Anodes, photo

Anodic photo currents for zinc oxide, 470 Anodic potential, oxidation of the polymer and, 395 Antimony... 

Zinc oxide, anodic photo currents for, 470 Zinc oxide layers, spotted, 471 Zinc oxide-electrolyte interfaces, electron transfer rate and its exponential increase at, 512... 

Fig. 9. Comparison of the experimental and calculated dependences of the anodic photo-cunent quantum yield Y on the potential of a W03 electrode (ND = 4 x 1015 cm"3) in 1 M CH3COONa. Solid line—calculation, dots—experiment. [From Butler (1977).]...

In the above argument, we only considered the electron-transfer reactions between the semiconductor surface and redox species in solution. Actually, the electrons and holes at the semiconductor surface often cause another type of redox reaction leading to semiconductor corrosion. For n-type semiconductors such as n-Si and n-GaAs, for example, the following anodic photo corrosion reactions occur.3,4)... 

FIGURE 22.22 Polarization curves for localized corrosion of a photoexcited p-n heterojunction electrode of GaAs in 0.5 kmol m-3 sulfuric acid solution [15] ta(ph) = anodic photo-dissolution current and lc(ph) = cathodic photo-hydrogen current. 

Some problems have to be solved relating to the stability of the photo-anode (photo-degradation) m8. 33.63 .mo) and to the cell performance >. Stability criteria for the choice of appropriate redox couples were recently discussed by... 

Anodic photo-oxidation ITR induced by laser M + zHzO + hv MO + 2zH+ + 2ze ... 

In the presented case CdS, as a prominent example for nano structured n-VI semiconductors, were used since their lower band gaps compared to corresponding oxide materials make them attractive candidates for visible light assisted photocatalytic hydrogen generation. The hole scavenger TEA protects the CdS from anodic photo corrosion (caused by oxidation reactions, i.e. CdS + 2h Cd - + S) by swiftly... 

The removal of the second electron by electron injection was foimd in the oxidation of germaniiun and silicon, not on GaAs and CdS or other semiconductors with band gaps >1.2 eV. The injection of electrons could be seen in the quantum yield of anodic photo ciurents at n-type specimens which exceeded one and reached nearly two in several cases. The energetics of the two steps in the photooxidation of n-type substrates are shown in Figure 2.36... 

Electrolysis of a water solution of KI gives a saturated solution of iodine at the anode (see photo at top of page 499). 

The photo-Kolbe reaction is the decarboxylation of carboxylic acids at tow voltage under irradiation at semiconductor anodes (TiO ), that are partially doped with metals, e.g. platinum [343, 344]. On semiconductor powders the dominant product is a hydrocarbon by substitution of the carboxylate group for hydrogen (Eq. 41), whereas on an n-TiOj single crystal in the oxidation of acetic acid the formation of ethane besides methane could be observed [345, 346]. Dependent on the kind of semiconductor, the adsorbed metal, and the pH of the solution the extent of alkyl coupling versus reduction to the hydrocarbon can be controlled to some extent [346]. The intermediacy of alkyl radicals has been demonstrated by ESR-spectroscopy [347], that of the alkyl anion by deuterium incorporation [344]. With vicinal diacids the mono- or bisdecarboxylation can be controlled by the light flux [348]. Adipic acid yielded butane [349] with levulinic acid the products of decarboxylation, methyl ethyl-... 

The (photo)electrochemical behavior of p-InSe single-crystal vdW surface was studied in 0.5 M H2SO4 and 1.0 M NaOH solutions, in relation to the effect of surface steps on the crystal [183]. The pH-potential diagram was constructed, in order to examine the thermodynamic stability of the InSe crystals (Fig. 5.12). The mechanism of photoelectrochemical hydrogen evolution in 0.5 M H2SO4 and the effect of Pt modification were discussed. A several hundred mV anodic shift of the photocurrent onset potential was observed by depositing Pt on the semiconductor electrode. 

The preparation of Titania nanocoils has been yet not investigated in literature. However, quite recent results258 show that the effective structure of Titania nanotube likely produced by controlled anodization process is that of a helical (compressed) nanocoil. Fig. 11 shows this concept. It was also demonstrated that the formation of these helical nanocoils improves the photo-generated current compared to samples after short anodization where only a Titania layer is formed. 

Anodic dissolution of n-Si can also proceed at a polarization under illumination. The maximum current is limited by illumination intensity when the saturation photo current density is lower than the critical current, Ji. The characteristics of i-V curves of n-Si under a high illumination intensity, when the reaction is no longer limited by the availability of photo generated carriers, is identical to that for p-Si. Similar also to p-Si, formation of PS on n-Si occurs only below the critical current, Jx 24... 

Photoemissive tubes are necessary for work in the ultraviolet range and they show greater sensitivity and precision than photoelectric cells. A simple photo-emissive tube consists of two electrodes in a vacuum. A silver cathode coated with an alkali metal is maintained at a potential difference of about 100 V from the anode, which is a plain silver wire and serves to collect the electrons (Figure 2.26(a)). 

An example of the effect of photon irradiation on the flat band potential is shown in Fig. 10-18 this figure compares a Mott-Schott plot with the anodic polarization curve of the dissolution reaction of a semiconductor anode of n-type molybdeniun selenide in an acidic solution in the dark and in the photoexcited conditions. In this example photoe dtation shifts the flat band potential from Em in the dark to pii) in the photoexcited state is about 0.75 V more positive than Em. This photo-shift of the flat band potential, Emi )-Em, corresponds to the change in the potential, of the compact layer due to photoexcitation as defined in Eqn. 10-23 ... 

Pig. 10-18. (a) PolarizatioD curves of anodic dissolution and (b) Mott-Schottky plots of an n-type semiconductor electrode of molybdenum selenide in the dark and in a photo-excited state in an acidic solution C = electrode capacity (iph) = anodic dissolution current immediately after photoexdtation (dashed curve) ipb = anodic dissolution current in a photostationary state (solid curve) luph) = flat band potential in a photostationary state. [From McEv( -Etman-Memming, 1985.]... 

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