Semiconductor chlorophyll-coated

Photoelectrochemical Systems Involving Chlorophyll-Coated Semiconductor and Metal Electrodes... 

Chlorophyll-Coated Semiconductor Electrodes. Chi has first been employed by Tributsch and Calvin (55,56) in dye sensitization studies of semiconductor electrodes. Solvent-evaporated films of Chi a, Chi b, and bacteriochlorophyll on n-type semiconductor ZnO electrodes (single crystal) gave anodic sensitized photocurrents under potentiostatic conditions in aqueous electrolytes. The photocurrent action spectrum obtained for Chi a showed the red band peak at 673 nm corresponding closely to the amorphous and monomeric state of Chi a. The addition of supersensitizers (reducing agents) increased the anodic photocurrents, and a maximum quantum efficiency of 12.5% was obtained for the photocurrent in the presence of phenylhydrazine. 

Photoelectrochemlcal conversion from visible light to electric and/or chemical energy has been investigated with chlorophyll thin membranes deposted on semiconductor or metal electrodes (71). Chlorophyll-coated metal (platinum) electrodes derived cathodic photocurrent in acidic electrolyte solutions, although the photocurrent efficiencies tend to be low compared to those of chlorophyll-semiconductor electrodes. The cathodic photoresponse may result from the p-type photoconductive nature of a solid chlorophyll layer and/or the formation of a contact barrier at the metal-chlorophyll interface, which contributes to light-induced carrier separation and leads to photocurrent generation. 

LB chlorophyll monolayers (25 mN/m 1.4 nm2/molecule) on Pt electrodes showed low photoactivity, possibly caused by a quenching of excited states by the metal electrode or by total reversibility of electron exchange. Addition of electron acceptors, e.g., quinones, had no effect. The optically transparent tin oxide semiconductor electrode proves to be a much better subphase for the generation of photocurrents. Chlorophyll-coated Sn02 combined with a platinum electrode gave approximately 100 nA/cm. Similar results were obtained with photovoltaic systems of the form mercury droplet/buffer solution/chlorophyll a monolayer/electron acceptor monolayer/aluminum (Fig. 6.9.3). The quantum yield of such monolayer arrangements never exceeded 10" in any of these systems and is thus far away from competitive inorganic semiconductor cells (Norris and Meisel, 1989). 

Dyes such as erythrosin B [172], eosin [173-177], rose bengal [178,179], rhodamines [180-185], cresyl violet [186-191], thionine [192], chlorophyll a and b [193-198], chlorophyllin [197,199], anthracene-9-carboxylate [200,201], perylene [202,203] 8-hydroxyquinoline [204], porphyrins [205], phthalocyanines [206,207], transition metal cyanides [208,209], Ru(bpy)32+ and its analogs [83,170,210-218], cyanines [169,219-226], squaraines [55,227-230], and phe-nylfluorone [231] which have high extinction coefficients in the visible, are often employed to extend the photoresponse of the semiconductor in photoelectro-chemical systems. Visible light sensitization of platinized Ti02 photocatalyst by surface-coated polymers derivatized with ruthenium tris(bipyridyl) complex has also been attempted [232,233]. Because the singlet excited state of these dyes is short lived it becomes essential to adsorb them on the semiconductor surface with... 

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