Kinetics Photogeneration

Considering the difficulties in measuring simple ion-transfer kinetics, photogeneration of ions close to the interface and observation of the photocurrent associated with their transfer was proposed. In this way, the interfadal polarization is kept quasi-constant, and the photocurrent does not suffer from the RC lime constant of the interface. This concept was pioneered by Kotov and Kuzmin that published a series of papers on the subject [143-146]. Two kinds of photochani-cal systems were used protoporphyrin (excitation 540-580 nm) quenched by quinones, and quinones (excitation 313-365 nm) quenched by telraphenylborale. Similarly, Samec et al. have studied photoinduced ion-transfer reactions nsing tetra-arylborate and tetra-arylarsonium as the chromophore, thereby generating the more hydrophilic bridged tetra-aryl intermediate [147,148]. 

The application of the Braun s kinetic photogeneration model, which is a particular case of the Noolandi-Hong model, enables... 

To interpret the magnitude of the PIA signal, the rale equation for the photogeneration of states will be discussed. It describes the change of the number of states, n, with respect to time, /, depending on the generation rate kp (A=const., excitation density) and the decay rate In1 /=inverse lifetime, (/reorder of kinetics) and can be written as follows ... 

Magnetic field effects on the reaction kinetics or yields of photochemical reactions in the condensed phase have been studied [20-23]. They have proved powerful for verifying the mechanism of photochemical reactions including triplet states. Previously, we obtained photogenerated triplet biradicals of donor-acceptor linked compounds, and found that the lifetimes of the biradicals were remarkably extended in the presence of magnetic fields up to 1T [24]. It has been reported that Cgo and its derivatives form optically transparent microscopic clusters in mixed solvents [25,26]. The clustering behavior of fullerene (C o) is mainly associated with the strong three-dimensional hydrophobic interactions between the C o units. Photoinduced... 

One additional problem at semiconductor/liquid electrolyte interfaces is the redox decomposition of the semiconductor itself.(24) Upon Illumination to create e- - h+ pairs, for example, all n-type semiconductor photoanodes are thermodynamically unstable with respect to anodic decomposition when immersed in the liquid electrolyte. This means that the oxidizing power of the photogenerated oxidizing equivalents (h+,s) is sufficiently great that the semiconductor can be destroyed. This thermodynamic instability 1s obviously a practical concern for photoanodes, since the kinetics for the anodic decomposition are often quite good. Indeed, no non-oxide n-type semiconductor has been demonstrated to be capable of evolving O2 from H2O (without surface modification), the anodic decomposition always dominates as in equations (6) and (7) for... 

The overall process performance, as measured by photon efficiency (number of incident photon per molecule reacted, like the incident photon to current conversion efficiency, or IPCE, for PV cells), depends on the chain from the light absorption to acceptor/donor reduction/oxidation, and results from the relative kinetic of the recombination processes and interfacial electron transfer [23, 28]. Essentially, control over the rate of carrier crossing the interface, relative to the rates at which carriers recombine, is fundamental in obtaining the control over the efficiency of a photocatalyst. To suppress bulk- and surface-mediated recombination processes an efficient separation mechanism of the photogenerated carrier should be active. 

Fig. 16.2 Simplified kinetic model of the photocatalytic process. ps represents the light absorbed per unit surface area of the photocatalyst, e b and h+b are the photogenerated electrons and holes, respectively, in the semiconductor bulk, kR is the bulk recombination rate constant and /R the related flux, whatever recombination mechanism is operating A is the heat resulting from the recombination kDe and kDh are the net first-order diffusion constants for fluxes Je and Jh to the surface of e b and h+b in the semiconductor lattice, respectively e s and h+s are the species resulting from...

The photodecarboxylation of nitrophenyl acetate in aqueous media was also investigated recently89 -92, especially with respect to the kinetic and spectral properties of the photogenerated p-nitrobenzyl carbanion its triplet state (Xmax ca 290 nm) was identified to have a lifetime of 90 nanoseconds at pH > 5.0. The proposed reaction mechanism following 266-nm laser excitation of p-nitrophenyl acetate is summarized in Scheme 792. 

The potential, E, for the onset of the photoexdted reaction relative to the equilibrium electrode potential E of the same reaction can also be derived in a kinetics-based approach [Memming, 1987]. Here, we consider the transfer of anodic holes (minority charge carriers) at an n-type semiconductor electrode at which the hole transfer is in quasi-equilibrium then, the anodic reaction rate is controlled by the photogeneration and transport of holes in the n-type semiconductor electrode. The current of hole transport, has been given by Eqn. 8-71 as a function of polarization ( - ,) as shown in Eqn. 10-20 ... 

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... 

The diffusion length of photogenerated charge carriers is one of the important parameters governing the efficiency of a solar cell. In conventional cells, this is an intrinsic property of the semiconductor and its purity [34]. However, in DSSCs, the diffusion length is a function of the rate of reaction (4) and, thus, varies with different redox couples, surface treatments, and so forth. When the oxidation of R [reaction (2)] is chemically irreversible, the diffusion length of electrons is effectively infinite, whereas with kinetically fast, reversible redox couples (see Section VI), it approaches zero with unpassivated interfaces. 

Although several single-crystal, wide-band gap semiconductors provide electrochemical and optical responses close to those expected from the ideal semiconductor-electrolyte model, most semiconducting electrodes do not behave in this manner. The principal and by far overriding deviation from the behavior described in the previous section is photodecomposition of the electrode. This occurs when the semiconductor thermodynamics are such that thermal or photogenerated valence band holes are sufficiently oxidizing to oxidize the semiconductor lattice [8,9]. In this case, kinetics routinely favor semiconductor oxidation over the oxidation of dissolved redox species. For example, irradiation of n-CdX (X = S, Se, or Te) in an aqueous electrolyte gives rise exclusively to semiconductor decomposition products as indicated by... 

On the fundamental side, the research on photocatalysis has focused on several topics, including a) the primary processes involved in the production and trapping of photogenerated electrons and holes, using pulsed femtosecond or picosecond laser techniques, b) measurements on the kinetics of the photodecomposition processes on longer time scales, and c) measurements on the kinetics on small size scales. For the first topic, the reader is referred to several recent publications.69-7 This work is of great practical importance, because it helps to point out the critical factors involved in the photocatalytic materials themselves. 

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