Photoanodic current, semiconductor electrodes

Ey is the photovoltage obtained for the derivatized n-type semiconductor photoanodes. We assume E° to be the values given in brackets and Ey is the extent to which the peak of the photoanodic current is more negative than E° under >Eg illumination. Data are from references given in (a). cWe assume E° to be the same on the n-type semiconductors as on metallic electrodes but these values have not been measured, since the n-type semiconductors generally are not reversible. 

This chapter considers photoanodes comprised of metal oxide semiconductors, which are of relatively low cost and relatively greater stability than their non-oxide counterparts. In 1972 Fujishima and Honda [1] first used a crystal wafer of n-type Ti02 (rutile) as a photoanode. A photoelectrochemical cell was constructed for the decomposition of water in which the Ti02 photoanode was connected with a Ft cathode through an external circuit. With illumination of the Ti02 current flowed from the Ft electrode to the... 

To compare quantitatively the current-voltage characteristic of an illuminated electrode, given by formula (31), with experimental data, Butler (1977) and Wilson (1977) measured the photocurrent, which arises in a cell with an n-type semiconductor photoanode ( 2, W03) when irradiated with monochromatic light at a frequency satisfying the condition ha>> Eg. In this case a light-stimulated electrochemical reaction of water oxidation with oxygen evolution... 

N-type semiconductors can be used as photoanodes in electrochemical cells Q., 2, 3), but photoanodic decomposition of the photoelectrode often competes with the desired anodic process (1 4 5). When photoanodic decomposition of the electrode does compete, the utility of the photoelectrochemical device is limited by the photoelectrode decomposition. In a number of instances redox additives, A, have proven to be photooxidized at n-type semiconductors with essentially 100% current efficiency (1, 2, 3, 6>, ], 8, 9). Research in this laboratory has shown that immobilization of A onto the photoanode surface may be an approach to stabilization of the photoanode when the desired chemistry is photooxidation of a solution species B, where oxidation of B is not able to directly compete with the anodic decomposition of the "naked" (non-derivatized) photoanode (10, 11, 12). Photoanodes derivatized with a redox reagent A can effect oxidation of solution species B according to the sequence represented by equations (1) - (3) (10-15). 

In a PEC, light is incident on the n-type semiconductor/electrolyte junction (photoanode), where light absorption occurs and an electron-hole pair is formed. The pair is separated by the strong electric field found just beneath the semiconductor surface, and the hole is driven towards the interface between semiconductor and electrolyte. Charge transfer to the redox species A contained in the electrolyte results in the oxidation to Conversely, the electron is driven to the metal/electrolyte interface (counter electrode), where the redox species is reduced. No net chenaical work is done and we can extract the energy as a current from the cell. 

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