Irradiated semiconductor surfaces reactivity

While the exploration of the implications of irradiated semiconductor surfaces for organic chemistry have only recently been attempted, there now exists a growing body of experiments illustrative of their power (283). A typical example of the contrasting oxidative reactivity observed on irradiated semiconductor surfaces can be seen in the different product distributions obtained by oxidation of 1,1-diphenylethylene photo-electrochemically on Ti02, electrochemically on Pt (an inert electrode material), and with thermal single electron transfer catalysts in homogeneous solution, eq. 89 (284) ... 

A final advantage offered by the irradiated semiconductor surface is illustrated by the contrasting reactivity shown in the photoelectrochemical and standard electrochemical oxidation of vicinal diacids, eq. 96 (302) ... 

Controlled Organic Redox Reactivity on Irradiated Semiconductor Surfaces... 

In principle, the arrangement of reactive intermediates generated by electron - hole pair capture by two redox couples on the semiconductor surface may allow for divergent reaction paths when the same reactive intermediates are generated on the irradiated surface and in an isotropic environment. If a particular reactive intermediate is quite stable, the overall chemistry observed may be... 

We see therefore that photoactive semiconductor particles provide ideal environments for control of interfacial electron transfer. Photoinduced electron-hole pairs formed on irradiated semiconductor suspensions, as in photoelectrochemical cells, allow for reactivity control not available in homogeneous solution. This altered activity derives from controlled adsorption on a chemically manipula-ble surface, controlled potential afforded by the valence band edge positions, controlled kinetics by virtue of band bending effects, and controlled current flow by judicious choice of incident light intensity. 

As a result of band bending, the surface of an irradiated -type semiconductor becomes electron-deficient and acts as a photoanode toward an oxidizable adsorbate. The conduction-band electron in an irradiated semiconductor is almost iso-energetic with the reduction potential for oxygen. In aerated solutions in which the semiconductor is saturated with adsorbed oxygen, the electron can often be trapped as superoxide, or when bound to the surface of the photocatalyst, it can trap other surface-confined reactive intermediates [26, 27]. 

The utility of visible- or UV-sensitive semiconductors as initiators for redox chemistry is limited by the instability of the surface under irradiation while in contact with an oxidizable substrate or an electrolyte. This decreased activity is caused by the chemical reactivity of the semiconductor itself, so that an insulating (blocking) layer is formed or the electrode quickly corrodes. This lack of stability is particularly troublesome with small-band-gap semiconductors that adsorb strongly in the visible region. 

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