Electron transfer charge recombination

The dynamics of electron-transfer at the interfaces of Ti02/Dye 2/electrolyte are summarized in Fig. 20.7. Two forward electron-transfer steps are much faster than the corresponding reverse electron-transfer (charge recombination) on the order of 103 to 106. The results well explain the high rjei value due to the efficient and vectorial electron-transfer in Dye 2-sensitized DSC. 

Fig. 16.8 Charge recombination lifetimes in the compounds shown in the inset in dioxane solvent. (J. M. Warman, M. P. de Haas, J. W. Verhoeven, and M. N. Paddon-Row, Adv. Chem. Phys. 106, Electron transfer—from isolated molecules to bio-molecules, Part I, edited by J. JortnerandM. Bixon (Wiley, New York, 1999). The technique used is time-resolved microwave conductivity (TRMC), in which the change in dielectric response of a solution is monitored following photoinduced electron transfer—a charge separation process that changes the solute molecular dipole. The lifetimes shown as a function of bridge length (number of a-bonds separating the donor and acceptor sites in the compounds shown in the inset) are for the back electron transfer (charge recombination) process.

The efficiency of this electron flow is almost 100% with negligible back electron transfer (charge recombination), which is indicated by the dynamics of the electron transfer where the forward electron transfer rate is higher than the backward one by almost two to three orders of magnitude. 

It should also be briefly recalled that semiconductors can be added to nanocarbons in different ways, such as using sol-gel, hydrothermal, solvothermal and other methods (see Chapter 5). These procedures lead to different sizes and shapes in semiconductor particles resulting in different types of nanocarbon-semiconductor interactions which may significantly influence the electron-transfer charge carrier mobility, and interface states. The latter play a relevant role in introducing radiative paths (carrier-trapped-centers and electron-hole recombination centers), but also in strain-induced band gap modification [72]. These are aspects scarcely studied, particularly in relation to nanocarbon-semiconductor (Ti02) hybrids, but which are a critical element for their rational design. 

This section shows that in order to separate klr and kTec it is necessary to carry out non-steady-state measurements. A simple example of a non steady-state measurement is switching the illumination on and off. The photogenerated flux of holes, g, towards the surface feeds into three processes surface charge storage, interfacial electron transfer and recombination. The magnitudes of the corresponding components of the total current density can be written in terms of the surface hole charge Qs ... 

The flux of minority carriers into the surface is linked to surface charging, electron transfer and recombination, so that... 

The energy of the 7) state can also be determined for soluble conjugated polymers by the technique of T-T energy transfer (Monkman et al., 2001). When the solvent in a polymer solution is ionised with an intense, pulsed electron beam, charge recombination gives rise to excited solvent molecules in... 

Figure 1. Schematic energy level diagram for a typical photoinduced electron transfer. Charge separation and recombination routes are shown—see text.

Rapid e / h recombination, the reverse of equation 3, necessitates that D andM be pre-adsorbed prior to light excitation of the Ti02 photocatalyst. In the case of a hydrated and hydroxylated Ti02 anatase surface, hole trapping by interfacial electron transfer occurs via equation 6 to give surface-bound OH radicals (43,44). The necessity for pre-adsorbed D andM for efficient charge carrier trapping calls attention to the importance of adsorption—desorption equihbria in... 

Although the electrostatic potential on the surface of the polyelectrolyte effectively prevents the diffusional back electron transfer, it is unable to retard the very fast charge recombination of a geminate ion pair formed in the primary process within the photochemical cage. Compartmentalization of a photoactive chromophore in the microphase structure of the amphiphilic polyelectrolyte provides a separated donor-acceptor system, in which the charge recombination is effectively suppressed. Thus, with a compartmentalized system, it is possible to achieve efficient charge separation. 

Recently, photochemical and photoelectrochemical properties of fullerene (Cto) have been widely studied [60]. Photoinduced electron-transfer reactions of donor-Qo linked molecules have also been reported [61-63]. In a series of donor-Cfio linked systems, some of the compounds show novel properties, which accelerate photoinduced charge separation and decelerate charge recombination [61, 62]. These properties have been explained by the remarkably small reorganization energy in their electron-transfer reactions. The porphyrin-Qo linked compounds, where the porphyrin moieties act as both donors and sensitizers, have been extensively studied [61, 62]. 

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 donor-acceptor complexes [Ir(/r-dmpz)(CO)(PPh2 0(CH2)2R )]2 exhibit photo-induced electron-transfer rate constants of 1012s—1 and charge recombination rates slower than 2 x 10los-1 when R = pyridine and 4-phenylpyridine.534 Further studies on these complexes revealed that recombination reactions were temperature dependent and slower for the deuterated acceptors.535... 

In alicyclic hydrocarbon solvents with aromatic solutes, energy transfer (vide infra) is unimportant and probably all excited solute states are formed on neutralization of solute cations with solute anions, which are formed in the first place by charge migration and scavenging in competition with electron solvent-cation recombination. The yields of naphthalene singlet and triplet excited states at 10 mM concentration solution are comparable and increase in the order cyclopentane, cyclohexane, cyclooctane, and decalin as solvents. Further, the yields of these... 

The mere exposure of diphenyl-polyenes (DPP) to medium pore acidic ZSM-5 was found to induce spontaneous ionization with radical cation formation and subsequent charge transfer to stabilize electron-hole pair. Diffuse reflectance UV-visible absorption and EPR spectroscopies provide evidence of the sorption process and point out charge separation with ultra stable electron hole pair formation. The tight fit between DPP and zeolite pore size combined with efficient polarizing effect of proton and aluminium electron trapping sites appear to be the most important factors responsible for the stabilization of charge separated state that hinder efficiently the charge recombination. 

The addition of a second species can cause a decrease in charge recombination and an increase in the TiOz photocatalytic efficiency. Such behavior was examined by loading a series of species on the surface or into the crystal lattice of photocatalysts inorganic ions [148-152], noble metals [153,154], and other semiconductor metal oxides [155], It was thus proven that modifications produced by these species can change semiconductor surface properties by altering interfacial electron-transfer events and thus the photocatalytic efficiency. 

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