Photogenerated behaviour

In conclusion, nanorods are a potentially interesting material, but present results still do not allow understanding of whether the nanostructure leads to an improvement of the intrinsic photocatalytic behaviour, or whether other factors (accessible surface area, enhanced adsorption, etc) are responsible for the observed differences. In ZnO nanorods have been shown quite recently by surface photovoltage spectroscopy that the built-in electrical field is the main driving force for the separation of the photogenerated electron-hole pairs.191 This indicates that the nano-order influences the photophysical surface processes after photogeneration of the electron-hole pairs. A similar effect could be expected for Titania nanorods. However, present data do not support this suggestion, mainly due to the absence of adequate photo-physical and -chemical characterization of the materials and surface processes. 

To understand the electrical behaviour of the LAPS-based measurement, the LAPS set-up can be represented by an electrical equivalent circuit (see Fig. 5.2). Vbias represents the voltage source to apply the dc voltage to the LAPS structure. Re is a simple presentation of the reference electrode and the electrolyte resistance followed by a interface capacitance Cinterface (this complex capacitance can be further simulated by different proposed models as they are described, e.g., in Refs. [2,21,22]). In series to the interface capacitance, the insulator capacitance Cj will summarise the capacitances of all insulating layers of the LAPS device. The electrical current due to the photogeneration of electron-hole pairs can be modelled as current source Ip in parallel to the... 

An interpretation of the observed behaviour is possible if we consider the combined actions of photogenerated electron hole pairs and the capture and recombination of the charge carriers on surface centres produced by chemisorption or on lattice defects in the interior (Eqs. 8 to 13). 

Under Ti02 electrode polarization slightly anodic from the Hatband potential (Vft), a cathodic current superimposed to the anodic photocunent (transient behaviour) can be observed (Fig. 1). This catohdic effect is attributed to the recombination with 6cb of holes trapped at surface species (mainly OH° radicals and H202 molecules) photogenerated at intermediate steps of oxygen evolution (Salvador, 1985). 

The behaviour of p-InP discussed in Section 8.9 was judged to be nonideal on the basis of a simplified theory for one electron transfer reactions. However, the reduction of protons to hydrogen by photogenerated electrons must be a two step process. There are two possible mechanisms ... 

The behaviour of photogenerated carriers in the space charge layer... 

Thus, it can be seen that a study of the steady state photoelectrochemistry of colloidal semiconductors with the ORDE can provide information relating to the energy distribution of the particle surface states, the photogenerated carrier density and the quantum efficiency of carrier generation. The next section describes how to obtain information pertaining to intraparticle charge carrier dynamics from a study of the behaviour of transient photocurrents at the ORDE. 

Theory of the transient photocurrent behaviour at the ORDE. The stationary optical disc electrode is assumed to be uniformly illuminated by parallel light which is switched on at time t = 0, and which produces a measurable concentration of photogenerated electrons on the particles denoted by c. The differential equation for the generation and transport of these electrons to the electrode surface, with concurrent homogeneous back reaction is set up with the following assumptions. 

Presented experimental data reveal that for CdSe/ZnS quantum dots with two ZnS monolayers values follow a monotonous function drastically decaying with the QD core diameter. From the physico-chemical point of view, we conjecture that upon interaction of P with QD surface, the electron wave function may be locally modified (via inductive and/or mesomeric effects [9]) forming a surface local state capable to trap the electron of the photogenerated exciton (Fig. 2A). Thus, we will consider the behaviour of the electron wave function at the interface to the functional pyridyl group of the attached porphyrin. The single-carrier envelope wave functions y/a in a spherical core/shell QD are determined by the Schrodinger equation... 

In this section we examine the case where there is an electric field in photocatalyst particle and ask what influence such an electric field might have on the photocatalytic behaviour of the photocatalyst particles. Accordingly, we now modify the earlier model (Fig. 5.29) to include the electric field, whose presence may affect both the magnitude and the direction of migration of the photogenerated carriers. We then query what factors govern the snrface concentration of carriers, and thus the quantum yield of the surface reaction nnder these new conditions. 

Photolysis of benzyl alcohol on alumina, surprisingly, allows the production of dibenzyl ether [11] this behaviour is not observed in solution. A plausible mechanism for the formation of the ether involves the photogeneration of a benzyl cation which subsequently reacts with benzyl alcohol [Eq. (23)]. 

The photoelectrochemical behaviour of p-InP has been studied by a number of authors [31, 35, 75, 162]. When a p-InP electrode is illuminated under depletion conditions, photogenerated electrons are driven to the surface where they can reduce protons to hydrogen. This process competes with surface recombination. Fig. 15 illustrates a set of experimental IMPS responses measured for p-InP in l.Omoldm H2SO4 [35] at different potentials. The measurements were performed using a small ac modulation of the illumination intensity superimposed on a larger steady component. 

As in P3HT, however, the second striking observation is the similar drop in the PL yield t], which at Aex = 248 nm is 10% of its value at Acx = 488 nm in DHO-PPV as well. The nature of this behaviour is probably similar to that of P3HT, and suspected to be due to photogeneration of free polarons, yielding fewer singlet excitons which are the source of the emission. 

The observed behaviour of the PA signals at 0.80 eV and at 0.95 eV (which is probably the vibronic satellite of the 0.80 eV peak) is in agreement with the predictions for bipolarons. On the contrary, the chopper frequency dependence of the 1.24 eV band is consistent with the photogeneration of a defect with lifetime values of the order of ms at 20K and substantially lower at 77K. The laser intensity dependence rules out the photocreation of defects with a simple monomolecular decay. On the other hand, this signal cannot be attributed to the photocreation of polarons and bipolarons, which are expected to recombine bimolecularly. ... 

By an extensive study by optical, Raman and photomodulation spectroscopy of samples of polyCPDA and polyDCHD we have been able to recognize that different electronic photoexcitations occur in these polymers. Long-lived metastable bipolaronic states are photogenerated in polyCPDA as inferred from the behaviour of the two PA bands exhibited by this polymer. On the contrary, the PA spectrum of polyDCHD... 

Note that in the decomposition of benzoyl chloroacetyl peroxide, the behaviour of photogenerated benzoyloxy radicals is different from those produced thermally. 

The development of photocathode materials for either single- or dual-absorber cells has also received considerable attention [80, 101, 102]. Thermodynamic equilibrium dictates that p-type semiconductors will exhibit upward band bending when in contact with a liquid electrolyte. This behaviour is the opposite to that of n-type semiconductors described previously, and will result in the movement of photogenerated electrons towards the semiconductor-electrolyte interface while the holes are driven into the bulk of the electrode, towards the electrical back contact. At the surface, provided that the energy carried by the electrons is sufficient, H2 is evolved. As discussed previously, one of the electronic properties of metal oxides that makes them suitable for water photo-oxidation purposes is the O 2p character of the valence electrons, which places the VB edge at potentials... 

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