Electron-transfer layer

An electrode interface has a layered structure in which a nonunifomi electric field (potential slope) is generated by polarization of the electrode. An extremely strong electric field of around 10 Vcm in the innermost layer, the so-called electron transfer layer, which is very thin, 10 A or less, might cause a variety of polar effects. Since not only the electron transfer step but also adsorption and some of the chemical steps involved in an electrolytic reaction take place in the electron transfer layer, the electrochemical reaction should be strongly influenced by polar factors. The orientation of polar adsorbed species, such as ions and dipoles, is electrostatically influenced, and consequently the stereochemistry of their reactions is also controlled by this kind of electrostatic factor. 

Under the appropriate conditions, monomers occupy virtually the entire space of pores or the whole interlayer space. Subsequent oxidative polymerization was carried out in the presence of molecular oxygen (as an electron acceptor) and a redox-active host that catalyzes electron transfer. Layered silicates containing metal ions are of particular interest because they initiate polymerization of an intercalated monomer. Thus when Na ions in hectorite are replaced by Cu or Fe, styrene can be polymerized both in the pores and on the smface. The polymer has a brushlike structure, which indicates that the inorganic smface possesses an orienting effect. This effect decreases as the chain grows away from the smface. 

Metal to ceramic (oxide) adhesion is very important to the microelectronics industry. An electron transfer model by Burlitch and co-workers [75] shows the importance of electron donating capability in enhancing adhesion. Their calculations are able to explain the enhancement in adhesion when a NiPt layer is added to a Pt-NiO interface. 

Much use has been made of micellar systems in the study of photophysical processes, such as in excited-state quenching by energy transfer or electron transfer (see Refs. 214-218 for examples). In the latter case, ions are involved, and their selective exclusion from the Stem and electrical double layer of charged micelles (see Ref. 219) can have dramatic effects, and ones of potential imfKntance in solar energy conversion systems. 

Within this framework, by considering the physical situation of the electrode double layer, the free energy of activation of an electron transfer reaction can be identified with the reorganization energy of the solvation sheath around the ion. This idea will be carried through in detail for the simple case of the strongly solvated... 

The situation in figure C2.8.5(b) is different in that, in addition to the mechanism in figure C2.8.5(a), reduction of the redox species can occur at the counter-electrode. Thus, electron transfer tlirough the layer may not be needed, as film growth can occur with OH species present in the electrolyte involving a (field-aided) deprotonation of the film. The driving force is provided by the applied voltage, AU. 

A type of molecular resonance scattering can also occur from the formation of short-lived negative ions due to electron capture by molecules on surfrices. While this is frequently observed for molecules in the gas phase, it is not so important for chemisorbed molecules on metal surfaces because of extremely rapid quenching (electron transfer to the substrate) of the negative ion. Observations have been made for this scattering mechanism in several chemisorbed systems and in phys-isorbed layers, with the effects usually observed as smaU deviations of the cross section for inelastic scattering from that predicted from dipole scattering theory. 

Of course, this simple picture constitutes only a crude approximation and should be valued only for showing that the completion of a metal layer around C o with 32 Ba-atoms is, indeed, plausible. More precise predictions would have to rely on ab initio calculations, including a possible change in bond lengths of Qo> such as an expansion of the double bonds of C o due to electron transfer to the antibonding LUMO (as was found in the case of QoLii2[I2,131T... 

The explicit mathematical treatment for such stationary-state situations at certain ion-selective membranes was performed by Iljuschenko and Mirkin 106). As the publication is in Russian and in a not widely distributed journal, their work will be cited in the appendix. The authors obtain an equation (s. (34) on page 28) similar to the one developed by Eisenman et al. 6) for glass membranes using the three-segment potential approach. However, the mobilities used in the stationary-state treatment are those which describe the ion migration in an electric field through a diffusion layer at the phase boundary. A diffusion process through the entire membrane with constant ion mobilities does not have to be assumed. The non-Nernstian behavior of extremely thin layers (i.e., ISFET) can therefore also be described, as well as the role of an electron transfer at solid-state membranes. 

The first realization of a conjugated polymer/fullerene diode [89] was achieved only recently after the detection of the ultrafasl phoioinduced electron transfer for an lTO/MEH-PPV/CW)/Au system. The device is shown in Figure 15-18. Figure 15-19 shows the current-voltage characteristics of such a bilayer in the dark at room temperature. The devices discussed in the following section typically had a thickness of 100 nm for the MEH-PPV as well as the fullerene layer. Positive bias is defined as positive voltage applied to the 1TO contact. The exponential current tum-on at 0.5 V in forward bias is clearly observable. The rectification ratio at 2 V is approximately l()4. 

Recall that Nemstian behavior of diffusing species yields a r1 /2 dependence, hi practice, the ideal behavior is approached for relatively slow scan rates, and for an adsorbed layer that shows no intermolecular interactions and fast electron transfers. 

Further improvements can be achieved by replacing the oxygen with a non-physiological (synthetic) electron acceptor, which is able to shuttle electrons from the flavin redox center of the enzyme to the surface of the working electrode. Glucose oxidase (and other oxidoreductase enzymes) do not directly transfer electrons to conventional electrodes because their redox center is surroimded by a thick protein layer. This insulating shell introduces a spatial separation of the electron donor-acceptor pair, and hence an intrinsic barrier to direct electron transfer, in accordance with the distance dependence of the electron transfer rate (11) ... 

Figure 7. Adsorption of an electronegative species from the gas phase onto a metal surface generates a dipolar layer due to electron transfer from the metal to the species. Adsorption of anions onto an electrode simulates the situation when the positive charge on the metal compensates for the adsorbed negative charge (zero diffuse-layer charge), and not when the charge on the metal is zero.

Zinc oxide, anodic photo currents for, 470 Zinc oxide layers, spotted, 471 Zinc oxide-electrolyte interfaces, electron transfer rate and its exponential increase at, 512... 

The individual structural features of the high-tem-perature superconductors are found in other substances. A substance with alternating metal-salt layers is Ag2F, with sequence FAgAgFAgAgF . Resonance between a covalent bond and a no-bond is found in B (l =3,Z. 6) and in metals and organometallic clusters. Hyperelectronic-hypoelectronic electron transfer occurs... 

Between the space charge layer establishes the potential (j>2 and the magnitude of this potential depends on and the ionic strength of the solution. It will be apparent that 2 will determine the concentrations of charged electroactive species, while will determine the rate of the electron transfer step if... 

Provided electron transfer between the electrode and solute species is not interrupted by the coating, even electroinactive films can offer interesting applications. Thus, a chiral environment in the surface layer may impose stereoselectivity in the follow-up reactions of organic or organometallic intermediates. Furthermore, polymer layers may be used to obtain diffusional permeation selectivity for certain substrates, or as a preconcentration medium for analyzing low concentration species. 

The oxidation or reduction of a substrate suffering from sluggish electron transfer kinetics at the electrode surface is mediated by a redox system that can exchange electrons rapidly with the electrode and the substrate. The situation is clear when the half-wave potential of the mediator is equal to or more positive than that of the substrate (for oxidations, and vice versa for reductions). The mediated reaction path is favored over direct electrochemistry of the substrate at the electrode because, by the diffusion/reaction layer of the redox mediator, the electron transfer step takes place in a three-dimensional reaction zone rather than at the surface Mediation can also occur when the half-wave potential of the mediator is on the thermodynamically less favorable side, in cases where the redox equilibrium between mediator and substrate is disturbed by an irreversible follow-up reaction of the latter. The requirement of sufficiently fast electron transfer reactions of the mediator is usually fulfilled by such revemible redox couples PjQ in which bond and solvate... 

---