Charge-transfer rate

An electric current flowing through an ITIFS splits into nonfaradaic (charging or capacity) and faradic current contributions. The latter contribution comprises the effects of both the transport of reactants to or from the interface, and the interfacial charge transfer, the rate of which is a function of the interfacial potential difference. By applying a transient electrochemical technique, these two effects can be resolved 

FIGURE 32.5 Cyclic voltammogram for the interface between solntions of 5mM tetra-butylammoninm tetraphenylborate in 1,2-dichloroethane and 5mM NaCl in water in the absence (dashed bne) and the presence (sohd bne) of 0.5 mM tetraethylammoninm in the aqneons phase. Sweep rate lOmV/s, interfacial area 0.02 cm.  

Irradiation of an ITIES by visible or UV light can give rise to a photocurrent, which is associated with the transfer of an ion or electron in its excited state. Alternatively, the photocurrent can be due to transfer of an ionic product of the photochemical reaction occurring in the solution bulk. Polarization measurements of the photoinduced charge transfer thus extend the range of experimental approaches to 

Apart from the study of physicochemical aspects such as ion solvation, and bio-mimetic aspects such as photosynthesis or carrier-mediated ion transfer (Volkov et al., 1996, 1998), there are several areas of potential applications of electrochemical IBTILE measurements comprising electroanalysis, lipophilicity assessment of drugs, phase transfer catalysis, electro-assisted extraction, and electrocatalysis. 

Studies of the polarized IBTILE provide a fundamental knowledge that makes it possible to explain phenomena occurring at the membranes of ion-selective electrodes. In addition, the rates of ion transfer and assisted ion transfer reactions are proportional to concentrations, which is a basis of an amperometric ion-selective (sensitive) electrode. 

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This means that the PMC signal will, apart from the generation rate of minority carriers and a proportionality constant, be determined by the interfacial charge transfer rate constant kr and the interfacial charge recombination rate sr... 

There is an additional simple relation between the surface concentration Aps of photogenerated minority carriers and the charge recombination and charge transfer rates sr and kr to be considered ... 

The interfacial charge-transfer rate constant can be determined when the PMC signal and the photocurrent are measured simultaneously. When the interfacial charge transfer is, on the other hand, very large and Aps negligible, the PMC value becomes... 

Figure 14. PMC potential dependence, calculated from analytical formula (18) for different interfacial rate constants for minority carriers S = 1 cm, minority carrier flux toward interface I,- 1 cm-2s 1, a= 780enr1, L = 0.01 cm, 0=11.65 cmV, Ld = 2x 0"3cm), (a) sr = 0 and different charge-transfer rates (inserted in the figures in cm s 1), (b) Constant charge-transfer rate and different surface recombination rates (indicated in the figure).

Figure 15. Effect of interfacial rate constants on PMC behavior and on the photocurrent (/0 = 1 cm-2), (a) Fast interfacial charge-transferrate, and (b) low interfacial charge-transfer rate.

Experimental evidence with very different semiconductors has shown that at semiconductor interfaces where limited surface recombination and a modest interfacial charge-transfer rate for charge carriers generate a peak... 

As outlined at the beginning of this chapter, combined photocurrent and microwave conductivity measurements supply the information needed to determine three relevant potential-dependent quantities the surface concentration of excess minority carriers (Aps), the interfacial recombination rate (sr), and the interfacial charge-transfer rate ( r). By inserting the... 

Interesting results have also been obtained with light-induced oscillations of silicon in contact with ammonium fluoride solutions. The quantum efficiency was found to oscillate complementarity with the PMC signal. The calculated surface recombination rate also oscillated comple-mentarily with the charge transfer rate.27,28 The explanation was a periodically oscillating silicon oxide surface layer. Because of a periodically changing space charge layer, the situation turned out to be nevertheless relatively complicated. 

With electrochemically studied semiconductor samples, the evaluation of t [relation (39)] would be more straightforward. AU could be increased in a well-defined way, so that the suppression of surface recombination could be expected. Provided the Debye length of the material is known, the interfacial charge-transfer rate and the surface recombination... 

Figure 42. Scheme comparing expected potential-independent charge-transfer rates from Marcus-Gerischer theory of interfacia) electron transfer (left) with possible mechanisms for explaining the experimental observation of potential-dependent electron-transfer rates (right) a potential-dependent concentration of surface states, or a charge-transfer rate that depends on the thermodynamic force (electric potential difference) in the interface. 

Thus the key experimental observation Equation (7.11), is satisfied in presence of spillover. When an external overpotential AUWR is applied, with a concomitant current, I, and O2 flux I/2F, although UWR is not fixed anymore by the Nemst equation but by the extremally applied potential, still the work function Ow will be modified and Equations (7.11) and (7.12), will remain valid as long as ion spillover is fast relative to the electrochemical charge transfer rate I/2F.21 This is the usual case in solid state electrochemistry (Figs. 7.3b, 7.3d) as experimentally observed (Figs. 5.35, 5.23, 7.4, 7.6-7.9). 

For semiconductor electrodes and also for the interface between two immiscible electrolyte solutions (ITIES), the greatest part of the potential difference between the two phases is represented by the potentials of the diffuse electric layers in the two phases (see Eq. 4.5.18). The rate of the charge transfer across the compact part of the double layer then depends very little on the overall potential difference. The potential dependence of the charge transfer rate is connected with the change in concentration of the transferred species at the boundary resulting from the potentials in the diffuse layers (Eq. 4.3.5), which, of course, depend on the overall potential difference between the two phases. In the case of simple ion transfer across ITIES, the process is very rapid being, in fact, a sort of diffusion accompanied with a resolvation in the recipient phase. 

According to the Marcus theory [9], the electron transfer rate depends upon the reaction enthalpy (AG), the electronic coupling (V) and the reorganization energy (A). By changing the electron donor and the bridge we measured the influence of these parameters on the charge transfer rate. The re-... 

Yeganeh S, Ratner MA, Mujica V (2007) Dynamics of charge transfer rate processes formulated with nonequilibrium Green s function. J Chem Phys 126 161103... 

The rate of this charge transfer (coulomb/second) determines the detector response (ampere). In other words, maximum detector response is obtained at maximum charge transfer rate. A measure for the charge transfer rate is the electrolytic efficiency the fraction of analyte that is electrolyzed during passage through the cell expressed in percentage of total amount injected. 

The photoreactivity of the involved catalyst depends on many experimental factors such as the intensity of the absorbed light, electron-hole pair formation and recombination rates, charge transfer rate to chemical species, diffusion rate, adsorption and desorption rates of reagents and products, pH of the solution, photocatalyst and reactant concentrations, and partial pressure of oxygen [19,302,307], Most of these factors are strongly affected by the nature and structure of the catalyst, which is dependent on the preparation method. The presence of the impurities may also affect the photoreactivity. The presence of chloride was found to reduce the rate of oxidation by scavenging of oxidizing radicals [151,308] ... 

T.W. Hamann, F. Gstrein, B.S. Brunschwig, N.S. Lewis, Measurement of the driving force dependence of interfacial charge transfer rate constants in response to pH changes at n-ZnO/ H20 interfacies, Chem. Phys. 326 (2006) 15-23. 

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