Semiconductor Photochemistry

Triplet excited state quenching by photoelectron transfer (PET) and energy transfer (ET) play a central role in biological photosynthesis, visual transduction, organic photochemistry, semiconductor photocatalysis and imaging. " PET and ET across the insulating walls of a hemicarcerand have recently been studied in order to improve our understanding of these photophysical processes. ... 

The combination of electrochemistry and photochemistry is a fonn of dual-activation process. Evidence for a photochemical effect in addition to an electrochemical one is nonnally seen m the fonn of photocurrent, which is extra current that flows in the presence of light [, 89 and 90]. In photoelectrochemistry, light is absorbed into the electrode (typically a semiconductor) and this can induce changes in the electrode s conduction properties, thus altering its electrochemical activity. Alternatively, the light is absorbed in solution by electroactive molecules or their reduced/oxidized products inducing photochemical reactions or modifications of the electrode reaction. In the latter case electrochemical cells (RDE or chaimel-flow cells) are constmcted to allow irradiation of the electrode area with UV/VIS light to excite species involved in electrochemical processes and thus promote fiirther reactions. 

An important aspect of semiconductor photochemistry is the retardation of the electron-hole recombination process through charge carrier trapping. Such phenomena are common in colloidal semiconductor particles and can greatly influence surface corrosion processes occurring particularly in small band gap materials, such... 

Kamat PV (1993) Photochemistry on nonreactive (semiconductor) surfaces. Chem Rev 93 267-300... 

Baral S, Fojtik A, Weller H, Henglein A (1986) Photochemistry and radiation chemistry of coUoidal semiconductors. 12. Intermediates of the oxidation of extremely small particles of cadmium sulfide, zinc sulfide, and tricadmium diphosphide and size quantization effects (a pulse radiolysis study). J Am Chem Soc 108 375-378... 

Spanhel L, Haase M, Weller H, Henglein A (1987) Photochemistry of colloidal semiconductors. 20. Surface modification and stability of strong luminescing CdS particles. J Am Chem Soc109 5649-5655... 

Hotchandani S, Kamat P (1992) Charge-transfer processes in coupled semiconductor systems. Photochemistry and photoelectrochemistry of the colloidal cadmium sulfide-zinc oxide system. J Phys Chem 96 6834—6839... 

Aruchamy, A. (Ed.), Photochemistry and Photovoltaics of Layered Semiconductors, Kluwer Academic Publishers, Dordrecht, 1992. 

There are new ideas and experiments on the rTCA cycle. A group from Harvard University studied some reaction steps in the rTCA cycle which were kept going by mineral photochemistry. The authors assumed that solar UV radiation can excite electrons in minerals, and that this energy is sufficient to initiate the corresponding reaction steps. In this photocatalytic process, semiconductor particles were suspended in water in the presence of a zinc sulphide colloid (sphalerite) the experiments were carried out in a 500 mL reaction vessel at 288 K. Irradiation involved a UV immersion lamp (200-410 nm) in the photoreactor. Five reactions out of a total of 11 in the rTCA cycle were chosen to check the hypothesis ... 

It is possible that colloidal photochemistry will provide a new approach to prebiotic syntheses. The work described previously on redox reactions at colloidal ZnS semiconductor particles has been carried on successfully by S. T. Martin and co-workers, who studied reduction of CO2 to formate under UV irradiation in the aqueous phase. ZnS acts as a photocatalyst in the presence of a sulphur hole scavenger oxidation of formate to CO2 occurs in the absence of a hole scavenger. The quantum efficiency for the formate synthesis is 10% at pH 6.3 acetate and propionate were also formed. The authors assume that the primeval ocean contained semiconducting particles, at the surface of which photochemical syntheses could take place (Zhang et al 2007). 

C. Minero, V. Maurino, E. Pelizzetti, Mechanism of the photocatalytic transformation of organic compounds, in V. Ramamurthy, K.S. Schanze (eds.), Semiconductor Photochemistry and Photophysics, Vol 10 of Molecular and Supramolecular Photochemistry, Marcel Dekker, New York, 2003, pp. 211-229. 

Adapted from A. Mills and S. Le Hunte, An Overview of Semiconductor Photocatalysis , Journal of Photochemistry and Photobiology A, 108 (1997) Elsevier... 

Kamat PV, Vinodgopal K (1998) Environmental photochemistry with semiconductor nanoparticles. In Ramamurthy J, Schanze K (eds) Molecular and supramolecular photochemistry. Dekker, New York, pp 307-350... 

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