiconductor NPs behave molecularly, blurring the distinction... 

The concentration profile of fixed oxidized and reduced sites within the film depends on the dimensionless parameter Dcjr/d2, where r is the experimental timescale, i.e. RT/Fv in cyclic voltammetry, and d is the polymer layer thickness. When Dcix/d2 1, all electroactive sites within the film are in equilibrium with the electrode potential, and the surface-type behavior described previously is observed. In contrast, Dcjx/d2 <3C 1 when the oxidizing scan direction is switched before the reduced sites at the film s outer boundary are completely oxidized. The wave will exhibit distinctive diffusional tailing where these conditions prevail. At intermediate values of Dcjr/d2, an intermediate ip versus v dependence occurs, and a less pronounced diffusional tail appears. 

A general mathematical formulation and a detailed analysis of the dynamic behavior of this mass-transport induced N-NDR oscillations were given by Koper and Sluyters [8, 65]. The concentration of the electroactive species at the electrode decreases owing to the electron-transfer reaction and increases due to diffusion. For the mathematical description of diffusion, Koper and Sluyters [65] invoke a linear diffusion layer approximation, that is, it is assumed that there is a diffusion layer of constant thickness, and the concentration profile across the diffusion layer adjusts instantaneously to a linear profile. Thus, they arrive at the following dimensionless set of equations for the double layer potential, [Pg.117]

All chemisorbed compounds are electroactive to at least some degree at sufficiently extreme applied potentials. The most common behavior is rapid, irreversible oxidation at relatively positive potentials [61, 72-76]. In particular, experiments have been performed in which a surface containing an adsorbed layer is oxidized in pure supporting electrolyte at electrode potentials shown to cause complete oxidative desorption of the layer. Measurement of the coulometric charge, Qox, and the background charge observed in the absence of an adsorbed layer, Qb, permitted calculation of the number of electrons removed in the oxidation of the adsorbed molecule, nox... 

This interpretation is tentative in the sense that it has not been proved directly. There is, however, indirect evidence that this kind of behavior is possible and even plausible, from the study of the oxidation of iodide in thin-layer cells. Tliere it was shown experimentally that the first layer of iodide ions adsorbed on the surface of platinum is not electroactive and iodine is formed from ions in solution, which presumably can be adsorbed on top of the first layer. It was also found that the first adsorbed layer can be oxidized at higher overpotentials, as proposed here for the h.e.r. 

In the earliest treatment of open-circuit potential-decay transients (729), C was identified with the double-layer capacitance, C, but it was recognized (cf. Refs. 105, 129) that this formulation did not account for changes in the coverage fractions by any electroactive intermediates involved. Conway and co-workers (126-128) were the first to treat the problem with allowance for changes in coverage of the adsorbed intermediate. However, C was interpreted as the sum of Cj, and C, and the potential-decay behavior for several... 

Conducting (via electrons or via permeation of electroactive species) adsorbed species usually introduce small disturbances in electrochemical behavior compared with the former class. Yet obviously the diffuse layer is extremely affected vis-a-vis the bare electrode. Thus the rate constants k° may be modified to a high degree because of large changes in the Frumkin correction. Such effects may easily explain, at least on a qualitative basis, the well-known dependence of k° on the size of the supporting electrolyte cation (reductions) or anion (oxidations), as well as on ionic additives [89]. 

Polymer films of various types have been studied with the EQCM technique, both from the standpoint of film creation as well as the final film properties hke redox behavior, ion and solvent transport, and energy dissipation level. Some of the classes of polymer films studied include micellar-polymer films, molecular imprinted polymer films used as chemical sensors of small molecules, layer-by-layer nanoarchitectured polymer films and electropoly-merized films, some containing electroactive functional groups. In this section, we briefly discuss some of the EQCM approaches taken with representative examples from each of these polymer film classes. 

Let us consider the case where adsorbed O, but not dissolved O, is electroactive (31-33). This could be the case when the sweep rate, u, is so large that O does not have time to diffuse appreciably to the electrode surface [i.e., Do(dCo(0, t)/dx =o drQ(t)/dt]. Alternatively, the wave for adsorbed O could be shifted to potentials well before the reduction wave for dissolved O. The conditions for such behavior will be given below. There are also cases where adsorption is so strong that the adsorbed layer of O can form even when the solution concentration is so small that the contribution to the current from dissolved O is negligible. We also assume that within the range of potentials of the wave, the F s are independent of E. Under these conditions, (14.3.1) becomes... 

Determination of the amount of species adsorbed, F, depends upon the electroactivity of the adsorbate. Consider the case where a molecule is irreversibly adsorbed and does not undergo an electrochemical oxidation at potentials where the dissolved species shows a cyclic voltammetric wave. An example of this type of behavior is hydroquinone (H2Q) in 1 M HCIO4. When an aliquot of solution containing a known concentration, C, is introduced into the thin-layer cell, FA moles of the H2Q will adsorb, so the new concentration in the solution, C, will be... 

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