Reactions photodecomposition

Trigonal, metallic selenium has been investigated as photoelectrode for solar energy conversion, due to its semiconducting properties. The photoelectrochemistry of the element has been studied in some detail by Gissler [35], A photodecomposition reaction of Se into hydrogen selenide was observed in acidic solutions. Only redox couples with a relatively anodic standard potential could prevent dissolution of Se crystal. 

Fujishima and Honda [16, 158] reported the photodecomposition of water using semiconductor photoelectrolysis cells (Figure 4.10). When the surface of the Ti02 electrode was irradiated with UV light, oxygen evolution was observed at the Ti02 electrode surface and hydrogen at the Pt black electrode. The overall water photodecomposition reaction ... 

Water is involved in most of the photodecomposition reactions. Hence, nonaqueous electrolytes such as methanol, ethanol, N,N-d i methyl forma mide, acetonitrile, propylene carbonate, ethylene glycol, tetrahydrofuran, nitromethane, benzonitrile, and molten salts such as A1C13-butyl pyridium chloride are chosen. The efficiency of early cells prepared with nonaqueous solvents such as methanol and acetonitrile were low because of the high resistivity of the electrolyte, limited solubility of the redox species, and poor bulk and surface properties of the semiconductor. Recently, reasonably efficient and fairly stable cells have been prepared with nonaqueous electrolytes with a proper design of the electrolyte redox couple and by careful control of the material and surface properties [7], Results with single-crystal semiconductor electrodes can be obtained from table 2 in Ref. 15. Unfortunately, the efficiencies and stabilities achieved cannot justify the use of singlecrystal materials. Table 2 in Ref. 15 summarizes the results of liquid junction solar cells prepared with polycrystalline and thin-film semiconductors [15]. As can be seen the efficiencies are fair. Thin films provide several advantages over bulk materials. Despite these possibilities, the actual efficiencies of solid-state polycrystalline thin-film PV solar cells exceed those obtained with electrochemical PV cells [22,23]. 

In the absence of photodecomposition reactions, the quantum yield for electron injection from the excited sensitizer to the semiconductor is given by Equation 17.11. 

First, a complete photodecomposition reaction usually consists of several successive stages. It may appear that the equilibrium potential of the rate-determining stage does not coincide with that of the complete reaction. In this case the analysis, similar to the above one, should be performed just for the rate-determining stage (Gerischer, 1978). 

The 03 that is formed in the troposphere is controlled approximately by the rate of its generation by N02 photodecomposition (reactions 18 and 19) and by the rate of its removal by reaction with NO the concentration is roughly [03] = jNo2[N02]/(fc17[NO]) (20). This balance is influenced by the presence of peroxy radicals because of the reaction shown in equation 16a. 

In conclusion, whereas Co(NH3)63+ is useless because it undergoes a fast photodecomposition reaction, the analogous Co(sep)3+ complex may be employed as an electron transfer photosensitizer because of its intrinsic stability in the excited state and in the reduced form. In the same way, one can think to use cage-type polypyridine ligands for Ru complexes, so as to prevent ligand dissociation reactions. 

The photosensitive (a,a-dimethyl-3,5-dimethoxybenzyloxy)-carbonyl group has proved to be useful in the protection of amines cyclohexylamine (89), for example, is irreversibly regenerated by photolysis of the urethane (90). Photodecomposition reactions of alkyl benzohydroxamates have also been examined. 

Other low-yield photodecomposition reactions involving elimination of small molecules have been reported.144-14 ... 

We were able to observe clear evidence for the chain-type mechanism in experiments, involving acetaldehyde decomposition in the gas-phase [98], similar to those already discussed for 2-propanol. With acetaldehyde, the values exceeded the maximum value obtained for a similar film for 2-propanol oxidation (0.28) (Fig. 6). As already discussed, the latter value may be considered to be an intrinsic maximum value for this particular film. Therefore, if < > exceeds the intrinsic maximum value, it indicates that radical chain reactions are important, that is, a single photon can cause more than one photodecomposition reaction. 

The photodecomposition reaction of the iodonium salt (in a simplified form) can be illustrated as follows ... 

Onium salts are decomposed by UV irradiation to produce acids. t-BCXZ (butoxycarbonyl) polyhydroxystyrene (t-BOCPHS) is decomposed by acid. t-BOCPHS added to oniiun salt is decomposed by acid produced from UV irradiation of the onium salt. Since acid is also produced by the photodecomposition reaction, heating the resist after irradiation (postbaking) gives further decomposition. Ito and Wilson reported a photochemical reaction, shown in... 

Nabar and co-workers (161) studied the photodecom-position of sodium chlorite solutions. They found that the extent of photodecomposition decreased with an increase in pH and an increase in the initial sodium chlorite concentration. Chloride ion had no effect on the photodecomposition reaction. The photodecomposition reaction was found to be much more rapid than the decomposition of acidified sodium chlorite solutions. For example, a chlorous acid solution that decomposed 2.5% in the dark in 3 hr decoit osed 100% in 1 hr in the presence of a carbon arc. Equations 48 and 49 qualitatively represent the stoichiometry found at pH 4.0 and 8.43, respectively... 

III. THE SEMICONDUCTOR ELECTROLYTE CONTACT UNDER ILLUMINATION AND PHOTODECOMPOSITION REACTIONS... 

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