Electron transfer across surface

Proposed intermediates in the above reaction include atomic hydrogen [27, 28], hydride ions [29, 30], metal hydroxides [31], metaphosphites [32, 33], and excitons [34]. In general, the postulated mechanisms are not supported by direct independent evidence for these intermediates. Some authors [35] maintain that the mechanism is entirely electrochemical (i.e. it is controlled by electron transfer across the metal-electrolyte interface), but others [26] advocate a process involving a surface-catalyzed redox reaction without interfacial electron transfer. 

Electrode reactions are inner-sphere reactions because they involve adsorption on electrode surfaces. The electrode can act as an electron source (cathode) or an electron sink (anode). A complete electrochemical cell consists of two electrode reactions. Reactants are oxidized at the anode and reduced at the cathode. Each individual reaction is called a half cell reaction. The driving force for electron transfer across an electrochemical cell is the Gibbs free energy difference between the two half cell reactions. The Gibbs free energy difference is defined below in terms of electrode potential,... 

An electron transferring across the metal surface first sustains the electrostatic coulomb potential due to the sinface potential, x, and then enters into the exchange and correlation potential field,, caused by the ion-electron and electron-electron... 

This process can be described in terms of a heterogeneous reaction in which ferri-cyanide (or hexacyanoferrate(III)) ions, [Fe(CN)g], are formed. At the beginning of the voltammetric peak, the current is controlled by the kinetic of the electron transfer across the electrode/electrolyte barrier so that the current increases somewhat exponentially with the applied potential. The value of the current is controlled 150-200 mV after the voltammetric peak by the diffusion rate of ferrocyanide ions from the solution bulk toward the electrode surface. 

From a description of the geometric structure of electrified interfaces we moved to a description of models for electrochemical electron transfer across an electrode interface. The science of atomic scale electrochemistry was presented with an emphasis on the bonding of water molecules and anions on electrode surfaces. Subsequently, we presented an in-depth description of the role of surface bonding in a number of important electrocatalytic processes for energy conversion. We have attempted to illustrate how closely surface bonding and catalytic activity are related. 

Let us note in conclusion that the thermodynamic approach has widely been used to describe the kinetics of electrochemical reactions at an illuminated semiconductor electrode (see, for example, Gerischer, 1977c Dog-onadze and Kuznetsov, 1975). Clearness and simplicity are an unqualified advantage of this approach, but the use of the quasilevel concept is not justified in all the cases. In particular, conditions (48) alone appear to be insufficient to substantiate the applicability of the quasilevel concept to the description of the processes of electron transfer across the interface (for greater details, see Pleskov and Gurevich, 1983 Nozik, 1978). Obviously, if photogeneration of the carriers occurs mainly near the surface, at which a... 

We can infer that the band positions of the irradiated semiconductor are greatly influential in controlling the observed redox chemistry and that formation of radical ions produced by photocatalyzed single electron transfer across the semiconductor-electrolyte interface should be a primary mechanistic step in most such photocatalyzed reactions. Whether oxygenation, rearrangement, isomerization, or other consequences follow the initial electron transfer seem to be controlled, however, by surface effects. 

As follows from the data from Sect. 2, the primary photochemical stage in the majority of the membrane systems studied is the redox quenching of the excited photosensitizer by an electron acceptor or donor leading to electron transfer across the membrane // water interface. For electron transfer to occur from the membrane-embedded photosensitizer to the water soluble acceptor, it is necessary for the former to be located sufficiently close to the membrane surface, though the direct contact of the photosensitizer with the aqueous phase is not obligatory. For example, Tsuchida et al. [147] have shown that electron transfer to MV2 + from photoexcited Zn-porphyrin inserted into the lecithin membrane, is observed only until the distance from the porphyrin ring to the membrane surface does not exceed about 12 A. 

The nature of the platform also impacts on the electrochemical and photophysical properties of the interfacial supramolecular assembly (ISA). For example, the density of states within gold, platinum and carbon electrodes are different, so causing subtle changes in the rate of electron transfer across the electrode/ISA interface. In addition, in terms of the photophysical properties, the nature of the platform can radically change the excited-state properties of a molecule upon adsorption. For example, if a adsorbate is located close to (<10 nm) a metal surface and is then pumped into an electronically excited state, efficient energy or electron transfer is expected which will lead to quenching of the excited state. This process can dramatically increase the photostability of compounds that would ordinarily photodecompose in solution. 

Reaction 5.1 is meant to represent a nonspecific electrostatic interaction (presumably responsible for double-layer charge accumulation) Reaction 5.2 symbolizes specific adsorption (e.g., ion/dipole interaction) Reaction 5.3 represents electron transfer across the double layer. Together, these three reactions in fact symbolize the entire field of carbon electrochemistry electric double layer (EDL) formation (see Section 5.3.3), electrosorption (see Section 5.3.4), and oxidation/reduction processes (see Section 5.3.5). The authors did not discuss what exactly >C, represents, and they did not attempt to clarify how and why, for example, the quinone surface groups (represented by >CxO) sometimes engage in proton transfer only and other times in electron transfer as well. In this chapter, the available literature is scrutinized and the current state of knowledge on carbon surface (electrochemistry is assessed in search of answers to such questions. 

Fig. 4 shows a simple phase diagram for a metal (1) covered with a passivating oxide layer (2) contacting the electrolyte (3) with the reactions at the interfaces and the transfer processes across the film. This model is oversimplified. Most passive layers have a multilayer structure, but usually at least one of these partial layers has barrier character for the transfer of cations and anions. Three main reactions have to be distinguished. The corrosion in the passive state involves the transfer of cations from the metal to the oxide, across the oxide and to the electrolyte (reaction 1). It is a matter of a detailed kinetic investigation as to which part of this sequence of reactions is the rate-determining step. The transfer of O2 or OH- from the electrolyte to the film corresponds to film growth or film dissolution if it occurs in the opposite direction (reaction 2). These anions will combine with cations to new oxide at the metal/oxide and the oxide/electrolyte interface. Finally, one has to discuss electron transfer across the layer which is involved especially when cathodic redox processes have to occur to compensate the anodic metal dissolution and film formation (reaction 3). In addition, one has to discuss the formation of complexes of cations at the surface of the passive layer, which may increase their transfer into the electrolyte and thus the corrosion current density (reaction 4). The scheme of Fig. 4 explains the interaction of the partial electrode processes that are linked to each other by the elec-... 

Under steady state conditions, the concentration of holes at the surface is determined by the rate of their arrival from the bulk and on the rate of their removal by electron transfer and surface recombination. It is usually assumed that the potential distribution across the semiconductor/electro-lyte junction is not affected by illumination under potentiostatic conditions,... 

For this purpose an electron transfer across the bilayer boundary must be accomplished (14). The schematic of our system is presented in Figure 3. In this system an amphiphilic Ru-complex is incorporated Into the membrane wall. An electron donor, EDTA, is entrapped in the inner compartment of the vesicle, and heptylviolo-gen (Hv2+) as electron acceptor is Introduced into the outer phase. Upon illumination an electron transfer process across the vesicle walls is initiated and the reduced acceptor (HVf) is produced. The different steps involved in this overall reaction are presented in Figure 3. The excited sensitizer transfers an electron to HV2+ in the primary event. The oxidized sensitizer thus produced oxidizes a Ru located at the inner surface of the vesicle and thereby the separation of the intermediate photoproducts is assisted (14). The further oxidation of EDTA regenerates the sensitizer and consequently the separation of the reduced species, HVi, from the oxidized product is achieved. In this system the basic principle of a vectorial electron transfer across a membrane is demonstrated. However, the quantum yield for the reaction is rather low (0 4 X 10 ). 

The simplest model of electron transfer across a semiconductor/metal interface assumes that the current depends linearly on the concentration of electrons at the semiconductor surface, s. It also assumes that the concentrations of acceptor and donor states in a metal are extremely large thus, these variables can be assumed to be constant and can be incorporated as time-independent quantities into the appropriate rate equations. This assumption is extremely reasonable for the moderate current densities that flow through typical semiconductor/metal interfaces. ... 

Both Cgo and C70 embedded in a lipid membrane can act as efficient electron acceptors from excited electron donors adsorbed on the surface of the membrane [70]. Very interestingly, Cgo and C70 can act not only as photosensitizers for electron transfer from a donor molecule but also as mediators for electron transfer across a membrane with very high efficiencies [71],... 

Films grown in this manner are usually characterized by techniques such as el-lipsometry, atomic force microscopy (AFM) and cyclic voltammetry. Ellipsometry gives the thickness of the layers and AFM the surface uniformity. Cyclic voltam-metric measurements can give information about electron transfer across the film. 

If a defect-free molecular assembly covers an electrode surface completely, the assembly is an efficient electrochemical blocker [23, 30, 46, 47]. Electron transfer through the SAM to a solution redox probe is substantially slower than the rate of diffusion of the probe to the surface, and the voltammetric response is a quasiexponential increase in Faradaic current resulting from electron transfer across the chains of the SAM [30, 46]. This type of response qualitatively supports the presence of a monolayer low in defect sites. A more quantitative measure of the EBE of a modified electrode is expressed as the percentage of total blocking achieved using a bare electrode as the standard [48] ... 

The Marcus Theory can also be applied for heterogeneous electron transfer reaction at electrode surfaces [24 and references therein]. The electronic coupling between the protein and the electrode can be varied using different self-assembled monolayers controlling the orientation of the redox active protein on the surface and the distance between the redox active site of the protein and the electrode. The driving force is related to the appHed potential and the redox potential of the protein. In many cases the rate of electron transfer across the protein-electrode interface is limited by conformational reorganization. This has focussed the efforts of many groups on tailored interaction between proteins and enzymes and electrode surfaces. 

Recently, Du et al. reported an AChE electrochemical sensor based on enzyme-induced growth of gold nanoparticles (AuNPs) without the addition of any gold nano-seeds [17], They have successfully used [Fe (CN)6]3 74 as a probe to indicate the process of electron transfer across the interface and also analyze the enzyme inhibitor quantitatively. Initially, cleaned gold electrode was coated with 0.5 % (w/v) chitosan solution and AChE was later immobilized onto this chitosan modified gold electrode. CV results show that, AuNPs presence increases the peak current and decreases the peak separation. This confirms the presence of AuNPs on the chitosan modified electrode surface. However, the peak current of... 

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. 

Sezer, M., Feng, J.J., Ly, H.K., Shen, Y.F., Nakanishi, T., Kuhlmann, U., Hildebrandt, P., Mohwald, H., and Weidinger, LM. (2010) Multi-layer electron transfer across nanostructured Ag-SAM-Au-SAM junctions probed by surface enhanced Raman spectroscopy. Physical Chemistry Chemical Physics, 12, 9822-9829. 

As already mentioned in the previous section, any electron transfer across the semiconductor-liquid interface occurs via the energy bands. There may also be an electron transfer via surface states at the interface the electrons or holes, however, must finally be transported via one of the energy bands. This is possible by capturing an electron from the conduction band or a hole from the valence band in the surface states. In the present section the basic rules for the charge transfer will be given, in particular, physical factors which determine whether an electron transfer occurs via the conduction or the valence band, will be derived. For illustration, the Gerischer model will be used here because it best shows the energetic conditions. 

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