Trapped hole yield

The energy estimated from the analysis of the emission assuming pair recombination may suffer the same problem but it would be, again, an overestimate. Thus it is concluded that the relaxed, self-trapped hole lies, at most, 400 meV above the valence band. The data from the EPR studies of the decay of the self-trapped holes yielded a thermal trap depth equal to or greater than lOOmeV [164],... 

Table IV. Variation of the Relative Quantum Yield of the Trapped Holes with the Concentration of HCIO4 and CIO< in Ultraviolet Irradiated Ce+4 (0.02M, as the...

Figure 5- Phenomenological representation of the thermal release of trapped electrons and their recombination with trapped holes to yield TL.

The chemical nature of the trapped holes has not been clearly clarified yet. Older reports assume that the holes are trapped at the titanium dioxide surface in adsorbed hydroxy groups yielding weakly adsorbed hydroxyl radicals (reaction (7.6)) [14,15]. 

Around neutral pH, by far the most common Fe(III) species are the oxides and hydroxides. Of these, a-Fc203 and /I-FeOOH have been most studied from a photochemical point of view [88-92], They are semiconductor oxides and irradiation in the visible promotes electrons from the valence band to the conduction one, leaving holes (h+) in the valence band [90,91]. Electrons and holes can either thermally recombine or migrate to the oxide surface, where they can be trapped by surface species and react with dissolved molecules [93]. In aerated solution, trapped electrons are likely to reduce oxygen to superoxide, while trapped holes can oxidise various molecules. Quite interestingly, when irradiated in the visible, g -Fc203 and /J-FcOOH are not able to transform phenol. However, in the presence of phenol and nitrite, quantitative yield in nitrophenols is observed as a consequence of the oxidation of nitrite to nitrogen dioxide [85]. 

Hole capture by a solute (S) is probably not sufficient to explain the Ps yield enhancement as recombination might well occur as easily between the electron and either M+ (the hole) or S+ (the trapped hole). An explanation to the phenomenon is probably that e+ cannot react with the electron once recombined with M+ whereas it can pick up an electron shallowly trapped by S+. The weakness of recombined S+/e pairs has been shown in some instances in pulse radiolysis experiments, where a delayed formation of e s has been observed from such a state [16]. The concentration range necessary for efficient enhancement of Ps formation is similar to that related to total inhibition, indicating that the processes involved occur on a very short time-... 

All these photocorrosion processes are, of course, undesirable and it is obvious that their relative importance depends strongly on the presence of surface states which may facilitate recombination or redox reactions with adsorbed substrates. It is well known from ESR [69, 70, 94] and emission spectra [94] that most of these metal sulfide powders contain surface states. They are introduced during preparation of the powder as a result of lattice defects [72, 96], trapped holes [94], surface impurities [97] and metallization [38], and during the actual catalytic reaction as a consequence of irradiation and substrate adsorption. The stabilizing effect of plati-nization is exemplified by Figure 6 for the ZnS-catalyzed reduction of water in the presence of sodium formate [98]. Note that platinum does not accelerate the reaction but doubles the time of constant catalytic activity from 1 to 2 days. Similarly, the apparent product quantum yield of the 2,5-DHF dehydrodimerization is not increased but slightly decreases when platinizes ZnS is the photocatalyst [97]. 

Afterglow is due to the phenomenon that radiant recombination of electrons and holes is sometimes considerably delayed due to trapping of electrons or holes. Figure 3.28 gives a simple illustration. A semiconductor contains, next to the luminescent centers, also centers which trap electrons. Excitation with energy above E yields free electrons and holes. Let us assume that the holes are trapped by the luminescent center, whereas the electrons in the conduction band recombine with the holes yielding emission. 

Solid and liquid sulphur are included in this section because below 160°C sulfur consists of Sg rings, while above this temperature a fraction of the rings polymerizes into long chains with a mean length of about 10 atoms. Orthorhombic sulfur crystals These molecular crystals are composed of Sg rings. Mobility measurements of holes yielded values of = 10 cm V s" at room temperature. At lower temperatures the hole mobility is an activated process. Above the valence band, hole traps exist with a depth of 0.19 eV (Adams and Spear, 1964). Similar conclusions were reached by Lohmann and Mehl (1967) who derived a trap depth of 0.13 eV from SCL-injection currents. An electron mobility of 4.5 x 10 cm V s" was estimated by Miller and Kershaw (1979). Studies of the photoconductivity yielded a value of E 4.2 eV for the intrinsic generation of electrons and holes (Spear and Adams, 1966). A strong increase of the conduction current observed in liquid sulfur above 3 x 10 V/cm was... 

Fig. 7. Electron carrier quantum yields at 255 nm as a function of applied field. Curve a is for a previously unirradiated or virgin crystal sample. Curve b illustrates the reduced low-field yield that occurs when trapped holes, left behind in the experiments shown in curve a are present in the excited volume of the crystal. Note that curves a and b actually superimpose at high fields but that curve b has been shifted arbitrarily for clarity of presentation.

Tamaki Y, Furube A, Murai M, Kara K, Katoh R, Tachiya M (2006) Direct observation of reactive trapped holes in Ti02 undergoing photocatalytic oxidation of adsorbed alcohols evaluation of the reaction rates and yields. J Am Chan Soc 128(2) 416-417... 

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