Holes surface-trapped

In many semiconductors the majority of the photoelectrons are produced by excitation from the valence band, the process thus simultaneously producing holes. Surface traps may act as recombination centers for electron-hole recombination and a change in the number or energy of these surface traps, or a change in the height of the surface barrier, may change the rate of recombination. For example Bube (9,10) has concluded that it is through this effect that the adorption of water vapor influences the photoconductivity of cadmium sulfide. 

No clear picture of the primary radical intermediate(s) in the HO2 photooxidation of water has appeared. The nature of the observed radical species depends on the origin and pretreatment of the HO2 sample, on the conditions and extent of its reduction, on the extent of surface hydroxylation, and on the presence of adventitious electron acceptors such as molecular oxygen (41). The hole is trapped on the terminal OH group (54). 

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... 

In the process of photocatalysis, the electrons and holes produced on photoirradiated Ti02 powders are trapped at the particle surface to form unpaired-electron species (step (4) in Fig.D.3). Photocatalytic reactions are actually the reactions of these radicals with reactant molecules at the Ti02 surface. Electron spin resonance (ESR) spectroscopy has been used for the detection of the photoproduced radicals on Ti02 at low temperatures such as 77 K. It has been reported that photoproduced electrons are trapped at various different sites titanium atoms on the surface or inside the particles, or oxygen molecules adsorbed on the surface. On the other hand, photoproduced holes are trapped at lattice OAygen atoms near the particle surface or at surface hydroxyl groups. We analyzed these radical species for several Ti02 photocatalysts that are commercially available, and found that the differences in the photoproduced radicals resulted from different heat-treatment conditions and the reactivity with several molecules.17)... 

Fig. 5.6 Idealized chemical structure for the radicals observed on anatase Ti02 by means of low temperature ESR spectroscopy.17 A, trapped hole with OH group (Ti O- Ti4+OH ) B, surface trapped electron(Ti3+) C. adsorbed superoxide radical (02 )> U, hole trapped at surface with less OH groups (Ti4+02 Ti4+0J) E, inner trapped electron (Ti3 ).

The breakdown at a pH higher than 10 of the parallelism between the Us(redox) and the redox potential of the redox couple in solution can be explained by assuming that the rate of the dissolution reaction, caused by the attack of H2O or OH- on the surface trapped hole, is so high in this pH range that the electron exchange equilibrium at the interface is no longer achieved. 

For the (111) face of n-GaP, the measured Us(redox) changed almost linearly with the pH, showing that the surface-trapped hole is less stable than that for the case of the (111) face above mentioned. ... 

All these results can be explained in terms of the model proposed above (cf. Fig. 11). Namely, with ferrous oxalate having a standard redox potential E° (Ox/R) of —0.2 V (SCE), which is a little more negative than the E of the surface trapped hole located ca. 0.5 V above E , the surface trapped hole is effectively quenched by the rapid reduction, and the photoanodic current flows without decomposition. With ferrocyanide, having an E(0x/R) of 0.2 V (SCE), which is more positive than the E of the surface trapped hole, the surface trapped holes are accumulated to the extent that the surface potential created will level it down to the E(0x/R) of the redox couple. At this point, the rates of nu-cleophillic attack of H2O and OH to the surface trapped holes are still low and the electrode decomposition is prevented. 

When the temperature was raised to 200 K the signal for the surface trapped electrons at the sites chelated with two cysteine molecules (Fig. 1.4) disappeared, while the signal for surface trapped electrons at the sites chelated with one cysteine molecule increased four times relative to the trapped hole signal ... 

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]. 

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