Electroactive layers monolayers

Other common techniques have been applied to the assembly of layers or films of TTF-derived molecular conductors. Compound 29 is an example of an amphiphilic TTF derivative. It forms conducting Langmuir-Blodgett charge transfer films with the acceptor TCNQF4 (30).98 Self-assembly of compound 31 on gold by electrochemical methods yielded an electroactive monolayer which was remarkably stable to electrochemical cycling.99... 

Film deposition refers to the preparation of polymer (organic, organometallic, and metal coordination) films which contain the equivalent of many monomolecular layers of electroactive sites. As many as 10 monolayer-equivalents may be present [9]. The polymer film is held on the electrode surface by a combination of chemisorptive and solubility effects. Since the polymer film bonding is rather nonspecific, this approach can be used to modify almost any type of... 

There are several reasons for the appeal of polymer modification immobilization is technically easier than working with monolayers the films are generally more stable and because of the multiple layers redox sites, the electrochemical responses are larger. Questions remain, however, as to how the electrochemical reaction of multimolecular layers of electroactive sites in a polymer matrix occur, e.g., mass transport and electron transfer processes by which the multilayers exchange electrons with the electrode and with reactive molecules in the contacting solution [9]. 

Chemical modification of electrode surfaces by polymer films offers the advantages of inherent chemical and physical stability, incorporation of large numbers of electroactive sites, and relatively facile electron transport across the film. Since th% polymer films usually contain the equivalent of one to more than 10 monolayers of electroactive sites, the resulting electrochemical responses are generally larger and thus more easily observed than those of immobilized monomolecular layers. Also, the concentration of sites in the film can be as high as 5 mol/L and may influence the reactivity of the sites because their solvent and ionic environments differ considerably from dilute homogeneous solutions [9]. 

Surface redox reactions — or surface -> electrode reactions, are reactions in which both components of the -> redox couple are immobilized on the electrode surface in a form of a -> monolayer. Immobilization can be achieved by means of irreversible -> adsorption, covalent bonding, self-assembling (- self-assembled mono-layers), adhesion, by Langmuir-Blodgett technique (- Langmuir-Blodgett films), etc. [i]. In some cases, the electrode surface is the electroactive reactant as well as the product of the electrode reaction is immobilized on the electrode surface, e.g., oxidation of a gold, platinum, or aluminum electrode to form metal oxide. This type of electrode processes can be also considered as surface electrode reactions. Voltammetric response of a surface redox reaction differs markedly from that of a dissolved... 

We are investigating the effects of binding non-electroactive molecules to electrode surfaces. The attached layer will be sufficiently thin (ca. 1 monolayer) that electron transfer across the electrode/electrolyte interface will not be inhibited. However, other surface properties may be advantageously modified. For semiconductor electrodes, desirable changes include suppression of the photo-activated surface corrosion and shifts in the flatband potential. We are seeking to improve the performance of semiconductor liquid-junction solar cells by these means. 

On the other hand, it is possible to study electron transfer to an electroactive species held at a fixed distance (10-30 A) from the electrode surface by a suitable spacer, such as an adsorbed monolayer (Section 14.5.2) (75, 76). One approach is based on the use of a blocking monolayer, such as a self-assembled monolayer of an alkane thiol or an insulating oxide film, to define the distance of closest approach of a dissolved reactant to the electrode. This strategy requires knowledge of the precise thickness of the blocking layer and assurance that the layer is free of pinholes and defects, through which solution species might penetrate (Section 14.5). Alternatively, the adsorbed monolayer may itself... 

Impedance methods have been more useful in studying electron-transfer kinetics in electroactive monolayers in the absence of an electroactive solution species (71-73), such as alkylthiol layers with tethered electroactive groups (Section 14.5.2). The equivalent circuit adopted is shown in Figure 14.3.18, where the adsorbed layer is represented by Cads = (F AT)/4RT and the electron-transfer kinetics by = (2RT)/F ATkf, so that... 

Figure 14.3.18 Equivalent circuit for an electroactive monolayer. Rfi = solution resistance, = double-layer capacitance, = charge-transfer resistance, and Cads capacitance of the adsorbed layer.

Studies of tethered electroactive species are less sensitive to pinholes than experiments with solution reactants and blocking layers, although heterogeneity and roughness of the substrate and film defects can still play a role. The rate constant, k, in this case has units of a first-order reaction (s ). Rate constants can be determined by a voltammetric method as described earlier for electroactive monolayers (Section 14.3.3). In addition potential-step chronoamperometry can be employed, in which case the current follows a simple exponential decay (88, 90, 91) ... 

Fig. 2.18 An equivalent circuit representing an electrode/solution interface. The electrode surface is covered by a monolayer of a redox-active species. e ac potential across the faradaic unit of equivalent circuit, Ca double-layer capacitance, Rs -uncompensated solution resistance, Zf impedance representing solely the electron transfer reaction process of the monolayer, )> ac current due to the faradaic process, Z, total impedance of the whole system, ks. heterogeneous electron transfer rate constant of the monolayer of electroactive species, R charge transfer resistance, Q capacitance associated with the redox reaction of the adsorbed species.

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