Illuminated electron transfer

Fig. 5.2 The n-Cd(Se,Te)/aqueous Cs2Sx/SnS solar cell. P, S, and L indicate the direction of electron flow through the photoelectrode, tin electrode, and external load, respectively (a) in an illuminated cell and (b) in the dark. For electrolytes, m represents molal. Electron transfer is driven both through an external load and also into electrochemical storage by reduction of SnS to metaUic tin. In the dark, the potential drop below that of tin sulfide reduction induces spontaneous oxidation of tin and electron flow through the external load. Independent of illumination conditions, electrons are driven through the load in the same direction, ensuring continuous power output. (Reproduced with permission from Macmillan Publishers Ltd [Nature] [60], Copyright 2009)...

Instead of postulating Zn," as intermediate, as it has a highly negative potential and is possibly unstable in ZnO, one may write the above mechanism with Zn e pairs. The blue-shift in the absorption upon illumination was explained by the decrease in particle size. The Hauffe mechanism was abandoned after it was recognized that an excess electron on a colloidal particle causes a blue-shift of the absorption threshold (see Fig. 19). In fact, in a more recent study it was shown that the blue-shift is also produced in the electron transfer from CH2OH radicals to colloidal ZnO particles When deaerated propanol-2 solutions of colloidal ZnO were irradiated for longer times, a black precipitate of Zn metal was formed. In the presence of 10 M methyl viologen in the alcohol solution, MV was produced with a quantum yield of 80 %... 

FIG. 6 SHG intensity as a function of time (ON) with and (OFF) without illumination of the interface by a probe ETV pulse. The increased SHG intensity during illumination arises from the production of the species I at the interface by photoinduced electron transfer, see text for more details. (From Ref. 103, copyright American Chemical Society.)... 

Between 0.20 and 0.30 V, a decay of the initial photocurrent and a negative overshoot after interrupting the illumination are developed. This behavior resembles the responses observed at semiconductor-electrolyte interfaces in the presence of surface recombination of photoinduced charges [133-135] but at a longer time scale. These features are in fact related to the back-electron-transfer processes within the interfacial ion pair schematically depicted in Fig. 11. 

The most direct evidence for surface precursor complex formation prior to electron transfer comes from a study of photoreduc-tive dissolution of iron oxide particles by citrate (37). Citrate adsorbs to iron oxide surface sites under dark conditions, but reduces surface sites at an appreciable rate only under illumination. Thus, citrate surface coverage can be measured in the dark, then correlated with rates of reductive dissolution under illumination. Results show that initial dissolution rates are directly related to the amount of surface bound citrate (37). Adsorption of calcium and phosphate has been found to inhibit reductive dissolution of manganese oxide by hydroquinone (33). The most likely explanation is that adsorbed calcium or phosphate molecules block inner-sphere complex formation between metal oxide surface sites and hydroquinone. 

All the surface recombination processes, including back reaction, can be incorporated in a heavy kinetic model [22]. The predicted, and experimentally observed, effect of the back reactions is the presence of a maximum in the donor disappearance rate as a function of its concentration [22], Surface passivation with fluoride also showed a marked effect on back electron transfer processes, suppressing them by the greater distance of reactive species from the surface. The suppression of back reaction has been verified experimentally in the degradation of phenol over an illuminated Ti02/F catalyst [27]. 

However, a very limited number of studies focused on the effect of solvent dynamics on electron transfer reactions at electrodes.Smith and Hynes" introduced the effect of electronic friction (arising from the interaction between the excited electron hole pairs in the metal electrode) and solvent friction (arising from the solvent dynamic [relaxation] effect) in the electron transfer rate at metallic electrodes. The consideration of electron-hole pair excitation in the metal without illumination by light seems unrealistic. 

Trifluoroethyl chloride, bromide, and iodide (but not fluoride) react with thio-late ions in DMF under laboratory illumination at 30-50 °C to give high yields of 2,2,2-trifluoroethyl thiol derivatives. Various features of the reactions show that they occur by the 5 rnI mechanism. The initiation may be spontaneous or thermal electron transfer between thiolate and halides, because the reactions can occur in the dark. 

Without illumination, the reaction proceeds slowly, but by no means it is negligible 8% of phenylpinac-olin is formed, with no regard to the prolonged duration. The addition of ferrous chloride in amounts of 40% to molar equivalent of Phi results in incisive acceleration of this reaction. The disappearance of Phi is observed within 20 min, replaced by 74% of the substitution product, PhCH2COCMe3, and ca. 10% of the disubstitution prodnct, Ph2CHCOCMe3. The authors cite diverse data, theorizing that iron(II), associated with the enolate ion, acts as an electron-transfer relay between the enolate and phenyl iodide (Scheme 5.25). 

Under anaerobic conditions with a low partial pressure of hydrogen and under low intensity illumination, hydrogen evolution takes place and the overall reaction can be represented by (2.4.1) [172] the electron transfer route is as follows [141] ... 

ZnS is a semiconductor with a full valence band and empty conduction band. When the phosphor is illuminated, electrons are promoted to the conduction band. Because the orbitals in this band are delocalised, the energy can easily be transferred to other parts of the crystal, particularly to the dopant atoms. 

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