Trapped holes

Zuo J, Pandey R and Kunz A B 1991 Embedded-oluster study of the lithium trapped-hole oenter In magnesium oxide Phys. Rev. B 44 7187-91... 

Pulse radiolysis results (74) have led other workers to conclude that adsorbed OH radicals (surface trapped holes) are the principal oxidants, whereas free hydroxyl radicals probably play a minor role, if any. Because the OH radical reacts with HO2 at a diffusion controlled rate, the reverse reaction, that is desorption of OH to the solution, seems highly unlikely. The surface trapped hole, as defined by equation 18, accounts for most of the observations which had previously led to the suggestion of OH radical oxidation. The formation of H2O2 and the observations of hydroxylated intermediate products could all occur via... 

Other models consider the steps up to and including Ag formation to be rapid in both the forward and reverse directions, thereby setting up an equihbrium condition involving, Ag, and Br, where is a trapped hole and Br is the result of Br neutralization by. In estabUshing... 

Vo) in the crystal. (Vo) can catch electrons to form F and centers. (Pb) is also able to attract electrons while (Vb)" can trap holes to give rise to color centers. They vdll make a contribution to the X-ray irradiation-induced absorption. Of course, the charge balance of the crystal is kept by charge compensation among these defects. Regretfully, the detailed characterization of these defects is too difficult to cover here and further experiments need to be performed. 

In our experiment, photocatalytic decomposition of ethylene was utilized to probe the surface defect. Photocatalytic properties of all titania samples are shown in table 2. From these results, conversions of ethylene at 5 min and 3 hr were apparently constant (not different in order) due to the equilibrium between the adsorption of gaseous (i.e. ethylene and/or O2) on the titania surface and the consumption of surface species. Moreover it can be concluded that photoactivity of titania increased with increasing of Ti site present in titania surface. It was found that surface area of titania did not control photoactivity of TiOa, but it was the surface defect in titania surface. Although, the lattice oxygen ions are active site of this photocatalytic reaction since it is the site for trapping holes [4], this work showed that the presence of oxygen vacancy site (Ti site) on surface titania can enhance activity of photocatdyst, too. It revealed that oxygen vacancy can increase the life time of separated electron-hole pairs. 

Another intermediate of the photolysis of TiO was observed in experiments with platinized particles (in the absence of polyvinyl alcohol). The spectrum shown in Fig. 22 is prraent immediately after the laser flash. The signal decays as shown by the inset in the figure. The rate of decay is not influenced by oxygen but is increased by oxidizable compounds such as Br ions in the solution. The broad absorption band in Fig. 22 with a maximum at 430 nm was attributed to trapped positive holes. Chemically, a trapped hole is an 0 radical anion. In homogeneous aqueous solution, 0 ... 

The reaction of the trapped holes with Br ions adsorbed at the colloidal particles was used to determine the absorption coefficient of the holes. In the presence of Br less absorption of the holes is observed after the flash, as the Br ions react with some of the holes during the laser flash. On the other hand, the absorption of Br, i.e. the oxidation product of Br , can be seen. By comparing the decrease in h absorption after the flash with the absorption of Br, , and knowing the absorption coefficient of Br, the absorption coefficient of the hole, = 9.2 x 10 cm , was cal-... 

Since the original studies of F centers many other color centers have been characterized that may be associated with either trapped electrons or trapped holes. These are called electron excess centers when electrons are trapped and hole excess centers when holes are trapped. 

The color center is the [A104]4 group, which can be thought of as [A104]5 together with a trapped hole. The color arises when the trapped hole absorbs radiation. 

D. Lawless, N. Serpone, D. Meisel, Role of OH. radicals and trapped holes in photocatalysis. A pulse radiolysis study,... 

CSH, A (PC31 Above Trapped holes and occupied impurity levels Depletive... 

In xerographic measurements, as illustrated in Fig. 5.3, the sample is corona-charged to a voltage Vq and then exposed to a short wavelength (absorption depth S L) step illumination. At the end of the illumination, there is a measurable surface potential, termed the residual potential V because of the bulk trapped charges. If positive charging is used, then is due to trapped holes in the bulk of the specimen. 

Hole centre, COT A self trapped hole forms a planar molecule, C03 (D3h symmetry) with 23 electrons (AB323 -type) gives... 

A series of papers has recently appeared in the literature concerning the dynamics of photo-induced formation of electron and hole centres at the surface of MgO powders.12 14 Monochromatic excitation of the sample with 282 nm photons leads to the creation of well-separated electron and hole centres at the surface which were monitored via EPR in term of a trapped hole (O ion) and a trapped electron according to the following equation ... 

Fig. 14. Some Auger processes involving one-free carrier (boles as illustrated) The case of two trapped electrons on the same center is shown in (a), and the situation for trapping on nearby centers is shown in (b). The case of an exciton (isoelec-tronic) type center, with electron recombination to the trapped hole is shown in (c), and recombination with a free hole in (d) [note that in practice these two processes have to be considered in parallel (see, for example, Neumark, 1973)].

---