Photodissolution

It was concluded from this and related works that suppression of the photodissolution of n-CdX anodes in aqueous systems by ions results primarily from specific adsorption of X at the electrode surface and concomitant shielding of the lattice ions from the solvent molecules, rather than from rapid annihilation of photogenerated holes. The prominent role of adsorbed species could be illustrated, by invoking thermodynamics, in the dramatic shift in CdX dissolution potentials for electrolytes containing sulfide ions. The standard potentials of the relevant reactions for CdS and CdSe, as well as of the sulfide oxidation, are compared as follows (vs. SCE) [68] ... 

Such dissolution reactions usually contain several steps and are complicated. An important example is silicon. In aqueous solutions this is generally covered by an oxide film that inhibits currents and hence corrosion. However, in HF solutions it remains oxide free, and p-type silicon dissolves readily under accumulation conditions. This reaction involves two holes and two protons, the final product is Si(IV), but the details are not understood. A simpler example is the photodissolution of n-type CdS, which follows the overall reaction ... 

Table I. Initial Rates of Photodissolution of y-FeOOH and am-FeOOH Suspended in 0.01M NaCl Using a Simulated Solar Spectrum of Total Radiation Output 300 pEinsteins cm-2 min-1.

At low pH, where resorption reactions are minimal, the photodissolution process may be represented as a two-step process involving adsorption of ligand L to metal oxide surface sites followed by detachment of reduced metal ions that is, for an iron oxyhydroxide ... 

Table II. Measured and Fitted Parameters Obtained in Studies of the Photodissolution of f-FeOOH Suspended in pH 4, 0.01M NaCl Containing Various Organic Acids. Illumination provided by a Hg arc lamp source and 365 nm band-pass filter (total radiation output 85 pE cm-2 min- ).

Except for phthalic acid, all other carboxylic acids studied induce considerable increases in the light compared to the dark values (the relatively high rate of iron oxide dissolution induced by oxalic acid has been extensively studied (5,8). Phthalic acid actually appears to stabilize the iron oxide against photodissolution despite the solution phase complex exhibiting some photoactivity. 

Litter, M.I. Baumgartner, E.C. Urmtia, G.A. Blesa, M.A. (1991) Photodissolution of iron oxides. 3. Interplay of photochemical and thermal processes in maghemite/carboxylic acid systems. Environ. Sci. Technol. 25 1907-1913... 

Beydoun D, Amal R, Low GKC, McEvoy S. Novel photocatalyst titania-coated magnetite. Activity and photodissolution. J Phys Chem B 2000 104 4387- 396. 

IMPS has been used to investigate the involvement of electron injection in the photodissolution of n-Si [33, 34], In the case of the photodissolution n-Si in fluoride solutions, the quantum efficiency varies from 4 at low light intensities to 2 at high light intensities. A quantum efficiency of 4 corresponds to a mechanism in which the capture of one photogenerated hole is followed by the injection of three electrons into the conduction... 

Fig. 8.18. The upper plot shows the experimental IMPS response measured at very low light intensities for the photodissolution of n-Si in 1.0 NH4F at pH 4.S. The lower plot is the calculated best fit. Note that the photocurrent efficiency varies from 4 at low frequencies to 1 at high frequencies as expected for the scheme shown in Fig. 8.16.

Practically, however, quantum yields somewhat lower than 2 are usually measured because step 34b or 34d competes with the further photooxidation or photoreduction of these intermediates, respectively. This is true especially at high photon flux values. Even multiplication factors as high as 4 are possible as in the photodissolution of n-Si in NH4F media [268-271]. 

Photocurrent multiplication has been observed for a variety of semiconductors including Ge [269], Si [268-271], ZnO [272-278], Ti02 [279-282], CdS [283, 284], GaP [285], InP [286] and GaAs [287-289]. These studies have included both n- and p-type semiconductors, and have spanned a range of substrates, both organic and inorganic. As in the Si case, this phenomenon can also be caused by photodissolution reactions involving the semiconductor itself. The earlier studies have mainly employed voltammetry, particularly hydrodynamic voltammetry (see, e.g.. Ref. [282]). 

Figure 30. Experimental and simulated IMPS plots for the photodissolution of n-Si in 1.0 M NH4F at pH 4,5. (Reproduced with permission from the authors of Ref. [9].)...

J. Stumper and L. M. Peter, A rotating ring-disc study of the photodissolution of n-Si in ammonium fluoride solutions, J. Electroanal. Chem. 309, 325, 1991. 

Linak WP, Miller CA (2000) Comparison of particle size distributions and elemental partitioning from the combustion of pulverized coal and residual fuel oil. J Air Waste Manage Assoc 50 1532-1544 Litter Ml, Villegas M, Blesa MA (1994) Photodissolution of iron-oxides in malonic-acid. Can J Chem 72 2037-2043... 

Hutton R. S., Peter L. M., Batchelor R. A. and Hamnett A. (1994), The potential distribution across the semicondnctor-electrolyte interface—a stndy by electrolyte electroreflectance spectroscopy of GaAs and Gai AbAs nnder conditions of photodissolution , J. Electroanal. Chem. 375, 193-201. 

Fig. 8.9 Comparison of experimental and calculated intensity-modulated photocurrent spectroscopy (IMPS) plots for the photodissolution of n-Si (111) in 6.5 M NH4F at low light intensities (photo quadrupling regime) The data used for the theoretical plot are k -2-10 s= 500 s ...

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